Studies in Surface Science and Catalysis 116 CATALYSIS AND AUTOMOTIVE POLLUTION CONTROL IV
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Studies in Surface Science and Catalysis A d v i s o r y Editors: B. Delmon and J.T. Yates
Vol. 116
CATALYSIS AND AUTOMOTIVE POLLUTION CONTROL IV Proceedings ofthe Fourth International Symposium (CAPoC4), Brussels, Belgium, April 9-11, 1997
Editors N. Kruse, A. Frennet and J.-M. Bastin
Chimie Physique des Surfaces - Cata/yse H&t~rogbne, Universit~ Libre de Bruxe//es, Brussels, Belgium
1998 ELSEVIER A m s t e r d a m - - L a u s a n n e - - - N e w Y o r k - - Oxford - - S h a n n o n --- S i n g a p o r e - - Tokyo
ELSEVIER SCIENCE B.V. Sara Burgerhartstraat 25 ~.O. Box 211, 1000 AE Amsterdam, The Netherlands
Library of Congress Cataloging in Publication Data. A catalog record from the Library of Congress has been applied for.
ISBN 0-444-82795-1 © 1998 Elsevier Science B.V. 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 Science B.V., Copyright & Permissions Department, RO. Box 521, 1000 AM Amsterdam, The Netherlands. Special regulations for readers in the U.S.A.- This publication has been registered with the Copyright Clearance Center Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923. Information can be obtained from the CCC about conditions under which photocopies of parts of this publication may be made in the U.S.A. All other copyright questions, including photocopying outside of the U.S.A., should be referred to the copyright owner, Elsevier Science B.V., unless otherwise specified. No responsibility is assumed by the publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein. Transferred to digital printing 2005 Printed and bound by Antony Rowe Ltd, Eastbourne
CONTENTS PRELIMINARIES Foreword ...........................................................................................................................xi Introductory Remarks and Outlook ................................................................................ xiii Acknowledgments ............................................................................................................ xv Financial Support ............................................................................................................ xvi Organizing committee ................................................................................................... xvii Scientific advisory board .............................................................................................. xviii
GENERAL LECTURES
Global trends in motor vehicle pollution control. a 1997 update M.P. Walsh .........................................................................................................................3
Contribution offossil fuels and air pollutants emissions in Belgium since 1980. The role of traffic W. Hecq .............................................................................................................................. 5
Auto Emissions after 2000. The Challenge for the Catalyst Industry R.A. Searles ......................................................................................................................23
Diesel engine development routes towards very low emissions P.L. Herzog ....................................................................................................................... 35
THREE WAY CATALYSTS
Novel Pd-based three-way catalysts R. van Yperen, D. Lindner, L. Muflmann, E. S. Lox, T. Kreuzer .................................... 51
Comparative behaviour of standard Pt/Rh and of newly developed Pd-only and Pd/Rh three-way catalysts under dynamic operation of hybrid vehicles S. Tagliaferri, R.A. K6ppel, A. Baiker ............................................................................. 61
Comparative three-way behaviour of Pt, Pd and Rh single and combined phases in a full gas mixture with oscillating feedstream
J.R. Gonz~ilez-Velasco, T.A. Botas, R. Ferret, M.A. Guti6rrez-Ortiz .............................. 73
Effect of alkaline addition on hydrocarbon oxidation activities of palladium three-way catalyst H. Shinjoh, N. Isomura, H. Sobukawa, M. Sugiura ......................................................... 83
Ethanol oxidation on three-way automotive catalysts. Influence of Pt-Rh interaction A. Marques da Silva, G. Corro, P. Marecot, J. Barbier .................................................... 93
Reduction of NO by CO on manganese promoted palladium catalysts
J.F. Trillat, J. Massardier, B. Moraweck, H. Praliaud, A. Renouprez ............................ 103
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Light-offperformance over cobalt oxide- and ceria-promoted platinum and palladium catalysts M. Skoglundh, A. T6rncrona, P. Thorm~.hlen, E. Fridell, A. Drewsen, E. Jobson ........ 113 CATALYST AGEING- POISONING
Influence of catalyst deactivation on automotive emissions using different cold-start concepts
T. Kirchner, A. Donnerstag, A. K6nig, G. Eigenberger ................................................. 125
Measurement of the ceria surface area of a three-way commercial catalyst after laboratory and engine bench aging E. Rogemond, N. Essayem, R. Fr6ty, V. Perrichon, M. Primet, S. Salasc, M. Chevrier, C. Gauthier, F. Mathis .................................................................................................... 137
The effect of the ageing procedure upon the activity of a three way catalyst working under transient conditions
R. Roh6, V. Pitchon, G. Maire ....................................................................................... 147
Causes of deactivation and an effort to regenerate a commercial spent three-way catalyst T.N. Angelidis, M.M. Koutlemani, S.A. Sklavounos, Ch.B. Lioutas, A. Voulgaropoulos, V.G. Papadakis, H. Emons ............................................................ 155
Pb poisoning on Pd-only TWC catalysts S. Sung, R.M. Smaling, N.L. Brungard .......................................................................... 165
Effect of ageing on the redox behavior of Ce in three-way catalysts
S. Irusta, A. Boix, J. Vassallo, E. Mir6, J. Petunchi ....................................................... 175
The CeO2-Zr02 system. redox properties and structural relationships
G. Vlaic, R. Di Monte, P. Fornasiero, E. Fonda, J. Kaspar, M. Graziani ...................... 185
DENOx NOBLE CATALYSTS
Kinetics of the reduction of NO by C3H6 and C3H8 over Pt based catalysts under lean-burn conditions
R. Burch, T.C. Watling ...................................................................................................199
N20 and NO2formation during NO reduction on precious metal catalysts P. Bourges, S. Lunati, G. Mabilon .................................................................................213
Mechanistic investigation on the selective reduction of NO with propene in the presence of oxygen over supported platinum S. Eckhoff, D. Hesse, J.A.A. van den Tillaart, J. Leyrer, E.S. Lox ............................... 223
Platinum-titania-sepiolite monolithic catalysts for the reduction of nitric oxide with propene in lean-burn conditions P. Avila, J. Blanco, C. Knapp, M. Yates ........................................................................233
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DeNOx mechanism on platinum based catalysts V. Pitchon, A. Fritz, G. Maire ........................................................................................243
Electrochemical promotion in emission control catalysis: the role of Na for the Ptcatalysed reduction of NO by propene I.V. Yentekakis, A. Palermo, M.S. Tikhov, N.C. Filkin, R.M. Lambert ....................... 255
Promoting effect of zinc in DeNOx reaction over Pt/A1203 A. Bensaddik, N. Mouaddib, M. Krawczyk, V. Pitchon, F. Garin, G. Maire ................ 265
Catalytic properties of Palladium exchanged ZSM-5 catalysts in the reduction of nitrogen monoxide by methane in the presence of oxygen: nature of the active sites P. G61in, A. Goguet, C. Descorme, C. L6cuyer, M. Primet ........................................... 275
Influence of the platinum-support interaction on the direct reduction of NOx unde lean conditions F. Acke, B. Westerberg, L. Eriksson, S. Johansson, M. Skoglundh, E. Fridell, G. Smedler ............................................................................285
DENOx BASE CATALYSTS
A comparative study of the activity of different zeolitic materials in NOx reduction from simulated diesel exhausts M. Guyon, V. Le Chanu, P. Gilot, H. Kessler, G. Prado ............................................... 297
The effect of Al and Cu content on the performance of Cu/ZSM-5 catalysts at the exhaust of high efficiency spark ignition engines P. Ciambelli, P. Corbo, M. Gambino, F. Migliardini ..................................................... 307
Kinetic study of the selective catalytic reduction of nitric oxides with hydrocarbon in diesel exhausts B. Westerberg, B. Andersson, C. Ktinkel, I. Odenbrand ................................................ 317
Steady state and transient activity patterns of Cu/ZSM-5 catalysts for the selective reduction of nitrogen oxides J. Connerton, R. W. Joyner ............................................................................................327
Selective reduction of nitrogen oxide with hydrocarbons and hydrothermal ageing of Cu/ZSM-5 catalysts P. Denton, Z. Chajar, N. Bainier-Davias, M. Chevrier, C. Gauthier, H. Praliaud, M. Primet ........................................................................................................................335
Transient kinetic study of NO decomposition on Cu-ZSM-5 catalysts Z. Schay, I. Kiricsi, L. Guczi .........................................................................................347
Stability of cerium exchanged zeolite catalysts for the selective catalytic reduction of NOx in simulated diesel exhaust gas W.E.J. van Kooten, H.P.A. Calis, C.M. van den Bleek ................................................. 357
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Study on copper and iron containing ZSM-5 zeolite catalysts: ESR spectra and initial transformation of NO J. Varga, J. Halfisz, D. Horvfith, D. M6hn, J.B. Nagy, G. Sch6bel, I. Kiricsi ................ 367 KINETICS- MECHANISMS
The use of isotope transient kinetics within commercial catalyst development J.C. Frost, D.S. Lafyatis, R.R. Rajaram, A.P.Walker .................................................... 379
Kinetic study of the ethene oxidation by oxygen in the presence of carbon dioxide and steam over Pt /Rh /Ce02 /y-Al203 R.H. Nibbelke, R.J.M. Kreijveld, J.H.B.J. Hoebink, G.B. Marin .................................. 389
Three-way catalytic converter modelling. numerical determination of kinetic data C. Dubien, D. Schweich ................................................................................................. 399
NO + CO -+1/2 N: +CO: differentiated from 2NO + CO -+ N20 + CO: over rhodia/ceria catalysts using 15N180 and ISC160 reactants or time-resolution of products J. Cunningham, N.J. Hickey, F. Farrell, M. Bowker, C. Weeks .................................... 409
Investigation on the role of rhodium on the kinetics of the oxidation of CO by NO over Pt-Rh catalysts P. Granger, J.J. Lecomte, C. Dathy, L. Leclercq, G. Mabilon, M. Prigent, G. Leclercq .............................................................................. 419 MODEL SYSTEMS ] STUDIES
CO oxydation on Pd (11 O) M. Bowker, I.Z. Jones, R.A. Bennett, S. Poulston ......................................................... 431
In- situ ESR of Rh/y-Al20s and Rh/ZSM-5 S.G. Lakeev, A.V. Kucherov, M. Shelef ........................................................................ 441 MISCELLANEOUS
Substrate contributions to automotive catalytic converter performance: the role of channel shape on catalyst efficiency J. Paul Day ...................................................................................................................... 453
Evaluation and characterization of catalysts for alternative-fuelled vehicles. A study of the influence of catalyst composition on activity and by-product formation L.J. Pettersson, A.M. Wahlberg, S.G. J~irfis ................................................................... 465
SHS catalysts for purification of exhaust gases from internal combustion engines E.H. Grigoryan, I.P. Borovinskaya, A. G. Merzhanov .................................................. 477
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Catalytic decomposition of high-concentration nitrous oxide N20 H.C. Zeng, M. Qian, X.Y. Pang ..................................................................................... 485
Structure an activity of Cu/Cr/Sn02 environmental control catalysts P.G. Harrison, W. Azelee, A.T. Mubarak, C. Bailey, W. Daniell, N.C. Lloyd ............. 495
Preparation and study of thermally stable washcoat aluminas for automotive catalysts Z.R. Ismagilov, R.A. Shkrabina, N.A. Koryabkina, D.A. Arendarskii, N.V. Shikina...507
Ensuring substrate retention. Part 2 4 J.Kisenyi, K. Soe, P. Leason, C. Tooby, D. Pritchett, G. Morgan, M. Zillikens ........... 513 STORAGE: NOx AND OXYGEN
A catalytic NOx management system for lean burn engines J. Feeley, M. Deeba, R.J. Farrauto ................................................................................. 529
Investigations of NOx storage catalysts E. Fridell, M. Skoglundh, S. Johansson, B. Westerberg, A. T6rncrona, G. Smedler .... 537
Oxygen storage capacity of three-way catalysts : a global test for catalyst deactivation R. Taha, D. Duprez, N. Mouaddib-Moral, C. Gauthier .................................................. 549
NO Reduction by CO over Pd /CeO:-Zr02-Al:Os Catalysts R. Di Monte, P. Fornasiero, J. Kaspar, A. Ferrero, G. Gubitosa, M. Graziani .............. 559
Comparative sulfur storage on Pt catalysts: effect of the support (CeOz Zr02 and CeO2-Zr02) P. Bazin, O. Saur, J.C. Lavalley, A.M. Le Govic, G. Blanchard ................................... 571
Oxygen storage capacity in perovskite-related oxides. the role of over-stoichiometric oxygen in three-way catalysis N. Guilhaume, M. Primet ............................................................................................... 581
Influence of ceria dispersion on the catalytic performance of Cu/(CeO2)/Al203 catalysts for the CO oxidation reaction A. Matinez-Arias, J. Soria, R. Catalufia, J.C. Conesa, V. Cort6s Corberfin ................... 591
Some surface chemical features of Pt catalysts supported on Al20s and Ce02 /Al20s G. Magnacca, G. Cerrato, C. Morterra ........................................................................... 601
Fundamental Properties of new cerium- based mixed oxide as TWC component S. Bernal, G. Blanco, M.A Cauqui, P. Cochardo, M. Pintado, J.M. Rodriguez-Izquierdo, H. Vidal ............................................................................... 611
DIESEL
Improved soot oxidation by fuel additives and molten salt catalysts S.J. Jelles, J.P.A. Neefi, B.A.A.L. van Setten,, M. Makkee, J.A. Moulijn ................... 621
Investigation of copper-cerium oxide catalysts in the combustion of diesel soot D. Courcot, E. Abi-Aad, S. Capelle, A. Abouka'fs ......................................................... 625
Catalytic ceramic filter for diesel soot removal: preliminary investigations P. Ciambelli, V. Palma, P. Russo, S. Vaccaro ................................................................ 635
Catalytic oxidation of model soot by chlorine based catalysts G. Mul, J.P.A. Neefl, M. Makkee, F. Kapteijn, J. A. Moulijn ....................................... 645
Copper catalysis for particulate removal from diesel exhaust gas. Copper fuel additives in combination with copper coatings J.P.A. Neefl, S.J. Jelles, M. Makkee, J. A. Moulijn ....................................................... 655
Supported liquid phase catalysts : a new approach for catalytic oxidation in diesel exhaust particulate emission control S.J. Jelles, B.A.A.L. van Setten, M. Makkee, J. A. Moulijn .......................................... 667 AUTHOR INDEX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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LIST OF PARTICIPANTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 681
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FOREWORD In spite of the energy crises and the recession, an explosive growth of the world vehicle population from 50 to 700 million vehicles in 50 years was observed, as analyzed by Michel Walsh in his introductory lecture. On the other hand, in spite of the poor yield of internal combustion engines associated to the inevitable production of some gazeous pollutants, economical reasons essentially have made their use to continue and probably for still an important number of years. The resulting increase of gazeous pollutants in our atmosphere coming from exhaust gas of automobile vehicles has enhanced the problem of the elimination of these pollutants produced by internal combustion engines. This is why there has been continuing interest in the organization of meetings on the depollution problem. Catalysis was considered and has been proven to be the best solution to lower the content of exhaust ges in pollutants The use of catalytic processes started to be studied in the United states already in the early seventies. That research was mostly conducted by the two giant american auto industries: General Motors and Ford. During many years no need for international scientific exchange on the problem was considered. In the eighties, the european countries started to show some interest to that problem. It is only in june 1984 that the EC Commission proposed standarts of permissible pollutants in the exhaust gas from motor vehicles to be introduced in Europe ; these standarts were approved by the Ministers of the Environment one year later. Very quickly, a number of Academic research laboratories started working on the subject, and namely on the development of new catalysts. We thought that a need for exchange of results and of ideas had appeared and I have initiated the organization of international meetings on this topics at the University of Brussels under the title "Catalysis and Automotive Pollution Control" associated with the acronym CAPoCFour meetings have been organized in Brussels in 1986 (CAPoC1), in 1990 (CAPoC2), in 1994 (CAPoC3) and in 1997 (CAPoC4). The proceedings were published as an issue of the series " Studies in Surface Science and Catalysis" published by Elsevier, respectively as vol. 30 for CAPoC1, vol. 71 for CAPoC2 and vol. 96 for CAPoC3. The present volume contains the proceedings of the last of these meetings, CAPoC4, that took place at the University of Brussels on april 9-11, 1997. I have been the organizer and the chairman of the three first of these meetings and the Honorary President of this fourth one. I would like to take the opportunity to thank some persons for their special contribution in the organization of these meetings and the publishing of their proceedings. First of all, my colleague Andre Crucq who joined me to start these meetings and who took in charge the heavy and important role of secretary of CAPoC 1 and CAPoC2. This function has succesfully been taken over by Jean-Marie Bastin for CAPoC3 and CAPoC4.
xii It is also a pleasure to stress the continuing interest and the enthousiasm of some colleagues who were active members of the organizing committee of all the four meetings namely: Andr6 Pentenero, Michel Prigent, Ginette and Lucien Leclercq, Walter Hecq and Georges Poncelet. Finally, I am glad that after my retirement in 96 from my position heading the catalysis group at the ULB, my successor, Prof. N. Kruse, accepted to continue the organization of this series of meetings. I also thank him to have accepted to be the chairman of this fourth meeting. From the startpoint, these meetings had an important succes, in spite of the otherwise very restricted topics. These last years, the problem of pollution by the emission from the engines of automobile vehicles has been examined in one of the sessions of several more general meetings devoted to transportation problems or to fuels production. The continuing succes of the CAPoC's meetings that comes out the following table puts well in evidence the still large importance of the topic. CONGRES CAPoC1 CAPoC2 CAPoC3 CAPoC4
EUR
USA
127 197 223 212
20 23 18 10
PARTICIPANTS Other Total Industry 30 177 106 40 260 160 38 279 133 11 232 97
Acad. Labs. 71 100 119 135
PAPERS Submitted Accepted
38 66 131 88
28 42 79 68
It appears that most of the participants come from Europe. The total number of participants is rather constant from CAPoC2 to CAPoC4. On the other hand the number of participants from industry is progressively decreasing whereas that from academic laboratories increases.
A. FRENNET Honorary President CAPoC4
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INTRODUCTORY
REMARKS
AND OUTLOOK
The fourth congress on catalysis and automotive pollution control (CAPoC4) was held in Brussels from 9-11 April 1997. Following the habit of its predecessors, this congress started with a number of keynote lectures both surveying the field on the whole and covering aspects ranging from vehicle/catalyst technology to legislative regulations. M. Walsh, in his presentation, elaborated on the continuing growth of global vehicle population with the highest rates being found in developing countries. Accepting that pollution knows frontiers, a clear need has been demonstrated for a worldwide move to pollution control. W. Hecq reviewed the EU emission regulations from 1970 up to now and examined the impact that they had on the emissions of the main pollutants from road vehicles. Based on measurements of pollutant concentrations in Belgium and, more specifically, in urban areas like Brussels, it became clear that certain improvements on a per-car-basis are destroyed by a general growth of the car fleet, especially diesel cars. Given the EU proposals for 2000 and 2005 emission standards of gasoline/diesel fuelled vehicles, R. A. Searles reviewed the state-of-the-art aftertreatment technology for the control of emissions. He also emphasized that in order to meet the 2005 standards, further technological improvements are necessary in catalyst performance, trapping and adsorption along with an optimization of engine managements and control systems. Based on the fact of increasing proportions of diesel fuelled engines and respective problems in achieving legislative standards, P. L. Herzog reviewed the main parameters influencing the emissions of NOx and particulates. Even in consideration of remarkable improvements in engine and combustion technology as well as in electronic control, P. L. Herzog sees the development of highly effective exhaust gas aftertreatment systems playing a key role in future development routes. There is no doubt to me that the four keynote lectures enjoyed great esteem and gave the prelude to a number of most interesting communications on various subjects in the field. The large number of accepted papers (68) made it necessary to shift some of them into a poster session. As a rule, poster and oral contributions were equally assessed and no discrimination was made in the proceedings. As its predecessors, CAPoC4 proved to be a most suitable platform for discussing technological improvements and developments along with future perspectives and challenges. In the light of new results and further legislative regulations, the following topics were intensely discussed: • low light-off behaviour based on improved catalysts and substrate formulations • efficient adsorber systems for storage of hydrocarbon emissions • electrically heated catalyst systems ahead the main catalyst or, alternatively, close coupled catalysts (at the manifold of the engine)
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lean DeNOx catalysts allowing for decomposition of NO× in the oxygen-rich exhaust of direct injection gasoline engines and high speed injection diesel engines or, alternatively, NOx trapping/reduction in a hybrid approach collection and destruction of dry particulates or soot. During the conference a poll was made on the structure of the congress. Although the tenor was to keep the general format (3 day meeting, every 2-3 year's), opinion was expressed to introduce short oral communications of 10 minutes duration (plus 5 minutes for discussion). The organizing committee will take care of this point and make respective arrangements for CAPoC5. Stimulating suggestions were made on future topics. Accordingly, all participants seemed to agree that the search for new catalyst materials is of high priority in view of tighter legislative regulations. More attention should also be given to questions related to catalyst or trap deactivation due to the presence of compounds containing sulphur. The need for more research on the recovery of noble metals and the development of sensors was likewise recognised. Last but not least, participants from industry requested the production of more engine data, performance of real test cycles and development of integrated systems. There is no doubt that clean vehicle technology is a vital part of improving air quality. Challenges remain and call for technological answers. The job is not done! Catalytic air pollution control is still an area providing a considerable incentive for innovative work. It would be a pleasure for the organizers if the outcome of this research would be part of CAPoC5 subjects.
N. Kruse Chairman CAPoC4
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ACKNOWLEDGEMENTS
The organizers thank the Rector of the Free University of Brussels, Mr. J. L. Vanherweghem, for his interest in the meeting and the words of welcome that he addressed to the participants of CAPoC4. We are indebted to the members of both committees for their important work. The success of a congress like CAPoC4 which covered so many fields round about catalyst technology and related issues, depends on the knowledge and advise of experts. It was our privilege that a number of the most distinguished experts accepted our invitation to participate in the scientific organization of the congress and/or the selection of submitted papers. We like to thank the four keynote lecturers, W. Hecq, P. L. Herzog, R. A. Searles, M. Walsh, for their excellent presentations in the introductory session. Special thanks are due to all coworkers, members and friends of the Chair of Inorganic Chemistry at our University. Their helpfulness and motivation have largely contributed to run the congress as smooth as possible and let CAPoC4 become a most successful event. Of course, it is difficult to render prominent the particular credits of a single person of "the team". Nevertheless, we would like to address our gratitude to Mrs. Parmentier- Depuydt for taking care of whatever you was approaching. Last but not least, the organizers recognize that CAPoC4 has succeeded in attracting and gathering experts from all over the world. A number of high quality contributions were made initiating most vivid discussions either in the lecture-hall or during poster sessions. Thanks to all participants for having contributed to a most successful CAPoC4 meeting.
The organizers, J-M. Bastin A.Frennet N. Kruse
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F I N A N C I A L SUPPORT
The following companies have provided financial support to this Congress. The Organizers express their gratitude to these companies for their generosity.
AlliedSignal Inc. Automobile Emissions Control by Catalyst (AECC) Degussa A G Engelhard Co Ford Motor Co Johnson Matthey Ltd Rh6ne - Poulenc Terres Rares et Gallium Shell
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ORGANIZING COMMITTEE Executive Chairman:
KRUSE N. Universit6 Libre de Bruxelles, B. Honorary President :
FRENNET A. Universit6 Libre de Bruxelles, B. Secretary :
BAST1N J-M. Universit6 Libre de Bruxelles, B. Members :
BELOT G. PSA Peugeot Citro6n, F. CUCCHI C. ACEA, B. HECQ W. Universit6 Libre de Bruxelles, B. JANNES G. Institut Meurice, B. LECLERCQ L. Universit6 de Lille 1, F. LEMAIRE J. Rhone Poulenc, F. MAIRE G. Universit6 de Strasbourg, F. MONTIERTH M. Coming Keramik, D. NIEUWENHUYSB. Rijksuniversiteit Leiden, N1. PENTENERO A. Universit6 de Nancy, F. PONCELET G. Universit6 Catholique de Louvain, B. PRIGENT M. Institut Frangais du P6trole, F. SEARLES D. AECC, B. WEBSTER D. Johnson Matthey LTD, GB.
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SCIENTIFIC ADVISORY BOARD :
All members of the organizing committee, and BAIKER A. Swiss Federal Institute of Technology, CH. BURCH R. University of Reading, GB CAMPINNE M. Ecole Royale Militaire, B. FARRAUTO R.J. Engelhard Corporation, USA IWAMOTO M. Hokkaido University, J. KONIG A. Volkswagen AG, D. LEDUC B. Universit6 Libre de Bruxelles, B. LOX E. Degussa AG, D. PALMER F.H.C.E.C., B. ROBOTA H.J. AlliedSignal, USA SHELEF M. Ford Motor Co., USA SCHWEICH D. CNRS - CPE, F. VAN DEN BRINK P.J. Shell, N1.
General Lectures
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CATALYSISAND AUTOMOTIVEPOLLUTIONCONTROLIV Studies in Surface Science and Catalysis,Vol. 116 N. Kruse, A. Frennet and J.-M Bastin (Eds.) 91998Elsevier Science B.V. All rights reserved.
Global trends in motor vehicle pollution control : a 1997 update M.P. Walsh
3105 N, Dinwiddie street, Arlington, Virginia 22207 USA
ABSTRACT Four trends continue to drive the global market for motor vehicle pollution control equipment 9 9 The continued growth in the world's population 9 The rising affluence of many rapidly industrializing developing countries, increasing the affordability of motor vehicles 9 The increasing number of health studies showing adverse effects at lower and lower levels of pollution 9 The response of governments by adopting more and tighter emissions standards for new vehicles or other incentives to stimulate the introduction of pollution controls on vehicles. As we approach the 21 ~t century, the global vehicle population exceeds 700 millionalmost 500 million light duty vehicles, about 150 million commercial trucks and buses and another 100 million motorcycles. Each year, the vehicle population is growing by about 12 million automobiles, 3.7 million commercial vehicles and 2.5 million motorcycles per year. While the growth rate has slowed in the highly industrialized countries, population growth and increased urbanization and industrialization are accelerating the use of motor vehicles elsewhere. One result is that air pollution is an increasingly common phenomena necessitating aggressive motor vehicle pollution control efforts. The purpose of this report is to survey what is presently known about transportation related air pollution problems, to summarize the adverse impacts which result, to review actions underway or planned to address these problems, and to estimate future trends. Based on these trends, this study will assess the large and growing vehicle pollution control market, expeeially with regard to exhaust after treatment systems.
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CATALYSIS AND AUTOMOTIVE POLLUTION CONTROL IV Studies in Surface Science and Catalysis, Vol. 116 N. Kruse, A. Frennet and J.-M Bastin (Eds.) 91998 Elsevier Science B.V. All rights reserved.
Contribution of fossil fuels and air pollutants emissions in Belgium since 1980 The role of traffic* W. Hecq CEESE-ULB, Avenue Jeanne, 44 1050 Brussels, Belgium For my first presentation made here in 1991 (HECQ, W., 1991), I began by recalling the role of the Rome Club Report, United Nations Conference in Stockholm, oil shocks which had revealed the dangers threatening western economies, i.e.: pollution, natural resources depletion and a strong economic dependence on imported oil. Six years later, Earth Summit + 5 Session will start in just a few weeks but the above mentioned essential problems identified six years ago stay the same. Pollution and resources depletion belong to today's concerns and dependency towards fuel imports lies on numerous uncertainties. Only wording has changed : it is a case of "sustainability" and more specifically, within the frame of this congress, an issue of "sustainable mobility". However, if the issues have remained the same throughout the years, many things have been achieved in favour of the environment. Parallel knowledge of environmental systems has improved and new issues came into sight. I shall begin this presentation by reminding you of some of the major air pollution issues in which traffic plays a major role. The second part of this presentation will give an overview of the EU legislations enforced until now and which obviously concern traffic. The third part will show and discuss results obtained in terms of emissions from vehicles in Belgium. The fourth part will consider air pollution as it is encountered in a European city in comparison with Brussels. The fifth and last part will conclude with a few suggestions. 1. M A J O R A I R P O L L U T I O N A S P E C T S I N V O L V I N G T R A F F I C Essentially, these aspects are energy related and if we refer to a geographical scale classification, three forms of pollution are a cause of concern : 9 local and urban levels; 9 local up to continental level; 9 global level.
* Walter J. HECQ
Centre for Economic and Social Studies on the EnvironmentUniversit6 Libre de Bruxelles
Local and urban levels
At local and urban levels, it is essentially a question of vicinity pollution. Several air pollutants are concerned : sulphur dioxide, SO2 (primary pollutant); nitrogen oxides NOx (primary or secondary pollutants); particulate matter PM (primary and secondary pollutants); carbon monoxide, CO, (primary pollutant); (volatile) organic compounds, HC (or VOCs) (primary and secondary pollutants), and photochemical oxidants, 03, PAN (secondary pollutants). Organic compounds account for a wide range of hydrocarbons and are found in solid, liquid, and gas forms. Effects of these pollutants are investigated today all over the world. Some research findings can be recalled. Firstly, for some of these pollutants, the WHO guidelines, when they exist, are exceeded episodically and more specifically in the urban areas. Secondly, above certain thresholds, significant health effects are observed. In fact, pollutants such as PM10, PM2.5, NO2, 03, can deeply penetrate the respiratory tracks with the consequence of a rise in medical consultations, of hospital admission, bronchitis amongst children younger than 14.... In fact, more and more people living in cities are breathing more CO, NO2 than what is good for them. Other effects such as the deterioration of buildings and monuments which increases either the repairing rate of material damaged with acid gases or the cleaning of facades soiled with sooty particles. Lastly, damage associated with this pollution is a cause of social costs which are born by the community and which are not taken into account in the benefit provided by a car trip. These costs, described as "external", are not included in the transportation prices, a factor which causes imbalances and a non-optimal distribution of resources. Their extent is undisputed but, in many cases, has still to be quantified. A number of initiatives are looking into this. As shown in studies carried out in my Centre (Bres, 1995; HECQ, W. and ALPI, I. 1995), this environmental damage, including on bronchial diseases, costs hundreds of $ million per year and this for the Brussels-Capital region only. An amount in which traffic plays a significant role. As indicated in other studies (Infras, 1995; Ecoplan, 1996; UNIPEDE, 1996), these costs are typical for European cities and concern especially public health and materials. They do not include other impacts like olfactory discomfort from VOCs, long-term effects like risk of mutagen and carcinogen diseases. Local up to continental level
At local up to continental level, pollution takes the form of acid deposition and photoxidant phenomena. On a large scale over Europe, critical loads are exceeded. Degradation of crops, forests, materials, terrestrial ecosystems ..... is produced here too. Damage from these forms of pollution is also extensively studied nowadays and assessed in physical (RENTZ, O., 1993) or monetary terms (ExternE, 1996). Global level
At a global level, we also have to take into account the global warming and ozone layer depletion. Here too, damage estimates as sea level rise, climate change .... are assessed. For the greenhouse effect, estimates range between 120 and 250 ECU/toe (Holland, M., 1996). Last, but not least, we still have to take into account resources scarcity :fossil fuels for which oil reserves, the cheapest ones and those of best quality, are concentrated in politically unstable areas of the planet and do not remain superseding today for the great majority of the vehicle fleet.
2. R E V I E W O F EU V E H I C L E E M I S S I O N R E G U L A T I O N S Obviously, decision makers did not remain unconcerned, especially considering traffic emissions. The first regulation came into force by September 1971, a few months before the first earth summit : the Stockholm conference. It was the starting point of a long sequence of amendment steps towards more and more stringent emission limit values, also associated with numerous technical standards (monitoring procedures, test cycle profiles, vehicle fleet typology,
...). Emission regulations for Europe were first introduced in order to assume a uniformity of technical prerequisite amongst car producers. This initiative belongs to the United Nations Economic Commission for Europe (UN-ECE). The European Commission found this to be a good opportunity to adopt a first vehicle standard with the Directive of 20 March 1970.
European motor emission standards for light vehicles As far as light vehicles are concerned, table 1 gives an idea on the evolution of standards for light duty vehicles.
Directive 70/220/EEC Adopted on 20 March 1970, it is the first directive concerning the reconciliation of Member State legislations relative to measures to be taken against air pollution by exhaust gases from vehicles with starting engines. It defines the relative prescriptions for the conformity of vehicles and fixes standards for 9 classes of vehicles, from less than 750 k~ to more than 2,150 kg. Only CO and HC are regulated. The standards are expressed in g/test.
Directive 74/290/EEC Adopted on 28 May 1974, it is the first amendment of the regulation of the EEC. It also only concerns CO and HC emissions. It lowers CO and HC emissions in respect to the base level.
Directive 7 7 / 1 0 2 / E E C Adopted on 30 November 1976, it fixes, for the first time, a limit value for NOx emissions, which, just like CO and HC, have a great influence on our health and on the environment. To simplify matters, nitrogen oxides are expressed in NO2 equivalent.
Directive 78/665/EEC Adopted on 14 July 1978. It is the third amendment of the first directive and its emission standards for CO, HC, NOx are more severe.
* 3 main procedures for the approval : Type I test cycle (HC and CO), before the test period, the vehicle is soaked for 6 hours at a temperature of between 20 and 30 C~ Type II test, CO determination test at low speed after fourth cycle type I; Type III test, crankcase emission procedure on chassis dynanometer.
Table 1 European 9 motor emissions standards ,for vehicles
VEHICLE TYPE
DIRECTIVE
DESCRIPTION
Light duty vehicles
70/220/EEC
Light duty vehicles
74i290/EEc (first amendment) 77/102/EEC (amending 70/220/EEC) 78/665/EEC (third amendment) 83/351/EEC (fourth amendment)
Base directive setting emission limits for c o and HC More stringent emission limits for CO and HC Introducing limits for NOx
Light duty vehicles Light duty vehicles Light duty vehicles
Light duty vehicles
88/76/EEC (fifth amendment)
Cars with engine capacity of less than 1.4 litres
89/458/EEC (amending 70/220/EEC)
Light duty vehicles
91/44i/EEC (amending 70/220/EEC)
Light duty vehicles
94/12/EC (amending 70/220/EEC)
More stringent emission limits for CO~ HC, and NOx Introducing new methods 0f HC and NOx measurements Emission limits for diesel engines More stringent emissions for co, HC and NOx Introducing particulate emission limits for diesel. Three vehicles types in function of cubic capacity Tightening iimit values for gaseous emissions set by 70/220/EEC (as amended) Consolidating Directive applying Stage 1 limits (tightening the limits imposed by Directive 70/220/EEC and its amending Directives) AND introducing requirements for evaporative emissions and durability of emissionrelated components Applying more stringent Stage 2 limits for hydrocarbons, carbon monoxide and nitrogen oxides, with separate limits for petrol and diesel cars and limits for particulates 9 from 1.1.96 for new models 9 from 1.1.97 for vehicles entering into service
Directive 8 3 / 3 5 1 / E E C Adopted on 16 June 1983. Up until then, directives concerning CO, HC and NOx emissions were only valid for gasoline fuelled vehicles. However, given the extent of the development of diesel vehicles, the EEC decided to submit them to the same standards as those of gasoline fuelled cars. So, up until now, emissions of CO, HC and NOx, from diesel vehicles are regulated. On the other hand, in the directives that follow, emissions of NOx are no longer regulated as such, but in combination with unburned hydrocarbons. This manner to regulate these two pollutants gives, to the manufacturers, the choice to reduce either NOx or HC. Directive 8 8 / 7 6 / E E C Directive 88/76/EEC, also called "Agreement of Luxembourg", fixes standards that are even more severe for gasoline and diesel fuelled vehicles of up to 3.5 t. This directive distinguishes two types of standards : type approval standard and conformity of production standard. Application dates for these standards vary according to three engine capacity categories : 9 vehicles with an engine capacity of below 1400 cm3; 9 vehicles with an engine capacity of between 1400 and 2000 cm3; 9 vehicles with an engine capacity higher than 2000 cm 3. What's more, these standards are as much applicable for gasoline fuelled cars as they are for diesel engined cars with a certain modulation and they take into account particulate emission. However, concerning emissions from vehicles with an engine capacity lower than 1400 cm 3, the decision was only taken on 18 July 1989 and brought into practice in Directive 89/458/EEC. Directive 9 1 / 4 4 1 / E E C This directive called the "Consolidated Emission Directive" was adopted on 26 June 1991. It replaces Directives 88/76/EEC and 89/458/EEC. Vehicle emissions are no longer measured with the same ECE 15 test cycle. In fact, the new test cycle combines the existing urban test cycle (ECE 15) with a test cycle (new ECE 83) simulating driving conditions outside urban areas (EUDC). It concerns a reinforcement to the extent that NOx emissions increase rapidly at high speed. These limit values with the new test cycle, make it very difficult for a gasoline fuelled car to satisfy the directive without requiring three way catalysts. Moreover, the directive anticipates a supplementary test in order to guarantee the durability of anti-pollution systems. Vehicles will have to take the test after 80,000 km and will have to comply to the same standards as those applicable to new cars. At last, limits for vehicles evaporative emissions are also given. Directive 9 4 / 1 2 / E E C In December 1993, more stringent limits from 1996 are programmed (stage 2). They are adopted in Directive 94/12/EC. With these new standards, it must be noted that production conformity must comply with the type approval limit. To summarise, thanks to this sequence of more and more stringent regulations, emission for new vehicles could be reduced by more than 95% between 1970 and now (Figure 1).
10
Figure 1: Evolution of gasoline car emission standards in E.U. Emission standards for diesel engines and other vehicles At the beginning, European regulations concerning diesel engine emissions were only effective for three 'classic' pollutants for light duty vehicles (Directive 70/220/EEC modified by Directive 83/351/EEC) and on black smoke emissions (Directive 72/306/EEC). This black smoke represents a potential danger for health. It is thus better to limit total emissions of particulates from these engines. The new limit values for particulates were reformulated according to three categories of vehicles: light duty, light commercial and heavy duty. The limit values reformulated for particulates for light duty vehicles are defined in Directive 88/76/EEC and those that follow (table 1) for the other vehicles. As far as light commercial vehicles are concerned, Directive 88/436/EEC, modifying Directive 70/220/EEC is published and specifically concerns emissions for diesel vehicles except small engines. This one is extended to the three other gaseous pollutants by Directive 93/59/EEC and Directive 91/441/EEC (for M<1250 kg). This directive dated 28 June 1993, distinguishes three categories of vehicles according to their mass. New reductions are forecasted in Directive 96/69/EC adopted on 8 July 1996.
11 Table 2: Key European motor emissions from vehicles (light commercial and heavy duty). Diesel motor Diesel diesel
engines for use in vehicles cars except for small engine vehicles
Diesel engines for use in motor vehicles (excluding passenger cars)
72/306/EEC 88/436/EEC
91/542/EEC (Amending 88/77/EEC)
Light commercial vehicles, passenger vehicles for more than 6 passengers or with mass over 2,500 kg
93/59/EEC (Amending 70/220/EEC) (revised by 96/69/EC - see below)
Light commercial vehicles
96/69iEC (Adopted 8.7.96)
Heavy duty diesel engines for use in motor vehicles Heavy diesel engines for use in motor vehicles
88/77/EEC Directive 96/1/EC (amending 88/77/EEC)
Introducing emission limits for black smoke Introducing a first phase of emission limits for particulates from diesel engines Two-stage introduction of limit values, including limits for particulates, as follows : EURO-1 9 from 1.7.92 for new models 9 from 1.10.93 for vehicles entering into service EURO-2 9 from 1.10.95 for new models 9 from 1.10.96 for vehicles entering into service Applying emission limits to 3 classes according to reference mass : Class I : up to i250 kg Class I1: 1250 kg - 1700 kg Class II1: over 1700 kg Stage 1 emission limits are applied to Class I vehicles and less stringent requirements to Class II and III vehicles : 9 from 1.10.93 for new models 9 from 1.10.94 vehicles enterin~ into service Applying the Stage 2 limits for passenger vehicles 9 from 1.1.96 for new model Class I vehicles 9 from 1.1.98 for new model Class !1 and III vehicles Introducing limits for CO, HC and NOX Introducing a new statistical procedure for the evaluation of conformity of production assessment AND a derogation for smaller engines from the particulate values
12
Concerning heavy duty vehicles, table 2 takes into account Directive 91/542/EEC in two steps, aiming at new types of heavy vehicles (diesel and others weighing more than 3.5t) and replaces Directive 88/77/EEC for gaseous emissions and Directive 72/306/EEC for black smoke from diesel vehicles. This directive is the result of a revaluation following the Council of Ministers on 1 October 1991. New implementation dates, as well as new standards were established. Directive 91/542/EEC is known under the term "clean lorry" directive.
Other regulations Accompanying these limit values of emissions, other regulations must be recalled : 9 fuel quality regulation: lead contents of gasoline, aromatic, such as benzene, oxygenates, sulphur contents of diesel .... ; 9 standards on evaporative emission, CO2 and fuel consumption, fuelling emissions; 9 air quality regulations specifying concentration limit values in the ambient air for SO2, NOx, particulates, 03; 9 and next to these "command and control" regulations : economic instruments like motor fuel tax (VAT, excise) and vehicle taxation (on sales, initial registration .... ) without forgetting differential tax between leaded and unleaded gasoline ..... etc.
3. HISTORY OF EMISSIONS V E H I C L E S IN B E L G I U M
OF SEVEN POLLUTANTS
FROM ROAD
The progressive implementation of all these regulations had, between 1980 and 1995, positive effects on pollutant emissions from traffic. From the results assessed in my Centre* and from the last available emission function (COST 319 Workshop, 1996), it has resulted in significant cutting in emissions of CO, NOx and VOC from gasoline powered vehicles. For CO emissions (figure 2), where road traffic is the main contributor and which is involved in local and urban pollution, annual emissions from gasoline vehicles fell by 45.0% (from 548 kt to 301 kt) between 1980 and 1995 whereas the contribution of diesel vehicles rose by 50.6% (from 47 kt to 71 kt) during the same period. Fortunately, as diesel vehicle contribution is lower than that of gasoline powered vehicles, the overall effect is largely positive.
* Without taking into account cold start over emissions.
Figure 2: CO emissions from fossil fuel combustion in Belgium.
13
14
Figure 3: NOx emissions from fossil fuel combustion in Belgium.
Figure 4: VOC emissions from fossil fuel combustion in Belgium.
15
16 As far as NOx and VOC are concerned (figures 3 and 4), which act as "acid" or ozone precursor pollutants, NOx dropped between 1980 and 1995 by 3.0% (from 113.2 kt to 109.8 kt) and VOC rose by 2.5% (from 65.8 kt to 67.5 kt). Meanwhile, emissions from diesel vehicles rose respectively by 40.4% for NOx (from 62.1 kt to 87.2 kt) and 50.9% for VOC (from 10.7 kt to 16.2 kt). For NOx and VOC, the same observation can be made as for CO emissions in the sense that the weight of gasoline vehicles in the overall emission is higher than for diesel vehicles. If some of these results are encouraging for gasoline and diesel road vehicles, related emissions still represented a relatively large contribution amongst the overall exhaust emissions from fuel consumption by 1995 it9 accounts for 74.0% for CO; 48.0% for NOx and 77.5% for VOC respectively. As for the other pollutants and with the exception of N20 (figure 5), the role of gasoline powered vehicles is rather limited : it accounts for less than < 10%. The case of N20 emissions must be distinguished. This greenhouse gas also plays a role in the stratospheric ozone layer depletion. Its emissions growing rate results in a perverse effect of catalyst in the conversion of NOx. The figures shown in figure 5 are worrying but must be confirmed because of the lack of available data. The contribution of diesel road vehicles for these kinds of pollutants is also limited with the exception of particulates matters (PM) for which emissions represent 27.3% amongst the total emissions from other sources. If diesel vehicles play a lesser role in these pollutant emissions (PM excepted) than gasoline vehicles, emissions from diesel cars are growing more than those of gasoline vehicles. Explanations can be found with the growth of diesel vehicles in the Belgian vehicle fleet. For diesel vehicles, indeed, the amount of cars rose from 170 thousands of cars to 1,370 thousand units between 1980 and 1995. A slower growth is also observed for commercial and heavy duty vehicles, whereas the number of gasoline fuelled cars almost did not change during the same period (2,850 thousands of cars). These trends can also be noticed whilst considering the evolution of diesel and gasoline consumption between 1980 and 1995. Annual consumption of diesel doubled between 1980 and 1995 (from 2.1 Mtoe to 4.25 Mtoe) whereas gasoline consumption stayed about the same (3.1 Mtoe in 1980 and 3.0 Mtoe in 1995) (EUROSTAT, 1980-1997). These factors explain the mitigated results on air pollutant emissions from traffic and we should focus on finding answers to the right question : "What about air quality, especially in urban areas"? 4. T R A F F I C AND AIR P O L L U T I O N IN T H E CITIES Measurements made in Brussels by the monitoring network in place on a high traffic density street give an idea of the relationship between traffic density and semi-hourly values of the concentration of pollutants in the ambient air. For CO and NO the link between air quality and mobility is easily seen in figure 7. Both morning and evening rush hour impacts are noticeable whether it is winter or summer time. In the latter, building heating has a small impact. Similar comments can be made whilst considering air pollution during the weekend, in comparison with "working days". The problem which remains is that however numerous the regulations, high levels of concentration are still measured without showing any clear tendency towards an improvement or a worsening over the last fifteen years (figure 8).
17
(*) Without cold start overemisslons
Figure 5 N20 9 and Particulate (PM) emissions from fossil fuel combustion in Belgium.
18
Figure 6" SO2 and CO2 emissions from fossil fuel combustion in Belgium.
19
Figure 7: CO and NO concentrations (semi-hourly) in a high traffic density street of Brussels.
20
!
0 Z r~
0
J
0
0
~
0
0
0
~
0
0
s
0
V
0
0
0
A
0
m
,L~
_
O~ OX
ol ox ox
0 o~ o~
oo oo 0"~
~o oo c~
oo ox
ol oo o~
~_ o oo o
Figure 8: Evolution of yearly average concentration of NO in a few streets of Brussels.
21 It is not surprising during unfavourable weather conditions (poor dispersion) and within the "sensitive" city areas (canyon street, high traffic) to see outdoor mean concentrations of CO, black smoke, but also NO2, 0 3 rise above the WHO guidelines or EC limit values, keeping in mind the very high concentration levels observed over short periods, inside cars or inside road tunnels. The reason of this is well known. Even when, emissions are very strictly reduced for new vehicles sold on the market (more than 95% since 1970) the benefits are annihilated with the various following factors : 9The growth of the car fleet, and especially diesel cars, because, as a rule of thumb, the main determinant of mobility is the level of car ownership in any area. 9The slow rate of replacement of the car fleet. New cars play a minor role in the emissions but one or two tenths of % of old, rundown cars can produce more than 1/2 the total vehicle emissions and yet, the lifetime of cars, particularly diesel cars, is rising. 9 A large % of journeys take place in urban areas where the average length is for Brussels of 5-6 km meaning that catalysts are often not warm enough to work efficiently. 9The rise of activities and jobs in urban areas and consequently commuters, without forgetting the rise in the number of shopping centres located in the outskirts, far and often from any public transportation station. Mobility rose by 32% for the last ten years in Brussels. 9 Finally, the fashion: as written in the Economist, "people are choosing more power than smaller fuel bills" : four wheeled cars, pick-up trucks, space wagons fitted with air conditioner or other energy consuming devices and why not heavy trucks? I know about them. This must be connected also with the low impact of taxation the 9 continuous rise in fuel prices did not really cut down consumption; in fact they did not change at purchasing power parity. Besides, each time fuel consumption per vehicle is decreasing, more kilometres by road are travelled. This is called a "rebound effect" and is widely observed in developed countries. 5. IN C O N C L U S I O N What can we learn from all this and particularly for catalyst makers? Cars have become a cultural phenomenon. They give people the freedom to go anywhere at anytime, they are a pleasure to drive, they are fun to look at and they are a pride for their owners. The fashion for cars is not new. Even as far back as our ancestry, the Celts, chariots were so much liked that they were buried with their owners. Now, each time income per head rises, car sales also rise and if you remember that the main determinant of car traffic growth is the level of car ownership you can understand why society builds partly around car usage. In this frame, it is logical that car emissions are so difficult to reduce. If positive results can be obtained for gasoline cars and significant cuts have to be made thanks to catalyst, these improvements are not presently sufficient. In this respect I shall finish my presentation by pointing out five challenges to overcome for car catalyst makers : 9firstly, problem of cold starts overemission; 9secondly, exhaust emission control of diesel cars; 9thirdly, long run durability of three way catalysts; 9fourthly, problems brought about with the introduction of reformulated gasolines; 9lastly, N 2 0 production control. In brief, there is no shortage of research for catalyst scientists and work for catalyst makers.
22 ABSTRACT
An introduction of air pollution problems in relationship with catalyst technology challenges is given through four aspects : 9major air pollution issues involving traffic at different geographical scales; 9review of EU vehicle emission regulations from 1970 up until now; 9history of emissions of seven pollutants from road vehicles in Belgium between 1960 and 1995 in relation with the regulation put in place; 9and finally, a few comments on results of air pollution monitoring network in cities during the last few years Conclusions on challenges for catalyst sciemists and makers are drawn up within the framework of this overview. BIBLIOGRAPHY
1.
Concawe (1995) Motor 9 vehicle emission regulation and fuel specifications in Europe and the United States 1995 update. Report 5/95. 203 p. 2. Concawe (1994) : Motor vehicle emission regulation and fuel specifications 1994 update. Report n ~ 4/94. 234 p. 3. ECOPLAN (1996) : Mon6tarisation des cofits extemes de la Sant6 imputables aux transports. Rapport de Synth6se - 6VF. Mandat SET n~ + annexes. 4. EPE (1996) : Transport Communication and Urban Issue. The EPE Workbook series for implementing sustainability in Europe. June 1996. 132 p. M. Moussel Ed., Paris. 5. EUROSTAT (1980-1997) :"Energy Balance Sheet". Statistical document 4 C (ECSCEC-EAEC), Brussels-Luxembourg. 6. FALLY S., JOANNES D., LEDUC D. et SCHARLL M-F (1995): "La pollution atmosph6rique, ses effets et ses coots en R6gion de Bruxelles-Capitale". Etude mini-arc. BRES Dossiers 95, n~ 67 p., IRIS Ed. Brussels. 7. HECQ W. et ALPI I. (1995): "Consommation de Combustibles Fossiles et Impacts l~conomiques de la Qualit6 Particulaire de l'Air sur le B~ti et la Sant6 en R6gion Bruxelloise" l~tude r6alis6e pour le Secr6tariat d't~tat ~ l'l~nergie de la R6gion de Bruxelles-Capitale. Rapport Final: premi6re et deuxi6me partie. 142 p. - Brussels. 8. HECQ, W. (1991) : Energy and air pollutants in Belgium. The contribution of automotive traffic since 1980 - in Catalysis and Automotive Pollution Control II. Elsevier, pp 5-15. 9. HOLLAND, M. (1996) : The ExtemE methodology : status and future perspectives" EC/DG XII Seminar : Belgian ExtemE Forum, 24 p. - Leuven. 10. INFRAS-IWW (1995) : Effets extemes dfis aux transports - Union intemationale des Chemins de Fer (UIC). 364 p., Paris. 11. RENTZ, O. (1993) : Critical loads/targets. Reduction of emissions. Results of the RAIN model under the scope of the UN/ECE. Pollution Atmosph6rique - Special issues, pp 6980. 12. UNIPEDE (1996) : What Price the Environment? Environmental Seminars. Background Papers. Nov. 1996. 71 p., Paris.
CATALYSISAND AUTOMOTIVEPOLLUTIONCONTROLIV Studies in Surface Science and Catalysis,Vol. 116 N. Kruse, A. Frennet and J.-M Bastin (Eds.) 91998Elsevier Science B.V. All rights reserved.
23
Auto E m i s s i o n s after 2000: The Challenge for the Catalyst Industry R A Searles AECC - Automobile Emissions Control by Catalyst, Avenue de Tervueren 13A, 1040 Brussels, Belgium
1. INTRODUCTION AECC is an international association, based in Brussels, whose members are European companies in the business of making the technologies for automobile exhaust emissions control. The members are Allied Signal Environmental Catalysts, Coming, Degussa, Emitec, Engelhard Technologies, Johnson Matthey, NGK Europe and Rh6ne-Poulenc Chimie. AECC members are specifically engaged in the development, testing and manufacture of ceramic and metallic substrates, autocatalysts and speciality materials, including ceria based oxygen storage components, that are incorporated into the catalytic converter and also in catalyst and filter based technologies to control diesel emissions. AECC members have their European manufacturing and development facilities in Belgium, France, Germany, Italy, Sweden and the United Kingdom. The automotive emission catalyst industry was born out of the 1970 US Clean Air Act and European based companies have been active in the business since that time with the supply of the catalysts and ceramic and metallic substrates that are now universally used in Europe. It is however little more than 10 years since the first catalyst equipped cars appeared on European roads and only 4 years since the European Union first legislated that all new cars must meet emission control regulations that effectively mandated the fitment of emission control catalysts. Now more than 250 million of the world's 500 million cars are fitted with autocatalysts. [ 1] 2. E U R O P E A N E M I S S I O N L E G I S L A T I O N
The survival and growth of the autocatalyst industry are linked to the existence and strengthening of exhaust emission legislation. The 1993 European Union emission limits for passenger cars have already been lowered from 1996 and the process is underway to finalise the emission standards for the European Union for 2000 and 2005. Proposals for passenger cars emission standards and fuel composition, including sulphur levels, were issued by the European commission in late 1996 and published only in March 1997 (Tables 1 and 2). [2]
24 Table 1 European Commission Proposal for Gasoline Fuelled Vehicles Year 1993/94 1996/97 2000/01 Step 1 Step 2 Step 3 (Present) (Proposed) Standard ( ~ ) Dir. 94/12 Scenario 2 of (corrected Auto Oil values) CO 2.72 2.2 (2.7) 2.3 HC
.
.
HC§
0.97
NOx
.
.
.
.
(0.341) 0.5
.
.
0.20
.
.
HC + NOx
0.97
.
1.0 0.10
-(0.252)
0.15
Table 2 European Commission Proposal for Diesel Fuelled Vehicles Year 1993/94 1996/97 2000/01 Step 1 Step 2 Step 3 (Present) (Proposed) Dir. 94/12 Scenario 2 of Standard (gm/km) Auto Oil (corrected values) 0.64 CO 2.72 1 (1.06) HC
2005/06 Step 4 (Indicated) Scenario 4 of Auto Oil
0.08
2005/06 Step 4 (Indicated) Scenario 4 of Auto Oil 0.50
. 0.7/0.9w
0.56
0.30
(0.71/0.91w NOx
--
(0.63/0.81w
0.50
0.25
PM
0.14
0.08/0.1 w
0.05
0.025
Notes: 1. wDirect injection diesel vehicles are allowed a more relaxed standard for Step 2. 2. The corrected values in Step 2 take account of the new test procedure from 2000 that eliminates the 40 sec pre-test idle period allowed in 1993-1996 regulations. 3. The Commission has proposed that the indicated values be confirmed or adjusted by the end of 1998 atler an Auto Oil 2 programme.
25 The proposals for changes to fuel composition include: 1. Leaded gasoline will not be able to be sold after 2000, with a derogation for 3 years for member states who have difficulties in applying the ban. 2. The maximum lead content of unleaded gasoline is to be reduced to 0.005 grams/litre from the present 0.013 grams/litre. 3. Maximum sulphur content of gasoline is to be 200 ppm and diesel is to be 350 ppm from 2000. 4. Fuel composition for 2005 is to be determined by end 1998, after Auto Oil 2. 50 ppm Sulphur level is suggested as the likely quality of fuel required for 2005 based on the current status of research & development. 5. Maximum benzene content of gasoline is to be 2%. Equivalent reductions in permitted emissions have now been translated into proposed standards for Light Commercial Vehicles and later this year proposals will be made for Heavy Duty Vehicles. European regulations would then become equivalent in the level of emission control to those in the USA, including California, reflecting increasing concerns in Europe on the health and environmental effects of pollution from transport. This paper will review the influence that US regulations, which were met by European car companies exporting to the US, and subsequently European Union regulations have had on the evolution of autocatalyst technology. Ironically although European Union regulations requiring catalysts were introduced some 18 years after the US, European companies have led the technology in a number of key areas: 1. The first demonstration of a car that would meet the 1975 US standards was made in Europe. 2. The first car company to contract to source its supply of catalysts for the US market was a European company, who also introduced the first "closed loop" 3 way catalytic converter in 1978 using advanced European emission control technology. 3. Platimma/Rhodium catalysts were first used on European cars to reduce NOx emissions for the US 1975 emissions regulations, using "open loop" three way catalysis. 4. The technology to produce thin walled extruded ceramic substrates was first developed by a European company. 5. European owned companies lead on the supply of oxygen storage components in 3 way catalysts and thin foil metal substrates around the world. The present European Union regulations have effectively decreased the allowed emissions per vehicle kilometre by 96% from the levels of 1970 (Figure 1). [3] The challenge for the future is how to tackle the remaining 4% of emissions on the test cycle. This is necessary to meet air quality targets given the large increase in the number of vehicles on European roads since 1970 and the greater annual distances that we all drive each year.
26 When the limit values for 2000 and 2005 have been finalised, probably under the codecision procedure between the Council of Ministers and the European Parliament, they are likely to require a reduction of more than 50% or more in the residual emissions per vehicle on the prescribed test cycle.
Figure 1. Evolution of Community Passenger Car Emission Standards 3. T E C H N O L O G Y F O R T H E C O N T R O L O F E M I S S I O N S
There are a number of ways to achieve lower tailpipe emissions from internal combustion engines: 1. Reduce the base emissions from the engine by improvements to the combustion process and fuel management, addition of air injection or exhaust gas recycle or by changes to the type of fuel or its composition. 2. Decrease the time taken for the catalytic converter to reach its full operating efficiency. 3. Increase the conversion efficiency of catalysts at their working temperature. 4. Store pollutants during the cold start for release when the catalyst is working.. 5. Devise catalysts or strategies to destroy nitrogen oxides under lean (oxygen rich) operation. 6. Devise reliable ways to regenerate particulate filters. 7. Increase the operating lifetime during which autocatalysts and their supporting systems efficiently convert pollution.
27 This presentation will concentrate on ways in which the catalyst industry can help in achieving the last six cases.
3.1. Low light-off Catalysts Low light-off Catalysts allow a conventional catalytic converter to start working earlier by decreasing the exhaust temperature at which the catalyst commences operation. This reduces the amount of untreated exhaust emitted both during the standard emissions test as defined in legislation and also on short journeys in the real world. This is achieved by changes to the thermal capacity of substrates (see below) and type and composition of the active precious metal catalyst. This includes greater use of palladium which has been possible as lead and sulphur levels have become lower in fuels (Figure 2.). [4]
Figure 2. Effect of Catalyst Technology on Light Off Temperature
3.2. More Thermally Durable Catalysts Increased stability at high temperatures allows the catalytic converter to be mounted closer to the engine and increases the life of the converter, particularly during demanding driving conditions. To achieve this durability requires precious metal catalysts with stabilised crystallites and washcoat materials that maintain a high surface area at temperatures as high as 1000~ Of key importance is the oxygen storage component used to maximise the "window" of air-fuel ratio for three way operation and as an indicator of the "health" of the catalytic converter by the On Board Diagnostic (OBD) systems which European cars will require from year 2000. This also stabilises the surface area of the washcoat. Figure 3. shows the progress made with mixed cerium and zirconium oxides.
28 After air ageing at 9000C for 6 hours
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Figure 3. Improvements to Thermal Stability and Oxygen Storage Capacity (OSC)
3.3. Technology of the Substrates The substrate, onto which the active catalyst is deposited, is an area in which great progress has been made. On first introduction in 1974 ceramic substrates had a density of 200 cells per square inch of cross section or cpsi (31 cells/square cm.) and a wall thickness of 0.012 inch or 12 mil (0.305 mm). By the end of the 1970's the cell density had increased through 300 to 400 cpsi and wall thickness had been reduced by 50% to 6 mil. Now 400 and 600 cpsi substrates are commercially available and wall thickness has been reduced to 4 mil - a mere 0.1 mm. Further increases in cell density to 900 cpsi and reductions in wall thickness to 2 mil are promised for 2000 and beyond. In the late 1970's substrates derived from ultra thin foils of corrosion resistant steels were offered onto the market. The foils could be made from material only 0.05 mm thick allowing high cell densities to be achieved. In their early application they were mounted in locations inside the exhaust manifold but they are now used in other locations as well. Complex internal structures can be developed and wall thickness is down to 0.04 mm and cell densities of 600 cpsi standard production. This progress in ceramic and metal substrate technology has major benefits. A larger catalyst surface area can be incorporated into a given converter volume and this allows better conversion efficiency and durability. The thin walls reduce thermal capacity and avoid the penalty of increased pressure losses. Alternatively the same performance can be incorporated into a smaller converter volume. This makes the catalyst easier to fit as cars get more compact. It also allows the converter to be mounted closer to the engine, where space is at a premium, for faster catalyst light off. Light off times have been reduced from as long as one to two minutes down to less than 20 seconds.
29 3.4. Optimised systems Systems incorporating components from the at,.-we are already in development and limited production. The use of additional catalytic converters in, for example, the starter position (close to the exhaust manifold) gives a consequent reduction in cold start and total emissions. By using the improved substrate technology with highly thermally stable catalysts and oxygen storage components the close coupled light-off catalyst approach is able to meet the California Low Emission Vehicle (LEV) and the next stage Ultra Low Emission Vehicle (ULEV) regulations [5] as well as the proposed European Union 2000 standards. 3.5. Diesel exhaust emission control Diesel emission control has been a major feature in catalyst development in Europe. Growth in sales of diesel cars, which in 1996 reached 22% of car sales across the whole of Europe and around 50% in France, has been a factor in raising concern on the possible health effects of diesel emissions particularly ultra fine particles. The key areas for research in diesel exhaust control are Lean NOx control, which is covered later, and oxidation catalysts, particulate filters and selective catalytic reduction. 3.5.1. Oxidation Catalysts To meet the 1996 EU emission regulations all diesel engined cars are now fitted with oxidation catalysts. Some, but not many, heavy duty vehicles also use oxidation catalysts. The fitment of an oxidation catalyst allows overall reductions in particulate levels of.up to 50%, destruction of the organic fraction of the particulate and significant reductions in CO, HC and the characteristic diesel odour. Very low back pressures mean little effect on performance and economy. Operation with low sulphur fuel is desirable since sulphur inhibits the performance of the catalyst and catalyst activity must be reduced to avoid oxidation of sulphur to the acidic sulphur trioxide. 3.5.2. Particulate Filters Ceramic wall flow filters were developed to remove up to 90% of the particulate matter contained in diesel exhaust. Particulate includes the carbon core as well as the soluble organic fraction removed by oxidation catalysts. The filters catch the very fine particulates or soot (less than PM 2.5 or 2.5 microns mean diameter) for which concern on health effects has been expressed. Since the wall flow filter would readily become plugged with particulate material in a short time, it is necessary to "regenerate" the filtration properties of the filter by burning off the collected particulate on a regular basis. The following technologies can allow the engine designer to tune for lower levels of NOx produced in the combustion process and then to use after-treatment to remove the consequent increase in particulate levels. Several methods of achieving regeneration have been developed including: 1. Using burners or electrical heating to raise the trapped soot to its combustion temperature of over 600~ This requires a good control system and the burn off process is either carried out when the vehicle is out of service or valves switch exhaust between twin filters while one filter is regenerated.
30 2. Incorporating a catalytic coating on the filter to lower the temperature at which particulate bums. This is an elegant approach but one that has proved difficult to achieve in practice. 3. Using small quantities of additives, such as cerium oxide, incorporated in the fuel or injected into the exhaust ahead of the particulate trap. The additive, when collected on the filter with the particulate, allows the particulate to bum at normal exhaust temperatures to form carbon dioxide and water. This system is insensitive to sulphur and can be used with current European diesel fuel containing 500 PPM of sulphur. 4. Incorporating an oxidation catalyst upstream of the filter that, as well as operating as a conventional oxidation catalyst, also increases the ratio of nitrogen dioxide (NO2) to nitric oxide (NO) in the exhaust. Particulate then burns off at normal exhaust temperatures using the powerful oxidative properties of NO2. This continuously regenerates the trap. This system requires diesel fuel containing less than 50 PPM sulphur such as the City Diesel used in Sweden.
3.5.3. Selective Catalytic Reduction (SCR) This process, originally introduced on stationary engines, is now at the stage of field trials on heavy duty diesel engines. Ammonia is used as a selective reductant, in the presence of excess oxygen, to convert NOx to nitrogen. The ammonia is usually introduced by a controlled injection of a solution of urea in water into the exhaust gas upstream of the special SCR catalyst. 3.6. Maintenance and Durability of emission performance This is the key to gain the maximum environmental benefit from the application of advanced emission control technology. It is important that the technology utilised should be durable for the defined, reasonable life of the vehicle and that the vehicle should be regularly checked to ensure that the systems installed on the vehicle are working properly and that they have not been abused or subject to inadequate maintenance. AECC supports the implementation of improved Inspection and Maintenance programmes for Europe and the requirement that On Board Diagnostic Systems be installed to monitor the operation of the engine management and emissions control components, including the conversion efficiency of the catalytic converter. This is important as the performance and durability of the catalytic converter are very dependant on the systems of which it is a key part. The European Parliament Environment Committee has proposed an increase, from 80,000 km to 160,000 km or 10 years, in the defined lifetime of emission performance within the standards they have proposed for 2000 and 2005. [6] Catalyst technology has been developed to meet the US requirement that vehicles be certified to last for at least 100,000 miles (160,000 km) within the set emission limits and can be adapted for European driving conditions.
31 4. FUTURE T E C H N O L O G Y For the standards that may be introduced in 2005 new technologies are under investigation. These include:
4.1. Hydrocarbon Adsorber Systems These use an approach based on the incorporation of special materials, such as zeolites. The adsorber is generally mounted ahead of the autocatalyst, or the adsorber and catalyst are integrated into a single brick, so that hydrocarbon emissions are collected when exhaust temperatures are too low for effective catalyst operation. The hydrocarbons are then desorbed at higher temperatures when the catalyst has reached its operating temperature and is ready to receive and destroy the hydrocarbons. This technology, although still at the stage of research and development, has already shown the potential to reduce hydrocarbons to less that half the levels emitted from a conventional 3 way catalytic converter.
4.2. Electrically Heated Catalyst Systems 0EHC) EHCs are a small catalyst installed ahead of the main catalyst. The substrate, onto which the active catalyst material is deposited, is made from metal so that when an electric current is passed it will heat up quickly. This brings the catalyst to its full operating temperature in a few seconds. Early electrically heated catalyst systems required high levels of power input but development of the systems has improved efficiencies so that the power requirement has been reduced to a tenth. EHC systems are currently available that have been demonstrated to meet Californian ULEV standards using 1-2 kW of additional power and 610 watt-hours energy consumption during the start-up phase.
4.3. Lean Combustion As a development of gasoline engines and an essential element of diesel engines, Lean Combustion is an important issue for the future development of catalysts. It is closely tied to the question of carbon dioxide emissions, and the linked issue of fuel consumption. The European Union and national governments are seeking to encourage lower levels of fuel usage to counter the "greenhouse effect". The introduction in Germany of fiscal incentives to encourage the so-called "3 litre car" (3 litre per 100 km consumption) and engines emitting CO2 levels of 90 or 120 gm/km [7] needs radical new engine technologies if they are to be achieved. Technologies that are under development include: 1. Two stroke engines. 2. Direct injection gasoline engines. 3. Greater use of electronic management of diesel engines. 4. Direct injection diesel engines using unit injectors or common rail injection. All these systems use lean operation to achieve the improved fuel consumption. However as NOx emission standards are lowered to meet environmental concerns this presents a challenge to the conventional 3 way catalyst technology used on gasoline engines and to the technologies described above for diesel engines.
32 3 way catalysts require a stoichiometric air to fuel ratio - where air and fuel are in chemical balance - to "persuade" NOx to combine with HC and CO and eliminate the pollution. To reduce NOx in an oxidising environment requires a new approach to catalysis and presents one of the most exciting areas of current development. Lean NOx (or DeNOx) catalysts, that are currently being developed, tackle this through advanced structural properties in the catalytic coating to create a rich "microclimate". Here hydrocarbons from the exhaust can reduce the nitrogen oxides to nitrogen. The systems under development for diesel and gasoline engines are: 1. Passive - with no added reductant. 2. Active - with 2-3% additional fuel added upstream of the catalyst. Current prototype DeNOx systems are demonstrating 30-40 % reduction in NOx levels [8] . Research and development are now concentrating on widening the operating temperature "window" at which this conversion can be achieved and improving the thermal durability of the catalyst systems. Further promising developments for gasoline engines involve the incorporation of a NOx trap to adsorb and store NOx under lean conditions. The stored NOx is then released during a transient excursion to rich conditions and using a 3 way catalyst to destroy the NOx. Efficiencies approaching 90% have been demonstrated in the laboratory. 5. THE E F F E C T OF SULPHUR ON THE DURABILITY AND PERFORMANCE OF C A T A L Y S T B A S E D EMISSION C O N T R O L S The effect of sulphur on catalyst performance has become more critical as lower tailpipe emissions are targeted. The loss of catalyst efficiency caused by sulphur in the fuel has a larger impact at very low emission levels. Sulphur is not a catalyst poison like lead but strongly competes against pollutants for "space" on the active catalyst surface. This limits the efficiency of catalyst systems to convert pollutants at any sulphur concentration. The effect of sulphur as a competitor on the catalyst surface may be reversible but it can cause irreversible changes to the washcoat and some of the base metal components. Sulphur resistant catalysts are not an option because that necessitates trading off catalyst performance for the removal of other pollutants. In addition when particulate removal is required the conversion of sulphur to sulphate limits the total particulate reduction and can cause net increases in particulate. Unlike lead, where loss of performance of catalysts increases over time, sulphur has an almost immediate effect and one tankful of high sulphur fuel will immediately degrade the catalyst performance. However catalyst performance will normally be restored on reverting to a low sulphur fuel. There is considerable current work and concern on this topic in Europe and the USA. [9,10,11] The levels of sulphur in fuel, particularly diesel, are an important factor in the performance of DeNOx catalysts catalysts. The lower the sulphur level in the fuel the better
33 the catalyst performance that can be obtained. The move to diesel fuel containing a maximum of 500 PPM sulphur throughout the European Union last year will help catalyst performance but if maximum reduction of other pollutants is to be obtained then increasing quantities of diesel fuel containing less than 50 PPM of sulphur, such as City Diesel, will be required. These fuels are becoming more widely available, not only in Sweden, but also in Germany, United Kingdom, Denmark and Finland. 6. CONCLUSIONS There is now a larger range of potential gaseous and particulate emission control options, based on the use of catalytic, trapping and adsorption technologies, than at any time in the past. Europe has a strong autocatalyst industry, supported by universities and other academic institutions, and is well placed to help meet the challenge of future emission regulations. European companies probably lead the world in the development of catalystbased systems to control particulates and nitrogen oxides from diesel engines. This field is of increasing importance as the proportion of diesel vehicles on our roads grows and people drive more. The catalyst industry has invested heavily in facilities, advanced analytical techniques, testing equipment and people. Advanced catalysts, together with optimised engine management and emission control systems, will aid the achievement of future low emission standards that are deemed necessary to meet air quality goals. REFERENCES
1. "PGM Autocatalysts - The Spread of Global Emission Standards and their Impact on Autocatalyst Demand", R A Searles, Platinum Group Metals, IPMI, September 1995. 2. Official Journal, C77, Vol. 40, 11 March 1997. 3. European Commission Communication, COM (96), Final. 4. SAE 950259. 5. "Low Emission Vehicle and Zero-emisslon Vehicle Review", Mobile Source Division, Air Resources Board, California Environmental Protection Agency, November 1996. 6. P.E. 220.506/Final 7. Bundesministerium fiir Verkehr, Kraftfahrzeugsteur~inderungsgesetz 1997, 14 March 1997. 8. "Diesel/Lean NOx Catalyst Technologies", SAE Fuels & Lubricants, San Antonio, October 1996, SP- 1211. 9. SAE Papers on Impact of Sulphur: SAE 910814, 912321, 912323, 920329, 920557, 920558, 922245, 930137, 930385, 940783, 940928, 942001, 950255, 952421, 952561. 10.Other Papers on Impact of Sulphur: B J Cooper, Platinum Metals Review, 38, (1), 2-10. E S Lox, SAE TOPTEC, Meeting the Legislated Standards, 1993. Beck et al, Applied Catalysis B: Environmental, 3 (1994), 205-227, Elsevier Science BV, Amsterdam. 11."Ford Position on the Effects of Fuel Sulfur on Advanced Emission Control Technology", Ford Motor Company, March 25 1996.
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CATALYSIS AND AUTOMOTIVE POLLUTION CONTROL IV Studies in Surface Science and Catalysis, Vol. 116 N. Kruse, A. Frennet and J.-M Bastin (Eds.) 91998 Elsevier Science B.V. All rights reserved.
35
Diesel Engine Development Routes T o w a r d s Very L o w Emissions Peter L. Herzog AVL LIST GmbH, Austria
1) Introduction Modern commercial vehicles are almost exclusively driven by highly efficient direct injection diesel engines. With passenger car diesel engines, a remarkable change is currently taking place. After the introduction of the first direct injection diesel en#nes a few years ago, it is obvious that the high speed _direct injection (HSDI) diesel engine is increasingly penetrating the market. It is expected that HSDI diesel driven vehicles will have a share of about 85% of the diesel passenger cars in Europe at the turn of the century. The most challen~ng field in achieving the exhaust emission legislative standards is the NOx and particulates control. Fig. 1 shows the comparison of the European light duty (LD) and heavy duty (HI)) diesel engine NOx/particulate emission targets. From 1995/1996 on it is clear that the HSDI diesel has to simultaneously reduce NOx and particulates emission, whereas the HD diesel has to concentrate on NOx emissions control.
Fig. 1 Comparison of European LD and HD Diesel Engine NOx/Particulate Emission Targets
36
S Exhaust gas emissions HC, NOx, CO, PM 9 Noise
(Pass by)
9 Fuel consumption / CO2 I
9 Smoke Q Odour
9 Fuel consumption Q Durability
9 Noise (subjective)
9 Performance and Driveability
9Ease of manufacture 9Quality 9 Production cost
9 Reliability
Recycling (?) I
Maintenance
Q Price Fig. 2
Ecological and Economical Factors for a Successful Engine
Fig. 3 Main Parameters Effecting NOx and Particulate Emission of D[ Diesel Engines
37 However, besides exhaust emission control - Fig. 2 - various other aspects in engine development have to be taken into account. The most challenging parameters for the diesel engine are: noise, vibration harshness (NVH) control and the necessary technology versus cost required for competitive performance.
2) Main parameters effecting NOx and particulate emissions Fig. 3 summarizes the parameters mainly effecting NOx and particulate emissions. According to the certification test procedures, the emissions of passenger cars are distinctly influenced by vehicle design parameters, which is not the case with engines for commercial vehicles. The design approach concentrates on 9Utilization of modem design and analysis tools (e.g. bionic optimization process) to improve weight, friction, distortion and NVH. 9System integration (e.g. TC in manifold, etc.) and 9Consideration of new technology elements (e.g. variable valve timing, exhaust gas recovery, brake energy recovery, etc.).
Fig. 4 ECE R49 Emission Reduction Resulting from Replacement of a CEC Reference Diesel Fuel by a Low-Sulphur, Low-Aromatic Fuel
38 2.1)
Operating
fluid
The development concentrates on both, fuel and lubricant formulation. New types of fuels, such as water/fuel or compressed natural gas (CNG) are not seen as the main stream. Fig. 4 shows the e~aust gas emission reduction for a 10L DI-TCI diesel engine in the ECE R49 test when using a low aromatics (< 5% vol.) and low sulphur (< 0.003% wt.) fuel instead of a CEC-RF-03-A-84 reference fuel. For the example shown, the gaseous emissions can be reduced by about 10%, whereas the particulates are improved by about 40%. Typically achievable soot reduction versus fuel oxygen content is shown in Fig. 5. Even a small amount (e.g. 2%) results in a distinct soot reduction (-~ 20%). 100
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Fig. 5 Soot Reduction of Compression Ignition Engines due to Fuel Oxygen Content
Table 1 summarizes the fuel development trends and their respective benefits.
9 Trend: Reformulation - Improved boiling range Higher CN, CI Lower total aromatics - Extremely low sulfur - Oxyginates 9 Benefits - Lower emissions Extended operation of aftertreatment systems - Improved fuel consumption because of more efficient engine timing
39
2.2) Mixture preparation and combustion The combustion is governed by the mixing process, which is the interaction between the cylinder charge and the fuel. Low smoke emission requires a uniform fuel distribution in the combustion bowl (high air utilization) with good atomization, as well as high turbulence and sufficient oxygen to promote soot oxidation. The minimization of NOx-formation demands a low charge temperature, the use of exhaust gas recirculation (EGR), the reduction of the initial rate of injection and an increased compression ratio though detrimental to smoke control. Fig, 6 shows by example the influence of the number of valves (2V ~ 4V, improved breathing and more uniform fuel distribution) and the compression ratio on fuel consumption and soot particulates versus the NOx level in the ECE R49 test for a 1.5L/cyl. DI-TCI-HD diesel engine. The NOx reduction was achieved by a delayed start of injection. In this example, the advantage of the 4-valve technique primarily consists in the improved fuel consumption at the target low NOx level.
Fig. 6 Effect of Valves/Cylinder and Compression Ratio (CR) on BSFC/SOOT/BSNOx Trade-Off in ECE R49 13-Mode Test
The effect of higher fuel injection pressures (better atomization and higher turbulence) for a HSDI-TCI diesel engine with EGR is shown in ~ . The introduction of a high 12ressure commron rail (HPCR) fuel injection system with flexible fuel pressure levels makes it possible to shift the NOx/particulate trade-off towards substantially lower levels, since better fuel atomization improves the EGR tolerance of the combustion system.
40
Fig. 8
AVL's Part Load/Full Load Injector with Tandem Nozzle
* Numbers in parentheses designate the references at the end of the paper.
41 Variable orifice nozzles offer additional freedom in optimizing both mixture preparation and combustion. ~ shows AVL's part load/full load injector [9]* which is capable of operating with two different spray hole configurations. Results of initial tests carried out on a four-cylinder 2.0L naturally aspirated (NA) engine, without EGR or oxidation catalyst, to investigate the effect of a part load/fuU load nozzle under simulated FTP72 conditions are shown in the upper fight of Fi~_jg~9. It can be seen that using the same standard rotary injection pump there is significant potential for the reduction of NOx and particulate emissions due to the part load/full load injector concept. A vehicle fitted with AVL's Leader (demonstration) engine [10, 11] was tested to the MVEG cold test cycle. The four-cylinder 2.0L TCI engine having inherently lower emissions levels than that of the NA engine mentioned previously, was fitted with a set of tandem part load/full load injectors (as shown in Fig. 8) in combination with a high pressure rotary pump, EGR and an oxidation catalyst. The test results shown in the lower left of Fig. 9 indicate similar NOx/particulate emissions improvement as experienced with the NA engine.
Fig. 9
Effect of AVL's Two Stage Orifice Nozzle on Vehicle Emissions
Table 2 summarizes the key combustion technology elements for low emission DI diesel engines. 9Turbocharging (VGT, hybrid) and intercooling 9Cooled EGR with temperature management 94-valve technology with swirl and VVT (?) control 9Increased compression ratio and peak firing pressure 9High pressure injection with injection rate control. Trend toward variable orifice injector.
42
2.3) Control In order to comply with the future emission regulations, the introduction of electronic control is necessary. Not only the fuel injection system needs electronics for enhanced accuracy with regard to injection timing, fuelling etc., but also air management components like EGR, wastegated or variable geometry turbocharger need to be controlled flexibly and accurately. Dynamic control functions (e.g. for injection timing and EGR) are desired in order to improve both the engine's transient emissions and response. Furthermore, for the lowest vehicle fuel consumption, the selection of the optimum gear in any driving condition should also be performed by electronic controllers. The effect of two different gear shift strategies on fuel consumption and emissions in hot MVEG tests is shown in Fig, 10. The tested vehicle had an ITW of 4000 lbs. Both gear shift strategies (A, B) improve the fuel consumption in the city driving cycle by about 16% and in the extra urban driving cycle (EUDC) by about 7% relative to the baseline gear shift. This resuits in a total fuel consumption reduction of about 12% in the total MVEG test. The simultaneously measured NOx and particulate emissions however show a significant deterioration. The reason for that is that due to the selected gear shift strategies, the speed/load operation map quits the optimum emission map valid for the baseline gear shift strategy. It may be that a combustion system rematch could overcome or at least mitigate this deficiency. However, the example shown clearly indicates that a gear shift strategy towards optimum fuel consumption can deteriorate exhaust emissions and consequently burden the environment.
Fig. 10:
Effect of Gear Shift Strategy on Change in Fuel Consumptions and Emissions in MVEG Test
43 A control system capable of optimizing both fuel economy and exhaust gas emissions would be the ideal solution. The structure of such a system was generally shown in [23], where emissions have been considered on the basis of a neural network emission model. The more appropriate control seems to be based upon an exhaust gas sensor guided concept. This would not only allow the best compromise between driver demands, vehicle response, fuel consumption and emissions to be found, but also keep the vehicle's emission level at the lowest possible deterioration versus life time. F.ig, 11 shows schematically the structure of such a control system.
Fig. 11
Block Diagram of an Emission Controlled Engine and Gear Box Control
The development trends in automotive controls are summarized in Table 3. ,,, i ,
9Control technology - Closed loop Higher level of monitoring - Model based systems - Self-learning systems -
-
-
9System integration Aftertreatment integration Powertrain management
.
.
.
.
.
.
.
.
.
* Diagnosis - Higher degree of on board diagnosis (OBD) Integration of OBD into control -
44
2.4) Exhaust gas aftertreatment In principle, three strategies may be envisaged to comply with extremely stringent emission limits: 9 Continuous improvement and sophistication of engine technology (e.g. variable compression ratio, extremely high charge density for "cooler combustion", etc.) 9 Introduction of totally new combustion technologies such as homogenous combustion [14 to 17] 9Exhaust gas aftertreatment. Considering development and research status, complexity of technology and benefit to cost ratio, it appears that exhaust gas aftertreatment represents the most promising strategy. However, this does not mean that the worldwide diesel community will desist from progress in both, continuous development of todays combustion process and research into new combustion technologies. The application of exhaust gas aftertreatment methods is currently mainly focussing on reducing the soluble organic fraction of particulates by means of oxidation catalysts. The exhaust gas aftertreatment strategies that may be followed in the future are: 9 Reduction of the particulate emission to below the defined limit by measures within the engine and application of a DeNOx catalytic converter for the external NOx reduction, or 9 Reduction of the NOx emission to below the defined limit by measures within the engine and use of a particulate filter. In both cases, however, the use of an oxidation catalyst for the conversion of the gaseous emissions HC and CO (as well as of the organic soluble particulate fraction) will remain indispensable. Taking into account that with an efficient DeNOx catalyst the engine can be operated at highest fuel economy, it appears evident that highest development priority should be Nven to DeNOx catalysts. However, due to the cancerogenic effect of small particles (e.g. dust, aerosols, etc.) in general - which is not a diesel specific effect - particulate filter development should not be disregarded.
3) Development routes towards efficient low emission DI diesel engines shows the evolution of technology for HSDI diesel engines versus the stages of emission legislation in Europe. Currently, the third generation is on the market and the technology applied is: Flexible air management (e.g. variable geometry turbine), dynamic modulated EGR, high pressure rotary pumps, electronic control and oxidation catalyst. The fourth generation under development will involve mainly four-valve engines. Fuel injection pressures are further increasing and new types of fuel injection equipment, such as electronically controlled unit injectors and high pressure common rail systems will be introduced. The application of cooled EGR becomes mandatory for heavier vehicles. Eventually, the oxidation catalysts applied will have some passive DeNOx capability. However, it has to be mentioned that vehicles heavier
45 than 1590 kg may have problems in achieving the legislative emission targets with the technology described above. The EURO 4 (Stage 2005) emission limits in question will require new exhaust gas aftertreatment devices as discussed above for vehicles heavier than approx. I000 kg (2200 Ibs).
Fig. 12
European Emission Legislation and Development Potential of the High Speed D[ Diesel Engine
Fig. 13 shows the technologies envisaged for the future H-D-DI diesel engine and what levels of emission values and fuel consumption could thus be achieved on the basis of the ECE-R49 test. First of all, striking is that an increased fuel consumption will have to be borne for NOx reduction even with the future EURO 3 engine technology. Though it may be possible to maintain the fuel consumption level of todays EURO 2 engines, these engines will, as a whole, run at least 5% above their optimum fuel consumption. By introducing cooled EGR for NOx control, it would become possible to almost maintain best fuel consumption. Nevertheless, EGR is a completely new technology for heavy-duty diesel engines still to be established and tested for durability of the engine and its components. If NOx emission is to be reduced even further, higher EGR rates will become necessary. However, these rates will then lead to higher particulate emissions and thus require the use of a particulate filter to stay within the PM limit. At the same time, a slight increase in fuel consumption will have to be expected. Alternatively, the selective DeNOx catalytic converter with urea-water solution could be installed on EURO 3 en~nes with an optimum fuel consumption, due to the high conversion efficiency, thus shifting the NOx/soot trade-off to the far left at practically unchanged cycle fuel consumption in relation to the EURO 2 baseline.
46
Fig. 13
Development Directions of Efficient Low Emission HI) Diesel Engines
47 References Ill
P.L. Herzog, L. Biirgler, E. Winklhofer, P. Zelenka, W. Cartellieri: "NOx Reduction Strategies for DI Diesel Engines." SAE Paper 920470.
/2/
P.L. Herzog, L. Btirgler: "Recent Developments in HSDI Diesel Engines." JSAE Paper 9302574.
/3/
R. Cichocki, W. Ospelt: "Technologies for Future HSDI Passenger Car Diesel Engines." Int'l. Symposium "Powertrain Technologies for a 3-Litre Car", Nov. 4-5, 1996, Sienna, Italy.
/4/
K.M. Wojik, H. Carstensen, W. Cartellieri: "Progress in the Pollutant Reduction of Vehicle Engines, SIAT 1996, Pune, India.
/5/
K. Moil: "Worldwide Trends in Heavy-Duty Diesel Engine E~aust Emission Legislation and Compliance Technologies." SAE Paper 970753.
/61
P. Tfitthart: "Requirements for Petrol and Diesel Fuels." Conference "Engine and Environment", Graz, Austria 1991.
/7/
P. Tfitthart, R. Cichocki: "The Contribution of Fuel Specifications to Light Duty Diesel Engine Emissions." 1l th European Automotive Symposium AGELFI, Nov. 19-20, 1992, Sorrento, Italy.
/8/
M. Tamanouchi, H. Morihisa, S. Yamada, J. Iida, T. Sasaki, H. Sue: "Effects of Fuel Properties on Exhaust Emissions for Diesel Engines With and Without Oxidation Catalyst and High Pressure Injection." SAE Paper 970758.
191
D.W. Gill, G. Heimel, P.L. Herzog: "A variable nozzle concept for high speed DI diesel engines." IMechE Seminar on Diesel Fuel Injection Systems, 28-29 Sept. 1995.
1101
P. Wfinsche, K. Wojik: "AVL LEADER: The New Passenger Car Diesel Generation Designed for Low Emissions." 14th International Vienna Engine Symposium 1993.
/11/
K. Wojik, F. K6nig: "The HSDI Engine AVL LEADER in the Car." 16th International Vienna Engine Symposium 1995.
/12/
D.W. Gill, P.L. Herzog: "Fuel Injection Technology for Low Emissions HSDI Diesel Engines." SAE Paper 962369.
/13/
P.L. Herzog: "Status and Potential of Shaping Injection Rate Control in High Speed Direct Injection Diesel Engines." MTZ Worldwide 57 (1996) 12.
/14/
H. Suzuki, N. Koike, H. Ishii, M. Odaka: "Exhaust Purification of Diesel Engines by Homogenous Chargewith Compression Ignition Part 1: Experimental Investigation of Combustion and Exhaust Emission Behaviour Under Pre-Mixed Homogenous Charge Compression Ignition Method." SAE Paper 970313.
48 /15/
H. Suzuki, N. Koike, H. Ishii, M. Odaka: "Exhaust Purification of Diesel Engines by Homogenous Charge with Compression Ignition Part 2: Analysis of Combustion Phenomena and NOx Formation by Numerical Simulation with Experiment." SAE Paper 970315.
/16/
H. Yokota, Y. Kudo, H. Nakajima, T. Kakegawa, T. Suzuki: "A New Concept for Low Emission Diesel Combustion." SAE Paper 970891.
/17/
K. Nakagome, N. Shimazaki, K. Niimura, S. Kobayashi: "Combustion and Emission Characteristics of Premixed Lean Diesel Combustion Engines." SAE Paper 970898.
1181
P. Herzog, K. Gschweitl, J. Mayrhofer, R. Schneider: "Electronic Control of Vehicle Power Plant." Workshop "Low Emission Vehicles for the Next Future." 28.-29. April 1994, Capri, Italy.
/19/
F. Indra, A. Bambeck: "Das automatisierte Schaltgetriebe (ASG) - Entwicklung und Versuch." 17. Internationales Wiener Motorensymposium. 25.-26. April 1996. VDIReihe 12 Nr. 267.
/20/
R. Rrsch, M. Kttrschner, W. Zaiser, G. Wagner: "Das automatische Getriebe W5A 330/580 yon Mercedes Benz - ein Beitrag zur Reduzierung des Kraftstoffverbrauchs yon Personenwagen." 17. Internationales Wiener Motorensymposium. 25.-26. April 1996. VDI-Reihe 12 Nr. 267.
/21/
D. Ka'axner, C. Bristle, H. Striebich: "Einflul3 von Fahrzeug, Antrieb und Fahrer auf den Kraftstoffverbrauch- eine methodische Analyse." 2. Stuttgarter SymposiumKraftfahrwesen und Verbrennungsmotoren. 18.-20. Feb. 1997.
/22/
D. Hrtzer, U. Ersus: "Entwicklung einer verbrauchsoptimierten Schaltstrategie fftr ein automatisiertes Schaltgetriebe." 2. Stuttgarter Symposium- Kraftfahrwesen und Verbrennungsmotoren. 18.-20. Feb. 1997.
/23/
M. Deacon, R.W. Horrocks, C.J. Brace, N.D. Vaughan, C.R. Burrows: "Impact of alternative controller strategies on emissions from a diesel CVT powertrain - preliminary results." IMechE Seminar on "Application of Powertrain and Fuel Technologies to meet Emission Standards." 24.-26. June 1996, London.
/24/
G. Hettich, G. Alberter: "Architectures for Electronic Powertrain Control." SAE Paper 970024.
/25/
P. Schoeggl, R. Schneider, P.L. Herzog: "Aspects for Future Vehicle Drive Train Controls." 6th EAEC Int'l. Congress "Lightweight and Small Cars. The Answer to Future Needs." 2-4 July 1997, Cernobbio, Italy
/26/
P.L. Herzog, P. Zelenka: "Automotive Application of Exhaust Gas Aftertreatment Systems toward Clean Diesel Engines." CIMAC Conference (FISITA Session), May 1518, 1995, Interlaken, Switzerland.
Three Way Catalysts
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CATALYSIS AND AUTOMOTIVEPOLLUTIONCONqROL I v Studies in Surface Science and Catalysis, Vol. 116 N. Kruse, A. Frennet and J.-M Bastin (Eds.) 91998Elsevier Science B.V. All rights reserved.
51
Novel Pd-based Three-Way Catalysts R. van Yperen, D. Lindner, L. Mul3mann, E. S. Lox, T. Kreuzer
Degussa AG, Automotive Catalysts Division P.O. Box 1345, 63403 Hanau, Germany ABSTRACT Three-way catalyst development programs focusing on processing precious metals on washcoats and on the application of the precious metals on certain washcoat components were initiated with the goal of improving the performance of three-way catalysts so as to fulfill the planned future emission standards with reduced precious metal costs. The different programs included newly developed Pd-only and Pd/Rh catalysts and novel oxygen storage components. The research programs resulted in the development of novel preparation techniques for exhaust catalysts, which led to a general improvement of catalyst performance. Besides the enhancement in activity the new preparation techniques make it possible to be more flexible and prepare tailor-made catalysts. The application of the new techniques for the preparation of novel oxygen storage components led to a further increase in catalyst performance despite the lower amount of oxygen storage components used. t. iNTRODUCTION The legislation concerning the exhaust emissions of automobiles is continuously tightened, requiring more active and durable catalysts. The essential improved catalyst performance has to be reached with a competitive cost structure. Decreasing the precious metal loading and applying the relatively low-priced Pd as (partial) replacement of Pt reduces the catalyst costs [1-9]. However, to ascertain a high or even improved activity some of the precious metals have to be applied selectively onto certain washcoat components, where the best performance can be realized. Since three-way catalysts have to fulfill two main tasks, i.e., the oxidation and the reduction reactions, different active sites are required. This often asks for the application of one or two precious metals on different washcoat components. Furthermore, to improve the dynamic characteristics of the three-way catalysts efficient oxygen storage comnonents have to be used.
52 This paper gives an overview of three-way catalyst development programs focused on new ways to place precious metal onto the washcoat and on the application of the precious metals on certain washcoat components. The goal was to improve three-way catalyst performance so as to achieve the planned future emission standards with reduced precious metal costs. The different programs included newly developed Pd-only and Pd/Rh catalyst technologies as described earlier [10], and novel oxygen storage components (OSC). Firstly, the experiments concentrated on the fundamental aspects, which will not be discussed in this paper. To get a better understanding of the single phenomena taking place in three-way catalysts, model catalysts were prepared and model gas experiments were performed. Secondly, completely formulated three-way catalysts were produced and tested in model gas test rigs and on engines to investigate their dynamic performance under different operation conditions. Thirdly, the most promising three-way catalyst systems were tested on a vehicle in the European test cycle. 2. E X P E R I M E N T A L
2.1. Catalyst preparation For the preparation of the three-way catalysts several procedures were used, which are summarized in Table 1. The reference catalyst samples were prepared by coating monolithic ceramic substrates with a cell density of 400 cpsi and a wall thickness of 6.5 rail. After drying and calcination of the coated monoliths, they were impregnated with the precious metals, precious metal loadings and precious metal ratios of choice. This method will be referred to as preparation method A. The novel catalyst technologies were prepared by placing the precious metals directly onto the washcoat. To do so, several methods were used. Preparation method C was used to apply the precious metals on all the washcoat components. The methods B and D were used to apply the precious metals selectively on one or more of the washcoat components. Details of the catalysts are given in Table 2.
2.2. Aging procedures Several aging procedures were utilized to determine the stability of the catalysts. Hydrothermal aging was performed in a furnace with 10 vol% water vapor in air at 1233 K for l0 hours. Engine aging of the catalysts was performed on EFI (electronic fuel injection) gasoline engines using dynamic fuel cut aging cycles. The minimum aging time was 40 hours, the minimum exhaust gas temperature at converter inlet (T~act) was 1123 K. The gasoline used in this study corresponded to the current E.U. standard EN 228 with a maximum sulfur content of 500 ppm. The data that compare different catalysts refer to catalysts aged in parallel.
53 TABLE 1" Preparation procedures remarks
Preparation method A B C
Technique Monolith impregnation WC impregnation Selective WC impregnation type 1
D
Selective WC impregnation type 2
homogenous distribution homogenous and controlled distribution more flexibility highest flexibility,
TABLE 2 Catalyst type description Catalyst type I, Ia, Ib IIa, lib III IVa, IVb V VI VII, VIIb VIII IX X
Preparation , method A B C B, B A B D A, B C, B C, D
,
Precious Meta! Pd Pd Pd Pd Pd Pd Pd Pd/Rh Pd/Rh Pd/Rh
Ratio g/g
9/1 9/1 9/1
Loading g/if3 100 100 100 100 45 45 45 60 60 6O
Single Layer x x x
Double Layer
x x x
2.3. Catalyst screening For engine tests cores of 1.5 inch diameter and 6 inch length were drilled out of larger pieces or full size pieces were used. The engine activity tests were performed on EFI gasoline engines using different types of light-off tests and different engine sweep tests. In the static engine sweep test the engine was operated at a certain lambda value. After stabilization of the engine and the exhaust gas the conversion was measured, after which the next lambda value was set. In the dynamic engine sweep test the lambda value was continuously changed from 0.99 to 1.02 within 3 minutes, during which the inlet and outlet gas flow was analyzed. Details are given in [11]. In the vehicle tests the ratio of catalyst volume to engine displacement was 0.77. The gasoline vehicle used in this study is certified for the current Euro II legislation and has an inertia weight of 2500 lbs. The engine displacement was 1.6 liters. The proposed new E.U. driving cycle was applied, which in contrast to the present E.U. test cycle (MVEG-A) does not have a 40 second period of not sampling the exhaust gas at engine idling conditions before starting the test.
54 65
80
typel
.~ 50
......
I.
.:-.-: .... ,:- :-: ........
ii
.2 65
type lla
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..
ll~el
=
o 35 r~
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type lib -
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I 0,996
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I
-
J-
1,004
Z
1,008
typeIla
35
''
0,992
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1
Lambda
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I
1,008
Figure 1 Static engine sweep test results for catalyst types I and II. (100 g Pd/ft3; 40 h, 1273 K engine aged; SV = 60000 Nl/I/h, T i n i e r = 673 K; 1FIz +IA/F)
.c 60
~9 55 r#,j
.>
l
=
~45
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0
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i
, ,f
1
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,
-
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1
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Figure 2: Static engine sweep test results for catalyst types I and IV. (100 g Pd/ft3; 40 h, 1273 K engine aged; SV=60000 N1/l/h, W ~ c t = 673 K; 1Hz +IA/F)
~60
. . . . . . .
~
=
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._c 50
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~60 "~,
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. . . .
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.-o
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I.
11
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1
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0,996
1
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Figure 3 Dynamic engine sweep test results for catalyst types V, VI and VII. (45 g Pd/Pt3; 10 h, 1233 K ,,ydro~hermal ag o'4",.~.,SV-50000 ,N1/Fh. T~.,~t--723 K; 1Hz -~1-.A/F)
3. RESULTS AND DISCUSSION 3.1. Pd-only catalysts To further improve the activity of our standard Pd-only three-way catalysts the precious metal was in a first attempt applied on certain washcoat components with commonly known techniques, i.e., method B. In this way it was tried to elucidate the effect of the interaction of the precious metal with the different washcoat components on the overall performance of the catalysts. Figure 1 shows the engine test results obtained with the first catalysts, where the precious metal was applied on an AI203 component (type IIa) compared with a catalyst where the precious metal was applied partially on AI203 and partially on one of the oxygen storage components (OSC) (type IIb) and a standard catalyst of type I. The results clearly show that the application of the precious metal on certain washcoat components with commonly known preparation techniques does not necessarily bring about an improvement in activity of the Pdonly catalyst under these test conditions. Both the oxidation of CO as well as the reduction of NOx suffer from the preparation method used. Since the first attempt to use the known standard preparation techniques to apply the precious metal on certain washcoat components clearly did not result in an improved activity under our test conditions, preparation method B was slightly modified and utilized to apply the Pd on A1203 in close contact with an OSC. Figure 2 gives the engine test results with a double layer Pd-only three-way catalyst, where in the bottom layer the precious metal was applied on an Al203 compound (type IVa) or on an Al203 component in close contact with an OSC (type IVb) in comparison with a standard Pd-only catalyst of type I. From the results it is obvious that applying the precious metal onto the AJ203 component in close contact with one of the OSCs leads to an enhanced performance of the catalyst. The catalyst, where the precious metal was only applied on the Al203 compound with the modified preparation method B, displays a performance comparable with the standard catalyst. With the results shown in Figure 1 in mind this indicates that the method of applying the pi'ecious metal onto the Al203 component strongly determines the activity of the catalyst. This phenomenon was further investigated and led to a novel preparation method for applying precious metals on complete washcoat formulations or washcoat components for utilization in engine exhaust catalysts. Figure 3 gives the engine test results obtained with Pd-only catalysts prepared with the preparation method B (type VI) and a first trial using a novel preparation method D (type VII) compared with a standard catalyst prepared by method A (type V). The preparation method B indisputably results in an improved performance for this catalyst type under the given test conditions. The utilization of the novel preparation method D also definitely ameliorates the catalyst performance. However, in comparison with the preparation method B the activity for the CO oxidation stays somewhat behind. However, the novel preparation method D involves significantly more parameter, which can be controlled and which influence the final performance of the catalyst. This gives the possibility to adapt the catalyst performance by varying one or more of these parameters in a controlled way.
56
~60
80
type Vllb
~950
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!
i
....
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1 Lamlxla
.~'~ "-,'-.':2
t
tyrx, u
1,004
1,008
Dynamic engine sweep test results for catalyst types VI and VII. (45 g Pd/ft3; 10 h, 1233 K hydrothermal aged; SV=50000 Nl/l/h, T~,t = 723 K; 1Hz +IA/F) ~70
type III
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7=,
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"'"I
75 type III
L) 50 0,992
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:
Static engine sweep test results for catalyst types I and III (100 g Pd/ft3; 40 h, 1273 engine aged; SV=60000 Nl/l/h, Tinier = 673 K; 1Hz •
=~80
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Static engine sweep test results for catalyst types I and III (100 g Pd/ft3; 40 h. !273 K engine aged; SV=60000 N1/1/h, T;.~,= 673 K: l Hz _4-1AfF)
1,008
57 The result of such a change of one of the preparation parameters on the catalyst performance can be seen in Figure 4. This figure clearly shows an improvement in CO oxidation activity and NOx reduction activity for the catalyst prepared under the different preparation conditions. The catalyst prepared by the novel preparation method D with optimized parameters' for this catalyst technology and test conditions (type VIIb) is even better than the catalyst prepared by the preparation method B (type VI). The results of fundamental research and the results presented in this paper indicate that benefits can be obtained by applying the precious metal on several washcoat components, e.g., AI20 3 and on OSCs. Instead of applying the precious metal on different washcoat components separately, we developed a modified preparation method for applying the precious metal selectively on one or on several washcoat components. Figure 5 gives a comparison of engine test results between a standard catalyst prepared by method A and a catalyst, where the precious metal was applied on the complete washcoat formulation by the novel preparation method C. The results definitely show that the utilization of the novel preparation method C results in a clearly enhanced catalyst performance. This unique preparation method was also tested for other washcoat formulations with a modified OSC. Figure 6 shows the performance during an engine test of a standard catalyst prepared by method A (type Ia), a catalyst prepared by preparation method A with a modified OSC (type Ib), and a catalyst with the same modified OSC but prepared by method C (type III). The results clearly show the benefits of the modified OSC as well as the benefits of the novel preparation method and indicate the possibility of using the novel preparation method for all kinds of Pd-only catalyst technologies. The new types of catalysts were tested on a vehicle. Figure 7 shows the emissions during the proposed new MVEG-A test cycle of a state-of-the-art Pd-only technology prepared by method A (type I) and the same technology, however, prepared by method C (type III). The activity of the catalyst prepared with the novel preparation method for the oxidation of CO and HC as well as for the reduction of NOx was higher in all stages of the test cycle. The total performance of the catalyst prepared with the novel preparation method is very promising. Despite the low catalyst volume to engine displacement ratio, the tailpipe emissions are lower than the proposed Euro 3 limits.
Figure 7:
Vehicle test results for catalyst types I and III. (100 g Pd/ft 3 76 h. 1123 K fuel cut aged' Vc~t/V~,,gi,,~-- 0.77)
58
~80
8O
".
~975
.- 70 r~2
1.
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typeaX
9 ,,
,
qJ >. ~
c 70
r,.) 0
o~
C r,.) NI
- -,
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t
........ ! . . . . .
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8:
[
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Lambda
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2:65
-
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.... l l
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. . . . . .
0,996
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' " ' , , , ,
, i
1
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1,0(18
Dynamic engine sweep test results for catalyst types VIII and IX. (9Pd/1Rh 60 g/ft3; 100 h, 1123 K fuel cut aged; SV=60000 Nl/l/h, Ti~et = 673 K; 1Hz •
=
-=945
55
t. ~J
.-;;-
= O 40
ca
35
~,, x 9 .-. ,-.. '" 7"
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Figure 9:
0,996
.. ....
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45
...
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.
i
i
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1,008
Dynamic engine sweep test results for catalyst types IX and X. (9Pd/1Rh 60 g/ft3; 50 h, 1123 K fuel cut aged; SV=50000 N1/l/h, T ~ t = 673 K; 1Hz +IA/F)
.~9o
9o ' ~ 1 7 6 1 7 6
o= .~ so
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0
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~
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x
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. 2r ~ 8 0
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= 65 o
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o,~
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Figure 10: Dynamic engine sweep test results for catalyst types IX and X. (9Pd/1Rh 60 g/ft 3 96 h, 1123 K fuel cut aged; SV=60000 Nl/l/h, T~a.t= 673 K; 1Hz •
59 3.2. Pd/Rh catalysts
The Pd-only technology developed with the new preparation method C was also tested for Pd/Rh three-way catalyst technologies. Figure 8 shows the engine test results of a standard Pd/Rh three-way catalyst technology (type VIII) compared with the same washcoat technology, however, with applying the new preparation technology for the Pd containing washcoat layer (type IX). The Rh containing washcoat layer was for both catalysts the same and was prepared by the utilization of preparation method B. The results clearly show the benefits of the novel preparation technique for applying the precious metal onto the washcoat on the overall performance of the Pd/Rh three-way catalyst. For further improvement of the activity of the Pd/Rh catalysts it was also tried to utilize the novel preparation method D for applying Rh onto one or more washcoat components. Figure 9 shows the engine test results of a Pd/Rh three-way catalyst technology, where for all three catalysts the preparation method C for the Pd containing washcoat layer was utilized. For the Rh containing washcoat layer two preparation techniques were used, i.e. method B (type IX) and method D. The method D was used to prepare catalysts where Rh was applied onto an AIzO 3 component with two different preparation parameters (type X and Xb). The results show that method D leads to comparable if not improved performance of the Pd/Rh three-way catalyst compared with the commonly known method B. As already discussed the new preparation methods are more flexible and include more parameters that can be controlled to prepare tailor-made catalysts. This can also be seen in Figure 9, where with the catalyst encoded type Xb it is shown that by changing the preparation parameters it is possible to prepare catalysts with an improved CO activity. Another important advantage of the novel preparation techniques is that they cannot only be used for the application of precious metal onto one or more washcoat components but also for the manufacturing of novel OSCs. The new preparation techniques allow the use of a smaller amount of OSC without sacrifices in catalyst performance, on the contrary. Figure 10 shows the engine test results of a Pd/Rh catalyst technology with commonly used OSCs compared with a catalyst technology where in the Rh containing washcoat layer a novel OSC is used, which was prepared by one of the new preparation technique (method D). Although the catalyst prepared with the new technology contains five times less OSC, the performance of the catalyst is comparable with the catalyst with a standard Rh washcoat layer. The CO oxidation activity is somewhat lower but the performance for the NOx reduction has improved by applying the new technology. The addition of stabilizers and promoters to the newly developed OSC should result in a further improvement in catalyst performance and will be investigated in future development programs.
60 4. CONCLUSION The development of novel preparation techniques for exhaust catalysts has resulted in a general improvement of the catalyst performance. For Pd-only catalyst technologies a preparation method was developed to apply the precious metal homogeneously and selectivitely into more than one washcoat component. These new catalysts show a significant improvement in performance compared with the standard Pd-only catalyst technologies. The same technology used to prepare the Pd containing washcoat layer in Pd/Rh three-way catalysts also results in an enhanced activity for these types of catalysts technologies. One of the novel preparation technologies was also used to apply Rh on one or two washcoat components, which led to an improved performance of the Rh containing washcoat layer.
ACKNOWLEGMENT. The authors wish to thank colleagues and coworkers for the valuable discussions and for the high quality experimental work. REFERENCES
1. B.H. Engler, E.S. Lox et al., ~tRecent Trends in the Application of Three Metal Emission Control Catalysts )>, SAE Paper 940928 (1994) 2. B.H. Engler, E.S. Lox, D. Lindner, A. Schafer-Sindlinger and K. Ostgathe, tt Development of Improved Pd-Only and Pd/Rh Three-way catalysts ~) in tt Catalysis and Automotive Pollution Contro III, A. Crucq, Ed. Elsevier (1994) 3. J.C. Summers, J.F. Skowron, W.B. Williamson and K.I. Mitehel, ~tFuel Sulfur Effects on Automotive Catalyst Performance )~, SAE Paper 920558 (1992) 4. J.Hepbum, K. Patel, M. Meneghel and H.S. Gandhi, tt Development on Pd-only Threeway Catalysts, SAE Paper 941058 (1994) 5. D.J. Ball, tt A Warm-up and Underfloor Converter Parametric Study ~) SAE Paper 930386 (1993) 6. B.H. Engler, E.S. Lox, D. Lindner and K. Ostgathe, ~ Advances in Three-way Catalyst Design to Meet more Stringent Emission Limits )~, ISATA Conference, Aachen, Germany, October 31-November 4, Automation Limited, England (19.94) 7. A. Punke, U. Dahle, S.J. Tauster, H.N. Rabinowitz and T. Yamada, (t Trimetallic Threeway Catalysts )), SAE Paper 950255 (1995) 8. R.J. Bfisley, G.R. Chandler, H.R. Jones, P.J. Anderson and .J. Shady, tt The Use of Palladium in Advanced catalysts ~>SAE Paper 950259 (1995) 9. S. Matsura, A. Akimasa, K. Arimura adn H. Shinjoh, ~tDevelopment of Three-way catalyst with Using Only Pd as Activator )) SAE Paper 950257 (1995) 10. D. Lindner, E.S, Lox, R. van Yperen, K. Ostgathe, T. Kreuzer, ~tReduction of Exhaust Gas Emissions by Using Pd based Three-way Catalysts )) SAE Paper 960802 (1996) 11. B.H. Engler, E. Koberstein and P. Schubert, ~t Automotive Exhaust Gas Catalysts: Surface Structure and Activity )) App. Cat. 48 (1989) 71-92
CATALYSIS AND AUTOMOTIVEPOLLUTIONCONTROLI v Studies in Surface Science and Catalysis,Vol. 116 N. Kruse, A. Frennet and J.-M Bastin (Eds.) 91998 Elsevier Science B.V. All rights reserved.
61
Comparative Behaviour of Standard Pt/Rh and of Newly Developed Pd-only and Pd/Rh Three-Way Catalysts under Dynamic Operation of Hybrid Vehicles S. Tagliaferri, R.A. K6ppel and A. Baiker Department of Chemical Engineering and Industrial Chemistry, Swiss Federal Institute of Technology (ETH), CH-8092 Z0rich, Switzerland
ABSTRACT The suitability of newly developed non-promoted and ceria-promoted palladium and palladium-rhodium three-way catalysts for the exhaust gas control of a hybrid drive system has been tested with periodic changes of the feed stoichiometry (~-r and pulsed-flow operation. The performance of the catalysts under dynamic operating conditions has been compared to the behaviour of analogue samples based on standard platinum-rhodium-technology. Combined use of mass spectrometry and time-resolved FTIR spectroscopy allowed simultaneous monitoring of the exhaust components. The air plug preceding the exhaust pulse in the intermittent operation of the combustion engine had a crucial impact on the performance of the catalysts. The air pulse transfers the catalyst into a defined state, which corresponds to a fully oxidized surface. Controlled application of appropriate ~-r allows to compensate the negative effect of the air plug and to achieve sufficient conversion of NOx as well as CO and HC to harmless compounds. For application in the hybrid vehicle, ceria-promoted palladium catalysts proved to be superior to the standard platinum-rhodium technology.
1. INTRODUCTION In view of new regulations for low-emission vehicles, research on hybrid vehicle technology has been intensified recently in an attempt to optimize overall vehicle performance, fuel efficiency and emissions. Generally, hybrid drive systems make use of the synergetie combination of a combustion engine, which guarantees a high range of performance, with an electric motor, which allows local emission free driving over a limited distance. Hybride concepts can basically be divided into serial and parallel configurations [ 1] with the latter showing higher overall efficiency but being more demanding with respect to emission control. As part of an interdisciplinary study we are working on emission control catalysis for an extended parallel hybrid concept [ 1-5]. The main elements of the experimental car developed at the Swiss Federal Institute of Technology are the combustion engine, operating in a fixed cycle mode, a flywheel as a short term energy storage device, an electrical machine and a continuous variable transmission. One of the features of this hybrid vehicle is the so called intermittent
62 mode, where driving energy is taken from the flywheel, which is recharged by operating an internal combustion engine for 3 s in intervals of about 17 s. On engine start-up and shut-off, the cylinders are filled with air, which consequently passes through the catalyst at the beginning of engine operation. The intermittent operation leads to a pulsed-flow operation of the TWC (Fig. lb).
a) X-Cycling l
b) Intermittent Mode
I I I i / I
I
I
I
-;k+
I .... x9time
flow rat
"l
~,: const, or cycled
air plug
......
,, ,, 17s
~ time 3s
Figure 1. Time dependent ~,-value and flow pattern during: (a) ~,-cycling between ~+ and ~,- with constant or variable length of the half cycles and constant gas flow; (b) Intermittent mode, 17 s without exhaust gas flow (engine shut off), 0.2 s pure air (air plug at engine start-up), 3 s exhaust gas flow (engine operation). Catalytic converters in automobiles are periodically forced about the stoichiometric air-fuel ratio at a frequency of about 1 Hz and a small amplitude [6]. In a recent review, Silveston [7] concluded that this periodic forcing suppresses rather than enhances conversions under normal operating conditions in the 400-600~ temperature range. Other authors reported that under cycling conditions the catalytic activities of three-way automotive catalysts can be superior compared to static conditions, depending on temperature, cycling period and feedstream conditions [8-10]. Cycling at temperatures below the light-offtemperature was found to increase conversions of NOx, CO and hydrocarbons, whereas the effect of )~-cycling was negative at higher temperatures [ 11 ]. Base metal oxides such as ceria are added to catalyst formulations in order to buffer excursions into the lean or rich region [12]. Ceria was reported to stabilize precious metal dispersion and to be involved in the storage and release of oxygen as well as in the promotion of the water gas shift reaction and the steam reforming reaction [ 13-15]. Recently, economic factors as well as the favorable low temperature performance and hightemperature resistance of Pd have lead to an increased interest in palladium as main noble metal component for three-way catalysts. Several research groups have presented a new generation of palladium catalysts with and without addition of Rh [ 16-20]. The objective of our study was to gain information about the behaviour of palladium based three-way catalysts under dynamic operation, especially under pulsed-flow operation as occurs in the intermittent mode of a combustion engine used in a hybrid vehicle.
2. EXPERIMENTAL 2.1. Catalysts The catalysts tested were supplied by Degussa AG and consisted of a ceramic honeycomb carrier with 400 cells/in 2. The washcoat loading was 110 g 1-1 with the composition (wt %) as denoted in Table 1. The catalysts had a length of 15 cm and a diameter of 2.5 cm. To reduce the
63 void volume of the catalyst to 12.73 cm 3, the outermost channels were sealed with an inert ceramic paste. Before catalytic tests, the catalysts were conditioned for 5 h at 600~ in a simulated exhaust with Z, = 1.
2.2. Apparatus Experiments were carried out in a fully computer controlled apparatus, which has been described in more detail elsewhere [3]. A synthetic exhaust gas mixture containing CO and HE at a ratio of 3:l, C3H6(500 ppm), C3H8 (500 ppm), NO (2000 ppm), O2, CO2 (12 %), H20 (10 %) and N2 (balance) was used for laboratory tests. The gas flow rate was 10.625 I(NTP) min "l, giving a gas hourly space velocity of 50'000 h "1 with regard to the total catalyst volume. The Evalue of the gas mixture, which represents the ratio between the available oxygen and the oxygen needed for full conversion of the components to CO2,H20 and N2: 2,=
2c~176176176176 2cco + ci_i2+ lOcc~m + 9Ccm, + 2Cco, + Ctl,O
(1)
was altered by adjusting the CO/H2 and the 02 flows via fast switching valves. The gas analysis system consisted of an FT-IR spectrometer (Bruker IFS-66) with a heatable gas cell (100 cm"3volume) and a quadrupole mass spectrometer (Balzers GAM 400). NO, NO2, N20, NH3, CH4, C3H6, C3H8, CO, CO2, and H20 were analysed by FT-IR spectroscopy and O2 and HE by mass spectrometry. The analytical system permitted the quantitative analysis with a resolution of up to 15 measurements per second. Table 1 Composition and denotation of tested catalysts. Catalyst denotation
Washcoat composition / wt % Pd
Pt
Rh
A1203
Pd
1
-
-
99
Pd-Ce
1
-
-
87
Pd-Rh
1
-
0.2
98.8
Pd-Rh-Ce
1
-
0.2
86.8
Pt-Rh
-
1
0.2
98.8
Pt-Rh-Ce
-
1
0.2
86.8
CeO2
12 12
12
2.3. Experimental procedure L-cycling (Fig. 1a) and pulsed-flow (Fig. lb) experiments were carried out to study the dynamic behaviour of the catalysts. Forced L-cycling with different amplitudes and frequencies was achieved by periodically changing the stoichiometry of the feed composition. To simulate the intermittent operation of the combustion engine in the hybrid vehicle, pulsed-
64 flow experiments (Fig. 1b) were carried out at 400~ and 1.7 bar. The exhaust gas was pulsed with a flow rate of 10.625 I(NTP) min1 through the reactor for 3 s, followed by a period of 17 s, with no gas flowing through the converter. The X-value was either kept constant or cycled symmetrically or asymmetrically during 3 s of the pulse. In most experiments an air plug with a flow rate of 3.187 I(NTP) min 1 and a duration of 0.2 s preceded the exhaust pulse, simulating air which is transferred into the cylinders.
3. RESULTS
3.1. Experiments with X-cycling Time resolved cycling experiments were carried out at 310~ using an amplitude of X = 1 • 0.05. In Figure 2 the changes of concentration of the exhaust gas components CO, C3H8, NO,
0.201
. . . . . . .
o,4 f
~
i ," \ ,
0.101 "-l
~o=-I,
,'
...-,,
,
Z0.15
~
-
/
~
/
/
-
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:
~ooo I
'~
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0.8 0.6
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o . ~ : - ~ - = ; !
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o, 1
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~ \
~
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,
,,
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.
000 l ' ~ ~ ' t ' ~ 1 2 3 Time/s
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,,,
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/
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'
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.-"=-I O0 5
t
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~
~
'
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o.,ol A
.... ! o.o
_~ .
0.201 0.15t
_ ;,'A:, I^..
\;
~o.oo r ' - ~
A
;
1 1.0
'
~ '
oo
. . . .
R~
o.,o
i 1.0
to8
Ao.4
o.oo~ ~ ~ . - - - - !
1o.2oi 0.15
.
.
-=
I /-~
O.lO-I / "i / "* 0 "
0
\ \
.
5
.
.
.
~
I
'
,'
.-,
~
0.00 r i ' ' , - - - ~ " ~ , ' ~ ~ ' - - - I 1 2 3 Tirre/s
o.oI~ .
R ~
i,.o 0.8
a
,-l-ao
, /t ',." .... -.1 I"0"4 ,I 0.2 4
5
00
Figure 2. Change of the concentrations of the most significant exhaust gas components with time at 310~ during X-cycling with X = 1 + 0.05 and v - 0.3 Hz for catalysts Pd, Pd-Ce, Pd-Rh, PdRh-Ce, Pt-Rh and Pt-Rh-Ce. The arrow indicates one rich half-cycle. Symbols: (.... ) CO, ( - - - ) C3H8, ( ~ ) NO, ( .... ) NH3, ( . - . ) N20, ( .... ) NO2 NH3, N20 and NO2 with time are shown for a frequency of 0.3 Hz. Propane had been chosen as low reactivity HC component to compare the performance of the catalysts, because 100 %
65 conversion was reached with propene. The black arrow indicates one rich half-cycle of 1.67 s duration. Qualitatively similar cyclic concentration-time profiles were observed for all catalysts, with the concentration maxima of both CO and propane appearing at the end of the rich halfcycle. For C3H8 a second maximum, coinciding with the NO peak at the end of the lean halfcycle, was observed. This effect was most pronounced for catalysts Pd-Rh-Ce and Pt-Rh-Ce, which also showed highest propane conversion. For all catalysts except Pt-Rh, substantial amounts of NH3 were produced. Ammonia concentrations reached their maximum in the middle of the lean half-cycle when NO started to appear. Substantial amounts of nitrous oxide were formed with catalyst Pd, whereas only small amounts were observed for the other catalysts. NO2 was not produced in significant amounts. Interestingly, CO formation was reduced for the Pd containing catalysts upon addition of ceria, whereas the opposite effect was observed for the Pt containing catalyst. When the frequency of L-cycling was increased to 1 Hz, concentrations of the exhaust gas components generally decreased markedly. The concentration-time profiles for catalysts Pd-RhCe and Pt-Rh-Ce are depicted in Figure 3 as an example. Concentrations were very low and no
0.04
0.08 '
"1"
o
'
'
'
' Pd---Rh-Ce '
i
'
'
'
'
'--Rh-C~ .... Pt'
,~176
z
o;
"** .
0.03-
**
*-***..**...*'**
.....
**..***
.~
0.06
Z & 4
~)
II II il
0.02
9 Ir"~l
" :
o o
." :
8
:
0.01
: : I
l
"~ II
o it
t I
9
I
I
: " t
" I
"L
o a * i a
o i "
o
m t
0.04 ~I
*1
I o * o
o: ,~ m , m
II ,
II
,
t ~
" ~,
, , I
'
8
@mmD
l
"
0.021
**" . . . . ~ 1 7 6 1 7 6 1 7 6 1 7 6 1. 7 6
I
0.00 2
3
Time/s
4
5 1
2
3 Time/s
4
5
Figure 3. Change of the concentrations of the most significant exhaust gas components with time at 310~ during ~,-cycling with ~, = 1 + 0.05 and v = 1 Hz for catalysts Pd-Rh-Ce and Pt-RhCe. The arrow indicates one rich half-cycle. Symbols: ( .... ) CO, (----) C3H8, ( ~ ) NO, ( .... ) NH3, ( . - . ) N20, (.... ) NO2 cyclic effects were discernible for the fully promoted platinum catalyst, whereas oscillations with intervals of 1 s were still observed for the corresponding palladium based catalyst.
66
3.2. Experiments with pulsed flow Simulation of the pulsed-flow operation of the catalytic converters was performed at 400~ Although the thermally insulated catalyst is located close to the exhaust ports in the hybrid vehicle, the exhaust temperature is expected to be uncommonly low for TWC applications due to the intermittent operation of the combustion engine. The performance of the various catalysts was examined for the nine different intermittent operating modes A to I, listed in Table 2. For modes A and B a stoichiometric exhaust with constant Z-value was used with (B) and without (A) a preceding air plug, whereas in mode C the gas mixture was kept rich to compensate the effect of the air plug. For operating modes D to I the Z-value was cycled during the pulse with a frequency of 1 Hz and an amplitude of + 0.05. Experiments were performed in pairs where cycling started either with a rich or lean exhaust gas, and was either symmetric or asymmetric (Table 2). Table 2 Examined intermittent operating modes Operating mode
a)
Air plug
Z-value, v
First half-cycle
Rich half-cycle a)
A
no
1.0
-
-
B
yes
1.0
-
-
C
yes
0.985
-
-
D
no
1 + 0.05, 1 Hz
rich
50 %
E
no
1 + 0.05, 1 Hz
lean
50 %
F
yes
1 + 0.05, 1 Hz
rich
50 %
G
yes
1 + 0.05, 1 Hz
lean
50 %
H
yes
1 • 0.05, 1 Hz
rich
60 %
I
yes
1 • 0.05, 1 Hz
lean
60 %
Portion of the rich half cycle, i.e. 60 % means 0.6 s rich and 0.4 s lean at 1 Hz.
The performance of catalyst Pd-Rh-Ce for the intermittent operating modes A, B, and H is illustrated in Figure 4, which shows the time-dependence of the gas concentrations during an exhaust pulse. With mode A, CO broke through after 0.5 s, but was eliminated in the second half of the pulse. During the period of 17 s without exhaust gas flow, the catalyst approached chemical equilibrium and CO concentration was zero at the beginning of the exhaust pulse. About 500 ppm NH3, corresponding to ca. 25 % of the NO inlet concentration in the exhaust, were continuously produced during the pulse, with the concentration being slightly higher in the second half of the pulse. NOn, C3H8, and N20 were not detected in significant concentrations. A preceding air pulse (mode B) had a dramatic influence on conversions. CO and NH3 were quantitatively eliminated from the exhaust, whereas NO broke through immediately after the
67
0.20
!
'
9
I
'
A
0.15 0.10 #
0.05
~ ....
0.00 o..9`
0.20
" .o_
0.15
t,..,, t'-
q) r
*" o (9
oo
,,=~
I ,t1-o ~ . . . . . . . . .
.....
- --- = ' ~ - - -
I
~ 1 7 6 1 7 6 '~ q ' ,=
,~176
'
'~..,.,=
r'-"~-/~
'
0.10 0.05 0.00 0.20
--- ~~ ~
'
.....
- .....
,~i~----'-'---r,,~:--~,-~-_-- .....
!
'
I
'
0.15 0.10 0.05
0.00
__!
0
1
__
,
2
Time during exhaust gas pulse / s
3
Figure 4. Concentrations of exhaust components for intermittent operating modes A, B and H with catalyst PdRh-Ce at 400~ Influence of air plug and its compensation. A: exhaust pulse with L = 1; B" air plug followed by exhaust pulse with L = 1; H: air plug followed by asymmetric L-cycling with L = 1+0.05, 1 Hz, periods 0.6 s rich/0.4 s lean. Symbols: (.... ) CO, (----) C3H8, ( ~ ) NO, ( .... ) NH3, (--.) N20, (.... ) NO2
beginning of the pulse, reaching full inlet concentration after 1 s. During the remaining time of the pulse NO concentration was reduced almost completely, showing a second smaller maximum at the end of the pulse. Best performance of cataiygt Pd-Rh-Ce was observed for the operating mode H. In this case, asymmetric cycling, starting with a rich exhaust of 0.6 s, compensated the negative impact of the air pulse on conversion of NO to N2 almost completely, without affecting conversion of the other exhaust components. To compare the effect of the intermittent operating modes A to I on the behaviour of the different catalysts, the concentrations of the exhaust components were integrated over the period of the pulse and divided by 3 s. As the inlet concentration of CO was not constant for the experiments with cycled feeds, concentrations instead of conversions are given. Figure 5 depicts the average C3Hs-conversions, N2-yields and CO-concentrations during an exhaust pulse of 3 s at 400~ Depending on the operating mode applied, strongly different performance of the catalysts was observed, e.g. C3Hs-conversion showed a maximum of 80 % for catalyst Pd-Rh with mode D, but only 26 % with mode E. Similarly, N2-yield ranged between 0 % (modes A and B) and 8 2 % (mode H) for catalyst Pd-Ce. The results indicate, that by choosing appropriate operating modes high C3Hs-conversions and N2-yields as well as low CO-concentrations can be achieved with all catalysts.
68 For the palladium based catalysts the presence of ceria had a strongly positive effect on C3Hsconversion as well as CO-concentration, independent of the operating mode applied. Moreover, N2-yields increased upon addition of ceria to the catalyst formulation, except for the fully promoted palladium catalyst and mode F. Adding rhodium to the palladium based catalyst had generally a negative impact on CO concentrations as well as on N2-yields for modes E, G and I. Interestingly, C3Hs-conversion was also significantly lower for these operating modes starting with a lean exhaust composition, which should facilitate propane conversion. As regards the platinum based catalysts, addition of ceria had a less pronounced influence on CO-concentration, C3Hs-conversion and N2-yield. Similarly as with the Pd-catalysts operating modes E, G, and I negatively influenced N2-yield and C3Hs-conversion, whereas otherwise slightly positive effects were observed. These results are also supported by the data listed in Table 3, which show the operating mode resulting in best performance with regard to highest N2-yield, highest C3H8 conversion and lowest CO concentration for the different catalysts. Evidently, ceria had a positive effect on C3H8 conversion and CO concentration as well as on N2-yield for the palladium containing catalysts. 100 60
~
40 ~ 20 0
:
100,
~
:
0.4 0.2
:
:
:
0
:
:
:
:
:
r 1.o
, 0.0
O,
~o40 i oo
-1
0
100
60 ~ 40 20 0
Pd
``9` 0 0 : : : : : , Z~ ~1001~. O' >~' 40608~02_0 I~ ~ ~ ~ ~I1"0 o G00"802.0~(."64.)e.. 0
...
=
loo- i
=
=
=
:
0 0
1.o
0.6 !~_ 60 0.4 ,,~
-~. 6040~> 20-
0.6 0.4 0.2 0.0
. 0.0
0.4 0.2 7:~
Z 100
Ii.o0.8
60 40 20
0.6
-~ 40 "~, 20 ~"
lo8o!iol
1.0
--
’
,
i
’
:
:
I ~~l~I :
:
:
:
0.2
:
:
:
0.0
15
C)
i '~
,
. 0.6 0.4 0.2 1.0
0
_~
0.6
40
0.4
20
0.20 :
,mlmmmmmt
.
.
.
0.0
.
~ Pd
cO
0.6 0.4 0.2 0.0
', . . . . .
Pd-CePd-Rh Pd-Rh-CePt-Rh Pt-Rh-Ce
r---] C3H8
~
N2
~
CO
0.0
Pd-CePd-Rh Pd-Rh-CePt-Rh Pt-Rh-Ce
Figure 5. Average C3Hs-conversions, N2-yields and CO-concentrations during an exhaust pulse of 3 s at 400~ for intermittent operating modes A to I and catalysts Pd, Pd-Ce, Pd-Rh, Pd-RhCe, Pt-Rh and Pt-Rh-Ce.
69 Table 3 Intermittent operating modes affording best performance. Selection has been based on following priority of performance characteristics at the catalyst outlet: highest N2-yield- highest C3H8 conversion- lowest CO concentration. Catalyst
Operating Mode
Nz Yield / %
C3H8Conv./%
CO Conc./%
Pd
D
60
67
0.48
Pd-Ce
H
82
94
0.00
Pd-Rh
H
76
80
0.53
Pd-Rh-Ce
H
83
96
0.00
Pt-Rh
C
83
70
0.05
Pt-Rh-Ce
H
72
84
0.02
Note the overall good performance of the rhodium free catalyst Pd-Ce, which showed almost the same characteristics as the fully promoted palladium catalyst Pd-Rh-Ce. Ceria addition also increased C3H8 conversion of the platinum catalyst, but resulted in a lower N2-yield. Generally, the palladium catalysts Pd-Ce and Pd-Rh-Ce showed similar or even superior catalytic performance compared to Pt-Rh-Ce.
4. DISCUSSION The potential of ceria for the storage of oxidizing and reducing components has a marked influence on the dynamic behaviour of the catalysts. For the non-promoted palladium catalyst (Pd), highest and broadest concentration peaks of CO and NO were observed, which can be explained by the missing storing capacity of ceria. Upon addition of ceria to the palladium catalyst (Pd-Ce), CO and NO peaks became significantly smaller and more narrow. The concomitant increase in NH3 formation can be attributed to a promoting effect of ceria on steam reforming and water gas shift reaction, which results in an increased formation of hydrogen. A similar effect is observed by comparing the performance of catalyst Pd-Rh with Pd-Rh-Ce, whereas addition of ceria to platinum only increased ammonia formation without decreasing CO and NO concentrations. Promotion of the activity of precious metal catalysts for the water gas shift and the steam reforming reaction by ceria has been occasionally reported [15]. For pulsed-flow operation experiments, the air plug at the beginning of the exhaust pulse substantially influenced the performance of the catalysts for steady stoichiometric exhaust compositions. NOx conversion to N2 strongly decreased. However, by adapting appropriate ~,cycling during engine operation, the negative effect of the air plug can be compensated. Moreover, the preceding air pulse transfers the catalysts into a defined state. Knowledge of this state, which corresponds to a fully oxidized surface, can be beneficial to improve the ~-control algorithm used. As expected, NE-yields were usually lower for operating modes G and I, starting with a lean exhaust after the air pulse. Best catalytic performance was observed for asymmetric cycling, starting with a rich exhaust of 0.6 s.
70 cycling pattems has so far not completely been exploited for optimizing exhaust catalysis.
5. CONCLUSION The suitability of newly developed palladium- and palladium-rhodium catalysts and of standard platinum-rhodium catalysts for the after treatment of the exhaust of a hybrid drive system, resulting in pulsed flow operation of the catalytic converter, has been compared. It was demonstrated, that the air pulse preceding the exhaust pulse, strongly influences the catalytic performance. The apparently negative impact of the air pulse on catalytic behaviour was found to be beneficial by virtue of transferring the catalyst into a well defined state, which can be accounted for in the closed-loop ~-control. Applying a rich exhaust during engine operation increases N2 yield, but partly lowers CO conversion. The use of an asymmetric cycling pattern with longer rich half cycles results in CO and HC conversions as well as N2 yields higher than without an air pulse. From the data presented it becomes evident that the ceria promoted palladium catalysts Pd-Ce and Pd-Rh-Ce are able to outperform conventional Pt-Rh-Ce catalyst in hybrid vehicle application. ACKNOWLEDGEMENTS
Financial support of this work by the Schweizerisches Bundesamt fiir Umwelt, Wald und Landschafi is gratefully acknowledged. The authors wish to thank Degussa AG for providing the catalyst samples. REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
L. Kiing, A. Vezzini and K. Reichert, Symposium Proceedings 1lth International Electric Vehicle Symposium, 1992. P. Dietrich, H. HOrler and M.K. Eberle, Proc. Conference on Electric, Hybrid and Alternative Fuel Vehicles, Aachen, Germany 13th-17th Sept. 1993, p. 193. L. Padeste and A. Baiker, Ind. Eng. Chem. Res., 33 (1994) 1113. S. Tagliaferri, L. Padeste and A. Baiker, Stud. Surf. Sci. Catal., 96 (1995) 897. L. Padeste, S. Tagliaferri and A. Baiker, Chem. Eng. Technol., 19 (1996) 89. R.P. Canale, C.R. Carlson, S.R. Winegarden and D.L. Miles, SAE Technical Paper Series, No. 780205 (1978). P.L. Silveston, Catal. Today, 25 (1995) 175. H. Shinjoh, H. Muraki and Y. Fujitani, Appl. Catal., 49 (1989) 195. H. Muraki, K. Yokota and Y. Fujitani, Appl. Catal., 48 (1989) 93. E. Jobson, M. Laurell, E. H6gberg, H. Bernler, S. Lundgren, G. Wirmark and G. Smedler, SAE Technical Paper Series, No. 930937, (1993). B.K. Cho, Ind. Eng. Chem. Res., 27 (1988) 30. H.S. Gandhi, A.G. Piken, M. Shelef and R.G. Delosh, SAE Technical Paper Series, No. 760201 (1976). J.G. Nunan, H.J. Robota, M.J. Cohn and S.A. Bradley, J. Catal., 133 (1992) 309.
71 14. R.M. Heck and R.J. Ferrauto, Catalytic Air Pollution Control, Van Nostrand Reinhold, New York, 1995. 15. J. Cuif, G. Blanchard, O. Touret, M. Marczi and E. Qu6m6r6, SAE Technical Paper Series, No. 961906, (1996). 16. J. C. Summers, W. B. Williamson and J. A. Scaparo, SAE Technical Paper Series, No. 900495 (1990). 17. B.H. Engler, D. Lindner, E.S. Lox, A. Sch~ifer-Sindlinger and K. Ostgathe, Stud. Surf. Sci. Catal., 96 (1995) 441. 18. S. Matsuura, A. Hirai, K. Arimura and H. Shinjoh, Sci. Technol. Catal., 92 (1995) 445. 19. J. Dettling, Z. Hu, K. Lui, R. Smaling, Z. Wan and A. Punke, Stud. Surf. Sci. Catal., 96 (1995) 461. 20. D. Lindner, E.S. Lox, R. Van Yperen, K. Ostgathe and T. Kreuzer, SAE Technical Paper Series, No. 960802, 1996.
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CATALYSIS AND AUTOMOTIVE POLLUTION CONTROL IV Studies in Surface Science and Catalysis, Vol. 116 N. Kruse, A. Frennet and J.-M Bastin (Eds.) 91998 Elsevier Science B.V. All rights reserved.
73
Comparative three-way behaviour of Pt, Pd and Rh single and combined phases in a full gas mixture with oscillating feedstream J. R. Gonzhlez-Velasco, J. A. Botas, R. Ferret and M. A. Guti~rrez-Ortiz Department of Chemical Engineering, Faculty of Sciences, Universidad del Pals Vasco, P.O. Box 644, E-48080 Bilbao, Spain
ABSTRACT A series of palladium, platinum and rhodium (single and combined) catalysts supported on cerium-doped 7-alumina has been prepared. The monometallic catalysts were prepared by adsorption from the metal solution, and the multimetallic catalysts by joint adsorption as well as by physical mixture of those monometallic which allowed to obtain similar final metal composition. The three-way behaviour of the prepared catalysts has been tested with full synthetic gas mixtures composed of N2, CO2, CO, C3H6, NO, 02 and H20 under reducingoxidising cycled and stationary feedstream compositions.
1. INTRODUCTION Three-way catalysts (TWC) which perform, at the same time, oxidation of carbon monoxide (CO) and hydrocarbons (HC) and reduction of nitrogen oxides (NOx), seems to be, up to now, a satisfactory and efficient solution. Fine work of these catalysts requires a composition of the exhaust gaseous stream corresponding to the stoichiometric air-to-fuel ratio, i.e. A/F=14.63 for a fuel with a H/C ratio of 1.89, which should be precisely controlled. It has been proved that monometallic Pt catalysts present high activity operating about stoichiometry, even more than some Pt-Rh formulations [1]. The reason for the addition of Rh becomes apparent when studying selectivity of the reaction under reducing conditions [1,2]. In previous work we have developed a rhodium-free catalyst with adequate activity on the simultaneous control of NO, HC and CO [3]. An increasing interest to promote the use of Pd in these catalysts, due to its low cost and large availability, appeared in the last years, which marks a tendency to substitute Pt by Pd in the conventional Pt-Rh compositions, or even
74 to develop new Pd-only formulations [4,5], maintaining the activity and durability of the catalyst. Alloying between precious metals in three-way catalysts has been proposed to lead to both negative [6] and positive [7] effects on performance The impact of Pt-Rh and Pd-Rh alloying on performance as well as the cumulative effect of both metals on overall activity is being intensively investigated to provide valuable bases for designing new formulations with enhanced characteristics [8-10]. In this work, we compare the TWC behaviour of Pd, Pt, Rh, Pd-Pt, Pd-Rh, Pt-Rh, and Pd-Pt-Rh in a simulated stationary and/or cycled environment near that existing in automobile catalytic converters, trying to discover the relative merits of each metal on the overall performance of the catalyst. Comparison of activity obtained with multimetallic catalysts prepared by co-adsorption and those obtained with physical mixtures of monometallic catalysts will contribute significantly to our understanding of the impact of the interactions between metals on performance and could provide a valuable basis for designing new formulations with enhanced characteristics.
2. EXPERIMENTAL 2.1. M a t e r i a l s
The starting alumina was SAS-1/16 supplied by La Roche. After grinding to adequate particle size and calcination in air at 700~ for 4 hours, its properties resulted in: catalyst size, 0.5-1.0 mm; surface area BET, 200 m 2 g-l; pore volume, 1.0 cm 3 gl; average pore radius, 5.3 nm; mode pore radius, 6.1 nm; isoelectric point, 7.6. The cerium oxide was incorporated by the conventional incipient wetness method from an Ce(NO3)3.nH20 aqueous solution, at 40~ and 30 mmHg. Promoter-modified alumina samples were dried at 120~ for 2 hours and calcined in air at 700~ for 4 hours to decompose the nitrate to oxide. The active phases --Pd, Pt, and R h ~ were incorporated by adsorption from aqueous solution using their corresponding salts--PdC12, H2PtC16-nH20, and RhCl~-nH20-- using 40 cm3 of solution per gram of ceria-modified alumina. The multimetallic catalysts were prepared by joint adsorption of the corresponding metallic s a l t s - - P d - P t , Pd-Rh, Pt-Rh, and Pd-Pt-Rh-- and by physical mixture of the monometallic catalysts--Pd+Pt (50/50 wt.-%), Pd+Rh (50/50 wt.%), Pt+Rh (50/50 wt.-%), Pd+Pt+Rh (33.3/33.3/33.3 wt.-.%)--. The nominal composition of the prepared catalysts was 0.5 wt.-% Pd, 0.1 wt.-% Pt, and 0.02 wt.-% Rh as the most usual in catalytic converters. After drying in nitrogen for 1 hour at 120~ final activation of the precursors was made by calcination at 550~ in a nitrogen atmosphere for 4 hours and subsequent treatment in a H2/N2=90/10 stream for 2 additional hours. The final catalysts resulted in the compositions shown in Table 1.
75 Table 1 Composition of the prepared catalysts, wt.-% Component CeO2 Pd
Pt
Rh
Pd Pt Rh
7.29 8.83 8.84
0.47 -----
0.079 ---
--0.021
Pd-Pt Pd-Rh Pt-Rh Pd-Pt-ah
8.43 8.81 9.16 8.87
0.45 0.47 --0.50
0.088 --0.081 0.087
--0.017 0.021 0.017
Pd+Pt Pd+Rh Pt+Rh Pd+Pt+Rh
(5.94+9.02)/2 (5.94+8.54)/2 (9.02+8.54)/2 (5.94+8.85+9.74)/3
0.84/2 0.84/2 --0.84/3
0.20/2 --0.20/2 0.31/3
--0.041/2 0.041/2 0.047/3
Monometallic catalysts (adsorption)
Multimetallic catalysts (co-adsorption)
Multimetallic catalysts (physical mixtures of monometallic catalysts)
2.2. A c t i v i t y t e s t s Catalytic activity data were obtained by using a conventional fixed-bed reactor at atmospheric pressure. A stainless steel tube with an inner diameter of 12 mm was chosen as the reactor tube. Catalyst (3.5 cm 8, ca. 1.8 g) was placed on ceramic wall at the lower part of the reactor. The upper part of the catalyst bed was packed with 10 cm 3 of inactive ceramic spheres (2 mm O.D.) to preheat the gas feed. The furnace temperature was controlled with a maximum variation of 2~ by an automatic temperature controller. The gas exiting the reactor was led to a condenser to remove water vapour. The remaining components were continuously analysed by non dispersive infrared (CO and CO2), flame ionisation (HC), magnetic susceptibility (O2), and chemiluminiscence (NOx). The redox characteristics of the model gas mixtures can be identified by the air-to-fuel ratio, A/F 14.63 A/F = 1+ 0.02545{[CO1 + [H 2 ] + 3 n [ C n H 2 n ] + ( 3 n + 1)[CnH2n+2 ] _ 2[02] _ [NO]} (1) To investigate the TWC behaviour of the prepared samples in an environment which resembled the exhaust A/F fluctuations in a closed-loop emission control system we used a similar apparatus to that developed previously by Schlatter et al. [11]. Two fast-acting solenoid valves allowed one to cycle between the two following feedstreams prepared in two independent gas blending systems: Reducing feedstream (A/F=14.13). It was composed of 10% C02, 1.60% CO, 900 ppm NO, 900 ppm Call6, 0.465% 02, 10.0% H20, and a balance of N2.
75
Oxidising feedstream (A/F=15.17). It consisted of 10% CO2, 0.40% CO, 900 ppm NO, 900 ppm C~H6, 1.26% 02, 10.0% H20, and a balance of N2. The prepared catalysts were tested cycling both feedstreams, with a frequency of 1 Hz, an amplitude of +0.5 A/F, and a space velocity of 125,000 h -1 (STP). The temperature was increased from 100 to 600~ at a rate of 3~ min "1, and the conversion data were continuously measured. Thus, the light-off temperature which is necessary to achieve 50% conversion, Tso, and the stationary conversion at the normal running temperature of 500~ Xsoo, were determined from the obtained activity data. Once the conversion-temperature profiles obtained, the experiment was continued at 500~ but shifting the cycled feedstream to some stationary feedstreams with the following composition: 10% CO2, 1.00% CO, 900 ppm NO, 900 ppm C3H6, 0.448% to 1.510% 02, 10.0% H20, and a balance of N2. These different oxygen percentages in feedtream allow us to experiment with A/F=14.33, 14.53, 14.63, 14.73, 14.93, and 15.13. From these experiments one can determine the stoichiometric window, defined as the interval of A/F inside which the conversion is equal or above 70% for all three contaminants. 3. R E S U L T S AND DISCUSSION
3.1. Activity under cycled feedstream composition Figure 1 shows the obtained CO-conversion profiles for all the tested catalysts. From this figure and similar ones for CaH6-conversion and NOconversion profiles (Figures 2 and 3, respectively), the T~o and Xsoo were determined resulting in the values shown in Table 2. Table 2 Tso and Xsoo obtained in cycled conditions for the prepared catalysts CO NO C3H6 Catalyst Tso (~ X~oo (%) Tso (~ Xsoo (%) Tso (~ Pd 322 100 325 70 313 Pt 159 100 305 90 300 Rh 245 99 253 71 268 Pd-Pt 307 100 310 76 303 Pd-Rh 312 100 316 76 307 Pt-Rh 173 100 247 91 257 Pd-Pt-Rh 304 100 307 89 301 Pd+Pt 271 100 281 90 277 Pd+Rh 267 100 267 90 273 Pt+Rh 166 100 247 90 260 Pd+Pt+Rh 249 100 257 89 264
Xsoo (%) 100 100 100 100 100 100 100 100 100 100 100
77 100 The analysis of reaction data becomes complex due to the large 80 number of reactions involved in the o< system [12]. Nevertheless, the high 60 conversions at 500~ shown in Table ~ 4o 3 confirm a very good three-way behaviour at this temperature for all 0 u 2O the tested catalysts, which are able to achieve total oxidation of CO and 0 600 300 400 500 100 200 C8H6, and high activity for NO 100 reduction, especially with the platinum-containing formulations. 80 As previously reported for platinum catalysts [13], in the 60 Pd-Pt profiles corresponding to Pt, Pt-Rh Pd-Rh and Pt+Pd, Pt+Rh, and Pt+Pd+Rh ".. Pt-Rh 40 Pd-Pt-Rh catalysts two regions can be clearly 20 observed: (i) the direct oxidation (CO+Y209 ~CO2) at low temperature 0 200 300 400 500 600 100 (T<125-200~ enhanced by the 100 presence of water, and (ii) the oxidation with NO (CO+NO-~Y2N~ 80 +CO2) and the water-gas shift o< (CO+H~O-~CO~+H~) above 250~ ~o ---&---- Pd+Pt Oxidation of carbon monoxide is Pd+Rh ~Pt+Rh favoured at 150-170~ by the ~ 4o Pd§ o presence of water although water-gas 0 0 2O shift becomes important only at about 300~ [ 14,15]. 100 200 300 400 500 600 With propene (or other olefins) Temperature, ~ a stabilisation of the CO-conversion takes place at intermediate Figure 1. Temperature-programmed COtemperature as a consequence of some conversion profiles obtained with all self-poisoning: CO is known to be prepared catalysts strongly adsorbed on platinum surfaces which effectively inhibits the adsorption and decomposition of oxygen [16]. Partial thermal desorption of CO allows the adsorption of oxygen and the oxidation with remaining adsorbed CO. This inhibitory effect may be weakened by water [17] and/or the hydrocarbon disappears at higher temperature (>200~ when adsorbed HC and CO begin reaction with oxygen and/or NO. On the contrary, paraffins are weakly adsorbed on platinum surfaces [6,18]; in fact, if propene is substituted by methane in feedstream, the self-poisoning effect disappears from the CO-conversion profile. 0
-':
::
TM
I
: ~'
= : = .....
o
0
.....
78 The effect of the nature of hydrocarbon processed in feedstream was analysed in previous work [13]. The monometallic Pd-catalyst needs higher temperature (T~o=322~ than those needed by monometallic Pt and Rh catalysts (T~o=159/250 and 245~ respectively) to become active in CO removal. The co-adsorption of Pt and/or Rh with Pd does not improve the behaviour of the monometallic catalyst. However, co-adsorption of Rh with Pt enhances the CO-conversion in the second region (after direct oxidation) resulting in a positive synergic effect, i.e. the performance of the Pt-Rh catalyst can be described by a superposition of the performance features of the Pt-only and the Rh-only catalysts, and also very similar to the performance of the Pt+Rh catalyst. This suggests that the oxidation of CO at low temperatures (125-200~ occurs on platinum, while at higher temperatures this oxidation occurs mainly on the rhodium sites, being both functions accesible to the reactant. With the Pd-containing catalysts, the total oxidation of CO is reached in only one step, as observed with monometallic Pd and Rh catalysts, once the removal of propene has begun with oxygen (C3H6+4 89 and NO (C3H6+9NO-~4 89 The co-adsorption of Pt and/or Rh with Pd does not improve the behaviour of the monometallic Pd-catalyst, suggesting that with Pd-catalysts prepared by coadsorption the elimination of CO occurs mainly on Pd, which could be even covering part of the other metal surface. These results indicate that Pd may alloy with rhodium and segregate to the particle surface inhibiting the rhodium function as has already been postulated as a cause of deactivation in previous Pd-Rh catalysts [7-9]. The total CO-conversion is obtained once the olefin has been completely converted, as can be seen by joint observation of Figures 1 and 3. The CO-conversion reached during the first oxidation step with the multimetallic Pd+Pt, Pt+Rh, and Pd+Pt+Rh being lower than that obtained with the monometallic Pt catalyst is due to the fact that, although there is no difference in the platinum loading, each active phase is distributed in one part of the catalyst bed, which implies less metal dispersion and metal surface area to be involved in the reaction. The oxidation of propene by either 02 and NO (Figure 2) is well achieved with all the prepared catalysts, which begin to be active around 250~ followed by a sharp rise in activity to total conversion at temperature of 3000C and above. The Rh-only catalyst presents lower light-off temperature than the Pt-only catalyst which is similar to that of the Pd-only catalyst. The Pt-Rh prepared by co-adsorption gives the best light-off performance, the presence of Rh enhancing the effectiveness of Pt for HC performance. On the contrary, all the Pdcontaining catalysts prepared by co-adsorption present similar behaviour to that corresponding to the Pd-only catalyst. The performance of catalysts prepared by physical mixture can be described by a simple linear ponderation of the performance features of their corresponding single-metal catalysts, resulting in light-off curves practically coincident.
79
100 -
100 80 -
8O
iff o
60
>4) o'~ o 3: "
40
~
60 -
'~Pd' -...e--et
"~ ~ ~
" 40 -
_
~
.
~
20 0 100
200
'~176 -
300
400
500
2o 0 100
600
1~176
80
pt i Rh I
200
300
400
500
600
80
60 = :
40
Pd-Rh Pt-Rh
~
60
c
40
-" Pd-Pt -~' Pd-Rh ~Pt-Rh = Pd-Pt-Rh
0 100
200
300
400
500
0 100 r 100
600
100
200
300
400
500
600
80
_o :9 4) > r O O
A
60
I
40
-" Pd+Pt ". Pd+Rh v" Pt+Rh ~- ~Pd+Pt+Rh
4) ~= 0 o
/
40
~I
I -" :Pd+Pt
~
I~ P d + R h
2O
10o
200
300
400
Temperature,
500
600
~
Figure 2. Temperature-programmed C3H6conversion profiles obtained with all prepared catalysts
100
200
300
400
Temperature,
500
600
~
Figure 3. Temperature-programmed NOconversion profiles obtained with all prepared catalysts
The steam-reforming does not occur in our operational conditions, which also was experimentally proved by performing the propene steam-reforming reaction (removing the rest of components in feedstream) and noting that this reaction occurs at temperatures above 400~ [12]. Comparison of Figures 1 and 2 makes clear how the CO-conversion is restablished with the beginning of the propene oxidation due to a decrease in the inhibition caused by self-poisoning.
80 Concerning the light-off temperatures for C3H~ removal, although big differences cannot be observed, the best behaviour corresponds to the Pt-Rh catalyst, followed by the multimetallic catalysts prepared by physical mixture, whose CaH6-conversion profiles are practically coincident. In relation to the NO-conversion (Figure 3) notable differences were found when running with the prepared catalysts. The elimination of NO can occur through reaction with CO and with Call6 as was already mentioned above. The behaviour of each metal can be compared with results obtained from the monometaHic formulations: platinum is the most active metal at the running temperature (X~oo=90%) but needs higher temperature than rhodium to become active (T5o=305 for Pt vs. 253 for Rh), the latter allowing a conversion at 500~ of 70%. The palladium shows lower values of both T~o and Xsoo,presenting the most unfavourable NO-conversion profile in Figure 3 at all temperatures. Again the sinergic Pt-Rh interaction can be observed as this catalyst presents the best behaviour, making use of the advantages of both metals, with T5o=247~ and X5oo=91%. The rest of formulations prepared by co-adsorption present a behaviour closer to the monometallic Pd catalyst. This could be explained considering that the higher amount of palladium in the bimetallic formulations, Pd/Pt=0.45/O.088=5.1 and PdfRh=0.47/O.O17=27.6, could be responsible for covering some of the platinum or rhodium sites restricting accessibility of reactants. Finally, the physical mixture of monometallic catalysts presented good NO removal capacity, with X~oo=90%, and intermediate light-off temperatures, even when rhodium is not present in the formulation.
3.2. Activity under stationary feedstream composition Table 3 shows the limits and amplitude of the stoichiometric windows obtained for all the studied catalysts. The upper limit of the stoichiometric windows is marked in all cases by the capacity of the catalyst to reduce NO above 70% conversion. On the other hand, the lower limit is marked by the high oxidation capacity of the catalyst to oxidize both HC and CO. All the prepared catalysts obtained practically total CO-conversion under net oxidising and slightly reducing conditions, conversion decreasing with the reducing character of the feedstream. Under reducing conditions the formulations with platinum resulted more active for CO removal, followed by palladium and then by rhodium. The removal of C3H6 was total with all catalysts containing Pd and under all tested conditions, oxidising and reducing. The Pt and Rh-containing catalysts allowed high conversions under oxidising conditions, decreasing under reducing conditions till 60%. The NO-conversion under reducing conditions resulted close to 100% with catalysts containing Pd and/or Rh, in spite of the reported low capacity of palladium for the NO reduction reaction attributed to some self-poisoning by hydrocarbons [5,16]. This effect is minimised in the prepared catalyst due to
81
Table 3 Stoichiometric windows for the prepared catalysts Catalyst CO (lower) HC (lower) NO (upper) (upper=15.13) (upper=15.13) (lower=14.13) Pd 14.36 14.13 14.55 Pt 14.13 14.25 14.60 Rh 14.44 14.13 14.60 Pd-Pt 14.13 14.13 14.57 Pd-Rh 14.13 14.13 14.57 Pt-Rh 14.26 14.13 14.68 Pd-Pt-Rh 14.13 14.13 14.56 Pd+Pt 14.13 14.13 14.59 Pd+Rh 14.13 14.13 14.57 Pt+Rh 14.24 14.13 14.62 Pd+Pt+Rh 14.13 14.13 14.57
Overall L0wer.Upper 14.36- 14.55 14.25- 14.60 14.44- 14.60 14.13- 14.56 14.13- 14.56 14.26- 14.68 14.13- 14.57 14.13 - 14.59 14.13- 14.57 14.24- 14.62 14.13- 14.57
Amplitude 0.19 0.35 0.16 0.43 0.43 0.42 0.44 0.46 0.44 0.38 0.44
their high activity for C3H6-oxidation. Under net oxidising conditions low conversions were obtained with all prepared formulations. The monometallic formulations resulted in much shorter amplitude of the stoichiometric window than multimetallic formulations. None multimetallic formulation presented notable differences in the amplitude, except for some displacement to the lean conditions for Pt-containing formulations and to the rich conditions for Pd-containing formulations.
4. CONCLUSIONS All the prepared catalysts oxidised completely both C3H6 and CO at 500~ under cycled oxidising-reducing conditions, presenting differences only in the reduction of NO. The Pd-Rh catalysts have resulted in different characteristics in comparison with Pt-Rh catalysts. This characteristic of Pd-Rh catalyst is similar to that of Pd-only catalyst. Pt, Pt-Rh and Pd-Pt-Rh prepared by co-adsorption converted 90% of NO, whereas Pd, Rh, Pd-Pt and Pd-Rh converted around 7075%. All the physical mixtures of monometallic catalysts reached 90% NOconversion. The co-adsorbed Pt-Rh catalyst presented the lowest light-off temperatures for all three contaminants. For the studied TWCs the light-off performance of Pt-Rh is dominated to a large extent by the Rh function, whereas in the case of Pd-Rh systems alloying has appreciable more negative effects on performance and suppressed the Rh function for NO reduction.
82 ACKNOWLEDGEMENTS
The authors acknowledge the financial support by the Basque Government, the Spanish Education and Science Ministry (PI93-44 and AMB93-574 projects) and the University of Basque Country (EB076/94). J.A.B also acknowledges to the Basque Government by the grant to work in the present research. REFERENCES
1. Entrena, J., PhD Thesis, Universidad del Pals Vasco/EHU, Bilbao 1994. 2. C. Howitt, V. Pitchon, F. Garin and G. Marie, in "Catalysis and Automotive Pollution Control III", A., Frennet and J.-M. Bastin (editors), p. 149-161, Elsevier, Amsterdam 1995. 3. J.R. Gonzfilez Velasco, J. Entrena, J.A. Gonzfilez Marcos, J.I. Guti6rrez Ortiz and M.A. Guti6rrez Ortiz, Appl. Catal B, 3 (1994) 191. 4. H. Praliaud, A. Lemaire, J. Massadier, M. Prigent and G. Mabilon, in " l l t h International Congress on Catalysis - 40th Anniversary", J.W. Hightower, W.N. Delgass, E. Iglesia and A.T. Bell (editors), p. 345-354, Elsevier, Amsterdam 1996. 5. S. Matsuura, A. Hirai, K. Arimura and H. Shinjoh, SAE Technical Paper 950257 (1995). 6. J.T. Kummer, J. Phys. Chem., 90 (1986) 4747. 7. S.H. Oh and J.E. Carpenter, J. Catal., 98 (1986) 178. 8. H. Muraki, H. Sobukawa, M. Kimura and A. Isogai, SAE Thechnical Paper 900610 (1990). 9. J.G. Nunan, W.B. Williamson, H.J. Robota and M.G. Henk, SAE Technical Paper 950258 (1995). 10. R.J. Brisley, G.R. Chandler, H.R. Jones, P.J. Anderson and P.J. Shady, SAE Technical Paper 950259 (1995). 11. J.C. Schlatter, R.M. Sinkevitch and P.J. Mitchell, Ind. Eng. Chem. Prod. Res. Dev., 19 (1980) 288. 12. K.C. Taylor, "Automobile Catalytic Converters", Springer-Verlag, Berlin 1984. 13. J.R. Gonzfilez Velasco, J.A. Botas, J.A. Gonzfilez Marcos and M.A. Guti6rrez Ortiz, Apl. Catal. B, 12 (1997) 61. 14. B.J. Whittington, C.J. Jiang and D.L. Trimm, Catal. Today, 26 (1995) 41. 15. M. Mundschau and B. Rausenberger, Plat. Met. Rev., 35 (1991) 188. 16. T. Engel and G. Ertl, Adv. Catal., 28 (1979) 1. 17. Gonzfilez Velasco and cols., unpublished results. 18. S.H. Oh, P.J. Mitchell and R.M. Siewert, J. Catal., 132 (1991) 287. 19. Muraki, H. Shinjoh and Y. Fujitani, Appl. Catal., 22 (1986) 325.
CATALYSIS AND AUTOMOTIVEPOLLUTIONCONTROLI'V Studies in Surface Science and Catalysis, Vol. 116 N. Kruse, A. Frennet and J.-M Bastin (Eds.) 91998 Elsevier Science B.V. All rights reserved.
83
Effect of Alkaline Addition on Hydrocarbon Oxidation Activities of Palladium Three-way Catalyst H. Shinjoh, N. Isomura, H. Sobukawa, and M. Sugiura TOYOTA CENTRAL R&D LABS., INC. Nagakute-cho, Aichi-gun, Aichi-ken, 480-11, Japan ABSTRACT The effect of alkaline addition on hydrocarbon oxidation activities of Pd catalyst supported on ,/-alumina was investigated using simulated automotive exhaust gases. The hydrocarbon oxidation activity of the Pd catalyst with alkaline earth metal such as Mg, Ca, Sr, and Ba is higher than that with nothing added. On the other hand, the activity of the Pd catalyst with alkali metal such as K and Cs is lower than that with nothing added. From the results of the partial reaction orders in C3H6 oxidation, TPR, and XPS, it was concluded that the alkaline addition to the Pd catalyst increased electron density of Pd on the catalyst and weakened the adsorption strength of Pd with hydrocarbons. The addition of alkaline earth metal suppressed the hydrocarbon chemisorption on the Pd catalyst and therefore allowed the catalytic reaction to proceed smoothly. On the other hand, the addition of alkali metal caused such a strong oxygen adsorption on Pd that rejected the hydrocarbon adsorption and suppressed the reaction. 1. INTRODUCTION Automotive three-way catalysts consist of noble metals, supports with a large specific surface and some additives. Many kinds of additives, such as cerium oxide, nickel, and other compounds, are used to supplement the activity of noble metals and to improve durability of automotive three-way catalysts. Cerium oxide is generally added to store oxygen under oxidizing conditions and to release the stored oxygen under reducing conditions(1-8). Nickel compound is often added as a scavenger of hydrogen sulfide and its addition also improves catalytic activity(9-12). Recently, barium(Ba) compound has been added to some kind of three-way catalysts, for example, palladium(Pd) only three-way catalyst and NOx storage reduction three-way catalyst(13-15). In spite of the fact that automotive three-way catalysts containing such alkaline compounds are already in practical use, the effect of alkaline addition on the catalytic activities of the catalysts are not yet clear. The authors have been studying the effect of alkaline addition on the catalytic activity of automotive three-way catalysts. We have found that the addition of Ba to Pd or platinum(Pt) three-way catalysts is effective for improvement of catalytic activity under reducing conditions, and that the suppression of hydrocarbon(HC) chemisorption on the catalysts by the addition of Ba allowed the catalytic reaction to proceed smoothly (16,17). This paper systematically reports the effect of alkaline addition, that is, alkali metals and alkaline earth metals, on hydrocarbon oxidation activity of Pd three-way catalyst.
84 2. EXPERIMENTAL
2.1. Catalysts The Pd catalyst was prepared impregnating y-alumina powder (a BET area of 200m2/g) with aqueous solution of palladium nitrate. The powder was dried overnight at 110~ in air, followed by calcination at 600~ for 5h in air, pressed, crushed, and sieved into 0.5 to 1.0 mm particles. The Pd/X catalysts (X=Li, Na, K, Cs, Mg, Ca, Sr, and Ba) were prepared impregnating the Pd catalyst powder with aqueous solution of each nitrates. The powder was dried, calcined, followed by pressing, crushing, and then sieved into particles in the same procedures mentioned above. Pd and alkaline loading amounts were 0.024 and 0.17 mole to one molar y-alumina, respectively.
2.2. Catalytic activity measurements The laboratory reaction system used was a conventional flow system with a tubular fixedbed reactor as described elsewhere(18). The characteristic feature of this system is its ability to simulate various air to fuel ratios (A/F) of automotive exhaust gases using eight mass flow controllers. In this study, catalytic activity on the catalysts in simulated automotive exhaust gases was measured as a function of E, which is a normalized value of A/F by a stoichiometric -l
one in the simulated exhaust gas, at 300~ and 420,000 h space velocity. The compositions of the simulated exhaust gases for each ~, are shown in Table 1. Catalytic activity was expressed as percent conversions of NOx(NO+NO2), CO, and HC. C3H6 oxidation activity was also measured to decide the kinetic parameters on the catalysts using the same laboratory reaction system. The compositions of C3H6 and O 2 were changed from 0.017 to 0.133 vol% and from 0.1 to 1.3 vol%, respectively, and space velocity was the same as that mentioned above.
2.3. Temperature programmed reduction(TPR) measurement The TPR measurement was performed using a flow system with a fixed-bed tubular reactor as described elsewhere(19). The 5 vol% H2/Ar was used in this measurement. Space -1
velocity was 7,000 h and heating rate was a linear rate of 50~ from -30 up to 300~ The effluent from the reactor was analyzed by both a thermal conductivity detector and a quadrupole mass spectrometer.
2.4. X-ray photoelectric spectroscopy (XPS) measurement The XPS measurement was performed using PHI-5500MC with MgK~ radiation as -9 incident beam. The base pressure of the instrument was 1x 10 Tort. Before conducting the XPS analysis, a catalyst was heated in pretreatment chamber connected to XPS chamber under 10 Tort of 10vol% C3H6/N2 at 400~ and then the pretreatment chamber was evacuated to 10 Tort and the catalyst transferred into the spectrometer without exposure to air. The electronbinding energy scale was calibrated by assigning 74.2 eV to A1 2p peak position.
85 Table 1 Compositions of simulated exhaust gases (N2 balance). #:[HJCO]=I/3
~. H2/CO#
C3H6
NO
02
C02 H20
(%) 0.960
2.00
0.062
0.12
0.41
10.0
3
0.967
1.73
0.060
0.12
0.41
10.0
3
0.974
1.49
0.058
0.12
0.43
10.0
3
0.980
1.33
0.057
0.12
0.46
10.0
3
0.985
1.20
0.056
0.12
0.49
10.0
3
0.990
1.00
0.055
0.12
0.54
10.0
3
0.992
1.05
0.055
0.12
0.56
10.0
3
0.994
1.01
0.054
0.12
0.57
10.0
3
0.996
0.99
0.054
0.12
0.60
10.0
3
0.998
0.96
0.054
0.12
0.62
10.0
3
0.999
0.95
0.053
0.12
0.63
10.0
3
1.000
0.93
0.053
0.12
0.65
10.0
3
1.001
0.92
0.053
0.12
0.66
10.0
3
1.002
0.91
0.053
0.12
0.67
10.0
3
1.004
0.88
0.053
0.12
0.70
10.0
3
1.006
0.85
0.052
0.12
0.72
10.0
3
1.008
0.84
0.052
0.12
0.75
10.0
3
1.010
0.83
0.052
0.12
0.78
10.0
3
1.015
0.77
0.051
0.12
0.85
10.0
3
1.020
0.73
0.050
0.12
0.92
10.0
3
1.027
0.69
0.050
0.12
1.03
10.0
3
1.034
0.65
0.049
0.12
1.13
10.0
3
1.040
0.60
0.049
0.12
1.21
10.0
3
3. R E S U L T S A N D D I S C U S S I O N
The conversions of HC, CO, and NOx on the Pd and Pd/Sr catalysts plotted as a function of ~. in simulated exhaust gases at 300~ are shown in Figs.1 and 2, respectively. The catalytic activity on the Pd/Sr catalyst was superior to that on the Pd catalyst, in particular, under reducing conditions defined as ~.<1. The conversion of HC, as representation of hydrocarbon oxidation activity, on the catalysts with alkaline earth metals and that with alkali metals plotted as a function of ~. in simulated exhaust gases are shown in Figs.3 and 4,
86 respectively, in comparison with the Pd catalyst with no such additives. The hydrocarbon oxidation activity in the simulated exhaust gases under reducing conditions for the additives followed the order of Ba > Sr, Ca > Mg > none > Li > Na > K > Cs The alkaline earth metal addition to the Pd catalyst improved the hydrocarbon oxidation activity. Similar phenomena have been observed on Pd/Ba and Pd/La catalysts, and it is concluded that the suppression of hydrocarbon chemisorption on Pd by the addition of Ba or La allows the catalytic reaction to proceed smoothly under reducing conditions(16,20). On the other hand, the alkali metal addition, especially K or Cs, to the Pd catalyst deteriorated the hydrocarbon oxidation activity. In C3H6-O 2 reaction system, the hydrocarbon oxidation activity on the Pd catalysts with alkaline compound was measured to get further information. The rate of carbon dioxide formation V(CO2) in the reaction of C3H 6 with 02 is given by the following equation (1). (1)
V(CO2)=kxP(C3H6)mx P(o2)nxexp(-AE/RT)
where P(C3H6) and P(02) are the partial pressures of C3H6 and 02, m and n are the partial reaction orders of CBH6 and O 2, respectively. The values of m and n were determined from a conventional log-log relationship between V(CO2), obtained
i00
l'"
NOx
80cC3 O3 C_ [13 > CO C_3
60-
40-
/ I
20 --
I,
0.96
I
'
'
' i
/ /
H C/" ./
/
/
J
'1
/
/
CO
!
~
1.
0.98
!
_
1
i.O0
I
......
!.
.
.
1.02
.
1
, !
1.04
A Fig.1 Conversion efficiencies as a function of X in simulated exhaust gases at 300~ catalyst.
for Pd
87
100
c
0
/'
NOx
80
60
/
~
a
/
-r-t
r._. > o
rj
/ H C
40
/
/
/
/
/
/
/CO
20
0
_l
. . . . .
t
0.96
.....
I
I
t
o.g8
_
t
I
1.00
!
t.02
.,. f
,
1.04
A
Fig.2 Conversion efficiencies as a function of X in simulated exhaust gases at 300~ catalyst.
I00
....
,
'
80 C o .r,,4
60
r_
:> c o
40
I
!
.
.
.
.
.
.
.
.
for Pd/Sr
w
none
.Z/,//"
/
(.3 rj :2=
2O --
9
!
0.96
! ....
!
0.98
I
,
t
.....
1.00
t
.....
I,
1.02
t
....... f
1.04
A Fig.3 The effect of alkaline earth metal addition: HC conversion as a function of X in simulated exhaust gases at 300~ on the Pd catalyst.
88
t00
- i
I
1
"
1
'
'
I
.
.
.
.
I
"
i
.......
~
I
"-
I--
--
80 C o .r-,,t
50
no
""
""~'"~"
lkl C 1:3
40-
/ / i /..///
20-
.....
I
~
0.96
~
.
I
0.98
l
I
1.00
__
1
_l
1.02
_
!
f
,
1.04
A Fig.4 The effect of alkali metal addition: HC conversion as a function of X in simulated exhaust gases at 300~ on the Pd catalyst. under conditions of low conversion of usually less than 30%, and partial pressures of respective species. The partial reaction orders in the C3H6-O2 reaction system determined on the Pd and Pd/X catalysts (X=Li, Na, K, Cs, Mg, Ca, Sr, and Ba) are summarized in Table 2. The partial reaction order m in the equation (1), was a large negative order(-1.39) on the Pd catalyst. The m values on the Pd/X catalysts with the alkaline components were all larger than that on the Pd catalyst, even showing positive orders for the catalyst with K or Cs. On the other hand, the partial reaction order n was positive on all the catalysts except for the catalysts with K and Cs. However, it was nearly zero or a negative order on the catalyst with K or Cs, respectively. The relationship between the partial reaction order m and the hydrocarbon oxidation activity in the simulated reducing automotive exhaust gas at k=0.98 are shown in Fig. 5. It was found that the maximum order of m was a certain negative one(-0.6), that is, the hydrocarbon oxidation activity increased with increasing the order of m from -1.39 to -0.6 and decreased with increasing the order of m further than -0.6. The intensity of Pd 3d peak by XPS of Pd, Pd/Ba, and Pd/K catalysts plotted as a function of binding energy is shown in Fig. 6. The dashed line shows the position of Pd 3ds/2 peak at 335.0 eV, which is in very good agreement with that in other studies (21,22). The value of Pd 3d peak on the Pd/Ba catalyst was a little smaller than that for the Pd catalyst. The shift value was approximately -0.2 eV. On the other hand, The value of Pd 3d peak on the Pd/K catalyst was much lower than that on the Pd catalyst. These data indicates that electron density of Pd increases because of electron transfer from alkaline compounds to Pd on the catalyst. The shift of Pd 3d peak on the catalysts with alkaline earth metals was almost the same as that on the Pd/Ba catalyst, and that on the catalyst with Na or Cs were larger than those on the catalysts with alkaline earth metal.
89 Table 2 Partial reaction orders in C3H 6 oxidation on Pd catalysts. [V(CO2)=kxP(C3H6)mx P(o2)nxexp(-AE/RT)] m
iO0
--
Pd
-1.39
1.10
Pd/Li
-0.46
0.71
Pd/Na
0.00
0.45
Pd/K
1.21
-0.70
Pd/Cs
3.64
0.01
Pd/Mg
-0.81
0.85
Pd/Ca
-0.45
0.91
Pd/Sr
-0.93
0.96
Pd/Ba
-0.73
0.61
~-
-
Ba
Sr ~0 Ca
80 co s cD
> c o u
U
"1"
150 n o n e ~ ~ O Mo Na
40
0
K
20 0 __
-2
9
-1
I
i
....
i
2
,1
,
i
3
Fig. 5 The relationship between m, as the partial reaction order with respect to C3H6, and the HC conversion for ~=0.98 in simulated exhaust gases at 300~ for the Pd catalysts with alkaline compounds.
90 The H 2 uptake on the Pd and Pd/K catalysts in flowing 5 vol% H2/Ar plotted as a function of temperature obtained by TPR measurement using the quadrupole mass spectrometer are shown in Fig.7. The oxygen adsorbed on Pd of the Pd catalyst was reduced below 100~ On the other hand, that of the Pd/K catalyst was harder to reduce than that of the Pd catalyst. The molar ratio of H 2 uptake to Pd on the Pd and Pd/K catalysts were 0.5 and 2.0, respectively. It suggests that H 2 reduces not only Pd oxides but also potassium compound, perhaps potassium carbonate. ; .
._~.__~.~ '
.
.
.
.
.
.
.
'
. . . .
i
I
"
. . . .
~
. . . . . . .
Pd/K
I
*f-"l
C 4J C H
_
346
|
. . . . . . . .
342
|
,
,
334
338
Binding
330
Energy
326
(eV)
Fig. 6 XPS spectra of Pd 3d for Pd, Pd/Ba, and Pd/K catalysts. The dashed line shows the position of Pd 3d5/2 peak at 335.0 eV.
~
o
~
c~
Pd/K
Cq
.
I
tO0
.
.
.
.
.
.
!
....
200
'I'emp e ra ture (~) Fig. 7 H 2 uptake on the Pd and Pd/K catalysts obtained by TPR measurement.
300
91 4. CONCLUSIONS The effect of alkaline addition on the hydrocarbon oxidation activity of Pd catalyst loaded on ~,-alumina was investigated using simulated exhaust gases. The hydrocarbon oxidation activity of Pd catalyst with alkaline earth metal such as Mg, Ca, Sr, and Ba is higher than that with nothing added. On the other hand, the activity of Pd catalyst with alkali metal such as K and Cs is lower than that with nothing added. From the partial reaction orders in the C3H6-O2 reaction system and characterization by XPS and TPR on the catalysts, it was concluded that the alkaline addition to the Pd three-way catalyst weakened the adsorption strength of hydrocarbons on Pd. The addition of alkaline earth metal suppressed the hydrocarbon chemisorption on the Pd catalyst and therefore allowed the catalytic reaction to proceed smoothly. On the other hand, the addition of alkali metals, in particular K or Cs, caused such a strong oxygen adsorption on Pd that rejected the hydrocarbon adsorption and therefore suppressed the reaction. It was considered that the effect of the alkaline addition to the strength of adsorbed hydrocarbons on Pd was caused by the increase of electron density of Pd. REFERENCES 1. 2. 3. 4. 5.
H.S. Gandhi, A.G. Piken, M. Shelef, and R.G. Delosh, SAE paper 760201 (1976). H.C. Yao and Y.F. Yu Yao, J. Catal., 86, 254(1984). E.D. Su, C.N. Montreuil, W.G. Rothchild, Appl. Catal., 17, 75(1985). Y.F. Yu Yao and J.T. Kummer, J. Catal., 106, 307(1987). J.Z. Shyu, K. Otto, L.H. Watkins, G.W. Graham, R.K. Belitz, and H.S. Gandhi, J. Catal., 114,23(1988). 6. J.C. Schlatters, P.J. Mitchell, Ind. Eng. Chem. Prod. Res. Develop., 19, 288(1980). 7. J.C. Summers, S.A. Ausen, J. Catal., 58, 131 (1979). 8. L.C. Hegedus, J.C. Summers, J.C. Schlatter and K. Baron, J. Catal, 56, 321 (1979). 9. R.L. Klimisch, K.C. Taylor, Environ. Sci. Technol., 7, 127(1973). 10. B.J. Cooper, L. Keck, SAE paper 800461. 11. M.G. Henk, J.J. White, G.W. Denison, SAE papae 872134(1987). 12. J.S. Rieck, W. Suarez, and J.E. Kubsh, SAE paper 892095(1985). 13. S. Matsuura, A. Hirai, K. Arimura, H. Shinjoh, TOCAT2, 1-20(1994). 14. S. Matsuura, A. Hirai, K. Arimura, H. Shinjoh, SAE paper 950257. 15. N. Takahashi, H. Shinjoh, T. Iijima, T. Suzuki, K. Yamazaki, K. Yokota, H. Suzuki, N. Miyoshi, S. Matsumoto, T. Tanizawa, T. Tanaka, S. Tateishi, Kasahara, Catalysis Today, 27(1996), 63. 16. H. Shinjoh, K. Yokota, H. Doi, M. Sugiura, S. Matsuura, Nippon Kagaku Kaishi, 1995(10), 779. 17. H. Shinjoh, N. Takahashi, K. Yokota, M. Sugiura, Appl. Catal., in press. 18. H. Muraki, K. Yokota, Y. Fujitani, Appl., Catal., 48(1989), 93. 19. H. Muraki, H. Shinjoh, H. Sobukawa, K. Yokota, Y. Fujitani, Ind. Eng. Chem. Prod. Res. Dev., 25,202(1986). 20. H. Muraki, H. Shinjoh, Y. Fujitani, Applied. Catal., 325, 22(1986). 21. F. Bozon-Verduraz, A. Omar, J. Escard, and B. Pontvianne, J. Catal., 53, 126(1978). 22. T.H Fleisch, R.F. Hicks, and A.T. Bell, J. Catal., 87, 398(1984).
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CATALYSIS AND AUTOMOTIVEPOLLUTIONCONTROLIV Studies in Surface Science and Catalysis, Vol. 116 N. Kruse, A. FrennetandJ.-M Bastin (Eds.) 91998 Elsevier Science B.V. All rights reserved.
93
Ethanol oxidation on Three-way automotive catalysts. Influence of Pt-Rh interaction. A. Marques da Silva, G. Corro, P. Marecot and J.Barbier Universit6 de Poitiers. URA CNRS 350. Laboratoire de Catalyse en chimie organique. 40, avenue du Recteur Pineau. 86022 Poitiers C6dex.
ABSTRACT Ethanol oxidation was studied under lean conditions on platinum/alumina catalysts modified by rhodium and/or lanthanum oxide. The results on alumina support suggest that the formation of bimetallic Pt-Rh particles enhances the production of acetaldehyde, particularly after oxidizing thermal aging of the Pt-Rh/alumina catalyst prepared by coimpregnation. The addition of lanthanum oxide to alumina allows to avoid the formation of Pt-Rh alloy after high temperature treatment and therefore induces a decrease of the production of acetaldehyde. 1. INTRODUCTION The Clean Air Act amendments of 1990 requires increased use of oxygenated compounds such as alcohols and ethers in motor fuels. Oxygenates in gasoline reduce vehicle emissions and act as high-octane replacements for harmful components that must be removed from reformulated fuels. In areas where air quality problems are severe, there is interest in using pure oxygenate fuels such as methanol or ethanol. But the use of ethanol-fueled passager cars produces high emissions of aldehydes relative to gasoline- fueled cars, which lead to the formation of photochemical smog. Moreover, upon installation of three-way Pt-Rh catalysts and after approximately 5,000 km use, acetaldehyde emissions increased beyond the levels observed without catalyst (1). The present work deals with ethanol oxidation which was performed in a flow reactor under lean conditions. Catalysts were evaluated by studying their light-off behaviour and by determining the amount of acetaldehyde produced during the temperature programmed experiment from ambient temperature to 500~ 2. E X P E R I M E N T A L
2.1 Catalyst preparation The supports used were a ?-A1203 with a BET area of 100 m2/g and the same alumina modified by addition of lanthanum acetate in order to obtain an A1203-La203 support with 12 wt % lanthanum oxide after calcination at 500~ The alumina support is initially ground
94 and sieved in order to retain particles with sizes between 0.25-0.10 mm. Monometallic catalysts were prepared by coimpregnation of hexachloro-platinic acid or rhodium trichloride. Bimetallic catalysts were prepared by coimpregnation of the metal precursor salts. After drying, the different samples were calcined at 500~ for 4 h and reduced in hydrogen at 500~ for 4 h. Before activity measurements, the different catalysts were either dechlorinated at 500~ for 10 h in a stream of N2 + 10 % H20 or thermally aged at 900~ for 16 h in a stream of 1% 02, 10 % H20, 10 % CO2, N2, and then reduced at 500~ The dechlorination treatment was carried out in order to avoid the inhibiting effect of chlorine on the activity of metals for oxidation reactions (2). The chlorine content of dechlorinated samples was below 0.2 wt %. Metal and lanthanum oxide loadings of the various catalysts are reported in Table 1.
2.2 Ethanol oxidation. Ethanol oxidation was performed in a flow reactor system attached on-line to a gas chromatograph equipped with a flame ionization detector (FID). The feed composition was 0.1 vol % ethanol, 1 vol % 02 in nitrogen at a space velocity of 30,000 h "l. The catalyst weight was 50 mg diluted in 250 mg tx-A1203. Catalysts were evaluated by the temperatures at 50 % conversion and by the amounts of acetaldehyde produced in the course of the temperature programmed experiments from 20~ to 500~ at a heating rate of 3~ min 1. As an example, Figure 1 shows the conversion of ethanol and yields of acetaldehyde, carbon dioxide and methane for the fresh platinum/alumina catalyst. Acetaldehyde production reaches a maximum near 150~ and decreases sharply at higher temperatures as CO2 production increases. Table 1: Characteristics of the different catalysts.
Catalyst
Lanthanum oxide (wt %)
Platinum (wt %)
Rhodium (wt %)
Pt/Al203
....
1.0
Rh/A1203
.
Pt-Rh/A1203
....
1.0
Pt/Al203-La203
12.0
1.0
Rh/AI203-La203
12.0
....
0.2
Pt-Rh/A1203-La203
12.0
1.0
0.2
.
.
.
.
.
.
.
0.2 0.2
2.3. Propane-propene oxidation Catalysts were characterized by activity measurements for hydrocarbon oxidation. Hydrocarbon oxidation was performed in a flow reactor system equipped with a flame ionization detector. The reactant mixture was composed of 0.2 % propene and 0.2 % propane
95 in N2 with 2 % oxygen (5 % excess oxygen). The gas flow rate was set at ca. 15 dm 3 h "1. The catalyst weight was typically 50 mg diluted in 250 mg ct-A1203 Catalysts were evaluated by studying their light-off behaviour at a space velocity of 30,000 h -1.
Figure 1. Ethanol conversion and product yields in the course of the temperature programmed experiment on the fresh Pt/AI203 catalyst. 0: ethanol ;X: carbon dioxide; I1: acetaldehyde; A :methane.
2.4. Temperature programmed reduction experiments. Temperature programmed reductions were conducted with a mixed flow of argon + 1 % hydrogen on activated catalysts (calcined at 500~ and reduced at 500~ recalcined at 450~ under pure oxygen for 2 h and outgassed at the same temperature for 1 h under argon flow. After cooling at room temperature, the TPR experiment consisted in heating the sample with a ramp of 5~ min l up to 500~ The hydrogen uptake was monitored by a thermal conductivity detector. 3. RESULTS AND DISCUSSION.
3.1. Ethanol oxidation on alumina supported catalysts. The temperatures at 50% conversion on the different catalysts for two successive oxidation cycles are reported in Figure 2. The results show that the monometallic platinum catalyst is more active than the monometallic rhodium one and that the addition of rhodium by coimpregnation does not modify the activity of platinum on fresh and aged samples. Thus, the light-off temperatures are similar on the monometallic Pt/AI203, on the mechanical mixture of Pt/A1203 and Rh/A1203 and on the coimpregnated Pt-Rh/A1203 catalyst. The "mechanical mixture" corresponds to the mixture of equal amounts of platinum and rhodium catalysts (50 mg Pt/A1203, 50 mg Rh/AI203) diluted in 200 mg ct-Al203. On the other hand, acetaldehyde production increases on the coimpregnated Pt-Rh/A1203 catalyst while it is the same on the monometallic platinum and the mechanical mixture (Fig. 3). This effect is more obvious on sintered samples, the amount of acetaldehyde produced on the coimpregnated Pt-Rh/A1203 catalyst being quite similar to that observed on pure rhodium (Fig.3b). The acetaldehyde
96 production is defined as the ratio between the total amount of acetaldehyde produced on catalyst (i)(p AcH catal. (i))and the total amoum of acetaldehyde produced on the fresh Pt/AI203 catalyst (p AcH catal. Pt/A1203).
Figure 3. Acetaldehyde production in the course of the temperature programmed experiments on alumina supported catalysts. MM and CI as in Fig 2. a = fresh catalysts; b = aged catalysts. Columns as in Fig. 2.
97 Previous work has shown that oxidizing thermal treatment at high temperature (800900~ of bimetallic Pt-Rh catalysts prepared by coimpregnation would lead to the formation of Pt-Rh alloys with surface enrichment in rhodium oxides (3-7). In order to verify this hypothesis in our case, the coimpregnated Pt-Rh catalyst was characterized by temperature programmed reduction in hydrogen and by measure of the activity for the oxidation of a propane-propene mixture under lean conditions. The TPR profiles displayed in Fig. 4 show that the coimpregnated Pt-Rh catalyst leads to a single peak although the reduction peaks of monometallic platinum and rhodium catalysts appear in two different temperature ranges. This result means that rhodium clearly catalyzes the reduction of platinum species in the bimetallic catalyst and that the coimpregnated catalyst is composed of bimetallic particles. This conclusion is bome out by the measure of the activity for the oxidation of a propane-propene mixture under lean conditions after thermal aging at 900~ which shows that the coimpregnated Pt-Rh/A1203 catalyst is far less active than the monometallic platinum one for propane conversion (Fig 5). Indeed, rhodium exhibits a poor activity for propane oxidation while platinum is very active, the reaction being enhanced on large particles. Thus, when rhodium is alloyed with platinum, large ensembles of platinum atoms are destroyed leading to the poisoning of platinum activity by rhodium (3,4,8).
Figure 4. TPR curves of the fresh catalysts I1: Rh/AI203; O: Pt/AI203 ; A: Pt-Rh/AI203. Figure 5. Propane-propene oxidation under lean conditions (5 % excess oxygen) on aged alumina supported catalysts: 0: propene on Pt/A1203;A: propene on Pt-Rh/A1203; I1: propane on Pt/AI203; X" propane on Pt-Rh/AI203. 3.2. Ethanol oxidation on alumina-lanthanum oxide supported catalysts. The main role of rhodium in catalysts used for the control of automotive emissions is to promote the reduction of NOx (9). The high cost and the limited availability of this metal provide a strong incentive to develop methods for its more effective utilization. Indeed, it is well known that Rh supported on 3,-A1203, when exposed to high temperatures in oxidizing atmosphere, interacts with the support leading to the diffusion of a part of rhodium into
98 ~/-A1203 and therefore to a loss of activity (10). As previous work has shown that incorporation of La203 into 7-A1203 allows to prevent the dissolution of Rh +3 ions into alumina (11), we examined the catalytic behaviour of a bimetallic Pt-Rh/A1203-La203 catalyst for ethanol oxidation. The results reported in Figure 6 show that catalysts supported on A1203-La203 are slightly less active than alumina supported catalysts (Fig 2). On the other hand, on modified alumina (Fig.7), platinum produces less acetaldehyde than on pure alumina (Fig. 3). This effect of the support on the acetaldehyde production is more obvious in the case of the bimetallic catalyst. Indeed, the Pt-Rh/A1203 catalyst shows a Rh/A1203 like behaviour while the Pt-Rh/A12OBLa203 catalyst resembles the Pt/A1203-La203 catalyst. In order to evaluate the interaction between platinum and rhodium deposited on the alumina-lanthanum oxide, the different catalysts were characterized by temperature programmed reduction and measure of the activity for the reaction of propane-propene oxidation. The TPR experiments reported in Figure 8 indicate that the monometallic Pt/A1203-La203 and Rh/A1203-La203 catalysts are reduced by hydrogen in the same temperature range and therefore this technique would not allow to differenciate the rhodium and platinum species in the Pt-Rh/A1203-La203 catalyst. With regard to the oxidation of the propane-propene mixture under lean conditions, Figure 9 shows that the performances of the aged Pt/A1203-La203 and Pt-Rh/AI203-La203 catalysts are similar for propane oxidation, contrary to the results previously observed on alumina supported catalysts. This means that rhodium does not inhibit the activity of platinum for the reaction under consideration. Two hypotheses can be put forward in order to explain this behaviour: i) platinum and rhodium are not alloyed in the Pt-Rh/A1203-La203 catalyst prepared by coimpregnation, even after high temperature treatment,
Figure 6. Ethanol oxidation. Temperature programmed experiments on A1203-La203 supported catalysts. MM and CI as in Fig 2. a: fresh catalysts; b: aged catalysts. Columns as in Fig.2.
99
Fig. 8
Fig. 9
Figure 8" TPR curves of the fresh catalysts" m. Rh/A1203-La203; 0: Pt/A1203-La203. Figure 9. Propane-propene oxidation under lean conditions (5 % excess oxygen) on aged AIzO3-La203 supported catalysts" $: propene on Pt/AlzO3-La203 ;A: propene on PtRh/A1203-La203 ; m: propane on Pt/AlzO3-La203 ; X" propane on Pt-Rh/AlzO3-La203. ii) the CI catalyst is composed of bimetallic entities with surface enrichment in platinum, rhodium being buried into the metal particles. However, this last hypothesis is the opposite of that generally proposed in the literature for Pt-Rh/A1203 or Pt-Rh/A12Oa-CeO2 catalysts aged under oxidizing conditions at high temperatures since such treatment leads to bimetallic particles with surface enrichment in rhodium (3,4).
100 In order to choose between these two hypotheses, the aged Pt-Rh/AI203-La203 catalyst was tested for propane-propene oxidation under rich conditions (oxygen deficiency). Indeed, it was shown in previous work that propane oxidation was catalyzed by platinum (between 200 and 400~ until all oxygen is consumed while rhodium was the key-component in the transformation of propane by steam reforming (between 400 and 600~ with water produced by the direct oxidation (12,13). Therefore the steam reforming activity is a good indicator of the rhodium surface state since the activity systematically decreases when the metallic rhodium area decreases (14). The curves reported in Figure 10 show that the conversion of propane by steam reforming occurs on the Pt-Rh/A1203-La203 between 400~ and 600~ This result means that rhodium is accessible to the reactants and that the Pt-Rh/AI203-La203 catalyst is composed mainly of monometallic platinum and rhodium particles.
Figure 10. Propane-propene oxidation under rich conditions (15 % oxygen deficiency) on aged A1203-La203 supported catalysts: 0" propene on Pt/A1203-La203; propene 9 on PtRh/A1203-La203; l " propane on Pt/A1203-La203 ; X" propane on Pt-Rh/A1203-La203. A possible explanation of this phenomenon could be the formation of La-Rh compounds under oxidizing conditions at high temperature (11) which would avoid the Pt-Rh alloy formation. In conclusion, the results reported in this paper show that the addition of lanthanum oxide to alumina induces a decrease of the production of acetaldehyde on bimetallic PT6Rh catalysts in the course of ethanol oxidation-Lanthanum oxide would avoid the formation of PtRh alloy. REFERENCES.
1. A.H. Miguel, J.B.Andrade, J. Brz. Chem. Soc. 1(3) (1990) p. 124. 2. P. Marecot, A. Fakche, B. Kellali, G. Mabilon, M. Prigent and J.Barbier, Appl Catal.B, 3, (1994) p 283. 3. S.H. Oh and J.E. Carpenter, J. Catal._98 (1986) p 178. 4. I. Onal in " Catalyst Deactivation" (C. H. Bartholomew and J.B. Butt, Eds), Elsevier, Amsterdam (1991), p 621.
101 5. B.R. Powell, Appl. Catal., 53 (1989) p 233. 6. S. Kim, M. J. D'Aniello, Appl. Catal., 56 (1989) p 23. 7. L. Pirault, D. E1 Azami El Idrissi, P. Marecot, J.M. Dominguez, G. Mabilon, M. Prigent and J. Barbier in "Catalysis and Automotive Pollution Control III" (A. Frennet and J.M. Bastin, Eds.), Elsevier, Amsterdam (1995), p 193. 8. P. Marecot, A. Fakche, L. Pirault, C. Geron, G. Mabilon, M. Prigent and J. Barbier, Appl. Catal. B; 5 (1994) p 43. 9. N.K. Pande and A. T. Bell, J. Catal. 98 (1986) p 7. 10. K.C. Taylor, "Automobile Catalytic Converters". Springer-Verlag, New-York (1984). 11. R. K. Usmen, R. W. Mc Cabe, L. P. Haaek, G. W. Graham, J. Hepburn and W. L. H. Watkins, J. Catal. 134 (1992) p 702. 12. J. Barbier Jr. and D. Duprez, in "Catalysis and Automotive Pollution Control III" (A. Frennet and J. M. Bastin, Eds),Elsevier, Amsterdam (1995), p 73. 13. J.C. Schlatter, SAE Techn. Pap. Ser. n ~ 780199. 14. D. Duprez, Appl. Catal., 82 (1992) p 111.
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CATALYSIS AND AUTOMOTIVE POLLUTION CONTROLIV Studies in Surface Science and Catalysis,Vol. 116 N. Kruse, A. Frennet and J.-M Bastin (Eds.) 91998 Elsevier Science B.V. All rights reserved.
103
Reduction of NO by CO on Manganese promoted Palladium Catalysts J.F. Trillat a, J. Massardier a, B. Moraweck a, H. Praliaud b and A.J. Renouprez a aInstitut de Recherches sur la Catalyse, C.N.R.S., conventionne ~, l'Universit6 Claude Bernard Lyon I, 2 avenue Albert Einstein, 69626 Villeurbanne Cedex, France. bLaboratoire d'Application de la Chimie/l l'Environnement, Unit6 Mixte C.NR.S. - Universit6 Claude Bernard Lyon I, 43 boulevard du 11 novembre 1918, 69622 Villeurbanne Cedex, France.
ABSTRACT Various Pd based catalysts supported on silica and containing manganese have been prepared and characterized. Two Mn species have been detected, i) reduced Mn in direct interaction with Pd on the metallic particles ii) oxidised Mn layed on SiO2 and showing <~reactive oxygens >>. An increase in activity for reduction of NO is observed on the catalysts containing Mn with an optimum for a Pd/Mn atomic ratio of about one. This enhancement of activity is due to either the presence of Pd-Mn dual sites or to a bifunctional mechanism between reduced Pd and oxidised Mn at the vicinity of Pd. 1. INTRODUCTION The use of palladium in the three-way catalysts as a substitute for more expensive metals such as Pt or Rh is an important both economic and strategic objective. Indeed, in the commonly used Pt-Rh based catalysts, the Pt/Rh ratio (= 0.2) is much lower than that of the Pt mine, leading to a deficit in the Rh supplies. Actually, the main reason for the preferential use of Pt in these depollution catalysts is a lower sensitivity than Pd to poisoning by Pb (1-4). Nowadays, the residual Pb concentration level in unleaded gasoline is negligible and many efforts have been devoted in the last years to the development of Pd based catalysts. Compared to Pt or Rh, Pd presents specific properties, such as a good thermal stability and an ability to keep a good activity under an excess of oxygen. Nevertheless, the activity of Pd for the reduction of NO by CO is lower than that of Rh and attention paid to increase its activity for the NO reduction by additive effects. It has been shown recently (5) that on Pt-Mn/SiO2 bimetallic catalysts, NO must be dissociated by the partially reduced MnOx. Since this dissociation is generally considered as the determining step in the NO reduction mechanism (6,7), it can be expected that the Pd-Mn association would lead to new catalysts active for the NO reduction by CO. In this context, the present work has thus been undertaken to study the influence of manganese on the Pd catalytic properties. Series of Pd-Mn catalysts supported on silica with various Pd/Mn ratios have therefore been prepared and characterized. Their activity has been measured in presence of different CO-NO-O2-hydrocarbons mixtures.
104 The observed differences between the Pd-Mn samples and the monometallic Pd catalysts are discussed in terms of modification of Pd electronic properties induced by Mn and / or by the creation of Pd-Mn dual sites. 2. MATERIALS AND CATALYSTS CHARACTERIZATION
2.1. Preparation of the bimetallic catalysts Series P d - M n / SiO2 catalysts have been prepared from molecular complexes. The precursors, palladium (II) bis-acetylacetonate and manganese (II) bis-acetylacetonate, were purchased from STREM Chemicals Inc. In order to prepare the precursors of the monometallic catalysts, the adequate amount of Pd or Mn salts (bis-acetylacetonate) has been dissolved in toluene, in which the calculated amount of a Degussa Aerosil 200 silica was added. The mixture was stirred for 24 hours at room temperature and then filtered under vacuum and dried at 350 K for 15 hours. To prepare the precursors of the bimetallic catalysts, the same procedure was used but the Degussa Aerosil 200 silica was added to a mixture of the two Pd and Mn salts dissolved in toluene with the convenient proportions. The activation treatment of the precursors has been carried out by decomposition under flowing Ar up to 670 K (0.5 K/mn). Indeed, previous works on Pd and PdCu supported on silica have shown that such an Ar treatment leads to metallic Pd and Pd-Cu bimetallic phases, well dispersed on the support (8). To remove the carbonaceous residues, all the samples have been calcined under 02 at 720 K and reduced under H2 at 870 K. Their compositions have been controlled by chemical analysis Several silica supported catalysts have been prepared according to the previously described method with different Pd/Mn ratios from pure Pd to MnOx/ SiO2 including Pd90Mnlo, Pd65Mn35, PdsoMnso and Pd30Mn70/SiO2. 2.2. Characterization of the catalysts The first question which has to be answered is the possibility of formation of bimetallic particles with two elements of such different reducibility. The composition of isolated metallic particles was determined with a JEOL-GEM 2010 analytical microscope by measuring the intensities of the of Pd L and Mn K emissions. With this instrument, the size of the probe can be reduced down to l nm 2. On large particles( d ~ 10 nm), or when the analysis is performed with a large probe size, the Pd/Mn ratio is close to the nominal composition. But most of the particles with a diameter smaller than 5 nm have a lower Mn content than the nominal composition, as can be seen in Table 1. The remaining Mn is dispersed on the support probably under the form of small particles which are not observed by electron microscopy because of a too low contrast. The question is now to determine if the two elements are alloyed in the bimetallic particles. The Pd-Mn phase diagram is complex; well defined compounds such as MnPd, MnPd2 and MnPd3 can be formed, but above 1300 K, and they are not observed here by X-ray diffraction. Below 1000 K, a solid solution of Mn in Pd, the ~ phase, extending up to a Mn concentration of 30 % can be formed. It cannot however be evidenced by X-ray diffraction since no lattice parameter variation occurs. The only method able to detect a formation of alloy is EXAFS. Actually the experiments were performed at LURE, in transmission mode, above the Pd K edge. Additional experiments at the Mn K edge in fluorescence mode showed that a large proportion of MnOx is present
105 As shown in Table 1, experiments at the Pd K edge prove that Mn is present in the first coordination sphere of Pd. The comparison with the EDX performed on isolated particles tells that 60 at.% of the manganese present is under the oxidised form layed on the support whereas 40 at.% of Mn is inside the metallic particles. Table 1. Nomenclature of the samples and comparison between the characteristics of these materials deduced from TEM- EDX and EXAFS measurements
TEM-EDX
EXAFS
Samples Pd/SiO2
Atoms-pairs
d (A)*
N*
Pd-Pd
2.76
10.7
Pd-Pd
2.77
10.2
Pd90Mnl0 Pd-Mn
2.71
0.3
Pd-Pd
2.81
9.7
Pd65Mn35 Pd-Mn
2.79
Mn-O
d(nm)*
100
2.4
Pd (At.%) 100 , , .
98
3.7
~95 +2
87
3.7
80+5
1.4
,
MnOx/Si02
Pd (At.%)
,,
6
<1.0
* d ( A ) Pd- Pd bond length in A, N: number of nearest neighbours Pd-Pd, d (nm.): particle size of bimetallic aggregates 3. CATALYTIC CONVERSION OF CO-NO GAS MIXTURES The catalytic tests were performed under atmospheric pressure in a gas flow microreactor which has been previously described (9). The analysis was performed both by gas chromatography equipped both with a TCD detector for CO2, N20, 02, N2 and CO and with a flame ionisation detector for hydrocarbons. Moreover, a on- line IR spectrometer was used for NO and NO2 analysis. Whatever the experiments, which were carried out at relatively low 02 pressure, no NO2 formation was observed. The reaction was studied at increasing temperatures (22 K/ran) between 423 and 773873 K. The catalyst (10 rag) was diluted with 40 mg of inactive tx-A1203 in order to prevent both mass and heat transfer limitations, at least at low conversion. The hourly space velocity, 120000- 130000 h-~ is in the margin of the values found for the exhaust gas of the gasoline motors. The redox character of the feedstream is defined by the number: <<S >>={2[O2]+[NO]}/( [CO]+(2x+y/2)[CxHy]} which is varied around the stoichiometric mixture value, S = 1. The catalytic behaviour of these different materials has been measured in presence of gas mixtures of increasing complexity and the most complex gas composition CO,NO,O2, C3H6 or C3Hs (diluted with He) can be considered as well representative of the exhaust gas mixtures.
106 In the presence of a simple equimolecular CO-NO mixture, the PdMn samples are always more active than Pdt00 as shown on figure 1, where the temperature of half or total NO conversion is lowered ( AT~50~ ) on Pd6sMn35/ SiO2 with respect to Pd / SiO2. It is noteworthy that the NO conversion leads to N2 without N20 formation since the two curves, NO or CO conversions, are nearly superimposed. Conversion % .........
100
Ii/'~ ~-' ~ .o o o
o~>c.-.-
8O /
q
60 ~
/
/
40
8/
/
//
//
20-
200
*
- -
250
NO'pd'~176
----~-~--CO, Pd65Mn35
/11/
0 ,~r-~,, ~ -,--~ 150
------ CO, Pdlo0
v
"T
- - I
300
NO, Pd65Mns5
-
350
I .......
400
l
450
500
Temperature (~ Figure 1. NO and CO conversion on Pdlo 0 and Pd 65 Mn35 in presence of a stoichiometric CO-NO gas mixture Moreover, figure 2 shows the variations of the NO conversion according to the Mn content in the catalysts. The best activity is obtained for the Pd 65 Mn35 sample. 360
-
340 ....0-- T1/~(NO) ~ 320 k 300 280
260
\.~
~ ~
24O 1oo
80
60
40
2o
Pd (at.%) Figure 2 -Temperatures of half- conversion of NO vs. the Pd content (at.%)
107 When 02 is added to the gas mixture keeping a stoichiometry close to 1 (_--1.034), an enhancement of the NO conversion, leading mainly to N 2 (with only traces of N20) is still observed on the PdMn samples as shown on table 2. With such a mixture, the NO conversion cannot exceed 75%, as observed, taking in account the gas stoichiometry,. Table 2. CO, NO and O2 conversions on the different catalysts; gas mixture: [CO] = 5930 ppm, [NO] = 790 ppm, [02] = 2670 ppm; gas flow: 17 1 / h. Samples Pdloo/Si02
Pd90Mnlo/Si02
Pd65Mn35/SiO2
Pd3oMn7o/SiO2
MnOx/SiO2
Gas
.
CO NO 02 CO NO 02 CO NO 02 CO NO 02 CO NO 0 2
Half-conversi0n (~ 260 330 260 260 280 260 225 250 220 255 265 255 280 / .
.
.
.
.
.
.....
.....
270
.
.
.
.
Maximum of conversion (~ 100% 325 70% 400 .
100%
.
-
100% 70% 100% 100% .... 70% 100% 100% 70% 100% 90% <20% 100% -
325
290 290 300 260 290 260 275 275 275 350 500 400
....
In the exhaust gas, there are some unburned hydrocarbons. Thus, for a more realistic approach, the efficiency tests of the PdMn samples have to be performed in presence of unsaturated (C3H6) or saturated (C3Hs) hydrocarbons. Therefore, 1000 ppm of C3I-I6 or C3Hs have been added to the previously used CO-NO-O2 mixture. In order to maintain the same gas stoichiometry, (=1.04), the adequate amount of 02 has been introduced in the gas flow. The O2 concentrations are respectively 7322 or 7838 ppm in presence of 1000 ppm of C3H6 or C3Hs. The results are shown in figure 3 on which are compared the catalytic activity of Pd~00, Pd65Mn35 and Pd30Mn70/SiO2 samples. Whatever the studied catalysts, the NO conversion is inhibited without no changes in the products. Nevertheless, the inhibition is not directly connected to the presence of hydrocarbons but at the presence of an excess of unconsumed 02 in the temperature range where the CO oxidation has occurred whereas the hydrocarbon oxidation has not still started as already discussed (9). As a matter of fact, the inhibition is the most important in presence of C3H8 less reactive with respect to oxidation. Moreover, in figure 4 are given the curves concerning the respective oxidation of both propane and propene. It may be observed that, on Pd30MnT0, the inhibition is the weakest. Indeed, the NO conversion reaches at 350~ an higher value, -_-70%, (fig. 3c) than the maximal value expected (236%) at higher temperatures (_>400~) when the total amount of 02 has been completely consumed by
108 NO conversion (%)
...............
100
"9
80
Without hc ~ With CsH=
~
With C3H6
/r
1 O0 --~--
~--v---v---v--v - ~ - ,
60 -
/
Pdlo 0
80-
x~q
Pd65Mn35
o
40 20
-t
/
/
0 d~~>~ 200
~../
oo "i I
20
- , "-Z<>-~-,~,-~N>~< 300 400 500
-_--~... ~ ~
200 100
__~Sans hc
60
[
/. A t
500
400
-~- ~---~.___~----d
o..o...~~._%,
3b)/
I / 200
o_i/if'~ ~"~~-~ a u
300
~- ~ r
"%~-~I :o
.........~ - ~ 1
400
f
500
- ~ ~
o r
Whithouthc W~thq K With
60
GH8
f
80
60,. ;> ~
NO conversion (%)
80
300
/
.-~ .....- - . = ~ 7 . ~Y-o-~ ~-':~-"
100
.... : . "-.'r.~"
-~--hc = C3H6
- - h c = C3H8_~
20
,.:~i
Temperature (~
80
40-
../'/'"
4b)
r
....
..~2--~-~-~
40-
o
"x3---O....C
/.I ~II
Pd30MnTo
60
3c)
4a>
(-) 0 ....
V " ~ql'---~-
~..~,_.~.~i.~
200
~F---Y--Y
I /f/
:/ l
40
.,- /
../ "i- :,
300
i
/
- "-
,
,
Pdloo
,
400
Temperature (~
40
3a)
~ r
-~"~o,,,,Q
.d ;X
20
~
>>-~--13----E3----E3---C3---E~-C}---t~--
200
300
400
500
Temperature (~ Figure 3. NO conversion in absence or in presence of hydrocarbon (C3H 6 or C3H8) 3a) on Pd, 3b) on Pd 65Mn35 and 3c) on Pd30 Mn70 samples. Figure 4. Hydrocarbon oxidation on Pd and PdMn samples: 4a) C3H6 and 4b) C3H8.
500
109 the oxidation of both CO and C3H6, taking in account the composition of the gas mixture. Such an inhibition by excess of oxygen is not observed on this Pd30Mn70/SiO 2 sample since the two oxidations of both CO and C3H6 nearly occur at the same temperature, - 250~ The C3H8 oxidation is not enhanced in presence of PdMn samples. Therefore, the NO conversion is not expected to be better on the bimetallic samples than on Pd~00 as observed.
4. IR STUDIES OF ADSORBED PROBE MOLECULES: MIXTURE
CO, NO OR CO-NO
In order to tentatively clear up the specific catalytic behaviour of catalysts containing Mn, adsorption and coadsorption of CO and NO have been carried out at 300 and 573 K. The adsorbed species were characterized by IR spectrometry.The IR experiments were carried out on a Brucker IFS 110 FTIR spectrometry with a 4 cm -1 resolution in the 1000- 4000 cm -1 region.The samples were compressed up to 2.105 kPa in order to obtain a thin disc of about 15 mg and a 15 mm diameter. Before any IR study, all the samples were reduced <
>in the IR cell and evacuated at the reduction temperature then cooled under vacuum. The IR spectra of the reduced samples were firstly recorded, then the <<probe >>molecules (CO,NO or the CONO stoichiometric mixture) were introduced under about 1.3-2 kPa and the IR spectra were recorded either under gaseous atmosphere or after evacuation. After smoothing and substraction of a linear background, the IR spectra of gaseous and adsorbed molecules are given by the <~ difference ~> spectra between these spectra and the initial spectra of reduced samples. On the Pd reduced samples, the CO adsorption carried out at room temperature shows the three infra- red bands of adsorbed CO: linearly bonded CO at 2050 cm~, bridged bonded CO near 1950 cml and multibonded CO near 1850 cm"~. When increasing amounts of Mn are added to Pd~00, the CO species adsorbed on top (vCO at 2050 cml ) decrease and disappear quasi completely at higher Mn contents ( Mn at.% >_ 35 %). Moreover, the intensities of the two other infra-red bands at 1950 and 1850 crn~ decrease (Fig.5). These IR bands are attributed to CO adsorbed either on Pd, Mn (11) or Pd-Mn dual sites. Such a disappearance of the 2050 cm~ IR band on the bimetallic Pd-Mn samples at high Mn content is rather surprising since, in general, the relative intensity of the linear CO IR band increases when a second metal is added to Pd due to the surface Pd dilution by the second metal (12). In order to explain this unusual behaviour, it has been assumed that the Mn segregation occurs on the low coordination surface atoms which agrees with the theoretical approach of this surface segregation phenomenon (13). However, no quantitative conclusions on the amounts of bridged and multibonded CO species (vCO at 1950 and 1850 cm"~) can be drawn. Indeed, the extinction coefficients can be changed with the nature of the adsorption sites. On Pd-Mn samples, in addition to the usual IR bands between 1850- 2050 cm~ assigned to CO molecular adsorption, CO reacts with the ~ mobile )) oxygen atoms of MnOx to form carbonated species characterised by the IR bands between 1750 a 1400 cm~. For the adsorption of NO on Pdl00, three infra-red bands are recorded at 1750, 1650 and 1550 cm~ assigned respectively to linearly, bent and bridged bonded NO (14). The sequential adsorptions : CO, NO then CO, show that each gas (NO or CO) displaces the other, which means that NO or CO are adsorbed on the same sites. However, the CO IR bands are
110
........ Pd
Pd 1oo - -" PdgoMn 1o
-----Pd65Mn35
r162 r
<
0
.<
0 2200
0 2100
2000
1 9 0 0 1800
Wavenumber (cm "1)
1700
2400
2200
2000
1800
1600
Wavenumber (cm *)
Figure 5 -IR spectra of adsorbed CO at room Figure 6 -IR spectra after NO-CO (NO/CO = 1) co-adsorption at 573K. temperature on Pd~00 and PdMn samples lowered after the NO adsorption which has left on the surface some strongly adsorbed or dissociated NO species. On the bimetallic catalysts, the same three IR bands like on Pdl0o are recorded after NO adsorption. As on Pd~00, the sequential adsorptions show that NO (or CO) displaces CO (or NO) but the IR band near 1650 cm ~ assigned to bent NO after NO adsorption is still observed after CO consecutive adsorption. In presence of CO in the gas phase, the presence of this IR band is explained by the formation of carbonated species in the 1750-1400 cm ~ IR region. These results show that new adsorption sites, Mn and (or) Pd-Mn, play a role in the adsorption and co-adsorption of reagents at room temperature. Thus, the influence of these new adsorption sites has also been studied in the case of CO-NO co-adsorptions at higher temperatures (at azbout 300~ where the NO reduction by CO occurs. On figure 6, the IR bands after CO-NO co-adsorption at 300~ carried out both on Pdl00 and Pd65Mn35/SiO2have been represented. The infrared bands assigned to adsorbed CO and NO are not detected. This means that CO and NO would be, if adsorbed, only dissociatively adsorbed. Moreover, these spectra being recorded under gas atmosphere, the expected IR bands near 1875-1880 and 2120-2130 cm~ attributed respectively to gaseous NO and CO are not observed. However, two IR large bands are recorded between 2160-2400cm ~. Their intensities are greatly improved on Pd65Mn35/SiO2 comparatively to Pd~00/ SiO2. These IR bands are assigned between 2160- 2300 cm t at isocyanates species on metal (Pd, Mn or PdMn) and N20 in the gas phase. Between 2300-2400 cm1, they correspond at Si-NCO (2300 cm I) (15) and at the formation of CO2 near 2350-2370 cm "~. After evacuation of the gas phase, the IR bands at lower wavenumbers are drastically decreased : N20 has been evacuated. In the same way, the disapearence of the IR band at 2350-2370 cm ~ is explained by the CO2 evacuation. According to these results, it can be concluded that CO and NO, which are not detected either in the gas phase or as adsorbed species, are either dissociatively adsorbed, mainly on Pd~00 or consumed by the reactions leading to the formation of COz, NzO and
111 isocyanates species as clearly shown on Pd6sMn3s/SiO2. Therefore, the presence of Mn increases the dissociative adsorption of NO since the N20 and isocyanates formation implies the interaction between adsorbed NO or CO with one adsorbed nitrogen atom according to the reaction pathway : NOads or CO ads + Nads ~ N20 or -NCO. Such a behaviour is illustrated on figure 7, on which the amounts of C02, N20 and -NCO are reported on Pd~00, Mn and PdMn. Relative armur~ .< "------offonned "~ 1 species(a t) i
/
!
/ i
PdMn ~o
M-NCO Si-NCO
Figure 7. Respective amounts of CO 2, N20 and -NCO species formed on the various samples after NO-CO co-adsorption at 573 K (CO/NO=l) It is noteworthy that this N20 formation during the coadsorption experiments is unexpected since this product is not observed in the CO-NO reaction. Two reasons can be invoked to explain such a desagreement: the contact time which is strongly increased (static conditions) and the pression range which is an order of magnitude higher. 5. D I S C U S S I O N
AND CONCLUSION
The catalytic activity of PdMn samples is significantly improved with respect to Pd~00. The structural studies reported above and the CO and NO adsorptions and co-adsorptions can have been undertaken to tentatively explain the effect of Mn. From these characterizations, a picture of the Pd-Mn samples can be given. They are made up of both alloy particles, the size of which is about 5 nm and of small MnOx particles (~ 1 nm) on the silica support. The Mn content in the bulk of the alloy particles depends on the total Mn concentration in the material and significant segregation of Mn occurs on the outermost layer of the bimetallic particles. According the bibliographic results on the NO-CO reaction mechanism,the NO dissociative adsorption is generally considered either as the rate determining step or, at least, as a very important step. This step is favoured by the presence of Mn in the bimetallic particles or on the support as previously discussed. Two reasons can be invoked to explain the improvement of the NO reduction in presence of PdMn/SiO2 bimetallic samples: either the specific catalytic behaviour of dual sites
112 or by a specific role of MnOx species at the vicinity of the metallic particles. Indeed on the PdMn dual sites, this dissociative adsorption of NO would be improved by the bent adsorbed NO species with interaction between the oxygen atom of NO and a Mn atom as deduced of theorethical studies (16) conceming the comparison between the adsorption modes on Pdl00, Mnl00 and PdMn. With respect to MnOx entities layed on the silica support, the presence of <)on MnOx is ascertained by the CO2 formation after CO adsorption. Therefore, a bifunctionnal mechanism between metallic P d , Mn or PdMn sites and reactive oxygen of MnOx can be invoked with formation of oxygen vacancies (9) on which the NO dissociative adsorption would occur according to the scheme: Metallic Site--NO + --[3-Mn-O ==> Metallic Site--N + O--E3-Mn-O ( [3 for oxygen vacancy) From the results of this work, it is not possible to choose between these two explanations, PdMn dual sites or bifunctionnal mechanism. Other experiments, namely water gas shift or steam reforming reactions, occurring via a mechanism implying oxygen vacancies, could give informations for a better understanding of the Mn role in the reaction process. REFERENCES
1. M. Shelef, K. Otto and N. C. Otto, Adv. in Catal., 27 (1978) 311-365 2. H.S. Gandhi, W. B. Williamson, E. M. Logothetis, J. Tabcock, C. Peters, M. D. Hurley and M. Shelef, Surf. and Interface Analysis, 6 (1984) 149 3. W.B. Williamson, D. Lewis, J. Perry and H.S. Gandhi, Ind. Eng. Chem. Prod. Res. Dev., 23 (1984) 53 4. R.L. Klimish, J. C. Summers and J. C. Schlatter, Amer. Chem. Soc. Adv. Chem. Set.143, (1975) 103 and R.W. Me Cabe and R.K. Usmen, Proceedings of the 1 lth Int. Congr. on Catalysis, Studies in Surface Science and Catalysis ( J. W. Hightower, W. N. Delgass, E. Iglesia and A.T. Bell Eds.) Vol. 1 (1996) p. 13 5. Y.J. Mergler, A. van Aalst, J. Van Delft and B.E. Nieuwenhuys, J. of Catal., 161 (1996) 310-318 6. W.F. Egelhoff Jr., The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis, (D. A. King and D. P. Woodruff eds.) Elsevier Publ. Amsterdam,Vol. 4 (1984) 424 7. Se. H. Oh and J.E. Carpenter, J. Catal., 101 (1986) 114 8. A.J. Renouprez, K. Lebas, G. Bergeret, J. L. Rousset and P. Delichere, Proceedings of the 11th Int. Congr. on Catalysis, Studies in Surface Science and Catalysis ( J. W. Hightower, W. N. Delgass, E. Iglesia and A.T. Bell Eds.) Vol. 2 (1996) p. 1105 9. A. Lemaire, J. Massardier, H. Praliaud, G. Mabilon and M. Prigent, Studies in Surface Science and Catalysis, Catalysis and Automotive Pollution Control (CAPOC 3) (A. Frennet and J. M. Bastin Eds.) (1995) p. 97-108 10. J.F. Trillat and al., submitted to J. Catal. 11. G. Blyholder and M. C. Allen, J. Amer. Chem. Soc., 91, (1969), 3158-3162 12. A. E1 Hamdaoui, G. Bergeret, J. Massardier, M. Primet and A. J. Renouprez, J. Catal., 148, (1984), 47-55 13. J. L. Rousset, Pers. Communication 14. S.Moriki,Y. Inoue, E Miraki and J. Yasumori, J. Chem. Soc., Faraday Trans.I; 78 (1982) 171 15. W.C. Hecker and A.T. Bell, J. Catal., 85, (1984), 389 16. L. V6rit6, F. Delbecq and B. Moraweck, Surf.Sci., in press.
CATALYSISAND AUTOMOTIVEPOLLUTIONCONTROLIV Studies in Surface Science and Catalysis, Vol. 116 N. Kruse, A. Frennet and J.-M Bastin (Eds.) 91998 Elsevier Science B.V. All rights reserved.
113
Light-off performance over cobalt oxide- and ceria-promoted platinum and palladium catalysts Magnus Skoglundfla, Anders TOrncronaa'b, Peter Thorm~ihlen a'e'd, Erik Fridell a, Astrid Drewsen a'd and Edward Jobson a'e aCompetence Centre for Catalysis, Chalmers University of Technology, S-412 96 G6teborg, Sweden bDepartment of Engineering Chemistry, Chalmers University of Technology, S-412 96 G6teborg, Sweden CDepartment of Applied Physics, Chalmers University of Technology and G6teborg University, S-412 96 G6teborg, Sweden dDepartment of Chemical Reaction Engineering, Chalmers University of Technology, S-412 96 G6teborg, Sweden eVolvo Technological Development, S-405 08 G6teborg, Sweden
ABSTRACT Monolith catalysts containing Co, Ce, Pt and Pd supported on alumina were prepared and tested with respect to low-temperature activity for oxidation of CO and propene. The catalysts were either pre-oxidised or pre-reduced prior to evaluation with respect to light-off performance, using net oxidising and net reducing CO/C3H6/O2[N2 gas mixtures. Promotion of Pt and Pd with cobalt or cerium oxide, favoured the low temperature activity significantly. Pre-reduction of Co- and Ce-promoted noble metals shifted the conversion starts of CO and propene toward lower temperatures compared with pre-oxidised samples. Pre-reduction of cobalt oxide, without Pt or Pd, yielded a dramatic improvement of the low-temperature catalytic performance compared with pre-oxidation of the said oxide. The catalysts were characterised by temperature programmed desorption of CO and specific surface area measurements. The high activity over the pre-reduced cobalt containing catalysts is suggested to be due to the presence of reduced cobalt oxide sites on those samples. 1. INTRODUCTION More than 80% of the emissions from cars equipped with catalysts steam from the first three minutes of driving [1]. Substantial efforts are therefore made to develop catalysts possessing high activity at low-temperature conditions. In this study we have investigated how the low-temperature activity for oxidation of CO and propene over Pt and Pd is affected by three different parameters: 1) the composition of the reactant gas (net oxidising or net reducing), 2) the pre-treatment of the catalyst (pre-oxidation or pre-reduction) and 3) the addition of different promoters (cobalt oxide or ceria).
114 The kinetics of oxidation over noble metals is complex as the reactants inhibit the adsorption rate of each other [2]. By variation of the reactant gas mixture, the surface coverage of different species can be changed and thus the activity for oxidation of CO and hydrocarbons (HC) at low temperatures can be affected. Pre-conditioning, to achieve the most active surface state for oxidation of CO and HC, can be performed by pre-reduction. Lowtemperature activity for CO oxidation has been reported for Pt/Ce/A1203 and Pd/Ce/A1203 after exposing the catalyst to reducing atmospheres at temperatures above 300 ~ [3]. It is well known that addition of base metal oxides can enhance the catalytic properties of noble metals. Addition of CeO2 in three-way catalysts has improved the performance by the ability to store oxygen, promote the water-gas-shift reaction, stabilise the alumina support, suppress strong Rh-A1203 interactions and promote noble metal dispersion [4-5]. Beside the precious metals, oxides of the first row of transition metals are generally active as oxidation catalysts. By promoting Pt and Pd with cobalt oxide it has recently been shown that the activity for oxidation of CO and propene is significantly increased [6-10]. 2. EXPERIMENTAL PROCEDURE 2.1. Preparation of catalysts Cordierite monoliths were coated with an alumina washcoat and stabilised at 550~ Some of the samples were then immersed in either an aqueous solution of cerous or cobalt nitrate, dried and calcined in air at 550~ at which the metal nitrates decomposed into their oxides [11]. The samples were weighed and the procedure was repeated until 40 mg of the metal oxide had been deposited onto the alumina washcoated monolith samples. Pt and Pd were applied by direct impregnation using aqueous solutions of H2PtC16 and PdC12 followed by drying and calcination in air at 550~ [8]. The Pt and Pd loadings (2.0 and 1.09 mg, respectively) of the catalysts were equal on molar basis. The nominal composition of the eight catalysts prepared are listed in Table 1. Table 1. The nominal composition, BET surface area and amount CO adsorbed on reduced samples using CO-TPD, of th e ]prepared catalysts. Sample Alumina Metal oxide Noble metal BET area AdsorbedCO content [mg] content [mg] cont.ent[mg] [m2/g,cat.] [gmol] Ce/A1203 160 40 19.3 0.0 Co/A1203 160 40 20.3 4.2 Pt/A1203 200 2.0 25.4 0.5 Pd/A1203 200 1.1 28.9 0.7 Pt/Ce/AI203 160 40 2.0 19.2 2.5 Pd/Ce/A1203 160 40 1.1 22.8 1.1 Pt/Co/A1203 160 40 2.0 21.1 3.7 Pd/Co/A1203 160 40 1.1 19.4 5.0 ...........
2.2. TPD The CO-TPD measurements were performed in a flow reactor, described elsewhere [12]. A quadropole mass spectrometer continuously analysed the gas composition after the monolith
115 sample. The temperature was measured in the middle of one of the centre channels inside the monolith sample. Each sample was first pre-oxidised (10 vol.-% O2/Ar) at 600~ in order to clean the catalyst surface from adsorbed hydrocarbons, and then reduced (4 vol.-% HE/Ar) at 600~ for 10 min. The sample was then exposed to highly purified CO at 5~ The TPD measurements were performed in an Ar-flow (50 ml/min) while heating the reactor from 5~ to 600~ at a rate of 40~ with continuos recording of the CO, CO2, O2 and HE concentrations. 2.3. Specific surface area Specific areas of the catalysts were determined by nitrogen adsorption according to the 13ET method [8]. 2.4. Catalyst testing The activity measurements were performed in a flow reactor, described elsewhere [8]. Briefly it consists of a horizontal quartz tube in which the sample is sealed. Temperatures were measured before the catalyst and in one of the monolith channels close to the catalyst front. Reactant and product gases were analysed on-line with respect to CO and CO2 (IR) and total hydrocarbons (FID). All catalysts were initially reduced (4 vol.-% HE/N2) at 450~ and stabilised at 550~ for 1.5 h in a net oxidising mixture of 1.0 vol.-% CO, 0.15 vol.-% C3H6 and 1.38 vol.-% 02, balanced with N2, at a space velocity (SV) of 90000 h l. The light-off performance of each catalyst was studied in either a rich, net reducing, (1.0 vol.-% CO, 0.15 vol.-% C3H6 and 0.98 vol.-% O2, balanced with N2) or a lean, net oxidising, (1.0 vol.-% CO, 0.15 vol.-% C3H6 and 1.38 vol.-% O2, balanced With N2) reactant gas mixture at SV=90000 h ~. Before each activity test, the catalyst had been pre-treated at 550~ for 10 min in either reducing (4 vol.-% Hz/N2) or oxidising atmosphere (10 vol.-% O2/N2). The temperature was then lowered to about 50~ in the same atmosphere. During the light-off performance test, the conversions of CO and total hydrocarbons were measured and recorded as functions of the inlet temperature at a heating rate of 5~ The effect of pre-treatment atmosphere and reactant gas mixture on the light-off performance was studied for all eight samples (see Table 2).
3. RESULTS AND DISCUSSION 3.1. TPD studies The results of the CO-TPD measurements, made after the activity tests, are shown in Figure 1 and summarised in Table 1. During the TPD measurements only small amounts of CO2 desorption, in the order of a few percent of the CO value, were detected. The TPD measurements show that CO adsorbs neither on A1203 nor on Ce/A1203. The samples with Pt/A1203, Pd/A1203 and Pd/Ce/A1203 all give similar TPD spectra as shown in Figure 1. For these samples, CO desorbs within the temperature interval 50-350~ with a maximum slightly below 100~ The magnitudes of the desorption maxima differ somewhat between the three samples, indicating differences in noble metal dispersion. The amount of adsorbed CO on Pt/Ce/A1203 is about six times of that adsorbed on Pt/A1203. The TPD spectrum of Pt/Ce/A1203 shows an additional desorption peak at about 200~ as shown in Figure 1. The addition of ceria seems to stabilise the noble metal dispersion. The cobalt containing catalysts all give similar TPD spectra, as shown in Figure 1,
116 with desorption within the temperature interval 25-300~ The desorption maxima are located slightly below 100~ and there is an additional shoulder at about 200~ It is thus obvious that reduced cobalt oxide, without Pt or Pd, adsorbs large amounts of CO. The presence of Pt or Pd on cobalt oxide increase the peak magnitudes at 100~ but they act in different ways at 200~ Pt lowers the magnitude of the shoulder at 200~ whereas Pd increases it somewhat. 0.08
I
....
I
I I I .- ...... ".
-
0.06-
"'.~.
t/
j f"
,., 0.04- !
|
,=, 0 . 0 2 - -
E
..
9 0.00
)
.
.-
--"
_.,,.-.,''''-'"
.~"
,, 9"'-..,.,
"'~
I I
Ir
I
0.06--
I
O
,
~ .~ ~,
..- ' " ' " ' "
I"
,,.:"
t.
%*~ %. "~'.~._. " - . . ~ , ..................
Palladium
I ... , ' " '" '. . % "'-.x...~..
~..=...,.~..._ 9 "'.. %
..o
......
"'. ~
._.~.~.
..."
=
"... %
~'-j 100
--
%.. ~
",.._.
o.oo0
(b) _
pmol CO ---- Pd/A~3 (0.7) . . . . . Pd/Ce/A~ (1.1) --Pd/Co/A~O 3 (5.0) ......... Co/A~O3 (4.2) --
/::
j/
9 % "... "% "~ 9 " ' . '.~. "
',
'
-
..
I
9~
I "' / " ~.."
0.04-0.02--
~'-.
9
.:'/ I
(a)
lumol C O Pt/AI~O3 (0.5) . . . . . Pt/Ce/ALD 3 (2.5) --Pt/Co/A~O 3 (3.7) ......... Co/A~O 3 (4.2)
~""..
r 0.08-i,_ o
"~.
--'" ~
,0=,,
I
Platinum
200 Temperature
--'=":::"';~""r'"''--'-----',300
400
[~
F i g u r e 1. C O - T P D spectra after C O adsorption at 5~ and Pd c o n t a i n i n g samples (b).
on reduced Pt c o n t a i n i n g samples (a)
3.2. Specific surface area The BET surface areas of Pt/AI203 and Pd/A1203 are 25.4 and 28.9 m2/g catalyst, respectively, corresponding to specific surface areas of 141 respective 161 m2/g A1203 (see Table 1). The BET surface areas of the cobalt oxide and ceria containing samples are in the range 19-23 mE/g catalyst, corresponding to surface areas in the range 106-125 m2/g support (A1203+metal oxide). It can thus be concluded that the improved catalytic properties (see below), observed with ceria or cobalt oxide as promoters, can not be attributed to increased specific surface areas.
117 3.3. Flow reactor studies
The oxidation activities for CO and HC were studied for the eight samples ( Table 1). The dependence on pre-treatment (oxidation and reduction), metal oxide promotion (Ce and Co) and gas composition (net oxidising and net reducing) was investigated. The light-off temperatures (50% conversion of CO respective HC) for the four different test conditions are given in Table 2. Table 2. Light-off temperature (50% conversion) of CO and HC, after pre-oxidation (10 vol.-% O2/N2) and pre-reduction (4 vol.-% H2/N2), at 550~ respectively, using either a net oxidising or a net reducing CO/C3H6/O2/N2 feed. Sample Light-off temperature [~ Light-off temperature [~ Net oxidising feed Net reducing feed Pre-oxidised Pre-reduced Pre-oxidised Pre-reduced Ts0(co) TS0(HC) Ts0(co) TS0(HC) Ts0(co) Ts0(Hc) Ts0(co) TS0(HC) Ce/A1203 Co/A1203 Pt/A1203 Pd/AI203 P~Ce/AI203 Pd/Ce/AI203 P~Co/A1203 Pd/Co/AI203
549 340 304 247 247 255 237 246
537 364 299 245 247 256 237 246
550 189 311 263 221 263 177 169
550 199 307 258 220 255 181 177
>550 365 333 278 243 245 212 281
>550 454 348 278 251 247 237 285
>550 195 334 295 209 231 178 178
548 218 346 291 231 231 187 185
3.3.1. Effect of reactant gas composition on the low temperature-activity
The oxidation of CO starts at about 250~ for Pt/AI203 and at about 200~ for Pd/AI203 regardless of the reactant gas composition. The oxidation of CO starts at a lower inlet temperature (30-50~ than the oxidation of HC. The oxidation of CO and HC is most likely inhibited by an adsorbed layer of CO and hydrocarbons that prevents adsorption of oxygen at temperatures below about 150~ When adsorbed CO and hydrocarbons start to desorb, oxygen is activated by dissociative adsorption and CO and HC are then oxidised. The light-off temperatures for CO and HC are about 60~ higher for Pt/A1203 than for Pd/A1203 (see Table 2) This difference may be explained by a higher specific activity of Pd, compared with Pt, for oxidation of CO and alkenes [13]. Furthermore, the CO-TPD measurements (see Table 1) show that Pd adsorbs more CO than Pt when supported on alumina which probably is an effect of higher dispersion. This may also contribute to the higher activity of Pd/AI203 than Pt/A1203. Figure 2 shows the conversion of CO and HC for Pd/Ce/A1203 using a net oxidising respective net reducing gas mixture. Below light-off, the conversions of CO and HC are only affected by the gas composition to a minor extent. Above light-off, under net reducing conditions, the conversion of CO reaches a maximum whereas the conversion of HC continues to increase with increasing temperature. This selectivity reflects the competition for activated oxygen between CO and HC and is caused by the oxygen deficit in the net reducing gas mixture.
118 100
80 o
* ~,,,I
60 -
> 40 o ~
20 ;..__
100
150
200
250
300
350
Inlet temperature [~ Figure 2. Conversion of CO (circles) and HC (squares) over pre-oxidised Pd/Ce/A1203 using net oxidising (filled symbols) and net reducing (open symbols) CO/C3H6/O2/N2gas mixtures. 3.3.2. Effect of promoting oxides Figure 3 shows the conversion of CO and C3H6 for pre-oxidised Ce/A1203, Pd/A1203 and Pd/Ce/AI203 when using a net reducing feed. The conversions of CO and HC for pre-oxidised Ce/A1203 do not reach 50% within the temperature interval studied. However, there is a promoting effect of ceria on Pd for oxidation of CO and C3H6. The promoting effect of ceria on Pd can be due to formation of new active sites in the Pd/Ce/A1203 catalyst but may also be caused by a higher dispersion of Pd (see Table 1). For Pt/Ce/A1203 a clear promoting effect of ceria on the low-temperature activity is seen. The light-off temperatures for CO and HC over Pt/Ce/AI203 are about 100~ lower than for Pt/A1203 (see Table 2). The CO-TPD measurements (see Figure l) indicate that platinum has a significantly higher capability of adsorbing CO when supported on ceria compared with Pt supported on alumina, which may explain the higher activity for Pt/Ce/AI203 compared with Pt/A1203. With cobalt oxide as promoter an even more pronounced improvement of the activity is seen (see Table 1). Pt/Co/AI203 exhibits a much higher activity at inlet temperatures below 300~ compared with both Pt/A1203 and Co/A1203. Based on activity and CO-TPD measurements (see above), the promoting effect of cobalt oxide on Pt are consistent with a model involving weakly bound oxygen on cobalt sites adjacent to Pt and may be understood as follows: At about 100~ CO starts to desorb from the cobalt sites. This creates free sites for dissociative oxygen adsorption and subsequent reaction between CO and O. When activated oxygen is present close to the interface between Pt and Co, the activated oxygen may spill over to Pt and react with CO adsorbed on Pt. This, in turn, gives free sites for oxygen adsorption on Pt as the reaction products, i.e., CO2 desorbs. The enhanced activity may also be explained by the exothermic oxidation reactions which starts on
119
the cobalt sites, heat up the catalyst and hence decrease the inlet gas temperature necessary for the oxidation reactions on the Pt sites to start. Mergler et al. have earlier shown that a Pt/Co/SiO2 catalyst has a considerably lower lightoff temperature for CO oxidation than a corresponding Pt/SiO2 catalyst [6-7, 9-10]. The authors suggested three main models that may explain the high activity of the cobalt promoted Pt catalyst: 1) cobalt promote dissociative adsorption of 02 on Pt, 2) weakly bound oxygen on cobalt oxide reacts with CO on Pt at the Pt-Co interface or 3) by oxygen spill-over, on Pt. The promoting effect of cobalt may also be due to Pt-Co alloy formation [9-10].
100 80
o
60
r~
o> 40
o r,.) 20
100
150
200
250
300
350
Inlet temperature [~ Figure 3. Conversion of CO (circles) and HC (squares) over pre-oxidised Pd/A1203 (filled symbols), Pd/Ce/A1203 (open symbols) and Ce/A1203 (dashed line = CO, solid line = HC) using net reducing CO/CaH6/O2/N2 gas mixtures. Pd/Co/A1203 (when pre-oxidised) does not seem to exhibit an improved low temperature activity compared with Pd/A1203, suggesting that pre-oxidised cobalt oxide does not promote the activity of Pd. Thus, pre-oxidised cobalt oxide promotes the activity of Pt, but not of Pd, for oxidation of CO and propene. However for pre-reduced Pd/Co/A1203, there is an obvious promoting effect on the low-temperature activity (see below). There is a clear difference in catalytic activity between C0/A1203 and Ce/A1203 (see Table 2). Co/A1203 has a much higher activity for oxidation of CO and propene compared with Ce/AI203. This is in accordance with earlier reports where metal oxides which can shift between at least two oxidation states, and consequently participate in redox reactions, usually possess catalytic oxidation activity [14-16]. Oxides of the first row of transition metals are particularly active as oxidation catalysts [13, 16-17]. The activity for CO oxidation of the oxides of lanthanides, which, like the transition metals, have multiple valence states, is generally lower than that of transition metal oxides [ 13]. The activity for oxidation of CO and propene over the pre-oxidised pure oxide catalysts (Ce/AI203 and Co/A1203) is well in accordance with the data reported in the literature, e.g., [13, 16]. However, the catalytic activity of pre-reduced cobalt oxide was unexpectedly high (see below).
120
3.3.3.Effectofcatalystpre-treatment Figure 4 shows the conversion of CO and propene for Pd/AI203 in net oxidising reactant gas mixture after pre-reduction and pre-oxidation at 550~ No significant effect of the pretreatment atmosphere on the light-off performance is seen for this catalyst. In Figure 5, however, it is seen that the pre-treatment atmosphere has a significant effect on the low temperature activity of Pd/Co/A1203. The effect of catalyst pre-treatment is most pronounced for the cobalt oxide catalyst promoted with Pd or Pt (see Table 2). In lean reactant gas, pre-reduced Pd/Co/A1203 has light-off temperatures at 169~ and 177~ for CO and HC, respectively, whereas the light-off temperatures over the same catalyst, but pre-oxidised, are 246~ for both CO and HC. A clear effect of pre-reduction is also seen for Pt/Ce/A1203, whereas no obvious effect of the pre-treatment atmosphere on the oxidation activities for CO or HC is seen for Pd/Ce/A1203. An even more pronounced difference in light-off performance between pre-oxidation and pre-reduction is seen for Co/A1203 (see below). The reason that pre-reduced Pd/Co/A1203 and Pt/Co/AI203 start to react at lower temperatures than the pre-oxidised samples (see Figure 5) is most likely due to that the initial reaction on cobalt sites starts at lower temperatures for the pre-reduced samples than for the pre-oxidised catalysts. The pre-reduction may also induce alloy formation with lower adsorption energy of CO compared with the noble metal [18]. It is also possible that O-vacancies form on the cobalt oxide which promote the dissociative adsorption of 02. Reaction with CO will then proceed on the noble metal or at the interface between Pt (or Pd) and cobalt oxide [9-10].
o•0
mlllllllllllllllllllllllnllllllllllllllllllllllllllllllll[nl|l
100 80
0 0
oi,,,,,i 60 9
0
.s
;~ 40 o L) 20
0 ~_,,~ ..... ~ 100
150
l
I
200
250
I
300
I
350
Inlet temperature [~ Figure 4. Conversion of CO (circles) and HC (squares) over pre-oxidised (filled symbols) and pre-reduced (open symbols) Pd/AI203 using net oxidising CO/C3H6/O2/N2gas mixtures. No improvements of the light-off performance for Ce/A1203 as a result of the pre-treatment or reactant gas composition can be observed (see Table 2). Obviously, ceria itself is a very poor oxidation catalyst. The light-off performance for Co/A1203 is, however, dramatically changed after pre-reduction at 550~ The light-off temperatures (see Table 2) for CO and HC
121 are about 150-170~ lower for the pre-reduced catalyst than for pre-oxidised Co/A1203. Furthermore, for a net reducing feed the light-off temperature for HC is 236~ lower over the pre-reduced catalyst than over the pre-oxidised one (see Table 2). The pre-reduced catalyst exhibits a rapid light-off performance, similar to that which is normally observed for noble metal catalysts indicating a catalytic process involving exchange of electrons (compare with Pd/A1203 in Figure 4), whereas the pre-oxidised catalyst seems to perform as regular, catalytically active base metal oxides (creating and restoring oxygen vacancies) [2, 19].
lOO l
ro ................................................. ,,......................................................................
8o ~
.
60
~ 40 o r,.) 20 0 100
150
200
250
300
350
Inlet temperature [~ Figure 5. Conversion of CO (circles) and HC (squares) over pre-oxidised (filled symbols) and pre-reduced (open symbols) Pd/Co/AI203 using net oxidising CO/C3H6/O2fl~I2 gas mixtures. 4. CONCLUSIONS The influence of reactant gas composition, promotion by ceria or cobalt oxide and catalyst pre-treatment on the low temperature activity of alumina supported Pt and Pd has been studied. The reactant gas composition does not influence the light-off performance for CO and propene for Pt/A1203 or Pd/A1203. There is a strong promoting effect from Co on both Pt and Pd. In the case of Ce there is a promoting effect on Pt, though not as strong as in the Co case, while for Pd, there is a promoting effect only in the reducing gas mixture. The low temperature activity of the cobalt oxide containing catalysts is markedly improved in both lean and rich reactant gas by pre-reduction. Pre-reduced cobalt oxide itself, supported on alumina, exhibits a light-off performance similar to that observed for noble metal containing catalysts, indicating that another mechanism than the conventional redox cycle is involved in the oxidation of CO and HC. The TPD experiments show that reduced cobalt oxide adsorbs large amounts of CO. The activation of cobalt oxide by pre-reduction opens up for new possibilities to use base metal oxides, with moderate activity when oxidised, in the efforts to develop catalysts with high activity at low temperatures.
122 ACKNOWLEDGEMENTS
This work has been performed within the Competence Centre for Catalysis, which is financed by NUTEK - The Swedish National Board for Industrial and Technical Development, Chalmers University of Technology, AB Volvo, Saab Automobile AB, Johnson Matthey, ABB F1/ikt Industri AB, Perstol9 AB and AB Svensk Bilprovning. REFERENCES
.
3. 4. 5. 6.
,
10. 11. 12. 13. 14. 15. 16. 17. 18. 19.
E. Jobson, G. Smedler, P. Malmberg, H. Bemler, O. Hjortsberg, I. Gottberg and A. Ros6n, SAE Paper Series 940926 (1994). J. Wei, Adv. Catal., 24 (1975) 57. Y.F. Yu Yao and J.T. Kummer, J. Catal., 106 (1987) 307. B. Harrison, A.F. Diwell and C. Hallett, Plat. Met. Rev., 32 (1988) 73. S.E. Golunski, H.A. Hatcher, R.R. Rajaram and T.J. Truex, Appl. Catal. B, 5 (1995) 367. Y.J. Mergler, A. van Aalst, J. van Delft and B.E. Nieuwenhuys, Stud. Surf. Sci. Catal., 96 (1995) 163. Y.J. Mergler, A. van Aalst and B.E. Nieuwenhuys, ACS Symp. Series 587 (1995) 196. M. Skoglundh, H. Johansson, L. L6wendahl, K. Jansson, L. Dahl and B. Hirschauer, Appl. Catal. B, 7 (1996) 299. Y.J. Mergler, A. van Aalst, J. van Delft and B.E. Nieuwenhuys, Appl. Catal., B 10 (1996) 245. Y.J. Mergler, J. Hoebink and B.E. Nieuwenhuys, J. Catal., 167 (1997) 305. M. Skoglundh, L. L6wendahl, K. Jansson, L. Dahl and M. Nygren, Appl. Catal. B,3 (1994) 259. S. Lundgren, K.-E. Keck and B. Kasemo, Rev. Sci. Instrum., 65 (1994) 2696. J.T. Kummer, Prog. Energy Combust. Sci., 6 (1980) 177. G.K. Boreskov, Paper 71, Catal., Proc. Int. Congr., 5th, J.W. Hightower (ed.), North Holland, Amsterdam, 1973, pp. 981-996. Y. Morovka and A. Ozaki, J. Catal., 5 (1966) 116. Y.Y.-F. Yao, J. Catal., 39 (1975) 104. R. Prasad, L.A. Kennedy and E. Ruckenstein, Combust. Sci. Technol., 22 (1980) 271. U. Bardi, B.C. Beard and P.N. Ross, J. Catal., 124 (1990) 124. P. Mars and D.W. van Krevelen, Chem. Eng. Sci., 3 (1954) 41.
Catalyst Ageing Poisoning
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CATALYSIS AND AUTOMOTIVE POLLUTIONCONTROL IV Studies in Surface Science and Catalysis,Vol. 116 N. Kruse, A. Frennetand J.-M Bastin (Eds.) 91998 Elsevier Science B.V. All rights reserved.
125
I n f l u e n c e o f c a t a l y s t d e a c t i v a t i o n on a u t o m o t i v e e m i s s i o n s using d i f f e r e n t cold-start concepts
T. Krichnera,.A. Donnerstag b, .A. K6nig b and Eigenberger a a Institute of Chemical Process Technology, University of Stuttgart B6blinger Str. 72, D-70199 Stuttgart, Germany b Volkswagen AG, Drive train research, D-38436 Wolfsburg, Germany c Bayer AG, Corporate Technology, D-51368 Leverkusen (fonnerly a)
ABSTRACT In order to meet future legal requirements several concepts are proposed for reducing coldstart 6missions. In this contribution their performance is determined by detailed simulation studies based on a one-dimensional model. The dynamic behavior during start-up and the influence of local catalyst deactivation on the efficiency of the diff6rent cold-start concepts is described. An innovative concept is developed which shows lowest cold-start 6missions for fresh and aged catalyst in the simulation study as well as in cold-start experiments.
1. INTRODUCTION Three-way catalyses (TWC) require a minimum temp6rature of approx. 3 500C for proper catalytic combustion. Due to the heat capacity of the exhaust system it takes about 1 min after engine start until this temp6rature level is reached if the catalyst is only heated by the exhaust gas. The amount of toxics produced during this cold-start period presents a consid6rable fraction of the total amount during one test cycle [1]. Due to more stringent legal purification requirements several concepts were developed to reduce the catalyst heat up time. Presently the main approaches to lower the cold-start 6missions are the use of an electrically heated catalyst (EHC) [2], a burner heated catalyst (BHC) [3, 4] and hydrocarbon adsorber systems [5, 61. This contribution reports on a joint research project with Volkswagen AG to improve the start-up behaviour of automotive catalytic converter systems. Detailed simulation studies are used to evaluate the perfomance of diff6rent cold-start concepts for fresh and aged catalyses. The dynamic behavior during cold-start and the influence of local catalyst deactivation on the efficiency of the diff6rent cold-start concepts will be described. Within the scope of this study an innovative concept is developed, where the light-off of the monolith is induced by an exothennic reaction at the catalytic surface (combustion heated catalyst, CHC). It will be shown that in comparison with the other concepts the cold-start 6missions with the CHC-concept are lowest level and almost independent of local deactivation of the catalyst.
126 2. MATHEMATICAL MODEL The mathematical model for the catalytic converter used in the simulation study is onedimensional and incorporates separate energy and mass balances for gas and solid phases. 9balances for the gas phase (gas temperature Og, weight fraction wj, g) OOg
I?." Og " Cp,g " - ~
--" - a
e . Og" otoj, Ot g _ m -Gz
z
" ~ Oz
Ol~g
9Cp,g . -~z
02#g
-]" • " ~ , e f f " ~
+ e . Uef f
az 2
+ O l s , g ( Z ) " a o " (Lgs - - Lgg)
- - Og " ~ j ( z ) " ao " (Wj, g -- Wj,~)
9balances of the solid phase (solid temperature 0~, weight fraction w~,,) (1 - e ) " Os . Cp,s . ~
= (1 - e) . Xs 9~OZ2 + - ots,g(Z) . a o .
0 --- O g " [ 3 j ( z ) ' a v "
ax . z i l = l ( - A h r i )
" Ri
(Os - Og) - Ols,am b " a e x t " (tgs -- 19amb) + qel
(l13j,g - - l13j,s) - - a x "
Mj.
zil=l
l)ij. g i
9Danckwerts boundary conditions at the inlet and outlet G z "Cp,g Gz
9LgG,in - - G z " C p , g "
Og - - F," i~,ef f
9 Oz
WG,in 9 - - G z " wg - e . D e f y . Oz
a_~[ =0 OZ z=0 Oz z = L
Oz z=L m
Oz
z=L
The model considers the most important reactions for the start-up. The combustion of the key components CO, propene, propane and hydrogen is described by kinetic rate expressions according to the following reaction scheme: 1
+
~ 02
>
CO
+
1
~ 02
>
C02
C3H6
+
5 02
~
3 C02
+
3 H2 0
C3H8
+
502
> 3C02
+
4H20
HE
9
1-120
The time dependent inlet conditions concerning exhaust gas temperature, mass flow and composition were obtained from FTP 75-measurements. A more detailed description of the model with the underlying assumptions is given elsewhere [7]. Spatially non-uniform heat and mass transfer between exhaust gas and the catalytic surface is considered due to the transition from turbulent to laminar flow at the inlet of the monolithic converter 9
127
Figure 2: Cold-start behavior using an electrically heated pre-catalyst (all times are given in seconds)
3. COLD-START BEHAVIOR FOR FRESH CATALYSTS 3.1 Electrically heated pre-catalyst (EHC) As mentioned before, the heat-up of the automotive catalyst only by the exhaust gas needs approx. 1 min. In order to shorten the start-up period an electrically heated pre-catalyst can be used which is located in front of the main catalyst (Figure 1). The presently used EHC is a two-brick design. It consists of a short metallic monolith which is heated by the car battery and a second, larger but unheated monolith. This second monolith enlarges the catalytic surface area and ensures the mechanical stability of the whole EHC construction. The design of this EHC was obtained by optimization based upon extensive experimental and simulation studies [ 1,7]. It was shown that future legislative requirements can be fulfilled with this EHC. Figure 2 shows temperature and concentration profiles of CO, propene and propane at different times of the cold-start test cycle. Because of the time dependent conditions at the inlet of the converter, the concentration profiles are always normalized with respect to the inlet concentrations at each
128
Figure 4: Cold-start behavior using the BHC concept (all times are given in seconds) time step. The simulation starts with a temperature level of 20 ~ over the entire length of the exhaust pipe. Five seconds after the engine start the heated brick reaches a temperature of approx. 400 ~ At this time, the reduction of the hydrocarbon emmisions is still low because of the very small catalytic surface of the heated brick. As a result of the good convective heat transport, the second brick of the EHC reaches the ignition temperature level fast. Thus the conversion of the pollutants can be increased to 80-90 % within 9 s. The electrical heating of the first brick plus the reaction heat set free at the EHC help to warm-up the main catalyst and total combustion of the hydrocarbons is completed after only 25 s.
3.2 Burner-heated catalyst (BHC) The maximum available electrical power from the car battery is only approx. 1.5 kW. For this reason the use of a burner-heated catalyst (BHC, Figure 3) was investigated as a possible alternative. By burning the fuel directly the available power is much higher compared to the
129 EHC approach. Experimental studies are published in literature with burners of up to 15 kW [3]. Figure 4 shows the results of a simulation run for a 13 kW burner concerning the gas phase. Depending on the configuration of burner and catalyst, the inlet temperature of the catalyst is rising up to approx. 800 ~ within 20 s due to the mixing of the hot burner exhaust with the exhaust gas stream of the engine. This leads to a very fast onset of the conversion of all pollutants at the catalytic converter. Nevertheless, it takes a few seconds to move the temperature front into the monolith in order to widen the reaction zone. For this reason total conversion of the hydrocarbons is still limited to approx. 70 % after 7 s. Due to the strong power input, total combustion of CO and the hydrocarbons can be reached after 8 s and 15 s respectively. In fact the heat-up of the TWC is slightly faster compared to the EHC concept, but nevertheless the cold-start emissions especially of the hydrocarbons are much higher in the case of the BHC. This is due to the production of additional pollutants by the external burner during the ignition period which can not be avoided as the burner is usually operated at stoichiometric fuel/air ratio in order to keep NOx emissions at a low level. This was observed in own experimental investigations of the ignition behavior of an commercial burner. With the above concepts only the entrance of the monolith is heated up after engine start. Thus the efficiency of both concepts is very sensitive to local deactivation of the front part of the catalyst which will be shown later. In addition, the heat input is limited by the heat transfer from the gas stream to the catalytic surface.
3.3 Combustion heated catalyst (CHC) A combustion heated catalyst is heated by an auxilliary combustion reaction during the start-up phase. For this purpose a suitable reactant and air have to be fed to the catalyst prior to and during motor start-up. From many tested reactants only hydrogen was able to act as such an igniting fuel since it was the only fuel with a light-off temperature on nobel metal catalysts below ambient. As mentioned in literature the light-off temperature of the catalytic combustion of hydrogen shifts to approx. 150~ in the presence of exhaust gas [8]. For this reason the catalyst has to be heated with an air/hydrogen mixture for a few seconds before the exhaust gas may reach the monolith. In oder to improve the start-up behavior significantly the power input by the CHC-concept should be in the range of 5 to 6 kW. A sketch of the arrangement is given in Figure 5. In the simulation results of Figure 6 profile "0" marks the temperature profile obtained after preheating 10 s with a mixture of 5 % hydrogen in air. In the narrow zone of 0 >300~ carbon monoxide is burned completely, whereas the hydrocarbons are converted up to 50 %. In the sequence the temperature front moves towards the end of the catalyst and the hot area is widened by the heat of reaction produced at the catalytic surface. 5 s after the engine was started propene is totally burned and in the simulation run the complete combustion of the pollutants is reached after only 9 s. By optimizing this heating strategy the preheating period can be reduced to at least 3 s. This cold-start concept was tested in a Volkswagen vehicle (engine: 2.0 1; catalyst: 1.9 1, Pt/Rh (5:1) 50 _z_).f t3 Since the experimental setup could not be placed in normal underbody position the catalyst in the experiments was positioned at the end of the exhaust tail pipe behind the mufflers. In spite of these unfavourable conditions very good conversion behavior of the CHC-concept can be observed in the experiments. The left diagram of Figure 7 shows the axial temperature profiles measured by thermocouples during cold-start after preheating the catalyst with air/hydrogen over 10 s and the right one
130
Figure 6: Cold-start behavior using the CHC concept (all times are given in seconds) shows the time dependent hydrocarbon (HC)-conversion observed in the experiment. In good agreement to the simulation results, a maximum temperature level of 400-500 ~ is reached at the end of the preheating period ("0 s"). At this moment the engine is started and the catalytic combustion of the exhaust gas takes place. Thus the monolith is heated to approx. 700 ~ The following cooling of the monolith entrance is due to the fact that the inlet temperature of the catalyst is lower than 50 ~ during the whole experiment due to the engine-far position of the converter. In spite of the high catalyst temperature a break-through of the hydrocarbons occurs (Figure 7, right diagram). Hence the initial HC-conversion is about 40 %, which is again in agreement with the simulation. Due to the widening of the temperature front the total combustion of HC is reached after 15-20 s, a few seconds later than predicted in the simulation run. This can be explained by the unfavorable position of the catalyst far from the engine. In the simulations as well as in experiments the CHC-concepts shows the best performance for the reduction of cold-start emissions under the assumption of a fresh catalyst. For each start-up only a little amount of hydrogen (~ 0.5 g) is needed. Experience in production and
131
800
.
.
lOs
I ~
E Q)
.
~
6 ~ 1 7 6I "
.
.
20 S
.
100
.
80
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"~.
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400 E
200
0
20
0
3
0
6 9 12 length of catalyst [cm]
0
5
10
15
20 25 time [s]
30
35
40
Figure 7: Cold-start experiment with CHC-concept and fresh catalyst 100 ,
80 I-
,
,
,
,
,
,
,
,
[O~-Oveh!cle agedl
6O
~0
40
~
2o 0
I
0
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I
I
6 9 length of catalyst [cm]
I
12
15
Figure 8: Measured axial activity profiles over the length of the catalyst storage of hydrogen in vehicles is already available [9]. 4. INFLUENCE OF CATALYST DEACTIVATION After a certain running period a catalyst deactivation profile is observed in real automotive application. This is due to thermal aging where temperatures above 800 ~ lead to sintering effects in the washcoat and reduce the catalytic surface. The highest temperatures at the catalyst occur in the main reaction zone, which is usually located in the first few centimeters of the monolith. Local hot-spots with maximum temperatures up to 1400 ~ may occur in the front part of the catalyst due to highly transient inlet conditions [10]. In addition, poisoning of the catalytic surface by exhaust components such as sulfur dioxide intensify the enhanced deactivation at the catalyst inlet. For this reason the highest degree of catalyst deactivation is found in the front area of the catalyst, whereas an activity near to 100 % is still observed at the end of the monolith. Figure 8 shows measured activity profiles over the length of the monolith for a catalyst aged over 150000 km in a vehicle and an engine aged catalyst over 200 h with inlet temperatures up to 900 ~ The influence of catalyst deactivation on the cold-start behavior will be discussed with the help of simulation runs for the above cold-start concepts. Figure 9 shows the cold-start with
132
1000 ~
~ inert
o o...
750
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catalyst
main
"77. 9 1.0 26
catalyst
.
0
.c_ 32
~
c
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23
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z [m]
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0.20
z [m]
Figure 9: Cold-start behavior for an aged catalyst using the BHC concept (all times are given in seconds) the BHC-concept for an aged catalyst with a mean activity of 40 %. This catalyst has an inert front part of 3 cm and a linear increase up to 100 % activity towards the end of the monolith. The heating of the catalyst by an external bumer only effects the front inert part of the catalyst. For this reason catalytic combustion is only induced at the end of the heating period (20 s). The conversion of the pollutants does not start until the temperature front reaches the active part of the monolith. Due to the retardation of the complete combustion of the hydrocarbons the emissions increase by a factor of 2 for an aged catalyst compared to a fresh one (Figure 4). Similar results can be found for the EHC-concept since the prime heat input is into the deactivated front part of the monolith. In contrast, the heating with the CHC-concept results from the catalytic combustion of hydrogen at the active parts of the catalyst. The influence of this heating strategy is shown for a simulation run in Figure 10. Due to the selective heating of the catalyst in the active area an almost identical temperature profile is reached at the end of the preheating period (10 s) as for the fresh catalyst. Hence, CO is burned completely when the engine is started. The initial conversion rate of the hydrocarbons of 35 % is slightly lower compared to a fresh catalyst because of the lower catalytic activity in the heated area. As the temperature front moves towards the end of the catalyst with high activity complete combustion of hydrocarbons is obtained after 10 s to 15 s. This clearly indicates that the CHC-concept retains its functionality even in the case of strong deactivated catalysts. Figure 11 shows the quantitative influence of catalyst aging on the conversion behavior of automotive catalysts using different cold-start concepts. In this figure the cold-start emissions resulting from simulation study are given over the mean activity of the main catalyst. In order to
133 main catalyst
main catalyst
1000 ~ 1 ~
Z
1
750
7:1.0 (19 r-
10
500
(D c3
.o
25
0.5 /
t-"
250 0
_
(1) C). O k_
~ ,
ca. 0.0 0.00 0.05 0.10 0.15 0.20 i
,,
0.00 0.05 0.10 0.15 0.20
|
t
, , ,
I
0 o
o o
I
. . . .
z [m]
z [m]
1.0
0
-
.n-:.
o
1.0
-
~0.5
0.5 0
0.0 . . . . 1,, 0.00 0.05 0.10 0.15 0.20 L
I
. . . .
!
0.0 0.00 0.05 0.10 0.15 0.20
. . . .
z [m]
z [m]
Figure 10: Cold-start behavior for an aged catalyst using the CHC concept (all times are given in seconds) 3000
'
'
'
i
,
,
,
i
,
,
,
i
,
,
,
i
,
,
,
Fraw emissio
E 1::: t~
,~ 2000
"O
O co
main catalyst
.=_ "-.1 tO
"~
._~ E
1000
......:
0 -r
0
20
40 60 mean catalyst activity [%]
80
100
Figure l 1" Cold-start emissions dependant on the catalytic activity
134
800
'
'
i
,
,
i
5s /,\
600
/!
"~ 400
,
,
/~15
/ ",/
i
\
,
,
!
,
100
,
aged 0
t'-
3o
6 9 12 length of catalyst [cm]
._o
60
(D > tO
40
o "r" i
3
,. . . .
,__, 80 9-.
s
/ /" I' /
E 200
.....................................
15
20 0
0
5
10
15
20 25 time [s]
30
35
40
Figure 12: Cold-start experiment with CHC-concept and aged catalyst simplify the comparison between the different concepts it is assumed that the electrically heated pre-catalyst doesn't show any aging tendencies. Using only a fresh main catalyst the cold-start emissions of hydrocarbons are about three times higher compared to the future legal requirements (ULEV). With decreasing catalyst activity the cold-start behavior of the main catalyst gets worse which results in increasing HC-emissions. The BHC-concept reduces the cold-start emissions considerably but is most strongly influenced by catalyst deactivation. Due to the short front part of the catalyst which is heated to high temperature levels (Figure 4) the length of an inert front area of an aged catalyst has a strong influence on the HC-emissions. Future legal requirements can be fulfilled with the EHC-concept for fresh catalysts. As the main catalyst is only heated by the exhaust gas again a considerable increase of the coldstart emissions results from catalytic aging. Hence, the ULEV-limit can not be kept for lower activities than 40 %. The CHC-concept shows the lowest HC-emissions in comparison to the other cold-start concepts. This is due to the fast and direct heating of the catalytic surface where the hydrogen combustion only takes place at active parts of the catalyst. The selective heating of the active areas of the monolith by the CHC-concept can also be demonstrated experimentally. An automotive catalyst was aged artificially in a way that the first 3 cm were completely deactivated. With this aged catalyst the cold-start experiment from Figure 7 was repeated. The heating by the hydrogen combustion results in a sharp temperature rise at the rear active part of the monolith (left diagram of Figure 12). Because of the inert entrance of the catalyst the temperature profiles are shifted downstream but are comparable in hight to the experiment with fresh catalyst. For this reason high conversion rates are reached after 10-15 s (right diagram of Figure 12). The obtained maximum HC-conversion of 95 % is probably due to inaccuracies of the gas analyser because at the measured temperature level of 500 ~ to 600 ~ total combustion of the pollutants is usually obtained. 5. CONCLUSIONS In detailed simulation studies the dynamic behavior of automotive catalytic converter systems during start-up is described for the EHC-, BHC- and the innovative CHC-concept, where light-
135 off of the monolith is induced by the catalytic combustion of hydrogen. In the simulation studies all concepts show a good performance for the reduction of the cold-start emissions as long as fully active catalyst is considered. Thereby, the necessary power input varies from 1.5 kW (EHC) tp approx. 13 kW (BHC). The results change drastically if a catalyst deactivation profile is assumed as observed in real application after a certain running period. Due to the selective heating the CHC-concept gives the best conversion behavior during start-up for an aged catalyst, whereas the B HC- and EHC-concept failed to reach future legal requirements for deactivated catalysts. The simulation results are confirmed in a number of specific cold-start experiments. NOTATION aext ao ax
Cp D Gz AhR
L
m 2
[~-~] m 2 [~] m 2 ,
[-~-] kJ [ kT_~.K] [m~-~.s] [ mk2---~].s kJ [Y~7] [m]
Mj
[ kgj 1
R
r kmol ]
t
[sl
wj Z
[m]
Ol
[ kW
e ~.
[-] [ kw m---~] [-]
tkmolj j
v o
L m2t.s J
[~1 m-r~.K]
[oc]
external surface to volume area ratio of monolith geometrical surface to volume area ratio of monolith catalytic surface to volume area ratio of monolith specific heat capacity dispersion coefficient specific mass flow heat of reaction length of monolith molar weight of component j specific heat flux reaction rate time weight fraction of component j spatial coordinate heat transfer coefficient mass transfer coefficient void fraction thermal conductivity stoichiometric coefficient density temperature
Indices amb g i
ambient gas reaction step i inlet component j solid
136 REFERENCES
1. W. Held, A. Donnerstag, E. Otto, P. Ktiper, B. Pfalzgraf ,and A. Wirth : The System Development of Electrically Heated Catalyst (EHC) for the LEV and EU-III Legislation. SAE Technical Paper Series Nr. 951072, 1995. 2. A. Donnerstag, A. Degen, W. Held and K. Korbel: Erftfllen der ULEV-Norm durch elektrisch beheizten Katalysator. VDI Fortschrittsberichte Reihe 12: Verkehrstechnik/Fahrzeugtechnik, (239), 1995.16. Intemationales Wiener Motorensymposium. 3. P. Oser, E. Mtiller, G.R. H/artel and A.O. Schtirfeld Novel 9 Emission Technologies with Emphasis on Catalyst Cold Start Improvements - Status Report on VW-Pierburg BumerICatalyst Systems. SAE Technical Paper Series Nr. 940474, 1994. 4. K. Kollmann, J. Abthoff and W. Zahn: Concepts for Ultra Low Emission Vehicles. SAE Technical Paper Series Nr. 940469, 1994. 5. B.H. Engler, D. Lindner, E.S. Lox, K. Ostgathe, A. Sch/ifer-Sindlinger and W. Mtiller : Reduction of Exhaust Gas Emissions by Using Hydrocarbon Adsorber Systems. SAE Technical Paper Series Nr. 930738, 1993. 6. M.D. Patil, W. Hertl, J.L. Williams and J.N. Nagel : In-Line Hydrocarbon Adsorber System for ULEV.SAE Technical Paper Series Nr. 960348, 1996. 7. T. Kirchner and G. Eigenberger: Optimization of the Cold-Start Behaviour of Automotive Catalysts Using an Electrically Heated Pre-catalyst. Chem. Eng. Sci., 51(10): 2409-2418, 1996. 8. S.E. Voltz, C.R. Morgan, D. Liederman and S.M. Jacob: Kinetic Study of Carbon Monoxide and Propylene Oxidation on Platinum Catalysts. Ind. Eng. Chem. Prod. Res. Develop., 12(4): 294 301, 1973. 9. Y. Kanada, M. Hayasi, M. Akakaki, S. Tsuchikawa and A. Isomura" Hydrogen Added After-Bumer System. SAE Technical Paper Series Nr. 960346, 1996. 10. T.Kirchner and G. Eigenberger: On the Dynamic Behaviour of Automotive Catalysts. Catalysis Today, 1997. (in press).
CATALYSIS AND AUTOMOTIVE POLLUTION CONTROL IV Studies in Surface Science and Catalysis, Vol. 116 N. Kruse, A. Frennet and J.-M Bastin (Eds.) 91998 Elsevier Science B.V. All rights reserved.
137
Measurement of the ceria surface area of a three-way commercial catalyst after laboratory and engine bench aging. E. Rogemond a, N. Essayem b, R. Fr6ty a, V. Perrichon a*, M. Primet a, S. Salasc a, M. Chevrier c, C. Gauthier c and F. Mathis c.
aLaboratoire d'Application de la Chimie ~ l'Environnement (LACE), UMR 5634, CNRS/Universit6 Claude Bernard Lyon I, 43 boulevard du 11 Novembre 1918, 69622 Villeurbanne Cedex, France. Fax : 334 78 94 19 95. e-mail : pe rrich@cp mol. univ -lyon 1.fr bInstitut de Recherches sur la Catalyse, 2 avenue Einstein, 69626 Villeurbanne Cedex, France. cR6gie Nationale des Usines RENAULT, Direction de ring6nierie des Mat6riaux, 8-10 avenue Emile Zola, 92109 Boulogne-Billancourt Cedex, Centre de Lardy, 1 all6e Cornuel, 91510 Lardy, Direction de la Recherche, 9-11 avenue, du 18 Juin 1940, 92500 Rueil Malmaison, France.
ABSTRACT The ceria surface area of a commercial Pt-Rh three-way catalyst was determined after laboratory hydrothermal aging at 1173-1373 K and after 200 h on engine bench. It was measured by X-ray diffraction (XRD) line broadening analysis and by a method based on the exploitation of the hydrogen temperature programmed reduction (TPR) profiles. In this case, the hydrogen uptakes below about 900 K include the ceria surface reduction and t h a t of the oxidized noble metals. They are analyzed and discussed, assuming two possiblities for the metals oxidation state. Compared to the fresh catalyst, the TPR profiles are deeply modified by the aging treatments. The ceria seems to sinter more t h a n alumina, particularly between 1173 K and 1273 K. After aging at 1273-1373 K, the calculated ceria surface area is only 15-10 m2g -1 washcoat, which represents 20% of the BET area, instead of 40% initially. A stabilization t r e a t m e n t at 823 K under reactants leads to an additional ceria sintering, even for the more aged system. Finally, the m e a s u r e m e n t s on the engine bench aged catalysts seem to indicate a better resistance of ceria to sintering in working conditions. The presence of a pollutant layer, containing phosphorus, zinc and calcium, did not modify the accessible ceria surface area measured by TPR.
138 1. I N T R O D U C T I O N Determining the surface area of ceria in three-way catalysts is an important problem for the characterization of these systems. Indeed, cerium oxide is a key component which enhances the global activity, particularly through the oxygen storage capacity (OSC), essential characteristic for regulating the oxidative power of the catalyst in a real catalytic converter [1-3]. Moreover, it has been proposed on model systems that the ceria support may become itself the active phase [4-7]. Accordingly, there is a great practical interest to find methods which are easily usable and allow a correct estimation of this parameter for fresh as well as for aged catalysts. In preceding papers, we have described a methodology to measure the exposed surface area in model metal/ceria-alumina catalysts [8-10]. It is based on the use of temperature programmed reduction (TPR) with hydrogen. It was shown that the reduction peaks for T lower than about 900-950 K, could be attributed to the reduction of both the oxidized precious metals and the superficial ceria layer. It has been established that 3.9 ~tmol H2 are necessary to reduce 1 m 2 of CeO2 [9]. The objective of this study is to extend this methodology to the case of a commercial three-way catalyst deposited on a ceramic monolith and to follow the evolution of the ceria surface area at different stages of its life-time, including long time engine-bench testing. In this case, it is important to examine the impact of poisons on TPR curves.
2. F ~ P E R I M E N T A L All characterizations were carried out on the same commercial catalyst (COM1). The active phase was constituted of 0.152 wt.% Pt and 0.031 wt.% Rh, the corresponding Pt/Rh weight ratio being 5/1 with a 32 g ft ~ loading. The ceramic was constituted of cordierite which contained iron as impurity. The hydrothermal laboratory aging (LA) consisted in treating the fresh catalyst during 5h at 1173, 1273 or 1373 K under a 6 1.h1 nitrogen flow containing 10% H20 introduced with an automated syringe. These four catalysts were stabilized (St), i.e. treated lh at 823 K under stoichiometric reactants synthetic mixture (CO, C3H6, C~Hs, NO, 02, CO2, H20). Moreover, the catalyst was studied after aging on engine bench (EB) during 200 h, with a air/fuel equivalence ratio oscillating around stoichiometry (~ ~ 1). Table 1 gives the results of the chemical analysis performed on the initial (fresh) monolith system. The figures remained nearly the same (within + 10%) after laboratory or engine-bench aging. We used X-ray diffraction technique to determine the CeO2 crystaUite sizes from the line broadening and the hydrogen TPR to calculate the ceria surface area according to a method developed on model catalysts [9,10]. In the case of TPR, the samples were ground before characterization. 250 mg were necessary for each run. They were treated lh at 673 K under air, and then lh30 at 773 K under argon flow before the TPR run. The heating ramp was 20 K min -1 up to
139 about 1073 K, temperature which was kept constant during 45 min. All the values will be given per gram of actual waschcoat (WC) Some XPS and SEM-EDX analysis were also realized on the engine bench aged catalysts in order to measure the surface composition of the catalytic washcoat which was modified by the poisons layer deposit. Table 1 Chemical analysis of catalyst COM 1 (wt. % basis). Washcoat 30.75
Pt
Rh
Ba
Ce
C1
Fe
La
S
0.152
0.031
0.66
6.3
0.145
0.45
0.3
0.15
2. R E S U L T S 2.1. Ceria m e a n p a r t i c l e size from X ray d i f f r a c t i o n Each catalyst was studied by XRD. To improve the intensity of the diffraction lines, the washcoat was scraped off the cordierite before analysis. The XRD spectra show the presence of alumina, ceria and some residual cordierite. The precious metals are never detected, as metal or oxide. For the fresh catalysts, the lines are broad meaning poorly crystallized phases. The resolution is greatly improved after aging. No other phase like cerium aluminate or other transformation product were evidenced on the XRD spectra. The calculation of the particle size of ceria was done on the broadening of the line at 20 = 56.37 ~ which is the best resolved and does not interfere with alumina. The results are given in Table 2.
Table 2 Ceria particle size determined from XRD diagram (line 20 = 56.37~ Catalyst
Fresh LAl173 LA1273 LA1373
particle size of CeO2 (nm)
S CeO2-XRD(m~g~)"
before stabilization
after stabilization
before stabilization
after stabilization
8.5 11.8 17.8 22.2
10.3 13.9 18.1 24.4
25 18 12 10
21 15 12 9
* S = [ 6,000 / (7.15 * Diameter in nm) ] ceria 9 percentage in the washcoat
140 In the fresh state, the ceria is rather well dispersed with a mean size of around 8 nm. During the aging up to 1373 K, this size increases from 8.5 to 22.2 nm, which evidences an important sintering of the ceria particles. This evolution is identical for the stabilized catalysts, with however slightly higher sizes. The corresponding surface areas per gram of washcoat are also given in Table 2. They were calculated by assuming spherical particles and a theoretical density of 7.15 g.cm -~for ceria. 2.2. S e l e c t i v e m e a s u r e m e n t o f the c e r i a s u r f a c e a r e a by TPR
Catalysts before stabilization
Figure 1 shows the TPR profiles of the catalyst as received and after laboratory aging at 1173, 1273 and 1373 K. The main features correspond to those observed on model catalysts [ 10]. In the fresh state, there is a well-resolved peak at 570 K, ascribable to the reduction of the oxidized precious metals and the ceria surface. After aging, the curve becomes flattened with much lower H2 uptakes. When the aging temperature is increased from 1173 to 1373 K, the intensity of the first peak is reduced to nearly zero, whilst two other curve inflexions or small peaks become more distinct at around 800 and 970-1000 K.
A 570 K
--*--Fresh
~LAl173
" m " LA1273
-*"LA1373
v
O
4)
e ..r
300
400
,500
600
700
800
900
I000
1100
Tern perature (K) Figure 1. H2 TPR of the catalysts before stabilization. Heating rate 20 9 K min ~
Although the separation between surface and bulk reduction is not always straightforward, the hydrogen consumption quantities were determined for temperatures lower than 900-950 K. They are given in Table 3. From them, it is possible to calculate the ceria surface areas of each catalyst, provided that some hypothesis are done on the mean oxidation state of the precious metal before starting the TPR experiment. In this study, the calculations were made with two different hypothesis" hypothesis 1) the metals are under the Rh 3§ and Pt 2§ states, hypothesis which was found valid for the fresh systems [9] and hypothesis 2)
141 rhodium is in a 3+ state whereas platinum is in a metallic state with a very small O/Pt ratio and set to 0 in the present study [10]. This second hypothesis has been supported by a separate TPR measurement performed on a Pt/A12Oa catalyst hydrothermally aged at 1323 K. Compared to the alumina support aged in the same conditions, no additional hydrogen consumption was detected during the TPR. The experimental hydrogen uptakes for the low temperature peaks and the calculated ceria surface areas are given in Table 3 with the BET areas. Table 3 Ce~a surface area s, per gram of washc0at, measured by the TPR method. Catalyst
SBET
He exp. --~900 K }imp1 g.1
Sc~o~ (hypo 1)a m~l
Sc~o9 (hypo 2)b m2g.1
Reduction extent r
(%)
.....
Fresh LAl173 LA1273 LA1373
167 101 77 54
309 163 75 55
69 30 7.5 2.4
75.5 38 15 10
128 121 44 41
Fresh St. LA1173 St. LA1273 St. LA1373 St.
139 96 72 41
251 124 74 49
54 20 7.2 1
61 28 15 8.5
126 74 48 29
ahypothesis 1 "O/Pt = 1 and OfRh = 1.5 bhypothesis 2 O/Pt 9 = 0 and OfRh = 1.5 csee text The difference between hypothesis 1 and 2 is about 6-8 m2g"1 and corresponds to the hydrogen quantity needed to reduce Pt 2+ into Pt ~ For the fresh catalyst, hypothesis 1 is the most appropriate and leads to a ceria surface area of 69 m2g~. For the aged catalysts, the relative uncertainty between the values of SCeO2 obtained with the two hypothesis is acceptable for LAl173, but not after aging at 1273 and 1373 K. As said above, hypothesis 2 seems the most reasonable, and the calculated values of the ceria surface areas are 15 and 10 m 2 ~ 1 after aging at 1273 and 1373 K. They are effectively in better agreement with the XRD results, than with hypothesis 1. To have more informations on the evolution of the support after the aging treatments, it is possible to follow an other parameter which is the reduction percentage of the catalyst at the end of the TPR. It corresponds to the ratio between the experimental hydrogen consumption during the whole TPR, including the 45 mill step at 1073 K, and the maximum theoretical H2 consumption necessary for the reductions (CeO2 ") CegO~; PtO ") Pt~ Rh20~ -') Rh o ). For fresh and LAl173, this percentage is higher than 100% (128 and 121%) and close to that of the washcoat without precious metals (131%). It
142
decreases to 44 and 41%, for LA1273 and LA1373 respectively. We can deduce that "i) in the initial solid and after mild aging (1173 K), some reducible species other than precious metals and ceria are present in the system, and ii) the main modification of the catalyst during the aging treatments occurs between 1173 and 1273 K. In this respect, LA1173 can be considered as a weakly aged catalyst. To explain the reduction extent higher than 100%, several hypothesis were considered but were not verified. The reduction of the iron oxide present in the cordierite was not observed during a separate TPR. The presence of barium sulfate was also evidenced by XPS. In the TPR conditions, BaSO4 begins to be reduced at about 1000 K. However, the hypothesis of its reduction in the catalysts was not kept, since the reduction percentage of a platinum catalyst supported on a ceria-alumina modified with BaSOa was a little lower than that performed in absence of barium sulfate. The assumption of the hydrogenation of some carbonates species, as surface lanthanum carbonates, was also rejected, since, as evidenced by mass-spectrometry, there is no relationship between the excess reduction percentage and the formation in the gas phase at very low concentration of CH4, or even CO and CO9. The question is still under study.
Catalysts after stabilization Figure 2 exhibits the TPR profiles of the previous catalysts, after 1 h at 823 K under the reactants, and Table 3 presents the results. For the fresh catalyst after stabilization, the initial low temperature peak is split into two peaks of lower intensity. For the aged catalysts, the stabilization leads also to profiles with a lower intensity during the whole TPR. Accordingly, the hydrogen uptakes are lower than those of the initial systems. If one supposes that the metals after reoxidation at 673 K have the same mean oxidation state before and after stabilization, which means no change in their size and their state during the stabilization, the calculated ceria surface areas are lower after stabilization (Table 3). Thus, the stabilization results in an additional ceria surface loss. , =
-e-Fresh
.
St,
.
.
.
.
.
.
- - ~ , - L A l 1 7 3 St,
..e.- LA1273 St,
--e-- LA1373 St,
~
515 K
01
e
:
- - - - -
.
.
.
.
.
.
"o
2: 300
I 400
I 500
I 600
I 700
Temperature
: 800
: 900
1000 1100
{K)
Figure 2. H2 TPR of the catalysts after stabilization. Heating rate 20 K min -1. (hydrogen uptake scale about two times higher than in Fig. 1)
143 2.3. Study of the catalyst after 200 h a g i n g on an engine-bench. One of the difficulties to study this catalyst is the possible influence of the poisons deposited on the active phases during the test and originated from the gasoline or motor oil components such as Si, Ca, P, Zn, S ... [11,12]. In particular, the TPR study may become totally erroneous if additional reducible compounds are present. To take into account this influence and to evidence an eventual aging gradient along the axis of the monolith, three samples were selected after the test, at the inlet, in the middle and at the outlet of the monolith. The analysis and distribution of the poisons were done by SEM coupled with an EDX analysis. A macroporous layer of pollutants was evidenced on the surface. The analysis was done on the elements of the support (AI, Ba and Ce) as well as the poisons usually found after such a treatment, i.e. P, Ca and Zn [11]. Sulfur was searched for but was not detected. In the front side of the converter, the poisons were the only elements detected, with almost 50% Zn, 40% P and 10% Ca. It means that the poison layer is thicker than that analysed by EDX, i.e. about l~m. The Zn concentration decreased quickly in a few millimeters axially and then was not detected (<1%), whilst P and Ca were always found in high proportion. However their concentration decreased also continuously along the axis of the converter, from 20 to 7% for Ca, and from 40 to 30% for phosphorus. It results that, after an engine bench test, the washcoat is covered by a thick layer of poisons which becomes progressively thinner when arriving to the outlet. These results are in agreement with those obtained by XPS on the superficial composition of the three selected samples. As shown in Table 4, Zn is detected only at the inlet, phosphorus is evenly distributed along the whole length, and the calcium content decreases between the inlet and the outlet. Conversely, the washcoat Constituents concentrations increase continuously along the axis. Table 4 Surface composition (atomic percentage) of the washcoat after an engine bench aging, as determinedby XPS. . . . . . . . . Analysed zone Inlet Middle Outlet
A1
Ce
Ba
O
C
Ca
P
Zn
7.6 11.6 14.2
1.6 2.5 3.1
1.3 2.1 2.2
54.3 50.2 54.2
9.0 12.1 8.1
8.3 7.8 5.5
12.8 13.8 12.8
5.2 0 0
A TPR study was performed on the same samples, but after grinding. As shown on Figure 3, there is no significant difference between the three samples, indicating that the poisons are not reduced in the TPR conditions. The TPR profile was attenuated compared to the fresh catalyst, but less than the aged and stabilized solids. Table 5 gives the quantitative results obtained, as above, from the hydrogen uptakes up to ~900 K.
144
A - P - EB
ei
--*-- E B . - e - EB
t~ Q.
Inlet Middle Outlet
745 K
530 K
c o
.o
qD "I-
300
400
500
600
700
800
900
1000
1100
Temperature (K) Figure 3. H2 TPR of engine bench aged catalysts. Heating rate" 20 K min 1. (hydrogen uptake scale about four times higher t h a n in Fig.l)
Table 5 Ceria surface areas m e a s u r e d by the TPR method. Catalyst
Sceo2
SBET m2g-1
H2 exp. ~900 K Fmol g-'
(hypo 1)a m2g-,
66 66 60
101 104 103
16 17 16
EB Inlet EB Middle EB Outlet
ahypothesis 1 O/Pt 9 = 1 and O/Rh = 1.5 r text
Sceo2 Reduction (hypo 2) b extent c m.2.g:1 . . . . . . . . . . (%).. 22 23 23
52 43 43
bhypothesis 2 - O/Pt = 0 and O/Rh = 1.5
It appears t h a t the engine bench aging effects are comparable to those obtained in laboratory at 1273 K for the BET area, but seem less severe for Sceo2. Moreover, there is no significant difference in SBET and Sceo2 from the inlet to the outlet of the converter which confirms the absence of negative effect of the pollutants layer on the m e a s u r e m e n t of the ceria surface area.
3. D I S C U S S I O N The ceria surface area of a commercial three-way catalyst has been m e a s u r e d by XRD and by TPR at different stages of its evolution. The reliability of the results has to be discussed. It can be recalled t h a t Sceo2.xaD corresponds to the sum of the surfaces of each crystaUite, whereas TPR measures the area accessible to hydrogen. For the fresh and LAl173 catalysts, Scoo2-weR is higher
145 t h a n SCeO2-XRD.It is likely t h a t a noticeable proportion of the ceria in the initial solid is poorly crystallized. During the aging, ceria becomes more and more crystallized, and bigger CeO2 particles are formed which can be composed of several crystaUites. In these conditions, SCeO2-XRDbecomes closer to the actual celia surface and m a y even become higher t h a n SceO2-TPRwhich measures only the external surface of these big particles. In other respects, Sc~O2.TPR depends on the hypothesis made on the mean oxidation state of the precious metals. We saw t h a t hypothesis 1 can be valid for fresh catalysts and also after aging at 1173 K. For higher aging temperatures, the m e a n oxidation state of platinum tends towards zero when supported on alumina. The same hypothesis is also probable for ceria-alumina. The relative good agreement between the values of SceOg-TPR and Sc~og.-xRo supports this assumption. A decrease in the reducibility of the rhodium ions in interaction with alumina is also quite possible but difficult to be quantified in the case of a ceria-alumina support. Accordingly, a s u m m a r y of the most probable data is presented in Table 6. For engine bench aging, hypothesis 2 was chosen, considering that, in this case, the reduction extent is closer to that of the most aged systems. Table 6 S u m m a r y of the results obtained by TPR and XRD on the catalyst after various aging t r e a t m e n t s . . . . . . . . . . . . . Catalyst
Sceo2. m2g-I
TPR XRD
Fresh St. 69 ~ 25
54 ~ 21
LA1173 St. 30 ~ 18
~hypothesis 1 : O/Pt = 1 and O/Rh = 1.5
20 ~ 15
LA1273 St.
LA1373 St.
15b 12
l0 b 10
15b 12
8.55 9
Inlet 22 b
EB Outlet 23 b
bhypothesis2 : O/Pt = 0 and O/Rh = 1.5
Finally, to describe the evolution of the ceria surface areas, if both methods are necessary, the TPR method seems more reliable for fresh and weakly aged catalysts. After aging at 1273-1373 K, the ceria surface area is only 15-10 m2g ' instead of 69 m2g -1 initially, which represents 20% of the BET area instead of 40% initially. It results t h a t ceria sinters more t h a n alumina in the washcoat. Even ff the engine bench test was considered as not very severe, with a catalyst t e m p e r a t u r e of about 1130 K, the BET areas and the reduction extents t a k e n as a criterion of the aging, seem to indicate t h a t the EB catalyst was submitted locally to t e m p e r a t u r e s at least higher t h a n 1273 K. Indeed, the BET surface area was found lower t h a n for the catalyst aged at 1273 K, 66-60 instead of 77-72 m2g -'. However the calculated ceria surface remained rather high, 22-23 m2g 1 for the u p s t r e a m and the downstream face. Since we observed that stabilization u n d e r reactants leads to lower ceria surface areas, this less severe
146 ceria sintering during the engine bench test cannot be attributed to the redox processes occurring during the three-way catalysis. According to previous results on the t h e r m a l stability of pure ceria [13], it could be tentatively interpreted as due to the stabilizing effect at high temperature of CO2 present at around 10 vol.% in the exhaust gases. Indeed, it could preserve the ceria specific area by forming stable carbonate species on the ceria.
4. CONCLUSION It is shown t h a t the TPR method developed in the case of ceria-alumina supported Pt-Rh model catalysts can be used to estimate the ceria surface area of commercial three-way monolithic catalysts. Upon hydrothermal aging, a strong decrease of the ceria surface area was evidenced, the main modification of the catalyst being observed between 1173 and 1273 K. As shown by the SBET and SCeO2-TPRvalues, a stabilization treatment at 823 K under reactants leads to an additional sintering, even for the more aged system. That shows that redox conditions are able to modify the catalyst surface state even at temperatures much lower t h a n during aging. Engine bench test was found equivalent to an hydrothermal aging at 1273 K, with however a better resistance of ceria to sintering. The surface of these engine bench aged samples is covered by a more or less compact layer of pollutants. Its thickness decreases from the inlet to the outlet of the converter. However, the presence of these poisons does not modify the accessible ceria surface area measured by the TPR method. REFERENCES
~
6. 7. 8.
.
10. 11. 12. 13.
B. Harrison, A.F. Diwell and C. Hallett, Platinum Met. Rev., 32 (1988) 73 W.B. Clemmens, M.A. Sabourin and T. Rao, S.A.E. paper, 900062 (1990). J.W. Koupal, M.A. Sabourin and W.B. Clemmens, S.A.E. paper, 910561 (1991). S.E. Golunski, H.A. Hatcher, R.R.Rajaram and T.J. Truex, Appl.Catal.B : Environmental, 5 (1995) 367. J.G. Nunan, H.J. Robota, M.J. Cohn and S.A. Bradley, J. Catal., 133 (1992) 309. C. Hardacre, R.M. Ormerod and R.M. Lambert, J. Phys. Chem., 98 (1994) 10901. C. Hardacre, G.M. Roe and R.M. Lambert, Surface Science, 326 (1995) 1. E. Rogemond, R. Fr6ty, P.J. L6vy, V. Perrichon, V. Pitchon, M. Primet, N. Essayem, M. Chevrier, C. Gauthier and F. Mathis, Stud. Surf. Sci. Catal., 96 (1995) 405. E. Rogemond, R. Fr~ty, V. Perrichon, M. Primet, S. Salasc, M. Chevrier, C. Gauthier and F. Mathis, J. Catal., 169 (1997) 120. E. Rogemond, R. Fr6ty, V. Perrichon, M. Primet, M. Chevrier, C. Gauthier and F. Mathis, Appl. Catal. A : General, 156 (1997) 253. S. Kim and M.J. D'Aniello Jr., Appl. Catal., 56 (1989) 23. J.C. Summers, J.F. Skowron, W.B. Williamson and K.I. Mitchell, S.A.E. paper, 920558 (1992). V. Perrichon, A. Laachir, S. Abouarnadasse, O. Touret and G. Blanchard, Appl. Catal. A: General, 129 (1995) 69.
CATALYSIS AND AUTOMOTIVE POLLUTION CONTROL IV Studies in Surface Science and Catalysis, Vol. 116 N. Kruse, A. Frennet and J.-M Bastin (Eds.) 91998 Elsevier Science B.V. All rights reserved.
147
The effect of the ageing procedure upon the activity of a three way catalyst working under transient conditions R. Roh6, V. Pitchon, G. Maire LERCSI, Laboratoire d'Etudes de la R6activit6 Catalytique, Surfaces et Interfaces, URA 1498 du CNRS - Institut le Bel, Universit6 Louis Pasteur, 4, Rue Blaise Pascal, 67070 Strasbourg, FRANCE. (Fax: 33 (0)3 88 41 61 47, e-mail: [email protected]).
ABSTRACT The mode of ageing of a model three-way catalyst has been studied. Ageing with fixed and oscillating as constituents differs from conventional thermal ageing. The catalysts were characterised by their reactivity under transient conditions, by Temperature Programmed Reduction and by TEM. Under conditions of oscillating gas constituents, wich are near the actual use conditions, and even at high temperature, almost no deactivation was observed while thermal ageing led to a severe sintering of the metallic phase Pt-Rh.
1. INTRODUCTION Air pollution due to the automotive exhaust has become a major problem in residential areas. This high concentration of pollutants has encouraged the authorities to control and reduce air degradation wherever possible. The solution as adopted by the automotive manufacturing industries, has been to supply vehicles with a catalytic converter equipped with a three-way catalyst which oxidiseS hydrocarbons (HC) and carbon monoxide (CO) into CO2 and H 2 0 and reduces nitrogen monoxide (NO) into nitrogen. Platinum, in association with Rhodium, is able to perform such reactions. To be efficient, the catalyst must operate under gas flow conditions in which the stoichiometric ratio is close to unity. For this reason, the engines are coupled to an oxygen sensor. This device along with the acceleration/deceleration phases, ensure that the engine never works under stationary conditions. The question of ageing for the three-way catalyst is a real problem. The literature reports the importance of the mode of preparation [1], and of the ageing conditions [2], but few studies have been carried out in alternating flow conditions at high temperatures. Authors have only studied the effects of such fluctuations using simplified gas mixtures (02fl----I2) or very low frequencies of oscillation between the two gas compositions [3,4,5]. Usually, to mimic the ageing phenomenom of a real engine, in the laboratory, high temperatures (>900~ and a wet oxygen flow are used. Of course, to know whether this type of procedure is representative of a real engine under transient conditions is questionnable. For this reason, the present study has been undertaken. In this work the influence of temperature and flow composition upon the ageing of the Pt-Rh/AI203 and upon its activity under transient
148 conditions has been studied while temperature programmed reduction has been used to characterise the catalyst.
2. EXPERIMENTAL
2.1. Catalyst preparation The monometallic and bimetallic catalysts in this study were prepared either by the Institut Frangais du P6trole (IFP) (1% Pt and 1% Pt-0.2% Rh) or in our laboratory (1% Rh) by impregnation of a y-alumina in pellet form of 2 to 4 mm in diameter. The precursor salts were H2PtCI 6 and RhC13, and the co-impregnation method was employed to prepare the bimetallic. Following evaporation and drying, the catalysts were calcined in air at 350~ for 4 hours and then reduced under hydrogen at 500~ for 8 hours. They were then crushed and sieved. The fraction of 80-250 ~tm was retained for the experiments.
2.2. Catalytic tests. For the catalytic tests, 100mg of the bimetallic catalyst Pt-Rh/A1203 were placed in a tubular flow reactor and fed with a complex gas mixture containing CO, NO, C3H 8 and 02, with N 2 as the diluent. The flow composition oscillated between two values of SN which represented the ratio between oxidising and reducing species in the reactive medium and can be defined as: SN=(2[O2]+[NO])/([CO]+ 10[Calls]) The two gas compositions were controlled by fast acting solenoid selector valves located at the inlet of the reactor, the switching frequency between the two compositions being 0.06 Hz. The catalysts were tested with SN=I_+.0.1, i.e. between SN=0.9 (slightly reducing) and SN=I.1 (slightly oxidisisng). The corresponding concentrations are reported in Table 1. The catalysts were heated from room temperature to 450~ with a ramp rate of 4~ The concentrations of the reactants and products were measured every 15 seconds using infrared detectors for CO, C3H8,CO2 and chemiluminescence analysers for NO. Table. 1 : Gas flow composition 02
CO NO HC
SN=I.1 6640 ppm 5157 ppm 2050 ppm 878 ppm
SN=0.9 5684 ppm 6131 ppm 2050 ppm 878 ppm
2.3. Ageing procedure. Several ageing procedures were performed to study the influence of heat treatment and to compare ageing under stationary and oscillating conditions. Initially, the catalysts were aged under conditions of oscillating composition at a variety of temperatures between 500~ and 900~ employing the same gas composition as for the
149 catalytic tests (SN=I+0.1). Subsequently the catalysts were aged at 900~ with different parameters. For comparison, the catalyst was submitted to a more classical ageing process, i.e. by heating it up to 900~ under a 4.5% oxygen flow. In some cases, the temperature was held at 900~ for 4 hours. All of the test procedures are summarised in Table 2.
Table 2: Ageing tests aAgeing C900 $900 T900 a C
--
Oscillating X
Stat!onary X X
Flow SN= 1+/-0,1 SN=I 02
cycling, S = stationary, T = thermal
2.4. Electron microscopy Electron microscopy was performed using a TOPCOM EM002B transmission microscope. When operating at 200 kV, this microscope provided an interpretable resolution of 0.18 nm. The samples were prepared by deposition and evaporation, over a copper grid, of a suspension of the catalyst into ethanol with a carbon support film. Some of them were treated by a hydrofloric acid solution, before deposition, to separate alumina and metallic particles. Samples were photographed with a direct magnification of 590,000x, 200,000x, 100,000x, and 49,000x.
2.5. Temperature programmed reduction (TPR) Temperature programmed reduction is a classical method for characterising supported catalysts. The analyses were performed using an apparatus of the Z-sorb range commercialised by Gyra. Prior to reduction the catalyst, placed in a U-shaped reactor, was calcined in an air flow at 400~ (10~ for 1 hour. Then, after cooling down to room temperature the catalyst was reduced in a flow of 1% H2 in argon. The temperature was programmed to increase at a constant rate of 8~ up to 500~ The gas composition was analysed by means of a catharometer and the data were collected on a computer. In Table 3. the characteristics of the catalysts employed in this procedure are given.
Table 3" Characteristics of the catalysts tested in TPR Name Rh Pt Pt-Rh $900
% Rh (wt) 0.2 0 0.2 0.2
%Pt(wt) 0 1 1 1
Weight (g) 1 0.5 0.5 0.5
T900
0.2
1
0.5
treatment Fresh Fresh Fresh Oscillating ageing Thermal ageing
150 3. RESULTS
3.1. Microscopy The fresh and C900 catalyst present homogenous particle size distributions compared to the distribution for T900 catalyst. Most of the particles of the fresh catalyst have a size ranging from 10A to 20A with the largest being 45A. The average size was 16A. The size distribution proved quite different for the C900 catalyst with a range of particle from 10A to 25A yet with several particles larger than 55A were observed ; This led to an average size of 19A. Two kinds of particles proved highly distinct in the T900 catalyst: the small ones which were not bigger than 40A, and the larger ones which ranged from 150A to 400A. The smaller particles, as with the fresh catalyst and the C900 catalyst, were seen to possess a spherical shape while the larger ones were multifacial. Calculation of a statistical average size was impossible since the population of the large particles was not high enough; the number of particles required to calculate a size being about 800. However it was possible to distinguish atomic planes on several particles of the T900 catalyst and the C900 catalyst. Nevertheless, owing to the size of the particles of the fresh catalyst, no observation of any atomic planes could be made, therefore no strong conclusion with regard to their amorphous nature was reached. 3.2. Catalytic results Oscillate ageing. A conversion profile for the three pollutants is represented in Figure 1. Usually, all of the CO is converted at about 210~ while NO and HC are converted simultaneously at a higher temperature. Activity is represented by Ts0, the temperature at which 50% of the reactant has been converted. The values for the catalysts aged under the oscillating gas composition regimen at different temperatures are given in Table 4, the standard test being carried out up to a temperature of 500~ The activity of these catalysts decreased for both NO and HC conversion as the ageing temperature increases, while the conversion of CO is not affected by this ageing treatment. Nevertheless, this loss of activity remained negligible for the case of an oscillating ageing procedure. Table 4" Ts0 depending on ageing temperature during gas composition oscillations. Ageing temperature 500 600 700 900
CO 196 204 200 200
T5o NO 283 287 292 291
HC 276 278 286 287
151 lOO
-1
'
.,J.
,,
,
,
.
-
L _ . A A
,
A
---r. ,, ,.,.T_._,.,.,_
,:
A
c 0
"~
50
c 0 0
y~
/ -~ "
.....
cCO
- - - -
cNO
, i,
0
v
v_
iii
100
-
ii
cHC
-
200
300
Temperature (*C)
4OO
500
Fig 1" Conversion profile of the pollutants during an oscillate test The comparison of catalysts aged at 900~ ageing under different mode. The results are presented in Table 5. The C900 ageing catalyst was rather less active than its $900 counterpart for both NO and HC conversions. Here again, the CO conversion remained unaffected by a treatment under the gas mixture at 900~ whether cycling or not. The T900 ageing catalyst however exhibited a completely different behaviour. A large deactivation was observed as revealed by the values of the Ts0 and in the case of the all three pollutants. The loss is of 53~ for CO and about 60~ for NO and HC. It is clear that a thermal treatment under oxygen only leaves to a completely different catalyst that the one treated under a complete mixture of gas although the temperature of treatment was the same.
Table 5" Ts0 dependence upon ageing conditions. Catalyst C900 $900 T900
CO 200 200 253
Ts0 NO 291 282 360
HC 287 273 355
3.3. TPR For the two monometallic samples, a single reduction peak was observed, as in Figure 2, corresponding to the reduction of either of the metal oxides as described by the following equations: PtO2 + 2H2 ~ Pt~ 2H20 [6] Rh203 + 3H2 ~ 2Rh + 3H20
152
I
- - -Rh ~
<~
1
Pt
----- Pt-R h
o
-
0
-
-
-
-
-
~
-
-
-
50
-
-
t
-
.
.
.
.
100
.
.
~
-
150
-
-
-
-
-
t
--
200
-,
250
------
Temperature*C
,
300
.....
;
350
400
450
fig 2: TPR diagram of Pt(1%)/A1203, Rh(0.2%)/A1203, Pt(1%)-Rh(0.2%)/A1203 The maximum of the peaks are, in both cases, located around 110~ However the TPR diagram of the bimetallic reveals two different reduction peaks. The first one with a maximum at about 100~ corresponds to the simultaneous reduction of platinum and rhodium oxides, while the second, around 210~ and weaker in intensity, is probably representative of further metallic oxide reduction, including perhaps an alloy. Following the oscillate ageing (fig. 3), the second peak disappeared from the TPR diagram while, subsequent to the thermal ageing, only a slight deviation from the base line was observed around 100~
/~
:5
0
50
100
150
A
200
250
Temperature ~
300
--
T900I
_ ::
Pt-R
350
c,ool ]
400
450
fig 3" TPR Diagram of Pt(1%)-Rh(0.2%)/A1203 flesh (--) and aged under either oscillating (A) or stationary conditions (--)
153 4. DISCUSSION The experimental data indicate clearly that the catalytic behaviour of an aged Pt-Rh/A1203 catalyst depends more on the gaseous atmosphere to which it is submitted than the temperature to which it is exposed. This is rather surprising since usually temperature and sintering are associated. Nevertheless, it is known that the mode of sintering differs between platinum and rhodium. Platinum oxide segregates into its constituent elements [7] at temperatures in excess of 600~ and will begin to sinter as the temperature approaches 800~ The sintering importance is not dependent upon the existence, or not, of a step of composition but on the composition gas flow. Indeed, the sintering is faster when the flow is oxidising rather than reducing [8]. Rhodium will also undergo sintering when it is heated under an oxidising flow [9]. However, it can be redispersed by subsequent heating in a reducing flow [4]. At this point, it is interesting to acknowledge that although the reactivity of these two catalysts are comparable, their respective TPR profiles are very different, since the disappearance of one peak is observed for the C900 when compared to a fresh catalyst. What are the species signified by this peak? For the present they remain unidentified, yet whatever they are, the conclusion can be drawn that they are not active for the reactions involved. They could well be due to an alloy or a bimetallic phase which would be destroyed by the reacting mixture perhaps by segregation of the two metals. Due to the differences between the heat of adsorption on Pt and Rh several authors have proposed [ 10] a phase segregation of the metals by hydrogen chemisorption, by oxidation-reduction cycles [11 ], or other treatments [12]. Another possibility, could be that this second peak represents free rhodium upon the support which when submitted to the reacting mixture at high temperature, would migrate toward the interior of the support. This idea has been proposed earlier for cases of stronger oxidative mixtures [9, 13]. This rhodium must be very well dispersed and should exist only in the form of Rh203 which is impossible to reduce, since as has been under conditions where, SN = 1 +/- 0.1, the reduction during the rich transition is only possible when Pt and Rh are in proximity in such a way that Pt is able to assist the reduction of rhodium [ 14]. When a catalyst is exposed to a stoichiometric gas mixture, whether oscillating or stationary, the resulting deactivation is very moderate when the temperature of the pretreatment is raised from 500~ to 900~ This deactivation can only be proven by the values of the T50 recorded for the reaction between NO and HC and not for that between CO and 02. This confirms that the CO/O2 reaction is structure insensitive. Indeed, a moderated sintering of the metallic phase, as seen by microscopy, has no influence upon the reaction when compared to a catalyst aged from 500~ to 900~ The implication is that the best way to observe an effect related to sintering is to consider the reactions involving HC or NO since they are usually structure sensitive [15, 16]. Nevertheless, the catalytic behaviour of the T900 catalyst reveals a far more important change since if it was the case of a simple sintering then no changes should be observed in the values of the T50 for CO. During thermal ageing, all of the particles of the catalyst have sintered to form larger particles of greater stability. This renders them unavailable for reoxidation and as a result no reduction peak is observed in the TPR diagram. This sintering
154 phenomenon is surely accompanied by a profound modification of the interaction phase between Pt and Rh. CONCLUSION In this paper, the mode of ageing of a model three-way catalyst has been studied. The results show that the heating of a catalyst under a complex gas mixture or in a flow which alternates between two compositions leads to a very moderate sintering and to the disappearance of a bimetallic phase present before the reaction. Meanwhile, an ageing under high temperature and very oxidizing gas flow leads to a severe sintering of the metallic phase and a strong deactivation. The results presented prove that the conventional procedure used at the laboratory scale to age a catalyst is not representative of the various phenomena occuring upon a real catalytic converter. REFERENCES
1. P.Marecot, A.Fakche, L.Pirault, C.Geron, G.Mabilon, M.Prigent and J.Barbier (1994), Appl. Catal., B, 5, 43. 2. J.C.Schlatter, R.Sinkevitch and P.J.Mitchell (1983), Ind. Eng. Chem. Prod. Res. Dev. 22, 51. 3. S.T.Schmieg and D.N.Belton (1995), Appl. Catal., B, 6,127. 4. D.D.Beck and C.J.Carr (1993), J. Catal., 144, 296. 5. R.Taha and D.Duprez (1992), Catal. Letters, 14, 51. 6. H.C.Yao, M.Sieg and H.K.Plummer (1979), J. Catal., 59, 365. 7. T.Huizinga, J. van Grondelle and R.Prins (1984), Appl. Catal., 10,199. 8. D.D.Beck and C.J.Carr (1988), J. Catal., 110, 285. 9. H.C.Yao, S.Japar and M.Shelef (1977), J. Catal., 50, 407. 10. N.Savargaonkar, B.C.Khabra, M.Pruski and T.S.King (1996), J. Catal., 162, 277. 11. T.Wang and L.D.Schmidt (1981), J. Catal., 70, 187. 12. R.E.Lakis, C.E.Lyman and H.G.Strenger (1995), J. Catal., 154, 261. 13. J.Barbier, D.Duprez, CAPOC III, (1995), 96, 73. 14. C.Howitt, V.Pitchon, G.Maire, (1995), J.Catal., 154, 45. 15. K.Otto, J.M.Andino, C.L.Parks, J. Catal., (1991) 131,243. 16. S.H.Oh, G.B.Fisher, J.E.Carpenter, D.W.Goodman, J. Catal., (1986), 100, 360).
CATALYSIS AND AUTOMOTIVE POLLUTIONCONTROL IV Studies in Surface Science andCatalysis,Vol. 116 N. Kruse, A. Frennet and J.-M Bastin (Eds.) 91998 Elsevier Science B.V. All rights reserved.
155
Causes of deactivation and an effort to regenerate a commercial spent three-way catalyst T.N. Angelidis a, M.M. Koutlemani a, S.A. Sklavounos a, Ch.B. Lioutas a, A. Voulgaropoulos a, V.G. Papadakis b and H. Emons c aAristotle University, Box 114, 54006 Thessaloniki, Greece bInstitute of Chemical Engineering and High Temperature Chemical Processes, P.O. Box 1414, 26500 Patras, Greece CInstitute fur Angewandte Physikalische Chemie, Forschungzentrum Julich GmbH, LeoBrandt-Strafie, 52428 Julich, Deutchland
ABSTRACT The main aim of the present paper is to summarize the research work of our group with respect to the deactivation and regeneration of the three-way automotive catalysts (T.W.C.). Measurements describing the effect of the thermal deactivation and contaminants accumulation on the structure and the efficiency of an a vehicle aged T.W.C. are presented. A variety of techniques, such as SEM-EDS, XRD, CTEM, ICP-AES, ICP-MS and AAS, were applied in this effort. Since, thermal deactivation is more or less irreversible, special attention was given to the nature and the distribution of the contaminants accumulated on the T.W.C. active surface. Based on these observations, a leaching procedure with acetic acid was tested and optimized in laboratory scale for the removal of the contaminants. The leached samples of the T.W.C. were examined with respect to their catalytic activity (CO and C2H4 oxidation, NO reduction and N2 production). The leaching procedure seems to improve considerably the catalytic activity of the aged catalyst. 1. INTRODUCTION During operation the three-way automotive catalysts (T.W.C.) display a general loss of their catalytic activity. Deactivation of the T.W.C. can result from many processes. The processes fall into four general categories: chemical, thermal, fouling and mechanical [ 1]. During vehicle operation, any or all of the categories can contribute to catalyst deactivation. With current catalyst package technology in combination with properly-tuned engine operation and emission control, deactivation resulting from carbonaceous deposits fouling and mechanical categories is minimal. Thus, catalyst deactivation during normal vehicle operation typically results from mechanisms in the chemical and/or thermal categories. Thermal deactivation includes the
156 following mechanisms [2]: noble metals sintering, noble metals alloying, support changes, noble metal-base metal interactions, metal/metal oxide-support interactions, oxidation (alloy segregation), noble metal surface orientation changes and metal volatilization. The methods to study thermal deactivation mechanisms are direct or indirect. The direct methods include microscopic studies by various techniques of the constituents of the catalyst's washcoat (active metals, promoters, stabilizers, contaminants and support constituents) and macroscopic physical characteristics (total and active surface, pore size and volume) [3-8]. Indirect methods include comparison of the catalytic activity between fresh and aged catalyst [2,7,9]. The catalyst is usually aged in laboratory scale by simple heating in a controlled atmosphere furnace at high temperatures (800~176 which induce thermal deactivation mechanisms. Extrapolation of results obtained from laboratory reactor experiments to the actual exhaust environment should be done with caution [4]. The aim of the present research was to examine in what extent XRD (X-Rays Diffraction) and CTEM (Conventional Transmission Electron Microscopy) may give information on the thermal deactivation mechanisms (mainly precious metals sintering and support changes) of a commercial T.W.C. aged under vehicle operating conditions. This is important, since during laboratory aging normally the conditions are selected to induce only a specific deactivation mechanism, while under vehicle operation conditions the situation is more complex, due to the simultaneous presence of all deactivation mechanisms. Chemical deactivation is the result of contaminants accumulation on the active surface of the T.W.C. and their action as poisons. There are three types of poisoning mechanisms [ 10]: chemical poisoning (the poisons interact chemically with the substrate or the active metals) physical poisoning (the poisons interact mainly as foulants by clogging the pores of the substrate) inhibition (instantaneous and active for the time period while the poison is present in the exhaust gas flow. The most detrimental contaminants are those originating from fuel and engine oil [11-12]. Most lubricating oils for engine use contain additives designed to improve such properties as lubricity, detergency, oxidation resistance and viscosity. The common lubricating additive ZDP (zinc dialkyldithiophosphate) [13-15] is the main contributor of phosphorus, zinc and partially sulphur contaminants. Other possible contaminants originating from engine oil are barium, calcium, magnesium and boron. Although the presence of sulphur and lead in automotive fuels was minimized the last decades due to environmental restrictions, the presence of small quantities is unavailable. So the fuel is a potential contn'butor of such contaminants. Another possible fuel originating contaminant is Mn, from the fuel additive MMT (methylcyclopentadienyl manganese tricarbonyl added to unleaded gasoline in order to improve the performance of the engine [16-17]. The engine and exhaust system construction material can also contribute by hamafid contaminants, i.e. iron, copper, nickel and chromium. Iron is the major component of the debris retained by the catalyst; nickel and chromium are usually components used to fabricate high-temperature resistant materials such as engine parts and the exhaustion system itself, copper may originate in engine bearings or in the copper lines used for air injection. Chemical quantitative analysis by ICP-AES (Inductively-Coupled Plasma/Atomic Emission Spectrometry) and ICP-MS (Inductively-Coupled Plasma/Mass Spetrometry) were applied to determine the contaminants accumulation and longidinual distribution and estimate precious metals losses on the aged T.W.C.. The main compounds formed by the contaminants at the T.W.C. operational conditions are phosphates, sulphates, aluminates and oxides [12-13,17]. The contaminants are selectively -
-
-
157
accumulated on the external surface of the washcoat [ 18]. Most of these compounds are readily dissolved in acidic environment. Thus, an acid-leaching procedure is expected to remove substantial quantities of the contaminants from the catalyst surface. The use of strong acids is not recommended since, they will possibly attack the useful constituents of the substrate and weak organic acids (acetic or oxalic) are prefered. Oxalic acid has been applied for the regeneration tests of a hydrodesulphurization catalyst [19] and acetic acid for a Pt/NiO industrial catalyst [20]. Acetic acid (pK~=4.72) is prefered since, is a weaker acid than oxalic acid (pK, l=l.21). Preliminary leaching tests using acetic acid combined with SEM photos and EDS surface analysis shown a substantial removal of contaminants from the surface of the catalyst [18]. A summary of our comprehensive work [21] on regeneration of T.W.C. by acetic acid leaching, in an effort to optimize the leaching conditions and to check the catalytic activity of the leached material is cited in this presentation 2. STUDIES OF THERMAL DEACTIVATION 2.1. Experimental
A fresh and an aged commercial monolithic catalytic converter of the same origin were used for the XRD and CTEM studies The aged catalyst was derived from an automobile with 60000kin mainly under urban conditions The main characteristics of the examined catalysts were~
- shape: oval - dimensions: length 15.2 cm, max major axis 14.5cm, minor major axis 6.7cm - channels density: 62.2 channels/era2 - wall thickness: 0.16mm - hydraulic diameter: 1. l mm The same fresh and aged T.W.C. were used in all experiments included in this presentation. Qualitative Energy Dispersion (EDS) analysis, performed by a JEOL 840 (LINK AN10S, ZAF 4) scanning electron microscope, show that the external surface of the new and the aged T.W.C. mainly consists of AI (Al2Oa) and Ce (CeO2) as support, Zr (ZrO2 as promote0, Ba (BaO or BaSO4 as stabilizer). Calcium and phosphorous detected in the aged catalyst are contaminants originating from the lubricants and the fuel. Material derived by scraping of the wash coat of the fresh and the aged T.W.C. was ground down to fine powder before XRD (SIEMENS D-500 X-ray generator with a CuK/t X-ray source) and CTEM (JEOL 120CX operating at 120kV) analysis. For the CTEM analysis, ethanol was added in a few finelly ground grains of the catalyst washcoat and the resultant suspension was treated in an ultrasonic bath before deposition of a few drops on a 200mesh copper grid coated with a holey carbon film 2.2. Results and discussion
Preliminary XRD studies were carried out in the range of 300<20<50 ~for the fresh and the aged catalyst samples in the range of 30~ 0<50 ~ in order to include the main Pt [111] and [200] reflections at d=2.265A (I/Io=100) and 1.962A (I/Io=53) respectively. The X R pattern of the fresh catalyst show pronounced reflection peaks representative of T-A12Oa, CeO2, ZrO2 and cordierite (small quantities scraped during the washoat separation procedure). A reflection peak at d=2.2733A was a possible indication of Pt[lll], but was confused with the v-A12031222] reflection peak at d=2 28A The reflection peak of Rh was not expected to have
158 measurable intensity, since Rh is in the form of very small particles [7]. The X R pattern of the aged catalyst show significant changes. New high temperature A1203 phases (8 and 0A1203) are present [4]. The It[ 111] reflection peak is again confused with the y-A12031222] and the 8-A1203 at d=2.28A reflection peaks. The position where the Pt/Rh alloy exists [6] is covered by the pronounced alumina and cerium oxide reflection peaks. The characteristic reflection peaks of the compound CeAIO3 observed by Hubert et al.[6] at temperatures higher than 950 ~ were not derived during this study, since, posssibly under the specific vehicle conditions, temperatures in this range were not achieved. In the wash-coat of fresh catalyst the existing phases were recognized by Selected Area Diffraction (SAD) using 1 ~tm aperture. Two discrete cases can be distinguished as shown on the Fig.l(a) and (b). In the first one, the characteristic polycrystalline material rings are identified, corresponding to the intense reflections of ZrO2 and 3t-A1203, with an accuracy of 1%, respected to the X-rays diffraction data. They are shown with big and small arrows respectively on the Fig. l(a) and (b). The second case presents a mixture of CeO2 and y-AI203. The tings that corresponds to the CeO2 diffraction intensity are shown by big arrows and the small arrows indicates the positions of the y-AI203 tings. The electron di~action patterns from the aged catalyst samples are shown on Fig.2(a) and (b) that correspond to the above mentioned cases. From the spots size on Fig.2(a) it is clear that the size of the ZrO2 particles is not affected considerably by ageing. On the other hand the spot size on the Fig.2(b) directly reveals the change of CeO2 and y-Al203 particles into larger ones. Another interesting result is the presence of very weak extra spots, indicated by double arrows on Fig.2. These spots are invisible on E.D. patterns from the samples of the flesh catalyst and they are rarely present on E.D. patterns from the aged material. The presence of the extra spots on the E.D. patterns of the aged material on the position that exactly corresponds to that of the strongest Pt[111] reflection suggest the origin of these spots to be the sintered Pt particles. Although the vicinity of the Pt [ 111] spot to that of CeO2 and y-Al203 tings does not allow the direct imaging of the Pt particles by using Dark Field (D.F.) mode, due to the high contrast support (A1203). The average particle size was estimated to be less than 10 rim. The use of CTEM enables particle size to be measured and in several cases make possible the distinguish between their nature, using dark field imaging mode. As already mentioned the existence of Pt particles is suggested from ED patterns but the diffraction conditions restrict the direct imaging. The CeO2 and y-Al203 particles were directly presented on D.F. images. On E.D. patterns on Fig.1 and Fig.2 the small circles indicate characteristic positions of the objective aperture for the selective D.F. imaging of the several phases. The corresponding images are presenting in Fig.3 and in Fig.4 for flesh (a) and aged catalyst (b) respectively. On Fig.3 the D.F. mierographs imaging ZrO2 and y-AI203 crystallites are shown. The objective apertures DF11 (on Fig. l(a)) and DF21 (on Fig.2(a)) were used, corresponding to [11 !1 ring of ZrO2. In the same way the mierographs on Figs.3 represent the particles of y-Al203 and Fig.4 the CeO2 erystallites. The size ofZrO2 crystallites was found to be from 4 to 9 nm for the flesh catalyst and from 6 to 12 nm for the aged material. The size for CeO2 erystallites was from 5 to 15 nm and 10 to 45 nm respectively. In the case ofy-Al203 particles from 10 to 17 nm and from 20 to 45 nm were measured, although large crystallites ofy-Al203 with size up to 250 nm were detected. It is obvious that the size of CeO2 and y-A1203 particles is nearly duplicated as the main result of the sintering effect, while the ZrO2 particles remain almost unchanged.
159
FIGURE 2. E.D. patterns from the washcoat of the aged T.W.C.. The small circles indicates the positions of the objective aperture used for D.F. imaging. The small arrows show the diffraction tings due to 7-A1203 and big arrows the position of ZrO2 and CeO2 rings.
160 Although XRD and CTEM permits the study of thermal deactivation due to the crystallic changes of the major washcoat constituents (in our case y-AI203, CeO2 and ZrO2), poor indications were obtained with respect to precious metals thermal changes (mainly sintering and alloying). The main cause was the low content of the precious metals and the interference from the major washcoat constituents. Further investigation is in progress with respect to the selective dissolution of alumina from the washcoat samples by acids, in order to prevent interference during the XRD and CTEM studies.
Figure 3. Selective D.F. imaging of ZrO2 and T'-AI203 particles from the fresh (a) and the aged (b) T.W.C.. The objectives apertures used for the D.F. imaging are indicated in Figs. l&2
161
Figure 4. Selective D.F. imaging of CeO2 particles from the fresh (a) and the aged (b) T.W.C.. The objectives apertures used for the D.F. imaging are indicated in Figs. l&2
3. CONTAMINANTS ACCUMULATION AND DISTRIBUTION 3.1. Experimental Selected samples from the center of the flesh T.W.C. and the inlet, center and outlet of the aged T.W.C. were ground down to fine powder and mixed thoroughly to be homogenized. The homogenized samples were heated at 550~ for 6h to remove any volatile carbonaceous material. Then they were digested under pressure for five hours at 180 ~ in PTFE containers (200rag of material with 3ml HNO3 65%+1ml HCi 37%+0.5ml H1= 40%). All chemicals used were at least of analytical grade. Twenty two elements were quantitavely detected. Zn, Ba, Mn, Fe, V, Ti, St, Ca, Na, K, P and S were measured by ICP-AES (Perkin Elmer 400), with the cross flow nebulizer and scandium as internal standard. Cr, Co, Ni, Cu, Zr, Nb, Rh, Hf, Pt and Pb were measured by ICP-MS (Perkin Elmer ELAN 6000), with cross flow nebulizer and indium as internal standard. The presented results are the mean of four separate measurements for samples of the same origin. 3.2. Results and dissussion Some of the measured elements (Ti, Zr and Ba) are mainly components of the T.W.C. acting as catalytic activity promoters or washcoat stabilizers and their content does not change considerably between the fresh and the aged T.W.C.. The same happens with another group of elements (Co, Nb, Hf, V, St, K and Na), which seems to be rather impurities of the raw materials used in the construction of the T.W.C. than contaminants. Sulphur was detected in both aged and fresh catalyst almost in stoichiometry with BaSO4, which is the possible form of Ba used as washcoat stabilizer, and it was difficult to separate the quantity of sulphur accumulated as contaminant. The rest of the elements (Cr, Ni, Cu, Zn, Mn, Fe, Pb~ P and Ca)
162
Figure 5. Comparison between the longidinual contaminants distribution and the content of the respective elements in the fresh T.W.C. ((a) in g/kg and (b) in mg/kg). as well as the active metals show a significant difference between the contents in the samples of the fresh and the aged catalyst. The longidinual distribution of the content of the contaminants is shown in Fig.5 and for the precious metals in Table 1. For all contaminants the near exhaust gas inlet content is considerably higher than the respective contents of the fresh and the other parts of the aged T.W.C.. The same results were obtained by SEM-EDS analysis [ 18]. The content of the active metals seems to be considerably lower in the aged than in the fresh T.W.C., especially at the gas inlet section. Although a part of these differences may be attributed to weight changes (due to contaminants accumulation and crystaUic changes of the washcoat), losses of active metals for mechanical reasons is possible. Table 1. Precious metals content in the fresh and longidinual distribution in the aged one (mg/kg). Metal
Fresh
Inlet
Center
Outlet
Pt
4100
3400
3600
3600
Rh
794
680
700
710
4. REGENERATION PROCEDURE
Cylindrical specimen (length 2.2cm, diameter 1.6cm) derived from the inlet of the aged catalyt, where the main quantity of contaminants was detected, were used for the leaching
163 tests. The leaching tests were accomplished in a glass column, while the leaching solution was continually pumped upstream through the specimen channels and recycled. After leaching the specimen were thoroughly washed by deinonized water to remove remaining traces of acetic acid. Before and after each leaching test, part of the specimen was treated with aqua regia by heating at atmospheric pressure in order to dissolve the major part of the washcoat and the contaminants accumulated on the washcoat surface (the extremely refractory cordierite substrate is not affected by these dissolution conditions). The products of this dissolution procedure were analyzed for selected contaminants (P, Zn, Fe and Pb) to determine the main contaminants removal yield of the process. Zn, Fe and Pb were analyzed by AAS (Perkin Elmer 2380) and P by UV spectroscopy (Hitachi U-2000), applying the ammonium molybdate method. As series of experiments at various acetic acid concentrations, temperatures and leaching solution rates was accomplished in order to determine the optimum leaching conditions. The as above determined optimum conditions were: 5% w/w solution of acetic acid, feed rate 18ml see"~ (space velocity based on the open front area of the T.W.C. specimen 21600h ~) and temperature 40~ The contaminants removal yields achieved at the optimum conditions were: 78.6% for P, 85.1% for Zn, 64.5% for Fe and 69.5% for Pb. A number of specimen leached at the optimum leaching conditions, were used in the catalytic activity tests. The tests were accomplished in a laboratory scale apparatus. A gas mixture, which simulates the stoichiometric composition of common car emissions (0.8%CO, 0.1%C2H4, 0.2%NO and 0.6%02 with He as inert gas) was fed to the catalytic reactor. The total flow rate was kept at 1000 ml min~. The analysis of the reactants and products was carried out by means of gas chromatography (Shimadzu GC-14B with two parallel columns, Poropak Q and Molecular Sieve 13X) combined with vacuum chemiluminescence spectrometry for NOx analysis (Teledyne Anal. Instrument-NOx stack gas emissions analyzer, model 911/912). The conversion yield of pollutants and nitrogen production yield measured at Table 2. Light-off temperatures for CO and C2H4, NO reduction and N2 production (~ Leached means that these specimen have been washed with the acetic solution at the optimum conditions. Reaction
Fresh Leached
Fresh
Aged Leached (1)
Aged Leached (2)
Aged
CO
198
200
229
232
258
C2H4
239
233
237
252
265
NO
234
226
232
243
257
N2
256
264
261
265
279
Reprinted from: T.N. Angelidis and V.G. Papadakis, Applied Catalysis B: Environmental, 12 (1997) 195-206 [21 ].
164 various temperatures were used for the light-off temperature determination. The results obtained are summarized in Table 2, where the light-off temperatures for flesh, acetic acid leached and aged T.W.C. specimen are compared. The results show a significant improvement of the catalytic activity for the studied ssubstances. After verification by long term activity tests and real size experiments, may be applicable in practice. The procedure is simple and does not require the removal of the T.W.C. from the exhaust system. The acetic acid solution could be pumped through the exhaust system using the dissmantled gas inlet and outlet. The highly corrosive resistant construction of the exhaust system (chromium steel) is not expected to be attacked by the weak acetic acid. REFERENCES
1. Catalyst Deactivation, E.E. Peterson and A.T. Bell, eds., Marcell-Dekker, Inc., pub., 1987. 2. L.A. Carol, N.E. Newman and G.S. Mann, SAE 892040. 3. Characterization of Heterogeneous Catalysts, F. Delannay, ed., Marcell-Dekker, Inc., pub., 1984. 4. F. Maire, M. Capelle, G. Meunier, J.F. Beziau, D. Bazin, H. Dexpert, F. Garin, J.L. Schmitt and G. Maire, Catalysis and Automotive Pollution Control III, 96 (1995) 749. 5. M. Skoglundh, L.O. Lowendahl, P.G. Menon, B. Stenbom, J.P. Jacobs, O.van Kessel and H.H. Brongersma, Catalysis Letters, 13 (1992) 27. 6. S. Humbert, A. Colin, L. Monceaux, F. Oudet and P. Courtine, Catalysis and Automotive Pollution Control III, 96 (1995) 829. 7. J.M. Bart, M. Prigent and A. Pentenero, Catalysis and Automotive Pollution Control III, 96, (1995) 813. 8. P.J.F. Harris, Journal of Catalysis, 97 (1986) 527-542.1. Catalysis Under Transient Conditions, A.T. Bell and L.L. Hegedus eds., ACS Symposium Series 178, 1982. 9. R.K. Usmen, R.W. McCabe and M. Shefer, Catalysis and Automotive Pollution Control III 96 (1995) 789. 10. Catalyst Poisoning, L.L. Hegedus, R.W. McCabe, Marcell Dekker, Inc., pub., 1984. 11. Shelef, M.; Otto K. and Otto N.C., Advances in Catalysis, Academic Press Inc., 27 (1978) 311. 12. McArthur, D. P.,, Adv. Chem. Ser., 143 (1975) 85. 13. WilliamsonW.B., Perry J., Goss R.L., Gandhi H.S. and Beason R.E., SAE841406. 14. Joy G.C., Molinaro F.S. and Homeier E.H., SAE85099. 15. Brett P.S., Neville A.L., Preston W.H. and Wdliamson J., SAE890490. 16. Furey R.L. and Summers J.C., SAE780004. 17. WilliamsonW.B., Gandhi H.S. and Weaver E.E., SAE821193. 18. T.N. Angelidis and S.A. Sklavounos, Applied Catalysis A: General, 133 (1995), 121. 19. J.O. Hernandez, paper presented at the Symposium on Recovery of Spent Catalyst, Div.Pet.Chem., American Chemical Society, Kansas City, USA, 1982. 20. T.N. Angelidis, V.G. Papadakis and E.Pavlidou, Applied Catalysis B: Environmental, 4 (1994) 301. 21. T.N. Angelidis, V.G. Papadakis, Applied Catalysis B: Environmental, 12 (1997) 195
CATALYSIS AND AUTOMOTIVEPOLLUTIONCONTROLIV Studies in Surface Science and Catalysis, Vol. 116 N. Kruse, A. Frennet and J.-M Bastin (Eds.) 91998 Elsevier Science B.V. All rights reserved.
165
Pb Poisoning on Pd-only T W C Catalysts S. Sung, R. M. Smaling, N..L. Brungard Engelhard Corporation 101 Wood Ave., Iselin, N.J. USA, 08830
Abstract: The Pd-only catalyst has made a strong comeback in both TLEV and LEV applications. However, it has been found that Pd-only catalysts are susceptible to Pb poisoning, despite the high Pd loading thereon. Several commercially available Pd-only catalysts were evaluated in this study. After aging with leaded fuel, lead deposition was discovered on these catalysts. The lead appears to be front-loaded and concentrated on the surface of washcoat coatings. It can also penetrate into the washcoat. The impact on catalyst performance is mainly on NOx conversion reduction. TEM elemental mapping and XPS results suggest that Pb preferentially associates with Pd. The hypothesis is proposed that Pb deposition onto Pd occurs because the Pd is competing with Pb for sulfur. Rh-containing catalysts may prove to be strong candidates in compensating for the compromised NOx performance observed in Pd-only catalysts aged with leaded fuel.
1. INTRODUCTION Advances made in TWC catalyst development have made the use of the Pd-only catalysts[ 1] possible in TLEV and LEV applications, replacing the traditional Pt/Rh catalysts. While Pd-only TWC catalysts had encountered some difficulties with regard to their use in NOx conversion in the past, recent developments have suggested that the NOx emission reduction engendered by the Pd-only catalyst is as noteworthy as that of the Pt/Rh catalyst, while HC and CO conversions are generally better than those obtained using Pt/Rh catalysts [2], despite that the Pd-only catalysts were aged with sulfur-containing fuel [3]. The superior performance of the Pd-only catalyst can, however, be undermined by Pb-poisoning. This paper will cover the Pb poisoning effect on different Pd-only TWC catalysts, Pb association with Pd particles, and the alternative Pb-resistant TWC catalysts.
2. EXPERIMENTAL
2.1 Catalyst Preparation iwo kinds of Vd-only I'WC catalysts were made. One catalyst contains oxygen storage compound (OSC); the other does not. Precious metal and substrate sizes are as follows:
166 Table 1. The Pd-only catalysts used in this study Catalyst PM, g/if3 Washcoat OSC Layers
Substrate
A
200
Single
None
3.66"Dx2.5"
B
100
Double
Yes
3.66"Dx4.5"
The Pd/Rh catalyst, also used in this study, has a PM loading of 80 g/if3 and Pd/Rh ratio ofll/1. Smaller core samples, representing catalysts A and B, were also prepared. Core sample size was 1.5"Dx3"L. All substrates were Coming standard: 400 cpsi and 6.5 mil wall thickness.
2.2 Catalyst Aging A converter system consisting of catalysts A+B was aged with a Pb-containing fuel at Pb concentration of 4mg/gallon ibr 100 hours under oxidation and reduction cycles at an inlet temperature of 820~ The aging cycle used was a four mode schedule referred to as RAT-A 820. The RAT A schedule is described by Sims and Johri [4]. Another converter, with the same catalyst configuration, was aged with prime unleaded fuel under the same protocol. Core samples were aged in the same way.
2.3 Catalyst Evaluation Two vehicles were used fbr catalyst evaluation. These were a MY 1996 truck with a 5.7L V-8 engine and modified exhaust system, and a MY 1995 passenger car with a 2.2L V4 engine. FTP-75 was the standard procedure. For the core samples, evaluation was conducted using engine dyno under the fbllowing conditions: Air Fuel Ratio: +_0.2,_+0.1 and Stoichiometric Frequency: 1 Hz Temperature: 500 ~C Space Velocity: 100 K Fuel: Prime Unleaded and 4mgPb/Gallon Fuel
167
2.4 Analytical Several analytical tools were used to arrive at an understanding of Pb impact on the Pd catalysts: XPS. XPS (X-ray photoelectron spectroscopy) was performed on an SSI Model E206/SSX100 ESCA system using monochromatized A1 K-alpha (1486.6eV) excitation, a spot size of 600 microns and a pass energy of 50eV. Fisons M-Probe software was used fbr data collection and processing. Relative peak areas and Scofield sensitivity factors [5] were used to quantify data. Binding energies, unless otherwise indicated, were referenced to C ls = 284.6eV. Sample flakes were taken from various distances from the inlet face such that the washcoat remained intact and analyzed face on. XRF. XRF (X-ray fluorescence) analysis was performed with a Philips PW1404 sequential spectrometer fitted with a scandium target x-ray tube and using UniQuant software (semi-quantitative, ppm to 100 weight percent sensitivity). A portion of the intact catalyst brick was removed from the inlet to the outlet and ground to a powder. The powder was then sandwiched between polypropylene films for analysis. EMP. EMP (electron microprobe) analysis was performed with a Cameca sx-50 microprobe equipped with WDS detectors. K-alpha fines were used to analyze A1, Si, P, S and Zn, and L-alpha lines were used for Ce, Pb and Pd. Data was collected using a 20kV accelerating voltage, 60nA beam current, 1 micron spot size and 3 micron step increments across the cross-sectioned, polished fillettes. Elements were calibrated against their respective standards, supplied by Micro-Analysis Consultants Limited. XRD. XRD (X-ray di~action) analysis was per~brmed on a Philips APD 3720 diffraction system with Cu K-alpha 1,2 radiation and a diffraction beam monochromator. The washcoat was scraped fi-om the honeycomb, ground and mounted on a glass slide tbr analysis between 3.0 and 80.0 ~ 2-theta using a voltage of45kV, a current of 40mA and a 0.02 ~ 2-theta step size. TEM elemental mapping was performed at Lehigh University on a VG603 300 Kv dedicated STEM equipped with a Link exL EDS detector. Elemental maps with 128x128 resolution were Collected with a l nm spot size and 100 ms dwell time. Pd L-alpha and Pb L-alpha spectral windows, along with a Pb background window (adjacent at slightly higher energy) were collected and processed using exL software. Pb elemental maps are defined as the difference (after subtraction) between Pb L-alpha and background maps. No alterations were made to Pd L-alpha maps. A Pb L-alpha window was used because of the S K-alpha line interference.
3. RESULTS AND DISCUSSION: Pb poisoning on noble metal automotive catalysts has been noted for at least 15 years [3, 6-11 ]. The poisoning effects can be summarized into two phenomena: Pb deposition on the precious metals, as claimed by Williams and Baron [8], and Pb diffusion into oxidized Pd to form
168 a solid solution of PbPda, as claimed by Gandhi, et al. [ 10]. The majority of Pb poisoning studies have been done on oxidation catalysts. Work on more advanced commercial three-way conversion (TWC) catalysts appeared only recently [ 12]. Although definitive analytical results on the nature of the Pb poisoning t'unction are not included in the Brisley et al.[12] study, the activity results gathered from different catalyst formulations confirm that Pd is more susceptible to Pb poisoning, especially with regard to NOx reduction, than other platinum metals. This research began with the evaluation of two commercially available Pd-only catalysts in an attempt to ascertain whether a combined catalyst system could meet MDV2 LEV regulations, regardless of whether leaded (4 mg/gallon) or unleaded fuels were used during aging. The catalyst system consisted of a front brick Pd-only catalyst A @ 200 g/ft3 Pd with a volume of 26.3 in3, and a rear brick catalyst B with a Pd loading of 110 g/ft3 and a volume of 47.3 in3. The MY1996 truck was used for FTP evaluation. This vehicle is equipped with two exhaust gas lines. Each line consists of a catalytic converter of 73.6 in3, located 14" away from the manitbld. FTP results from catalysts aged with unleaded fuel are shown in Figure 1. It is clear that after 100 hours RAT-A aging (with an inlet exhaust gas temperature of 820~ this catalyst A+B system meets LEV standards.
Figure 1. After aging with unleaded fuel, the Pd-only catalyst can meet LEV standards for MDV2 applications.
Figure 2. NOx performance deteriorates the most tbr the Pd-only catalyst, after aging with leaded fuel.
However, FTP results from the same catalysts aged with lead-containing fuel indicate a severe deterioration in NOx performance, as shown in Figure 2. Although the aging with a leaded fuel deactivates the Pd-only catalyst activity in all sections, it is clear that Pb impedes NOx performance the most, confirming Brisley et al. findings. If one examines the second-by-second FTP data, it is clear that catalysts suffer NOx activity loss in both cold and hot light-offs (Bags 1&3), and hot per~brmance (Bag 2), as shown in Figure 3.
169 1O0
75
II! i'l~lii!i li I~l I:, ll!i' !:ili~l
~ s0
llll: II~
o z
It:, ~,!~i !,~' ~! I~,!
it.
25
i i :,
|
!11,
I
..
~ ,.'.~
~,1 ,'!
I l -'--Aged ~th UnleadedFuel 1 -
0
- -Aged 0 0
,
with Leaded Fuel
0 0
0 0
0 0
I 0 0
FTP
0 0
0 r.-
O
O
r-
r162 r
Time, sec
Figure 3. Sample aged with unleaded fuel performs well in NOx
I O0
..................................................
75
--
N
O x
o
z
~
i I '" -
0
' O
O
O
-t'__A0.~~0~0~,i ..i.A0o0OLe 0, F ,, ~ O
FTP
' O
' O
I Q
Time, sec
Figure 4. Sample aged with unleaded fuel has NOx light-off advantages The use of leaded fuel has delayed the NOx light-off by about 30 seconds (Figure 4). This is different from the HC light-off curves where the change of fuel from unleaded to leaded has no impact upon HC light-off, as shown in Figure 5.
170
100 o
75 50
(J -r 25
100
I
1 I
o
80
--Aged
^.
V,
:A
/ . o
w)
f__
with Unleaded Fuel
_
- - "Aged with Lea ed ue
.
.
.
.
o
o
.
.
o
~';
.3 o
o
....
o
~
.-
....
I-HO, Le",
?
I- ~ c , ~ " ' ~ ~ I
l " " mOx,Le~ed I
2o
,__., L w.:o,.v ~,,,o:~ed ._,_. j
0
-0.2
-0. I
S'toich.
, 0. I
0.2
AIF: R~lo
FTP Time, sec
Figure 5. H C Hght-off curves show no difference between the different fuel aged samples
.--'---.
Figure & Pb impact is greatest when the MF is slightly rich
The deterioration observed in HC performance when leaded fuel is used is therefore mainly from Bag 2, as shown above in the engine dyno Sweep results (Figure 6), using the catalyst A core samples. Since the converter used in the FTP consisted of both A and B catalysts, a separate study was conducted to differentiate which catalyst is more resistant to Pb-poisoning. Both A and B catalysts were separately aged under RAT-820~ for 50 hours. To enhance the Pb impact, a 12mgPb/gallon fiael was used for aging; additionally, the converter volume was kept relatively small at 26.3 in3. These two converters were evaluated using a 2.2L L4 vehicle. To further track and differentiate the Pb impact between these two Pd-only catalysts, only Phase 2 of FTP was conducted. Results are shown in Figure 7:
Figure 7. Leaded fuel im poses sign ificant NOx perform ance reduction
It is apparent that both Pd-only TWC catalysts suffered a marked loss in NOx reduction. Nonetheless, with a Pd loading of 200 g/it 3for the A catalyst, twice as much as the B catalyst, the A catalyst shows more deterioration in NOx performance than does the B catalyst. To ascertain su~,.~e,~,,e why the Pd-ordy ,.,t,~,y~L . . . . ' . . . . .is. . .more .. :~' to Pb poisoning, and why there ;s a marked difference in Pb resistance among the Pd-only catalysts, we conducted analytical studies on these samples using XPS, XRD, XRF, EMP and TEM elemental mapping.
171 XPS results, shown in Table 2, indicated that the Pb deposition on the catalytic converter is front loaded. XRF analysis confirmed this finding. Table 2 XPS and XRF results show Pb deposition to be front loaded. ....Catalyst A (Front Brick) Catalyst B (Rear Brick) Inlet Outlet Inlet XPS (Atom Ratio): Pb/Al 0.13 0.01 Trace
x ~ (%): Pb S
0.45 0.44
(Whole Brick) (Whole Brick)
Non-Detectable (Whole Brick) 0.67 (Whole Brick)
Despite that there were two substrates, back-to-back, in the converter, Pb compound was found mainly on the front brick. The front loading phenomena suggests that because the Pb compound is heavy in density, it can therefore settle comparatively rapidly once the linear velocity of the exhaust gas drops from the turbulent flow region at the endcone of the converter to the laminar flow region inside the honeycomb channel. Results from the electron microprobe (EMP) analysis indicated that the Pb has penetrated the en'tire washcoat depth, as shown in Figure 8. 45
-
0.45
,
36
i
11
27
- ~ Pbi
~--Fe
-
"--t
t
---;a.~-~,
"%7r
~- 0 . 3 6
i
,. t , . j q . . . .
"
18 ~
.....
-
.
....
7-
. N~{-7
0.27
0.18
.....
9 ._,.~!~/i_~’~.....il.... --~’~i~-Fe’-: --i’~~’~’-..... -/~-i0~ o
0
0
0
Counts Figure 8. Lead penetrates through the washcoat
0
".y
0
!i 0
172
Figure 9: TEMEM results suggest Pb-Pd association. Left: Pd map. Right: Pb-background map Both XRD and TEM elemental mapping (TEMEM) results indicated an agglomeration of the Pd particles to a size of 200-300 Angstrom. Interestingly, TEMEM results, as shown above in Figure 9, also indicated a likelihood of Pb-Pd association, i.e. Pb appears to preferentially deposit on Pd particles. XPS results fi'om the aged core samples also suggested Pb-Pd association. While Pd is deposited on the A1203 washcoat and therefore would ostensibly be associated with A1, XPS analysis showed that, in the catalyst A, Pb and Pd can be detected in areas with little or no detectable AI, suggesting a preferential association of Pb with Pd, despite that other contaminates (P, S, Zn, etc.) might have covered the AlzO3 washcoat. From this study's findings, it appears that Pb deposition on Pd, rather than the formation of PbPd-,, solid solution [9], is the more likely cause of the NOx perfbrmance deterioration. Since NOx light-off has been compromised in the catalyst A, then the Pb must be deposited in very close proximity to the Pd particle in order to hinder the NOx-NOx dissociation-recombination process. XPS data from the core samples indicated that Pd ~2compound was observed on the inlet of the catalyst A sample while only Pd ~was observed on the inlet of the catalyst B samples, suggesting that a deactivation mechanism for the NOx performance may be related to the Pd ~-'. Other work has shown that Pt & Rh are more S resistant than Pd [3,13], and also more Pb resistance than Pd [6,12-14]. Our hypothesis proposes that Pb will form Pb-S-Pd compounds which deactivate the Pd activity, during aging. Because the NOx reduction is greatly dependent on the Pd dispersion, and is effective only when Pd particles or the available Pd surf'ace active sites are close to each other, any steric hinderance generated on the Pd surface that would block the N-N bond fbrmation will compromise the NOx reduction activity. When the Pb-S-Pd compound is tbrmed, illustrated hypothetically below, the closeness of two NOx molecules required so that the N-N bond can be fbrmed (NO+NO->N2+O2) is no longer [,easible. Consequently, the NOx reduction function of the Pd catalyst is hindered by the physical steric effect. Since neither HC or CO oxidations require tbrmation of a C-C bond, or the proximity of two same molecules, to complete their reactions, the Pb-S-Pd compound intereference is therefbre less predominant.
173 NOx+NOx=N2+O2x
Steric hinderance has less impact on CO and HC oxidations.
Requires two NOx molecules at close proxim ity
Figure 10. Steric hinderance caused by Pb decreases the Pd catalyst's NOx reduction performance
Since Ce is known to enhance Pb resistance[ 15], this can explain why the catalyst B has a higher Pb resistance than the catalyst A, which does not contain OSC compounds. Ultimately, a Pb resistant catalyst should include other precious metals. A Rh-containing catalyst, such as the Pd/Rh catalyst, tbr example, offers high Pb resistance, especially in NOx performance, as shown in Figure 11.
100 i 6
Conversions,
%
[lUUnleadedFuel
~ Leaded Fuel I
...............................................................................................................................................
92
t
88
~,,..~,-v,m@:~l ~.~;,. .........
.~.,.~-.~
84
80
~..~..~.~ ..
THC
CO
Figure 11. Pd/Rh catalyst displays high Pb resistance
174 REFERENCES
1. R. Heek and R. Farrauto, Automotive Engineering, Feb, 1996, p93. 2. T. Sekiba, et. al., Catalysis Today, Vol. 22, 1994, p 113. 3. Giacomazzi, R.A., Homfeld, M.F., "The Effect of Lead, Sulfur, and Phosphorous on the Deterioration of Two Oxidizing Bead-Type Catalysts" SAE Meeting, Detroit, May, 1973, SAE 730595. 4. Sims, G. S. and Johri, S., "Catalyst Performance Study Using Taguchi Methods", SAE 881589, 1988 5. Scofield, J. H., J. Electron Spectros. Relat. Phenom. 9, 1976, pp129-137 6. Gallopoulos, N.E. Summers, J.C., Klimisch, R.L., "Effects of Engine Oil Composition on the Activity of Exhaust Emissions Oxidation Catalysts" SAE Meeting, Detroit, May, 1973, SAE 730598. 7. Lester, G.R., Brennan, J.F., Hoekstra, J., "The Relative Resistance of Noble Metal Catalysts to Thermal Deactivation", Catalysts for the Control of AutomQtive Pollutants, ACS Advances in Chemistry Series 143. 1975, pp24-31. 8. Williams, F.L., Baron, K., "Lead, Sulfur and Phosphorous Interactions with Platinum and Palladium Metal Foils". Journal of Catalysis, 40, 1975, pp 108-116. 9. Monroe, D., "Phosphorous and Lead Poisoning of PeUeted Three-Way Catalysts", SAE, Dearborn, June, 1980, SAE 800859. 10.Gandhi, H.S., et. al., "Affinity of Lead for Noble Metals on Different Supports", Surface and Interface Analysis, 6, No:4,1984, pp149-161. 11 .Williamson, W.B, et. al., "Durability of Palladium Automotive Catalysts: Effects of Trace Lead Levels, Exhaust Composition, and Misfueling", Industrial Engineering Chemistry, Prod. Res. Dev. 23, 1984, pp531-536. 12. Brisley, R.S, et al. "The Use of Palladium in Advanced Catalysts", SAE, Detroit, February, 1995, SAE 950259. 13. Klimisch, R.L. et. al., Catalysts for the Control of Automotive pollutants, Advances in Chemistry Series, No 143, American Chemical Society, 1975 14. Doolp, L., et. al., Catalysts for the Control of Automotive Pollutants, Advances in Chemistry Series, No 143, American Chemical Society, 1975 15. Diwell, A.F., and Butler, G, GB 2,122,912, 1984.
CATALYSIS AND AUTOMOTIVEPOLLUTIONCONTROL IV Studies in Surface Science and Catalysis, Vol. 116 N. Kruse, A. Frennet and J.-M Bastin (Eds.) 91998Elsevier Science B.V. All rights reserved.
175
Effect of ageing on the redox behavior of Ce in three-way catalysts S. Irusta a, A. Boix a, j. Vassallo b, E. Mir6 a and J. Petunchi a. a Instituto de Investigaciones en Catfilisis y Petroquimica- INCAPE (FIQ,UNL-CONICET) Santiago del Estero 2829 - 3000 - Santa Fe. Argentina. Phone: 54-42-536861. Fax: 54-42571162. e-mail: [email protected] b Vega & Camji SAIC. 14 de Julio 344-1427 - Buenos Aires. Phone: 54-01-5513750 ABSTRACT. A strong decrease of the oxidating activity of the aged catalysts in the rich region and a reducibility decrease of Ce20 3 are found. The incorporation of La and Zr does not modify the catalytic behavior of the fresh samples, but strongly affects the oxidating capacity of the aged catalysts in the rich region. We suggest that a modification in the Ce/Pt,Rh interaction is responsible for this effect.
1. INTRODUCTION The Environmental Protection Agency requires that On-Board Diagnostics (OBD-II) be used to monitor the hydrocarbons conversion efficiency of the catalytic converter [1]. The current method is the dual oxygen sensor approach where a comparison of the responses of front and rear sensors is a measure of the oxygen storage capacity of the catalytic converter. It is well known that the amount of oxygen storage in three-way catalysts correlates with the ceria content in the catalysts [2,3], although the alumina and precious metal can provide smaller amounts of oxygen storage capacity [4]. Inherent in this monitoring method is the assumption that the HC conversion activity of the catalyst is correlated with its oxygen storage capacity [5,6]. This is true for ceria-containing catalysts when both the storage capacity and the catalyst activity are very high. However, the correlation is not so clear for aged converters where the storage capacity can be quite low but the catalyst activity is still sufficiently high to pass OBD-II and emission standards [7]. Several authors have studied the relationship between oxygen storage capacity and catalyst activity [8,9]. Nunan et al. [8] in their study of the effect of high temperature lean ageing on the performance of Pt, Rh/CeO 2 and Pt/Rh/CeO 2 catalysts, doped with rare earth and alkaline earth, found that by doping the CeO 2 lattice with rare earths such as La, a severe loss of activity followed ageing. Using TPR and in situ XPS they suggested a correlation between the loss of activity and the decoupling of the low temperature synergistic reduction of the noble metal (Pt/Rh) and surface Ce. In this study the results reported were obtained on powder y-A120 3 supported catalysts. Schmieg and Belton [9] using a commercial automotive catalyst containing Pt, Rh, Ni and Ce concluded that the
176 ageing of the catalyst causes a loss of oxygen storage capacity due to the sintering of the ceria particle which reduces the ceria/noble metal interaction and does not allow Ce to cycle between the oxidation states. These authors only used XPS to measure changes in the oxidation state of the Ce present in the washcoat. Within this frame, the goal of this work is to carry out a systematic study of the volumetric and surface oxide-reduction properties of Ce on three-way catalysts by temperature-programmed reduction (TPR) and XPS. Studies were also made to determine how these properties were affected by thermal ageing. The effect of promoters such as La, Ba and Zr on the same properties was analyzed, as well. Among the samples prepared in our laboratory, we have selected those with better catalytic behavior. The redox capacity was correlated with the catalytic activity obtained with synthetic mixtures.
2. EXPERIMENTAL Three-way catalysts were prepared by the impregnation of a cordierite monolith (Coming) of 62 channels/in 2. A thin layer of ~,-A120 3 Ketjen (dp_< 74 ~t) promoted with Ce, La, Ba and Zr (either from nitrates or oxides, see Table 1) was deposited on them, through the immersion of the monolith into the A1203 suspension and its subsequent drying at 200~ and calcination at 400~ After successive impregnations the active phase was incorporated (Pt, Rh). The precursor salts were H2PtC16 and RhC13. Solids were calcined in air at 450~ Table 1 lists the catalysts used in this study. XRD measurerements were taken using CuK~ radiation at 30kV and 30 mA and scanning rate of 1o min-l, fresh and aged catalysts were studied. Table 1 Catalysts characterization Catalyst Washcoat CeO 2 promoters (%)
La203 (%)
BaO (%)
ZrO 2 (%)
Pt (%)
Rh (%)
Ni (%) -
ME2
nitrates
28.0
8.0
-
10.0
0.20
0.04
IP4
oxides
30.0
-
8.0
-
0.31
0.06
-
IP5
oxides
28.0
3.0
3.0
7.0
0.20
0.06
0.45
The XPS measurements of all samples were carried out at room temperature using an ESCA 750 Shimadzu Electron Spectrometer (Non-monochromatic MgK X-ray radiation); C 1s was used as reference (284.6 eV). Fresh and aged samples were reduced at 550~ for 1 h in a reaction chamber attached to the equipment. TPR experiments of samples pre-oxidized at 550~ were performed in a flow instrument with thermal conductivity detector. 0.1 g of catalyst, and a H 2 5%/Ar current as reducing agent were used. Heating was performed from room temperature up to 550~ at 10 ~ In order to quantify the amount of reduced Ce, temperature-programmed experiments were performed with the following sequence: temperature-programmed reduction from room temperature up to 350~ and kept at this temperature for 18 h . It is considered that Pt, Rh and Ni were completely reduced and also some Ce. The temperature-programmed reduction then continued up to 550~ The reaction was performed using 0.5 g of monolith placed on a fixed-bed reactor flow. The feed was made
] 7'1
up of a mixture of 1000 ppm of NO, 1000 ppm of CO, 500 ppm ofC2H 4and different amounts of 02, in order to obtain the S values shown in Table 2, with the balance at 1 atm He. Solids were evaluated at GHSV = 20.000 h -1. Conversions of CO, NO and C2H4 were measured using a conventional chromatographic system with TCD detector. The solids ageing was performed in an oxygen 3% (in nitrogen) current at 1000~ during 5 h.
3. RESULTS AND DISCUSSION Fig. 1.a and b shows the effect of ageing (5 h under dilute oxygen stream at 1000~ on the light-off temperature (temperature at 50% conversion) of the IP4 catalysts which have not been doped with La or Zr. (Table 1). A higher increase in the oxidation temperature of CO and C2H 4 ( A T = 25 and 37~ than in the NO reduction (A T~ 8~ can be observed. This could suggest that the thermal treatment at high temperatures more markedly affected the oxidating capacity of the catalyst. The incorporation of La and Zr (Table 1) does not modify the light-off temperature of the NO reduction in the fresh catalysts, and at the same time, it increases the one corresponding to the oxidation of CO and C2H 4, respectively lOOI
a
100 -
80
b
~
!~
80 73~
6~
60
40
40
.
co
200 250 300 350 400 450 500 Temperature (~
20 -
2/
-.fl_l
/,
9 co 9
c2.4
200 250 300 350 400 450 500 Temperature (%")
Figure 1 Effect 9 of ageing on the light-off temperature of the IP4 catalyst (a) fresh; and (b) aged. Reaction conditions: NO and CO" 1000 ppm, C2H4:500 ppm; stoichiometric amount of 0 2 and balance to 1 atm. He. GHSV: 20.000 h -1.
1'i8 If compared to the fresh one, the aged IP5 catalyst does not undergo a significant change in the light-off temperature when it is operated in stoichiometric ratio. In order to obtain better information on the effect of the oxygen storage capacity of Ce on the catalytic activity and as the latter could be affected by thermal ageing, experiments with different O2/reactant ratios were performed. The results obtained at 275~ reaction temperature are summarized in Fig. 2 and Table 2. The conversion of CO, C2H4 and NO was plotted against parameter S, obtained by dividing the sum of the equivalent oxidising components of the feed by the sum of the corresponding reducing components. Table 2 Effect of ageing (1000~ 5 h in 02 3%) on the reactants conversion at different oxygen concentrations in the feed stream a Samples Reactants Sb < 1 (0.7) S b =1 S b > 1 (1.7) IP4
IP5
Fresh
Aged
Fresh
Aged
Fresh
Aged
CO
92.8
55.74
100
96
100
97
C2H4
21.86
4.71
79.66
9.42
100
32.4
NO
77.9
76
100
100
68.31
80.6
CO
80
37
100
96.6
100
100
C2H 4
24
0.63
100
43.7
100
88.6
NO
93.42
78.22
100
100
50.87
77.08
Reaction conditions: Temperature: 275~
NO and CO: 1000 ppm, C2H4:500 ppm,
GHSV = 20.000 h-1. S= (2[02] + [NO])/([CO] + 6[C2H4] ). It is interesting to analyze the solids behavior alter ageing in dilute oxygen flow for 5 h at 1000~ 9In both IP4 and IP5 an important decrease of the oxidating activity should be remarked. This is more pronounced for C2H4 than for CO. CO conversion noticeably drops in the rich region of the feed stream but it is kept almost constant in the lean region. 9The reducing activity of NO is not strongly affected by any of the variables under study. The results obtained on fresh catalysts were as expected for this type of system and they are in agreement with what has been reported by other authors [10]. The incorporation of La and Zr to the washcoat does not significantly modify the catalytic behavior of the fresh samples for any of the S values under study.
179
Figure 2" Effect of 0 2 on the feed stream on the CO, C2H 4 and NO conversion of fresh and aged catalyst. Conditions: see Table 2. These results suggest that the oxygen release capacity of CeO 2 has been modified by the ageing treatment, as reported by Yao and co-workers [ 11 ] and Schmieg and Belton [9]. To corroborate these ideas, a systematic study of the redox properties of the catalysts here employed was performed by TPR and XPS.
tO tar) tO
0 E
I
C ......
i
I
200
,
I
300
,
I
400
~
500
I
,
600
I
,
I
,
isothermal
......
Temperature (~ Figure 3 Reducibility 9 of fresh and aged IP4 catalysts: (a) fresh catalyst (b) fresh catalyst reduced at 350~ during 18 h., (c) aged catalyst after the same procedure as (b).
180 Fig. 3.a. shows the TPR profile of the flesh IP4 sample. Two main peaks can be observed in it, with maxima at 350~ and 510~ and a third less defined peak with a maximum of about 200~ These results somehow differ from those published by Nunan et al. [12] who, for Pt(0.75) Rh(0.04) Ce(6 ) / A120 3 (numbers between brackets represent weight percents), reported a sharp low temperature peak centered at 175~ a shoulder at 280~ and a broad peak centered at 720~ This discrepancy could be attributed to the Ce content [ 11 ] and to the preparation method which conditions the particle size and, probably, the degree of contact between the noble metals and the ceria particles.The high temperature peak would correspond to the reduction of a bulk oxygen anion which is bonded to two Ce +4 ions in the bulk ceria. Instead, the low temperature peaks are associated with the reduction of a surface capping oxygen anion which is attached to a surface Ce +4 ion in an octahedral coordination, as suggested by Yao and Yao [11] plus a synergetic reduction of the Pt,Rh active phase.The incorporation of La and Zr does not substantially modify the TPR profile of the solid. A downscale temperature shift was observed from 350 to 310~ and the shoulder at 200~ was not detected. The high temperature peak was not modified. In order to obtain a better peak quantification of the TPR profile, these experiments were conducted in two stages (see Experimental). Figs. 3.a and 3.b clearly show the separation of the peaks between the low temperature (25 to 350~ and the high temperature (350-550~ regions. The hydrogen consumption in both regions is indicating that a 100% reduction of the active phase to Pt ~ and Rh ~ has been achieved as well as the passing of total Ce +4 (CeO2) to Ce +3 (Ce203). (Table3).
v e0
e0 0 e-
2 "1-
J
/
"/X,
J I
200
Temperature (~
I
300
I
200
Temperature (~
i
300
Figure 4: Reducibility of fresh (a, b) and aged (c,d) catalysts; a, c IP4 ; b, d IP5. Running conditions from 25 to 350~ A substantial change in the reducibility of the solids was observed after they were treated in
181 0 2 s t r e a m for 4 h at 1000~ In effect, a resolution of the TPR peaks are the most behavior with respect to their reducibility IP4 which contains no La or Zr, a 50%
substantial decrease in H 2 consumption and a poor remarkable aspects in the modification of the solids (Figs. 3.c and 4.c and d and Table 3). In the case of decrease in H 2 consumption of the first peak was
verified, while the decrease of the second peak was dramatic (Table 3. See first and second line on the ninth column). The incorporation of La and Zr affects even more pointedly the reducibility of CeO 4 (Table 3, third and fourth lines, eighth and ninth columns) which is in agreement with what was reported by Nunan et al. [8]. Comparable results were obtained when Ce(NO3) 4 (and not CeO2) was employed for the incorporation of Ce to the washcoat. (Table 3). Table 3 Effect of ageing on the reducibility of the 3-way catalyst a CAT gmoles in the sample b gmoles H 2 consumed
d
e
% Ce reduced
Tmax
Ce
Pt
Rh
Ni
(25/350) c
(350/550)
(25/350) (350/550)
IP4F
35
0.3
0.04
-
9
9.1
50
49
200
360
550
IP4 E
35
0.3
0.04
-
4.5
0.9
24
5.1
75
150
400
IP5 F
32
0.2
0.04
1.5
8.2
6.2
41
38
310
-
550
IP5 E
32
0.2
0.04
1.5
0.5
0.7
0
3.7
100
250
550
ME2F
32
0.2
0.03
-
n/a
7.3
n/a
45
130
220
520
ME2E
32
0.2
0.03
-
n/a
0.7
n/a
4.6
n/a
-
-
F" fresh, E" aged catalysts Amount of sample used in the TPR experiments, 100 mg. Temperature range (~
in which the H 2 consumption was considered.
Reduced Ce percent, considering that the metals (Pt, Rh, Ni) are completely reduced in the lower temperature range. TPR maxima temperature. The decrease in the reducibility of CeO 2 is consistent with the results obtained by XRD. In fact, the fresh catalyst shows an estimated particle size of 140 A and 300 A for IP4 and IP5, respectively. It was determined from the measured full width half maximum of CeO 2 [13]. After the ageing of the samples, particle sizes of approximately 288 A and 450 A were obtained in such catalysts. The increase in particle size of CeO 2 would cause a decrease of its reducibility as shown by measurements made by Nunan et al. [8,12] and also a decrease in Pt/Ce interaction which could cause a loss of synergistic Pt and surface Ce as suggested by Nunan et al. [ 12]. In order to obtain additional information on the systems under study, XPS experiments were performed in fresh and aged catalysts.
182 Only Ce, A1 and O could be detected. The remaining elements (La, Ba, Zr, Pt and Rh) were probably not observed due to a low surface concentration. Fig. 5 shows the spectra of fresh and aged IP5 catalyst, both oxidated and after being reduced in situ with H 2 for 1 h at 550~ It also shows the spectrum of pure CeO 2 (Ce +4) which was taken as reference for the characterization of the signals. Fig. 5.e corresponds to the spectrum of pure CeO 2 with 3d5/2 and 3d3/2 core level peak at 889.5 eV and 908.2 eV, respectively. The peaks at lower binding energies (BE) 883.5 eV and 901.5 eV are shake-down satellites, and at higher BE, 899.1 eV, it is a shake-up satellite. (The other shake-up satellite at approximately 917 eV is not observed in the figure). The split between the core levels is 19.1 eV, and the split between the core levels and the shakedown satellite (5.6 eV) together with the binding energies described above are in agreement with what was reported by Schmieg and Belton [9]. In the fresh IP5 catalyst (Fig. 5.a) the peaks observed at 909.1 eV and 889.1 eV corresponding to Ce 3d 5/2 and 3/2, respectively, and the shake-down satellites at 884.0 eV and 900.0 eV and shake-up satellite at 909.1 eV coincide with the Ce+4 signal observed for pure CeO 2 .
l
I~
Ce 3d I
e d
c
910
900
890
880
Binding Energy (eV) Figure 5 : XPS spectra oflP5; a) fresh, b) fresh reduced at 550~ for lh, c) aged, d) aged reduced at 550~ for l h, e) CeO2. When the IP5 solid is reduced in the treatment chamber attached to the instrument with H 2 at 550~ a spectrum is obtained which is shown in Fig. 5.b. The core level peaks appear at 903.5 eV and 885.0 eV and they correspond to 3d 3/2 and 5/2, levels of the Ce +3 (Ce203),
183 respectively and the shake-down satellites are observed at 900.0 eV and 883.5 eV. The shakeup satellites are not very well defined, consequently it is not possible to determine the BE. In the oxidated aged catalyst (Fig. 5.c) the signal corresponding to Ce3d 5/2 is masked in a broad peak whereas the other signals are in agreement with the fresh catalyst. The spectrum obtained when reducing the aged catalyst resulted in a very low signal in which it was not possible to identify the main signals. In all IP5 catalysts the A12p signal resulted between 74.5 eV and 74.6 eV corresponding to y-A120 3 whereas the Ols signal resulted between 532.0 eV and 531.8 eV. The Ce/AI surface ratios of the fresh IP5 are 0,033 and 0,032 for the oxidized and for the reduced sample, respectively. In the aged sample the Ce/AI ratio considerably decreased to 0,02. The O/A1 ratio was about 1 for the fresh solids and 0,92 and 1,17 for the aged ones, oxidated and reduced, respectively.
Table 4 Effect of ageing on the surface concentration in 3-way catalysts CATALYST
TREATMENT a
Ce/A1)s
O/A1)s
IP5
fresh-oxidated
0.033
1.10
fresh-reduced
0.032
1.01
aged-oxidated
0.020
0.92
aged-reduced
n.s.d.
1.17
fresh-oxidated
0.12
1.60
IP4
Treatments in the sample: (a) oxidation at 500~ 8 h with 0 2 flOW in a reactor. (b) reduction at 550~ 1 h with H 2 in the reaction chamber attached to the spectrometer. Surface Ce/A1 and O/A1 calculated from XPS data. The XPS spectrum of the fresh, oxidated IP4 was similar to Ce +4 and the Ce/A1 surface ratio was notably higher, 0,12 (Table 4). However, in the fresh catalyst (reduced), and in the aged catalyst (oxidated and reduced), it is not possible to observe the Ce signal. The same happens with the ME2 catalyst. The A12p and O ls signals of the IP4 and ME2 catalysts were not modified with respect to those observed in IP5. From the above it can be seen that in the fresh IP5 catalyst (doped with La and Zr) the surface reduction follows the same behavior as in the bulk. When the solid was aged at 1000~ the Ce signals of the reduced solid were practically not detected. From the results obtained in this work it can be concluded that: 9The incorporation of La and Zr does not modify the catalytic behavior of the fresh samples but notably affects the oxidating capacity of the aged catalysts in the rich region (Table 2). 9The incorporation of lanthanum and zirconium causes a strong decrease of the reducibility of the surface capping oxygen anions attached to surface Ce +4 in the
184
aged catalyst (Table 3). 9In the catalysts without La and Zr, the ageing process originates a surface modification of the solid leading to the total masking of CeO 2. The increase in size of the CeO 2 crystals in the aged catalysts and the changes observed at surface level would allow us to suggest that a modification in the Ce/PtRh interaction is responsible for the oxygen storage capacity loss of CeO 2 and, consequently, of the oxidating activity of the catalysts in the rich region, in agreement with Schmieg and Belton [9]. 4. A C K N O W L E D G M E N T S This work has been performed within the framework of the agreement "Vega & Camji S.A.I.C. - UNL - CONICET". Thanks are given to Elsa I. Grimaldi for the English edition. REFERENCES
.
3. 4. 5. 6. 7. 8.
9. 10. 11. 12. 13.
Nunan, R. Silver and S. Bradley, "Catalytic Control of Air Pollution", ed. R.G. Silver, J.E. Sawyer, J.C. Summers, ACS Symposium Series 495, 83 (1992). P. L66f, B. Kasemo and K.E. Keck, J. Catal., 118 (1989) 339. E.C. Su, C.N. Montreuil and W.G. Rothschild, Appl. Catal., 15 (1985) 75. J.R. Theis, W.J. La Barge and G.B. Fischer, SAE Paper N ~ 932666 (1993). J.W. Koupal, M.A. Sabourin and B. Clemmens, SAE Paper N ~ 910561 (1991). W.B. Clemmens, M.A. Sabourin and T. Rao, SAE Paper N ~ 90062 (1990). J.J. Hepburn and H.S. Gandhi, SAE Paper N ~ 9200831 (1992). J.G. Nunan, M.J. Cohn and J.T. Donner, Catal. Today, 14 (1992) 277. J.J. Schmieg and D.N. Belton, Appl. Catal. B 6 (1995) 127. B. Engler, E. Koberstein and P. Schubert, Appl. Catal., 48 (1989) 71. H.C. Yao and Y.F. Yao, J. Catal., 86 (1984) 254. J. Nunan, H.J. Robota, M.J. Cohn and S.A. Bradley, J. Catal., 133 (1992) 309. B.D. Cullity in B.D. Cullity (editor) Elements of X-Ray Diffraction, Addison Wesley1978, p. 102.
CATALYSIS AND AUTOMOTIVEPOLLUTIONCONTROLIV Studies in Surface Science and Catalysis, Vol. 116 N. Kruse, A. Frennet and J.-M Bastin (Eds.) 91998 Elsevier Science B.V. All rights reserved.
185
The CeO2-ZrO2 System: Redox Properties and Structural Relationships. G.Vlaic a, R. Di Monte, P.Fornasiero, E.Fonda, J.Ka~par, and M.Graziani Dipartimento di Scienze Chimiche, Universit/t di Trieste, Via Giorgieri 1, 34127 Trieste (Italy); a also Sincrotrone Trieste, Padriciano 99, 34100 Trieste, (Italy). CeO2-ZrO 2 mixed oxides show improved oxygen storage property compared to CeO 2 due to the involvement of the bulk of the oxide in low temperature redox processes. In the present work we investigated the M-O local structure by means of EXAFS and Raman spectroscopy. Evidence is found that significant distortions of the oxygen sublattice are induced by introduction of Zr 4+ into the CeO 2 lattice. It is suggested that these distortions generate mobile oxygens which are responsible for the improved redox properties at moderate temperatures. 1.
INTRODUCTION CeO2-ZrO 2 mixed oxides are important innovative materials for the automotive exhaust
catalysis. Addition of ZrO 2 to CeO 2 significantly increases the thermal stability with respect to CeO 2 and it improves its ability to store and release oxygen (OSC) in the reaction conditions. Development of thermally stable efficient oxygen storage systems is an important aspect of the cold-start technology. In the last few years, we have investigated redox properties and catalytic activity in the NO-CO reaction of noble metal/CeO2-ZrO 2 mixed oxides, which is responsible for the NO removal from the exhausts. In particular, we found
(1-5) that addition of ZrO 2 to CeO 2
significantly improves the catalytic performances compared to the CeO 2 systems. High OSC was observed at low temperatures due to the fact that reduction in the bulk of the mixed oxides occurred at 500-800 K (1,2). We have attributed such an enhancement of the reducibility to an increased oxygen mobility in the bulk induced by the addition of ZrO 2 into the CeO 2 lattice. Consistently, we observed a strong dependence of the redox behaviour on the nature of the CeO2-ZrO 2 phase and ZrO 2 content (2). The occurrence of an easy reduction in the bulk gives unusual redox properties to these systems: at variance with the traditional systems, a sintering of Ceo.5Zr0.502 in reducing conditions at 1273 K improves the redox behaviour compared to the initial high surface area sample (1,3). While the superior performances of these systems are established, there is no direct experimental evidence how the structural modification of the CeO 2 induced by the presence of ZrO 2 may be correlated to the above behaviour. In the present work, EXAFS, XRD and Raman techniques were employed to correlate the structural properties of the Rh/CexZrl_xO 2 catalysts with the redox ones. Different
186 compositions were analyzed in order to rationalize the previously observed variation of the reduction temperatures with ZrO 2 content. The aim is to provide sound scientific basis for the design of thermally stable, highly efficient CeO2-ZrO 2 based OSC systems. The main point addressed is the modification of the oxygen distribution around the metal center in order to understand how it may influence the oxygen mobility. Secondly, we wished to ascertain the homogeneity at atomic level of CeO2-ZrO 2 solid solution which is still matter of debate.
2. EXPERIMENTAL 2.1. Materials, EXAFS, XRD and Raman Spectra The catalysts were from the previous studies (1,2). The low surface area (LSA, surface area 1 m 2 g-l) samples were prepared by a solid state synthesis, firing mixtures of ZrO 2 and CeO 2 of appropriate composition at 1673 K. The cubic Ceo.sZr0.50 2 (LSA) was prepared by firing the mixture at 1873 K for 1 h and then quenching it to room temperature (r.t.). The high surface area (HSA, surface area ~ 60-90 m 2 g-l) sample was prepared by a homogeneous gel route from Ce(acac)4 and Zr(O-Bu)4 precursors (Aldrich) (1). The supports were Rh-loaded (0.5% wt) by the incipient wetness method using a solution of RhCI 3 . nH20. Powder X-ray diffraction patterns were collected on a Siemens Kristalloflex Mod.F Instrument (Ni-filtered CuKo0. Cell parameters were determined by using the TREOR90 program which employs the Visser algorithm. FT-Raman spectra were performed on a Perkin Elmer 2000 FT-Raman. The laser power was 50-500 roW. For the EXAFS measurements at the Zr K edge, finely grounded powders were pressed in thin self-supporting pellets. For those at the Ce LIII edge, the powder was deposited from an acetone suspension on a graphite holder. The amount of sample was calculated to obtain a total absorption coefficient (tax) after the jump of about 2.5. The measurements were performed in the transmission mode at r.t. on the EXAFS-I line at the DCI storage ring (1.85 GeV, 300 mA) at LURE (Orsay, France), using a channel-cut Si (331) monochromator and two ionization chambers, the first filled with air and the second with argon or air for Zr and Ce respectively. At least three scans were recorded for each sample in the energy interval 1790018900 eV for Zr K and 5670-6160 for Ce LIII edge respectively, using steps of 2 eV. Each point was measured for 2. 2.2. EXAFS Data analysis The procedure is detailed elsewhere (6). The EXAFS spectra were analyzed according to standard procedures (7) using a program set written by A. Michalowicz.(8) The final signals were obtained by averaging the individual EXAFS data. The standard deviations were calculated point by point as measure of the experimental errors in the final functions. The Fourier filtered data in the k-space. The EXAFS signals multiplied by k 3 were then Fourier transformed in the limits 3.05-12.70 A -1 for Zr and 2.40-9.85 A -1 for Ce. The peaks present in the R-space were then back transformed to k-space. To model the first peak, we measured the EXAFS signals and deduced phases and amplitudes from BaZrO 3 and CeO 2 for Zr-O and CeO respectively. For more distant shells we used the McKale's functions (9), after their testing on reference samples of known structure (m-ZrO 2, Zr, CeO2, BaZrO3) to evaluate the photoelectron mean free path.
187 In all the fits, the number of free parameters Npar was kept smaller than the number of independent points Nind, defined as Nind=2ARAk/rt (AR is the width of the R space filter windows, Ak is the actual interval of the fit in the k space). Goodness of the fit was discriminated by using the reduced chi-squared term e,2v, as defined in refs. (10,11), where v is the degree of freedom (v - Nind - Npar). When the model is
properly evaluated, e2 v should approach unity. We used the F-test (12) to discriminate the models. 3. RESULTS AND DISCUSSION 3.1.
Low surface area Rh/CexZrl_xO 2 (LSA) catalysts
3.1.1. XRD and Raman studies For the sake of clarity, before discussing the results, we will recall some features of the CeO2-ZrO 2 phase diagram. Despite considerable efforts, the true nature of this diagram is still matter of debate due to the presence of a number of stable and meta-stable phases. Recently, Yashima et al, distinguished five different phases: monoclinic (m), tetragonal (t), (t '), (t") and cubic (c) on the basis of Raman and XRD studies (13,14). Of these, the t-form is a stable one formed through a diffusional phase decomposition, the t'form is obtained through a diffusionless transition and it is metastable, while the t"form is intermediate between t' and c. It shows no tetragonality and it exhibits an oxygen displacement from ideal fluorite sites system. Increasing the CeO 2 content in the solid solution progressively stabilizes the ZrO 2 leading to structures with higher symmetry.
t~ C c
,.,....
O N t~
E O
z
i
20
40
............~
,
....... J ...... )J ......... L
60
;
80
,
1 100
20 Figure 1. Powder XRD patterns of(l) c-Ceo.sZro.202, (2) t"-Ceo.6Zro.402, (3) t" Ceo.sZro.502, (4) t '-Ceo.sZro.502, (5) t-Ceo.2Zro.802.
188
5
100
300
500
700
900
Raman Shift (cm -1) Figure 2. Raman spectra of (1)
m~/Ceo.sZro.202, (2)
t"-Rh/Ceo.6Zro.402, (3) t"-Rh/Ceo.5Zro.502, (4) t' Rh/Ce0.5Zro.502, (5) t-Rh/Ceo.2Zro.802 .
Five samples Rh/CexZrl.xO 2 with x content varying between 0.2 and 0.8 were analyzed. The XRD patterns reported in Figure 1 allow the following phase attribution: 0.2 (t), 0.5 (t'), 0.5 - 0.8 (t" or c). Note that according to the synthesis conditions a tetragonal or a "cubic" phase can be prepared for x- 0.5. As seen in Figure 1, the distinction between the tetragonal and cubic phases is easily detected by the splitting of the peaks at about 33 ~, 48 ~, 57 ~ and 70 ~ (20). Due to the low scattering factor for the oxygen, the t" and c phases are distinguished only on the basis of their Raman spectra (13). For this reason both the t" and c form are often referred to as cubic phase. 6 Raman-active modes of A lg + 3 Eg + 2 B lg symmetry are observed for tetragonal P42/nmc),
ZrO 2 (space group while for the cubic
fluorite structure (space group Fm3m) only one F2g mode centered at around 490 cm q is Raman-active. In the pure CeO 2 which has also the fluorite structure, the F2g mode is observed at 465 cm -1. The Raman spectra of Rh/Ceo.8Zro.202 (Figure 2) shows a single band which is consistent with attribution to a c phase. Consistently with the attribution to a t phase, Rh/Ceo.2Zro.80 2 features 6 bands. Differently, the Ceo.5Zro.502 (cubic and t') and Ce0.6Zr0.402feature four bands which may suggest a different type of distortion of the oxygen sublattice.
3.1.2. EXAFS studies The samples were analyzed by EXAFS techniques. For every sample, the reliability of a large number of different models was examined. The procedure is briefly reported for the t"Rh/Ce0.sZro.502. For a complete discussion of this sample we refer to (6). Fig.3.1 reports the modulus of the Fourier transform (FT) of the EXAFS signal of the Rh/Ce0.sZr0.502 measured at the Zr K edge. The region 1.11-2.34 )t, is associated with the Zr-O bonding while the region 2.35-4.12 A is mainly associated with the Zr-M (M = Zr, Ce) interactions. CeO 2 has the fluorite structure with 8 oxygens as nearest neighbors (NN) located in the tetrahedral sites at a distance of 2.34 A. ZrO 2 is monoclinic (m) at r.t., however, the tetragonal (t) and cubic (c) phases can be stabilized at r.t. upon addition of host dopants. Three different local structures were reported for the M-O shell in the c-ZrO2: a single shell of 8 Zr-O bonds and two models each with two subshells with 4 + 4 or 6 + 2 oxygens respectively at a short
9
189
(Zr-O = 2.07-2.14) and long (Zr-O - 2.34-2.42) distance from Zr (15). The results obtained on t"-Rh/Ce0.sZr0.50 2 are reported in Table 1 for a few significant models. A perusal of these models and the application of the F-test reveal that the two subsheUs model (C.N. = 4 + 2) gives the best fit of the experimental data with reliability of at least 75%. The goodness of the fit is illustrated in Fig.3.2. In addition to the statistical consideration, the other models show some physically unreliable values such as the Debye-Waller factor. The observed C.N. = 4 + 2 means that the two "missed" oxygens are at a distance Zr-O _> 2.60 A. Consistently with our observation, examination of a cubic ZrO 2 stabilized with y3+ suggested the oxygen vacancies are located at the Zr 4+ decreasing the C.N. to 7 (16). To the contrast, the Ce-O NN local structure is best modeled by a single shell (C.N. - 8) contribution to the EXAFS signal at the Ce LIII edge, suggesting that the Ce maintains the coordination of CeO 2.
~It.. 4 "6
1
o.1-
!
2
"
0
4'
~
~
-0.2 0-'i"
'
0
i
'
2 R (A)
i
'
I
I
'
I
12
8
k (A-l) 3
F-It. 4 -
'
4
4
0.2
IJ..
~0.0
14.=-
O
-0.2 '
0
|
I
2
'
|
R (A)
I
4
i
|
|
4
k (A-l)
| ....
I
6
|
u
8
Figure 3. Zr K edge : (1) Modulus of the Fourier transform (FT) of the experimental data and the modeled FTs relative to the first and second shell (R space); (2) back FT of the first peak and the corresponding fit, Ce LIII edge: (3) Modulus of the Fourier transform (FT) of the experimental data and the modeled FT relative to the first shell (R space); (4) back FT of the first peak and the corresponding fit; (m) experimental, (-) fit.
190
Table 1. Local coordination in Ceo.5Zro.50 2 as determined from EXAFS measured at Zr K and Ce LII I edges. CN a
R (/~)
o (~)
AE (eV) b
p
~;v2
8 4 4
2.113 + 0.007 2.130 + 0.006 2.339 4- 0.007
0.142 + 0.010 0.073 4- 0.004 c
-3.4 + 0.6 5.2 4- 0.6
4 3
21.2 36.4
6 2
2.130 • 0.007 2.360 4- 0.016
0.110 4- 0.001 c
-1.0 • 0.7
3
7.6
4 2
2.115 • 0.008 2.324 • 0.011
0.078 4- 0.003 c
0.0 • 0.5
3
1.9
4 2 2
2.112 • 0.008 2.318• 2.60 4- 0.20
0.079 + 0.003 c
-0.5 • 1.0
1
3.6
Zr-Zr Zr-Ce Zr-O
6 6 24
3.64 • 0.04 3.72 4- 0.01 4.24 4- 0.03
0.105 • 0.006 0.095 • 0.007 0.140 4- 0.010
-10.7 + 4.7 -6.8 4- 1.5 -8.3 • 2.0
1
0.9
I shell
Ce-O
8 7.6 40.4
2.312 + 0.007 2.308 4- 0.008
0.092 + 0.005 0.085 4- 0.009
-0.3 + 0.3 -0.5 4- 0.4
3 2
0.6 0.4
II shell
Ce-Zr Ce-Ce Zr-O
6 6 24
3.75 + 0.03 3.72 4- 0.01 4.50 d
0.106 + 0.08 0.095 + 0.007 0.086 + 0.025
15 + 4 -6.8 4- 1.5 14a
1
0.9
Zr K edge I shell
II shell
Zr-O
0.214 4- 0.068
Ce LII I edge
a After preliminary testing of the proper C.N., the values were kept costant in the final fittings; 6 C.N.: coordination number is allowed to vary in the fitting; c the same values of cy were employed for the two subshells; d the value was kept costant in the fitting. The data concerning the Zr-O and Ce-O shells are summarized in Table 2. For all the samples a C.N. = 8 is found for the Ce-O shell suggesting that, independently of the ZrO 2 content, CeO 2 always maintains intact its coordination sphere. The contraction of the cell parameter (6) upon insertion of the smaller Zr 4+ cation (ionic radii - Zr 4+, 0.84 A; Ce 4+ 0.97 A) accounts for the decrease of the Ce-O bond lengths with increasing the ZrO 2 content. In contrast, the Zr-O local structure depends on the nature of the phase and on the ZrO 2 content. In all the samples, ZrO 2 tends to conserve the typical Zr-O bond and the variation of the bond lengths are also consistent with a contraction of the cell parameter. The number of NN oxygens depends on ZrO 2 content. Both the C.N. = 4 + 3 and 4 + 2 are statistically equivalent in the Rh/Ceo.sZro.202, while for the Rh/CexZrl_xO 2 (x = 0.6-0.5), the C.N. = 4 +2 model is preferred on the statistical bases. The C.N. = 4 + 4 model is the only one acceptable for the Rh/Ceo.2Zro.802.
191 Table 2. Local coordination of the Zr-O shell in CexZrl.xO 2 as determined from EXAFS measured at Zr K. mixed oxide
CN a
R (,~)
6 (h)
AlE (eV) b
v
~Sv2
4
2.159 + 0.015
0.075 + 0.017
0.9 + 0.8
2
0.97
3
2.318 • 0.037
0.101 • 0.033
t-Ce0.6Zr0.40 2
4 2
2.139 • 0.004 2.336 • 0.010
0.073 • .005 c
1.4 + 0.2
3
1.3
t" Ce0.sZr0.50 2
4 2
2.115 + 0.008 2.324 • 0.012
0.078 • 0.003 c
0.4 • 0.8
3
1.9
t' Ce0.sZr0.502
4 2
2.138 • 0.005 2.382 • 0.009
0.084 • 0.003 c
2.0 • 0.5
3
5
t-Ce0.2Zr0.80 2
4
2.089 • 0.007
0.062 • 0.008
0.5 + 0.4
2
0.7
4
2.306 • 0.012
0.102 • 0.011
Zr-O c-Ce0.8Zr0.20 2
Ce-O c-Ceo.8Zro.20 2
8
2.327 + 0.004
0.077 + 0.005
-0.1 + 0.4
1
1.3
/-Ce0.6Zr0.402
8
2.312•
0.080+0.005
-0.5 +0.1
1
0.7
t" Ceo.sZr0.50 2
8
2.312 + 0.007
0.092 + 0.005
-0.3 + 0.3
1
0.6
t' Ceo.5Zro.50 2
8
2.308 + 0.004
0.078 + 0.005
-0.7 + 0.2
1
1.3
/-Ce0.2Zr0.802
8
2.291 + 0.008
0.081 + 0.005
-0.9 + 0.4
1
1.0
a After preliminary testing of the proper C.N., they were kept costant in the final fittings; b The same AE values were employed for both the subshells; c The same o values were employed for both the subshells.
Formation of homogeneoues solid solutions is confirmed by the modeling of the peaks at 2.35-4.12/~ (Zr, K edge, Fig.3). This is exemplified for t" Rh/Ceo.sZr0.50 2 in Table 1 and Figure 3.1-3.4. This region cannot be modeled with a Zr-Zr shell only. Use of Zr-Zr and Zr-Ce subshells with appropriate C.N.s (6+6) is necessary to obtain a reasonable fit. This suggests that homogeneous solid solution were formed. Inclusion of a Zr-O subshell further improves the quality of the fit. The validity of all the fits at the Zr K edge is confirmed in Fig.3 which shows an excellent agreement of the experimental data and the model both in R- and k-space.
3.1.3. Redox property- Structural correlation for the LSA samples As written above, the investigation of TPR behavior of the LSA Rh/CeO2-ZrO 2 disclosed an important improvement of the reduction of the solid solutions at low temperatures (2). Figure 4 reports the peak temperatures for the reduction of the LSA Rh/CeO2-ZrO 2 samples as a function of the CeO 2 content. Remarkably, as the amount of ZrO 2 inserted into the CeO 2 lattice increases in the c and t" samples, the temperature of the reduction decreases. The reverse is true for the t' and t samples. We recall, that observed reduction process are related to the bulk of the solid solution. In the presence of the supported Rh, the activation of H 2 is
192 easy which suggested that the peak temperatures are related to the facility of the oxygen migration in the bulk (2). As shown by Raman, XRD and EXAFS characterization, the oxygen sublattice of the CeO 2~" 1000 ZrO 2 LSA strongly depends on the phase nature and ZrO 2 content. The Raman and EXAFS data clearly I 9 ~ 800 point out that in the case of the cubic samples (c and E 9 9 t"), the insertion of ZrO 2 leads to an apparent decrease I-f, 6oo of the Zr 4+ coordination number. No evidence for a .._... I._
4-,
4--' 0
strong non stoichiometry is found. There are 8 NN oxygen to the metal cation in the fluorite structure. 0 20 40 60 80 100 This means that lack of observation of the NN oxygens CeO2 content ( mol %) close to the Zr 4+ must be attributed to a high structural disorder of these oxygens. The impossibility of Figure 4. Peak Temperatures for the distinguishing between the C.N. = 4 + 3 and C.N - 4 reduction of the Rh/CexZrl_xO2 (x= +2 for the Rh/Ce0.8Zr0.202, while this is possible for 0.1-1) LSA (2)(e) c and t", (11) t the Rh/CexZq_xO 2 (x= 0.6-0.5), suggests that the and t' phases. structural disorder increases as the amount of inserted ZrO 2 increases. Apparently, the increase of the structural disorder represents a mechanism for release of the stress generated by the insertion of the smaller Zr 4+ cation into the CeO 2 lattice. We infer that the pushing of the oxygens to a non bonding distance should generate mobile oxygens in the lattice. More mobile oxygens should be therefore expected as the amount of ZrO 2 increases. It should be noted, however, n,'
400
that such a picture is true as long as the dominant structural factor is the CeO 2 e.g. t" and c phases. When the ZrO 2 becomes dominant, the t-phase is formed. Under these conditions, the release of the stress, above discussed, occurs by a different mechanism, e.g. by a tetragonal distortion of the cation sublattice. This leads to a modification of the oxygen sublattice, which now presents the C.N. = 4 + 4 coordination which is typical of tetragonal zirconias. Accordingly, we found that the pychnometric density decreased in the cubic region, with a minimum found for the t"-Rh/Ceo.sZro.502, while, despite a contraction of the cell volume, it was almost constant in the t' and t" samples (2). We do not have evidence for a significant difference of the Zr-O local structure in the t' and the t"-Rh/Ceo.5Zro.502 samples except that the former sample shows a somewhat longer Zr-O distance. This may be easily attributed to a distortion of the cell along the c direction induced by the tetragonalization (cell parameter ratio c/a = 1.008). Such a distortion may represent a mechanism for the release of the stress in the lattice, accounting for the higher reduction temperature of this sample compared to the t"Rh/Ce0.sZr0.502. Summarizing, the present data suggests a straight forward interpretation of the TPR results reported in Figure 4: In the cubic phases, the easiness of the oxygen migration in the bulk appears related to the property of Zr 4+ to maintain a less crowded coordination. Consequently as the cell parameter decreases, more and more labile oxygens are generated in the lattice making the reduction process easier. In contrast, in the tetragonal region, the labile oxygen comes closer to the Zr 4+ since the tetragonal C.N.- 4 + 4 coordination becomes prevalent.
193 Under these conditions, as the oxygens are found at a bonding distance, the reduction process is hindered. 3.2.
Redox property - Structural correlation for the HSA samples
The above findings prompted us to investigate the redox property of a high surface area Rhloaded and Rh-free Ceo.5Zro.502 (1). For a full discussion we refer the reader to ref (1,17). Some aspects related to the present results are discussed here.. The Raman spectrum of the flesh sample (Figure 5) features a strong broad band centered at 465 cm -I with a shoulder at about 550 cm -1 and two bands at 313 (w) and 140 (vw) cm -1 which are attributed to a t" phase (13). The higher symmetry of the spectrum strongly suggests that smaller distortions of the oxygen sublattice are present compared to the above LSA samples. This is attributed to the presence of smaller particles in the HSA sample. Decreasing the particle size of ZrO 2 the tetragonal and cubic phases are progressively stabilized. A shoulder at about 550 cm -1 in CeO 2 doped with trivalent cations was attributed to the presence of oxygen vacancies. Accordingly, by supporting the Ceo.5Zro.502 with Rh, the intensity of this shoulder increases in the calcined sample (Figure 5, spectrum 3). The TPR behavior of the flesh bare and Rh-loaded Ceo.5Zro.502 is reported in Figure 6. After, the initial TPR, the samples were oxidized at 700 K and subjected to a further TPR experiment. Remarkably, the reduction features at high temperatures disappear and the overall reduction process is shifted to lower temperatures compared to the fresh catalysts. The ability of the supported Rh to promote the reduction process by a spiUover of the H 2 over the support is well represented in the TPR of the fresh Rh-loaded Ceo.5Zro.502. Note the shift of the peak at 880 K in the Rh-free sample to approximately 620 K in the presence of Rh. In the H 2 treated Ceo.5Zro.50 2 a strong decrease of surface area was observed: after reduction at 1000 K, the surface area decreases to 18 and 10 m 2 g-I respectively for the metal free and Rh-loaded sample. Even if a quantitative evaluation cannot be given, the Raman spectra of the recycled samples (Figure 5) are in a good agreement with the results observed for the LSA samples. As the sintering of the Ceo.sZro.50 2 under H 2 proceeds, the symmetry of the M-O local structure is progressively lost as denoted by the disappearance of the band at 465 cm -1 attributed to the T2g mode. This should result in progressive generation of mobile oxygen in the lattice, making the reduction in the bulk of the solid solution easier. Accordingly, the reduction in the recycled samples occurs at lower temperatures. Such an interpretation, of course, must include the role of the supported metal. On the bare support, the activation of H 2 occurs through the surface OH group. Upon sintering, less OH groups should be available for the H 2 activation, accounting for the higher reduction temperatures compared to the Rh-loaded samples. In the latter case the H 2 activation is promoted by the metal.
194
465 140
v
.o 31_.~ 3/'!\'E 273
2x-J p 'd \
2
,J , / ' ,
f,"........../ ~s'~
"',
Y
'
-
3
ZZL~Z !
i
300
J
!
500
........ i . . . .
700
;
O
o c"
'
/\
100
2
,J"~ .~05 ..............
;
900
R a m a n Shift (cn5t )
s P~ 1-
4 |
300
9
l
500
i
i
700
~
|
900
s
" l ....
1100
1300
T e m p e r a t u r e (K)
Figure 5. Raman spectra of (1) fresh Ceo.sZro.502, (2) Ceo.sZro.502 reduced at 1073 K and oxidized at 700 K, (3) fresh Rh/Ceo.sZro.502, (4) Rh/Ceo.sZro.50 2 reduced at 1273 K and oxidized at 700 K. Figure 6. TPR of (1) fresh Ceo.sZro.502, (2) Ceo.sZro.50 2 recycled from run (1) and oxidized at 700 K, (3) fresh Rh/Ceo.sZro.502, (4) Rh/Ceo.sZro.50 2 recycled from run (3) and oxidized at 700K. Summarizing, also the data obtained on the HSA samples strongly suggest that the redox properties of the CeO2-ZrO 2 solid solutions can be related to the metal-oxygen local structure.
4. CONCLUSIONS The local structure of the metal-oxygen ions in the CeO2-ZrO 2 mixed oxides appears to be strongly related to their redox properties. The distortions generated by the insertion of ZrO 2 into the cubic lattice of CeO 2 are indicated as the dominating factor which generates mobile oxygens in the structure. These oxygens are responsible for the unusual redox properties of these mixed oxides. Finally, the formation of a truly homogeneous solid solution was confirmed, suggesting that the adoption of an appropriate synthesis methodology may avoid formation of domain type structures.
ACKNOWLEDGMENTS
Ministero dell'Universit~t e della Ricerca Scientifica (MURST 40% and 60%), Universith di Trieste, EU Large Installation Program and European Commission (TMR Program Contract N ~ ERB FMRX-CT96-0060) are acknowledged for financial support.
195 REFERENCES 1.
~
8. ,
10. 11. 12. 13. 14. 15. 16. 17.
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DeNOx Noble Catalysts
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CATALYSISAND AUTOMOTIVEPOLLUTIONCONTROLIV Studies in Surface Science and Catalysis,Vol. 116 N. Kruse, A. Frennet and J.-M Bastin (Eds.) 91998Elsevier Science B.V. All rights reserved.
199
Kinetics of the Reduction of NO by C3H6 and C3H8 Over Pt Based Catalysts Under Lean-Burn Conditions R. Burch and T.C. Watling Catalysis Research Centre, Chemistry Department, University of Reading, Whiteknights, Reading, RG6 6AD, UK The effect of temperature, contact time and reactant concentration on the kinetics of NO reduction by C3H6 and by C3Hs over Pt/AI203 under lean-burn conditions have been investigated and kinetic models which satisfactorily fit the data have been developed. The results suggest that with C3H6 the Pt surface is dominated by carbonaceous species, while with C3Hs adsorbed atomic oxygen is the main species on the Pt surface. This difference in the state of the Pt surface results in different mechanisms for NOx reduction. Thus, with C3H6, NOx reduction seems to occur via the dissociation of adsorbed NO on the Pt surface, while with C3H8, NOx reduction appears to occur via spill-over of NO2 from the Pt metal onto the A1203 support where it reacts with C3Hs-derived species to form N2 and N20.
I. INTRODUCTION NOx removal under lean-burn conditions has not yet been fully demonstrated although Pt based catalysts show considerable promise. However, very little detailed information is available on the nature of the active sites, the reaction mechanism or the effectiveness of different hydrocarbon reductants. This paper presents a detailed kinetic analysis of the reaction between NO and C3H6 or C3Hs under lean-burn conditions and provides a rationalisation for the differences between hydrocarbons which compete successfully and unsuccessfully with oxygen for adsorption sites on the catalyst.
2. EXPERIMENTAL The platinum on 7-alumina catalyst used in this study was prepared by incipient wetness impregnation using dinitrodiammine-Pt. The sample was calcined at 500~ for 14 h and had a 1 wt.% Pt loading, a dispersion of 69% (by H2 chemisorption) and a grain size of 250-850 tam. Catalyst testing was carried out using a quartz tubular downflow reactor (I.D. 5 mm) operating at atmospheric pressure. The sample (100 mg) was held between plugs of quartz wool. Reactant gases were fed from independent mass flow controllers. Unless stated otherwise the feed consisted of 1000 ppm C3H6 or C3H8, 1000 ppm NO and 5% 02 in He and the total flow was 200 cm3 min~ (corresponding to a space velocity of about 87 000 h"~, based on reactor volume). The reactor outflow was analysed using a Perkin Elmer Autosystem gas chromatograph with a TCD detector, a Signal Series 2000 IR CO2 analyser and a Signal Series 4000 chemiluminescence NOx analyser, as described in more detail elsewhere [ 1]. No reaction
200 was observed over quartz wool, provided the temperature was below 600~ Changing the catalyst grain size had no effect on the conversions, indicating freedom from intra-particle transport limitation. Hydrocarbon and NOx conversions were linear in contact time for C3I-I6 conversion up to 60% and C3Hs conversion up to 30% (Fig. 2 and 8), indicating differential conditions and freedom from inter-particle transport limitation. For experiments in which the temperature was varied, the temperature was increased stepwise with constant feed composition and total flow. For determining the effect of contact time, measurements were made at a series of total flows between 50 - 300 cm3 min~ with either 100 or 250 mg of catalyst. The flow was varied in a random way rather than sequentially, to avoid any bias in the data. Similarly, for determining the effect of reactant concentration, the concentration of one reactant was varied in a non-sequential manner, while the concentration of the other reactants was kept constant. The catalyst was treated with 20% o2/ne for 15 min at 480~ between experiments using different hydrocarbons to remove any hydrocarbon residues from the catalyst.
3. RESULTS AND DISCUSSION The effect of temperature, contact time and reactant concentration on the reaction rate/conversion and products of the of the C3Hs-NO-O2 and CaHr-NO-O/reactions over 1% Pt/AI203 are given in Fig. 1-11. It is clear that there are a number of differences in behaviour dependant on whether an C3H8 or C3H6 is used as the reductant, viz., 9 C3H6is much more reactive than Calls, 50% hydrocarbon conversion being reached at 265 and 400~ respectively. In addition, C3H6 lights-off considerably more rapidly than Calls. C3H6 is a more effective NOx reductant and is active at a lower temperature than C3H8, C3H6 gives a maximum NOx conversion of 45% at 270~ compared with 25% at 430~ for C3Hs. Maximum NOx conversion is coincident with 100% hydrocarbon conversion being reached with C3H6but not with C3Hs.
NO2 is produced at all temperatures above 170~ with C3H8, but is only produced after 100% hydrocarbon conversion was reached with C3H6. The kinetics of oxidation of the two hydrocarbons are very different; with C3H6 the rate increases strongly with 02 concentration, is inhibited by NO and is zero order in C3H6 while, alkane oxidation is at least first order in C3H8, slightly inhibited by NO and is inhibited by 02 at lower concentrations, but independent of 02 concentration at higher concentrations. The kinetics of NOx removal with C3H6 are very similar to those of C3H6 oxidation, while with C3H8 the kinetics of NOx removal are very different to those of Calls oxidation, e.g. the former reaction is enhanced by increasing 02 and NO concentrations while the latter reaction is inhibited by O2 and NO. The differences in reaction behaviour observed between CsHs and C3H6 suggests that the reaction mechanism is different depending on the choice of reductant. To further understand this, realistic reaction mechanisms based on the observed kinetics have been proposed for the
201 two reactions and from these kinetic models have been developed which satisfactorily predict the observed kinetics. These proposed mechanisms are summarised in Schemes 1 and 2. In these models it is assumed that all the sites on the Pt surface are equivalent, that there are no adsorbate-adsorbate interactions other than those due to chemical reaction and that NO adsorption is at equilibrium. 3.1. C3H6-NO-O2 Reaction Developing a kinetic model based on a realistic reaction mechanism proved to be more difficult for the C3H6-NO-O2 reaction than the C3Hs-NO-O2 [2] reaction, simply because very little work has been published on the oxidation of alkenes over Pt group metal catalysts. While it is generally accepted that the rate determining step in alkane oxidation over Pt group metal catalysts is the breaking of the first C-H bond [3], no such consensus exists for alkene oxidation. The proposed mechanism is summarised in Scheme 1. For clarity the key reaction steps will be discussed in this section, while the derivation of the final expressions for the rates of reaction is given in Appendix 1. CsH6(o) 02(o)
.~ CHx* + 2*
NO(o) " ~ *. /
~
~.~,.)
3CO2(o) + 3H20(o)
/ > 20*
NO* I
+ * ) N* + O* I
+ N*-2" -2"
,, ,,,,> N2(g) >
N20(g)
Scheme I. Mechanism for the C3H6-NO-O2reaction. All reactions occur on the Pt surface.
C3H6 oxidation was found to exhibit a high order in 02 (1.8) and to be zero order in C3I-I6 (Fig. 3 and 5), suggesting that the coverage of oxygen on the Pt surface is small, while that of C3H6-derived species is close to saturation. It is also known that C3H6 fragments to CHx type species on a clean Pt surface at temperatures above 200~ [4]. Thus a plausible mechanism involves C3H6 adsorbing and cracking irreversibly on the Pt surface to form various carbonaceous species. Since the desorption of adsorbed atomic oxygen is reported to occur only at temperatures above 400~ [5], oxygen adsorption must be irreversible. Thus, oxygen adsorbs dissociatively and irreversibly on the Pt surface to give adsorbed O atoms which rapidly react with the adsorbed carbonaceous species to form CO2 and H20. The high order in 02 can be explained as follows. Increasing the 02 concentration increases the rate at which carbonaceous species are burnt off, resulting in a reduction in the coverage of CHx species and a corresponding increase the number of vacant sites at which oxygen can adsorb. Thus the order of C3H6 oxidation is considerably greater than unity. 02 adsorption can be represented as:
202 100
100
.
o~
80
g
6o
> 40-
~
4o-
o 20-
o
20-
N 80 .~ 60 e--
~
r o
~
o
.
.
.
/ .~
a
-
,.
o |
i
200
i
i
0
250 300 Temperature /~
~
350
Fig. 1. The effect of varying temperature on the C3H6-NO-O2 reaction. Lines are fit to kinetic model. (0 C3H6, @ NO, A NOx, O NO to N2, l"l NO to N20, A NO to NO2)
,sl
~
. . . . . . . . . . . .
0
1
2
3
4
103w/f (g min cm3)
5
Fig. 2. The effect of varying the reciprocal space velocity, w/f, on the C3I-I6-NO-O2 reaction at 240~ Key as for Fig. 1. Feed: 1000 ppm C3H6, 1000 ppm C3H6 and 5% O2. 15
o~ 10 u..
5
~
[]
o I-~'o
[]
,r
5
&
i...I
1000 2000 C3H6 Concentration / ppm
3000
Fig. 3. The effect of varying C3H6 concentration on the CaH6-NO-O2 reaction at 240~ Lines are fit to kinetic model. Key as for Fig. 1. Feed: 1000 ppm NO and 5% 02.
,s!
,
0
=
I i
|
,
|
,
i i
|
|
i
i
I !
|
|
|
|
500 1000 1500 NO Concentration / ppm
2000
Fig. 4. The effect of varying NO concentration on the C3H6-NO-O2 reaction at 240~ Lines are fit to kinetic model. Key as for Fig. 1. Feed" 1000 ppm C 3 H 6 and 5% 02. 100 > 8
80 60
z t'~
5
E 40
-'
.E 20
o
~
i
0 ~
0
0,'
I
'
2
4
O2 Concentration / %
6
Fig. 5. The effect of varying 02 concentration on the CaH6-NO-O2 reaction at 240~ Lines are fit to kinetic model. Key as for Fig. 1. Feed: 1000 ppm NO and 1000 ppm C3H6.
.....
........,
,-
'
'
200 250 300 350 400 Temperature of Maximum NOx C o n v / ~ Fig. 6. Correlation predicted from the kinetic model for the C3H6-NO-O2 reaction (see text). Points are calculated results while the line is to guide the eye.
203
kox Co2 0v
O2(g) + 2* --) 20*
(1)
where * represents a site on the Pt surface and the expression on the right gives the rate of the elementary step in which kox is a rate constant, c02 the gas phase concentration of O2 and 0v the fractional coverage of vacant sites. A number of models were tried for the adsorption and cracking of C3H6 but none was capable of fitting the data even remotely (i.e. the calculated rate had the opposite curvature to the experimental points when the concentration of at least one of the reactants was varied). Instead, the coverage of carbonaceous species will be assumed to be given by: 0c = 1 - A Co2
for CC3H6> 0
0C = 0
f o r CC3H6 = 0
(2)
where A is a constant. This expression predicts that in the absence of 02 the surface will be completely covered by carbonaceous species and that the coverage of these species will decrease with increasing O2 concentration. Provided C3H6 is present, the oxygen coverage is negligible. While eqn 2 appears to be somewhat arbitrary, it does allow the logical development of expressions which satisfactorily predict the rates of both Call6 oxidation and of NO reduction to N2 and N20. The kinetics of NOx reduction were found to be very similar to those of CaI-I6 oxidation; both reactions are 1.8 order in 02, zero order in C3H6 and inhibited by NO (Fig. 1-5). This is consistent with NOx reduction occurring via the dissociation of molecularly adsorbed NO on vacant Pt sites, followed by the formation of N2 and N20 by the combination of adsorbed N and NO. This mechanism is supported by our earlier TAP study [6]. It is also in agreement with the observation that the ratio of N2 : N20 formed is independent of contact time (Fig. 2) indicating that N2 and N20 are formed from parallel routes. This can be represented as: NO (g)+
9~
NO*
KNO-
0NO CNo 0v
(3)
NO* + * - , N* + O*
kNONoOv = kNKNo CNoOv2
(4)
N* + N* - , N2tg) + 2*
kN2 0~2
(5)
N* + NO* - ) N20(g) + 2*
kN2oON0NO= kN20KNO CNOOVON
(6)
Under conditions at which complete C3H6 oxidation occurs, oxidation of NO to NO2 is observed (Fig. 1 and 2). This suggests that only in the absence of C3H6 is the coverage of oxygen on the Pt surface significant. Note that no NOx reduction occurs in the absence of 02 (at least under the conditions used here) (Fig. 5), in which case the Pt is covered by carbonaceous species, or in the absence of C3I~ (e.g. Fig. 2 shows NOx conversion does not increase with contact time after 100% C3H6 conversion is reached), as the Pt surface is covered by oxygen. In both cases there are no available sites for NO adsorption/dissociation. The model also correctly predicts that the maximum in the NOx conversion is coincident with 100% C.ffI6 conversion being reached. This can be understood as follows. Oxygen is deposited onto the Pt surface from dissociative O2 adsorption and from NO dissociation. This oxygen is rapidly removed from the Pt surface by reaction with the CaH6-derived species present. The competition between NO dissociation and 02 adsorption determines the NOx
204 conversion (related to NO dissociation) obtained for a given C3H6 conversion (related to both 02 adsorption and NO dissociation). In fact NO competes quite successfully with 02; with a feed of 5% 02 and 1000 ppm NO there are 100 times more oxygen atoms as 02 than NO present in the gas phase and yet at 240~ the amount of oxygen deposited on the surface from NO is only about a factor 10 smaller than that from 02. One reason for this is that molecularly adsorbed NO is a significant surface species while molecularly adsorbed 02 is not (Note that NOx and C3H6 conversion are inhibited by NO and not by 02). Thus, release of oxygen from the former species is favoured over that from the latter (for comparable gas phase concentrations). As the temperature is increased the rate of NO dissociation increases less rapidly than the rate of dissociative 02 adsorption, partly as a result of the decrease in NO coverage with temperature, resulting in NOx conversion increasing less rapidly than C3H6 conversion. Once the temperature for complete C3H6 oxidation is exceeded, C3H6 is only present in the initial part of the catalyst bed and hence reduction of NOx only occurs in this part of the bed. In the remainder of the bed, the oxygen coverage on the Pt surface is high and hence oxidation of NO to NO2 is the only reaction occurring. Further increase in temperature results in a decrease in the volume of the bed in which deNOx occurs. Since the rate of NOx reduction increases more slowly than the rate of C3H6 oxidation with temperature the NOx conversion falls off with increasing temperature. Thus maximum NOx conversion is coincident with 100% C3H6conversion being reached. The model can also explain our earlier observation [7,8] that when the maximum NOx conversion is plotted against the temperature at which this maximum occurs for a series of Pt/AI203 catalysts a remarkably good straight line is obtained. By varying either the weight of catalyst, the dispersion or the contact time inputted into the model a number of conversion versus temperature curves were calculated using the parameters given in Table 1. Figure 6 shows the maximum NOx conversion obtained from these curves as a function of the temperature at which the maximum occurred. With the exception of the last two at high temperature, the points lie on a straight line, i.e. the model has reproduced the empirical correlation. A consequence of the method of calculation is that the ratio of the rate of NOx reduction and the rate of C3H6 oxidation is constant for a given temperature. Thus it can be concluded that while changing the Pt loading, dispersion, or Pt precursor, changes the activity of the Pt/AI203 it does not alter the effectiveness of NOx reduction (i.e. the fraction of the total amount of reductant consumed used for NOx reduction) and so, at least for the precursors and preparation method used, Pt/AI203 cannot be further improved as a deNOx catalyst with C3I-I6.
3.2. C3Hs-NO-O2 Reaction The model for the C3Hs-NO-O2 reaction has been published in detail elsewhere [2], hence only the main points will be summarised here. Scheme 2 outlines the proposed mechanism. The lines drawn in Fig. 7 and 9-11 are calculated from the kinetic model derived from the mechanism. The kinetics of the C3Hs-NO-O2 reaction are very different from those of the C3H6-NO-O2 reaction. The rate of C3Hs oxidation is greater than first order in C3Hs and is inhibited by 02 at lower 02 concentrations (1 < [02] < 3%) and zero order in 02 at higher 02 concentrations ([02] > 5%). This indicates that the oxygen coverage on the Pt surface is high and so increasing the 02 concentration results in blocking of surface sites from C.-Hs and hence a
205 reduction in the rate of C3Hs oxidation. At higher 02 concentrations, the surface oxygen coverage reaches saturation but there are still sites available for C3Hs to react and hence the reaction is zero order in 02. So with the C3Hs-NO-O2 reaction oxygen is the dominant species on the Pt surface, while with the C3H6-NO-O2 reaction carbonaceous species dominate. .f--*
02 (g) " + . /
02*
+.
-
) 20* + OH*-O*-* r - FAST "f H20(g)
C3H8 (g)
+*+O*
...... > 0 3 H 7 " + O H *
I+ 9.50*-10.5 * > 3 CO2 (~) + 3.5 H20 (g) FAST
NO(g)
. , . . . ~ NO*
L
- 7 ~ .....
~ ~ - - - - - " ~ + O* - O*~.....~~ NO2(g)
...........................
C3H8(g)
f "
§ *. . ~ NO2*
-
--.\...
~> CxHy \~
...........................................
> N2(g), N20(g), CO2(g), H20(g)
...........
A!203 support
Scheme 2. Proposed mechanism for the C3Hs-NO-O2 reaction. Reactions above the dotted line occur on the Pt surface, while reactions below occur on the A1203 support.
The fact that the order of C3H8 oxidation in C3Hs is greater than unity is consistent with dissociative chemisorption of C3Hs involving the breaking of a C-H bond being the rate determining step, as is generally accepted [3]. Increasing the C3H8 concentration increases the rate at which adsorbed oxygen (the dominant species) is removed from the Pt surface, resulting in an increase in the number of vacant sites at which C3H8 can adsorb. This in turn results in an increase in the rate of C3Hs oxidation in addition to that due to the gas phase concentration of C3Hs resulting in the order in C3Hs being greater than unity. Adsorption of NO and NO2 on the Pt surface results in blockage of reaction sites and hence inhibition of C3Hs oxidation. Oxidation of NO to NO2 is observed over a wide temperature range (Fig. 7) and (unlike the C3H6-NO-O2 reaction) occurs in the presence of the reductant. This is consistent with a high coverage of oxygen on the Pt surface. The conversion of NO to NO2 was independent of contact time (Fig. 8) suggesting that the rate of NO oxidation to NO2 is so fast that a pseudo equilibrium was established between NO and NO2 even at the shortest contact time used. For this reason the formation of NO2 is expressed as a conversion rather than as a TOF. However, NO and NO2 are not in thermodynamic equilibrium since the conversion of NO to NO2 (about 30%) is much less than that predicted for equilibrium under these conditions (79%). In addition, the fall in conversion to NO2 with increasing C3H8 concentration (Fig. 9) is not predicted by thermodynamics. The oxidation of NO to NO2 and of dissociation of NO2 to NO was modelled using eqns 16 and 17.
206 100-, 0~80 -
o~60 e-o
=60 0
"F940
~40
(9 > e--
0
,k
,
,
&
~
0o20
o 20 150
250
350
Temperature/ ~
450
0.0
550
Fig. 7. The effect of varying temperature on the C3Hs-NO-O2 reaction. Lines are fit to kinetic model. (0 C3Hs, 9 NO, = NOx, O NO to N2, I"1 NO to N20, A NO to NO2) 4
80
3
60~
o F ~,~ 0
i ,,-:-: 1000
~:;
,:,~ o
2000
Carte Concentration / ppm
3000
Fig. 9. The effect of varying C3Hs concentration on the C3Hs-NO-O2 reaction at 310~ Lines are fit to kinetic model. Key as for Fig. 7. Feed: 1000 ppm NO and 5% 02. 4
80
3
60~..,
2
4o i
o 1
20 8
0
0 0
2
4
6
0 2 Concentration / %
8
10
Fig. 11. The effect of varying 02 concentration on the C3Hs-NO-O2 reaction at 310~ Lines are fit to kinetic model. Key as for Fig. 7. Feed: 1000 ppm NO and 1000 ppm C3Hs.
0.5 1.0 1.5 10~ w/f (g ~ n crn~)
2.0
Fig. 8. The effect of varying the reciprocal space velocity, w/f, on the C3Hs-NO-O2 reaction at 310~ Key as for Fig. 7. Feed: 1000 ppm C3Hs, 1000 ppm C3Hs and 5% 02. 4
80
%3
o
60..
0
500
1000
1500
NO Concentration / ppm
o
2000
Fig. 10. The effect of varying NO concentration on the CaHs-NO-O2 reaction at 310~ Lines are fit to kinetic model. Key as for Fig. 7. Feed: 1000 ppm C3Hs and 5% O2.
207 Attempts were made to fit the data for the rate of NOx reduction to kinetic models based on the various reaction mechanisms proposed in the literature, viz.: (i) the oxidation of NO to NO2 which then reacts with the hydrocarbon [9, 10, 11], (ii) the formation of an oxidised hydrocarbon intermediate [12,13]; (iii) reduction of the metal surface followed by NO dissociation on the reduced surface [6], possibly with the NO dissociation being assisted by other adsorbed species [1]; (iv) the formation of an isocyanate surface species as an intermediate [14]. Combinations of these mechanisms have also been suggested, such as the oxidation of NO to NO2 which then reacts with an oxidised hydrocarbon intermediate [ 15,16]. However, none of these models satisfactorily fitted the data [2]. The only correlation that was found with the rate of deNOx was with the NO2 coverage, i.e. rt~o,, oc 0t~02. This can be interpreted in terms of a mechanism in which the rate determining step is the spill-over of adsorbed NO2 onto the A1203 support. This NO2 then reacts with C3Hs-dedved species deposited on the support, possibly located close to or at the metal-support interface, to give N2 and N20. This mechanism suggests that the nature of the support should be important, and indeed, while Pt/Al203 shows deNOx activity with a CaHs-NO-O2 feed, little [8, 17] or no [ 18] deNOx activity is observed with Pt/SiO2 with the same feed. Recently, Hamada and coworkers have reported that physical mixtures of A1203 and Pt/SiO2 are active for deNOx with a C3Hs-NO-O2 feed [18]. This is consistent with the mechanism suggested above, although in this case the reaction presumably occurs by gas phase transfer of NO2, produced by oxidation of NO on the Pt surface, to the surface of the A1203, where it reacts with Calls derived species. Since Calls oxidation (by O2) seems to occur on the metal, while the deNOx reaction occurs on the support and/or at the metal-support interface the kinetics of these two reactions are very different (see above). The final expression used to fit the rate of deNOx was:
knox 0No2
rNO~= 1 + B e c~RT
(7)
where B and C are constants. The dominator of this expression allows for the fall in concentration of C3Hs-derived species on the support with increasing temperature (presumably) as a result of reaction with 02. The concentration of these species appears to be independent of reactant concentration. Without this term the model did not predict the maximum in the NOx conversion.
4. CONCLUSIONS The kinetics of NO reduction by C3H6 and by C3Hs over Pt/Al203 under lean-burn conditions have been investigated and kinetic models which satisfactorily fit the data have been developed. The state of the Pt surface depends on the relative activities of the reductant and 02. Thus, C3H6 is more reactive than O2 and hence the Pt surface is predominantly covered with C3H6-derived species, while Cans is less reactive and the Pt surface is mainly covered with atomic oxygen. This difference determines which reaction pathway for NO reduction is possible. Thus, NO reduction via NO dissociation on the Pt surface (as appears to occur with C3H6) is favoured by the negligible oxygen coverage obtained in the presence of C3I-I6, but is presumably inhibited by the high oxygen coverage in the presence of C3H8. Conversely, NO reduction via reaction of spilt-over NO2 with carbonaceous species on the A1203 (as seems to
208 occur with C3Hs) requires the high oxygen coverage obtained with C3Hs to facilitate the oxidation of NO to NO2, but cannot occur with C3H6 since NO2 formation is only possible in the absence of C3H6. The mechanism of NOx reduction needs to be known when considering how the catalyst could be improved. If NO reduction occurs on the A1203 support then the deNOx activity may be enhanced by modifying the support to better facilitate the reaction between NO2 and hydrocarbon-derived species, perhaps by adding a basic component to trap NO2 or to aid hydrocarbon activation on the support. Lowering the temperature of hydrocarbon activation by the support so that it coincides with the maximum in NO2 production may also be beneficial. Alternatively, if NOx reduction occurs via NO dissociation on the Pt, then deNOx activity can be improved by modifying the Pt to increase the rate of NO dissociation relative to 02 adsorption. This does not appear to be possible by changing the Pt dispersion or precursor (see above), but may be possible by adding a suitable promoter.
ACKNOWLEDGEMENT We are grateful to the EPSRC for financial support for this work through grant GR/KO1452. APPENDIX 1: RATE EQUATIONS FOR THE C3H6-NO-O2 REACTION
In this appendix, expressions for the rates of reaction of the C3H6-NO-O2 reaction are derived from the mechanism discussed in section 3.1 and the method of determining the parameters outlined. Applying the stationary state approximation to 0~ (using eqns 4-6) gives, 0 = dON = kNKNO CNO0V2 - kmo KNO CNO0V ON- 2kN20N2 dt
(8)
This is a quadratic in 0r~/0v the solution of which is: .
+
ON 0V
.
.
.
+
.
.
(9)
4kN2
The number of surface sites is assumed to be constant, i.e., 1 = 0v +0NO + ON + 0c
(10)
Note that in the presence of C3H6 the coverage of oxygen is believed to be negligible (see above) and hence does not appear in eqn 10. Substituting in eqns 2 and 3 gives, 0v =
A co2
1+ KNocr~o + 0 ~ / 0 v
for cc3n6 > 0
(11)
The rate of the deNOx reaction is the rate at which NO is converted into N2 and N20 (eqns 5 and 6), i.e., rNox = 2kN2 0N2 + 2kN2oONONo
(12)
209 Substituting in eqns 3 and 11 gives: rNox =
2 AE(0N / 0 v ) Co2 2 (kN( 0 N /0 v ) + kmoK No CNo) (1 + KNo CNo + ON / Ov) 2
forcc3m>0
(13)
The term in ONhas been left as 0N/0~ rather than substituting in eqn 9 for simplicity. Since adsorbed oxygen does not desorb and 9 0 atoms are required to totally combust one C3H6 molecule, the rate of C3H6 oxidation is given by (using eqns 1 and 4): rc3u6 = (2ko~ Co20v + kNKNo CNo0v2)/9
(14)
Substituting in eqn 11 gives: 1
2 2 ko~ A Co2
+
.kN . . .KN . . . .~ A 2 CN~ CO2
The oxidation of NO to NO2 and the dissociation NO2 to NO in the absence of Call6 can be represented as: NO(g) + O* --'>NO2*
kNoE,fCNO00
(16)
NO2* --> NO(g) + O*
kNo2,b 0NO2
(17)
Under these conditions the conservation of sites is: 1 = 0v + 0NO + 0NO2 + 00
(18)
Thus, the net rate of NO oxidation to NO2 is given by (from eqns l, 3, 16-18): 2 kox k NO2,fCO2CNO FNO2 kNo2,f CNo (1 + KNo CNO + KNO2 CNO2) + 2kox CO2 =
+ kNo2,b KNO2 CNO2
for CC3H6= 0 (19)
The effect of temperature on the rate constants and adsorption coefficients was assumed to be given by the Arrhenius equation and Van't Hoff isochore respectively, i.e., In (k-k~l)= - ~E'(~22 - ~-]]
(20)
lnCK-~-~)-
(21)
AH'~(-~2 ~ ) R --~-
where k~ & k2 and KI & K2 are rate constants and adsorption coefficients at temperatures T, & T2, T~ and T2 are thermodynamic temperatures, R is the molar gas constant, E, is the activation energy for the appropriate reaction step and AH~ is the standard enthalpy of adsorption of the appropriate molecule. The reactor was assumed to exhibit plug flow and transport limitations were assumed to be negligible. For experiments at constant temperature, the reactor was assumed to behave differentially, while in experiments in which the temperature was varied the effect of nondifferential conditions was included by allowing for concentration gradients along the catalyst
210 bed as described elsewhere [2]. The parameters given in Table 1 were determined using the method described previously [2]. Table 1 Parameters . . . obtained . . fr0m . . fittin . 8 the CaH6-NO-OE reaction model to experimental data. Parameter
Value at 240~
Ea/kJ tool"1(a)
8.26
x
104 ppm 1 s]
2 kox
1.46
x
10"1 0~ "l
kN2oKNo
8.92 x 10"4 ppm 1 s-I
14to)
kN2
7.33 x 10"1
14tO
KNO = KNO2to)
5.53 x 10-3 ppm "1
A (d)
7.28 • 10"2 %-1
0
kNo2,f
5.26 x 105 ppm q S"1
0
kNo2,b
1.88 x 10"l s"l
0
kNKNo
S"l
AH~
,.
kJ mol q(b)
14(c) 107
-96.7
(a)" Activation energy for the appropriate reaction step; (b) Standard enthalpy of adsorption of the appropriate species; (c)" Set to be equal to reduce the number of parameters; (d): Set equal to kox.
LIST OF SYMBOLS A B C CNO
[%-1] [-] [J mol "1] [ppm] [ppm]
co2
[%]
kN kN2 kN20 KNO KNO2 kNo2,f kNo2.b kNOx kox R
[S"l] IS "l] [S"l] [ppm "1] [ppm "1] [ppm "I s"l] [S"l] [S"l] [%-1 s-l] [J mol q K "1]
rc3.6
[s"]
rNo2 rNox t T
[s"1] [S"] [s] [K]
0i
[-]
CC3H6
Constant used in expression for 0c, defined by eqn 2. Constant in eqn 7, for rate of NOx reduction with C3H8. Constant in eqn 7, for rate of NOx reduction with Calls. Gas phase concentration of C3H6. Gas phase concentration of NO. Gas phase concentration of 02. Rate constant for NO dissociation, defined by eqn 4. Rate constant for N2 formation, defined by eqn 5. Rate constant for N20 formation, defined by eqn 6. Adsorption coefficient for NO adsorption, defined by eqn 3. Adsorption coefficient for NO2 adsorption. Rate constant for oxidation of NO to NO2, defined by eqn 16. Rate constant for dissociation of NO2 to NO, defined by eqn 17. Rate constant for NOx reduction by C3Hs, defined by eqn 7. Rate constant for oxygen adsorption, defined by eqn 1. Molar gas constant. TOF for C3H6 combustion. Net TOF for oxidation of NO to NO2. TOF for NOx reduction. Time. Thermodynamic temperature. Fractional coverage of species i on the Pt surface.
211
0~ 0v
[-] [-]
Fractional coverage of carbonaceous species on the Pt surface. Fractional coverage of vacant sites on the Pt surface.
REFERENCES
1. 2. 3. 4. 5. 6. 7.
8. 9. 10. 11. 12. 13. 14. 15. 16.
17. 18.
R. Burch and T.C. Watling, Catal. Lett., 37 (1996) 51. R. Burch and T.C. Watling, J. Catal., 169 (1997) 45. R. Burch and M.J. Hayes, J. Mol. Catal. A, 100 (1995) 13. R.A. Van Santen and J.W. Niemantsverdriet, Chemical Kinetics and Catalysis, Plenum Publishing Co., New York, 1995. T. Engel and G. Ertl, Adv. Catal., 28 (1979) 1. R. Butch, P.J. MiUington and A.P. Walker, Appl. Catal. B: Env., 4 (1994) 65. G.P. Ansell, S.E. Golunski, J.W. Hayes, A.P. Walker, R. Butch and P.J. Millington, Stud. Surf. Sci. Catal. 96 (Catalyst and Automotive Pollution Control III), A. Frennet and J-M. Bastin (eds.), Elsevier, Amsterdam, 1995, p. 577. P.J. Millington, PhD Thesis, University of Reading, UK, 1995, oh. 5. S. Naito and M. Tanimoto, Chem. Lett., (1993) 1935. T. Tanaka, T. Okuhara and M. Misono, Appl. Catal. B: Env., 4 (1994) L 1. A. Obuchi, A. Ogata, H. Takahashi, J. Oi, G.R. Bamwenda and K. Mizuno, Catal. Today, 29 (1996) 103. A. Obuchi, A. Ohi, M. Nakamura, A. Ogata, K. Mizuno and H. Ohuchi, Appl. Catal. B: Env., 2 (1993) 71. M. Sasaki, H. Hamada, Y. Kintaichi and T. Ito, Catal. Lett., 15 (1992) 297. G.R. Bamwenda, A. Obuchi, A. Ogata and K. Mizuno, Chem. Lett., (1994) 2109. G. Zhang, T. Yamaguchi, H. Kawakami and T. Suzuki, Appl. Catal. B: Env., 1 (1992) L15. B.H. Engler, J. Leyrer, E.S. Fox and K. Ostgathe, Stud. Surf. Sci. Catal. 96 (Catalyst and Automotive Pollution Control III), A. Frennet and J-M. Bastin (eds.), Elsevier, Amsterdam, 1995, p. 529. R. Burch, and T.C. Watling, Catal. Lett., 43 (1997) 19. M. Inaba, Y. Kintaichi and H. Hamada, Catal. Lett., 36 (1996) 223.
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CATALYSIS AND AUTOMOTIVE POLLUTIONCONTROLIV Studies in Surface Science and Catalysis,Vol. 116 N. Kruse, A. Frennetand J.-MBastin (Eds.) 91998 Elsevier Science B.V. All rights reserved.
213
N 2 0 and NO2 formation during N O reduction on precious metal catalysts P. Bourges, S. Lunati and G. Mabilon Institut Frangais du P6trole, 92506 Rueil-Malmaison C6dex, France ABSTRACT The comparison between the NO x reduction activity of Pt, Pd, Ir, Ru, Rh on alumina and Cu on ZSM-5 shows that only platinum and copper present high activity. Copper practically does not give any N20, but platinum is very selective for N20 formation. Moreover, platinum is the only active catalyst where NO 2 formation is important. But NO reduction doesn't produce simultaneously N 2, N20 and NO 2. If NO 2 is an intermediate in NO reduction mechanism, it could participate only as adsorbed species. On platinum catalysts, the kinetics of HC oxidation, NO reduction and NO oxidation are strongly dependant on the hydrocarbon nature. Those mechanisms occur at lower temperature with long chain alkanes than with olefins and these alkanes lead to a higher N20 selectivity than unsaturated molecules. 1. INTRODUCTION Diesel and lean gasoline engines are very attractive because of their low fuel consumption. But their development could be limited by the difficulty to comply with future NO x emission regulations: new depollution techniques have to be developed. The catalytic reduction of NO x by hydrocarbons has been studied over a large number of transition metal catalysts. Cu-ZSM-5 allows high conversion rates and good selectivities to N 2 but only above 350~ This is too high for diesel application where exhaust gas temperature varies generally between 100 and 450~ Precious metal catalysts show a higher potential for low temperature reduction. Platinum is especially attractive because it starts NO reduction at 150 - 200~ A large number of reaction mechanisms have already been proposed. They include NO decomposition prior to hydrocarbon oxidation by adsorbed O species [1], NO insertion in hydrocarbon to form N-containing organic species that are further decomposed by oxygen [26], NO oxidation to NO 2 that could be reduced by hydrocarbons to N 2 but also N20 or NO [7-9]. As precious metals can catalyze NO oxidation, we decided to investigate the influence of NO 2 formation in the global NO reduction process and to correlate N20 formation to the characteristics of the catalysts and of the reacting medium.
214 2. EXPERIMENTAL Catalysts were prepared by impregnation of alumina coated cordierite monoliths. Cylindrical catalyst samples (O = 30 mm, L = 76 mm) were placed in a down-flow reactor. The reaction mixture contained 3 to 9 components among NO, 0 2, NO 2, HC, CO, CO 2, H20, N2, SO2" in most cases NO concentration was 600 ppm with 3000 ppmC HC with a space velocity of 50 000 h-1. On-line analysis was performed by chemiluminescence for NO and NO 2, IR for N20 and CO, FID for HC. Catalysts were activated during 2 hours at 600~ under reaction conditions. Tests were performed under temperature ramp at 5~ between 150 and 500~ 3. RESULTS AND DISCUSSION 3.1. NO oxidation by 0 2
3.1.1. Influence of the catalyst nature Oxidation of NO by 02 in the presence of water, CO 2 and SO2 strongly depends on the nature of the catalyst (Fig. 1). With rhodium or iridium and more particularly with palladium or copper NO oxidation is very limited below 400~ The only catalyst which is very active even at low temperature is platinum.
-- Cu
,~ 60 t"q
50
---o-- Ru
o
-,--,
40
~Rh
O
30
--o- Pd I
Z
9
.-, 9
I-.i
~ 2o
~Ir
I
o
~ 10 9 2; 0 100
200
300
400
500
Temperature (~ Figure 1. Comparison of copper and noble metal catalysts for NO oxidation by 0 2 (GHSV=50 000h ~, 600ppm NO, 5% 0 2, 10% CO 2, 7% H20, 20vpm SO2). 3.1.2. NO oxidation over Pt/A120 3
3.1.2.1. Influence of the space velocity Below 300~ NO oxidation over platinum is very sensitive to the space velocity. Above that temperature the conversion is limited by the thermodynamic equilibrium (Fig. 2).
213 lO0
r
0 Z
80
=
60
~'
40
oo 0 2:
20
s
- - a - - GHSV 10 000
~
~
~r
__.J"~
F
--.--GHSV 50000
n I=t"
I ~
.,,.,,,"
rrrr ii
I
0 200
11
300
400
500
Temperature (~ Figure 2. Comparison of NO oxidation thermodynamics limit and NO oxidation rate over Pt/AI20 3 at different space velocity (GHSV = 10 000 or 50 000 h l ; 600ppm NO, 20vpm SO 2, 5% 0 2, 10% CO 2, 7% H20 ).
3.1.2.2. In the presence of reductants On Pt/A120 3, when CO is introduced at low concentration in a gas feed containing NO and 0 2, NO is not reduced and NO 2 is detected in the gas phase at slightly higher temperatures than in the absence of CO (Fig. 3).
1oo
~ i - ( f [
[
9 9[ [
9[ [ [ [ [ [ [
9 9 9[
9 9[ [
9[ - [ - l ' ~ [ - [ - I ( - [ - [ - [ - l " [ l l / ~
]
=
80
NO (no CO) ---o--CO
~,,,,4 9
60
_-7
0
9
40
N
2o
0 Z
0
r..)
No ( c o )........
!
100
.
i
!
i
!
a
i
150
200
250
300
350
400
450
500
Temperature (~ Figure 3. NO and CO oxidation rates in NO 2 and CO 2 over Pt/A120 3. Tests without CO or with 500 ppm CO (GHSV = 50000 h l ; 0 or 500 ppm CO, 600ppm NO, 5% 0 2, 10% CO2, 7% H20, 20vpm SO2). The introduction of hydrocarbons in the feed has a larger effect on NO oxidation and reduction. In the presence of n-decane (Fig. 4), NO is not oxidised before 230~ while in the presence of ethylene (Fig. 4), NO 2 is detected only above 290~ As in the presence of CO, NO oxidation is delayed until most of the reductant is eliminated. But hydrocarbons are oxidised at higher temperatures than CO and the nature of the hydrocarbon strongly affects its own oxidation: n-decane is oxidised around 220~ while ethylene is oxidised around 280~
216
r.,/3
Q
lOO
-,~-_,_,_,'_,,','_,'-mrrtr~ .. i~()
80 60
[
"~
20
9 2:
0
i ~--~'-,
~ ~,40
100
200
300
NO --o- HC 1 ~ NO --,-- HE
400
(no HC) (decane) (decane) (ethylene) (ethylene)
500
600
Temperature (~ Figure 4. NO and HC oxidation rates in NO 2 and CO 2 over Pt/A120 3. Tests without HC or with 6000 ppmC of decane or ethylene (GHSV = 50000 h 1 ; 0 or 6000 ppmC HC 500 ppm CO, 600ppm NO, 20vpm SO2, 5% 0 2, 10% CO 2, 7% H20 ). As in the absence of reductant, NO2 concentration goes through a maximum when temperature increases, but this maximum is clearly below that observed without reductant. This can be explained if we consider that the thermodynamic equilibrium for NO oxidation should more appropriately be expressed with the reactor outlet temperature than to the reactor inlet temperature: under adiabatic conditions the temperature increase is about 120~ for the combustion of 6000 ppmC hydrocarbon. This temperature shift is well suited to explain the NO oxidation curve in the presence of ethylene. It is less adapted in the presence of n-decane probably because decane oxidation is diffusion limited and reaches total conversion only at high temperature: NO oxidation is not at equilibrium. The temperature shift for NO oxidation in the presence of reductants could have several origins. NO 2 could not be formed on the catalyst surface until the reductant concentration in the adsorbed state is very low or NO 2 could be formed but rapidly reduced by hydrocarbon or carbon monoxide. NO 2 reduction could occur on the catalyst surface between adsorbed species or in gas phase by homogeneous reaction with the reductant. NO 2 reduction in gas phase has been studied in the absence of catalyst in a small test device equipped with on-line mass spectrometer (Fig. 5). The reduction occurs at low temperature but at very low space velocity. The maximum conversion rate is about 50%. At higher temperature ethylene is oxidised by 0 2 which accounts for the decrease of NO 2 reduction rate. As NO 2 reduction rate is 50 % at maximum at GHSV of 6 000 h "l, it must be very lower at 50 000 h "1 and could not significantly contribute to the elimination of NO 2 in the gas phase. Although NO 2 can be formed at low temperature on platinum, it is not detected in the presence of a reductant. The NO oxidation sites can be blocked when their covering by reductant is high or NO 2 adsorbed species are reduced in presence of carbon monoxide and hydrocarbons before their desorption. Therefore the absence of NO 2 in the gas phase does not necessarily means that it is not present as adsorbed species.
217 600 o
3500
500
.v~,4
~
400 El
CO 2
..................... i
NO 2
300
3000
E~
25oo
&
2000
O
15oo
"~
1000
o
500
r..)9
9. Z
200
0 Z
lOO
/
/'
~
NO
/
- r - - w ,,~
0
100
i
I
200
i
300
400
u
500
0
600
Temperature (~ Figure 5. Formation of NO and CO 2 during ethylene oxidation by NO 2 and 0 2 in absence of catalyst (GHSV = 6 000 h l , 600ppm NO 2, 3000ppmC ethylene, 1% 02). 3.2. NO reduction by hydrocarbons 3.2.1. Influence of the nature of the metal The comparison between the NO x reduction activity of Pt, Pd, Ir, Ru, Rh on alumina and Cu on ZSM-5 shows that only platinum and copper present high activity (Fig. 6). Platinum allows NO x reduction by n-decane in the temperature range 180 to 300~ with a maximum conversion of 70% near 220~ Copper allows 70% NO x conversion above 380~ In the presence of water, the percentage of reduction of NO x stays inferior to 15% on palladium, iridium, ruthenium and rhodium. These results are in accordance with those proposed by Obuchi et al [ 10] who tested the activity of precious metal catalysts with Diesel exhaust. lO0 ,--.,
1
80
=
60
~
40.,
---o-- Ru
!
--w- Rh --o-Pd I
9 Z
20 u
100
150
200
250
v
300
r
350
400
450
500
Temperature (~ Figure 6. NO reduction by n-decane on different metals (GHSV = 50 000 h "1 ; 6000 ppmC n-decane, 600ppm NO, 500 ppm CO, 20vpm SO2, 5% 02, 10% CO2, 7% H20 ).
218 The comparison (Fig. 1 and 6) between NO oxidation by oxygen in the absence of hydrocarbon and NO reduction in the presence of C 10H22 and 0 2 shows that the oxidation and the reduction of NO are comparable and occur in the same temperature range with platinum or ruthenium catalysts, the conversion rates being very low with ruthenium. With rhodium or iridium and more particularly with palladium or copper NO oxidation is very limited at the temperature where NO reduction occurs. The selectivity to N 2 0 is sensitive to the nature of the catalyst (Table 1). Copper practically does not give any N20, but platinum is very selective for N20 formation in the presence of decane [3,11 ]. Table 1. N 2 selectivity and NO x maximum conversion for NO reduction by decane on precious metal over alumina and Cu/ZSM-5 catalysts.
copper ruthenium rhodium palladium iridium platinum
N20 selectivity at NO x maximum conversion in % 3 2 20 26 3 75
NO x maximum conversion in % 76 6 23 17 22 64
Platinum is therefore the only active catalyst where NO 2 formation is important and could compete or favour NO reduction. Moreover platinum is the more selective for the N20 formation. Therefore, we decided to study especially the formation of NO 2 and N20 on platinum catalysts during NO reduction. 3.3.2. NO reduction on alumina supported platinum catalysts
3.2.2.1. Influence of temperature
In the presence of n-decane in the feed, NO is reduced to N 2 and N20 at low temperature. The reduction rate goes through a maximum at 220~ and then decreases slowly to become nil near 320~ Above 230~ NO oxidation and NO reduction occur simultaneously. For example, at 260~ 31% of NO is reduced (9% in N 2 and 21% in N20 ) and 14 % of NO is oxidise in NO 2 (Fig. 7). In order to discriminate if oxidation and reduction of NO are simultaneous or consecutive we studied the influence of the contact time. This was achieved by cutting the monolith at different lengths: 19.38 to 76 mm. The dynamics of the system was kept constant by adding an inactive catalyst to maintain the same reactor length. Results obtained at 220~ and 260~ are presented on figure 8. They are expressed as a function of the relative length of active catalyst.
219 50 -"-N2
40-
/'~
----N20
30~o t,..,i
o 9 2; 0
20
/i
10-
|I
I
_,,~ ~-
0 100
~ ,,r,-',.,,=,,,,,,,.",,,'"'"-"
,~/ -~,__.
200
,
300
Ham
400
500
Temperature (~ Figure 7. NO conversion in N 2, N20 and NO 2 in presence of n-decane on platinum catalyst (GHSV = 50 000 h l ; 6000 ppmC n-decane, 600ppm NO, 500 ppm CO, 5% 0 2, 10% CO 2, 7% H20, 20vpm SO2).
& o
~9 (1.) e,~
o:
700
7000
600
-6000
7OO 600
C
500
- 5o00
~
400
-4ooo
8 ~
300
r 3000
o1,,.~ 9
_.
L~ 100 ~ o o
50
lOOO ,
N20
0
N2
lOO
Relative lengths (%)
;
--t:~ NO2 ---o- HC
6000
& = 500
2000~_._NO
2oo
7000 r,.)
"~ o
r,.)
..~
c~
5000 . .
400
4000
300
3000
200
2000 r,j
lOO
1000
=
(1) O o
,~o 0
50
100
Relative lengths (%)
Figure 8. N2, N20 and NO 2 formation as a function of the relative length of active catalyst at 220 and 260~ (GHSV - 50 000 h -I ; 6000 ppmC n-decane, 600ppm NO, 500 ppm CO, 5% 02, 10% CO2, 7% H20, 20vpm SO2). When the temperature is such that HC oxidation is about 90%, N 2, N20 and NO 2 are observed at the outlet of the catalyst. But this results from an integral effect: indeed NO reduction to N 2 and N20 occurs at the monolith inlet while NO oxidation to NO 2 occurs at the monolith outlet. On platinum catalyst, NO reduction and NO oxidation do not occur in the same conditions. NO 2 is observed at the outlet of catalyst only when hydrocarbon concentration in the feed becomes low. But NO is no more reduced in these conditions. In the first part of this work we concluded that NO 2 reduction by hydrocarbon in the gas phase is negligible in our reaction conditions. Therefore, NO 2 is not desorbed in gas phase
220 during NO reduction. If NO 2 is an intermediate in NO reduction mechanism, it could participate only as adsorbed species.
3.2.2.2 Selectivity to N20 and N 2 On platinum catalysts, NO x reduction produces N 2 and N20. We studied the influence of the nature of the hydrocarbon on N 2 0 selectivity and N 2 yield in case of alkanes, alkenes and aromatics of various chain length (% N20 selectivity = 100 - % N 2 selectivity). The N 2 0 selectivity seems to be strongly dependant on the hydrocarbon nature [12]. On platinum catalyst, it varies from 34 to 80 % as a function of the hydrocarbon type (Table 2). Burch et al. [ 12] indicated that N 2 selectivity is 100% (0% of N20 ) in the absence of water if NO reduction is carried out with toluene. But in the presence of a complete mixture including water, CO 2 and SO2, similar N 2 0 selectivities are obtained with ethylene and toluene (34%). Table 2. Selectivity in N 2 0 and yield in N 2 as a function of the hydrocarbon nature. NOx maximum conversion % n-octane n-decane decaline dodecane ethylene propylene toluene xy,!ene
50 59 36 30 23 25 39 26
N20 selectivity N 2 yield at at NO x the NO x maximum maximum conversion conversion % % 80 73 75 65 37 40 38 65
10 16 9 10 15 15 24 9
Temperature at NO x maximum conversion ~ 220 217 230 260 305 311 295 288
HC halfconversion temperature ~ 221 218 222 290 302 320 281 285
N 2 0 selectivity is higher with alkanes than with unsaturated molecules except xylene. N 2 0 selectivity is no much dependent on the temperature for maximum NO x conversion. NO x reduction occurs at the beginning of HC oxidation. HC oxidation characteristics on platinum catalyst depends strongly on their adsorption strength [13,14]. This explains why the temperatures of HC oxidation and NO reduction vary similarly as a function of hydrocarbon nature. N 2 0 selectivity stays constant as a function of temperature during NO reduction by one hydrocarbon type. It seems to depend strongly on HC oxidation mechanism. Long chain alkanes lead to a higher N20 selectivity than unsaturated molecules even if NO reduction occur in the same temperature range. The choice of hydrocarbon nature determines N 2 and N20 selectivities. But the temperature range of NO conversion and the yield in nitrogen are function of the chain length (Table 2). On platinum catalysts, the best nitrogen yield are obtained with long chain alkanes at low reduction temperature and with unsaturated hydrocarbons like toluene and ethylene at higher reduction temperature.
221 at low reduction temperature and with unsaturated hydrocarbons like toluene and ethylene at higher reduction temperature. 3.2. 2. 3. Kinetics of NO reduction The influence of the reactant concentrations on the reaction rates has been studied at low conversions on platinum catalysts in order to determine the partial reaction orders and apparent activation energies both for NO reduction and HC oxidation. A negative order is obtained for NO, a positive order for 0 2 and either a negative order for HC if it is an olefin or a positive if it is an alkane [ 15] (Table 3). Table 3. Activation energies and partial reaction orders for NO reduction by ethylene or decane.
Ea (Kcal/mol) Partial order HC NO 02
ethylene HC oxidation NO reduction 34 26 - 0,9 - 0,9
- 0,7 - 0,3
n-decane HC oxidation NO reduction 43 37 0,8 - 1,0 1,8
1,0 - 1,0 1,6
Hydrocarbon oxidation is more actived than NO reduction. This explains that whatever the reductant, NO reduction rate increases more slowly with temperature than HC oxidation rate. NO shows an inhibiting effect on HC oxidation and NO reduction whatever the nature of the reductant. Ethylene is more strongly adsorbed than n-decane and shows inhibiting effects both on HC oxidation and NO reduction. Large ethylene concentrations cause a shift of NO reduction to high temperatures. Decane is more smoothly adsorbed and promotes NO reduction: the temperature for NO reduction decreases when decane concentration increases. Strongly adsorbed hydrocarbons induce an inhibiting effect on NO reduction and on HC oxidation. Their covering of metal surface is high so that the reaction of NO reduction, HC oxidation and NO oxidation are shifted to higher temperatures (Fig. 4, Table 2). Moreover, the N 2 selectivity is higher with olefins than with long chain alkanes. N20 and N 2 formation could depend on the covering of metal by hydrocarbon. Hydrocarbon adsorption strength may be an important parameter in the reduction selectivity. 4. CONCLUSION Under Diesel exhaust gas conditions, only platinum and copper supported catalysts allow high NO reduction activity. Copper practically does not give any N20, but platinum is very selective for N 2 0 formation. Unlike other transition metals, platinum is a good catalyst both for NO oxidation by oxygen and NO reduction by hydrocarbons. In the presence of hydrocarbons, NO 2 is observed at low HC concentration. But NO is not reduced under these conditions. If NO 2 is an intermediate in NO reduction mechanism, it could participate only as adsorbed species. On platinum catalysts, the kinetics of HC oxidation, NO reduction and NO oxidation are strongly dependant on the hydrocarbon adsorption strength. These mechanisms occur at higher temperature with olefins than with long chain alkanes.
222 N20 selectivity is higher with long chain alkanes than with unsaturated molecules. N20 and N 2 formation could depend on hydrocarbon nature and on the metal covering by hydrocarbon.
ACKNOWLEDGEMENTS Part of this work was carried out with the financial support of ECE (Brite Euram project BRE2-CT92-0192) REFERENCES
1. R. Burch, P.J. Millington and A.P. Walker, Applied Catalysis B, 4 (1994) 65. 2. N.W. Hayes, R.W. Joyner, E.S. Shpiro, Applied Catalysis B, 8 (1996) 343. 3. G.R. Bamwenda, A. Ogata, A. Obuchi, J. Oi, K. Mizuno, J. Skrzypek, Applied Catalysis B, 6 (1995) 311. 4. T. Beutel, B. Adelman, W.M.H. Sachtler, Catalysis Letters, 37 (1996) 125. 5. F. Poignant, J. Saussey, J.C. Lavalley, G. Mabilon, Catalysis Today, 29 (1996) 93. 6. C. Gaudin, D. Duprez, G. Mabilon, M. Prigent, Journal of Catalysis, 160 (1996) 10. 7. K.A. Bethke, C. Li, M.C. Kung, B. Yang, H.H. Kung, Catalysis Letters, 31 (1995) 287. 8. T. Tanaka, T. Okuhara, M. Misomo, Applied Catalysis B, 4 (1994) L 1. 9. M. Guyon, V. Le Chanut, P. Gilot, H. Kessler, G. Prado, Applied Catalysis B, 8 (1996) 183. 10. A. Obuchi, A. Ohi, M. Nakamura, A. Ogata, K. Mizumi, H. Ohuchi, Applied Catalysis B, 2 (1993) 71. 11. A. Obuchi, A. Ogata, H. Takahashi, J. Oi, G.R. Bamwenda, K. Mizuno, Catalysis Today, 29 (1996) 103. 12. R. Burch, D. Ottery, Applied Catalysis B, 9 (1996) L 19. 13. G. Mabilon, D. Durand, Ph. Courty, Catalysis and automotive pollution control III, Studies in surface science and catalysis, A. Frennet and J.-M. Bastin (Eds.), Elsevier, 96 (1995) 775. 14. Y.F. Yu Yao, Journal of Catalysis, 87 (1984) 152. 15. G. Mabilon, D. Durand, Catalysis Today, 17 (1993) 285.
223
Mechanistic investigation on the selective reduction of NO with propene in the presence of oxygen over supported platinum S. Eckhoff a, D. Hesse a, J.A.A. van den Tillaart b, j. Leyrer b, and E.S. Lox b
a Institute for Technical Chemistry, University of Hannover, 30167 Hannover, Germany b Automotive Catalysts Division, Degussa AG, P.O. Box 1345, 63403 Hanau, Germany
ABSTRACT The selective reduction of NO with propene in the presence of oxygen over a Pt/alumina catalyst has been investigated using TAP and model gas equipment. Experiments with different gas compositions (stoichiometric and overstoichiometric with respect to the complete oxidation of propene) were carried out at temperatures between 473 and 673 K. Additionally, the NO decomposition on reduced and oxidised Pt/alumina was studied. It is shown that N2 is generated due to NO dissociation and following recombination of Nadatoms. Associatively adsorbed NO needs to be present on the surface to form N20. 1. INTRODUCTION The emission of nitrogen oxides (NOx) from automotive and stationary sources causes serious environmental concern. Automotive exhaust gas aftertreatment systems are commonly based on precious metal catalysts (three way or diesel oxidation catalysts). One undesired effect during NOx reduction with these catalysts is the formation of N20, which is now considered to be an environmental pollutant also [ 1,2]. In this report the generation of N2 and N20 during NOx decomposition or reduction on Pt/alumina is investigated. It has long been established that platinum is active for the decomposition and reduction of NO [3-6] and that this reaction is inhibited by oxygen [7-10]. The formation of N2 is reported to take place over platinum in the presence of NO and 02 according to the following elementary reaction steps [3,4,7]: NO NO* 2 N*
+ +
* ~ * ~ --~
NO* N*
+
O*
(1) (2)
N2
+
2*
(3)
In these reaction equations a free surface site is represented by an asterisk, *. The formation of N20 is not well studied until now [7] but it is reported that N20 is only weakly adsorbed on platinum [11] and that platinum is not an active N20 decomposition catalyst [ 1,16]. Different mechanistic pathways for the formation of N20 over platinum in the presence of NO and 02 are considered in general:
224 2 N* N* NO* N*
+ O* + NO + NO* + NO*
~_~ ~~ ~
N20* N20* N20* N20*
+
2*
+ +
O* *
(4) (5) (6) (7)
It is shown that some of the reaction paths above can be excluded from the results reported in this study.
2. EXPERIMENTAL 2.1. Catalyst preparation and characterization A 1 % - w t platinum supported on alumina catalyst was used throughout this study. This catalyst was prepared by a proprietary incipient wetness method with tetraammine platinum (II) hydroxide, Pt(NHa)4(OH)2, as precursor. After drying in air at 393 K for 2 hours the catalyst was calcined in air for 3 hours at 623 K and reduced at 803 K in flowing hydrogen for 3 hours. The average platinum particle diameter, measured by both CO-chemisorption and TEM, amounted to 2 nm. The total exposed surface area of the platinum amounts to 1.4 mE/g. The BET surface area of the used A1203 was 92 m2/g. 2.2. TAP set-up Most of the experiments described in this study were performed in a so called TAP (Temporal Analysis of Products) apparatus. This apparatus consists essentially of a micro reactor, two high speed pulse valves, and a fast detecting mass spectrometer together with the necessary data acquisition and control systems. The pulse valves can generate up to 40 pulses per second of 1013 up to 1019 molecules with a pulse width of typically 1 ms. With the fast detecting mass spectrometer at the reactor outlet the signals of reactants, products, and intermediates can be monitored in time. A more detailed description is given by Gleaves and co-workers [12]. The TAP reactor was loaded by placing 200 mg of the catalyst granulated to 250-500 ~tm particles between two beds of a-alumina and two stainless steel meshes. In general the amount of molecules introduced per pulse was adjusted in the range of 0.5 - 1 % with respect to the total number of platinum surface atoms. All experiments were carried out with 15NO to differentiate between N2 and CO as well as between N20 and CO2. The NO and N2 responses were corrected to account for fragmentation of N/O in the mass spectrometer to NO and N2. Also the CO response was corrected to account for the CO fragment of CO2. Three different pulse techniques were used in this study: a) single pulse, b) multipulse experiments, where a series of pulses is introduced, and c) pump-probe experiments, where two different pulses are alternately introduced at a user-specified time interval At. All single pulse and pump-probe experiments consisted of 40 pulse cycles of 3 seconds duration. Directly before the measurement 5 initial precycles were given. The responses of the 40 cycles were averaged to improve the signal/noise ratio. Two different gas mixtures were used in the experiments (see Table 1). Argon was used as the internal standard.
225 Table 1. Composition of gas mixtures used in TAP experiments (molecular amounts relative to C3H6) and in model gas experiments (concentrations). mixture TAP experiments Model gas experiments
A istoichiometric)
B (overstoichiometric)
valve A 15NO
valve B 02 / C3H6
1 3
4/ 1 12 / 1
NO
02
[vppm] 600 600
[vppm] 2400 2400
C3H6 ,
[vppm] 600 200
2.3. Model gas test setup The model gas test setup used in this study has already been described in literature [13]. The gas compositions used are given in Table 1. A substrate (NGK, 62 cell/cm2, wall thickness 200 lxm) was coated with the active Pt/A1203 powder (120 g/l) and measured in the model gas setup with a space velocity of 50000 hr 1. Nitrogen was used as the carrier gas.
3. RESULTS 3.1.NO pulses over reduced and oxidised catalyst Figure 1 shows the responses for NO, N2 and N20 during a multipulse NO experiment over a reduced catalyst at 473 K. The catalyst was reduced in-situ in flowing hydrogen at 473 K. No NO2 and 02 was observed during this experiment.
Figure 1. N2, NO and N20 responses during a NO multipulse experiment on a prereduced catalyst at 473 K. Initially, NO adsorbs dissociatively on the reduced surface forming N- and O-adatoms. At this time no NO is detected at the reactor outlet. The N-adatoms recombine to form N2. The O-adatoms remain on the surface as no 02 respons can be observed during this experiment. After a certain induction time, NO is quantitatively converted into N2. The occurrence of an induction time indicates that the concentration of N-adatoms increases until a pseudo steady state is reached. After about 100 NO pulses the signals for NO and N20 increase
226 simultaneously. At this point the active surface is probably almost completely covered with oxygen adatoms from the dissociative NO adsorption. The dissociative NO adsorption becomes increasingly inhibited by the decrease of unoccupied surface sites. Consequently, the N2 formation decreases due to a lower N-adatom concentration. In contrast, the production of N20 increases at this time. This indicates that associatively adsorbed NO is necessary for the generation of N20. Reactions 4) and 5) can therefore be excluded because N20 production would also occur over a reduced surface for these reactions. After about 160 NO pulses the N20 production reaches a maximum. If N20 is formed via reaction 7) the N20 production could indeed be maximal when at this moment the product of the concentrations of associatively adsorbed NO and N-adatoms is maximal. However, when the concentration of associatively adsorbed NO reaches a maximum at this point, N20 formation via reaction 6) can not be excluded. The same trends were observed for a similar multipulse experiment at 673 K. When NO was pulsed at 673 K over an oxidised catalyst, upto 5% conversion into N2 was observed. This observation is in sharp contrast with the observations from Butch et al. [3] who did not observe any N2 production over an oxidised catalyst. Figure 2 shows the normalised N2 formation over a reduced and an oxidised platinum surface. The N2 formation takes place more slowly over an oxidised surface compared with a reduced surface. On a reduced catalyst the N2 signal is close to the Ar signal (N2 comes earlier than Ar because of mass discrimination due to the total diffusion processes) implying a very fast production of N2. NO reacts immediately at contact with the reduced platinum surface. The shape of the N2 peak over an oxidised catalyst resembles closely the corresponding NO peak shape. This indicates that the N2 formation takes place after, on average, very many contacts of NO with the surface.
1.0 (~
0.8
"~
0.6
E: (/)
0.4 0.2
Z
(pre-reduced)
o.o 0.0
0.1
0.2 0.3 0.4 0.5 Time [s] Figure 2. Normalised (peak height=l) N2 formation during a NO pulse over an oxidised and a reduced surface at 673 K. The surface area of the N2 response over a preoxidised surface amounts to only 5% of that on the prereduced surface.
227 To understand the effect of 02 during NO decomposition, the catalyst was first oxidised insitu with 1802. Then NO was pulsed, followed by an 1802 pulse 1.5 seconds later to stabilise the 180-adatom coverage on the platinum. At 513 K the NO left the reactor continuously (baseline increase of NO signal on the MS). At 673 K almost all NO left the catalyst within 5 seconds. At 513 K and 673 K about 10% of the NO coming out of the reactor was NI80. This result suggests that NO adsorbs mainly associatively on an oxidised surface as dissociation and subsequent reassociation should yield a very high isotope exchange. The peak shapes of NO and Nt80 are identical at 673 K. This suggests that the reaction leading to the product N180 is a fast process compared to the total retention processes of NO in the reactor. Otherwise the N180 signal should come somewhat retarded to the NO signal. Although no NO2 could be observed during the TAP experiments, oxygen exchange due to the formation of NOl80 and subsequent decomposition can not be excluded here. However, the formation of N180 by dissociation of adsorbed NO (reaction (2)) and subsequent reassociation of the adsorbed N-adatom with an 180-adatom is more likely (reverse of reaction (2)). 3.2. NO reduction in the presence of 02 Figure 3 shows the responses of the nitrogen containing species during a pump/probe experiment with a NO pulse followed by a propene pulse over a surface preoxidised with 1802"
N2
NO ~ t/) t" --
N2 .z "
2
tO
t"
~ 0.6-
\\
I
~N20 ~ ~ , , ~ ~
1
\
~
0
o
~
T,meIs]
18 I
0.0
,
I
0.5
,
I
1.0
,
I
1.5
,
I
2.0
,
I
2.5
,
I
3.0
Time [s]
Figure 3. Responses of N-containing species during a pump/probe experiment (At=ls) with NO and C3H6 respectively over a platinum surface preoxidised with 1802 at 673 K. The inset shows the normalised responses during the NO pulse. The amount of NISo formed is much higher than the amount formed in the single pulse experiments over an oxidised surface. At 573 and 673 K about 50 % of the outcoming NO is detected as NI80. At 513 K 30 % of the outcoming NO is detected as N180. As the pump/probe pulses are cycled for signal averaging the surface will be partly reduced in this experiment. This result clearly demonstrates that on a partly reduced platinum surface NO will
228 more often adsorb dissociatively than on an oxidised platinum surface as more N2 and N180 is formed. These results do still not unambiguously differentiate between the two proposed mechanisms of NO/NlsO isotope exchange. However, because the oxidation of NO to NO2 is thought to be more probable over an oxidised surface than over a partly reduced surface, these results support the dissociative mechanism for the NO/N180 isotope exchange. As both NO and NlsO are present, one would also expect that N2180 is formed. However, almost no N2180 is detected. At 573 K also little N2180 was observed. However, at this temperature a clear delay of NlSO compared to NO was observed. This indicates that at 573 K the formation of N180 is a slow process compared to both the total diffusion processes of NO through the reactor and to the formation of N20 and N2. As N20 is only formed during the initial phase of the NO pulse, no N2180 will be formed as NISo is formed later in the process. The formation of N~80 at 673 K is probably also slow compared to the formation of N20 and N2, but is faster as the total diffision processes of NO, hence no retardation of Nl80 compared to NO is observed. Figure 4 shows the responses of all components observed during a typical pump/probe experiment. In this experiment propene/O2 from the stoichiometric mixture A was pulsed followed by a NO pulse one second later at 573 K. The sharp 02 peak during the propene/O2 pulse indicates that propene is not completely converted at this temperature. CO2 is formed mainly during the propene/O2 pulse and only a small extra amount is generated during the NO pulse. This increased CO2 formation during the NO pulse is probably caused by the direct reaction of NO with carbonaceous residues on the surface or due to the reaction of O-adatoms, formed by the dissociative NO adsorption, with the carbonaceous residues [14,15]. Some NO comes through the reactor during the NO pulse and a small amount of NO is released during the propene/O2 pulse. N2 and N20 are mainly formed during the NO pulse but small amounts are also generated from the remaining N-adspecies during the propene/O2 pulse. N2 is formed during the propene/O2 pulse in a double peak suggesting two different reaction mechanisms. No propene could be detected at the outlet of the reactor.
2.5 i,---i
~>' t-. Q.) e-
.--.
O2/2
Ar/50
2.0 N2j
1.5 1.0 0.5 0.0
~C3H6 I
0.0
,
I
0.5
,
I
1.0
I
'
1.5
I
Time [s]
......
21.0
i
21.5
i
3.0
Figure 4. Responses of observed components in a pump/probe experiment with NO and O2/C3H6 from stoichiometric mixture A over an oxidised platinum surface at 573 K.
229 0.20
-
0.15
••--•at rl
At = 0 . 0 1 s
"~ ~.,-, r- 9 0.10 0.05
= 0s
V
zxt = l s ~
I
0.00 ~ ~ 0.0
1 0.5
1.0
~ 1.5
2.0
l 2.5
3.0
Time [s] Figure 5. N2 formation in a pump/probe experiment with O2/C3H6 and NO from overstoichiometric mixture B over an oxidised platinum surface at 573 K as a function of the offset time, At, between the two pulses. The Nz response after a single pulses of NO/Ar over an oxidised catalyst is depicted for reference. Figure 5 shows the N2 production at different offset times between the O2/propene and the NO pulse with mixture B at 573 K. It was found that the amount of N2 (and also N20) formation decreases with increasing offset time At from 0 to 1 second for the lean mixture B. The amount and peak shape of the N2 formed during the NO pulse at At = 1 second are identical to those observed when NO is pulsed over a preoxidised surface. This indicates that one second after the propene/O2 pulse no residual carbonaceous species are present on the surface which can reduce NO. When the next propene/O2 pulse enters the catalyst, two seconds after the NO pulse, still some N containing adspecies are on the surface as some N2 (and also N20) formation is visible. At At = 0 and 0.01 seconds more oxygen leaves the catalyst after the propene/O2 pulse then at larger offset times. Also more N2 is formed during the NO pulse at At = 0 and 0.01 seconds. This effect can be caused by a competition between O2 and NO for a direct reaction with propene or reaction products of propene. Another possibility is that 02 and NO compete for reduced adsorption sites. Rottlander et al. [ 14] recently reported similar results with TAP experiments on a Pt/ZSM-5 catalyst. They proposed that carbon containing surface species, formed from propene, are mainly responsible for the NOx reduction at T < 600 K. With the stoichiometric mixture A the amount of N2 and N20 formed was independent from the time interval between the propene/O2 pulse and the NO pulse. Burch et al. [3] observed from similar TAP experiments with lean mixtures of propene/O2/NO that the N2 yield did not change significantly with increasing At.
230 3.3. Correlation between model gas and TAP experiments TAP experiments are performed at operating conditions which are quite far from operating conditions in automotive converters; pulse conditions, low pressure (10 -4 - 10-5 Pa) and low absolute concentrations. To find out in how far conclusions from TAP experiments are still applicable to the automotive converter, correlation experiments were performed on the model gas test setup using gas compositions as given in Table 1. temperature TAP measurements [K] 400 100
450 i
500 ,
550
i
,
600
i
, I 13. ............ ...'"
80 84
.
. "'"":
temperature TAP measurements [K]
700
650 ,
i'
0-4000 1
,"
-.....
~
t
J
i o
40-
---o-- TAP mixture B
.".
O ~ 20Z
xl"
N2
""-.
13.:"
s~o
,
500
,
~,o
,
~o |
,
o! ..... , " ........... |
700
i
7so
~
,
"
"-. "'or'"
" o ....
........
~O/o] 40-
~."
O
550
,
600 i
,
650
| ....~
700
qt o,
temperature model gas measurements [K]
""'?~
.
....tr. MG mixture A ....c~... MG mixture B + TAP mixture A = TAP mixture B
200
~oo
" .......
.... 9". .. . . ......
ivit 60"
....: .... MGpmi~Urr: B
.... (/
,
o ,, ....
ect
:"
..."
500
!
"'"'[3
sel
t'-- 60.0
450
,
9
500
5~o
'
6~o
'
6~o
7~o
7~o
~oo
temperature model gas measurements [K]
Figure 6. NOx conversion (left) and N2 selectivity (right) for both model gas (MG) and TAP measurements as function of the temperature. Two different temperature scales are used to show correlation. Figure 6 shows the NOx conversions and N2 selectivities for both model gas and TAP experiments with the stoichiometric and lean mixture as function of the temperature. The TAP measurements were carried out by simultaneously pulsing both valves. No NO2 formation was observed in TAP experiments in contrast with model gas experiments, therefore conversion and selectivity were calculated for the reaction ofNOx (NOx - NO + NO2) to N2 and N20. The equilibrium of NO and NO2 in 02 is given by: 2NO
+
02
,~
2NO2
(8)
This equilibrium will shift to the left side at increasing temperature and decreasing pressure. This could explain the absence of NO2 in the TAP measurements. When the temperature scale for the TAP measurements is adjusted to higher temperatures by approximately 100 K, a quite good correlation between the model gas and the TAP experiments is found for both the NOx conversion and the N2 selectivity and for both the stoichiometric and the lean gas mixture. The NOx conversion increases for the stoichiometric mixture with increasing temperature. The NOx conversion for the lean mixture shows a maximum in conversion at approximately 575 K (model gas temperature). The selectivity towards N2 shows for both gas mixtures a minimum around 575 K. This is of course equivalent to a maximum in N20 selectivity. In contrast to the conversion of NOx, the selectivity towards N2 is not strongly depending on the composition. From these figures it appears that the higher 02 concentration in the lean gas mixture compared to the
231 stoichiometric mixture only influences the NO• conversion over a platinum catalysts at temperatures higher than 575 K. 4. CONCLUSIONS It was verified in this study that NO adsorption, dissociation and adsorbed N-atom recombination is the reaction path for N2 formation on reduced supported platinum catalyst. Two clear tendencies were observed for the formation of N20: decreasing temperature and increasing oxygen adatom coverage on the catalyst increase the selectivity to N20. Both observations correspond with a higher coverage of associatively adsorbed NO on the catalyst and indicate that associatively adsorbed NO leads to N20 generation on polycrystalline platinum. At high temperatures associatively adsorbed NO will desorp quickly. At low oxygen coverages molecular adsorbed NO will dissociate. Further it is shown that the NOx conversions and the selectivities for N2/N20 observed within TAP experimems correlate well with the results from model gas tests. Therefor the mechanisms of N2 and N20 formation are probably identical in both model gas and TAP experiments. REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
Y.Li and J.N. Armor, Appl. Catal. B: Environmental, 1 (1992) L21-L29 M.A.K. Khalil and R.A. Rasmussen, Tellus, 35B (1983) 161 R. Burch, P.J. Millington, and A.P. Walker, Appl. Catal. B: Environmental, 4 (1994) 65-94 R.I. Masel, Catal. Rev.-Sci. Eng., 28 (2&3) (1986) 335-369 E. Shustorovich and A.T. Bell, Surf. Sci., 289 (1993) 127-138 R. Burch and P.J. Millington, Catal. Today, 29 (1996) 37-42 K.J. Lim, D.G. LOftier, and M. Boudart, J. Catal., 100 (1986) 158-166 H. Miki, T. Nagase, T. Kioka, S. Gugai, and K. Kawasaki. Surf. Sci., 225 (1990) 1-9 M.J. Mummey and L.D. Schmidt, Surf. Sci., 109 (1981) 29-42 A. Amirnazmi and M. Boudart, J. Catal, 39 (1975) 383-394 N.R. Avery, Surf. Sci., 131 (1983) 501-510 J.T. Gleaves, J.R. Ebner, and T.C. Kuechler, Catal. Rev.-Sci. Eng., 30 (1988) 49 J.A.A. van den Tillaart, J. Leyrer, S. Eckhoff, and E.S. Lox, Appl. Catal. B: Environmental, 10 (1996) 53-68 14. C. Rottlander, R. Andorf, C. Plog, B. Krutzsch, and M. Baerns, Appl. Catal. B: Environmental, 11 (1996) 49-63 15. S. Lacombe, J.H.B.J. Hoebink, G.B. Marin, Appl. Catal. B: Environmental, 12 (1997) 207-224 16. S.Imamura, N. Okamoto, Y. Saito, T. Ito, H. Jindai, Sekiyu Gakkaishi, 39, (5) (1996) 350-356
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CATALYSIS AND AUTOMOTIVEPOLLUTIONCONTROLIV Studies in Surface Science and Catalysis, Vol. 116 N. Kruse, A. Frennet and J.-M Bastin (Eds.) 91998 Elsevier Science B.V. All rights reserved.
233
Platinum-titania-sepiolite monolithic catalysts for the reduction of nitric oxide with propene in lean-burn conditions P. Avila, J. Blanco, C. Knapp and M. Yates Instituto de Catfilisis y Petroleoquimica. CSIC. Camino de Valdelatas s/n. 28049 Madrid
In this work the influence of the support preparation variables on the activity of Pt-titaniasepiolite monolithic catalysts for the reduction of nitrogen oxides with propene in lean-bum conditions was studied. The influence of raw materials were studied using five types of titania. The catalysts activities showed significant differences in the NOx conversions, which were related to the textural properties of the catalysts. Heat treating the support at 800~ induced a fall in the catalyst's activity compared to that treated at 500~ XRD and TG-DSC measurements indicated that this decrease was not due to phase changes either of the titania, or of the sepiolite. MIP and nitrogen adsorption/desorption results indicated that it was due to a large decrease of the surface area. EPMA-WDS line profiles of Pt, Ti, Mg and Si across a monolith wall section indicated that platinum is selectively deposited on titania and homogeneously dispersed within the wall. The influence of area velocity and steam on the activity were also studied. 1. INTRODUCTION The control of NOx emissions in lean-bum gasoline and diesel engines has become one of the most important challenges in environmental catalysis due to the difficulty of reducing nitrogen oxides in their typically humid, oxygen rich exhaust streams. Reduction with hydrocarbons is an attractive means of converting NO to N2 [ 1]. However, no industrially practical catalyst has been reported to date. Since the discovery by three groups working independently that Cu-ZSM-5 catalyses the catalytic reduction of NO by various hydrocarbons, much research has been carried out with this material [234]. However, these catalysts present major problems in terms of their thermal stabilities and sensitivities to water [5]. Another important system that is also being studied intensively is Pt/A1203, and generally Pt on different metal oxide supports [6], since this is already used in conventional (stoichiometric) exhaust gas cleanup and has proven stability and tolerance to typical potential poisons in the engine exhaust. Other systems such as Co-ZSM-5, Ga-ZSM-5, Cu-ZrO2 and H-ZSM-5 have been found to be active [7], but their industrial application has not been achieved yet. Most existing studies concentrate on the selection of active phase and support, and improvement in preparation methods. However, little attention has been paid to the possible influence of industrially produced raw materials' origin and nature, that could be of great importance looking forward to industrially applying a catalyst.
234 The aim of the present work was to study the activity of Pt/titania/sepiolite monolithic catalysts prepared with different kinds of titania, in the reduction of NOx with propylene under oxidising conditions. The influence of the source of titania, titania content, heat treatment, presence of water vapour in the gas exhaust and area velocity on the activity, and on the structure and properties of the catalysts were investigated. 2. EXPERIMENTAL Steady-state activity measurements were made in a monolithic reactor of 2.54 cm ID operating in an integral regime. The composition of the inlet gas was: 1000 ppm NO, 1000 ppm propene, 10% oxygen and 0-3% water. The area velocity ranged from 6 to 13 Nmh "I and the reaction temperature from 150 to 275~ Analysis of the inlet and outlet gas concentrations were made using specific analysers for each gas. Thus, the NO~ concentration was determined by chemiluminescence with a Beckman analyser model 951A, the oxygen by paramagnetic analysis in a Beckman 755 analyser and the hydrocarbons by means of a Beckman flame ionisation detector model 400A. The catalysts were prepared as monolithic structures of parallel channels of square section with a density of 8 cells/cm2 and wall thickness of 0.90 mm. All the monolithic supports were subsequently heat treated at 500~ if not otherwise stated, for 4 hours in an air atmosphere. A platinum content of 0.1 wt% was achieved by impregnation of the monolithic supports with an aqueous solution of chloroplatinic acid and subsequent reduction. The surface areas were measured by nitrogen adsorption at -197~ using a Micromeritics 2000 ASAP, and determined by application of the BET equation [8], taking the area of the nitrogen molecule as 0.162 nm2 [9]. The samples were outgassed overnight at 140~ to a vacuum of <10 .4 torr to ensure a dry clean surface, free from any loosely held adsorbed species. The pore size distribution, pore volume and surface areas were determined by use of mercury intrusion porosimetry (MIP) using Fisons Instruments Pascal 140/240 porosimeters. For these measurements the values recommended by the IUPAC [9] of contact angle 141 o and surface tension 484 mNm ~ were used. Powder X-ray diffraction (XRD) patterns were recorded on a Phillips PW1710 powder diffractometer in the 5-75 ~ (20) region using CuKot radiation: ~, - 0.1518 nm. The TG-DSC curves were measured on a Netzsch 409 EP Simultaneous Thermal Analysis device. Approximately 20-30 mg of powdered sample were heated in air at a rate of 5 ~ "~ from ambient to 1000~ using an o~-alumina reference. Electron Probe Microanalysis by Wavelength Dispersion Spectroscopy (EPMA-WDS) measurements were carried out in a Jeol JXA-8900M device. The monolith samples were previously set in a resin and polished with diamond gel. 3. RESULTS AND DISCUSSION
The results of this study have been divided into four sections. In the first, the influence of the titania nature and sepiolite content on the catalysts activities were studied. The second deals with the influence of the heat pretreatment on both the textural properties of the support and activity of the catalysts. The phase distribution of platinum on the titania-sepiolite supports
235 was studied in the third section. Finally, the effect of the area velocity and presence of water vapour in the exhaust were investigated. 3.1. Influence of the titania nature and content
To study the influence of the titania used in the support preparation on the properties of the catalyst, five different commercial titanias produced in industrial scale were used to prepare catalysts with 0.1 wt.% Pt supported on sepiolite/titania, 50:50, wt/wt, monoliths heat treated at 500~ The properties of the raw materials used to prepare the supports are shown in Table 1. With the exception of type A, which is a hydrated non-heat treated titania, all other types were calcined in the industrial production process and therefore no significant changes in the BET area with heat treatment were observed. The titania particle sizes were calculated from the BET surface areas, assuming that the heat treated titania was constituted by non-porous spherical particles. Their XRD patterns (not shown) indicate that they all had the anatase structure. Table 1 Textural properties of the raw materials Heat treated at 110 ~ Material Titania A " B " C " D " E Sepiolite
Heat treated at 500 ~
BET surface area, m2gq 330 81 55 12 90 277
75 70 55 10 90 149
Aggregate diameter*, nm
Particle diameter~, nm
800 700 75 350 600 150
20 22 28 65 9 -
* Calculated from MIP results of the powders # Calculated from the BET surface area From mercury intrusion porosimetry (MIP) measurements of the titania powders treated at 500~ the sizes of the aggregated particles were calculated assuming that the intruded volumes found corresponded to interparticulate pore voids between spherical "units" (see Table 1) [ 10]. The sizes obtained were, in four cases, more than one order of magnitude larger than the primary particle size, and were therefore assigned to larger aggregated units. The capability of titania to form aggregates in which the original particles maintain their identity has been described by the authors from scanning electron microscopy (SEM) results in a previous study [11 ]. Therefore, the unit size determined by MIP should correspond to aggregates formed by several particles that were not desegregated during the mercury intrusion. In the case of titania type C the MIP intrusion curve (not shown) followed a continuous volume increase with increasing pressure with an inflexion only at high pressures, indicating a much smaller
236 aggregate diameter. This could be assigned to a gradual destruction of the aggregates almost until total desegregation during pressurisation. The activities of the catalysts prepared with the five titanias containing 0.1 wt.% Pt supported on 50:50, wt/wt, titania-sepiolite monoliths are shown in Figure 1. These displayed variations of the maximum conversion, by more than 15%, and maximum conversion temperature, of 25 ~ As all supports used were prepared with the same sepiolite content and the same platinum quantity, the activity differences might be attributed to the differences in the titanias used in the support preparation.
60 o-e, 50
.o_ 40
,? "",
r>
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150
/."
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.
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,
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Figure 1. NOx conversion of catalysts containing different kinds of titania. Titania types (--) A, ( - - - - ) B, (---) C, ( - - - - - ) D and (. . . . ) E.. Textural properties of the supports were studied in order to understand the activity differences amongst the monolithic catalysts prepared with the different titanias. The mercury intrusion porosimetry curves of the supports are shown in Figure 2. These results indicated that the nature of the titania used strongly influenced the total pore volume, surface area and pore size distribution of the support. Following the model proposed by the authors in a previous study [11], the titania aggregates desegregate during the kneading process and the particles enter the swollen sepiolite fibrous structure, producing pores that are not present in a sepiolite monolith. Table 2 summarises the main textural properties of the five titania-sepiolite, 50:50, wt/wt, supports (named after the titania type used in their preparation). The BET surface area gives the total surface area, while the MIP surface area is due to pores larger than 7 nm in diameter. Thus, the differences between both areas were due to narrow mesopores of less than 7 nm diameter. Characteristic mesopores of the sepiolite [12] were detected for A, B and E, which also showed a bimodal MIP pore size distribution. From these results, taking into account the conversion order, A ~. B > E > C > D, observed in Figure 1, the highest activity achieved with support A and the lowest obtained with support D could be explained taking into account that their BET surface areas and total pore volumes were, respectively, the greatest and least of the series. However, the NOx conversions achieved with the other three catalysts, as their BET surface areas were similar, indicate a complex interaction of the textural parameters. Catalyst B showed a higher activity than should be
237 expected from its total pore volume and BET area, but not from its MIP surface area. From the high total pore volume and MIP surface area of C a higher conversion should be expected. The high activity of A, B and E might indicate that the improved activity was due to their bimodal pore size distributions due to the presence of sepiolite mesopores (10-18 nm diameter). From the results in Table 2, titanias C and D were shown to be more easily desegregated and therefore could block these mesopores in the supports. 0.8
0.7
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Pore Diameter, nm
Figure 2. Mercury intrusion porosimetry of titania-sepiolite 50:50, wt/wt, supports prepared with five different types of titania and heat treated at 500~ Table 2 Textural properties of titania-sepiolite 50:50, wt/wt, supports Mercury Intrusion Porosimetry Titania type A B C D E
Total Pore Volume, mEg "1
0.69 0.62 0.69 0.51 0.61
Surface Area, m2g"1 95 110 97 56 91
N2 adsorption
Pore Diameter, nm 18 18 10
65 40 32 55 67
BET Surface Area, m2g"l 133 111 100 73 113
These results give a picture of the complex influences of the parameters that could affect preparation of catalysts at industrial scale. Firstly, as was observed, the nature of the raw materials strongly influenced the textural properties of the support, affecting the activity of the catalyst. Secondly, not only the BET surface areas of the raw materials and catalyst were of
238 importance when trying to improve its activity, but also its porosity total pore volume, surface area and pore size distribution. These results indicate that a high surface area, large total pore volume and bimodal pore size distribution (mesopores + macropores) should be beneficial to improve the catalyst's activity, but the interaction between all three parameters is not clear and should be further studied. Additionally, the influence of other parameters, e.g. the surface hydration degree, the presence of additives added by the producer, etc., could be of importance and is currently being studied. The influence of the titania content was studied by measuring the activity of three catalysts containing 30, 50 and 70 wt.% of titania type B. The resulting NO~ conversions (not shown) indicated no major differences between the activities of these three catalysts. These results indicated that the Pt dispersion was not adversely affected by a decrease in the available titania surface. Taking into account the previous results, a catalyst with 0.1 wt.% Pt supported on a 50:50, wt/wt, sepiolite/titania A support, designated catalyst A, was selected for the following studies. 3.2. Effect of heat treatment: Pore volume vs. surface area
The heat treatment of support A previous to impregnation of the active phase has a strong effect on the catalyst activity. Figure 3 shows a decrease of the NOx reduction activity when the support is pre-treated at 800~ compared to that pre-treated at 500~ 60 o--*, ~- 50 .o 40
-
(D
> 30
t-
o 20 O
z
/I 111~
10 0 150
175 200 225 250 Temperature, ~
275
Figure 3. NOx conversion of Pt supported on support A treated at (--) 500 and (---) 800~ Although it is well known that the anatase to futile phase change takes place in the range 550-700~ previous studies have shown that the presence of sepiolite retards this process [12]. The XRD spectra of support A treated at 500, 800, 1000, and 1200~ are shown in Figure 4. Up to pretreatment at 800~ the titania phase present is anatase. However, after heating at 1000~ the phase change: anatase --~ rutile was not completed. From calculation of the relative intensities of the principal peaks for anatase and rutile the conversion under these conditions was found to be only ca. 25% [13]. Even treatment at 1200~ was only sufficient to cause an 80% conversion to rutile. The other main component in the support, sepiolite, can also undergo transformation to enstatite above 830~ [14]. The XRD patterns of the support pre-treated at temperatures below 1000~ (Figure 4) showed no enstatite peak. In order to further study if this transformation could be responsible for the effect of heat pretreatment on the activity, the TG-
239
DSC curves of sepiolite and support A from 25 to 1000~ were recorded, and are shown in Figure 5. The peak observed at 830~ in the DSC curve of sepiolite and of the support indicates that the sepiolite to enstatite transformation takes place above the treatment temperature of both catalysts (500 and 800~ and is not affected by the presence of titania.
I --.-E dR ~ it} t"
(d)
(P r ..==.
=>
(c)
~
tli n," .===.
~ 0
(
a
i
i
10
20
) i
i
30 40 Angle, 20
i
i
50
60
,
70
Figure 4. XRD patterns of support A treated at (a) 500, (b) 800, (c) 1000 and (d) 1200~ showing the principal peaks for anatase (A), futile (R) and enstatite (E).
i.,
,TG
!
!
,
!
9
!
9
"k~--
\._~.~._~____ (a)
-10
0
,
-20
t 1
~
es
~ O
.,o
-20 , ~ . D S C 0
,,
,
,
200 400 600 800 1000 Temperature, ~
Figure 5. TG and DSC curves of (a) sepiolite and (b) support A.
240 From the MIP curves of support A heat treated at 500 and 800~ (not shown), although the total pore volume was maintained, the mesopores associated with sepiolite (18 nm) were lost and a shift in the pore size of the wider pores due to titania (65 to 90 nm) was observed. An associated fall in the MIP and BET surface areas from 95 to 46 m2/g and 133 to 60 m2/g, respectively, was found. These changes in the support would explain the decrease in activity, pointing out the importance, not only of the surface area, but also of the pore size distribution and the presence of sepiolite mesopores. A change in the available titania surface in the support should not affect the Pt dispersion as indicated by the results with different titania contents in Section 3.1. 3.3. Platinum phase distribution
Intensity, Counts Mg
I
3oo0 2000 1000
mm
_.
I
0.0
.....
I
........
I
__
I
I
i 0.5
,
0.5
I
I
I
I
i
_
1.0
Intensity, Counts
si 6oof mm
ha.
"1
"'1
j.
~176
,
0.0
,,
!
,
Ill 1.0I
,
_1
Intensity, Counts
Ti
,ooooL/ /
mm
t
11
t
0.0
.
.
0.5
.
.
Intensity, Counts Pt
IL
90 L .
'Jil
uL
-
-
t
L_
t
-1
-
1
I ......
J
1 ......
i
1.0 I
60 30
mm
0.0
,I,
0.5
"1
1.0
I
Figure 6. EPMA-WDS line profiles of Pt, Ti, Mg and Si across a wall section of catalyst A. Previous studies on the particle size and dispersion of Pt on TiO2 catalysts indicated that the noble metal was well dispersed on titania [15]. In order to study the platinum phase
241 distribution on the support, EPMA-WDS line profiles of Pt, Ti, Mg and Si across a wall section were recorded (Figure 6). It could be observed that the platinum distribution was not constant, but regular across the wall, indicating no concentration gradient. As the Pt profile followed that of the titanium and behaved opposite to that of Si and Mg, which were the dements present in high concentrations in sepiolite, it was clearly indicated that the platinum was selectively deposited on titania. 3.4. Effect of area velocity and steam
In order to evaluate the behaviour of the catalysts in conditions closer to those of real diesel exhaust, the linear, gas hourly and area velocities were increased, and water was added to the inlet gas mixture. Figure 7 shows the activity of the selected catalyst, A, in the absence and presence of 3 % water in the inlet gas mixture, at a GHSV of 11000 h"1 and an area velocity of 13 Nmh ~. The area velocity increase from 6 to 13 Nmh -~ (GHSV from 4400 to 11000 h~) induced a decrease in the NOx conversion of c a . 15%. The presence of water vapour induced a 5% decrease in activity. This was higher than expected from the observations of Hamada [ 16], who reported that the NO reduction with propene in oxygen excess on Pt/AI203 was not affected by water.
100
=0
,f-
80
11 ///d ,i I , "I i (a)
so
,/,~ i
w 40
d z
.~
l/
~L
20 o 150
200
250
300
Temperature, ~
Figure 7. Influence of area velocity and steam on the NOx (--) and C3I-I6 (---) conversions of catalyst A: (a) without steam at AV = 6 Nmh l, (b) without steam at AV = 13 Nmh ~, and (c) in the presence of 3% water at AV = 13 Nmh 1. 4. CONCLUSIONS The results presented in this study on Pt-titania-sepiolite monolithic catalysts indicated that their activity in the reduction of nitrogen oxides with propene in the presence of 10% oxygen was strongly affected by the properties of the titania selected for the support's preparation. Thus, different industrially produced titanias had different textural properties, and these differences affected the textural properties of the titania-sepiolite monoliths (MIP total pore volume, MIP surface area, MIP pore size distribution and BET surface area) and the NOx
242 conversions achieved with the Pt catalysts prepared with them. A complex interaction of the textural parameters was observed for the catalysts prepared with the other three types of titania (B, C and E). However, the results indicated that the presence of mesopores due to the sepiolite could be of importance. No major influence of the support's sepiolite content on the activity was observed. XRD, TG-DSC and MIP results showed that the fall in activity produced by heat treating the support of catalyst A at 500 and 800~ should not be due to phase changes in either the titania or sepiolite, but to sintering of the materials which reduced the surface areas and changed the pore size distribution, although the support's total pore volume was maintained. Platinum was shown by EPMA-WDS line profiles to be selectively deposited on titania and homogeneously dispersed throughout the wall, explaining the small influence of the supports' titania content on the activity. A 55% percent NOx conversion was achieved at an area velocity of 6 Nmh~ in the presence of 10% oxygen. However, this parameter showed to have a strong influence on the NOx conversion, lowering it to 40% when approximately doubling the area velocity. The catalyst's activity was also slightly steam sensitive, falling by ca. 5% due to the presence of 3% water. ACKNOWLEDGEMENTS We are grateful for the financial aid from the CICYT (Spain), Project AMB93-0244, the CAM (Spain), Project 0057/94 and the EC, Project EV5V-CT94-0558. REFERENCES 1. M. Shelef, Chem. Rev., 95 (1995) 209. 2. M. Iwamoto, H. Yahiro, S. Shundo, Y. Yu-u and N. Mizuno, Appl. Catal., 69 (1991), L15. 3. W. Held, A. KOnig, T. Richter and L. Puppe, SAE paper 900496 (1990). 4. C. Kawaki and H. Muraki, Jap. Patent No. 265 649 (1990). 5. K.C.C. Kharas, D.J. Liu and H.J. Robota, Appl. Catal. B: Environmental, 2 (1993) 225. 6. R. Burch and P.J. Millington, Catal. Today, 26 (1995) 185. 7. M. Iwamoto and H. Yahiro, Catal. Today, 22 (1994) 5. 8. S. Brunauer, P.H. Emmett and E. Teller, J. Am. Chem. Sot., 60 (1938) 309. 9. J. Rouquerol, D. Avnir, C.W. Firbridge, D.H. Everett, J.H. Haynes, N. Pernicone, J.D.F. Ramsay, K.S.W. Sing and K.K. linger, Pure and Appl. Chem., 66 (1994) 1739. 10. W.C. Conner, C. Blanco, K. Coyne, J. Neil and J. Pajares, J. Catal., 106 (1987) 202. 11. P. Avila, J. Blanco, A. Bahamonde, J.M. Palacios and C. Barthelemy, J. Mater. Science, 28(1993)4113. 12. J. Blanco, P. Avila, M. Yates and A. Bahamonde, Stud. Surf. Sci. Catal., 91 (1995) 755. 13. R.A. Spurr and H. Myers, Anal. Chem., 29 (1957) 760. 14. U. Shuali, S. Yariv, M. Steinberg, M. Muller Vonmoos, G. Kahr and A. Rub, Thermal Analysis Proc. Ninth ICTA Congress, (1988) 291. 15. R.T.K. Baker, E.B. Prestidge and R.L. Garten, J. Catal., 56 (1979) 390. 16. H. Hamada, Catal. Today, 22 (1994) 21.
CATALYSIS AND AUTOMOTIVEPOLLUTIONCONTROLIV Studies in Surface Science and Catalysis, Vol. 116 N. Kruse, A. Frennet and J.-M Bastin (Eds.) 91998 Elsevier Science B.V. All rights reserved.
243
DeNOx mechanism on platinum based catalysts *V.Pitchon, A.Fritz, G.Maire. LERCSI, Laboratoire d'Etudes de la R6activit6 Catalytique, Surfaces et Interfaces, URA 1498 du CNRS
-
Institut le Bel, Universit6 Louis Pasteur, 4, rue Blaise Pascal, 67070 Strasbourg,
FRANCE. (Fax: 33-(0)3.88.41.61.47, e-mail: [email protected]).
ABSTRACT The selective reduction of NO by hydrocarbon in an excess of oxygen was studied using a platinum catalyst supported upon La203-ZrO2 prepared by a sol-gel technique. Following a standard pre-treatment with oxygen and water, it was found that the platinum was existing in a mixture of both oxidised and reduced forms, yet during the course of the reduction reaction, all of the metal became oxidised. A systematic study of the parameters influencing the reaction demonstrated that NO2 is not a reaction intermediate. Certain conclusions concerning the mechanism of the reaction were drawn: a partially oxidised hydrocarbon could be associated with noble metal-type catalysts.
1. INTRODUCTION In respect of the severe regulations imposed, the motor car manufacturers have decreased considerably the NOx emissions from their vehicles. Initially this was by a modification of the engine's combustion chamber. More recently these measures have proved less than sufficient and so to prevent pollution an exhaust after-treatment has become necessary. With this in mind, several technologies have been developed in order to decrease the harmful NOx emissions. One of which is selective catalytic reduction by hydrocarbons (HC-SCR). An enormous amount of catalytic materials have now been developed [1] and an appropriate choice would seem to be supported noble metal catalysts [2]. A high activity is generally observed at low temperature while the efficiency remains little affected by water. The effects
* Corresponding author
244 of the support have been reported by Bamwenda et al [3] but as yet, no study has discussed the effect of a combination of several supports upon the activity. In this paper, we present results of a catalyst prepared by impregnation of a support made from a combination of zirconia and lanthanum oxide, themselves prepared by a sol-gel technique. A systematic study of the parameters influencing the reaction was performed in order that participation in the debate on-going in the literature might be undertaken and to prove that the structure of the catalyst could ascertain or eliminate certain assumptions relative to the mechanism detailed in recent publications.
2. E X P E R I M E N T A L
2.1. Catalyst preparation The catalyst was made by impregnation of an ethanolic solution by Pt(NH3)2(NO2)2 of a La203-ZrO2 support prepared in the following manner: the method consists of a simultaneous precipitation of La(NO3)3, 6H20 and ZrO(NO3)3, with 6H20 dissolved in ethanol by oxalique acid. The solvent was then evaporated and the remaining powder dried overnight in an oven at 120~ and then calcined in air at 550~ for 3 hours, with a ramp rate of 5~
1. Following
impregnation of the platinum salt, the catalyst was calcined again under the same conditions but only for 1 hour. The atomic absorption analysis (SCA, Solaize, France), implied a 1/1 ratio between La and Zr and a percentage inclusion of Pt equal to 1. Under those conditions, an XRD pattern was not observed either for ZrO2, or for La203, and neither for a well-defined structure between Zr and La. The catalyst, characterised by BET, possessed a surface area of 12 mE.g"l with a dispersion of platinum, measured by pulse CO chemisorption of 3 5%.
2.2. Catalyst eharacterisation XPS spectra were recorded on a VG-ESCA III spectrometer with A1 Kcx X-rays as photon source (1486.6 eV). The apparatus allows in situ treatments under oxygen or hydrogen at 1 Atm. and up to 900~
The power of the X-Ray source was set at 100W and the time to
record all the transitions of interest was 90 minutes. A flood gun was used to balance the charge effects due to ion milling. The sample in the preparation chamber was systematically submitted to a calcination under 6% O2/N2 for 2 hours. Binding energies (BE), were referenced from the C(1 s), peak at 284.5 eV Pt 4f photoemission lines were determined using a curve-fit. Binding energies corresponding to the core levels of Pt 4f7/2 and 4t"5/2proved characteristic.
245 2.3. Catalytic test procedure
The reaction was carried out in a dynamic flow reactor using a synthetic gas mixture. The flow rates were adjusted using Tylan mass flow controllers and the effluents were analysed using IR or UV analysers from Fisher-Rosemount for NO, NO2 and N20 and FID chromatography for the hydrocarbons. The gas was humidified by passage through a water saturator regulated at 50~
Before the analytical section, a Perma Pure dryer was installed to
remove selectively, water vapour from the gas stream. The data were collected every 10 seconds on a computer using a " purposely-written" software. The catalyst was pre-treated in a mixture of 6% O2 and 12% H20 in nitrogen. The temperature was increased to 450~ with a ramp rate of 4~
l and was maintained for 30
min. The temperature was then decreased to 150~ and the reacting gas mixture introduced. The standard comprised 500 ppm NO, 220 ppm C3H8, 110 ppm C3H6, 350 ppm CO, 6% 02, 10% CO2, 12% H20 and the SHV was 60.000 h l. For the mechanistic studies some of the concentrations were changed. Details of these variations will be given in the text where appropriate. The N-containing products of the reaction are N2, N20 and NO2. N2 was not analysed therefore the results are expressed by the conversion of NO (XNo) and the production of NO2 (XNo2) and N20 (XN20). The conversion of NO into N2 is given by: XN2 -" XNo-XNo2-XN20.
The conversion of hydrocarbon is calculated from the amount of unreacted HC in the gas phase and all are expressed as percentages (%).
3. RESULTS AND DISCUSSION 3.1.The state of the platinum before reaction
Figure 1 shows the XPS spectrum of binding energies corresponding to the core levels of Pt 4f7/2 and 4f5/2 following treatment under 6% O2 at 450~
The result of the mathematical
manipulation indicated clearly that the platinum was existing mainly in the form of PtO (80 %), with the remaining 20 % in the metallic form.
246
\ /! . . . . . .
~
"
. .
Figure 1: XPS spectrum at the 4f level of Pt/La203-ZrO2 calcined at 550~
3.2.
Activity of the catalysts with a standard mixture of reactants.
The conversions of NO and HC into N2, NO2 and N20 are shown in Figure 2 (CO and C O 2 are not displayed). The conversion of NO goes through a maximum of c.a.70% at 200~
The
main product is N2 which follows the NO curve with a maximum of 66%. The production of NO2 or N20 never exceeded 13%. Perhaps noteworthy is that the combustion of HC starts at low temperature and is complete at c.a. 300~
l~176 I 90 80 70
~
"*-NO ~N2 ~NO2 -x-N20
60 50 403020 10 -~ 100
-~-'H_C
150
200
250
300
350
400
T (~ Figure 2: Conversion of NO on l%Pt / La203-ZrO2 (500 ppm NO, 330 ppm HC, 350 ppm CO, 6% 02, 12% H20, 10% CO2)
247 As this catalyst exhibited quite a high activity for the conversion of NO into nitrogen using standard conditions, a series of experiments, designed in an attempt to determine and understand the mechanism were performed, their description can be found in the following sections. 3.3. Mechanistic
studies
3.3.1. Carbonaceous deposit One of the principal assumptions for the mechanism of the reaction over Cu-ZSM5 concerned the presence of a carbonaceous deposit as an active species [4, 5]. In order to verify this hypothesis the following experiments were performed: The catalyst was heated in a gas stream containing 660 ppm of hydrocarbons and 6% of O2 for 2 hours up to 450~
At this
temperature, the gas composition was then switched to pure oxygen and the temperature raised further to 500~
Oxygen was introduced into the gas flow, if carbon deposits were
observed, then parallel formation of CO2 should be anticipated. The same experiment was repeated in the presence of 500 ppm of NO during the first step. In both cases, the formation of CO2 remained undetected. 3.3.2. Reactivity with NO 2 The activity for SCR was measured using NO2 instead of NO as described when using standard conditions. The Figures 3 and 4 show the evolution of the concentration of NO2 with temperature in both the absence and presence of oxygen respectively.
500
450 [~ 400 t~ 350 = 300 ~ 250 = 200 o o 150 tm o
"9
'
--
__•_•
+
NO2 NO
100
50 O-
75
100
125
150
,.
~
175
200
....
~
.
225
250
t9 275
Toc
Figure 3: Conversion of NO2 on l%Pt / La203-ZrO2 (500 ppm NO2, 330 ppm HC)
* 300
248 In the absence of oxygen, from 100~ by 175~
NO2 is reduced to NO, this reaction being complete
At this temperature, the concentration of NO is 500 ppm. Above 200~
the
concentration decreases because selective reduction by HC begins. There is also a small production of NO2 and N20 starting at 250~
(not represented in Figure 3), which never
exceeded 50 ppm. In the presence of 6% oxygen, initially, NO2 is entirely reduced into NO which then becomes the substrate for reaction with HC leading to nitrogen formation as the main product. On the basis of this evidence, it appears clearly that NO and not NO2 is the reactive species.
5OO 450 400 "~350
.t
300
250 200 g 150 lOO
50 075
.b
100
125
150
175
200
225
250
275
300
Toc Figure 4:Conversion of NO2 on 1%Pt / La203-ZrO2 (500 ppm NO2, 330 ppm HC, 6%02) 3.3.3. The state of the platinum after the reaction In paragraph 3.1., it was suggested that the platinum was in an oxidised state after the pretreatment. Although it is less than probable than the oxide could be reduced under the gas stream because it contains 6% 02, this point has been verified. The catalyst underwent 2 reactions and was then transferred into the XPS spectrometer (Figure 5). Since the only reducing agent in the reacting media was HC, the catalyst was submitted to an HC flow with a concentration equivalent to those used during the reaction (330 ppm), for 2 hours in the pretreatment chamber of the spectrometer (Figure 6).
249
I ~176 9 ':' t;
./
61.00
71.00
6
~
",
83.00
81.00
Figure 5" XPS Spectrum of the catalyst after 2 reactions at 550~
Figure 6: XPS Spectrum of the catalyst pretreated by 330 ppm of HC at 550~
The figures reveal that during the reaction the platinum remained in the form of the oxides PrO and PtO2. The reactants were unable to reduce it. Therefore, it is more than probable that the selective reduction of NO by HC will occur over platinum oxide and that reduced platinum is not an influencing factor. 3.3.4. The influence of each reactant upon the activity This section of the work was undertaken in order to determine the influence of each individual reactant on the overall reaction. The concentrations of oxygen, HC and finally NO were studied by maintaining the concentrations of 2 of these constant while effecting a variation of the third. i) Oxygen The reaction mixture contained 500 ppm of NO, 220 ppm C3Hs, 110 C3H6 while the 02 concentration was varied between 0 and 10%. The results are presented in Figure 7. In the absence of oxygen, NO was converted between 200~ and 300~
the temperature at
which the conversion reaches 100%. In the presence of oxygen, the temperature at which the reaction started was lower and the conversion followed a curve profile with a maximum at c.a. 200oc.
250 Conversion in N2 (%) 100 90 80 70 60 50 40 30 20 10
-c
0 150
100
200
250
300
350
400
Temperature (~ ---
0% 02 - - •
1% 02
"
2% 02 ---o--- 6% 02 ---ty-- 10% 02
Figure 7" Conversion into N2 with a variation in the 0 2 concentration (Composition: 500 ppm NO, 330 ppm HC and 02) The conversion increased with the concentration of oxygen. This increase is not due to the formation of NO2 but was the result of the formation of nitrogen. Both the dependence upon the oxygen concentration and the shape of the conversion curve invokes the hypothesis suggested previously about catalysts such as Cu-ZSM5 with a reactive intermediate formed by a partial oxidation of the hydrocarbon adsorbed onto the surface as proposed by certain other authors [6,7,8]. This intermediate should react directly with NO: C3H6-S +
89 0 2 ---~C3OH6-S
C3OH6-S + 8 NO-S ---) 4 N2 + 3 CO2 + 3 H20 The exact structure of the oxygenated intermediate is not described in the literature. Nevertheless, compounds such as aldehydes, alcohols, ketones or carboxylates have been proposed. ii) Hydrocarbon The reacting mixture contained 500 ppm of NO, 6% 0 2 while the HC concentration was varied between 165 ppm and 1320 ppm at constant propane propene ratio of 2. The results are presented in Figure 8.
251
NO Conversion (%) 100 90 80 70 60 50 40 30 20 10 0" 100
7
150
200
250 T~ture
"--
165ppmHC - - •
330ppmHC
300
350
400
(~ 9 660ppmHC ---o-- 1320ppmHC
Figure 8: NO Conversionwith variation in the HC concentration (Composition: 500 ppm NO, 6% 02 and HC) The conversion of NO was independent of the HC concentration between 165 ppm and 660 ppm and reached c.a. 75%. With 1320 ppm, the conversion proved more efficient below 220~
than
above this temperature. The reactions, between C3H6 or C3H8 and NO respectively could be written as: 9 NO + C3H 6 "-) 4.5 N2 + 3 CO2 + 3 H20 10 NO + C3H8 ")5 N2 + 3 CO2 + 4 1-120 Therefore, to reduce 500 ppm NO, only 50 ppm of C3H8 are required. The working conditions employed a large excess of HC compared to the stoichiometry. In the domain 165-660 ppm, there was no competition between NO and HC for the reaction sites. The competition which appeared at higher concentration is accounted for in the presentation of results in another paper [9]. iii) NO The reaction mixture contained 330 ppm of HC, 6% 02 while the NO concentration was varied between 250 ppm and 1000 ppm. The results are presented in Figure 9. The conversion of NO increased with its concentration for all the temperature range studied suggesting a competitive adsorption between NO and HC. At 200~
in the presence of 1000 ppm of NO the conversion into
nitrogen was equal to c.a. 60% which decreased to c.a. 40% when 250 ppm of NO was used.
232 Cam,ersion in N2 (%) 100 90 80 70 60 50 40 30 20 10 0 100
150
200
250 T
[]
250 I~nNO ~ x ~
~
300
350
400
(~
500 pgnNO
"-
1000I:pmNO
Figure 9: Conversion into N2 with variation of the NO concentration (Composition: 330 ppm HC, 6% 02 and NO)
CONCLUSION The aim of this work was to prepare a catalyst made of platinum housed upon a new type of support, to investigate its physical characteristics and to submit it to catalytic testing in order to understand the mechanism involved in the DeNOx reactions. The following points have been discussed: - NO2 is not involved in the selective reduction of NOx by HC in the case of platinum. - In the presence of oxygen, there is no formation of a carbonaceous deposit, probably because the removal of the latter is strongly favoured by the Pt itself. - The selective reduction occurs upon an oxidised form of Pt. However, at this point, neither of the 2 oxides PtO or PtO2 can be eliminated. - The mechanism often invoked on zeolitic-type catalysts with a partially oxidised hydrocarbon as a possible intermediate of the reaction could account for these results. Indeed, there are similarities such as the conversion of NO which follows a volcano-type curve and there is an increase of activity parallel to an increase of the oxygen concentration in the reacting mixture. -
A systematic study of the influence of each reactant (NO, 02, HC), upon the conversion of NO,
seemed to indicate that there was competition for adsorption onto the reactive sites between NO and HC. A more detailed study upon the selectivity and of the kinetic parameters will follow [9].
253 REFERENCES
1. A.Fritz and V.Pitchon, Appl. Catal., B, accepted for publication (1997). 2. A.Obuchi, A.Ohi, M.Nakamura, A.Ogata, K.Mizuno, H.Ohuchi, Appl.Catal., B, 2 (1993) 71 3. G.R.Bamwenda, A.Ogata, A.Obuchi, J.Oi, K.Mizuno, J.Skrzypek, Appl.Catal., B, 6 (1995) 311 4. G.P.Ansell, A.F.Diwell, R.R.Rajaram, A.P.Walker, Appl.Catal. B, 2 (1993) 81 5. C.J.Bennet, P.S.Bennett, S.E.Golunski, J.W.Hayes, A.P.Walker, Appl. Catal. A, 86 (1992) L 1 6. C.N.Montreuil, M.Shelef, Appl. Catal., B, 1 (1992) L1 7. J.L.D'itry, W.M.H.Sachtler, Catal. Lett., 15(1992) 289 8. Y.Torikai, H.Yahiro, N.Mizuno, M.Iwamoto, Catal. Lett., 9 (1991) 91 9. A.Fritz and V.Pitchon, to be published (1997).
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CATALYSISAND AUTOMOTIVEPOLLUTIONCONTROLIV Studies in Surface Science and Catalysis,Vol. 116 N. Kruse, A. Frennetand J.-M Bastin (Eds.) 91998Elsevier Science B.V. All rights reserved.
255
E l e c t r o c h e m i c a l P r o m o t i o n in Emission Control Catalysis: The Role of Na for the Pt-Catalysed Reduction of N O by Propene I.V. Yentekakis a, A. Palermo b, M.S. Tikhov, N.C. Filkin and R.M. Lambert Chemistry Department, Cambridge University, Cambridge CB2 1EW, England. a
Dept of Chemical Enginnering, University of Patras & ICE-HT/FORTH, GR-26500, Greece.
b Institute of Material Science ant Technology (INTEMA), UNMPD-CONICET, J.B. Justo 4302,(7600), Mar del Plata, Argentina. Conventional heterogeneous metal catalysts are commonly enhanced by the addition of socalled promoter species that are used to modify intrinsic metal surface chemistry with respect to activity and/or selectivity. Electrochemical promotion (EP) provides an in situ, reversible and efficacious means of catalyst promotion and it allows for a systematic study of the role of promoters in heterogeneous catalysis. EP studies relevant to the "three-way" catalytic chemistry i.e., control of automotive CO, NO and hydrocarbons emissions, demonstrate that major enhancements in activity of Pt catalyst supported on ~"-A1203 (a sodium ion conductor) are possible when Na is electrochemically pumped to the catalyst surface. In the case of the important reactions involving NO reduction by CO or by hydrocarbons, major enhancements in selectivity towards N2 (from 15 to 70%) have been also achieved. The promotional effect of Na is due to enhanced NO chemisorpion and pronounced NO dissociation on the Pt surface. The Pt-catalysed reduction of NO by propene, which is of particular importance with respect to emission control catalysis, exhibits strong electrochemical promotion by spillover of Na from the 1~"-A1203 support. In the promoted regime rate increase by an order of magnitude are achievable. At sufficiently high loading of Na, the system exhibits poisoning, and excursion between the promoter and poisoned regimes are fully reversible. XPS was used in order to shed light on several important questions emerging for the role and the form of Na promoter on the catalyst surface. We have also demonstrated that EP of Pt catalyst very substantially enhances the ability of NO to oxidise propene even in the presence of oxygen.
1. INTRODUCTION The increasing volume of air pollutants such as carbon monoxide (CO), hydrocarbons (HC) and nitrogen oxides (NOx) has become a serious global environmental problem. Currently the "three-way" catalytic converters, which contain Rh, Pt and Pd noble metals supported on a ceramic monolith, are used for the simultaneously control of all the above pollutants from
256 internal combustion engines. Three-way catalysts (TWCs) are very effective in oxidising CO and HC in the oxygen rich exhaust gas, however they are substantially less effective in reducing the NOx emissions. Reactions involving the catalytic reduction of NO are today of major environmental importance with respect to the removal of toxic emissions from both stationary and automotive sources [1 ]. Because of its ability to chemisorb NO dissociatively, Rh is the key catalyst in TWC converters for the reduction of NOx. Due to its rarity in nature in comparison with the other noble metals Pt and Pd (~ 1"15) and its consequent significantly higher cost, a reduction in the amount of Rh present in automotive exhaust catalytic converters, via appropriate enhancement (promotion) of the catalytic activity of the other noble metals components (Pt or Pd) would be highly desirable. The performance of conventional heterogeneous metal catalysts is commonly enhanced by the addition of promoters that are used to modify the intrinsic metal surface chemistry. Their exact role in industrial heterogeneous catalysis is well understood for a limited number of systems [2], while most fundamental studies have addressed their role in chemisorption only under ultra-high-vacuum (UHV) conditions [2,3]. This is primarily due to the difficulty of achieving in situ control of promoter concentration on the catalyst surface under reaction conditions. Electrochemical promotion (EP) provides a novel in situ and high controllable means of catalyst promotion. In brief, it has been found that solid electrolytes can be used as reversible in situ promoter donors or poison acceptors. These active supports affect the catalytic activity and/or selectivity of metals deposited on them in a very pronounced, reversible and, to some extent, predictable manner. During the last ten years, EP has been studied for over forty catalytic reactions on Pt, Ph, Pd, Ag, Ni and IrO2 catalyst films using a variety of solid electrolytes i.e.,: 1. ZrO2(8mol%Y203) (YSZ), an 02. conductor [8,15] 2. 13"-A1203,a Na + conductor [4,8-11,20,21,22] 3. CaF2, a F" conductor [5], 4. protonic conductors [6] 5. TiO2, a mixed electronic-ionic conductor [7]. The behaviour of these systems may be quantitatively rationalised in terms of changes in adsorption energies (and therefore reaction activation energies) caused by changes in catalyst work function which result from backspillover of electrochemically pumped ions from the solid electrolyte to the active metal component [8]. The Electrochemical Promotion literature has been renewed recently [8]. A potentially important application of EP could be in enhancing the catalytic activity and selectivity of metals in the reduction of NO to N2. In fact, it was recently found that the use of pQ"- AI2O 3 to reversibly dose Na on Pt leads to very significant enhancement of reactions related to three-way catalytic chemistry, namely CO [4] and hydrocarbons (C2I~) oxidation [9], and, more importantly, NO reduction by C~H4 [10] and by CO [11]. In the latter case, significant enhancements in N2 selectivity from 15 to 70% were also observed [ 11]. In other words Pt could be induced to behave like Rh. This paper is a continuation of this work to a potentially more important system: EP of the Pt-catalysed reduction of NO by propene. Propene is a major constituent of the hydrocarbon componem in lean-bum exhaust and the industry standard for catalyst testing. Despite the extensive recent literature on the catalytic chemistry of NO+ hydrocarbon
257 reactions [12,13] very few studies on the effects of alkali promotion have been carried out. "~ support as Na supplier provides an Electrochemical promotion of a Pt film using pf~rr -A l 21,)3 effective way of examining the effects of Na promotion in a controllable and reversible manner. Large catalytic rate enhances and substantial gains in selectivity towards N2 production were achievedover the clean surface. Our results strongly suggest that NO dissociation is the key reaction initiating step. XP spectroscopy data confirm that the mode of operation of the electrochemically promoted Pt film indeed involve reversible pumping of Na to or from the solid electrolyte. It was also confirmed that the electropumped Na and the vacuum-deposited Na behaves identically on the surface of the Pt catalyst-electrode. We have also demonstrated that EP of Pt catalyst substantially enhances the ability of NO to oxidise propene even in the presence of oxygen, i.e., at conditions relatively similar to that which exists in lean-bum engine exhaust. This is of potential technological significance.
2. EXPERIMENTAL The experimental apparatus used for the atmosphere pressure kinetic studies provides a combination of on-line gas chromatography (Shimadzu-14B) and on-line mass spectrometry (Balzers QMG 064) for the analysis of reactants and products. N2, N20, CO, CO2 and C3I-I6 were measured via G.C., while NO at AMU 30 was continuously monitored via mass spectrometry. AMUs 28 (N2--{-CO), 44 (CO2+ N20), 32 (02), 41 (propene) and 17 (NH3) were also continuously monitored. Reactants were pure NO and propene diluted in ultrapure (99.999%) He. Their fluxes were controlled by mass-flow controllers. The total flowrate of the reactant gas mixture was kept essentially constant for all experiments at 1.3x10"4 mol/s (190 cm3 STP/min), with partial pressures PNo, Pc3n6 varied between 0-6.5 kPa and 0-0.4 kPa respectively, and Pm bringing the total pressure to 1 atm in every case. Conversion of the reactants was restricted to <15%, in order to avoid mass transfer effects. The catalyst (working electrode, W) consisted of a porous but continuous thin (5-20 ~m) Pt film deposited on the a 20 mm diameter of [3"-A1203 disc (Ceramatee) as described in detail elsewhere [5]. Au reference (R) and counter (C) electrodes were attached to the other face of the solid electrolyte wafer (Fig. 1) by vacuum evaporation producing good adhesion and extremely low resistance electrodes. Control experiments confirmed that Au electrode films were catalytically inert for the reaction under consideration at all conditions used in this study. The true, catalytically active surface area of the Pt catalyst electrode film, was estimated by comparison of the rate of a model reaction (CO oxidation) under standard conditions (6kPa 02, 1.8kPa CO, 623K and VWR=800mV) with published rates [4] obtained using samples whose surface area had been accurately determined using a surface titration technique [14]. This gave a value of 5x10 "7 mols of Pt (or 197cm2) for the active metal area. The Pt/[Y'-AI2OjAu sample was suspended in a 115 cm3 quartz, atmospheric pressure, well-mixed reactor. The behaviour of such a single pellet, continuous stirred tank reactor (CSTR), with all three electrodes exposed to the reactant gas mixture, has been described and discussed in detail elsewhere [ 15,11 ]. An AMEL 553 galvanostat-potentiostat was used in order to supply constant currents (galvanostatic operation) or potentials (potentiostatic operation) [8] between the working and counter, or the working and reference electrodes, respectively. Most experiments were carded
258 out in potentiostatic mode by following the effect of the applied catalyst potential VWR on the reaction rate. The galvanostatic transient behaviour of VWR was used to calibrate the Na coverage scale [4]. Changes in the catalyst potential, VwR, are directly related to changes in catalyst work function, e~, (eAVwR-A(e~) [16]) as predicted theoretically [8] and established experimentally [16] by in situ measurement of e~ with a Kelvin probe. XPS measurements were carried out in a VG ADES 400 UHV spectrometer. The sample arrangement and other instrumentation and construction details are given elsewhere [221.
3. RESULTS 3.1. Electrochemical promotion under transient conditions The results of a typical EP experiment under transient conditions are shown in Fig. 1. This depicts the effects of applying a constant negative current (Na supply to the catalyst) on catalyst potential (VwR) and on the AMU 28 (N2) and 44 (CO2+N20) m.s. signals. The only detectable reaction products were CO2, N2, N20 and H20. The experimental procedure was as follows: First, the surface was electrochemically cleaned of Na by application of a positive potential (0.3V) until the positive current (Na removal from the catalyst) vanished [4]. This current corresponds to the reaction: Na(Pt) --> Na+(~"-AI203) + e
(1)
The potentiostat was then disconnected (I=0 at t =- l min) and VWRrelaxed to the value imposed by the gaseous composition. Then, at t=0 the galvanostat was used to impose a constant current I =- 100pA, This pumped Na to the catalyst surface (the reverse of the reaction (1)) at a rate I~=l.04xl0 "9 mol Na/s. The corresponding Na coverage, 0Na, starts increases according to Faraday's law: 0Na= - It/FN
(2)
where t is the time of current application, F is Faraday's constant and N is the number of available surface Pt sites (5x10 "7 mol Pt). Under these conditions, i.e., Na pumping to the catalyst surface, a pronounced decrease in the catalyst potential VwR, and consequently in the catalyst work function (eAVwR=A(eO)) and a very pronounced increase (by a factor of 7) in the production rate of N2 was observed. The rates pass through a maximum when VwR is about -0.3V (0Na~0.3-0.4) (Fig. 1). The CO2+N20 signal (AMU 44) is given in arbritary units in Fig.1 because the separate contributions of CO2 and N20 could not be measured under transient conditions. Potentiostatic imposition of the initial VWR restored all rates to their initial, unpromoted, values corresponding to clean Pt, thus demonstrating that the system is perfectly reversible. Galvanostatic transients are also required for establishing the relationship, 0~a(VWR), between VWR and Na coverage 0~a under reaction conditions, simply by following the time dependence of VWR and using eq.(2) (Fig.l). In general, this relationship depends also on gaseous composition as observed previously [4,11 ].
259 The "volcano-type" behaviour of the rates shown in figure 1 is also manifested in the steady-state mode and it is remarkable that similar promoting-poisoning behaviour of Na has been also observed in the cases of CO [4] and ethylene [9] oxidation and NO reduction by CO [11 ] under appropriate conditions.
ao I
|Vu=0.3
I--0
< ._ '/
= ~ - - ~ = --
I
i"-
o.o
I=-100 pA
o.,
-] o 5
12/
~
.n
|
"
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~ ...........F.-.................. ._ t o,;..,~.,,;,,.i..,;,,,;..,i,,,,,o
J"
G)
[_
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J
I pno=l.3kPa
/
' , _ - - 0 . ~ , .io.8d
~
0.0
o~
" } O
"0"
~
"
t, min Figure 1. Transient effect of a step change of the applied current on the ainu 28 (N2) and 44 (CO2+ N20) m s signals (solid lines) and on the catalyst potential VWR(dotted line); PNo=l.3kPa, Pc3H6=0.6kPa, T=648K.
11"~
T --375~
~
'"'" o O
"',~.
N,o
Vwe
. ,
--"
O
1
V
Figure 2. Effect of catalyst potential VWR on CO2, N2 and N20 formation rates and on N2 selectivity. Conditions: T=375~ PNo=l.3kPa, Pc3H6=0.6kPa.
3.2. Electrochemical promotion under steady-state conditions
Effect of catalyst potential on reaction rates: Figure 2 shows the steady-state effect of catalyst potential (Vws) on the rates of CO2, N2 and N20 formation and on the selectivity towards N2 which is defined as: SN2 = rN2/(rN2+rN2o)
(3)
As noted above, VWR values >0.2 V correspond to a Na-free Pt surface. Decreasing VWR (thus supplying Na to the Pt surface) causes a dramatic enhancement in CO2, N2and N20 rates by factors of 7, 10 and 3 respectively at VWR~-0.3V. The rate enhancement ratios Pco2, Ps2 and PN2o defined as: px = r~ / r,~o
(x: CO2, N2, N20)
(4)
where r~o depicts the unpromoted (Na-free Pt) rate, are about 7, 10 and 3, respectively. Note especially that the selectivity towards N2 increases from 60% on the clean Pt surface to 80% on the Na promoted (0Na~0.3) Pt surface at VwR ~-0.3 V.
260
Effect of gaseous composition on unpromoted and promoted rates: The dependence of the steady-state CO2, N2 and N20 rates on the partial pressure of propene, PC3H6, at constant NO partial pressure and temperature, for three different values of catalyst potential (different Na coverages), is illustrated in Figure 3a. For VWR=0.3V the Pt surface is Na-free and the "unpromoted" reaction rates are low. All rates exhibit a maximum, i.e. typical Langmuir-Hinshelwood-Hougen-Waston (LHHW) behaviour, at P*c3n6~0.02 kPa (hereaRer P*C3H6denotes the propene partial pressure Pc3n6 value which maximises rates). Decreasing Vwa to more negative values, (supply of Na to the catalyst), causes a pronounced increase in the rates and a systematic shift of P*c3u6 to higher values (up to 0.12 kPa).
T=375"C P~=l.4kPa
20 [ a
~0151
20
Pc~L=O.27kPa
15
g]
/
~"'''
"~
-300 mV
T=375~
~-300 mV
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20[
.
.
.
.
.
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20
-
15
9
o ?
~
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a
I
I
I
I
. . . . . .
~
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'
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o
9
Z
8.0
0.1
0.2
Pc~.~.
0.3
0.4
O0
I
2
3 4 5 PNO, k P a
6
Figure 3. Dependence of CO2, N2 and N20 production rates on PC3H6 (a) and PNO (b), at several fixed values of catalyst potential VwR.
261 The corresponding dependence of the CO2, N2 and N20 rates on the partial pressure of NO is shown in Figure 3b for the same three fixed values of catalyst potential. It is again obvious a pronounced enhancement in catalytic activity occurs upon decreasing the catalyst potential from 0.3 to -0.3 V.
a
.
.
Na ls
.
.
.
omv
b , 300 K; as deposited
]2
.~400 K; open circuit
J
r
Na ls
~ ~
~
~
-
1
2 /
~ 500
1066
~~
1068 1070 1072 1074 1076 1078 Binding Energy, eV
1066
K; VWR=+IO00m V ~
"~~_
~
/
1068 1070 1072 1074 1076 1078 Binding Energy, eV
Figure 4. Na ls XPS. (a): Showing effect of catalyst potential in pumping sodium to~from the Pt film under UHV conditions at 600K. (b): Illustrating electrochemical pumping of vacuumdeposited Na away from the surface under UHV conditions at sufficiently high temperature and positive catalyst potential.
3.3. XPS studies of the electrochemical promotion catalysts Figure 4 shows Na ls spectra acquired at 600K in ultra high vacuum under electrochemical bias conditions that replicate those in the reactor. The electrochemically cleaned surface (Vwa=+500mV) exhibits a feature at 1074 eV BE which is due to Na in the 13"-A1203visible through cracks in the porous Pt film. At Vwa=-600 mV, two features are now present, that at 1073.5eV being due to Na pumped to the Pt surface. Note that the 13"-A1203derived peak has shifted to ~1075.5eV, i.e, by an amount about equal to the change in Vwa (1.1eV), as it was expected [22]. The Pt film is at ground potential and Na ls photoemission from the Pt should of course appear at fixed kinetic energy, independent of Vwa. Thus our XP spectra dearly reveal the location of relationship between the two types of Na sampled by photoemission. For intermediate values of Vwa the amount of Na on Pt varies monotonically with catalyst potential and the (constant intensity) Na ls emission from the 13"-A1203 shifts with Vwa. This spectral behaviour was reproducible and reversible as a function of Vwa. Fig. 4b demonstrates that under UHV conditions, electro-pumped Na is identical in behaviour and in chemical state with Na supplied by vacuum deposition from a Na evaporation source. Spectrum (1) shows the XPS of the catalyst film when Na was vacuum-deposited. Heating to 400 K under open circuit conditions caused no change- spectrum (2).
262 20
At 500 K (when the ~ ' - A ] 2 0 3 becomes appreciable conducting) and with VWR=+1000mV, the deposited Na was strongly pumped away from the surfacespectrum (3).
Pno=O.BkPa Pc.a.=O.4kPa
T='~75~
oo 0
| 10 I
. 5 ....
~ , , ....
? c~ ~2
3.4. EP of the NO+C3H6 reaction in the presence of oxygen.
,Q~--O
,~,,,,
'
Figure 5 shows preliminary results of electrochemical promotion experiments on the oxidation of propene by NO in the presence of oxygen, i e , under conditions relatively similar to that which exists in lean-burn engine exhausts. The figure shows the effect of catalyst potential on CO2, N2 and N20 formation rates in the presence of oxygen (at an inlet P02=0.63 kPa) and compares the behaviour of the system with that in the absence of oxygen: sodium promotes the CO2, N2 and N20 formation rates for both cases in a similar manner. Work is now in progress for the study of EP under simulating lean-burn engine exhaust conditions.
C)--@
2:
/=,
0 3
,
,
,
,
I
,
I
I
I
,
I
,
I
,
,
I
1
1
1
1
ww
--2 O ?
r~ -J
-1.0
,
v
t
i
,
-0.5
,
,
t
i
,
0.0 Vws , V
,
,
,
i
0.5
I
O i
i I
1.0
Figure 5. Effect of Vws on CO2, N2 and N20 rates in the presence (black symbols, Po2=0.63 kPa) and in the absence (open symbols) of 02.
4. DISCUSSION The present results show that the catalytic activity and selectivity of Pt for the reduction of NO by C3H6 can be altered dramatically and reversibly by dosing Na on the Pt surface. The maximum gain in the rates over the clean surface rate is of the order of a factor of 13 and 25 (Fig.3) for the N2 and CO2 production, respectively. In the discussion that follows the term "Na coverage" is used; this does not imply that the promoter is thought to be present in the form of chemisorbed metallic sodium as it would be in vacuum. The reactive gas atmosphere is expected to lead to the formation of surface compounds of Na, and XPS studies under single crystal or EP conditions indicate that stable Na-CO complexes [10,17] or Na carbonates and nitrates [10,22] can be formed, depending on the composition of the ambient gas. Adsorbed polar alkali compounds lead to large decreases in work function, quite similar to
263 those produced by the alkali metal itself, so the general theory of electrochemical promotion [8] is nevertheless applicable. It is evident from Figs 1 and 2 that there is a precipitous fall in rates as the coverage of Na species increases beyond a critical value, i.e. the regime of electrochemical promotion (0 to -350 mV) is followed by a regime of strong poisoning. The strong poisoning behaviour at high Na coverages is understandable after a detailed XPS study of the system [22], indicating that large amounts of Na compounds at very negative potentials are responsible for the surface poisoning. In other words, coverage of active Pt sites by overloading with promoter is a likely major cause of poisoning [22]. We have shown that surface compounds can be electrochemically destroyed by pumping Na away from the Pt film [22], so that excursions between the promoter and poisoned regimes are fully reversible. The observed dependence of the CO2, N2 and N20 formation rates on propene partial pressure for fixed NO partial pressure (Fig.3) demonstrates that the system exhibits classical Langmuir-Hinshelwood kinetic behaviour, i.e., a characteristic rate maximum reflecting competitive adsorption of the two reaetans. The rate maximum shifts systematically to higher propene partial pressures as the sodium coverage is increased (i.e., as catalyst potential is decreased), reflecting the increase in the binding of NO relative to propene with increasing 0Na: sodium enhances NO ehemisorption vs C3H6 ehemisorption. This kind of behaviour is exactly what one would expect in the case of an eleetropositive promoter: the chemisorption strength of electron donors (propene) should be decreased whereas the ehemisorption of electron acceptors (NO and its dissociation products) should be enhanced. Sodium also enhances NO dissociation on the Pt surface: this is a consequence of the above observation and is apparent from the dramatic enhancement in selectivity towards N2 with increasing Na coverage, an affect that is observed in all EP studies involving NO reduction on Pt. In fact, increases from 15% to 70% and from 40% to 80% have been reported for the eases of NO reduction by CO [11 ] and by 1-12[20], respectively. In present study an increase in N2 selectivity from 60% to 80% was observed. The rates of production of N~ and N20 (and thus N~ selectivity) depend on the degree of NO dissociation NO(a) ---> N(a) + O(a)
(5)
which is followed by the elementary steps: N(a) + N(a) --->N2
(6)
N(a) + NO(a) --> N20
(7)
The role of CO, 1-12 or hydrocarbon is to scavenge atomic oxygen resulting from the NO dissociation. The observed increase in the selectivity towards N2 is a consequence of increased NO dissociation, i.e. a decreased amount of molecular NO, and an increased amount of atomic N on the surface. Both factors favours reaction (6) over reaction (7). This dissociative mechanism is the generally accepted pathway under ultra high vacuum conditions [18]. However, a recent study by Klein et al [19] has questioned the validity of the dissociative mechanism under atmospheric pressure conditions in favour of a non-dissociative mechanism. A particular difficulty with the non-dissociative mechanism is that it cannot readily accoum for
264 the lack of reactivity of low index planes of Pt. Unpromoted low index planes of Pt, Pt(111), are relatively inert towards NO dissociation, the process we propose as the key reactioninitiating step. Our EP results strongly suggest that the dissociative mechanism holds, even in the high pressure regime. The catalyst film consists of large polycrystalline Pt particles whose surfaces are dominated by low index planes that are inactive for NO dissociation. The low rates observed at high positive catalyst potentials (Na-free system) may be ascribed to defects and high index planes that are inevitably present at crystallite edges. Both Nz and N20 are produced in this region as there is a mixture of molecular NO plus atomic N and O. Na supplied to the Pt surface strongly enhances the overall activity by inducing NO dissociation on the otherwise ineffective low index planes in accord with both theory and experiment. REFERENCES .
2. 3. 4.
10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.
K.C. Taylor, Catal. Rev.-Sci. Eng., 35 (1993) 457 M. Kiskinova, Studies in surface Science and Catalysis, 70 (1992) 1 J.M. Campbell, Catalysis at Surfaces, NewYork, Chapman and Hall, 1988 I.V. Yentekakis, G. Moggridge, C.G. Vayenas and R.M. Lambert, J. Catal., 146 (1994) 292 I.V. Yentekakis and C.G. Vayenas, J. Catal., 149 (1994) 238 T.I. Politova, V.A. Sobyanin and V.D. Belyaev, React. Kinet. Catal. Lett.,41 (1990) C. Pliangos, I.V. Yentekakis, S. Ladas and C.G. Vayenas, J. Cat.,159 (1996) 189 C.G. Vayenas, S. Bebelis, I.V. Yentekakis, and H.-G. Lintz, Catal. Today, 11 (1992) 303 I.R. Harkness, C. Hardaere, R.M. Lambert, I.V. Yentekakis and C.G. Vayenas, J. Catal., 160 (1996) 471 I.R. Harkness and R.M. Lambert, J. Catal. 152 (1995) 211 A. Palermo, R.M. Lambert, I.R. Harkness, I.V. Yentekakis, O. Marina and C.G. Vayenas, J. Catal., 161 (1996) 471 X. Zhang, A.B. Waiters and A. Vannice, Appl. Catal. B: Env., 4 (1994) 23 7 R. Burch, P.J. Millington and A.P. Walker, Appl. Catal. B: Env., 4 (1994) 65 I.V. Yemekakis, S. Neophytides and C.G. Vayenas, J. Catal., 111 (1988) 152 I.V. Yentekakis and S. Bebelis, J. Catal., 137 (1992) 278 C.G. Vayenas,~S. Bebelis and S. Ladas, Nature, 343 (1990) 625 J.C. Bertolini, P. Delichere and J. Massardier, Surf. Sci. 160 (1985) 531 B.A. Banse, D.T. Wickham and B.E. Koel, J. Catal., 119 (1989) 238 R.L. Klein, S. Schwartz and L.D. Schimdt, J. Phys. Chem., 89 (1985) 238 O.A. Marina, I.V. Yentekakis, C.G. Vayenas, A. Palermo and R.M. Lambert, J. Catal., 166(1997)218 R.M. Lambert, M.S. Tikhov, A. Palermo, I.V. Yentekakis and C.G. Vayenas, Ionies, 1 (1995) 366 I.V. Yentekakis, A. Palermo, N. Filkin, M. Tikhov and R.M. Lambert, J Phys. Chem. B, 101 (1997)3759
CATALYSIS AND AUTOMOTIVE POLLUTIONCONTROLIV Studies in Surface Science and Catalysis,Vol. 116 N. Kruse, A. Frennet and J.-M Bastin (Eds.) 91998 Elsevier Science B.V. All rights reserved.
265
Promoting effect of Zinc in DeNOx reaction over Pt/AI203 A. Bensaddik, N. Mouaddib, M. Krawczyk a, V. Pitchon, F.Garin, G. Maire LERCSI, URA 1498 CNRS, ECPM, Institut Le Bel, Universit6 Louis Pasteur, 4, rue Blaise Pascal, 67070 Strasbourg-France. a Institute of Physical Chemistry, Department of Applied Surface Science. Polish Academy of Sciences, Kasprzaka 44152, 01-224 Warszawa, Poland. ABSTRACT , The selective reduction of NO by hydrocarbon in an excess of oxygen was studied using a platinum catalyst doped or not with zinc. Successive impregnetion or co-impregnation of Pt and Zn on alumina were made. In some cases, in the presence of Zn, the NO conversion is increased in parallel with N2 formation. A better conversion of hydrocarbons was also observed. EXAFS experiments and N20 decomposition experiments have been carried out to explain these observations.
1. INTRODUCTION Organo-zinc compounds are commonly present in nearly all commercially lubricating oils which are intended for vehicle use. Unfortunately, these compounds are involved in the combustion process in the engine and are converted mainly into zinc oxide before entering the catalytic converters. This form of zinc usually deactivates the vehicle exhaust catalysts [ 1]. Catalytic experiments performed with 3.3wt% Pt/AI203 catalyst over which 0.5wt% of zinc was added have shown, for skeletal rearrangement of hydrocarbons, that the activity was nearly completely suppressed. This result indicated that zinc deposition, from Zn(NO3)2followed by air calcination 2 hours at 550~ is not statistically performed onto the whole surface area. If it were the case the activity should have been decreased by about 10% only [2]. Zinc seems to be preferentially located near platinum aggregates. Contrary to these reforming reactions involving only hydrogen and hydrocarbons, we noticed that light-off experiments performed with a complete gas mixture (CO, CO2, 02, NO, HC and H20), either under rich or lean conditions, gave better conversions for the three pollutants on S-[Pt-Zn] catalysts than on platinum catalysts.These very encouraging results for three way catalysts led us to investigate these alumina supported S-[Pt-Zn] catalysts for DeNOx reactions as these catalysts gave promising results for NO reduction under lean conditions.
266 The aim of this paper is to understand the influence of zinc on platinum catalytic behaviour. The added metal can either deactivate or provoke an increase in the catalytic activity of platinum either for reforming reactions or depollution reactions respectively, even when the gas atmosphere is always reductive. We shall study the influence :-i) of the mode of preparation ,-ii) of the zinc loading and -iii) of the kinetic parameters, on the activity of S-[Pt-Zn] catalysts in DeNOx reactions.The catalysts have been characterised by TPR, chemisorption and EXAFS and tested in the reaction of selective catalytic reduction (SCR) using diesel conditions.
2. EXPERIMENTAL
2.1 Preparation The catalysts were prepared by wet impregnation of 7 A1203 (GC0 64 RP, 206 m2g -1) by a solution of H2PtC16 or Zn(NO3)2. The concentrations of these solutions were calculated in order to have l wt%Pt and 0.5wt%Zn. Two sets of catalysts were elaborated : by coimpregnation of both precursor salts (named C) or by successive impregnations (named S). For the latter, the zinc was introduced on a Pt/A1203 catalyst. Between each preparative step, after the solution had been evaporated and the powder dried at 110~ overnight in an oven, the catalyst was calcined under oxygen at 500~ for 4 hours prior to hydrogen reduction performed in situ at 500~ for 4 hours. Before any catalytic test, the catalyst was pre-treated under a flow containing 10% H20 and 6% 02 in nitrogen at 450~ for 30 min. and cooled down to 150~ to start the experiment. 2.2. Characterization BET surfaces were determined by nitrogen adsorption at 77 K, in an automated volumetric set up after a vacuum desorption at 383 K for 1 hour. Dispersion was determined by chemisorption of CO at room temperature using the pulse technique. Prior to chemisorption, the catalysts were reduced under an hydrogen flow at 500~ and flushed with helium at the same temperature for 30 min. In Table 1 are given the metal content, the BET surface and the dispersion of the platinum metal for the fresh catalysts, i.e. catalyst calcined in oxygen at 500~ and reduced in H2 at 500~ for 4h, then purged in nitrogen flow for 30rain. The BET surface is between 148 to 195 m2.g -1 for all the catalysts prepared and the dispersion of the platinum metal, for the fresh catalysts is between 13% and 35%. The BET surface and the numbers of exposed platinum atoms decrease with the amount of added zinc.
26'/ Table 1 : Characterisation of the catalysts Alumina supported Pt wt% catalysts Pt 1.2 S- [Pt-Zn] 1.2 C- [Pt-Zn] 1.2 C- [Pt-Zn] 1* C-[Pt-Zn] 1*
Zn wt%
BET surface
Dispersion %
(mE.g "l ) 0 0.5 0.5 2.7 10
195 174 181 185 148
35 13 29 24 13
* Theoretical values The TPR (Temperature Programmed Reduction) experiments were carried out on some CPtZn and on the S-Pt-Zn catalysts. The samples were heated in 1%H2 in Ar (30 ml/min) from room temperature to 500~ at 8~ while H2 consumption was recorded. From the TPR profiles of Pt and Pt-Zn coimpregnated or successively impregnated samples only one peak was observed at about 280~ even in the presence of zinc. Quantitatively the amount of H2 consumed corresponds to the reduction of PtO2 to metallic Pt. The reduction temperature was the same in all case i.e. in the presence or absence of zinc. This can signify that there is no interaction between platinum and zinc probably due to the small quantity of added zinc in the catalyst (about 0.5% of Zn). The absence of a peak of hydrogen due to the reduction of ZnO and the stability of the temperature of reduction of the platinum may suggest that there is formation of a zinc aluminate. It is important to have an idea on the oxidation state of the platinum in order to know the nature of the adsorbed intermediates. For this reason we have characterised the platinum oxidation state of the S-[Pt-Zn] catalyst using the X-ray atomic absorption technique. The experiments were carded out on the Exafs 4 spectrometer at LURE-DCI, running at 1.85 GeV with an average current of 250 mA. The Exafs data were collected using a conventional step-by-step set up with a channel cut monochromator Si(111) for Pt and two ion chambers as detectors, with Pt foil used to calibrate the monochromator. The spectra were recorded above the Lm edge of platinum (11,560 eV). The results are displayed in figure 1. A simple comparison between the reference spectra of Pt foil, bulk PtO2 and Pt-Zn/A1203 spectrum (Figure 1) allows the conclusion that after the pre-treatment, the platinum is fully oxidised. It is less than likely that it could be reduced during the course of the reaction since this one occurs in a very oxidising media (6% OE). This result is very important in the sense that NO does not dissociate on platinum oxide and therefore implying that one of the intermediate is likely to be an adsorbed hydrocarbon species reacting with NO.
268
2.3. Catalytic experiments 2.3.1. Apparatus The reaction was carried out in a dynamic flow reactor using a synthetic gas mixture. The flow rates were adjusted using Tylan mass flow controllers and the effluents were analysed using IR or UV analysers from the Binos range for NO, NO2 and N20, and FID chromatography for the hydrocarbons. The gas was humidified by passage through a water saturator regulated at 50~ Before the analytical section, a Perma Pure dryer was installed to selectively remove water vapour from the gas stream. The data were collected every 10 seconds on a computer using a "purposely-written" software. 2.3.2. Reactions studied Several types of experiments were performed such as" - Kinetic studies were undertaken with a simplified gas composition (NO 9 1000 ppm, propane 440 ppm, propene 220 ppm, 6%02 and N2) and we mainly studied influence of the space -1
-1
velocity between 60000 h to 15000 h on the catalytic conversion; the catalyst weight ranges between 100 to 400mg. - Reaction tests, for which experimental conditions were as follows. The space velocity -1
was kept at 60000 h ; the catalyst weight was 100 mg, the reaction temperature varied from 150 -1
to 550~ with a temperature ramp of 4 ~ , and the reactant gas mixture was : NO 500 ppm, CO 350 ppm, CO 2 10%, 02 6%, H20 12%, propane 220 ppm and propene 110 ppm.
3. RESULTS AND DISCUSSION
3.1. Influence of the zinc loading and of the method of preparation The conversion of NO as a function of the zinc loading for the C-[Pt-Zn] catalysts is described in figure 2. The activity is almost independent of the zinc content from 0.5% to 10%. Moreover, when compared to a monometallic platinum the presence of zinc inhibits the activity towards NO conversion. The value of the conversion does not exceed 20% in the range of temperature studied with co-impregnated Pt-Zn catalysts while it reaches c.a. 33% with the Pt/A1203. On the other hand, when platinum and Zn are added by successive impregnation there is a noticeable enhancement of the activity below 300~ The maximum of conversion observed is 42% This catalyst is stable with time as evidenced by figure 3. Over a period of 700 min, the conversion remains very stable. This figure represents also the selectivity into the N-containing product when using S-[Pt-Zn] with a space velocity of 30.000 h -1 at 280~ the temperature at which the maximum of conversion was observed.
269
Figure 2: Influence ofthe zinc loading or of the method of prepadon on the NO conversion Conditions: SV:60.000h-', 100 500 ppm, CO 350 ppm, CO, 1OOm. O26%. 12%
PPane~oppm~propene~~oppm
Figure 3: Selectivity and stability versus time on S-[Pt-Zn] catalyst
270
3.2. Influence of space velocity During the catalytic reactions involved in NO reduction versus the temperature we observed the following points: - The NO conversion goes through a maximum versus the temperature. - N20 is formed and its evolution follows a volcano shape curve versus the temperature. The maxima of these two volcano curves are more or less flat. - NO2 is formed and its development follows a plateau versus the temperature. In Table 2, the maxima conversion in NO and N20 in percent are reported as well as the temperature of these maxima. For NO2 formation is reported the temperature at which the plateau is attained and its conversion in percent. The catalyst used was S-[ Pt-Zn]/A1203. From Table 2, we can observe that on S-[Pt-Zn] catalysts more NO is transformed and less N20 and NO2 are formed than on Pt catalyst, except when 200mg of S-[Pt-Zn] is used. The temperatures at which these maxima are observed are about 20~ higher on S-[Pt-Zn] than on Pt but the summits of these volcano curves are more flat for NO conversion on S-[Pt-Zn] which explains why in a range of 20~ we have the same NO conversion. Moreover at 35% NO conversion a larger domain of temperatures is observed as mentioned in Table 3.
Table 2" Conversions versus th e reaction temperature. Catalyst Catalysts weight Max. NO (VVH h "l) conversion (%) temperatureT~ Pt 38% at 300~
100mg
Max. N20 conversion (%) temperatureT~ 17% at 200~
NO2 conversion (%).at T~
8% at 250~
2% at 320~
17% at 210~
3% at 320~
5% at 2600C
(60 000h q) S-[Pt-Zn]
42% at 320~ 42% at 300~ 45% at 260~
Pt 200mg (30 000h 1) S-[Pt-Zn] Pt
58% at 280~ 56% at 260~ 48% at 280~
15% at 225~
18% at 320~
21% at 200~
10% at 280~
60% at 280~
4% at 200~
8% at 300~
400mg (15 000h 1) S-[Pt-Zn]
Table 3: Domain of temperatures for a constant NO transformation of 35%. VVH in h" 1 AT in ~ on S-[Pt-Zn]/A1203 AT in ~ on Pt/A1203 60000 120~ 50~ 15000 21 o~ 1oo~
271 Now we are going to analyse how NO is transformed, at a constant temperature of 300~ versus the space velocity. The nitrogen mass balance is: [NO]0 = [NO]t + 2[N20] + [NO2] + 2[N2] and the conversion is defined as" ([NOlo- [NO]t)/[NO]o = 2[N20]/[NO]o + [NO2]/[NO]o + 2[N2]/[NO]o where [NO]0 and [NO]t are the concentration respectively of NO initially and at time t at one defined temperature. The results reported in Table 4 prove that more N2 is formed on S-[Pt-Zn] catalysts than on Pt catalyst at 300~ Moreover, when the space velocity decreases, N2 selectivity increases mostly at the expenses of N20. This fact would tend to prove than N20 is a reaction intermediate. Table 4: Nitrogen mass balance at 300~ Alumina Catalysts NO supported weight conversion catalysts (VVH h l ) in % Pt 38% 100mg (60 000h ~) S-[Pt-Zn] 42% Pt
N20 Formation in % 5%
NO2 N2 Formation Formatio in % n in % 5% 28%
8%
2%
32%
40%
5%
2%
33%
56%
5%
16%
35%
44%
5%
10%
29%
55%
2%
8%
45%
200mg (30 000h l ) S- [Pt-Zn] Pt 400mg (15 O00h1) S-[Pt-Zn]
3.3. The influence of oxygen. 3.3.1. Effect of the concentration of oxygen The conversion of NO increases with oxygen concentration (Figure 4) but this increase is more marked between 0% and 2% 02 The initial slope from the NO conversion curve versus the percentage of 02 is 1.5 higher on [Pt-Zn] than on Pt catalyst, when the reaction is performed at 250~
272
60
--
50
40
20
10
0 0
I
2
3
4
5
6
Oxygen conc. (%)
Figure 4 : Effect of the oxygen concentration upon NO conversion into N2 on S-[Pt-Zn] 3.3.2. Reaction in the absence of NO In order to investigate the role of hydrocarbon on the NO conversion the activity of Pt and S[Pt-Zn] catalysts was measured using a gas mixture: 500 ppm NO or in the absence of NO in the gas stream, 330 ppm HC, 6% 0 2 and 12%H20. It can be seen from the figures 5a and 5b that the HC conversion was improved by the addition of NO over the S-[Pt-Zn] catalyst; for example, the HC conversion was only 42% at 300~
in the absence of NO, whereas the conversion was
reached about 78% when NO was added. While over Pt catalyst the addition of NO in the feed apparently inhibits the HC oxidation. This enhancement of activity observed on S-[Pt-Zn] can be attributed to the presence of zinc oxide which needs further investigations to precisely define its role. 3.4. The decomposition of N20 This part of the research was undertaken in order to prove whether or not N20 was an intermediate in the reaction of NOn reduction and to demonstrate a possible effect of zinc in the surprising low production of N20. Indeed, it is well known than under TWC conditions one of the by-product when using noble metal type catalysts is nitrous oxide [3]. Two types of experiments were carried out (in both cases, the catalyst was pre-treated under 6 % 02, 12 % H20): i) Feed composition: 0.5 % N20 in nitrogen, ii) Feed composition: 0.5 % N20 + 6 % 02 in nitrogen. The conversions of N20 at 480~ are respectively reported in tables 5 and 6.
273
100
100 90
Pt/Alumina
.-.. 80
/
~
~
- -:
~
---70 e.o
60
r>
50
ul L
e.-
'
__
_
90 8(1
70
~ so e ao4~
o 40 c.) 0 30 'r"9 20
!2w"|' No I
..,.
.. W i t h o u t NO[ 20
10
10
0 150
200
250
300
350
400
450
500
550
0
600
,
150
Temperature (*C)
2{10
,
,,
250
300
350
Temperature
,
u ,,,
400
,
450
500
550
(~
Figure 5b" Conversion of I-IC wltn or without NO on S-[Pt-Zn]/A1203
Figure 5a" Conversion of HC with or without NO on Pt/AI203
Table 5' D.ecomposition of N,O at 480~ .... Conversion % ........ _ ~Catalyst . . . . . . . . . . .
1%Pt
11
S-l%Pt-O.5%Zn C-l%Pt-O.5%Zn C- 1%Pt-2.7%Zn C- 1%Pt- I O%Zn
1 7
Table 6: Decomposition 0fN20 in the presence of oxygen at 480~ Catalyst Conversion % 1%Pt S-1%Pt-0.5%Zn 3 C-l%Pt-0.5%Zn 1 C-l%Pt-2.7%Zn 0 C- 1%Pt- 10%Zn
When N20 is passed on the catalyst, the decomposition reaction never occurs at temperature lower than 430~ The addition of zinc has almost no effect on the activity. The conversions are very low in the temperature range of 430-500~ and are even lower in the presence of 6% oxygen. This suggest several remarks: i) Oxygen inhibits the decomposition reaction, the expression of the reaction rate is [4]: -d[N20]/dt = K x PN20 X P02 "!/2 it) The absence of N 2 0 formation under Lean NOx is not explained by a formation/decomposition mechanism. Nevertheless, we do not exclude the fact that an intermediate of N20 could be formed on the adsorbed state from NO following this scheme, as
274 pointed out by the results in table 4, where the adsoption of N20 from the gas phase during the reaction of decomposition would be strongly unfavoured: 2NO
~
(N20)ads + (O)ads --~
N2 + ~ 02
:r fN20)ga~
4. CONCLUSION From these results we may understand that, surprisingly, under certain conditions that zinc could be a promoter of the platinum rather than a poison for reactions involved in a catalytic exhaust device. Several points have been fotmd: In the presence of Zn, the NO conversion is increased in parallel with the N2 formation. Moreover, the range of temperature in which N2 is formed is larger than in the absence of zinc. Also, this catalyst is very stable in time over a period of 10 hours. From EXAFS characterisation, the presence of platinum oxide was established which has several implications on the possible mechanism. As in the case of Cu-ZSM5 catalyst type, a promoting effect of the oxygen concentration was observed [5] as well as a volcano shape curve [6] for the NO conversion. On the contrary, an inhibiting effect of the partial pressure of oxygen was found in the case of the reaction of N20 decomposition which would indicate that N20 is not a rection intermediate. Another important point was the occurrence of a promoting effect of zinc for the HC conversion in the presence of NO. All these remarks recall the work on several system where an oxygenated HC is involved as a reaction intermediate in the DeNOx process. Zn has a promoting effect in an oxidising atmosphere contrary to the reactivity under hydrogen where inhibiting effects are usually observed[7]. The role of Zn (or ZnO) could be to favour the adsorption and formation of the reaction intermediate. The nature of the interaction between Pt and Zn still need to be elucidated. REFERENCES
1. 2. 3. 4. 5. 6. 7.
W.Fitzgerald, J.V.D.Wilson, SAE 750447 (1975) C.Serre, PhD dissertation, Universit6 Strasbourg (1991) B.K.Cho, B.H.Shanks, J.E.Bailey, J.Catal, 115, 486, (1989) E.R.S.Winter, J.Catal., 19, 32 (1970) C.N.Montreuil, M.Shelef, Appl.Catal., B, 1 (1992) L1 M.Iwamoto, Proc.meet. Cat. Technol.Removal of NO, Tokyo, Jan. 1990, 17 B.Coq, F.Figueras, J.Mol. Catal., 40, 93, (1987)
CATALYSIS AND AUTOMOTIVEPOLLUTIONCONTROLIV Studies in Surface Science and Catalysis, Vol. 116 N. Kruse, A. Frennet and J.-M Bastin (Eds.) 91998Elsevier Science B.V. All rights reserved.
275
Catalytic Properties of Palladium Exchanged ZSM-5 Catalysts in the Reduction of Nitrogen Monoxide by Methane in the Presence of Oxygen: Nature of the Active sites P. G61int , A. Goguet I , C. Descorme I , C. L6cuyer2 and M. Primer I ~Laboratoire d'Application de la Chimie ~ l'Environnement, UMR CNRS 5634, Universit6 Claude Bernard Lyon I, Bat. 303, 43, Boulevard du 11 Novembre 1918, 69622 Villeurbanne Cedex France, Fax : +33 478 94 19 95 2GAZ DE FRANCE, Direction de la Recherche, CERSTA, P.O.Box 33, 93211 La Plaine Saint-Denis Cedex, France, Fax +33 9 149 22 49 67 The catalytic activity of Pd-H-ZSM-5 catalysts containing 0.18 to 1.56 wt.- % Pd in the reduction of NO by CH4 in the presence of excess 02 (lean mixture) was measured and the adsorption of NO was studied by FTIR. For all samples, NO adsorption results in the formation of a Pd + mononitrosyl complex (v NO = 1881 cm l ) and adsorbed NO2 (v NO = 2136 cm -1) interacting with both Pd ions and acidic hydroxyl groups of the zeolite. The formation of NO2 arises from the reduction of isolated Pd 2+ cations and/or Pd 2+ hydroxyl complexes bonded to the oxygen atoms of the zeolite framework into Pd + complexes still anchored to the framework. The amount of Pd complexes increases linearly with Pd content up to 0.6 wt.-% Pd. The catalytic activity measurements indicate that two competitive reactions occur: the reduction of NO by CH4 and the combustion of CH4 by 02. Samples containing less than 0.5 wt.-% Pd exhibit high selectivity for NO reduction and the conversion of NO increases with the amount of Pd nitrosyl complexes detected by IR. For samples with higher Pd contents, the combustion of CH4 is favored. The coexistence of isolated Pd complexes active for the SCR reaction and PdO aggregates active for the CH4 combustion is suggested to explain the catalytic properties. 1. INTRODUCTION In the past few years there has been growing interest in studying the catalytic reduction of nitric oxide by methane in the presence of oxygen over various ion metal-exchanged zeolites including Co, Fe, Ni, Mn, Ga, I n , Pd (1-29). Most of the work was devoted to Co-ZSM-5 because of its high activity (2-14). Attempts to describe the mechanism by which methane reduces NO over these catalysts revealed specific catalytic behaviors depending on the zeolite structure and/or the exchanged metal. For example, Cu-ZSM-5 was shown to form nitro and nitrate species reactive with C3+ alkanes but non reactive with methane (12), which could explain the inactivity of this catalyst in the reduction of NO by methane. The activity of Ga- or In- ZSM-5 catalysts was found highly sensitive to water vapor in the feed, much more than Co-ZSM-5 catalysts (18). Much work is still needed to fully understand the mechanism of the
276 reaction and the specificity of the various zeolite catalysts. Among these, Pd-H-ZSM-5 and Pd-H-MOR catalysts were recently found even more active than Co-ZSM-5 and Co-Ferrierite in the reduction of nitric oxide by methane under lean conditions (24, 25, 29). We present here a study of Pd-H-ZSM-5 catalysts in the reduction of NO by CH4 in the presence of excess 02 coupled with a detailed IR investigation of their reactivity towards NO at room temperature. The surface species formed upon NO adsorption were identified. The influence of the Pd content on the catalytic and adsorptive properties of these catalysts was examined. A relationship between the catalytic activity in the reduction of NO and the formation of surface adsorbed species observed by FTIR upon NO adsorption was obtained. 2. E X P E R I M E N T A L
2.1. Samples preparation The Pd-H-ZSM-5 zeolite samples were prepared by conventional exchange of H-ZSM-5 (PQ Zeolites B.V., CBV 5020, Si/A1 = 25) with aqueous stirred solutions of tetrammine Pd(II) nitrate at 50~ for 6 hours. The amount of Pd(II) salt was adjusted so as to vary the final Pd loading. After exchange, the preparations were thoroughly washed with deionized water, filtered and dried at 120~ overnight. In order to decompose the Pd complexes, the Pdexchanged zeolite samples were subsequently activated under flowing oxygen from 25~ up to 500~ (linear ramp of 0.5 ~ minl). Five samples were prepared, containing respectively 0.18, 0.38, 0.49, 0.66 and 1.56 wt.-% Pd (measured by atomic absorption analysis) and named Pd-H-ZSM-5(x) with x = wt.-% Pd.
2.2. Catalytic activity measurements The experimental details for activity measurements were described elsewhere (25). Briefly, the catalytic activities for the conversion of NO and CH4 were measured using a U-shaped quartz reactor (16 mm ID) operating in a steady-state plug flow mode. Typically, 200 mg of catalyst were activated in-situ in oxygen flow at 500~ (linear ramp rate of 0.5~ purged for one hour at 500~ by flowing helium and cooled to 250~ before being contacted with the reactants. The reaction mixture was adjusted so as to examine the catalytic activity under lean conditions: 2000 vpm NO, 1000 vpm CH4, 6240 vpm 02; helium as balance; total flow rate = 167 cm3/min. [The apparent gas hourly space velocity (GHSV) was 30,000 h l , based on the apparent bulk density of the zeolites, ca. 0.5 g/cm3.] The catalytic activity was measured as a function of temperature in the range 250- 600~ by using a linear heating rate of 1~ The stability of the catalytic activity was examined during two additional hours at 600~ before cooling the sample down to 250~ (linear ramp of 1~ The same sequence under reaction mixture was applied again to check for possible irreversible changes of the catalyst. The effluent gases were analyzed using two gas chromatographs equipped with TCD and FID detectors and NOx infrared analyzers. Carbon and nitrogen balances were checked. The NOx conversion was determined according to the following equation" NOx conversion % = ([NO]0 + [NO2]0 - [NO] - [NO2]) * 100/([NO]0 + [NO2]0) where [NO]0 and [NO2]0 are the inlet concentrations of NO and NO2 respectively and [NO] and [NO2] the outlet concentrations. The NO2 formation was low in the whole range of temperature ([NO2]< 40 vpm), almost independent on the temperature and ascribed to the NO
2'/'/ oxidation in the dead volume of the apparatus. The CH4 conversion was determined from the consumption of CH4.
2.3. X-ray diffraction measurements X-ray diffraction measurements were performed using CuK~ radiation on a Philips PW 1710 diffractometer. 2.4. FTIR measurements The IR studies of NO or NO2 adsorption were performed using self supported samples wafers (18 mm diameter, weight of 30 mg ) introduced into a home made IR cell allowing in situ studies at varying temperatures under controlled atmosphere (30). The samples were pretreated in situ in flowing oxygen at 500~ for 30 min (linear ramp rate of 10 ~ The cell was subsequently connected to a UHV system allowing a base pressure as low as 108 Torr (1 Torr = (101 325/760) Pa) and the sample evacuated at 500~ for 3 hours before being cooled down to 25~ and contacted with NO. The IR spectra were recorded at a resolution of 4 cm -I on a Nicolet Magna-IR 550 FTIR spectrometer. All the reported spectra have been corrected for the baseline (spectrum of the sample activated in situ and evacuated under vacuum at 25~ 3. RESULTS
3.1. Catalytic activity Figure 1A shows the effect of temperature on the conversions of NO and CH4 over Pd-HZSM-5(0.38). Both conversions begin around 300~ and increase with temperature up to 600~ the curves exhibiting an inflection point around 500~ It must be pointed out that NO is converted only to N2 in the whole range of temperature. Over Co-ZSM-5 catalysts, two reactions, NO reduction by CH4 (1) and CH4 combustion (2), have been suggested to occur (2-5): CH4 + 2 NO + 02 "--} CO2 + N2 + 2 H20 (1) CH4 + 2 O2 --+ CO2 + 2 H20 (2) Reaction (1) was based upon the fact that the catalyst was not active for NO reduction in the absence of 02 (2-6) below 500~ The same was observed on Pd catalysts (26, 32). Accordingly, the selectivity towards NO reduction, SSCR, defined as the fraction of methane involved in reaction (1), can be written: SSCR -" 0.5 [NO]o CNO / [CH4]0 CCH4,
where CNO and CcH4 are the conversions of NO and CH4, [NO]o the inlet concentration of NO and [CH4]o the inlet concentration of CH4. In our experimental conditions, 0.5 [NO]0 = [CH4]0, and SSCR= CNO / CCH4. The selectivity for NO reduction over Pd-H-ZSM-5(0.38) was plotted as a function of temperature (figure 1B). The selectivity is high (70%) and constant below 500~ then slightly decreases above 500~ indicating that the CH4 combustion would be slightly favored at high temperatures. At 400~ the selectivities of Co and Pd catalysts for NO reduction are comparable. However, the selectivity of the Pd-H-ZSM-5 catalyst does not vary in the whole range of temperatures, while it decreases rapidly from 75% down to 13% between 400 and 500~ over Co-ZSM-5 (5). Upon ~ lempemtt~ rarnl~ the conversions
278
40
--
-
A
50
_ .
B 0.8-
40
30-
T O
t-
o o 0.6z o
t.-
30 o
ffl L_
> tO
20
v
> t-O
o T
o
$"
~
20 o O z 10
,.
10
u.
250
...
>,
.--- 0.4-O
.,..,.
o9 0.2
-
0
I
I
I
350
450
550
400
Temperature / ~
I
I
500
600
Temperature / ~
Figure 1" Catalytic activity of Pd-H-ZSM-5 ( 0.38 wt.-% Pd ) as a function of temperature. (A) Conversions of NO ( II, [21 ) and CH4 ( O, O ), (B) Selectivity for NO reduction (CNo / CCH4 ). Solid line = 1st run, broken line - 2 nd run.
A
100 --
o~
,,
. t 50
1 -
- 40
.~ 0.8 -
80
=
t-.
- 30 .o ~
.9 60 > tO
o T 0
>t-.-
o.9. o
o 0.6-
v >,
40
-- 20 0o
'~ -= 0.4
20
t
10
co 0.2
/
0
0 250
,
I
I
350
450
550
Temperature / ~
B
o
0 400
I
I
500
600
Temperature / ~
Figure 2: Catalytic activity ofPd-H-ZSM-5 ( 1.56 wt.-% Pd ) as a function of temperature. (A) Conversions of NO ( II, UI ) and CH4 ( Q, O ), (B) Selectivity for NO reduction ( CNO / CCH4 ). Solid line = 1st r u n , broken line - 2 nd run.
279 curves were the same, indicating no irreversible change of the catalyst upon reaction up to 600~ Figure 2A shows the conversions of NO and CH4 over Pd-H-ZSM-5(1.56) as functions of temperature. The NO conversion exhibits a volcano shape curve with a maximum around 500~ At this temperature, the conversion of CH4 reaches 95% and upon increasing further the temperature CH4 is totally consumed and NO conversion declines. The selectivity curve plotted in figure 2B as a function of temperature exhibits a continuous decrease with temperature from 50% at 400~ down to 14% at 600~ the decrease being more severe above 500~ Above 500~ the decline of the selectivity could be ascribed to the disappearance of CH4 through the combustion reaction (reaction 2), as already observed over Co catalysts (4-6). Indeed, the NO reduction is known to be dependent on the partial pressure of CH4 (5, 31). The NO conversion is therefore expected to decrease upon decreasing amount of CH4 due to increasing rate for CH4 combustion with temperature. In the low temperature range, the selectivity of Pd-H-ZSM-5(1.56) appears less than Pd-H-ZSM-5(0.38). Moreover, it decreases with temperature. This might indicate the presence, in Pd-H-ZSM-5(1.56), of catalytic sites only active for the CH4 combustion by 02 in addition to those active for both reactions. Upon repeating the experiment, the Pd-H-ZSM-5(1.56) sample still shows a volcano curve of NO conversion but with much less activity, the selectivity for CH4 combustion being still increased. This would indicate strong irreversible modifications of the catalyst upon reaction at 600~ favoring the combustion reaction. 3.2. XRD The X-ray pattern of the Pd-H-ZSM-5(1.56) catalyst reveals a small peak at 33.9~ characteristic of PdO ([ 101 ] plane), indicating the presence of large particles of PdO. For all the other samples, no peak at 20 = 33.9 ~ could be observed, confirming the high dispersion of palladium in the catalysts. Palladium is supposed to be atomically dispersed in the channels of H-ZSM-5 in the form of Pd(II) or Pd(II) hydroxyl complexes anchored to the zeolite framework (25, 28, 29), presumably at the channels intersections.
3.3. IR Studies of NO adsorption NO was adsorbed at 20~ on all the prepared Pd-H-ZSM-5 catalysts (with varying Pd content). For all the samples, two bands at ca. 2136 and 1881 cm 1 are clearly developing simultaneously when incrementing NO pressure. Figure 3 shows the IR spectra obtained after contacting each Pd catalyst with 0.5 Torr NO at 20~ The 2136 cm -I band, observed also upon adsorption of NO2 on H-ZSM-5, is ascribed to adsorbed NO2 ~+ probably interacting with acidic hydroxyls (28). Upon evacuation under vacuum at increasing temperature, the 2136 cm l band disappears progressively between 50 and 180~ while NO2 is detected in the gas phase, corroborating its attribution to some form of adsorbed NO2 (29). It is noteworthy that NO2 adsorbed on H-ZSM-5 is already removed by pumping off at room temperature. This would indicate that NO2 formed upon reaction of NO with Pd-H-ZSM-5 is much more strongly held to the surface. Therefore it would be ascribed to NO2 ~+ associated with both acidic hydroxyls and Pd cations, in agreement with previous studies (14, 28, 33). The 1881 cm ~ band was attributed to a Pd nitrosyl complex (28). The ratios NO/Pd = 1.5 measured upon NO adsorption and NO2/Pd = 0.5 measured upon thermodesorption in flowing helium were consistent with the reduction of Pd(II) into Pd(I) and subsequent formation of
280 Pd(I) mononitrosyl complexes (28). These complexes are highly stable upon heating under vacuum since the intensity of the 1881 cm 1 band is almost not affected up to 400~ (29). However the nature of this band could be multiple since shoulders at about 1940 and 1840 cm I can be observed too. The 1940 cm "l shoulder behaves exactly as the 1881 cm l one and does not find any ascribment yet. The 1840 cm l band readily disappears upon evacuation under vacuum at 20~ while the intensity of the 1881 cm 1 band slightly decreases too. This would suggest that the 1840 cm "l band and some band masked by the strong 1881 cm -1 absorption characterize NO weakly bonded to the catalyst, tentatively ascribed to NO adsorbed on very small PdO particles. A broad envelope of very weakly intense unresolved peaks between 1670 and 1610 cm "l is observed too, ascribed to nitrate species formed upon reaction between NO2 and the zeolite surface since it readily forms upon contacting H-ZSM-5 with NO2 gas phase at 20~ (28). This ascribment is confirmed by the following. When evacuating Pd-H-ZSM-5(0.49) up to 100~ after NO adsorption, the feature around 1650 cm "1 develops (its intensity keeps weak). At this temperature, adsorbed NO2 is removed, as indicated by the decrease of the 2136 cm -1 band, and it is expected that NO2 moving along zeolite channels reacts partially with energetic sites to form nitrate species. These species progressively decompose into NO in the 150400~ range (29). 1881 cm a
A =0.05 A=0.1
(1)
=o
O t..Q I...
2136 c m l
2200
2000
O
~
d~
1800
1600
1400
Wavenumber / cm 1
Figure 3" IR spectra of Pd-H-ZSM-5 contacted with 0.5 Torr NO at 20~ and containing: (a) 0.18, (b) 0.38, (c) 0.49, (d) 0.66, (e) 1.56 wt.-% Pd.
i
a
.......... "---1
2300
1900 Wavenumber / cm -1
1500
Figure 4: IR spectrmn of Pd-H-ZSM-5(0.49) contacted with 15N180 at 20~
The reduction of Pd(II) upon reaction with NO at room temperature is confirmed by adsorption of 15N180 on Pd-H-ZSM-5(0.49). The IR spectrum in vNO region shown in Figure 4 shows 4 bands at 2093, 2043, 1844 and 1799 cm 1. These bands are shifted from 2136 and 1881 cm "1 towards lower frequencies according to the variation of reduced mass expected for 15N160 and 15N180 respectively. This indicates that Ol6-1abeled NO2 was formed upon reacting Ol8-1abeled NO with ol6-containing Pd species of the catalyst, and therefore explains
281 the reduction of Pd(II) ions. Adsorbed O16-NO2might exchange with O18-NOgas phase either at the Pd or H + center (both sites are available for competitive adsorption of NO and NO2). Therefore as O 16- NO2 forms progressively, the NO gas phase is enriched in 016. At equilibrium, the ratio of O16-to ol8-containing species is the same for adsorbed NO2 and Pd complexes, reflecting the O 16 to O 18 composition of the NO gas phase. In order to know whether the Pd ions or complexes are anchored to the zeolite framework or not, the IR framework vibrations of Pd-H-ZSM-5(0.49) were investigated (Figure 5). After activation under O2, a weak band at 930 cm ~ forms. Upon NO adsorption, the 930 cm 1 band disappear while a new band appears at 980 cm ~ These bands are attributed to asymmetric internal stretching vibrations of T-O-T bonds (T = Si or A1) perturbed by Pd ions. The higher the perturbation, the lower the frequency. Therefore, the 930 cm 1 band could be related to anchored Pd(II) ions or complexes formed upon decomposition of exchanged complexes, and the 980 cm 1 band could be due to Pd(I) nitrosyl entities formed upon NO contact. Similar observations were found on Cu-ZSM-5 catalysts (34). 4. DISCUSSION For all the Pd-H-ZSM-5 samples prepared in this study, IR results indicate the main formation of Pd(I) mononitrosyl species in the presence of NO. These complexes are linked to the zeolite framework and characterized by a sharp intense band at 1881 cm "l. In Figure 6, the integrated intensity of this band (measured with 0.5 Torr NO) is plotted as a function of Pd content. Up to 0.5 wt.-% Pd, a linear relationship is observed. This result is consistent with the existence of isolated Pd(II) ions / Pd(I) nitrosyl complexes as catalytic sites for NO reduction similarly to Co 2+ cations as active sites in Co-ZSM-5 (1, 3). As the Pd loading is further increased, the curve bends over, indicating that an increasing fraction of exchanged Pd does not form Pd nitrosyl complexes. Accordingly, XRD patterns indicate the presence of large PdO particles (and the sample turns to the gray color characteristic of PdO instead of beige pink for low Pd contents). It can be concluded that, in spite of low Pd exchange levels, the ZSM-5 structure cannot maintain Pd cations in highly dispersed state above 15-20 % exchange (equivalent to 0.5-0.7 wt.-% Pd content). The ability of Pd-H-ZSM-5 catalysts to form Pd(I) nitrosyl species was related to their specific behavior of selectively reducing NO to N2 (25). This statement finds support in the curve of NO conversion versus Pd content (Figure 7A). Indeed, for reaction temperatures less than 500~ NO conversion clearly increases with Pd content, in a manner similar to the amount of Pd nitrosyl complexes versus Pd content. Above 500~ volcano shape curves are observed and NO conversion decreases for Pd content higher than 0.5 wt.-%. This can be easily explained by the simultaneous total conversion of CH4. The absence of reductant in the feed is expected to decrease the rate of NO reduction. This implies that CH4 participates to two distinct reactions, SCR reaction and methane combustion by 02, which compete at high temperatures. This competition is confirmed by the selectivity results, which indicates that the combustion is strongly favored above 500~ The question arises to know whether these two reactions are catalyzed by the same types of sites.
282
700
I A = 0.05
600 980 cm -~
r o E t~ .13 I,.,. O
'7,
/~
o (71
930 cm 4
+
500
~E 400 o
.13
<
>, 300
C
c E
200 100
1020
920
I
820
0
Wavenumber / cm -~ Figure 5: IR framework vibrations of (a) activated H-ZSM-5, (b) Pd-H-ZSM-5(0.49) after activation and (c) after subsequent adsorption 0.5 Torr of NO at 20~
4o--
B
600 ~
80 o
E ..O .,.
r-
.9 6 0 -
01"~176
L_
m > 20c O
o 0 z
100 --
~
o--9, 3 0 -
1.5
Figure 6: Integrated intensity of the 1880 cm l band (obtained upon contacting with 0.5 Torr NO) versus Pd content.
A
550 ~
I
I
0.5 1 wt.-% Pd
~
10-
o o
J: ,,x
0 0
40-
20I I 0.5 1 wt.-% Pd
I 1.5
0
I
0
I
0.5 1 wt.-% Pd
I
1.5
Figure 7: Influence of Pd content on catalytic activity of Pd-H-ZSM-5 samples at different temperatures. (A) NO conversion, (B) CH4 conversion.
283 For low reaction temperatures and low Pd contents, the selectivity for NO reduction never reaches unity, which suggests that the two reactions, SCR and combustion, are competing or coupled even at low temperature. On Co-ZSM-5, these reactions were shown to be coupled (13). The striking feature in the case of Pd catalysts is that the selectivity is almost constant up to 600~ and this strongly contrasts with the catalytic behavior of Co catalysts. These two catalysts are suggested to exhibit different mechanisms. On Co- catalysts, NO2, formed by reaction of NO with 02, would initiate both reactions (13), and the bending over of NO conversion curve with temperature would be ascribed to a decrease of NO2 concentration with temperature. This would be related to the decomposition of NO2 into NO + 8902 favored above 500~ The rate determining step of the SCR reaction was attributed to the activation of CH4 into CH3 radicals (1, 8, 11-13). On Pd- catalysts, NO2 is formed too but it does not depend on the equilibrium with NO/O2. Its formation would result in the reduction of Pd(II) cations to the +1 oxidation state. This might explain the specific behavior of Pdcatalysts compared to Co- ones, i.e. the constant and high selectivity of Pd- catalysts for NO reduction. For higher Pd contents, the selectivity for NO reduction clearly decreases and increasing the temperature strongly favors the combustion of methane. This suggests the presence of sites active for methane combustion but not (or little) active for the SCR reaction. This interpretation is also supported by the non linear relationship between Pd and Pd nitrosyl amounts. Since PdO particles do form, it is suggested that Pd catalysts might contain two types of sites: (i) Pd cations atomically dispersed in exchange positions and, upon NO adsorption, forming nitrosyl complexes anchored to the zeolite framework and adsorbed NO2: these sites are thought to be responsible for selectively reducing NO to N2 in the presence of 02 and also catalyzing the combustion reaction at a smaller rate; (ii) PdO aggregates, able to catalyze mainly methane combustion: their size would depend on experimental factors such as Pd exchange level and possibly exchange and/or activation conditions. Unfortunately the latter sites are not revealed by IR study of NO adsorption and further characterization studies are needed. Complementary TPD experiments are under work in order to confirm these statements and evaluate the relative proportions of these two types of sites and characterize their adsorptive properties. REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
J.N. Armor, Catalysis Today, 26 (1995) 147. Y. Li and J.N. Armor, Appl. Catal., B : Environmental, 1 (1992) L31. Y. Li and J.N. Armor, Appl. Catal., B : Environmental, 2 (1993) 239. Y. Li and J.N. Armor, Appl. Catal., B : Environmental, 3 (1993) L1. Y. Li, P.J. Battavio and J.N. Armor, J. Catal., 142 (1993) 561. F. Witzel, G.A. Sill and W.K. Hall, J. Catal., 149 (1994) 229. Y. Li and J.N. Armor, J. Catal., 150 (1994) 376. Y. Li, T.L. Slager and J.N. Armor, J. Catal., 150 (1994) 388. J.N. Armor and T.S. Farris, Appl. Catal., B : Environmental, 4 (1994) L11. Y. Li and J.N. Armor, Appl. Catal., B Environmental, 9 5 (1995) L257. A.D. Cowan, R. Dtimpelmann and N.W. Cant, J. Catal., 151 (1995) 356.
284 12. B.J. Adelman, T. Beutel, G.-D. Lei and W.M.H. Sachtler, J. Catal., 158 (1996) 327. 13. D.B. Lukyanov, J.L. d'Itri, G. Sill and W.K. Hall, l lth International Congress on Catalysis - 40th Anniversary, Studies in Surface Science and Catalysis, 101, J.W. Hightower, W.N. Delgass and A.T. Bell (Eds), Elsevier, Amsterdam, 1996, p. 651. 14. A.W. Aylor, L.J. Lobree, J.A. Reimer and A.T. Bell, l l t h International Congress on Catalysis - 40th Anniversary, Studies in Surface Science and Catalysis, 101, J.W. Hightower, W.N. Delgass and A.T. Bell (Eds), Elsevier, Amsterdam, 1996, p. 661. 15. K. Yogo, M. Ihara, I. Terasaki and E. Kikuchi, Chem. Lett., (1993) 229. 16. E. Y. Li and J.N. Armor, J. Catal., 145 (1994) 1. 17. E. Kikuchi and K. Yogo, Catalysis Today, 22 (1994) 73. 18. T. Tabata, M. Kokitsu and O. Okada, Appl. Catal., B : Environmental, 6 (1995) 225. 19. E. Kikuchi, M. Ogura, I. Terasaki and Y. Goto, J. Catal., 161 (1996) 465. 20. T. Tabata, M. Kokitsu and O. Okada, Catal. Lett., 25 (1994) 393. 21. M. Ogura and E. Kikuchi, 1 l th International Congress on Catalysis - 40th Anniversary, Studies in Surface Science and Catalysis, 101, J.W. Hightower, W.N. Delgass and A.T. Bell (Eds), Elsevier, Amsterdam, 1996, p. 671. 22. A. Fakche, B. Pommier, E. Garbowski, M. Primet and C. L6cuyer, French Patent Application 93 08 006. 23. Y. Nishizaka and M. Misono, Chem. Lea., (1994) 2237. 24. C. Descorme, A. Fakche, E. Garbowski, M. Primet and C. L6cuyer, 1995 International Gas Research Conference, Cannes (France), 6-9 Nov. 1995, Preprints Vol. IV, p. 505. 25. C. Descorme, P. G61in, M. Primet, C. L6cuyer and J. Saint Just, Studies in Surface Science and Catalysis, 97, Zeolites : A refined Tool for Designing Catalyst, L. Bonneviot and S. Kaliaguine, Eds, Elsevier, Amsterdam, 1995, p. 287. 26. C.J. Loughran and D.E. Resasco, Appl. Catal. B : Environmental, 7 (1995) 113. 27. H. Uchida, K. Yamaseki and I Takahashi, 2nd Japan-EC Joint Workshop, JECAT'95, Catalysis Today, 29 (1996) 99. 28. C. Descorme, P. G61in, M. Primer and C. L6cuyer, Catal. Lett., 41 (1996) 133. 29. C. Descorme, P. G61in, C. L6cuyer and M. Primet, Appl. Catal., B : Environmental, (1997), to be published. 30. N. Echoufi and P. G61in, J. Chem. Soc., Faraday Trans, 88 (1992) 1067. 31. C. Descorme, Thesis, Claude Bernard Lyon 1 University, 1996. 32. C. Descorme, A. Fakche, E. Garbowski, M. Primet, unpublished results. 33. T.E. Hoost, K.A. Laframboise and K. Otto, Catal. Lett., 33 (1995) 105. 34. G.D. Lei, B.J. Adelman, J. Sarkany and W.M.H. Sachtler, Appl. Catal., B : Environmental, 5 (1995) 245.
CATALYSIS AND AUTOMOTIVEPOLLUTIONCONTROLIV Studies in SurfaceScience and Catalysis, Vol. 116 N. Kruse, A. Frennet and J.-M Bastin (Eds.) 91998 Elsevier Science B.V. All rights reserved.
285
Influence of the p l a t i n u m - s u p p o r t interaction on the direct reduction of NOx under lean conditions Filip Acke a'b, Bj6rn Westerberg a'c, Lars Eriksson ~'d, Stefan Johansson a'e, Magnus Skoglundh a, Erik Fridell a and Gudmund Smedler a aCompetence Centre for Catalysis, Chalmers University of Technology, S-412 96 G6teborg, Sweden bDepartment of Inorganic Chemistry, G6teborg University, S-412 96 GOteborg, Sweden CDepartment of Chemical Reaction Engineering, Chalmers University of Technology, S-412 96 G6teborg, Sweden dDepartment of Engineering Chemistry, Chalmers University of Technology, S-412 96 G6teborg Sweden eDepartment of Applied Physics, Chalmers University of Technology and G6teborg University, S-412 96 G6teborg, Sweden
ABSTRACT Catalysts containing Pt supported on SiC, A1203 and ZSM-5 were prepared and studied for NO• reduction by C3H6 in 02 excess under transient (temperature ramps) and steady-state conditions. The maximum NO• reduction activity in the heating ramp experiments was similar for Pt/SiC and Pt/ZSM-5, while Pt/A1203 showed higher maximum activity. Both N2 and N20 formation was observed for all catalysts, although the respective amounts varied with the investigated system. Highest N2 selectivity was observed for Pt/A1203. When the NOx reduction activity was studied under steady-state conditions the activity of Pt/A1203 decreased substantially (mainly due to a loss in N2 production). Pt/ZSM-5 became somewhat more selective towards N2 production whereas the activity and selectivity of Pt/SiC remained at about the same values as for the heating ramp experiments. Adsorbed species on the surface of the different catalysts were investigated using in-situ FTIR in order to obtain information about the reaction mechanisms. The adsorption of species on Pt/SiC was negligible, while a number of absorption bands were observed for Pt/A1203 (N and C containing species, and -NCO) and Pt/ZSM-5 (HC).
1. INTRODUCTION Good fuel economy and durability are factors that have made the diesel engine the dominating source of power for the transport industry [ 1]. A setback with diesel technology is the emission of pollutants, in particular nitrogen oxides (NOx) and particulate matter (PM).
286 The emissions of PM can be reduced using filter technology, where the continuously regenerating trap (CRT) system appears promising [1]. The emission levels of NOx are however only affected by the CRT technology to some extent [1 ]. The large oxygen excess in the diesel exhaust obstructs the catalytic reduction of NOx by hydrocarbons. In order to reduce NO• under these conditions, there is a need to develop catalysts that possess high selectivity towards NOx reduction to N 2. Catalysts based on platinum have been pointed out as potential candidates for this process [2-3]. It has been concluded that Pt has the highest NO• reduction activity among the platinum group metals [23]. However, this high activity is accompanied by low N2 selectivity, i.e., large quantities of nitrous oxide (N20) are formed [2]. By appropriate choice of support material, e.g., introducing acidic groups on the surface, the selectivity towards N2 can be enhanced [4]. The objective of this investigation is to study the effect of the platinum support material in the lean reduction of NOx using propene as the reducing agent. For this reaction we describe differences in total activity and selectivity between platinum supported on three different materials with increasing acidity; SiC, A1203 and ZSM-5. The activities of the catalysts are studied in flow reactors under both transient (temperature ramps) and stationary conditions. Adsorbed species on the surface of the catalysts are characterised using in-situ Fourier transformed infrared spectroscopy (FTIR). Different reaction mechanisms and the nature of adsorbed species are discussed.
2. EXPERIMENTAL
2.1. Design of catalysts In accordance with our objective to study the influence of support acidity, our catalyst sample design approach was to keep the total surface area constant at 40 m 2 and total Pt loading constant at 2.0 rag, yielding a surface area based Pt loading of 50 gg/m 2 for all three supports under investigation. Through this catalyst design, we expect to compensate for the different specific areas that would have resulted from a constant support weight, as well as for the different amount of Pt per unit area that would have resulted from a constant Pt loading per unit mass of support.
2.2. Preparation of catalysts The support materials; SiC, T-AlaO3 and H-ZSM-5 (SIO2:A1203 = 34) were initially calcined in air at 600~ for 2 h. The specific surface areas of the calcined support materials were measured using nitrogen adsorption [5] and are included in Table 1. Pt was deposited on the support materials using the method described by Axelsson et al. [6]. A slurry of the respective support material was prepared by dispersing the support in distilled H20 under stirring. Specific amounts of the Pt-solution was added to the respective slurry under continuous stirring in order to obtain a constant Pt-content per square meter support (see Table 1). The three slurries were then freeze dried and finally calcined in air at 550~ for 1 h.
287 Table 1. The nomina! comPosition and BET-surface area of the prepared catalysts. i
Sample Pt/SiC Pt/AI/O3 Pt/ZSM-5
Sample weight [mgJ 1600 200 106
Pt weight {rag] 2.00 2.00 2.01
,
Pt content [mg/g support] 1.25 10.00 18.85
Surface area [mZ/g] 24.9 200 377
Pt/surface area [10 -6 ~/m2] 50 50 50 i
2.3. Flow reactor studies
The flow reactor used in the activity studies is described elsewhere [7]. Briefly it consists of a vertical quartz tube in which the sample is supported on a sintered quartz filter. Gases are introduced via mass flow controllers and the temperature is measured after the catalyst bed. Reactants and products are analysed using a quadrupole mass spectrometer and a photo acoustic FTIR gas analyser. The bed material consisted of a mixture of the powder sample and quartz sand in order to obtain a constant space velocity (25000 h l ) for all tested catalysts. The gas composition used in the experiments was: 10% O2, 405 ppm NO and 911 ppm C3H6, balanced with Ar to yield a total flow of 420 ml/min. The samples were initially reduced in 5000 ppm HE at 400~ for 15 min and stabilised in the reaction mixture at 525~ for 1 h. The samples were then cooled down to room temperature under an Ar flow. At this temperature, the catalyst was exposed to the reaction mixture under 15 min before starting the heating ramp up to 525~ at a constant rate of 6~ The steady-state experiments were performed by subsequently lowering the temperature in steps of 50~ starting from the final ramp temperature and the products were analysed after approximately 90 min. In order to facilitate the interpretation of the flow reactor and FTIR results the model gas was simplified by omitting H20 and SO 2 (which would have been present if a diesel exhaust was used). 2.4. F T I R studies The FTIR experiments were performed using thin discs (approximately 15 mg/cm2) of catalyst in a reaction chamber with CaF2-windows [8]. A disc was fixed in between folded tungsten grids placed in the centre of the reaction chamber. The temperature was measured with a thermocouple, in contact with the grid, and controlled via the voltage applied over the grid. The reaction chamber was placed in a FTIR spectrometer. All spectra were measured with 1 cm/s scan speed and a resolution of 4 cm l. The fresh catalysts were reduced in 30% H2 in N2 (total flow rate of 100 ml/min) at 450~ for 30 min., stabilised in a gas mixture with 5% 02, 1000 ppm NO and 3000 ppm C3H6 in N2 (total flow rate 1000 ml/min) for 30 rain and finally degassed in N2 (1000 ml/min) at 550~ for 30 min. The in situ FTIR measurements were performed with 5% 02, 400 ppm NO and 900 ppm C3H6 in N2 and at a total flow rate of 1000 ml/min (this means that the reaction cell operates as a differential reactor). The experiments started at 450~ and the temperature was then lowered in steps of 50~ with a 5 minute interval. Spectra from an average of 50 scans were taken the last minute of each interval. Reference spectra were taken with pure N2 in an otherwise identical sequence.
288 I
I
Nbal I
I
3500
I
4OO 300
g
3000
8
2500
C) ~
2000 200
100
/~c.~~!~.. _
_
0 ~
100
~
"~%~,
_
'
NO2
-
~klO N2
200
___. I
300
1500
~.
1000
o
500
400 .....
0
500
Temperature (~
Figure 1b. Concentration traces over a Pt/ZSM-5 catalyst in a heating ramp.
-"-I
T
1
I
3500
400
E ~
3OOO 0 0 .
.
.
.
.
.
.
.
.
2500 0
-
o
300
cO ~ c(1) r to
O
2000
o P-I,,
200
1500 m m .
o
100 -
1000
.~.
500
..,..,.-
"o -o
3
l
100
200
300
400
500
Temperature (~
Figure 1c. Concentration traces over a Pt/SiC catalyst in a heating ramp. The Pt/SiC catalyst (Fig. 1c) shows light-off around 220~ and NOx reduction in the same temperature range as for the other catalysts. The N20 formation maximises around 235~ and is of similar magnitude as for the Pt/ZSM-5 catalyst. The NO 2 formation rate increases rapidly around this temperature and shows a maximum at 320~ Adsorption of neither hydrocarbons nor NOn on the Pt/SiC sample is obvious from Fig lc. In Table 2 the NOn reduction efficiency and the selectivity towards N2 and N20 formation are summarised for the flow reactor experiments. Both the NOn reduction activity and the N2 selectivity of the Pt/SiC and Pt/ZSM-5 system appear to be similar, while Pt/A1203 shows a higher peak reduction value for the heating ramp experiments.
289 3. RESULTS 3.1. Flow reactor studies
3.1.1. NOx reduction-Activity and selectivity In the flow reactor study, the NO reduction activities and the N2 and N20 selectivities of the respective powder samples were investigated both by heating ramp (increasing temperatures) and steady-state experiments. The results of the heating ramp experiments are displayed in Figures 1a to 1c, where the outlet concentrations of NO, NO 2, N20 and CO2 are shown as functions of ramp temperature. Included is also the nitrogen balance, calculated as the sum of the concentrations of all detected nitrogen containing species, ([NO] + [NO2] + 2[N20]), and referred to as "Nbal". The by-pass value of all N containing species should be about 405 ppm and deviation from this value is due to adsorption and desorption phenomena and/or N2 formation by NOx reduction. No traces of other nitrogen containing compounds as, e.g., ammonia were detected. 3500 N
400
b
a
l
~
3000 O
0
E
.....
300
_~ i ~ ~ t . - ' ~ -
=~ ==.,-=..-~.=~HNN~,~ 2500 C) O
2000 o O
tO
tO O tO
O
200
m
1500 ~~ ~ 0
1000 ~.
100
500 100
200
300
400
"1:3 "13
3
500
Temperature (~
Figure 1a. Concentration traces over a Pt/A1203 catalyst in a heating ramp. The Pt/A120 3 catalyst (Fig. la) reaches 50% conversion of propene, around 230~ In connection to the light-off, there is a NOx reduction window between approximately 180 and 320~ The formation of N20 has a maximum around 260~ The NO2 formation proceeds above this temperature with a maximum around 370~ Around 150~ the Nbal level is higher than the inlet value of NO indicating desorption of NO adsorbed at lower temperatures. For the Pt/ZSM-5 catalyst (Fig. l b) there is light-off at somewhat lower temperature (210~ and a significant over-shoot in the CO 2 formation just above light-off. This behaviour is probably connected with combustion of hydrocarbons adsorbed at lower temperatures. The NOx reduction window occurs around the same temperatures as for Pt/A1203 but is less pronounced. The maximum in N20 formation is somewhat higher in magnitude than for Pt/A1203 and occurs at a lower temperature (230~ The NO2 formation starts at about this temperature and has a maximum around 340~ There is no desorption of adsorbed NOx below light-off.
290 Table 2. Maximum NOx reduction activity and selectivity (at the temperature for maximum reduction) during heating ramp and steady-state experiments, for a feed containing 405 ppm NO, 10% 02 and 9!1 ppm C3H6 at a flow of 420 ml/min. Sample Heating ramp experiments Steady-state experiments NOx Corresp. Yield [%] NO• Yield [%] red. [%] temp [~ N2 N20 red. [%] N2 N20 Pt/siC 56.1 232 20.8 35.3 50.9 19.2 31.7 Pt/A1203 85.8 250 61.3 24.5 53.1 35.8 17.3 Pt/ZSM-5 58.6 225 23.3 35.3 61.0 30.9 30.1 ii
iiii
i
i
i
i i
Interesting is that the increased reduction activity is accompanied by a high N 2 selectivity. The lowest temperature for maximum reduction is observed for the Pt/ZSM-5 system, followed by Pt/SiC and Pt/AI203. 3.1.2. NO2 formation Comparison of the NO2 formation for the investigated materials, as displayed in Figures la to lc, shows a difference between Pt/SiC on one hand, and Pt/AI203 and Pt/ZSM-5 on the other. The former system shows a fast increase in NO2 formation in the temperature interval of maximum reduction compared with the A1203 and ZSM-5 supported systems. Note the corresponding decrease in the NO signal. 3.1.3. Adsorption of reactants Differences in adsorption behaviour are observed for the investigated systems. No NO or NO2 desorption peaks are observed for Pt/SiC or Pt/ZSM-5, while a clear desorption of NO, with a maximum at 158~ can be observed for Pt/A1203. The Pt/SiC system is also inert towards hydrocarbon adsorption, while Pt/ZSM-5 adsorbs a substantial amount of hydrocarbons. It can be observed that for all tested catalysts, the CO2 formation and the NOx reduction are closely correlated: the maximum in NOx reduction is observed at almost complete hydrocarbon oxidation. 3.1.4. NOx reduction under steady-state conditions When the three materials are tested under steady-state conditions, a different picture is obtained. The results of the NOx reduction activity and the N2 and N20 selectivity, under steady-state conditions, are included in Table 2. The maximum NOx reduction activity and the selectivity towards N2 for the Pt/AI203 system are lower than in the heating ramp experiment. Under steady-state cbnditions, the Pt/ZSM-5 system shows the highest overall NO reduction activity and a somewhat higher selectivity towards N 2 compared with the heating ramp experiments. For Pt/SiC there are no significant differences between the two types of experiments. It was observed that also under steady-state the maximum NOx reduction coincides with almost complete CO2 conversion.
291 3.2. FTIR studies Since all samples only contain small amounts of platinum, we do not expect that absorption of IR radiation due to species adsorbed on platinum sites is strong enough to be detected in the measurements. Thus the absorption bands seen are likely to be connected with molecules adsorbed on the support. Figure 2a shows the spectra of Pt/A1203 when exposed to the reactant gas (see above) at different temperatures. A double band at 2235-2255 crn"1 can be seen at temperatures up to 250~ This feature can be ascribed to isocyanate (-NCO) adsorbed on the support [9-10]. Several bands in the 1200-1700 cm "l region are also observed. Three of them, 1465, 1575 and 1660 cm l , are attributed to carbonate species on the support [11-12]. The remaining bands are attributed to disparate nitrate, nitrite and nitro groups adsorbed on the support [13]. These bands can be observed together with the isocyanate bands up to 250~ The only bands that can be detected above this temperature, are the carbonate bands at 1465 and 1575 cm "l. The interpretation is obvious. Above 250~ the catalyst is ignited and as the reactants are consumed their coverages decrease. The CO2 that is formed is on the other hand still available for adsorption. Below 250~ the catalyst is not ignited and the reactants remain on the substrate.
0.30
Pt/AI20 3
0.25 oc -
0.20
'O
0.15
<
0.10
g.j c~
100 ~ 400
\~
300 ~ 200 ~ o~/ 250 ~ / 150 ~
\
0.05
0.00
I 2200
I 2000
! 1800 Wavenumber
I 1600
I 1400
....
II 1200
(cm "1)
Figure 2a. FTIR spectra from the Pt/A1203 sample when exposed to the reactant gas mixttn'e at different temperatures. For clarity, the curves are off-set (by 0.01) relative each other. Figure 2b shows the spectra of Pt/ZSM-5 when exposed to the reactant gas at different temperatures. Broad bands can be observed between 1300 and 1900 cm "1. As the bands are difficult to separate, no specific assignments can be made. However, when decreasing the concentration of propene at 250~ the absorbance in this spectral region declines. No effect was observed when varying the NO concentration. From these observations the majority of the bands can be attributed to different modes of C-H bending. Between 100 and 150~ no difference between the spectra in Fig. 2b can be seen. Above 150~ the absorbance of the whole region diminishes as the temperature increases. From 350~ and above no bands can be
292 observed. This reflects the immense ability of ZSM-5 to adsorb hydrocarbon. Even above light-off, that occurs around 225~ a considerable amount of hydrocarbon remains adsorbed. I
0.30
I _
Pt/ZSM-5
0.25
150oc
I
loo
to
c- 0.20
t~ .Q !.._.
o 0.15
r ..Q
<
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400oC
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-
-
I
I
2000
1800
I-
1600
350~~ I
1400
--'---I1
1200
W a v e n u m b e r (cm 1)
Figure 2b. FTIR spectra from the Pt/ZSM-5 sample when exposed to the reactant gas mixture at different temperatures.
/
0.30
I
i
I
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i
Pt/SiC
0.25 t-t~
0.20 -
100 ~
\150~
200~~ 250 oC
\
/
3oooc 3sooc
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.-Q
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<
0.10 _~ ----__-L_Z_--_Z _-7T_.~-_L-_k -_-J- _- ES ~-i.L-7_- 7_S~--7 . . . . . . . . . .
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_
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=S,,7"1
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!
2200
,,
! . . . .
2000
I
1800
, I
1600
I
1400
u
1200
W a v e n u m b e r (cm "1)
Figure 2c. FTIR spectra from the Pt/SiC sample when exposed to the reactant gas mixtm'e at different temperatures. The curves are off-set (by 0.02) relative each other. Figure 2c shows the spectra of Pt/SiC when exposed to the reactant gas at different temperatures. No absorption bands can be seen at any temperature. This is expected as SiC is considered to be an inactive support and the Pt content is low.
293 4. DISCUSSION The steady-state experiments for the Pt/SiC catalyst show that NO does not react at low temperatures, that it is mainly oxidised to NO2 at high temperatures, but that it is reduced to N2 and N20 in a temperature window closely connected with the propene light-off as observed by the rapid increase in CO2 formation. The in situ FTIR experiments show that adsorption on the Pt/SiC catalyst support is negligible at all temperatures and that SiC can be considered to be an inert support. From these studies we conclude that the NO oxidation and reduction reactions occur on Pt. The products from the lean NOx reactions on Pt include NO2, N20, N2 and recombined NO. The role of platinum is then manifold. Three important catalytic functions of Pt are: dissociation of oxygen, activation of the hydrocarbon and dissociation of NO. The extent of these reactions will in part determine the ratio between the coverages of N, O and NO on the Pt surface and hence, control the formation of NOz, N20 and N2. Other important factors determining the product yields are, e.g., sticking coefficients, activation energies for desorption and for formation of products and intermediates, and spillover of various species between Pt particles and the support. The heating ramp experiments for Pt/SiC revealed no significant difference in NO reduction activity or N2 selectivity compared with the steady-state experiments (see Table 2). For Pt/A1203, on the other hand, both the activity for NO reduction and the N2 selectivity were significantly higher in the ramp experiments compared with the steady-state experiments. For Pt/ZSM-5 the activity was similar in the two cases whereas the N2 selectivity was higher in the steady-state experiments. This raises the question why the steady-state experiments and the heating ramp experiments are so different. The FTIR experiments for Pt/A1203 and Pt/ZSM-5 show, in contrast to Pt/SiC, the presence of significant amounts of surface adsorbates on these samples. The adsorbates on A1203 seem to include, among others, isocyanate, nitrate, nitro and nitrite species. For ZSM-5 the adsorbates are indistinguishable but seem to include mainly hydrocarbons. On the basis of these results we hypothesise that some of the adsorbates on A1203 and ZSM-5 are thermally activated and participate in reactions with nitrogen and oxygen containing species on the Pt surfaces. The reactions with these adsorbates will then influence the ratio between the N, O and NO coverages on the Pt surface and thus change the rates of the reactions that determine the NO reduction activity and the Nz selectivity. As a result, the heating ramp experiments will show different product yields compared with the steady-state experiments. Secondly, these support adsorbates may participate directly in the NO reduction, for example by providing unpaired nitrogen atoms, which participate in the reduction [14]. In this connection isocyanate has been suggested as an adsorbate that may contribute with nitrogen during a heating ramp [15]. In our FTIR experiments on the Pt/A1203 sample there is an isocyanate absorption present at low temperatures which is not seen above light-off (thermal desorption of this species is known to take place above 300~ [10]). Also the magnitude of several other peaks, connected with carbonate species, are much lower at temperatures just above light-off compared to just below light-off. A third possibility for the influence of the substrate adsorbates on the reduction yields may be that the adsorbed species participate in the reduction of the platinum surface by reacting with adsorbed oxygen, and indirectly increase the probability for NO dissociation [16]. It is known that NO cannot dissociate on an oxygen covered surface [ 17-18]. The influence of the support on the activity
294 and selectivity in the lean reduction of NO can be attributed to its ability to provide intermediates or reductants for the reactions taking place on Pt. The acidity of the support probably affects the ability to store or form suitable adsorbates for the lean NO reduction. The relatively low Pt loading in our samples may result in an inadequately low activity for hydrocarbon oxidation, which in turn would result in hydrocarbon accumulation on the support in the pre light-off region. This effect is expected to be advantageous for the NOx reduction up to a certain level of hydrocarbon accumulation, but would, at very high hydrocarbon adsorption levels, lead to inhibition of the catalyst activity. Since this inhibition effect would be stronger for the more acidic support materials, it is quite likely that the results obtained in the present study would look different at a higher Pt loading.
ACKNOWLEDGEMENTS This work has been performed within the Competence Centre for Catalysis, which is financed by NUTEK - The Swedish National Board for Industrial and Technical Development, Chalmers University of Technology, AB Volvo, Saab Automobile AB, Johnson Matthey, ABB Flakt Industri AB, Perstorp AB and AB Svensk Bilprovning. One of us (GS) gratefully acknowledges the support from Johnson Matthey, Catalytic Systems Division.
REFERENCES ~
2. 3. 4. 5. .
7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.
P.N. Hawker, Plat. Met. Rev., 39 (1995) 2. R. Butch and P.J. Millington, Catal. Today, 26 (1995) 185. H. Hamada, Catal. Today, 22 (1994) 21. G. Zhang, T. Yamaguchi, H. Kawakami and T. Suzt~, Appl. Catal. B, 1 (1992) L 15. M. Skoglundh, H. Johansson, L. L6wendahl, K. Jansson, L. Dahl and B. Hirschauer, Appl. Catal. B, 7 (1996) 299. I.-M. Axelsson, L. L6wendahl and J.-E. Otterstedt, Appl. Catal. 44 (1988) 251. F. Acke and O. Lindqvist, Proc. of the 14th Fluidised Bed Combustion, accepted. P. Basu, T.H. Ballinger and J.T. Yates Jr., Rev. Sci. Instrum., 59 (1988) 1321. Y.J Mergler and B.E. Nieuwenhuys, J. Catal., 161 (1996) 292. F. Solymosi, L. V61gyesi and J. Sarkany, J. Catal., 54 (1978) 336. G. Bamwenda et. al., React. Kinet. Catal. Lett., 56 (2) (1995) 311. F. Solymosi and J. Sarkany, Appl. Surf. Sci., 3 (1979) 68. M. Schraml-Mgrth, A. Wokaun and A. Baiker, J. Catal., 138 (1992) 306. M. Shelef, Chem. Rev., 95 (1995) 209. Bamwenda, Chem. Lett, (1994) 2109. R. Burch, Appl. Catal. B, 4 (1994) 65. P. L66f, B. Kasemo, S. Andersson and A. Frestad, J. Catal., 130 (1991) 181. E. Fridell, M. Skoglundh, S. Johansson, B. Westerberg, A. T6mcrona and G. Smedler, this volume.
DeNOx Base Catalysts
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CATALYSIS AND AUTOMOTIVEPOLLUTIONCONTROLIV Studiesin Surface Science andCatalysis,Vol. 116 N. Kruse, A. Frennet and J.-M Bastin (Eds.) 91998 Elsevier Science B.V. All rightsreserved.
297
A Comparative Study of the Activity of Different Zeolitic Materials in NOx Reduction from simulated Diesel Exhausts M. Guyon a V. Le Chanu b p. Gilot a, H. Kessler b and G. Prado a a Laboratoire Gestion des Risques et Environnement, Universit6 de Haute Alsace, Ecole Nationale Sup6rieure de Chimie de Mulhouse, 25 rue de Chemnitz, 68200 Mulhouse, France
b Laboratoire de Mat6riaux Min6raux, Ecole Nationale Sup6rieure de Chimie de Mulhouse, URA CNRS, 3 rue Alfred Werner, 68093 Mulhouse Cedex, France ABSTRACT Cerium-, copper-cerium coexchanged ZSM-5, copper-MCM-22, copper- and cerium-EMT type zeolite, copper-FAU type zeolite and copper-Beta exhibit an activity of the same order as that of copper-ZSM-5 in NOx reduction under simulated Diesel exhaust conditions. Propene was used as the reducing agent. The catalysts were used in a powder form and their activities tested in a fixed-bed flow reactor at a space velocity of 50 000 H ~. Copper-SAPO-34 and cerium- and gallium-EMT type zeolite have a moderate activity under the same conditions. The presence of water vapor inhibits the activity of EMT-zeolites. When an ageing procedure is carried out on copper-EMT type zeolite, dealumination occurs. The increase of the Si/A1 ratio of the zeolite does not limit the dealumination process. The exchange of the zeolite with lanthanum prevents the zeolite from dealumination but leads to a loss of the catalytic activity.
1. INTRODUCTION Diesel engines, which operate at high temperatures and under oxygen-rich environment lead to the formation of large amounts of nitrogen oxides in the form of NO. Very stringent regulations have appeared for mobile applications [1]. Since the improvement of engine design will not be enough to meet these regulations, a procedure to cleanup the exhaust is required. New catalysts have to be developed. They have to be effective under high-oxygen conditions (about 10%) and high space velocities. The space velocity is defined as the ratio of volumetric flow rate to catalyst volume (powder form) or monolith volume (wash-coated powders). Space velocities in the range 50 000-150 000 h l are currently used for laboratory or engine-bench testing. Catalysts have to be active over a wide temperature range corresponding to urban and highway driving. They also have to be thermally stable, to keep their activity in the presence of a high concentration of water (about 10%) and to present a weak activity for the conversion of SO2 to SO3 to avoid sulfate formation [2]. Much experimental work has been performed using the copper-exchanged zeolite ZSM-5 (Cu-ZSM-5) as a catalyst for lean NOx reduction [3-7]. It shows high efficiency in the reduction of NOx by various hydrocarbons, even under oxygen in excess (10 %), especially in
298 the absence of water and at low space velocity. Under real exhaust conditions (10 % of water vapor and high space velocity of about 100 000 h 1) the NOx reduction rates decrease considerably. Reaction mechanisms have also been proposed [8-11]. Results regarding this copper-doped ZSM-5 as well as other exchanged-metal zeolites are summarized in a review [12]. A more general review, including performances of other non zeolitic catalysts such as oxides doped with metal ions, has also been published [ 13 ]. This contribution deals with the catalytic reduction of NOx under severe conditions (presence of water and high space velocity of 50 000 h -1) using other zeolitic materials than Cu-ZSM-5. The materials which are considered in this work are cerium and co-exchanged (Cu and Ce) ZSM-5, MCM-41 and MCM-22 exchanged with copper, copper-, cerium- and gallium-EMT type zeolites, copper FAU-type zeolites, copper-SAPO-34 and copper-Beta. The criteria for the selection of these materials were: good thermal stability, variability of the Si/A1 ratio, hence of the cation exchange capacity, and pore openings large enough for the adsorption of hydrocarbons. The performances of all these materials are compared to that of Cu-ZSM-5 which is considered in this work as a reference material. Performances of aged EMT-type samples were also determined and interpretations of their loss in activity are given. Some solutions to improve their stability were experimentally investigated and the results are discussed.
2. EXPERIMENTAL 2.1. Catalyst preparations The cerium and cerium-copper exchanged ZSM-5 samples were supplied by IRMA (Lorient, France). Zeolite MCM-22 was synthesized using a variant of the first synthesis of Rubin and Chu [14]. A gel with the molar composition 1 SiO2:0.033 A1203:0.09 Na20:0.35 R :45 H20 (R: hexamethyleneimine) was heated at 150~ for 8 days. The solid was filtered, washed, dried and calcined in air at 540~ for 16 h to remove the organic template. The molar Si/A1 ratio was 15.3. The organized mesoporous material MCM-41 [15] was obtained by combining at room temperature colloidal silica with a sodium aluminate solution in the presence of cetyltrimethyl-ammonium surfactant cations. After filtration, washing and drying, the solid was calcined in air at 200~ (4 h) followed by 10 h at 540~ The Si/A1 ratio was 23. The EMT- and FAU- type samples were prepared according to Delprato at al. [16] in the presence of the crown ethers 18-crown-6 and 15-crown-5, respectively. A gel of molar composition 1 SiO2:0.1 A1203:0.21 Na20:0.07 R :14 H20 (R :18-crown-6 or 15-crown-5) was aged at room temperature for 24 h, then heated at 110~ for 15 days. The solids were filtered, washed, dried and calcined in air at 450~ for 6 h. The Si/A1 ratio was 3.8. The silicoaluminophosphate SAPO-34 [ 17] was synthesized by heating a gel of the molar composition 0.8SIO2:1A1203:0.6P206 :ITEAOH :IHF :100H20 (TEA :tetraethylammonium) at 200~ for 13 days. The solid was separated by filtration, washed, dried and calcined in air at 500~ for 12 h. The chemical formula of the dry calcined material was Si0.11A10.5 P0.39 H0.11. Zeolite Beta [18] was prepared from a gel of molar composition 1 SiO2:0.05 A1203:0.7 TEAOH :0.085 Na20:28 H20 by heating at 140~ for 15 days. The dried zeolite was calcined at 550~ during 10 h in air. The Si/AI ratio of the solid was 10.
299 The calcined solids were ion-exchanged with an aqueous solution of the desired cation (Cu2+,Gaa+,cea+,La 3+) at various temperatures and durations to yield given exchanged levels. The ratio cm 3 solution / g solid used was 70. The conditions for the Cu E+ exchange and the corresponding exchange degree are given in Table 1. Table 1 Conditions for the
MCM-22 MCM-41 EMT(Si/AI=3.8) FAU(Si/AI=3.8) SAPO-34 Beta a
exchange and corresponding exchange degree Cu(NO3)2 cone. T Duration (mol.1 -I) (~ (h)
C u 2+
0.3
80
0.3 0.3 0.3 0.025 0.3
80 80 80 80 80
2 2(x2) b 2(x2) 3(x3) 3(xl 0) 3(x3) 2
Exchange degree a % 56 100 100 91 c 95 37 82
mol. Cu mol. Cu Expressed as 2 mol.A1 xl00. For SAPO-34 as 2 mol.Si xl00.
b A first exchange for 2 h was followed by another for 2 h with a fresh Cu(II) solution. c Cu-EMT-3.8-10.3 was obtained by exchange with a 0.012 mol 1-I Cu(CH3COO)2 solution at room temperature for 5 h. The Ce (III), La (III) and Ga (III) exchanges were performed with nitrate solutions at 70~ The La (III)/ Cu (II) exchanged zeolites were obtained after a first exchange with a La (Ill) solution followed by an exchange with a Cu (II) solution.
2.2. Activity measurements The measurements of the catalytic activity of the different materials were carried out in a fixed-bed flow reactor (16.5 mm inner diameter). The catalyst was crushed and sized and the grains with a diameter ranging from 250 to 400 gm were placed over the fritt to obtain a bed height of around 6 mm. Only experiments with catalysts in a powder form were conducted. A gas flow rate of 64 1 h -I (NTP) led to a space velocity of 50,000 h -l. The inlet gas composition was 300 ppm propene, 300 ppm NO, 10% 02 and 10% of water. The carrier gas was nitrogen. The concentrations of NO, NO2, N20 and propene were measured at steady state in the outlet gas flow using analysers based on chemiluminescence for NO (Cosma Topaze 2000), UVVisible absorption for NO2 (Rosemount Binos 1004) and Infrared absorption for N20 (Cosma Beryl 100). The rate of conversion of NO to N2 was obtained from the difference between the total conversion rate of NO and the rate of NO oxidation. N20 was never detected at a significant level. Hydrocarbon concentrations were measured with a FID (Cosma Graphite 55).
300 3. RESULTS AND DISCUSSIONS The results regarding Cu-ZSM-5, the reference catalyst, are presented in Fig. 1 where two curves are drawn, the first one concerning the percentage of NO reduced to N2 and the second one related to the percentage of NO transformed. Since no N20 was produced, the difference between the extents of NO reduction and NO transformation gives the percentage of NO oxidized to NO2. The reduction curve is typical with a maximum of 30% at 300~ followed by a decrease due to intense oxidation of propene by oxygen. Above 400~ the reduction efficiency falls under 15%. The extent of NO transformation reaches 55% at 400~ meaning that, at this temperature, 40% of NO are oxidised to NO2.
60 50
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150
200
250
300
350
400
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Temperature
.........,
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Figure 1. Effect of temperature on reduction efficiency of Cu-ZSM-5-27.5-100; &, NO conversion ; l , NO reduction to N2. Conditions 300 9 ppm NO, 300 ppm C3H6, 10% 02, 10% H20. The experimental results obtained with the different catalysts are summarized in Table 2. In this table, the maximum reduction rate of NO to N2 and the corresponding temperature are given for each catalyst. The temperature range of activity corresponds to the range where the reduction rate is higher than half the maximum rate. In this table Cu-ZSM-5-27.5-100 means that Si/A1 = 27.5 and that the Cu exchange level is 100 %. 3.1.
Cerium-
and
copper-cerium
coexchanged
ZSM-5
zeolite
These catalysts exhibited a significant activity, especially cerium-ZSM-5 which returned 38% of NO reduction at 400~ when the exchanged level was 98%. However, the temperature range of activity was slightly too high since the NO reduction rate fell down to 23% at 3000C. Hydrocarbons were totally converted at a temperature as high as 500~ These results can be compared to those obtained by Misono [19] who reported 80% of NO reduction
301 at 350~ with Ce-ZSM-5-23.3-19. The absence of water in Misono's experiments explains the higher efficiency of the catalyst. The sample with an ion exchange ratio of 98% seems to perform better than the sample with a corresponding ratio of 120%. This result shows that the relation between the exchange ratio and the NO to N2 conversion is complex, as stated by Sato et al. [4]. ZSM-5 exchanged with both copper and cerium showed a decrease of the reduction rate with a maximum of 20% of reduction at 300~ The extent of NO reduction was only 10% at 400~ Hydrocarbons were totally converted at 300~ The performances of the two coexchanged ZSM-5 were very similar whatever the Si/A1 ratio and the copper exchange level. It also appears that the activity of a coexchanged ZSM-5 is not close to the sum of the activities of the monoexchanged corresponding materials. Table 2 Summary of the efficiencies of the different catalysts Maximum extent Temperature of Material this maximum of reduction (%) (~
Temperature range of activity (~ *
Cu-ZSM-5-27.5-100 30 300 Ce-ZSM-5-27.5-98 38 400 Ce-ZSM-5-27.5-120 32 400 Cu-Ce-ZSM-5-27.5-82-83 20 300 Cu-Ce-ZSM-5-27.5-56-94 20 300 Cu-MCM-22-15.3-56 19 350 Cu-MCM-22-15.3-100 20 300 Cu-MCM-41-23-100 3 350 Cu-EMT-3.8-10.3 8 400 Cu-EMT-3.8-91 23 300 Cu-EMT-5.8-92 22 300 Ga-EMT-3.8-70 10 600 Ce-EMT-3.8-69 15 500 Cu-FAU-3.8-95 23 300 Cu-SAPO-34-37 15 500 Cu-Beta- 10-82 25 350 * Range in which the reduction rate was higher than half the maximum rate.
250-500 280-500 350-520 200-500 200-550 270-500 250-550 270-400 300-600 250-550 250-600 570-700 400-700 250-550 400-700 250-600
3.2. Copper MCM-22 and MCM-41 The catalyst Cu-MCM-22-15.3-56 exhibited only a moderate activity with a maximum extent of reduction of 19% at 350~ When the exchange level was increased from 56 to 100%, for a similar Si/A1 ratio, the maximum extent of NO reduction was not significantly changed but occurred at 300~ instead of 350~ corresponding to a wider temperature range of activity. The catalyst Cu-MCM-41-23-100 showed a very weak activity and a narrow temperature range of activity. Such a low activity may be due to the high sensitivity of the mesoporous
302 material to steam. Further study is needed to determine, in particular, whether all Cu 2+ is on exchange position for the copper-richest sample. 3.3. Copper-, Gallium- and Cerium-EMT type zeolite In the absence of results about the activity of these materials in NOx reduction under Diesel exhaust conditions, experiments were first conducted with a copper-EMT type zeolite in a gas mixture free of water. The experiments were carried out with a Cu-EMT-3.8-86 zeolite and the effect of the space velocity was investigated in the range 25 000-200 000 h -1. Figure 2 shows that a high level of NO reduction was obtained, of the same order as that with ZSM-5 in the absence of water. The effect of the space velocity was significant only for temperatures in the range 200-350~ Above 400~ the propene was probably totally converted in the inlet part of the catalyst bed and then was not available for NO reduction in the rest of the bed, explaining the absence of effect of the space velocity for high temperatures. At 300~ the extent of NO reduction rose from 10 to 50% when the space velocity was decreased from 200 000 to 25 000 h -~. Moreover, the temperature corresponding to the maximum reduction activity increases with the space velocity. Another interesting point is that this material tends to decrease the extent of NO oxidised into NO2, compared to ZSM-5 samples.
60 5O
r
Z =
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9
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o
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he,,.
/ d'
20
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~
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•
200
300
400
500
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Figure 2. Effect of the space velocity on reduction efficiency of Cu-EMT-3.8-86. 0 , 25 000 h 1. [3, 50 000 h 1 A, 9 100 000 h "~ X9 200 000 h 1. Conditions 300 9 ppm NO, 300 ppm C3H6, 10% Oa. The catalytic activity of the same material was investigated in the presence of water and also in the absence of water but after a test under wet conditions. The maximum reduction rates are
303 given in Table 3 for comparison. This table shows a dramatic decrease of the activity of the catalyst when water is present in the gas mixture. However, when a new test without water was performed on the same material, the catalyst recovered a part of its previous activity, especially at high temperature. This deactivation of the catalyst when exposed to water vapor is attributed to dealumination. The initial Si/AI ratio of 3.8 increased to 5.5 after exposure to water. This ratio was measured by 29Si magic angle spinning NMR. An attempt to avoid the deactivation of the catalyst in the presence of water was made by increasing the initial Si/A1 ratio from 3.8 to 5.8 while keeping constant the copper exchange level at about 90%. No significant change in the extent of NO reduction was observed (see Table 2). Lanthanum is known to improve the hydrothermic stability of Y-type zeolites [20]. A second attempt to stabilize the catalyst was made by preparing La-exchanged EMT-type samples. The dealumination process was stopped but this catalyst did not present any catalytic activity in NO reduction. A La-Cu-EMT-3.8-70-10 sample was also prepared. A maximum extent of reduction of 15% was measured at 500~ This low activity in NO reduction is probably due to the very low copper content. Table 3 Maximum extent of NO reduction by Cu-EMT-3.8-86 under dry or wet conditions. 10% 02, 300 ppm propene, 300 ppm NO, 10% water (when present), carrier gas: N2 Dry conditions Wet conditions Dry conditions after a test under wet conditions 300~ 28.5 11.0 14.3 350~ 45.7 9.9 25.0 400~ 48.7 8.6 34.0 450~ 33.5 9.3 32.0 The catalyst Cu-EMT-5.8-92 was aged, during 6 hours, at 700~ under the same gas mixture as used during the catalytic tests (in the presence of 10% of water). The following sequence was used for the tests: first test in the presence of water, ageing procedure, second test in the presence of water, ageing procedure again and finally third test in the presence of water. The maximum extent of NO reduction decreased from 22 to 17% between the first and the third test and the corresponding temperature shifted from 300 to 600~ These observations were also made by Kharas et al. [21 ] for Cu-ZSM-5. This loss of the catalyst activity was related to the hydrocarbon consumption occurring at a higher temperature. The Si/A1 ratio was determined after the second test for different Cu-EMT catalysts with various initial Si/A1 ratios and the same copper-exchanged level. The results are given in Table 4. Dealumination is effective whatever the initial Si/A1 ratio although a higher stability of the zeolite was expected for the highest initial Si/AI ratio. The Si/A1 ratio of the catalyst Cu-EMT-5.8-92 was determined after the third test, leading to a value of 20 and showing that dealumination continued during the successive tests.
304 Table 4 Si/A1 ratio of the fresh catalyst and of the catalyst after one ageing procedure and a second test under wet conditions (see the sequence described in the text) Catalyst Si/A1 ratio of the fresh Si/A1 ratio of the catalyst catalyst after ageing and a second test Cu-EMT-3.8-90 3.8 6.9 Cu-EMT-4.8-92 4.8 9.2 Cu-EMT-5.8-92 5.8 14.0 Cu-EMT-9.9-92 9.9 15.0 A cerium-exchanged EMT sample (see Table 2) exhibited a lower maximum of activity (15%) than copper-exchanged ones (with exchange levels of about 90%) at a temperature as high as 500~ The latter temperature is in relation with the fact that hydrocarbons were completly converted at a temperature as high as 600~ A consequence of this high conversion temperature of hydrocarbons was that no NO2 was produced. A galliumexchanged EMT zeolite (see Table 2) exhibited only a maximum extent of reduction of 10% at a high temperature of 600~ 3.4. FAU-type zeolites The catalyst Cu-FAU-3.8-95 exhibited a moderate activity in NO reduction. The maximum extent of reduction was 23% at 300~ The hydrocarbon conversion was completed at 300~ The effect of the copper content was also investigated. When the Cu-exchanged level was decreased to 20%, the hydrocarbon conversion was 100% at only 700~ leading to a drop of the catalytic activity at temperatures less than 400~ The maximum reduction rate was less than 15% at around 400~
3.5. Copper SAPO-34 zeolites The catalyst Cu-SAPO-34-37 was active in NO reduction only at high temperature since the maximum extent of reduction (15%) was reached at 500~ and the temperature range of activity extended to 700~ No formation of NO2 was detected, in relation with a total conversion of hydrocarbons above 600~ 3.6. Copper-Beta zeolites A maximum extent of reduction of 25% was exhibited by the catalyst Cu-Beta-10-82 at 350~ A reduction rate above 12% was maintained up to 600~ At 450~ about 30% of NO was oxidized to NO2. 4. CONCLUSIONS Many catalysts exhibit a catalytic activity for NO reduction, under conditions of Diesel exhausts, not very inferior to that of ZSM-5, especially copper-MCM-22, copper-EMT, copper FAU-type zeolites and copper-Beta. EMT-catalysts which present a good potential for Diesel exhaust after-treatment suffer from deactivation in the presence of water as it is the case for ZSM-5 catalysts. Dealumination was a significant cause of this loss of activity when the catalysts were aged. It was not possible to avoid dealumination while keeping the catalyst
305 activity. EMT-catalysts prepared with a higher Si/AI ratio also suffered from dealumination during the ageing process. Investigations of the hydrothermal stability of catalysts such as copper-MCM-22, copper FAU-type zeolites and copper-Beta zeolite should be carried out. Some catalysts such as Ce-EMT, Ga-EMT and Cu-SAPO-34, although presenting a significant activity, seem to be non suitable for this application since the temperature range of activity is too high. However, if thermally stable, they could be associated with a lowtemperature catalyst. ACKNOWLEDGEMENTS
This work was supported by the French Automotive Constructors Renault SA and PSA, as well as the "Minist6re de l'Enseignement Sup6rieur et de la Recherche". We greatly appreciated discussions within the group working on the project "VPE". We also thank M. Hamon (IRMA) for having supplied us with some catalytic materials.
REFERENCES
1. European directive 70/220. 2. G. Smedler, G. Ahlstr6m, S. Fredholm, J. Frost, P. L66f, P. Marsh, A. Walker and D. Winterborn, SAE Technical Paper No. 950750 (1995). 3. M. Iwamoto, H. Yahiro, S. Shundo, Y. Yu-u and N. Mizuno, Applied Catal., 69 (1991) L 15-L19. 4. S. Sato, H. Yu-u, H. Yahiro, N. Mizuno and M. Iwamoto, Applied Catal., 70 (1991) L 1-L15. 5. M. Konno, T. Chikahiza, T. Muruyama and M. Iwamoto, SAE Paper No. 920091 (1992). 6. C. Yokoyama and M. Misono, Chemistry Letters (1992) 1669. 7. W. Held, A. Konig, T. Richter and L. Puppe, SAE technical Paper No. 900496 (1990). 8. H. Hamada, Y. Kintaichi, M. Sasaki, T. Ito and M. Tabata, Applied Catal., 70 (1991) L 15. 9. K.C.C. Kharas, Applied Catalysis B: Environmental, A2 (1993) 207. 10. G. P. Ansell, A. F. Diwell, S. E. Golunski, J. W. Hayes, R. R. Rajaram, T. J. Truex and A. P. Walker, Applied Catalysis B: Environmental, 2 (1993) 81. 11. M. Guyon, V. Le Chanu, P. Gilot, H. Kessler and G. Prado, Applied Catalysis B: Environmental, 8 (1996) 183. 12. P. Gilot, M. Guyon and B. R. Stanmore, Fuel, 76 (6) (1997) 507-515. 13. M. Shelef, Chemical Reviews, 95 (1995) 209. 14. M.K. Rubin and Chu, US Pat. 4954325 (1990). 15. J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T-W Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins and J.L. Schlenker, J. Am. Chem. Soc., 114 (1992) 10834. 16. F. Delprato, L. Delmotte, J.L. Guth and L. Huve, Zeolites 10 (1990) 546. 17. S.T. Wilson, in <>, Studies in Surface Science and Catalysis, Vol.58, H.van Bekkum, E.M. Flanigen and J.C. Jansen (eds.), Elsevier, Amsterdam (1991 ) p. 137. 18. R.L. Wadlinger, G.T. Kerr and E.T. Rosinski, US Pat. 3308069 (1967).
306 19. C. Yokoyama and M. Misono, J. of Catal., 150 (1994) 9-17. 20. D. Keir, E. F. T. Lee and L. V. C. Rees, Zeolites, 8 (1988) 228-231. 21. K. C. C. Kharas, H. J. Robota and A. Datye, Environmental Catalysis, Armor J. N., ACS Symposium Series 552, 4 (1994) 39-52.
CATALYSIS AND AUTOMOTIVE POLLUTIONCONTROL IV Studiesin SurfaceScienceand Catalysis,Vol. 116 N. Kruse, A. Frennet and J.-M Bastin (Eds.) 91998 ElsevierScience B.V. All rightsreserved.
307
The effect of A! and Cu content on the performance of C u Z S M 5 catalysts at the exhaust of high efficiency spark ignition engines. P. Ciambelli a, P. Corbob, M. Gambino b, F. Migliardini b Dipartimento di Ingegneria Chimica e Alimentare, Universitfi di Salerno, 84084 Fisciano (SA), Italy b Istituto Motori del CNR, via G.Marconi 8, 80125 Napoli, Italy
a
Catalytic performance of copper catalysts based on ZSM5 structure (MFI) were investigated at the exhaust of a lean-burn engine for the NOx reduction. The presence of both Cu and A1 resulted indispensable to have catalyst activity in real conditions. While activity remained unchanged for the over-exchanged catalysts, the durability always increased with copper content. A1 enhanced both activity and durability. Fast deactivation rate resulted correlated to segregation of small CuO particles dispersed into zeolite channels, as evidenced by characterisation of deactivated catalysts. 1. INTRODUCTION The compliance with emission legislative limits makes indispensable the use of catalytic converters at the exhaust of gasoline cars. The commercial "three-way" catalysts for automobiles are very effective in simultaneously removing NOx, HC and CO, but require the air/fuel ratio to be maintained very close to the stoichiometric point (air/fuel mass ratio, A/F=I 4.6). In the last years economical and environmental motives have determined a strong interest of automobile industries toward "lean-burn" engines, able to assure best fuel economy. In these conditions CO and HC oxidation on noble metal catalysts is obviously favoured, while NOx removal would be hardly realised, resulting in elevated selectivity to N20 [1]. Therefore, the formulation of new catalysts, able to reduce NOx to N2 in the presence of oxygen, is necessary. As the perspective of stricter future regulations let foresee the necessity to adopt exhaust catalytic treatment of NOx for Diesel engines, the development of such catalysts would find an immediate practical application in the treatment of Diesel exhaust. One of the ways for removing NOx from high efficiency engine exhaust (Diesel engine and lean burn S.I. engines) is the selective catalytic reduction (SCR) by hydrocarbons. Copper ion-exchanged ZSM5 zeolites are widely accepted as potential de-NOx catalysts for lean exhaust gases, but at present the deactivation problems of these materials in real conditions have not been overcome [2-5]. Several causes of deactivation have been suggested, such as zeolite dealumination [6], copper oxide segregation [7,8], copper ions migration to inaccessible sites [9]. The role of NO2, as a reaction intermediate [10,11], and the effect of selective hydrocarbons [12-14] have been investigated, but the mechanism of lean NOx reduction on CuZSM5 catalysts has not been unambiguously understood [9,15].
308 The aim of this work was to investigate the effect of Si/A1 ratio and metal loading on activity and durability of Cu-ZSM5 catalysts in NOx SCR by hydrocarbons. For this purpose two ZSM5 samples (Si/Al=25 and 80) and a sample of Silicalite were used as starting materials. All tests were performed in real reaction conditions, at the exhaust of a gasoline fuelled engine. The physico-chemical characterisation (XRD, DRS) of the catalysts was carried out before and after reaction. The reactivity and selectivity of the different hydrocarbons present in the exhaust were also evaluated during the catalytic tests. The results of the experiments effected in real conditions were compared with those obtained on the same catalysts with synthetic NO/O2 mixtures, to investigate the possible role of NO/in the reaction mechanism. 2. EXPERIMENTAL
Several catalysts with different copper contents were prepared by the ion-exchange method described in details elsewhere [ 16]. The careful control of the preparation method allowed to obtain copper exchange levels higher than 100% (the theoretical 100% exchange level corresponds to 1 Cu 2+ per 2 A1 atoms). Either copper exchange at defect sites, such as "nested silanols" at Si vacancy defects [ 17] or ion exchange of polymeric chains, such a s Cux(OH)y(2xY)+in the zeolite pores [ 18,19], are usually invoked to explain the over-exchange phenomenon. Copper was introduced into Silicalite up to 2.86 wt% by the same experimental technique used for ZSM5 ion exchange. In Table 1 the samples tested are reported together with their copper exchange and weight percentages. The abbreviations used in the following are also indicated. Table 1 Catalysts and their copper loading. Catalyst HZSM5 CuZSM5 CuZSM5 CuZSM5 CuZSM5 HZSM5 CuZSM5 CuZSM5 CuZSM5 CuZSM5 Cu-Silicalite Cu-Silicalite
Si/A1 ratio 25 25 25 25 25 80 80 80 80 80 -
...... Ion exchange, %
Cu weight %
67 85 102 166
1.28 1.62 1.94 3.12
70 221 536 648 -
0.46 1.44 3.49 3.89 0.18 2.86
Abbreviatio n HZ(25) Z(25)67 Z(25) 85 Z(25) 102 Z(25) 166 HZ(80) Z(80)70 Z(80)221 Z(80)536 Z(80)648 S-0.18 S-2.86
X-ray diffraction (XRD) pattems of samples were obtained with a Philips automated PW 1729 diffractometer. Diffuse Reflectance spectra (DRS) of samples were recorded using a
309 Cary 5 spectrometer with a diffuse reflectance accessory. The characterisation was performed before and after catalytic tests. The catalytic properties were evaluated at the exhaust of a lean-burn S.I. engine (1350 cm 3 displacement, air/fuel mass ratio A/F=I 8, 2000 rpm, 17 kW), in temperature programmed tests (from room temperature to 550~ and isothermal durability tests (400 ~ Some tests were performed after water removal by a cold trap before feeding the reactor. The exhaust gas average composition was the following: O2=4%, CO2=11%, H20=12%, HC (as propane)=410 ppm, NOx=1220 ppm, CO=1310 ppm, Nz=balance. The experiments were effected at space velocity of 30000 h 1. NOx, HC, CO and O2 concentrations were measured by on-line Rosemount analyzers: chemiluminescence for NOx, flame ionisation for total HC, infrared for CO, and electrochemical for O2. N20 was measured by on-line Hartmann & Braun infrared analyzer. An Applied Automation on-line gas-chromatograph, with a FID detector, was adopted to analyse the individual hydrocarbon concentrations. Other details of the experimental apparatus are described in [ 16]. 3. RESULTS AND DISCUSSION
3.1. Catalyst activity measurements. In Figure 1 the results of programmed temperature tests effected on ZSM5 based catalysts for different copper contents are reported. While on the zeolites in acidic form NOx reduction was not detected, the introduction of copper gave them activity. The NOx conversion profiles (Figures 1a,b) resulted very similar for all samples, showing a maximum value at about 400 ~ After this temperature HC oxidation by oxygen became predominant with respect to NOx reduction by hydrocarbons. The highest NOx conversions for CuZSM5(80) were obtained on the over-exchanged catalysts; in particular, values ranging from 24 to 27% were achieved varying ion exchange from 221 to 648%. For CuZSM5(25) the maximum NOx conversion (33%) was reached on the Z(25)85 sample, and remained unchanged up to 166% of exchange level. It should be noticed that Z(80)221 and Z(25)85 had very close copper content (see Table 1). These results evidenced the superior performance of the catalysts prepared from the zeolite at lower Si/AI ratio with respect to those derived from HZ(80). The electronic properties of the zeolitic matrix, related to the aluminium content, seem to play an incisive role in determining the catalyst activity. Referring to HC, both the starting zeolites resulted active, but slightly higher conversions were obtained on HZ(25) (Figures lc,d). Copper loading enhanced the activity, but varying metal content some significant differences were observed in dependence of Si/A1 ratio. In fact, while for CuZSM5(80) the HC oxidation activity increased with the ion exchange level, the introduction of copper into HZ(25) over 67% of ion exchange determined a decrease of HC conversions. Furthermore, the sample Z(25)67 resulted the most active among all the catalysts considered (Figures 1c,d). This result can be attributed to the combined action of copper and of residual acidity deriving from the low Si/A1 ratio. Higher copper loading into HZ(25) did not compensate the reduction of zeolite acidity. On the other hand, by reason of the lower acidity of the starting zeolite, the oxidation capacity of CuZSM5(80) catalysts depended only on the Cu concentration. Considering the CuZSM5(25) samples, which were more active in the NOx reduction (Figure l b), the catalyst Z(25)166 resulted more selective as gave high NOx conversions coupled with lower HC oxidation (Figure 1d). As regards CO oxidation, negative apparent conversions were obtained on the HZSM5 samples, due to partial HC oxidation. This effect was also observed on Z(80)70 and Z(80)221
310 at lower temperatures. However, the general behaviour of CuZSM5(80) was an increase of CO conversion with the ion exchange level (Figure 1e).
CuZSM5(80) 30
* A =
O
~ r,,,I
~
CuZSM5(25)
648 536 221 70
30
*
166
"
102
=
20
85 67
b /~,/
~-~
20
8
O
>~ IO 9
N 10 9
Z
2; o 200
300
400
Temperature, lOO "i
" ~-
648 5 3
200
500
400
lO0i
Temperature,
~ C 6
300
~
500 ~
-'_ lo2
.~ 75 / . ~ . i / ) /
75 /
/
~- 5o
/ -~-: 221 " 70
25
85
"
~ 5o 25
o
...
200
300
400
500
Temperature, 100 ~ ff
536
200
~
300
400
Temperature,
. -. -. = _-_-. e
75
~
f
100
d
O
500
75
166 102 85 67
O
,v-4
9r,,,t
m 50
r~
"-"
25 O
221 70
~ 25
HZ(80)
~ 0 9 r..) -25
0 9 r.D -25
~ i
200
i,,,
300
i
i
400
500
Temperature,
50
~
O v
200
300
400
Temperature,
HZ(25)
500 ~
Figure 1. NOx, HC and CO conversion profiles during temperature programmed tests at S.V. = 30000 h ~ for ZSM5 based catalysts at different copper content, a, c and e Si/AI=80 9 ; b, d and f-Si/Al=25.
311 On the other hand, the high conversions obtained on all the CuZSM5(25) samples, in particular on Z(25)67 (Figure If), evidenced also for this reaction the effect of the aluminium content on catalyst performance. 100 75
empty symbols: S-0.18
fullsymb~ I
CO
_ /HC
25
0 200
300
400
500
Temperature, ~ Figure 2. NOx, HC and CO conversion profiles during temperature programmed tests at S.V. = 30000 h -1 for Cu-Silicalite at two different copper content. Empty symbols: Cu=0.18 wt%; full symbols : Cu=2.86 wt%. The role of the chemical properties of the zeolitic matrix is confirmed by the results of temperature programmed tests effected on two samples of Cu-Silicalite and reported in Figure 2. XRD analysis did not detect bulk copper oxide on both samples, then metal introduction in Silicalite was attributed to phenomena similar to those considered responsible of the overexchange in zeolites (see Experimental), occurring only on crystal defective sites. The Figure 2 shows that on both Cu-Silicalite samples NOx conversion was zero at all temperatures, and significant HC and CO oxidation was reached after 300 ~ only on S-2.86. This again underlined the effect of copper content on the oxidation reactions, while the absence of activity toward NOx reduction seems associated to the absence of framework aluminium.
3.2. Catalyst durability and relative characterisation. The results of isothermal tests, effected at 400 ~ and S.V.=30000 h l at the engine exhaust, are presented in Figure 3 in terms of NOx conversions versus time on stream. A positive effect of copper loading on durability properties was observed for both zeolites. In particular, while the initial activity was totally depressed on Z(80)70 and Z(80)221 after about 3 and 12 h, respectively, the deactivation rate of the sample Z(80)536 resulted much lower with respect to the other two catalysts. Similar trend was presented by Z(25)67 and Z(25)166, the last sample resulting the most stable between those considered. Physico-chemical characterisation was performed to have some indications about the changes undergone by the catalysts because of deactivation. All the fresh copper zeolite samples presented the typical XRD pattern of the parent zeolites, without any signal due to bulk copper oxide. DRS spectra showed the characteristic absorption (650-1000 nm) due to the d-d transitions expected for Cu 2+ in octahedral environment of O-containing ligands [20].
312
30
~~~q~~
(80)221, no water in the feed
~%...
"i~-
o o--
,..~....
Z*( % ~ 5 ) 166
~ 20 o ~L ~ 10 J'~, N
9
Z
~_
~
~
" Z(80)221 ~
"-" Z(25)67
o 0
10
20
30
40
Time on stream, h Figure 3. NOx conversion vs time on stream during durability tests at 400 ~ 30000 h -1 for CuZSM5 catalysts.
and S.V. =
As an example, the comparison between DRS spectra of HZ(25) and Z(25)67 is shown in Figure 4, curves a and b. The spectrum of Z(25)67 after deactivation by durability test at 400 ~ (Figure 4, curve c) shows an absorption edge between 450 and 650 nm, which can be attributed to segregation of CuO particles. As XRD did not detect bulk CuO, the particles evidenced by DRS must be not larger than 30-40 A. On the other hand, the spectra of all copper catalysts after temperature programmed tests resulted very similar to those of fresh samples. This result suggests the hypothesis that the loss of activity of CuZSM5 catalysts is mainly caused by the disappearing of active sites, due to the sintering of the small CuO particles inside the zeolite channels. This hypothesis seems confirmed by the DRS spectrum of Z(25)166 after durability test at 400 ~ As shown in Figure 3, after 40 h this samples retained about 60% of initial activity, and its spectrum presented a less intense absorption due to CuO segregation (Figure 4, curve d). A comparable correlation between deactivation rate and CuO segregation was observed also on CuZSM5(80) catalysts. XPS analysis, previously effected [21 ], confirmed the formation of such small particles in the over-exchanged samples after catalysis. Then, the longer durability observed at elevated exchange levels suggests that the large copper amount dispersed into the zeolite may limit metal migration, responsible of sintering. It should be noticed that there was not significant difference in copper weight percentage between Z(25)67 and Z(80)221, as well as between Z(25)166 and Z(80)536. Then the superior durability of Z(25)166 can be attributed to the higher A1 content, whose influence results more effective rising copper concentration in the zeolite. The hypotheses is advanced that the more elevated ionic character of the HZ(25) framework, with respect to HZ(80), could reduce CuO sintering phenomena by limiting metal mobility inside the channels. In this view Cu and A1 concentrations acted in the same direction, increasing the catalyst durability. Because of the presence of high water concentration in the engine exhaust some experiments were effected to evaluate its influence on catalyst performance. In Figure 3 the result of an isothermal durability test effected on Z(80)221 after water separation from the exhaust is also reported. It can be noticed that water elimination affected both the catalyst initial activity and its deactivation rate. In fact, in the absence of water the maximum NOx
313 conversion started from 33%, and slowly decreased reaching an apparent steady state value (27%) in about 20 h. As DRS spectrum of Z(80)221 after durability test effected without water in the feed did not show significant CuO segregation, this result evidences the primary role of water in favouring copper migration and sintering.
d
d
I
I
500
I
1000 1500 nm
I
I
2000
2500
Figure 4. DRS spectra for a) HZ(25), b) fresh Z(25)67, c) Z(25)67 after durability test (12 h at 400 ~ d) Z(25)166 after durability test (40 h at 400 ~
Z(80)536
Z(25)166 a
~9
12
~
--o-
b
12
9
v-4 ,
~o .~
9
~
6
|
-r--4
ne
3
15
F-
._ e~,one
~
O
~
0 300
400
500
Temperature, ~
/
toluene _ _ ~ .ethYlene butanes benzene
3
0
r pentanes -" propylene
v
I
300
400
500
Temperature, ~
Figure 5. Reactivity of the most selective hydrocarbons (expressed a s C l) as evaluated during temperature programmed tests at S.V = 30000 h ~ on Z(80)536 and Z(25)166.
314 3.3. Individual h y d r o c a r b o n reactivity and selectivity.
The reactivity and selectivity of the different exhaust hydrocarbons were evaluated by online gas-chromatographic analysis effected during the catalytic tests. It was evidenced that on all the catalysts tested paraffins were less reactive than olefins and aromatics, in agreement with other literature data [ 14], while inside the different classes the reactivity increased with the carbon atom number. This order of reactivity remained unchanged during the test. Referring to the NOx light-off temperature, the hydrocarbons not completely converted at this temperature were considered potentially involved in NOx reduction. Applying this criterion to the catalyst Z(80)536 the most selective HC resulted light paraffins, ethylene, benzene and toluene (Figure 5a). As seen before (w 3.1) the higher NOx removal activity of Z(25)166 corresponded to lower hydrocarbon conversions. The analysis of single compounds showed that in this case also the contribute of propylene should be taken into account (Figure 5b). 3.4. Role of NO-->NO2 oxidation in the overall S C R - H C reaction.
Recently much consideration has been addressed to the role of NO---~NO2 oxidation in the overall reaction scheme of NOx SCR by hydrocarbons [2,10,11]. In the present work some experiments were performed with synthetic NO/O2 mixtures at the same concentrations as in the engine exhaust. The results, represented in Figure 6, evidenced the following : - NO oxidation was not appreciably detected on the zeolites in acidic form ; - NO2 formation was favoured by copper introduction ; - all conversion curves showed a maximum at about 400 ~ ; - the equilibrium conversion value (42%) was reached at 400 ~ on Z(25)166 while lower NO conversion was observed at the same temperature on Z(80)536 (32%) and on S-2.86 (13%), even if they had a copper concentration very close to that of Z(25)166. 50
Z(25)166 \ Z(80)536
40 -
~
equilibrium S- 2 .86
~9 30
~ 2o O
lO
0 I
I
I
I
I
200
300
400
500
600
Temperaham, ~ Figure 6. NO-+NO2 conversion versus temperature for some ZSM5 and silicalite based catalysts, compared with the equilibrium values. Initial concentrations: NO=1200 ppm, O2=4%, N2 balance. S.V. = 30000 h q.
315 All these points indicated a parallelism between NOx reduction in real conditions and NO--~NO2 oxidation, suggesting that this last reaction could be the first step of the global SCR reaction, in agreement with other literature data [2,10,11,22]. 4. CONCLUSIONS The kinetic experiments effected at the engine exhaust on MFI catalysts at different Cu and AI content, and the relevant characterisation measurements, evidenced the following" 9copper introduction gives activity to the ZSM5 samples (Si/AI=25 and 80), but not to Silicalite samples ; 9catalyst activity significantly increases with copper content only for the samples at copper exchange lower than 100% ; 9the presence of A1 in the MFI structure is essential to have copper catalysts active toward NOx reduction ; the catalysts based on HZSM5(25) result more active than those based on HZSM5(80), because of superior metal dispersion connected to the A1 concentration ; 9all the catalysts underwent fast deactivation in real conditions; 9the excess of copper loading over 100% of ion exchange, and the higher A1 content of the samples based on HZSM5(25), increases catalyst durability ; 9catalyst characterisation suggests that deactivation can be associated to small CuO particles segregation, which is strongly favoured by exhaust water vapour ; 9the CuO formation is retarded by high A1 and Cu content, which appears to limit copper mobility.
ACKNOWLEDGMENT
The authors gratefully acknowledge Dr. G. Moretti (Centro SACSO-CNR Roma) for catalyst preparation, and Mr. G. Minelli (Centro SACSO-CNR Roma) for characterisation measurements. REFERENCES
1. 2. 3. 4. 5. 6. 7. 8.
R. Burch and P.J. Millington, Catal. Today, 26 (1995) 185. M. Iwamoto and H. Hamad, Catal. Today, 10 (1991) 57. M.Shelef, Chem. Rev., 95 (1995) 209. J.N. Armor, Catal. Today, 26 (1995) 99. A.P. Walker, Catal. Today, 26 (1995) 107. R.A.Grinsted, H.-W.Jen, C.N.Montreuil, M.J.Rokosz and M.Shelef, Zeolites, 13 (1993) 602. K.C.C.Kharas, H.J.Robota and D.Liu, Appl. Catal. B:Environmental, 2 (1993) 225. W.Joyner and E.S.Shpiro, Symposium NOx reduction, 20th National ACS Symp. San Diego, CA, Division Petroleum Chemistry. Preprints, vol. 39, N. 1 (1994) 103. 9. T.Tanabe, T.Iijima, A.Koiwai, J.Mizuno, K.Yokota and A.Isogai, Appl. Catal. B.:Environmental, 6 (1995) 145. 10. F. Witzel, G.A. Sill, and W.K. Hall, J. Catal., 149 (1994) 229.
316 11. M. Guyon, V. Le Chanu, P. Gilot, H. Kessler and G. Prado, Appl.Catal. B: Environmental, 8 (1996) 183. 12. B.K. Cho, J. Catal., 155 (1995) 184. 13. R. Burch and P.J. Millington, Appl.Catal. B : Environmental, 2 (1993) 101. 14. B.H. Engler, J. Leyrer, E.S. Lox and K.Ostgathe, SAE Technical Paper 930735 (1993). 15. R.H.H. Smits and Y. Iwasawa, Appl. Catal. B : Environmental, 6 (1995) L201. 16. P. Ciambelli,P. Corbo,M. Gambino,G. Minelli,G. MorettiandP. Porta Catal.Today,26 (1995) 33. 17. L. Woolery, L.B. Alemany, R.M. Dessau and A.W. Chester, Zeolites, 6 (1986) 14. 18. J. S~rrkany, J.L. d'Itri and W.M.H. Sachtler, Catal. Lett., 16 (1992) 241. 19. Y. Kuroda, A. Kotani, H. Maeda, H. Moriwaki, T. Morimato and M.Nagao, J. Chem. Sot., Faraday Trans., 88 (1992) 1583. 20. A. Schoonheydt, Catal. Rev. Sci. Eng., 35 (1993) 129. 21. G. Moretti, G. Minelli, P. Porta, P. Ciambelli, P. Corbo, M. Gambino, F. Migliardini and S. Iacoponi, Prepr. 11 o Intern. Cone on Zeolites, Seoul (1996). 22. K.A.Bethke, C.Li, M.C.Kung, B.Yang and H.H.Kung, Catal. Lett., 31 (1995) 287.
CATALYSIS AND AUTOMOTIVEPOLLUTIONCONTROLIV Studiesin Surface Science and Catalysis, Vol. 116 N. Kruse, A. FrennetandJ.-M Bastin (Eds.) 91998 Elsevier Science B.V. All rights reserved.
317
Kinetic study of the selective catalytic reduction of nitric oxides with hydrocarbon in diesel exhausts Bj6rn Westerberg 1'3, Bengt Andersson 1, Christian Kiinkel2 and Ingemar Odenbrand 2 IDepartment of Chemical Reaction Engineering, Chalmers University of Technology, S-412 96, G6teborg, Sweden 2Department of Chemical Engineering II, Lund University, Institute of Technology, P.O.Box 124, S-221 00, Lund, Sweden 3Competence Center for Catalysis, Chalmers University of Technology, S-412 96, G6teborg, Sweden
ABSTRACT The kinetics of the selective catalytic reduction of nitric oxides (NOx) on a proprietary high temperature catalyst with diesel as the reductant have been studied. The objective was to derive a kinetic model that can be used for real time simulation of the catalyst. In the extension, the real time simulation will be used when controlling the injection of reductant. This is a requirement for achieving a high efficiency and a low fuel penalty. The response time and the NOx conversion level upon transient diesel injection was found to be dependent on the temperature. At temperatures below 570 K very low or no NOx conversion was observed. Above 570 K a small conversion was observed. No direct response upon diesel injection could be distinguished and the NOx conversion was independent on the hydrocarbon concentration. As the temperature was increased the response became apparent and then faster and the conversion level gradually became more dependent on the hydrocarbon concentration. Above 700 K the response was immediate (response time less than 15 s) and the conversion level was directly dependent on the hydrocarbon concentration. It was concluded that the NOx reduction proceeds via the formation of a hydrocarbon intermediate and the successive reaction between the hydrocarbon intermediate and NOx. When this reaction mechanism was modeled many features of the catalyst behaviour were reproduced.
1. INTRODUCTION The diesel engine has many advantages when used as the power source in heavy transport vehicles, but a disadvantage is the emission of pollutants. As the exhaust emission limits become stricter, the need for more effective emission control systems becomes urgent.
318 Particulates, carbon monoxide and hydrocarbons can be removed with particulate filters and oxidation catalysts, but none of these systems can effectively reduce the NOx emissions. High NOx conversion levels (60-70%) have been achieved with systems that use ammonia [ 1] or urea as a reductant, but this technology has some disadvantages. A distribution chain to supply the reductant, and a reservoir to keep it onboard the vehicle will be required. Another necessity is a reliable control system [2] or an oxidation catalyst [3] to avoid ammonia slip. A more attractive alternative may be a system that uses the fuel, already available, to reduce NOx emissions. In this study the kinetics of the NOx reduction on a proprietary high temperature catalyst with diesel as the reductant was examined. The objective was to derive a kinetic model that can be used for real time simulation of the catalyst. In the extension, the real time simulation will be used when controlling the injection of reductant. This is a requirement for achieving a high efficiency and a low fuel penalty.
2. EXPERIMENTAL Transient experiments were performed on a 12 1 heavy duty diesel engine, with a 24 1 monolithic catalytic converter connected to the exhaust pipe. The catalytic converter contained two different catalysts, both supplied by Johnson Matthey. These were an 18 I high temperature active (HT) catalyst placed upstream and a 6 1 low temperature active (LT) catalyst placed downstream. The HT catalyst provides the main capacity for NOx reduction. The LT catalyst combusts unreacted hydrocarbon from the HT catalyst and contributes with some NOx reduction at lower temperatures. As this study only concerns the performance of the HT catalyst, the LT catalyst will not be discussed further, and the HT catalyst will be referred to simply as the catalyst. Diesel was injected with an air assisted spray nozzle placed 2 m upstream of the catalyst. A 1.5 mm K-type thermocouple provided the exhaust temperature before the catalyst and a 2 mm K-type thermocouple provided the temperature after the catalyst. Sampling of the exhaust were done before and after the catalyst. The sampled gas was led through heated pipes and passed through a J.U.M. Engineering model 222 heated gas pre filter before passed to the analyzing equipment. With a switching valve before the gas filter, sampling before or after the catalyst was selected. The NOx content was determined with a TECAN CLD 700 EL ht chemiluminescence detector and the hydrocarbon content was determined with a J.U.M. Engineering model VE5 FID detector. To evaluate the catalyst and to provide data for a kinetic model a specially designed test cycle was used. The engine was run at different speeds and loads as specified in table 1. The space velocities ranged from 35 000 to 150 000 h -l. The first step in the cycle was selected to provide equal starting conditions between different runs. Desorbing accumulated hydrocarbon, burning off carbonaceous deposits was done by heating up the catalyst to a temperature equal between different runs. Step 2-6 in the test cycle were selected to provide different NOx concentrations and mass flows and a temperature that varied during the steps. During each step hydrocarbon transients, with a duration of one or two minutes, were introduced by injecting diesel before the catalyst. Two runs of the test cycle were performed. In the first run the flow of injected hydrocarbon (as Cl) was twice the NOx flow in mole/s, and in the second run this ratio was four.
319 Table 1. LSpecificmion of the test cycle. Step Time Speed (min) (rpm) 1 0-10 1920 2 10-20 1920 3 20-30 1920 4 30-40 1260 5 40-50 1260 6 50-60 1260
Load (Nm) 900 100 500 100 500 900
Exhaust Flow (mole/s) 11.2 7.5 9.0 4.5 5.0 6.0
Temperature (K) 450-810 810-530 530-680 680-470 470-590 590-730
NOx (ppm) 450 140 340 170 600 880
3. RESULTS AND DISCUSSION Figure 1 shows the hydrocarbon concentration at the catalyst inlet and outlet during the first and the second run of the test cycle. The hydrocarbon conversion during diesel injection varied from 40% at low temperatures, to 85% at high temperatures. The response in hydrocarbon concentration at the outlet when diesel was injected was slow at low temperatures and fast at high temperatures. When the injection was interrupted a tailing was observed that was more pronounced at lower temperatures. This indicates that the hydrocarbon both adsorb and desorb from the catalyst.
12000
E O. v
10000
8000
0 ..~
6000 0
o
?
0
"1"
Inletlst
4000
~: __ Ou__ tlet2nd
2000
r-n n
r]
/'~ ~
f'~
ra
t--t
n
I LJL_J .
.
.
.
I
I I
Outlet 1st
0
-
0
, - . 10
-,-_.r20
-
,----30
r'L.f~ 40
50
60
Time (min)
Figure 1. Hydrocarbon concentration at catalyst inlet and outlet during the first and the second run of the test cycle. The outlet 2nd, the inlet 1st and the inlet 2nd curves are offset by 2000, 4000 and 6000 ppm respectively.
Figure 2 shows the temperature at catalyst inlet and outlet during the first and the second run of the test cycle. The outlet temperature followed the inlet temperature with a lag of about one minute. Due to external heat losses, the outlet temperature never reached the inlet
320
temperature, except during diesel injection. At high temperatures an outlet temperature peak could be seen during diesel injection. This was an effect of the evolved heat from the hydrocarbon conversion. When injection was done in the 40-50 minutes' interval, this effect could barely be seen. The temperature here was between 560 and 590 K. These temperatures are just below the reported value of the ignition temperature for a fresh Cu zeolite catalyst [4]. Below this temperature no increase in the outlet temperature could be seen during diesel injection. 1100 1000
~
9oo
Inlet
.~ 800
~-~ 700 600 500 400 0
10
20
30
,
,
40
50
,
60
Time (min)
Figure 2. Temperature at catalyst inlet and outlet during the first and the second run of the test cycle. The outlet 2 nd and the inlet curves are offset by 100 and 200 K respectively.
Figure 3 shows the NOx concentration at the catalyst inlet and outlet during the first and the second run of the test cycle. At the end of the 0-10 minutes' interval the temperature was just above 800 K and the response time when diesel was injected was less than 15 s. The NOx conversion was 8% in the first run and 18% in the second run. In the beginning of the 10-20 minutes' interval the temperature fell rapidly towards 550 K and only during the first three minutes a small NOx conversion (9% in both runs) was observed. In the 20-30 minutes' interval the temperature rise to 680 K and a NOx conversion of 15%, in the first run, and 20%, in the second run, was observed before any diesel had been injected. When diesel was injected the NOx conversion remained at 15% in the first run, and increased to 24%, in the second run. The response time was 45 s. The 30-40 minutes' interval had the lowest temperatures in the test cycle (below 500 K at the end of the interval) and only during the first injection a small NOx conversion were observed (6 and 9% in respective run). In the 40-50 minutes' interval the temperature was increased to 590 K. No direct response upon diesel injection could be distinguished, but a continuos and increasing NOx conversion (from 5 to 10% in both runs) could be observed during the interval. In the last 10 minutes' interval the temperature was increased to 720 K. The first minutes, before any diesel injection was done, a relatively high NOx conversion could be observed. The conversion peaked at 20%, in the first run, and at 26%, in the second run. When diesel was injected the conversion was 20 and 35% in
321 respective run. The response time was 40 s for the first injection and 25 s for the second injection. 2000
E 1600 ck t.--~ 1200
otO
800
0 Z
400
Inlet
~ ........
I_
tJ-- Outlet 2 n d ~
?
,,-------a.r-
0
0
_
_
Outlet, 1st ~
,
..........
l
I
I
I
10
20
30
40
,
I
50
60
Time (min) Figure 3. NOx concentration at catalyst inlet and outlet during the first and the second run of the test cycle. The outlet 2 nd and the inlet curves are offset by 400 and 800 ppm respectively.
An interesting trend can be observed when examining the catalyst behaviour at different temperatures. At temperatures below 570 K very low or no NOx conversion was observed. Above 570 K a small NOx conversion was observed. No direct response upon diesel injection could be distinguished and the conversion was independent on the amount of hydrocarbon injected. As the temperature was increased the response became apparent and then faster and the conversion level gradually became dependent on the amount of injected diesel. Above 700 K the response was immediate and the conversion level was directly dependent on the amount of injected diesel. From these observations it can be concluded that the NOx reduction does not proceed via the direct reaction between NOx and hydrocarbon. Instead they suggest that the hydrocarbon first form an intermediate, and that NOx is reduced when it reacts with this intermediate. At lower temperatures the formation of the intermediate is slow, but the consumption is even slower, so the intermediate will accumulate on the surface. When the temperature is increased the accumulated intermediate is consumed accompanied by a simultaneous NOx reduction. If the coverage of the intermediate is high, as it will be after extended times at low temperatures, the NOx reduction can be quite significant. At elevated temperatures the formation and consumption of the intermediate balances. A prolonged time at the same conditions will yield a steady state coverage. As a consequence the response to a change in the hydrocarbon concentration will be slow. At high temperatures the consumption of the intermediate is faster than the formation and th e coverage will be small. The rate limiting step in the NOx reduction is the formation of the intermediate. These observations agrees with the findings of Ansell et. al. [5] in their study of the classical Cu/ZSM-5 catalyst. They observed that carbonaceous deposits (coke) is deposited on
322
the catalyst when exposed to a lean propene/oxygen feed and that the coke is active in the lean NOx reaction. They also showed that the deposited coke is burnt off in oxygen. They assumed that coke is formed on the acidic sites of the zeolite and that the NOx reduction takes place when NO2 reacts with the coke. They also assumed that NO is converted into NO2 on the Cu sites, and that oxygen is essential in this process. Bennet et. al. [6] found earlier that the NOx conversion on a Cu/ZSM-5 catalyst is first order dependent on the propene pressure and zero order dependent on the NO pressure. They suggested a mechanism in which the hydrocarbon generates a reactive intermediate capable of reducing NO. An attempt was made to model the studied catalyst. It was assumed that hydrocarbon (HC) adsorbs and forms an intermediate (HC*) which either reacts with NOx or oxygen. It was also assumed that NO and NO2 can be treated as the same species, i.e. NOx. This can be justified if the NOx reduction proceeds via the reaction between the hydrocarbon intermediate and NOz, and if the conversion of NO into NO2 is not a rate limiting step. The oxidation products were assumed to be CO2 and H20. Formation of CO and the successive oxidation to CO2 probably occur as well, but has been omitted in the model. The model also assumes that the reaction rates are independent on the oxygen concentration. S 1 + H C ---> S~ - H C
r~ = k~c~cO,,~
(1)
S l - H C --> S~ + H C
r2 = kEOl,nc
(2)
S~ - H C --> S~ - H C *
r 3 = k3OI,Hc
(3)
r4 = k4Ol,ttc,
(4)
r5 = k s c Nox O...c.
(5)
S1
-
-
HC* + 0 2
--> S 1 dl- C O 2 -~- HEO
S~ - H C * + N O x --~ S~ + C O z + 1 1 2 0 + N 2
When fitting this mechanism only, large residuals were attained for the hydrocarbon concentration. In order to obtain a better fit a second site with hydrocarbon adsorption and oxidation was introduced. It consisted of the following steps: S 2 + H C ---) S 2 - H C
r 6 ---- k6Cl_1CO2,v
(6)
S 2 - H C --) S 2 + H C
r7 = k702.,c
(7)
S 2 - HC + 02 --, S 2 + CO z + H 20
r8 = k802,,c
(8)
The monolith was modeled with a one dimensional model. The following simplifications have been made in the model: a) b) c) d) e) f)
uniform radial flow distribution negligible radial temperature and concentration profiles no axial diffusion or heat conduction no gas phase accumulation no diffusion resistance in the washcoat transfer of mass and energy between gas and solid is accounted for by coefficients derived from the correlation obtained by Tronconi and Forzatti [7] g) the monolith is treated as a series of continuously stirred tank reactors
323
The following equations were used to model a differential axial monolith segment: Gas mass balance: (9)
Fi,~_ l - Fi, k - k ~ A k (Cg,i,k -- Cs,i, k ) -- 0
Surface mass balance:
kcAk (cg.~.k --C.,..,,k) = ~_~ vi,,,r, mwc,k
(lO)
n
Gas energy balance: F~c p,i ( T~ ,k-, - T~ ,~, ) - hA~, ( T~ ,k - ~,k ) =0
(12)
i
Solid energy balance:
" Ot - hAk (T~'k - ~"k ) + ~-'r"mc'k ( - A H " ) - k f A[ (T"'k - T")
(13)
n
The preexponential factors and the activation energies of the reactions were fitted to the experimental data of the second run of the test cycle. The values of these parameters can be found in table 2.
Table 2. Parameters obtained from fitting the model to experimental data. Reaction Preexponential factor Activation energy number (k J/mole) 1 7.8 x 10 ~ m 3 kg cat. l s l 14 2 9.4 x 101 mole kg cat. -l s -l 51 3 2.4 x 103 mole kg cat. -l s l 69 4 4.0 x 104 mole kg cat. ~ s -I 71 5 2.7 x 108 m 3 kg cat. -l s ~ 97 6 8.4 x 10 3 m 3 kg cat. l s -l 32 7 1.8 x 101 mole kg cat. l s l 26 8 3.9 x 102 mole kg cat. l s l 60
Figure 4 shows the observed and the simulated hydrocarbon concentration at the catalyst outlet during the second run of the test cycle. The standard deviation for the residual is 118 ppm or 18% of the mean HC concentration. The modeled concentration follows the observed
324
with some exceptions. During injection in the 0-10 minutes' interval and during the second injection in the 20-30 minutes' interval the model predicts too low conversion. In the 10-20 minutes' interval the model predicts too low conversion between the injections. There is also a slighter deviation in the conversion during the first and second injection in the 10-20 minutes' and in the 30-40 minutes' interval. There are also deviations at the flanks of the hydrocarbon transients in the 40-50 and the 50-60 minutes' interval. One explanation to the deviations could be that the model treats all hydrocarbon as a single compound. This is a coarse simplification. The diesel fuel itself consists of a variety of larger hydrocarbons which are cracked into shorter ones in the catalyst. All these different hydrocarbons have different adsorption properties and reactivities. An improved model would need to distinguish between different hydrocarbons or at least groups of them. Another improvement would be to account for variations in the oxygen concentration or even include oxygen adsorption in the model.
400
200 E 4000 El. Ex co
3000
c
2000
o
1000
8 0 -r"
20
30
40
50
E
o. (D.
0
-~
-200
32 tO o n,
-400
10
.-.
60
Time (rain)
Figure 4. Observed and simulated hydrocarbon concentration at catalyst outlet during the second run of the test cycle. The simulated curve is offset by 2000 ppm.
Figure 5 shows the observed and the simulated NOx concentration at the catalyst outlet during the second run of the test cycle. The standard deviation for the residual is 33 ppm or 8% of the mean NOx concentration. The model manages to predict the NOx conversion that onsets before diesel injection in the beginning of the 20-30 minutes' and the 50-60 minutes' interval. During injection in the 0-10 minutes' interval and during the second injection in the 20-30 minutes' interval the model predicts too low conversion. This coincides with a predicted too low hydrocarbon conversion. The model also predicts a slightly too low conversion at the beginning and a slightly too high conversion at the end of the 40-50 minutes' interval. In the 50-60 minutes' interval the model deviates at the flanks of the hydrocarbon transients. There are also deviations at the end of each 10 minutes' interval, when the NOx inlet concentration is changed. These deviations could indicate that NOx adsorption and desorption occurs. The agreement between the modeled and the observed NOx concentration is to a large extent influenced by the deviations between the observed and the
325 modeled hydrocarbon concentration. An improvement of the model's ability to predict the hydrocarbon concentration would probably result in better predictions of the NOx concentration. Another improvement would be to distinguish between NO and NO2.
lOO AE 9
50
0
E
n Q.
v
t-
-50 "~
1200
-100
O L_
G) 0 tO
o X
m "10
n,
(---- Simulated
800
(---- Observed
400
o Z
0 0
10
20
30
40
50
60
Time (rain)
Figure 5. Observed and simulated NOx concentration at catalyst outlet during the second run of the test cycle. The simulated curve is offset by 400 ppm.
4. CONCLUSIONS It has been concluded that the reduction of NOx on a high temperature catalyst proceeds via the formation of a hydrocarbon intermediate and the successive reaction between the hydrocarbon intermediate and NOx. When this reaction mechanism was modeled many features of the catalyst behaviour were reproduced.
5. N O M E N C L A T U R E A AP
c Cp Cp F h kc kf
mwc ms N
Channel wall area in monolith Peripheral area of monolith Gas concentration Gas heat capacity Solid heat capacity Molar flow Heat transfer coefficient Mass transfer coefficient Heat loss coefficient Mass of washcoat Mass of solid Number of sites
m2 m2
mole/s J/mole K J/kg K mole/s W/m E K m/s W/m 2 K kg kg mole/kg
326 r t Ta Tg Ts -All 0 v
Reaction rate Time Ambient temperature Gas temperature Solid temperature Heat of reaction Coverage Stoichiometric coefficient
mole/s kg washcoat S
K K K J/mole
Index: i j k n v
Specie number Site number Section of monolith Reaction number Vacant site
ACKNOWLEGDEMENTS The authors would like to thank: Johnson Matthey for supplying the catalysts for this study. AB Volvo for providing admittance to their engine laboratory. Bengt Cyr6n and Martin Bruszt for technical assistance. NUTEK for financial support.
REFERENCES
1. S.L. Andersson, P.L.T. Gabrielsson and C.U.I Odenbrand, AIChE J., 40(11) (1994) 1911. 2. L. Andersson., "Mathematical Modeling in Catalytic Automotive Pollution Control", Ph.D. thesis, Department of Chemical Reaction Engineering, Chalmers University of Technology, Sweden, 1995. 3. C. Havenith, R.P. Verbeek, D.M. Heaton and P. van Sloten, SAE Technical Paper Series 952652 (1995). 4. K.M. Adams, J.V Cavataio and R.H. Hammerle, Appl. Catal. B, 10 (1996) 157. 5. G.P. Ansell, A.F. Diwell, S.E. Golunski, J.W. Hayes, R.R. Rajaram, T.J. Truex and A.P. Walker, Appl. Catal. N, 2 (1993) 81. 6. C.J. Bennet, P.S. Bennet, S.E. Golunski; J.W. Hayes and A.P. Walker, Appl. Catal. A, 86 (1992) L1. 7. E. Tronconi and P. Forzatti, AIChE J., 38(2) (1992) 201.
CATALYSIS AND AUTOMOTIVEPOLLUTIONCONTROLIV Studies in Surface Science and Catalysis,Vol. 116 N. Kruse, A. Frennet and J.-M Bastin (Eds.) 91998Elsevier Science B.V. All rightsreserved.
327
Steady state and transient activity patterns of Cu/ZSM-5 catalysts for the selective reduction of nitrogen oxides Jan Connerton and Richard W. Joyner Catalysis Research Laboratory, Department of Chemistry and Physics, The Nottingham Trent University, Clifton Lane, Nottingham NG11 8NS, UK.
The high activity for NOx reduction by hydrocarbons under lean conditions exhibited by ion exchanged copper/zeolite catalysts is well recognised, as is the high selectivity to nitrogen. This paper reports our continuing studies of this important catalyst system, discussing both steady state and transient kinetics, and relating these to the mechanism of reaction, and in particular the possible role of small, ionic copper clusters. We have studied the kinetics of reaction and determined the turnover numbers of a series of otherwise identical catalysts with different copper contents. The turnover number is roughly constant at copper contents < ca 90% exchange, and then increases by about a factor of two at 100% exchange, remaining constant up to the highest nominal extent of exchange studied, ca 160%. These results suggest that both isolated copper ions and small metal/oxygen clusters, including dimers, catalyse the SCR reaction, with the dimeric species being roughly twice as active per copper/on. In earlier studies we have drawn attention to the possible catalytic importance of hydrocarbon deposits, which can be formed on the catalysts at temperatures of ca 600 K or lower, and we now report non-steady-state results for catalysts with different copper loadings. For each catalyst it was found that a similar amount of reactive deposit was formed, and that it decomposed NO according to rather straightforward kinetics. Significant differences in reactivity were however noted, with the rate of reaction reaching a maximum at 680 K on catalysts with 100% degree of copper exchange, compared to almost 100 K higher for 54% exchanged materials. These results show a variation in the stabifity of the hydrocarbon deposit towards NO, suggesting that the acidity of the host zeolite may be one of the factors determining the way in which catalytic activity varies with copper loading. 1. INTRODUCTION The exceptional activity exhibited by ion-exchanged copper ZSM-5 zeolite catalysts for nitric oxide (NO) decomposition, and for the selective catalytic reduction (SCR) of nitrogen oxides (NOx) in the presence of excess oxygen is well documented [ 1-10]. The nature of the active copper species in the SCR reaction however still remains uncertain. We and others have recognised that there are two different types of copper species within the ZSM-5 zeolite channels [ 11]. Isolated copper ions exist in low symmetry environments, and small clusters, where the copper atoms are linked by extra-lattice oxygen species such as [Cu(II)-O-Cu(II)] 2+ dimers, are also present. Recent studies have also suggested that the isolated copper ions in ZSM-5 occupy two types of sites [ 11], which may have different SCR reactivity. It is likely
328 ~:hat the relative concentration of different isolated copper sites and dimers vary with the zopper content of the zeolite, so this work examines the relationship between the copper zontent of a series of catalysts and the turnover number of the SCR reaction. Dimeric and ~ther copper clusters are likely to be formed preferentially at high copper loadings, so if they are catalytically important we expect to see turnover numbers increasing with copper content. We note that Moretti et al have shown that for NO decomposition the turnover number versus loading curve has an S shape, with a very rapid increase above 2 wt % copper [12 - 14]. In studies of the SCR reaction mechanism much interest now centres on the nitrogen nitrogen bond forming step, with spectroscopic evidence emerging both for oxidised nitrogenous species such as nitro groups [15 - 17] and also for reduced species, including cyanides [17 - 19]. Less attention has been paid to the characterisation of the hydrocarbon moieties which must also be involved, although it is recognised that long lived carbonaceous species can be formed within the pores of the zeolite, with possible mechanistic importance, as discussed by ourselves and others [16,20]. ~ Here we also report on the reactivity of hydrocarbon species deliberately deposited on catalysts with different copper contents. We have previously shown that nitrogen is released from these deposits by reaction with oxygen [ 16], and now show that these deposits are also able to activate NO and oxygen directly.
2. EXPERIMENTAL Catalysts were prepared from H-ZSM-5 obtained from Catal Intemational, Sheffield, with a framework Si/A1 ratio of 25, using conventional ion-exchange techniques [11]. Copper exchange levels ranged between 54% and 160%. Catalytic experiments were carried out using a fixed bed microreactor which has been described previously [11]. Product analysis was performed by a chemiluminescent NOx analyser (Signal Instruments Model 4000) and a gas chromatograph fitted with a thermal conductivity detector (Pye UNICAM PU 4550). All catalysts were activated at 773 K for 1 hour under a stream of 2% oxygen/helium. Steady state rates of reaction were measured over the temperature range 773 - 473 K, using a reactant gas mixture of 2000 ppm NO, 1220 ppm propene and 2% oxygen, balance helium, at a GHSV of 30,000 h l. Turnover numbers are expressed as molecules of NOx (NO + NO2) converted to dinitrogen per copper atom per second. Before studying the reactivity of the carbonaceous deposit, the reactant mixture above was passed over the catalyst, a procedure known to form hydrocarbon or coke deposits [20, 16]. The catalyst was then heated to 773 - 823 K at 10 K min 1 in a mixture of NO (2000 ppm), 02 (2%) balance helium. The extent of NOx conversion was monitored continuously by the chemiluminescent analyser. 3. RESULTS AND DISCUSSION 3.1. Turnover number studies The Cu/ZSM-5 catalysts showed the expected performance in the SCR reaction, as indicated typically in Fig. 1. Selectivity is entirely to dinitrogen, with no nitrous oxide observed at the sensitivity of our GC analysis.
329 !00
9
90-8070=
60--
I,,,,,,,
50-
>
40
o
o
o
30
--
20
-
10
-
0 :z 473
573 Temperature
673
773
/ K
Figure 1. Steady state rate of the SCR reaction with a 164% exchanged Cu/ZSM-5 catalysts, tested under the conditions described in the text. Squares, conversion of NOx to dinitrogen; Diamonds, conversion of propene to carbon dioxide.
Turnover numbers have been determined for the series of catalysts with difference copper contents at 573,598 and 623 K, and the results are shown in Fig. 2. Nitric oxide conversions in these studies range from 3%, for the lowest copper content at the lowest temperature at which turnover numbers are shown, to 45%, observed with the highest copper loading at the highest temperature shown in Figure 2. Despite this considerable range of conversion, a consistent picture emerges, indicating two turnover regimes. At each of the three temperatures studied, the turnover number is approximately constant up to about 90% copper exchange. Above this degree of exchange, the turnover number increases, approximately doubling at the two higher temperatures studied, and then remains constant as the copper content increases further. The decline observed at high degrees of exchange and 623 K is due to the onset of excessive propene oxidation. The simplest interpretation of these results is that two copper species, isolated ions and small clusters, are both active in the SCR reaction. Although more complicated explanations can be envisaged, the approximate doubling of the turnover number between 90% and 100% copper exchange suggests that in this range of copper loading new types of copper entity are created, with greater unit activity than is found in the zeolite at lower copper loading. The constancy of the turnover number at low copper contents suggests that the active species are isolated copper ions, which EXAFS and other techniques show to be present [ 11 ]. A recent careful study by Lamberti et al [21], using a range of techniques including X-ray absorption spectroscopy and IR spectroscopy of adsorbed CO, has suggested that introduction of Cu[I] into a ZSM-5 with a very low aluminium content, from vapour phase copper [I] chloride, generates isolated copper ions in two different sites, as also suggested by others
330 [22]. The present results suggest that these different isolated sites are probably of similar activity in the SCR reaction.
0.002
:~ 0.001
g
E
?.
9
_ _
50
. . . .
'
'
70
90
_
.
_
,. . . . .
.
:
110
....
130
150
.
170
Degree of Exchange / % Figure 2. Turnover number (NO molecules per Cu atom per second) measured as a function of copper content in Cu/ZSM-5 SCR catalysts at 573 K, (triangles); 598 K (squares); and 623 K, (diamonds).
As the copper content is increased, the turnover number doubles, and then becomes constant in the exchange range ca 100 = 165%. This constancy is very significant, as it suggests that there are only two types of active copper entity, and that these continue to increase in concentration up to a maximum copper content studied. Since the catalysts are prepared by exchange and not by impregnation, this indicates that the second type of active species is a dimer or other small metal/oxygen cluster, and not simply isolated copper ions in a different type of site. Stoichiometry dictates that degrees of exchange above 100% cannot be achieved by isolated Cu(II) ions (irrespective of the nature of the site occupied), but must instead involve the formation of metal/oxygen clusters such as [Cu- O - Cu] 2+. The entities which are introduced into the catalyst by copper overexchange are thus dimers or other small dusters and these must therefore by the species with the higher turnover number. It is interesting to note that, since the turnover number is calculated per copper ion, the doubling indicates that the activity of a dimer cluster is about four times that of an isolated copper ion.
331 Our results for the SCR reaction are similar to but much less dramatic than those reported by Moretti et al [ 12 - 14] for the NO decomposition reaction. These authors observed an Sshaped relationship between activity and copper content. As in this study, three regions of activity were reported, below ca 80% exchange - where the catalyst activity was negligible, between 80 and 100% exchange - where the catalyst activity increased by nearly a 100 fold and above ca 100% exchange, where the activity again remained almost constant. From this it was concluded that not all of the copper sites are equivalent in their NO decomposition activity, and that the high catalytic activity of Cu-ZSM-5 is due to the very last fraction of copper exchanged in the zeolite framework (20% of the total copper at most) immediately below 100% exchange. Li and Hall had earlier reported a two fold increase in activity for NO decomposition as the copper exchange level was increased form 76 to 166% [4].
3.2. REACTIVITY OF CARBONACEOUS DEPOSITS We now report on the reactivity of carbonaceous deposits which can be laid down on the Cu/ZSM-5 catalysts by exposure to the reaction mixture at low temperatures. As in our earlier study [16], carbonaceous material referred to for simplicity as coke was deposited by exposing the catalysts to the reaction mixture at 473 K. The reactivity of material deposited was then examined by exposing the catalyst at 473 K to the same concentration of NO and oxygen as present in the reaction mixture, (but no propene), and then heating to ca 920 K at 10 K min ~. The conversion of NO was monitored by the chemiluminescent detector, which has a response time of < 1 s. Fig. 3 shows the results of ramping experiments carried out on two catalysts, with respectively 54% and 100% copper exchange. The shapes of the curves are similar, as is the total amount of nitric oxide converted, in each case corresponding to a minimum of 25 mg g-l catalyst of coke. This amount is also close to our previous observation of 30 mg of carbonaceous deposit per gram of catalysts [16]. The reactivity is, however, significantly different for the two catalysts studied here, with maximum reactivity being observed at a much lower temperature at the higher degree of exchange. A simple kinetic model has been formulated in an effort to understand the differences in reactivity between catalysts with different degrees of copper exchange. The model assumes: 1. That the rate of reaction is first order in the amount of hydrocarbon deposit remaining on the catalyst as the temperature is increased: 2. That the rate of reaction is first order in the NO concentration, taken to be the mean of the NO concentration on entering and leaving the catalyst bed: 3. That the reaction is zero order in oxygen, since this is present in substantial excess at all stages of the experiment: 4. That the reaction is described by a single activation energy. 5. That the rate of reaction may be taken as constant over a 5 K interval, and that the rate at any temperature is obtained by numerical integration over all of the 5 K intervals up to that temperature, from the initial temperature of 473 K. The model has 3 disposable parameters, the activation energy for reaction, and two constants which have the nature of pre-exponential factors. One of these normalises the calculated NO concentrations to those which are observed, while the other relates to the consumption of the hydrocarbon deposit.
332
2500 T I
2OOO -
E
1500 -
X
0 z
1000 -
500 -
0
03 l'.-,q,-
03 ("4 t..O
03 1",.t..O
03 ("4 (,.0
03 I".-r
03 t'4 P'-.-
I
03 r~
I
O3 t"N O0
I
cO !".-.. oo
I
I
Temperature/K
2 5 0 0 --
2000
E 1500 X
0 z
1000 I
500
,
03 r'-~1"
I
03 04 If)
!'
I
03 r'-LO
r
I r
I 1'~
I ~
I-
I::
o3
TemperaturelK
Figure 3. Observed and calculated NOx concentrations during heating in NO - oxygen after formation of a hydrocarbon deposit, as described in the text. A) 54% Cu exchanged catalyst: B) 100% Cu exchanged catalyst. Squares, experimental: Circles, calculated.
333 As well as the experimental results, Fig. 3 shows the NO concentrations calculated from the model, and very reasonable agreement can be seen given the simplicity of the modelling approach. The model indicates that the greater activity of the catalyst with the higher copper content is mainly due to a lower activation energy, 82 kJ mol "1, compared with 92 kJ mollfor the catalyst with lower copper content. The model also shows that all of the hydrocarbon deposit is consumed during each of the ramping experiments. These results show a variation in the stability of the hydrocarbon deposit towards NO, which is inversely dependent on copper content. This suggests that the residual acidity of the zeolite could be a factor in determining how catalytic activity varies with copper loading. The results imply that having too many Bronsted zeolite acid sites inhibits the NOx reduction reaction. We have shown previously that relatively few Bronsted acid sites remain in the fully exchanged catalysts which are most active for the NOx reduction reaction [23], and which appear to form the most active coke. The present study compares a catalyst of low degree of exchange with one having little residual Bronsted acidity. Where these materials are pre-treated under conditions which allow the acid sites to have maximum influence in hydrocarbon activation, namely at low temperature, the result on the more acid catalyst is a hydrocarbon deposit which is less active in NOx reduction.
ACKNOWLEDGEMENTS We are grateful to Johnson Matthey PLC for their support of this work, through a PhD studentship granted to JC. We also acknowledge very useful discussions with Drs. Jack Frost, Alan Diwell, Raj Rajaram, Janet Fisher and Andy Walker of Johnson Matthey, as well as Dr. Olga Tkachenko of the Zelinsky Institute of Organic Chemistry, Moscow, who also prepared some of the samples used in this study. Experimental assistance was provided by Messrs. Chris Angell and Tim Shaw as part of their MSc projects. The late Professor Efim Shpiro also contributed much to this programme of study. REFERENCES ~
2. 3. 4. 5. 6. 7. 8.
9. 10. 11.
12.
M. Iwamoto, Stud.Surf.Sci.Catal.,54 (1990) 121. M. Iwamoto, H. Yahiro, S. Shundo, Y. Yu-u and M. Mizuno, Appl.Catal., 69 (1991) L 15. M. Iwamoto and H. Hamada, Catal. Today 10 (1991) 57. L. Li and W.K. Hall, J Catal., 129 (1991) 202. J. Valyon and W.K. Hall, J.Phys. Chem., 97 (1993) 1204. W. Held, A. Koenig, T. Richter and 1 Pupper, SAE Tech. Pap. Ser., 1990 900496 M. Iwamoto, H. Yahiro, Y. Mine, S. Kagawa, Chem. Lett., 1989, pp. 213. C.N. Montreuil and M. Shelef, Appl. Catal., B 1 (1992) L 1. J.O. Petunchi and W.K. Hall, Appl. Catal., B2 (1993) 303. K.C.C. Kharas, Appl. Catal. B, 2 (1993) 207. W. Griinert, N.W. Hayes, R.W. Joyner, E.S. Shpiro, M.R.H. Siddiqui and G.N. Baeva, J. Phys. Chem., 98 (1994) 10,832; B. Wichterlova, J. Dedecek, Z. Sobalik, A. Vondrova and K. Klier, J. Catal., 169 (1997) 194. G. Moretti, Catal. Lett., 23 (1994) 135.
334 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.
M.C. Campa, V. Indovina, G. Minelli, G. Moretti, I. Pettiti, P. Porta and A. Riccio, Catal. Lett., 23 (1994) 141. G. Morretti, Catal. Lett., 28 (1994) 143. T.Tanaka, T. Okuhara and M. Misono, Appl. Catal. B, 4(1994) L 1. N.W. Hayes, W. Griinert, G.J. Hutchings, R.W. Joyner and E.S. Shpiro, JCS Chem. Commun., 1994 pp 531. N.W. Hayes, R.W. Joyner and E.S. Shpiro, Appl. Catal. B, 8 (1996) 343. F. Radtke, R.A. Koeppel and A. Baiker, JCS Chem.Commun., 1995, pp 427. A.W. Aylor, L.J. Lobree, J.A. Reimer and A.T. Bell, Stud. Surf. Sci. Catal., 101 (1996) 661. G.P. Ansell, A.F. Diwell, S.E. Golunski, N.W. Hayes, R.R. Rajaram, T.J. Truex and A.P. Walker, Appl. Catal. B, 2 (1993) 81. C. Lamberti, S. Bordiga, M. Salvalaggio, G. Spoto, A. Zecchna, F. Geobaldo, G.Vlaic and M. Belltreccia, J. Phys. Chem., 101 (1997) 344. I.C. Hwang, D.H. Kim and S.I. Woo, Catal. Lett., 42 (1996) 177. J. Connerton, M.B. Padley and R.W. Joyner, JCS Faraday Trans., 91 (1995) 184
CATALYSIS AND AUTOMOTIVE POLLUTIONCONTROLIV Studies in Surface Science and Catalysis,Vol. 116 N. Kruse, A. Frennet and J.-M Bastin (Eds.) 91998 Elsevier Science B.V. All rights reserved.
335
Selective reduction of nitrogen oxide with hydrocarbons and hydrothermal aging of Cu-ZSM-5 catalysts P. Denton a, Z. Chajar a, N. Bainier-Davias b, M. Chevrierc, C. Gauthier d, H. Praliaud a, M. Primet a aLaboratoire d'Application de la Chimie ~ l'Environnement, Unit6 Mixte CNRS-UCB n~ Universit6 Claude Bernard Lyon I, 43 Bd du 11 novembre 1918, 69622 Villeurbanne Cedex, France, Tel (33) 4 72 43 15 87, Fax (33) 4 78 94 19 95. bRenault Automobiles, Direction de la Recherche, 9-11 Av. du 18 Juin 1940, 92500 Rueil Malmaison, France ~ Automobiles, Direction de l'Ing6nierie Mat6riaux, 8-10 Av. Emile Zola, 92109 Boulogne Billancourt Cedex, France dRenault Automobiles, Centre de Lardy, 1 All6e Comuel, 92510 Lardy, France
ABSTRACT This paper deals with the hydrothermal deactivation, under an air + 10 vol. % H20 mixture between 923 and 1173 K, of Cu-MFI solids, catalysts for the selective reduction of NO by propane. Fresh and aged solids were characterized by various techniques and compared with a parent H-ZSM-5 solid. The catalytic activities were measured in the absence and in the presence of water. The differences between fresh and aged Cu-ZSM-5 catalysts (destruction of the framework, extent of dealumination...) were shown to be small in spite of the strong decreases in activity. Cu-ZSM-5 is more resistant to dealumination than the parent H-ZSM-5 zeolite. The rate of NO reduction into N2 increases with the number of isolated Cu2+/Cu+ ions. These isolated ions partially migrate to inaccessible sites upon hydrothermal treatments. At very high aging temperatures a part of the copper ions agglomerates into CuO particles accessible to CO, but these bulk oxides are inactive. Under catalytic conditions and in the presence of water, dealumination is observed at a lower temperature (873 K) than under the (air + 10 % H20) mixture, because of nitric acid formation linked to NO2 which is either formed in the pipes of the apparatus or on the catalyst itself. 1. INTRODUCTION The selective reduction of NO by hydrocarbons in oxygen rich atmosphere has been reported for zeolite-based catalysts, especially Cu-ZSM-5 solids (1), but their low thermal stability limits their use for treatment of emissions from Diesel and lean-burn engines. An
336 understanding of the deactivation of Cu-ZSM-5 solids could, however, facilitate the improvement of their stability and the search for a more durable catalyst. A number of causes of deactivation have been invoked: structural collapse, dealumination (2), partial dealumination with the corresponding loss of exchangeable sites (3, 4), agglomeration of copper ions with the formation of CuO clusters (5, 6), migration of copper ions to inaccessible sites (7) and change in the nature of Cu species (8), especially change in coordination (9). In this work the catalytic activities of fresh and hydrothermally treated Cu-MFI solids (Si/Al = 19, 78, 130, 151, 319) are measured in the absence and in the presence of water. Fresh and aged Cu-MFI catalysts and a parent H-ZSM-5 solid are characterized by various techniques in order to understand the modifications of the copper ions and of the zeolite itself, as well as the relationship between these modifications. 2. EXPERIMENTAL The main starting material was a commercial H-ZSM-5 zeolite from Degussa with a Si/A1 ratio of 19 (1-14.8A14.8Si91.2O192) (crystals of size inferior to 0.6 ~tm with some spherical aggregates of diameters ranging between 1.0 and 4.0 ~tm). Copper was introduced by conventional ion exchange with an aqueous solution of copper nitrate. After calcination under an oxygen flow at 773 K (heating rate 1 K min1) the solids contained 1.2, 1.77 and 1.96 wt. % Cu. If one assumes that one Cu 2§ ion replaces two protons, 1.2 wt % Cu corresponds to an exchange level of 44 %. For comparison some other MFI zeolites with high Si/A1 ratios (78, 130, 151, 319), either commercially available or hydrothermally synthesized, were also considered. In those cases copper (1 or 4 wt % atter calcination) was introduced by impregnation. All the prepared solids showed the XRD patterns of the parent zeolite and the CuO phase was not detected. Thermal treatments were performed at 923, 973, 1073 and 1173 K in the presence of water (10 vol. % H20 in air) for 24 hours (total flow rate 10 1 h-1for 5 g solid). Catalytic measurements were made using 100 mg catalyst diluted with 400 mg of inactive otA1203 in a fixed-bed flow reactor. The typical gas mixture consisted of 2000 vpm NO, 2000 vpm C3Hs, 10 vol. % O2, balance He, without or with 10 vol. % water (total flow rate 10 1 h-l). In the absence of water in the reactant mixture, the temperature was increased from 300 to 773 K (or 873 K) (heating rate 2 K min-1) and then decreased to 423 K. Water was then added at 423 K and the temperature increased and decreased again as above. The analysis was performed by gas chromatography with two columns (porapak and molecular sieve) and a TCD detector for CO2, N20, 02, N2 and CO, and with a porapak column and a flame ionisation detector for hydrocarbons. Moreover, on-line IR and UV analyzers were used for NO, NO2, CO2, and N20 analysis. The NO conversion was calculated from the N2 production and the nitrogen balance was checked. The solids, particularly the 1.77 %-Cu-H-ZSM-5(19) and the parent H-ZSM-5, were characterized before and alter steam treatments by various techniques: powder X-ray diffraction (Siemens diffractometer with CuKcx radiation), SEM (Hitachi $800 with a 10 nm resolution), N2 adsorption (BET and pore volume with a laboratory-made automatic apparatus), FTIR spectroscopy (framework vibrations with KBR dilution and CO probe molecule in an in situ cell), 29Si and 27A1MAS NMR (BRUKER DSX 400).
337 For FTIR spectroscopy of adsorbed CO the calcined samples (thin discs of known weight) were degassed at 773 K for 1 hour and the background spectrum was recorded after cooling at 300K. After introduction of CO (around 50 Torr) at 300 K as well as after evacuation at 300 K the spectra were recorded as a function of time, as already described (10, 11, 12). Spectra were recorded on a Nicolet 550 spectrometer (2 cm-1 resolution). The optical densities of the bands were normalized by taking into account the amount of copper. 3. RESULTS
3.1.Catalytic activity The main products are N2, CO2 and NO2. The formation of CO and NzO is negligible. In the presence of oxygen, NO2 is formed at 298 K in the pipes of the apparatus (13). The fresh Cu-H-ZSM-5 (Si/AI = 19) solids are active for SCR in the presence of excess 02. In the absence of water in the mixture, no deactivation is observed upon increasing and decreasing the temperature, whether the temperature reached is 773 or 873 K (Fig. 1, Table 1). Upon addition of water to the stream the activity of the fresh Cu-H-ZSM-5(19) solids clearly decreases (Figure 1) but this effect is fully reversible if the water is suppressed and if the temperature does not exceed 773 K, as already noticed (14). Most probably an competitive adsorption between H20, NO or C3Hs, is invoked. When the temperature reaches 873 K in the presence of water the catalyst is deactivated irreversibly (Table 1). Kharas et al (5) have also noticed that a working temperature of 873 K induces a deactivation contrarily to 773 K. Furthermore, Yan et al (15) have noticed that the deactivation is faster with a complete [hydrocarbon, NO, 02, H20] mixture than if a component is missing. In fact, we observed more deactivation under catalytic conditions and in the presence of water than under the air + 10 % H20 mixture at the same temperature (873 K). We will explain this phenomenon later on, in paragraph 3.2.
100
NO/N2 conversion ( % )
without H20 50
423
623
823
T(K)
Figure 1. Conversion of NO into N2 as a function of the temperature with the Cu(1.96 %)-HZSM-5(19) solid. The reaction is performed up to 773 K in the absence and in the presence of water in the feed. The arrows indicate the increase and decrease in the reaction temperature.
338
The activity strongly decreases when the aging temperature reaches 923 K (Table 1). When water is added to the mixture the activity decreases again but to a lesser extent than in the case of the fresh solid. With the solids previously aged at 923 and 973 K (air + 10 vol. % H20 mixture) an irreversible deactivation under the reactant mixture is still observed if the temperature reaches 873 K. After aging at 1073 or 1173 K the activity is very weak, even in the absence of water in the feed.
Table 1. Conversion of NO into N2 for the flesh and aged Cu(1.96 %)-H-ZSM-5(19) solids in the absence and in the presence of water in the feed. The reaction temperature reaches 773 K or 873 K. Ts0 light-off temperature in K (temperature to reach 50 % conversion). C0nv623, c0nv773, c0nv873: conversions at 623, 773, 873 K. Max: maximum NO conversion at T(K). Ts0 Max C0nv623 C0nv773 C0nv873 Fresh solid Up to 873 K without H20 with H20 ~a) suppression H20
598 _ 593
66 % at 638 K 30 % at 823 K 58 % at 633 K
63 2 56
58 35 30
Up to 773 K without H20 with H20 , suppression 1-120
593 593
70 % at 623 K 45 % at 683 K 68 % at 623 K
65 28 65
60 37 59
493 -
54 % at 810 K 40 % at 810 K 45 % at 813 K
20 8 15
50 36 44
23 15 18
-
24 % at 833 K 15 % at 773 K 20 % at 830 K
12 6 10,
21 15 18
10 8 9
38 13 17
Aged solids Up to 873 K Aged 923 K without H20 with H20 (a) suppression H20 Aged 973 K without H20 with HRO (") suppression HzO
_
~a) Because of the deactivation observed in the presence of water in the feed stream at 873 K, the values reported here are measured during the decrease in temperature.
339
3.2. Physicochemicai characterizations
3.2.1. After aging treatments at 923, 973, 1073 and 1173 K. The modifications of the Cu(1.77 %)-H-ZSM-5(19) solid upon aging have been compared to those of the parent H-ZSM-5 zeolite. We detect no significant modification of the SEM pictures and no modification of the X-ray diffractograms, i.e., no destruction of the zeolite framework and no loss of cristallinity, even after the treatment at 1173 K. However traces of CuO with X-ray peaks at 2.52 and 2.32 A are detected after aging at 1073 or 1173 K. The nitrogen adsorption isotherms are characteristic of microporous solids. The aging treatments cause a clear decrease in the micropore volume and in the microporous surface. The decreases are nevertheless smaller for the Cu-ZSM-5 solid (variation A = 0.04 after aging at 1173 K) than for the parent zeolite (A= 0.07) (Table 2). An apparent BET surface area has been reported though the BET theory is not applicable to microporous materials since the pore condensation isotherm is interfering with the multi-layer adsorption isotherm. Table 2. Apparent BET surface area (SBETin mE/g), micropore volume (laVol in ml/g) and microporous surface (laS in m2/g) for the fresh and aged H-ZSM-5(19) and Cu(1.77 %)-H-ZSM-5(19) solids Solid. . . . . SBEa' ........ ~Vo1 laS H-ZSM-5 fresh H-ZSM-5 aged 973 H-ZSM-5 aged 1073 H-ZSM-5 aged 1173 Cu-ZSM-5 fresh Cu-ZSM-5 aged 973 Cu-ZSM-5 aged 1073 Cu-ZSM-5 aged 1173
347 330 323 299 343 297 314 295
0.15 0.13 0.11 0.08 0.15 0.11 0.11 0.11 .
.
.
.
.
.
.
.
.
292 257 222 150 287 216 232 215
Table 3. Variations of the FTIR band at 1227-28 cml as a function of the treatments for the H-ZSM5(19) and Cu(1.77 %)-H-ZSM-5(19) solids. H-ZSM-5 Cu-ZSM-5 Fresh Aged 923 K Aged 973 K Aged 1073 K Aged 1173 K
1228 1227 1232 1235 1235
1227 1228 1230 1231 1230
340 It is known (16) that there is a linear relationship between the IR wavenumbers of the T-O vibrations of the zeolite lattice and the aluminum content. The aging treatments shift the FTIR bands towards higher wavenumbers which indicates a partial dealumination of the lattice (Table 3). The variations remain however small because of the low initial Si/A1 ratio (for the 1227-28 cm ~ band and after aging at 1173 K: Av = 7 cm~ for H-ZSM-5 and 4 cm ~ for CuZSM-5). Let us recall that, when the Si/AI ratio decreases from 319 to 19, the 1235 cm~ band shifts to 1220 cml (A= 15 cm-1). From the 2VAlM R spectroscopy it is possible to follow the amounts of lattice and extra-lattice A1 in the flesh and aged samples studying the signals corresponding to tetrahedral Td AI (at around -5 5 ppm) and octahedral Oh A1 (at around 0 ppm) (17).
7
H-ZSM-5
I
I
80
I
40
100
I
I
i
0
100
0 K
Cu-ZSM-5
,,I
80
,,
I
40
.....
S
I . . . . .
80
I
40
.
.
.
.
.
.
ppm
Figure 2.27A1 NMR signals for the flesh, 973 K-aged and 1073 K-aged H-ZSM-5 solids and for the fresh and 1073 K-aged Cu(1.77 %)-ZSM-5(19) solids.
341 Both the flesh samples show only the signal of T~ AI. After the hydrothermal treatments the signal of Oh extra-lattice A1 appears in the case of the parent H-ZSM-5 zeolite (Figure 2). The quantitative determination is not very accurate, but however, it may be noticed that the quantity of Oh AI reaches approximatively 30 % after aging at 1073 or 1173 K, instead of 0 for the flesh solid. The decrease in Td A1 is not really clear. With Cu(1.77 %)-H-ZSM5(19), no Oh A1 appears after aging at 923, 973 and 1073 K, contrarily to H-ZSM5, for which extra-lattice A1 appears as soon as 923 K. A small peak at 0 ppm (Oh A1) is observed only after aging at 1173 K. From the 29Si NMR signal it is theoretically possible to discriminate Si linked to 4 Si (and 0 AI) at around -111 ppm from Si linked to 3 Si (and 1 AI) at around -105 ppm but a problem arises from the contribution of SiOH at -106 ppm (17), which prevented meaningfull interpretation of the spectra. By studying the properties of Cu/AI203 and Cu-ZSM-5 solids with electronic and vibrational spectroscopies, we have already concluded that Cu 2§ and Cu ~ are not detected by the FTIR spectroscopy of the adsorbed CO probe molecule. The IR bands belong to CO adsorbed on Cu + ions, these surface ions being generated by the reduction of Cu 2§ ions under vacuo and/or by the CO probe itself (12). Furthermore the zeolite framework acts as a host for isolated Cu "§ ions (10). In fact for a ZSM-5 zeolite a ZO-(CuOH) § species may be formed during the exchange process, where ZO- represents the zeolite framework; Cu2+(OH) is thus linked to only one AI atom (18). We have already shown (10) that, for various flesh solids, the NO reduction rate into N2 (activity expressed as moles NO transformed into N2 per gram Cu and per second) correlates with the optical density (normalized by taking account the weight of copper) of the 2152-57 cm~ band, i.e., with the number of superficial isolated copper ions accessible to CO. Furthermore the reduction into Cu ~ and the agglomeration of the Cu "+ isolated ions into bulk oxides induce strong decreases in the reaction rate (10,12). From the present work, after aging at 923 K or 973 K, the IR spectrum of the adsorbed CO is the classic one (Figure 3), characteristic of isolated Cu + ions (10). The band at 2177 cm-1 vanishing upon evacuation at 300 K is assigned to the vs mode of the dicarbonyl Cu+(CO)2. The second band due to the vas mode overlapping with the band due to the Cu+CO species is not detectable. The band at 2151 cm-1 (2157 cm ~ after evacuation at 300 K) is assigned to the Cu+CO species, the Cu + ions being isolated. By comparison with the flesh solid the intensities of the bands decreases during the aging treatments. For instance, for the Cu(1.77 %)-ZSM5(19) solid and after treatment at 923 K, the absorbance of the 2151-2157 cm1 band, and therefore the number of the isolated Cu n* ions accessible to CO, has decreased by a factor of 3 to 4 (3 in the presence of CO, 3.8 after evacuation at 300 K). It can be concluded that, atler aging at 923 K (or 973 K), some copper ions become inaccessible but the spectrum is not qualitatively modified, namely an agglomeration is not detected. After aging at 1173 K a vCO band is detected at 2138 cm~ (2139 cm~ after evacuation at 300 K) (Figure 3). This band may be attributed either to CO adsorbed on isolated Cu + ions in a new environment or to non-isolated Cu + ions resulting from a partial reduction of a bulk CuO (11), which supposes a previous copper agglomeration. It may be noticed that partial
342 reductions of a model CuO oxide and of a high-copper loaded-Cu/AlzO3 solid in which CuO has been detected by XRD lead to a vCO band at 2135-2125 cm 1 (11).
A g e d 973 K
A g e d 1173 K i,-,
9,
2200
2100
I
I
2200
2100
I
’
,,,
cm-~
Figure 3. Infrared spectra of CO adsorbed on the Cu(1.77 %)-ZSM-5(19) solids previously calcined under oxygen at 773 K and evacuated at the same temperature. (a) upon contact with 50 Torr of CO at 300 K for 20 h, (b) previous sample evacuated at 298 K for 4 h. 3.2.2. After treatment at 823 K under catalytic conditions in the presence o f water. When the temperature reaches 873 K, the deactivation observed under catalytic conditions in the presence of water is not only a hydrothermal effect and it is not due to coking. In spite of the relatively low temperature, a dealumination is observed leading to the appearance of the 27Al NMR signal of octahedral AI (appearance of 10 to 20 % of extra-lattice A1), to a shift of the FTIR band from 1227 to 1230 cm1 and to decreases in the microporous volume and in the microporous surface from 0.15 to 0.11 ml/g and from 287 to 215 m2/g, respectively. This dealumination occurs at 873 K under the reaction mixture (in the presence of water) but not under an air + 10 % H20 mixture at the same temperature. This is due to the presence of an acidic component, HNO3, produced by the reaction of NO2 (and 02) with H20. It may be also noticed that, in the presence of water, the quantity of disappeared NO exceeds the quantities of N2 and NO2 formed (by a factor of 10-20 % for temperatures under 623 K) and the pH of the trapped water reaches 1. We have already noticed (13) a NO2 formation in
343 the pipes of the apparatus, at room temp6rature and in the absence of catalyst. The acid formation is linked either to this NO2 formation or to a NO2 formation on the catalyst itself. This formation increases when the NO reduction into N2 decreases.
4. DISCUSSION AND CONCLUSION Most of the physico-chemical measurements (XRD, SEM, N2 adsorption, framework vibrations) show little difference between fresh and hydrothermally treated (air + 10 % H20 mixture) Cu-ZSM-5 solids. There is no clear destruction of the zeolite framework; the decrease in micropore volume remains moderate and it is difficult to observe dealumination in the aged solids, even alter treatment at 1073 or 1173 K. Significant changes in catalytic activity are observed, however, even after treatment at 923 K, and the activity becomes negligible at~er treatment at 1073 or 1173 K. From the infrared spectroscopy of adsorbed CO it appears that aging treatments, as low as 923 K, lead to a migration of the active isolated copper ions to inaccessible sites. In these conditions an agglomeration is not detected but, after aging at 1173 K, an agglomeration is evidenced both by XRD and by the infrared bands of CO adsorbed on partially reduced bulk CuO oxide. These accessible copper oxide crystallites are probably located at the external surface of the zeolite and are inactive. In fact, the activity remains correlated to the number of Cu2+/Cu+ isolated ions deduced from the infrared spectra of adsorbed CO and located in the zeolite structure. This correlation holds whatever the treatment and whatever the Si/AI ratio (Table 4).
Table 4. ,Ratios of the activities of the fresh and 923 K-aged solids and ratios of the optical densities of the fresh and 923 K-aged solids. Activities measured at 623 K and with 10 vol. % 02 and without water. Optical densities (O.D.) of the vCO bands at 2151-2157 cm1 (isolated Cu§ ions) under CO and after evacuation. Two solids are considered, the Cu(1.3%)-MFI(21) one .and the Cu(4%)-MFI(130) one. .... Cu(1:3 %)-MFI(21) Cu(4 %)-MFI(130) Activity fresh solid/activity aged solid 3.7 2 O.D. fresh solid/O.D, aged solid CO, 298 K, l h 3.2 1.4 Vacuo, 298 K, lh 3.5 2.5
We were expecting that a loss of the isolated copper ions would be linked to a loss of exchangeable sites via dealumination. The possibility of a local dealumination, not evidenced by the physico-chemical characterizations performed here, cannot be excluded. Under catalytic conditions and in the presence of water, a dealumination is observed at relatively low temperature (873 K) and the deactivation of the solid is thus stronger than under the (air + 10 % HE0) mixture at the same temperature. This dealumination is attributed to the formation of acid resulting from the reaction of H20 with NO2 formed in the pipes of the apparatus. NO2 could also be formed on the catalyst.
344 In this work it is also shown that Cu-ZSM-5 is more resistant to dealumination by steaming than the parent H-ZSM-5 zeolite (Tables 2 and 3, Figure 2). Such a phenomenon has already been reported for Cu, Zn (4, 5, 20) and Cr (19). It may be supposed that the presence of another cation would neutralize part of the dealumination and would thus impede the migration of the exchanged copper. We have for this reason studied the effect of cocations on the thermal resistance of Cu-ZSM-5 solids. Among the cocations studied, the silver cation, active by itself, with a relatively weak affinity for water and an oxide unstable enough to probably allow an NO dissociation, is the most promising. The thermal stability is, however, a function of the order of exchange and of the preparation procedure. In conclusion the differences between flesh and aged solids (destruction of the zeolite framework, extent of dealumination...) are small in spite of the strong decreases in activity. Furthermore Cu-ZSM-5 is more resistant to dealumination by steaming than H-ZSM-5. The rate of NO reduction into N2 is correlated with the number of isolated Cu2+/Cu§ ions located in the zeolite structure; this number decreases with the aging treatments. A partial migration of copper to inaccessible sites seems more important than the degradation of the zeolite itself. At higher aging temperatures a part of the copper ions agglomerates into CuO particles accessible to CO but these bulk oxides are inactive.
REFERENCES
.
3. 4.
.
10. 11.
M. Iwamoto and H. Hamada, Catal. Today, 10 (1991) 57. M. Iwamoto, Catal. Today, 29 (1996) 29. M.D. Amiridis, T. Zhang and R.J. Farrauto, Appl. Catal., B, 10 (1996) 203 A.P. Walker, Catal. Today, 26 (1995) 107 J.O. Petunchi and W.K. Hall, Appl. Catal., B, 3 (1994) 239 M. Shelef, Chem. Rev., 95 (1995) 209. R.A. Grinsted, H.W. Jen, C.N. Montreuil, M.J. Robosz and M. Shelef, Zeolites, 13 (1993) 602. K.C.C. Kharas, H.J. Robota and D. Liu, Appl. Catal. B, 2 (1993) 225. W. Joyner and E.S. Shpiro, Symposium NOx reduction, 207th National ACS Symp. San Diego, CA, Division Petroleum Chemistry. Preprints, vol. 39, n~ February 1994, p. 103. T. Tanabe, T. Ijima, A. Koiwai, J. Mizuno, K. Yokata and A. Isogai, Appl. Catal., B, 6 (1995) 145. S. Matsumoto, K. Yokota, H. Doi, M. Kimura, K. Sekizawa and S. Kasahara, Catal Today, 22 (1994) 127. A.V. Kucherov, C.P. Hubbard, T.N. Kucherova and M. Shelef, Appl. Catal., B, 7 (1996) 285 A.V. Kucherov, C.P. Hubbard and M. Shelef, J. Catal., 157 (1995) 603. A.V. Kucherov, J.L. Gerloch, H.W. Jen and M. Shelef, Catal. Today, 27 (1996) 79 Z Chajar, M. Primet, H. Praliaud, M. Chevrier, C. Gauthier, F. Mathis, Appl. Catal., 4 (1994) 199. Z. Chajar, M. Primet, H. Praliaud, M. Chevrier, C. Gauthier, F. Mathis, CAPOC III, Studies in Surface Science and Catalysis, Vol. 96, A. Frennet and J.M. Bastin eds., Elsevier, 1995, p. 691.
345 12. 13. 14.
15. 16. 17. 18. 19. 20.
H. Praliaud, S. Mikhailenko, Z. Chajar, M. Primet, submitted to J. Catal. Z. Chajar, M. Primet, H. Praliaud, M. Chevrier, C. Gauthier, F. Mathis, Catal. Letters, 28 (1994) 33. Y. Zhang, T. Sun, A.F. Sarofim and M. Flytzani-Stephanopoulos, Symposium NOx reduction. 207th National ACS Symp. San Diego, CA, March 1994, Division of Petroleum Chemistry, Preprints, Vol. 39, n~ February 1994, p.171 J.Y. Yan, G.D. Lei, W.M.H. Sachtler and H.H. Kung, J. Catal., 161 (1996) 43. E.M. Flaningen, Zeolite Chemistry and Catalysis, ed. J.A. Rabo, ACS Monograph 171, Washington D.C., 1976. P. Budi, E. Curry-Hyde and R.F. Howe, Catal. Letters, 41 (1996) 47. G. Centi and S. Perathoner, Appl. Catal., A, 132 (1995) 179 and J. Catal., 152 (1995) 93. R.L. Keiski, H. Raisanen, M. HarkOnen, T. Maumula and P. NiemistO, Catal. Today, 27 (1996) 85. T.Tabata, M. Kokitsu and O. Okada, Catal. Today, 22 (1994) 147.
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CATALYSISAND AUTOMOTIVEPOLLUTIONCONTROLIV Studies in Surface Science and Catalysis,Vol. 116 N. Kruse, A. Frennet and J.-M Bastin (Eds.) 91998Elsevier Science B.V. All rights reserved.
347
T r a n s i e n t kinetic s t u d y on N O d e c o m p o s i t i o n o v e r C u - Z S M - 5 catalysts Z. Schay, I. Kiricsi a and L. Guczi Department of Surface Chemistry and Catalysis, Institute of Isotopes of the Hungarian Academy of Sciences, P. O. Box 77, H-1525 Budapest, Hungary aApplied Chemistry Department, J6zsef Attila University, Rerrich B. t6r 1, Szeged, H-6720, Hungary ABSTRACT Decomposition of NO was studied on Cu-clinoptilolite and Cu-ZSM-5 zeolites of different Si/AI ratio and copper ion exchange rate. During the first contact at 600oC with NO an irreversible oxygen uptake by the catalysts was observed. On the long pulse of the NO concentration in the transient stage carried out under isothermal condition, overshoots in N 2 and 0 2 concentration were observed at the leading and falling edge, respectively. In the TPD experiments after NO adsorption a surface complex formulated as NO 3 is decomposed at about the reaction temperature into equimolar amounts of NO and 0 2. The role of the surface complex in the NO decomposition is discussed. 1. INTRODUCTION Since the discovery of the activity of ion-exchanged Cu-ZSM-5 zeolites in the catalytic decomposition of NO by Iwamoto and co-workers [1 ], the nature of the active sites and the reaction mechanism is still conflicting. Even review articles do not consider changes in the state of the catalyst during the start up period [2, 3]. Although the catalysts have been characterized by sophisticated techniques before the NO decomposition, still little effort has been made to study the catalysts after the reaction [3]. Nevertheless, there are some pieces of evidence available about the change of the structure of catalyst itself under reaction conditions. Previously we have reported in a transient kinetic study an overshoot in the formation of N 2 and 0 2 at the beginning and at the end of a long NO pulse, respectively [4]. In the present study we demonstrate that transiem stages in the N 2 and 0 2 formation are also present in other Cu-zeolites, as well as the very first pulse of NO at the highest reaction temperature results in a drastic change in the state of the catalysts. 2. EXPERIMENTAL Two copper containing ZSM-5 catalysts with Si/AI ratio of 24 and 66 and Cu/AI ratio of 0.5 and 1, respectively, have been prepared by conventional ion exchange of sodium with copper. The third catalyst was a c o ~ ion exchanged dinoptilolite, a natural zeolite of small pore size.
348 Transient kinetic studies were performed in a fixed-bed quartz tubular flow reactor of 4 mm inner diameter. 0.2-0.4 g catalyst of 0.5-0.25 mm sieve fraction was placed between quartz wool plugs. The catalysts were activated by heating it at 600oc in a stream of 25 cmJ/min argon for lb. The flow was then switched for 25 cm3/min 2 vol. % NO/Ar and after having reached a steady state (in about 10-40 rain) the was switched back to Ar. The effluent was analyzed by a QMS in multiple ion detection mode. A gas inlet system consisting of a heated stainless steel capillary differentially pumped by a rotary pump and linked via an orifice plate to a turbomolecular pump made the QMS signal proportional to the gas concentration in the effluent and ensured a high stability of the QMS calibration. The m/e values 18, 28, 30, 32, 38, 44 and 46 were recorded for measuring H20, N 2, NO, 02, Ar, N20 and NO2 concentrations, respectively. In Figures 1-3 only m/e values are given in which significant changes were observed. For calibration a mixture of 0.9 vol. % N 2 and 0.9 vol. % 02 in argon was used. To study the catalytic activity the catalysts were cooled in 2 vol. % NO/Ar mixture from 600oc to 300oc in about 30oc steps. In temperature programmed desorption (TPD) 25 cm3/min argon was used as carrier gas and a heating rate of 20~ was applied. If not stated otherwise, before TPD experiments the catalyst was cooled in 2 vol. % NO/Ar from reaction temperature to 200~ and purged with argon for 5 rain. An KRATOS XSAMS 800 XPS machine equipped with an atmospheric reaction chamber was used to characterize the valence state and surface composition of copper in the catalysts before and after the NO decomposition reaction. The binding energies were determined relative to Si 2p at 103.2 eV. For the surface composition signals of Cu 2p, O 1s, C Is, Si 2p and AI 2p were considered using the sensitivity factors given by the manufacturer. 3. RESULTS During the first contact of the catalysts with NO at 600~ an overshoot in N 2 formation along with a significant uptake of the 02 have been observed (see Fig. 1). At the same time the leading edge of the NO signal has been significantly leveled off and some N20 formed.. On switching back to argon, all signals with the exception of the 02 signal of very low intensity in Figure l a returned sharply to their background. A second contact to NO resulted in a fast response in all signals without any overshoot or delay and without any N20 formation. Even an extended purge in argon at 600~ was not able to restore the characteristic features given in Figure 1. This indicates an irreversible change in the catalysts during the first contact to NO. The amount of oxygen uptake is given in Table 1. Note that all quantities are about an order of magnitude less than those which correspond to the copper content in the catalysts. The starting temperature for the NO decomposition lies in the range between 340-360oc. On increasing the temperature an Arrhenius type temperature dependence has been observed up to 450~ with an apparent activation energy of about 90 kJ moleq. Between 500-550~ there was a maximum in the NO conversion followed by a marked decrease above 600oc. The highest conversion of about 50% was observed for the Cu-ZSM-5 Si/AI = 24 Cu/AI = 0.5 catalyst. The highest conversions for the other Cu ZSM-5 and Cu-clinoptilolite catalysts were about 10 % and 8 %, respectively. The transient measurements under isothermal condition at 400~ are shown in Fig. 2. The catalysts were cooled in argon from 600~ and at 400~ the flow was switched for a NO/At mixture for 5-10 min. At the beginning of the NO signal an overshoot in the N 2 concentration
349
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Figure 2. Isothermal transients 400 ~ a) Cu-clinoptilolitc b) Cu-ZSMS, Si/AI=66, Cu/AI=I c) Cu-ZSMS, Si/AI=24, Cu/AI=0.5
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Figure 3. NO TPD after cooling in NO/Ar from 600 ~ a) Cu-clinoptilolite b) Cu-ZSMS, Si/AI=66, Cu/AI=I c) Cu-ZSMS, Si/AI=24, Cu/AI=0.5 d) as c) but cooled in Ar and dosed by NO at 200 ~
.
.
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352 was observed together with a delay in the NO and 0 2 signals. The 0 2 signal was delayed to much lesser extent than that observed at the first contact with NO at 600~ At the end of the NO pulse, an 0 2 overshoot was observed. In contrary to the first NO pulse at 600oc these transients characteristics were the same on repeated NO pulses indicating reversible changes in the catalysts during the NO decomposition. The TPD experiments shown in Fig. 3. were started at 200oc. Although at room temperature the NO molecule is adsorbed and its desorption starts already at about 25~ [6, 7], we believe that this interaction with the surface entirely differs from that measured under reaction conditions. Thus, the catalysts were treated in 2 vol. % NO/Ar mixture at the reaction temperature, cooled down to 200~ in the same mixture and the TPD started at that temperature to obtain information about the intermediates formed on the catalysts. It is remarkable that N 2 is produced in a very small quantity, no N20 is found among the desorption products (the trace for N20 is omitted for the clarity of the figure) and the NO and 0 2 peaks appear nearly at the same temperature for the Cu-ZSM-5 catalysts. In the TPD after room temperature NO adsorption a similar high temperature NO + 0 2 TPD spectrum was reported by Li and Armor after oxidative pretreatment of Cu-ZSM-5 [6]. On Cu-clinoptilolite a broad low temperature NO desorption peak was observed without any N 2 or 0 2 desorption. The high temperature NO peak together with the 0 2 peak are shifted by about 50~ towards higher temperature compared to that observed on Cu-ZSM-5 catalysts. When the latter sample was cooled first in argon, then contacted with NO at 200oc, the NO TPD drastically changed Table 1 Oxygen retardation and TPD Catalyst
Cu content (Ixmol/g)
02 1 uptake
TPD 2
(gmol/g) NO low temperature
NO high temperature
02 high temperature
peak
peak
peak
0maol/g)
(lamol/g)
(~tmol/g)
Cu-
clinoptilolite Cu-ZSM-5 Si/Al=66 Cu/AI=I Cu-ZSM-5 Si/AI=24 Cu/AI=0.5
350
12
45
52
42
450
35
4.2
3.8
4.1
320
27
6.5
70
60
28 3
26 3
30 3
1) First contact with NO at 600 oC 2) 2) Cooling in 2 vol% NO/Ar from reaction temperature to 200 oC 3) 3) Cooling in Ar and dosing with NO at 200 ~
353 resulting in the TPD profile similar to that found for the Cu-clinoptilolite catalyst. The low temperature peak increased and simultaneously the high temperature peak decreased (see Fig. 3c and d). This means that at 200~ about half of the NO adsorbs in molecular form, whereas during cooling in NO the amount of the molecular form drastically decreases together with a slight increase in the total amount of NO. In Table 1. the amounts of NO and 0 2 detected in the TPD are presented. It is worth mentioning that the NO to 0 2 ratio is in all cases close to one indicating the decomposition of a NO 3 instead of a Cu-NO 2 type surface complex proposed in ref. [6]. Note also that above 400~ the highest amount of NO + 0 2 is released and the low temperature NO peak is the smallest for the most active Cu-ZSM-5 catalyst (Si/A1 = 24, Cu/AI = 0.5). The total amount of NO is always much less than the copper content indicating that only a small fraction of the copper adsorbs NO under reaction conditions. XPS results shown in Fig. 4. demonstrate how the copper sites change in the reaction. Although the binding energy of Cu 2p at 934.2 eV is unchanged being characteristic of Cu 2+, there is a drastic drop in the surface copper concentration from about 3 at. % to 0.9 at. % as well as a change in the structure of the satellite peak. This is indicative of a change in the environment of the copper sites [8] as the satellite structure originates from the paramagnetic properties of Cu 2+. x 10"s . 934.2 eV 5.6 5.4 5.0 Q o
4.8
-
~
4.6 ""J
it i
4.4
b) "'~,%
4.0 9
965
'.
955
1
' d
945
935
.
925
Binding energy (eV)
Figure 4. Cu XPS signals on Cu-ZSM5 Si/AI=24 Cu/AI=0.5 a) as prepared b) 600 ~ Ar lh followed by 450 ~ NO/Ar lh
354 4. DISCUSSION Although the three catalysts are quite different in respect to the copper content and zeolite structure, they show similarities in the NO decomposition. The catalyst activities are developed during the first contact with NO when the oxygen atoms resulted from NO decomposition are trapped and some extra-lattice oxygen is formed. Most probably during the first heating in argon a part of Cu 2+ ions is reduced to Cu + [6, 9] and on contact with NO they are reoxidized. Simultaneously, trace amount of N20 is formed, a situation similar to the room temperature NO adsorption [6]. The copper ions either migrate inside the zeolite cages or form larger oxide particles at the external surface of the zeolite crystals. This structure is rather stable as on extended purge in pure argon at 600oc there is no additional oxygen trapped or N20 formed when the catalysts are again contacted to NO at 600oc. This indicates that in argon Cu 2+ is not reduced into Cu+ ions in the active catalysts. In the 350-450~ temperature range the catalysts are active and oxygen rich intermediates or poison are developed. The poisoning is evidenced by the N 2 overshoots shown in Fig. 2. As the oxygen rich surface species are formed the N 2 evolution decreases. The peak in the N 2 concentration appears considerably above the state characteristic of steady state. The oxygen rich surface species are stabilized by NO, as on switching off the NO flow they decompose resulting in an overshoot in the 0 2 signal. The presence of this oxygen rich surface species is also evidenced by TPD results. The high temperature the desorption peak at 450-500~ this complex decomposes into NO and 0 2 without formation of N 2. If NO molecule were adsorbed on the surface at this temperature, it should have decomposed, at least partially into N 2 and 02. The amount of the complex depends on the "prehistory" of the catalyst. When NO is adsorbed at 200~ most of it is in molecular form and desorbs at about 300~ Only a part of NO forms oxygen rich complex which decomposes at 450-500~ When the catalyst is cooled in NO, there is practically no molecular NO adsorption and only the oxygen rich complex forms. The amount of this complex is considerably less than the copper content of the catalysts and there is no correlation between this and the initial oxygen uptake. On the Si/AI = 24 Cu-ZSM5 catalyst the nature of this complex was studied by FT-IR [5]. In agreement with the present study it was shown that only a small fraction of the copper ions is in interaction with this complex. In the present work we have shown that the same effect is measurable also on other Cu-zeolites. We propose that the intermediate is also the same as it was suggested in [5], namely a Cu2+(O)(NO)(NO2) type complex. The amount and the bond strength of the complex determines the catalytic activity.
ACKNOWLEDGEMENTS The financial support of this research by Grant OTKA T-017047 is acknowledged.
355 REFERENCES
1. 2. 3. 4. 5. 6. 7.
M. Iwamato and H. Hamada, Catal. Today, 10 (1991) 57 W. K. Hall and J. Valyon, Catal. Lett., 15 (1992) 311 G. Centi and S. Perathoner, Appl. Catal. A, 132 (1995) 179 Z. Schay and L. Guczi, Catal. Today, 17 (1993) 175 Z. Schay, H. Kn6zinger, L. Guczi and G. P~-Borbdy, Appl. Catal. B, to be published Y. Lee and J. N.Armor, Appl. Catal.,76 (1991) L1 G. P. Ansell, A. F. Diwell, S. E. Golunski, J.W. Hayes, R. R. Rajaram, T.J. Truex and A.P. Waker, Appl. Catal. B 2 (1993) 81 8. W. Grunert, N. W. Hayes, R. W. Joyner, E. S. Shpiro, M. R. H. Siddiqui, G. N. Baeva, J. Phys. Chem., 98 (1994) 10832 9. Y. Lee, W. K. Hall, J. Catal. 129 (1991) 202
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CATALYSIS AND AUTOMOTIVEPOLLUTIONCONTROLIV Studies in Surface Science and Catalysis, Vol. 116 N. Kruse, A. Frennet and J.-M Bastin (Eds.) 91998 Elsevier Science B.V. All rights reserved.
357
Stability of cerium exchanged zeolite catalysts for the selective catalytic reduction of NOx in simulated diesel exhaust gas W.E.J. van Kooten, H.P.A. Calis and C.M. van den Bleek Delft University of Technology, Department of Chemical Technology and Materials Science, Julianalaan 136, 2628 BL Delft, The Netherlands
This paper describes the activity and the stability of several Ce exchanged zeolite SCR catalysts. NH3 is used as the reducing agent. CeNa-MOR is very active and reaches NOx conversions up to 100%, at a GHSV of 43000 hl and temperatures between 300 and 500~ The stability however, especially when SO2 is added, appeares to be poor. CeH-ZSM-5 on the contrary is less active but shows SO2 resistance, at least for the relatively short time it is investigated (37 hours) with SO2 concentrations up to 450 ppmv. CeH-ZSM-5 extruded with 50 wt% alumina suffers from irreversible deactivation when the catalyst is exposed to SO2 concentrations higher than 300 ppmv.
1. INTRODUCTION Earlier research [ 1-6] has shown that Ce exchanged zeolites are very promising catalysts for the selective catalytic reduction (SCR) of NOx using NH3 or urea as reducing agent because of their high activity and selectivity. These catalysts perform very well over a large temperature range, i.e. 300 to 600~ They can be operated with an e x c e s s amount of NH3 (up to 30% excess), the excess NH3 being converted to N2 rather than NOx or N20. The Ce zeolites are only slightly active in the oxidation of SO2 to SO3, which is advantageous for a diesel deNOx catalyst because SO3 adds to particulate emission, corrosion and formation of salts, which plug the catalyst. These features make the Ce zeolite catalyst in combination with urea as reducing agent a suitable candidate for a deNOx process for stationary diesel engines, such as marine diesel engines. Regarding the composition of diesel exhaust gases (containing amongst others water and SO2), developing a stable, zeolite based diesel exhaust deNOx catalyst is a challenging task. Zeolites can show dealumination under hydrothermal conditions accompanied by a loss of active material; furthermore SO2 can also cause deactivation. Many authors already have reported on the hydrothermal stability of zeolite SCR catalysts [e.g. 7-9] and also some papers exist on the stabilization with respect to hydrothermal deactivation of zeolite SCR catalysts by the choice of proper cations [ 10-13]. A small number of articles describes the influence of SO2 on zeolite SCR catalysts [ 14-17]. The current paper gives the results of measurements on both the short term hydrothermal stability and the influence of SO2 on CeNa-MOR and CeH-ZSM-5 zeolite catalysts. For application of zeolite SCR catalysts a monolith type reactor can be used, which is however
358 relatively expensive. Cheaper low pressure drop reactors, such as a Radial Flow Reactor (RFR) or a Lateral Flow Reactor (LFR) can also be used. For the RFR and the LFR as well as for a traditional fixed bed reactor, the zeolite should be extruded with a binding material to obtain proper, mechanically strong particles. To investigate the influence of a binding material, some zeolites were extruded with alumina as this is an often used binding. Results are shown of measurements on the activity and stability of an (50/50 wt%) extruded CeH-ZSM-5 with A1203.
2. EXPERIMENTAL CeNa-MOR was prepared by exchanging Na-MOR (PQ Zeolite, CBV-10A) with an aqueous solution of Ce2(SO4)3 at 80~ CeH-ZSM-5 was prepared by first exchanging Na-ZSM-5 (Uetikon, PZ-2/40) with an aqueous NH4NO3 solution at 80~ and next with a Ce(Ac)3 solution in water also at 80~ The ion-exchange process is subject of current research and will be described elsewhere [ 18]. After ion exchange CeNa-MOR and CeH-ZSM-5 contained 3.2 wt% (i.e. 41% ion exchanged) and 0.64 wt% (i.e. 23% ion exchanged) Ce respectively, as determined with ICP-AES. The zeolite samples were pelletized and crushed to a sieve fraction of 0.8-1.0 mm particles. Furthermore CeH-ZSM-5 was extruded with A1203 (50/50 wt%). The extrudates were also crushed to a sieve fraction of 0.8-1.0 mm. The Ce zeolites were tested for their catalytic activity for the SCR reaction in repeated temperature program runs: from 200~ up to 600~ and back to 200~ in steps of 50 or 100~ with a 2 hours dwell at each temperature level. The activity measurements were performed in a tube reactor made of quartz under plug flow conditions at a GHSV of 43000 hl, using 0.45 gram of catalyst particles. The standard feed composition was: 900 ppmv NO, 900 ppmv NH3, 5 vol.% 0 2 , 0 , 7 or 10 vol.% H20, 0, 100, 300 or 450 ppmv SO2 and balance nitrogen. Before the gases entered the reactor, they were mixed in a stainless steel gas mixing chamber (150~ water was added to this chamber by using a peristaltic pump. The stainless steel sampling lines had also a temperature of 150~ An ECO-physics CLD 700 EL-ht NOn-analyzer, based on the chemiluminescence principle, was used to monitor the NOx conversion. At each temperature the NOn concentrations of the inlet gases and outlet gases of the reactor were analyzed. The ammonia concentration was analyzed by a microwave process gas analyzer (Siemens, M52033-A901). The N20 formation was examined using ECD Gas Chromatography. In the temperature range of 200600~ the maximum amount of N20 produced was 1.5 ppmv at a temperature of 300~
3. RESULTS 3.1 SCR activity in presence of H20 The deNOx activity test sequence was the same for all three samples (CeNa-MOR, CeH-ZSM5 and CeH-ZSM-5/A1203), see Table 1. In this paper we only show the activity curves of the 'dry' and the 'wet' experiments as those activities are more representative for the catalyst activity than the activity during the pretreatment . The activities at ascending temperatures during the pretreatment were always a little higher (0-10%) than at the descending temperature phase of the pretreatment. The activity at the descending temperature phase of the pretreatment (step 2) always coincided with the ascending temperature activity of the dry experiment (step 3).
359 Table 1 Description of the test sequence of all catalYStsamples. Step # Temperature program Gas composition 1 200 to 600 ~ No H20, no SO2 2 600 to 200 ~ No H20, no SO2 3 200 to 600 ~ No H20, no SO2 4 600 to 200 ~ No H20, no SO2 5 200 to 600 ~ 10 vol.% H20, no SO2 6 . . . . . . 600 t o 200 ~ .......! 0 vol.;% .H20, n o S O 2 "-"
100
g
8o
o
60
~
o
~
T
.
.
.
,
,.,,,
,
.
40 2o
0
200
300 -
I
400 ~-
9 Dry gas, Tup Wet gas, T up
500
600
Temperature (~ O Dry gas, Tdown E] Wet gas, T down
Fig. 1 NOx conversion as a function of temperature for CeNa-MOR. (GHSV = 43000h", 900 ppmv NO, 900 ppmv NH3, 5 vol.% 02, 0 vol.% H,O (dry), 10 vol.% H:O (wet) and balance nitrogen)
100 l
o
i
80
O
o 0
d
60 40
2: 20 200
I
300
4
I
’
400
i,
I
- ~
500
..
,.J
,.,
.
,,,.,
Experiment . . . . . designation Pretreatment, Asc. T Pretreatment, Desc.T 'Dry' exp., Asc. T 'Dry' exp., Desc. T 'Wet' exp., Asc. T. 'Wet' exp., D.esc: T
I
600
Temperature (~ Fig. 2 NO, conversion as a function of temperature for CeH-ZSM-5. (GHSV - 43000h", 900 ppmv NO, 900 ppmv NH3, 5 vol.% 02, 0 vol.% H20 (dry), 10 vol.% H20 (wet) and balance nitrogen) (for legend see Fig. 1)
360 Figs. 1 to 3 show deNOx activity versus temperature for the three zeolite samples (CeNa-MOR, CeH-ZSM-5 and CeH-ZSM-5/AI203) during steps 3 to 6 indicated in Table 1. Fig. 1 shows that CeNa-MOR in the dry run possessed high activities already at low temperatures. A maximum conversion of 100% was reached between 300 and 500~ The descending temperature curve is shifted to higher temperatures, particularly in the low temperature region. When 10 vol.% water was added to the gas stream, the wet ascending temperature curve more or less coincided with the dry descending temperature curve. Again high conversions were reached. The wet descending temperature data showed a deactivation, chiefly at temperatures of 300 to 400~ Further experiments indicated that this deactivation was only partly reversible. The decrease in activity of all curves at high temperatures is probably caused by the oxidation of a small part of the ammonia, which is a common phenomenon for deNOx catalysts. Fig. 2 displays the results for CeH-ZSM-5. Though the catalyst only contains 0.64 wt% Ce, the activity reached peak conversions higher than 80%. At high temperatures there is a small NOx conversion decrease. The presence of water shifted the maximum conversion to higher temperatures in both the ascending and descending temperature curve. No further deactivation was noticed, as a second wet experiment (not shown here) gives the same activities as the first 'wet' experiment which is shown in Fig. 2.
100 80 60 40
20 0 200
300 ....
400
500
600
, Temperature (~
Fig. 3 NOx conversion as a function of temperature for CeH-ZSM-5/A1203. (GHSV = 43000h !, 900 ppmv NO, 900 ppmv NH3, 5 vol.% 02, 0 vol.% H20 (dry), 10 vol.% H~O (wet) and balance nitrogen). (for legend see Fig. 1) Fig. 3 shows that the extrudate CeH-ZSM-5/A1203, which in fact is a diluted catalyst, had only reasonable activity at temperatures higher than 400~ in the dry experiment. The ascending and descending temperature curve exhibited about the same data. The presence of water shifted the curve to higher temperatures. The low temperature activity of the catalyst was very poor. In the presence of water the conversion at 300 ~ reached only about 10%. A reversible deactivation for the wet descending temperature line was found.
361 3.2 S C R activity in the presence of SO2 Fig. 4 shows the influence of 50 ppmv SO2 on the NOx conversion for CeNa-MOR. The experiment was carried out with 7 vol.% water at a constant temperature of 387 c C. Directly from the start of the SO2 addition, the NOx conversion decreased and continued decreasing as long as SO2 was supplied. After stopping the SO2 addition, the deNOx activity did not recover significantly.
,-, 100 150 ppm SO=
~
80
-
-
(400"C)
0
d Z
6o
T
40
CeNa-MOR (387"C)
50 ppm SO= .........
,
I'
t
t
5 Time (h)
10
Fig. 4 The influence of SO: on the NO~ conversion of CeNa-MOR and CeH-ZSM-5 at 4000C. (GHSV = 43000h t, 900 ppmv NO, 900 ppmv NH3, 5 vol.% O:, 7 vol.% H20 for CeNa-MOR, 10 vol.% H:O for CeH-ZSM-5, SO2 as indicated in the figure and balance nitrogen.) 100 =
Q
500
90
400 ~
. ....~
Q
'-.'
80
o
70
200
o" 7;
6O
100 o
;> CJ
T 5o o
...-4
j
300
;o
20 Time (h)
2 00
2=
9
T
Fig. 5 The influence of SO: on the NO~ conversion of CeH-ZSM-5 at 500~ (GHSV = 43000h "t, 900 ppmv NO, 900 ppmv NH3, 5 vol.% 02, 10 vol.% H~O, SO~ as indicated in the figure and balance nitrogen.)
362 In the same figure the activity of CeH-ZSM-5 is plotted during a similar experiment with an even higher SO2 concentration of 150 ppmv and with 10 vol.% water at a temperature of 400 * C. The NOx conversion before S02 addition was about 80 % (which is in agreement with the data of Fig. 2) and upon SO2 addition it dropped to about 70% NO• conversion in about half an hour. No further activity decline was observed within 4 hours. Removal of SO2 from the feed restored the initial catalyst activity within about 1 hour. Further experiments revealed that the catalyst was resistant to SO2 concentrations up to 450 ppmv maintaining a NOx conversion of 70% at 400 o C during 16 hours. Repeating this experiment at 500" C, we found a rather surprising effect. The presence of SO2 in the feed gave rise to an increase in the NOx conversion as shown in Fig. 5. Going from 0 to 100 ppmv SO2, the NOx conversion increased from about 80% to about 90%. Further increase of the SO2 concentration did not show any significant influence. After making the feed SO2 free, the NOx conversion decreased to 80%, the value it was before SO2 exposition. Thus the CeH-ZSM-5 catalyst used, did no show any irreversible deactivation during the 3 experiments with SO2 in the feed, which lasted a total time of 37 hours. ~" 50
500 ,~,
o 40
400
r/l
300 .~
> 30
200
0
~ 2o
tl.,)
I00
Z
T
10
+
0
10
t
20 ; time (h)
30
4O
0
= Q
d
T
Fig. 6 The influence of SO2 on the NOx conversion of CeH-ZSM-5/A1203 at 500~ (GHSV = 43000h 1, 900 ppmv NO, 900 ppmv NH3, 5 vol.% 02, 10 vol.% 1-/20, SO2 as indicated in the figure and balance nitrogen.) o
The same type of experiment was carried out on CeH-ZSM-5/AI203 at 400 C in presence of 10 vol.% water. In Fig. 6 the results are plotted. The initial catalyst activity was lower than for the others and started at about 40% NOx conversion. Increasing the SO2 concentration to 100 or even 150 ppmv caused no significant deactivation on this time scale. A further increase of the SO2 concentration to 300 ppmv and 450 ppmv caused a significant decrease in NOx conversion. The deactivation was slow though it went on until SO2 was no langer added to the feed, at t = 18 hours. Only a slight recovery of the catalyst activity could be noticed.
4. DISCUSSION 4.1 Influence of H20 CeNa-MOR showed a very high activity but suffered from deactivation in 10 vol.% H20. Partly, the deactivation could be caused by dealumination. Especially the low temperature deNOx activity of the zeolite is positively influenced by the presence of Ce cations [1,5]. Thus
363 dealumination mainly caused deactivation in the low temperature region, which indeed can be seen in Fig. 1. Dealumination is a common problem for zeolite catalysts, depending not only on the type of zeolite, the gas feed and the temperature but also on the kind of cations. Some rare earth metals (such as lanthanum) are thought to inhibit dealumination [ 13] but so far this could not be proven for cerium cations. This study also can not confirm that Ce is an inhibitor for dealumination, at least not for CeNa-MOR. The activity of CeH-ZSM-5 is not very high but seems to be stable. Thus unlike CeNa-MOR, CeH-ZSM-5 did not suffer much from dealumination. More severe conditions are necessary to test the long term hydrothermal stability of CeH-ZSM-5. Increase of the amount of Ce to about 100% of the Ion Exchange capacity would surely benefit the low temperature activity [18]. Extrusion of this catalyst with A1203 has a clear effect on the activity. In the SCR reaction in the absence of H20 there is no deactivation but the low temperature activity has decreased compared to the non extruded zeolite. This is due to the dilution of the catalyst with A1203, a material which has no low temperature activity (A1203 started to show deNOx activity itself from about 400~ under our test conditions.). The high temperature activity of the extrudate still is reasonably good compared to non extruded CeH-ZSM-5 and CeNa-MOR. The experiment with H20 in the feed results in a clear reversible deactivation. Generalizing, the effect of water in the feed is twofold. Water in the feed causes a shift of the conversion maximum to higher temperatures (clearly visible in Figs. 2 and 3). This temperature shift is noticeable already at low temperatures. The shift is caused by a reversible inhibition of active sites by water. Water in the feed can also cause irreversible deactivation by dealumination (CeNa-MOR) which only occurs at high temperatures and results in a decreased deNOx activity mainly in the low temperature region. 4.2 Influence of S02 In general, addition of S02 to the feed can cause several problems. Oxidation of S02 to S03 adds to the emission of particulates and moreover 803 reacts with H20 to give HESO4,which causes corrosion in the exhaust pipe. Oxidation of SO2 to sulfates, e.g. ammonium sulfates, is also highly undesirable, because of possible blocking of pores of the catalyst resulting in catalyst deactivation. It is known [ 19] that ammonium sulfates decompose easily at moderate temperatures depending on the concentration of NH3 and SO3. We did not find any indication on the oxidation of SO2 to SO3 or sulfates in the sampling lines (stainless steel, 150~ Our results show that CeNa-MOR was more susceptible to deactivation than CeH-ZSM-5. CeNa-MOR probably formed sulfates which clog the catalyst. The influence of SO2 on CeHZSM-5 was quite different. Upon SO2 addition a 10% activity decay occurred in one discrete step. The catalyst regained its full activity within one hour of SO2 free feed. This could point to a reversible site-blocking effect. The formation of a sulfate salt is less likely because there was no continuing deactivation during SO2 exposition. The results of the experiment at 500~ confirm this hypothesis. At this temperature even an increase in deNOx activity was found. We think that it is due to an effect that is comparable to the influence of water. Fig. 2 shows that the influence of H20 on the NO• conversion at 400~ was slightly negative but that at 500~ and 600~ a small increase was obtained, i.e. a shift of the conversion maximum to higher temperatures. Others also have reported that a zeolite of the type ZSM-5, used as SCR catalyst, could withstand SO2 at temperatures higher than 400~ [20]. In case of the extrudate (Fig. 6) a deactivation at SO2 concentrations higher than 300 ppmv on the time scale of hours could be noticed which is probably related to a reaction of the A1203, as
364
dealumination mainly caused deactivation in the low temperature region, which indeed can be seen in Fig. 1. Dealumination is a common problem for zeolite catalysts, depending not only on the type of zeolite, the gas feed and the temperature but also on the kind of cations. Some rare earth metals (such as lanthanum) are thought to inhibit dealumination [13] but so far this could not be proven for cerium cations. This study also can not confirm that Ce is an inhibitor for dealumination, at least not for CeNa-MOR. The activity of CeH-ZSM-5 is not very high but seems to be stable. Thus unlike CeNa-MOR, CeH-ZSM-5 did not suffer much from dealumination. More severe conditions are necessary to test the long term hydrothermal stability of CeH-ZSM-5. Increase of the amount of Ce to about 100% of the Ion Exchange capacity would surely benefit the low temperature activity [18]. Extrusion of this catalyst with A1203 has a clear effect on the activity. In the SCR reaction in the absence of H20 there is no deactivation but the low temperature activity has decreased compared to the non extruded zeolite. This is due to the dilution of the catalyst with A1203, a material which has no low temperature activity (A1203 started to show deNOx activity itself from about 400~C under our test conditions.). The high temperature activity of the extrudate still is reasonably good compared to non extruded CeH-ZSM-5 and CeNa-MOR. The experiment with H20 in the feed results in a clear reversible deactivation. Generalizing, the effect of water in the feed is twofold. Water in the feed causes a shift of the conversion maximum to higher temperatures (clearly visible in Figs. 2 and 3). This temperature shift is noticeable already at low temperatures. The shift is caused by a reversible inhibition of active sites by water. Water in the feed can also cause irreversible deactivation by dealumination (CeNa-MOR) which only occurs at high temperatures and results in a decreased deNOx activity mainly in the low temperature region. 4.2 Influence of SO2 In general, addition of SO2 to the feed can cause several problems. Oxidation of SO2 to SO3 adds to the emission of particulates and moreover SO3 reacts with H20 to give H2SO4, which causes corrosion in the exhaust pipe. Oxidation of SO2 to sulfates, e.g. ammonium sulfates, is also highly undesirable, because of possible blocking of pores of the catalyst resulting in catalyst deactivation. It is known [ 19] that ammonium sulfates decompose easily at moderate temperatures depending on the concentration of NH3 and SO3. We did not find any indication on the oxidation of SO2 to SO3 or sulfates in the sampling lines (stainless steel, 150~C). Our results show that CeNa-MOR was more susceptible to deactivation than CeH-ZSM-5. CeNa-MOR probably formed sulfates which clog the catalyst. The influence of SO2 on CeHZSM-5 was quite different. Upon SO2 addition a 10% activity decay occurred in one discrete step. The catalyst regained its full activity within one hour of SO2 free feed. This could point to a reversible site-blocking effect. The formation of a sulfate salt is less likely because there was no continuing deactivation during SO2 exposition. The results of the experiment at 500~C confirm this hypothesis. At this temperature even an increase in deNOx activity was found. We think that it is due to an effect that is comparable to the influence of water. Fig. 2 shows that the influence of H20 on the NOx conversion at 4 0 0 ~ C was slightly negative but that at 500~C and 600~C a small increase was obtained, i.e. a shift of the conversion maximum to higher temperatures. Others also have reported that a zeolite of the type ZSM-5, used as SCR catalyst, could withstand SO2 at temperatures higher than 4 0 0 ~ C [20]. In case of the extrudate (Fig. 6) a deactivation at SO2 concentrations higher than 300 ppmv on the time scale of hours could be noticed which is probably related to a reaction of the A1203, as
365 the non-extruded ZSM-5 clearly shows other behavior. The formation of pore plugging sulfates is the probable cause for the observed deactivation. Using the catalysts at low SO2 concentrations of about 20 ppmv (which is the SO2 concentration in the exhaust when standard European diesel fuel containing about 0.05 %S is used), a deactivation is expected on the long term. So the use of a 50/50 wt% extrudate with alumina seems no suitable choice for diesel deNOxing. Extrudates with less alumina or with other binding materials should be investigated.
5. CONCLUSIONS In absence of H20 and SO2, CeNa-MOR is a very active SCR catalyst. CeNa-MOR however suffers from deactivation under hydrothermal conditions and in presence of SO2. CeH-ZSM-5 is slightly less active but is a much more stable catalyst both in the presence of H20 and SO2. Application of this latter zeolite using a 50/50 wt % extrudate with A1203 seems not appropriate due to poisoning by SO2 at concentrations higher than 300 ppmv. To maintain the good properties of CeH-ZSM-5, extrudates with less alumina or other binding materials should be used.
ACKNOWLEDGMENT The authors would like to thank Dr. J. Nieman of AKZO NOBEL for arranging the manufacturing of the extrudates and for helpful discussion.
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9. 10. 11. 12.
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CATALYSIS AND AUTOMOTIVEPOLLUTIONCONTROLIV Studies in Surface Science and Catalysis, Vol. 116 N. Kruse, A. Frennet and J.-M Bastin (Eds.) 91998 Elsevier Science B.V. All rights reserved.
367
Study of Copper - and Iron - containing ZSM-5 zeolite catalysts: ESR spectra and initial transformation of NO J. Varga a, J. Hal~sz a, D. Horv~tha, D. M6hn a, J. B.Nagy b, Gy. Sch0bel a and I. Kiricsi a aApplied Chemistry Department, J6zsef Attila University, Rerrich t6r 1 H-6720 Szeged, Hungary bLaboratoire de RMN, Facult6s Universitaires Notre-Dame de la Paix, rue de Bruxelles 61, B-5000 Namur, Belgium
ABSTRACT For a better understanding of the first steps of the reaction of NO over Cu-ZSM-5 and FeZSM-5 zeolites the following measurements were performed: (i) the products of the gas phase interactions were followed by MS, (ii) the valence state and coordination of transition metal ions in zeolites by ESR spectroscopy. Catalysts were prepared both by conventional and solidstate ion-exchange methods and pretreated in vacuum, in oxidative and in reductive atmosphere. The conventional ion-exchanged samples are more active in NO decomposition than the solidstate exchanged ones. Over the reduced catalysts the first step consists in N20 formation and the oxidation of Cu + ~ Cu 2+ (Fe2+ ~ Fe 3+) followed by N20 reduction to N2 (in these conditions 02 release was not detected). These results are consistent with a simple redox mechanism in which NO adsorption and decomposition on active (reduced) sites leads to the formation of gaseous nitrogen and adsorbed oxygen. Over oxidized samples NO transformation is slower than over reduced ones, and the first step is to be the reduction of metal ions (Cu2+, Fe 3+) with NO as reducing agent.
1. INTRODUCTION The removal of nitrogen oxides from mobile and stationary sources remains an important environmental technology task for solving acid rain and other pollution problems [1-3]. Transition metal exchanged MFI zeolites, particularly Cu-ZSM-5, were proven to be active in catalytic reduction of NO (and NO2) by ammonia [4] or hydrocarbons [5,6] as well as in the direct decomposition of NO to molecular nitrogen and oxygen in "lean-burn" engine conditions [7,8]. Numerous ESR measurements were carried out to identify the oxidation states and coordinations of transition metals - especially Cu - in ZSM-5 zeolite structure [9-12]. Moreover Kucherov et al. performed in situ ESR studies to clarify the redox properties of copper exchanged zeolites [ 13,14]. Although much effort was devoted in the recent years to understand the mechanism of these reactions, involving the explanation of the exact role of the catalyst, the information on the character of the metal active sites and their interactions with the zeolite structure and the reactant
368 molecules are contradictory and incomplete. In the present work, the role of copper and iron content of the catalysts (prepared by conventional and solid-state ion-exchange methods) was investigated in the initial elementary steps of the direct decomposition of NO by ESR spectroscopy and catalytic measurement.
2. EXPERIMENTAL Cu2+- and Fe3§ containing ZSM-5 zeolites were prepared by conventional and solid-state ion-exchange methods described in ref. [ 15]. The Si/AI ratio of parent Na-ZSM-5 was 40. The samples were characterized by X-ray diffraction, IR spectroscopy, thermal analytical method and BET measurement. Related data are shown in Table 1. Table 1
Sample characterization Sample
Cu-ZSM-5
Exch. method
Conventional
Fe-ZSM-5
Solid-state
Conventional
Solid-state
Metal ion content (wt %)
1.14
3.05
1.19
5.11
Ion-exchange degree %
93
250
54
190
BET area m2/g
370
318
367
318
ESR spectroscopy is a powerful tool to monitor the valence state and coordination of cupric and iron ions in zeolites. After different pretreatment procedures (listed in Table 2) and/or NO adsorption spectra were recorded on a Bruker BER-420 spectrometer with a TE~o2 rectangular cavity - at both ambient temperature and 77 K. Table 2
Pretreatment procedures and designation of ESR samples Pretreatment
Designation
I.
One hour evacuation at ambient temperature
II.
Evacuation at 773 K for 2 hours
V
III.
Oxidation by 100 Torr (13.33 kPa) 02 at 573 K for 2 hours
O
IV.
Reduction by 100 Torr (13.33 kPa) 1-12at 373 K for 2 hours
R
V.
Heating in air at 773 K for 2 hours
L
The kinetic measurements to study the transformation of nitrogen oxides were carried out in a recirculatory batch reactor with mass spectrometric analysis (see details in [16]). To eliminate the
369 effect of the gas phase oxygen for the initial steps of NOx reactions the measurements were carried out in conditions where the molar ratio of active centres (supposedly metal ions) to reactant was as high as 3. The catalyst samples (0.5 g) were heated at 773 K in vacuum (0.1 Pa) for two hours in every case followed by oxidative - at 573 K in 100 Torr (13.33 kPa) 02 for 2 hours - or reductive - at 373 K, 100 Torr (13.33 kPa) H2 for 2 hours - treatment. The gas-phase concentrations of reactants (NO, NO2 and N20) and products were measured by mass spectrometry. Mass numbers 30 of NO, 46 of NO2, 44 of N20, 28 of N2 and 32 of O2 were used for the quantitative analysis. It is important to emphasize that no detectable 02 was found in NO decomposition at the reaction conditions used.
3. RESULTS AND DISCUSSION The catalytic transformations of nitrogen oxides are considerably affected by the oxidation state of the metal ions occupying exchange position in the zeolite. The ESR technique is a useful method to follow the oxidation states and changes of catalysts in a different way, however, the real reactions could be characterized by measurements in actual conditions. 3.1. ESR Measurements 3.1.1. Characterization of Cu-ZSM-5 samples For copper in Cu-ZSM-5 zeolites three different coordinations are given in the literature [914,17]: square planar (g = 2.27), square pyramidal (g = 2.33), and octahedral (g = 2.38). Additional exchange ions present in zeolites, or water content of the samples influence these potential distributions. Furthermore, a given coordination can be realized by involving different number of framework oxygens, exchange cations or other extra framework ligands such as water or hydroxyl groups. Considering the general rules the following main features can be drawn (more detailed explanation is found in [18]). In hydrated CuL and CuS samples (see designations in table 3) the Cu 2§ ions occupy octahedral coordinations. Evacuation causes partial or full dehydration, and a decrease of symmetry from water molecules assisted as ligands of copper ions. CuLV and CuSV spectra can be considered as superpositions of two spectra - one for square pyramidal and one for square planar coordinations. Oxidation treatment caused no change in the spectra just as it was expected. Reduction had to be made cautiously as Cu2+ can be readily reduced by hydrogen above 573 K. This treatment at 373 K resulted in an increase in the symmetry of copper ions. The signal intensities of spectra CuLR and CuSR are lower due to the fact that Cu + and Cu ~ are ESR silent species. By reduction water molecules can be formed, which can be coordinated to the remaining 2+. Cu Ions forming octahedral complexes, different from that of the hydrated samples. Upon NO adsorption some broadening and simultaneously, some increase in the intensity of the signals took place, which indicate a complex redox transformation on the surface metallic sites of the catalysts. In the interaction between NO and Cu-ZSM-5 mainly Cu 2+ ions of lower symmetry (square planar or square pyramidal) are involved.
370 3.1.2. Characterization Fe-ZSM-5 samples The general rules listed in case of Cu-ZSM-5 are still valid for Fe-ZSM-5. In zeolite structure four main coordinations for the Fe species can be found: octahedral (g - 2.06), oxo-hydroxo species (g = 2.89), tetrahedral (g = 4.34), and distorted tetrahedral samples (g = 5.4 - 6.8) are mentioned in the literature [ 19]. The ESR parameters of samples are collected in Table 3. Table 3
ESR parameters determined for different Fe-ZSM-5 samples Sample*
gB
g
g
g
FeL
2.442
5.383/5.403
4.380
2.904
FeLNO
2.442
6.534/5.442
4.385
2.894
FeS
2.433
5.405
4.338
2.896
1.999
FeSNO FeLV FeLVNO FeLL FeLLNO FeSL FeSLNO
2.435 2.658 2.579 2.436 2.438 2.447 2.443
6.908/5.719
2.892
6.248/5.815 6.302/5.743 6.22/5.943 5.861
4.346 4.340 4.347 4.320 4.324 4.341 4.336
2.907 2.899 2.907 2.893
2.058 2.004 2.007 2.062 2.066 2.064 2.061
FeLO
2.384
6.269/5.812
4.304
2.899
2.159
FeLONO
2.385
6.249/5.838
4.318
2.897
2.157
FeSO FeSONO FeLR FeLRNO FeSR FeSRNO
2.444 2.443 2.442 2.441 2.439 2.440
6.194/5.940 5.430 6.749/5.345 5.384 6.286/5.869 5.876
4.339 4.313 4.312 4.284 4.336 4.340
2.394 2.885 2.891 2.897 2.897 2.900
2.022 2.022 2.062 2.004 2.055 2.054
*Designation of samples:
g
1. chemical symbol of exchanged metal (Cu, Fe) 2. ion-exchange method used (Liquid or Solid state) 3. designation of the pretreatment procedure (see in table 2) 4. +NO if spectrum was taken after NO adsorption on the sample
As can be concluded from the spectra of Fig. 1 (FeL and FeS, respectively) both peaks at g - 2.06 and g - 4.34 are more intense for samples prepared by solid state ion-exchange than for those prepared by conventional method. These deviations can be explained by the different quantities of iron-content. Indeed, the spectrum of sample pretreated in vacuum (FeLV) is extremely simple. During the pretreatment some extent of the iron present was reduced to Fe ~ ferromagnetic properties of which disturbed the magnetic field applied.
371
FeL
FeS
FeLL
FeSL
FeLV-C----~
FeSV
FeLO
~,vf.~
FeSO
FeL
f
/4 FeSR
f f
Fig. 1" The ESR spectra of Fe-ZSM-5 samples prepared byconventional (L) and solidstate (S) ion-exchange method.
FeSO
FeSONO~.,//q
Fig. 2. The effect of NO adsorption on the ESR spectra of Fe-ZSM-5 samples.
372 Upon NO adsorption (Fig. 2) the intensities of the distorted tetrahedral peaks decreased which lead us to the conclusion that adsorption took place on these sites, hi situ adsorption measurements showed increases in line intensities indicating the oxidation of iron ions I ' I i I ' I ' I 3.2. Catalyticmeasurements100 ~k_~,T_ -(3- N2 The transformation of NO and NO2 as main _~ 80 ~ -0- NO components of NOx pol,~, lution in exhaust gases, ~ ~-\ -El- N20 and N20 as stable inter~ 60 mediate in these reactions were investigated ~ 40 over the Cu- and Fe-conr taining ZSM-5 zeolite catalysts. ~ 20
t
3.2.1. NO
tion
decomposi-
0
0 10 20 Time (rain) For the NO decomposition reaction the CuFig. 3: Decomposition of NO over reduced Cu-ZSM-5 ZSM-5 prepared by con(prepared by conventional ion-exchange) at 573 K. ventional ion-exchange method and pretreated in reductive atmosphere proved to be an effective catalyst, as can be seen in Fig. 3, where the results obtained at 573 K are presented. The first and very fast step is the NO transformation into 1'420 over the reduced Cu centres: 2 N O + Cue0 "~ N 2 0 + 2 CuO,
which is followed by the relatively slow decomposition of N20 into nitrogen and oxygen. At the end of the reaction only nitrogen can be detected in the gas phase, the oxygen reacts with Cu § centres resulting in Cu2+. Over catalyst pretreated in oxidative atmosphere similar reaction can be observed (see Fig. 4), however, the rate is much lower. The quasi steady-state N20 concentration can be explained by the reaction of Cu2§ with NO in forming Cu § ions, i.e. the NO is capable to reduce the oxidized metal ion. As can be seen in Fig. 5 and 6, over catalysts prepared by solid-state ion-exchange the rate of NO conversion is the same, however, the N2 formation is lower both in the reduced and oxidized samples corresponding to those prepared by conventional method. Considering the kinetic behaviour, similar results could be obtained in the reaction over FeZSM-5 samples, however, the conversion of NO was lower at a notable extent.
373 I
100
~
I
J
I
.... I
.... I" "
-0- N2
8O ~
7
-@- NO -E]- N20
o 60
4a O
r~ 20 v
20
0
40
~e
60
(rain.)
Fig. 4: Decomposition of NO over oxidized Cu-ZSM-5 (prepared by conventional ionexchange) at 573 K.
100t I
I ''
I
"1"'
!
I
J
I"
l'
I
I
~80 ~9
-<3- N2
60
- 0 - NO2
,o h e~ 9 r j 2o
-D- N20
I..d
--
0
0
20
40
60
80
" ~ m e (rain')
Fig 5: Transformation of NO over reduced Cu-ZSM-5 sample (prepared by solid state ionexchange) at 573 K. 3.2.2. N20 decomposition The N20 decomposition resuked in N2 and 02 with 2 " 1 molar ratio. The reaction is practically independent both on the catalyst preparation method and on the pretreatment process. The measured kinetic curves can be seen on Figures 7 and 8. These results are characteristic for the oxidized catalysts, and practically no differences could be observed using reduced catalysts.
374
I
l
I
~
I
i
I
i
I
i
I
i
I
-0- N2 - l - NO
8o
-s
so
N20 ,,
4a o
20 0
0
20
40
G0
80
100 Ti.me (rain)
Fig 6 Transformation of NO over oxidized Cu-ZSM-5 sample (prepared by solid-state ionexchange) at 573 K.
I
3.2.3. NO2decomposition 100 On Fig. 9 the kinetic curves of NO2 decom~ 80 position into N2 are shown over the reduced form of the catalyst, via ~ 60 N20 as intermediate. In this reaction only the ca40 talysts pretreated in ~u reductive atmosphere are o active, the oxidative prer,.) 20 treatment leads to deactivation of the catalyst in0 dependently either on the metallic form or the ion-
I']--
0
I
I
I
t
I
!
I
i
I
I
I
I
I
I
I
I'l
- ( ~ N2 - 0 - 02
I t
! I'"l
i
_ -
20 40 G0 80 100 120 140 160 180 T i m e ( r a i n )
exchange procedure. Fig. 7: Decomposition of N20 over oxidized Cu-ZSM-5 sample This behaviour can be (prepared by conventional ion-exchange) at 523 K. explained by the fact that NO2 due to its high oxidation state is not capable to reduce the CuE+or Fe3+ ions.
375
_100
i-.-o2 I
:~ 60 IV
i ~ ,~o 1
r..) 20 0
0
20 40
60
80 100 120 1 4 0 T i m e ( r e ' i n )
Fig. 8: Decomposition of N:O over oxidized Cu-ZSM-5 sample (prepared by solid-state ionexchange) at 523 K. 40 30
!
i
i .... ,
i
!"'
' ..... i
,
I
i
i
'i .......
9
0 or-,t
~
2O (
! *-"0
i....
o 10 rj
0
20
"~
40 Time (mJn)
Fig. 9 Decomposition of NO2 over reduced Cu-ZSM-5 sample (prepared by solid-state ionexchange) at 523 K.
4. CONCLUSIONS On the basis of ESR investigations and the catalytic measurements performed at a relatively high molar ratio of the assumed active sites to the reactants, the following statements can be made for the initial steps of the NO and NO2 decomposition: i) the conventional ion-exchanged samples are more active in NO decomposition than the solid state exchanged ones, however, in the transformation of NO2 the behaviour is reversed; ii) over the reduced catalysts the first (and very fast) step consists in N20 formation and the
376
iii)
iv)
oxidation of Cu § to Cu2+ followed by N~O reduction to N2; at a copper to NO ratio much smaller than unity molecular oxygen cannot be detected in the gas phase; these results are consistent with a simple redox mechanism in which NO adsorption and decomposition on active (reduced) sites leads to the formation of gaseous nitrogen and adsorbed oxygen; over oxidized catalysts NO transformation is much slower and the first step is the reduction of the Cu2+(or Fe3+) to Cu+ (or Fe2+), meaning that NO acts as a reducing agent; NO2 transformation over the reduced catalysts first results in N20 formation, the subsequent steps are the same as in the NO transformation.
In conclusion, the copper or iron containing MFI zeolite exhibits significant activity in the NO conversion due to the redox properties of the exchanged cations, and these catalysts can be utilized for treatment of lean exhaust gases. REFERENCES
1. 2. 3. 4.
Bosch, H. and Janssen, F., Catal. Today, 2 (1988), 369. Iwamoto, M., Stud. Surf. Sci. Catal., 54 (1990), 121. Shelef, M., Chem. Rev., 95 (1995), 209. Komatsu, T., Nunokawa, M., Moon, I.S., Takahara, T., Namba, S. and Yashima, T., J. Catal., 148 (1994), 427. 5. Li, Y. and Armor, J.N., Appl. Catal. B: Environm., 2 (1993), 239. 6. Adelman, B.J., Beutel, T., Lei, G.-D. and Sachtler, W.M.H., J. Catal., 158 (1996), 327. 7. Hall, W.K. and Valyon, J., Catal. Letters 15 (1992), 311. 8. Iwamoto, M., Catal. Today, 29 (1996), 29. 9. Kucherov, A.V., Kucherova, T.N. and Slinkin, A.A., Catal. Lett., 10 (1991), 289. 10. Schoonheydt, R., Catal. Rev.-Sci. Eng., 35 (1993), 129. 11. Larsen, S.C., Aylor, A., Bell, A.T. and Reimer, J.A.: J. Phys. Chem., 98 (1994), 11533. 12. Beutel, T., Shrk/my, J., Lei, G.-D., Yan, J.Y. and Sachtler, W.M.H., J. Phys. Chem., 100 (1996), 845. 13. Kucherov, A.V., Gerlock, J.L., Jen, H.-W. and Shelef, M., J. Phys. Chem., 98 (1994), 4892. 14. Kucherov, A.V., Gerlock, J.L., Jen, H.-W. and Shelet~ M., Zeolites, 15 (1995), 9. 15. Varga, J., Fudala, A., Hal~isz, J., Sch6bel, Gy. and Kiricsi, I., Stud. Surf. Sci. Catal., 94 (1995), 665. 16. Ha!~isz, J., Varga, J., Sch6bel, Gy., Kiricsi, I., Hemfidi, K., Hannus, I., Varga, K. and Fejes, P., Stud. Surf. Sci. Catal., 96 (1995), 675. 17. Kucherov, A.V. and Slinkin, A.A., J. Mol. Catal., 90 (1994), 323. 18. Varga, J., B.Nagy, J., Hal~isz, J. and Kiricsi, I., J. Mol. Structure,/in press/ 19. Wichterlovh, B., Kubelkovh, L., Jiru, P. and Kolihovh, D., Collect. Czechos. Chem. Commun., 45 (1980), 2143. ACKNOWLEDGEMENT
The financial support of the National Science Foundation of Hungary (OTKA No. T 007601) is gratefully acknowledged.
K i n e t i c s - Mechanisms
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CATALYSIS AND AUTOMOTIVEPOLLUTIONCONTROLIV Studies in Surface Science and Catalysis, Vol. 116 N. Kruse, A. Frennet and J.-M Bastin (Eds.) 91998 Elsevier Science B.V. All rights reserved.
379
The Use of Isotope Transient Kinetics Within Commercial Catalyst Development JC Frost, DS Lafyatis, RR Rajaram and AP Walker* Johnson Matthey Technology Centre, Blount's Court, Sonning Common, Reading RG4 9NHIUK
The overall catalytic activity of a catalyst is the product of two independent properties: the total number of active sites on the catalyst and the intrinsic activity per site (turnover frequency) of each of these sites. The only technique capable of measuring both the number of active sites and their turnover frequency simultaneously under actual operating conditions is the Isotope Transient Kinetics (ITK) technique. This makes it one of the most powerful techniques available to the catalytic chemist today. This paper describes the principles underlying the ITK approach, and shows how the technique can be used to guide catalyst design strategy. Specifically, ITK has been used to elucidate the fundamental processes which occur during the ageing of Pt/Rh and Pd catalysts. It is now well established that Pt/Rh and Pd catalysts respond differently to different ageing treatments; the gas phase stoichiometry during the ageing treatment can have a pronounced effect on the subsequent activity of the respective catalysts. In addition, the response of Pt/Rh- and Pd-based technologies to the presence of sulphur is very different under rich conditions, with Pd catalysts undergoing more severe deactivation via sulphur poisoning.
1. INTRODUCTION Over the course of the last twenty years there has been a dramatic increase in the number of techniques available to help catalytic chemists address the issues they face. Many of these advances have aimed to increase our understanding of catalysis at the molecular level, via the development of sophisticated surface science techniques to investigate the fundamental aspects of processes such as chemisorption. In addition, EXAFS is used routinely to obtain information about critical details of the local structure within catalytic systems (eg [ 1, 2]), and the power of in situ EXAFS for the study of catalytic systems is now well established [3, 4]. The development of the TAP technique [5] has increased our understanding of the mechanisms by which certain reactions proceed [6, 7], and it appears that both STM and AFM have exciting things to offer to the world of catalysis in the near future. However, while such techniques can undoubtedly increase our understanding of some of the important factors governing catalyst performance, it is still the case that the process of catalyst development is extremely time-consuming, and in most cases relies upon approaches based on trial and error. * to whom correspondence should be addressed
380 The overall rate of a catalytic process is governed by two factors: the activity per site (ie :umover frequency) of the active sites and the number of active sites present under reaction conditions. This latter qualification is a critical one; the number of accessible active sites at reaction temperature and in the presence of a number of other competitively-adsorbing species is often very different from the number of "active sites" measured at room temperature under well-defined conditions using either the CO or H 2 chemisorption approach. Traditionally, this difficulty in providing a realistic assessment of the number of active sites under reaction conditions has misled a number of investigators who have attempted to assign "turnover frequency" values to their catalysts by simply dividing the rate of reaction (measured under one set of conditions) by the "number of sites" measured using CO chemisorption (under a completely different set of conditions). The turnover frequency values thus derived are invariably incorrect. However, catalytic chemists should not despair, because there is a technique which is capable of measuring both the turnover frequency of the active sites and the number of these sites simultaneously under real reaction conditions. This technique is Isotope Transient Kinetics (ITK) and uses a simple experimental approach to obtain very powerful data. This paper will introduce the ITK technique and outline its potential by demonstrating how it has been used to identify and address problems pertaining to autocatalysis. The use of Pd-based technologies within the autocatalyst area is growing rapidly, and clear performance benefits can be seen for such catalysts over more conventional Pt/Rh catalyst formulations within certain applications and under certain conditions (see eg [8]). Many of the differences between Pt/Rh- and Pd-based catalysts are associated with their respective responses to ageing treatments. Specifically, the catalysts respond in different ways to ageing under oxidising (ie lean) and reducing (ie rich) conditions; ageing Pt/Rh catalysts under lean conditions usually leads to poorer performance than does ageing the catalyst under rich conditions. The converse often applies to Pd-based catalysts. In addition, the presence of sulphur during the ageing of autocatalysts is known to lead to a significant deterioration in performance under certain conditions. Specifically, the rich-side NOx performance of Pd-based catalysts is often significantly reduced by rich ageing treatments in the presence of sulphur. Such poisoning can occur in at least two ways. First of all, it is possible that the number of active sites is lowered by the retention of reduced sulphur species on the active Pd sites. Secondly, it is possible that the formation of reduced sulphide species within the bulk of the Pd leads to a decrease in the turnover frequency of the active sites. The formation of such species within the bulk of Pd particles has been recently demonstrated [9], and it is possible that the electronic modification of the Pd particles induced by these sulphide species could reduce the NO dissociation probability on the Pd sites and hence reduce the effectiveness of the catalyst within the NOx reduction reaction. The Isotope Transient Kinetics (ITK) technique has been used to examine these thermallyinduced and sulphur-induced ageing effects in more detail, and specifically to assess whether the deactivation induced under a defined set of conditions is brought about as a result of a decrease in the number of active sites, or whether it is brought about by a decrease in the turnover frequency of the active sites. The CO-NO reaction was selected to probe these effects, since this reaction is known to be particularly sensitive to sulphur poisoning.
381 2. E X P E R I M E N T A L The experimental apparatus required to perform 1TK experiments is relatively simple (Figure 1). The two isotopic feeds are routed via a four-way valve to either a vent or the reactor. Their flow rates are set as closely as possible using standard mass flow controllers. The pressures of the two gas streams are then equalised using a differential pressure transducer and fine needle valve at the vent outlet. The reactor effluent is monitored continuously by a quadrupole mass spectrometer (Fisons Instruments GasLab 300) via a capillary sampling system.
Figure 1: Schematic Representation of the ITK Apparatus An ITK experiment is performed as follows. Steady state catalysis is established using the standard gas feeds (eg ~2CO + NO + He carrier gas) ensuring that conversion levels are low to maintain differential conditions (ie below 10%). At a specified time, the appropriate feed is exchanged for its isotopically labelled analogue (eg ~2CO to 13CO). The ensuing transients are then recorded by the mass spectrometer. Ideally the switch in feeds should be a step function but clearly this cannot be achieved experimentally. The switch cannot be instantaneous but more importantly there are delays in the transport of gases through the reactor system and the mass spectrometer. For this reason, all volumes downstream of the four-way valve have been minimised to give maximum time resolution. To compensate for these gas transport delays an inert gas, normally argon, is included at a trace level (typically 5%) in one of the feeds. Argon is free from any interaction with the catalyst and is only prone to the aforementioned delays. Therefore, its decay provides a measure of these effects. Figure 2 shows the response curves obtained during a typical ITK experiment.
382
Transients in CO2 following 2 CEJ] CO switch Pt/alumina, 200~ C Response% 100
1,13~.002 80 60 40 20 Ar ~~...112-CO2 I
10
20
......................................................................
I
I
30 40 Time (seconds)
I
50
60
Figure 2: Typical Data Generated during an ITK Experiment Note that the Ar trace decays very rapidly; note also that the 12CO2and 13CO2traces mirror each other, as expected for a perfect switch which does not perturb the steady state catalysis. The two basic measurements available from an 1TK experiment are: (i) 00, the number of species (containing the initial isotope) present on the catalyst at the time of the switch. This can be calculated from the chromatographic delay in their elution; (ii) k, the specific rate at which these species emerge from the catalyst. This is described by the decay function. Therefore, an 1TK experiment allows the simultaneous definition of the number of active sites on the catalyst and the specific rate of reaction over these active sites. A full description of the data analysis procedures used to interpret 1TK data is beyond the scope of this article, but such details can be found in the literature (see eg [ 10]). Within the experiments described herein, isotopic switches between IECO/Ar[NO and 13CO/NO were used under rich conditions. Three catalysts were studied: 1% Pt-0.2% Rh/support, 2.5% Pd/support and 2.5 %Pd/A/support where A is a proprietary JM additive and where the support phase was a commercial material based upon A1203, CeO 2 and ZrO 2. These catalysts were studied in their fresh states (ie following calcination at 500~ and following a range of different ageing treatments. These ageing protocols have been given the following abbreviations: LHA900C: LHSA900C: RHA900C: RHSA900C:
aged aged aged aged
at 900~ at 900~ at 900~ at 900~
under under under under
1% 02, 10% H20, balance N 2 for 2 hours 1% 02, 10% H20 , 20ppm SO 2, balance N 2 for 2 hours 5% CO, 10% H20, balance N2for 2 hours 5% CO, 10% H20, 20ppm SO 2, balance N 2 for 2 hours
383 (The abbreviations stand for Lean Hydrothermal Ageing, Lean Hydrothermal Sulphur Ageing, Rich Hydrothermal Ageing and Rich Hydrothermal Sulphur Ageing.)
3. RESULTS AND DISCUSSION 3.1 Performance of the aged Pt/Rh catalyst The impact of the ageing treatments described above upon the performance of the Pt/Rh catalyst is summarised in Table 1, where the parameters derived from the fresh sample are reported for comparison. ,,,
Catalyst Condition
Rate (pmol/s/g)
,
Fresh
2.22
1.25
1.78
LHA900C
0.52
1.0
0.52
LHSA900C
0.45
0.95
0.48
1.00
O.95
1.05
1.0
0.98
. . . . .
RHA900C
. . . .
RHSA900C
0.98
....
!
Turnover [ No. of Active Frequency (/s) Sites (lamol/g)
,,
Table 1" ITK Analysis of the Pt/Rh Catalyst (Fresh and Aged) within the CO-NO Reaction at 250~ It is clear from the data presented in Table 1 that lean ageing is more severe than rich ageing for the Pt/Rh catalyst. ITK analysis reveals that this is n o t because of changes in the specific activity of the sites, since the measured turnover frequency following lean and rich ageing is the same (within experimental error). This turnover frequency is, however, below that observed over the fresh sample. Instead, the catalyst performs more poorly after lean ageing because there are fewer active sites on the catalyst after the lean ageing treatments. Experience teaches that the majority of the NOx conversion occurring over aged Pt/Rh catalysts is associated with the Rh component, so transformations in the effectiveness of this component for NOx reduction are probably underlying the trends apparent from Table 1. We can begin to understand the data within the table by considering that high temperature lean ageing of catalysts containing Rh and A1203 often leads to dramatic decreases in performance because the oxidised Rh species interact very strongly with the alumina support following such ageings (see eg [ 1]). Under rich ageing conditions, the strong interaction between Rh and alumina does not develop, and this is reflected in the fact that the number of active sites on the catalyst is significantly higher following the rich ageing treatments than following the lean ageing treatments. Note that the presence of sulphur during the ageing has very little effect on any of the parameters (rate, turnover frequency, number of active sites), irrespective of whether the ageing is carried out on the rich side or on the lean side.
384
3.2 Performance of the aged Pd-only catalyst Table 2 summarises the data obtained over the Pd-only catalyst following the various ageing treatments. The fresh catalyst data are also included in the table, for reference. J .
.
.
Catalyst Condition
.
.
Rate (iamol/s/g)
.
!
Turnover ] No. of Active Frequency (/s) [ Sites (larnol/g)
Fresh
94.38
1.8
52.43
LHA900C
9.79
2.3
4.26
LHSA900C
7.90
2.3
3.43
RHA900C
4.06
0.75
5.41
0.40
3.82
..........
RHSA900C
1.53
Table 2: ITK Analysis of the Pd-only Catalyst (Fresh and Aged) within the CO-NO Reaction at 250~ Table 2 reveals that ageing the catalyst at 900~ dramatically decreases the rate of reaction over this material. ITK analysis shows that this deactivation is primarily associated with the very large reduction in the number of active sites present on the catalyst. This effect is more pronounced over the Pd-only catalyst than it was over the Pt/Rh catalyst (Table 1) - over the Pdonly catalyst the number of active sites falls by a factor of 10-15, while over the Pt/Rh catalyst this factor is only 2-4. However, it should be noted that the aged Pd catalyst is still significantly more active than the aged Pt/Rh catalyst, following any given ageing protocol. Rich ageing reduces the performance of the Pd-only catalyst far more than lean ageing. Analysis of the ITK data reveals that the increased severity of the deactivation on the rich side is brought about by the dramatic decrease in the turnover frequency of the active sites, since the number of active sites on the catalyst is pretty similar irrespective of the stoichiometry of the ageing feed. In fact, it is interesting to note that lean ageing at 900~ actually increases the turnover frequency of the active sites with respect to the turnover frequency observed over the fresh catalyst. The presence of sulphur within the lean ageing feed leads to a small decrease in the activity of the catalyst. This is because the sample aged in the presence of sulphur contains a lower number of active sites. Note that the turnover frequency of the active sites is not affected by the presence of sulphur in the lean ageing feed. In contrast, the presence of sulphur within the rich ageing feed leads to a substantial decrease in the activity of the sample. There are two factors responsible for this decrease in activity. First of all, the turnover frequency of the active sites is substantially lower following rich ageing in the presence of sulphur than it is following rich ageing in the absence of sulphur. Secondly, the number of active sites is lower when the ageing is conducted in the presence of sulphur. These effects can be understood in terms of the known sulphur chemistry of Pd. Reduced sulphur species are known to form on both the surface of the Pd particles and within their bulk. The ones on the surface can clearly act to reduce the number of active sites by merely site blocking, but the fact that the remaining active sites operate with a lower turnover frequency than those aged under the same conditions but in the absence of
385 sulphur indicates that there is some electronic modification of the Pd sites associated with the presence of the reduced sulphur species. This suggests that either the sulphur on the surface of the Pd particles is modifying the local electronic structure, or that the sulphur within the bulk of the Pd is inducing this electronic modification. In either case, it appears that the reduced sulphur species are modifying the local electronic structure of the Pd in such a way as to bring about a decrease in the NO dissociation probability of the Pd particles. This rich-side sulphur poisoning of Pd is clearly an important effect, and any promoter species capable of alleviating this effect would be expected to lead to a significant improvement in catalyst performance. We have used ITK to characterise such promoters.
3.3 Performance of the aged, promoted Pd/A catalyst Table 3 summarises the data collected over the promoted Pd/A catalyst following the various ageing treatments. The data obtained over the fresh catalyst are also included, for reference.
Rate (lamol/s/g)
Turnover Frequency (/s)
No. of Active Sites (~tmol/g)
37.81
1.8
17.87
LHA900C
8.48
1.4
6.06
LHSA900C
6.87
1.4
4.91
RHA900C
3.64
0.80
4.55
Catalyst Condition Fresh ,,
......
RHSA900C
2.65
0.65
4.07
Table 3" ITK Analysis of the Promoted Pd/A Catalyst (Fresh and Aged) within the CO-NO Reaction at 250~ The data within Table 3 show that ageing reduces the performance of the Pd/A catalyst. This decrease in activity is brought about by reductions in both the turnover frequency of the active sites, and in the number of active sites. The extent of the reduction in performance is lower over this catalyst than it was over the Pd-only material, although the overall performance of the Pdonly catalyst still exceeds that of the promoted Pd/A sample when the rates are expressed in terms ofpmoUs/g. Note, however, that one gram of the Pd-only sample contains more Pd than one gram of the promoted Pd/A sample due to the effect of the promoter. When the aged activities of these two samples are compared at the same Pd content, the activities are fairly similar, with the exception of the RHSA data, where the promoted catalyst is significantly more active. It is clear that rich ageing is more detrimental to the performance of the promoted Pd/A catalyst than is lean ageing. The same observation was made over the Pd-only catalyst. The rich aged samples are less reactive because the active sites on the rich aged catalysts are characterised by much lower turnover frequencies than are the active sites over the lean aged catalysts. Once again, the same trend was observed with the Pd-only catalyst.
386 The presence of sulphur in the lean ageing feed leads to a decrease in the activity of the promoted Pd/A sample because it reduces the number of active sites in the catalyst. The turnover frequency of the active sites is not modified by the presence of sulphur during ageing on the lean side. Similar behaviour was observed over the Pd-only catalyst. The presence of sulphur in the rich ageing feed leads to a decrease in the activity of the sample, and, as with the Pd-only catalyst, this decrease is made up of two contributions. First of all, the turnover frequency of the active sites is reduced by the presence of the sulphur, and secondly, the number of active sites is decreased when sulphur is present in the rich ageing feed. However, the magnitude of both of these effects is reduced by the presence of the promoter. That is, the turnover frequency only drops from 0.80 to 0.65 s-1 when sulphur is added to the rich ageing feed over the promoted Pd/A catalyst, while it drops from 0.75 to 0.40 s~ over the Pd-only sample. In addition, the number of active sites decreases by a much smaller amount when the promoter is present. Both of these effects indicate that the promoter is significantly modifying the interaction of the sulphur with the Pd under rich ageing conditions. It appears that less sulphur is retained on/in the Pd when the promoter is present, which mediates its damaging effect on both the number of active sites (via site blocking) and turnover frequency (via electronic modification).
4. CONCLUSIONS Detailed Isotope Transient Kinetics (ITK) studies of the effects of ageing on Pt/Rh and Pd catalysts have revealed a number of interesting general features about thermal ageing and sulphur poisoning effects within autocatalysts. The major observations can be summarised as follows: (a) within the model CO-NO reaction under laboratory conditions, Pd-only and promoted Pd/A catalysts are significantly more active in their fresh states than a corresponding Pt/Rh catalyst. This difference is principally associated with the number of active sites, which is always significantly greater on the Pd catalysts. However, it should also be noted that, in its fresh state, the turnover frequency of Pd within the CO-NO reaction significantly exceeds that of the active sites in the Pt/Rh catalyst, ie Pd is intrinsically more active than Pt/Rh within this reaction. Upon ageing at 900~ the performance of the Pd catalysts decreases far more than does the performance of the Pt/Rh material, although the overall activity of the Pd catalysts still exceeds that of Pt~h. This very large decrease in activity observed over the Pd catalysts is principally associated with the dramatic decrease in the number of active sites which is induced by the ageing treatments. (b) the measured turnover frequency of the active sites within the Pt/Rh catalyst decreases slightly when the sample is aged at 900~ However, the value of the turnover frequency over the aged catalysts is independent of the ageing stoichiometry (rich or lean) and of the presence/absence of sulphur in the ageing feed. Lean ageing has a greater detrimental effect on performance, because this leads to a larger decrease in the number of active sites on the catalyst than does rich ageing. The presence of sulphur in the ageing feed has virtually no effect on the performance of this catalyst, (c) over the unpromoted Pd-only catalyst, rich ageing leads to more extensive deactivation than does lean ageing. This is because the turnover frequency of the active sites is much lower following rich ageing. The number of active sites is approximately the same following lean and rich ageing. The addition of sulphur to the lean ageing feed induces a small deterioration in
387 performance, because it leads to a reduction in the number of active sites. However, the incorporation of sulphur into the rich ageing feed leads to a dramatic loss of activity, associated with both a large drop in the turnover frequency of the active sites (from 0.75 to 0.4 s-~) and with a significant decrease in the number of active sites. It appears that rich ageing in the presence of sulphur reduces the NOx performance of the catalyst both by blocking active sites at the surface and by electronically modifying the surface Pd to decrease the NO dissociation probability, (d) over the promoted Pd/A catalyst, rich ageing is more detrimental than lean ageing. As was the case with the Pd-only material, this is principally because the turnover frequency of the active sites is much lower following rich ageing. Sulphur has a small deactivating effect during lean ageing, associated with a small decrease in the number of available active sites. During rich ageing, sulphur also leads to poisoning of the catalyst, but the extent of the poisoning is much lower than was the case over the unpromoted Pd-only sample. The promoter appears to mediate the sulphur effects in two ways. First of all it reduces the extent to which the sulphur blocks the active sites, and secondly it reduces the extent to which the sulphur decreases the turnover frequency of these sites. These two effects are clearly linked.
REFERENCES
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DD Beck, TW Capehart, C Wong and DN Belton, J. Catalysis, 144 (1993) 311. W Grunert, NW Hayes, RW Joyner, ES Shpiro, MRH Siddiqui and GN Baeva, J. Phys. Chem., 98 (1994) 10832. AP Walker, T Rayment, RM Lambert and RJ Oldman, J. Catalysis, 125 (1990) 67. GP Huffman, N Shah, J Zhao, FE Huggins, TE Hoost, S Halvorsen and JG Goodwin Jr., J. Catalysis, 151 (1995) 17.
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JT Gleaves, JR Ebner and TC Keuchler, Catal. Rev.-Sci. Eng., 30 (1988) 49.
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GP Ansell, SE Golunski, JW Hayes, AP Walker, R Burch and PJ Millington, in A Frennet and J-M Bastin (Editors), Catalysis and Automotive Pollution Control I~, Stud. Surf. Sci. Catalysis Vol. 96, (1995) 577, Elsevier, Amsterdam. RJ Brisley, GR Chandler, HR Jones, PJ Andersen and PJ Shady, SAE Paper 950259, Detroit, Michigan, February 1995.
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CATALYSIS AND AUTOMOTIVEPOLLUTIONCONTROLIV Studies in Surface Science and Catalysis, Vol. 116 N. Kruse, A. Frennet and J.-M Bastin (Eds.) 91998 Elsevier Science B.V. All rights reserved.
389
Kinetic Study of the Ethene Oxidation by Oxygen in the Presence of Carbon Dioxide and Steam over Pt/Rh/CeO2/~,-AI203 R.H. Nibbelke, R.J.M. Kreijveld, J.H.B.J. Hoebink, G.B. Marin a Eindhoven University of Technology, Schuit Institute of Catalysis, Laboratorium voor Chemische Technologie, PO Box 513, 5600 MB, Eindhoven, The Netherlands current address: Universiteit Gent, Laboratorium voor Petrochemische Techniek, Krijgslaan 281, B-9000, Gent, Belgium a
ABSTRACT The oxidation of ethene by oxygen in the presence of steam and carbon dioxide over a commercially available Pt/Rh/CeO2/),-A1203 three-way automotive catalyst was studied. Experiments were carried out in a fixed-bed micro reactor under intrinsic conditions, i.e. in the absence of external and internal mass and heat transport limitations. A kinetic model based on elementary reaction steps was developed for the following range of experimental conditions: a total pressure of 110 kPa, temperatures between 463 and 483 K, ethene inlet partial pressures between 0.04 and 0.14 kPa, oxygen inlet partial pressures between 0.25 and 1.10 kPa and steam and carbon dioxide inlet partial pressures equal to 10 kPa. For these conditions, only the total oxidation of ethene to carbon dioxide and steam was observed. The reaction rate was found to decrease with increasing ethene partial pressure and decreasing oxygen partial pressure. Both carbon dioxide and steam were found to inhibit the reaction rate. The activity of CeO2 can be neglected at the investigated conditions. The kinetic model comprises the following elementary steps: irreversible adsorption of oxygen and reversible adsorption of ethene on the noble metal surface, followed by a surface reaction between adsorbed ethene and oxygen. The values of the kinetic parameters, i.e. preexponential factors and activation energies, were estimated by non-linear regression of the ethene conversion and found to be physically meaningful. 1. INTRODUCTION Automotive exhaust catalysis for vehicles powered by Otto engines aims at the simultaneous treatment of three types of pollutants, i.e. carbon monoxide, hydrocarbons and nitrogen oxides. For this purpose the so-called three-way catalyst was developed. Further improvements in the conversion of the pollutants are needed in order to meet the more stringent future standards. Hence, there is an urgent need for realistic kinetic models based on reliable experimental data. In the present paper, a kinetic model based on elementary reaction steps is developed for the oxidation of ethene by oxygen over a commercially available Pt/Rh/CeO2h/-Al203 three-way catalyst in the presence of steam and carbon dioxide, the latter constituting a significant fraction of automotive exhaust. The study is carried out at
390 temperatures representative for the cold start, as the largest CO and hydrocarbon emissions occur in that period [ 1]. A large number of different hydrocarbons is present in automotive exhaust gas. The composition and amount strongly depend on the mode of driving. According to Impens [2], the most abundant hydrocarbons present in a typical exhaust gas are ethene, 25 mol% of the total hydrocarbon amount, ethyn, 20 mol%, methane, 20 mol% and aromatics, mostly toluene, 10 mol%, and benzene, 5 mol%. Because of its large quantity, ethene was chosen as model hydrocarbon for the present study, instead of propene or propane, which are often used for investigating the oxidation of hydrocarbons in exhaust gas catalysis. Moreover, benzene and toluene display similar oxidation behaviour as ethene [3,4]. It must be noticed, however, that for a complete understanding of hydrocarbon oxidation, other hydrocarbons, e.g. ethyn and methane, must also be considered. Although much attention has been paid to the oxidation of hydrocarbons, the underlying kinetics are still unclear. For alkenes, a negative partial reaction order in the alkene and a positive partial reaction order in 02 is usually found for Pt and Pd catalysts [5,6,7,8,9]. Only in large excess of Oa, a positive partial reaction order in the alkene is found [10,11 ]. In contrast to Pt and Pd, a positive partial reaction order in the alkene and a negative partial reaction order in 02 was found for Rh catalysts [5,8,9]. A model based on elementary reaction steps was developed by Sant et al. [12] for the the ignition and extinction behaviour of the C2H4 oxidation by 02 over Pt/SiO2. Voltz et al. [6] modelled the CO and C3H6 oxidation by 02 over Pt/A1203 in the presence of NO using empirical rate equations with adsorption terms. Montreuil et al. [ 13] carried out an extensive kinetic study using a full synthetic automotive exhaust, with C3H8 and C3H6 as model hydrocarbons, over three-way catalysts. The latter model, however, was not based on elementary steps either. 2. EXPERIMENTAL The experimental set-up consists of a feed, a reactor and an on-line gas analysis section. The feed section consists of a gas blending manifold including a series of thermal gas massflow controllers and a HPLC pump to feed water to an evaporator located downstream of the flow controllers. The reactor section consists of a tubular preheater and a reactor which is contained in an oven. The reactor employed is a stainless steel (type 316) fixed-bed laboratory reactor. The catalyst bed, retained by a sintered quartz plate, is diluted with non-porous tx-A1203 pellets of the same average diameter as that of the catalyst pellets in order to minimise temperature gradients in the reactor. A typical catalyst bed contained 0.5 g of catalyst and 1.6 g diluent. The length of the catalyst bed is 10 mm and its diameter is 13 mm. The reactor in- and outlet were filled with cz-A1203 beads, 1 mm in diameter, in order to maintain an isothermal bed and to enhance radial mixing of the inlet stream. The reactor is heated by two infra-red radiators placed in line with the reactor. Nitrogen, serving as an internal standard component for the chemical analysis, is added to the reactor effluent directly downstream of the reactor. The on-line gas analysis section contains two gas chromatographs (Carlo Erba Instruments GC 8340 and GC 6000) and a quadrapole mass spectrometer (VG Sensorlab 200D). The gas chromatographic analysis enables on-line quantitative analysis of H2, CO, NO, 02, CO2, N20, CH4, C2H2, C2H4 and C2H6, C3H6, C3H8, C4H8 and Call10 using N2 as an
391
Table 1. Properties of Pt/Rh/CeO2/y-Al203. as is the BET surface area, dp the pellet diameter and wi the mass fraction.
intemal standard. Water is separated by passing the sample stream through a Hayesep Q column. The mass spectrometer was used only to verify a stable water flow r 1.25 105 m 2 kg'lcAT rate and to verify the absence of products dp 0.15-0.21 10-3 mp other than the ones detected by the gas chromatographic analysis. A more detailed LNM 13.0 mols kg-lcAT description of the experimental set-up can wpt 3.98 10-3 kg kg-lcAT be found elsewhere [ 14,15]. WRh 7.98 10-4 kg kg'lcAT The catalyst used is a commercially available Pt/Rh/CeO2/7-A1203 three-way Wceo2 2.8 10-1 kg kg'lCAT catalyst as used for coating monoliths, provided by Degussa A.G.. Prior to the experiments the catalyst was pre-treated in order to obtain reproducible kinetic data. The catalyst was heated to 773 K in a flow of 1.7 10-3 mol si He. Then, the catalyst was oxidised during 4 ks by a stream of 7.0 10-4 mol s~ containing 27.5 kPa 02 in He. Next, the catalyst was kept under a flow of 1.7 10-3 mol sl He at 773 K for 2 ks in order to purge reversibly adsorbed oxygen, followed by reduction in a stream of 8.84 10-4 mol s"l containing 5.5 kPa H2 in He at 773 K for 8 ks. Finally, the catalyst was allowed to cool down to reaction temperature under a stream of 8.15 10-4 mol s1 containing 10 kPa H20 and 10 kPa CO2 in He. The latter conditions were applied for 20 hours. Next, kinetic experiments were carried out for a period of only 10 hours. Within this period the decrease in catalyst activity is smaller than 10%. The above procedure was applied for each new batch of catalyst. The experiments are complicated by the fact that the activity for a certain experimental condition depends on the conditions applied prior to the experiment. Chemical changes of the catalyst, such as oxidation and reduction of ceria or rhodium and the appearance and disappearance of crystal surfaces, are thought responsible for the latter observation. The importance of the dependence of the catalyst structure and composition on the experimental conditions was reported previously for automotive exhaust catalysts [16] and is in the present paper referred to as extrinsic relaxation. In order to obtain reproducible experimental data, a so-called standard condition was applied in between two subsequent experiments. The conditions corresponding to a given experiment were applied for only 10 min. Next, the standard condition was applied for at least 30 min. In such a way, the catalyst changes due to extrinsic relaxation were avoided and changes in the Table 2. Range of experimental catalyst activity as compared to the activity of the conditions used for the developstandard condition were the result of the kinetics ment of the kinetic model only. Indeed, following the above procedure, PTOT 110 kPa reproducible kinetic data could be obtained. The 0.04-0.14 kPa P0C2H4 standard condition used for the experiments p~ 2 0.25-1.10 kPa discussed in the present paper is as follows: p0C2H4/p002 0.08-0.4 p0C2H4=0.1 kPa, P~ kPa, p0H20=p0co2=10 kPa at reaction temperature, where p~i is the reactor inlet 0H20 10 kPa P partial pressure of component i [Pa]. In all 10 kPa P~ experiments, helium was used as a balance. The T 463-483 K range of experimental conditions covered for the model development is shown in Table 2. The
392 reaction temperatures are representa-tive for the cold start. The partial pressures are typical for an automotive exhaust, although during the cold start more reducing conditions may be expected. For larger C2H4/O2 ratios, however, no accurate experimental data could be obtained because of too low conversions. Unless explicitly mentioned, only the total oxidation of ethene to CO2 and H20 was observed. All experiments were carried out under intrinsic conditions, i.e. in the absence of external and internal mass and heat transport limitations. On bed scale, pressure drop and axial and radial temperatures gradients could be neglected. 3. M O D E L L I N G The degrees of surface coverage of the considered species were calculated from the corresponding continuity equations, which in the steady state can be represented as follows:
N Evi,j rw,j =0
(1)
J where j is the number of the reaction step, N the total number of reaction steps considered, vii the stoichiometric coefficient of surface species i in reaction step j and rwj the reaction rate of reaction step j [mol kg-lCAT S'I]. The reaction rates of the elementary steps are calculated via the law of mass action and under the Langmuir assumptions of identical active sites, absence of interactions between adsorbates and confinement of adsorption to a monolayer. Equation (1) forms a set of algebraic equations with the degrees of surface coverage of the considered species as only unknowns for a given gas phase composition, pressure and temperature. Subsequently, the ethene disappearance rate can be calculated from the degrees of surface coverage. Since the fixed-bed can be regarded as an ideal plug flow reactor, the fractional ethene conversion can be calculated by integration of the following ordinary differential equation:
dXc2H 4 dz
_WcAT PTOT Rw, C2H4 FTOT pOc2H4
(2)
where XC2H4is the fractional ethene conversion [-], defined as the difference between the inand outlet divided by the inlet ethene partial pressure, z is the dimensionless reactor length [-], defined as the cumulative catalyst mass divided by the total catalyst mass, WCAT [kgcAT], PTOT is the total pressure [Pa], FTOTis the total molar flow rate [mol sq] and Rw,c2H4 is the ethene production rate [mol kgqcAT Sq]. The estimation of the kinetic parameters was performed by non-linear regression of the ethene conversion, using a single-response Levenberg-Marquardt algorithm [17,18]. The parameter estimates were tested for global significance on the basis of their individual tvalues. The statistical significance of the global regression was expressed by means of the socalled F-test. Discrimination among rival models was based on this statistical testing, whenever it was not possible by direct observation or by physico-chemical laws. In order to avoid strong correlation between the Arrhenius parameters, the Arrhenius equations were reparametrised according to Kitrell [ 19].
393 75
P~ kPa, T=463 K WcAr=O.5 g; FTOT=6mmol s -j !"1 P~ kPa, T=463 K WcAT=0.5 g; Fror=5 mmol s 1 @ P~ 60 kPa, T=473 K WcAr=0.5 g; FTOF=5 mmol s j P~ 85 kPa, T=463 K WcAF=O.6 g; Frov=5 mmol s j ~,. P~ 60 kPa, T=483 K WcAr=0.5 g; Frov=6 mmol s j 9
50 o" )< 25
.03
i
i
0.06
0.09
i
0.12
0.15
p0C2H,/kPa
6O
9 P~ kPa, T=473 K WCAT=0.5 g; Fror=4 mmol s ~ I-! P~ 08 kPa, T=463 K WcAv=0.5 g; FTor=5 mmol s "~ @ P~ kPa, T=463 K s-i WcAv=0.5 g; Frov=4 mmol P~ kPa, T=463 K WCAT=0.5 g; Fvov=5 mmol s -~
4O
0
0.00
.
0.20
0.40
0.60
0.80
1.00
1.20
pO%/kPa 0
Figure la. XC2H4 YS. p C2H4" Figure lb. Xc2tt4 vs. p~ 2. p~ = p~ kPa. Markers represent the experiments. Full lines were calculated with the parameter estimates given in Table 3 by integration of Eq. (2) with Eq. (5) as the C:H4 disappearance rate. 4. E X P E R I M E N T A L RESULTS
The ethene conversion decreases monotonically with increasing ethene inlet partial pressure, as shown in Figure l a. The partial reaction order in ethene is negative, indicating that the reaction is inhibited by ethene. Steam reforming can be neglected at these low temperatures [20]. Indeed, experiments carried out in the absence of oxygen showed significant ethene conversions at temperatures higher than 600 K only. The ethene conversion increases monotonically with increasing oxygen inlet partial pressure, as shown in Figure lb. In Figure 2, the influence of the steam and carbon dioxide inlet partial pressures on the ethene conversion is shown. Especially steam strongly inhibits the reaction rate, the effect
394 60 i
9p~ P~ 9P~ p~
P, 1 i i
o~2
40
i
i
l
"0-._. 0 . . . . . . . . . . . .
0
X
201
kPa, kPa, p~ kPa, kPa, p~
kPa =10 kPa
_ . 9. . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . .
.............. I ........ .1
0.0
I
r
I
2.5
5.0
7.5
10.0
p0H2O, p0co = /kPa
Figure 2. Xc2144vs. p~
(l) andp~
(ll) for T=463 K, WCAT=0.5g, Fror=5 mmol S "1.
being most pronounced at low steam partial pressures. Mar6cot et al. [21 ] found inhibition due to steam of propane and propene oxidation over Pt/7-A1203. Bart et al. [22] found inhibition of the propane oxidation over a three-way catalyst for reducing conditions and rate enhancement for oxidizing conditions. The inhibition by steam is in contrast to the oxidation of CO by 02 over the same catalyst, where steam was found to strongly enhance the reaction rate [15]. Carbon dioxide also inhibits the reaction rate, although the inhibition is much smaller. The inhibition by H20 and CO2 may be explained by adsorption on noble metal sites. At large partial pressures, e.g. 10 kPa, the partial reaction order in H20 and CO2 is approximately zero. In order to investigate the activity of the CeO2 carrier, the C2H4 conversion was measured as a function of temperature, both for CeOz/7-A1203 and ~-A1203. For ~-A1203, significant ethene conversion is only found at temperatures larger than 700 K, probably due to gas phase reactions. For CeOE/7-A1203, significant ethene conversion is found for temperatures higher than 550 K. In both experiments, a small amount of CO is formed at temperatures higher than 700 K, either due to steam reforming or partial oxidation [20]. The higher activity for CeOJT-AI203 as compared to c~-A1203 indicates than ceria catalyses the conversion of ethene at high temperatures. At the temperatures for which the kinetic model was developed, however, i.e. 463-483 K, the activity of CeO2/7-A1203 can be neglected. 5. MODELLING RESULTS AND DISCUSSION A kinetic model was developed for the experimental conditions shown in Table 2 and was based on the reaction steps listed in Table 3, where ~ denotes the stoichiometric number of the reaction steps [23]. A '*' denotes a noble metal site, where no distinction is made between platinum and rhodium. Although 10 kPa H20 and 10 kPa CO2 was fed during the experiments, their influence is not explicitly taken into account in the elementary reaction steps. The latter is justified because at 10 kPa the partial reaction orders in H20 and CO2 are both approximately zero, as can be seen in Figure 2. Reaction step 1 describes the molecular adsorption of oxygen, followed in step 2 by
395 Table 3. Reaction steps considered for the Mnetic model.
its dissociation. The adsorption and nr. Reaction steps dissociation of oxygen in two steps instead of a 1 02 + 9 k[ > 02 * single step, was 2 02 * + 9 k~ ,> 2 0 * concluded from a kinetic study of the CO oxidation by 02 over the 3 C2H 4 + * ( > C2H4" same catalyst [15]. The third step describes the 4 C2H4" + 6 0 * k~ > 2CO 2 + 2 H 2 0 + 7 " reversible molecular adsorption of ethene on C2H 4 + 3 02 --> 2 CO 2 + 2 H20 a noble metal site. The ethene molecule is known to adsorb in a di-~ or arc mode. The di-~ mode involves two metal atoms, while the mode only involves one metal atom. The n adsorption mode prevails on low coordination atoms, while the di-r~ mode is more stable on high coordination atoms [24]. On Pt/A1203, both adsorption modes were observed [25]. Reaction step 4 describes the surface reaction between adsor-bed ethene and adsorbed oxygen. This reaction step consists of more than one step and is therefore not an elemen-tary reaction step. The surface reaction may involve decomposition species, which are also formed upon heating a surface covered with ethene, e.g. ethylidyne (*C-CH3), ethylidene (*CH-CH3) or vinylidene (,C=CH2) [26]. If it is assumed that the rate determining step of reaction step 4 is the oxygen assisted abstraction of the first hydrogen atom, the rate of reaction step 4 is first order both in the ethene and the oxygen surface coverage, leading to the following expression for the rate of C2H4 disappearance:
k[
(3 k~ PC2H, " k[ PO2)(1 - k--~ Po, )-Rw'C2H4 = LNM
k[ PO2 k3b
3 k~ PC2H4 " k[ PO~ + 3 k b
k4f
kf PO2 kb "3 k~ PC:H~ " k[ PO2
(3) Although the experimental data can be described adequately by Equation (3), not all kinetic parameters can be determined in a statistically significant way. Therefore, Equation (3) must be simplified in order to obtain statistically significant parameter estimates only. From the kinetic study of the CO oxidation by 02 over the same catalyst [ 15] it was concluded that the dissociation of adsorbed oxygen, step 2, is potentially much faster than its adsorption, step i, thus kf>>kfl Po2, implying that 002<<00, where 0j is the surface coverage of species j [-]. If the latter assumption is applied, Equation (3) reduces to: (3 k~ -Rw'C~H~ = LNM '
PC2H 4 -
kf Po: )-
k[
kf
3 k~ PC2H~ " k[ PO: + 3 k b
k[ Po~ kb "3 k~ PC:H~ " k[ PO~ (4)
The modelling results indicated that the surface reaction is potentially fast as compared to the
396 adsorption steps. Indeed, no statistically significant parameter estimate for the surface reaction could be obtained. If it is assumed that 0o<<1, Equation (4) is simplified to:
kf -Rw'CEH4
-
-
3 k3f PC2H, - k{ PO2 + 3 k~
(5)
If Equation (5) is used for the regression analysis, the estimates of the activation energies of reaction steps 1 and 3, i.e. molecular adsorption of oxygen and ethene, are not significantly different from zero. This can be understood Table 4. Parameter estimates with their as molecular adsorption on noble metals is 95% confidence intervals, obtained by usually not activated [27]. The estimates of regression of all 50 experimental data the kinetic parameters with their 95% confipoints using Eq. (2) and Eq. (5). dence intervals are shown in Table 4. The kfl (1.7 _+0.3) 10-3 Pa -I s"l calculations were carried out with the value kf3 (1.5 _+0.7) 10-2 P a "l s -1 of LNM shown in Table 1. If the inhibition by H20 and CO2 is caused by occupation of Ab3 (1.0 + 0.3)* 1019 S"l noble metal sites, the latter value may be too Eb3 172 _ 40 kJ mol l large, which would mean that the true values * Confidence interval based on of kfl, k f and Ab3 are larger than those reparametrised parameter shown in Table 4. In Figures l a and l b the 60 ethene conversion as calculated by Equations (2) and (5) with the estimated parameter values given in Table 4 is shown together '-'~ 40 " 9 with the experimental data. Notice that in Figure l b, the calculated ethene conversion is also shown for oxygen inlet partial -~ 20 o pressures at which no experimental verification was carried out. In Figure 3, the parity diagram of the calculated versus the 00 20 4'0 60 observed ethene conversion is shown for all Observed X C2H 4 /% 50 data points considered for the modelling. The experimental data is described Figure 3. Parity diagram of all data used adequately, although especially as a function for the model development. of the oxygen partial pressure deviations between the calculated and measured ethene con-version can be seen. The non-activated adsorption rate coefficients and the preexponential factor of C2H4 desorption are within the ranges as predicted by transition state theory [28]. Since molecular ethene adsorption is not activated, the activation energy for ethene desorption is equal to C2H4 minus the standard enthalpy of ethene adsorption,-AHaa s . Yeo et al. [26] measured an . r "~
j
9
0
0
initial heat of C2H4 adsorption on a Pt { 111 } surface equal to 174 kJ mol -l, a maximum value of 184 kJ mo1-1 at intermediate coverage and a value of 70 kJ mo1-1 towards saturation coverage. The initial heat of adsorption is in excellent agreement with the estimated value of Eb3 equal to 172 kJ mo1-1 shown in Table 4. Szulczewski and Levis [29] measured a binding energy of C2H4 adsorbed in the di
397 adequate kinetic model it is required that the rate of C2H4 adsorption is first order in the fraction of free noble metal sites. Second order adsorption, either molecular or dissociative, does not allow an adequate description of the experimental data. As the di-cy mode is associated with two noble metal sites, first order adsorption suggests that the rate determining step of C2H4 adsorption is associated with only one site, e.g. C2H4 adsorption in n mode, as was indeed proposed by Szulczewski and Levis [29]. For PCEn4=0.1kPa, Po2=0.6 kPa, PnEO=PCO2=10kPa and T-463 K, the simplified model corresponding to Equation (5) calculates a ethene surface coverage equal to 0.75. Remember that the oxygen surface coverage is zero as a result of the simplifying assumption that 0o<<1. For the latter conditions, a turnover frequency of the global reaction is equal to 0.086 s"l , that of the C2H4 adsorption is equal to 0.380 s-1 and that of C2H4 desorption is equal to 0.294 s-l. Thus, the adsorption and desorption rate are respectively a factor 4.4 and 3.4 larger than the global rate of ethene oxidation. The latter indicates that ethene adsorption is close to quasiequilibrium, although statistically significant estimates of both the forward and backward rate coefficients can be obtained. For the considered experimental conditions, the oxidation of C2H4 by O2 can be described by a kinetic model involving the noble metals only. For the oxidation of CO by O2 in the presence of H20 and CO2 over the same catalyst, the experiments could only be described adequately by a kinetic model incorporating two parallel reaction paths: a monofunctional reaction path, catalysed by the noble metals only, and a bifunctional reaction path, involving a reaction at the noble metal/ceria interface between CO adsorbed on the noble metal and oxygen from ceria [15]. The importance of the bifunctional path increases at increasing CO/O2 ratios. The apparent absence of a bifunctional path available for the C2H4 oxidation can be explained by the lower C2H4 coverage on the noble metal surface, e.g. 0.75 for the case discussed above, as compared to CO coverage on the noble metal surface, which is always close to 1. Therefore, it can be expected that a bifunctional path becomes important for C2H4/O2 ratios much larger than considered in the present paper. At higher temperatures, the oxidation of C2H4 catalysed by ceria only must also be considered, as well as C2H4 conversion due to steam reforming. 6. CONCLUSIONS The oxidation of C2H4by 02 over a commercially available Pt/Rh]CeO2/7-A1203threeway catalyst in the presence of 10 kPa H20 and 10 kPa CO2 at temperatures representative for the cold-start period can be described adequately by a kinetic model comprising the following elementary reaction steps: irreversible molecular adsorption of oxygen and reversible molecular adsorption of ethene on the noble metal surface, followed by a surface reaction to CO2 and H20. Statistically significant and physically meaningful estimates for the kinetic parameters corresponding to the ad- and desorption steps could be obtained. For the investigated conditions, the surface reaction is potentially instantaneous. The reaction is inhibited by C2H4 adsorbed on the noble metal surface, resulting in a positive partial reaction order in 02 and a negative partial reaction order in C2H4. Both steam and carbon dioxide inhibit the reaction rate, probably due to adsorption on active sites. For the investigated conditions, the oxidation activity of ceria can be neglected.
398 ACKNOWLEDGEMENT
Dr. E. Lox of Degussa A.G. is gratefully acknowledged for providing the catalyst samples. REFERENCES .
2. .
.
5. 6. o
8.
9. 10. 11. 12. 13. 14. 15. 16. 17. 18.
19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29.
A.L. Boehman and S. Niksa, Appl. Catal. B: Env., 8 (1996) 41. R. Impens, in "Catalysis and Automotive Pollution Control" (A. Crucq and A. Frennet, Eds.), Elsevier, Amsterdam (1987) 11. G. Mabilon, D. Durand and Ph. Courty, in "Catalysis and Automotive Pollution Control III" (A. Frennet and J.-M. Bastin, Eds.), Elsevier, Amsterdam (1995) 775. J.M. Bart, A. Pentenero and M.F. Prigent, ACS Symposium Series, 495 (1992) 42. N.W. Cant and W.H. Hall, J. Catal., 16 (1970) 220. S.E. Voltz, C.R. Morgan, R.Morgen, D. Liederman and S.M. Jacob, Ind. Eng. Chem. Res. Develop., 12 (1973) 294. J.R. Hawkins and S.E. Wanke, Can. J. Chem. Eng., 57 (1979) 621. Y.-F.Y. Yao, J. Catal., 87 (1984) 152. H. Shinjoh, H. Muraki and Y. Fujitani, Appl. Catal., 49 (1989) 195. L. van de Beld, M.P.G. Bijl, A. Reinders, B. van der Werf and K.R. Westerterp, Chem. Eng. Sci., 49 (1994) 4361. C.G. Vayenas, B. Lee and J. Michaels, J. Catal., 66 (1980) 36. R. Sant, D.J. Kaul and E.E. Wolf, AIChE J., 35 (1989) 267. C.N. Montreuil, S.C. Williams and A.A Adamczyk, SAE Technical Paper Series 920096 (1992). M.A.J. Campman, Ph.D. dissertation, Eindhoven University of Technology (1996). R.H. Nibbelke, M.A.J. Campman, J.H.B.J. Hoebink and G.B. Marin, accepted for publication in J. Catal. B. Engler, E. Koberstein and P. Schubert, Appl. Catal., 48 (1989) 71. G.F. Froment and L.H. Hosten, in "Catalysis, Science and Technology" (J.R. Anderson and M. Boudart, eds.), Springer-Verlag, Berlin (1981) 98. P.T. Boggs, R.H. Byrd, J.E. Rogers and R.B. Schnabel, "ODRPACK Version 2.01, Software for Weighted Orthogonal Distance Regression," National Institute of Standards and Technology, Gaithersburg (1992). J.R. Kitrell, Adv. Chem. Eng., 8 (1970) 97. B.I. Wittington, C.J. Jiang and D.L. Trimm, Cat. Today, 26 (1995) 41. P. Mar6cot, A. Fakce, B. Kellali, G. Mabilon, M. Prigent and J. Barbier, Appl. Catal., 3 (1994) 283. J.M. Bart, A. Pentenero and M.F. Prigent, ACS Symposium Series, 495 (1992) 42. M.I. Temkin, Int. J. of Chem. Eng., 11 (1971) 89. J.-F. Paul and P. Sautet, J. Phys. Chem., 98 (1994) 10906. S.B. Mohsin, M. Trenary and H.J. Robota, J. Phys. Chem., 92 (1988) 5229. Y.Y. Yeo, A. Stuck, C.E. Wartnaby and D.A King, Chem. Phys. Lett., 259 (1996) 28. M. Boudart and G. Dj6ga-Mariadassou, "Kinetics of Heterogeneous Catalytic Reactions", Princeton University Press, Princeton (1984). V.P. Zhdanov, J. Pavli[]ek and Z. Knor, Catal. Rev. Sci. Eng., 30 (1988) 501. G. Szulczewski and R.J. Levis, J. Am. Chem. Soc., 118 (1996) 3521.
CATALYSIS AND AUTOMOTIVE POLLUTION CONTROL IV Studies in Surface Science and Catalysis, Vol. 116 N. Kruse, A. Frennet and J.-M Bastin (Eds.) 91998 Elsevier Science B.V. All rights reserved.
399
Three-way catalytic converter modelling. Numerical determination of kinetic data C. Dubien and D. Schweich Laboratoire de G6nie des Proc6d6s Catalytiques - CNRS-CPE 43, Bd du 11 Novembre 1918, BP 2077, F-69616 Villeurbanne cedex, France A numerical approach for obtaining quickly and simply kinetic data from light-off curves is proposed. It is applied to methane oxidation which exhibits light-~ at a much higher temperature than the other pollutants. Sensitivity analysis shows that heat loss is secondary and a simple 1-D adiabatic model can be user Conversions ~ t e d from the estimated kinetic parameters are close to experimental values. Finally, the eff~s of the apparent activation energy and frequency factor are discussed through a relation that links the rate constants to the light-off temperature.
1. I N T R O D U C T I O N Numerical simulations of the three-way catalytic converter could be advantageously substituted for expensive experiments with an engine on a test bench. However, the l~ability of numerical results depends on the quality cs the physical and chemical parameters involved, ~ y the kinetic model [1]. Because rate data are scarce, the simplified reaction scheme and rate expressions ofVoltz et aL [2] are used in most models. In this historical work, CO, C3H6 and H2 oxidation in a lean environment on a platinum catalyst was dealt with. The rate expressions are of the ~gmuir-Hi~helwood type and account for the inhibition of NO. Unfortunately, there is no mc~:hanism supporting the e x p ~ o n s that should be c o n s i ~ as fitting rate laws. For instance, the heat of adsorption of NO has the wrong sigrL Moreover, H2 oxidation rate is the same as CO which has been contested by Zygourakis and Aris [3]. The rate expre~on for NO reduction by CO was first descnbed by Subramanian and Varma [4]. Here again, the expression is of the ~ u i r - H i n s h e l w o o d type, but it also involves fractional orders which are of empirical nature. Surprisingly, although e m p ' ~ these rate laws give saliffac~ry restdts in simulations. This suggests that simple rate expressions are probably ~ n t for simulation purposes. However, introducing new reactions (new hydrocarbons, steam-reforming, etc.) in the model would require time~onsuming experiments. The same problem arises when adapting the frequency factor, koi, and the apparent activation energy, Eai, to the catalyst under use. There is thus a need for a simple method allowing the estimation of
400 rate p a r a m e t e r s from readily available experimental data like the light-off (LO) curve. 2. N U M E R I C A L D E T E R M I N A T I O N OF KINETIC DATA A l t h o u g h the LO curves lump t o g e t h e r the chemical a n d physical processes, and the experimental conditions, they can be used to estimate the r a t e p a r a m e t e r s provided t h a t a reliable model accounting for all the nonchemical characteristics is available. We consider a simple mixture of reactive species subject to a single reaction: A + B -~ products
(1)
Schweich [1] showed t h a t the LO curve is essentially sensitive to the a p p a r e n t activation energy Ea and frequency factor k0, and to a lesser extent to the exact structure of the rate expression. Consequently, the simple expression:
-Ea)
r = k 0 exp
RT
(2)
XAXB
is chosen to simulate the LO curve with a s t a n d a r d converter model [5]. By fitting laboratory and simulated LO curves, Ea and k0 are adjusted using the Downhill Simplex m e t h o d [6]. The main problem of the methodology is to account for the competition between reactions. In the next example, we will show t h a t special features of the LO curve allows one to find the conditions u n d e r which rate p a r a m e t e r s can be determined. Then, a sensitivity analysis will provide further results. 3. A P P L I C A T I O N TO METHANE OXIDATION Several h u n d r e d hydrocarbons (HC) are found in e x h a u s t gases [7-8]. These HC are more or less easy to oxidize. The gas can be schematically describe by a mixture of fast oxidizing C3H6 and slow oxidizing CH4: g1 C3H6
+
3
0 2 --> CO 2 + H 2 0
CH 4 + 2 0 2 -~ CO 2 + 2 H 2 0
AH = 6.42 x 105 J mo1-1 9
(3)
AH = 8.03 x 105 J. mo1-1
(4)
where each HC is expressed in equivalent carbon. The rate expression for C3H6 oxidation is given by Voltz et al. [2]. However, the literature does not provide reliable laws for CH4 oxidation. Bart [9] studied CH4 oxidation in a stoichiometric mixture of CO, H2, NO, CO2, H20, N2, and O2 (Table 1, left column). The experimental LO curves of CO, H2 and CH4 (Figure 1) show t h a t CH4 oxidation takes place only when CO and
401 H2 are burned. Assuming t h a t NO does not react with CH4, CH4 oxidizes as if t h e r e w e r e no c o m p e t i n g reaction. However, CH4 reacts at the imposed p r o g r a m m e d t e m p e r a t u r e plus the adiabatic t e m p e r a t u r e rise of CO and H2 oxidation and NO reduction which occurred earlier. Consequently, the CH4 LO curve can be i n t e r p r e t e d as if there were a single reaction between CH4 (= A) a n d t h e r e m a i n i n g 0 2 (=B) which t a k e s place at a shifted t e m p e r a t u r e Tin + ~ATad,i. The r i g h t column of Table 1 s u m m a r i z e s these o p e r a t i n g conditions. U s i n g the r a t e expression (2), the following p a r a m e t e r s are obtained: k 0 = 1.555 • 101~ mol.s-l.m -2 PtRh E a = 1.555 • 105 J.mo1-1 Table 1 E x p e r i m e n t a l and simulated conditions E x p e r i m e n t and simulation with the Simulation complex m i x t u r e CH4 oxidation only Inlet ga s tl ;mperature Slow rise from 400 to 1500 K Slow rise from at 3 K/min 400 + ZATad,i = 475 K to 1500 + ]~ATad,i = 1575 K at 3 K/min Inlet gas composition (50000 h -1) CO 0.0061 H2 0.002 CH4 0.0015 02 0.003 CH4 0.0015 NO 0.00048 N2 balance 02 0.00681 N2 balance Reactions 1 CO + - 0 2 --> CO 2 2 1 NO + CO ~ CO 2 + ~ N 2 CH 4 + 2 0 2 --> CO 2 + 2 H 2 0 1 H 2 + ~ 02 --) H 2 0 CH 4 + 2 0 2
-~
CO 2 § 2 H 2 0
Then, t h e s e v a l u e s are used to s i m u l a t e the behavior of the complex m i x t u r e , u s i n g for CO, H2, and NO reactions the classical r a t e expressions given in A p p e n d i x A. The a g r e e m e n t for the LO t e m p e r a t u r e of CH4 is excellent (Figure 1.). The discrepancy between the observed and calculated CH4 LO curves at high t e m p e r a t u r e is due to the oxidation of CH4 by NO and the s t e a m reforming reaction which were ignored. Since the kinetic p a r a m e t e r s for CO/H2 o x i d a t i o n are not a d j u s t e d , it is not s u r p r i s i n g t h a t the corresponding LO curves differ slightly (about 20 K on TLO).
402
100-
80•
!
...........
",,t" '-
...........
~
-
,
'
/
~ ................
...........t . . . . . . . . . . . . . . . . . . . . . . . |
........... ~-
'
0-
I
400
',
~.....................
.
:
! ......................
:
!.....................:."......................!
--i ...................... i ..................... i ...................... i
:!:!i'i!i!i!!i!iiii!!ii /
!
600
i
I
i
800 1000 Tginlet (K)
I
1200
1400
F i g u r e 1. E x p e r i m e n t a l and s i m u l a t e d LO curves of CO and CH4 in a complex mixture.
4. D I S C U S S I O N The model a s s u m e d adiabatic conditions in the e x p e r i m e n t . Because of h e a t loss, the fitted p a r a m e t e r s m a y be biased. An e s t i m a t e of the overall h e a t t r a n s f e r coefficient, h, is given by: 1 h
1
Rm
hex
)~r
~ +
~
(5)
w h e r e Rm = 2.5 cm is the monolith radius, hex = 20 W.K-I.m -1 is the wall h e a t t r a n s f e r coefficient, )~r--0.2 W.K-I.m -2 is the radial conductivity of the ceramic m o n o l i t h . This r e s u l t s in h - - 6 W.K-I.m -1 w h e r e a s h = 0 was a s s u m e d in the model. Table 2 and Figure 2 show t h a t the kinetic p a r a m e t e r s and the LO curve are unaffected by h e a t loss even if h is 10 times g r e a t e r t h a n the r e a s o n a b l e e s t i m a t e . T h u s a simple o n e - d i m e n s i o n a l a d i a b a t i c model a c c o u n t i n g for e x t e r n a l h e a t a n d m a s s t r a n s f e r limitations can be used to e s t i m a t e the r a t e parameters. Table 2 Effect of h e a t loss on the adjusted p a r a m e t e r s Curve h Ea (W.K-I.m -1) (J.mol-1)
ko (mol.s-l.m -2 PtRh)
1
0
1.555 x 105
1.555 x 10 l~
2
10
1.540 x 105
1.540 x 10 l~
3
50
1.510 x 105
1.510 x 10 l~
403
11.113
~
o
~ m
: ..............................
: ...............................
r .............
so
.............................................................
60
.........
I
I
I
---::--:........ ....--....2.
~ simulations ] e x p e r i m e n t ................ h 0-h=50
~ ..............................i...........
~
I
I
I
600
7O0
......i..............................i..............................i 9
I
I
800 Tginlet (K) Figure 2. Effect of h e a t loss on LO curves.
9O0
1000
If the initial guess for the p a r a m e t e r s is far from reality, any fitting algorithm will fail. This is because conversion varies in a narrow t e m p e r a t u r e i n t e r v a l only. The steeper the LO curve, the more accurate the initial guess m u s t be. This is especially crucial for the frequency factor which may vary over several decades depending on metal dispersion and loading and on the n a t u r e of the r e a c t a n t s . On the other h a n d Ea is known to be roughly of the order of 105 J. mo1-1. For a given initial guess of Ea it is possible to estimate an efficient initial guess of k0 from the LO t e m p e r a t u r e by (see appendix B):
a oex /
R
RTLo
<0,
SpteV ( - AH) XAinXBin
(7)
TLO - W ~ p + ~ AWad,i
F r o m F i g u r e 1, ~exp "LO = 790 K ~ATad,i = 75 K. Assuming Ea = 105 J.mo1-1, one obtains ko = 4.884 x 106 mol.s-l.m -2 PtRh. Curve 2 of Figure 3 shows t h a t this initial guess is close to the optimal curve, and t h a t it ensures convergence of the fitting algorithm. Table 3 Kinetic p a r a m e t e r s for Figure 3 - Initialization of kinetic p a r a m e t e r s Curve Ea ko TLO (J.mol "1) (mol.s-l.m -2 PtRh) (K) 1
1.555 x 105
1.555 x 101~
856
2
1.000 x 105
4.884 x 106
887
404
100~ i!.............................. I i1~!............................. ~ . . - ~ ' "....:"- :-J:-~~:~"'~':r -v, .............................. -! 80 ....... curve ............. _v:.~ .............i
~
40
....................................................................
r/1
r
0
I
I
i
6OO
80O
. . . . . . . . . . . . . . . .
I
I
1000 1200 1400 Tginlet (K) Figure 3. Initialization of kinetic parameters for optimization. Finally, the values of ko and Ea given in Table 4 show that for sensitivity reason one may estimate ko(To) and Ea such as: r=ko(To)ex p
1/)
R
-~oo
(8)
XAXB
where To is an average value of the temperatures which can reasonably be chosen equal to TLO. Table 4 Sensitivity of LO curve to ko and Ea Rate expression r
Ea ko (J.mo1-1) (mol.s-l.m -2 PtRh) estimated values 1.555 x 105 1. 555 x 1010 Correlated Values (see equation (6))
k0 e x p ( - RT E a ) xCH4xO2
(1.555 x 1 0 5 ) / /0. .8
1.237 • 1012
( 1.555 x 105 ) x 0.8 1. 163 x 108 estimated values
k 0 (T 0) exp
( a(1 R
T-
1.555 • 105 7.062 correlated values (see equation (6)) xcn4 xO2
(1.555 x 1 0 5 ) / /o. .8
2.336
( 1.555 x 105 ) • 0.8
3.650
405 5. CONCLUSIONS The proposed method allows a simple determination of the frequency factor and activation energy from standard light-off curves using the simplest type of monolith model and readily available experiments. It can easily be used provided that the observed curve corresponds to a single reaction as in the case of CH4 oxidation. When the LO curves of various species (CO and H2 for instance) are very close to each other, the method cannot be used because the reaction with the lowest light-off temperature triggers the next reaction(s). It is thus impossible to estimate the shifted temperature. The method applies only when the next LO curve begins when the previous one has reached complete conversion. This demonstrates indirectly that it is impossible to extract rate parameters of competing reactions from LO curves which are not separated by a t e m p e r a t u r e interval larger than the adiabatic temperature rise. The only simple method is to study simple mixtures which lead to non-competing reactions. In the l a t t e r case the competitive adsorption effects cannot be observed simply. This problem will be dealt with in a next paper. ACKNOWLEDGEMENTS M. Bart is gratefully acknowledged for fruitful discussions and providing the experimental data. The authors also acknowledge the "Institut Fran~ais du P~trole" and the Ademe ("Agence de l'Environnement et de la Maitrise de l'Energie") for their financial support. LIST OF SYMBOLS Cp Ea h hex ki k0i kaj kaoj Q Qm R
Rm ri Sit T
to tR0 V X
Heat capacity [J.kg-l.K -1] Activation energy [J.mo1-1] Global heat loss coefficient [W.K-I.m -1] Wall heat loss coefficient [W.K-I.m -1] Reaction rate constant [see appendix A] Frequency factor for rate constant i [see appendix A] Adsorption constant [-] Pre-exponential factor for adsorption constant [-] Volumetric flowrate [m3.s -1] Mass flowrate [kg.s -1] Ideal gas constant [J.mol-l.K -1] Radius of the monolith [m] Rate of reaction i [mol.s-l.m -2 of noble metal] Area of noble metal per unit volume of monolith [m -1] Temperature [K] Mean residence time Is] Characteristic reaction time constant [s] Volume of the monolith [m 3] Mole fraction [-]
406 Greek letters AHi AHaj ATad E
~r P
Enthalpy of reaction i [J.mo1-1] Heat of adsorption [J.mo1-1] Adiabatic temperature rise [K] Fraction of open frontal area [-] Radial thermal conductivity [W.K-I.m -2] Gas density [kg.m -3]
Subscripts in s
i
J
LO 0
Inlet conditions In contact with the wash-coat Reaction Adsorption constant Light-off Reference
Superscripts exp
Experimental
RE~'ERENCES 1.
D. SCHWEICH, in Catalysis and Automotive Pollution Control III, ed. A. Frennet and J. M. Bastin (Elsevier, Amsterdam, 1994), p. 55. 2. S . E . VOLTZ, C. R. MORGAN, D. LIEDERMAN and S. M. JACOB, Ind. Eng. Chem. Proc. Des. Devel., 12 (1973) 294. 3. K. ZYGOURAKIS and R. ARIS, Chem. Eng. Sci., 38 (1983) 733. 4. B. SUBRAMANIAN and A. VARMA, Ind. Eng. Chem. Proc. Des. Devel., 24 (1985) 512. 5. S. SIEMUND, D. SCHWEICH, J. P. LECLERC and J. VILLERMAUX, in Catalysis and Automotive Pollution Control III, ed. A. Frennet and J. M. Bastin (Elsevier, Amsterdam, 1994), p. 887. 6. W . H . PRESS, S. A. TEUKOLSKY, W. T. VETTERLING and B. P. FLANNERY, Numerical Recipes in FORTRAN: The art of scientific computing, 2nd Edition. Cambridge University Press, New York, USA, 1992. 7. K . C . TAYLOR, Automobile catalytic converters. Catalysis: Science and Technology, Springer-Verlag, Berlin, 1984. 8. P. DEGOBERT, Automobile et Pollution, Technip, Paris, 1992. 9. J . M . BART, Th~se: Sci.: Universit~ de Nancy, 1992. 10. J. VILLERMAUX, G4nie de la r~action chimique, conception et fonctionnement des r~acteurs, Edition Lavoisier, Tec & Doc, 1993.
407 A p p e n d i x A~ Voltz [2] a n d S u b r a m a n i a n [4] kinetic models Table A1 Reactions and rate expressions Reaction AH (J.mo1-1) 1 CO + - 0 2 --) CO 2 2 -2.83 x 105 1 C3H6 +
3
02 -~ CO 2 + H 2 0
1 H2 + 2 O2 --~ H20
Rate expression r1 =
- 6 . 4 2 x 105
r2 =
Fl(%,x~)
k2xc3H6sXO2s
Fl(%,x~)
klXH2sXO2s r3 = Fl(Ts,xs )
- 2 . 4 2 x 105
1
N O + C O --> CO2 + ~ N 2
klXCOsXO2s
k4xcOs1"4xo2s 0"3XNOs0"13 -3.74 x 105
,
F I ( T s x s) = Ts (1+ k a l x c o s + ka2XCHxs
)2
r4 =
F2(%,x~)
0.7 (1+ ka3xcos2XCHxs 2)(1+ ka4xNO ~ )
F2 (Ts, Xs ) = Ts-O.17 (Ts + ka5xco s )2 k i=kOiexp -~s) kaj = kaoj exp Table A2 Rate constants Reaction i
R'rs
1
Activation energy Eai (J.mo1-1) 1.040 x 105
6.699 x 1013 mol.K.s-l.m -2 PtRh
2
1.210 x 105
1.392 x 1015 mol.K.s-l.m -2 PtRh
3
1.040 x 105
6.699 x 1013 mol.K.s-l.m -2 PtRh
4
7. 177 x 104
3.061x 1012 mol.Kl-S3.s-l.m-2 PtRh
Table A3 Adsorption constants Specie j Adsorption heat AHaj (J.mol-1)
Rate factor koi
Adsorption factor kaoj (dimensionless)
1 2 3
-7. 990 x 103 -3.000 x 105 -9.650 x 104
6.550 x 101 2.080 x 103 3.980
4
3.100 x 104
4.790 x 105
408 Appendix
B: Theoretical
relation
k0(Ea, TI~)
The light-off phenomenon is a safe runaway. There is a lot of articles devoted to the prediction of runaway condition in the chemical engineering literature. Villermaux [10] showed that runaway occurs in a batch reactor at a time given by:
tRoWo2
(B1)
to = ATadEa / R
where To is the initial temperature and tR0 a characteristic reaction time defined below. If we assume that plug flow prevails in the monolith and that chemical expansion is negligible, equation (B1) applies when the mean residence time of the fluid is to. tR0, to, and ATad are given by: AWad = ( - AH) C0A pCp
, to =
aV , C0A ~ -Q- tR~ = r 0
(B2)
where CA0 is an arbitrary reference concentration. The reaction rate ro [mol.s-l.m "3 of fluid] is given by:
(Ea)
r 0 = koSpt exp - ~ - 0
X0AsX0Bs
(B3)
where Spt is the area of noble metal per unit volume of monolith. Combining (B1) to (B3) gives:
k 0 --R--exp - R ~ o
= eVSpt (-AH)X0AsX0Bs
(B4)
It r e m a i n s to define the reference composition, X0As, X0Bs, and t e m p e r a t u r e TO. The light-off phenomenon takes place over a narrow temperature range. It is thus concluded that To = TLO. A few degrees below the light-off temperature, the reaction is sufficiently slow to assume t h a t the composition of the fluid in contact with the wash-coat is close to the inlet composition. (B4) thus becomes:
a0ex/ / R
RTLo
Spt eV (-AH) XAinXBin
which is identical with (6). (7) results from the fact t h a t the real LO temperature inside the monolith is the observed temperature plus the adiabatic temperature rise.
CATALYSISAND AUTOMOTIVEPOLLUTIONCONTROLIV Studies in Surface Science andCatalysis,Vol. 116 N. Kruse, A. Frennet and J.-M Bastin (Eds.) 91998Elsevier Science B.V. All rights reserved.
409
NO + CO --> ~/2 N2 + CO2 differentiated from 2NO + CO --> N20 + CO2 over rhodia/ceria catalysts using lSN180 and 13C160 reactants or timeresolution of products Joseph Cunningham a, Neal J. Hickeyb, Frank Farrell c, M. Bowkerd and Colin Weekse. a'b'CChemistry Department, University College Cork, Ireland. d'eCatalysis Research Centre, University of Reading, UK.
ABSTRACT Profiles versus Ramp-temperature for isotope exchange between 15Nl80 and 0.5% RhO• CeO2 or Rh203 materials within a recirculatory reactor system indicate unique low-temperature lability of ~60-surface species at rhodia-ceria perimeter positions upon preoxidised 0.5% RhO• and its absence there from after LTR. Onset temperatures and relative efficiencies are likewise compared for conversion of 15N180 plus 13C160 mixtures to isotopicaliy distinguishable forms of N20, N2 and CO2 products over preoxidised and prereduced materials. Complementary insights into the time-sequence for appearance of N20 and N2 products in the gas phase over the materials at selected temperatures in the range 125298~ are provided by results carried out in an alternative micro reactor system which allowed introduction of individual 10s pulses of CO into a continuous flow of NO plus helium over preoxidised or prereduced aliquots of the materials.
1. INTRODUCTION Synergisms between Rhodium and Ceria components of three-way catalytic converters (TWC' s) are widely considered important for continuing efficient operation of the latter, with the zero-valent metallic state being favoured for the dispersed Rhodium component. Transitory existence of at least some part of the dispersed rhodium component in oxidised R h n+ form (hereinafter denoted by RhO• is, however, probable at various times during TWC operation and most especially during engine <<warm-up from cold start>>[1,2]. Twin objectives of the experiments here performed in a recirculatory reactor system were: (a) to determine what catalytic activity and selectivity is associated with 0.5% RhO• in respect of the conversions NO + CO --> 1/2 N2 + CO2 and 2NO + CO --> N20 + CO2 across such warm-up temperature range, and (b) to compare such activities/selectivities with those of the same catalysts after prereducing, firstly, the rhodium component to Rho, (LTR) and secondly, the surface-ceria component to CeO2.x (HTR). In view of previous reports of facile oxygen isotope exchange between N180 and the lattice oxygens of some metal oxides
410 [3,4] and in order to gain background information necessary for meaningful interpretation of present studies of the NO + CO reactions, preliminary studies of the interactions of 15N180 alone over the CeO2, RhaO3 and 0.5% Rh/CeO2 materials in their pre-oxidised LTR and HTR states were undertaken. Subsequently, an equimolar mixture of 15N180 and 13C160 was used for mass spectrometric based comparisons of the relative efficiencies and selectivities of the various materials in promoting 15N180 + 13C160 ~ 1/2 15N2+ 13CO2and/or 2 15NlSo + 13C160 15N2180 d- 13C160180 conversions in the recirculatory reactor system. Such experiments clearly could not provide direct insights into aspects of TWC operation during switching between exhaust gases with 'lean' and 'rich' stoichiometry- e.g. the time-sequence for isothermal evolution of N20 and other products from such reactions at warm-up temperatures. Such insights were sought instead using an alternative experimental arrangement which allowed injection of individual 10s duration pulses of normal CO into a continuous flow of helium plus normal NO over the catalysts and was equipped with fast MS detection to delineate the time sequence and profiles for product evolution. 2. EXPERIMENTAL Origin and properties of the Rh203 and CeO2 powders have elsewhere been described in full, as also the wet impregnation of CeO2 with non-aqueous Rhodium III acetyl acetonate (5,6). In addition to prior, ex-situ, calcinations in a flow of pure, dry O2 at 823K for 6-15 h, each sample introduced into the high-conductance section of the recirculatory reactor system received an in-situ calcination for 3 h at 823K under 100 torr 02 recirculating through a liquid N2 cooled tap. This was followed by cooling to RT in O2 and evacuation of the 02. Samples in that condition are termed preoxidised, whereas those subsequently subjected to in-situ reduction by H2 at 423K or at 773K are designated by LTR and HTR respectively. Comparisons were then made at several temperatures between activities of those variously pretreated materials in promoting: (a) isotope exchange between 5 mbarr 15Nl80 and 1602" lattice oxygens to yield 15N160 at various temperatures, and (b) in similar manner, conversions of equimolar (15Nl80 + 13C160) mixtures to yield various possible isotopic forms of N20, NO and CO2.
3. RESULTS AND INTERPRETATION
lSNlSo conversions on pre-oxidised samples: Some adsorption of the 15NlsO together with small extent of conversion(s) were measured upon introducing the gas, at room temperature (RT) and 5 mbarr pressure into the recirculatory reactor system (total volume ~ 1.5 dm 3) wherein was positioned a 300 mg aliquot of pre-oxidised CeO2, or Rh203 or 0.5% RhOx/CeO2. The most unique and clear-cut of these conversions of 15N180 at room temperature was a limited yield of 15N160(g) over the 0.5% RhOx/CeO2 aliquot only. This was reminiscent of reports that oxygen isotope exchange (0.i.x) occurred with surprising ease at RT between 14NlSO(g) and the 1602-(s) anions of iron or nickel oxides. In those cases such 0.i.x was envisaged to proceed via triatomic surface intermediates resulting from additional coordination of chemisorbed NO to 1602-{s)of the metal oxide [3,4]. If the 15N180 --> 15N160 exchange here observed at RT only over pre-oxidised 0.5% RhOx/CeO2 is assumed to have
411 occurred via a similar mechanism, then an essential role for 1602" at microinterfaces between dispersed RhOx and the CeO2 support in the formation of such surface intermediates seemed probable in view of additional present observations that neither CeO2 alone nor Rh203 alone yielded any detectable 0.i.x at RT when in pre-oxidised condition equivalent to that of 0.5% RhOx/CeO2. A further difference between mass spectra of the gas phase present over the preoxidised materials after 30min recirculation of 15NlSO over each at RT was the detection of a gradual but very limited increase in signal level @ m/e - 30 above background over CeO2 and Rh203 but not over 0.5% RhOx/CeO2. Thus the RT o.i.x, activity detected over the latter was replaced by limited conversion of 15N180 to 15N2 as the preferred RT chemical conversion over pre-oxidised CeO2 and Rh203. Changes from the gas phase composition attained by 0.5h contact at RT between 15N180(g) and the pre-oxidised CeO2, Rh203 or 0.5% RhOx/CeO2 samples, were monitored by MS at 30 see. intervals, whilst ramping temperature of the reactor at 10~ min -1. As illustrated by fig. 1a, readily detectable rates of decrease of 15NlSO(g) from its RT - pre-equilibrated value were observed to onset at 120~ over pre-oxidised 0.5% RhOx/CeO2 (cf fig.la) or over CeO2 (not shown), together with comparable increases in 15N160(g). In both cases, equality of the 15NlSO(g) and 15Nl60(g) signals was reached at ramp temperature ca. 530~ Clearly, activation of the 15N180(g) ~ 15Nl60(g) isotopic exchange to similar extents was the predominant effect over both oxidised ceria-based materials at 120 - 530~ Such similarity in 0.i.x activity across that temperature range was in sharp contrast to observations above that only pre-oxidised 0.5% RhOx/CeO2 was effective in promoting a limited amount of 0.i.x at RT. That contrast could be rationalised on the basis of (i) limited availability only on preoxidised RhO2/CeO2 of labile 16on"or 1602n" species capable at RT of undergoing exchange with 15N180 in low activation energy events; and (ii) shared capability of pre-oxidised CeO2 surface regions on both materials at 120-580~ to promote 0.i.x events requiring significant thermal activation. Since no significant decrease in 15N180 or increase in 15N160 was detected at RT or upon applying the temperature ramp to ~5N180 in contact with pre-oxidised Rh203, neither type of site for 0.i.x appeared to be available on pre-oxidised Rh203. Comparisons of 15N180conversions over LTR samples (cf. fig. lb): On the basis of TPR profiles measured upon pre-oxidised samples [5,6], the following redox conditions were expected for the LTR materials: (a) extensive reduction of the rhodium content of 0.5% RhOx/CeO2 to metallic rhodium, allied to limited reduction of adjacent ceria by hydrogen spillover, but with much unreduced CeO2; (b) reduction of CeO2 to very limited extent, since the <<surface-reduction>> feature of CeO2 in TPR is usually delayed until ca. 450~ [7]. In line with (b), no significant differences were detected in the interaction of 15N180 with LTR-CeO2 relative to that with pre-oxidised CeO2. Behaviour similar to that reported for NO over metallic rhodium [8] was to be expected over LTR 0.5%RhOx/CeO2 within the context of (a) above. Results in Fig. l b consistent with this include more extensive decrease in 15NlSO(g) during 1 hour contact with LTR- 0.5% RhO• at RT than with the pre-oxidised material, allied to small but clearly detected yields of 15N2and 15N2180. These could arise from some re-oxidation of mainly metallic rhodium surface, 15N 18O + Mm ~ 15N(s)+ JsO/Mm, followed by reaction of 15N~s)with one another to give 15N2, or with adsorbed 15N180 to yield 15N2180. Observations in Fig. l b that increase in the latter became noticeable at 150~ under temperature ramp, whereas the yield of ~5N2 did not increase until 300~ also resembled temperature dependences reported for N20 formation and dissociation over metallic rhodium
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[9]. A further experimental observation, which was not expected but which could be rationalised within the context of 16on" or 1602n" species having been removed from rhodia/ceria contact perimeters by prior LTR of RhOx/CeO2, was the non-appearance of detectable 15NlSO(g) ---> 15Nl60(g) conversion over the LTR 0.5% RhOx/CeO2 samples either at RT or under temperature ramping [cf. fig. lb with 1a]. Comparisons of lSNlsO conversions over HTR CeO2 and 0.5% Rh/CeO2(ef. fig. lc): The principal additional redox changes expected to result from such pre-treatment under H2 at 550~ followed by evacuation at 550~ to remove any H20 formed, were more extensive reduction of surface/sub-surface regions, allied to increased numbers and types of oxygen anion vacancies in the reduced ceria support [10]. Concentrations of the latter seemed likely to be greatest in regions of HTR 0.5% Rh/CeO2. adjacent to rhodium metal particles, as a consequence of hydrogen spillover [11]. However, the MS studies indicated zero formation of 15Nl60(g) at RT or at ramp temperature < 250~ over either material, despite significant decreases in 15NlSO(g) accompanied by smaller invariant yields of tSN2(g) and 15N2180(g). No further increase in those limited yields of 15N2(g) or 15N2180 occurred until ramp temperature ca. 200~ over HTR-CeO2 or 300~ over HTR-0.5% RA/CeO2 (cf. data for RhO• in Fig. l c). However, those likewise represented the respective temperatures for onsetplus-continued growth of 15Nl60(g), allied to decreases in 15NlSO(g), thereby demonstrating recurrence of temperature-activated 15NlSO(g) ---> 15Nl60(g) (o.i.x.) over both HTR materials. This contrasted markedly with absence of such 0.i.x over the same materials when in the LTR condition (fig. l b). Apparently, the much enhanced extent of ceria reduction after HTR - with resultant large increases in concentrations of surface and sub-surface oxygen anion vacancies and of co-ordinatively unsaturated
Fig. 1" T-ramp induced changes in gas phase composition, subsequent to RT equilibration between 5 mbarr 15N 180 and 300 mg of 0.5% RhOx/CeO2 when in preoxidised (la); LTR (lb) or HTR (lc) condition. [Vertical scale MS peak heights in mutually consistent a.u.; bottom scale ~
413 Ce 3+ ions adjacent to oxygen anion vacancies [ 10] - had created new ceria-related active sites which opened up new thermally activated pathways for the 15NlsO -~ 15N160 isotope exchange at such defect sites. ISNlsO plus 13C160 over pre-oxidised materials: The trace amounts of 15N2and 15N160 produced over pre-oxidised CeO2 by RT contact with equimolar 15N180 + 13C160 were similar to those observed from RT contact with 15NlsO only. Under T-ramp the first significant difference from the latter to emerge in contact with the equimolar mixture was a delay of ca 100 ~ in onset of ramp-induced increase of 15N160(g) from its initial trace level. Such delay until 250~ could be understood in terms of competition by 13C160 against 15N180 for reaction with 1602"s species rendered labile on ceria at ~160~ and above. The fact that onset of ramp-induced increases in either 13C1602(g) or 13C160180(g) did not become apparent until ~450~ made it clear that CO2 products from CO oxidation on preoxidised ceria were retained by the CeO2 surface until such temperatures. Decreases in 15NlSO(g) at 260-450~ over preoxidised ceria appeared to originate from the isotopic exchange process 15NlgO(g) ~ 15N160(g), since 15N160 was the only 15N-containing species observed to increase at 260 ~ 450~ Subsequently, the 15Nl60(g) signal levelled off and then decreased at 450 ~ 650~ consistent with 15N160 species then reacting with chemisorbed CO or CO2. Isotopic analysis of the then-observed increase in gas-phase CO2 products showed AI3cI602/AT to be three fold greater than AI3C160180/AT, consistent with 15N180 + 13C160 --~ 13C160180 + 1/215N2being less efficient than reaction of adsorbed 13C160 with lattice 1602" or with 15N160(s) to yield 13C1602. No evidence for 15NE160(g) or 15NE180(g) was found over the pre-oxidised CeO2. Over pre-oxidised Rh203 the most notable difference observed under the equimolar 15NlSO h- 13C160 mixture was that uptake of 15NlSO(s) commenced at ~280~ which was ca. 220~ lower than onset of a much smaller decrease over the same material under 15N180 only. Furthermore that onset at ~280~ was accompanied by a parallel decrease in 13C160(g) and by onset of increases in CO2 products, thereby indicating that the pre-oxidised Rh203 sample promoted reaction(s) between 15NlSO(g) and 13C160(g) at temperatures 280 ~ 600~ Since no 15N2160or 15N2180 was detected, reaction with 1"1 stoichiometry appeared to be favoured so that 13C160180 could be expected as the primary product from 15N180 + 13C160 ~ 1/215N2 -t- 13C160180. Increases in 13C160180 and 15N2were observed but isotopic composition of the CO2 product - with 13C1602(g) increasing at similar rate to that for 13C160180(g) and 13C1802(g) increasing at only one-quarter of that rate pointed to more efficient reaction of CO with lattice ~602 of the rhodia surface than with lSo from 15Nt80. Comparison between the composition profiles versus ramp temperature observed over preoxidised 0.5% RhOx/CeO2 when in contact with 15N180 + 13C160 (el. fig. 2a) rather than with 15N180 only (cf. fig l a) reveals the absence of detectable 15N180 ~ 15N160 in the former. Furthermore, the profiles in fig. 2c confirm the lack of any evidence for the oxygen isotope exchange process over the 0.5% Rh/CeO2 sample when in the HTR condition in contact with the equimolar mixture, thereby doubly emphasising the inhibitory role of 13C160 co-reactant upon the 0.i.x process observed over the same material equivalently pre-treated but in contact only with 15NlsO. Blockage or removal by CO of the sites or species active for o.i.x, on 0.5% RhOx/CeO2 is implied by these results, but details thereof are as yet unclear. Other notable differences in conversions of the equimolar 15NlsO + 13C160 mixture over pre-
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oxidised CeO2 or Rh203, were the definite detection of 15N2180 over RhOx/CeO2 when ramp temperature reached 150~ and the subsequent doubling of that signal at 150-> 350~ without significant increase in 15N2. For preoxidised materials this low temperature, formation of 15N2180 was observed only over 0.5% RhOx/CeO2 which implied that selective promotion of the conversion 215N180 + 13C160 --> 15N2180 +13C160180 required synergism between the RhOx and CeO2 components -such as could be envisaged at contact perimeters between them. Remarkably, the onset temperature for release into the gas phase of CO2 products from conversions of the equimolar mixture over the preoxidised 0.5% RhOx/CeO2 were halved relative to those observed over preoxidised ceria oxidised 0.5% RhOx/CeO2, relative to the abovenoted conversions over pre-oxidised CeO2 or Rh203, were the definite detection of 15N2180 over RhOx/CeO2 when ramp temperature reached 150~ and the subsequent doubling of that signal at 150 ~ 350~ without significant increase in 15N2.above (i.e. to 225~ in Fig. 2a from 450~ over CeO2). Onset and growth of CO2 products in the gas phase over preoxidised RhOx/CeO2 at 225--->450~ were thus markedly enhanced relative to those over preoxidised ceria and significantly relative to preoxidised rhodia Evidently the 0.5% RhOx content of RhOx/CeO2 was especially effective in facilitating release of CO2 product to the gas phase, yielding much more 13C1602 than 13C160180. Whilst MS peak heights for both those species continued to increase 300-450~ that for 15N180 levelled off and began to decrease at T>300~
Fig. 2. Changes in gas phase composition following RT introduction of 5 mbarr each of 15NlsO and 13C160 and T-ramp to 700~ over 0.5% RhOx/CeO2 when pre-oxidised (2a); LTR (2b); and HTR (2c). (Vertical scale MS peak height in mutually consistent a.u., except for small upward displacements of CO2 profiles to avoid overlap) Bottom scale ~
415
This would be consistent with catalysed conversion NO + CO changing over from 2:1 to 1:1 stoichiometry, [11 ] or possibly with onset of catalysed N20 dissocation [9]. 15N180 + 13C160 over L T R materials: Over LTR 0.5% RhOx/CeO2 in presence of the equimolar gas mixture, onset of a low yield of 15N2180 was observed at ramp temperature -~ 100~ with subsequent growth at 100-300~ The contrast between this and strong predominance of 15N160 production from 15N180 alone over LTR CeO2 in that temperature range implied that the 13C160 component of the equimolar mixture, allied to 0.5% Rh upon LTR CeO2, somehow blocked/removed the ceria-related sites responsible for o.i.x, on LTR CeO2 and replaced them by sites active for 215N180 + 13C160---~ 15N2180§ 13C160180. Evidence in support of this was that the constant rate of decrease of 15NlSO(g) observed across the ramp temperatures 120 --~ 600~ was approximately twice the observed rate of decrease in lacl60(g) (cf. fig. 2b). Escape of the CO2 products into the gas phase once more again become evident at ramp temperatures > 230~ The fact that 13C1602(g) increased three-fold faster than 13C160180(g) at 230 --~ 600~ pointed again to substantial isotope exchange, 13C160180(S) + 1602"(S) --} 13C1602(S) + 1802"(S) before escape. 15N180 plus 13C160 over HTR materials: The above noted, mechanistically significant ratio of 2:1 between (-AISNISO/AT) and (-AI3CI60/AT) over LTR 0.5% Rh/CeO2 was not reproduced when a similar run was carried out over the same 0.5% Rh/CeO2 material following its re-oxidation for lh in O2 at 550~ plus lh reduction in HE at 550~ and lh evacuation at 550~ On the contrary, the MS peak heights for 15N180 and 13C160decreased in parallel across 150 -~ 600~ ramp temperatures with equal slopes (cf. fig 2c), suggesting that 15NlSO(g) § 13C160(g) ---} 1/215NE(g)+ 13C160180(g) was favoured over this HTR sample rather than reaction with 2:1 stoichiometry. Support for this came from observations of increasing release of CO2 and N2 products in approximately a 2:1 ratio at ramp temperatures > 200~ A much smaller increase in 15N2180, from its trace level after RT contact with the equimolar mixture, was observed at ramp temperatures 100 --~ 300~ after which it decreased. Conversions from pulsing 12C160 into continuously flowing 14N160: For all samples investigated by this method one of two pretreatment regimes was employed: (i) pre-oxidation conducted at 300 ~ in 100% 02 for 1 hr; or (ii) pre-reduction conducted at 200~ in 100% H2 for 1 hr (LTR). The results depicted are those obtained from the fourth pulse of CO injected, by which time stable pulse profile were established which could be fully reproduced from the fifth and subsequent pulses. Such measurements thus provided insights into the time sequence for CO-pulse initiated growth of product-related MS peaks (upward displacements) and subsequent flushing away, by the continuous NO/helium flow, of gas phase products and any unconverted CO. 0.5% Rh / CeO2 : Figure 3a depicts the time-profiles of responses observed at m/e = 44 over pre-oxidised 0.5% RhOx / CeO2 at various temperatures between 100 ~ and 175 ~ Across this <>temperature range onset of activity was observed as low as 125 ~ whilst by 148 ~ and 175 ~ there is a clear division of responses into a prompt peak and a slow eluting peak. Time-profiles of responses observed at m/e = 44 for the same sample under the same conditions at high temperatures between 175 ~ and 297 ~ are depicted in Figure 3b.
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Temperature increase in this range results in decreasing magnitude of the prompt response whereas the slow component remains high and very broad. The following points emerge from cross-correlation between these m/e = 44 time-profiles and those observed at other m/e values over the same sample from the same pulse: (1) Decreases at m/e = 30 were observed indicative of 14N160uptake during the CO pulse. However time-profiles of that uptake did not exhibit a <> nature but more closely corresponded to the slow m/e = 44 response. (2) The response profiles observed at m/e = 12 showed that at and above 148 ~ there were no features paralleling the prompt feature in Figures 3a and 3b, thereby eliminating CO2 as their source and indicating that the prompt feature at m/e = 44 was exclusively 14N2160; (3) Profiles of small responses at m/e = 28 closely resembled that of the prompt m/e = 44 signal, an observation which when allied to the absence of prompt signal at m/e = 12, pointed to a disproportionately small yield of prompt N2 product at 148 and 174~ relative to the signal size expected if the overall uptake of CO and NO had reacted with 1"1 stoichiometry over the preoxidised RhOx/CeO2. Between 175 ~ and 297 ~ the 14N2 peak progressively increases in magnitude and broadens resulting in some overlap with the slow rn/e = 44 component. The overall behaviour of the prompt 14N2160and 14N2responses were compatible with the low-temperature activity of preoxidised RhOx/CeO2 having high initial selectivity towards 14N2~60 but with some increase in selectivity towards 14N2 at high temperature. The slow component of 14Nl60 loss (see below) likewise appeared to favour 14N2160.
Fig. 3: MS-response time profiles @ m/e = 44 upon injection of the fourth 10s duration pulse of CO into continuous flow of NO/helium over preoxidised 0.5% RhOx/CeO2 after 440s onstream therein: 3a and 3b, fixed P(NO) = 15.2 torr and T=100 --~ 175~ or 175-+297~ resp.; [3c, T fixed at 162~ P(NO) varied as indicated]
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417
Figure 3c illustrates the time-profiles obtained at rn/e = 44 over oxidised 0.5% RhO• / CeO2 from isothermal introduction of identical CO pulses when different partial pressures of 14N160 were established in the continuous flow of NO/helium over the sample before admitting CO pulses. Decreasing that variable clearly resulted in decreases in magnitude of the prompt and slow responses and in a shift to longer times for maximum of the slow component. Data for uptake at rn/e = 30, recorded simultaneously with the data in Figure 3c, generally mirrored that observed for the m/e = 44 slow component in Figure 3c but with no prompt uptake feature being observed. Overall behaviour of the slow-eluting features, appeared consistent with contribution towards slow formation of products by post-CO-pulse surface processes involving 14N160 from the continuous flow reacting with 12C160 retained on the surface from the pulse. Size variations of the prompt, m/e = 44, component evidenced in Fig. 3c may be understood in terms of different extents of interaction of an incoming CO pulse with different surface coverages, 0NO, of the preoxidised RhOx/CeO2 material established under NO/helium flows having different PNo before CO-pulse injection. Prior formation of N20 from NO over preoxidised RhOx/CeO2 at 162 ~ being unlikely (of. Fig. 1a), these prompt effects of CO in each incoming pulse appeared to include CO(g) + NO(ads) --->CO2(ads) + N(ads) and facilitation of N(ads) + NO250~ and came to resemble that of HTR CeO2.x This pointed to HTR induced production of labile oxygen species associated with defect sites on ceria. Delayed onset-temperatures, allied to substantial overall inhibition of the above-noted levels of 0.i.x towards 15N180 alone, were evidenced whenever equimolar ISNl80 + 13C160 was introduced over equivalently pre-treated 0.5% RhOx/CeO2, thereby pointing to efficient scavenging of labile 160-containing surface species by 13C160, especially at T < 200~ Yields of 15N1180 observed at those temperatures over 0.5% RhOx/CeO2 in LTR and HTR condition could be understood, as an indirect consequence of such scavenging: by virtue of surviving ISN fragments reacting via 15NlSO + lSN ~ lSN21sO, thereby redirecting selectivities for lSNlSO conversions away from o.i.x, and towards N20 formation. Contact perimeters between RhOx and
418 CeO2 appeared the likely locations for such selectivity modifications, since equivalent effects were not observed over CeO2 alone or Rh203 alone. Evidence yielded by the pulsed experiments for predominance of N20 in the 'prompt' product detected from CO pulse contact with NO//0.5% RhO2/CeO2 could likewise be understood in terms of the ~2C~60 pulse having scavenged oxygens from the laN~60-covered surface, thereby facilitating N20 formation through ~4N + ~4N160 --->N20 reaction events. ACKNOWLEDGEMENTS Mobility of researchers between the laboratories involved has been aided by support under EC contracts SC1 CT91, 0904 and ERB CH RX CT 92 0065 and Eolas/British Council Grants '91 and '92. UCC workers also gratefully acknowledge the access given to pulsedreactant equipment at University of Reading and the expert assistance there received in applying it for the present studies. REFERENCES
1. (a) B. Harrison, A. F. Diwell and C. Hallet Plat. Met. Rev., 32 (1988) 73 (b)K.C. Taylor, fatal. Rev. Sci. Eng., 35 (1993) 457. 2. C.H.F. Dedon, D.N. Belton and S.J. Schmeig, J. fatal., 15 (1995) 204. 3. K. Otto, M. Shelef and J.K. Kummer, Z. Phys. Chem., N.F., 72 (1970) 316. 4. K. Otto and M. Shelef, J. fatal., 35 (1974) 460. 5. Trovarelli fatal. Reviews, 38 (1996) 439. 6. F.D. Farrell PhD Thesis NUI (1996); J. N. Hickey PhD Thesis NUI (1997) 7. J. Cunningham, D. Cullinane, F. Farrell, M.A. Morris, A. Datye and D. Kalakkad <
CATALYSIS AND AUTOMOTIVEPOLLUTIONCONTROLIV Studies in Surface Science and Catalysis, Vol. 116 N. Kruse, A. Frennet and J.-M Bastin (Eds.) 91998 Elsevier Science B.V. All rights reserved.
419
Investigation on the role of Rhodium on the kinetics of the oxidation of CO by NO over Pt-Rh catalysts P. Granger, J.J. Lecomte, C. Dathy, L. Leclercq, G. Mabilon a, M. Prigent a and G. Leclercq Laboratoire de Catalyse H6t&og6ne et Homog6ne, URA CNRS 0402, Universit6 des Sciences et Technologies de Lille I, 59655 Villeneuve d'Aseq, France alnstitut Franeais du P6trole, l-4 avenue de Bois-Pr6au, B.P. 311, 92506 Rueil-Malmaison cedex, France
ABSTRACT The kinetics of the CO+NO reactions has been studied at 300~ over a fresh and a deactivated bimetallic Pt-Rh/AI203 catalyst. Two kinetic models have been examined including competitive and non-competitive adsorptions of the reactants. The discrimination between these two assumptions has been achieved by using graphic and mathematical methods. From the comparison of kinetic and thermodynamic constants calculated from these methods with those previously obtained on Rh/AI203 and on Pt/AI203, we believe that the kinetic data obtained on the flesh Pt-Rh/AI203 catalyst can be modelled by non-competitive adsorptions of the reactants assuming a preferential adsorption of NO on Rh and CO on Pt. By contrast NO and CO competitive adsorptions can only occur on the deactivated Pt-Rh/AI203 catalyst, which to assume that the active surface is mostly composed of Rh. 1. INTRODUCTION Rh is essential to promote the reduction of NO by CO in three-way catalysts [ 1, 2]. Many investigations have shown that rhodium exhibits a greater ability to dissociate NO than platinum or other noble metals [3-5]. According to this finding, the procedure for the preparation of bimetallic Pt-Rh catalysts usually used in three-way catalytic converters have been optimised in order to preserve the individual adsorption properties of these two metallic components [6]. Unfortunately, earlier studies have clearly shown that alloying is typical in TWCs, in particular when sintedng reactions take place at high temperatures. It is usually observed that both metal particle growth and Rh enrichment proceed at the same time [7-9]. In addition partial oxidation of metallic Rh can occur, yielding oxidized rhodium species inactive in CO+NO reactions. The kinetics is an important tool to investigate the nature of interactions between accessible metal sites and adsorbed molecules, providing useful informations on the type of mechanism as well as on the role of the specific active phases. The following mechanism has
420 been earlier established from a kinetic study of CO+NO reactions performed at 300~ over Pt catalysts supported on ), alumina, chromium carbide Cr3C2 and silicium nitride Si3N4 [l 0]: NO + * CO + * NO* + * NO* + N* NO* + N* N* + N* CO* + O*
r r --+ --+ --4 --+ --4
NO* CO* N* + O* N2 + O* + * N20 + 2 * N2 + 2 * CO2 + 2 *
(1) (2) (3) (4) (5) (6) (7)
where * denotes an adsorption site. This mechanism includes reversible adsorptions of NO and CO, steps (1) and (2), and the dissociation of adsorbed NO, step (3) as rate determining step. The values of the rate constant of step (3) and of the equilibrium adsorption constants of CO and NO determined on these different Pt catalysts were discussed in terms of changes in the adsorption properties of Pt induced by support effects [10]. Hence kinetics could be useful to state on the modifications in the extent of such interactions when Rh is added to Pt, in particular when the deactivation proceeds during the CO+NO reactions. This study reports kinetic data on a fresh and on an aged bimetallic Pt-Rh/A1203 catalyst which have been further interpreted with kinetic models including competitive adsorptions of NO and CO on a single kind of active site as well as non-competitive adsorptions in accordance with preferential adsorptions of the reactants on Pt and Rh sites as suggested by Van Slooten and Nieuwenhuys [11]. 2. EXPERIMENTAL
2.1. Catalysts preparation and characterization y alumina (1O0 m 2 g-l) was impregnated with aqueous solutions of rhodium trichlodde and hexachloroplatinie acid in order to obtain 1 wt% Pt and 0.2 wt% Rh. The experimental procedure used for the preparation of bimetallic catalyst was the classical coimpregnation of the support with aqueous solution of the metallic salts. The precursors obtained were dried at 120~ and calcined at 450~ prior to reduction under flowing hydrogen at 500~ The metal dispersion was calculated from hydrogen chemisorption measurements. The values obtained on monometallie Pt/A1203, Rh/A1203 and a fresh bimetallic Pt-Rh/Al203 were respectively 0.55, 0.93 and 0.64. A sample of the fresh Pt-Rh/A1203 catalyst was sintered by submitting it to a gas mixture containing H20, 10 vol%, diluted with N2 at 800~ for 16 hours. This sintered catalyst was subsequently aged in a reactant mixture containing CO, NO and 02 for two weeks at 280~ The fresh and the aged bimetallic Pt-Rh/Al203 catalysts following this procedure have been labeled repectively Pt-RlffR, and Pt-Rh/D.
2.2. Catalytic testing The experimental set up has been described in details in an earlier study [10]. The kinetic measurements were performed at 300~ in differential flow reactor conditions by recycling the outlet gas mixture. The reactants (NO, CO) and reaction products (N2, N20 , CO2) were analysed by means of a chromatograph HP5850 equipped with a thermal conductivity detector. Their separation was achieved on a column CTR1 supplied by Alltech.
421 The reaction was studied in the following experimental conditions: 0.2 g of catalyst was mixed with 0.8 g of tx alumina, the global flow rate was 10 L h "~. The space velocity was kept constant at 25000 h "~ during the catalytic testing. 3. KINETIC MODELS
3.1. Competitive adsorptions Up to now, most of the kinetic models proposed in the literature on Rh [12], Pt [13] and bimetallic Pt-Rh [14] catalysts assume (i) competitive adsorptions of NO and CO according to reaction steps (1) and (2) at equilibrium, (ii)the dissociation of adsorbed NO, step (3), as rate limiting, (iii) NO and CO as the most abundant adsorbed species on the active surface. With these assumptions, Eq. (8) is found"
k 3 ;~NO PNO r = k 3 0NO 0 v
(1 + ~'NO PNO + ~'CO PCO) 2
(8)
where 0r~o and 0v stand respectively for the NO coverage and the fraction of vacant adsorption sites, k3, Eco and ),No are respectively the rate constant for the dissociation of NO and the equilibrium adsorption constants of the reactants. Eq. (8) can easily be linearized into Eq. (9).
~
PNo
1+ L NO PNO + ~,CO PCO
r
4k3 ~NO
(9)
3.2. Non-competitive adsorptions An alternative model can be proposed for describing the CO oxidation by NO which assumes a preferential adsorption of NO on Rh and of CO on Pt as mentioned by Van Slooten and Nieuwenhuys [11 ] who investigated this reaction on a Pt-Rh/SiO2 catalyst by means of infrared spectroscopy measurements. However this situation leads to a more complex description of the rate limiting step (3) as far as the nature of the vacant nearest neighbour adsorption site to dissociate NO is concerned. This one can be Pt alone, Rh alone or either Pt or Rh. According to these different assumptions the following reactions steps for the dissociation of NO can be considered. NO* + *' NO* + * NO* + (* or *')
--+ N* + O*' --+ N* + O* --~ N* + O(* or *')
(10) (11) (12)
* and *' represent metallic rhodium and platinum adsorption sites respectively. The following equations can be derived from steps (10), (11) and (12),
r = k 10 0NO (1-0CO) =
k 10 ~,NO PNO
(1 + ~NO PNO)(1 + )~CO PCO )
from reaction step (10)
(13)
422
r = k 11 0NO (1-0NO ) =
k I l )~NO PNO
from reaction step (11)
(14)
from reaction step (12)
(]5)
(1 + )~NO PNO ) 2 r = k'12 0NO (1- 0NO ) + k12 0NO (1 -0CO ) kl2 )~no PNO = k'12 )~NoPNo + (1 + LNO PNO) 2
(1 + ~LNOPNO)(1 + ~,CO PCO)
where the rate constants k!2 and k'j2 represent the contribution of a Pt and of a Rh site respectively on the overall dissociation of adsorbed NO. One can notice that only Eq. (13) can be linearized yielding Eq. (16). PNO (1 + ~CO PCO) (1 + Z,NO PNO ) r kl0 ~,NO
(16)
The assumptions of competitive and non-competitive adsorptions of the reactants (Eqs. (9) and (16) respectively) can be discriminated by plotting (P~o/r) ~ and PNo/r versus PNo and Pco. The valid equation should give a linear plot. Moreover, the specific rate constant for the dissociation of NO, k., and the equilibrium adsorption constants of NO and CO (kNo and kco) can be estimated from the slopes and intercepts of the straight lines. A mathematical procedure based on the least square method was also carried out in order to calculate these different parameters, in particular when the rate law cannot be linearized, as for Eqs. (14) and (15). The adjustment of the values for k,, kr~o and Lco was achieved when the summation of the square deviations between experimental and calculated rates, tx tends towardsthe lowest value.
ct =
i=l
ri exp - ri calc
)
(17)
4. RESULTS 4.1. kinetic measurements
Two series of experiments were performed on bimetallic Pt-Rh/R and Pt-Rh/D catalysts by varying Pco from 2.8x 10-3 to 8.8x 10.3 arm while Pr~o was kept constant at 5x10 "3 atm on the one hand, then by varying PNo from 1.5• 10.3 to 8.6• 10.3 atm at a constant CO partial pressure of 5x10 "3 atm on the other hand. For example the steady-state activities measured in stoichiometric conditions (PNo = Pco = 5x103 atm) at 300~ on these two catalysts are reported in Table 1 with those previously measured on Pt/A1203 [10] and Rh/AI203 [15] catalysts in similar experimental conditions. Their comparison shows an increase in the specific activity within the following sequence. As expected Pt-Rh/D exhibits a lower activity than that of Pt-Rh/R, nevertheless higher than that of Pt/A1203. Pt/AI203 < Pt-Rh/D < Pt-Rh/R << Rh]A1203
423 A more significant comparison is obtained from the intrinsic activities expressed per surface metal atom which clearly show a sharp increase in the intrinsic activity of Rh/Al203 compared to that of Pt/A1203 (three order of magnitude). Rh incorporation to Pt induces an increase in the intrinsic activity of Pt-Rh/R but in a lesser extent. The reaction orders obtained on these catalysts with respect with the power law expression of the type, r = kPl~o P~O are listed in Table 1. Table 1 Rates and reaction orders for the CO+NO reactions at 300~
Catalyst
Pr~o atm x 10 .3
Pt/A1203 a
1.5 0.6 1.5 2.2
Rh/AI203 b Pt-Rh/W Pt-Rh/D ~
-
5.7 1.1 5.6 8.6
Pco atm x 10 J
5.03.0 3.02.8 -
9.0 14.0 8.0 8.8
Specifie d activity
Intrinsic c activity
2.3x104 2.6x 10 -2 2.1x10 3 5.1 x 10 4
8 1423 46 -
tx
0.93 0.36 0.40 - 0.50
13
-0.81 - 0.32 - 0.38 - 0.25
Rate = kxPNo~xPco 13- a see ref. [10], b see ref. [15], c tlfis study d CO mol.h'~.g "I of catalyst calculated at T = 300 ~ and PNo = Pco = 5x 10.3 arm c CO molee, h~.(surface metal atom) ~. Negative CO orders are obtained on all catalysts which evidence the well known CO inhibiting effect on the CO+NO reactions. On the contrary, positive NO orders are obtained, except for Pt-Rla/D which exhibits negative order in NO.
* Competitive adsorptions of NO and CO We have examined in this section the assumption of a single kind of active site for the adsorption of NO and CO. As seen in Figures 1 and 2, the plots of (PNo/r)~ V.S. PNo and Pco for Pt-Rh/R and Pt-Rh/D catalysts give correct straight lines in spite of the lack of accuracy in determining the experimental rates. The values of k3, kco and L~o derived from this graphic method are reported in Table 2. Their comparison with the corresponding values obtained from the optimisation method show a good agreement. As far as the intrinsic rate constant for the dissociation of NO, k'a, is concerned, only small variations are observable on Pt/Al203, Pt-Rh/R. By contrast k'3 is considerably greater for Rh/Al203 (two order of magnitude). The equilibrium adsorption constants of CO, kco, shifting from 92 to 120-130 atm ~ respectively on Rh/A12Oa and Pt-RlffD, probably vary within the margin of error. By contrast Rh incorporation to Pt induces a large increase in ~No which shifts from 15 on Pt/AI2Oa to 874-984 atm ! on Pt-Rh/D.
424
Table 2 Kinetic and thermodynamic constants calculated on Rh and Pt based catalysts at 300~ ease of competitive adsorptions of CO and NO
in the
Catalyst
k3 d
k'3c
~.No atm'~
Lco atm "~
Pt/AI~O3a Rl~AI203 b Pt-Rh/R ~
8.9x 10 "3 245x 10 "3 12.9x 10 "3f 12.5x10 "3g 4.7x10 "3 f 5.0x 10 3 g
316 13554 285 f 276 ~ -
15 480 191 195 874 984
127 115 92 f 92 g 120 f 130 g
Pt-Rh/D ~
f g f g
"see ref. [10], b see ref. [15], c this study d specific rate constant for the dissociation of NO (CO mol.hl.g a o f catalyst) c intrinsic rate constant for the dissociation of NO (CO molee, ha.surface metal atom "z) f calculated from the graphic method g calculated from the optimisation method.
1.7-
.8
'
'
~
a
1.6-
b
1.7
1.5
1.6-
1.4 "~z 1.3
1.2
~
1.1
[] .
'
0
1.4
T-3 '
0.9
'
"
'
1.5
I
--
. . . .
o.oo2
|
'
0.004
PNo, a~n
T = 300~
" '
' ' .
6.0
1.3
''' 0
i
| i
B
0.0025
- P I = 5 10 "3,atm 9
9 |
i
9 !.
0.005
j
|
|
0.0075
P c o , a~n
Figure 1. Plots o f ( P ~ r ) ~ versus Pm (a) and Pco Co) on Pt-RNR in the CO+NO reactions at 300~
i
I
|
0.01
425
3.3
,
.
a
.
.
.
.
.
.
b
.
,0
3.1
d
o
}
3
. 2.9
2 T
'
T=300C ,
1. 0.005
2.7
P~o
47!03,atrr~,
'
0.01
0
0.005
PNO, alan
0.01
Pco, arm
Figure 2. Plots of(PNo/r)~ versus Pr~o(a) and Pco (b) on Pt-Rh/D in the CO+NO reactions at 300~
* Non-competitive adsorptions of NO and CO In this seetion we have examined the situation where NO preferentially adsorbs on Rh and CO on Pt. Right now one earl discard the requirement of a vacant Rh site for the dissociation of adsorbed NO sinee Eq. (14) is unable to take into account the CO partial pressure dependency of the reaction rate. Figures 3 and 4 show linear plots of P~o/r v.s. P~o or Pco respectively on Pt-Rh/D and Pt-Rh/R. However the negative intercept of the linear plot in Figure 3-a allows to discard the assumption of non-competitive adsorptions of NO and CO on Pt-Rh/D.
20.,
11,
[]
a
b
16slope = 2515 intercept = - 3.2
12-
m
f
8---
Z
4--
.
z O~
T =
T - 3 ~ 0 ; C - Pco = 4.7 10.3 atm i
I
i
I
! I
I
0.005 PNO, atm
9
!
7
I
0.01
l
0
- PNo = 4.7 10 .3 a t m
300~ I
e
I I
I
!
I
0.005
Pr
arm
Figure 3. Plots of Pno/r on Pt-Rh/D v.s. Pr~o(a) and Pco (b) for the CO+NO reactions at 300~
.l
0.01
426
3.2
b
2.5
2.7z
1.5 i
0.5
2.2-
T = 300~ i
0
I
i
i
= 5 10 3
I I
i
!
0.002
i
I
I I
i
atm I
0.004
i
T = 300~ |
1.7
0.006
t
0
t
t
t
| I
9 i
0.002
PNO, atm
- P ~ o = 5 10 "3 atm i
t
I |
I
0.004
i
l
t
I !
t
|
0.006
!
I
| !
I
0.008
Pco, atm
Figure 4. Plots of P~/r on Pt-Rh/R v.s. PNo (a) and Pco (b) for the CO+NO reactions at 300~ As seen in Table 3, the resolution of Eq. (15) by using the optimisation method leads to the same values for the kinetic and thermodynamic constants as those obtained from Eq. (13), furthermore the rate constant k'~2 is equal to zero. Accordingly, the dissociation of adsorbed NO requires a vacant nearest neighbour Pt site. Table 3 Kinetic and thermodynamic constants calculated on Rh and Pt based catalysts at 300~ in the case of non-competitive adsorptions of CO and NO
Catalyst
rate equation
lq ~
Pt-Rh/R
( 13)' (15) b
8.47x 10.3a 4.74x10 "3 4.74x10 "~
(13) ~
<0
Pt-Rh/D
k'~2r
0
~ o atm'l
~.co atm'l
449 d 505 505
129 a 122 122
<0
<0
a Pt as the vacant nearest neighbour site - b either Pt or Rh the specific rate constant for the dissociation of NO (mol.lfl.g 1 of catalyst) d calculated from the graphic method. It is worthwhile to mention that kco on Pt-Rh/R is close to that obtained on Pt/AI~Oa, within the margin of error, whereas ~ o is approximately the same as for Rh/A1203 when the adsorptions of the reactant are assumed non-competitive.
i
427 5. DISCUSSION The partial pressure dependencies of the CO+NO reaction rate on monometallic Pt/AI203, Rh/AI203 and bimetallic Pt-Rh/R, Pt-Rh/D catalysts can be discussed in terms of competitive as well as non-competitive adsorptions of the reactants (NO and CO). In the case of bimetallic Pt-Rh/AI203 catalysts these two kinetic models have been discriminated using graphic and mathematical methods. The comparison between kinetic and thermodynamic constants obtained from these two methods with those previously determined on monometallic Pt/AI203 and Rh/A1203 catalysts allows us to state on the role of Rh incorporation to kinetic behaviour of Pt. Kinetic performances of Rh/AI203 and Pt/A1203 catalysts have been examined using the competitive adsorption model. The intrinsic rate constant of NO dissociation k'3, and the equilibrium adsorption constant of NO Z.No, are considerably higher on Rh/AI203 than on Pt/AI203. This result corroborates earlier observations on the adsorption of NO on Rh (111) and Pt (111) surfaces showing that NO dissociates easier on Rh than on Pt [3-5]. Before arguing on the reasons for changes in the kinetic behaviours of Pt-Rh/R and Pt-Rh/D in the CO+NO reactions, let us mention that a simple calculation, based on the Rh content and the metal particle sizes (1.4 and 3.4 nm respectively for Pt-Rh/R and Pt-RND) estimated from hydrogen chemisorption measurements, allows us to show that the surface Rh distribution cannot exceed 50% on Pt-RNR while the active surface of the intermediate sintered catalyst and the subsequent aged Pt-RND could be composed of only Rh atoms [15]. Such prediction have been supported by further spectroscopic characterizations which reveal a low Rh surface composition on Pt-Rh/R while Pt-RND exhibit a Rh like surface. According to XPS measurements we should also mention the formation of unreduced Rh species on this last catalyst. The comparison of the kinetic and thermodynanaic constants on Pt-Rh/R calculated from these two kinetic models is in favour of non-competitive adsorptions of the reactants, kso on Pt-Rh/R being close to that on Rh/A1203 and ~co on Pt-Rh/R similar to that on Pt/AI203. Hence, these results are consistent with preferential adsorptions of NO on Rh and CO of Pt according to Nieuwenhuys and Van Slooten [11 ]. Furthermore, thi~ participation of a vacant nearest neighbour Pt site to dissociate NO seems the most realistic situation on Pt-Rh/R which is in good accordance with previous spectroscopic observations [15]. All these trends are in agreement with the fact that both Rh and Pt in the bimetallic Pt-RNR catalyst preserve their individual adsorption properties. On the contrary only competitive adsorptions of CO and NO can model the reactant pressure dependencies of the rate of CO oxidation by NO on Pt-Rh/D. The large value of kr~o is in good agreement with the fact that the active surface of this catalyst is mostly composed of Rh atoms. As expected Pt-RND behaves like Rh/AI203 in the CO+NO reactions. Unfortunately the specific rate constant of NO dissociation k3, on Pt-RND is significantly lower than that of Rh/A1203. It seems uneasy to discuss on the value of Xno on Pt-RND compared to those obtained on Rh]AI203. We believe that this value of kno is mostly due to experimental error rather than to electronic modifications of the surface atoms. Hence, the low specific activity of Pt-RND compared to that of Rh/A1203 could be explained by the decrease in the specific rate constant for the dissociation of adsorbed NO k 3, based on a lower metal dispersion and the formation of unreduced Rh species unable to dissociate NO [ 16]. In such a case, one can envisage that the probability for adsorbed NO molecules to find a vacant neighbour Rh atom able to dissociate NO is significantly lower on Pt-RND than on Rh/AI203.
428 The drastic decrease in the rate constant for the dissociation of adsorbed NO from 245x 10.3 on Rh/A1203 to 4.7x10 3 - 5x10 3 mol h1 g-i of catalyst on Pt-Rh/D agrees with such interpretation. 6. CONCLUSION Kinetic study of the CO+NO reactions at 300~ on Pt-Rh/R and Pt-Rh/D catalysts show significant changes in their kinetic behaviour due to deactivation. We have examined both competitive and non competitive adsorptions of NO and CO assuming a preferential adsorption of NO on Rh and CO on Pt. The comparison of kinetic and thermodynamic constants calculated on Pt-Rh/R by using both graphic and optimisation methods with those obtained on monometallic Pt/A1203 and Rh/AI~O3 catalysts is in good agreement with noncompetitive adsorptions previously suggested by Van Slooten and Nieuwenhuys [11]. Moreover, according to a low surface Rh atoms on Pt-Rh/R, the most probable situation is a vacant nearest neighbour Pt site for the dissociation of adsorbed NO. On the other hand competitive adsorptions of the reactants can only fit the kinetic data obtained on Pt-Rh/D. This kinetic behaviour is explained by an active surface covered by Rh atoms as suggested the bigh adsorption equilibrium constant of NO. The decrease in the specific activity of Pt-Rh/D compared to that of Rh/A1203 can be related to the lower value of the specific rate constant for the dissociation of NO. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
M. Shelef, G.W. Graham, Catal. Rev. Sci. Eng., 36 (1994) 433. K.C. Taylor, Catal. Rev. Sci. Eng., 35 (1993) 144. T.W. Roots, L.D. Schmidt, and G. B. Fisher, Surf. Sci., 134 (1983) 30. R.J. Gorte, L.D. Schmidt and J.L. Gland, Surf. Sci., 109 (1981) 367. D.T. Wickham, B.A. Baure and B.E. Koel, Surf. Sci., 243 (1991) 83. P. Mar6cot, A. Fackche, L. Pirault, C. G6ron, G. Mabilon, M. Prigent, J. Barbier, Appl. Catal. B : Environmental, 5 (1994) 43. S. Kim and M.J. D'Aniello Jr., Appl. Catal., 56 (1989) 23. B.R. Powell, Y.L. Chen, Appl. Catal., 53 (1989) 233. J.O. Main and J.O. Bovin, Microsc. Microanal. Microstruct., 1 (1990) 387. P. Granger, C. Dathy, J.J. Lecomte, M. Prigent, G. Mabilon, L. Leclercq and G. Leclercq, submitted to J. Catal. R.F. Van Slooten and B.E. Nieuwenhuys, J. Catal., 122 (1990) 429. D.N. Belton and S.J. Schmieg, J. Catal., 144 (1993) 9. D. Lorimer and A.T. Bell, J. Catal., 59 (1979) 223. K.Y. Simon Ng, D.N. Belton, S.J. Sclunieg and G.B. Fisher, J. Catal., 146 (1994) 394. P. Granger, J.J. Lecomte, L. Leclercq, G. Mabilon, M. Prigent mad G. Leclercq, in preparation. R. Dictor, J. Catal., 109 (1988) 89.
Model Systems I Studies
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CATALYSIS AND AUTOMOTIVEPOLLUTIONCONTROLIV Studies in Surface Science and Catalysis, Vol. 116 N. Kruse, A. Frennet and J.-M Bastin (Eds.) 91998 Elsevier Science B.V. All rights reserved.
431
CO Oxidation on Pd(llO) Michael Bowker, Isabel Z. Jones, Roger A. Bennett and Stephen Poulston. Reading Catalysis Research Centre, Department of Chemistry, University of Reading, Whiteknights Park, Reading, RG6 6AD, UK. Palladium offers some scope as an alternative precious metal for use in catalytic removal of pollutants from automobile exhaust gases. An understanding of the operation of this metal depends partly on developing a fundamental knowledge of the surface properties for adsorption and reaction. We have used surface science methods to investigate the adsorption and reaction of CO and 02 on Pd(110) using a molecular beam reactor, temperature programmed desorption and scanning tunnelling microscopy (STM). Both 02 and CO stick efficiently on the surface (S02 = 0.42 + 0.02, SCO = 0.52 + 0.02, at room temperature) and react with high probabilities at temperatures as low as 310 K, providing that the CO coverage is low; at high CO pre-coverages the surface reaction is poisoned. There appears to be an initial CO coverage (0.64 + 0.1 monolayers) above which the 02 sticking on the surface is drastically reduced. Experiments with a mixed CO and 02 beam, run with a step wise increase in temperature, show low steady state rate in CO2 production at low temperature as a result of CO poisoning the surface. The rate goes through a maximum at 396 K and then diminishes at high temperatures due to a low equilibrium coverage of CO. Transient 'Lightoff' is observed at 396 K due to self acceleration of the reaction when the CO begins to desorb. STM images of the oxygen c(2x4) structure have been obtained at 520 K. The surface is grossly reconstructed and roughened by oxygen and there is considerable inhomogeneity in the overlayer structure. Both features may be linked with subsurface oxygen formation. CO clean off of the oxygen at 300 K leaves a (1 x2) reconstructed Pd surface.
1. INTRODUCTION The CO oxidation reaction has been studied because of its importance in automobile exhaust gas catalysis. These three way catalysts remove pollutants (CO, NOx and hydrocarbons) from the exhaust gas; CO is converted to CO2 by reaction with 02. Rh and Pt are commonly used in such catalysts. However, Pd is continually being developed for use in new three way catalysts which meet more stringent legislative requirements [1 ]. Although Pd is less efficient at NOx reduction and more susceptible to sulphur and lead poisoning than Rh and Pt [2], Pd has good low-temperature performance and high-temperature resistance. It is also of interest due to low cost and high availability [3]. Surface science techniques have been used to obtain vital information about reactions at a molecular scale [4]. We have employed a molecular beam reactor and STM to obtain kinetic and topographic data. The reaction is carried out on an atomically clean Pd (110) single crystal, under ultra high vacuum (UHV) conditions. Thus when 02 and / or CO are
432 -7
introduced in the pressure region of 10 mbar they adsorb on clean Pd and under these welldefined conditions accurate kinetic information can be obtained. The data can be related to catalytic studies under atmospheric conditions and are important for modelling catalytic performance. The CO oxidation reaction has been extensively studied on Pd(ll0) using other techniques, for examples see references [5, 6, 7 and 8 ]. Many studies concentrate on CO oxidation oscillatory behaviour at pressures above 1 x 10-3 mbar [9, 10 and 11]. He et al. have investigated the interaction of oxygen with Pd(110) and their results indicate three adsorption states one of which is weakly held molecular oxygen, desorbing at 100 K [12]. There are further two states due to recombination of atomic oxygen desorbing at around 700 and 780 K, the one which desorbs at the lower temperature is believed to be associated with subsurface oxygen. The formation of subsurface oxygen is achieved at coverages greater than 0.5 ML. He et al. have shown that CO adsorbs in 5 different states [13], the sharp desorption peak (labelled a3) is associated with a phase transition (2xl) A2 (4x2) which involves Pd-atom displacements. We have studied the temperature dependence of sticking for CO and 02 on Pd(110). Transient CO oxidation has been investigated by means of sequential dosing in the molecular beam: CO has been dosed onto an oxygen covered surface at different sample temperatures, and 02 has been dosed onto a CO pre-dosed surface with varying CO precoverages. Mixed CO and 02 dosing onto the clean sample at 310 K has been carried out with a CO:O2 ratio of 1:1. The temperature was subsequently increased in a step wise fashion. The reaction has also been followed by imaging the surface using STM. The surface was imaged before and after an 02 covered surface was dosed with CO.
2. EXPERIMENTAL
The molecular beam system has been described elsewhere [ 14]. In brief, it consists of a main analysis chamber which is equipped with facilities for argon ion sputtering, low energy electron diffraction (LEED), Auger electron spectroscopy and a V. G. sensorlab quadrupole mass spectrometer. The chamber is under UHV at a base pressure 2 x 10-10 mbar. The molecular beam is attached to the analysis chamber via a gate valve. It is able to produce a beam of gaseous molecules via two collimator and pumping stages, with an in beam pressure of approximately 1.6 x 10-7 mbar when 20 mbar is used in the source. The beam has a diameter of 2.9 mm at the sample and has a flux of around 5 x 1013 molecules cm -2 s-1 . The Pd(110) sample was mounted on tungsten wire and could be heated up to 1000 K. The sample was cleaned with cycles of argon sputtering, annealing and heating in oxygen at 830 K. The presence of the a3 peak at 338 K in the CO desorption spectrum, and Auger surface analysis were used to test for surface contaminants. Exposures of dosing gas were measured in Langmuirs (L)where 1 L = 1.33 x 10-6 mbar.s. The surface was imaged using a variable temperature Oxford Instruments STM and details of this instrument can be found elsewhere [ 15]. The STM is contained within a UHV chamber which is equipped with similar facilities as described for the molecular beam system. Up to 550 K the thermal drift is sufficiently small such that the same area can be scanned
433 many times; each scan takes around 40 s. The STM tip is made from 0.2 mm diameter wire consisting of 97% W and 3 % Re.
3. R E S U L T S A N D D I S C U S S I O N
3.1 CO and 02 sticking Figure 1 shows the sticking probability dependence upon coverage for 02 and CO adsorbing on the clean surface at two different surface temperatures. The initial sticking probability, at 315 K, for 02 and CO was 0.42 + 0.02 and 0.52 + 0.02 respectively. The sticking of CO was more severely affected by temperature than 02; at 472 K the initial sticking for CO was ca. 0.29 • 0.02 and dropped rapidly with coverage as the adsorption / desorption equilibrium was established whereas oxygen sticking remained similar in profile to the lower temperature result. CO has the lower desorption temperature of the two; 460 K compared to 780 K for 02. Thus at 472 K adsorption of CO competes with desorption, under these low pressure conditions, resulting in less net adsorption. Higher temperatures are required before 02 sticking is affected.
. -- ~
]~ CO at 315K I ~ CO at 472K I ---Q~ 02 at 315K
oI-4
r
0.1
0
0.1
0.2 0.3 0.4 0.5 0.6 Coverage / Monolayers
0.7
0.8
Figure 1" The sticking probability of CO (triangles, filled and open at a sample temperature of 315 and 472 K respectively) and 02 (circles, filled and open at 315 and 450 K respectively) as a function of coverage, in monolayers.
3.2 Transient CO oxidation Figure 2a shows the rate of CO2 (m/e - 44) formed when CO was dosed onto an oxygen pre-covered surface. The oxygen was dosed at 310 K to a coverage of 0.5 ML, the oxygen gas was pumped away before the sample temperature was increased to the appropriate value (which is indicated on the figure) and CO introduced. CO2 was produced at 310 K and the
434
reaction rate maximum was immediately reached, this was followed by a slow decrease and the reaction was nearly complete after 1 minute. Increasing the temperature to 460 K increased the average rate and time taken for the maximum to be reached. At 673 K the initial reaction rate was similar, however less overall CO2 was produced. CO sticks and reacts on an oxygen saturated surface with high efficiency and CO2 was produced even at 310 K. CO must adsorb onto the oxygen pre-adsorbed surface in order to react with the dissociated oxygen to form CO2. This type of kinetics is known as LangmuirHinshelwood and can be represented as: RCO 2 = k[CO(a)l[O(a)] where k is the rate constant for the surface reaction. At the lower temperature the rate was relatively slow as the reaction between CO(a) and O(a) to form CO2 was rate limiting, thus the value of k was low. At 460 K, the rate was greater due to the increase in temperature and thus the value of k increased. At 670 K the initial rate was high, again due to the higher value of k. However, less net CO2 was produced, a result of oxygen starting to desorb between 02 and CO dosing when the temperature was increased; temperature programmed desorption (TPD) measurements show that oxygen starts to desorb below this temperature.
/
/5
I
"
46~
II
o
rr
'
'
"'~
'1
~
"
'
'"
2'0
,
9
,
,
3b
,
,
Time / s
w
,
40 I
'
'
'
'
5b .... 60
Figure 2a: The rate of formation of CO2, in arbitrary units (a.u.), as a function of time for transient CO oxidation; 02 was dosed to a coverage of 0.5 ML at 310 K and CO was dosed at various sample temperatures. Figure 2b follows the reaction rate of CO2 production when 02 was beamed onto a CO predosed surface. The reaction was carried out at 310 K and the surface was pre-dosed with different amounts of CO which are shown on the figure, in monolayers. The data show that the reaction was extremely dependent on the pre-coverage of CO. At 0.10 ML CO the reaction rate was high and the rate maximum was immediately reached before a relatively rapid decrease, a result of 02 cleaning off all the adsorbed CO. At 0.46 ML the initial reaction rate was low and gradually increased until 190 s had passed, after which the rate decreased. A CO pre-coverage of 0.64 ML yielded no CO2; 02 was unable to adsorb, dissociate and react,
435 thus CO completely blocks the surface at this coverage. At 0.46 ML CO a less severe blocking effect was observed which was evident from the slow initial reaction rate.
i ~. 0.1MLCO / I/~ ..... ~ MLCO I ;. II "k . ,...',v',~':';..'J.'~ I I
~
II II
.~,'~ .,'"'~
0
40
-"11o tY
r' ~'~'
~ ~'
' \, ,v: ' ',,
80
120 160 Time / s
200
240
280
Figure 2b: The rate of formation of CO2 as a function of time for transient CO oxidation at 31 OK; CO was dosed to a coverage of 0.1 ML and 0.46 ML which is followed by 02 dosing It is interesting to compare the two combinations of sequential dosing: pre-adsorbed oxygen has no blocking effect on CO adsorption unlike the effect of CO pre-adsorption on oxygen. This observation can be explained in terms of the site requirements for the adsorption of the second gas. Molecular adsorption of CO requires only one vacant site. However oxygen adsorbs dissociatively, thus two adjacent sites are required for 02 adsorption which indicates that there becomes a critical coverage of CO (0.64 ML from our data) after which the sticking on to the surface is drastically reduced.
3.3 Mixed CO and 02 Dosing Experiments A mixed beam was introduced onto clean Pd(110) with a CO:O2 ratio of 1:1, at room temperature. The mixture was continually dosed while the temperature was increased stepwise. Figure 3 shows the rate of CO2 (m/e = 44) production with the changes in temperature against time; this is a pseudo-steady state experiment with steady state being approached before a temperature jump. The mixed dose impinges onto the clean surface at point A. There was immediate production of CO2. The rate went through a maximum, then subsequently decreased and remained at essentially a zero rate until around 372 K. There was then a sudden increase in the rate and 'light-off occured at 396 K, point C. This is a transient ignition phenomenon and the rate goes through a sharp maximum in the heating region, then decreases to a lower steady state level. Further temperature increases decreased the rate until the reaction rate was very low at ca. 640 K. CO2 was produced on introduction of the mixture as the surface was clean and both CO and 02 can adsorb and react. However the rate rapidly dropped as the surface became dominated by CO, such that it poisoned the surface. This was confirmed by stopping the dosing at point B, carrying out a TPD experiment and only CO was desorbed from the surface.
436
A surge in CO2 production occured at 372 K. This was at a temperature near the beginning of CO desorption. Thus more CO desorbs than is possible to adsorb and this change in equilibrium results in sites becoming available for 02 adsorption. The reaction becomes selfaccelerating as oxygen creates more sites for itself by removing further CO. The subsequent slow decrease in rate is a result of there being a low coverage of CO, again this was confirmed by TPD as only oxygen desorbed. The rate remained relatively high however, compared to the low temperature region, due to CO being able to reactively stick on an oxygen covered surface.
650 CO 2 production -.
C
- 600
I
O
rr
-
550
-
500
3
"o
I11
A Temperature
- 450
"-,
- 400
~
)
- 350 ....
I .... 10
I .... 20
I .... 30
I,
,
300
40
Time / mins
Figure 3" The rate of CO2 production and changes in temperature are shown as a function of time. A mixed CO and 02 beam was dosed onto clean Pd(110) at 310 K and the temperature was increased stepwise up to 640 K 3.4 STM Figure 4 is a 300 x 300 A image of an oxygen covered surface taken at 520 K. LEED analysis, just before the sample was placed in the STM, showed that the structure was c(2x4). A constant background pressure of 5 x 10-9 mbar of 02 was used throughout scanning to maintain the oxygen c(2x4) structure otherwise background gases in the chamber, mainly H2, reactively remove the oxygen. Upon 02 adsorption, Pd(110) reconstructs to a (lx2) missing row structure, the missing rows are the darker vertical rows in-between the lighter ones. Figure 6 represents the surface model of this transition from the (lxl) to the (lx2) missing row reconstruction. In addition mesoscopic islands are formed, i.e. the surface is highly stepped and the terraces are small, typically less than 100 A and this is characteristic of the whole imaged area. Niehus et al. have observed this phenomenon at room temperature [16] and our results show that the reconstruction is stable up to 520 K. In fact there was surprisingly little oxygen and Pd diffusion despite the high temperature. Missing row reconstructions of oxygen on Rh(110) have been observed, the mesoscopic islands were formed [17]. The surface is particularly heterogeneous in the bottom left hand comer and a high number of defects were observed in various parts of the image. These defects could be linked to the mesoscopic island formation or the formation of subsurface oxygen.
437
F i g u r e 5" A n S T M i m a g e o f C O d o s i n g o n t o a n o x y g e n pre-covered c(2x4) surface at 300 K leaving (lx2) reconstructed P d ( 1 1 0 ) . S a m p l e b i a s = + 2 V, t u n n e l c u r r e n t = 1 nA a n d s c a n t i m e = 2 8 s.
438 Figure 5 shows an STM image taken at 310 K following CO dosing (310 K, 5 Langmuirs) onto the oxygen c(2x4) surface. The separation of the bright rows in the [001] direction is equivalent to twice the inter-row spacing of Pd(ll0) indicating that the (lx2) missing row reconstruction is maintained together with the mesoscopic island formation. Tanaka et al. have shown that D2 reactively removes oxygen, again leaving the heavily reconstructed surface [18]. A change in the LEED pattern from c(2x4) to (lx2) indicates that no oxygen remains on the surface, though we believe the surface is CO covered as molecular beam studies indicate that CO cleans off the oxygen and when all the oxygen has been removed the surface is CO covered. Following CO clean off at 520 K the (lx2) reconstruction was not observed though the mesoscopic islanding was still present. This suggests that at 300K it is the presence of CO on the surface that helps maintain the (lx2) reconstruction. More detailed studies of this system using STM are continuing.
Figure 6: Structural model of Pd(110) showing the reconstruction from (lxl) to (lx2) missing row. The top layer is represented as the darker filled circles, the second layer as the lighter dotted circles and the third layer as the open circles.
4. C O N C L U S I O N S We have measured the efficiency of CO and oxygen adsorption on Pd(110), and of their reaction to produce CO2. Both molecules stick with high probability, though the adsorption of CO (in contrast to oxygen) is strongly surface temperature dependent due to the low desorption temperature of molecular CO. In sequential dosing experiments it is clear that oxygen coverage does not poison CO sticking, whereas CO coverage above a certain level completely blocks oxygen sticking. In these transient types of experiment CO2 is produced from the surface even at 310 K. In a steady state situation CO2 production is negligible at this temperature due to site blockage by CO and CO2 is only produced at measurable rates at elevated temperatures. We observe a transient 'light-off at 396 K (exact temperature is strongly dependent on gas ratio and total pressure) due to autocatalytic self acceleration as the CO begins to desorb at a significant rate, allowing oxygen onto the surface. The steady state rate goes through a maximum then diminishes again at high temperatures due a to low CO coverage. STM reveals gross surface reconstruction induced by oxygen, which is not removed by the CO clean-off experiment at room temperature, but is annealed out in high temperature experiments. These structural changes have a significant effect on the rate of reaction. These data are important for elucidation of the detailed kinetics of the reaction process on real auto catalysts. We intend to establish the effect of different surface structures and
439 surface promoters on the reaction on well defined surfaces and then extend this to the study of real supported materials. At the same time we will develop the model for this reaction which should extend from the low pressure environment to the high pressure one. It is clear that many processes are involved in this, including surface reconstructions and atom mobility, though the rate determining step under any set of particular set of condition may be a simple one.
ACKNOWLEDGEMENTS We would like to express thanks to EPSRC, Johnson Matthey plc, The IRC in Surface Science at The University of Liverpool and The University of Reading for their financial support of this work.
REFERENCES
~
8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.
Platinum Metals Rev., 39 (1995) 73. Platinum Metals Rev., 38 (1994) 173. H. Muraki, H. Sobukawa, M. Kimura and A. Isogai. Toyota Central R. & D. Labs., Inc. 900610. G. A. Somorjai. Introduction to Surface Chemistry and Catalysis. (John Wiley & Sons, New York, 1994). S. Ladas, R. Imbihl and G. Ertl. Surf Sci., 280 (1993) 14. J. Goschnick, M. Grunze, J. Loboda-Cackovic and J. H. Block. Surf. Sci., 189 (1987) 137. T. Engel and G. Ertl., Advances in Catalysis, 28 (1979) 1. K. Fukui, H. Miyauchi and Y. Iwasawa., J. Phys. Chem., 100 (1996) 18795. R. Imbihl and G. Ertl., Chem. Rev., 95 (1995) 697. T. Yamamoto, H. Kasai and A. Okiji., J. Phys. Soc. Japan., 60 (1991) 982. M. R. Bassett and R. Imbihl., J. Chem. Phys., 93 (1990) 811. J.-W. He, U. Memmert, K. Griffiths and P. R. Norton., J. Chem. Phys., 90 (1989) 5O82. J.-W. He and P. R. Norton., J. Chem. Phys., 89 (1988) 1170. M. Bowker P. D. A. Pudney and C. J. Barnes., J. Vac. Sci. Technol., A 8 (1990) 816. L. Kuipers, R. W. M. Loos, H. Neerings, J. ter Horst, G. J. Ruwiei, A. P. de Jongh, and J. W. M. Frenken., Rev. Sci. Instrum., 66 (1995) 4557. H. Niehus and C. Achete. Surf. Sci., 369 (1996) 9. F. M. Leibsle, P. W. Murray, S. M. Francis, G. Thornton and M. Bowker., Nature, 363 (1993) 706. H. Tanaka, J. Yoshinobu and M. Kawai., Surf. Sci., 327 (1995) L505.
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CATALYSISAND AUTOMOTIVEPOLLUTIONCONTROLIV Studies in Surface Science and Catalysis,Vol. 116 N. Kruse, A. Frennet and J.-M Bastin (Eds.) 91998Elsevier Science B.V. All rights reserved.
441
I n - s i t u E S R of Rh/y-AI203 and R h / Z S M - 5
S. G. Lakeev*, A.V. Kucherov** and M. Shelef Ford Research Laboratory, Ford Motor Company, MD3179/SRL, POBox 2053, Dearbom, MI 48121 USA Permanent Addresses:*Karpov Institute of Physical Chemistry, Moscow, Russia; **Zelinsky Institute of Organic Chemistry, Moscow, Russia Keywords: ESR, Rh/AI203, Rh/ZSM-5, Rh 2+ stabilization
1. INTRODUCTION Rhodium supported on 7-A1203 is an important component of 3-way automotive catalysts and has been studied by a wide variety of methods [1-5] including ESR. In the last 15 years Rh-species introduced into zeolites of different types (Y, X, L, A, SAPO) have also been examined by several techniques [6-9]. However, most of these methods were applied after the specimens were removed from actual reaction conditions and transferred into the respective characterization instruments and the state or behavior of the catalyst in-situ was arrived at indirectly by inference. Also the deactivation processes or the effect of modifiers is seldom, if ever, determined by direct in-situ observations. We have previously devised a method for high-temperature measurement of ESR-active ions under flow conditions and applied it to characterize specimens containing Cu 2+ [ 10] or Cr5+ [ 11]. We have extended this method now to specimens containing Rh2+. Here, we summarize the results of a study of the interaction of R.h/y-A1203 and Rh/ZSM-5 with different gases and gas mixtures (NO, NO2, CO, propene, 02, H20) at 120-573 ~ The amount of Rh 2+ present in the samples is evaluated quantitatively. The effect of copper and lanthanide addition on the stabilization of Rh 2+ by the zeolitic matrix was also investigated.
2. EXPERIMENTAL
2.1. Sample preparation Two Rh/ZSM-5 samples, with 1.0 and 0.25 wt% Rh, were prepared by incipient wetness impregnation of (NH4)ZSM-5 (Si/A1- 25) by water solutions of [Rh(NHa)6](NO3)3 (0.8 cm3 per 1 g of dried zeolite). A different 0.25%Rh/ZSM-5 sample was also prepared by the same method starting from HZSM-5 with a Si/A1 ratio of 40. A Rh/y-A1203 catalyst with 2.2 wt% Rh was prepared by impregnation of y-A1203 precalcined at 600~ and having a surface area of 130 mE/g by a solution of the same complex. The samples were pressed without binder, crushed into 0.1-0.2 mm particles and placed in a quartz cell for ESR measurements. Dried samples were precalcined at 500-800~ in an [He+l 0%02] stream for 2-5 h.
442 The Gd/ZSM-5 sample, with 4.0 wt% Gd, was prepared by two-fold incipient wetness impregnation of (NH4)ZSM-5 (Si/A1 = 25) by a water solution of Gd nitrate, dried at 130~ and calcined in an air stream at 500-800~ For the study of the effect of a second cation o n R h 2+ distribution in the ZSM-5 matrix, copper (1.5 wt%) and gadolinium (up to 4 wt%) were introduced into the catalyst by incipient wetness impregnation of precalcined l%Rh/ZSM-5 using a solution of Cu(NO3)2 or Gd(NO3)3. Water solutions used for the calibration of ESR signals by cupric ions (0.0315-2.0 wt% Cu) were prepared by dissolving analytical grade CuSO4.6H20 in twice distilled water. 2.2. ESR
measurements
The in-situ ESR spectra of paramagnetic Rh-species, at 120-573~ were taken in the X-band (3.4 GHz) on a Bruker ESP300 spectrometer, equipped with either a high temperature cavity ER 4111 HT-VT or a low temperature cavity st8410.91 and a co-axial quartz gas flow cell [10]. In some experiments the cell, being connected to a gas flow system via long capillaries, was taken out from the low-temperature spectrometer resonator, placed in a high-temperature furnace, calcined in a gas stream, placed back in the resonator, cooled in-situ under gas flow and the ESR spectra were taken at 120~ The Bruker ESP300E software and the special Bruker program WIN-EPR (version 901201) were used for data treatment. The Origin 3.5 program for Windows was used for the treatment (baseline correction, double integration, and deconvolution) of the recorded spectra (resolution 4096 points). The ESR signals were registered in the field region of 200-4200 G. Resonances for various levels of microwave power were recorded to verify the lack of sample saturation. The gas flow was regulated by a 4-channel readout mass flow controller (Model 247C, MKS Instruments). This system permitted to change the composition of the gas mixture and to regulate the flow from 1.5 to 18 cm3/min. Pure helium (99.999%) and the mixtures [ 10%O2+He], [0.4%NO+He], [0.4%propene+He], [3%CO+He], and [5%H2+He] were used for in-situ sample treatment. Frozen water solutions of CuSO4 (2.0, 1.0, 0.5, 0.25, 0.125, 0.063, and 0.0315 wt% of Cu) were used for the absolute calibration of Cu 2+ ESR signals. Spectra were taken for weighed samples of approximately equal volume placed in quartz ampoules (sample height 20 mm; 80 - 100 mg of solution). To record the ESR spectra, the sample-containing ampoule was placed in the cavity and cooled to 120~ by flowing nitrogen through a Dewar filled with liquid nitrogen. 3. RESULTS AND DISCUSSION 3.1. Rh/7-AI203 In oxidized Rh/7 -A1203 weak ESR signals from paramagnetic Rh-species are observed even at 120~ (Fig. 1a). Sorption of either pure water or of concentrated NH4OH solution at 293 ~ results in a sharp increase of the ESR signal intensity (Fig. lb). This interaction cannot affect either the nuclearity of the paramagnetic Rh aggregates or the Rh-support interaction. It provides evidence of a rather weak coupling between ESR-visible surface R h 2+ ions which constitute a small fraction of the total Rh (vide infra). The adsorption of the polar H20
443 molecules weakens the dipole-dipole interaction between the paramagnetic Rh-species on the imperfect surface of the support. Most likely the Rh-H20 (-NHaOH) bonding successfuly competes with that of Rh-Rh bonding. In the absence of water the surface [Rh3§ ] species have no ESR signal owing to the large spin coupling between these species. Adsorption of CO, at 293~ on Rh/? -A1203 produces a strong ESR signal from the adduct formed (Fig. l c). It confirms once more a rather weak interaction between the surface Rh 2§ ions accessible to gas phase molecules. Heating of the sample in [He+CO] flow at 200~ is accompanied by a gradual irreversible loss of the ESR signal intensity due to the reduction of the Rh 2§ surface species. Adsorption of NO on Rh/y-A1203 produces an ESR signal at 120~ only. Adsorption of propene and CO at 120~ is accompanied by appearance of ESR signals (Figs. 1c and 2b) due to low-temperature formation of paramagnetic adducts with similar symmetries. However, heating of the sample with propene to 293~ results in irreversible disappearance of the ESR signal due to the loss of the paramagnetic adduct by the reduction of the surface Rh(II). The problem of the ESR data interpretation for Rh/alumina catalysts is complicated by the fact that only a small fraction of the supported metal (<5%) contributes to the ESR signal. Spin concentration of the paramagnetic species in Rh/A1203 is a small percentage of the total supported rhodium. It is plausible that this relative fraction may be higher at lower Rh loading.
I,
Ik
b.
_,- ~ _
. . . . . . . . .
,,,,!
-%
,,
~
'
~
.
I:.
1000
2t~
30~
0
G
Fig. 1 Effect of H20 and CO sorption at 293~ on ESR signal taken at 120 ~ from 2.2% Rh/?-A1203: (a)-precalcined in air at 700~ (b)-after water adsorption (c)-after CO adsorption. [g!• g2•
Fig. 2. Effect of C3H6sorption in-situ at 120~ on ESR signal from 2.2% Rh/7-A1203 calcined at 700~ (a)-in [He+O2] flow; (b)-15 min after switch to [He+C3H6] c)-after 15 h at 293~
[g•
444
3.2. Rh/ZSM-5 The matrix of low-loaded high-silica ZSM-5 can stabilize a high fraction of isolated paramagnetic Rh-species as these are exchanged at the protonic sites [12]. Hence, Rh/ZSM-5 samples may serve as models for the ESR study of high-temperature interaction between Rh active sites and different molecules. This requires quantitative evaluation of Rh 2§ First let us consider the Rh/ZSM-5 interaction with oxygen and water since it is important in Rh 2+ quantitation. 3.2.1. Interaction of Rh 2+ in Rh/ZSM-5 with 02 and H20 Among the various Rh oxidation states only Rh(0) (d9), Rh(II) (d 7) and Rh(IV) (d 5) are paramagnetic with S = 1/2. Fresh Rh/ZSM-5 samples, containing ammino-complexes with diamagnetic, low-spin Rh (III) (d6), show no ESR signal. Oxidative calcination, at 300-800~ is accompanied by appearance of an ESR signal from paramagnetic Rh-species, and spin concentration of Rh-species reaches a maximum value at Tcalr = 700~ In our case all paramagnetic species generated by oxygen activation are probably Rh(II); the occurence of neutral atoms, as well as stabilization of non-charged Rh(0) in cationic positions, is highly improbable. The existence of Rh(IV) may also be excluded because the ESR signals assigned to Rh(IV) are different from ours. Both the signal intensity and form depend strongly on the presence of oxygen (Fig. 3). Sample evacuation at 293~ results in transformation of the signal 3a into 3b. The principal gvalues of spectrum 3b (gl = 2.037; g2 = 2.005; ga = 1.985) correspond to Rh 2+ species located in orthorhombic symmetry sites [13,14]. A rather weak interaction with 02 molecules gives rise to a more intense ESR signal, with gl = 2.016 and g2 = 1.935, from a paramagnetic [Rh2++O2]-adduct where the unpaired electron is partially located on 02 (Figs. 3a, 4b). In turn, saturation of the samples with water results in a sharp change in the signal shape (Fig. 4a, gll = 2.075; ~ = 2.009) and an additional two-fold increase in the signal intensity (Fig. 4). Thus, hydrated paramagnetic [Rh2++O2+H20]-species are formed, and the signal intensity reaches a maximum value. The intensity rise due to hydration can be attributed not to a change in the valence state of some part of the rhodium but to a change in the spin-lattice interaction and relaxation conditions of the ions. A dehydrated evacuated sample contains Rh 2+ ions positioned near channel walls in coordinatively unsaturated environments, i.e., the cations are in part covalently bonded to the zeolitic lattice. Oxygen bonding and further hydration, with coordination of up to four water molecules, weaken the interaction of cations with the zeolitic lattice. Redistribution of electronic density in the formed paramagnetic species also takes place. Therefore, the change in the Rh 2+ ESR signal intensity (Figs. 3 and 4) can be attributed to a change in the local configuration of the Rh 2+ site.
445
IQ
BWW
bQ
Fig. 3. ESR spectra, at 120~ of Rh/ZSM-5 calcined at 600~ (a)-[Rh2++O2]; (b)-after evacuation at 293 K
~
__-
n
Fig. 4. ESR spectra, at 120~ of Rh/ZSM-5 calcined at 700~ (a)-[Rh2++O2+H20];
(b)-[Rh2++O2].
3.2.2. ESR quantitation of Rh 2§ in Rh/ZSM-5 In ESR spectroscopy the intensity of the signal is usually expressed in arbitrary units. For absolute calibration of the signal it is necessary to compare the double integral (DI/N) of the ESR spectrum of the sample with a known standard measured under identical conditions. In the case of Rh 2§ ion no appropriate water-soluble salt exist for quantitation by direct comparison. As a surrogate, frozen diluted solutions of inorganic copper salts may be used as a reference, because in the solid state these salts form crystallohydrates with magnetically separated cupric ions, and this separation by hydrate shells is even more effective in diluted water solutions. The Bohr magneton number for Cu2§ in diluted water solutions of sulfate, nitrate or chloride is close to its theoretical value, 1.73, which confirms the effective magnetic separation of the ions. Freezing of the solutions produces a solid specimen with isolated Cu2§ ions simulating quite well the CuZSM-5 zeolite saturated with H20. Itwas demonstrated earlier, that for these water-saturated zeolitic samples calibration with frozen water solution is correct and gives quantitative results [ 15,16].
446
~ - O.15RhTZSM-t; - 1RhtZSlVl-5.
10
0.1
1
Nmuhn' o/copper or ~ m
I0
lan h the pra~ x 10tl
Fig. 5. Quantitation, at 120~ of"ESR-visible" [Rhe++O2+H20] complexes in Rh/ZSM-5, precalcined at 700~ by calibration with CuSO4 solutions. Figure 5 gives, in logarithmic coordinates, the relationship between the DI/N values of the ESR spectra and the copper concentration in the water solutions. On a linear plot this dependence passes through the origin and confirms the absence of Cu2+ ion aggregation in reference solutions up to 2.0 wt% Cu. Hence, a direct calibration of the ESR signal from the RhZSM-5 sample saturated with water, such as in Fig. 4a, is warranted. The estimate of the ESR-visible rhodium concentration derived from the data (Fig. 5) gives a value of 0.15 wt% for I%Rh/ZSM-5 and 0.12 wt.% ofparamagnetic species for 0.25%RNZSM-5. Quantitation of Rh 2§ shows that the proportion of ESR-visible Rh 2§ approaches 50% for the most diluted 0.25%Rh/ZSM-5 sample. Conversely, as noted above, in the more concentrated sample [2.2%Rh%,-A1203 + H20] the proportion of ESR-visible Rh 2§ does not reach even 5%.
447 3.2.3. In-situ ESR of interaction between Rh2§ and different molecules In-situ monitoring of isolated paramagnetic Rh-species in a flow of gas mixtures allows to clarify the state of active sites in supported Rh-catalysts under more realistic, i.e. closer to the working state, conditions. Changes of the ESR signal accompanying the interaction of Rh/ZSM-5 with propene at 293-323~ are shown on Fig. 6. Switching the gas flow to [0.5%C3H6+He] results in an immediate signal change, 6a --~ 6b, which is stable at room temperature. Heating of the sample in the flow at 323~ for 10 min leads to a noticeable decrease in the signal intensity but after a backswitch to [He + 02] there is noted a partial restoration of the signal (6c --~ 6d). Therefore, propene adsorption in zeolitic channels at room temperature results in a change in the symmetry of paramagnetic Rh-species due to formation of an adduct without noticeable reduction of the Rh 2+. It is interesting to note. that further heating of the sample, with propene sorbed, in [He + 10% 02] flow at 473~ is accompanied by a steep decrease of the ESR signal (Fig. 6e). This maybe due either to reduction of the divalent Rh-ions by residual, strongly sorbed olefin in the zeolitic channels or perhaps by reoxidation to Rh 3+ which is thermodynamically stable in this temperature range. To clarify this we treated the sample in a [propene+O2] gas mixture at different temperatures. In-situ treatment of Rh/ZSM-5 at 473~ in oxidizing mixture [He+5%O2+0.2%C3H6] (more than a two-fold over-stoichiometric 02 excess) sharply lowers the ESR signal from pararnagnetic Rh-species. Increase of the temperature to 573~ leads to total disappearance of the ESR signal. Therefore, the reoxidation is relatively slow, and the state of the active sites in reaction conditions even in hydrocarbon-deficient mixture differs substantially from original Rh 2+ species positioned in cationic sites after high-temperature oxidative calcination of Rh/HZSM-5. Oxidative calcination of the sample at 600-700~ results in complete restoration of the original Rh 2§ ESR signal. The interaction of RNZSM-5 with paramagnetic NO and NO2 molecules, in [0.4%NO+He] and [0.2%NO2+5%O2+He] flows, respectively, at 293-473~ produces weak, broad ESR signals. Some information can be acquired by monitoring the effect of water on the sample before and after interaction of the catalyst with NO2. H20 sorption at 293~ after saturation of the sample by NO2 molecules does not produce an intense ESR signal shown on Fig. 4a, as distinct from the starting sample. Thus, a strong complex is formed as a result of NO2 bonding, quenching the formation of paramagnetic [Rh2§ Changes in the ESR signal accompanying the in situ RNZSM-5 interaction with CO at 293~ are shown on Fig. 7. Switch of the gas flow to [3%CO+He] results in change of the signal 7a to a complex spectrum 7b. Subsequent backswitch to pure He flow results in complete disappearance of one of the ESR lines (7b -> 7c). Therefore, CO adsorption at room temperature results in formation of three forms of adducts with CO, one of them being weak. Interaction of the two more strongly bonded adducts with 02 at 293~ does not lead to restoration of the starting ESR signal, i.e. full elimination of CO by 02 does not take place. Heating of RNZSM-5 in [He+CO] flow at 473~ is accompanied by a gradual irreversible disappearance of the ESR signal due to reduction of the Rh2§ 3.2.4. Effect of Cu 2§ ions o n R h 2+ in Rh/ZSM-5 after heat treatment Cupric ions introduced at the 1.5% level, by impregnation of precalcined 1%RNZSM-5 with Cu(NO3)2 solution, change markedly the coordination of isolated Rh 2§ ions in cationic positions inside the zeolite. Calcination of the bi-cationic sample in air at 550~ for 4-5 h results in complete disappearance of the ESR signal from Rh paramagnetic species. This is
448 accounted for by displacement of R h 2+ from the original positions in the ZSM-5. At the same time an intense ESR signal typical of isolated Cu 2+ ions, stabilized in cationic positions, appears. The spectra coincide very well with those obtained earlier [17-21] in CuZSM-5 samples and show that the bi-cationic sample contains isolated cupric ions located in two discrete coordinations (square-planar and square-pyramidal). Quantitation of the Cu 2+ by comparison with frozen solutions of copper sulfate (from Fig. 5), demonstrates that practically all the copper in calcined CuRhZSM-5 becomes ESR active. Therefore, Rh 2+ and Cu 2+ may be located at the same cationic sites and migration and redistribution processes proceed concurrently, similarly to other bi-cationic systems in ZSM-5 [12]. The Cu 2+ ion is bound much more strongly in the cationic positions than the Rh 2+ ion and the majority of the latter is replaced by the former as a result of a solid-state reaction. Also, the concentration and coordination of Cu 2+ cations, introduced into the Rh/ZSM-5 sample by a solid-state exchange, coincided completely with those of Cu 2+ in CuZSM-5 [ 17-20].
L
IL
bQ
k. CO
d.
eO
I
I
2000
2500
__
~
I
3000
3500
....
I
4000
IG] Fig. 6. In-situ ESR spectra at 293~ of Rh/ZSM-5; (a)-in [He+O2]; (b)-at~r switch to [He+C3H6] flow (gl = 2.001); (c)-after 10 min at 323~ in [He+C3H6]; (d)-backswiteh to [He+O2] flow (e)-after 30 min in [He+O2] at 473~
__
t
'
,,
I
I
tel
Fig. 7. In-situ ESR spectra at 120~ of Rh/ZSM-5 (a)-treated at 293~ in [He+O2]; (b)-after 40 min in [He+CO] flow at 293~ (c)-after evacuation at 293~ for 10 min; (d)-after ingress of dry air.
449 3.2.5. Effect of lanthanide ions o n R h 2+ in Rh/ZSM-5 Most lanthanide ions are ESR-silent but there are several that exhibit paramagnetism. One of the most useful for ESR study is Gd3+: (1) the signal from Gd3+ can be easily registered in a wide temperature range and (2) the form of the Gd3+ ESR signal depends on both the ion aggregation and coordination. For these reasons we chose this ion for our study. An impregnated and dried Gd/ZSM-5 sample gives an intense, broad ESR signal typical of coupled Gd3+ ions. Calcination of the sample at 500-600~ in air is accompanied by sharp changes in the ESR spectrum: the broad signal disappears completely and a complex spectrum with narrow lines is formed, pointing to the formation of isolated Gd3+ ions in coordinatively unsaturated environments. Subsequent introduction of Rh, by impregnation of Gd/ZSM-5 with a rhodium ammino-complex and by a repeated calcination, does not lead to a measurable change in the ESR spectnun of the isolated Gd3+ ions. At the same time, the signal typical of isolated R h 2+ species appears (same as in Figs. 3 and 4) being only-~2-fold less intense than the ESR signal of the calcined Rh/ZSM-5 sample. The Gd 3+ modifier introduced at a level of up to 4%, by impregnation of precalcined 1%Rh/ZSM-5 with Gd(NO3)3 solution, does not change the coordination of isolated Rh2+ ions in the cationic positions inside the zeolite, but reduces their quantity by a factor of 1.5 - 2. The complex ESR signal from isolated Gd3+ ions appears after calcination being superimposed on the signal from paramagnetic Rh-species. Thus, in both cases calcination of bi-cationic (Gd+Rh)/ZSM-5 samples in air at 550~ for 4-5 h results in appearance of the same superposition of the two ESR signals: one from cationic Rh2§ paramagnetic species and the other from isolated Gd 3+ ions stabilized by the ZSM-5 matrix. Hence, the solid state migration and redistribution of ions when both Rh 2§ and Gd3§ are present in the ZSM-5 differ from those typically taking place as when Rh 2§ and Cu2§ are present in the same matrix. This suggests that Gd 3§ ions are stabilized mainly in non-cationic positions and do not impair completely the bonding of paramagnetic Rh 2§ species in the cationic sites of ZSM-5. It makes this bi-cationic system attractive for further study of the stabilizing action of lanthanide additives on the catalytic activity of Rh/ZSM-5. Moreover, since there is still interest in the stabilization of the catalytic activity of zeolites containing cupric ions by additives of lanthanides [22], in-situ ESR investigations of specimens containing Cu 2§ and Gd 3+ may offer insight into the stabilization mechanism.
4. CONCLUSIONS (1) In a moderately loaded, 2.2%Rh/7-A1203, the vast majorty of the Rh, ca. 95%, is ESR inactive. The rest, a small proportion, can be made ESR-visible by room temperature sorption of water or other polar molecules. This indicates that the interaction of these ESR active ions with the support is weak. This is corroborated by the formation of ESR active CO adducts at 20~ on these Rh ions. Using ESR, no information is gained on the major part of the Rh in the specimen. (2) The quantitation of R h 2+ in ZSM-5 shows that the fraction of ESR-visible ions approaches 50% for the most dilute 0.25%Rh/ZSM-5 sample. This makes low-loaded Rh/ZSM-5 samples useful model catalysts for the ESR study of high-temperature interaction between Rh active sites and different molecules.
450 (3) Rh 2+ and Cu 2+ are located at the same cationic sites in ZSM-5. At high temperature these ions migrate and redistribute concurrently. Cu 2+ ion is bound much more strongly in the cationic position than the Rh 2+ ion and the majority of isolated paramagnetic Rh 2+ species is replaced by Cu 2+ by solid-state reaction. (4) Rh 2+ and Gd 3+ migration and redistribution in ZSM-5 differ from those occurring in samples containing Rh 2+ and Cu 2+. It appears that Gd 3+ ions occupy mainly non-cationic positions and do not impede the bonding of paramagnetic Rh 2+ in the cationic sites of ZSM-5. ACKNOWLEDGMENTS
We thank John Gerlock and H.-W. Jen for help with the ESR measurements.
REFERENCES 1. 2. 3. 4. 5.
M. Shelef and G. W. Graham, Cat. Rev.-Sci. Eng., 36 (1994) 433. C. Wong and R.W. McCabe, J. Catal., 119 (1989) 47. H.C. Yao, S. Japar, and M. Shelef, J. Catal., 50 (1977) 407. H.C. Yao and M. Shelef, Stud. Surf. Sci. Catal., (1981) 329. A. Gervasini, F. Morazzoni, D. Strumolo, F. Pinna, G. Strukul and L. Zanderighi, JChem. Soc., Faraday Trans. 1, 82 (1986) 1795. 6. A. Sayari, J.R. Morton and K.F. Preston, J. Chem. Soc., Faraday Trans.1, 84 (1988) 413; 93 (1989) 2093. 7. J.S. Bass and L. Kevan, J. Phys. Chem., 94 (1990) 1483; 4640. 8. D. Goldfarb and L. Kevan, J. Phys. Chem., 90 (1986) 264; 90 (1986) 5787. 9. V.D. Atanasova, V.A. Shvets and V.B. Kazanskii, React. Kinet. Catal. Lett., 9 (1978)349. 10. A.V. Kucherov, J.L.Gerlock, H.-W. Jen and M. Shelef, J. Phys. Chem., 98 (1994) 4892. 11. A. V. Kucherov, C. P. Hubbard and M. Shelef, Catal. Lett., 33 (1995) 91. 12. A.V. Kucherov, and A.A. Slinkin, J. Molec. Catal., 90 (1994) 323. 13. A. Sayari, J.R. Morton, and K.F. Preston, J. Phys. Chem., 91 (1987) 899; 93 (1989) 2093. 14. D. Goldfarb, L. Kevan, M.E.Davis, C. Saldarriaga, and J.A. Rossin, J. Phys. Chem,91 (1987) 6389. 15. S.C. Larsen, A.Aylor, A.T. Bell and J.A. Reimer, J. Phys. Chem., 98 (1994) 11533. 16. A.V. Kucherov, J.L. Gerlock, H.-W. Jen and M. Shelef, Zeolites, 15 (1995) 9; 15. 17. A.V. Kucherov, A.A. Slinkin, S.S. Goryashenko and K.I. Slovetskaya, J. Catal., 118 (1989) 459. 18. A.V. Kucherov, T.N. Kucherova and A.A. Slinkin, Catal. Lett,, 10 (1991) 289 19. A.V. Kucherov, J.L. Gerlock, H.-W. Jen and M. Shelef, J. Catal., 152 (1995) 63. 20 a J. Dedecek and B, Wichterlova, J. Phys. Chem., 98 (1994) 5721. 20 b A.V. Kucherov, C.P. Hubbard, T.N. Kucherova and M. Shelef, Appl. Catal., B: Environmental, 7 (1996) 285. 21. M. J. Rokosz, H.-W. Jen, A. V. Kucherov and M. Shelef, Catal. Today, in prim.
Miscellaneous
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CATALYSISAND AUTOMOTIVEPOLLUTIONCONTROLIV Studies in Surface Science and Catalysis, Vol. 116 N. Kruse, A. Frennetand J.-M Bastin (Eds.) 91998Elsevier Science B.V. All rightsreserved.
453
S u b s t r a t e C o n t r i b u t i o n s to A u t o m o t i v e C a t a l y t i c C o n v e r t e r P e r f o r m a n c e : T h e R o l e o f C h a n n e l S h a p e on C a t a l y s t E f f i c i e n c y
J. Paul Day Science and Technology Division, Corning Incorporated, SP-DV-2-1, Corning, New York, 14831 USA ABSTRACT The catalyst support plays an integral part in the performance of the automotive catalytic converter system. Although the precious metal catalyst is the primary contributor to the conversion efficiency, the substrate role is to properly distribute and support the washcoat and precious metal catalyst in order to utilize the properties of these components most fully. By optimizing the substrate properties as well as the precious metal and washcoat systems, optimum benefit from the catalyst system can be obtained. The automotive catalyst support is composed of the material and the structure. Previous papers have dealt with the materials. This paper will describe the structure of the catalyst support, specifically the channel shape, and present the structure-related heat transfer, mass transfer, and pressure drop properties for the laminar flow condition for various channel shapes. After these relationships have been established, a comparison will be made between the catalyst system performance of two of the channel shapes available commercially.
1. I N T R O D U C T I O N A previous paper (1) discussed the heat&mass transfer and pressure drop contributions of two specific channel structures. The paper showed that the channel shape can significantly influence the catalyst performance by effectively canceling the high surface area with a low cell shape factor. This paper extends this discussion to several other shapes and defines, within these sets, the limits on performance that they could reach under the same coating and testing conditions. Periodic cellular structures of the type used for almost all automotive catalyst supports are described by a few simple parameters (2, 3). If the cell density, wall thickness, and channel shape are known, the surface area, open area, channel hydraulic diameter, friction factor, Nusselt Numbers, and Sherwood Number are all obtained. And from these expressions the various properties of interest for
454 the performance of the catalyst are derived. Several papers have been written which describe these parameters for square and triangular cell structures. This paper is intended to generalize this description to three sets of structures and also to give a framework through which different structures (cell density, wall thickness, and channel shape combinations) can be compared. The cell density (N) is defined as the number of cells or channels per unit of cross-sectional area perpendicular to the axis of the channel. This is usually expressed in units of cells per square inch, and abbreviated cpsi. The open frontal area (OFA) is equal to the open area of an individual channel multiplied by the cell density and is usually expressed as a percent. The geometric surface area (GSA) of a cellular structure is derived by establishing the surface area per unit length of an individual channel that is then multiplied by the cell density. This represents a surface area per unit volume and is expressed as cm2/cma, m2/liter, or some other appropriate set of units. The total surface area (TSA) of a structure is then the geometric surface area multiplied by the volume (V) of the structure under consideration. The hydraulic diameter (Dh) is defined as four times the open area divided by the wetted perimeter which is equivalent to four times the open frontal area divided by the geometric surface area.
2. HEAT T R A N S F E R , MASS T R A N S F E R , AND P R E S S U R E D R O P A discussion of the derivation and limits on the use of the Heat&Mass Transfer and Pressure Drop Factors has been published previously (1). A summary of the derivations is contained in the next paragraphs. 2.1 Heat&Mass
Transfer
Factor
The Heat&Mass Transfer Factor is that portion of the heat and mass transfer equations which relates to the cellular structure. This factor is defined by use of the equation for energy flux as follows;
(I)
E = h- TSA- AT- t = H - V- k- AT- t,
where h is the heat transfer coefficient, AT is the temperature difference between the gas and the solid, and t is the time interval of interest. Employing the definitions of h (4) and TSA, the second portion of Equation 1 is derived, where k is the thermal conductivity of the gas and H, the Heat Transfer Factor (and by analogy M (5, 6), the Mass Transfer Factor) is defined as: Nu'GSA H&M
-
Nu'GSA 2 =
Dh
.
4-OFA
(2)
455 Here Nu is the average of the constant temperature and constant heat flux Nusselt Numbers, related to the cellular channel shape. Using this factor the relative performance of different cellular substrate designs can be compared for both heat and mass transfer, thereby suggesting the impact these designs would have on light-off and steady state catalyst performance.
2.2. Pressure Drop Factor The pressure drop requirements could act in direct opposition to the requirements for quick heat-up, for example in the light-off converter application where small diameter would be most beneficial. Hence, an understanding of the characteristics that influence pressure drop is important for the overall design. The pressure drop through a cellular catalyst support is composed of two parts (7), the first (referred to as the core pressure drop) is proportional to the flow rate while the second (composed of entrance and exit effects) is proportional to the square of the flow rate. At flow rates of interest for most automotive applications the linear part dominates. The core pressure drop is (8): 2-f*Re'wX AP =
.Q.
(3)
A-Dh2.OFA In this equation f'Re is the Friction Factor for the channel shape of interest, X is the channel length, A is the cross-sectional area of the cellular part, ~ is the viscosity of the gas, and Q is the flow rate. The Pressure Drop Factor represents the cellular structural contribution to the pressure loss down the length of the flow path and can be thought of as the cellular contribution to the core pressure drop coefficient (8). The Pressure Drop Factor is defined as: 2.f'Re P=
= Dh2-OFA
f*Re-GSA2 .
(4)
8.OFA 3
The Heat&Mass Transfer and Pressure Drop Factors will be used to evaluate the relative performance of various channel shapes that might be used for catalyst supports. In these applications it is desirable to maximize the heat and mass transfer and to minimize the pressure drop. As their derivations show, these factors represent the cellular contributions to these phenomena. Therefore, if the outer dimensions (length, cross-sectional area, and volume) and the characteristics of the environment (temperature, fluid viscosity and density) are held constant, these two factors determine the heat transfer, mass transfer, and pressure drop behavior of the system.
456
3. CHANNEL SHAPES The structures of interest for this paper are regular polygons (of which the square and the equilateral triangle are two members); triangles (of which the equilateral triangle is an end member); and sine ducts (of which the wrapped ceramic and metal structures are members). The following sections will discuss the Heat&Mass Transfer and Pressure Drop Factors of catalyst supports of various channel shapes through the identification of the open frontal area, the geometric surface area, and the channel shape-related quantities of Friction Factor and Nusselt Number.
3.1 Regular Polygons The complete set of regular polygons is comprised of all structures having n equal sides and n equal angles, where n is any integer from three to infinity. At n = 3, the structure is an equilateral triangle, n = 4 describes a square, and so forth, to n = ~, which represents a circle. The properties of each of these structures can be established by analysis of the set of identical triangles formed by radiating lines from the center of the polygon to each of the corners. The geometric surface area and open frontal area for the regular polygons are expressed by the following equations: GSA = 2- n- N . h ctn[90-(180/n)] 9 OFA = n . N.
hZ.ctn[90-(180/n)]
(5) (6)
where h is the distance from the center of the polygon to the center of an inside open side and n is the number of sides. For this class of structures, the hydraulic diameter is the same as the diameter of the largest cylinder that can be inserted into the channel. Although these relationships can be written for all members of this set, only three regular polygons - the triangle, square, and hexagon - can be assembled at arbitrary values of open frontal area. Hence, although it is interesting to observe the relationships for the other structures of this set, only these three are of practical importance for the automotive catalyst support application. In all of the figures relating to the regular polygons, the values will be given as discrete points without connecting lines because the regular polygons are represented by a series of integers and not, as with the other sets of shapes discussed, a continuous set of real numbers. The remaining information necessary to derive the heat&mass transfer and pressure drop relationships for the regular polygons are the Nusselt Number and Friction Factor (9) for each of these shapes. A set of data for these two shape related quantities is given in Figure 1, where the Friction Factor and average N u s s e l t N u m b e r are p l o t t e d a g a i n s t a s h a p e v a l u e [1/(1+1/n)], w h e r e n
457 is the number of sides. In Figure 1 the values of n are 3 through 10, 20, and infinity. The Heat&Mass Transfer and Pressure Drop Factors for the regular polygons are given by combining Equations 5 and 6 with the definitions in Equations 2 and 4. The expressions are:
H&M = N u - N . n ctn[90-(180/n)] 9
(7)
f ' R e - N n9 ctn[90-(180/n)] 9 P =
(8)
.
2 OF_.&' 9 -' Note that the Heat&Mass Transfer Factor is dependent on the shape of the channel and the cell density but independent of the open frontal area while the Pressure Drop Factor is dependent on the open frontal area of the cellular structure as well. Figure 2 shows the Heat&Mass Transfer and Pressure Drop Factors at 100% open frontal area for 400 cpsi regular polygons as functions of the shape. These graphs show that both the Pressure Drop Factor and the Heat&Mass Transfer Factor decrease as the number of sides of the regular polygon increases. At a given cell density and open frontal area, the triangle has the best heat and mass transfer; unfortunately, the triangle also has the highest pressure drop. '-"
JQ
4.2 F~-~-~ 7~--~-~--~ ~--...... ,. . . . . . | 4 1-
E P = 3.8 ~ ~ . . . . . . . . . . . . . Z L ~3.6! 9 ,~ 3.4 = Z ~ >
3.2
9
9 ~
9n " E]
9
9
~. . . . . . . 16.5 Circle 9 116 'J
L~
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< 2.8
Triangle
....
". . . . .
~
......
~--~--~-
E 21[.-
+- 1 5 5 - n =. m. 15 Pq" o--1 14.5-n ~4~
2.6 . . . . . . . . . . . . . . . . . . . . . . . . 0.75 0.8 0.85 0.9 0.95 Shape [1/(1+1/n)]
~"E- " 2 2 ' ~ ~ - ~ -T~f-l'aenr g l
o
13.5 113
Figure 1: The Friction Factor and Average Nusselt Number for Several Regular Polygons.
g
"-r "Ill
: 8.8~
"- 2 0 [
98.6 ~ t:.
-4
u_~. 19 I s 18 ~17F
16[-&~ ~
] 8.4~ 8.2 = .,
'
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15 r . . . . 0 . . . . . . . . . . . . . . . . . . . . '0.75 .8 0.85 0.9 0.95 Shape [1/(1+1/n)]
" 7.6 3 1 3 ----
Figure
2" The Pressure Drop and Heat&Mass Transfer Factors for Several Regular Polygons.
3.2. Isosceles Triangles An equilateral triangle is an end member of the set of regular polygons (n = 3) and also a member (and special case) of the set of all isosceles triangles.
458 The equations fbr the open i~ontal area and geometric surface area of an isosceles triangular channel of unit length are given by the following equations: (9)
OFA = a~- N-sinc~ cosa 9
(10)
G S A - 2 - a- N (1 9 + cos(z)
where a is the inside open dimension of each of the two equal sides and a is the base angle. The Friction Factor and average Nusselt Number for several isosceles triangles (9) are given in Figure 3 as functions of the shape, where here n is the ratio of the inside open height to the inside open width of the triangle. The Heat&Mass Transfer and Pressure Drop Factors are derived by applying Equations 9 and 10 to Equations 2 and 4, with the following results: Nu" N ' ( 1 + cos(z)2
H&M
(11)
-
sina - cosa
fire-N-(1
+ cosa) ~
p =
(12) 2 - O F A " . sin(z - cosa
These relationships are shown in Figure 4. It is notable that both the Heat&Mass and Pressure Drop Factors reach minimum values at the equilateral triangle (shape - 0.464), which represents the poorest of the isosceles triangles for heat and mass transfer but also the isosceles triangle lowest in pressure drop. 2.8 ,, ~,. ..a 2.6 E "~------/'~'~ "~' ", Z= 2.4 //c.,~ ................... "~.., ~ 2.2
13.4 13.2 13
z
12.4 m
0
/,/
12.6128~=6
if)
2
0
~t~ 1.8 L.. ID
~: 1.6, 1.4
0
/~
'
~..,'
0.2
~
,
,
,
~
0.4 0.6 0.8 Shape [1/(1+1/n)]
2.2 s 2 11.8 1
Figure 3" The Friction Factors and Average Nusselt Numbers for Several Isosceles Triangles.
7O
--r-~
i
,
,
,
!
,
,
,
i
,
,
,
22 n"
,
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'
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"-4
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~ 30 a_ 20
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0.8
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Figure 4: The Heat&Mass Transfer and Pressure Drop Factors for Several Isosceles Triangles.
3,
459 3.3. S i n e D u c t s The sine ducts were chosen for this analysis because they appear to be a very good representation of the wrapped or con'ugated structure (1). The derivations of the geometric surface area and open frontal area of this structure are based on the equation for a sine curve: y = a + a . s i n [ ( T r x / b ) - (rt/2)]
(13)
where a is the amplitude of the sine wave and 2-b is the period. The surface area of a sine duct is composed of the surface area of the sine curve plus the surface area of the associated flat sheet. The determination of the surface area of a sine curve involves the solution of the integral for the length of the curve (A) given by: u/2 A - 2~(1 + ,~-cos2x)l/2dx. 0
(14)
With proper manipulation, this equation can be expressed as a Complete Elliptic Integral of the Second Kind, which has a numerical solution for each value of the quantity a (10). The open area of the sine duct is likewise defined by an integral, which is constructed by integrating Equation 13 over the full cycle. The solution of the integral for the open area is 2.a.b, where 2-a and 2.b are the height and width of the open area of the sine duct, respectively. This solution is, for the case of 100% OFA, the reciprocal of the cell density. For structures of finite wall thickness, the quantity 2 . a . b is the open frontal area divided by the cell density. The values of Friction Factor and average Nusselt Number (9) are shown in Figure 5 for selected sine ducts as functions of the relative shape, where n here is the ratio of the inside open height to the inside open width of the duct. Combining the information in Figure 5 with the calculated open frontal area and geometric surface area values and through the use of Equations 2 and 4, the Heat&Mass Transfer and Pressure Drop Factors can be calculated for the sine ducts. These results are shown in Figure 6.
460
.
(D
"
E
2.5
i
i-
~
,
/
I
-~ i it,...(,)
, , , ,, ,..~ ~
[
t
,
--,
"'~\ , ~\',
/
._.5O
16
14-n
=.
m 30
-4--'
\ 2
z
/
(-)
:/
~i
1.5
0
12 -n
,\
I
0.2
,
,
,
I
0.4
Shape
,
.
,
!
,
,
,
~!
u... I
0.6 0.8 [1/(1+1/n)]
,
11
,~10 ,.....,
")9
1
Figure 5 The Friction Factors and Average Nusselt Numbers for Several Sine Ducts.
0
s
/d'
13 2 O
/ ./ 1o / ,
~ erl 1 0 0
m
110~.
131_
13_
20
15 -~4
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/1
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0
9
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Figure 6 The Heat&Mass Transfer and Pressure Drop Factors for Several Sine Ducts.
4. O V E R A L L S T R U C T U R A L C O M P A R I S O N S In order to achieve the best possible catalyst conversion efficiency at a constant volume, while minimizing the power drain due to excessive pressure drop through the converter, one would maximize the heat and mass transfer with respect to the pressure drop. In other words, in the graph shown in Figure 7 for 100% open frontal area, where the Heat&Mass Transfer Factor is on the xaxis and the Pressure Drop Factor is on the y-axis, the slope of the curve should to be as shallow as possible. All of the channel shapes evaluated here tend to fall close to the same line. However, as the open frontal area decreases from 100%, the Pressure Drop Factor increases while the Heat&Mass Transfer Factor remains constant so that the relative attractiveness of some channel structures will be improved as the OFA is taken into account.
461 7O
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Figure 7: Comparison of Pressure Drop and Heat&Mass Transfer Factors for Three Sets of Shapes at 400 cpsi and 100% Open Frontal Area.
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Figure 8: The Conversion Efficiency of a 400 cpsi Square Cell Ceramic System Compared to a 400 cpsi Sinusoidal Cell Metal System.
5. CATALYST P E R F O R M A N C E The derivation outlined in Section 2 demonstrates that the Heat&Mass Transfer Factor can be treated as the cellular support contribution to the heat and mass transfer. This suggests that it can serve to predict relative steadystate catalyst performance rather than utilizing the geometric surface area for this purpose, as has been the practice in the past (2, 11, 12). Although apparently directionally correct for channels of the same shape, these calculations show that the geometric surface area is not sufficient to make comparisons among significantly different channel structures. One need only consult direct catalyst performance comparisons between square and sinusoidal shaped channel structures (13, 14) to see that, when the channel shapes are different, higher geometric surface area (different by 35%) does not necessarily result in better performance. In these references the same performance was realized with these dramatically different structures when the external conditions (volume, washcoat, catalyst, aging, etc.) were kept constant. One of these sets of data is discussed below. 5.1. Comparison of Square and Sinusoidal Channel Shapes Using the measured data for the coated 400 cpsi extruded ceramic and wrapped metal structures shown in Table 1 (1), the Heat&Mass Transfer Factors are calculated to be 7.96/mm 2 for the square and 7.68/mm ~- for the sinusoidal channel structures. The difference between these two values is within the error associated with the assumptions of the analysis. These numbers suggests that the steady-state catalyst performance of these two structures, when tested under the same conditions, will be similar.
462 Data for such a direct comparison have been published elsewhere (13), along with a complete description of the tests. A summary of the results is shown in Figure 8, where the conversion efficiency for each of the square channel substrates is plotted against the conversion efficiency for each of the sinusoidal channel substrates, for the particular FTP test portion and for each of the three measured gases (HC, CO, and NOd. Here, only the latter part of the first test cycle (bag 1B), all of the second cycle (bag 2), and only the latter part of the third test cycle (bag 3B) are included in the comparison. The slope of the best-fit line to these data is 1.004+0.003, demonstrating that the test results are similar for the two structures, as the Heat&Mass Transfer Factor predicts. Table 1 Cellular Properties of 400 cpsi Square and Smusoidal Channel Substrates (1) Square Channel
Sinusoidal Channel
Geometric Surface Area (m2fliter)
2.49
3.31
Open Frontal Area (%)
63.5
77.4
Hych'auhc Diameter (mm)
0.102
0.094
Nusselt Number
3.26
2.18
H&M Transfer Factor (/mm2)
7.96
7.68
6. CONCLUSIONS The heat&mass transfer and pressure drop relationships have been developed for three sets of channel shapes which could reasonably be expected to be encountered in catalyst support applications. Using the Heat&Mass Transfer and Pressure Drop Factors, comparisons can be made among different shapes and descriptions of cell structures in order to optimize the heat transfer and catalyst performance with respect to the pressure drop. The three sets of structures evaluated here have similar relationships between the Pressure Drop Factor and the Heat&Mass Transfer Factor. However, the extent of the regular polygons is very limited and even more so because only three of them can be packed efficiently. The calculated Heat&Mass Transfer Factors for 400 cpsi square and sinusoidal cell products are almost identical, even with the very much higher geometric surface area of the sinusoidal channel. The near identity of this factor
463 for these two very different shaped structures is the result of a much higher Nusselt Number for the square channel. Analysis of a set of catalyst performance data which represent a direct compalison between the 400 cpsi square and sinusoidal cell structures indicates almost identical steady-state performance for the catalysts on the two structures. These data suggest that the Heat&Mass Transfer Factor can in fact be used to predict relative catalyst performance when other aspects of the experiment such as washcoat loading, substrate volume, and test conditions are held constant. REFERENCES
1. J. Paul Day, ,Substrate Effects on Light-Off- Part II Cell Shape Considerations,, SAE Paper # 971024, February 1997. 2. J.S. Howitt, ((This Wall Ceramics as Monolithic Catalyst Supports~,, SAE Papel~800082, February 1980. 3. Suresh T. Gulati, ((Cell Design for Ceramic Monoliths for Catalytic ConverterApplicatiom,, SAE Paper #881685, October 1988. 4. W.M. Kays, Convective Heat and Mass Transfer, McGraw-Hill Book Company, NY, NY, 1966, p 109. 5. J.P. Holman, Heat Transfer, McGraw-Hill Book Company, NY, NY, 1963, p 111,262-263. 6. Kays, op. cit., p311-314. 7. W.M. Kays and A.L. London, Compact Heat Exchangers, McGraw-Hill BookCompany, NY, NY, 2 "d Edition, 1955, p33. 8. J. Paul Day and Louis S. Socha, Jr., ((The Design of Automotive Catalyst Supportsfor Improved Pressure Drop and Conversion Efficiency~), SAE Paper #910371, February 1991. 9. W.M. Rohsenow, J.P. Hartnett, and E.N. Ganic, ed., Handbook of Heat Transfer Fundamentals, McGraw-Hill Book Co., NY, NY, Second Edition, 1993, p7-91. 10.Milton Abramowitz and Irene A. Stegun, Handbook of Mathematical Functions, Dover Publications, Inc., New York, Ninth Edition, 1972, p609. l l.K. Nishizawa, K. Masuda, H. Horie, and J. Hirohashi, ((Development of Improved Metal-Supported Catalyst)~, SAE Paper #890188, February 1989. 12.R. Brtick, F.W. Kaiser, and C. Kruse, ((Anforderungen an zuktinftige Katalysatoren-Entwicklungspotential am Beispiel des metallischen Katalysatortrfigers,, Haus der Technik, Essen, Germany, November 1996. 13.J. Paul Day and Louis S. Socha, Jr., ((Technique for the Analysis of FTP Emissions~), SAE Paper #920724, February 1992. 14.J. Paul Day, ((Analysis of Catalyst Durability Data from the Standpoint of Substrate Surface Arem), SAE Paper #952397, October 1995.
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CATALYSIS AND AUTOMOTIVEPOLLUTIONCONTROLIV Studies in Surface Science and Catalysis, Vol. 116 N. Kruse, A. FrennetandJ.-M Bastin (Eds.) 91998 Elsevier Science B.V. All rights reserved.
465
Evaluation and characterization of catalysts for alternative-fuelled vehicles. A study of the influence of catalyst composition on activity and byproduct formation. L.J. Pettersson, A.M. Wahlberg and S.G. J~ir~s Royal Institute of Technology (KTH), Department of Chemical Engineering and Technology, Chemical Technology, S-100 44 Stockholm, Sweden, Tel. +46 8 790 82 59, Fax +46 8 10 85 79
ABSTRACT An experimental study of catalytic oxidation of ethanol in a laboratory flow reactor is presented. The miniature catalyst samples consisted of monolithic cordierite substrates onto which various combinations of washcoat material and active material were applied. Oxides of Cu and Cu-Mn, as well as different combinations of precious metals were evaluated as active material supported on various washcoat materials. The experimental conditions were chosen in order to simulate the exhaust from a diesel engine fuelled by neat ethanol. Catalyst characterization included measurements of BET surface area and pore size distribution as well as temperature-programmed reduction analysis. While combining two precious metals as active material, a positive synergistic effect has been observed. The light-off temperature (Ts0) is considerably lower for some of these combinations than for the corresponding monometallic catalysts. The base-metal oxide catalysts tested were more selective for oxidation of ethanol to carbon dioxide and water. The results also indicate that the oxidation of nitric oxide to the more hazardous nitrogen dioxide can be suppressed by using a suitable combination of active material and washcoat material.
1. INTRODUCTION The OECD transport sector produced nearly three times as much carbon dioxide in 1990 as it did in 1960 (IEA, 1993). There is a growing concern that these increasing carbon dioxide emissions will create a greenhouse effect on our planet (Boer et al., 1990). One way of reducing the emissions of CO2 would be to introduce renewable fuels, such as alcohol fuels or biogas. Carbon dioxide, which is produced by combustion of biomass-derived fuels, is naturally recycled and consumed in the photosynthesis. This means that there will be no net increase of CO 2 in the atmosphere when using, for example, ethanol produced from biomass. This is valid if biomass-derived fuels or chemicals are used in all parts of the production chain. Due to its fast volume growth, the transport sector is responsible for an increasing part of total emissions, despite the development of purification technology (Swedish Government,
466 1991). A great concern is the increasing concentration of ozone close to the ground, which, among other things, threatens the vegetation. Ozone is formed when reactive hydrocarbons and nitrogen oxides from mobile sources are acted on by sunlight. The Swedish Environmental Research Institute (IVL) estimates that the concentration of tropospheric ozone has doubled in Sweden during the 20th century (Hasund et al., 1990). One approach to lowering the motor vehicle contribution to ozone formation is to decrease the reactivity of the tailpipe emissions by the use of alternative fuels (Nichols, 1994). The use of diesel engines in vehicles is growing in popularity worldwide. One of the reasons for this is the high efficiency of the engine, which leads to low fuel consumption. In Europe, all of the heavy-duty vehicles are equipped with diesel engines. About 60 % of the light-duty commercial vehicles and approximately 20 % of the passenger cars are propelled by diesel engines (Summers et al., 1996). In some countries such as France, Belgium and Italy the latter figure is considerably higher. Exhaust gas catalysts has been widely used since the launching of the 1970 Clean Air Act in the USA and especially after the introduction of stricter regulations in 1981. At present, one of the fastest growing areas of catalyst-based technology is automotive pollution control. All gasoline-fuelled vehicles sold in the USA, Japan and in the European Community must be equipped with exhaust aftertreatment in order to meet the emission standards. Oxidation catalysts for heavy-duty vehicles have only been used for a short period, but following the tightening emission standards there will be an increased demand for such systems. To address air pollution problems in the Swedish big city areas, local transport companies have set up fleets with alternative-fuelled vehicles. Currently ca 550 buses are operating on alternative fuels in Sweden (KFB, 1996). The largest portion of these buses is running in Stockholm, where Stockholm Transport has 180 neat ethanol vehicles in inner city operation. The exhaust gas from ethanol-fuelled vehicles contains above all unburned ethanol and acetaldehyde (Goodrich, 1982). Acetic acid, which has a characteristic smell and can be experienced as an irritant far below directly hazardous concentrations, can be formed either directly from ethanol or via partial oxidation of acetaldehyde. In order to achieve the large potential emission advantages of alcohol fuels, catalysts should be designed to minimize specific fuel-related emissions. On the whole, vehicle test results show that the potential for obtaining lower emission levels with a bio-based engine fuel is considerable if the fuel is used in a well developed engine system which includes a catalyst (Egeb~ick et al., 1996). A health risk assessment of ethanol as a bus fuel has recently been performed by Bostr6m et al. (1996). The driving pattern of inner city buses includes a lot of idling and many starts and stops. Hence, the exhaust gas temperature will be quite low, occasionally below 200~ The first generation of commercial catalysts has not complied with the demands for contemporaneous high catalytic activity and low by-product formation at low temperatures. Due to problems with high amounts of unregulated compounds, such as acetaldehyde, acetic acid, a catalyst research programme was initiated. The activities include both evaluation in a laboratory-scale flow reactor using miniature catalyst samples, and full-scale evaluation utilising engine dynamometer tests. In this paper, only results from the laboratory reactor tests are described. The catalyst development is performed using two parallel lines concentrating on catalysts possessing two different types of properties: Type 1. Catalyst with a very high activity at low temperatures and aldehyde peak below the normal operating temperature of a diesel engine Type 2. Highly selective catalyst with negligible by-product formation over the entire
467 temperature range. A somewhat lower activity than for type 1 can be accepted. The engineering target is to eventually create a catalyst that is both highly selective and highly active over the entire operating temperature range of the diesel engine. This paper concentrates on evaluation and characterization of base-metal oxide catalysts and precious metal catalysts for total oxidation of ethanol. The first phase of this investigation was presented in Pettersson et al. (1995). The paper concentrated on precious metal catalysts. For detailed reviews of catalysts for alcohol vehicles refer to Pettersson (1991 ), and Pettersson and J~irfis (1994).
2. E X P E R I M E N T A L Miniature catalyst samples prepared according to standard techniques at Svenska Emissionsteknik (Johnson Matthey) in Gothenburg, Sweden, were studied. The catalysts consisted of a ceramic monolithic honeycomb-shaped substrate made of cordierite (2MgOe5SiOze2A1203), onto which either of the washcoat materials A1203, CeO2, SiC, SiO2, or TiO2 had been applied. The cordierite monoliths were manufactured by Coming and had a cell density of 62 cells/cm 2 (400 cells/in2). The catalyst volume was ca 12 cm 3 and the wall thickness 180 pm as measured by scanning electron microscopy. The active materials tested in this work were different combinations of precious metals, either monometallic or bimetallic, and oxides of Cu and Cu-Mn (molar ratio 1:4). The preparation of the precious metal catalysts was made on an equimolar basis calculated per unit volume of monolith. The same principle was used for the base-metal oxide catalysts, where in this case the metal loading was an order of magnitude higher than that of the precious metal catalysts. The catalyst samples were tested in a tubular reactor made of either quartz or sintered alumina installed in an electrically heated furnace. The inner diameter of the reactor was ca 25 mm. The gas stream composition, given in Table 1, simulates ethanol combustion at 100% excess air (k=2). All feed gas flows were regulated by mass flow controllers. Most of the gases were fed through a gas mixer/preheater, but the reactive gases (CO, NO and C2HsOH) were added to the gas stream close to the catalyst, in order to avoid homogeneous reaction of the gases before they had reached the catalyst. The test set-up is described in Figure 1. Ethanol was evaporated by bubbling nitrogen through two consecutive ethanol-containing glass cylinders with porous glass filters. The temperature of the liquid ethanol was kept Table 1 Test gas composition in the ethanol combustion experiments Compoun d concentration 02 10 vol.% H20 10 vol.% CO2 6.5 vol.% N2 73.4 vol.% CzHsOH 200 ppm NO 600 ppm .,..CO 300 ppm
468
Figure 1. Experimental set-up. FID: flame ionization detector, CLD: chemiluminescence detector, GC: gas chromatograph, TCD: thermal conductivity detector, NDIR: non-dispersive infra-red instrument, PC: personal computer. constant at room temperature by a water bath. The space velocity was 100 000 h "1 and the temperature was raised from 70 to 500~ during the experiment. All catalysts were subjected to a standardized pre-treatment procedure in three steps in situ in the reactor before the evaluation, in order to eliminate possible differences in the catalyst preparation. Continuous analysis instruments equipped with flame ionization, chemiluminescence, and infrared detectors were used to measure the concentrations of total hydrocarbons, nitrogen oxides and carbon monoxide, respectively. The concentration of total hydrocarbons was measured by a JUM FID 3-300 hydrocarbon analyzer with a flame ionization detector. NO, NO2 and NOx was measured by an ECO Physics CLD 700 EL-ht chemiluminescence detector. CO was measured with either a Beckman Industrial Model 880 non-dispersive infrared instrument or an NDIR instrument from Maihak (UNOR 6N). Gas chromatographs equipped with flame ionization and thermal conductivity detectors gave detailed information on concentrations of oxygenated compounds, hydrocarbons etc., online in a semi-continuous manner. For the analysis of the hydrocarbons either a Varian Star 3400 CX or a Shimadzu GC-3BF with flame ionization detectors was used. N2, O2 and CO2 were measured with a Shimadzu GC-3BT equipped with a thermal conductivity detector.
3. RESULTS AND DISCUSSION The most important reactions that occur in catalytic aftertreatment of emissions from an ethanol-fuelled diesel engine are listed in Table 2. For a catalyst involved in this type of pollution control one of the most important qualifications is the selectivity towards complete oxidation of ethanol. This is indicated by the formation of acetaldehyde, which is the major by-product formed. Over some of the catalysts other by-products such as acetic acid, diethyl ether, methane and ethylene are also formed, but to a much lower extent. The low-temperature
469 Table 2 Important reactions C2HsOH + 302 --~ 2CO2 + 3H20 2C2HsOH + 02 "--)'2CH3CHO + 2H20 C2HsOH + 02 --~ CH3COOH + H20 2CH3CHO + 02 --~ 2CH3COOH
(1) (2) (3) (4) (5) (6) (7)
2C2H5OH --~ C2HsOC2H5 + H20 C2H5OH --~ C2H4 + H20 2NO + 02 -~ 2NO2
activity of the catalyst can be expressed as the light-off temperature (Ts0), that is the temperature at which 50% of the ethanol is converted. A third feature is how much of the nitric oxide, NO, is oxidized to the even more hazardous nitrogen dioxide, NO2, which should be avoided at street levels. In Figure 2, the light-off temperatures for precious metals A and C, and a combination of these metals, AC, supported on the same washcoat material, are compared. A positive synergistic effect between these metals can be found, since the light-off temperature for the combination is lower than for the corresponding monometallic catalysts. On the other hand, more by-products are formed at low temperatures. The maximum yield of acetaldehyde is higher, even though only at very low temperatures (below 150~ Yield is defined as the amount of acetaldehyde formed per mole of ethanol in the feed. The influence of the washcoat material on the catalytic performance of the combination of A and C is quite clear. There is a great difference in both activity and selectivity between the catalysts presented in Figure 3. AC supported on silica gives a light-off temperature slightly below 100~ while the same active material supported on titania gives a light-off temperature of about 160~ The amount of acetaldehyde produced as well as the NO2 formed also varies a great deal. This indicates that there is an interaction between the active material and the
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Figure 2. a) Conversion of ethanol over precious metals A, C and AC supported on alumina. b) Yield of acetaldehyde over the same catalysts.
470
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Figure 3. a) Conversion of ethanol over the bimetallic combination AC supported on various washcoat materials, b) Yield of acetaldehyde over the same catalysts.
support material. Catalysts with SiO2 and SiC act quite similarly, both in the conversion of ethanol and in the formation of acetaldehyde. On the other hand, in the formation of acetic acid, SiC gives a distinct maximum at approximately 200-250~ while the silica-supported catalysts exhibit a more uniform profile. This is shown in Figure 4, where the yield of acetic acid over two different combinations of precious metals supported on SiC and SiO2 is compared. This amount is also about twice as high as the maximum yield for the corresponding SiO2-supported catalysts. Neither of the base-metal oxide catalysts tested were active at low temperatures (see Figures 5 and 6), but CuO supported on alumina and silica exhibit rather low formation of acetaldehyde. This corresponds to the results presented by Rajesh and Ozkan (1993) who have tested the activity of catalysts containing either oxides of copper or chromium and also a combination of these two metal oxides supported on ),-alumina pellets. The formation of acetaldehyde is slightly higher over CuO-MnO2. Catalysts supported on titania show the lowest light-off temperature for all the base-metal catalysts tested, but also the highest formation of acetaldehyde.
!----ACmC 1 i ..... ~B,S!C /
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~
200 300 Temperature [~
400
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Figure 4. Yield of acetic acid over AC and AB supported on SiO2 and SiC.
471 100
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Figure 5. a) Conversion of ethanol over CuO supported on various washcoat materials. b) Yield of acetaldehyde over the same catalysts.
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Figure 6. a) Conversion of ethanOl over CuO-MnO2 supported on various washcoat materials. b) Yield of acetaldehyde over CuO-MnO2 supported on various washcoat materials. Interactions between the precious metal and support influence the performance of the catalyst. Bell (1987) has defined metal-support interaction as depending on contact between the metal particle and the support which can be a dissolution of the dispersed metal in the lattice. The interaction could also depend on the formation of a mixed metal oxide, or the decoration of the metal particle surface with oxidic moieties derived from the support. It is possible that in this study, the differences in catalytic performance of the same active material supported on different washcoats can be attributed to any of these phenomena. Another explanation could be that the support materials exhibit different acid-base properties. According to the Bronsted and Lewis definitions, a solid acid shows a tendency to donate a proton or to accept an electron pair, whereas a solid base tends to accept a proton or to donate an electron pair. The tendency of an oxide to become positively or negatively charged is thus a function of its composition, which is affected by the preparation method and the precursors used. Refer to the section "Catalyst characterization" for further discussion on the influence of support material on catalyst performance. To thoroughly examine the influence of the support
472 on the reaction mechanisms further studies are needed. However, this was beyond the scope of this work. The BET surface areas of the studied catalysts have been compared, but no correlation has been found between surface area and activity. On the contrary, supported on the rather low surface area SiC (only ca 20 m2/g washcoat) and SiO2 (ca 55 m2/g washcoat), AC showed a much higher activity than supported on the relatively high surface area A1203 (about 125 m2/g washcoat). The formation of NO2 is generally larger over the precious metals than over the base-metal oxides, and it also starts at lower temperatures. However, there is no evident correlation between the formation of NO2 and the conversion of ethanol, when comparing catalysts containing the same active material but different washcoats. The amount of NO2 formed seems to be affected both by the active material and the support material. Some precious metal catalysts supported on ceria show relatively low formation of NO2, while the opposite effect has been observed on ceria-supported CuO and CuO-MnO2. Among the most interesting combinations tested are AC/SiO2 and AC/SiC, which have very low light-off temperatures. The acetaldehyde yields are high, but only at temperatures below 200~ The formation of NO2 is average. These catalysts can be rated as the Type 1 catalyst described in the introduction. CuO/A1203 has a relatively low by-product formation, but the light-off temperature is as high as 330 ~ The maximum NO2/NOx ratio is only 9 %. This is a catalyst that is close to Type 2.
4. CATALYST CHARACTERIZATION In this study, the temperature-programmed reduction (TPR) technique was used on CuO and CuO-MnO2 supported on various washcoat materials, and on a reference sample containing pure copper oxide. A TPD/TPR 2900 Chemisorption Analyzer from Micromeritics was used in these experiments. The samples were submitted to a gas stream of 45 cm3/min containing 10 vol.% hydrogen, as the reducing agent, and 90 vol.% argon. The quartz reactor containing the sample was heated at 10 ~ starting at 35 ~ The temperature was governed by a temperature-programmed controller. Before the product gas reached the thermal conductivity detector, it was passed through a cooling trap with a mixture of isopropanol and liquid nitrogen. This procedure ensured the separation of water formed, which could disturb the analysis. At a certain temperature, substances on the surface were reduced by hydrogen. This temperature is characteristic for each substance. The TPR results were only used for a qualitative comparison between the different catalysts. There is a marked difference between the TPR profiles for the different catalysts (see Figure 7). The TPR profiles of supported CuO catalysts are strongly dependent on the support. Centi and Perathoner (1995) point out that the nature of the copper species depends clearly on the specific kind of washcoat material used. CuO supported on TiO2 is more readily reduced than CuO supported on A1203, CeO2 or SiO2. In the case of the CuO/A1203 catalyst, the support seems to act solely as a dispersing agent, while enhancing the reactivity of the copper oxide towards reduction at lower temperatures. Pure unsupported CuO has a peak at ca 300 ~ CuO supported on TiO2 exhibits a low-temperature peak shifted to ca 170 ~ This can be interpreted as meaning that the titania promotes the reactivity of the copper oxide towards reduction (Robertson et al., 1975). Titania influences the number of peaks in TPR
473 42 225
269
CuO/CeO2
248 277
i,, !--
0
a)
c0o,s
_
CuO/Al
CuO i
',
100
200
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300
Temperature (~
',
I
400
500
......
0
100
200
-----T----
300
.... - t - - - - - - - t
Temperature (~
400
500
b)
Figure 7. TPR profiles for unsupported CuO (reference) (a), CuO-MnO2 (a) and CuO (b) supported on various washcoats.
(cf W611ner et al., 1993). Both titania-supported CuO and CuO-MnO2 exhibit three peaks. These catalysts have a high-temperature peak at a temperature over 350 ~ Strong metalsupport interaction (SMSI) for TiOz-supported catalysts has been described by Tauster et al. (1978) and Chang et al. (1985). There seems to be a correlation between TPR profile and pore size distribution for TiOzsupported catalysts. The low-temperature peak on the titania-supported catalyst can be attributed to small pores, while the high-temperature peaks can be associated with large pores. This finding is in line with the observations of Mile et al. (1988) studying supported nickel catalysts. In Figure 8 the bimodal pore size distributions for CuO and CuO-MnO2 supported on titania are shown. The pore volumes were 0.076 and 0.074 cm3/g, respectively. Measurements of pore size distribution, pore volume and BET surface area were performed on a Micromeritics ASAP 2000 by nitrogen adsorption using a volumetric method. Gentry and Walsh (1982) have shown that copper oxide supported on silica can exist in two forms. These are a dispersed cupric-oxide phase and a combined cupric-oxide-support phase (copper silicate). These two phases can be reduced with varying degrees of difficulty. It is possible that their findings could explain that, in this study, more than one peak can be distinguished in the TPR profiles of supported copper oxide. Another explanation has been given by, among others, Dow and Huang (1996). They point out that bulk-like copper oxide can exhibit a two-step reduction: Cu 2+ ~ Cu § ~ Cu ~
474
0.02
?.
0.015
_ /iA E
0.01
r-~cuomo2
1
nO2/TiO2J
0> 0 0.005
10
1O0 1000 Pore diameter (A)
10000
Figure 8. The pore size distribution for CuO and CuO-MnO2 supported on TiO2. When comparing the TPR profiles with the light-off curves from the ethanol oxidation experiments, we have found an indication of a correlation between activity and reducibility of the catalyst. Copper oxide supported on titania is the most active towards ethanol oxidation among the copper oxides tested. It is also the catalyst in which the reduction starts at the lowest temperature. The results obtained in the TPR experiments strengthen the hypothesis that there is a considerable interaction between the support and the active material.
5. CONCLUSIONS Among the most interesting catalysts tested for complete ethanol oxidation are the bimetallic precious metal combinations AC/SiO2 and AC/SiC, which exhibit very low lightoff temperatures, <100 ~ These catalysts can be rated as the Type 1 catalyst described in the introduction. The selectivity, however, does not meet the engineering target. Even though they form large amounts of acetaldehyde, it should be noted that the emissions will not be very high at operating temperatures above 200 ~ The formation of NO2 is average. A synergistic effect between the precious metals A and C has been found, since the activities of the bimetallic AC catalysts are higher than for the corresponding monometallic catalyst. The basemetal oxide catalysts tested were more selective towards oxidation of ethanol to carbon dioxide and water. CuO/A1203 has a relatively low by-product formation, but the light-off temperature is as high as 330 ~ The maximum NOz/NOx ratio is only 9 %. This is a catalyst that is close to Type 2. The yield of acetaldehyde, which is the major by-product formed, as well as the yield of acetic acid, are strongly influenced by the choice of both support and active material. The metal-support interaction also has a certain influence on the oxidation of nitric oxide to nitrogen dioxide. It has been demonstrated that the propensity for the catalyst to produce nitrogen dioxide at elevated temperatures can be selectively inhibited by the combination of appropriate washcoat and active materials. The number of peaks in temperature-programmed reduction was shown to be affected by the support through an interaction with the active material. This interaction will in turn affect the by-product formation. A correlation has also been found between the activity for ethanol oxidation and the reducibility of supported CuO catalysts.
475 ACKNOWLEDGEMENTS
Financial support to this work given by the Swedish National Board for Industrial and Technical Development (NUTEK) and the Swedish Transport and Communications Research Board (KFB) is gratefully acknowledged. REFERENCES
1. A.T. Bell, Supports and metal-support interactions in catalyst design, in L.L. Hegedus, (Ed.), Catalyst Design. Progress and Perspectives, Wiley-Interscience, New York, 1987, 103. 2. M.M. Boer, E.A. Koster, and H. Lundberg, AMBIO, 19 (1990) 2. 3. C.-E. Bostr6m, P. Camner, K.-E. Egeb~ick, L. Ewetz, S. Ljungquist, G. Omstedt, L.J. Pettersson, M. T6rnqvist, R. Westerholm, Health Risk Assessment of Ethanol as a Bus Fuel, KFB Report No 1996:19, KFB, Stockholm, 1996. 4. G. Centi and S. Perathoner, Appl. Catal. A, 132 (1995) 179. 5. T.-C. Chang, J.-J. Chen, and C.-T. Yeh, J. Catal., 96 (1985) 51. 6. W.-P. Dow and T.-J. Huang, Appl. Catal. A, 141 (1996) 17. 7. K.-E. Egeback, L.J. Pettersson, and R. Westerholm, Proc. XI International Symposium on Alcohol Fuels, Sun City, South Africa, April 14-17, (1996) 750. 8. S.J. Gentry and P.T. Walsh, J. Chem. Soc., Faraday Trans. 1, 78 (1982) 1515. 9. R.S. Goodrich, Chem. Eng. Prog., 78 (1) (1982) 29. 10. K.P. Hasund, L. Hedvhg and H. Pleijel, Economic Consequences of the Influence of the Tropospheric Ozone on Agricultural Crops, SNV 3862, Swedish Environmental Protection Agency, Solna, 1990. In Swedish. 11. International Energy Agency [IEA], Cars and Climate Change, OECD, Paris, 1993. 12. KFB, Clean Traffic, Newsletter 1996, v. 34, KFB, Stockholm, 1996. In Swedish. 13. B. Mile, D. Stirling, M.A. Zammitt, A. Lovell, and M. Webb, J. Catal., 114 (1988) 217. 14. R.J. Nichols, J. Eng. Gas Turbines Power, 116 (1994) 727. 15. L.J. Pettersson, Catalytic Treatment of Emissions. State of the Art for Alcohol and Natural Gas-Fueled Vehicles, Department of Chemical Technology, Royal Institute of Technology, Stockholm, Sweden, ISRN KTH/KT/FR--91/9--SE, 1991. 16. L.J. Pettersson and S.G. J/argts, Exhaust Gas Catalyst for Alcohol Vehicles, Report 92-265742, KFB, Stockholm, Sweden, 1994. 17. L.J. Pettersson, S.G. J~hs, S. Andersson, and P. Marsh, Stud. Surf. Sci. Catal. 95 (1995) 855. 18. H. Rajesh and U.S. Ozkan, Ind. Eng. Chem. Res., 32 (1993) 1622. 19. S.D. Robertson, B.D. McNicol, J.H. de Baas, S.C. Kloet, and J.W. Jenkins, J. Catal., 37 (1975)424. 20. J.C. Summers, S. Van Houtte, and D. Psaras, Appl. Catal. B, 10 (1996) 139. 21. Swedish Government, A Good Living Environment. The Government Bill 1990/91:90, Stockholm, Feb. 7, 1991. In Swedish. 22. S.J. Tauster, S.C. Fung, and R.L. Garten, J. Am. Chem. Soc., 100 (1978) 170. 23. W611ner, F. Lange, H. Schmelz, and H. Kn6zinger, Appl. Catal. A, 94 (1993) 181.
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CATALYSIS AND AUTOMOTIVEPOLLUTIONCONTROLIV Studies in Surface Science and Catalysis, Vol. 116 N. Kruse, A. Frennet and J.-M Bastin (Eds.) 91998 Elsevier Science B.V. All rights reserved.
SHS Catalysts for Combustion Engines
Purification
of
Exhaust
477
Gases
From
Internal
E.H. Grigoryan, I.P. Borovinskaya, and A.G. Merzhanov Institute of Structural Macrokinetics, Russian Academy of Sciences Chernogolovka, Moscow Region, 142432 Russia Fax: +7(095) 962 8025; E-mail: [email protected]
1. INTRODUCTION
Efficiency of reducing the level of pollutant emission from internal combustion engines and industrial convertors depends mainly on the properties of catalysts introduced in the combustor. Catalysts for purification of exhaust gases from internal combustion engines should meet some specific requirements. On the one hand, they are to provide efficient oxidation of carbon monoxide and hydrocarbons and simultaneously reduction of nitrogen oxides at variable flow rate, gas composition, and temperature, on retention of high catalytic activity after prolonged use. On the other hand, the catalysts should be wear- and heatresistant as well as provide required heat and mass exchange. Drawbacks of virtually all catalytic convertors [1 ] are (i) involvement of expensive precious metals and inaccessible natural materials for catalyst carriers; (ii) multistage, ecologically hazardous, and energy-consuming production method; (iii) poor service parameters, etc. [2]. In this work, we used oxynitride block catalysts produced by self-propagating hightemperature synthesis (SHS) [3,4] for purification of the gaseous mixtures modeling exhaust gases from carbon monoxide, hydrocarbons,and nitrogen oxide.
2. SHS OF CATALYSTS AND SUPPORTERS SHS is a promising energy-saving, and ecologically safe method for production numerous inorganic materials, including catalysts and catalyst supporters [5]. In essence, SHS is a hightemperature redox reaction between metals and nonmetals, metals and oxides, etc. occuring due to high heat release. After ignition (by an electric wire spiral or higly inflammable mixtures) the combustion wave propagates through the mixture of powder reagents. The temperature in the condensed phase can attain a value of 4000K [6]. Obviously, SHS products have high heat capacity and thermal stability. Many inorganic compounds (borides, carbides, silicides, nitrides, phosphides, oxides, chalcogenides, hydrides, intermetallides, as well as multicomponent compounds) can be prepared by this technique in a one-stage process at high
478 conversion degree [7]. Energy-consuming and low-output conventional furnace technique (not always providing high degree of conversion and quality of the products) can hardly compete with SHS production method. As a rule, other methods for production of catalysts and carriers are multistage, labor-consuming, and ecologically hazardous. The main advantages of the SHS production method are low energy constmaption, simplicity of facilities, short processing time (a few seconds), high productivity, ecological safety, and purity of the products [4]. Moreover, a production of high-purity off-stoichiometric phases, direct production of multicomponent solid solutions, synthesis of metastable phases and intermediates, etc. are among the tasks that can be resolved by SHS [4 ]. The physicomechanical properties (hardness, strength, wear-resistance, etc.) of SHS materials are much better than those of the materials produced conventionally. During SHS, the particle size is reduced [4], thus increasing the material strength and resistance to thermal shock [8]. High rates of heating and cooling, another specific feature of SHS, allow one to obtain nonequilibrium phases and quenched materials with modified properties [4]. High heat and wear resistance, thermal stability, resistance to chemically aggressive media, hardness, high melting points (up to 4300K), low coefficient of friction in air, superconductivity, semiconducting or dielectric properties, etc. can exemplify unique properties of SHS materials. Many of the properties listed above are valuable for developing novel carriers for catalysts. Items of any desired shape (cylinder, spheres, honeycomb-structure blocks, etc.) can be produced by net-shape SHS. For this to attain, the charge is preshaped by cold pressing or extrusion. By varying synthesis conditions, one can produce SHS items of desired porosity (total porosity, 5-85%; fraction of open pores, up to 99.7% ) and specific surface (30-90m2/g). Pore size (5-250~tm) can be controlled by varying process parameters or by doping with foaming agents [4]. Mechanical strength and thermal stability of porous SHS materials are an order of magnitude higher than those of conventionally produced materials. Due to high thermal stability, the shape, size, porosity, and developed surface of SHS materials remain unchanged even at very high temperatures. To date, a number of corrosion-resistant, thermally stable, and highly active SHS catalysts (including honeycomb structures) with excellent mechanical characteristics [5] have been applied for oxidation and hydrogenation of organic molecules [9-13].
3. OXINITRIDE SHS CATALYSTS FOR PURIFICATION We developed and studied some new catalytic systems for purification of exhaust gases. These are oxynitride SHS ceramics in the form of powder (specific surface, Ssp=27m2/g), granules (Ssp=l.3ma/g), and cylindrical blocks with a honeycomb structure (Ssp =5m2/g; diameter, 34mm; cell dimension, 1.0xl.0mm; wall thickness, 0.1mm). The blocks had a compression strength (in axial direction) of 50 kg/cm2 and heat resistance up to 1300~ total porosity of 20-25 cm3/g; weight by volume of 1,1 g/cm3.
479
Figure 1. The conversion degree for the gaseous mixture containing 1% CO, 1% CH4, 1% C3H8 in the presence of SHS oxynitride ceramic powder as a function of temperature(W= 18-103hl).
Figure 2. The degree of conversion for CO at different content of bound oxygen of the oxinitride SHS ceramics as a function of temperature. W=45.103h1. x =0,3([~);1,0((~ ;1,5(A); 2,4(V); 2,9((~);3,8(-~.
We used a laboratory-made flow reactor. A gaseous mixture (1-10% CO, 1-2% CH4 and/or C3Hs, 0.44% NO, 3-16% 02, N2 up to 100%) was allowed to pass through (at 100-750~ a tube reactor containing the catalysts (flow rate 6.103-100-103 h-l). The content of carbon monoxide and hydrocarbons was determined by chromatography using molecular sieve (5 A) columns and aluminum oxide (helium, carrier gas; katharometer). Unreacted nitrogen oxide was determined by mass spectrometry. Upon passing the gaseous mixture (1% CO, 1% CH4, 1% C3H8, 9% 02, 88% N2) through the powder of oxynitride SHS ceramics (W=I 8.103h-l), carbon monoxide and propane are oxidized partially at 170~ and completely at 300~ (Fig. 1). Methane, the most stable hydrocarbon, is oxidized at 200-550~ For the gaseous mixture containing 4% CO, 1.5% C3H8, 16% 02, N2 up to 100%, we studied effect of bound oxygen in the oxynitride SHS ceramic granules on their catalytic activity in the oxidation process. A quantity of the bound oxygen in the SHS oxynitride ceramics, general formula M M"OxNy, was determined by method [14]. We found that the bound oxygen markedly affected the catalytic activity (Figs.2 3). With decreasing of the bound
480
Figure 3. The degree of conversion for C3H8 at different content of bound oxygen of the oxinitride SHS ceramics as a function of temperature. W = 45.103h l. x =0,3([-]);1,0((~ ;1,5(A); 2,4(q7); 2,9((~);3,8(-~.
Figure 4. The degree of conversion for CO(1.5%) and C3H8 (1.5%) the mixture contained 10% 02, W=70 -103h-1 in the honeycomb SHS catalyst as a function of temperature.
oxygen content, the temperature of complete oxidation of carbon monoxide and propane diminished by 100-150~ However, the oxygen-free charge yielded catalytically inactive SHS ceramics. Of particular interest are the block honeycomb-structure SHS catalysts. In these catalytic systems, the gas-dynamic resistance is much lower than in conventional ones, the catalytic layer is immobilized, and the active surface is used more efficiently. The data on the oxidation of carbon monoxide and propane in the block oxynitride SHS catalyst (1.5% CO, 1.5% C3H8, 10% 02; W=70 103 9 h 1) are presented in Fig. 4. Note, that at high flow rates, the conversion degree for carbon monoxide and propane attains 100% at 450-500~ The temperature of complete oxidation can be lowered upon immobilization of the 3d transition metals (Co, Ni, Cr, and Fe) oxides on the catalyst surface. Efficiency of the catalysts with immobilized Co and Ni oxides (0.2%) for the oxidation of carbon monoxide and propane is shown in Fig. 5. In this case, carbon monoxide is oxidized at 400-450~ while propane is oxidized at 125-175~ We studied the effect of some modifying agents (NH4C1, alkali earths, rare earths, etc.) on the catalytic activity of oxynitride SHS ceramics. In some cases, the catalytic activity increases while the temperature of complete oxidation of hazardous contaminations
481
Figure 5. The degree of conversion for CO and C3Hs in the honeycomb SHS catalyst with immobilized Co and Ni oxides as a function of temperature; W = 10.103h"1.
Figure 6. The degree of NO conversion for gas mixture with 2% CO, 2% CH4, 0,4% NO, 6% 02 at different flow rates: W=(1)l.103h-l; (2) 4.103h "l and (3) 6.103h-1.
decreased by ~100~ Our experiments showed that the oxygen deficiency (< 10%) in the model mixture results in higher amount of carbon monoxide in the reaction products. The oxidation of propane occurs via intermediate formation of carbon monoxide that can be oxidized to carbon dioxide only in the presence of excess oxygen (> 15%). We also studied reduction of nitrogen oxide in the block honeycomb-structure oxinitride SHS ceramic catalysts upon variation in the flow rate of the model gaseous mixture (Fig.6). Mass spectrometry data showed that nitrogen oxide is reduced to nitrogen by carbon monoxide and hydrocarbons. In the absence of 02, the conversion degree for nitrogen oxide decreases sharply. This suggests that the reduction of nitrogen oxide involves the products of hydrocarbon oxidation as intermediates. We investigated behaviour of the catalytic systems under extreme conditions because the temperature of the catalyst during some modes of convertor operation can attain values of 1000-1100~ Oxynitride SHS ceramics exhibited high heat (1000 h at 150-1000~ and corrosion resistance. Prolonged exposure of the SHS ceramic catalytic systems to gaseous mixtures containing sulfur dioxide and oxygen at 700-1000~ did not cause a noticeable decrease in the catalytic activity.
482 4. CONCLUSION The catalytic activity of the SHS ceramic catalysts in the processes of incineration was found to be virtually identical to that of widespread precious metal catalysts. The ceramic catalysts prepared by promising SHS technique may be a challenge to conventional catalysts for purification of engine exhaust and other waste gases.
ACKNOWLEDGEMENT We thank N. Pershikova, V. Mikhalkin, V. Loryan, V. Semenova and K. Smirnov for the contribution their share to the experimental part of the work.
REFERENCES 1. Prasad R., Kennedy L.A., and Ruckenstein E., Catalytic Combustion, Catal. Rev.Eng., 26, No. 1(1984) 1.
Sci.
2. Panchishnii V.I., Catalytic Render Harmless of Exhaust Gases of Internal Combustion Engines, In: Problems of Kinetics and Catalysis, Nauka, Moscow, 18(1981) 145(Rus). 3. Merzhanov A.G., Shkiro V.G., Borovinskaya I.P., SU Patent No. 255 221 (1967); US Patent No. 3 726 643 (1974); Jap. Patent No. 1 098 839 (1982) 4. Merzhanov A.G., "Self-Propagating High-Temperature Synthesis: Twenty years of search and findings",( Chernogolovka, 1988) In: Combustions and Plasma Synthesis of HighTemperature Materials / Eds. Z.A.Munir, J.B. Holt.N.Y. etc. VCH Publ., 1990 (1-53). 5. Grigoryan E.H., Bokiy N.N., "Prospects of Self- Propagating High-Temperature Synthesis Application for the Technology of Catalysts Preparation",Publ. in Series "Information for the Review" by Scientific and Technical Center "INFORMTECHNICA", Moscow, 1993, Review No 5314 (Rus). 6. Merzhanov A.G., SHS-Process: Combustion Theory and Practice, Arch. Combust., 191, No. 1/2 (1981) 23. 7. Merzhanov A.G., Combustion Problems in Chemical Technology and Metallurgy, Usp. Khim., 45, No. 5 (1976) 827 (Rus). 8. Zavitsanos P.D., Morris J.R., Synthesis of Titanium Diboride by a Self-Propagating Reaction, Ceram. Eng. Sci. Proc., 4, No.7-8 (1983) 624. 9. Marchenko L.S., Zhuk S.Ya., Kir'yakov N.V., Nersesyan M.D., Grigoryan E.H., Oxidative Dehydrodimerization of Methane Over Complex Oxide Catalysts Prepared by Self-Propagating High-Temperature Synthesis, Catal. Today, 13 (1992) 593.
483 10. Borovinskaya I.P., Loryan V.E., Grigoryan E.A., Salnikova E.N., Pershikova N.I., SHS Oxynitrides as Catalysts for Carbon Monoxide Oxidation, Intern. J. of SHS,1, No. 1 (1992) 131. 11. Grigoryan E.H., Blumberg E.A., Borovinskaya I.P., Merzhanov A.G., SHS Catalysts of Oxidation, 6th Intern. Symp. on the Activation of Dioxygen and Homogeneous Catalytic Oxidation, The Netherlands, 1996. 12. Merzhanov A.G., Grigoryan E.H., Pisarev R.,V., Nayborodenko Y.S., Filatov V.M., Lavrenchuk G.V., Lunin V.V., Sichev N.N., and Meshcheryakova E.V., RU Patent No.2 050 192 (1992). 13. Lunin V.V., Sichov N.N., Kruglova M.A., Grigoryan E.H., Nayborodenko Y.S., Effect of Method of Starting Alloies Preparation on Physicochemical Properties of Iron-Containing Catalysts for Ammonia Synthesis, Zhurn. Phiz. Khim.,69, No. 6 (1995) 987 (Rus). 14. Barinov Y.N., Kustova L.V., Vishnyakova G.A., Egorova N.E., Analysis of total and bound oxygen in SHS powders of refractory compounds. Preprint. Chemogolovka,1989.
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CATALYSISAND AUTOMOTIVEPOLLUTIONCONTROLIV Studies in Surface Science and Catalysis,Vol. 116 N. Kruse, A. Frennet and J.-M Bastin (Eds.) 91998Elsevier Science B.V. All rights reserved.
Catalytic
485
decomposition of high-concentration nitrous oxide N 2 0
H.C. Zeng ~ M. Qian, X.Y. Pang
Department of Chemical Engineering, Faculty of Engineering National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260
High-concentration N20 has been decomposed catalytically with four metal oxide systems which contain s, p, d-block elements. In all cases, the decomposition reaction can be treated as first order with respect to partial pressure of N20, considering that decomposition product 02 exhibits no or little inhibition effect on the reaction. Optimal preparation conditions for the catalysts have been explored. In particular, chemical interaction between surface metals and carriers has been investigated. The catalysts have been characterized by various analytical techniques and general material issues have also been addressed. Among the four systems studied, Ru/AI203 and Mg-Co mixed oxides give best catalytic performance. For example, approximately 6 moles of N20 per kg of the hydroxide/hydrotalcite can be decomposed at 350~ within an hour.
1. INTRODUCTION The catalytic decomposition of nitrous oxide (N20) has become an important sub-field of de-NO~ research because of the severe greenhouse effect of the gas and its contribution to the catalytic stratospheric ozone destruction. Man-made nitrous oxide may arise as a coproduct from some chemical processes, such as the use of circulating fluidized beds for combustion, automotive exhaust emissions, and nylon production. To tackle N20 emission, a great variety of catalysts and processes have been recently developed. For example, zeolitebased catalysts, such as Cu-ZSM-5, Fe-ZSM-5, Co-ZSM-5, Ru-NaZSM-5, and Na-ZSM-5, have been studied extensively over the last ten years [1-7]. Transition metals and their oxides, in forms of either simple or complex oxides, have also been investigated thoroughly [ 1,8]. As a result of continuous search for new catalysts, hydrotalcite-like compounds, which show a significant improvement in lowering decomposition temperature, have been synthesized and tested very recently [9]. In this paper, we review four different metal oxide systems which have been recently investigated for the high-concentration N20 (27-29 mol%) decomposition [ 10-13].
2. EXPERIMENTAL The catalytic materials were prepared by impregnation method in Co-Ni/ZrO 2, PbZr/Al203 and Ru/Al203 [10-12] and by coprecipitation method in Mg-Co oxides [13]. Briefly, the preparation involved impregnating carrier pellets with a metal containing aqueous solution
486 (or organometallic sol-gel [11]). After the coating treatment, the wet materials were calcined in a furnace (Carbolite) with static air at various elevated temperatures. The N20 gas was continuously supplied from a gas cylinder (27-29 mol% N20, balanced with He) to a tubular reactor packed with catalytic material (V) and situated in a vertical furnace (Carbolite). The decomposed gases were cooled through a cooling coil and then vented through a scrubber. The inlet and outlet compositions of gas were analysed by the gas chromatography to determine N20 decomposition conversion (%) [10-13]. The catalytic materials were studied with Fourier transform infrared spectroscopy (FTIR), electron scanning microscopy (SEM), energy-dispersive analysis of x-rays (EDAX), x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), the differential scanning calorimetry (DSC), elemental analysis (EA) and surface area determination (BET).
3. RESULTS AND DISCUSSION 3.1 Co-NYZr02 The steady state conversion of N20 on Co-Ni/ZrO2 system is reported in Table 1. For as-received monoclinic ZrO2 (CA), a T of 525~ should be maintained to achieve the 99.5% conversion. With catalyst CB, similar conversion can be obtained at 474~ which is 50~ lower than that in the case of blank ZrO2 (CA). Further reducing of temperature can be seen in the decomposition study using catalyst CC, which results in a 117~ lowering from 525~ The decrease in catalytic activity is observed when the metal loading exceeds that of catalyst CC. For example, indicated by T = 455~ catalyst CD shows a deactivation in N2O decomposition. It is interesting to realize that ZrO2 is a solid electrolyte in the reaction temperature range used, which may generate new electronic states and surface compositions for these catalysts. In particular, the catalytic activity arising from this catalyst-support modification can be ascribed to the decrease Zr-O-Zr interaction and increase M-O-Zr (M = Co 2§ or Ni z§ interaction. As in the case of catalyst CD, an overloading in metal content should promote M-O-M interaction, causing a reduction in the number of active surface sites for N20 adsorption which is a crucial step in decomposition reaction [10]. Table 1. Compositions and preparation procedures for Co-Ni/ZrO2 catalysts. Catalysts CA CB CC CD
Preparation ZrO2 support ZrO2 in 1.4 M Co2§ ZrO2 in 2.0 M Co2§ ZrO 2 in 2.6 M Co2§
z§ 2§ 2+
Heat Treatment
T for 99.5% Conversion
700~ for 2 h 700~ for 2 h 700~ for 2 h
525~ 474~ 408~ 455~
Concentration (M) is based on total metal ions in aqueous solution; Co:Ni GHSV (F/V) = 300 h"l.
= 1:1;
29% N2O,
Based on results of XPS, it is found surface carbon species can be removed from catalysts through the reaction with N20 decomposition intermediate products, for example, adsorbed O or N. Since no N ls photoelectron was detected by XPS, the depletion of surface carbon can be described by the following two reactions [10]:
487 ZrC + 20"(ad) ~ ZrO + CO(g) ZrC + 02 (ad) ~ ZrO + CO(g)
(1) (2).
The loaded metal cobalt Co2p3a XPS spectra measured from Co/Ni/ZrO 2 systems at room temperature are presented in Figure 1. According to a recent surface analysis for cobalt oxide system [14], surface Co304 is thermodynamically stable form at room temperature and a Co304 to CoO transition occurs at around 620 K. For an air-formed cobalt oxides, it is often found that a Co(OH)2 film forms on the outer Co304 oxide surface [15]. In Figure 1, XPS spectra for catalyst CC reveal a Co2p3n binding energy of 780.8 eV and a shake-up satellite 5.3 eV from the primary peak. This would likely indicate either a formation of Co(OH)2 on the outer oxide surface or a surface cobalt oxide in its complex form Co-O-Zr, compared with the literature values and analyses [15]. Nevertheless, since no OH" was detected in FTIR, the Co2p3r2 observed here could be attributed to the chemical shift resulting from the modification of substrate ZrO 2. The surface species arising from this modification, Co-O-Zr, would cause Co2p3t2 shifted to higher binding energy [16]. Therefore, XPS spectra reported in Figure 1 suggest a predominant presence of Co304 oxide on the surface. Although EDAX shows the coexistence of both Co and Ni in the ZrO 2 support, nevertheless, the Ni2p3a photoelectrons are not in a detectable level, compared to that of Co2p3t2 in the same XPS study. In our latter XPS work on Ni 2§ diffusion in sol-gel derived ZrO 2, it is found that atomic ratio of Ni/Zr decreases significantly over the temperature range of 400 to 700~ [17]. This observation as well as the failure in detecting Ni2p3~2 photoelectrons here can be attributed to the formation of solid solution NiO-ZrO2 in surface region resulting from an in-diffusion of Ni 2+ to ZrO 2 matrix. Co2p 9
.. .-.
9
.....~. 9....
. . . . - ' ; .
....
-;:,
03 c..
O
0
:~'..
9, ."
;9 --
-..
.
...
9 "".,.
.. .
.
761
......
.
_
.
.
93
Figure 1. C02p3t2 XPS spectra measured from optimal metal loading Co-Ni/ZrO2 (CC, Table 1) before (top) and after (bottom) NzO decomposition reaction.
Binding E n e r g y (eV)
3.2 Pb-Zr/AI203 Conversion-versus-temperature curves measured for blank AI203 (CE), supported ZrO 2 (catalyst CF, i.e. ZrO2/A1203), and catalyst CG (PbO-ZrO2/Al203 with Pb:Zr atomic ratio = 1:6) are displayed in Figure 2. Also displayed in the figure is the conversion rate measured from the empty reactor, which corresponds to the gas phase N20 decomposition. It is clear that catalyst CG gives the best catalytic performance among the three tested materials. For example, at 420~ catalyst CG has shown catalytic activity, indicated by a conversion rate of 8% for the high concentration N20 gas. Similar conversion rate for CF and CE starts only at around 490~ Among the two types of supports (CF versus CE), the ZrO 2 shows a better catalytic activity towards N20 decomposition, although the specific surface area for AI203 is higher [ l 1].
488
80
v r
.o_ ctJ CD C: 0
Figure 2. The conversion-vs-temperature curves for catalytic activity comparison, using CE ( , ) , CF (zx) and CG (x), and the empty reactor (m); GHSV = 577 fit.
60
4O
C) 20
0420
470
520
570
620
Temperature
670
720
770
(~
To correlate Pb 2§ concentration to catalyst activity, Figure 3 presents a Pb-uptake curve and effect of Pb 2§ content on conversion. Up to 0.151 M, the Pb-content rises monotonically with the increase of concentration. The turning point (0.151 M) here has an atomic ratio Pb:Zr = 1:1, corresponding to the formation of PbZrO 3 compound [18,19]. It is important to recognize that the curve shown in Figure 3 in fact is not truly "uptake", since the catalysts were heat-treated at 650~ before measurement. The deviation from linearity for higher concentration samples can be attributed to decomposition and evaporation of the excessive PbO, which has been observed in a number of PbO-involved materials, including the PbOZrO2 system [20]. The high catalytic activity and surface area observed for the catalyst CG can be explained by the formation of Pb involved active sites and stabilization of low temperature tetragonal phase for the carrier material ZrO 2 (XRD results [ 11]). High oxidation state of Zr(IV) and low Pb:Zr ratio will favour the variation of oxidation state of counter cation Pb to higher ones, creating certain active sites (M"+/M~§ M = Pb; or MWM '"§ M = Pb, M'= Zr) for N20 decomposition. The actual oxidation states of the metals are investigated with XPS [21]. The possible active sites for the reaction may also be related to lattice oxygen anion defects formed after cation incorporation [22]. For high Pb:Zr ratio catalysts, such as in the case of Pb:Zr = 1:1, the catalytic activity is substantially reduced. This can be attributed to change of ZrO2 structure to perovskite type and thus to demolishment of cation pairs with multiple oxidation states that are essential for facilitating the decomposition of N20 gas [23-25]. It is interesting to note that, in this material system, 02 behaves differently over the various reaction temperatures. At the temperature of 450~ the mixing 02 into the gas stream increases the conversion. Since the conversion rate at this temperature is rather low, the partial pressure of 02 generated from the decomposition products in the outlet gas is insufficient in keeping certain chemical equilibria between catalyst (solid) -oxygen (gas) [20]. Due to this the adding of 02 in the first place would help the catalyst surface to maintain fight oxidation states for Pb cations. However, at 500~ 02 shows a small degree of inhibition effect on the reaction, indicating the retention of 02 gas hampers decomposition of N20 gas at this temperature. At higher temperature range (550 and 600~ the inhibitive role of 02 gas in decomposition is greatly weakened. This is suggested by the constant conversion rates found over the increasing O2 concentrations.
489 It has been suggested for many oxide systems that the N20 decomposition is first order with the gas adsorption as rate-controlling step [25,26]. The decomposition mechanism of this type involves an electron transferring from a low oxidation state metal cation to an adsorbed molecule N20 [27,28]. In this oxide system, it has been illustrated that the inhibitive role of 02 over the high conversion rate regime is insignificant. Therefore, the rate law of such a reaction regardless of the inhibition of the product gas O 2 can be given as [ 17]: -dPN2o/dt = kPmo
(3).
The integration of Eq. (3) gives: (4).
In {ln(Pi~2o~e~2o) } = lnA - In(F/V) - EJRT
where V is the volume of the catalyst bed, F is gas flow rate, and A is Arrhenius preexponential factor, and E, is apparent activation energy of the decomposition [25]. Eq. (4) is used in the current work to evaluate the catalytic activities and to get some sense of kinetic behaviours of the materials. Plotting tn{ln(Pg.l~2o/Py.mo)} against 1/T for materials CE, CF, and CG, one can found that the data of CE and CG are fitted nicely into straight lines. Using the same experimental data, other kinetic models such as surface reaction control mechanism give less linearity [25,27,28], compared to the above adsorption control case. It is therefore concluded that the decomposition of N20 on CE and CG follows the first order reaction with NzO adsorption as rate-limiting step. The apparent activation energies calculated are 144.1 and 111.1 kJ/mol for CE and CG respectively. However, using Eq. (4) for CF carrier, the linearity of data reduces noticeably, suggesting the existence of different reaction mechanisms from the current adsorption control type. This observation is in agreement with a postulated reaction mechanism for blank ZrOz matrix surface [10], in which the electron transfer involved is not directly from a metal cation.
S
5 ,.-.,
v ...., t-
(b) 90
v
.o r-
3
tO
i,_ 0 > cO
(,3 r n
80
t/}
2
7o
60
I
0
!
,!
0.1
0,2
,
i
i
0.3
0.4
Pb(NOa)2 Concentration (mol/L)
50 0.5
0
0.1
0.2
0.3
0.4
0.5
Pb(N03)2 Solution Concentration (mol/L)
Figure 3. (a) Pb content (wt%) in the catalysts against Pb(NO3) 2 concentration (M) used in the catalyst preparation, and (b) N20 conversion at reaction temperature 580~ ( N 2 0 28%, GHSV = 577 ht) versus Pb(NO3) 2 concentration (M) used in the catalyst preparation.
490 3.3 Ru/AI203 The general trend observed for specific surface areas of the CH-CQ catalysts in Table 2 is that with increase of ruthenium content on the surface of alumina, specific surface area reduces rather linearly. This can be understood in terms of modifications of original carrier porosity by the metal and its related surface species [12]. With Ru-loading on alumina, the catalytic activity is significantly enhanced. For example, the as-treated alumina (CH) gives only conversion of 6% at 480~ while for most Ru-loaded catalysts a 100% conversion can be achieved at the same temperature or below. The activity for the current catalyst system is high, based on the reaction rate of 4.8x 104 lamol (N20).g-t .fit at 400~ calculated for the catalyst CN. In Figure 4(a), conversion-versustemperature curves for different surface metal loadings are displayed. While specific surface area data show a monotonic decrease with the increase of surface Ru-loading, actual catalytic activity of N20 decomposition for CH-CQ varies markedly. The most active catalyst is CN (Table 2), whose ruthenium content is somehow only "intermediate". The discontinuous feature in Figure 4(a) reveals a drastic change in surface structure across a critical loading at Ru wt% = 0.20. This is further suggested by the linear increase of conversion in each discrete stage, i.e. either low-Ru-loading or high-Ru-loading, for these curves. Table 2. Preparation parameters for Ru/A1203 catalysts. Catalyst
Ru-red dissolved Ru-red(g)/I-t20(ml)
Ru-red concentration Thermal-treatment (g/ml) ~
Ru-loading (wt%)
CH CI CJ CK CL CM CN CO CP CQ
0.0000/20.0 0.0883/20.0 0.1766/20.0 0.2650/20.0 0.3500/20.0 0.4000/20.0 0.4500/20.0 0.5000/20.0 0.7500120.0 1.0000/20.0
0 4.4x 104 8.8x 10.3 1.3 x 10.2 1.8x 102 2.0x 10-2 2.3 x 102 2.5x 102 3.8x 10.2 5.0x 10"2
0.00 0.07 0.11 0.15 0.18 0.19 0.20 0.21 0.24 0.26
200(2)/500(4) 200(2)/500(4) 200(2)/500(4) 200(2)/500(4) 200(2)/500(4) 200(2)/500(4) 200(2)/500(4) 200(2)/500(4) 200(2)/500(4) 200(2)/500(4)
Notes: (i) For each batch catalyst, 10.0 g of as-received AlaO 3 support was soaked in 20.0 ml Ru-containing solution indicated above; and (ii) Ru-loading (wt%) on AlzO 3 support was determined by EA method. The above observation on catalytic reactivity suggests two surface structures may be responsible for the varied catalytic performances. This postulation is validated by the study on the uptake behaviour of the catalyst series. The Ru-loading (uptake curve) on alumina carrier is shown in Figure 4(b). It is clearly identified from these data that there are two distinct regions corresponding to low- and high-Ru-content catalysts. In low-Ru region, the increase in ruthenium content is quite sharp, while in the high-Ru region, the slope changes pronouncedly, indicating a levelling-off. Obviously, two different types of surface structure can be expected, judging from the slopes of the Ru-loading versus Ru-red dissolved. It is important to mention that, the breaking point, which is bridging the two different regions,
491 happens to be the Ru wt% = 0.20. Once again, the juncture point feature of the catalyst CN is illustrated. For a rough estimation, if Ru:A1 atomic ratio in the low-Ru part of Figure 4(b) is determined by submonolayer of adsorbed Ru-red molecules, using Langmtiir chemisorption model, that of the high-Ru part should be thus ascribed to the monolayer adsorption of Rured. It is noted that in previous studies of N20 decomposition on 1 wt% Ru/AlzO3, the activities at temperatures below 350~ are found negligible [1,2]. Following our "uptake" study, apparently, the low activities reported for the relatively high Ru-content catalysts (1 wt% Ru/AlzO3) [1,29] should be ascribed to the N20 decomposition over the region two of Figure 4(b). 100
---
0.3
(b)
(a)
8o
.~.
g
g 0
~
0.2
60
= 360
40
*390
20
- 420
0
~
o
~ -
o.1
0~ 0.1
0.2
Ru-Loading (wt%)
0.3
: 0.0
-
, 0.2
..... - . . . . . . . , ..... ~.4
-
, 0.6
-
, ....... 0.8
, 1.0
Ru-Red Dissolved (g, in 20 ml Water)
Figure 4. (a) Conversion against ruthenium loading for different reaction temperatures, 330, 360, 390, and 420~ (28% 1'420, GHSV - 1000 ht), and (b) Actual ruthenium loading (wt%) in the catalysts CH-CQ against Ru-red (g) dissolved in 20.0 ml H20 for the catalyst preparation. It has been reported in the literature for the Ru/A1203 system that the ruthenium present on alumina carrier is in the form of RuOz [30] or RuO/ [1,2], under the similar preparation conditions. No obvious bimetallic phase between Ru and A1 is found, which suggests the direct interaction between the two metals is not significant [30]. The phase of RuO 2 is not expected to be altered after N20 decomposition, since it is formed when ruthenium metal reacts with N20 gas [31,32]. The ruthenium dioxide or its related oxides RuO x [2], that has been known a good electron conductor with high chemical stability [33,34], is thus thought to be active component in N20 decomposition. Based on ruthenium coordination chemistry, the cation pair Ru(m)/Ru(IV) has been proposed to be responsible for the N-O bond weakening and charge transfer in the N20 decomposition [2,29]. According to the existing decomposition models in the literature, the decomposed product oxygen has been often related to reaction rate in the form of (Po2)(~176 [35]. This gas also shows a small effect on the decomposition rate in the current catalyst system. The effect, however, is only significant in low temperature reaction (320~ For higher decomposition temperatures (360 and 400~ this effect is almost negligible, though it still shows a decline trend. It should be mentioned the lowering in conversion with increasing 0 2 is not due to increase of space velocity [12]. Eq. (4) has also been employed here to get some sense of kinetic behaviours of the catalyst series Ru/AI203. The In{In(Pi.mo/Pi.mo) } term is plotted against I/T for three representative catalysts CI, CN, and
492 CQ. It is found that the data of CN is fitted nicely into a straight line. For other "less than ideal" catalysts, such as low Ru-content catalyst CI and high Ru-content catalyst CQ, the linearity of the fits is much worse [12]. As the current study serves only as an initial exploratory work on this catalyst system, further detailed investigation on the reaction mechanisms using other kinetic models such as surface reaction control and 02 desorption control [35] will not be carried out here.
3.4 Mg-Co Oxides Two catalyst precursors, CR and CS which were synthesized from coprecipitation method [13], have been studied in this material system. Revealed by the EA study, CR has an atomic ratio of Co:Mg = 4.37 and CS Shows a Co:Mg = 2.35. The atomic ratio of Co:Mg in the precipitates depends mainly on the aging temperatures received [13]. The formation of mixed hydroxide in CR and CS is confirmed with FTIR investigation. For example, the OH absorption at 3644 cm"L (CR; high Co:Mg ratio) is closer to 3631-3632 cm "L of Co(OH)2 while the 3654 cm "t (CS; lower Co:Mg ratio) is nearer to 3700-3698 cm ~ of Mg(OH)v With higher content of Mg in the double hydroxide (CS), the brucite-like sheet would be generally anticipated [36]. Furthermore, since the CS is hydrothermally treated at 60~ for 18 h, higher crystallinity will also be expected. These two points are actually confirmed by the SEM morphological study [13]. Better crystallinity, greater grain sizes, more regular aggregates, and flatter surfaces for CS are indeed observed. However, the calcination treatment at higher temperatures had obviously led to considerable modifications in surface morphology, which will be addressed shortly. The formation of hydrotalcite-like phase is confirmed by the XRD investigation. As shown in for the anion rich sample CR, the broad diffraction peaks are indeed characteristics of hydrotalcite-like compounds [36-38]. In the spectrum of CS, however, this hydrotalcite feature is not observable; only pronounced brucite-like compound diffractions can be seen. This observation is consistent with the low anion content found in the elemental analysis. The presence of hydrotalcite-like phase in the CR samples indicates the formation of the trivalent cation (Co 3§ due to the presence of air atmosphere during precipitate formation [39]. In line with the XRD results, the DSC/FFIR study of CR also suggests the formation of hydrotalcite-like phase [13]. High Mg-containing CS is more thermally stable than CR. With increase in calcination temperatures, the FTIR spectrum evolution reveals the formation of mixed metal oxides, i.e. spinel-type phase [40]. The decomposition of catalyst precursors is also reflected in the XRD study which shows that the resultant oxides are largely amorphous although weak spinel-type feature can be observed [41]. Figure 5(a) shows results of BET measurement for the above two calcined sample series. When the hydroxide/hydrotalcite phases are converted to metal oxides, the surface area varies substantially. For example, the surface area of CR peaks at 250~ and later declines at higher temperatures since it is Co-rich and decomposes readily. In the CS series, however, the surface area maximizes only at 300~ since it is more stable to sustain thermal treatment. In both cases, maximum surface area is obtained when hydroxide/hydrotalcite framework collapses, i.e., it occurs at temperatures of metal oxide formation. The decrease in surface area at higher calcination temperatures can be explained as grain growth of oxides. All resultant Mg-Co oxides studied exhibit reasonable catalytic activities for N20 decomposition. Both CR and CS give a notable activity at temperatures as low as 250-275~ and 275-300~ respectively. Under these experimental conditions, approximately 6 moles of
493 NzO per kg of the hydroxide/hydrotalcite can be decomposed at 350~ within an hour, which is comparable to some of the most active catalysts reported so far [ I-9]. It is recognized that the activity difference between CR and CS is not due solely to surface area variation. For example, the surface area of CS is about 40% higher than that of CR after calcination (Figure 5(a)), whereas CS is about 200% more active than CR at 350~ Since atomic weight for Mg is much lighter than that of Co, the total amount of metal cations in CS catalyst is higher than that of the CR. It is thus suggested that the higher catalytic activity observed for CS be attributable to more active sites formed by Mg-O-Co with an appropriate atomic ratio of Mg:Co. Apparently, the optimization of Mg:Co ratio and other precessing parameters is needed in further study. 60
20O
5o.
(a) .-.
150
9
I
(b)
CS
CS ~"
CR
3o
R
o
:j
50 10-
0
', .
.
.
.
9
200
100
9
'"'
=
3OO
calcination tem0erature/*C
0
9
400
100
~-~ 150
200
250
300
3,50
400
, 450
Ti*C
Figure 5. (a) BET measurements for the calcined CR and CS sample series, and (b) Catalytic activity evaluation for catalysts (27% NzO, GHSV = 3000 ht); prior the catalytic test, asprepared CR or CS was decomposed in situ under the flow of the reaction mixture at 350~
CONCLUSIONS High-concentration N20 has been decomposed catalytically with four material systems. In all cases, the adsorption of N20 can be described as controlling step in the decomposition reaction. The decomposition product, 02 , exhibits no inhibition effect in high conversion rate range. These studies show the presence of chemical interaction between surface metals and carriers and their combination effects on the optimization of catalysts. The general material issues have also been addressed. Among the four systems studied, Ru/AlzO3 and Mg-Co mixed oxides give best catalytic performance, which are among the most efficient catalysts reported in literature for NzO decomposition at low reaction temperature.
REFERENCES
.
.
Y. Li and J.N. Armor, Appl. Catal. B 1 (1992) L21; Appl. Catal. B 3 (1993) 55; US Patent 5 149 512 (1992); US Patent 5 171 553 (1992). Y.-F. Chang, J.G. McCarty, E.D. Wachsman, V.L. Wong, Appl. Catal. B 4 (1994) 283. L.M. Aparicio and J.A. Dumesic, J. Mole. Catal. 49 (1989) 205.
494
~
5. ,
7. 8.
9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41.
I. Valyon, W.S. Millman, and W.K. Hall, Catal. Lett. 24 (1994) 215. K. Ebitani, M. Morokuma, J.-H. Kim, and A. Morikawa, Bull. Chem. Soc. Jpn. 66 (1993) 3811. G.I. Panov, V.I. Sobolev and A.S. Kharitonov, J. Mol. Catal. 61 (1990) 85. L.M. Aparicio, M.A. Ulla, W.S. Millman and J.A. Dumesic, J. Catal. 110 (1988) 330. B.W. Riley and J.R. Richmond, Catal. Today 17 (1993) 277. S. Kannan and C.S. Swamy, Appl. Catal. B 3 (1994) 109. H.C. Zeng, J. Lin, W.K. Teo, J.C. Wu, and K.L. Tan, J. Mater. Res. 10 (1995) 545. X.Y. Pang, H.C. Zeng, J.C. Wu and K. Li, Appl. Catal. B 9 (1996) 149. H.C. Zeng and X.Y. Pang, Appl. Catal. B (1997) in press. M. Qian and H.C. Zeng, J. Mater. Chem. (1997) in press. M. Oku and Y. Sato, Applied Surface Sci. 55 (1992) 37. D.B. Mitton and J. Walton and G.E. Thompson, Surf. Inteff. Anal. 20 (1993) 36. T.L. Barr, J. Vac. Sci. Technol. A 9 (1991) 1793. H.C. Zeng, J. Lin, W.K. Teo and K.L. Tan, J. Non-Cryst. Solids 181 (1995) 49. J.C. Shaw, K.S. Liu, and I.N. Lin, J. Am. Ceram. Soc. 78 (1995) 178. C.S. Hwang and H.J. Kim, J. Am. Ceram. Soc. 78 (1995) 329. J.H. Moon and H.M. Jang, J. Am. Ceram. Soc. 76 (1993) 549. H.C. Zeng, X.Y. Pang, J. Lin, and K.L. Tan, unpublished work. P. Li, I.W. Chen, and J.E. Penner-Hahn, J. Am. Ceram. Soc. 77 (1994) 118; J. Am. Ceram. Soc. 77 (1994) 1281; and J. Am. Ceram. Soc. 77 (1994) 1289. G.D. Lei, B.J. Adelman, J. Sarkany, W.M.H. Sachtler, Appl. Catal. B 5 (1995) 245. A.K. Ladavos and P.J. Pomonis, Appl. Catal. B 2 (1993) 27. R. Sundararajan and V. Srinivasan, Appl. Catal. 73 (1991) 165. A. Cimino, La Chimica el Industria 56 (1974) 27. S. Akbar and R.W. Joyner, J. Chem. Soc., Faraday Trans. 1, 77 (1981) 803. D.D. Eley, A.H. Klepping and P.B. Moore, J. Chem. Soc., Faraday Trans. 1, 81 (1985) 2981. Y.-F. Chang, J.G. McCarty, and E.D. Wachsman, Appl. Catal. B 6 (1995) 21. J.M. Rynkowski, T. Paryjiczak, and M. Lenik, Appl. Catal. A 126 (1995) 257. R. Klein and R. Siegel, Surf. Sci. 92 (1980) 337. R. Klein, R. Siegel, and N. Erickson, J. Vacuum Sci. Technol. 18 (1979) 489. A.F. Wells, Structural Inorganic Chemistry 5th Ed. (Clarendon Press, Oxford, 1984) p. 541. J.F. Tressler, K. Watanabe, and M. Tanaka, J. Am. Ceram. Soc. 79 (1996) 525. R. Sundararajan and V. Srinivasan, Appl. Catal. 73 (1991) 165. F. Cavani, F. Trifiro and A. Vaccari, Catal. Today 11 (1991) 173. W.T. Reichle, Solid State Ionics 22 (1986) 135. K.A. Carrado, A. Kostapapas and S.L. Suib, Solid State Ionics 26 (1988) 77. H.C. Zeng, M. Qian and Z.P. Xu, unpublished work. G. Busca, F. Trifiro and A. Vaccari, Langmuir 6 (1990) 1440. G. Fornasari, S. Gusi, F. Trifiro and A. Vaccari, Ind. Eng. Chem. Res. 26 (1987) 1500.
CATALYSISAND AUTOMOTIVEPOLLUTIONCONTROLIV Studies in Surface Science and Catalysis,Vol. 116 N. Kruse, A. Frennet and J.-M Bastin (Eds.) 91998Elsevier Science B.V. All rights reserved.
495
Structure and Activity of Cu/Cr/SnO2 Environmental Control Catalysts Philip G. Harrison*, Wan Azelee, Ahmed T. Mubarak, Craig Bailey, Wayne Daniell and Nicholas C. Lloyd Department of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD, (U.K.)
1. INTRODUCTION The driving force for the development of non-platinum exhaust emission catalysts is the price, strategic importance and low availability of the platinum group metals. Development work on alternative catalytic materials can either be directed towards finding a material that completely replaces platinum group catalysts or towards finding compounds that allow a reduction in the amount of precious metals needed in each catalyst. Non-noble metal materials comprising copper- and chromium-promoted tin(IV) oxide (Cu/Cr/SnO2 catalysts) exhibit excellent three-way catalytic activity. - activity which is comparable to that shown by noble metals dispersed on alumina [1]. In this paper we focus upon the nature of these oxide materials with respect to composition and calcination history using powder XRD, TEM, EXAFS and XPS. Some catalytic activity data for the oxidation of CO and propane are also described. 2. PREPARATIVE DETAILS Catalyst materials were prepared by the coprecipitation method. The appropriate quantities of copper(II) and chromium(Ill) nitrates were dissolved and homogenised in a vigourously stirred solution of tin(IV) chloride in triply distilled water. To this solution was added dropwise concentrated 33% aqueous ammonia solution to a final pH of 4. The resulting gelatinous precipitates were washed free from chloride ion by repeated centrifuging and redispersing in triply distilled water. The solid Cr(III)/Cu(II)/SnO2 gel was then allowed to dry in air at 60~ for 2-3 days. Portions of the gels were calcined at temperatures of 300, 400, 600, 800 and 1000~ for 24 hours. Target and actual elemental analysis together with appearance and nomenclature data are collected in Table 1. 3. X-RAY DIFFRACTION STUDIES Information regarding the crystalline phase(s), at calcination temperatures of _>600~ and average particle size data determined by line broadening were obtained by powder X-ray diffraction. Prior to calcination, all of the Cr(III)/Cu(II)/SnO2 materials exhibit four very broad peaks characteristic of small particulate SnO2. XRD patterns for the 60Sn:20Cr:20Cu system after calcination at 600, 800 and 1000~ exhibit a second phase other than tetragonal SnO2 due to the tetragonal spinel CuCr204. This phase is observed initially after calcination
496 at 600~ and Rietveld analysis of the material after calcination at 1000~ gave peak positions for the CuCr204 spinel phase at interplanar spacings (d/A) of 4.765 {101 }, 3.018 {200}, 2.876 {112}, 2.550 {211},2.384 {202}, 2.384 {103}, 2.134{220}[2].
Table 1. Analytical and physical data for the catalyst materials. Catalyst metal Calcination temperature / ~ atom ratio a (Target metal atom ratio in parentheses) As prepared 300 600
1000
61.1Sn: 18.4Cr:20.7Cu (60Sn:20Cr:20Cu)
Dark green
Black
Black
Grey
70.0Sn:21.0Cr:9.0Cu (70Sn:20Cr: 10Cu)
Dark green
Black
Black
Grey
71.0Sn:9.5Cr:19.5Cu (70Sn: 10Cr:20Cu)
Dark green
(a)
Light green
Red/brown
Pink/grey
Analysis by X-ray fluorescence.
Diffractograms for the 70Sn:20Cr: 10Cu material, where chromium is in excess of copper, after calcination at temperatures in excess of 600~ show CuCr204 as well as Cr203. Similar observations have been made by Castiglioni et al.[3] for various copper-zinc-cadmium chromite catalysts where, with excess chromium, Cr203 and tetragonal CuCr204 were observed after calcination at 500~ The diffractograms for the 70Sn:10Cr:20Cu material, where copper is in excess, after calcination at >600~ again exhibit CuCr204 with small amounts of. However, after calcination at 1000~ although monoclinic CuO can still be detected, copper chromate, CuCrO2, which possesses a spinel-like structure with the most intense peaks at interplanar spacings (d/A)of(2.851) {006}, (2.549) {101 }, (2.468) {012}, {104} (2.207), {018} (1.645), { 110} (1.488) [4], is also present. These observations are in accordance with the literature [57], i.e. the high-temperature heating of Cu/Cr oxide catalysts enhances the formation of CuCrO2, especially with an excess of copper. Recent studies by Chien et al. [8] on Cu/Cr/~/alumina catalysts for carbon monoxide and propene oxidation have also shown that with an excess of copper phase transformation from the CuCr204 spinel to CuCrO2 occurs readily on calcination at 900~ Investigations into the possibility of incorporation of chromium or copper into the tin(IV) oxide were made by indexing and refining the XRD patterns by the Rietveld method. No lattice incorporation is induced by thermal treatment. Mean crystallite sizes of the SnO2 particles (D) (Table 2) were calculated using the Scherrer equation. Particles for all three compositions remain small (<25A) for calcination
497 temperatures up to 400~ However, after calcination at 600~ a ten fold increase in size is observed. Further increase in size occurs at higher calcination temperatures. 4. TEM STUDIES
Microscopy was performed on the 60Sn:20Cr:20Cu material as prepared and after calcination at 300, 400, 600, 800 and 1000~ Data was obtained using a JEOL 2000FX transmission electron microscope with specimens mounted on holey carbon nickel coated TEM grids. There is a good agreement with the observed particle sizes obtained by TEM with those calculated from X-ray line broadening. The system appears to be generally amorphous in nature up to calcination temperatures of 400~ with agglomerations of particles ranging from 15A (as prepared) up to 250A (600~ in dimension (Table 2). A large increase in particle size is observed after calcination at 800~ and higher, with the particles possessing the appearance of rounded hexagons but displaying a wide variation in size (ca. 200A). At 1000~ the particles are ca. 120 times greater in size than for the uncalcined material. EDX analysis confirmed that the Sn, Cr and Cu compositions remained uniform in good agreement with the XRF elemental analysis.
Table 2. Mean particle sizes for Cr(III)/Cu(II)/SnO2 catalyst materials. .... Material
Mean Particle Size (D) / A
As Prep
300~
400~
600~
800~
1000~
60Sn:20Cr:20Cu a
13
13
17
165
364
1494
60Sn:20Cr:20Cu b
15+5
15+5
20+5
250+80
400+200
70Sn:20Cr: 10Cu a
12
13
21
131
342
1186
882
2896
70Sn: 10Cr:20Cu a 13 (a) From X-ray line broadening.
13
25 159 (b) From TEM micrographs.
1800i200
5. EXAFS STUDIES
Chromium K-edge EXAFS data were obtained at DRAL, Daresbury. for the 70Sn: 10Cr:20Cu material (uncalcined and after calcination at 300, 800 and 1000~ and also for the 60Sn:20Cr:20Cu material after calcination at 300~ and 1000~ and the 70Sn:20Cr: 10Cu material after calcination at 1000~ A pre-edge peak is observed for the materials after calcination at 300~ at ca. +4.1-4.3eV from the Cr K-edge which disappears on calcination at higher temperatures. This peak arises from a dipole allowed 1s--->3d electronic transition and is characteristic of Cr 6+ in a tetrahedral co-ordination environment [9,10]. Observation of this feature demonstrates that oxidation from Cr(III) to Cr(VI) occurs in the catalyst materials on mild calcination. Numerical fits to the EXAFS data for the 70Sn: 10Cr:20Cu catalyst material "as prepared" prior to any calcination treatment, and after calcination at 300, 800 and 1000~ are listed in Tables 3-6.
498 Priot to calcination the chromium species for 70Sn: 10Cr:20Cu catalyst material is present as ),-CrOOH (Table 3). This compound has a polymeric structure [11], comrising edgesharing (Cr ..... Cr 3.04/~) and comer-sharing {CrO6} octahedra (Cr ..... Cr 3.92A), with Cr ..... O distances of 1.98A which are characteristic of Cr 3+ species. The fit between the predicted and experimental curves was again greatly improved with the inclusion of a tin atom showing that the chromium compound is strongly sorbed onto the surface of the tin(IV) oxide particles. Although EXAFS data was not recorded for the other uncalcined compositions, it is believed that the same surface species exists for the other Cr(III)/Cu(II)-doped SnO2 materials.
Table 3. Refined structural parameters from Cr K-edge EXAFS data for the uncalcined 70Sn: 10Cr:20Cu catalyst material (R = 27.5). Atom type Coord.No.
Debye-Waller Factor 2c~2 / A 2 Radial distance / A y-CrOOH/A ll
O
6
0.006
1.980
1.990
Cr
2
0.024
3.040
2.990
Sn
1
0.015
3.170
3.380(Si)
Cr
1.5
0.008
3.920
3.980
Calcination at 300~ results in the formation of a CuCrO4 phase (Table 4), where the chromium is present in a Cr 6+ valence state (cf the presence of the pre-edge feature in the XANES region at this calcination temperature). This phase comprises distorted {CrO4} tetrahedra, with two oxygen atoms in the first two shells with Cr-O bond lengths of 1.602A and 1.731A, which are connected by slightly irregular {CuO6} octahedra [12]. Studies by Chien et al. [6] for Cu/Cr/v-alumina catalysts have shown that crystalline CuCrO4 readily forms on alumina after treatment at 300~ This phase is also believed to be present in the Cr(III)/Cu(II)-doped SnO2 materials after calcination at 400~ (infrared data not shown). Bulk decomposition of CuCrO4 into amorphous CuCr204 occurs over the temperature range of 480-500~ [15]. Powder XRD studies show that at calcination temperatures of 600~ and higher, the dopant metals exist as phase separated CuCr204 for all the compositions studied. For the 70Sn:10Cr:20Cu catalyst material, this phase is also identifiable in the EXAFS after calcination at 800~ (Table 5), with good agreement with the literature values, supported by a fit correlation of R = 30. However, with an excess of copper present, high temperature heating (>900~ results in the transformation of copper chromite, CuCr204, into crystalline CuCrO2 as shown by the EXAFS data for the 70Sn:10Cr:20Cu material calcined at 1000~ (Table 6). Powder XRD studies have shown that small amounts of phase separated CuO also exist after calcination at 1000~ Unfortunately however, no Cu K-edge data were recorded to verify the XRD observations.
499 Table 4. Refined structural parameters from Cr K-edge EXAFS data for the 70Sn: 10Cr:20Cu catalyst material ca!cined at 300oc (R = 61). ................ Atom type Coord No.
Debye-Waller Factor 202 / A 2 Radial distance / A CuCrO4/A 12
O
2
0.022
1.602
1.599
O
2
0.013
1.752
1.731
Cu
4
0.011
3.328
3.301
O
4
0.009
3.519
3.539
Cu
2
0.008
3.592
3.634
O
2
0.009
3.712
3.736
Cr
2
0.006
3.748
3.751
O
2
0.009
4.092
4.122
O
4
0.047
4.508
4.493
Cr
4
0.068
4.554
4.552
Table 5. Refined structural parameters from Cr K-edge EXAFS data for the 70Sn: 10Cr:20Cu catalyst material calcined at 800~ (R = 30). Atom type Coord. No.
Debye-Waller Factor 2~r2 / A 2 Radial distance / A CuCr204/A 13
O
6
0.005
1.977
2.000
Cr
4
0.005
2.911
2.896
Cr
2
0.007
3.041
3.035
Cu
2
0.021
3.128
3.289
O
2
0.009
3.298
3.310
Cu
2
0.011
3.371
3.385
O
2
0.005
3.412
3.480
Cu
2
0.003
3.657
3.644
O
2
0.023
3.759
3.780
O
2
0.010
3.791
3.850
Cr
2
0.002
4.692
4.750
EXAFS data also show that the crystalline CuCr204 phase is formed in the 60Sn:20Cr:20Cu and 70Sn:20Cr: 10Cu catalyst materials after calcination at 1000~ in good agreement with the XRD analyses. With an excess of chromium(III), however, a small amount of phase separated Cr203 is detectable in the powder XRD after calcination at 1000~ This is reflected by the higher R value (R = 36.5) for the 70Sn:20Cr: 10Cu material. As the EXAFS signal averages all of the Cr absorber sites, slightly shorter radial distances
500 than expected are incurred for the fitted CuCr20 4 phase. This is due to a contribution from the phase separated Cr203
Table 6. Refined Structural parameters from Cr K-edge EXAFS data for the 70Sn: 10Cr:20Cu catalyst material calcined at 1000~ (R = 32). .
.
.
.
Atom type Coord. No.
Debye-Waller Factor 2or2 / A 2 Radial distance / A CuCrO2/A14
O
6
0.007
1.982
1.989
Cr
6
0.007
2.986
2.975
Cu
6
0.019
3.313
3.327
O
6
0.045
3.620
3.580
Cu
6
0.020
4.452
4.463
6. XPS STUDIES This section describes the Cr 2p, Sn 3d and O l s photoelectron spectra obtained for the 70Sn: 10Cr:20Cu and 60Sn:20Cr:20Cu catalyst materials "as prepared" and after calcination at temperatures in the range 300-1000~ Spectra were recorded under UHV conditions (10 -8 torr) uaing Mg Koc (13kV, 20 mA) as the primary radiation. Since Cr 6+ species undergo photoreduction in the X-ray flux to form a Cr 5+ species, data were collected in the sequence of a survey scan to determine the C 1s reference to account for charging effects, followed by scans in the sequence Cr 2p, Ols and Sn 3d. Both FT-IR and EXAFS analysis demonstrate that phases containing Cr 6+ exist in the materials under mild calcination conditions (300400~ and so exposure time was kept to a minimum. Binding energy and the spin-orbit splitting data of the Cr 2p region are used to distinguish the valence states of chromium in these materials. Data for the 70S:10Cr:20Cu catalyst material is shown in Table 7; that for the 60Sn:20Cr:20Cu material is similar. Since EXAFS data for both compositions show that similar dopant species and phases are present for calcination temperatures up to 600~ the XPS data for both materials calcined at these temperatures are discussed together. For the as prepared material, the Cr 2p doublet can be resolved into four individual peaks. The maximal peaks (Cr 2p3/2, 577.0eV) in both manifolds exhibit a spin-orbit splitting of 9.8eV, characteristic of a Cr 3+ species and assigned to Cr 3+ in the y-CrOOH phase characterised by EXAFS, with the binding energy position in good agreement with studies by Brooks et al. [16] (577.0eV). Analysis of the O ls peak, described later, further supports this assignment. The peaks at a slightly higher binding energy to the maximal peaks have a spin-orbit splitting of 10.7/8eV which corresponds to the Cr 3+ satellite. After calcination at 300 and 400~ four peaks can be resolved within the Cr 2p doublet, both with Eso values of ca. 9.0eV, much lower than that expected for Cr 3+ species (ca. 9.8eV). The peak at higher binding energy in each manifold (ca. 580eV Cr 2p3/2, 589eV Cr 2pl/2) is assigned to the Cr 6+ present in the previously characterised CuCrO4 phase which remains unaffected by photoreduction in the X-ray flux. The peaks at lower binding energy in
501 each manifold (ca. 577eV Cr2p3/2, 586eV Cr 2pl/2) are assigned to the Cr 5+ species formed by photoreduction of the Cr 6+ species in CuCrO4 under UHV conditions.
Table 7. Parameters obtained by deconvolution of XPS spectra for the Cr 2p doublet of the 70Sn: 10Cr:20Cu catalyst material (FWHM in parentheses). Treatment
Binding energy / eV
AEso / eV 2p3/2 area Assignment
2p3/2
2pl/2
577.0(3.2) 579.4(3.6)
586.8(3.5) 590.1(3.5)
9.8 10.7
1265
Cr 3+ Cr3+ sat.
300~
577.7(3.5) 580.4(3.9)
586.7(3.5) 589.7(3.8)
9.0 9.3
1431
Cr 5+ Cr6+
400~
577.5(3.5) 580.4(3.7)
586.6(3.4) 589.4(3.9)
9.1 9.0
1502
Cr 5+ Cr 6+
600~
576.6(3.5) 579.7(3.8)
586.3(3.4) 589.6(3.9)
9.7 10.7
1543
Cr 3+ Cr 3+ sat.
800~
576.6(3.4) 579.9(4.0)
586.4(3.6) 590.6(3.9)
9.8 10.7
1676
Cr3+ Cr 3+ sat.
576.4(3.4) 579.5(4.1)
586.2(3.5) 589.3(3.9)
9.8 10.8
1715
Cr3+ Cr3+ sat.
As prepared
1000~
/ counts
After calcination at 600~ the binding energy positions and spin-orbit splittings for the peaks in the Cr 2p manifolds are all in accordance with Cr 3+ being the sole chromium valence state present in the materials. The assignments could not be assigned to a specific phase. Nevertheless, the spin-orbit splittings of 9.8eV for the maximal peak and 10.7/8eV for the satellite are indicative of Cr 3+. Analysis of these materials have shown that copper chromite (CuCr204), where chromium exists in a +3 valence state, readily forms after calcination at this temperature via a transformation from crystalline CuCrO4. In the case of the 60Sn:20Cr:20Cu material, the CuCr204 phase is still believed to be present in the material after calcination at 1000~ (EXAFS and XRD data). However, these two techniques have both demonstrated that when copper is in an excess of chromium (i.e. the 70Sn:10Cr:20Cu material), calcination at 1000~ results in the formation of the CuCrO2 phase. The binding energy position in the Cr 2p3/2 manifold (576.4eV) for this system after calcination at 1000~ is in good agreement with that of Allen and Tucker [18] (576.4eV) for CuCrO2. However, as the difference between this value and that obtained for this material after calcination at 800~ (576.6eV, due to CuCr204 (EXAFS and XRD)) is practically negligible (ca. O. 1-0.2eV), the XPS does not provide conclusive evidence for the existence of the CuCrO2 phase. This is because of the high charging effects observed with all tin(IV) oxide
502 based materials. Nevertheless, the EXAFS and XRD provide conclusive evidence that this phase does indeed exist after calcination at elevated temperatures. The peak areas under the Cr 2p3/2 manifold increase progressively as the calcination range up to 1000~ is ascended. This is a reflection of dopant migration to the surface and aggregation as the crystallite sizes of the copper/chromium phase-separated species increase with increased calcination. This effect is accompanied by a decrease in the Sn 3d5/2 peak area for the Sn 3d doublet region above 600~ (Table 8) as surface coverage of SnO2 particles by the secondary phase(s) increases. The binding energy positions are in good agreement with those of Ansell et al. [ 19] (3d5/2, 486.3eV).
Table 8. Parameters obtained by deconvolution of XPS spectra for the Sn 3d doublet of the 70Sn: 10Cr:20Cu catalyst material (FWHM in parentheses). Treatment Binding energy / eV Sn 3d5/2 area 3d5/2 3d3/2 / counts As prepared
486.3(2.2)
494.8(2.1)
2013
300~
486.1 (2.2)
494.4(2.1)
2288
400~
486.4(2.2)
494.7(2.1)
3990
600~
486.4(2.3)
494.6(2.2)
4265
800~
486.3(2.2)
494.6(2.1)
3020
1000~
486.4(2.1 )
494.6(1.9)
2874
Binding energy data for the O l s peak are presented in Table 9 for the 70Sn: 10Cr:20Cu material; that for the 60Sn:20Cr:20Cu material are similar. Two peaks can be fitted in the O ls peak for both uncalcined materials, with the maximal peak at ca. 530eV corresponding to tin(IV) oxide lattice oxygen. The second peak at higher binding energy (ca. 532eV) is characteristic of surface hydroxyl oxygen species, present both on the tin(IV) oxide particles and in the ),-CrOOH phase. At calcination temperatures of 300~ and above, three peaks can be fitted into the O 1s peak for both materials, with the two higher energy peaks assigned as before. The peak at lower binding energy to the tin(IV) oxide lattice oxygen peak becomes marginally greater in energy (528.8/529.3eV). This is in accordance with oxygen ions in a phase containing Cr 6+ (CuCrO4) transforming thermally to a phase containing Cr 3+ (CuCr204/CuCrO2).
503 Table 9. Parameters obtained by deconvolution of XPS spectra for the O l s peak of the 70Sn: 10Cr:20Cu catalyst material (FWH M in parentheses). Treatment ..... O ls Binding energy / eV As prepared 300~ 400~ 600~ 800~ 1000~
528.8(2.0) 528.9(1.9) 528.1(2.0) 529.2(1.8) 529.3(2.0)
530.4(2.2) 530.4(2.2) 530.3(2.0) 530.4(2.0) 530.2(1.9) 530.4(2.0)
531.9(2.2) 531.8(2.0) 531.9(2.1) 532.0(2.1) 531.8(2.2) 532.0(2.0)
7. CATALYTIC ACTIVITY Catalytic activity data for both CO and propane oxidation were obtained using a conventional continuous flow microreactor. The catalyst sample (0.5g) is situated in a pyrex glass tube located within a stainless steel heated block. Catalyst samples were activated by in situ preheating in the reactor for 2 hours under a flow of air. The catalysts were then allowed to cool to ambient temperature still under the air flow before acquiring %conversion versus temperature data. Input gas mixture compositions, which were controlled by mass flow controllers, and flow rates are shown in Table 10.
Table 10. Microreactor details. Reaction
Gas mixture composition
Flow rate / ml min -1
CO oxidation
5.0%CO + 20.0%02 + 75.0%N2
88
Propane oxidation 0.8%C3H8 + 20.0%02 + 79.2%N2
98
Temperatures at which complete oxidation of carbon monoxide (Tloo(CO)) and propane (Tloo(C3Hs)) occurs are listed in Table 11. Prior to calcination, the 60Sn:20Cr:20Cu catalyst material exhibited a T100(CO) of 75~ However, calcination at 400~ for both 60Sn:20Cr:20Cu and 70Sn:10Cr:20Cu catalyst materials resulted in complete conversion occurring at ambient temperature. Calcination at higher temperatures caused a progressive deterioration in the T100(CO) value. The Tlo0(C3Hs) values are substantially higher with values in the range 240-265~ for calcination temperatures up to 600~ Calcination at 1000~ resulted in an increase of the T100(C3Hs) value up to ca. 350~
504 Table 11. T lOO Values for the oxidation of carbon monoxide and propane. Catalyst (precalcination temperature / ~
Tloo(CO) / ~
Tloo(C3H8) / ~
60Sn:20Cr:20Cu (60)
75
240
60Sn:20Cr:20Cu (400)
Ambient
245
60Sn:20Cr:20Cu (600)
75
265
60Sn:20Cr:20Cu (1000)
175
345
70Sn: 10Cr:20Cu (400)
Ambient
250
70Sn: 10Cr:20Cu (1000)
170
350
(a) Minimum temperature at which 100% conversion occurs. 8. S U M M A R Y AND CONCLUSIONS
Prior to calcination, the chromium(Ill) dopant in Cr(III)-Cu(II)-doped SnO2 catalyst materials is present as sorbed polymeric y-CrOOH. The copper species are believed to exist in the form of sorbed hexaaqua {Cu(H20)6} 2+ ions [20]. After calcination at 300-400~ these sorbed species are transformed (oxidised) into CuCrVlO4. Powder XRD and EXAFS show that, after calcination at 600~ and higher, chromium and copper are present as phase separated CuCr204, CuCrO2, CuO and Cr203 depending on the initial catalyst atomic stoichiometry. With a 1:1 copper:chromium atomic ratio, calcination at elevated temperatures (>600~ results in the transformation of crystalline CuCrO4 into tetragonal copper chromite, CuCr204. Similar behaviour is observed when the stoichiometry is 2Cr: 1Cu, together with small amounts of chromia, Cr203, after calcination at 1000~ When copper is present in an excess to chromium (i.e. 2Cu: 1Cr stoichiometry), the phase separated CuCr204 observed after calcination at 800~ undergoes a transformation to form copper chromate, CuCrO2, after calcination at 1000~ Optimum catalytic activity for the oxidation of carbon monoxide and hydrocarbons occurs for catalysts calcined at 300-400~ i.e. when the catalysts comprise CuCrVIO4 supported on particulate tin(IV) oxide. Although still high, calcination at higher temperatures decreases the catalytic activity somewhat but now the catalyst comprises CuCrIII204 supported on tin(IV) oxide.
505 ACKNOWLEDGEMENTS:
We thank the EPSRC (under research grant GR/J76026 and for beam time at the synchroton source at DRAL), the Commission of the European Union (under contract number CT92AV 10012 of the Avicenne Initiative), and the Malaysian Government (for a scholarship to W.A.) for support. REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9.
Harrison,P.G., Harris, P.J., U.S. Patent, 4,908,192 (1989). JCPDS Diffraction File 34-424. Castiglioni, G.L., Appl. Catalysis A, 123, 123 (1995). JCPDS Diffraction File 39-247. Apai, G., Monnier, J.R., Hanrahan, M.J., Appl. Surf. Sci., 19, 307 (1984). Monnier, J.R., Hanrahan, M.J., Apai, G., J. Catal., 92, 119 (1985). Patnaik, P., Rao, D.Y., Ganguli, P., Murthy, R.S., Thermochim. Acta, 68, 17 (1983). Chien, C.C., Chuang, W.P., Huang, Y.J., Appl. Catalysis A, 131, 73 (1995). Kutzler, F.W., Natoli, C.R., Misemer, D.K., Doniach, S., Hodgson, K.O., J. Chem. Phys., 73,327 (1980). 10. Penner-Hahn, J.E., Benfatto, M., Hedman, B., Takahashi, T., Doniach, S., Groves, J.T., Hodgson, K.O., Inorg. Chem., 25, 2255 (1986). 11. Fendorf, S.E., Lamble, G.M., Stapleton, M.G., Kelley, M.J., Sparks, D.L., Environ. Sci. Technol., 28, 284 (1994). 12. Seferiadis, N., Oswald, H.R., Acta Cryst. C, 43, 10 (1987). 13. Prince, E., Acta Cryst., 10, 554 (1957). 14. Hannhauser, W., Vaughn, P.A., J. Am. Chem. Soe., 77, 896 (1955). 15. Horvath, I., Hanic, F., Thermochim. Acta, 92, 177 (1985). 16. Brooks, A.R., Clayton, C.R., Doss, K., Lu, Y.C., J. Electrochem. Soc., 133, 2459 (1986). 17. Allen, G.C., Tucker, P.M., Inorg. Chim. Acta, 10, 41 (1976). 18. Ansell, R.O., Dickinson, T., Povey, A.F., Sherwood, P.M.A., J. Electrochem. Soc., 124, 1360 (1977). 19. Matar, K., Zhao, D., Goldfarb, D., Azelee, W., Daniell, W., Harrison, P.G., J. Phys. Chem., 99, 9966 (1995).
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CATALYSIS AND AUTOMOTIVEPOLLUTIONCONTROLIV Studiesin Surface Science and Catalysis, Vol. 116 N. Kruse, A. Frennet and J.-M Bastin (Eds.) 91998 Elsevier Science B.V. All rightsreserved.
507
Preparation and study of thermally stable washcoat aluminas for automotive catalysts. Z.R.Ismagilov, R.A.Shkrabina, N.A.Koryabkina, D.A. Arendarskii, N.V.Shikina Boreskov Institute of Catalysis, 630090, Novosibirsk 90, Lavrentieva 5, Russia
1. INTRODUCTION Porous and thermally stable washcoating layer on mechanically strong support is an important component in both oxidative and three-way catalysts used for car exhaust gas cleaning. The washcoat provides a high and stable surface area for dispersion of the active component of the catalysts consisting of platinum and /or paladium. Usually for the preparation of this layer aluminas modified by La, Ce, Zr, Si etc. are used [1-3]. As it was shown in [4-6] the properties of modified aluminas depend on the method of introduction of the additives In this work we present the results on the preparation and study of model alumina systems modified by La, Ce and Zr as well as of monolith supports washcoated by optimal compositions of alumina and additives. 2. EXPERIMENTAL y-Alumina with different concentration of La203 were used as model system to study the formation of lanthanum [3-aluminate and perovskite type LaA103 at the calcination. These systems were chosen because it is known that the modifying compounds may act as catalysts for total oxidation themselves. Furthermore, formation of Ce- and Zr-modified aluminas depending on the amount of additives and the calcination temperature have been studied. Additives were introduced into 3,-alumina by impregnating with aqueous solutions of nitrates. Optimal thermostable composition of alumina and additives were chosen for using as a washcoat on cordierite and alumosilicate ceramic honeycomb monoliths. For the preparation of washcoated monoliths the suspensions of sol-aluminium hydroxide with pseudoboehmite structure have been used. This sol formed during the reaction between the hydroxide and nitric acid serves both as a binder and a source of y-A1203 in the final product after calcination. Salts of additives were introduced into sol. The influence of the following parameters on the formation of thermostable washcoated layer have been studied: concentration of anhydrous alumina in the sol; amount of added HNO3; dipping time; number of dippings; drying and calcination duration. Attrition resistance of washcoated layer have been tested in specially developed equipment.
508 Two types of monolithic supports on the base of cordierite (SBET =0,3 m2/g, pitch 64 cell/cm 2) and dense alumosilicate (SBET= 0,6 m2/g, pitch 16 cell/cm2) were used. Catalytic activity was measured with a fixed bed flow reactor. The reaction mixture contained 1% CH4 diluted in air and the gas flow rate was 1000 h "1, unless otherwise specified. The conversion of methane after the reaction was analyzed by gas chromatography. 3. RESULTS AND DISCUSSION Table 1 presents the properties of aluminas modified by various additives. It is seen that lanthanum is the most effective additive considering stabilisation of phase composition and SBETof samples at 1100~ Table 1 The properties of aluminas depending on type and amount of temperature. N additive 600~ 4hrs 1000~ 1200~ content, SBET, Phase SBET, Phase SBET, wt.% mE/g composi m2/g composition mE/g tion La203 5 200 7-A1203 90 90%5+7 29 8 190 7-A1203 100 50%8+7 28 10 160 ~,-A1203 100 20%8+7+La* 26 12 160 ]'-A1203 100 10%~5+7+La 25 CeO2 5 180 7-CEO2 70 0-+(/,tr+ CeO2 9,5 12 190 ~/67 0-+ CeO2 12 +CeO2 ZrO2 70 5 170 y-A1203 90 46%5-+7-+ ZrO2 (cub) 8
9
140
100 +ZrO2 (cub) y-A1203 70
]t-A1203
210 *) La- LaA103; * *) La2 - La203-11 A1203
47%6+7-+ ZrO2 (cub)
80
6-+50%c~-
9
additives and calcination
Phase composition
70%5+~+Latr+La2** 60%8+~+La+La2 50%5+Ottr+La+La2 50%5+~tr+La+La2 ~-A1203 + CeO2 ~-A1203 + CeO2
10%c~- A1203 +0+ZrO2(cub)+ ZrO2 monokl. 0-A1203+ZrO2 (cub)+ZrO2 monokl. ct- A1203
Formation of perovskite type LaA103 proceeds at lower temperature by increasing of La3+ content. For the samples, with 10 and 12wt% La203 the phase of LaA103 is formed already at 1000~ As shown in [2], formation of LaA103 depends on the introduction method of La+3 13-aluminate is observed at T >1000~ Note, that at 1200~ LaA103 takes part in the formation ofLa203.11A1203, because it is shown that the amount of LaA103 is decreased while
509 [3-aluminate amount is increased. The stabilizing effect of lanthanum was assumed to result from the formation of intermediate X-ray amorphous compounds with the transient A1203 forms [2]. The data presented in Table 1 also show, that ceria is less effective additive. As discussed in [3] it is connected with the limited character of interaction of ceria with 7-A1203. The interesting results are obtained when Zr4+ is used as an additive. As it is seen in Table 1 at 8wt% ZrO2 the ~-A1203 phase is not formed at 1200~ while this phase is observed in all of other modified aluminas at this temperature. The value of SBETfor zirconia modified alumina at 1200~ is more than two fold than that for La/A1203 system. The Zr/A1203 and La/A1203 systems were used for the preparation of washcoated supports. Special washcoating procedure has been developed. The properties of washcoated supports are given in Table 2. The attrition test procedure includes the measurement of weight loss of the sample by abrasive powder in the air flow. Testing conditions were the following: - mass relation between abrasive powder (particle size -~ 60mkm) and support fragment being 4:1; air flow rate - 100 L per hour; total duration of the experiment to achieve the constant weight, during about one hour, with weight measurement every 10 minutes. -
-
Table 2 Properties of washcoated supports N Composition of Support washcoating 1 5%ZRO2/ ]t-A1203 cordierite 2 5%CeO2/7-A1203 cordierite 3 5% La203 / 7-A1203 cordierite 4 12% La203 / 7-A1203 cordierite 5 5% ZrO2/7-A1203 alumosilic. ceramic 6 12% La203 / 7-A1203 alumosilic. ceramic .
.
.
.
.
.
.
.
.
.
.
.
Content of washcoating, wt% 5.5 7.8 5.5 9.5 4.1
SBET, m2/g at 600~
5.5
4.9
9.5 19.8 8.5 12.3 7.7
It was shown that the used method of washcoating provides strong bound between monolith support and washcoated layer. So, for the samples (see Table 2) containing less than 10wt% of washcoat the weight loss was <0,001 wt% per hour. Impregnated Pt or Pt-Pd model catalysts have been prepared on cordierite and alumosilicate ceramic with zirconium-containing washcoat and tested in the reaction of methane oxidation. The results of activity testing are given in Figures 1,2. Content of Pt and Pd in these catalysts is shown on curves. Activity of catalysts was characterised as temperature of 50% of methane Conversion (T5~ Figure 1 shows the plots of methane conversion vs. temperature for catalysts supported on alumosilicate ceramics. It is seen that the highest activity in methane oxidation has the sample with the active component : 0,4%Pt + 0,2%Pd (T 5~ 440~ The lowest activity is exhibited by the sample containing only 0,3%Pt (T 5~ 550~
510
o•?00
0.4% Pt-t0.2%Pd/5O/oZrO24-A1203
60
0.2% Pt40.1% Pd/
~
_
40
/
/
/
20
0.3% Pt / 5~
h, -Al203
0
100
,
I
200
,
I
300
,
I
400
,
I
500
,
I
600
700
TEMPERATURE, ~ Figure 1. Methane conversion vs. temperature for catalysts on the base of alumosilicate ceramics.
100 0A% I~+02% Pd/5%7_rO2+t-Al20a
zs 8o
0.2% Pt+0.1% Pd/5%ZrO2~ -AI203
r~ 40 0.2% Pt / 5~
h, -Al20'
o
100
,
I
200
,
I
300
,
I
400
,
I
500
,
I
600
,
I
700
TEMPERATURE, oC Figure 2. Methane conversion vs. temperature for catalysts on the base of cordierite.
511 The plots of CH4 conversion vs. temperature for cordierite supported catalysts with the same washcoat are given in Fig.2. It is seen that the two fold increase of the content of noble metals has no substantial effect on the catalyst activity. Nevertheless the comparison of the activity of the catalyst supported on alumosilicate ceramics with that of the catalyst on cordierite with the same content of active component (0,4%Pt + 0,2%Pd) and the same washcoat composition (5%ZrO2+~-A1203) gives same results. These data show that developed washcoated supports are good candidates for the preparation of automotive catalysts.
4. CONCLUSION The main findings of this work can be summarized as follows. The introduction of CeO2, La203, ZrO2 stabilizes the support surface area at 1200~ ZrO2 is the most effective additive to stabilize surface area of alumina at high temperature. It was shown that the method of washcoating used provides strong bond between monolith support and washcoated layer. The catalysts prepared on the thermally stable washcoat composition and containing a mixture of Pt and Pd are more active than the Pt ones independent on the type of support (cordierite or alumiosilicate) used.
REFERENCES 1. 2. 3. 4. 5.
Burtin P., Brunelle J.P., Pijolat M. and Soustelle M., Appl. Catal., 34 (1987)225. Church J.S., Cant N.W., Trimm D.L., Appl. Catal. A., 101(1993)105. Beguin B., Garbowski E., Primet M., Appl. Catal, 7(1191) 119. Z.R.Ismagilov, R.A.Shkrabina, N.A.Koryabkina and F.Kapteijn, Catal. Tod., 24 (1995)269. R.A.Shkrabina, N.A.Koryabkina, V.A. Ushakov, E.M. Moroz, M.Lausberg and Z.R.Ismagilov, Kinet.katal., 37(1996) 116, (in Russian). 6. N.A.Koryabkina, R.A.Shkrabina, V.A. Ushakov, E.M. Moroz, M.Lausberg and Z.R.Ismagilov, Kinet.katal., 37(1996) 124 (in Russian).
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CATALYSIS AND AUTOMOTIVEPOLLUTIONCONTROLIV Studies in Surface Science and Catalysis, Vol. 116 N. Kruse, A. Frennet and J.-M Bastin (Eds.) 91998 Elsevier Science B.V. All rights reserved.
513
Ensuring substrate retention Part 2 4
J Kisenyi, K Soe, P Leason, C Tooby, D Pritchett a, G Morgan b, M Zillikens c Ford Motor Co., aGillet bArvin C3M ABSTRACT To meet the European emission legislation limits at 80,000 Kin, the catalytic converter must have, demonstrated thermal and mechanical durability, as well as excellent catalytic conversion efficiencies. As engine performance increases, and allowable tailpipe emission are reduced, the demand for thermal and mechanical durability is even greater especially for European driving conditions~'2'3. The system under discussion is a ceramic substrate mounted in the converter with an expanding (intumescent) support mat (See Schematic below). The primary functions of the support mat are : 9 To create the pressure that holds the substrate in position even when the can expands on heating. 9 To act as the gas-seal between the substrate and the can. 9 To insulate the can from excessive temperatures of the substrate. Therefore ceramic monolithic support is an important part of the emission control system. The design has been very successful in the past, however for some models a small number of substrates become loose after 25,000 to 30,000 miles in the field due to mat loss. Work was initiated to identify/eliminate the causes of mat loss and guarantee system durability. A range of design modifications have been investigated using the High Speed Dyno durability cycle. This paper is a fuller discussion of High Speed dyno durability cycle data for Catalyst designs with two 169.67 x 80.77 x 63.09 mm substrates. The design modifications investigated fell into two categories. (1) Mat mount density such as increasing the mount density from 4070 g/m 2 to 4300 g/m 2, gradual reduction in the mat gap, and internal ribs. (2) Gas barrier concepts comprised a range of seals. All these modifications resulted in mat loss. Controlling the can skin temperature to below 560 ~ was the only method of getting the assembly to survive the High Speed Dyno test.
514
Schematic of the converter assembly.
1. BACKGROUND In the earlier paper 4, we presented data from High Speed Dyno test on designs of catalyst support mat where a single layer of mat with a weight of 4070 g/m 2 is in a nominal gap of 4.25 mm. It had been noticed that for some models, the substrate becomes loose at around 25,000 to 35,000 miles. Warranty costs make it imperative to find a quick solution. The high speed dyno cycle, which is an engine durability cycle, has been identified as a very good means of reproducing mat loss. In this cycle, the engine runs at idle for 20 minutes and then at full power for 160 minutes. The full test is 60 cycles, 180 hours. This paper provides a further discussion of the data from the above design modifications.
1.2. How the Mat works The mat, which consists of ceramic fibres and vermiculite particles, is held in the gap between the can and the substrate by friction and compressive forces. On heating, the can expands much more than the ceramic substrate. The vermiculite particles try to expand. They are held in the gap, so they create the compressive pressure that holds the substrate in place. There is a maximum mat gap above which, there will be very little keeping the mat fibres together, so the mat would offer little resistance to vibration or gas impingement. The layer of mat that is in direct contact with the substrate, is exposed to much higher temperatures than 700 ~ the temperature at which the vermiculite irreversibly looses its chemically bound water, and thus ceases to create pressure. This layer only acts as an insulator to protect subsequent layers which continue to create pressure, and to insulate the can. It is important to minimize the layer of mat that has been exposed to temperatures higher than 700 ~ 1.2.Design issues * A disadvantage of the race track design is that the can has a relatively flat region and a curved region. The flat region, which is not as stiff as the curved, expands more on heating. This results in a bigger gap between the substrate and the mat in the flat region, thus a weaker area for the mat. This is further compounded by poor gas distribution where the highest velocities are close to the flat can area of the mat. In this design, the bottom of the
515 can has a heat shield for legal reasons. It has been observed that all failures initiate on the side of the substrate with the heat shield.
9 The other issue with this design is the substrate length of 63 mm. The substrate is covered with a 60 mm length of mat. The can has two external ribs 15 mm from either end. The mat under the ribs has minimum contribution to substrate holding. The loss of 7 mm of mat from either end, is therefore a substantial reduction in the holding area. The vehicles that were failing in the field had satisfied the standard sign-off tests, so their failure in the field showed the inadequacies of the sign-off tests. The long term aim of this work was to adopt a systems approach 3' 5 and define a test which once passed, guaranteed a robust design. The initial challenge was to identify a test that resulted in failure. The High Speed Dyno test originally designed to test engine durability, was identified as able to reproducibly result in catalyst failure due to mat loss. What the test did not reproduce are the patterns of mat loss. It was observed that after 60 cycles of this test, racetrack designs (4070 g/m 2 mat in a 4.25 mm gap) resulted into some form of mat loss. This is a very severe test since all dyno tested parts were failing, and yet the field complaint rate was well below 10%. The test is not based on vehicle data and correlation with field data is yet to be established. Though the test is not perfect, a design that withstands the test will have demonstrated a significant improvement in robustness. There is a resistance to having a radical change in catalyst can design because of the associated high tooling costs, and also the new can may not fit in the existing package envelope. When this work was initiated, it was known that a system with internal cones and two layers of 3100 g/m 2 mat, in a gap of 6 mm does not result in mat loss on the high speed dyno test, even on the race track design. The two layer design would require re-tooling and was thus not considered. 2. C O N C E P T S EVALUATED
Mat mount density increase Increase 9 the mount density from 4070 to 4300 g/m 2 Gradual 9 reduction of the mat gap Internal 9 ribs Gas Barrier methods: Rope 9 Seal .Z-seal Water-glass 9 seal The 9 Polymer seal A9 Metal baffle
516
This paper discusses the resistance to mat loss for each concept, on the high speed dyno test. 3. EXPERIMENTAL. 1.6 L CVH, 1.8 L Zeta and 2.0 L Zeta engines were selected for this work. For each concept, two identical parts were canned and made up to a full production-intent exhaust assembly. The parts are instrumented with at least five thermocouples all located in the "flat" region of the can. Figures 1 to 18 show output ~om the thermocouples T1, T2, T3, T4, and T5.
T1 records the Gas inlet temperature, it is located in the middle of the gas stream, 25mm from the front face of the substrate. T2 and T3 are located on the external surface of the can on the side without the heat shield. T2 is located on the cone edge, and T3, 25mm from the cone edge. T4 and T5 are in analogous positions to T2 and T3 but on the side with the heat shield. Minor axis can-skin dimensions were recorded before testing. The parts were then subjected to 60 cycles (180 hours) of the High Speed Dyno test. Temperatures T1, T2, T3, T4, and T5 were recorded during the test. Measurement of minor axis dimensions after ageing and cooling, indicates the extent of permanent can deformation. Parts would then be cut open to observe extent of mat loss. The object was to achieve no mat loss. During the testing, the status of the mat can be inferred from T4 and T5. If T4 is above 700 ~ it is reasonable to assume that the mat is no longer able to create pressure. This represents the onset of mat loss, but the more dramatic event is when T5 crosses the 700 ~ line. This indicates the point where the gases have penetrated 25mm of the mat. Beyond this point, the difference in temperature between T4 and T5 can only be attributed to the fact that the gases get to T4 before they get to T5. For this discussion, the system is defined as failed once T5 begins to rise. Other methods of detecting the status of the mat during the high Speed dyno test, such as Thermo Imaging Cameras were also used, but there was sufficient data from thermocouple measurements to allow excellent prediction of mat status. The extent of can expansion after the test was recorded after allowing the can to cool. This gave the extent of permanent mat gap damage. It is inconclusive since with mat loss, the can is then exposed to unrepresentative temperatures. Some general observations: 9 The gas-in temperature is fairly stable at around 800 ~ 9 The temperature difference between the heat shield side and the side without, is about 200 ~
517
T2 Temp. ~
(T1, T2, T3, T4, T5)
VS
(X-axis)
(Y-axis)
_.~
_1
T1 T4
Fig 1.
Fig 2.
Current production can
900
T5
Mat gap reduction to 3.66 m m
900
800 ~
1
700 ]
800 ~ / ~ M ~ I ~ I I ~ I
700~
5
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
i
60O
~
600
T2
5O0 400
T3
No. of Cycles
500 ~
3 .
.
.
0
.
.
.
.
.
.
.
.
.
10
.
.
.
.
.
,.
20
.
.
400
.
30
................
40
2
~
3
-
0
+ ...... ~...... +. . . . +. . . . + - -
1.6 CVH engine
1.6 CVH engine
4.25 m m gap 4070 Series IV mat No seal
3.66 m m gap 4070 Series IV mat "Waterglass" Seal Improved flow pipe
Mat Loss
Mat Loss
Fig 3.
Mat gap reduction to 3.66 m m
Fig 4
Mat gap reduction to 2.65 m m
.........................................................
800
" "
600700
500 400
TI!
4
.................
~
. . . . . . . . . . . . . . . . .
0
I I
I
800
__.__"_____
_ _
-
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..........
..............
600 __,
___
~
.
-
.
.
~T2
-
.
.
-
500
3
T3
..... F---- F-- -4
10 20 30 40 50 60
4
10 20 30 40 50 60
400
0
10
20
30
1.6 CVH engine
1.6 CVH engine
3.66 m m gap
2.65 m m mat gap
4070 Series IV mat
4070 Series IV mat
"Waterglass" Seal
"Waterglass" seal
Improved flow pipe
Improved flow pipe
Mat Loss
Mat Loss
40
518
(Y-axis)
Temp. ~ (T1, T2, T3, T4, T5)
VS
T2
(X-axis)
T3
No. of Cycles T1 ' ~ ~ 'L 1~~""~
I
--.2)-
~
T4
Fig 5
Mat gap reduction to 2.65 mm
800 ~
1
'~176
400
~
300 0
I 20
~4 5
600 .15_ _ _ _ _ ~ . T 2
500 4 4
I 30
I 40
I 50
J 60
0
~ 0
-
1
300 . . . . . ~ 0 10
1.6 CVH engine 2.65 mm mat gap 4070 Series IV mat "Waterglass" seal Improved flow pipe M a t Loss Fig 7
lIT1
700
3
I 10
Saffil Edge Seal
8 0 0 1 ~ m l ~ ~ ~ l l l ~
600
500
Fig 6
T5
t 20
~
,
~T2 ~
3
t - - + - ..... ~-..... 30 40 50 60
1.6 CVH engine 3.60 mm mat gap 4070 Series IV mat Saffil edge seal Improved flow pipe Mat Loss
Saffil Edge Seal
Fig 8
Standard can, limit T4 to 580 oC
. _
800 L
800
.
.
.
.
1
700
600 500 _ -
6ooOO !!! -; ........ ii....
- ~ . . . .
400--I ~ ~ I ' ~ t ~ I I I I ~ / ~ - ~ T 2 T 3
400 " ~ l , , z _ _
300 ~..... ~ ..... +- .......~....... ~....... ~.......
300
0
10
20
30
40
50
60
1.6 CVH Engine 3.60 mm mat gap 4070 g/m2 Series IV mat Saffil Edge seal New engine at cycle 43 (first one blew up) No mat loss
-2 ......... .....
T2 T3
0
10
20
....~...... F - - ~ 30 40 50 60
1.6 CVH engine 4.25 mm mat gap 4070 g/m2 Series IV mat No Seal T4 limited to 580 oC Air cooled No mat loss
519
Temp. ~ (T1, T2, T3, T4, T5)
VS
(Y-axis)
T2
No. of Cycles (X-axis)
T3
~ ~~ . ~ .-..~l --~ [ . i| ~=" T1 ~ ' ~ - ~I : _ . ._ J~... T4
Fig 9
Standard can, limit T4 to 580 oC
Fig 10 900
800
800
700
700 I
600
600
500 400 300 200
-___L
500
~
-q-----l---
0
10
~
--
v
-
.
.
- --I------~--
20
30
....
40
60
Temp. T4 limited to 560 oC
-1--
I
t
I
10
20
30
40
Fig 12
1.8 Zeta
T4
.
I ----q
50
60
3MSeal
800 700 . _1
-
...L_--
nil
.
.
.
.
600 I l t m J l ~ = ' - ~ - - ' - ' ~ - ~ ' - - - - - ~ - i i - ' - T 4
- _%-T4
500
200
.
1.6 CVH engine 4.25 mm gap 4070 Series IV mat No seal T4 limited to 560 oC, Air cooled No mat loss
700
300
.
T2
200 I 0
--I-......
50
800
400
.
300
---
]Fig 11
--
4 0 0
------~---T2
1.6 CVH engine 4.25 mm mat gap 4070 g/m2 Series IV mat No Seal T4 limited to 580 oC Air cooled No m a t loss
600
T5
Temp. T4 limited to 560 oC
........
.=akT5
500
! l 0
r
400 - -- - " ~
T3
--I 10
i 20
i
I -- 3 ----t-----4 30
40
50
300 - -
60
....
0
~
i 10
-----'-=--_ - - " - - T 2 T
t
i
i
I
i
20
30
40
50
60
.......
1.6 CVH engine 4.25 mm gap 4070 Series IV mat No seal T4 limited to 560 oC, Air cooled No m a t loss
1.8 Zeta engine 4.25 mm gap 4600 Series 10D mat 3M Polymer Seal Improved flow pipe No mat loss
3
520
VS
Temp. ~ (TI, T2, T3, T4, T5) (Y-axis)
No. of Cycles
~
T2 T3 j , ~k
(X-axis)
T1
Fig 13
1.8 Zeta
Fig 14 2.0 Zeta
3M Seal
800
T4 T5 Front Rope Seal
1000
700 '
-
4
900
T5
800
-
600
700 ~
'T2
500 400
T
4
600 'X"
2"
500 = " ~ ' ~ ' = ' ~ T 2
3
400
300
300
200 -------+ 0 10
J 20
I 30
I 40
t ---~ 50 60
200 . . . . +- i 0 10 20
t 30
I 40
I 50
i
1.8 Zeta engine 4.25 mm gap 4600 Series 10 D mat 3M Ploymer Seal
2.01 Zeta engine 4.25 mm gap 4070 Carborundum mat Rope Seal
Serious Substrate&mat loss
Mat loss (Slight)
Fig 15 2.0 Zeta Front Rope Seal 1000 900 I l l l l ~ ~ l l ~ / ~ / i ~
1
800 700 l i ~ ~ l l k ~ l l a ~ T 4
600 ! 500 I__ 400 3
_ _
0
0
10
_ .
0
20
T2 ~
30
40
2.01 Zeta engine 4.25 mm gap 4070 Carborundum mat Rope Seal Mat loss
60
521 There is a can-skin temperature difference between the gas inlet and the midbrick part of the mat of 60 - 100 ~ This difference is reduced to less than 10 ~ once the mat is eroded and the can not insulated from the gases any more. 4. R E S U L T S AND D I S C U S S I O N
4.1. Mount Density concepts
Slight Increase in mount density. One difference between two layers of mat at 3100 g/m 2 in a 6 mm gap (1.03 g/cm 3) and a single layer of 4070 g/m 2 in a 4.25 mm gap (0.96 g/cm3), is that the two layers have a slightly higher mount density than the single layer. For equivalent mount density, the single layer needs to have a density of 4392 g/m 2 . Parts, with a mount density of 4300 g/m 2 in a 4.25 mm nominal gap, were canned for the 1.6 CVH engine. They were run on the High Speed Dyno test, and after 60 cycles, they showed more erosion than 4070 g/m 2 in a 4.25 mm gap. For both parts, a 2 cm width of the mat had been eroded in the fiat section for the entire length of the substrate, on the same side as the heat shield. This most unexpected observation was explained by the wide production tolerance in the manufacture of mat of + 8%. Although the overall roll is labeled 4300 g/m 2, the density of the parts tested could have been as low as 3956 g/m 2 or as high as 4644 g/m 2. The result ruled out 4300 g/m 2 since under normal production practice, there would be no opportunity to eliminate low density cuts.
Reduction in the gap between the Can and the Substrate
This has been achieved in a number of ways. -Gradual reduction of the gap in the fiat section from 4.25 mm to (a) 3.6 mm and (b) 2.65 mm -Three internal ribs were introduced into the flat section of the can. The mat gap for the "rib" section of the can was 3.3 mm
Mat-gap Gradual Reduction to 3.60, Series IV, Waterglass seal, Better flow pipe
Two assemblies where the mat gap had been gradually reduced from 4.25 to 3.6 were tested on the High Speed Dyno test. Two other ideas were tested in each of these cases, a new type mat edge seal called "Waterglass", and a new design catalyst gas inlet pipe to improve the gas distribution onto the substrate. Figure 2 shows the variation in skin temperature with number of cycles for the first assembly. It is worth noting that T4 starts close to 700 ~ so there is little chance of this assembly surviving all 60 cycles. T5 begins to rise after 15 cycles, this turns into a sharp rise after 40 cycles. The mat on the side without the heat shield was not affected. The second assembly (Figure 3), started off with more favourable but still high T4 and T5 temperatures. T5 started to rise after 22 cycles, and at 31 cycles T4 reached 700 ~ This coincided with the sudden rise in T5. These events are preceded by a slight rise for the thermocouples on the side without the heat shield. Opening the can, in each case, revealed significant erosion right through the mat on the side of the heat shield.
522
Mat-gap Gradual Reduction to 2. 65, Series 1V, Waterglass seal, Better flow pipe The first assembly (Figure 4), started off with high T4 and T5, and both temperatures started to rise after 5 cycles. It is interesting to note that failure on the heat shield side resulted in faster deterioration of the side without the heat shield. The test had to be terminated because the substrate could be heard rattling after 27 cycles. The second assembly (Figure 5), lasted a little longer, it took 25 cycles before T5 dramatic rise past 700 ~ The starting points are not very different from Figure 4. T4 and T5 both start to rise after 4 cycles Failure on the heat shield side had little effect on the non-heat shield side. In the second assembly the substrate is retained through the test, but the mat is eroded on the side with the heat shield. The plot of 'Mat Gap' and 'Cycles to Fail' for 11 different parts, shows that a mat gap of 2.65 is not to be advised. On the contrary, simply having a high mat gap does not guarantee a robust design otherwise all gaps greater than 3.66 would survive the test. M A T G A P vs F A I L P O I N T 60
--
5
5o
2~
d 2o lO
1 1
2
3
4
5
6
7
8
9
1
0
1
1
i Internal Ribs, Mat gap at each rib
: 3.3mm
A disadvantage of having a very narrow mat gap is that it reduces the thermal gradient across the mat, heat is transferred easily to the can, and the can expands sooner. The increase in can pressure due to the narrower gap doesn't compensate for the can expansion. Parts were canned and tested on the 1.8 Zeta engine. Both parts showed extensive erosion on the side with the heat shield. 4.2. Seals
Seals are used in an attempt to reduce direct gas impingement onto the support mat. They are used in a variety of designs. In some cases, the mat is cut a little shorter, and a seal is placed before the mat. The alternative is to wrap the seal around the edge of the mat, or to change the characteristics of the mat such that the mat edge seals the rest of the mat from gas impingement.
Rope Seal Parts were canned and tested on the 1.8 Zeta engine : Figure 14 and Figure 15. Both parts showed a little bit of erosion. Since the objective was zero erosion, this idea was not adopted,
523 but it gave a very good result bearing in mind that this engine was giving gas-in temperatures close to 900 ~ The seals are intact at the end of the test, indicating that the gases go over the seal and erode the mat. The seal therefore cannot be preventing gas impingement onto the mat. Carborundum mat used in these tests did not offer any additional benefits
Z- Seal
Z-seals, like rope seals, are placed before the mat. They have been used with great success in North America. The main difference between European and North American applications is the gas-in temperature of the catalyst. Europe runs at typically 800 ~ while North America runs at around 500 ~ Both of the parts tested showed complete erosion of the mat in the flat region on the side with the heat shield. Again the seals themselves were left intact, the gases were able to go over the seals to the mat.
"Waterglass" seal
The edge of the mat is treated with a material called "Waterglass". This renders the mat edge brittle and is therefore expected to create a gas barrier for the rest of the mat. This idea was tested in conjunction with the gradual reduction of the mat gap Figures 9 2, 3, 4, and, 5. All the parts tested showed extensive mat loss. The seal provided no resistance to gas impingement. The edge of the mat treated with 'Waterglass' became very hard during the test, and thus all its elasticity and its ability to create pressure. The gases are allowed to bypass the seal once the can warms up.
Polymer seal
This was tested on a 1.8 Zetec engine : Figure 12 and Figure 13. This engine gave low gasin temperatures 750 - 780 ~ The results looked excellent for one of the parts Fig 12, which had a cycle 1 T4 of 600 - 610 ~ The mat was intact though there was a slight crack in the seal. The other part : Figure 13, which had a cycle 1 T4 of 650 ~ showed a very unusual type of erosion where it wasn't just the mat that was eroded, but the brick as well. Some high velocity particles, thought to come from the exhaust manifold, are responsible for eroding both the substrate and the mat. In this case, the mat wasn't eroded preferentially even on the side with the heat shield. Erosion of the manifold resulting into substrate and mat loss is not discussed in this paper. The part th,:t showed no loss demonstrated that under the right conditions, mat loss can be overcome. Sinc~ other seals had already been demonstrated to provide little resistance to exhaust gases, it was unreasonable to attribute this success to the seal. The other factor had to be the lower running temperature of this engine. Figure 12 shows T4 around 620 ~
Ceramic Fibre Edge-Seai
Two parts were tested v~;th a new low shot ceramic fibre material, acting as an edge seal. This material comprises of lo 3se ceramic fibres which can be compressed, but have the ability to revert to the original loose structure once the compressive force is removed. There is no vermiculite, the fibres have suf~cient elasticity to revert to the original thickness, which is a lot greater than the mat gap thus protecting the mat. Part of the length of the mat both the inlet and outlet sides, was replaced by the seal. Two parts were tested : Figure 6 and Figuxe 7. First cycle T4 (700 ~ T5(600 ~ temperatures for Figure 6 were very high, but it was thought that the high thermal resistance of the ceramic fibre should support that. T5 started to rise after only 6 cycles. At 32 cycles, there was a dramatic event which affected both the heat shield side and the side without. When the part was cut open, there was an open channel through the mat on the side with the heat shield. The second part started with more favourable temperatures, T4(580 ~ and T5(510 ~ The
524 temperatures remained well below 700 ~ for the duration of the test. The mat was found intact when the part was cut open. The two parts that survived the test started with T4 close to 600 ~ in each case, there was an identical part which failed due to high operating temperatures. There is a need to control T4 and establish a value, below which, the part will always support the high speed dyno test. Once that value is known, then it can be engineered into the design. 4.3. Mechanism of mat loss Observations from Thermo imaging experiments suggest that mat loss proceeds by the gases causing preferential expansion of the can in the flat region such that the mat is unable to prevent gas contact with a little more of the can. So the can gets gradually separated from the mat. This separation progresses both from the front and the rear of the mat. During this process, no erosion of the mat, takes place. Monitoring this process allowed the de-canning of parts where the can had been completely separated from the mat, and the mat was found intact with carbonaceous deposits. Once there is a clear path over the mat, erosion proceeds quite quickly. This would explain why seals do not work, they do not prevent the separation process. A robust design must not only prevent gas impingement onto the mat but also the gradual separation of the can from the mat. 4.4. Heat Conduction from the Cone Substrate skin temperatures have been measured and been found to be lower than Can skin temperatures indicating that the mechanism of heating the part of the can with the mat is not by conducting heat through the mat, but rather by conducting heat from the cone part of the can which is exposed to exhaust gas temperatures to the part with mat which is insulated by mat. EFFECT
OF CONDUCTION
FROM CONE
12o 100 4 . . . . . . . . . . . . . . . . . 8O O o
60 40 20 0 1
2
3
4
5
6
7
8
9
10
11
This effect can be estimated by measuring T2 - T3 and T4 - T5. (This data was not generated for Tests 9 and 10.) The best design should have the largest differences. The conclusion is that conduction contributes but is not the only factor. Can expansion during the hot condition has got to be controlled by keeping the temperature below that required to separate the can from the mat. In a vehicle, the solution lies in having a compromise between the following:
525 o o o
Reduce gas-inlet temperatures. Cool the can from the outside Have sufficient quantities of mat to ensure a high THERMAL GRADIENT, so that there are always residual layers of mat which create pressure. Care must be taken not to permanently deform the can. A system with intemal(insulating) cones, two layers of 3100 g/m 2 mat in a 6.0 mm gap with an underside heat shield withstands the high speed dyno test. This suggests that, for each mat gap, there is a maximum temperature beyond which the can will be peeled away from the mat. The 6.0 mm gap will support much higher temperatures than the 4.25 mm gap (to give equivalent thermal gradient). Analysis of the performance of reduced mat gap on the high speed dyno test shows that the smaller the gap, the fewer the number of cycles survived by the part. This is consistent with insufficient thermal gradient leading to can-substrate separation.
4.5. Controlled Can Skin Temperature Experiments The last set of experiments (Figures 8, 9, 10, and 11) therefore concentrated on limiting T4. In tests represented in Figures 8 and 9, T4 is limited to 580 ~ This results in very slight mat loss. Reducing the T4 temperature further to 560 ~ results in no mat loss at all. Temperature plots are consistent, there are no dramatic events for T5. The plot below was generated from the Mat Gap experiments data. This also shows that the parts that survived the 60 cycle test were those with the can skin temperature T4 is maintained below 630 ~ EFFECT OF INITIAL T4 60
700
50
680 660
40
640
30
620 6O0
20
.....
10
.....
-
-
-
-
580
-
.,,..,,. .......
0
. 1
2
.
3
. 4
r
~
7
8
560 540
9
.
' 520 5
6
9
10
TEST No
I
~
Mount Gap mm ~ C y c l e s
to Fail ~
INITIAL T4oC 1 J
5. CONCLUSIONS. For a given mat gap, there is a critical can skin temperature below which the mat has sufficient thermal gradient to prevent initial can expansion. For a 4070 g/m Emat in a gap of 4.25 mm, that temperature is 560 - 580 ~ The maximum for two layers of 3100 g/m 2 in a 6.00 mm gap is yet to be determined but higher than 650 ~ At temperatures higher than the above, exhaust gases peel the can away from the mat and seal. That's why none of the seals tested above were effective against mat loss. Effective means of keeping the can below the critical temperature 9
526 Lowering the gas-inlet temperature Putting Louvers in the heatshield Eliminating the heatshield wherever possible. Increasing mat gap, and mat mount density. Canning materials with a lower coefficient of thermal expansion should be sought. The best design to avoid differential can expansion, is a round substrate. 9 The can expands uniformly around the substrate.(Assuming perfect gas distribution.) 9 More effective canning methods ensure even pressure distribution around the substrate 9 Can achieve better gas distribution The race track design in its current form is only maintained on some models for cost, and package reasons. The target for future models has to be round. After this work was done, a new design incorporating a low shot ceramic fibre in the main body of the support in the fiat region of the racetrack design was developed, and has been the subject of a recent publication 6. REFERENCES
.
.
J.P.Day, and L.S.Socha, "Impact of Catalyst Support Design parameters on Automotive Emissions." SAE Paper 881590. D.Kattge, "Advanced Canning Systems for Ceramic Monoliths in Catalytic Converters." SAE Paper 880284. P.D.Stroom, R.P.Merry, and S.T.Gulati, "Systems Approach to Packaging Design for Automotive Catalytic Converters." SAE Paper 900500 J.Kisenyi, K.Soe, P.Leason, C.Tooby, R.Hurley, D.Pritchett, G.Morgan, "Ensuring Substrate Retention in the Racetrack Design" Autotest'96 IDIADA 1996 D.Maret, S.T.Gulati, D.W.Lambert, U.Zink, "Systems Durability of a Ceramic Racetrack Converter" SAE Paper 912371. C.Kroeger, P.Thomas, S. Kohrs, "Patchwork Design Support Mat. A new Support Mat System for Ceramic Monoliths." SAE Paper 971131.
Storage : NOx and Oxygen
This Page Intentionally Left Blank
CATALYSIS AND AUTOMOTIVEPOLLUTIONCONTROLIV Studies in Surface Science and Catalysis,Vol. 116 N. Kruse, A. Frennetand J.-M Bastin (Eds.) 91998 Elsevier Science B.V. All rights reserved.
529
A catalytic NOx m a n a g e m e n t system for lean burn engines Jennifer Feeley, Michel Deeba and Robert J. Farrauto* Engelhard Corporation 101 Wood Avenue, Iselin, New Jersey 08830-0770 tel: 908-205-5306 fax: 908-321-0334 * author of correspondence ABSTRACT A new system, designed specifically for diesel engines, composed of a NO• adsorbent trap coupled to a lean NOx reduction catalyst is described. The trap adsorbs NO• between 150 and 500 ~ which is periodically desorbed thermally by a localized exotherm generated within the catalyst from the controlled injection and oxidation of diesel fuel (always maintaining the exhaust lean). The hydrocarbon injection temperature is controlled to allow for a downstream lean-NO• Pt catalyst to reduce the desorbed NO• Significant increases in NO• reduction are possible. The trap has good thermal stability but suffers from deactivation by sulfur oxides.
Keywords : NOx trap~catalyst system, lean burn engines, NOx reduction 1., INTRODUCTION Lean burn vehicles are stated to have a 20% fuel efficiency advantage and generate less CO2 than stoichiometric gasoline fueled engines. It is projected that about 30% of the vehicles sold in Europe will be diesels by the year 2000. A gasoline fueled lean burn engine, which meets emission standards, is also highly desirable. Gasoline vehicles have been equipped with three way catalysts since 1979 in the US for controlling CO, HC and NOx but are not effective for NO• when operated lean of the stoichiometric air to fuel ratio (~ > 1). Diesel cars and trucks have been equipped with catalysts since the early 1990's but these catalyst are only effective for oxidation of CO, HC and particulates [ 1,2]. No significant abatement of NOx occurs due to their lean mode of operation. Recently, however, it has been demonstrated [3] that 15% NO• reduction in a diesel passenger car is possible with a Pt containing diesel oxidation catalyst. The major challenge for lean burn engines continues to be NO• conversions greater than 70% using hydrocarbons derived from the fuel reductants. Catalysts that passively decompose NO• are the most desirable solution but no progress has been made in finding suitable candidates. The search for a highly selective lean burn catalyst for NOx reduction using on-board derived hydrocarbons has had only limited success. Pt and Cu/ZSM-5 based materials are the leading candidates but have serious limitations for practical use [4,5]. The Pt catalyst operates between 175 to 250 ~ [6] which is too narrow for
530 practical use temperatures. The Cu/ZSM-5 catalyst reduces NOx to N2 at temperatures in excess of about 350-400 ~ but is severely inhibited by sulfur oxides and has poor thermal stability [7]. A partial lean burn approach as an alternative to a full lean burn gasoline system was announced by Toyota [8]. They have developed an engine/catalyst hybrid system in which the control strategy allows lean burn operation during cruising modes delivering a 5-6% fuel economy benefit. A conventional TWC catalyst containing an alkaline component, i.e. BaO, adsorbs NOx during lean modes. The engine control strategy periodically drives the air to fuel ratio to stoichiometric or rich for a time sufficient to reduce the stored NOx over the TWC component. This system is commercially available in Japan where ultra-low sulfur fuel is readily available. For European and US markets, however, the higher fuel sulfur competes more strongly for the alkaline sites on the trap and is difficult to remove during the reduction. Consequently, deactivation is rapid and durability is extremely poor. Finally, this strategy is not designed for diesels which never see stoichiometric conditions. The current paper offers an alternative systems approach that broadens the temperature window for managing NO• in a full lean environment. The system has a trap component which adsorbs NOx over a temperature range where current lean NOx catalysts are not active. The trapped NOx is periodically desorbed and presented to a downstream lean NOx catalyst when conditions are optimal for its' reduction. The predominant species present in the exhaust is NO. The principle is to oxidize NO to NOx above 150 ~ to enhance its adsorption. The trapped or stored NOx is desorbed by an exotherm generated within the washcoat by oxidation of a small amount of injected hydrocarbon, i.e. diesel fuel while maintaining the environment lean and not significantly modifying the bulk gas temperature. The injection temperature is controlled to allow for efficient downstream reduction of the NOx over a lean NOx catalyst i.e. 200-250 ~ for Pt or above 400 ~ for Cu/ZSM-5. 2. E X P E R I M E N T A L The trapping component was formulated into a washcoat and supported on a ceramic monolith with 400 cells per square inch (cpsi). The trap material was chosen for NOn adsorption, regenerability, thermal stability and rate of adsorption/desorption. Platinum is incorporated within the trap to oxidize the NO and the injected hydrocarbon. The lean NOx catalyst was Pt (60 gft-3) deposited on 3'-A1203 on a 400 cpsi cordierite monolith. All experiments were carried out in a typical laboratory reactor consisting of quartz reactor tube loaded in a tube fumace. Flow rates and gas compositions were varied using mass flow controllers and the composition of the feeds and products were measured with a chemiluminescence detector for NOn and a flame ionization detector for total hydrocarbon. Isothermal NOn adsorption capacities were measured using varying amounts of NO as a feed in background gas [10% 02, 10% H20, 50 vppm SO2 and balance N2]. The space velocity was 25,000 h "l and measurements were taken over a variety of temperatures. Altematively, the NOn adsorption capacity properties of the trap were measured in a temperature programmed experiment where NOx was passed over the trap as the temperature was ramped at 10 ~ min l. This allowed a profiling of the sensitivity of the trap to gas inlet temperature. The concept of creating a local exotherm in the washcoat to effect NOn desorption was tested via the introduction of propylene at a variety of concentrations and inlet temperatures. Aging was carried out as indicated in the Results and Discussion Section.
531 Experiments were also carried out on the trap + lean NOx catalyst system to demonstrate proof of concept. The trap was maintained at 300 ~ (maximum adsorption) and the downstream lean NOx catalyst maintained at 210 ~ where its activity is a maximum. The space velocity was 25,000 hl and the inlet gas was 250 vppm NO in background gas [10% 02, 10% H20, 50 vppm SO2, with the balance N2] and 7000 vppm CI (as propylene) as the injected hydrocarbon. The injected hydrocarbon was cycled in an on/off manner to allow the trap to experience NOx adsorption and desorption. The feed to the downstream lean NOx catalyst was a steady addition of propylene at a 4:1 CI/NO ratio based on the feed NO. Thus the actual CI/NO ratio is less because of the contribution from the desorbed NOx. 3. RESULTS AND DISCUSSION The first function of the Pt is to oxidize the NO to a highly adsorbable species which will be called NOx*. It must be pointed out that NO2 generated upstream of the trap does not adsorb as well as when it is generated within the trap. For this reason it is called NOx*. In Figure 1 the
Figure 1" ADSORPTION IN TRAP VS. LEAN NO x REDUCTION OVER P t A N D CulZSM-5 AVERAGE % NOx REMOVAL
70
NOx ADSORPTION
60
I
Pt
4ol
H
l[
"~~
.~,~CulZSM-5
30 20 10 0
Ira=
100
150
200
250
300
350
400
450
500
TEMPERATURE (C)
Figure 1. ADSORPTION CONDITIONS: 250 PPM NOx,10% 02
IN N2, 10% H20, 50 PPM SO2. LEAN-NOX
REDUCTION WITH 4:1 CI/NO TEMPERATURE RAMP RATE = 10 C/MIN.
amount of NOx adsorbed from a gas composed of 250 vppm NO along with the background gases is dramatically improved above 150 ~ where the NO is oxidized. The average adsorption of NOx is significant up to about 500 ~ The decrease observed after the maxima is governed by the thermodynamics of the NO/NO2 equilibrium and the stability of the nitrate species formed. Independent experiments under isothermal conditions generate a similar adsorption profile as that generated by temperature ramping. Included in Figure 1 is a typical lean-NOx reduction profile for Pt at a 4:1 C ~/NOx ratio. Diesel exhaust temperatures typically are between 100 and 500 ~
532 so a significant portion of the NOx, which would not be reduced by the lean-NOx catalyst, can be stored in the trap. No NOx is stored on the lean-NOx catalyst. A schematic representation of the reactor system is shown in Figure 2, showing two beds and two points of hydrocarbon injection, HC 1 at the trap inlet and HC2 at the lean NOx catalyst inlet. For diesel applications, HC 1 and HC2 will be diesel fuel. In operation the trap serves to adsorb NOx over a wide temperature range. When it approaches saturation and the exhaust temperature reaches a value in which the lean NOx catalyst is active a small amount of diesel fuel, HC 1 is injected while maintaining the gas stream lean. This generates an exotherm in the trap which desorbs the NOx without significantly increasing the bulk gas temperature. Simultaneously, HC2 is added to provide a suitable C/NO ratio to effectively reduce the desorbed NOx. The amount of HC2 will be governed by the selectivity of the catalyst, i.e. Pt at about 200-220 ~ will give 60% NOx conversion at a CI/NO ratio of 4:1. An alternative approach would be to design the trap such that HC 1 breakthrough occurs providing the necessary reductant.
Figure
2: N O x T R A P + L E A N - N O SYSTEM - 9" " " ~ ~ ' ~ E X H A U
INJECTION OF HC 1
ST
NOx
4
INJECTION OF HC 2
TRAP LEAN-N O x REDUCTION CATALYST
The sequential operational steps for the trap (1-3) and the NOx reduction downstream (4) are Pt/trap NO
+
02
> NOx*...... trap
(1)
CO2 + H20 + HEAT
(2)
Pt HC
+
02
'>
NO• .... trap + HEAT ....... > NOx + trap
(3)
lean NOx catalyst NOx + HC + 02 ,> N2 (N20) + CO2 + H20
(4)
533 A fresh trap was initially treated with 1000 vppm of NOx in background gas at 300 ~ as shown on the left side of Figure 3. Following a N2 purge for 10 minutes a NOx desorption profile was generated by the injection of 6500 vppm Cl (propylene) in background gas at a trap inlet temperature of 300 ~ The profile is shown on the right side of Figure 3. In a vehicle the NO• would be adsorbed during a normal driving cycle whenever the exhaust temperature is between 150 and 500 ~ The injection of fuel is controlled so that the downstream catalyst is at about 210 ~ optimum for NOx reduction. The amount of NOx desorbed depends on the intensity of the exotherm which depends on the amount of hydrocarbon injected and oxidized. This is shown in Figure 4 where the amount ofNOx desorbed increases with an increasing amount of hydrocarbon fuel. Repeat desorption runs, after adsorption of NOx as in Figure 3, indicate that the trap is completely regenerated after an injection of 10,000 vppm hydrocarbon. The bulk gas phase temperature remains
Figure 3 NOx A D S O R P T I O N 1000 PPM NOx IN BACKGROUND GAS,
DESORPTION OF NOx BY HC INJECTION
300 C, SV= 25,000/h 400
PPM NOx REMOVAL 350
GAS PHASE NOx (PPM)
350 300
3O0
250
250
200
200
150
150 100
100
~__
~o 0
....
5
, ,,
10
TIME (MIN)
5O ,,,
0
i 20
ii 21
ii 22
23
30
35
TIME (MIN)
fairly constant because the desorption of NOx is endothermic which counters the exothermic heat of the oxidation reaction. The downstream lean-NOn catalyst temperature is at 210 ~ which gives a reduction of about 60% of the inlet NOx at a 4:1 C1/NO ratio. Additional hydrocarbon (HC2) must be available to the NO• reduction catalyst. This can be added immediately before the leanNOx reduction catalyst or alternatively the trap system can be under-designed to allow for sufficient hydrocarbon slip or breakthrough to accomplish the downstream NOx reduction. Results for a combination trap/lean-NOx system are shown in Figure 5. A continuous flow of 250 vppm NOx in background gas is passed over the trap at 300 ~ The rising portion of the curve on the left side shows the adsorption of NOx. At the maxima 7000 vppm Cl, as propylene, (HC 1 in Figure 2) is injected desorbing all of the adsorbed NOx as evident by the equal areas
534
F i g u r e 4" N O x DESORPTION BY HYDROCARBON INJECTION NOx DESORBED (PPM)
800 700
10,000 PPM C1 (C3He)
600
INJECTED
500 400
~,000
300
4,000
200 100 0
1,000 11
o
0.5
1
1.5
2
2.5
3
3.5
4
TIME (MINUTES)
Figure 5" T R A P + Pt LEAN NOx CATALYST % NOx REMOVAL 100
LEAN-NOx CATALYST C I l N O = 4"1 ( B A S E D ON
LEAN-NOx
CArA,XSr / ~
80 60
/~
F~D NOb
A
40 20 0 -20
TRAP INLET =300C C A T A L Y S T IN=210 C
-40 -60
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
TIME (ARBITRARY) N O x = 250 PPM IN B A C K G R O U N D G A S ( C O N T I N U O U S ) HC1 = 7000 PPM C1 ( C a l l s ) I N J E C T E D A T M A X l M I M S
18
535 above and below the zero point. When the cycle is repeated but with secondary HC2 injected at a 4:1 CI/NO (based on feed NO) the entire profile is elevated reflecting an average reduction of about 45%. Optimization of the Cl to NO ratio to account for the extra NO resulting from the desorption would further increase the NOx reduction possible. Naturally a more selective lean NOx catalyst would also be beneficial. The proprietary architecture of the trap system was evaluated for thermal stability as shown in Figure 6. Adsorption experiments were conducted with 250 vppm NO• in background gas (minus the SO2) at a ramp rate of 10 ~ The system maintains good adsorption capacity after aging at 700 ~ in air with 10% H20 for 100 hours and 50% of the original capacity is maintained after aging at 800 ~ This was encouraging because diesel engines are not expected to exceed 700 ~ exhaust temperatures in use.
70
F i g u r e 6" T H E R M A L A G I N G OF T R A P : C H A N G E IN ADSORPTION CAPACITY NOxADSORPTION
60 50 40 30 20 10 100
150
200
250
300
350
400
450
500
TEMPERATURE ADSORPTION:250
P P M N O • IN B A C K G R O U N D
GAS
AGING: AIR+10% H20 FOR100 HOURS
The presence of SO2 deactivated the trap as shown in Figure 7. The trap was initially treated with 1000 vppm NO• in background gas (minus the SO2) for 15 minutes. The desorption profile generated by injection of 6500 vppm Cl, as propylene, is shown as the fresh catalyst. After aging in 150 vppm SO2 in air and 10% H20 at 500 ~ for 24 hours the trap effectively loses all of its NOx adsorption/desorption capacity. Even with a calcination at 800 ~ in sulfur free air only about 30% of the initial capacity could be recovered. The absence of regeneration is primarily due to retained SOx which occupies NOx adsorption sites although some thermal deactivation at 800 ~ also occurs.
536 Figure 600
7:
AGING
IN SULFUR
DIOXIDE
NOx DESORBED (PPM)
500 400 300 200 100 0
0.5
1
1.5
2
2.5
3
3.5
4
TIME (MINUTES)
INITIAL DOSING: 1000 PPM, 300 C, B A C K G R O U N D G A S A G I N G : 150 PPM SO2, 500 C IN 10%H20,10%O2, N2, 24h NOx D E S O R P T I O N : Cl = 6500 PPM (C3He)
4. CONCLUSIONS 1) A system designed to manage NOx between 150 and 500 ~ in a lean bum environment has been explored as an alternative to current lean NOx catalyst limitations. 2) NOx desorption can be accomplished by the exotherm generated by the oxidation of injected hydrocarbon within the trap while always maintaining the environment lean. 3) The trap inlet injection temperature must be controlled to be compatible with the maximum activity of the downstream lean-NOx catalyst. 4) The oxides of sulfur compete more strongly than NOx for the trap sites leading to a permanent loss in NOx adsorption capacity. 5) A SOx tolerant trap component is needed for this approach to have commercial significance. REFERENCES 1. R.M. Heck and R.J. Farrauto, Catalytic Air Pollution Control: Commercial Technology. Van Nostrand Reinhold, NY, NY 1995, Chapter 7. 2. R.J. Farrauto and K.E. Voss, Applied Catalysis B: Environmental 10:1-3 (1996)29 3. Y.K. Lui, J. Dettling, O. Weldlich, R. Krohn, D. Neyer, W. Engeler, G. Kahman and P. Dore, SAE 962048 (1996) 4. M. Iwamoto, Catalysis Today 28 (1996)29 5. M. Amiridis, T. Zhang and R. Farrauto, Applied Catalysis B: Environmental 10:1-3 (1996) 203 6. K.M. Adams, J. Cavataio and R. Hammerle, Applied Catalysis B: Environmental 10:1-3 (1996)157 7. J. Feeley, M. Deeba and R. Farrauto, SAE 950747 (1995) 8. M. Miyoshi, S. Matsumoto, K. Katoh, T. Tanaka, J. Harada, N. Takahashi, K. Yokota, M. Sigiura and K. Kasahara, SAE 950809 (1995)
CATALYSISAND AUTOMOTIVEPOLLUTIONCONTROLIV Studies in Surface Science and Catalysis,Vol. 116 N. Kruse, A. Frennet and J.-M Bastin (Eds.) 91998Elsevier Science B.V. All rights reserved.
537
Investigations of NOx storage catalysts Erik Fridell a, Magnus Skoglundha, Stefan Johansson a'b, Bj6rn Westerberga'c, Anders T6rncrona~d and Gudmund Smedler a aCompetence Centre for Catalysis, Chalmers University of Technology, S-412 96 G6teborg, Sweden bDepartment of Applied Physics, Chalmers University of Technology and G6teborg University, S 412-96 G6teborg, Sweden CDepartment of Chemical Reaction Engineering, Chalmers University of Technology, S 412 96 G6teborg, Sweden dDepartment of Engineering Chemistry, Chalmers University of Technology, S-412 96 G6teborg, Sweden
ABSTRACT NOx storage catalysts are used to reduce nitrogen oxides from lean-bum vehicles. The nitrogen oxides are stored in the catalyst during lean conditions and subsequently released and reduced during short periods of rich conditions. In the present study, we systematically investigate the sequence of elementary steps in the NOx reduction cycle, and the extent to which these steps influence the maximum NOx reduction potential of the catalyst. As a model system, we use barium oxide as the NOx storing compound in a Pt/Rh/A1203 system. Kinetics of NO oxidation, NO and NO2 adsorption, NO and NO2 release and reduction are studied under controlled conditions with systematic variations of temperature, gas composition, and storing/release times. The transient experiments comprise a storing phase using a lean NO/C3H6/O2/N2 gas mixture, and a regenerating phase where the 02 flow is turned off. Experimentally, a significant amount of NOx is found to be stored in the Ba-containing material. A maximum in NOx storage is observed around 380~ For most of the experiments, there are clear NO and NO2 desorption peaks upon switching from the storing to the regeneration phase. TPD studies of NO and NO2 reveal a significant difference between prereduced and pre-oxidised samples where the former produce predominantly N2 and N20 at around 200~ while NO and 02 desorb from the latter around 500~ In situ FTIR spectra show nitrate peaks in the region 1300 - 1400 cm -~ when NOx is stored under lean conditions.
1. INTRODUCTION There is currently a great interest in improving automobile fuel economy and to reduce emissions of carbon dioxide into the atmosphere. One step in this direction is to use a technique where a gasoline engine is operated at lean-bum conditions. Using this concept it is possible to improve fuel economy significantly [1, 2] compared to the normal stoichiometric operation. A setback with lean-burn technology is that the common three-way catalyst is
538 unable to efficiently reduce nitrogen oxides (NOx) at oxygen excess resulting in significant emissions of NOx from the vehicle. One concept to solve this problem is the NOx storage catalyst. It was first presented in several patent applications by Toyota and later by Miyoshi et al. [2], Takahashi et al. [3] and B6gner et al. [4]. NOx reduction capability of 90% [3] has been reported from vehicle tests. The NOx storage catalyst usually contains noble metals for reduction and oxidation and a NOx storage compound (an alkaline earth compound) supported on alumina [2]. During leanburn conditions, NOx is being stored in the catalyst. To regenerate the catalyst, short periods of rich conditions are employed during which the stored NOx is released and subsequently reduced on the noble metal surfaces. The objective of this work is to investigate some key steps in the reaction mechanisms of catalytic reduction of NOx under oxygen excess by a systematic variation of the essential components for NOx storage and reduction. We describe the function of a model system for NOx storage catalysts containing Pt, Rh, A1203 and barium oxide as the storage component. The catalyst was made as simple as possible, yet containing the essential constituents for NOx storage and reduction, in order to investigate the mechanisms of the NOx storage concept. Also catalysts without noble metals or barium, respectively, were prepared and studied. The reaction mechanisms were investigated by transient reactor studies, temperature programmed desorption (TPD) and in situ Fourier transformed infrared spectroscopy (FTIR). TPD measurements were performed for both pre-reduced and pre-oxidised samples in order to elucidate the different surface conditions during the different phases of the transient storage reduction cycles. The reaction sequence and the nature of the stored NOx compound are discussed.
2. E X P E R I M E N T A L P R O C E D U R E
Flow reactor studies were performed using monolith samples with an alumina washcoat, a storage compound (BaO) and noble metals (Pt, Rh). In order to elucidate the importance of the various ingredients, samples were also prepared without storage compound or noble metals, respectively. For the FTIR studies, similar powder catalysts were pressed into thin discs. Three different monolith samples were prepared according to a procedure described elsewhere [5]; the first with 160 mg alumina, 40 mg barium oxide, 4.0 mg Pt and 2.0 mg Rh, the second with 160 mg alumina and 40 mg barium oxide and the third with 200 mg alumina, 4.0 mg Pt and 2.0 mg Rh. We do not know the exact nature of the barium compound in the catalyst but refer to it as BaO in this paper. The fresh catalysts were reduced in H2 at 450~ for 35 minutes and stabilised in the lean gas mixture (see Table 1) at 550~ for two hours. The flow reactor used in most experiments is described elsewhere [5]. Briefly, it consists of a horizontal quartz tube encased in a divisible tubular furnace. The catalyst is sealed in the middle of the heated zone with quartz wool and the gases are introduced via mass flow controllers. Reactants and products are analysed on-line with respect to N20, CO, CO2 (IR), and NO, NO2 (chemiluminescence) The TPD measurements were performed with the same monolith samples in a different quartz flow reactor described elsewhere [6-7]. The samples were either prereduced (4% H2 in Ar) or preoxidised (5% 02 in Ar) at 500 ~ for five minutes. They were then exposed to 1%
539 of NO or NO2 in Ar at a flow of 100 ml/min, for 5 minutes in room temperature prior to a heating ramp (40~ in Ar flow. The FTIR-experiments were performed with thin discs (approximately 15 mg/cm2) of catalyst in a reaction chamber with CaFE-windows [8]. Two different catalysts were used; one with Pt/BaO/A1203 (2% Pt, 20% BaO) and one with Pt/A1203 (2% Pt). The fresh catalysts were initially reduced in 30% H2 in N2 (total flow rate of 100 ml/min) at 450~ for 30 minutes, stabilised in a gas mixture with 5% 02, 1000 ppm NO and 3000 ppm C3H6 in N2 (total flow rate 1000 ml/min) for 30 minutes and finally degassed in N2 (1000 ml/min) at 550~ for 30 minutes. All FTIR-experiments were performed at 380~ at a total flow rate of 1000 ml/min with a scan speed of 1 cml/s and a nominal resolution of 4 cm -l. Table 1. Gas concentrations in the different exPeriments.., Experiment NO (NO2)[ppm] Flow reactor, lean phase 600-1100 Flow reactor, rich phase 600-1100 FTIR, lean phase 400 FTIR, rich phase 400 , ,
"
C3H6 [ppm] 900 900 900 900
,,
02 [ % ] ' 8.0 0.0 5.0 0.0
3. RESULTS 3.1. General
In order to obtain effective NOx storage the catalyst is used in connection with transients in the gas composition, i.e., during lean-burn periods NOx is stored in the catalyst and during short periods of rich conditions, the stored NOx is released and reduced. 1000
400 E i3_ o E O
~
’ I
:
800
Iklt'~ --~.......~-----~----._.,~ii., H
,
V
No
:
I
, I
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I --
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I
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,
’
l jr !
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I
I
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’ I
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I
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time
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i
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.
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I
I
'
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(s)
Figure 1. The NO and NO2 concentration traces during a transient with the gas compositions quoted in Table 1 over a Pt-Rh/BaO/A1203 catalyst. The dashed vertical lines mark the switches in gas composition where the rich phase is indicated by the double arrow.
540 Fig. 1 shows the NO and NO2 concentration traces in the product gas over a PtRh/BaO/A1203 catalyst, at an inlet temperature of 400~ during such transients with the conditions described in Table 1. The difference between the lean (NOx storage) phase and the rich (regenerating) phase is that the oxygen content in the gas is turned on and off, respectively (and compensated by changes in the N2 flow to maintain a constant flow rate). When 02 is switched on, at 320 s, for example, there is a relatively slow increase in both the NO- and NO2 signals. The reason for this slow increase is that NOx is being stored in the catalyst. The amount of stored NOx is obtained by integration of the area related to storage in this type of transients. For the experiment in Fig. 1, the ratio between stored NOx and barium is approximately 1:14. During the rich phase, all NOx is being effectively reduced by the propene. Also the NOx stored in the catalyst during the previous lean phase is released and reduced. Break-through peaks can be seen for both NO and NO2 when switching from lean to rich conditions. These desorption peaks will be commented upon below. The reduction of NOx under the lean phase is negligible at 400~ and the gas mixture used here (see Fig. 3 below). The formation of nitrous oxide (N20) can be observed both when switching from lean to rich conditions and vice versa (not shown). It is well known that N20 may be formed when switching from an oxidised to a reduced surface, for example, during light-off [9] (see also Fig. 3 below). The HC concentration in the product gas (not shown) is zero during the lean phase and reaches about 750 ppm during the rich phase. An HC break-through peak is present when switching from rich to lean conditions. There is a temperature decrease within the catalyst of about 15~ during the rich phase compared to the lean phase at the present conditions due to the lack of combustion during the rich phase (the magnitude of this decrease depends on the space velocity). When performing similar cycles with a catalyst containing no specific NOx storage component, i.e., a Pt-Rh/A1203 catalyst, no NOx storage can be observed. Break-through peaks of NO and NO2 similar to those in Fig. 1 can be observed also with this sample. For a sample without Pt-Rh but with BaO, no storage (with either NO or NO2 in the feed gas) and only minor reduction of NOx (during the rich phase) is observed. 3.2. Influence of temperature The NO and NO2 signals during storage transients are measured at different temperatures using the conditions in Table 1 (with NO in the feed). The amount of NO• stored during each lean phase is presented vs. temperature in Fig. 2 for cycles with 240 s in the lean phase followed by 60 s in the rich phase. In the temperature interval studied, a maximum in NOn storage is found at around 380~ and very low values above 500~ are measured. Also shown is the corresponding storage values when using NO2 in the feed gas rather than NO. The latter shows similar appearance as with NO.
541 ..... ...-..%
.~
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-sL. ~3
15
~3 L. O
10
-!-3 03
O Z
X
I
9
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"..... N O 2 in feed
__
X - ~
5 I
,.
I
300
,
350
I
I
I
400
450
Inlet Temperature
500
(~
Figure 2. The NOx storage vs. inlet gas temperature with either NO or NO2 in the feed (compositions are given in Table 1). The CO2, NO, NO2, N20 signals and the catalyst temperature, during a heating ramp with the lean gas mixture (i.e., without transients) over the Pt-Rh/BaO/A1203 catalyst, are shown in Fig. 3. Observe that there is a maximum in NO oxidation to NO2 around the temperature (375~ where there is a maximum in NOx storage (see Fig. 2). Light-off can be observed around 250~ which is in the same range where reduction to N20 and N2 take place. Similar experiments with the Pt-Rh/A1203 catalyst, with the same noble metal load, give a significantly more pronounced light-off behaviour. This may imply that either the dispersion is lower when BaO is present, that BaO partially covers the noble metal, or that BaO in the close vicinity of Pt/Rh inhibits the capacity of the noble metal to catalyse oxidation reactions. 600 -I
0
50o -
I
Pt/Rh/BaO/AI203 catalyst NO in feed - 9149 9 .:'" ;~_
..'\
400 -
F-~--" 300 E
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~
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"-............-.-".-
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3
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.~,~N20
" ,~.. I
300 Inlet T e m p e r a t u r e
.... (~
Figure 3. Concentration traces during a heating ramp (5~ catalyst in the lean gas mixture.
;,," I" -,-',- . . . . . . 400
I--
500
over the Pt-Rh/BaO/A1203
542 3.3. Influence of gas composition The storage capacity was measured as a function of 02 and C3H6 content in the feed gas. At very low 02 concentrations (i.e., a net reducing gas) there is no net storage can be observed. The storage yield in lean gas mixtures increases slowly with increasing 02 content. The magnitude of the NO and NO2 break-through peaks (see Fig. 1) also increase with increasing 02 concentration (much more rapidly than the storage yield). Regarding the C3H6 concentration, there is a maximum in storage around 500 ppm C3H6. There is storage observed even if the C3H6 is turned off but the regeneration and reduction is then much slower. This variation is probably connected both with the different temperatures in the catalysts caused by the different reaction heat dissipated at the different C3H6 flows and with the somewhat lower oxygen coverage at high propene concentration. There are NOx breakthrough peaks of similar magnitude observed at all C3H6 levels examined. 3.4. Influence of phase duration The influence of the storage and regeneration times on the NOx storage over a PtRh/BaO/AI203 catalyst at 400~ is studied. The catalyst is completely regenerated after 40 s, i.e., longer regeneration times do not influence the amount of NOn being stored. The storage component is saturated after about 200 s during these conditions. 3.5. Break-through peaks There are break-through peaks in NO and NO2 when switching from the storage phase to the regenerating phase. Regarding these peaks the following observations were made: 1) they show a significant increase with increasing temperature (below 500~ 2) they show an increase in magnitude with increasing oxygen content in the lean phase, 3) when the propene fraction in the feed gas was varied from 100 to 1300 ppm, the magnitude of the peaks remained constant, 4) peaks are also seen for a catalyst without BaO, 5) the storage time does not influence the peak heights, 6) the amount of NOn in the peaks for the data in Fig. 1 corresponds approximately to 3.10 -6 moles for each cycle. 3.6. TPI) studies TPD measurements were performed for the NOx-storage catalyst with Pt-Rh/BaO/A1203 and for samples without the storage medium (BaO) or noble metals, respectively 9TPD experiments were performed by exposing pre-reduced and pre-oxidised catalysts, respectively, to either NO or NO2 at room temperature. i i i I , , , , -t I i
30X10"3
NO-TPD prereduced Pt/Rh/BaO/A~O 3 sample
'..
i
.... ;
2O =.. < .-- 15 O
~.:-
--
i.
0
i
ii
.
..
....
X ,Oo
"'.. .... .....
i .... I 300 .....200 T e m p e r a t u r e (~ ,,.
100
-
NO
~.......’-t: 9
~
-
N2
<2
.I:: _
-
,.
.~ : :..,. .
10
.9. . .
";
i
N20
NO-TPD preoxidised Pt/Rh/BaO/A~O 3 sample
B 30x10: -
I 400
I 500
"; ...... 7 .... lOO
,:::
200 300 T e m p e r a t u r e (*C)
." 400
I 500
Figure 4. TPD after exposing the Pt-Rh/BaO/A1203 sample to NO at room temperature. A) pre-reduced catalyst, B) pre-oxidised catalyst 9
543 Fig. 4a shows the TPD spectra after dosing NO at room temperature on a pre-reduced PtRh/BaO/A1203 catalyst. As can be seen the NO is reduced on the surface as manifested in N2 and N20 desorption peaks around 200~ The only trace of NO is a small peak around 100~ The integrated amount from these curves correspond to an adsorbed amount of 7.1.10 .6 moles NO. The result of a similar experiment but with a pre-oxidised sample is shown in Fig. 4bl In this case there is no reduction of NO taking place. There is a small NO peak at around 90~ and a larger one at about 500~ There is also an 02 peak around 500~ It is likely that a chemisorbed oxygen layer on the noble metals prevents the dissociation of NO as observed by LO6f et al. [6]. When NO2 rather than NO is dosed at room temperature, there is a much larger quantity adsorbed. Further, pre-reduced and pre-oxidised samples show similar TPD spectra indicating that the strong oxidising agent NO2 oxidises the sample at room temperature. 3.7. FTIR
studies
Transmission FTIR spectra were taken in situ both for catalysts with and without BaO, respectively. Fig. 5 shows the region 1200 - 2300 cm -I taken during both the lean and the rich phases at 380~ There are peaks around 1550 - 1575 cm "z which are associated both with nitrate and carbonate species which are most intense for the BaO containing sample in the storage phase [10-12]. At around 1302 cm l there is strong absorption only for the BaO containing catalyst in the lean phase. This peak was seen also in a gas mixture of 02 and NO. We therefore attribute this peak to barium nitrate [12] formed from BaO, O and NO (se further below). These absorption bands disappeared when switching to a rich reactant gas composition. In situ FTIR studies of catalysts without barium, using the same reaction conditions, showed no (or very weak) presence of absorption bands in the region 1300-1400 cm 1. During the rich conditions there is a peak at 2226 cm l , which is very strong for the barium containing catalyst, not seen in the lean gas-mixture. This absorption band is attributed to surface bound isocyanate, -NCO [ 13-14]. 60x10
-3
I I"-
I
I
--
50~ | ~> / ~40}--. c--
I: I,:
"
".
--
I,
2200
iT ............
2000
'"
~! ~! 1'
J."
-
':
L:'' .;" ,. - : ~ . " ~ " , "."
.Vke41a'~,'B
i .... " ' " ' "
1800
Wavenumber
I
I
1600
:
"
^
.."-;
i."':
9
10~" .. :.....-.,.. .'.~r "-. 9 lI ' ~ i ' ~d ~l r ~ - - l ' ~ a i ~~ ' ~ ." I ' ] D e I U L . - . . - -"... __"o~,,,.~.,~.
o~-
I
..... P t / B a O / A I 2 0 3, rich p h a s e - - Pt/AI20 3, lean p h a s e ...__ . . . . Pt/AI20 3, rich p h a s e ~
t. :.:-:i -~
i/i
I
PtyBaOIAI20 ~, lean p h a s e
,'~.'; - *..
'
9
"'""."'....
9
,".--"
"" " " - 9
"'," ".. ,9 . ""
-
9 "" . . . . . .
i'"
1400
*" "
,
.
L
"" "" "'.
,
1200
c m -1
Figure 5. FTIR absorption spectra for two samples with and without BaO, respectively, in two different gas mixtures (see Table 1).
544 4. DISCUSSION
During a lean/rich cycle over the NOx storage catalyst there is storage of NOx, possibly preceded by NO oxidation. Nitrogen oxides are probably stored as a nitrate of the storage compound during the lean phase. This is followed by decomposition of the nitrate and reduction of NOx over noble metal sites during the rich phase. One NOx storage/reduction cycle is suggested to comprise the following steps:
.
2. 3. 4a. 4b. 5a. 5b. 6. 7.
lean c o n d i t i o n s lean conditions NO a + O a -~ NO2 at lean conditions 2NO + 30 + BaO --~ Ba(NO3)2 at lean conditions 2NO2 + O + BaO --~ Ba(NO3)2 at lean conditions Ba(NO3)2 --~ BaO + 2NO + 30 at rich conditions Ba(NO3)2 -~ BaO + 2NO2 + O at rich conditions C3H6 g ~ C3H6a at rich conditions 9NO + C3H6 --~ 9/2N2 + 3CO2 + 3H20 at rich conditions
N O g --~ N O a at 0 2 g --~ 2 0 a at
This is a simplified reaction scheme. For example NO2 adsorption and desorption and NO desorption are not included as well as N20 formation. Further, the NOx reduction, steps 6 and 7, are of course only an example of many possible pathways. The importance of NO2 as a precursor to stored nitrate was assumed by B6gner et al. [4] and Takahashi et al. [3]. From our data we know that the storage of NOx works equally well with either NO or NO2 in the feed gas. However, this fact does not tell us anything about the importance of step 1 in the proposed reaction scheme, because at the temperatures used here, there is quasi-equilibrium between NO and NO2 within the sample (see also Fig. 3). When comparing Figs 2 and 3 it can be seen that the temperature dependence of the NO to NO2 oxidation does not follow the NOx storage rate at high temperatures. It is, however, likely that the temperature dependence of the NO oxidation is important for the NOx storage rate at temperatures below 350~ The decrease in NOx storage at higher temperatures probably has to do with the stability of the metal nitrate in which the NOx is stored. Further indication of the importance of NO2 is the high adsorption yield at room temperature. The reason that no storage is observed above 500~ is most probably that NOx desorb around 480~ (see Fig. 4b). Considering that NOx storage requires the presence of noble metals, it is likely that the presence of atomic oxygen is important for the process. This species is present both in reactions 3, 4a and 4b above and the storage does not work for a sample without noble metals (i.e., a BaO/A1203 sample) even if NOx is in the form of NO2. Alternatively, the absence of NOx storage in the latter case can be caused by this sample not being easily reduced (and therefore not easily regenerated if barium nitrates were formed) because of the lack of noble metals. At reducing conditions two important things take place. The oxygen flow is turned off and therefore reactions 5a and 5b become important and NO and/or NO2 become present on the surface. Secondly, the noble metal sites are reduced which opens up for the dissociation and reduction of NO• in step 7 (which is just an example of many possible pathways for NOx reduction). The importance of the sites being reduced for step 7 to take place is well known
545 [8] and clearly demonstrated in the TPD measurements (Fig. 4) where recombination of dissociated NO to N2 and N20 takes place only on the pre-reduced sample in a NO TPD experiment. Among the pure nitrogen containing compounds barium nitrate, Ba(NO3)2, is the most likely candidate that can explain the observations in the transient reactor studies, in the in-situ FTIR spectroscopy studies and in the temperature programmed desorption studies. Barium nitrate is stable up to about 590~ where it melts and decomposes [15]. The formation of Ba(NO3)2 is thermodynamically favoured at high oxygen contents at lower temperatures. When switching the oxygen supply off, as during the rich phase in the experiments, the decomposition of barium nitrate is favoured. Other nitrogen containing compounds, e.g., barium nitrite and aluminium nitrate, decompose at lower temperatures, about 215 and 150~ respectively. Several complex compounds, e.g., barium aluminates and non-stoichiometric compounds, may have NOx storage capacity under the lean reaction conditions. Several surface complexes containing barium, alumina and nitrogen oxides, rather than bulk species, may also be formed. The nature of such complexes is difficult to elucidate. NOx may therefore be strongly bound on the catalyst surface and in the surface layers of the catalyst in several type of species. Two different types of break-through peaks are observed in the experiments. There is an HC peak when switching from rich to lean conditions. To understand this peak we must remember that the oxygen is turned off during the rich phase, i.e., there is little combustion proceeding, NOx being the only oxidising agent, which is manifested in the lowering of the catalyst temperature. When the oxygen is turned on, there will be a moving combustion front in the catalyst as the adsorbed hydrocarbon is oxidised. This may result in local hot zones where adsorbed HC will thermally desorb until the (local) oxygen coverage becomes high enough for all HC to react with oxygen. The NOx break-through peaks when switching from lean to rich conditions is by B6gner et al. [4] discussed in terms of a relatively slow reduction of noble metal sites compared to the decomposition of the barium-nitrate giving a sudden increase in the surface coverage of NOx. However, against this view can be argued that we observe these peaks also without a storage component in the catalyst. We rather believe that there is a fast reduction of noble metal sites. In this process, the adsorption properties for NOx on the noble metal surface change. This is manifested in the TPD results in Fig. 4. For the oxidised samples, the desorption of NO and 02 takes place above 400~ For the reduced sample, on the other hand, NO desorbs as N2 and N20 at much lower temperatures. This indicates that binding of NO is weaker on an oxygen covered surface than on a reduced surface. Root et al. have shown that coadsorption of oxygen and NO on Rh(111) lowers the desorption temperature for NO compared with NO adsorbed on a clean Rh (111) surface [16]. The switch from lean to rich atmosphere most probably causes a transformation to a surface where the NOx desorption rate is very high at these temperatures (400~ resulting in the break-through peaks seen in Fig. 1. Note that the amount of NOx in these peaks is less than what can be adsorbed on the noble metal sites (3.10 6 moles and 7.1-10 -6 moles respectively). It is also possible that displacement of adsorbed NO and NO2 by the suddenly large amount of C3H6 takes place and contribute to the breakthrough peaks. Further, a sudden local temperature increase at the end of the monolith sample, where adsorbed oxygen reacts with surplus C3H6, may contribute to NOx desorption. We thus believe that the reduction of noble metal sites is fast and therefore that the rate limiting step during the rich phase is the decomposition of barium nitrate.
546 6. CONCLUSIONS We have presented kinetic data on NOx storage for a well defined model sample. Further, TPD and FTIR experiments give an improved understanding compared to earlier studies. This investigation has shown that barium containing catalysts can store NOx under oxygen excess and at elevated temperatures. The stored NOx can be released and reduced under a subsequent rich period. The NOx storage capacity depends on several parameters and among these, temperature is of major importance. In this study maximum in NOx storage capacity was observed for both NO and NO2 at a catalyst inlet temperature of about 380~ which corresponds to a reaction temperature of about 395~ In this temperature region in situ FTIR studies of barium containing catalysts have shown strong absorption bands in the region 13001400 cm -1 and at 2226 cm -1, when using a lean reactant gas. Temperature programmed desorption studies of NO and NO2 have shown the presence of a high temperature (about 500~ NO desorption peak from pre-oxidised barium containing catalysts accompanied by an 02 peak. From a pre-reduced catalyst, mainly N2 and N20 desorb. ACKNOWLEDGEMENTS
This work has been performed within the Competence Centre for Catalysis, which is financed by NUTEK - The Swedish National Board for Industrial and Technical Development, Chalmers University of Technology, AB Volvo, Saab Automobile AB, Johnson Matthey, ABB Fl%okt Industri AB, Perstorp AB and AB Svensk Bilprovning. One of us (GS) gratefully acknowledges the support from Johnson Matthey, Catalytic Systems Division. REFERENCES
1. 2.
M. Shelef, Chem. Rev., 95 (1995) 209. N. Miyoshi, S. Matsumoto, K. Katoh, T. Tanaka, J. Harada, N. Takahashi, K. Yokota, M. Sugiura and K. Kasahara, SAE Technical Paper Series, 950809 (1995). 3. N. Takahashi, H. Shinjoh, T. Iijima, T. Suzuki, K. Yamazaki, K. Yokota, H. Suzuki, N. Miyoshi, S. Matsumoto, T. Tanizawa, T. Tanaka, S. Tateishi and K. Kasahara, Catal. Today, 27 (1996) 63. 4. W. B^gner, M. Kr%omer, B. Krutzsch, S. Pischinger, D. Voigtl%onder, G. Wenninger, F. Wirbeleit, M.S. Brogan, R. J. Brisley and D.E. Webster, Appl. Catal. B, 7 (1995) 153. 5. M. Skoglundh, H. Johansson, L. L6wendahl, K. Jansson, L. Dahl and B. Hirschauer, Appl. Catal. B, 7 (1996) 299. 6. P. L66f, B. Kasemo and K.-E. Keck, J. Catal., 118(1989)339. 7. S. Lundgren, K.-E. Keck and B. Kasemo, Rev. Sci. Instrum., 65 (1994) 2696. 8. P. Basu, T.H. Ballinger and Y.T. Yates Jr., Rev. Sci. Instrum., 59 (1988) 1321. 9. G.P. Ansell, S.E. Golunski, J.W. Hayes, A.P. Walker, R. Burch and P.J. Millington, in A. Frennet and J.-M. Bastin (eds.), Stud. Surf. Sci. Catal., 96 (1995) 255. 10. G. Bamwenda, A. Ogata, A. Obuchi, H. Takahashi and K. Mizuno, React. Kinet. Catal. Lett., 56 (1995) 311. 11. F. Solymosi and J. Sarkany, Appl. Surf. Sci., 3 (1979) 68.
547 12. 13. 14. 15. 16.
M. Schraml-Marth, A. Wokaun and A. Baiker, J. Catal., 138 (1992) 306. Y.J. Mergler and B.E. Nieuwwenhuys, J. Catal., 161 (1996) 292. F. Solymosi, L. V61gyesi and J. Sarkany, J. Catal., 54 (1978) 336. D.R. Lide (ed.) Handbook of Chemistry and Physics, CRC Press, Boca Raton, FLA, 1996. T. W. Root, L. D. Schmidt and G.B. Fisher, Surf. Sci., 134 (1983) 30.
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CATALYSISAND AUTOMOTIVEPOLLUTIONCONTROLIV Studies in Surface Scienceand Catalysis,Vol. 116 N. Kruse,A. Frennet and J.-M Bastin (Eds.) 91998Elsevier Science B.V. All rights reserved.
549
Oxygen Storage Capacity of three-way catalysts : a global test for catalyst deactivation. R. T a h a a, D. D u p r e z a*, N. M o u a d d i b - M o r a l b a n d C. G a u t h i e r b. a Laboratoire de Catalyse en Chimie Organique, CNRS et Universit6 de Poitiers, 40, Av. Recteur Pineau, 86022 Poitiers Cedex, France b Renault Automobiles, Centre Technique de Lardy, 1 All6e de Cornuel 91510 Lardy, France ABSTRACT Two commercial catalysts (PtRh and PdRh) were treated in N2+H20 (or Air+H20) at 900, 1000 or l l00~ (Lab. ageing) or aged on an engine bench for 25, 50 or 200h. OSC (transient CO oxidation) and catalytic activity for CO, HC and NO abatment (light-0ff temperatures and conversion at 450~ were measured over these aged catalysts. Correlations between OSC values and catalytic activity are discussed for a possible OnBoard Diagnostic application. 1. I N T R O D U C T I O N The role of ceria added to three-way catalysts is complex [1,2]: (i) it improves the stability of noble metals ; (ii) it promotes some catalytic reactions, particularly the NO reductionand the water gas shift reaction and (iii) it allows the catalyst to work in the oscillatory regime imposed by stoichiometry regulation (high frequency) and traffic perturbations (low frequency). This third role of ceria is a consequence of its oxygen storage capacity (OSC) : ceria can store oxygen u n d e r lean conditions and is able to release its oxygen under rich ones. The noble metal particles are then fed in oxygen species even when the Ox/Red ratio decreases in gas phase. Oxygen storage capacity of TW catalysts has been widely studied [3-10] and, recently, has been envisaged to serve as a global test of the catalytic activity for On-Board Diagnostic (OBD) [11,12]. In this paper, we shall attempt to correlate the OSC values with the catalytic activity of fresh and aged PtRh and PdRh commercial catalysts for a possible application to On-Board Diagnostic. 2. E X P E R I M E N T A L
2. 1. Catalysts Two commercial catalysts on s t a n d a r d ceramic cordierite were used throughout this study. Their chemical composition with respect to the total weight wash-coat+monolith is given in Table 1
550 Table 1 Catalysts composition (in wt.-%) Catalyst
Pt%
Pd%
Cat.1 (PtRh) Cat.2 (PdRh)
0.152 -
0.19
Rh%
Metal ratio 0.031 5 0.022 10
Loading (g ft-3) 31 36.5
Some m e a s u r e m e n t s were also carried out corresponding to Cat.1 before metal impregnation.
Additives Ce" 6.3%; La: 0.66% Zr: 4.6%; Ce: 1.9%; Ba:0.9% on a wash-coat (WC1)
2.2 A g e i n g The fresh catalysts (stabilized in reaction at 550~ were aged under laboratory conditions and under engine bench conditions (Table 2). Table 2 Ageing conditions Type
Gas
Laboratory " Engine
N2+10%H20 Air+10%H20 Exhaust gases
T(~ 900- 1000- 1100 900- 1000- 1100 950 max
Duration 5h 5h 25 - 50 - 200h
2.3. OSC m e a s u r e m e n t s OSC m e a s u r e m e n t s were carried out at 450~ in a chromatographic reactor previously described [9,10], according to a method developped by Yao and Yu Yao [3]. A portion of the catalyst sample (0.02 to 0.1g) was roughly crushed to ~ 0.20.4 mm, inserted in a U-tube reactor and heated in a flow of He (30 cm3 min-1 impurities < l p p m ) up to 450~ Five pulses of 02 followed by five pulses of CO and again five pulses of 02 (loop volume: 0.268 + 0.003 cm 3) were then injected on the catalyst sample. In all cases, the fraction of 02 or of CO consumed by the catalyst was smaller t h a n 50%. The amount of CO2 produced upon the first pulse of CO was t a k e n as a m e a s u r e m e n t of the Oxygen Storage Capacity. 2.4. C a t a l y t i c a c t i v i t y The activity of the fresh and aged catalysts for CO, HC and NO abatment was m e a s u r e d in a synthetic exhaust gas which simulated reaction conditions around the stoichiometry (R=I) with a pulsation frequency of 1Hz and an amplitude of • The TWC performance was measured with a catalyst inlet temperature of 450~ The light-off performance was determined in experiments where the e x h a u s t gas t e m p e r a t u r e at the catalyst inlet was increased continuously. 2.5. O t h e r c h a r a c t e r i z a t i o n s The BET area of all the fresh and aged catalysts were measured. The crystallite size of CeO2 in Cat. 1 was also determined by XRD.
551 3. R E S U L T S
3.1. General features in OSC m e a s u r e m e n t s (fresh Cat.1 catalyst) It was checked t h a t different portions of the catalyst (taken near the centre or n e a r the border of the monolith, crushed or not crushed) gave very close OSC values. The results obtained for each pulse at different temperatures are reported in Table 3. After each series of pulses of CO at T~ the catalyst sample is re-oxidized at 450~ Table 3 OSC m e a s u r e m e n t s on Cat. I and WC1. Effect of temperature and pulse number. First columns: CO consumption and CO2 formation (pmol g-l). Last column: 02 consumption (pmol at. 0 g 9 after CO reaction at 450~ T~ Pulse Nr
CO
350 CO2
CO
450 CO2
CO
550 CO2
CO
700 CO2
450 O2
1 2 3 4 5
88.3 19.9 13.5 9.7 6.1
71.8 9.7 6.8 6.2 6.1
92.6 17.3 13.6 12.3 11.7
91.8 17.0 13.6 11.8 10.4
103.7 23.5 18.7 15.5 13.4
103.7 22.9 16.8 13.6 12.0
119.3 40.8 28.1 22.7 19.7
119.3 37.8 24.3 19.1 16.1
134.1 2.0 0.5 0 0
WC1 ~ 1 2
14.3 9.3
14.2 9.3
47.4 15.6
46.7 15.0
77.4 17.6
77.4 16.4
101.5 30.5
101.5 28.1
71.0 2.2
a Wash-coat of Cat. I before metal impregnation. These results show some of the specific features of OSC measurements: a) the amount of CO consumed by the oxidized catalyst decreases slowly at each pulse while the reduced sample is virtually re-oxidized upon the first pulse of 09. This indicates t h a t the oxidation step is much more rapid t h a n the step of reduction by CO, in agreement with other studies [13,14]. b) there is a good accordance between the amounts of CO consumed and those of CO2 produced, except at the lowest t e m p e r a t u r e (350~ A likely hypothesis is t h a t a p a r t of the CO9 formed at this temperature remained adsorbed on the basic sites of CeO~. c) the OSC values increase slightly with the temperature, which shows t h a t the reaction is apparently not activated on the catalyst (4 k J tool-l). d) a different behavior can be observed with the bare wash-coat WCI: in the absence of any noble metal, the ceria present in the wash-coat seems to be reducible by CO with a relatively high activation energy (36 k J tool-l). The effect of the metals, predominant at low temperatures, is much less marked above 500~ In w h a t follows, only the CO9 values recorded upon the first pulse of CO at 450~ will be considered.
552 3.2. OSC m e a s u r e m e n t s on l a b o r a t o r y - a g e d c a t a l y s t s OSC measurements carried out on Cat.1 aged in N2+H20 and in Air+H20 are compared in Table 4. BET area and crystallite sizes of CeO2 are also given for the N2+H20 ageing. The decrease of OSC properties is definitely marked in wet nitrogen as in wet air. The important parameters of sintering are therefore the temperature and the presence of steam. In separate experiments carried out in the absence of steam (not reported here), we showed that, at a given temperature, the ageing was much less severe in dry gases. We can notice t h a t the catalyst ageing is accompanied by a moderate decrease of the BET area (factor 2.6) and by a definite increase of the particle size of ceria (factor 4.2). Table 4 Characteristics of the laboratory-aged catalysts. OSC (~mol CO2 g-i), ABET (mS g-l) and dceo2 (nm). T~
500 (fresh) 900 1000 1100
Cat. 1 Air+H20 OSC 91.8 33.6 20.8 9.7
OSC 91.8 24.9 21.6 8.0
Cat. 1 (PtRh) N2+H20 ABET 41.4 27.2 21.3 16.0
Cat.2 (PdRh) N2+H20 OSC ABET
dce02 5.5 13.3 16.0 23.2
59.7 35.9 29.1 17.2
35.6 24.9 22.1 17.4
OSC and BET area measurements carried out on Cat.2 after ageing in N2+H20 are also reported in Table 4. This catalyst is significantly more stable t h a n the PtRh (Cat.l) catalyst.
3 . 3 0 S C m e a s u r e m e n t s on e n g i n e b e n c h - a g e d c a t a l y s t s The characteristics of the two catalysts aged on an engine bench during 25, 50 and 200h are reported in Table 5. Table 5 Characteristics of the engine bench-aged catalysts (same units as in Table 4). Ageing time
(h) fresh 25 50 200
Cat.1 (PtRh)
OSC 91.8 22.6 14.3 7.7
ABET 41.4 23.8 21.8 17.9
dee02 5.5 15.0 16.0 18.3
Cat.2 (PdRh) AB~T OSC 59.7 34.7 28.9 23.8
35.6 21.4 17.4 16.5
The PtRh catalyst is severely affected by the engine bench-ageing while, again, the PdRh catalyst is much more resistant to this treatment. This point will be discussed further.
553
3.4. Effect of laboratory-ageing on the catalytic properties The catalytic activity was estimated by the light-off temperatures for CO, HC and NO and by the percentage of non-converted pollutant at 450~ (R=1~0.05, 1Hz). The results are given in Table 6. Table 6 Effect of the laboratory-ageing (N2+H20) on the catalyst performances. Catalyst Cat.1 (PtRh)
Cat.2 (PdRh)
Ageing T~ fresh 900 1000 1100 fresh 900 1000 1100
Light:off temp. (~ CO HC NO 186 210 204 250 261 236 251 262 233 277 294 254 207 270 273 293
216 282 281 304
210 256 252 266
Non-converted at 450~ (%) CO HC NO 0.8 0.4 5.1 4.6 1.5 15.9 10.4 2.9 28.3 17.0 5.8 35.3 2.6 10.8 14.0 28.1
0.9 5.4 5.9 9.6
0.9 23.4 31.0 36.3
If we compare the aged catalysts, the light-off temperatures are in the following order: TNo < Tco< THC for the two catalysts. By contrast, the reverse order can be observed with the percentages of non-converted pollutant. A high level of conversion is obtained with the hydrocarbons whatever the sintering temperature. This is not the case for CO and NO which are much less converted than the HC's at 450~
3.5. Effect of engine bench-ageing on the catalytic properties The results obtained with the engine bench-aged catalysts are reported in Table 7. Light-off temperatures are given for a 50% conversion. Table 7 Effect of the engine bench-ageing on the catalyst performances. Catalyst Cat.1 (PtRh) Cat.2 (PdRh)
Ageing time (h) 25 50 200 25 50 200
Light-off.temp. ( ~ CO HC NO 266 279 262 285 303 275 310 328 300 264 276 273
276 286 285
255 275 274
Non-converted at 4500C ( % ) CO HC NO 8.0 5.5 13.4 14.6 7.9 29.9 33.4 14.9 39.2 4.9 8.9 31.5
5.1 7.0 11.0
17.2 22.1 38.4
The catalytic results confirm that the PtRh catalyst is rapidly deactivated when it is aged in engine bench, while the PdRh catalyst keeps a better light-off
554 activity over a longer time of ageing. This latter catalyst is comparatively more resistant to an engine bench-ageing t h a n to a thermal ageing in N2+H20. 4. D I S C U S S I O N
4.1. Comparison between OSC of laboratory and engine bench-aged catalysts The changes in the OSC values with the temperature of ageing (lab-aged catalysts) are represented in F i g . l a (Cat.l) and Fig.lb (Cat.2). The levels of oxygen storage measured on the engine bench-aged catalysts are also indicated on these figures.
25 :
o
25h
40
2o
~o 3 0 "
2o
~" 10 r~
200h
9
I
0
. . . . . . ">oh't1 . . . .
"~
10 ....
900
1000
11O0
900
1000
T (~
W (~
(a)
(b)
1100
Figure 1. OSC of the catalysts aged at different temperatures (laboratory ageing; N2+H20;) and for different times (engine-bench ageing). (a) Cat.1 (PtRh); (b) Cat.2 (PdRh). The decrease observed on the OSC values gives a relationship between the t e m p e r a t u r e of t h e r m a l ageing and the duration of ageing in an angine bench. For Cat.l, a ll00~ corresponds to a 200h-run on an engine bench while for Cat.2, the same temperature corresponds to a catalyst sample having worked for more t h a n 200h. This stresses the good behavior of the PdRh catalyst when it is aged on an engine bench, viz under more realistic conditions 4.2. O x y g e n storage components There are essentially two components of the catalysts able to store oxygen: the noble metals the cerium oxide(s). Among the noble metals, rhodium and palladium should first be considered. It is well known t h a t the core of the p l a t i n u m particles cannot be re-oxidized by 02 in the absence of chlorine [15-17] so t h a t only surface Pt atoms are potential sites for oxygen storage. Cat.1 contains 7.7 ~mol Pt g-1 and the metal dispersion after sintering does not exceed 3-4% [17,18]. Platinum as a component of oxygen storage can therefore be
555 neglected (contribution to OSC of aged Cat.1 < 0.3~mol g-i). By contrast, the bulk of rhodium [17,19,20] and of palladium [21,22] can be re-oxidized at 450~ However, Rh ions can also diffuse in the sub-layers of alumina and are then difficult to be reduced [19]. Cat.1 contains 3 ~tmol Rh g-1 which, potentially, can store 4.5 ~mol at. O g-1 (Rh203). We showed that the presence of steam accelerated the diffusion of Rh ions into the alumina matrix [19,23] and that the proportion of rhodium having diffused was very high at low Rh content [21]. From the data reported in Ref. [23], a proportion of reducible Rh smaller than 20% for catalysts aged above 900~ can be expected. The contribution of the noble metals to OSC of Cat.1 would then be negligible (<1.2 ~tmol g-i). This is not the case of the PdRhcatalyst (Cat.2) which contains a relatively high proportion of palladium (18.6 ~mol g-l). This metal can potentially store one O per Pd atom so t h a t the noble metals can contribute for a great part to the OSC of the aged samples (Tables 4 and 5). Ceria is the main component of oxygen storage in Cat.1. This catalyst contains 7.74 wt.-% CeO2, i.e. 450 ~mol at.O g.1, which would correspond to a virtual OSC of 112 ~mol CO2 g-1 (Ce4§ ~_> Ce3§ if all the bulk atoms were reducible. This figure is greater t h a n the OSC values found for Cat.1 (even with the fresh catalyst), which shows that, most likely, only the surface atoms of ceria participate in the oxygen storage. The number of CeO~ particles N (assumed to be hemispherical) is given by: 12x N = particles g.1 (1) 100upd 3 where x is the wt.-%, p, the density (7.13x106 g m-3) and, d, the particle size of ceria (m). For Cat.l, Eq.(1) reduces to: 4.15 • 1019 N = with d in nm (2) d3 The surface area of ceria A can be calculated by: A =6 x which leads to A(m2eo2 g-l) = 65.____11 (3) 100dp d(nm) The n u m b e r of surface oxide ions is close to 1 3 . 6 0 nm -2 (cubic structure with a "d" spacing of 0.541nm), which gives a potential OSC of 5.64 ~tmol CO2 m -2, in agreement with a previous OSC study on pure ceria [9]. Johnson and Mooi [24] have calculated there was a surface concentration of 24.8 ~tmol O m -2 of ceria but, curiously, they estimated t h a t all these O atoms could be reduced by H2. It seems reasonable to consider, as we do, t h a t one O atom out of four can be captured by a molecule of CO. For Cat.l, the expected contribution of ceria to the OSC would then be: 367 g-1 (4) OSCth = d(nm) ~m~ Table 8 gives the values of N, A and of OSCth calculated from the particle size determined by XRD. There is an excellent agreement between the predicted and the experimental OSC values for the catalysts aged under the less severe conditions (900-1000~ or 25-50h). Owing to the presence of an amorphous phase
556 of CeO2 and to a good metal accessibility, we found OSCexp>OSCthfor the fresh catalyst. The reverse situation was observed for the catalyst aged under severe conditions. This can be explained by a loss of accessibility of ceria (which could be partly encapsulated in the alumina matrix) or to a decrease in the promotor effect by the metals ( which become unable to activate the ceria reduction). Table 8 Contribution of CeO2 to OSC of Cat.1 (calculated values). CeO2
Fresh
parameters N 2.5• particles g-1 A 11.8
Engine bench-ageing during
Laboratory-ageing at 900~ 1000~ 1100~ 1.8•
25h
1.0xl0 le 3.3x101
50h
1.2•
1.0•
200h 6.8x1015
4.9
4.0
5 2.8
4.3
4.0
3.5
27
22
15
24
22
20
2 g-1 mce02
OSCth ~tmolC02 g-i
66
4.3. Correlation b e t w e e n OSC values a n d c a t a l y t i c a c t i v i t y The activity data reported in Tables 6 and 7 for different pollutants show there is not a strict parallelism between the light-off temperatures and the percentages of non converted pollutant at 450~ However, for a given category of pollutant, the activity decay can be measured either by the increase of the lightoff temperatures or by the decrease of conversion at 450~
100 95
I~R-~
90 ~9 85 80
~..---o
HC D--.--'-
-
_
_
co ~
~
,,---
"la
.../
LPdRhl ....
9. . . . . . .
h""
. ....
9He 9C O
.~, NO o o
r~ 70 65
-
60 0
N O A. - ' ' ~
~-~
....
I
I
I
10
20 OSC Otmol/g)
30
40
Figure 3. Correlation between the conversion at 450~ and the OSC of the catalysts aged in N2+H~O (PtRh: full lines; PdRh: dashed lines). There exists a definite correlation between the conversion at 450~ of the different pollutants and the OSC of the catalyst. Conversion vs OSC plots are
557 represented on Figure 3 for the two catalysts aged in Ng+H~O. A similar picture was obtained with engine bench-aged catalyts. The effect of the deactivation is clearly more marked for NO conversion than for the other pollutants. The minimum level (Losc) of OSC required for the working catalyst will therefore be governed by the NO conversion. It is also clear that the adjustment of Losc to a reliable On-Board Diagnostic will depend on the nature and composition of the catalyst in use: for a given level of deactivation, the PdRh catalyst keeps a higher storage capacity than the PtRh catalyst, the difference being more marked for HC and CO (Aosc ~ 20~tmol CO2 g-l) than for NO conversion (Aosc ~ 10~tmol CO9 g-l). For a given catalyst, the type of ageing can also influence the deactivation profiles (Fig. 4). This is particularly evident for the PdRh catalyst which exhibits a severe engine bench deactivation whereas its storage capacity remains relatively high.
100
100
....e--j
95 HC
90
~ 85
0
"~ 80
. ~
CO
"
.:
95 90
HC r
y
85
&
"~ 80
."
'
&.-
7o
70 65 6O
N
65
"'
0
10 20 OS C (~mo l/g) Cat.1 (PtRh)
I A
60
i
30
10
I
20 30 OS C ( ~ m o l/g)
40
Cat.2. (PdRh)
Figure 4. Effect of the type of ageing upon the "Conversion vs OSC"profiles. Full lines: laboratory-ageing; Dashed lines: engine bench-ageing. 5. C O N C L U S I O N Oxygen storage capacity is an internal parameter of three way catalysts, depending on the surface state both of the noble metals and of the ceria. For this reason, there is a close correlation between OSC of three way catalysts and their catalytic activity. However, the "conversion vs OSC" profiles depend on several factors - in PtRh catalysts, the influence of ceria on OSC is predominant. The contribution of the noble metals to OSC of aged catalysts is negligible. OSC can fall to very low values.
558 - in PdRh catalysts, the contribution of the noble metals (particularly Pd) remains significant and the OSC values of aged samples are not so small as in the case of PtRh catalysts. - there is a complex effect of the mode of ageing (engine bench or thermal ageing), which suggests that the OSC threshold should be adjusted to a given type of engine. - NO conversion is very sensitive to catalyst ageing. REFERENCES ~
2. 3. 4. 5. 6. 7. 8. .
10.
11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.
B. Harrison, A. F. Diwell and C. Hallet, Plat. Met. Rev., 32 (1988) 73 J. Barbier J r and D. Duprez, Appl. Catal. B, 4 (1994) 105 H. C. Yao and Y. F. Yu Yao, J. Catal., 86 (1984) 256 E. C. Su, C. N. Montreuil and W. G. Rothschild, Appl. Catal., 17 (1985) 75 E. C. Su and W. G. Rothschild, J. Catal., 99 (1986) 506 B. Engler, E. Koberstein and P. Schubert, Appl. Catal., 48 (1989) 71 P. L55f, B. Kasemo and K.-E. Keck, J. Catal., 118 (1989) 339 T. Miki, T. Ogawa, M. Haneda, N. Kakuta, A. Ueno, S. Tateishi, S. Matsuuta and M. Sato, J. Phys. Chem., 94 (1990) 6464 S. Kacimi, J. Barbier Jr, R. Taha and D. Duprez, Catal. Lett., 22 (1993) 343 D. Martin, R. Taha and D. Duprez, in A. Frennet and J.-M. Bastin, Eds., Catalysis and Automotive Pollution Control CaPoC3, Stud. Surf. Sci. Catal., 96 (1995) 801 W. B. Clemmens, M. A. Sabourin and T. Rao, SAE Paper 900062 (1990) J. R. Theis, SAE Paper 961900 (1996) S. Bernal, J. J. Calvino, G. A. Cifredo, J. M. Gatica, J. A. P~rez Omil and J. M. Pintado, J. Chem. Soc., 89 (1993) 3499 A. Laachir, V. Perrichon, S. Bernal, J. J. Calvino and G. A. Cifredo, J. Molec. Catal., 89 (1994) 391 H. C. Yao, M. Sieg and H. K. Plummer, J. Catal., 59 (1979) 365 H. Lieske, G. Lietz, H. Spindler and J. VSlter, J. Catal., 81 (1983) 8 S. Kacimi and D. Duprez, in A. Crucq, Ed., Catalysis and Automotive Pollution Control CaPoC2, Stud. Surf. Sci. Catal., 71 (1991) 581 G. Mabilon, D. Durand and M. Prigent, ibid., p. 569 D. Duprez, G. Delahay, H. Abderrahim and J. Grimblot, J. Chim. Phys., 83 (1986) 465 D. Martin and D. Duprez, Appl. Catal., 131 (1995) 297 T. Paryjczak, W. K. J6zwiak and J. G6ralski, J. Chromatogr., 155 (1979) 9 T. E. Hoost and K. Otto, Appl. Catal., 92 (1992) 39 J. Barbier J r and D. Duprez, Appl. Catal.B, 4 (1994) 105 M.F.L. Johnson and J. Mooi, J. Catal., 103 (1987) 502
CATALYSIS AND AUTOMOTIVEPOLLUTIONCONTROLIV Studies in Surface Science and Catalysis, Vol. 116 N. Kruse, A. Frennet and J.-M Bastin (Eds.) 91998Elsevier Science B.V. All rights reserved.
559
NO Reduction by CO over Pd/CeO2-ZrO2-AI203 Catalysts.
R. Di Monte a, P.Fornasiero a, J.Ka~para, t , A.Ferrero b, G.Gubitosa b, and M.Graziani a a Dipartimento di Scienze Chimiche, Universit/~ di Trieste, Via Giorgieri 1, 34127 Trieste (Italy); b Magneti Marelli D.S.S., V.le Carlo Emanuele II, 150, 10078 Venaria Reale (Torino), (Italy). Pd-loaded Ce0.6Zro.402 supported on A120 3 are investigated as catalysts for the reduction of NO by CO. The attention is focused on the role of the support and of the Pd dispersion on the catalytic activity. The system shows a very high activity below 500 K which is almost independent on the Pd dispersion. The high activity is attributed to a promoting effect of the Ceo.6Zro.40 2 on the NO conversion. 1. INTRODUCTION In recent years much research has been focused on cerium oxide based transition metal catalysts because of their applications in different processes, particularly in the catalytic treatment of automotive exhausts. Ceria is indeed extensively added as a promoter to the current three-way catalyst (TWC). Several functions are attributed to this promoter (1-3), namely i) stabilization of metal dispersion and alumina support; ii) promotion of water gas shift and steam reforming reaction, and iii) oxygen storage and release capacity under respectively fuel-lean and fuel-rich conditions. The last point is of relevant technological importance since high oxygen storage capacity favors an enlargement of the operating air/fuel window increasing the overall efficiency of TWCs. The high oxygen storage capacity provided by CeO 2 plays a crucial role in enhancing the activity in reducing conditions, making more oxygen available for the oxidation processes (4). Furthermore, the oxygen vacancies associated with reduced ceria in the proximity of noble-metal particles were proposed as promoting sites for NO and CO conversion (3, 4). Recently, we reported (5, 6) that NO is effectively decomposed at the Ce 3+ sites in the Rhand Pt-loaded Ce-containing materials, suggesting a direct participation of the reduced support in the NO conversion. It was also observed that upon incorporation of ZrO 2 into a solid solution with CeO2, the reducibility of the Ce 4+ is strongly enhanced compared to pure CeO 2 in samples both with (7, 8) and without noble metal (5, 6). In the presence of H 2, Rh/CeO2-ZrO 2 mixed oxides were reduced even in the bulk in the range of temperature 440-673 K (7,8) with significant improvement of the catalytic activity in the reduction of NO by CO compared to Rh/AI20 3 (9) indicating that the nature of the support strongly affects the catalytic efficiency. t Correspondingauthor. E-mail : [email protected],fax: +39-40-6763960
560 Due to the high efficiency as oxidation catalysts, the application of Pd-based technology is gaining strong interest. Cordatos and Gorte (10) examined the adsorption of CO, NO and H 2 on Pd/ceria catalysts. Their results demonstrated a straight similarity of effects induced by celia with those found for Rh and Pt. It appears that the lattice oxygen can be transferred from the eeria to the metal surface favoring CO oxidation at low temperatures and producing new ceria sites on which NO is adsorbed and then readily decomposed to give N20 and N 2. In the present paper, the influence of the Pd loading on the catalytic behavior of high surface area Pd-loaded Ceo.6Zro.402-AI203 is addressed. In addition, the H 2 chemisorption is investigated. 2. EXPERIMENTAL Pd/Ceo.6Zr0.402(10 wt%)-Al203 samples were obtained from Magneti Marelli D.S.S. Nominal Pd content varied between 0.7 and 2.8 wt%. The catalysts were calcined at 873 K before use. Powder X-ray diffraction patterns were collected on a Siemens Kristalloflex Model F Instrument, (Ni-Filtered CuK~). The H2, CO and 02 chemisorption measurements were carried out on a Micromeritics ASAP 2000 instrument. Before chemisorption, the samples (0.1-0.3 g) were evacuated for 2 h at 673 K and then reduced in H 2 (flow rate 20 ml min"1) at 473 K or 1000 K for 2 h. Catalytic experiments were carried out in differential conditions using an U-shaped quartz microreactor (NO (1%) and CO (3%) in He, total flow 60 ml rain"1, 5-10 mg of catalyst) as previously described (6). 3. RESULTS AND DISCUSSION
3.1.
Sample characterization.
The samples were characterized by powder XRD technique. The powder XRD pattern of the Ceo.6Zro.402-A1203 support after a calcination at 873 K showed only a weak broad feature at about 30 ~ (20) attributable to a Ceo.6Zro.402 phase, apart from the peaks due to the AI203 phase. The low intensity is associated with a low crystaUinity of the sample. Upon increasing the temperature of calcination up to 1273 K, sintering of the Ceo.6Zro.402 phase occurs and the intensity of the peak at about 30~ (20) which is indexed as (111) plane (Figure 1), increases. No splitting of the peaks at 47 ~ and 59~ (20) is found. Accordingly, the reported pattern is indexed in the cubic Fm3m space group. A lattice parameter of 5.33 A is calculated which is in agreement with the presence of a solid solution (11). It is worthy to note that the Ceo.6Zro.402 phase favorably alters the sintering process of the 8-A1203. As in fact, no evidence of formation of o~-A1203 is found after calcination at 1273 for 5h, while in the absence of the mixed CeO2-ZrO2 oxides, significant amounts of ~-AI203 are found. Notably, the initial surface area of 107 m2 g-1 decreases after a calcination at 1273 K for 100 h to 8 and 67 m2 g-1 for respectively bare AI203 and Ceo.6Zro.402-AI203. This is consistent with previous observation that CeO 2 prevents sintering of alumina.
561
100 80
t.-.
8
1
60 4o
2
20
9
20
,
30
9
_
~ , ,
,.~.
40
,
50
,
.
60
70
20 Figure 1. Powder XRD pattems of (1) Ceo.6Zro.402-A1203 calcined at 1273 K and (2) Ceo.6Zro.402 obtained by subtraction of the A1203 contribution.
3.2. H 2 ehemisorption H 2 chemisorption has long been employed as a valuable technique to rapidly evaluate the exposed metal surface and hence the particle size of supported metals (12). This method has undergone severe criticism, since the underlying assumptions on the H/M stoichiometry and the particle geometry may not be totally reliable, especially for very small particles. Nevertheless, due to its simplicity and cheapness, it is certainly the most widely employed method. In the case of CeO 2 containing catalysts, there is a further difficulty in measuring the exposed metal surface due to an extensive adsorption of H 2 on the CeO 2 itself (13-16). Further, the interaction of H 2 with Pd differs from other noble metals because, in addition to chemisorption, it absorbs hydrogen in bulk forming hydrides (12). This interferes with the chemisorption and may cause severe errors in the determination of the dispersion unless appropriate adsorption conditions are employed. Specifically, room temperature adsorption at low H 2 pressures (< 10 torr) or alternatively adsorption at 373 K in the range o f H 2 100-160 torr were found to give reliable estimate of the exposed metal surface. Recently, a back sorption method was developed (17). We preliminary tested the two methods on the Pd/A1203 (Table 1). For sake of comparison, results on 02 titration and CO adsorption on the same sample are also included. The excellent agreement of the latter measurements with those obtained from "low pressure room temperature" H 2 adsorption confirms the validity of the proposed methodology for our catalysts. The applicability of this methodology to the 0.7 wt% Pd/Ceo.6Zro.402-A1203 catalyst was therefore examined in detail (Table 1). It is worthy to note that use of a low H 2 pressure intrinsically sfavors its adsorption on the support. As suggested by Bemal et al. (14), evacuation at high temperatures leads to suppression of rate of H spillover in Rh/CeO 2. We have, therefore, examined the effects of time of evacuation on the apparent H/Pd. The decrease of the apparent Pd dispersion with increasing the evacuation time to reach a constant value after about 15 h (Table 1) suggests that the thermal treatment minimizes the extent of H 2 spillover over the Ceo.6Zro.402-A1203. Consistently, thermal treatment at 900 K depressed the H 2 spillover in a Rh/Ceo.5Zro.502
562
catalyst (7). The lack o f H 2 adsorption over the metal-free Ceo.6Zro.402-A1203 even at 373 K further supports the idea that true H/Pd ratios are measured under the adsorption conditions here reported. The H/Pd ratios were measured for the 0.7, 1.4, 2.1 and 2.8 wt% Pd-loaded Ceo.6Zro.402-A1203 catalysts after a reduction at 473 and 1000 K. The results are shown in Figure 2. Table 1. Chemisorption measurements Pd/Ceo.6Zro.402-A1203. Sample
Pd/A1203
Pd/Ce0.6Zr0.402A1203
Ce0.rZr0.40 2A1203
carried
out
on
0.7
wt%
Pd/A1203
Treatment
and
0.7
wt%
Pd dispersion b
T (K)
Gas
Time (h)
473
H2
0.5
373
H2
0.1
473
H2
473 473
Evacuation at 673 K (h)
Technique a
(pressure range/ torr)
H 2, 373 K
44 (120-260)
0.25
02, 305 K
34 (100-600) c
0.5
5
CO, 305 K
35 (100-260)
H2
0.5
5
H 2, 305 K
34 (2-5)
H2
0.5
5
H 2, 305 K
80 (2-5)
10
H2, 305 K
75 (2-5)
15
H 2, 305 K
60 (2-5)
20
H 2, 305 K
60 (2-5)
10
473
H2
0.5
5
H2, 305 K
0 (>5)
473
H2
0.5
5
H 2, 373 K
0 (120-260)
a Adsorbate nature and adsorption temperature are reported. b The apparent Pd dispersions were obtained by back extrapolation of the linear part of the adsorption isotherm to zero pressure. Equilibrium time 1 min. The assumed reaction stoichiometries are: 2Pd + H 2 .... > 2 Pd-H Pd + CO .... > Pd-CO 2Pd-H + 3/20 2 .... > 2 Pd-O + H20 c equilibrium time l h For the catalysts reduced at 473 K, an increase of the metal loading leads to a decrease of the H/Pd ratio. This picture is reversed after reduction at 1000 K. The relative efficiency of the H 2 treatment at 1000 K in blocking the H 2 chemisorption is illustrated in Figure 3. For comparison the data for Pd/A1203 are included. H 2 chemisorption on noble metals (NM)/CeO 2 has been extensively studied (18) and special attention was generally given to the so called strong metal-support interaction (SMSI), e.g. the suppression of H 2 and CO chemisorption after a high temperature (usually 773 K)
563 reduction. This phenomenon which is attributed to the migration of the reduced support to cover the metal particles such as depicted in the scheme (19), was observed on Rh/CeO 2 only above 973 K (20). After reduction at 473 K, the Pd dispersion decreases with Pd content. The independence of the H 2 adsorption stoichiometry was reported for Pd/La203 for a range of Pd-loadings 0.25-8.80 wt% (21). Accordingly, the decrease of the H/Pd with Pd-loading suggests an increase of the Pd particle size.
CeO2_x Pd
7;-/7///'/////77///,I. ":". "
A reasonable interpretation of the variation of the H/Pd ratios after a reduction at 1000 K invokes a SMSI type phenomenon. Attribution of this effect only to a Pd sintering may be ruled out by the smaller relative decrease of H/Pd observed in the case of A1203 as support, compared to Pd/Ceo.6Zro.402-A1203. There are, indeed, some reasons which suggest that the SMSI type effect may be operative in our case: i) the reduction temperature (1000 K) is comparable to that observed for Rh/CeO 2 (20), ii) high surface area which favors the reduction process, and iii) an easy Pd encapsulation is expected for small Pd particles. 0.6
,--
0
100%
. ~ ,4...o
0.5
0
80%
t"q ,--,
60%
0.4
-r-~
0.3
o
0.2 0.1
r
0
tl} t/}
J 1
o
n 2
Pd loading (wt%)
3
"-1
40% 20%
0% 0
1
2
3
Pd loading (wt%)
Figure 2. H/Pd ratios measured over the 0.7, 1.4, 2.1 and 2.8 wt% Pd-loaded Ceo.6Zro.402A120 3 catalysts after a reduction at 473 (e) and 1000 K (m). Figure 3. Relative decrease of H 2 chemisorption induced by a reduction at 1000 K, (R) Pd/Ce0.6Zr0.402-A1203, (A) 0.7% Pd/A1203. Accordingly, Lavalley et at. showed that efficient Pd encapsulation was induced by a reduction of highly dispersed Pd supported on a high surface area CeO 2 (22). The reduction at 1000 K causes the sintering of the CeO2-ZrO 2 supports (7,23) which would favor metal encapsulation. However, the ZrO 2 may also well participate in determining the chemisorption properties of the present system. Infact, Lee et al. (24) found that the addition of ZrO 2 to
564 Pd/A120 3 catalyst resulted in a reduction of Pd 3d5/2 binding energy relative to that observed in metallic Pd suggesting that Pd had become negatively charged. Summarizing, the H 2 chemisorption studies here reported show a partial suppression of the H 2 adsorption after a high temperature reduction suggesting that, in addition to a metal sintering, significant metal particle encapsulation occurs. A possible methodology for the determination of the true H/Pd ratios is also disclosed. 3.3. NO reduction by CO. The catalytic activity in the reduction of NO by CO was investigated in a flow reactor in the range of temperatures 433-573 K. All the samples show a very high activity at moderate temperatures (< 500 K). Indeed, gas hourly space velocities as high as 1 x 106 h -1 and reaction temperatures of 433 K had to be employed to measure the reaction rates in absence of diffusional limitations. The reaction rates were measured under isothermal conditions after aging of catalysts in the reaction conditions for at least 15 h. During this period slow and partial deactivation of the freshly reduced catalysts is observed to reach a steady activity. Afterwards, the catalysts were subjected to a thermal cycle similar to that depicted in Figure 4 for the 0.7 wt% Pd/Ceo.6Zro.402-A1203. A light-off type behavior is observed. However, during the run-up part of the cycle, a partial deactivation occurs as denoted by the peak of activity centered at about 500 K (compare Figure 5). This effect is reversible and, remarkably, immediately after the end of the thermal cycle, the catalyst slowly regains its initial high activity upon aging at 473 K. No such behavior is observed either during the run-down part of the cycle or using 0.7 wt% Pd/A1203 as catalyst. All these results clearly point out an "active" state of the catalyst at low temperatures which is reversibly deactivated above 500 K.
800
100
700
80
,,g v
v
60 E 600
ID
40
tO
-= O
o
"5 500
20
n,' 400 0
1000
2000
3000
Reaction Time (min) Figure 4. NO-CO reaction catalyzed by 0.7 wt% Pd/Ceo.6Zro.402-A1203, ( l ) NO and (e) CO conversions, (-) temperature. Figure 5 compares the N20 and N 2 formation over the Pd/Ceo.6Zro.402-A1203 and Pd/A1203 catalysts. The peak in the N20 formation at about 650 K which is present on both
565 the catalysts is associated with the shift of the selectivity from the N20 production to the N 2 formation commonly observed over supported platinum group metals. The peak at 500 K observed in the picture 1 of Figure 5 is attributed to the low temperature "active" state of the Pd/Ceo.6Zro.402-A1203 catalyst which is promoted by the presence of the mixed CeO2-ZrO 2 oxide. The reaction rates measured in steady conditions at 433 K are reported in Table 2, e.g. under conditions when the "active" state of the catalyst is present. The activity of the catalysts is compared after a reduction at 473 and 1000 K. As a general trend, the reaction rate increases with metal loading. This increase is more pronounced on the catalysts reduced at 473 K where a four-times increase of the Pd-loading, from 0.7 to 2.8 wt% induces an eighttimes increase of activity. Also the TOFs slightly increase as the Pd particle size increases, indicating a slight structure sensitivity of the reaction, even though, there are some scattered data after reduction at 1000 K. The overall picture clearly indicates a favorable effect of the Ceo.6Zro.402 on the rate of the NO-CO reaction which does not appear to be related to a stabilization of higher dispersions induced by the support. Consistently, even after the high temperature reduction, despite a considerable decrease of the number of active sites, either due to a sintering and/or encapsulation of the metal particles, high TOF are observed on the Ce0.6Zro.40 2 containing catalysts compared to Pd/A1203. All the evidence points out an active role of the Ceo.6Zro.402 support which improves the catalytic efficiency of the supported metal. Consistently, recently we found evidence for a low temperature CeO2-ZrO 2 promoted catalytic cycle which favors the reduction of NO on Ce 3+ sites. In agreement with an active role of the support in the catalytic cycle, a very slight structure sensitivity is observed. Consistently with our observations, Gorte et aL recently reported that an interaction of NO with a reduced Pd/CeO 2 catalyst leads to enhanced N20 and N 2 production compared to a Pd/A1203 (10).
4.0E-05
o
~o 6.0E-05 >= 4.0E-05
E
8 2.o5-o5
O o
..,.,
o
~ 2.0E-05
E
1.0E-074O0
500
600
700
Temperature (K)
800
n~ 1.0E-07 400
500
600
700
800
Temperature (K)
Figure 5. Reaction rates vs temperature for ( I ) N20 , (11) N 2 formation and (A) NO conversion measured in the run-up cycles over (1) 0.7 wt% Pd/Ceo.6Zr0.402-A1203 and (2) 0.7 wt% Pd/A1203.
566 Table 2 Steady state reaction rates measured at 433 K over Pd/A1203 and Pd/Ce0.6Zr0.402-A120 3. Pd loading (wt%)
Reduction Temperature 473 K 1000 K Reaction rate a TOF b Reaction rate a 0.7 c 0.7 2.8 0.3 0.7 d 2.8 7.2 0.5 1.4 d 5.3 9.2 3.4 2.1 d 19 25 5.0 2.8 d 22 22 4.2 a MolesN O converted gcatalyst-1 s- 1 . 106; b MolesN O converted m~ exposed-1 s-1 * 102; c Pd/A1203; d Pd/Ce0.6Zr0.402_A1203.
3.4.
TOF b 2.3 8.8 20 14 7.9
In situ i.r. spectra of 2.8 wt% Pd/Ce0.6Zr0.402-AI203 In order to obtain further indication on the role of the support in the catalytic reaction, we investigated the interaction of the 2.8 wt% Pd/Ce0.6Zro.402-A1203 with both CO and NO under reaction conditions using an i.r. flow cell. After a thermal pretreatment at 773 K in flow of He, the interaction of flowing NO/CO with the 2.8 wt% Pd/Ce0.6Zr0.402-A1203 generates the spectra reported in Figure 6. Two sets of prominent band are observed in the 2500-2300 and 2300-2200 cm -1 regions (Figure 6.1) which are associated respectively to CO 2 and N20 produced during the reaction, both in the gaseous phase and adsorbed on the support. It is worthy of note that formation of CO 2 starts immediately, at about 313 K, while formation of N20 is observed above 400 K. The strong band at 2168 cm -1 may be associated to CO chemisorbed on the Ce 4+ sites (25,26). Its intensity is highest at 313 K and it disappears at about 400 K. The weak broad band at about 2140 cm -1 is a low frequency end of the roto-vibrational band of the CO (the high frequency one is covered by the band at 2168 cm -1) due to a non perfect subtraction of the gaseous phase. Its intensity slightly decreases with temperature as the CO conversion increases. At about 450 K, a new band at 2121 cm -1 suddenly forms. This band could be attributed to the presence of Ce 3+ sites (25,26). Its intensity decreases as the temperature approaches 473 K. As far as the species adsorbed on the metal are concerned (2000-1700 cm1), the weak band at about 1950 cm -1 which is formed above 400 K, is associated with bridged CO species bonded to Pd. The two weak broad bands at about 1900 cm -1 are due to gaseous NO, while the strong band at 1757 cm -1 is associated with NO linearly bonded to Pd. The intensity of the latter peak is highest at about 400 K. The peak maximum gradually shift from 1757 to 1745 cm -1 at 473 K. Above 473 K (Figure 6.2), no evidence for CO adsorption on the support is found, while the Pd is essentially covered by adsorbed CO as denoted by the broad weak band at about 1900 cm -1. Note the higher intensity of the peaks in the 2300-2200 cm -1 region compared to those at 2500-2300 cm -1.
567
T /
~-
l'lA
473 -
I
t
II
I
i
.2A
I
313--
1 i
2500
i
2
2 00
24oo
i
Wavenumber (cm-1)
,,,
1700
....
2 00
,
2 00 "1 2 00
, 1 00 Wavenumber (cm-1)
1700
Figure 6. I.r. spectra of 2.8 wt% Pd/Ceo.6Zr0.402-A1203 in flow of NO/CO mixture (1% each in He) measured increasing linearly the temperature (1 K min -1) from (1) room temperature to 473 K and, after aging at 473 K for 10 h, (2) from 473 K to 523 K. For the description of the indicated bands, compare text. Summarizing, the experiments reported in Figure 6, show an important evolution of the interaction of the gaseous reactants with both the support and the supported Pd. The coincidence of the lighting temperature for the catalytic NO/CO reaction and the disappearance of the CO adsorbed on the Ce4+ with consequent observation of Ce 3+ sites suggests that all these phenomena are related to a catalytic cycle mediated by the Ceo.6Zro.40 2 mixed oxide. Above 473 K, limited evidence for interaction with the support is found. Accordingly, we associate the high temperature activity to a metal catalyzed cycle where the contribution of the support is minimal. A possible reaction scheme which account for the above finding is reported in the following scheme. According to this scheme, reduction of NO to give N20 easily occurs at the Ce 3+ by a redox process involving the Ce4+/Ce3+ redox couple. Formation of the Ce 3+ sites which is a crucial step of the reaction may occur either by a direct interaction of CO with the support or by a reverse spillover of the lattice oxygen to the metal surface as found by Cordatos and Gorte (10). Two reasons suggest that the latter hypothesis might be more reliable. The interaction of CO with the Ceo.6Zro.402 leads to formation of carbonates. Their desorption which leads to creation of the Ce 3+ sites, generally occurs at high temperatures. On the contrary', upon desorption of NO from the Pd surface, CO starts being adsorbed on the metal and at the same time the catalytic reaction starts as denoted by the formation of N20 (Figure 6.1).
568 CO
Pd
Pd --- CO
C02 ~
C2 +
2 NO
C 4 -----+ 0
N20
4. C O N C L U S I O N S The present work reports a promoting effect of the Ceo.6Zro.40 2 mixed oxide on the catalytic reduction of NO by CO at moderate temperatures. This effect is attributed to the Ce4+/Ce 3+ redox couple which efficiently reduces NO. The interaction of the Pd/Ceo.6Zro.402-A120 3 catalysts with H 2 reveals that considerable metal particle encapsulation occurs after a high temperature reduction. This suggests that the choice of an appropriate metal particle size may be an important factor to avoid the catalyst deactivation. Finally, a methodology for the determination of the H/Pd ratios is discussed. Acknowledgments. Magneti Marelli D.S.S., Ministero dell'Universit~t e della Ricerca Scientifica (MURST 40% and 60%, Roma), CNR (Roma) and Universitg di Trieste are acknowledged for financial support. 5. R E F E R E N C E S
1.
K.C. Taylor, "Catalysis-Science and Technology" (J.R. Anderson and M. Boudart, Eds.), Chap.2, Berlin, Springer-Verlag (1984).
2.
K.C. Taylor, Catal. Rev. -Sci. Eng. 35 (1993) 457.
3.
B. Harrison, A.F. Diwell, and C. HaileR, Plat. Met. Rev. 32 (1988) 73.
4.
J.G. Nunan, H.J. Robota, M.J. Cohn, and S.A. Bradley, J. Catal. 133 (1992) 309.
5.
G. Ranga Rao, J. Ka~par, R. Di Monte, S. Meriani, and M. Graziani, Catal. Lett. 24 (1994) 107.
6.
G. Ranga Rao, P. Fornasiero, R. Di Monte, J. Ka~par, G. Vlaic, G. Balducci, S. Meriani, G. Gubitosa, A. Cremona, and M. Graziani, J. Catal. 162 (1996) 1.
7.
P. Fornasiero, J. Ka~par, and M. Graziani, J. Catal. 167 (1997) 576.
8.
P. Fornasiero, R. Di Monte, G. Ranga Rao, J. Ka~par, S. Meriani, A. Trovarelli, and M. Graziani, J. Catal. 151 (1995) 168.
569 9.
P. Fomasiero, G. Balducci, J. Ka~par, S. Meriani, R. Di Monte, and M. Graziani, Catal. Today, 29 (1996) 47.
10. H. Cordatos and R.J. Gorte, J. Catal. 159 (1996) 112. 11. M. Yashima, N. Ishizawa, and M. Yoshimura, J. Amer. Ceram. Soc. 75 (1992) 1541. 12. J.A. Anderson, "Structure of Metallic Catalysts", London, Accademic Press, (1975). 13. S. Bernal, F.J. Botana, J.J. Calvino, M.A. Cauqui, G.A. Cifredo, A. Jobacho, J.M. Pintado, and J.M. Rodriguez-Izquierdo, J. Phys. Chem. 97 (1993) 4118. 14. S. Bernal, J.J. Calvino, G.A. Cifredo, A. Laachir, V. Perrichon, and J.M. Herrmann, Langmuir 10 (1994) 717. 15. S. Bemal, J.J. Calvino, G.A. Cifredo, J.M. Gatica, J.A.P. Omil, and J.M. Pintado, J. Chem. Soc. Faraday Trans. 89 (1993) 3499. 16. A. Badri, C. Binet, and J.C. Lavalley, J. Chem. Soc. Faraday Trans. 92 (1996) 4669. 17. V. Ragaini, R. Giannantonio, P. Magni, L. Lucarelli, and G. Leofanti, J. Catal. 146 (1994) 116. 18. A. Trovarelli, Catal. Rev. -Sci. Eng. 38 (1996) 439. 19. D.E. Resasco and G.L. Hailer, J. Catal. 82 (1983) 279. 20. S. Bemal, F.J. Botana, J.J. Calvino, G.A. Cifredo, and J.A. Perezomil, Catal. Today 23 (1995)219. 21. R.F. Hicks, Q.J. Yen, and A.T. Bell, J. Catal. 89 (1984) 498. 22. A. Badri, C. Binet, and J.C. Lavalley, J. Chem. Soc. Faraday Trans. 92 (1996) 1603. 23. P. Fornasiero, G. Balducci, R. Di Monte, J. Ka~par, V. Sergo, G. Gubitosa, A. Ferrero, and M. Graziani, J. Catal. 164 (1996) 173. 24. B.Y. Lee, Y. Inoue, and I. Yasumori, Bull. Chem. Soc. Jpn. 54 (1981) 3711. 25. C. Binet, A. Badri, and J.C. Lavalley, J. Phys. Chem. 98 (1994) 6392. 26. C. Morterra, V. Bolis, and G. Magnacca, J. Chem. Soc. Faraday Trans. 92 (1996) 1991.
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CATALYSIS AND AUTOMOTIVEPOLLUTIONCONTROLIV Studies in Surface Science and Catalysis, Vol. 116 N. Kruse, A. Frennet and J.-M Bastin (Eds.) 91998 Elsevier Science B.V. All rights reserved.
571
Comparative sulfur storage on Pt catalysts : effect of the support (CeO2, ZrO2 and CeO2-ZrO2) P.Bazin a, O. Saur a, J.C. Lavalley* a, A.M. Le Govic b and G.Blanchard b a Laboratoire Catalyse et Spectrochimie, UMR 6506, ISMRA-Universit6, 6, Boulevard du Mar6chal Juin, 14050 CAEN-C6dex (France) b Rh6ne-Poulenc-Recherches, 52 rue de La Haie-Coq, 93308 Aubervilliers (France)
The effect of the support on the nature, amount and reducibility of the sulfate species formed from SO2 oxidation on Pt catalysts has been studied by IR spectroscopy and thermogravimetry. Only surface species are observed on pure zirconia while bulklike species are also formed on the CeO2-ZrO2 mixed oxide (Ce/Zr=64/36) as on ceria. It appears that the H 2 reducibility of the bulklike species does not depend on the support, CeO2 and CeO2-ZrO2. By contrast, on Pt catalysts, surface sulfate species which first appear from SO2 oxydation are more easily reduced on CeO2-ZrO2 than on CeO2. This shows the role of the surface sites for the formation and reduction of sulfate species.
1. I N T R O D U C T I O N Sulfur is present in all commercially available gasoline. It interacts with Three Way Catalysts (TWC), deactivates their reactivity and also leads to H2S emission during rich operations by reduction of sulfates stored on the catalysts during stoichiometric or lean conditions (1, 2). In previous works, we studied ceria sulfation and the effect of platinum on both formation, from SO2 and oxygen, and reduction by H2 of sulfates (3, 4). Ceria is a key component of TWC for the treatment of exhaust gas from automobiles due to its oxygen capacity storage (OCS) (5). High surface area ceria samples are essential to obtain a significant OCS since the redox processes essentially occur on the surface. Moreover, this high surface area has to be maintained at high temperature. Recently, it has been claimed that zircinia mixed with ceria stabilized the surface at high temperature (6,7) and, in the new generation of TWC, it is possible that ceria will be replaced by mixed oxides such as CeO2ZrO2. The purpose of this work is to compare the nature, amount and H2 reducibility of the sulfate species formed from SO2 oxidation on Pt catalysts supported on CeO2, ZrO2 and a CeOz-ZrO2 solid solution. Two techniques have essentially been used : thermogravimetry and IR spectroscopy, both in static conditions. * To whom correspondence should be addressed
572 Previous works showed the importance of the surface in the formation and reduction of sulfate species (3,4). So, in addition to high surface area samples, the study has been extended to highly calcined samples (900~ in order to study the ageing effect.
2. EXPERIMENTAL 2.1.Catalysts preparation The cerium-zirconium mixed oxide (Ce/Zr = 64/36, atomic ratio) as well as the pure ceria samples (both without and with Pt) were obtained by a Rh6ne-Poulenc proprietary process. The purity of the cerium used was higher than 99.5%. For the study of the interaction between Pt and cerium-zirconium mixed oxides, a hexachloroplatinic solution was used as a source of Pt. The OH groups on the surface of the mixed oxides were exchanged by PtC162- ions in water. The material obtained was dried in an oven at 120~ and calcined in a muffle furnace at 500~ For aged catalysts, the calcination was performed at 900~ for 6 hours. The Pt loading was 0.5% (weight %).
2.2. Catalysts characterization X-Ray diffraction was used to identify the major phases and to measure the lattice parameters of the cubic phases. A Siemens D500 diffractometer was used for all XRD measurements. The mean crystallite diameter was measured from XRD pattern using line broadening Warren et al's formula (8). The surface area of the oxides was measured by the BET method and are reported in table 1. Tablel Surface area of the samples Samples
Pt/ZrO2
CeO2
Pt/CeO2
Aged Pt/Ce02
CeO2 ZrO2
Pt/CeO2 -
Aged
ZrO2
Pt/CeO2ZrO2
Surf.Area (m 2 g,1)
38
2.3. Infrared study
170
147
22
137
107
32
For infrared studies, powders were pressed (1 ton.cm 2) and used as wafers of c.a.lO mg.cm ~ They were activated at 450~ under oxygen atmosphere for two hours and then evacuated at the same temperature. Sulfation was performed by adding 600 ~tmol.g-1 of SO2 and a large excess of 02 (Pe - 6.5 kPa) in the cell and then heating at increasing temperature. For the study of sulfate reduction, a large excess of H2 (Pe -~ 13 kPa) was introduced at room temperature (r.t.) in the cell and then the wafer was heated at increasing temperature until
573 disappearence of the sulfate absorption bands. All spectra were recorded at r.t. after evacuation of the sample at 400~ using a Nicolet Magna-500 spectrometer.
2.4. Thermogravimetric study For gravimetric measurements, the powders were pressed (c.a.400mg), activated and sulfated in the same conditions as for the IR study. A McBain thermobalance was used. The temperature was increased from r.t. to 450~ (0.5~ l ) and then kept at 450~ until a plateau was obtained for the weight. For the study of sulfate reduction, a large excess of H2 (Pe - 13 kPa) was introduced at r.t. in the thermobalance and then the sulfated sample was heated under H2 atmosphere at increasing temperature (0.5~
3. RESULTS
3.1. Formation and nature of sulfate species Previous IR studies of metal oxides sulfation have evidenced two types of sulfate species, surface species characterized by one or more bands in the 1410-1370 cm -1 frequency range, and bulklike species leading to wide bands in the 1200-1000 cm -1 range (3, 9-12). In a recent paper (4), it has been shown that Pt does not affect the sulfate formation nor the nature of adsorbed species. Spectra reported in Fig. 1 show that only surface sulfate species are present on Pt/ZrO2 (Fig.la) while both species are formed on Pt/CeO2 (Fig. 1b) and Pt/CeO2-ZrO2 (Fig. 1c).
,, iX:)
A b s d
t',q v-.
/
O r b a n ec ~ 4 0 5 ~ ~
I
,,
-c~ ~ ~ ~I
~176
a
....
1500
I
13'00
I
11'00
I
Wavenumber (cm -1)
"
960
t
I
Figure 1. IR spectra of sulfate species formed on various samples after evacuation at 400~ a) Pt/ZrO2; b) Pt/CeO2 ; c) Pt/CeO2-ZrO2; d) aged Pt/CeO2-ZrO2.
574 Moreover comparison of spectra l c and ld shows that the amount of bulklike species is higher while surface species are less numerous on the aged samples. As observed on ceria (3), the relative amount of the two types of sulfate species depends on the sample surface area.
65 60 q
-"
CeO 2 (a)
--~
Pt/(Ce/Zr)O 2 (e)
"
(Ce, Zr)O 2 (b)
~
Pt/CeO 2 - 900~
Pt/ceo~ (c)
-+-
Pt/(Ce, Zr)O~-900~ (g)
-o-
55 -
(f)
_
- - v - - p t / Z r O 2 (d)
_
50 _
_
45 _
_
~.40
:
35 N 3o ~25-
2o 10 5 00
100
200
temperature /
300
400
~
Figure 2. Mass gain of various samples (CeO2, CeO2-ZrO2, Pt/CeO2, Pt/ZrO2, Pt/CeO2-ZrO2, aged Pt/CeO2, aged Pt/CeO2-ZrO2) heated under SO2 + 02
In figure 2, we report the mass gain of different samples heated under SO2 (600 ~mol.g -1) + 02 (- 6.5 kPa) atmosphere versus temperature. The results on ZrO2 and Pt/ZrO2 show a small weight gain (- 13 mg. g-l) confirming that surface sulfate species are only formed. The curves are very similar for CeO2 and CeO2-ZrO2 and all the SO2 amount is oxidized at E] 200~ on the high surface area catalysts in the conditions used. No effect of platinum is clearly detected on the sulfate formation on Pt/CeO2. The difference between CeO2-ZrO2 and Pt/CeO2-ZrO2 shown in figure 2 may be due to a surface area effect (Table l).Indeed, in the case of the aged catalysts, the total SO2 oxidation rate is drastically reduced. This suggests that the oxidized species are formed on the surface and then migrate into the bulk.
575 In order to understand the sulfate formation mechanism, IR spectroscopy measurements have been performed.Fig. 3a shows the spectra of SO2 species formed from SO2 adsorption at r.t. on CeO2-ZrO2. Strong bands due to sulfite species are observed at 1005, 919, 830 cm-1; sharp bands at 1334 and 1144 cm -I are assigned to physisorbed SO2. After heating up to 150~ new bands appear, in particular at 1378 cm "1 (Fig. 3b), showing that surface sulfate species are formed. Concomitantly, the absorbance of the bands due to sulfite species decreases (negative absorbance near 800 cm -1, Fig.3e). Bands due to sulfate bulklike species (near 1170 cm -1) appear only at 250~ while the intensity of the band assigned to surface species (1389 cm -1) still increases (Fig.3c, 3f). Further heating at 350~ (Fig.3d, 3g) mainly provokes bulklike sulfates formation. This confirms that surface sulfate species first appear at the expense of sulfite adsorbed species. Diffusion of sulfate into the bulk then occurs and is favored by heating at higher temperature.
A
/
T,
\~ ~
0.5
5"-
A
'
sd
: Oct,
v 1388
I a
>(c-b) f
c b
-(b-a) ,
,
'14'00'
,
,
.
,
i
,
.
.
I
12'00 1000 800 Wavenumber (cm-')
,
,
I '
'
i
,
1400
,
,
!
,
'~
,
!
,
'
,
,
i
,
1200 1000 800 Wavenumber (cm")
Figure 3 - IR spectra of adsorbed species on CeO2-ZrO2 after SO2 + 02 addition in the cell at a) r.t. b) 150~ ; c)250~ ;.d)350~ ; e, f, g ) subtractions of spectra (a) from (b), (b) from (c) and (c) from (d), respectively.
3.2. R e d u c t i o n o f s u l f a t e s by H 2
Previous studies have been reported on adsorbed sulfate species on pure zirconia showing that they are thermally stable until 600~ (12). Thermal stability of adsorbed sulfate species on ceria or on Pt/CeO2 has also been previously studied (3,4) whereas studies are in progress on CeO2-ZrO2 mixed oxides. All these results show that sulfate groups do not decompose under vacuum before 400~ on zirconia or ceria based catalysts. Under H2 atmosphere, most sulfates adsorbed on oxides are less stable than under vacuum (3,9,11,12). In figure 4, we have reported the variation of the sulfated samples mass versus temperature during treatment under H2 atmosphere for CeO2 and CeOa-ZrO2 compounds with or without
576 Pt. Reduction begins at the same temperature for the CeO2 and CeO2-ZrO2 samples (- 380~ and the curves are very similar, the average temperature of reduction being about 430~ However, we can note that the total mass loss is larger for the CeO2-ZrO2 sample than for pure CeO2 although they initially contain the same amount of sulfates. Addition of platinum to ceria and ceria-zirconia samples decreases the sulfate reduction temperature of about 80~ It is worthwhile noticing that the mass of the sulfated Pt/CeO2ZrO2 begins to decrease between 150 and 350~ while it seems stable in the case of the Pt/CeO2 sample up to 200~ The mass loss is also more important for Pt/CeO2-ZrO2 than for Pt/CeO2 (Fig.4). As for Z r O 2 (results not shown), the sulfate reduction, in the conditions used, begins near 380~ and mainly occurs near 450~ Addition of Pt makes the reduction easier; in agreement with (13), it begins at 300~ and still occurs at 480~
5O 40 9
i~~.
~--~
30
' 2o 9
IIII
1/11
>
///I
7, lo
III
-10
Oxidation and evacuation at 450~ ,,,,,,,,,
0
i,,,,,,,,,
100
i,,,,,,,,,i,,,,,,,,,
i,,,,,,,,
200 300 400 temperature / ~
,1 . . . . . . . . . . . . . . . . . . . . . . .
500
Figure 4 - Variation of the weight of various samples (CeO2, Pt/CeO2, CeO2-ZrO2, Pt/CeO2ZrO2),heated under H2 atmosphere, at increasing temperature.
Experiments on ceria catalysts under H2 f l o w at 400~ have shown that sulfate reduction leads to H2S formation (3,4) .However, H2S titration showed that the amount evolved is far lower than expected. This can be explained by the remaining of sulfur species on the samples. To confirm such a result, the reduced samples have been treated by O2 at 450~ in the thermobalance. This treatment leads to a mass gain for all samples (Fig.4). This is due to the
577 reoxidation of sulfur species, which remain on the samples after H2 reduction, into sulfate as shown by the IR spectra reported in Fig.5 which evidence for sulfate formation.
tt~
0.4
~" co
o o
T-I t~
A b
r
co
t'~
v.-
v-I
S O
b a
n C e
'
'14'oo'
'
'12'oo'
'
'lo'oo'
Wavenumber (cml)
'
'8do
Figure 5 - IR spectra of species formed from the reoxidation at 450~ of the reduced sulfated samples'a) Pt/CeO2, b) Pt/CeO2-ZrO2). The IR study of sulfate reduction by H2 was also carried out on ceria and ceria-zirconia samples. It confirms the gravimetric results: -platinum favors the sulfate reduction, decreasing the reduction temperature for all samples, -addition of ZrO2 to CeO2 gives rise to a higher amount of sulfate species which are reduced between 250 and 350~ It has recently been published that ceria reduction leads to Ce 3+ ions giving rise to an IR band near 2120 cm -1 (14). This band appears during the H2 reduction of all sulfated samples (Fig.6) showing that ceria reduction concomitantly occurs with the sulfate reduction. It has been shown that the introduction of Z r O 2 i n t o C e O 2 strongly modifies its reduction behaviour in comparison with pure ceria (15). The gravimetric curves in figure 4 confirm that ZrO2 enhances the ceria reduction. Moreover the study of the intensity of the 2120 cm- 1 band (Fig. 6) also shows that the Pt/CeO2-ZrO2 samples are more easily reduced at a given temperature than Pt/CeO2 even in presence of adsorbed sulfate.
578
0.02
A b c S 0
n
e ' 2200' ' 2100' '
' ' 2200" 21'00'
' ' 2~00' 2;00' ' ' 2300' 21'00' Wavenumber (cm -l)
' '2200' 2~00'
'
Figure 6 - Variation of the intensity of the 2120 cm -1 band during heating, under H2 atmosphere, sulfated samples a) Pt/CeO2, b) Pt/CeO2-ZrO2, c) Aged Pt/CeO2-ZrO2
Study of the IR spectra of sulfate species during their reduction at increasing temperature in the case of high area samples(Fig.7) allows us to compare the reducibility of surface and bulklike species. It appears that, without platinum, heating under H2 atmosphere
A
0.5
S O
0.5
S
a
a
n
n
C
C
e
e II Ot'~ (D v--
~
15'00 afle~
'
13'00 ' 11'00 ' 960 Wavenumber (cm-l)
15'0o
'
13'00
'
0
11'oo
'
9oo
Wavenumber (cm-1)
Figure 7 - IR spectra of sulfate adsorbed on A) CeO2, B) CeO2-ZrO2 and heated under H2 atmosphere at" a) 250~ b) 350~ c) subtraction of spectra (a) from (b).
579 up to 350~ only some surface sulfate species are reduced on ceria (negative absorbance at 1404 and 1370 cm -1) (Fig. 7A). On CeO2-ZrO2, at this temperature, a higher amount of surface sulfate is decomposed and few bulklike species are also reduced (Fig.7B).
4. CONCLUSION Only surface sulfate groups are observed on zirconia by $02 oxidation, even in presence of platinum, while bulklike species are also formed on CeOz-ZrO2 mixed oxides, with or without platinum, as on pure ceria (3,4). Then the sulfate poisoning can be as important on Pt/CeO2 -ZrO2 catalysts as on Pt/CeO2. As previously reported for pure ceria (4), platinum favors the H2 reducibility of both sulfate species on all studied catalysts. In this work, we show that sulfate reduction occurs at a lower temperature on Pt/CeOz-ZrO2 than on Pt/CeO2. So, it appears that addition of zirconia, not only enhances ceria reduction (15) but also sulfate reduction. This result relative to the sulfate reducibility on Pt/CeO2-ZrO2 can be considered as a favorable factor for the COS recovery. We also show that the surface sulfate species first appear and are the first ones to be reduced by H2. Their formation but also their reduction involve the surface sites. Diffusion of the oxidized species into the bulk during their formation or towards the surface during their reduction then occurs. The textural properties of the ceria catalysts then play an important role and have to be stabilized.
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1. D.R. Monroe, M.H. Krueger, D.D. Beck, and M.J. D'Aniello, Stud. Surf. Sci. Catal. 71 (1991) 593. 2. A.F. Diwell, S.E. Golunski, J.R. Taylor and T.J. Truex, Stud. Surf. Sci. Catal., 71 (1991) 417. 3 . . M . Waqif, P. Bazin, O. Saur, J.C. Lavalley,G. Blanchard and O. Touret, Appl. Catal. B, 11 (1997) 193 4. P. Bazin, O. Saur, J.C. Lavalley, G. Blanchard, V. Visciglio and O. Touret, Appl. Catal. B,in press. 5. B. Harrison, A. Diwell and C. Hallett, Platinum Metals Rev., 32 (1988) 73. 6. M. Pijolat,M. Pfin, M. Soustelle,O. Totaet and P. Nortier, J. Chem. Sot., FaradayTrans., 91 (1995) 3941. 7. G. Sauvion, J. Caillod and C. Gourlaoen, Rh6ne Poulenc, Eur. Pat.,0207857, (1986); T. Ohata, K. Tsuchitani and S. Kitaguchi, Nippon Shokubai Kagaku, Jpn Pat.,8890311, (1988); N.E. Ashley and J.S.Rieck, Grace W R and Co-Conn, US Pat., 484727, (1991). 8. B.E. Warren and B.L. Averbach, J. Appl. Phys., 595 (1950) 21. 9. O. Saur, M. Bensitel, A.B. Mohammed Saad, J.C. Lavalley, C.P. Tripp and B.A. Morrow, J. Catal., 99 (1986) 104. 10. M. Bensitel, M. Waqif, O. Saur and J.C. Lavalley, J. Phys. Chem., 93 (1989) 6581. 11. M. Waqif, O. Saur, J.C. Lavalley, Y. Wang and B. Morrow, Appl. Catal., 71 (1991) 1373. 12. M. Bensitel, O. Saur, J.C. Lavalley and B. Morrow, Mat. Chem. Phys., 19 (1988) 147. 13. C. Morterra, G. Cerrato, S. Di Ciero, M. Signoretto, F. Pinna and G. Strukul, J. Catal., 165 (1997) 172. 14. C. Binet, A. Badri and J.C. Lavalley, J. Phys. Chem., 98 (1994) 6392. 15. P. Fornasiero, G. Balducci, J. Kaspar, S. Meriani, R. di Monte and M. Graziani, Catal. Today, 29 (1996) 47.
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CATALYSIS AND AUTOMOTIVE POLLUTION CONTROL IV Studies in Surface Science and Catalysis, Vol. 116 N. Kruse, A. Frennet and J.-M Bastin (Eds.) 91998 Elsevier Science B.V. All rights reserved.
Oxygen storage capacity in Perovskite-related oxides stoichiometric oxygen in three-way catalysis
581
9The role of over-
N. Guilhaume and M. Primer Laboratoire d'Application de la Chimie/l l'Environnement Universit6 Claude Bernard Lyon I, UMR CNRS 5634, Bat. 303 43 Boulevard du 11 Novembre 1918 F-69622 Villeurbanne Cedex, France.
Two
perovskite-related catalysts doped with small amounts of noble metals, and La2Cuo.gPdo.~O4+8, present high activity for three-way catalysis reactions including the simultaneous removal of CO, NO and C3H6. These catalysts exhibit oxygen storage properties, evidenced in step-change experiments where CO was oxidised by the lattice oxygen of the solids, at low temperature (400~ These properties allow them to compensate for the fluctuation of the feedstream stoichiometry, when the activity is evaluated under cycling conditions. LaMn0.976Rh0.02403+8
1. INTRODUCTION Because the exhaust composition in automobile catalytic converters fluctuates between oxidising and reducing conditions, oxygen storage is a particularly important property for three-way catalysts, which allows the solid to compensate for these fluctuations. This role is ensured by ceria, an essential additive in automotive emissions control catalysts. Among the different roles played by ceria in enhancing the catalytic performances (oxygen storage, promotion of reactions involving water, stabilisation of noble metals dispersion and of 7-A1203 [ 1-5]), the most significant one appears to be its ability to store and release oxygen, based on the easy and reversible change in the oxidation state of cerium between Ce4+ and Cea+. In a previous work, we have shown that the introduction of small amounts of noble metals in the perovskite-related oxides LaMno.976RM.024Oa+8 [6] and La2Cuo.gPd0.104+8 [7] leads to solids with high activity for the simultaneous removal of CO, NO and C3H6. These catalysts showed no deactivation when tested under periodically rich/lean conditions with low frequencies, suggesting that the lattice oxygen of the catalysts can compensate for the fluctuations in the composition of the reactant mixture. In the present study, we examined the response of these catalysts to step-changes in the composition, and how it is related to oxygen storage properties in the conditions of three-way catalysis.
582 2. RESULTS AND DISCUSSION
2.1. Experimental The experimental details concerning the preparation and characterisations of the catalysts have been described previously [6-7]. The main characteristics of the solids are presented in table 1. They were prepared by calcination of polyacrylamide gels in which the metal salts were incorporated. This method allows to obtain the mixed-oxide phases at moderate temperature (700~ with rather high surface areas. Table 1 Characteristics of the two per ovskite-type catalysts. Catalyst . . . . Calcination X-ray diffraction pattern temperature LaMno.976Rho.o2403+~ 700~ LaMnO3.,5 La2Cuo.aPdo.lO4+8 700~ La2CuO4+~(T)
Amount of noble metal 1 wt.% 2.41 wt.%
Specific surface area (m2/g) 27 15.8
The catalytic testing equipment is described in ref. [7]. The simulated exhaust compositions chosen for light-off experiments, under stationary and cycling conditions, are reported in table 2. In cycling tests, we chose a very low frequency (0.1 Hz) compared with the real cycling frequency in an engine (around 1 Hz), because the design of our apparatus and the flow rate are such that at 1 Hz the two streams mix together and the catalyst is submitted to an average composition. When cycling at 0.1 Hz, the streams reaching the catalytic bed correspond to 8085% of the two individual net-oxidising or net-reducing compositions. Table 2 Simulated exhaust gas composition used in stationary and cycling Light-off tests (total flow rate 10 l.hq). Composition (ppm) a Stationary . Cycled c O2 5600 3377 7823 CO 6200 10781 1619 NO 1000 1000 1000 C3I-I6 667 667 667 Sb 1 0.462 2.184 a The effective compositions were adjusted with an accuracyof + 1% around these values. b Stoichiometric factor S= (2 [O21+ INO])/([CO] + 9 [c~l). ~The average composition is stoichiometric, and identical to that used under stationary conditions. The step-change experiments were performed under isothermal conditions (400~ with single components (CO as the reducer, 02 or NO as the oxidiser) diluted in nitrogen. This temperature was chosen since it corresponds to 100% conversion of NO, CO and C3I-I~ in light-off experiments for the two catalysts. Two series of tests were performed, the "oxidation step" corresponding to: (1) dwell under CO, (2) flushing by nitrogen, (3) dwell under Oz (or NO), (4) flushing by nitrogen and (5) dwell under CO. The reverse experiment
583
(O2/N2/CO/N2/O2) corresponds to the "reduction step". Before starting the experiment, the catalyst was stabilised under the stream corresponding to the first dwell for one hour at 400~ 2.2. D y n a m i c evaluation of the redox properties of the catalysts
In order to evaluate how the catalysts change under oscillations in the feedstream composition, we examined the response of the solids to step-changes in the composition of the gas phase. Single components, CO and 02 (or NO) were sent alternatively on the solids at 400~ separated by intermediate flushing by nitrogen to remove the gas phase and weakly adsorbed species. 2.2.1. LaMn0.976Rho.02403+~catalyst . O x i d a t i o n s t e p (Fig. 1A) During the first dwell under CO (following one hour stabilisation under CO), no oxidation into CO2 is observed since the catalyst has been fully reduced. After 15 min flushing with nitrogen, the N2/O2 transition is accompanied by a very small COz peak (40 ppm at the maximum) which probably corresponds to the oxidation of traces of carbon formed during the first dwell under CO. The catalyst is then oxidised in oxygen for 30 min, flushed with nitrogen and CO is introduced again. A large CO2 peak appears immediately, whose trace has not completely returned to the baseline after 30 min dwell under CO. 6000 E
N~ '. I
c(
4000
i
i
I
I I ,
CO
I
co2
e
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'
i
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E 2000 O
0
15
30
45
60
75
90
105
Time (min) 6000
' N2J'!"
~" C:: .o
'
; ..,.,_.,_._...,-.1.
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i
,.
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*~9
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15
,i L _ 30
. I
45
,
::
v ~.
,,
60
,§ 75
(B) I
!
g0
105
Time (min)
Figure 1. Step-change activity of LaMno.976Rho.02403+8under successive streams of CO and O2 at 400~ (100 mg catalyst, total flow rate 10 1.h~). (A) oxidation step; (B) reduction step.
584 . R e d u c t i o , s t e p (Fig. 1B) The reverse experiment shows the same CO2 peak upon introduction of CO on the solid previously oxidised. These results show that : 9 The large CO2 peak corresponds to CO oxidation by the lattice oxygen of the catalyst, and not to the CO dismutation reaction (2 CO ~ CO2 + C) since there is nearly no CO2 formed upon introduction of oxygen on the solid reduced under CO. 9 This large CO2 peak is highly reproducible : in several cycles (up to 5 cycles were performed) of step-change experiments with changing the CO and 02 compositions and increasing dwells lengths, the same response of the catalyst is obtained. Integration of this peak gives an average amount of 55 + 6% lamoles CO2 for 100 mg catalyst, which corresponds to 0.133 mole CO2 per mole catalyst. This corresponds well to the amount of over-stoichiometric oxygen which can be accommodated in this Lal~Ino.976Rh0.02403.15 perovskite (0.15 mole [O] per mole catalyst). The CO2 evolved represents the oxygen storage capacity of this catalyst 9 The same experiments were performed with NO as the oxidiser instead of oxygen. The same CO2 amount is evolved upon introduction of CO, while a small amount of N20 is formed when NO is introduced on the reduced catalyst, showing that the lattice oxygen can be replenished by NO as easily as by 02. 2.2.2. La2Cu0.9Pd0.104§ catalyst Similar tests were performed on the La2Cuo.9Pd0.104+~ catalyst (Fig. 2). The same profiles are obtained as in the case of the LaMn0.976Rh0.02403+8catalyst, although the small CO2 peak due to the oxidation of carbonaceous deposits is somewhat bigger than previously (440 ppm CO2 at the maximum). The main difference with the previous catalyst is that the amount of CO/evolved on introduction of CO on the oxidised solid is much larger (about 400 lamoles), and corresponds in this case to 0.75 to 0.8 mole CO2 per mole catalyst. According to several authors, the La2CuO4 structure can also accommodate overstoichiometric oxygen [8] up to La2CuO4.13[9]. However the CO~ peak formed on the oxidised solid cannot be due only to the removal of the over-stoichiometric oxygen, since it corresponds to a nearly total reduction of Cu2§ and Pd2§ into the metals. This indicates that this catalyst undergoes much deeper reduction that the lanthanum manganite, probably because the copper can be reduced into the metal in the present conditions, while it is unlikely that the surface manganese ions should be reduced more than in a Mn2+ state during CO oxidation.
2.3. Characterization of the reduced and oxidised states of the catalysts The X-ray diffraction patterns of the fresh catalysts correspond to those of LaMnO3.15 in the case of LaMno.976Rh0.02403§ and to a tetragonal form of La2CuO4 in the case of La2Cu0.gPd0.~O4+~. After successive step-change experiments under CO/O2, the solids were stabilised under CO or 02 at 400~ cooled at room temperature under the same atmosphere and flushed with nitrogen before recording the X-ray diffraction patterns. The phases identified by XRD are presented in table 3. In both cases the diffraction lines are broad and noisy. The structure of the Rh-doped lanthanum manganite is modified when stabilised in the reduced state: the XRD patterns corresponds now to the stoichiometric form LaMnO3.00 (orthorhombic, ICDD n~ 35-1353), which is slightlydifferent from that of (hexagonal, rhombohedrally
585 distorted, ICDD n~ 32-0484). This corresponds very well to the amount of CO2 evolved in the step-change experiments (0.133 mole per mole catalyst), and shows that the structure can easily switch between LaMnO3.00 and LaMnO3.~s depending on the reducing or oxidising stoichiometry of the feedstreams. Several authors have studied the structure of the LaMnO3+a perovskite according to its oxygen content, which may simply depend on the calcination temperature (the solid tends to loose oxygen on increasing the calcination temperature, starting from rhombohedral-hexagonal LaM/103.09 at 800~ to reach the orthorhombic form LaM/IO2.99 at 1300~ [10]). The oxygen non-stoichiometry in LaMnO3+a has been shown to influence the activity of the catalysts in various reactions like NO reduction by CO and 1-12 [11] or N20 decomposition [ 12].
12000
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Figure 2. Step-change activity of La2Cu0.gPdo.lO4+a under successive streams of CO and 02 at 400~ (200 mg catalyst, total flow rate 10 l.hl). (A) oxidation step; (B) reduction step.
586 Table 3 Phases identified in the X-ray diffraction patterns of the catalysts in the fresh state and after step-change experiments, when stabilised in the oxidised or reduced state at 400~ Sample
Fresh catalyst
LaMno.976Rho.02403+~i
LaMnO3.15
La2Cuo.9Pdo.iO4+~
La2CuO4 (T)
Catalyst after step-change tests Reduced state Oxidised state LaMnO3.00 LaMnO3.15 La2CO5 Cu (very weak) Cu20 (traces)
La2CO5 CuO (very weak)
The XRD patterns of the La2Cu0.9Pd0.104§ catalyst after stabilisation under oxidising or reducing feedstreams explain the results of the step-change experiments: under our conditions (CO, 400~ the catalyst is not stable, leading to the formation of lanthanum oxycarbonate and reduced copper (and probably palladium). It must be reminded that the La2CuO4 structure, often called 'perovskite-type', consists in (LaCuO3) perovskite blocks separated by (LaO) § layers. It is also well established that lanthanum oxide carbonates rapidly even at room temperature, leading to the formation of LazCOs. This carbonate is quite stable (up to 1000~ under CO/). It is not decomposed during the oxidising dwell under oxygen at 400~ while the copper (and probably also the palladium, although not depicted by XRD) is fully oxidised into CuO. The same phenomenon has been observed with LazCuO4+8by other authors, in the case of the simultaneous NOx reduction and soot oxidation reactions [13]. We suppose that this collapse of La2CuO4+6 under CO is connected with the presence of (LaO)+ sheets which have a strong affinity for CO2, and destabilise the structure: CO is oxidised on surface Cu 2+ and Pd 2§ ions, leading to reduced copper and palladium while the CO2 formed is incorporated into the La203 lattice until saturation and total collapse of the mixed oxide structure. This does not occur in the pure perovskite La(MnRh)O3+8, where the lanthanum ions are surrounded by manganese only, and manganese ions cannot be reduced more than in a Mn 2+ state under these conditions. We had observed previously [7] by XPS and 1R study of CO adsorption that the La2Cu0.gPdo.lO4+~catalyst was deeply modified atter light-off tests in the presence of CO, NO and C3I-I6, even though the reactant gas mix is stoichiometric: it is transformed into highly dispersed copper and palladium species in various oxidation states (Cu 2+, Cur, Pd 2+ and Pd~ in a lanthanum oxycarbonate matrix. This instability is shown here to be connected with the reaction of the solid with CO. It is interesting to note that, since the lanthanum oxycarbonate is not decomposed under oxygen at 400~ and cannot thus be taken into account for the formation of carbon dioxide, this means that the amount of CO2 evolved in the step-change experiments (about 0.75 to 0.8 mole per mole catalyst) correspond to the nearly total and reversible reduction or oxidation of the two transition metals (Cu and Pd), at a mild temperature.
2.4. Evaluation of the catalytic activity under cycling conditions The ability of the catalysts to supply oxygen for the oxidation of CO and HCs when the exhaust is fuel-rich, and to remove excess oxygen when it is lean is evidenced in the light-off tests under cycling conditions. We chose a low cycling frequency (0.1 Hz) between two
587 strongly oxidising (S=2.184) and reducing (S=0.462) compositions, which minimises the mixing of the two feedstreams before they reach the catalytic bed: we checked that under these conditions the catalyst is in contact with 80-85% of the initial compositions. The light-off activities for CO, NO and C3H6 conversions are shown in Fig. 3. For comparison, the results obtained with a classical Pt-Rh/CeO2-AI203 (1.13 wt.% Pt, 0.19 wt.% Rh, 19.3 wt.% Ce) are included. 100
100
-
=> 50
,
,
.._
_
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~
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~
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8 o
o 150
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350
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.
o 9 50.} d
.
.
.
.
350
450
.
;'//
o 250
250
Temperature (*C)
,, 150
150
350
Temperature (*C)
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Figure 3. Light-off activity for CO, NO and C3H6 conversion in cycling conditions over LaMno.976R~.02403+8 (m, full line), La2Cu0.9Pd0.104+8 (B, dashed line) and Pt-Rh/CeO2-AI203 (C, thick line) catalysts. (Conditions : v=0.1 Hz, total flow rate 170 ml.mn~, 100 mg catalyst A, 200 mg catalysts B and C, G.H.S.V.: 13000hl).
Clearly, the Mn and Cu-based catalysts are very active for CO and HC oxidation reactions, a behaviour which is well known for these two metals, their activity being higher than that of the reference Pt-Rh catalyst. The assistance of small amounts of noble metals (1 wt.% Rh or 2.4 wt.% Pd respectively) improves the activity in NO reduction, which is total around 400~ Monceaux et al. [ 14] also observed a pronounced promoting effect of Pt and / or Rh dopes on the three-way catalytic activity of La0.sSr0.2MnO3+8 perovskites. Rh was shown to be particularly important to obtain a nearly total conversion of NO at 500~ with a high space velocity (100 000 h1) under stationary conditions. The interesting point is the comparison of the light-off activity under stationary or oscillating compositions (table 4). It is clearly seen that, despite the low cycling frequency chosen (the rich and lean feedstreams are switched for each other every 5 seconds), the catalysts show no loss of activity when compared with the light-off tests under stationary composition.
588 Table 4 Comparison of the light-off activity under stationary and cycling conditions, expressed as the temperatures (~ for 10, 50 and 80% conversion of CO, NO and C3H6. LaMno.976Rho.02403+~i
LaECUo.9Pdo.lO4+~
Pt-Rh/CeO2-Al203
Stationary CO
Tlo Tso Tso
190 230 245
<150 205 215
<150 250 300
NO
Tlo Tso Tso
240 265 290
225 320 340
225 310 335
C3H6
Tlo Tso Tso
240 265 280
185 245 270
220 300 340
Tlo Tso T80
185 235 255
155 210 230
<150 255 300
NO
Tlo Tso T80
235 260 295
225 280 370
235 290 315
C3H6
Tlo Tso T8o
235 270 295
190 250 280
220 295 315
Cycling CO
In the case of the LaMn0.976Rh0.02403+~perovskite, we have found that the overstoichiometric oxygen is quickly available and replenished when the feedstream becomes rich or lean. The basic perovskite structure remains unchanged and simply accommodates more or less oxygen, the over-stoichiometry corresponding to the presence of some Mn 4+ ions. Recently, supported manganese oxides have received attention as new oxygen storage components with high oxygen storage capacity [15], which can be used for CO and CH4 removal in natural gas fuelled vehicles emissions. With the assistance of a noble metal (Pt or Pd), the catalysts are also active for NO reduction. The OSC, measured by TPO/TPR, is based here on the redox couple Mn3+/Mn2§ which presents significant oxygen uptake and release at temperatures below 600~ In our case, the OSC was measured in milder conditions, and seems to correspond to the reversible change between Mn4+ and Mn3+ in the perovskite structure. In the case of the 'La2Cuo.9Pdo.lO4+8'catalyst, we have seen that the mixed oxide structure collapses in the presence of CO, and although the easy reduction and oxidation of the welldispersed copper and palladium seems to be sufficient to compensate for the variations in the composition, it is difficult to invoke real oxygen storage properties for this solid.
589 3. CONCLUSION The difference in oxygen stoichiometry between the two perovskite forms LaMno.976Rho.o2403.15 and LaMno.976Rl~.02403.00 can be used for oxygen storage. The perovskite structure remains stable under successively oxidising and reducing reactants, and simply adapts its oxygen stoichiometry to the composition of the reacting mixture. The most striking feature is that the amount of oxygen evolved or replenished corresponds to all the over-stoichiometric oxygen available, suggesting that this process is not limited to the surface of the solid, but involves the whole catalyst mass. In the case of LazCu0.gPdo.lO4+8, the structure is not stable in the presence of CO, leading to well dispersed Cu and Pd species in a lanthanum oxycarbonate matrix. This is supposed to be due to the different crystal structure of this latter solid, where layers of La-O are present, and to the easy reduction of Cu 2§ and Pd 2§ ions by CO into the metallic state. However the copper and palladium are easily reduced and reoxidised under mild conditions (400~ these reactions being fast enough to supply and retain oxygen when the composition oscillates between rich and lean. REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.
B. Harrison, A. F. Diwell and C. HaUett, Platinum Met. Rev., 32 (1988) 73. S.H. Oh and C. C. Eickel, J. Catal., 112 (1988) 543. J.G. Nunan, H. J. Robota, M. J. Cohn and S. A. Bradley, J. Catal., 133 (1992) 309. H.C. Yao and Y. F. Yu Yao, J. Catal., 86 (1984) 254. M. Shelefand G. W. Graham, Catal. Rev.-Sci. Eng., 36 (1994) 433. N Guilhaume and M. Primet, J. Catal., 165 (1997) 197. N. Guilhaume, S. D. Peter and M. Primet, Appl. Catal. B-Environ., 10 (1996) 325. K. Sekizawa, Y. Tanako, H. Takigami, S. Tasaki and T. Inaba, Jpn. J. Appl. Phys., 26 (1987) 840. J.E. Schirber, B. Morosin, R. M. Merrill, P. F. Hlava, E. L. Venturini, J. F. Kwak, P. J. Nigrey, R. J. Baughman and D. S. Ginley, Physica C, 152 (1988) 121. Y. Takeda, S. Nakai, T. Kojima, R. Kanno, N. Imanishi; G.Q. Shen, O. Yamamoto, M. Moil, C. Asakawa and T. Abe, Mat. Res. Bull., 26 (1991) 153. R.J.H. Voorhoeve, J.P. Remeika, L.E. Trimble, A.S. Cooper, F.J. Disalvo and P.K. Gallagher, J. Solid State Chem., 14 (1975) 395. S. Louis Raj and V. Srinivasan, J. Catal., 65 (1980) 121. V. Duriez, L. Monceaux and P. Courtine, Stud. Surf. Sci. Catal., 96 (1995) 137. L. Monceaux and P. Courtine, J. Chim. Phys., 92 (1995) 1544. Y.-F. Chang and J. G. McCarty, Catal. Today, 30 (1996) 163.
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CATALYSIS AND AUTOMOTIVE POLLUTION CONTROL IV Studies in Surface Science and Catalysis, Vol. 116 N. Kruse, A. Frennet and J.-M Bastin (Eds.) 91998 Elsevier Science B.V. All rights reserved.
591
Influence of Ceria Dispersion on the Catalytic Performance of CB/(CeO2)/Ai203 Catalysts for the CO Oxidation Reaction. A. Martinez-Arias, J. Sofia, R. Catalu~ J.C. Conesa and V. Cort6s Corber/m. Instituto de Cat/disis y Petroleoquimica. CSIC. Campus UAM. Camino de Valdelatas s/n. Cantoblanco, 28049 Madrid. Spain.
Cu/CeOE/Al203 catalysts containing different amounts ofceria, as well as ClffAl203 and Cu/CeO2 samples, have been studied in thermal programmed CO oxidation experiments, and characterized with EPR and FTIR. The presence of copper-ceda contacts is revealed by analysis of superoxideEPR spectra in a Cu/CeO2/Al203 sample. These interactions stabilize copper in a reduced state, as indicated by the formation of carbonyls giving bands at 2106-2098 cm1 in the FTIR spectra of CO adsorbed on the calcined samples. On the basis of the reactivity profiles obtained it is proposed that copper-ceda contacts largely promote catalytic reactivity, enhancing the formation of active reduced Cu species, and that the heterogeneity of the different supports, where tridimensional ceria entities appear (together with bidimensional patches) as the ceria loading is increased, leads to the presence of different copper entities whose properties change as a function of their interactions with the different parts of the support, the final catalytic activity seems to be higher for Cu species interacting with 3-D ceria than for those contacting more dispersed 2-D ceria entities.
1. INTRODUCTION Main challenges of present three-way catalysts (TWC) concern the availability of precious metals, lowering of the ignition temperature of the converters and maintenance of a high temperature stability [ 1]. Development of non-noble-metal alternatives, able to reach the activity levels achieved by usual Pt-Rh-Pd catalysts, is particularly desirable and supported copper catalysts are possible candidates to attain the mentioned requisites. Particularly considering ceria-related systems, formation of intimate copper-ceda contacts is thought to be crucial to explain the remarkably high activities shown for methanol synthesis [2] or for CO oxidation [3]. When using CeO2/AI203 supports, several questions arise in principle from the fact that the structure of cefia can change as a function of its dispersion and/or its interactions with the alumina carder [4, 5], possibly leading to different metal-support interactions when a metal is incorporated on these supports. Previous work has shown that at least two different ceria entities can be discerned in Al203-supported ceria systems [4, 5]. One is related to highly dispersed ceria entities (bidimensional patches, here called 2D-Ce); these would experience important interactions with the alumina support and would consequently show significant differences in their chemical bdmit~, inregx~ to Ix~ r Theother e n f i t i e s a r e ~ la~tfidinmsional ceria p ~ s
592 (3D-Ce), these would present properties closer to those of pure ceria, although one could still expect some differences between different celia particles as a function of their size [7]. Improvement of the catalytic performance of these systems requires a good comprehension of the phenomena o c c ~ g at a microscopic level, including any copper-support interactions able to affect their catalytic reactivity. Here we present catalytic activity data for the CO-Oz reaction on different supported copper samples; the results are correlated with the existence of specific coppersupport interactions, related to structural changes affecting to cerium oxide as a function of its dispersion on the alumina support.
2. EXPERIMENTAL CeO2/AI203 supports (xCA samples, x = wt. % CeO2) were prepared by incipient wetness impregnation ofy-Al2Oa (Condea, 1.8 mm spheres, SBEX=200m2gq) with aqueous C~qO3)3x6H20 solutions, followed by drying and calcination in dry air for 2 h at 773 K. Cu/CeO2/AI203, Cu/CeO2 (CeO2 from Rh6ne-Poulenc, S~Ea=285 m2gq) and Cu/A1203 samples (called, respectively, CuxCA, CuC and CuA; final Cu load: 1 wt. %) were prepared by impregnation of the corresponding supports with aqueous solutions of Cu(NO3)2-3H~) followed by drying and calcination in dry air for 2 h at 773 K. In all the cases, except for catalytic tests where 0.4-0.5 mm particles were selectedaRer sieving crushed spheres (previous data show the absence of internal diffusion effects for this case [8]), the samples were ground into powder with an agate mortar. All the gases used were of commercial purity and, for adsorption experiments, further purified by vacuum distillation methods before storage in glass recipients attached to the vacuum system. Thenml programmed reaction tests were carried out in a pyrex gas flow reactor. The feed and outlet streams were analyzed with a Perkin-Elmer FTIR spectrometer mod. 1725X, coupled to a multiple reflection transmission cell (Infrared Analysis Inc. "long path gas miniceU", 2.4 m path length, ca. 130 cm3 internal volume), for 02, a paramagnetic analyzer (Servomex 540 A) was used. Before the tests, the catalysts were subjected in the system to a standard pretreatment consisting of heating for 1 h under a 3% O2:N2 flow at 773 K, cooling in the same flow to room temperature (RT) and then purging briefly (5 min) with N2. EPR spectra were taken at 77 K with a Bruker FaR 200 D system working in the X-band and calibrated with DPPH (g=2.0036). Computer simulations were used as needed to cheek spectral parameters. Portions of ca. 40 mg of sample were placed inside a quartz probe cell with greaseless valves and handled in a conventional high vacuum line for the different treatments. For the 02 adsorption experiments at T~77 K, doses of about 70 I~nol of 02 per gram of catalyst were used. CO reduction treatments were made in static conditions using 100 Torr of CO, heating during 1 h at the chosen reduction temperature (Tr) and then outgassing at the same temperature for 0.5 h. Absolute EPR intensity measurements were made by comparison with the spectrum of a copper sulphate standard (CuSO4-5H20). FTIR spectra were recorded at RT with a Nicolet 5ZDX spectrometer, using a resolution of 4 cmq and taking 128 scans for every spectrum. Thin self-supporting discs (ca. 10 mg cm'2), prepared by pressing the powders, were handled in standard greaseless cells, where they could be subjected to thermal or adsorption treatments.
593 3. RESULTS
3.1. Preliminary characterization data None of the copper-containing samples showed in their XRD diffractograms peaks which may be ascribed to Cu-containing phases, xCA or CuxCA (x=10, 39) samples showed features due to the CeO2 and 3/-A1203 phases, with broad peaks due to the small crystallite size. Somewhat narrower CeO2 peaks were detected upon increasing ceria content of these samples. Only peaks due to the CeO2 structure were observed for the CuC catalyst. Raman spectra showed the presence in ceria-containing samples of a peak at ca. 460 cm -1, due to relatively large ceria particles [4], which increases with the ceria loading. 3.2. Thermal programmed reaction measurements. The samples were tested for their reactivity in a gas mixture containing 1% CO and 0.5 % 02 (i.e. of stoichiometric redox composition) in the 1',I2carrier at a flow rate equivalent to 30,000 h-1 space velocity; after a short time on stream (in order to ensure a steady gas flow on the catalyst), heating of the oven at a 5 K min-~ rate up to 823 K was started. CO and 02 are observed to be consumed in a parallel way with simultaneous CO2 production. The results of catalytic CO conversion obtained for the Cu-containing samples are shown in Figure 1. Pure alumina presents some catalytic activity only at T > 673 K, so that the conversion observed for CuA below that temperature must be attributed to copper. The CO conversion curve corresponding to this sample shows that the reaction starts at 323 K and increases slowly up to ca. 573 K; above this temperature a much faster increase is observed. This variation in the conversion rate may be associated to changes in the nature of the copper entities during the reaction. The curve corresponding to Cul0CA shows a marked increase in conversion, showing that addition of 10% CeO2 to the catalyst decreases the isoconversion temperature at 50% conversion ('I"50)from 643 K for CuA to 443 K for Cul0CA (ATs0=-200 K). A higher increase in the Ce content up to 39 % further decreases Tso (but the additional change is comparatively lower: 100 T5o=383 K) and the conversion curve slope increases for any .-. 80 conversion level (below 80 %). The CuC sample is the most .~ 60 active one, presenting ca. 20% conversion already at RT. --- C u A o 40 Logarithmic plots (not 9 ~ CA shown) of the conversion data r~ 20 ~ Cu39CA vs. T -1 in the conversion + CuC interval 15-35% yield nearly 0 straight-line sections the slopes 273 373 473 573 673 773 of which correspond to apparent T (K) activation energies of 8.5, 10.0 and 12.7 Kcal/mol for samples Cul0CA, Cu39CA and CuC respectively. Tests on the xCA Figure 1. CO conversion profiles of the Cu-containing catalysts. r~
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.
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.
.
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,
,
,
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594 or CeO2 supports (not shown) yield Ts0 values ca. 250 K higher than the corresponding Cucontaining catalysts. 3.3. E P R results
EPR spectra of the starting calcined samples show significant differences between the Cu-containing samples, Figure 2. Thus, CuC shows mainly an axial signal, showing four-line hyperfine splittings in each of its features, with parameters g ll =2.265, g . = 2 . 0 4 0 , All = 16 x 10-3 cm -1 and A ~ = l . 3 x 10-3 cm -1, signal A, due to isolated Cu 2§ ions [9] in a ceria environment; other smaller peaks at the magnetic field zone typical of the g II component of these signals reveal the presence of smaller amounts of similar isolated Cu2§ ions, the differences between them being most likely due to small variations in the coordination environment of the corresponding ions. On the other hand, for CuxCA or CuA, at least two different signals can be discerned: one presents g ll =2.321 and g . = 2 . 0 5 7 and four-line hyperfine splittings with All---17.1 x 10-3 cm -~ and A~= 1.9 x 10 -3 cm ~, signal B; this signal is similar to that found earlier in other Cu/A1203 samples and attributed to isolated Cu 2§ ions in a square pyramidal environment [10]. The other signal is significantly broader and presents extremes at g=2.24 and g=2.05, signal C; it must be ascribed to Cu 2§ ions into an oxidized copper-containing dispersed phase (like copper oxide or aluminate), the higher linewidth being due to dipolar interactions between Cu 2+ ions. The fact that these species are detected in the ceria-free sample suggests they are due to A1203-based Cu 2§ species. Small changes are however observed between the spectra of CuxCA samples and CuA, indicating the presence of other smaller Cu 2§ signals in CuxCA samples, whose parameters cannot be measured with certainty, and which might be related to Cu 2§ species interacting with the ceria component of these samples. It is noteworthy that double integration of the spectra shows that the fraction of total copper detected as Cu2§ increases as CuC (25 %) < Cul0CA (53 %) - Cu39CA < CuA (63 %), suggesting that copper-ceria interactions favour formation of diamagnetic species (reduced copper or antiferromagnetically coupled Cu 2+ ions). Introduction of a small amount of 02 at 77 K on reduced ceria-containing samples 2.265 2 040 leads to the formation of superoxide species I I II I ~ (formally Of-Ce 4+) [5, 11], as shown in the spectra of Figure 3; these were obtained by subtraction of spectra after and before adsorption, in order to cancel contributions of Cu 2§ signals. The characteristics of these species change as a function of the amount of ceria present in the catalyst, which I reveals changes in the environments of the corresponding superoxide species [5]; Table 1 summarizes the characteristics of these signals. Thus, CuC shows signal O1, 2oo G ~1~2.057 presenting a g tensor close to that shown by some of the signals observed in outgassed pure ceria [11], except for a slightly Figure 2. EPR spectra of the starting calcined lower gz value, which might be due to the samples.
595 Table 1. Summary of O2-Ce4§ signals detected by EPR in this study. More details on axis assignment or signal attributions can be found elsewhere [5, 11]. signal O1
EPR parameters
proposed assignment
g,=2.031-2.029, gx=2.014-2.013, gy-- 2.011
O:-Ce 4+ located on relatively large tridimensional ceria particles (3D-Ce) .
,
02
g,=2.028-2.027, gx=2.017, gy=2.011
.
.
gj_=2.027-2.026, gll =2.012
.
Of-Ce4+ at the surface of bidimensional ceria patches (2D-Ce) dispersed on A1203
,,
03
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.
.
.
O,-Ce 4+ located at ceria-alumina borderline locations (like 2D-Ce edge sites) .
.
.
.
.
influence of carbonate species in the environment of the adsorption center. For the CuxCA samples, the maximum of the first derivative spectra is shifted to lower g values which is due to the overlapping of increasing contributions of signal 02 and a smaller one of 03, as the ceria loading decreases. Thus, for reduced Cul0CA, most of the O~-Ce 4+ species are formed at 2D-Ce patches where the Ce cations have aluminium cations as close neighbours, indicating that the amount of 3D-Ce particles is small. Comparison of the contributions, evaluated by computer simulations, of O2-derived signals (obtained after 02 adsorption at 77 K on the samples reduced in CO) as a function of T~, between Cul0CA and 10CA is presented in Figure 4. It shows that the presence of copper favours the formation of O2 species at the surface of the 2D-Ce patches but hinders the formation of these species at the edges of these bidimensional patches and at the 3D-Ce particles; some effect of copper in promoting ceria dispersion (but blocking edge sites at the 2D-Ce patches) should not be 1.0
--
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CulOCA:
CulOCA_J. \ 2 or2 .... 2-o2
I j
I OCA:
xl
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open symbols/;21 full symb
,.,.,o 0.5
2.029 5O G 0.0
1
373 r Figure 3. EPR spectra after oxygen adsorption at 77 K on the samples reduced at Tr =573 K.
473 (K)
573
Figure 4. Comparison of the contributions of superoxide signals vs. temperature of reduction in CO for Cul0CA and 10CA.
596 discarded to explain this latter point. 3.4. FTIR results Figure 5 shows IR spectra in the CO stretching region after CO adsorption at RT on the initial calcined samples. Sample CuA displays two very weak bands at 2113 and 2097 cm -~. As the Ce amount is increased an important increase of bands at 2106-2098 cm ~ is produced. Besides, for CuC, other small bands or shoulders are detected at 2115 and 2051 cm ~. A recent literature report has shown that changes in the stability of the different stable oxidation states of copper can be produced upon interaction with CO-O2 mixtures [12]. Thus, in order to examine the changes produced in the copper species characteristics upon interaction of the calcined samples with a CO-O2 mixture, the evolution of carbonyl bands has been monitored for CuA and Cul0CA, as shown in Figure 6. Two different temperatures were selected for these experiments at 373 and 573 K. The samples were heated using a 10 K rain -~ ramp until the corresponding temperature is reached, and subsequently cooled to RT, in a stoichiometric flow similar to that used for the catalytic reactivity tests shown above. Then, after prolonged outgassing at RT, a known CO pressure is admitted in the cell. Two bands are mainly formed on CuA at 2114 and 2097 cm -~. Upon increasing reaction temperature, an increase of the intensity of these bands is produced with hint of a shoulder
~ ,061 i/
98
/
CuC
Cu39CA
~/
/~
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\
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T
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2113 2097 CuA
~
x8 1
2150
2100
2050
Wavenumbers (cm- 1) Figure 5. FTIR spectra following adsorption of 10 Torr of CO at RT on the calcined samples. (Ordinate axis in absorbance units).
597 appearing at 2125 cm -~, Figures 6a-b. Certain differences, in respect to the CuA sample, are observed for the Cul0CA sample contacted with the mixture at 373 K, since maximum absorption is detected in this case at 2100 cm -t, with a shoulder at higher wavenumber, Figure 6c. In order to check whether new bands are formed in comparison to the CuA sample treated in the same conditions, the spectrum of Figure 6a has been partially subtracted from that of Figure 6c. The criterion used to perform this subtraction operation is to assume that the bands observed for CuA in Figure 6a are present as overlapping components in the spectrum of Figure 6c; thus the spectrum 6a subtracted from spectrum 6c is gradually increased until the point where some amount of negative band begins to be detected, the final result being shown in Figure 6e. It shows that two new bands at 2104 and 2089 cm1 can be present for the Cul0CA sample treated at 373 K. For the Cul0CA sample treated at 573 K, Figure 6d, the main bands are detected at 2097 and 2114 cmt, the spectrum being quite similar to that observed for CuA treated in the same conditions, except for a somewhat higher absorption at about 2100 cm -t. It is generally acknowledged that carbonyl bands at wavenumbers lower than about 2110 cm ~ are due to carbonyl species adsorbed on metallic copper particles [13], the variations in their wavenumbers being related to changes in the nature of the exposed metallic copper faces [13]. On this basis, bands appearing at 2097 cm -1 in the CuA or CuxCA samples can
2104- .. /-'-'-._~'
e)
x2
2114 2097
I
d) Cul0CA
0.03 a.u.
c) CulOCA
b) CuA a) CuA
~ '
2150
.
~ +
i
2100
+
2050
Wavenumbers (cm- 1) Figure 6. FTIR spectra after adsorption of 10 Torr of CO at RT on the samples treated in CO+O2 at 373 K (a,c) and 573 K (b,d). (e) Partial subtraction (c)-0.6(a). (Ordinate axis in absorbance units).
598 be ascribed to carbonyl species adsorbed on metallic copper, while the low wavenumber of the smaller bands at 2089 and 2051 cm-~ shown by ceria-containing samples can be attributed to similar species, in which the copper particles are affected by interactions with the basic ceria support, as proposed for a Cu/MgO catalyst [14]. Some doubts can exist in respect to the nature of the adsorption center for the bands observed in ceria-containing samples at 2106-2098 cm 1. Although they are in the wavenumber range of Cu~ carbonyls, an alternative assignment might be made on the basis of the existence of interactions with the support [14] and the possibility that they are due to Cu § carbonyls cannot be fully discarded. On the other hand, bands appearing at 2113-2115 cm -~ can be attributed to Cu § carbonyls, on the basis of previous reports [13-15] or to metallic copper carbonyls adsorbed on open sites of the particles [13].
4. DISCUSSION As pointed out in the introduction, it is recognized that ceria can adopt at least two different configurations when dispersed on alumina [4, 5]; well-dispersed 2D-Ce patches, showing important interactions with the alumina support and relatively large 3D-Ce particles. Characterization of the former is difficult; it has been claimed that some small peaks in Raman spectra might be due to these species, but there are doubts in respect to their possible attribution to other species like transitional alumina phases [4]. In our Raman spectra it was not possible to discern unambiguously other peaks in addition to that of large ceria particles located at ca. 460 cm -~. This can be due either to the non-existence of well-defined Raman peaks for these highly dispersed ceria entities or to the fact that in our case relatively mild thermal pretreatments have been performed, leading to a poor crystallization of this superficial phase (so called CeA103 precursor [4]). For similarly prepared CeO2/A1203 samples, more reliable XPS results [4] point towards a predominance of 2D-Ce entities for ceria loadings lower than ca. 3/~mol/m2 CeO2/A1203 while larger 3D-Ce particles would predominate for higher loading; it is worth recalling that even for the lower ceria loading used in that work (0.58 ~mol/m2 CeOE/A1203) the presence of 3D-Ce entities was noticed [4]. In our case, 10CA is close to the breakpoint (3.2/~mol/m2 CeO2/A1203) while 39CA is well above this point and thus one would expect the predominance of large ceria particles for this latter. In this sense, both Raman and XRD evidence the presence of 3D-Ce particles which seem to increase both in amount and in size as ceria loading increases from 10% to 39%. On the other hand, EPR spectra after oxygen adsorption following a method developed in [5] show the formation of characteristic superoxide signals due to the presence of 2D-Ce entities in the catalysts based on aluminasupported ceria, which give a higher contribution for the lower ceria loading. In such situation, when copper is deposited on these supports one may expect the presence of at least three kinds of copper entities differing in their interactions with the underlying support and which may be designated as copper-alumina, copper-2D-Ce and copper-3D-Ce. The chemical behaviour of the first and the last of these entities can be assumed in principle to be close to that of copper supported on the pure supports. For the calcined samples, in the case of the CuA catalyst two different Cu E* entities have been identified on the EPR spectra differing in their dispersion degree, with a fraction of Cu remaining undetected. On CuC, the observable species is clearly different from those
599 detected on CuA; besides, an important part of the copper is not detected by EPR. For CuxCA samples, the spectra show important similarities with that of CuA, thus suggesting that a substantial part of the copper in these samples interacts with alumina; however, a lower amount of Cu 2+ is detected for these samples, approaching in this sense the behaviour of CuC; this points to an interaction of part of the copper in these samples with the ceria component. The non-detection by EPR of part of the copper present in the calcined samples may correspond to the formation of EPR-non-observable Cu 2+ species (due e.g. to diamagnetic coupling) or to the stabilization of reduced states of copper. Previous reports propose, on the basis of XPS results, that ceria can stabilize copper, already for preoxidized samples, as Cu + species [3]. The important increase in the amount of reduced copper carbonyls (giving a band at 2106-2098 cm-~) when ceria loading is increased, as shown in Fig. 5, suggests that this would be the case as well in our samples, even in the calcined state (this being then the reason for the lower EPR intensity); thus, formation of these carbonyl bands must be taken as indicative of the existence of direct copper-ceria contacts in these samples. Further evidences on the existence of these interactions are obtained by comparative analysis of the O2-Ce4+ species obtained for Cul0CA and the corresponding copper-free 10CA support, Figure 4. Thus, the relatively lower intensity of superoxide species at 2D-Ce entities for Cul0CA treated at T~<473 K can be interpreted in terms of the existence of copper-ceria contacts affecting to the corresponding adsorption positions, hindering their observation due either to electronic interactions or to blocking effects; the increase of these signals for higher T~ might be related to copper reduction with subsequent migration and sintering of metallic Cu particles [16]. Similarly, the absence of signal O1 for Cul0CA treated at T~=573 K indicates the existence of contacts between copper and 3D-Ce particles. The reactivity profiles shown by the samples suggest on their own that copper-ceria interactions lead to a large promotion of CO oxidation. The activity observed for CuA can be interpreted in terms of the formation of important amounts of more active [12] metallic copper by interaction of the catalyst with the reactant mixture, as evidenced by the FTIR experiment of Figure 6b, for a temperature close to 573 K. At lower temperature, the presence of less active dispersed Cu 2+ ions [17] or of smaller amounts of reduced copper entities leads to low activity levels for this sample. The important decrease of isoconversion temperatures observed already upon addition of a relatively small amount of ceria must be due to changes in the nature of the active centers and/or in the reaction mechanism involved. As discussed above, copper-ceria interactions stabilize reduced states of copper, which are known to be more active than Cu 2+ [12], thus implying a redox state-type promotion of copper activity. The fact that in the CuxCA samples an important fraction of copper seems to interact with alumina rather than with ceria indicates that a small amount of copper interacting with ceria can produce a large activity enhancement. It may then be considered whether, apart from the increase in the number of active Cu species, a specific mechanism involving both copper and ceria sites at interfacial regions (bifunctional promotion), as proposed for other M/CeO2/AI203 systems [18], operates in our catalysts. For example, ceria could intervene in activating and transforming 02 (possibly via Of radicals) into the reactive adsorbed oxygen species and/or in transferring it towards the CO-Cu adsorption complex. If it were so, the activity per site should be influenced by the chemical properties of the ceria component. This seems to be the case here: indeed the different slopes in the conversion plots indicate different intrinsic activities (with different activation energies)
600 depending on the kind of Cu-Ce active center involved. Recent results of this laboratory, to be published in more detail in a forthcoming paper, reinforce this model in which both kinds of promotion (redox state and bifunctional) are present in these systems. Overall, it can be presumed that the profile observed for Cu39CA, being closer to that displayed by CuC, reflects to a larger extent the behaviour of copper interacting with 3 D - C e particles, while copper interacting with 2D-Ce would be involved in a higher degree in the activity levels found for Cul0CA. Thus, these results suggest that both 3D-Ce and 2D-Ce entities could promote copper activity in these systems, the 3D-Ce species producing a higher effect. These differences in promoting efficiency are likely to be related to differences in the redox properties of both kinds of ceria entities [5-7]. In this respect, the easier oxidation by 02 shown (after vacuum reduction) in the case of 3D-Ce entities [5] could play an important role, taking into account the detrimental effect of oxygen on copper reactivity [12].
ACKNOWLEDGEMENTS Financial help from CICYT (project Nr. 94-9835-CO3-O2) and the Comunidad de Madrid (project Nr. 06M/085/96) is gratefully acknowledged.
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
17. 18.
P.R. Courty and A. Chauvel, Catal. Today, 26 (1996) 3. E.A. Shaw, T. Rayment, A.P. Walker and R.M. Lambert, Appl. Catal., 67 (1990) 151. a) W. Liu, A.F. Sarofim and M. Flytzani-Stephanopoulos, Chem. Eng. Sci., 49 (1995) 4871. b) W. Liu and M. Flytzani-Stephanopoulos, J. Catal., 153 (1995) 304. J.Z. Shyu, W.H. Weber and H.S. Gandhi, J. Phys. Chem., 92 (1988) 2561. J. Sofia, J.M. Coronado and J.C. Conesa, J. Chem. Soc. Faraday Trans., 92 (1996) 1619. C. Morterra, V. Bolis and G. Magnacca, J. Chem. Soc. Faraday Trans., 92 (1996) 1991. H. Cordatos, D. Ford and R.J. Gorte, J. Phys. Chem., 100 (1996) 18128. R. Catalufia. PhD Thesis. Universidad Polit6cnica de Madrid (1995). J. Sofia, J.C. Conesa, A. Martinez-Arias and J.M. Coronado, Sol. St. Ionics, 63-65 (1993) 755. P.A. Berger and J.F. Roth, J. Phys. Chem., 71 (1967) 4307. J. Sofia, A. Martfnez-Arias and J.C. Conesa, J. Chem. Soc. Faraday Trans., 91 (1995) 1669. G.G. Jemigan and G.A. Somorjai, J. Catal., 147 (1994) 567. P. Hollins, Surf. Sci. Rep., 16 (1992) 51. A.A. Davydov, Kinet. Katal., 26 (1985) 157. Y.A. Lokhov, V.A. Sadykov, S.F. Tikhov and V.V. Popovskii, Kinet. Katal., 26 (1985) 177. A. Martinez-Arias, R. Catalufia, J.C. Conesa and J. Sofia (in preparation). T-J. Huang, T-C. Yu and S-H. Chang, Appl. Catal., 52 (1989) 157. A. Trovarelli. Catal. Rev. Sci. Eng., 38 (1996) 439, and references therein.
CATALYSIS AND AUTOMOTIVEPOLLUTIONCONTROLIV Studies in Surface Science and Catalysis,Vol. 116 N. Kruse, A. Frennet and J.-M Bastin (Eds.) 91998Elsevier Science B.V. All rights reserved.
601
Some s u r f a c e c h e m i c a l f e a t u r e s of Pt catalysts supported on A!203 and CeO2/A1203 G. Magnacca, G. Cerrato and C. Morterra (*) Department of Chemistry IFM, University of Turin, via P. Giuria 7, I-10125 Turin, Italy
Pt catalysts supported on alumina and ceria-alumina have been investigated by the adsorption/eoadsorption of CO, 1-120, and 1-12. Gas-volumetric and FTIR spectroscopic data indicate that, on pure alumina-supported systems, multiple-vacancy sites exist that tend to be eliminated in the presence of eeria; this is attributed to a strong Pt/CeO2 interaction. In the presence of large amounts of CeO2, the catalysts are active in oxidation processes also in reducing conditions. If the alumina component of the support of Pt-containing catalysts undergoes the thermal ~/~/5,0-A1:O3 phase transition, all the adsorptive capacity of Pt is lost.
1. INTRODUCTION A three-way conversion catalyst is normally made up of several components [1], among which: (i) a metal component with good selectivity towards NO reduction, also in oxidizing conditions (Rh and Ir are known to possess this quality); (ii) a metal component with good hydrocarbon oxidation activity (Pt is normally employed for this goal); (iii) an oxygen-storage agent, capable of emending the activity of the catalyst in both oxidizing and reducing conditions. (In "lean" conditions, the agent should be oxidized and remove the eeeess oxygen from the gas stream, whereas in normal reducing conditions the agent should revert to a lower oxidation state and release oxygen). Usually the washcoat support of a three-way converter consists of an active transition alumina (either 7-A1203 or /5,0-A1203), and of a variable amount of eeria. The CeO2 component has been suggested to: (i) participate in the catalytic process as an oxygen-storage agent, due to the formation of reactive 02 radicals [2,3]; (ii) favour the dispersion of the active noble metal(s) through a sort of strong-metal-support-interaction [4]. (The interaction between Pt and the ceria moiety of the support has been revealed in the Raman spectrum through the formation of Pt-O-Ce bonds absorbing at 550 and 690 em~); (iii) act as a phase stabilizing agent for the active alumina support [5]. Actually, recent contributions have shown that the action of the CeO2 additive towards the alumina support does not modify the alumina phase transitions, but is beneficial with respect to the acidic properties of the transition alumina support [6-8]. Aim of the present contribution is the investigation of the surface chemical role played by the presence of CeO2 in some Pt catalysts supported on alumina, i.e., some model catalysts fairly similar to those actually working in three-way converters.
602 The surface chemical features of Pt-containing systems will be mainly tested following the IR spectral behaviour of CO adsorbed on the catalysts at ambient temperature (RT). The interaction of CO with reduced Pt (Pt ~ is a widely studied and long known subject: CO uptake normally yields two Vco bands of odd intensity, centred at ~2080 and ~1845 cm~, that have been attributed to linear (Pt-CO) carbonyls and bridged (Pt2CO) carbonyls, respectively (e.g., see [9], and references therein). CO frequencies like these, located far below the spectral position of the free CO gaseous molecule (2143 cm"l) are typical of surface carbonyl-like species in which both a-donation and a strong rt-backdonation are acting simultaneously. When the noble metal becomes oxidized (Pt"+), the frequency of linear carbonyls formed upon CO uptake increases (up to ~2120 cml), due to a decreased and possibly null contribution of the 7t-backdonation contribution. Also the ambient temperature adsorption of H2 will be used for the characterization of the surface chemical features of Pt-containing systems: in fact it has been long known that, of the various forms of adsorbed hydrogen, the strongly held one (i.e., the species irreversible at RT) corresponds to a dissociative adsorption, in which there is an H/Pt ratio of 1:1 [10]. The irreversible 1-12uptake is thus a suitable tool for the evaluation of the noble metal dispersion.
2. EXPERIMENTAL 2.1. Materials
Three starting Pt catalysts were prepared, containing each 1% wt Pt, and differing for the composition of the support that was, in the three cases, pure ~/-A1203 (A), CeO2/3,-AI203 in a 3:100 wt ratio (ACe3), and CeO2h/-Al203 in a 20:100 wt ratio (ACe20). The preparation of the supports has been described in detail elsewhere [6,7], and was carried out starting from pseudo-boehmite (Disperal, Condea Chemic) and, where required, titrated solutions of Ce nitrate. The suspensions were first dried at 353 K, and then oven fired at 773 K for 3 hrs in order to convert 3,-AIOOH into y-AI203. The deposition of Pt on the ~,-Al203-based supports was obtained by impregnation with dosed amounts of a diluted solution of H2PtCI6, followed by dessiccation at 353 K, a second oven calcination at 773 K, and reduction at 823 K in a 5% H~q2 stream. These catalysts are hereafter referred to as APt773, ACe3Pt773, and ACe20Pt773, respectively. Activation temperatures at Tact up to 1073 K were explored, in order to simulate the thermal conditions normally experienced by a working catalyst; the temperature (K) at which the samples were vacuum activated prior to adsorption studies and/or IR spectra recording are indicated in the text and figures as a numeral following the symbol of the catalyst. In order to simulate the most severe conditions sometimes experienced by a working catalyst, three high-temperature catalysts were prepared starting from the original ones by carrying out the second calcination at 1273 K, so that 7-A1203 is converted into 6,0-A1203. These systems are hereafter referred to as APt~273, ACe3Pt~273, and ACe20Ph273, respectively. 2.2. Methods
Physical, structural, and morphological characteristics of the various supports have been reported previously [6]. The corresponding features of the Pt-containing catalysts were determined by the usual means: (i) BET areas with a conventional all-glass gas-volumetric appartus, using N2 uptake at 77 K and the t-plot method [11]; (ii) crystal phases with XRD
603 data from a conventional Bragg-Brentano diffractometer (Philips PW 1050); (iii) electron micrographs with a High Resolution TEM microscope (Jeol JEM 2000 EX, 200 kV acceleration), as described previously [6]. XRD data indicate that: (i) also in the presence of Pt, the alumina phase transitions occur at the expected temperatures; (ii) at low CeO2 contents the diffractograms are basically those of the alumina support, whereas at high CeO2 contents the diffractogram of the CeO2 moiety becomes dominant; (iii) for firing temperatures up to 1073 K there is no evidence for the reflections due to Pt. This is indicative of a good dispersion of the metal on the ~,-Al203-based support, as confirmed by the HRTEM images (not shown for the sake of brevity) where only small and poorly defined dark dots indicate the presence of the noble metal component. H2 and CO adsorption quantitative data were obtained in the form of adsorption isotherms (at 298 K) with an all-glass gas-volumetric apparatus; the amounts of irreversibly adsorbed 1-12 and CO species were calculated as the difference between primary isotherms (overall uptake) and secondary isotherms (reversible uptake). IR spectra (2 cm 1 resolution) of adsorbed species were obtained at RT in a strictly in situ configuration, using an FTIR spectrometer (Bruker l13v) equipped with MCT cryodetector.
3. RESULTS AND DISCUSSION 3.1. BET areas. CO and H2 uptake Surface area data are summarized in Table 1. If compared with the data relative to the pure supports that were reported in a previous note [6], it is quite evident that in the ),-AI203based systems the presence of Pt and the treatments needed for its reduction do not affect much the textural features of the support. In particular, low amounts of well dispersed CeO2 [12] favour an increase of surface area, whereas high amounts of CeO2 induce a moderate decrease. When the transition to ~5,0-A1203 occurs, the presence of CeO2 and of Pt leads to a drastic decrease of surface area. Table 1 - BET data (m 2 g-l) sample
773
calcination temperature T (K) 1073
1273
APtT ACe3PtT ACe20Ph.
178 198 138
132 163 121
108 41 20
Table 2 reports some data relative to the irreversible adsorption of CO, H2, CO on preadsorbed 1-12(CO/H2), and H2 on preadsorbed CO (H2/CO) relative to typical Pt-containing catalysts, fired at 773 K and 1273 K respectively, and activated at Tact up to 1073 K. The data in table 2 deserve some comments: (i) When the alumina support component still belongs to the y-AI203 phase, increasing the activation temperature does not modify (or increases slightly) the adsorptive properties of APt catalysts, whereas decreases appreciably the adsorptive properties of ACenPt catalysts. This is a first indication of a sort of strongmetal-support-interaction, involving the CeO2 moiety. Also, on APt catalysts CO
604 preadsorption reduces slightly the adsorptive capacity towards H2 and viceversa, but on ACenPt catalysts H2 preadsorption does not reduce much the further adsorption of CO, whereas CO preadsorption inhibits completely the adsorption of H2. Table 2 - CO and H2 irreversible adsorption at 298 K (mol per nm 2)
sample
APt773 773 APt773 1073 ACe3Pt773 773 ACe3Pt773 1073 ACe20Pt773 773 ACe20Pt773 1073 ACe3PtI273 1073 ACe20Ptj273 1073
CO
1-12
0.080 0.095 0.075 0.058 0.080 0.060 0.0 0.0
0.035 0.050 0.040 0 025 0.040 0.025 0.0 0.0
CO/H2
H2[CO
0.075
0.035
_
0.050
0.0
_
0.058
0.0
_
_
As the dissociative adsorption of H2 requires the presence of multiple-vacancy sites (at least two vacancies on the same metal atom, or two adjoining vacancies on nearby atoms are needed), this is a further indication of a strong CeO2/Pt interaction that renders virtually null the probability of multiple-vacancy sites; (ii) When the y-AI203 ---~,0-A1203 phase transformation occurs in the presence of Pt, all the adsorptive capacity of ACenPt catalysts is lost. Although the surface area of the catalyst still remains appreciable (see Table 1), the transformation of the porous texture (dealt with in a previous work [6]) and the modified metal-support interaction "buries" all the available Pt, so that the catalysts become inactive.
3.2. FTIR adsorption data 9The adsorption of CO. Figure 1 shows the spectral features in the range 2150-1950 cm1 of CO adsorbed at RT on some Pt catalysts. Curve 1, relative to the system APt773300, indicates the presence of a weak band at ~2115 cm l , ascribable to CO uptake onto oxidized Pt (most likely Pt 2§ [13]), and a strong band due to Pt ~ centered at 2065 cm-~ with a shoulder at ~2085 cm -1. (In view of the low extinction coefficient of the vco mode of CO on Pf~+, the band at ~2115 cm -~ corresponds to an appreciable amount of oxidized Pt). After vacuum activation to 1073 K (curve 2), the spectrum of CO on APt7731073 presents only a sharp strong band at 2085 cm l. The component due to oxidized Pt has disappeared, and this is no surprise in view of the reducing effect of vacuum treatment and thermal desorption of CO. To confirm that the complex structure of the band due to CO/Pt ~ in curve 1 was due to the high hydration state of the starting catalyst, the system first treated at 1073 was rehydrated, evacuated at 300 K (APt1073300), and further contacted with CO. The spectra of curves 3 were so obtained, in which is even more evident than in curves 1 that, on a highly hydrated APt catalyst, CO yields two components, the one located at higher wavenumbers being formed first (i.e., at low CO coverages) and being ascribed to CO ligands not perturbed by the copresence of a nearby 1-120 ligand. Curves 4 and 5 are relative to CO adsorbed onto ACe3Pt773300 and ACe20Pt773300, respectively. It can be seen that the high v component due to oxidized Pt is scarce in the first case, and totally absent in the second: it is thus confirmed that the presence of CeO2 protects the metal from oxidation, also in oxidizing conditions. The broad band due
605 to CO adsorption onto Pt ~ sites in an hydrated system is in an intermediate position in the case of ACe3Pt773300, and is in the high v position (~2075 cm ~) in the case of ACe20Pt773300; moreover, the CO/Pt ~ band on the latter system becomes sharper but almost does not change in position further to activation to 1073 K (curve 6), confirming the hypothesis that, in the presence of CeO2, the presence of multiple-vacancy sites capable of chemisorbing two ligands (CO and H atoms) becomes unlikely.
2065 2085 / N ,
0.1 flat
2075
~-~-- 2066
6 ,
i_
, |
2100
l
~ i
~ J
2000 wavenun~
!
2100
I
,
I
...... I
2000
~n ~
Figure 1. CO adsorption onto: 1) APt773300; 2) APt7731073" 3) APt1073300(the sample of (2) was exposed to water vapour, and then outgassed at 300K); 4) ACe3Pt773 300;5) ACe20Pt773300; 6) ACe20Pt773 1073. [Pco = 1 Torr (broken lines) and 100 Torr (solid lines)] 9CO and water coadsorption. To confirm the interpretation proposed, small H20 doses were gradually adsorbed on APt7731073 and ACe20Pt7731073 samples that preadsorbed CO (figure 2). In the first sample (left-hand spectra), the first small 1-120 doses bring about a gradual decrease of the strong CO band originally present at ~2080 cm~ with parallel formation of another band at ~2055 cm "1 and an isobestic point in between. At higher H20 coverages, when a thorough rehydration of the support and the formation of a physisorbed H20 layer begin, the newly formed band (ascribed to mixed CO-H20 complexes) is gradually brodened and shifted to lower v (~ 2040 cml), due to the increased polarity of the surface layer. Unlike that, in the CeO2-containing system (right-hand spectra), the first H20 doses decrease slightly the intensity but do not modify the spectral position of the strong CO band at ~2075 cm~,
606 meaning that a partial ligand-displacement can occur but no multiple-vacancy sites are available to lodge a second ligand (1-I20). At high 1-120 coverages, an abundant spectral broadening and a large downward shift of the only CO band present are produced. The behaviour of the low CeO2-1oaded system ACe3Pt is somewhat intermediate (no spectra are shown: a tiny shoulder band still forms at ~2050 cm 1 with small H:O doses), though more similar to that of ACe20Pt: multiple-vacancy Pt sites are not eliminated yet by a 3% CeO2 component, but nearly so.
20 8 0 ---.,.~
,,,.--'- 2 0 5 7
~ i
0,
lk",
r,\
[
/,,2075
i
i.....-
l a.u i
, 0, 00,400,
2~
:
\
\
rl .., '
'!
2200
"
'
I'
...
'x
..:-
,.-
~ 0e~
~
-i )
'
]
,
' I
2000 2200 w a v e n u m b e t cm -~
'
"~
\
'". %
~-"
2000
Figure 2. Effect of dosing 1-t20 on CO irreversibly adsorbed on APt7731073 0eli-hand section) and ACe20Pt7731073 (right-hand section). Inset: changes in the 6Hon region, as a funcion of H20 coverage. [Dotted lines: pure CO uptake; solid lines: small H20 doses added; broken lines: 12 Torr 1-120added] 9CO/H2 and H~/CO coadsorption. The presence/absence of multiple-vacancy Pt sites in the absencJpresence of CeO2 in the support has been also checked through the competitive adsorption of other probe molecules, among which H2 (normally used to test the dispersion of supported Pt) and CO. Some quantitative aspects of the competitive adsorption of these probes was reported in table 2; here the spectroscopic aspects of their coadsorption will be dealt with. Figure 3 reports the spectra of CO adsorbed at RT on preadsorbed H2 (The appearance of weak bands due to Pt hydrides formed upon H2 uptake has been reported in the literature [14], but in the present case no direct spectroscopic evidence was ever obtained for the dissociative adsorption of H2). On APt7731073 (top spectra), at low CO coverages a band forms at-~2085 cm 1 (i.e., in the same position of the strong CO band that would form on a bare surface), whereas at high coverages a second band forms at lower v (~2075 cm1) indicative of a weak perturbation between CO and H ligands on multiple-vacancy sites. In the case of CO adsorbed on ACe3Pt7731073 (middle curves) and on ACe20Pt7731073 (bottom curves) that preadsorbed H2, only the band at ~2080 cm -~ due to single CO oscillators is
607 formed with unchanged frequency and slightly decreased intensity, consistent with the quantitative data reported in table 2. This means that the sites that on CeO2-containing systems are still capable of dissociating and chemisorbing 1-I2cannot host a second CO ligand. (Note that the preadsorption of H2 has no effect at all on both position and intensity of the CO bands formed, at high CO coverages, on coordinatively unsaturated AI3+ (~2210 cm"l) and C e 4~- sites (~2185 cm "1) of the support [7]).
2085J~'~x'2075
0.2
2085
a.u.
~ ' ~ ' " 2078 2185 ~ ~k
2210
0.2
.i 2078
.....
II
............. :
.
2200 waven~
-
-
|
. . . . . . . . . . . . . . . . . . . . .
2(D3 crn ~
Figure 3. Effect of CO uptake onto 1-I2 preadsorbed on APt7731073 (1), ACe3Pt7731073 (2), and ACe20Pt7731073 (3). [Dotted lines: pure CO uptake; broken lines: 1 Torr CO added on I-I.2;solid lines: 100 Tort CO added on 1-12].
!
t
t_
'
l !
.
.
.
.
.
.
.
.
.
.
.
;
.
.
. .
.
.
.
.
I I
_
~_I
2100 2000 w a v e n ~ crn~ Figure 4. Effect of H2uptake onto CO preadsorbed irreversibly on J~ket7731073 (1), ACe3Pt7731073 (2), and ACe20Ptvv31073 (3). [Dotted lines: irreversible CO uptake; solid lines: 100 Torr Hz added].
Figure 4 reports the bands of CO that remains adsorbed after RT evacuation of the eccess CO (and thus also of CO reversibly adsorbed on AI3§ and Ce 4§ sites) on the same samples dealt with in fig. 3, and the bands of CO observed after the subsequent adsorption of H2. It can be seen that, on the APt catalyst (top curves), H2 adsorption leads to the formation of the usual twin CO band at lower v (~2070 cm"I) ascribed to CO in mixed two-|igands (COH) surface complexes, whereas in the case of ACenPt catalysts (middle and bottom curves) almost no perturbation or no perturbation at all is produced on the CO band, meaning that CO
608 preadsorption blocked nearly all or all of the sites at which H2 can dissociate so that no appreciable 1-12uptake occurs, consistent with the volumetric data of table 2.
3.3. Spectral evidence for the redox properties of Ce02 The easy formation of active oxygen species at the surface of CeO2-containing systems has been reported recently [15], and the interaction between active oxygen species and supported Pt will be dealt with elsewhere. In this contribution it will be shown that, in the presence of an abundant (tridimensional) CeO2 phase [6,7,12], ACePt catalysts possess oxidizing properties also when operating in reducing conditions. Figure 5 shows the effect produced on the CO species adsorbed irreversibly on the catalysts dealt with in fig. 3 and 4, when the systems are heated in a static vacuum (heating in vacuo a system containing adsorbed CO is a typically reducing condition).
2076 0.1 a.tl.
00
21 O0
1950
21 O0 1950 wavenumber cm ~
21 O0
1950
Figure 5. Effect of heating treatments in a static vacuum on CO adsorbed on Pt catalysts. Set A: APt7731073, heated (from left to right) at 373, 473, 573 and 773K, respectively; set B: ACe3Pt7731073, heated (from left to right) at 373, 473, 573 and 773K, respectively; set C: ACe20Pt7731073, heated (from top to bottom) at 373, 473, 573 and 773K, respectively. Inset: band of CO2 formed on ACe20Pt7731073 upon CO oxidation. The lett-hand curves, relative to the APt7731073 system, and the middle curves, relative to the ACe3Pt7731073 system, indicate that heating at increasing temperatures broadens and shifts downwards the band due to single CO oscillators bonded to Pt ~ sites (from ~2075 cm 1 to ~2060 cm-1). This effect has been observed previously and explained [9] as due to the gradual migration of CO molecules, originally adsorbed in islands on the Pt particles as a consequence of the high sticking probability of CO on Pt ~ toward well separated sites so
609 that the mutual interactions are gradually reduced to zero (the lowest v reached would thus correspond to the frequency of the singleton Pt-CO oscillator). Unlike that, in the case of the ACe20Pt7731073 catalyst (fight-hand curves) heating the isolated system brings about the gradual oxidation of CO to CO2 (see in the inset the band at ~23 55 cm -1 due to the E4u mode of CO2 coordinated to surface cationic sites of the support). This confirms that the CeO2-rich catalyst can act as an oxygen-storage agent, and release active oxygen when operating in reducing conditions.
4. CONCLUSIONS It was observed that, when supported Pt catalysts reach temperatures as high as -1273 K, the phase transition occurring in the major support component (~/- -~ ~5,0-A1203) has lethal effects on the adsorptive properties of the supported noble metal. It was also observed that, when a supported Pt catalyst reaches temperatures higher than those at which the catalyst was first fired and/or reduced, but still lower than those needed for the y- --~ ~5,0-A1203 phase transition, the Pt ~ adsorptive capacity of pure-alumina-supported catalysts is somewhat increased, whereas the capacity of ceria-containing catalysts is appreciably decreased. This effect was ascribed to an increased strong interaction between Pt particles and the ceria component of the support. The combined use of gas-volumetric adsorption measurements and of IR spectroscopy of adsorbed species indicated that, on pure-alumina-supported Pt catalysts, a large number and variety of Pt~ sites exist. These are characterized by the frequent presence of multiplevacancies, so that the coadsorption of various probes becomes possible, and the preadsorption of some probes does not reduce much the adsorbing capacity towards other probes. The mentioned situation was found to be no longer true in the case of ceria-containing Pt catalysts. In fact in the presence of ceria, multiple-vacancy sites tend to be eliminated by the strong interaction of the support CeO2 component with the noble metal and, as a consequence, the coadsorption of different probe molecules tends to be no longer possible. In particular, CO adsorption on Pt~ in CeO2-containing catalysts prevents entirely the possibility for these catalysts to dissociate 1-12.It is thus concluded that the presence of CeO2, that is highly beneficial for the dispersion of the noble metal and in preventing losses by metal oxydation/volatilization [ 16], reduces appreciably the adsorptive potentiality of the supported noble metal. Finally it was observed that, in the presence of large amounts of CeO2 (i.e., when a real CeO: phase starts being present in the supported Pt system), oxidation of Pt becomes difficult also in oxidizing conditions, whereas the catalyst exhibits good oxidizing properties also when operating in reducing conditions. These are significant aspects of the oxygenstorage activity played by the CeO2 component in CeO2-containing Pt-AI203 catalysts.
610 REFERENCES
1. H.S. Gandhi, A.G. Piken and M. Shelef, SAE Technical Paper Series, N. 760201 2. H.S. Gandhi and M. Shelef, in Catalysis and Automotive Pollution Control, A. Crucq and A. Frennet Eds., Elsevier (Amsterdam) 1987, p. 199. 3. J.Z. Shyu, K. Otto, W.L.H. Watkins, H.S. Gandhi, G.W. Graham and R.K. Belitz, J. Catal, 114,23(1988). 4. M.S. Brogan, T.J. Dines and A.J. Cairns, J. Chem. Soc. Faraday Trans., 90, 1461 (1994). 5. J.G. Nunan, H.J. Robota, M.J. Cohn and S.A. Bradley, in Catalysis and Automotive Pollution Control II, A. Crucq and A.Ed., Elsevier (Amsterdam) 1991, p. 221. 6. C. Morterra, G. Magnacca, V. Bolis, G. Cerrato, M. Baricco, A. Giachello and MFucale, in Catalysis and Automotive Pollution Control III, Studies in Surface Science and Catalysis, vol. 96, A. Frennet and J.-M. Bastin Eds., Elsevier (Amsterdam) 1995, p. 361. 7. C. Morterra, V. Bolis and G. Magnacca, J. Chem. Soc. Faraday Trans., 92, 1991 (1996). 8. C. Morterra and G. Magnacca, J. Chem. Soc. Faraday Trans., 92, 5111 (1996). 9. M. Primet, J. Catal., 88, 273 (1984). 10. P.G. Sminiotis and E. Ruckenstein, J. Catal., 140, 526 (1993). 11. S.J. Gregg and K.S.W. Sing, Adsorption, Surface Area and Porosity, 2nd ed., Academic Press, London, 1982, p.94. 12. J.Z. Shyu, W.H. Weber and H.S. Gandhi, J. Phys. Chem., 92, 4964 (1988). 13. L. Marchese, M.-R. Boccuti, S. Coluccia, S. Lavagnino, A. Zecchina, A. Bonneviot and M. Che, in Structure and Reactivity of Surfaces, Studies in Surface Science and Catalysis, vol. 48, C. Morterra, A. Zecchina and G. Costa Eds, Elsevier (Amsterdam) 1988, p. 653. 14. T. Szilagyi, J. Catal., 121,223 (1990). 15. J. Sofia, A. Martinez-Arias and J.C. Conesa, J. Chem. Soc. Faraday Trans., 91, 1669 (1995); J. Sofia, A. Martinez-Arias, J.M. Coronado and J.C. Conesa, Coll. Surf. A, 115, 215 (1996). 16. J.M. Schwartz and L.D. Schmidt, J. Catal., 138, 283 (1992).
CATALYSISAND AUTOMOTIVEPOLLUTIONCONTROLIV Studies in SurfaceScienceand Catalysis,Vol. 116 N. Kruse,A. Frennetand J.-M Bastin(Eds.) 91998ElsevierScienceB.V. All rightsreserved.
611
Fundamental properties of a new cerium-based mixed oxide alternative as T W C component S. Bernal*, G. Blanco, M.A. Cauqui, P. Corchado, J.M. Pintado, J.M. Rodriguez-Izquierdo and H. Vidal Departamento de Ciencia de los Materiales e Ingenieria Metaldrgica y Quimica Inorgfinica. Universidad de Cfidiz. Puerto Real. 11510. SPAIN.
ABSTRACT This work reports on the redox behaviour of a CeTbOx mixed oxide. As revealed by TPR and TPD experiments, the oxide, with 80% Ce-20% Tb atomic composition, exhibits interesting properties for TWC applications. It is shown that its redox behaviour is much less sensitive than that of ceria to successive high temperature reduction/reoxidation cycles, thus indicating that the inherent sintering effect does not imply a drastic loss of chemical activity. Upon comparison of the TPR diagrams for CeTbOx and CeO2, it is shown that their redox properties are rather complementary, the Tb containing mixed oxide allowing the aim of lowering the operative temperature window. This may improve the TWC response under cold start operation conditions. Our Cefrb mixed oxide also shows interesting redox properties in a flow of both inert gas and O2(5%)/He.
INTRODUCTION The development of the next TWC generation, the one fitting the future more restrictive regulations on exhaust emissions from vehicles, is demanding new materials with better adapted OSC (Oxygen Storage Capacity) properties. Improvements are required not only in their quantitative OSC capability, but also in the temperature range at which they are effective. Accordingly, the investigation of new oxygen buffering systems allowing to diversify t h e current offer of available materials is highly desirable. In this way, it would become easier to develop oxide blends fitting more closely the specific requirements of different types of automobiles, motor designers, or exhaust regulations. Ceria based mixed oxides have proved to be interesting candidates [1-14] to enlarge the panoply of available OSC materials. In this work we report on the * Corresponding author, e-mail address: [email protected]
612 redox behaviour of some new terbia-containing mixed oxides. Their general properties will be compared to those of the classic TWC redox promoter: CeO2. As will be shown here, CeTbO~ and CeO~ exhibit rather complementary redox behaviours, thus widening very significantly the temperature window at which effective redox operation would be achieved.
EXPERIMENTAL The samples studied herein are a 99.9% pure cerium dioxide (sample C) prepared as described elsewhere [15,16], a pure terbia showing a composition T b O ~ (sample T), and a ceria/terbia mixed oxide with 20 mole percent of terbia (CT-80/20), from our laboratory. In this latter case, the nominal chemical composition was further confirmed by ICP analysis of a series of previously dissolved oxide samples. The preparation of the CT-80/20 sample was carried out by precipitation with ammonia from an aqueous solution with the proper concentrations of Ce and Tb nitrates. The precipitate was washed, dried in an oven overnight at 373 K and calcined in air at 873 K for 2h. Before the different types of experiments, the samples were always pretreated '~n situ" for 1 hour in a flow of Ch(5%)/He (60 craB-rain9, and cooled slowly under the oxidizing mixture up to 398 K. At this temperature, the gas flow was switr~ed to He, the oxides being fitaher cooled to 298 K under flowing inert gas. This standard pretreatment guarantees a common well defined starting redox state throughout our whole study. Following this pretreatment the surface concentration of hydroxyl groups is negligible in comparison with the amounts of water evolved on fitr~er reduction treatments. The TPR-MS and TPD-MS experiments were performed with a VG Sensorlab 200D Mass Spectrometer as analytical device. The gas flow rates of either I-I2(5%)/Ar (TPR) or He (TPD) were always 60 cmS-min1, and the heating rate 10 K-rain-1. We have also run TPO-like experiments in a flow (60 cm3-min-9 of (:h(5%)/He. In this latter case, the analytical tool consisted of a TCD detector; the heating rate was 10 K-rain"~. The X-ray Diffraction (XRD) p a ~ were obtained on a Philips, PW 1820, diifractomete~, CuIQ radiation and a Ni filter were user The BET specific sur~ce area of the samples was determined by N2 adsorption at 77 I~ Except for terbia (7 m2.gg, the S~r values for the stmcdng oxygen-pretreated samples, h e m a ~ r referred to as HS (High Surface area) oxides, were rather close to each other.. C: 51 m2.g'; and CT-80/20:47 m2.g1. The BET surface areas for the samples submitted to several suc~ssive high temperature (1223 K) reduction/r~xidation cycles, h e m a ~ r referred to as "stabilisecr', LS (Low S ~ area) samples, were always well below 5 m2.g'. The EDX m_icroanalytical studies were performed on a I2nk AN10000 analyser a ~ e d to a JFA)L 820-SM Scanning Electron Microscope. The space analylic~ resolution was 1 ~m. The 'light aft' curves for methane oxidation were recorded in a flow u-shaped reactor. The amount af oxide catalyst was always 100 rag. The stoichiometric reaction mixture was prepared with the help of mass flow controllers; it consisted of: 30 cm3-min '' CH4(5%)/He and 60 cm3-min -' of 02 (5%) 02/He. The space velocity was therefore: 85,000 h".
613 3. RESULTS AND DISCUSSION In accordance with the XRD study, the HS O2(5%)/He pretreated samples show the cubic fluorite-like structure, with the following lattice parameters: C: 0.5411 nm; T: 0.5220 nm; and CT-80/20:0.5369 nm. As already shown for several other ceria-based mixed oxides [2,3,5,14], the TPR-MS study of the oxide samples investigated here provides a first indication of the promoting effect of the added aliocation (La, Gd, Zr,...) on the ....... i" i " 'i" ' i reducibility of ceria. In these studies, the behaviour of pure ceria will be used as a reference helping us to evaluate the potential advantages and the complementariness of the redox properties of the mixed oxide. Fig. 1.a shows the TPR-MS trace for water E (m/e: 18) corresponding to the ceria C-HS sample. It is in good agreement with those r-reported elsewhere [14,15,17,18]. The water ._~ released from 773 up to 973K is interpreted as cO due to the surface reduction process, whereas the second peak, the one featuring at 1100K being related to the ceria bulk reduction. Fig I I I I , 1.b accounts for the behaviour of ceria after 473 673 873 1073 successive (up to 4) reduction (1223 Temperature (K) K)/reoxidation (standard 02 pretreatment) Figure 1.- Water traces corresponding to cycles. The major difference between these two successive TPR-MS experiments on pure ceria, a) first run, b) second and I i I I successive runs. traces consists on the disappearance of the feature associated to the surface reduction d process in Fig. lb. This effect has been interpreted as due to sintering induced on the starting ceria sample (C-HS) by the first reduction treatment at 1223 K [ 16,19,20]. Fig 2 shows the TPR-MS diagrams for the oxygen-pretreated CT-80/20-HS (a) and CTco 80/20-LS (b) samples. As deduced from them, the evolution of water starts in both cases at a temperature as low as 600K, taking place I I I I throughout two broad overlapping peaks. It 473 673 873 1073 would also be noticed that, compared to HS Temperature (K) and LS ceria samples, the TPR-MS traces for the corresponding CT-80/20 oxides are much Figure 2.- Water traces corresponding to closer, the main differences between them successive TPR-MS experiments on CTconsisting of the disappearance of a weak 80/20. a) first run, b) second and feature at 500K as well as the lower intensity of successive runs v
9 o
o,.=.
614 the peak appearing in the 600 K - 750 K range observed on the CT-80/20-LS diagram. Figure 3 depicts the TPR-MS profiles for the stabilised samples: a) C-LS, b) CT-80/20-LS, and c) T-LS. These stabilised samples were submitted to several TPR-reoxidation cycles, showing a reproducible behaviour after the second one. Accordingly, the traces in Fig. 3 can be considered as representative of oxides aged under severe thermal/chemical conditions. As references for the behaviour of our CT mixed oxide, pure ceria and terbia have been included in the same figure. The analysis of Fig. 3 shows that the main TPR peak for CT-80/20 cover a temperature range rather complementary to pure ceria. Thus, it suggests that proper combinations of these oxides would allow to broaden the temperature window for effective redox operation of TWCs very significantly. Upon integration of the TPR water traces in Fig 3, it is possible to estimate the amount of water evolved from C-LS and CT-80/20-LS. These samples were kept in flowing H2(5%)/Ar for 2 h, at 1223K. The total amounts of evolved H20 was the same in both cases, being equivalent to 0.1 mole of 02 per mole of oxide. In the case of pure terbia, the equivalent released oxygen was 0.16 moles per mole of oxide. There is, however, a drawback: the process takes place in a rather narrow temperature range.
(J
E t-
._o) (/)
L__J A I
I
I
I
473
673
873
1073
Temperature (K)
Figure 3.- TPR-MS reducibility studies on the stabilised samples: a) C-LS, b) CT-80/20-LS, and c) T. Traces for water (m/e: 18) after a first cycle of reduction followed by reoxidation. The diagrams are reproducible upon successive reduction-reoxidation cycles.
615 The TPD-MS studies provide some further pieces of information about redox properties of interest for TWC applications. Figure 4 reports on the 02 (m/e: 32) traces for: a) C-HS, b) CT-80/20-LS, c) CT-80/20HS, and d) T-LS. In accordance with Fig. 4, under flowing He, the evolution of oxygen Z~ from ceria is almost negligeable. This is so, v in spite of showing the TPD-MS trace for C(b) ) HS samples. In good agreement with some r earlier studies [2], terbia reduction takes E place throughout two well defined steps characterised by rather narrow peaks at 700 to~ K and 973 K. The behaviours exhibited by O9 the mixed oxide are again worth of outlining, O9 because of the ability of the CT-80/20 samples to release oxygen, at moderate temperatures, under very mild reducing conditions. The oxygen loss of the CT-80/20-LS samples, as determined by integration of the (d) . . . . . J _ J L__ corresponding TPD-MS trace, is equivalent to 0.05 moles of 02 per mole of the oxide. I I I I Compared to CT-80/20-LS, the TPD 473 673 873 1073 diagram for CT-80/20-HS, Fig. 4c, shows Temperature (K) that 02 evolution starts at even lower temperature, 450K. This suggests that, Figure 4.- TPD-MS study of 02 (m/e:32) though not having a significant effect on the evolution from: a)C-HS, b)CT-80/20-LS, total amount of evolved 02, the surface area c) CT-80/20-HS, d) T. The oxygen is of the CT mixed oxides does influence on traced through the signals corresponding the rate of oxygen evolution at the low to m/e ratio 32. temperature range. Consequently, it would be desirable to develop preparation procedures leading to CT mixed oxides with improved textural properties. The redox behaviour of our CT-80/20-LS under 02 pressure (40 Torr) has also been checked. The experiment consisted of tracking the signal for 02 upon heating the oxide from 773 K to 873 K in a flow of O2(5%)/He (60 cm3.min 1 ). In accordance with Figure 5, oxygen evolution does occur, being also worth of noting the fast response of the oxide to the temperature changes. This implies that the oxygen content of the CT-LS sample rapidly reaches the equilibrium composition determined by both the temperature and the oxygen partial pressure.
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823 "P,.i,, c-
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798
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773 0
5
10
15
20
25
30
Time (rain)
Figure 5.- Oxygen evolution upon programmed heating of the CT-80/20-LS sample in a flow of 5%O2/He. a) TCD signal for 02, b) thermocouple, and c) derivative of the temperature with respect to the time. In effect, Fig. 5 shows an excellent fitting between the curves accounting for the oxygen signal (a), and the derivative of the temperature with respect to the time (c). In relation to Fig. 5 there are two observations to be outlined: 1) the inflexion point occurring at 10 minutes heating is coincident with the maximum concentration of oxygen evolved from the sample; and 2) the slight overheating (less than 3 K) occurring at that maximum temperature setting (t: 20-25 min), is followed by the oxide as revealed by the minimum observed in the TCD trace for 02. This behaviour of our CT is remarkable, suggesting very interesting properties as oxygen buffer in a domain of temperatures relevant for TWC applications. One of the questions arising when these oxides are submitted to severe thermal treatments, is the likely occurrence of phase segregation. To check this particular point, we have carried out EDX microanalysis studies on both HS and LS CT mixed oxides. Ten randomly selected areas of the two samples mentioned above were analysed. The results are reported in Fig. 6.
100
t'b;
80 c-
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0 Tb
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,-- 40 13.. 20
0000000000 I
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Figure 6.- Results of the EDX microanalysis in 10 different points of the CT-80/20-HS fresh sample, (a), and CT-80/20-LS stabilised sample (b). Atomic percentages of Ce and Tb.
617 As a comment of that, we conclude that phase segregation is not apparent. In good agreement with the XRD studies, we conclude form Fig. 6 that phase segregation does not seem to occur. It would be noted, however, that the intrinsic resolution of the microanalytical essays (1 micron) does not permit to conclusively ruling out such hypothesis. Regarding the catalytic essays, the total oxidation of methane has been studied. The "lightoff" temperatures associated to 50% conversion of methane on the C-LS, CT-80/20-LS and TLS were found to be 778 K, 667 K and 692 K, respectively. This result, also in agreement with those reported above, gives us a further indication of the synergistic improvement of performance achieved in the mixed oxide with reference to that exhibited by the ceria and terbia mixed oxides. In accordance with the ensemble of results discussed above, the incorporation of terbium cations into the fluorite-like structure of ceria would promote, at moderate temperatures, the creation of oxygen vacancies, probably associated to Tb 3+ ions [21]. Compared to terbia, however, the content of anion vacancies would be smaller, thus preventing the strong interactions among them and subsequent loss of mobility [22,23]. In addition to this, the lattice parameter of the mixed oxide is shortened with reference to that of ceria, the lattice shrinking being obviously much lower than that for pure terbia. Both structural and electronic factors, the later probably related to the higher reducibility of the Tb3§ 4+ couple, and to the intimate Ce/Tb contact in the mixed oxide, might well be relevant to interpret the likely origin of the Ce/Tb synergetic effects observed in the mixed CT oxide. 4. CONCLUSIONS We have reported on a new ceria based mixed oxide containing 20 mole % of terbia. It has been shown that it exhibits interesting redox properties for TWC formulations. The reducibility of CT-80/20 is much higher than that shown by pure ceria and pure terbia. The temperature window in which the reduction process of CT-80/20 takes place is very broad, with a maximum oxygen release at lower temperature than that determined for the other single oxide samples, used as references. Reduction is not only observed in a flow of Hz(5%)/Ar, but also in flowing inert gas, at moderate temperatures. The redox behaviour of our ceria-terbia mixed oxide, though sensitive to the high temperature reduction/reoxidation cycling, is not strongly disturbed by this ageing treatment. All these observations allow us to conclude that this new mixed oxide has interesting oxygen buffer properties, complementary with those exhibited by ceria in a higher temperature range. In accordance with its singularities, the CT mixed oxides can be considered as promising components for new TWCs with improved performance, specially under low temperature operation conditions like those occurring during the cold start of engines. ACKNOWLEDGEMENTS. The authors thank the financial support received from CICYT (Project MAT95-960931) and DGICYT (Projects PB94-1305 and PB95-1257).
618 REFERENCES
1.
S. Bemal, G. Blanco, F.J. Botana, J.M. Gatica, J.A. P6rez Omil, J.M. Pintado, J.M. Rodriguez-Izquierdo, P. Maestro, and J.J. Braconnier, J. Alloys Comp, 207/208 (1994) 196 2. S. Bernal, G. Blanco, G.A. Cifredo, J.A. P6rez Omil, J.M. Pintado, and J.M. RodriguezIzquierdo, J. Alloys Comp., (1997) 3. A.D. Logan and M. Shelef, J. Mater. Res., 9 (1994) 468 4. S. Torng, K. Miyazawa, and T. Sakuma, Mater. Sci. Tech., 11 (1995) 130 5. G. Balducci, P. Fornasiero, R. di Monte, J. Kaspar, S. Meriani, S., and M. Graziani, Catal. Lett., 33 (1995) 193 6. P. Fornasiero, R. di Monte, G.R. Rao, J. Kaspar, S. Meriani, A. Trovarelli, and M. Graziani, J. Catal., 151 (1995) 168 7. G.R. Rao, J. Kaspar, S. Meriani, R. di Monte, and M. Graziani, Catal. Lett., 24 (1994) 107 8. C. de Leitenburg, A. Trovarelli, F. Zamar, S. Maschio, G. Dolcetti, and J. Llorca, J. Chem. Soc., Chem. Commun., (1995) 2181 9. F. Zamar, A. Trovarelli, C. de Leitenburg and G. Dolcetti, J. Chem. Soc., Chem. Commun., (1995) 965 10. B.K. Cho, J Catal, 131 (1991) 74 11. Y. Zhang, S. Andersson and M. Muhammed, Appl. Catal. B:Environmental, 6 (1995) 325 12. F. Zamar, A. Trovarelli, C. de Leitenburg and G. Dolcetti, Stud. Surf. Sci. Catal., 101 (1996) 1283 13. J. Cuif, G. Blanchard, O. Touret, M. Marczi and E. Qu6m6r6, E. SAE paper 96/906, 73 (1996) 14. A. Trovarelli, Catal. Rev. Sci. Eng., 38 (1996) 439 15. S. Bernal, J.J. Calvino, G.A. Cifredo, J.M. Gatica, J.A. P6rez Omil and J.M. Pintado, J. Chem. Soc. Faraday. Trans., 89 (1993) 3499 16. S. Bernal, F.J. Botana, J.J. Calvino, M.A. Cauqui, G.A. Cifredo, A. Jobacho, J.M. Pintado and J.M. Rodriguez-Izquierdo, J. Phys. Chem., 97 (1993) 4118 17. V. Perrichon, A. Laachir, G. Bergeret, R. Frety, L. Tournayan and O. Touret, J. Chem. Soc. Faraday Trans., 90 (1994) 773 18. A. Laachir, V. Perrichon, A. Badri, J. Lamotte, E. Catherine, J.C. Lavalley, J. E1 Fallah, L. Hilaire, F. Normand, E. Qu6m6r6, G.N. Sauvion, G.N and O. Touret, J. Chem. Soc. Faraday Trans., 87 (1991) 1601 19. S. Bernal, F.J. Botana, R. Garcia, Z.C. Kang, M.L. L6pez, F. Ramirez and J.M. Rodriguez-Izquierdo, Catal. Today, 2 (1988)653 20. S. Bernal, F.J. Botana, R. Garcia, F. Ramirez and J.M. Rodriguez-Izquierdo, Mater. Chem. and Phys., 18 (1987) 119 21. M.Y. Sinev, G.W. Graham, L.P. Haack and M. Shelef, 1 lth International Congress on Catalysis, Baltimore, (1996) Po- 12 22. H. Yahiro, K. Eguchi and H. Arai, Solid State Ionics, 36 (1989) 71 23. H. Inaba and H. Tagawa, Solid State Ionics, 83 (1996) 1
Diesel
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CATALYSIS AND AUTOMOTIVEPOLLUTIONCONTROLIV Studies in Surface Science and Catalysis, Vol. 116 N. Kruse, A. Frennetand J.-M Bastin (Eds.) 91998 Elsevier Science B.V. All rights reserved.
621
I m p r o v e d s o o t o x i d a t i o n by fuel a d d i t i v e s and m o l t e n salt catalysts S.J. Jelles, J.P.A. Neeft, B.A.A.L. van Setten, M. Makkee, and J.A. Moulijn Delft University of Technology, Section Industrial Catalysis, Julianalaan 136, 2628 BL, Delft, The Netherlands The removal of soot from diesel exhaust gas is preferably done catalytically. Fuel additives and supported molten salts are promising catalyst for this application. NOx in the exhaust gas can be used to increase the soot oxidation rate. 1.
INTRODUCTION
The reduction of emissions by diesel engines is a topic of great interest. On one hand diesel engines are fuel efficient and reliable, on the other hand the soot and NOx emissions of diesel engines are high, compared to those of gasoline engines equipped with a three way catalyst. For the abatement of NO• emissions, the selective reduction with NH3 or urea is in a commercial stage for stationary sources. For the removal of soot, the solutions are not yet ready for large scale implementation. Several possible systems for soot removal are currently under investigation. Basically, there are two main techniques of soot emission reduction. The first technique is the application of a 'flow through catalyst', a monolith that does not capture the soot, but where part of the particulate matter is burnt during the short pass time through the monolith. In practice, these catalysts can achieve a reduction of particulate mass up to 50% [ 1], mostly as a result of the combustion of the organic fraction adsorbed on the soot. The number of particulates are less affected. It can, therefore, be considered as an intermediate solution. Total removal of particulates should be the ultimate goal. The second technique is based on a filter to capture the soot particulates. Common filters are wall flow monoliths or ceramic foams. Cordierite wall flow monoliths are probably currently the most used particulate traps. They can capture diesel particulates with an efficiency of 99%. At normal diesel engine exhaust gas temperatures, the captured soot is not reactive enough to prevent build up on the filter, with an intolerable high pressure drop over the exhaust system as a result. The oxidation rate of the soot should, therefore, be increased which can be achieved by increasing the temperature of the filter, resulting in higher fuel consumption and thus making this solution unfavourable. The other possibility is catalytic oxidation of the collected soot. Several catalytic systems will be discussed. 2. CATALYTIC SOOT OXIDATION
2.1. The contact between soot and catalyst From earlier work, it can be concluded that the contact between soot and catalyst is of great importance on the oxidation rate that can be achieved with any catalyst [1, 2]. When there is no good contact between soot and catalyst, the oxidation rate is low. The contact between the
622 catalyst on a coated monolithic filter and the soot collected on this filter is so called 'loose contact', and this is comparable with the contact between soot and catalyst powder when they are mixed with a spatula. This contact is not sufficient for a high enough oxidation rate. Better contact between soot and catalyst is obtained when the catalyst is applied in the form of a fuel additive, or when the catalyst has wetting properties.
2.2. The application of catalytic fuel additives The high soot oxidation activity that can be obtained with fuel additives can be explained as follows. The fuel additive contains a metal, in the form of a soluble organo-metallic compound. This compound decomposes in the engine combustion chamber and the metallic part is integrated in the soot, from the moment of nucleation, while the organic part is combusted. The metal is finely dispersed in the soot. Transmission electron microscopy proved that copper particles in soot collected from an engine running with 100 ppm copper in the fuel were smaller than 1 nm, while the soot particles were 30 - 40 nm in diameter [3]. Obviously, the almost atomic distribution of the metal through the soot creates a large interaction surface between carbon matter and catalyst and the result is a very high oxidation rate compared to catalytic coatings of the same metal. Besides copper fuel additives in combination with wall flow monoliths, the influence of a copper coating on the performance of the system has been extensively studied. It was found that the catalytic activity of the copper coating is negligible compared to the activity of the copper additive. Nevertheless a copper coating might be useful. It was found that the thermal conductivity of the trap material can be of great influence on the stability of the system. High thermal conductivity of the trap, for example of silicon carbide, contributes to stable operation. Regenerations of the trap, during which all the soot collected on the trap is burnt, occur without extreme temperature excursions, whereas with cordierite traps temperatures up to 1200 K can be observed. Continuous operation of a system with a copper fuel additive and a wall flow monolith is feasible at temperatures above 625 K.
2.3. The application of molten salt catalysts Molten eutectic salt mixtures have been reported to be active catalysts in graphite and coal char gasification [4, 5]. A major reason probably is the contact between soot and catalyst which is increased by wetting of the soot with the catalyst. The liquid state of the catalyst might also result in higher particulate capture by materials that are normally poor soot filters, for example ceramic foams. The micro porous structure can retain the molten salt, while the surface is covered with liquid catalyst, causing the soot particulates to stick. Oxidation experiments with a flow reactor and with a thermobalance confirm that molten salt catalysts are more active than solid single oxide catalysts [6]. Catalysts investigated are among others eutectic mixtures of C s 2 M o O 4 - V 2 0 5 and CsVO3 - MOO3. Oxidation occurs at a rate that makes operation at temperatures of 625 and higher feasible. A major advantage of the molten (oxide) salts are their favourable wetting properties in combination with low volatility, compared to for example chlorine based catalysts, that are active due to evaporation and, therefore, tend to deactivate. Of course, potential loss of catalyst is a point of concern for all catalysts exhibiting mobility.
623 The technology that is needed to prevent the loss of catalyst through the gas stream still has to be developed. Flow-through systems could be considered, next to ceramic wall flow monoliths and ceramic foams. 2.4. The influence of NO on catalytic soot oxidation
The NO• that is present in diesel exhaust gas, can be an advantage in the oxidation of soot. Several studies report an important increase of catalytic soot combustion by addition of NO to the gas phase [2, 7]. Probably an oxidation cycle with NO as an intermediate for oxygen transfer plays an important role. This cycle can be catalysed at one or more stages. The oxidation of NO to NO2 can be catalysed, but also the subsequent oxidation of soot by NO2 can be catalysed. This observation is confirmed by very recent experiments with bi-metallic fuel additives [8]. REFERENCES 1. 2. 3. 4. 5. 6.
J.P.A. Neeft, Catalytic oxidation of soot, PhD thesis, 1995 G. Mul, Catalytic diesel exhaust purification. PhD thesis (1997) J.P.A. Neeft, S.J. Jelles, M. Makkee, and J.A. Moulijn, CAPoC 4 (to be published) D.W. McKee et al., FUEL 64 (1985) 805 D.W. McKee, Carbon, 25 (1986) 587 S.J. Jelles, M. Makkee, B.A.A.L. van Setten, and J.A. Moulijn, CAPoC 4 (to be published) 7. P.N. Hawker, Platinum Metals Rev. 39 (1995) 2 8. S.J. Jelles, M. Makkee, and J.A. Moulijn, unpublished results
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CATALYSIS AND AUTOMOTIVE POLLUTION CONTROL IV Studies in Surface Science and Catalysis, Vol. 116 N. Kruse, A. Frennet and J.-M Bastin (Eds.) 91998 Elsevier Science B.V. All rights reserved.
625
Investigation of copper-cerium oxide catalysts in the combustion of diesel soot D.Courcot, E. Abi-Aad, S. Capelle and A. Abouka'fs Laboratoire de Catalyse et Environnement, Universit6 du MREID, 145, route du Pertuis d'Amont, 59140 Dunkerque, France
Littoral-C6te
d'Opale,
ABSTRACT The combustion of diesel soot has been studied in presence of copper-cerium oxide catalysts. These catalysts were prepared by impregnation of copper nitrate on ceria and calcination under air up to 1073 K. The solids were widely characterised by electron paramagnetic resonance (EPR) technique. For an atomic ratio Cu/Ce-1 and a calcination temperature of 673 K, the catalyst seems to be the most active in the combustion of diesel soot, when compared to other catalysts. The Cu monomers are more concerned than the dimers in this combustion. I. INTRODUCTION Cerium oxide based catalysts have been widely studied during the last two decades. Main catalytic application was the elimination of automotive exhaust emissions (1,2). The catalytic properties of this oxide has o~en been related with the mobility of oxygen vacancies in the solid (3,4) and hence with its capacity to release stored oxygen under reducing conditions tests (5,6).Moreover, A.F. Ahlstr6m and C.U.I. Odenbrand (7) reported the deactivation by sulphur dioxide of supported copper oxide during the oxidation of soot. In addition, recent works on Cu-Ce-O systems have shown that copper oxide tends to form monomers, dimers and clusters of Cu E+ ions in ceria (8,9). It has been evidenced that the monomers are the precursors of the C a 2+ ion pairs which are stabilised in the ceria lattice once its crystallisation occurs. The coupling of the dimer is enabled by an oxygen bridge and the distance between the two copper ions was evaluated to 3.6 ~: 0.3 A. Furthermore, the reactivity of oxygen species around the supported copper oxide phase was studied after a mild H2-He treatment of the solid. The reduction at low temperature of dimers and monomers revealed the high mobility of the oxygen coupling the dimer and of the oxygen species around the isolated Cu 2§ species. Copper-cerium oxide may be considered as interesting catalysts in oxidation reactions (10). The purpose of this work is to study the catalytic properties of different copper-cerium oxides in the combustion of diesd soot and to correlate their activities with the nature of copper species present in the catalyst.
626 2. EXPERIMENTAL
2.1. Catalysts Samples of Cu-Ce oxides are prepared by incipient wetness impregnation method. Cerium hydroxide is precipitated by adding cerium nitrate to a concentrated solution of NaOH. The precipitate is thoroughly washed with warm water, filtered and dried at 373 K before its calcination at 673 K for 6 hours in a flow of dry air. Copper nitrate of different amounts is then impregnated, at room temperature, on the oxide support. The samples are dried at 373 K and calcined at 673, 873 or 1073 K for 6 hours in a flow of dry air. Samples are denoted by 1CuxCe673, 1CuxCe873 and 1CuxCe1073 where x indicates the atomic ratio of cerium to copper in the solid, and the last number indicates the calcination temperature value of the sample. Table 1 Chemical composition and specific areas of samples Samples
Chemical composition
Specific areas (BET) m2.g-I calcination temperatures 673 K
873 K
1073 K
1Cul00Ce
0.96Cu looCe 201.220
124
78
13.5
1Cu 10Ce
0.99Cu 10Ce 21.080
116
86
13.5
1Cul Ce
1.02Cu 1Ce 3.110
94
53
9
2.2. Combustion of diesel soot The soot is collected on a diesel exhaust. The chemical analysis revealed a weight percentage %wt. : 66.1% C, 3.6% H, 2.4% N, 21.4% O and 2.0% S (C/H atomic ratio = 1.53), which indicates a high hydrogen and sulphur contents for this soot compared with normal diesel soot (11). Diesel soot-catalyst mixtures (0.1 g soot and 0.4 g catalyst) are obtained by thorough mechanical mixing using an agate mortar. The combustion of diesel soot is studied by Differential Thermal Analysis (DTA), with a heating rate of 5 K .minl under air flux (5 L.hl). 2.3. Electron Paramagnetic Resonance (EPR) EPR spectra are recorded at 293 K and 77 K with a Bruker EMX spectrometer using the Xband microwave frequency. A dual cavity is used and the g-values are determined by measuring the magnetic field, H, and the microwave frequency. All the thermal treatment of the samples are carried out in a microflow reactor, which is assembled with a quartz EPR tube to allow the introduction of the solid into the resonance cavity without exposure to air.
627 3. RESULTS AND DISCUSSION
3.1. Catalytic combustion of diesel soot Figure 1 shows the DTA curve obtained when a sample of diesel soot is burned in the absence of a catalyst. With increasing the temperature, four exothermic peaks appear. These peaks can be attributed to the combustion of different types of hydrocarbons constituting the soot (11). Indeed, it is known that a real soot consists of a volatile fraction, which is more active than a carbonaceous solid fraction. The composition of the volatile fraction can also vary depending on the quality of fuel and the engine's mode of operation. In some cases, aromatic, oxygenated and paraffinic compounds can be present, as well as residual coke of the lubricant (12). In our case, the low value of C/H atomic ratio calculated for the soot could be correlated with an important volatile fraction. For this reason, the first two DTA peaks are attributed to the combustion of unburned HC compounds and the third peak, at 750 K, to the residual coke of the lubricant. The carbonaceous solid fraction is burnt off last, giving the fourth peak at 883 K.
640
750
diesel s o o t
883
dslCe02
273
I
I
I
373
473
573
,,
I
I
673
773
"'
I
I
873
973
temperature K Figure 1. DTA profiles obtained for diesel soot (ds) and soot-CeO2 mixture.
1073
628 When the soot is burnt in presence of CeO2 as a catalyst, no modification of DTA peaks is observed (Fig. 1). Such a result indicates that the ceria is not active in the combustion of soot. On the contrary, when Cu E+ ions are added on ceria, the mixed oxides become active and their activities depend on the previous calcination temperature of the catalysts before the combustion test and on the copper concentration.
589 640
750
883 without catalyst
742 574 ~
273
I 373
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,I
I
473
573
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673 773 temperatureK
ds11Cu100Ce673
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1073
Figure 2. DTA profiles obtained for soot-lCul00Ce mixture samples with different catalyst calcination temperatures. Figure 2 illustrates the DTA curves obtained when the combustion of soot is performed at different temperatures in the presence of 1Cu100Ce as a catalyst. Compared to that obtained without a catalyst, these results show clearly that the presence of Cu 2+ ions in small amounts on CeO2 shifts the peaks position by 70-140 K towards the low combustion temperatures. In addition, by comparison of the DTA curves obtained at different calcination temperatures, it seems that the 1Cu100Ce673 is the most active. This large activity can be due to the presence of not well localised Cu 2+ ion monomers in ceria because the solid remains in a not well crystallised state at this calcination temperature (9, 10). Indeed, with the increase of the calcination temperature from 873 to 1073 K, the solid is perfectly crystallised and the Cu 2§ ions are present on the solid in form of monomers and dimers. In this case the catalyst is less active than in the first case (Fig.2). Consequently, a previous calcination of the solid at 673 K
629 gives the best catalyst in the diesel soot combustion. For this reason, Figure 3 shows the DTA curves obtained for catalysts previously calcined at 673 K and containing different copper concentrations. The curves evidence that the activity of catalysts increases with the amount of Cu 2+ ions present in the solid. Indeed, for the 1Cul Ce673 catalyst, the combustion of soot is performed at relatively weak temperatures when it is compared to other catalysts. As mentioned above, this activity seems to be correlated to the presence of Cu 2+ ion monomers. Indeed, it was shown in previous works (10,13) on 1CulCe oxides that the intensity of the EPR signal relative to the Cu 2+ monomers reaches a maximum when the solids are calcined under air at 673 K. At this temperature, dehydration of the solid is complete and allows the formation of isolated Cu 2+ species, whereas the crystallisation of ceria gives rise to the formation of Cu 2+ dimers at higher calcination temperatures. The formation of the dimers implies a decrease in the intensity of the EPR signal related to the monomers since these latter are the precursors of the Cu 2+ ion pairs (9). In order to reveal the participation of Cu 2+ sites in the combustion of diesel soot, EPR studies were performed on the calcined catalysts before and after the catalytic reaction.
589 640
750
883 without catalyst
742 574
~7/~29~
Cu100Ce673
73
e673
I 273
373
473
573
673
773
873
973
1073
temperature K
Figure 3. DTA profiles obtained for soot-catalyst samples with different copper content after catalyst calcination under air at 673 K.
630 3.2. EPR measurements of calcined catalysts before the catalytic combustion
Figure 4 shows EPR spectra of 1Cul Ce oxide sample dried at 373 K and calcined under a flow of dry air at 673, 873 and 1073 K. With increasing temperature, the spectrum changes from a single broad signal to a complex signal consisting of well-resolved narrow lines. These spectra were widely described elsewhere (8-10). Four signals denoted by A', A, O and K were detected. The A' signal centred at g=2.078 (AH~300 G) is attributed to Cu2+ in form of clusters. The A signal, characterised by g//=2.237; g• A//=160 G and A• G, is attributed to isolated Cu 2+ ions located in octahedral sites in ceria with a tetragonal distortion and surrounded by more than six ligands (14,15). The broad signal (AH~950 G) designated by O, and centred at g=2.089, is assigned to Cu2+ ions in presence of strong dipolar interactions. The presence of dispersed small particles of CuO in CeO2 can provide this signal (10).
Gain 1CulCe373
1.3
1CulCe673
lculc K
1CulCe1073
1000
--
~
1
I
I
I
I
1500
2000
2500
3000
,
I
I
3500
4000
4500
H (Gauss)
Figure 4. X-band EPR spectra of 1Cul Ce sample dried and calcined under air at 673,873 and 1073 K.
631 Finally, the K signal, constituting by a fine and a hyperfine structures and the presence of a weak signal intensity at half magnetic feld, was attributed to Cu 2+ ion pairs formed in ceria (10). The EPR signals change with increasing the calcination temperature (Fig.4). The A' and A signals seem to be closely correlated, since with the dehydration of the oxide, the A' signal transforms into the A signal. At 873 K, the O signal disappears, while a transformation of small particles of CuO in CeO2 into large CuO aggregates occurs. The intensity of K signal increases from 673 to 1073K ; Cu 2+ ion dimers being progressively stabilised in substitution positions of two Ce 4+ ions in the ceria lattice (10). Similar signals have been observed on 1Cul0Ce and 1Cul00Ce samples with different proportions of the detected Cu 2§ species (13,16). 3.3. EPR measurements during the catalytic combustion of soot An EPR study is also performed during the catalytic combustion of diesel soot on 1CulCe1073 sample. Before EPR acquisition, the soot-catalyst mixture is heated at different temperatures in similar conditions as the catalytic test, i.e. with a heating rate of 5 K.min 1 under air flow (5L.h-~). Figure 5 exhibits the evolution of the EPR spectra of the soot1CulCe1073 mixture during the increase of the temperature. Until a temperature of 573 K, the EPR spectra (Fig.5) are the superposition of 1CulCe1073 signals and the characteristic signal of the soot. This latter consists of a narrow symmetric line, centred at giso -- 2.003 and assigned to the graphite free electron present in soot. With increasing temperature, a decrease of the narrow line intensity is observed and it disappears at 773 K. This observation reveals the complete oxidation of the soot graphite at this temperature. This result is in good agreement with the DTA data which shows the last exothermic peak related to the combustion of soot at 740 K on this catalyst. During the soot combustion, the relative intensity of isolated Cu 2+ ions signal, in comparison to the Cu R+ ion dimers, decreases at 673 K and seems partially restored once the complete oxidation of the soot occurs. This phenomenon demonstrates that isolated Cu 2+ monomers are more concerned in the soot oxidation than the dimers. Then, the oxygen species surrounding them are of high mobility and responsible of this reaction. Moreover, table 1 shows that, for a given temperature, the specific areas of catalysts decrease when the copper concentration increases. Since the 1CulCe673 is the most active among all the catalysts (Fig.3), it is then evident to deduce that the activity is closely correlated to the copper concentration and not to specific areas. The new broad signal (AH-1670 G) detected at 673 K could arise from the agglomeration of isolated copper species in form of aggregates during the soot combustion. Once this combustion is finished (773 K), the broad signal intensity decreases, whereas, an increase of the monomer intensity is observed. It seems that a rearrangement of Cu 2+ species in the catalyst occurs during and after the soot combustion.
632
Gain 3
1CulCe1073
/g=2.003
(1CulCe1073 + diesel soot)573
s o o t ) 6 7 3 -.--.___ J t _(1CulCe1073 . ~ _ . ~+ diesel -
(lCulC~diesel
I 1000
1500
~
.... 10 =
so
,,.
I
I
I
I
I
2000
2500
3000
3500
4000
4500
H (Gauss) Figure 5. Evolution of the EPR spectrum of a soot-1Cu1Ce1073 mixture during the soot combustion as a function of temperature. 3.4. Effect of SO2 poisoning on the copper oxide phase. The presence of sulphur in diesel exhaust gases or particles has to be considered as a poisoning agent for the catalysts used in soot combustion reactions. Copper oxide has been reported to be sensitive towards sulphur dioxide (7) which implies a deactivation of the solid and then eventual modifications of its surface properties. In this way, 1CulCe1073 sample was treated in a microflow reactor under SO2 flow (2L.hl ) at room temperature for 30 minutes. Figure 6 shows the EPR spectra of the catalyst before and after this treatment. Compared to the spectrum of 1CulCe1073 sample, a new signal, centred at g=2.166 with Z~H,~280 G appears in the second spectrum without affecting the intensities of signals attributed to Cu E+ ion monomers (A) and dimers (K). The new signal is better observed after a simple subtraction from the second spectrum the first one. A similar signal is recorded when a flow of SO2 is introduced into CuO particles at room temperature. Moreover, the same signal is
633 detected for a CuSO4,5H20 sample. From these results, it is then evident to attribute the new signal to the formation of CuSO4 phase when the 1Cu1Ce1073 catalyst is treated with SO2 at room temperature. Indeed, it was already demonstrated that CuO phase is present in such a catalyst previously calcined at 1073K (9, 10). Therefore, when SO2 is in contact with the catalyst, the CuSO4 phase is formed following the mechanism (17) Cu 2+ + 20(surface) + SO2 (g)"~ Cu E+, SO4 (ads)
Gain 1Cu1Ce1073
1
1Cu1Ce1073*_~So2
1
..~,~.
subtraction
.
.
.
.
.
.
1
CuSO4,SH20
.........
1000
I
I
I
I
1500
2000
2500
3000
"
I'"
3500
I 4000
4500
H (Gauss)
Figure 6. EPR spectra of the 1Cul Ce1073 after 802 treatment. 4. CONCLUSION In this work, it is shown that the copper-ceritma oxide catalysts are active in the diesel soot combustion reaction. The isolated Cu E+ ions seem to be the most active sites in such catalysts. The activity depends on the copper concentration and the pre-treatment of solids. Among all the tested catalysts, the 1CulCe673 oxide, calcined at a relatively low temperature and containing the highest copper concentration, is the most active.
634 ACKNOWLEDGEMENTS The authors would like to thank the Conseil G6n6ral du Nord and the R6gion Nord - Pas de Calais for financial support of this work. REFERENCES
1. J.C. Summers, A. Ausen, J. Catal., 58 (1979)131. 2. K.C. Taylor, Catalysis, Science and Technology, Ed. J.R. Anderson, M. Boudart, Springer Verlag, Berlin, 5 (1984) 119. 3. G.J. Van Handel, R.N. Blumenthal, J. Electrochem. Soc., Solid State Sci. Technol., 121 (1974) 1198. 4. H.L. Tuller, A.S. Nowick, J. Electrochem. Soc., Solid State Sci. Technol., 122 (1975) 225. 5. T.Takahashi, Physiques of Electrolytes, Ed. J. Hladik, Academic Press, Orlando, 1972, vol.2, p.989. 6. J.L.G. Fierro, J. Soria, J. Sanz, M.J. Rojo, J. Solid State Chem., 66 (1987) 154. 7. A.F. AhlstrSm and C.U.I. Odenbrand, Appl. Catal., 60 (1990) 143. 8. J. Soria, J.C. Conesa, A. Martinez-Arias and J.M. Coronado, Solid States Ionics, 63 (1993) 755. 9. A. Aboukais, A. Bennani, C.F. Aissi, G. Wrobel and M. Guelton, J. Chem. Soc., Faraday Trans., 88 (1992) 1321. 10. A. Aboukais, A. Bennani, C. Lamonier-Dulongpont, E. Abi-Aad and G. Wrobel, Colloids and Surfaces A, 115, (1996) 171. 11. A.F. AhlstrSm and C.U.I. Odenbrand, Carbon, 27 (1989) 475. 12. G. Hunter, J. Scholl, F. Hibbler, S. Bagley, D. Leddy, D. Abata and J. Johnson, Soc. Automot. Eng., SP 1981, SP 484, p . l l l . 13. A. Bennani, Thesis, the University of Lille, 1994. 14. A. Aboukais, R. Bechara, D. Ghoussoub, C.F. A:issi, M. Guelton and J.P. Bonnelle, J. Chem. Soc., Faraday Trans., 87 (1991) 631. 15. G. Martini, V. Basseti and M.F. Ottaviani, J. Chim. Phys., 77 (1980) 311. 16. A. Aboukais, A. Bennani, C.F. Aissi, G. Wrobel, M. Guelton and J.C. Vedrine, J. Chem. Soc., Faraday Trans., 88 (1992) 615. 17. A. Galtayries, J.Grimblot and J.P.Bonnelle ; Surface and Interface Analysis, 24 (1996) 345.
CATALYSIS AND AUTOMOTIVE POLLUTION CONTROL IV Studies in Surface Science and Catalysis, Vol. 116 N. Kruse, A. Frennet and J.-M Bastin (Eds.) 91998 Elsevier Science B.V. All rights reserved.
Catalytic ceramic investigations
filter
for
Diesel
soot
635
removal:
preliminary
P. Ciambelli% V. Palma ~, P. Russo ~ and S. Vaccaro b aDipartimento di Ingegneria Chimica e Alimentare, Universit~ di Salerno, via Ponte Don Melillo - 84084 Fisciano (Salerno), Italy. tel. +39 89 964151- FAX +39 89 964057. bDipartimento di Chimica, Universit~ di Napoli "Federico Ir', via Mezzocannone 4- 80134 Napoli, Italy. The catalytic combustion of Diesel soot was studied performing reactivity tests of soot-catalyst mixtures in a tubular flow reactor. The dependence of the reaction rate on the temperature was found. With respect to the uncatalysed combustion the reactivity of the soot in the presence of catalyst increased of some orders of magnitude while the apparent activation energy was found to be less than half. Complementary tests were carried out for studying the regeneration process of ceramic sintered filter samples by uncatalysed and catalysed combustion of the accumulated carbon particles. With respect to the uncatalysed case, the presence of catalyst reduces the carbon ignition temperature so favouring spontaneous filter regeneration. However, the catalyst activity appears to be lower than that observed in the reactivity tests. The results of both series of tests were discussed and compared in order to assess the role of carbon-catalyst contact and of catalyst preparation on its performances. 1. I N T R O D U C T I O N
Soot emitted from Diesel engines is hazardous for human health since it is made of inhalable particles [1] and contains gases and liquids adsorbed on its surface, some of which (Polycyclic Aromatic Hydrocarbons) are suspected to be cancerogenic [2]. Virtually, soot-free Diesel exhaust may be obtained combining reduction of soot formation in the combustion chamber with exhaust gas treatment [3]. This latter is generally performed by a ceramic wall-flow filter that collects the carbonaceous particles while the filter regeneration is achieved by post-combustion of collected soot [3, 4]. An important step of the above mentioned soot removal process is the filter regeneration that could lead to filter failure by melting or breaking when high temperature spots or large temperature gradients occur [5, 6]. These phenomena
636 might be prevented by using a catalyst that lowers the carbon ignition temperature, thus allowing more frequent filter regeneration at lower temperatures. Catalytic applications for soot burn out include the use of precursor compounds soluble in Diesel fuel, of catalytic compounds injected upstream the soot loaded filter, and of catalytic coating on the filter itself [7]. Various catalysts with specific activity towards the soot carbonaceous matrix have been so far developed [8-14]. However, commercial systems for Diesel exhaust cleaning are only suitable to convert the hydrocarbon compounds, as they are similar to the catalysts employed for gasoline engine exhausts [15]. Indeed, in spite of the encouraging results obtained in fundamental studies with supported catalyst particles in micro scale systems, a satisfactory soot combustion was difficult to achieve with ceramic filters under practical conditions [16]. A number of reasons cause this discrepancy. One of these is the different dispersion of the catalytic species over the support surface. Indeed, with powders as catalyst support, the surface is completely available for catalyst deposition (laboratory conditions), while with a ceramic monolith (real conditions), the surface of deposition is not easily accessible to the catalyst during the preparation. Another reason is the different contact that can be realised between the carbon and the catalyst in laboratory tests and in a real environment. In the first case, intimate contact is generally obtained by thorough pounding of the two components in a mortar, while in the second case, a looser contact is achieved by natural deposition of the soot particles over the filtering surfaces when the gas stream crosses the filter. Finally, an additional reason is the lack of control in the real case of the relative amount of catalyst and carbon which is an important variable in determining both the contact between carbon and catalyst and the features of the filter regeneration. Thus critical points are: 1) the preparation of the catalytic ceramic filter by deposition of the catalytic layer over the filtering surfaces; and 2) the possibility of realising an effective carbon-catalyst contact during normal operating conditions. These aspects may affect the feasibility itself of the overall process [17], and, in addition, their knowledge may be crucial to avoid possible trap failures [9, 15]. Only recently these aspects were considered in literature [7, 14, 17]. In particular, we have reported [14] on the performances of a Cu-V-K based catalyst (137AA) in the combustion of carbonaceous materials, underlining the importance of the carbon-catalyst contact in the performances of the catalyst during trap regeneration. Neeft et al. [7] remarked the differences between the activity of powder catalysts and that of filter supported catalyst prepared with the same active species. In this paper results of soot reactivity tests and of catalysed and uncatalysed ceramic filter regeneration experiments are presented. The aim is to investigate the influence of the catalyst features and of the carbon-catalyst contact on the catalyst performances. This is accomplished by comparing the results of combustion tests of mixtures of carbon and catalyst particles, specifically prepared by thorough pounding of the two components in a mortar, with those attainable in a more realistic filter regeneration system.
637
2. E X P E R I M E N T A L 2.1. C a r b o n a c e o u s m a t e r i a l s Three carbonaceous materials were employed in the experiments: i) commercial (Degussa) amorphous carbon black (CB-330) with specific surface area (s.s.a.) of 82 m2/g (BET) and apparent density of 375 kg/m3; ii) Diesel Soot (DS), collected at the exhaust of a Lombardini, single cylinder, D.I., 325 cm 3 displacement, 18:1 compression ratio Diesel engine, operated at an air/fuel mass flow ratio of about 20. The BET s.s.a, of fresh soot was 90 m2/g and the apparent density 375 kg/m3; iii) soot (BS) generated by an heating gas-oil burner. The fuel mass flow rate fed to the burner was 1.6 kg/h while the air/fuel mass flow rate ratio was 18 and, correspondingly, the soot concentration in the exhaust gas was about 1000 ppm. 2.2. T h e r m o g r a v i m e t r i c tests (TG) Air flow temperature programmed oxidation tests were performed on carbon materials alone, on carbon-powder catalyst mixtures and on carbon-filter mixtures. These latter were obtained by pounding in a mortar a piece of a carbon covered filter. Such tests were carried out with catalysed and uncatalysed filters using a thermogravimetric analyser (Netzsch TA 209).
f
i
coco0.o il!
mfc CONTROL UNIT
Figure 1. Apparatus for kinetic tests.
2.3. R e a c t i v i t y t e s t s A Cu-V-K based catalyst (137AA) was used in the experiments [18]. The catalyst was prepared by impregnating a-A1203 powder (s.s.a.=2.7 m2/g,) with aqueous solution of different salts: NH4VO3, CuC12-2H20 and KC1 (Baker Chemicals). The catalyst was then dried at 393 K and calcined at 973 K overnight [12].
638 The a p p a r a t u s for kinetic tests is shown in Figure 1. It comprises a quartz t u b u l a r flow reactor 300 mm height and 20 mm internal diameter, heated by an electrical furnace. The reactor t e m p e r a t u r e was controlled by a programmercontroller (Ascon). Cylinder air (99.999 % purity) was fed to the reactor and the flow rate was controlled by mass flow controllers (Hi-Tec). Exhaust gas concentrations were determined by H a r t m a n n & Braun continuous analysers: U r a s 10E (for carbon monoxide and carbon dioxide) and Magnos 6G (for oxygen). The signals from the analysers were acquired and processed by a personal computer which also performed the control of the experiment. These tests were carried out using DS as carbon material. Mixtures of DS and catalyst were prepared by thorough pounding of the two components in a mortar thus resulting in a very effective carbon-catalyst contact. The DS alone or mixed with catalyst was diluted with 180-350 m m quartz particles up to a mass ratio between reacting blend and quartz of 0.01, to avoid localised t e m p e r a t u r e rises during the tests. The diluted mixture was loaded in the reactor, filling about 3 cm in length of its central zone. The remaining p a r t of the reactor was filled with 700-2000 m m quartz particles. The test was started by raising the t e m p e r a t u r e to the desired value in nitrogen flow. The gas feed to the reactor was t u r n e d to air to start the oxidation. Carbon burn-off was followed by continuous monitoring of carbon monoxide and dioxide produced during the combustion. Carbon mass balance was verified within a 5% tolerance range for all tests. These experiments were performed u n d e r constant operating conditions and the reactor was operated u n d e r differential conditions. The feed air flow rate was 500 Ncm3/min and the operating pressure 101 kPa. The t e m p e r a t u r e was varied in the range 593-693 K in the tests carried out in the presence of catalyst, and in the range 773-873 K in the absence of catalyst. The initial mass of carbonaceous m a t t e r was changed in the range 3-15 mg while the initial mass ratio of catalyst to soot was 1.
2.4. Filter r e g e n e r a t i o n tests The filter was a porous alumino-silicate 29 m m OD and 13 mm ID tube (Goodfellow) with a porosity of 45% and a mean pore size of 90 ~m. The BET specific surface area was 0.7 m2/g. For the catalytic regeneration tests the filter was impregnated with aqueous solution of the salts used for the preparation of the catalyst 137AA, t h e n dried at 393 K and calcined at 973 K overnight. In the uncatalysed regeneration the filter was used as received. These tests were carried out using BS or CB-330 as carbon material. Filter covering with BS was achieved by isokinetic sampling of p a r t of the b u r n e r exhaust gas stream and by forcing the sampled stream to pass through the filter over which the soot was collected. The sampling line and the filter were kept at constant t e m p e r a t u r e of about 420 K to avoid water and hydrocarbon condensation. Filter covering by CB-330 was performed at ambient t e m p e r a t u r e by forcing a carbon particle-laden air flow to pass through the filter. In both cases the carbon-catalyst contact is virtually the same of t h a t attainable in practical conditions.
639 Regeneration tests of BS or CB-330 covered filters, impregnated or not with catalyst, were performed by temperature programmed oxidation (TPO) tests in the quartz flow reactor similar to that described above with 35 mm OD. As in the case of the reactivity tests, the carbon oxidation rate and the filter regeneration degree (Xa) were calculated from the concentrations of carbon oxides in the reactor outlet gas. Air flow was 500 Ncm3/min. X-ray diffraction and EDS analysis of the alumina supported catalyst and of the catalytic filter were performed in order to compare their features. Scanning Electron Microscopy (SEM) analysis was carried out on catalysed and uncatalysed filters before and after the regeneration tests. --
80
_ ~
3. R E S U L T S
The air flow TG analyses of the three 40 carbonaceous materials Ds in the absence of CB catalyst show that their ~~~_ combustion behaviour ......... is very similar. In fact, . . . . 0 Figure 2 evidences "7, that, under the same "~: operating conditions, Ds -10 ~ all the carbonaceous materials start to burn o ~ around 790 K while the temperature at which the reaction rate is I I I I -20 600 800 1000 maximum is 828 K for Temperature, K BS, 840 K for CB-330, and 860 K for DS. Figure 2. Weight loss curves and their derivatives (DTG) of carbonaceous materials. Air flow" 20 Ncm3/min; heating rate 10 9 K/min. o .J J= r ,el
3.1 C a t a l y s t f e a t u r e s Some differences between the features of the alumina supported catalyst and the filter supported catalyst were found comparing the results of X-ray diffraction (XRD) analysis performed on both catalysts. The XRD spectrum of the catalytic filter shows a greater number of signals with respect to the catalyst 137AA. Due to the complexity of the interaction between the catalyst components and the support, the identification of such signals is difficult and is still object of study. Furthermore, it is important to remark the absence in the catalytic filter spectrum of the signals due to KC1 that are, instead, characteristic of the catalyst 137AA [19].
640 SEM pictures of filter samples are shown in Figure 3a-d. After the catalyst deposition, the appearance of the filter changes from that of an almost homogeneous aggregate of grains (Figure 3a) to a very heterogeneous system (Figure 3b). The size of the catalyst particles is at least one order of magnitude lower than that of the filter grains. Correspondingly, the specific surface area of the filter changed from 0.7 m2/g to 1.1 m2/g. After the filter regeneration, the presence of the small catalytic particles is less evident and the grain surface appears smoothed off.
Figure 3a-d. SEM pictures of the ceramic filter a) as received; b) after catalyst deposition; c) after catalyst deposition and covering with soot; d) as c) after regeneration. 3.2 R e a c t i v i t y t e s t s Results of uncatalysed and catalysed DS combustion tests, carried out at constant temperature in the tubular flow reactor, are outlined in the Arrhenius plot of Figure 4. Overall and partial reactivity is defined as the rate of change of the overall carbon conversion (X) and of the partial conversions to carbon monoxide (Xco) and to carbon dioxide (Xco2). From Figure 4 it is evident that under the same operating conditions the catalysed reaction rate would be several order of magnitude higher than the uncatalysed rate. Furthermore, the apparent activation energy for the uncatalysed reaction, evaluated from data in Figure 4 is
641 150_+12 kJ/mol while it is 100_+8 kJ/mol in the presence of catalyst. Figure 4 also shows t h a t the catalyst strongly promotes the carbon conversion to carbon dioxide. Such findings conform to the results obtained by us with different carbonaceous materials [20].
-~
Catalys~d CO
G
ca~ty,~TOT
~j~
Uncatalysed CO
u,,~,,t,b',~d co 2
r, 4~
"~
'0
0.00
-2.00
9
Uncatalysed TOT
e4 o r
_a
-4.oo -
0 r
"~
-6.00 I 1.00
1.20
1.40
~
I
1.60
,
1.80
1000/T, 1/K Figure 4 Comparison between Arrhenius plots for uncatalysed and catalysed combustion of DS. 3.3. Filter
regeneration
tests
Typical results of filter regeneration tests attainable with fresh prepared catalytic filters are shown in Figure 5 where the filter regeneration rate, expressed as the time derivative of (XR), is reported as a function of the temperature. For comparison a regeneration rate curve of a BS covered uncatalytic filter is also shown in Figure 5, from which it is evident t h a t the presence of catalyst lowers the b u r n out temperature of about 200 K. In particular, without catalyst the carbon material starts to burn at about 530 K and the complete regeneration is achieved at about 940 K. These values are very similar to those obtained by TG analysis of BS alone reported in Figure 2. This implicitly suggests t h a t the presence of the uncatalytic filter does not modify the soot reactivity. Of the two curves in Figure 5 one was attained with catalytic filters one was attained by regenerating a CB-330 covered catalytic filter and the other by regenerating a BS covered catalytic filter. In spite of the change of carbon material the combustion features do not vary significantly. In both cases the carbon material starts to b u r n at about 500 K and the complete regeneration is achieved at about 800 K. In addition, the regeneration rate curves show a dominant peak at about 660 K in the case of CB-330 and at about 690 K in the case of BS.
642
r
-,0
cB.$$o,catalyutd tits,catalysed
/ | ~
--0.00k 400
9
,~ I, 600 800 Tempertature, K
- - "
1000
Figure 5. Regeneration rate of catalytic and uncatalytic filter samples during TPO test. Air flow: 500 Ncm3/min; heating rate- 10 K/min.
0.00 400
cs-~3e,eatalysM,Srdcycle
.
_
' 600 800 Temperature, K
1000
Figure 6. Comparison of the catalytic regeneration rate curves after various carbon depositionregeneration cycles. Air flow: 500 Ncm3/min; heating rate : 10 K/min.
Catalyst durability was investigated by performing six cyclic tests of carbon deposition and filter regeneration. Some cycles were performed using CB-330 while others using BS. In Figure 6 the regeneration rate curves obtained in the first, the third and the sixth cycle are reported being those relevant to the other cycles very similar to the others. All the curves are bimodal with peaks of reactivity at about 660 K and 750 K, respectively. However, while in the first cycle the peak at lower temperature is dominant, in the following cycles the second peak is the most important. In addition, the regeneration rate curves obtained from the second to the sixth cycle are very similar. 4. D I S C U S S I O N
The results in Figure 4 show that the catalyst has an outstanding effect on the reactivity of soot. In addition, previous results of TPO tests [19] carried out with various carbonaceous materials including DS and CB-330, showed that the burnoff temperatures were lowered by about 250-300 K in the presence of the catalyst 137AA. The comparison of the results in Figures 5 and 6 show that the catalytic filter is less active. Indeed, with respect to the uncatalysed regeneration the decrease of burn out temperature, due to the presence of catalyst, is about 200 K during the first regeneration and about 100 K for the following regeneration.
643 The differences in reactivity between the filter supported catalyst and the o alumina supported catalyst could be due to a lower 40 intrinsic catalytic activity or a less effective carboncatalyst contact. In the case of the alumina supported catalyst an intimate carbon.=_ catalyst contact was insured through pounding of the two -10 (~ components in a mortar (reactivity tests), while a relatively loose carboncatalyst contact resulting ! , I I I, _ -20 400 600 800 from the natural deposition Temperature, K of soot particles by filtration a gas flow Figure 7. TG and DTG curves of a) DS-137AA from mixture; b) DS-ceramic filter supported catalyst (regeneration tests) was mixture. Air flow : 20 Ncm3/min; heating rate: 10 achieved in the case of the filter supported catalyst. K/min. L
- -
,
_
~)
-
m
|
In order to discriminate between the two possible limiting stages (catalytic activity or effectiveness of the carbon-catalyst contact), a mixture of carbon and catalytic filter was prepared by pounding in a mortar a piece of DS covered filter. Figure 7 reports the comparison between TG tests carried out with catalytic mixtures of DS-powder catalyst and of DS-filter catalyst. It is immediately evident that the reactivity of the DS-filter catalyst mixture is markedly lower than that of the DS-powder catalyst mixture, the maximums of the weight loss rate being 580 K and 700 K, respectively. This occurs in spite of the procedure of sample preparation, made in both cases by thorough pounding of the components in a mortar. Therefore it is more likely that the relatively modest performances of the filter supported catalyst in the soot combustion should be attributed to the intrinsic lower catalytic activity rather than to ineffective carbon-catalyst contact. The comparison of the curves of consecutive regeneration rate tests in Figure 6 suggests that during the first cycle there is a loss of catalytic activity but also that further carbon deposition/regeneration cycles do not influence significantly the activity of the catalyst. It is worth noting that in the first four cycles the filter was coated with CB-330 while in the last two cycles the filter was covered with BS. This implicitly demonstrates that the regeneration does not depend on the nature of carbon material deposited neither on the procedure of deposition but it is strictly affected by the catalytic effect of the filter.
644 5. C O N C L U S I O N S TG analysis showed that in spite of the different origin of the carbonaceous materials their combustion behaviour is very similar allowing the comparison of experimental results obtained with different carbon materials and techniques. The catalyst 137AA strongly decreases the burn out temperature of Diesel soot. However, significant differences were found between the reactivity of mixtures of soot and 137AA catalyst, prepared by thorough pounding of the component in a mortar, and of soot covered alumino-silicate filter supported catalyst, obtained by soot deposition by filtration. These differences seem mainly due to the lower intrinsic catalytic activity of the filter supported catalyst with respect to the catalyst 137AA. In addition, the effect of the carbon-catalyst contact also plays a significant role. ACKNOWLDGMENT The work was fmanced by Progetto Finalizzato Trasporti 2 of Research National Council (CNR) of Italy. REFERENCES
1. 2. 3. 4. 5. 6. 7.
Fang C. P. and Kittelson D.B., SAE Paper No 840362 (1984). Funkebush C.F., Leddy D.G. and Johnson J.H., SAE Paper No 790418 (1979). Lox E.S., Engler B.H. and Koberstein E., Studies Surf. Sci. Catal., 71 (1991) 291. Weaver C.S., SAE Paper No P-140 (1984). Higuschi N., Mochida S. and Kojima M., SAE Paper No 830078 (1983). Gulati S. T., SAE Paper No 860008 (1986). Neeft J.P.A., van Pruissen O.P., Makkee M. and Moulijn J./L, Appl. Catal. B:Environmental (1997) 227. 8. AhlstrSm A.F. and Odenbrand C.U.I., Appl. Catal. 60 (1990) 143. 9. Xue E., Seshan I~, van Ommen J.G. and Ross J.R.H., Appl. Catal. B:Environmental, 2 (1993) 183. 10. Hoffmann U. and Rieckmann T., Chem~ Eng. Technol., 17 (1994) 149. 11. Mul G., Neeft J.P./L, Kapteijn F., Makkee M. and Moulijn J.A_, Appl. Catal. B:Environmental, 6 (1995) 339. 12. Ciambelli P., Parrella P. and Vaccaro S., Thermochimica Acta, 162 (1990a) 83. 13. Ciambelli P., D'Amore M., Palma V. and Vaccaro S., Combust Flame, 99 (1994) 363. 14. Ciambelli P., D'Amore lVL,Palma V. and Vaccaro S,. XXVI Symp. (Int.) on Combustion, The Combustion Institute, Pittsburgh, PA (1996) 1789. 15. Miiller E., Wiedemann B., Preuss/LW. and Schfidlich H.I~, Au~mobiltechnische Zeitschritf, 91 (1989) 674. 16. Neeft J.P.A., Makkee M. and Moulijn J./k, Fuel Process. Technol., 74 (1996) 1. 17. Need, J.P_4., Makkee 1VLand Moulijn J ~ , Appl. Catal. B:Environmental, 8 (1996) 57. 18. Ciambelli P., Corbo P., Scialb M.R. and Vaccaro S., Italian Patent No 1221416 (1990). 19. Ciambelli P., Parrella P. and Vaccaro S., Studies Surf. Sci. Catal., 71 (1991) 323. 20. Ciambelli P., Palma V. and Vaccaro S., Combust. Sci. Tech., 103 (1994) 337.
CATALYSIS AND AUTOMOTIVEPOLLUTIONCONTROLIV Studies in Surface Science and Catalysis,Vol. 116 N. Kruse, A. Frennet and J.-M Bastin (Eds.) 91998Elsevier Science B.V. All rights reserved.
645
Catalytic oxidation of model soot by chlorine based catalysts G. Mul, J.P.A. Neeft, M. Makkee, F. Kapteijn and J.A. Moulijn Delft University of Technology, Faculty of Chemical Engineering and Materials Science, Department of Industrial Catalysis, Julianalaan 136, 2628 BL Delft.
Different oxidic compounds potentially present in a highly active Cu/K/Mo/CI soot oxidation catalyst (copper-molybdates, potassium-mol~tes, and a mixed copper-potassium-molybdate (K2Cu2(MoO4)3)have been tested individually on their activity in soot oxidation. Although the oxides induce oxidation between 685 K and 720 K after intensive mixing in a ball mill, i.e. in 'tight contact' conditions, in 'loose contact' the oxides are hardly active and only show a high activity above approximately 790 K. DRIFT and XRD analyses have shown that reaction of KCI with CuMoO4 (two compounds present in the Cu/K/Mo/C1 catalyst), results in the formation of potassium containing molytx~tes and volatile copper and chlorine containing compounds (such as CuCl and K2CuCI4). These volatile compounds possess a high 'loose contact' soot oxidation activity between 600 and 690 K. Several other metal chlorides have been screened on their catalytic activity in the oxidation of model soot (Printex-U) in 'loose contact'. Only PbCl2, CuCI2, and CuC1 are very active. The activity of these metal chlorides is thought to be induced by in-situ formation of intimate contact between the soot and the metal chloride via 'wetting' and/or gas-phase transport. A correlation between the melting point and the catalytic activity was found. Furthermore, a catalytic cycle is proposed involving activation of oxygen on the surface of the metal (oxy)chloride, followed by transfer of activated oxygen to the soot surface. Practical application of metal chlorides for the removal of soot from diesel exhaust is not recommended, because they suffer from instability or high vapor pressures.
1. INTRODUCTION Soot emission from diesel engines can be reduced in various ways. Collection of soot in a ceramic filter combined with simultaneous oxidation, seems to be the best option to minimize the contamination of the environment. However, soot oxidation takes place at temperatures of 825 875 K, whereas the temperature of diesel exhaust is typically 500 - 675 K. Hence, a catalyst is needed to prevent accumulation of soot in the monolithic filter. Several investigators have focused their attention on the application of oxidic materials to lower the oxidation temperature of soot. Reported active soot oxidation catalysts are, among others, PbO, Co304, V205, MOO3, and CuO [1, 2], copper vanadates [3], and perovsldte type oxides [4]. Also noble metals, in combination with metal oxides have been tested as soot oxidation catalysts [5]. Ciambelli et al. [6, 7, 8] report a catalyst containing copper, potassium, and vanadium compounds stabilized on an Al203 support. This catalyst is prepared by impregnation of the A1203 carrier with a solution containing KCI, CuCI2, and NH4VO3. A similar catalyst is described by Watabe et al. [9], based on KCI, CuCI2 and
646
(NH4)6MoTO24and supported on TiO2. These two catalysts are able to lower the soot oxidation temperature from 825-875 K to 600 K, thus being active in the temperature range of interest. Moreover, they are the most active catalysts reported so far. Yuan et al. [10] give a possible explanation for the increase in activity in soot oxidation after addition of a potassium compound to a TiO2 supported copper catalyst (i.e. stabilization of the support material). In three recent publications [11, 12, 13] we have presented various experimental details concerning the soot oxidation activity of the CtgK/Mo/CI catalyst. In this paper we will summarize the results and show that chloride and copper containing compounds are the active species. In order to find design rules for an active catalyst in soot oxidation, the catalytic activity of several other metal chlorides is evaluated with respect to their melting point, as well as to the necessity of a chloride ion for the activation of oxygen. Finally the applicability of this system as a catalyst for diesel soot oxidation is discussed. 2. EXPERIMENTAL The potassium molybdates (K2MoO4, K2M02OT,K2M03Olo, and K2Mo4013) and copper molybdates (CuaMo209 and CuMoO4) were prepared by solid state reaction of K2CO3and MoO3, and CuO and MoO3, respectively [ 11]. A OgK/Mo/CI catalyst supported on ZrO2 was prepared by wet impregnation according to the procedure given by Neett [12]. The metal chlorides and oxychlorides investigated were purchased or prepared as described elsewhere [13]. Since it is difficult to obtain diesel soot with constant properties (the composition depends on the engine load) a model soot was applied (l~tex-U, a flame soot kindly provided by Degussa). This soot has a N2-BET surface area of 96 m2g"l and contains approximately 5 wt% of adsorbed hydrocarbons and 0.2-0.4 wt% sulfur. Catalytic soot oxidation temperatures were determined in a thermobalance (STA 1500H). About 4 mg catalyst, 2 mg soot and 54 mg SiC were applied as a sample. A heating rate of 10 K/rain and a flow rate of 50 ml/min 21 vol% 02 in N2 were used. The maximum of the DSC curve was defined as the oxidation temperature. Samples referred to as 'tight contact' were intensively milled in a ball mill for one hour, before dilution with SiC and thermal analysis, whereas 'loose contact' was established by simply mixing of the catalyst and soot with a spatula. TG analysis of CuCl, as well as Cu2OCl2 was performed in the thermobalance under the exclusion oi~ and in the presence of air, and either with or without soot. Partially converted CuCl, and FeOCl samples were prepared isothermally in the thermobalance at 550 K (CuCl), and 645 K (FeOCI) [13]. Diffuse Reflectance Infrared Fourier Transformed (DRIFT) spectra were recorded on a Nicolet Magna 550 spectrometer, equipped with a Spectratech DRIFT accessory. Samples were analyzed ex-situ, after partial conversion of the soot in the thermobalance and dilution with KBr (1:100 by weight). A resolution of 8 cm"~and 256 scans were applied to obtain the spectra. 3. RESULTS AND DISCUSSION Soot oxidation profiles as obtained for K2MoO4 in 'tight contact' and 'loose contact' are shown in figure 1. The maximum of the exothermic heat effect is located at 685 K for the ball milled sample ('tight contact') and 790 K for the spatula mixture ('loose contact'). Apparently the milling procedure lowers the catalytic soot oxidation temperature by approximately 100 K, and is essential for a high soot oxidation activity. Non-catalytic soot oxidation occurs at 875 K. Neefi et al. [14]
647 and v. Doom et al. [ 15] reported earlier on the effect of contact on the catalytic performance of metal oxides. They also found considerable differences in soot oxidation temperatures depending on the mixing procedure between the catalyst and soot. 40
Heat Flow (mJ/s)
Weight (mg)
65
685 K - 63
18-
TG
-,,.. -5
-
---9
-61
790K ~ -
59
DSC -28 -
-50
5C0
I
600
I
700
I
800
I
900
57
55
1000
Temperature (K) Figure 1. TG and DSC profiles for soot oxidation catalyzed by K2MoO4 in 'tight' (A) and 'loose' contact(B) in the presence of synthetic air. Heating rate: 10 K/min. An extensive TG/DSC analysis of the soot oxidation activity of the various molybdates, potentially present in the CtfK/Mo/CI catalyst (in 'fight contact'), showed that they are active in the temperature range of 660-720 K [11]. The best results are obtained with a mixed potassium copper molybdate. It should be emphasized that the molybdates showed this activity only after ball milling, whereas the oxidation temperature of the Cu/K/Mo/CI-ZrO2 catalyst has been determined to be 675 K without such a procedure [11]. So, the high 'loose contact' activity of the supported CtgK/Mo/CI catalyst cannot be caused by the molybdates, which are present in this catalyst. Addition of KCI to CuMoO4 significantly enhances the 'loose contact' activity (the DSC maximum is located around 650 K) of CuMoO4, whereas KCI itself is hardly active in catalytic soot oxidation. XRD analysis after reaction of the mixture showed that various potassium molybdates and a mixed potassium copper chloride was formed [11]. The following reactions can be proposed: 4 KCI + 7 CuMoO4
--~
K2MO3Olo+ 2 Cu3M0209 + K2CuCl4
(3)
-->
2 K2Cu2(MoO4)3+ CuMoO4 -FCuCl2
(4)
2 KCI + K2M03Olo + 2 Cu3Mo209
648 The activity of K2CuCI,2H20 was determined to be similar to the activity of the KCI/CuMoO4 mixture, as well as to" the activity of the sublimed material out of a Cu/K/Mo/CI-ZrO2 catalyst. Oxidation temperatures of catalytic mixtures of CuMoO4 (or CuWO4 or Cu2V2OT)with KCI, CsCI, and LiC1 (without pre-calcination), were also detemained to be in the range of 655-665 K, whereas addition of K2CO3 did not significantly enhance the activity of CuMoO4 [13]. Apparently a chloride ion is essential for the high activity, and the presence and formation of copper chlorides results in the high soot oxidation activity of Cu/K/Mo/CI- and related catalysts. The only function of the Cuanion (i.e. molybdate, tungstate or vanadate) is to react with potassium and to s t a b l e the system. 1000
Oxidation Temperature (K)
900 ............................................................................................................................................ BaCI~........ 9
9
HgCI2 8oo
~eCaCI2 ................................................... c i S c i ~ . ~
...............................
700
Cu(ll)Ci2
600
'
500 500
/
~
FeOCI
~
9
~
o-
/
..............~
.... MoO3
.
.
.
.
.
.
.
.
.
.
.
.
.
NiCI2
e//~
Cu(I)Cl
I
600
i
700
I
800
,
I 900
t
1000
I
1100
~
1200
9 1300
Melting or Decomposition Point (K)
Figure 2. Correlation between the melting point of metal chlorides and the corresponding soot oxidation temperature. Solid dot: well defined melting point. Open dot: decomposition or oxidation takes place before melting. The horizontal solid line indicates the non-catalytic soot oxidation temperature. The activity of various metal chlorides, expressed by the soot oxidation temperature, is shown in figure 2. The non-catalytic soot oxidation temperature is indicated by the horizontal solid line. Several metal chlorides have hardly any effect on the soot oxidation temperature. HgCl2, BaCI2 and CaCI2 only show a small catalytic effect. Upon heating, COC12.6H20 and NiCI2.6H20 lose crystal water; in air they are (partially) converted into the corresponding oxides. MoC15 is completely oxidized in air at relatively low temperatures. The activity of oxidized MoCls is equal to MoO3 in 'tight contact'. FeCIs.6H20 and hydrated BiCI3 are fist converted into FeOCl and BiOCl, respectively [16,17]. The catalytic activity of these oxychlorides are given in figure 2. C u C I 2 and
649 PbCb are quite active, and CuCI even has a higher activity: the soot oxidation temperature is lowered by 285 K. Figure 2 also shows a correlation between the 'loose contact' activity and the melting point of a transition metal chloride. Unfortunately, several metal (oxy) chlorides do not have a well defined melting point in air. As previously discussed, they are (partially) transformed before they melt into the corresponding oxide and 02 or decompose otherwise. Therefore, the decomposition temperatures given by Knacke [18] are plotted in the figure, except for Mo, whose melting point of the oxide (MOO3[18]) was used. These metal chlorides are indicated with an open circle in figure 2, while a solid circle indicates that the metal chlorides (and MOO3) have a well defined melting point. Metal (oxy)chlorides with high melting points (CaCb, BaCI2, NiCI2 and COC12)are less active in 'loose contact' than metal chlorides with relatively low melting points and high vapor pressures. Although HgCb does have a well defined melting temperature, it is not on the curve, obviously because HgCI2 has evaporated before it can exert its catalytic influence. Milling a metal chloride and soot hardly effects the catalytic soot oxidation temperature. The observations suggest that the high catalytic activity of several metal chlorides in 'loose contact', is due to an in-situ distribution of the chlorides over the soot surface (resulting in 'tight contact'). Whether the in-situ 'tight contact' formation occurs by 'wetting' or gas phase transport has yet to be established. Xie et al. [19] have investigated the spreading (or 'wetting') behavior of many inorganic salts on several carrier materials (like Al203, TiO2 and activated carbon), and found that CuCb was able to wet the surface of alumina [19]. Previously, it has been shown that gas phase transport of copper chlorides also occurs [ 11]. Weight (mg)
60-1
|
Heat Flow (mJ/sec)
160 -120
"-
..~, i " \
T5 -80
| 58-1
I,
/
..... ---
-'--f--'f--'~-'V. .....
I /
\
~.. :,'k.
-40 "'"--. A
-0 --40
9od8O
5 Temperature (K)
Figure 3. TG/DSC analysis of the Cu2OCh catalyzed soot oxidation (curves A), the CuCI catalyzed soot oxidation (curves C) and the oxidation of CuCI without soot (curves B). TG: dashed lines, DSC: solid lines.
650 It should be mentioned, however, that the activity of the (oxy)chlorides of Cu, Pb, Fe and Bi is even higher than that of their corresponding oxides in 'tight contact' [11]. This might be the result of an even better contact obtained by 'wetting' or condensation than obtained after ball-milling, but a chlorine ion might also effect the activation of oxygen and the redox properties of transition metal oxides. Therefore, a thorough TG/DSC and DRIFT analysis of CuCl and FeOCI was carried out, in order to reveal the mechanism by which transition metal (oxy)chlorides are active in soot oxidation. The TG and DSC profiles of soot oxidation catalyzed by Cu~OC12 (A) and CuCI (C), and the oxidation of CuC1 in air (4 mg, without soot, but diluted with SiC (1:15)) ~), are shown in figure 4. The CuCI and Cu~OCI2 catalyzed soot oxidation temperatures are located at approximately 595 K (T1) and 625 K (T5), respectively. After 100 % soot conversion, an increase in weight can be observed in the range of 660 K - 690 K, accompanied by two heat effects (T2 and T3): one of which is positive, due to oxidation, and one a superimposed negative heat effect, due to melting. Similar heat effects are observed for pure CuCI. The interpretation is the (re)oxidation of CuCI into Cu2OC12, which is thermally stable in air up to 740 K (T4). At this temperature of 740 K a weight decrease is observed, due to decomposition of the oxychloride, yielding CuO and gaseous chlorine. In the TG profile of soot oxidation catalyzed by Cu2OC12, an increase in weight can be observed after complete soot conversion at 650 K - 680 K. Apparently, even in the presence of 20% 02 in N2, an in-situ conversion of Cu2OC12 into CuCI during soot oxidation has taken place. This is corroborated by a TG analysis of heating CuCI in air, which revealed a weight increase at the same temperature where the weight increase of the Cu~OC1Jsoot sample atter 100 % soot conversion (figure 3) occurred. Also the heat effects are similar. Furthermore, the TG profile of heating Cu2OCI~ and soot in nitrogen showed a weight decrease around 600 K, indicating carbothermic reduction of Cu2OC12by soot [11,13]. Therefore, we conclude that during soot oxidation Cu2OC12 is reduced to CuCI.
A b
1605
0.04
905
810 -"
Soot T
2000
1500
"r
1000
Wavenumber (cm "1)
Figure 4. DRIFT analysis of partial converted soot samples. Non-catalytic (3% soot conversion), CuCI catalyzed (50 %), and BiOCI catalyzed (20 %).
651 DRIFT spectra of a partially converted CuCl/soot mixture (50~ conversion), and a FeOCl/soot mixture (60% conversion) are depicted in figure 4. The spectrum of soot after an identical heat treatment is shown for comparison. The DRIFT spectra contain three main absorptions located at 1738 cm"l, 1607 cml and centered around 1257 cm"l. The 1607 cm"l absorption is caused by aromatic stretching vibrations of the soot, which are enhanced by polar functional groups like quinone [20]. The other two absorptions have been assigned to oxygen complexes formed on the soot surface: lactones ( 1738 cm"l) and ether-like complexes (1257 crn'l), respectively [21,22]. Clearly CuCI causes an enhancement of the amount of surface oxygen complexes. Pure CuCI has no infrared absorptions in the spectral region recorded (400-4000 cm'~). However, the absorptions located at 905, 855 and 810 cm~ can be ascribed to water adsorbed on CuCI [23]. A strong band below 600 cml is indicative for Cu2OC12. As this band is not present in the spectrum displayed in figure 6, the DRIFT analysis shows that transformation of CuCI into Cu2OC12 does not occur during catalytic soot oxidation at 550 K. The DRIFT spectrum of soot partially converted in the presence of FeOCI is also included in figure 6. FeOCI catalyzes the formation of surface oxygen complexes. Similar absorptions as for the CuCI sample can be observed. The absorption band at 810 cm"l can be ascribed to FeOCI [24]. Apparently, during soot oxidation FeOCI is not reduced into FeCI2 or other iron chlorides, as was confirmed by a TG/DSC analysis [ 131. The following catalytic cycle is proposed for the activity of CuCI and other transition metal (oxy)chlorides. CuO + ~ Ch
I
CuCI-O* C
C0/C02
C-O,* - ' - - " ~ R3
02 CuCI 5
CuCh Starting soot oxidation with CuCI, the first step is oxygen activation on the surface of CuCI ~1). Transfer of activated oxygen (indicated by O*) occurs according to reaction R2. The DRIFT analysis shows that this results in the formation of Surface Oxygen Complexes (SOCs), indicated by C-O,*. Decomposition of the oxygen complexes results in the formation of CO and CO2. (Surface)
652 oxidation of CuCI has already been proposed in the seventies as an important step in the catalytic conversion of HCl to C12,called the Deacon reaction, and also in the oxy-chlorination of e.g. ethene [25]. Bulk oxidation of CuCI does not occur during soot oxidation, as was discussed previously. Instead, carbothermic (bulk) reduction of Cu2OC12 occurs around 600 K (if this compound is applied as the starting material), yielding CuCI and CO and CO2. Cu2OC12carbothermally reduces at lower temperatures than CuO (600 K vs. 685 K), and even in the presence of oxygen (figure 4). This indicates that a chlorine ligand affects the reducibility of CuO. Once CuCI is formed, it exerts its activity according to scheme (1). Starting soot oxidation with CuCI2 results in a higher soot oxidation temperature, than starting with CuCI or Cu2OC12, because decomposition of CuCb (reaction R5), which is apparently essential for an efficient activation of oxygen (on CuCI), takes place at temperatures higher than 700 K. Reaction R4 indicates the possible decomposition of the active (oxy)chloride into the oxide and chlorine by reaction with the activated oxygen. For CuCI this reaction does not occur in the temperature region, where it is catalytically active (550-650 K). However, for other metal chlorides bulk oxidation occurs at relatively low temperatures, even in the presence of soot, as was discussed previously. At these low temperatures transfer of activated oxygen to the carbon (soot) surface is not fast enough to compensate for reaction R4. As FeOCI was not carbothermally reduced, it is suggested that FeOCI activates and transfers oxygen, just like CuCI. Interestingly, the application of FeOCI leads to the formation of surface oxygen complexes, whereas catalytic soot oxidation by Fe203 does not [19]. This is another indication that chlorine chemically affects the catalytic soot oxidation activity of metal oxides and that the previous scheme also holds for FeOCI. Evaluating the experimental results, the high activity of metal (oxy)chloride based catalysts in the oxidation of soot is induced by a high mobility or volatility of the metal chlorides. Chlorine modification of transition metal oxides might also induce beneficial oxygen activation properties and/or transfer of activated oxygen to the soot surface. A priori, it is to be expected that a major problem of the application of a soot oxidation catalyst based on copper (oxy)chloride, such as the Cu/K/Mo/(CI) catalyst, will be deactivation by evaporation and/or decomposition of the active compound. Although copper chlorides can be reformed by reaction of KCl with CuMoO4 and a high stability has been reported [7,8], Neet~ has demonstrated that the 'loose contact' activity of the Cu/K/Mo/CI-ZrO2 catalyst significantly decreases after treatment in air at 975 K for 24 hours [12]. Moreover, CtrtK/Mo/Cl catalysts supported on ZrO2 and TiO~ were tested in the exhaust gas of a diesel engine, and a profound deactivation was observed [12]. Another aspect of soot oxidation catalyzed by (copper)chloride based catalysts worthwhile mentioning is the possibility that carbon-chlorine bonds are formed by an oxychlorination like reaction. When this reaction is followed by reaction with oxygen, very toxic compounds can be formed. Luijk et al. [26] have demonstrated that during oxidation experiments of an activated carbon catalyzed by CuCb, a relatively small bum-off of the chlorinated carbon surface gives rise to the production of chlorinated compounds such as chlorobenzenes and chlorophenols. Especially chlorophenols are very reactive precursors in the formation of polychlorinated dibenzo-pdioxines at carbon surfaces. In this respect chlorine containing soot oxidation catalysts are less attractive for practical applications. Since the results described in this paper show a correlation between the melting point and catalytic activity of transition metal chlorides, a possibility to eliminate the contamination of the environment by diesel soot particulate is to use oxidic cocktails with low melting points. Also other solutions to overcome the contact problem of transition metal
653 oxides, such as the application of diesel fuel additives [27] or the use of the NOx/soot reaction [28] need to be evaluated. 4. CONCLUSIONS 9Molybdates of copper and potassium have a moderate activity in (diesel) soot oxidation, even in 'tight contact'. 9The high soot oxidation activity in loose contact of catalytic systems containing an alkali metal chloride (KCI or CsCI or LiCI) and CuMoO4 (or CuWO4 or copper vanadates) can be ascribed to the formation of volatile copper chlorides. 9The metal (oxy)chlorides ofCu, Pb, Fe, and Bi appear to be more active in the oxidation of soot than their corresponding oxides in 'loose contact'. 9The high activity of metal chlorides can be explained by the in-situ formation of intimate contact between the soot and the active metal chloride by 'wetting' or through the gas phase. 9The high catalytic activity of metal chlorides and metal oxychlorides can be further explained by the activation of oxygen, followed by a transfer of the activated oxygen to the soot surface, resulting in the formation of surface oxygen complexes (SOCs). Decomposition of SOCs results in CO and CO2 formation. 9Although copper chlorides can be formed by reaction between KCI (which serves as a chlorine supplier) and CuMoO4, the application of the C ~ o / C I - Z r O 2 catalyst and other chloride based catalysts is questionable, because loss of activity due to evaporation and decomposition of the active species will occur eventually. REFERENCES
1. J.P.A. Neefl, Fuel Proc.Techn., 47 (1996) 1. 2. J.P.A. Neefl, O.P.v. Pruissen, M. Makkee and J.A. Moulijn, in A. Frennet, and J-M. Bastin, (Editors), Preprints of the Third International Congress on Catalysis and Automotive Pollution Control, April 20-22, 1994, Brussels, Belgium, 1994, p. 355. 3. A.F. Ahlstr6m and C.U.I. Odenbrand, Appl. Catal., 60 (1990) 157. 4. Sri Rahayu, W.L. Monceaux, B. Taouk and P. Courtine, in A. Frennet, and J-M Bastin, (Editors), Preprints of the Third International Congress on Catalysis and Automotive Pollution Control, April 20-22, 1994, Brussels, Belgium, 1994, p. 365. 5. A. L6we and C. Mendoza-Frohn, Appl. Catal., 66 (1990) L11. 6. P. Ciambelli, V. Palma and S. Vac.c,aro, Studies Surf. Sci. Catal., 71 (1991) 323. 7. P. Ciambelli, V. Palma and S. Vaccaro, Catal. Today, 17 (1993) 71. 8. P. Ciambelli, M. D'Amore, V. Palrna and S. Vacxaro, Combustion and Flame, 99 (1994) 413. 9. Y. Watabe, C. Yamada, K. Irako and Y. Murakami, Catalyst for use in cleaning exhaust gas particulates, European patent application 0092023, (1983). 10. S. Yuan, P. M6riaudeau and V. Perrichon, Appl. Catal. B: Env., 3 (1994) 319. 11. G. Mul, J.P.A. Neeft, F. Kapteijn, M. Makkee and J./L Moulijn, Appl. Catal. B, Env. 6 (1995) 339. 12. J.P.A. Neefl, G. Mul, M. Makkee, J.A. Moulijn, Appl. Catal. B: Env., (1996) accepted. 13. G. Mul, F. Kapteijn and J.A. Moulijn, Appl. Catal. B: Env., (1996) accepted. 13b.A. Goreaud, M. Goreaud and L. Walter-L6~, Bull. Soc. Chim. France, (1970) 2789.. 14. P.A.Neefl, M. Makkee, J.A. Moulijn, Appl. Catal. B: Env. 8 (1996) 57.
654 15. J. van Doom, J. Varloud, P. M&iaudeau and V. Perfichon, Appl. Catal. B: Env. 1 (1992) 117. 16. Gmelins Handbuch der Anorganischen Chemic, 8. Auflage, Verlag Chemic, GMBH.,Weinheim, Fe, 59 B (1932) 279. 17. Gmelins Handbuch der Anorganischen Chemic, 8. Auflage, Verlag Chemic, GMBH., Weinheim, Bi, 19 Erg. (1964) 689. 18. O. Knacke, O. Kubaschewski and K. Hesselmann, Thermochemical properties of inorganic substances, Vol. I and 1I, Springer-Verlag, Berlin, 1991. 19. Y-C Xie and Y-Q Tang, Adv. Catal., 37 (1990) 1. 20. C. Morterra and M.J.D. Low, Spectrosc. Lett., 15 (1982) 689. 21. Q.-L. Zhuang, T. Kyotani and A. Tomita, Energy & Fuels, 8 (1994) 714. 22. G. Mul, F. Kapteijn and J.A. Moulijn, in Proceedings of 22nd Biennial Conference on Carbon, San Diego, USA, 1995, pp. 554-555. 23. C.F. Ng, K.S. Leung and C.K. Chan, J. Ca'tal. 78 (1982) 51. 24. R.A. Nyquist and 1LO. Kagel, Infrared Spectra of Inorganic Compounds, Academic Press, New York, 1971. 25. J.A. Allen and A.J. Clark, Rev. Pure and Appl. Chem. 21 (1971) 145. 26. R. Luijk, D.M. Akkenn~ P. Slot, K. Olie and F. Kapteijn,Environ. Sci. Technol., 28 (1994) 312. 27. J.P.A. Neett, 'Catalytic oxidation of soot-Potential for the reduction of diesel particulate emissions', Ph.D. Thesis TU Deltt (1995), chapter 9. 28. G. Mul, F. Kapteijn, J.A. Moulijn, in preparation.
CATALYSIS AND AUTOMOTIVE POLLUTION CONTROL IV Studies in Surface Science and Catalysis, Vol. 116 N. Kruse, A. Frennet and J.-M Bastin (Eds.) 91998 Elsevier Science B.V. All rights reserved.
655
Copper catalysis for particulate removal from diesel exhaust gas. Copper fuel additives in combination with copper coatings. J.P.A. Neeft, S.J. Jelles, M. Makkee, and J.A. Moulijn Delft University of Technology, Section Industrial Catalysis, Julianalaan 136, 2628 BL, Delft, The Netherlands
An exploratory study was carried out with respect to the performance of a copper fuel additive in combination with monolithic wall flow filters for the removal of soot from diesel exhaust gas. Cordierite filters, copper coated cordierite filters, and silicon carbide filters were studied. Model experiments have been performed to investigate the influence of contact between soot and catalyst on the oxidation rate. The observation that the effect of a copper coating on the filter performance is marginal compared to the influence of the copper additive is supported by the model experiments. The contact between soot and catalyst is a crucial parameter for the performance of a catalyst. The contact between a copper coating applied to a filter and soot collected on this filter is not sufficient to achieve continuous, complete soot oxidation under realistic diesel engine exhaust conditions. I. INTRODUCTION Diesel engines are reliable, fuel efficient, and relatively clean. Since the application of three-way catalysts for otto engines, the emissions of diesel engines are subject to discussion. The emissions that are the main topics of concern are those of NOx and particulates. In this paper, the removal of particulates from diesel exhaust gas will be discussed. Particulates in diesel exhaust gas, often called 'soot', are agglomerates of spherical particles, 10 to 30 nm in size, and composed around graphitic spheres. In and on these spheres, hydrocarbons, water, and sulfate are condensed. In the hydrocarbon fraction among others polynuclear aromatics (PAH) are present and these are thought to have carcinogenic properties. The most convenient way to remove particulates from the exhaust gas is combustion. Diesel particulates are, however, relatively unreactive and oxidation occurs moderately at normal exhaust temperatures for both passenger cars and heavy-duty trucks. In the non-catalytic as well as the catalytic oxidation of soot the reaction rate under all circumstances is rather slow in comparison to the residence time in the exhaust system. To achieve complete oxidation, the reaction
656 conditions have to be optimized. The residence time has to be increased by using a filter. The reactivity of the particulates can be increased by either increasing the temperature of the filter system or by activation with a catalyst. Since the increase of temperature is an economically unattractive option, a catalytic solution is preferred. Straightforward application of an oxidation catalyst to a flow through monolith will result in the oxidation of adsorbed hydrocarbons and partly the graphitic nuclei of the soot. This system is able to reduce particulate emissions up to 50 % and is considered to be an intermediate solution [1]. The ultimate goal will be the complete removal of the soot. In general, in soot oxidation two solids, namely catalyst and soot, and a gas phase are involved. The interaction between the catalyst and the soot under practical conditions is poor. To achieve sufficiently high oxidation rates, this contact between catalyst and soot has to be improved [2]. Up to now, the most effective way to achieve sufficiently good contact between the catalyst and the soot is the application of a metal fuel additive [2]. The high activity can be explained as follows. The organo-metallic compound in the additive decomposes and the organic part is completely oxidized in the engine combustion chamber. The residual metallic part is enclosed in the soot at the moment of nucleation and subsequent particulate growth. This results in a high dispersion of the metal in the soot and thus in the ultimate contact between catalyst and soot. Metal based fuel additives, such as Ce [3], Cu [4, 5] , Fe [6], and Mn [7, 8, 9] have been studied for use in combination with a particulate filter. In general, a wall flow monolith is applied as a particulate filter in this kind of operations. In this study the performance of a copper fuel additive in combination with a cordierite wall flow monolithic filter is compared with that of copper coated wall flow monolithic filters. Furthermore, the performance of a silicon carbide wall flow monolithic filter was compared with that of a cordierite one. 2. EXPERIMENTAL 2.1.
Materials
Cordierite wall flow filters were made from segments of 20 mm in diameter and 40 mm in length cut out from an EX 47 / 100 filter, supplied by Corning Glass Works, and consisted of about 36 channels. From these segments, the alternate channels were plugged using ceramic glue (Ceramabond 569 supplied by Gimex B.V.). Silicon carbide filters with identical dimensions were supplied by Stobbe Engineering. Four different types of copper coating were applied to cordierite filters. Type 1 coating was applied by impregnation of a cordierite filter with a saturated Cu(NOa)2 solution, followed by drying and calcination for I hour at 575 K. Type 2 coating was similar to type 1 coating, but instead of one impregnation the filter was sequentially impregnated three times, each time followed by drying and calcination. Type 3 coating was prepared by impregnation of the cordierite filter
657 with a Cu(OH)2 slurry, followed by drying and calcination. With the calcination, the copper compounds are decomposed to copper oxide. Type 4 coating was a proprietary copper coating from ECS and was a gift from Lubrizol. The fuel used was reference diesel CEC-RF-03-A-84 (Haltermann). The lubrication oil used was Valvoline 10W40. The copper additive used was a gift of Lubrizol. 2.2. Procedures 2.2.1. Model experiments Four types of contact were studied. Printex-U, a model soot, was mixed with CuO using a spatula, resulting in 'loose contact' or using a ball mill, resulting in 'tight contact'. Further, Printex-U was impregnated with a solution of Cu(NO3)2 followed by decomposition of the nitrate at 675 K into CuO by heating in nitrogen. A fourth sample was soot collected from the engine described in p a r a g r a p h 2.2.3. with 100 ppm copper in the fuel. With these four samples, flow reactor and TGA experiments were performed. For experimental details see [2] and [10]. 2.2.2. Characterization of soot Soot samples were collected on a glass fibre filter with the engine running on fuel containing 100 ppm copper and analyzed by TEM and XRD. TEM analysis has been carried out with a Philips CM30, high resolution transmission electron microscope operating at 300 kV. This apparatus is integrated with energy dispersive analysis of X-rays (EDX). Alumina sample grids were used for the detection of copper in the soot particulates. XRD has been performed with a FR552 Guinier camera. 2.2.3. Performance m e a s u r e m e n t s The engine used was a Y a n m a r L90E 4 kW, direct injected, naturally aspired, air cooled, one cylinder diesel engine, and had a built-in generator set. The electrical power generated was converted into heat by a resistance bank and was used to control the engine output power. All experiments were performed at an engine load of 3 kW. The engine rotational speed was set at 3000 rpm. At the applied load of 3 kW, the particulate emission is 4.7 g/h [11]. The filters were glued onto a quartz tube with an outer diameter of 20 mm and a wall thickness o f - 1 . 5 mm using Ceramabond 569. The quartz tube was then placed in the oven, as illustrated in Figure 1. A side stream of the engine exhaust gas, generally ~12 1/min, was pumped through the filter. The oven temperature was controlled. The t e m p e r a t u r e within the filter was normally about 20 K below the oven temperature. Before each experiment, the pressure drop over the clean, unused trap was measured with air at a filter temperature of~575 K. The total gas throughput was measured with a wet gas meter. A guard filter, placed downstream of the test filter (item 9 in Figure 1) was used to check for leakage. If, after the experiment, the guard filter was not sufficiently clean, the experiment was considered to be a failure.
658 During the experiment, the pressure drop over the filter was continuously measured and recorded in combination with the temperatures upstream, downstream of, and within the filter. At the end of a test, the filter was usually regenerated by heating the oven with a controlled rate of 25 K/min to a temperature of 875 K, resulting in the complete combustion of the collected soot on the filter. During these regenerations the pressure drop over the filter and the temperatures within and downstream of the filter were recorded.
Figure 1. Experimental set-up XRF analyses have been performed with a Philips PW 1400 spectrometer to get a n indication of the amount of copper present in different trap types. Five traps were analyzed. The matrix of l~Iter test experiments is shown in Table 1. These tests were performed to compare the performance of catalytic coatings, of fuel additives, and of the combination thereof. These filter experiments were all performed with the oven temperature set at 650 K.
659 Table 1. Matrix of experiments (referred to by the run number) performed for comparison of filters. The oven temperature was set at 650 K for all experiments. a dditive concen tra tion filter type
0 ppm
100 ppm
blank cordierite
12
3-6, 14, 20
blank silicon carbide
11
10, 18, 19
cordierite, coating #1
1
cordierite, coating #2
2
cordierite, coating #3
7
cordierite, coating #4
22
200 ppm
23
The influence of the oven temperature on the collection behaviour has also been investigated. Several experiments have been performed with blank traps of cordierite and of silicon carbide, at different collection temperatures. For all these experiments, the additive concentration was 100 ppm. These experiments are listed in Table 2. Table 2. Matrix of experiments (referred to by the run number) performed with different external filter temperatures. The additive concentration was 100 ppm for all experiments. Oven t e m p e r a t u r e (K) l~lter type
blank cordierite
650
blank silicon carbide
3, 6, 14, 20 10, 18, 19
cordierite, coating #4
23
3.
675
700
725 13, 21
17
16
15 24, 25
RESULTS AND DISCUSSION
3.1. Model experiments
From the model experiments performed by Mul [10], comparison of contact modes results in an order of activity of i m p r e g n a t i o n > tlght contact > loose contact. Results from Neeft et al. [12], give an order of activity of addltlve >> t i g h t contact > loose contact. The difference in activity is several orders of magnitude. From Kissinger plots, derived from TGA/DTA data, the activation energy Ea and the pre-exponential factor ko can be determined. For soot with copper from an additive and soot with CuO in tight contact, the Ea is the same but the ko differs [11]. This indicates that the copper in its environment of soot is
660 the same in both cases, but the number of sites of interaction differs. In practice, the contact between soot and a catalytic coating applied on a filter, resembles loose contact. 3.2.
Soot characterization
Figure 2a shows a TEM micrograph of a collected soot sample generated with 100 ppm copper in the fuel. The agglomerates and the individual soot spheres of 30 40 nm can be distinguished. Copper particles should appear as dark spots. Although EDX analysis shows t h a t in the background copper is present all over the sample, dark spots are not observed. From the line resolution of the microscope of 0.16 nm, it can be concluded t h a t the copper in the particles is almost atomically dispersed in the soot and t h a t the sizes are generally smaller t h a n 1 nm. the sample contains at least 1 wt % copper. These results, therefore, confirm the TEM/EDX results. The copper is highly dispersed present within the soot. 3.3. Elemental composition of the filters after use
In Table 3, the analyzed traps are listed in combination with the amount of exhaust gas t h a t had been pumped through the filter and the copper concentration in the fuel used. Table 3.
sample 1
The history of several traps and their results of XRF analysis. life* (1)
fuel (ppm copper)
MgO (wt%)
A1203 SiO2 (wt%) (wt%)
CuO (wt%)
trap type blank cordierite (fresh) blank cordierite
-
-
9.2
48.9
38.9
-
4100
200
7.6
52.4
35.3
0.1
cordierite, coating #1
3800
100
8.5
42.4
29.0
17.3
cordierite, coating #2
3100
100
8.2
45.3
30.6
12.1
cordierite, coating #3
5000
100
7.6
50.2
33.1
5.3
*Life : Amount of exhaust gases in litre passed over the filter device XRF is a semi-quantitative technique and it gives a good impression on the relative amounts. The amount of copper present in the blank, cordierite trap (sample 2) is roughly two orders of magnitude lower t h a n the amount t h a t is deposited on such a trap by impregnation. The numbers in Table 3 show t h a t the copper coated filters contain high amounts of copper. In view of the fact that cordierite is a meso/macro porous material, the copper dispersion is expected to be low and reproducibility is also expected to be poor. The low number for exCu(OH)2 coating #3 (sample 5 in Table 3) is not surprising in view of the low solubility of Cu(OH)2 in the impregnation solution.
561
Figure 2a. TEM micrograph of soot particulates from a 100 ppm fuel additive run. No individual copper particles can be observed. - - " " - - : 100 nm
Figure 2b shows a micrograph of a r a t h e r unique spot in the sample, where amorphous copper particles around the soot can be distinguished by EDX. In the lower right corner, a large lump of copper can be seen. Both phenomena are probably the result of sintering of copper from soot, which has been accumulated in the exhaust system. Figure 2b. TEM micrograph of soot particulates from a 100 ppm fuel additive run. Copper particles are dark. -----:
100 nm
With XRD analysis no copper reflections were found. The smallest crystalline particles t h a t can be detected with the used camera are typically 5- 10 nm, when
662 the sample contains at least 1 wt % copper. These results, therefore, confirm the TEM/EDX results. The copper is highly dispersed present within the soot. 3.4. Filter performance tests The pressure drop over the clean, unused filters was around 20 - 30 mbar for silicon carbide and around 50 mbar for non-coated EX 47. For coatings of type #1 and type #2, the clean pressure drops were around 80 mbar. For type #3 coating, it was around 50 mbar and for the type #4 coating it was around 130 mbar. The reported pressure drops have been corrected for the contribution of the clean filter. In Figure 3, the pressure drop curves are presented for experiments performed with EX 47 filters at a temperature of 650 K. These curves show the
500 q no copper
,~'~ 400
/ r u n 12
~o
.-..-..run 14 ~ 3
///
"~ 300
/ //~/t///
200
__ l OOpp m
~
/copper
/ ..... .~--
run8
100
0 0
........
2 0 0 p p m copper
!
+
~-
I
2
4
6
8
.........
t 10
time (hours)
Figure 3. The influence of the additive concentration on the pressure drop. For every run, a fresh EX47 trap was used. The oven t e m p e r a t u r e was 650 K. dramatic influence of the additive concentration on the system performance. Without the additive the pressure drop increases steeply, whereas in the presence of the additive a constant level is reached. At higher copper concentrations, the pressure drop is lower. The experiments performed with 100 ppm copper in the fuel (3, 14, 20) show t h a t the reproducibility in these pilot bench experiments is remarkably good.
663
=o I t t
-I
~,
l OOppm copper
m
"u
~3oo
~ 2oo
~
100
0 0
2
4
time (hours)
Figure 4. The effect of regenerations on pressure drop curves. For all runs, the same EX47 trap was used. The filters were regenerated after each run. The oven temperature was 650 K, the additive concentration 100 ppm.
500
no copper
]'
i......................................
!
r'~ 400
1
i
run 12/ ....
E X 47
sic
/
/run
11
run 14 -----'--
/ |=
~ ~
run 19 - - - -
.
.
.
.
.
.
E X 47
_
l OOppm copper
_
__J
200
~
100
0
.................................. 0
-t- . . . . . . . . . . . . . . . . . . . . . 2
-4- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A
4
6
t ~ e (hours)
Figure 5. The influence of the filter material on the pressure drop. Thin lines represent runs with an EX47 trap. Thick lines represent runs with a SiC trap. The oven temperature was 650 K. In Figure 4, the influence of regeneration on the performance of the system is shown. The difference between the curves is not significant. This implies that there is no plugging of the filter by the copper collected on the filter within the experimental time. On the other hand, it is clear t h a t the copper that is built up
654 on the filter during a collection period, does not exhibit measurable activity after a regeneration probably due to sintering during the regeneration procedure. During the regeneration process of filters, the combustion of the soot collected on the filter proceeds very rapidly and causes a temperature runaway. The t e m p e r a t u r e in the filter can be as high as 1200 K during these events. This imposes high demands on the filter. Exceptions for this observation are silicon carbide filters and cordierite filters with coating #4. Regenerations of these filters are not accompanied by thermal runaways. Silicon carbide has a better thermal conductivity and this property possibly has a strong impact on the regeneration behaviour. In Figure 5, the pressure drop curve is plotted for the two different trap materials used. The build-up of pressure drop occurs less steep with silicon carbide filters t h a n with EX 47 filters. In presence of 100 ppm copper in the fuel, the performance of the two different trap materials are almost identical. 500
no copper
"~ 400 ~o
run 12/,'-~..run 22 S c ~
no coatina
3X) ~ / / ~ I
~'/
~
~
~ I~// o 100 ~
0
o
__
,.
no coaung
run7 run2
y /
0
run 14
~
coating#3 a
t
i
n
g
~
.
.
run 3
~ 2
l OOppm
oating #
.
.
.
copper
.
no coating
~-...... 4
200ppm copper
6
t~e (hours) Figure 6. The performance of copper t e m p e r a t u r e was 650 K for all runs.
coatings.
The
oven
The effect of a copper coating on the performance of the system is shown in Figure 6. The activity of a copper coating is very low compared to that of the copper additive. In all cases, the copper coated traps show a slightly lower equilibrium pressure drop, which might be favourable. It should, however, be t a k e n into account t h a t the pressure drop over clean, copper coated traps is in most cases higher t h a n t h a t of clean cordierite traps. Therefore, the reduction of the pressure drop by a coating is only relative. In an absolute sense, a copper coating results in a higher equilibrium pressure drop. In summary, coating of a trap in additive based systems will only be useful when the manufacture of the coated traps is done in such a way t h a t the coating does not contribute to the pressure drop.
665 All pressure drop curves in Figure 6 show a maximum except for the curve for the uncoated cordierite filter with 100 ppm copper (run 14). This maximum in pressure drop suggests that there is a build-up of activity on the filter. Two explanations can be given for this increased activity. A possible explanation is the formation of small copper particles that are stabilized by the soot. These particles have a high mobility and can be active [11]. A second explanation is also based on the formation of small copper particles. These particles can act as a catalyst for the oxidation of NO into NO2. The formed NO2 subsequently oxidizes the soot, forming NO. This catalytic cycle with NO as a key intermediate is described for platinum by Hawker [13] and for copper, chromium and molybdenum by Mul [10]. Although complete oxidation of diesel soot with only an NO-oxidation catalyst such as platinum may prove not to be possible, the catalytic cycle with NO as intermediate should be considered crucial for every catalytic process for oxidation of diesel soot.
NIO~
"~400
o
300
200 ~.~ r~ o
100
................
0
0
4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1
2
3
4
5
6
time (hours)
Figure 7. The performance of several filters at elevated temperature. The oven temperature was 725 K for all runs, the additive concentration 100 ppm. To get information on the temperature dependence, also experiments at high collection temperature were performed. Figure 7 gives results for 725 K and 100 ppm copper additive. Spontaneous regenerations can be observed with the cordierite filter without coating. These regenerations occurred stochastically, and are reproducible. The peak temperature measured in the filter during these regenerations were as high as 1200 K. With silicon carbide traps or a cordierite trap with coating #4, this phenomenon was not observed and stable operation seemed to be possible. An important difference between silicon carbide and cordierite is the thermal conductivity. Probably due to the high thermal conductivity of silicon carbide, hot spots in the filter can be avoided and
666 runaways do not occur. The stable operation characteristics of the copper coated cordierite trap could be explained by catalytic activity of the copper coating or by the increased thermal conductivity. The latter explanation is preferred by comparing the smooth regeneration behaviour of silicon carbide traps one of cordierite with coating #4, although a catalytic influence is not excluded.
4. CONCLUSIONS The additive concentration is of great influence on the pressure drop build-up with any filter coating or trap material. Copper coated EX47 filters reduce the pressure drop build-up over a filter, but this effect is limited compared to the effect of a higher additive dose. Further, the gain in corrected pressure drop is reduced by the fact that the clean pressure drop is higher for coated filters. The limited influence of the copper coating on the pressure drop is confirmed by model experiments. The activity of copper as an additive is several orders of magnitude higher than that of copper in loose contact, the latter resembling the contact between soot and a coating on a filter. Silicon carbide is a promising material for use in diesel filters. NO can play an important role in the catalytic oxidation of soot from diesel exhaust gas. REFERENCES
1. 2. 3. 4. 5.
J.P.A. Neeft, M. Makkee and J.A. Moulijn, Fuel Process. Technol. 47 (1996) 1 J.P.A. Neeft, M. Makkee and J.A. Moulijn, Chem. Eng J. 64 (1996) 295 K. Pattas et al., SAE Paper 920363 (1992) J. Saile, G.J. Monin and D.T. Daly, IMechE, (1993) 171 D.T. Daly, D.L. McKinnon, J.R. Martin, and D.A. Pavlich, SAE Paper 930131 (1993) 6. E. Miiller, B. Wiedemann, A.W. Preuss, and H.K. Sch~idlich, ATZ 91, (1989) 674 7. A.G. Konstandopoulos et al., SAE paper 880009 (1988) 8. E.D. Dainty et al., SAE Paper 870014 (1987) 9. B. Wiedemann and K.H. Neumann, SAE Paper 850017 (1985) 10. G. Mul, Catalytic diesel exhaust purification. Thesis (1997) ll.J.P.A. Neeft et al., IMechE (1996) 233 12.J.P.A. Neeft, M. Makkee and J.A. Moulijn, Appl. Catal. B:Environ 8 (1996) 57 13.P.N. Hawker, Platinum Metals Rev. 39 (1995) 2
CATALYSIS AND AUTOMOTIVE POLLUTION CONTROL IV Studies in Surface Science and Catalysis, Vol. 116 N. Kruse, A. Frennet and J.-M Bastin (Eds.) 91998 Elsevier Science B.V. All rights reserved.
667
Supported liquid phase catalysts: A new approach for catalytic oxidation in diesel exhaust particulate emission control. S.J. Jelles, B.A.A.L. van Setten, M. Makkee, and J.A. Moulijn Delft University of Technology, Section Industrial Catalysis, Julianalaan 136, 2628 BL, Delft, The Netherlands
An exploratory study was carried out with respect to the performance of molten salts as diesel soot oxidation catalyst. The activity of two binary eutectic salts, with a low melting point, was measured and compared with the activity of two very active solid single oxides. Also the influence of NO on the oxidation rate was investigated. It was observed that the molten salt catalysts are highly active compared to the solid single oxide catalysts, probably as a result of the increased contact area due to wetting of the soot by the mobile catalyst. The oxidation rate is strongly increased by the presence of NO in the gas phase. 1.
INTRODUCTION
In the abatement of the emissions of diesel engines, particulate removal is an essential step. Catalysis is a promising technology but a fundamental problem is the poor contact between particulate and catalyst in filters, which are currently under development. Diesel fuel additives are at this moment the best way to achieve optimal contact between catalyst and soot. The organo-metallic compound in the additive decomposes in the engine combustion chamber and the metallic part is integrated within the soot at the point of nucleation. Copper and ceria are successfully used as a fuel additive. Topics complicating the introduction of fuel additives on a large scale are numerous. The additive should be added to the fuel at some point, leading to either an additional distribution network for the fuel or a complicated dosage system. Another point is the emission of the metal or the catalytic side reactions which might occur. For example, polyaromatic hydrocarbons from the soot and chlorine, present in traces in the fuel, could be converted into dioxins by the catalyst. Copper is an active catalyst for this reaction. Finally, the additive could cause engine wear or oxidation of the fuel in the tank. In summary, catalytic solutions other than fuel additives could be favourable. The application of a catalytic system which becomes liquid at the average lower exhaust temperature, can have the possibility to achieve sufficient contact between catalyst and soot. The interaction between catalyst and soot is than no longer based on the poor interaction between two solids, but a good contact between a solid and a liquid. The use of a liquid catalyst is new in the field of exhaust gas cleaning and there are many unanswered questions and unsolved problems. In this exploratory work, it will be shown that the application of molten eutectic salt mixtures as liquid catalysts might result in sufficiently
668 active oxidation catalysts for continuous diesel soot removal. The most relevant topics will be discussed. 2. BACKGROUNDS 2.1. Contact between soot and catalyst In earlier work, [1, 2] it has been shown that the contact between soot and catalyst is crucial for the performance of the catalyst. McKee et al. reported that eutectic salt mixtures can be more active than their individual pure components in the catalysed gasification of graphite and coal char [3]. McKee also reported that the activity of a eutectic mixture increases dramatically above the melting point [4] and argued that this was caused by migration of the small salt droplets over the carbon surface. In diesel soot oxidation, the situation can be similar, but more complex. Migration of the catalyst over the soot surface can easily lead to loss of catalyst in the exhaust gas stream. Ideally the wetting of soot should occur without the loss of contact between the salt and the material by which the liquid catalyst is supported. In Figure l a, the interaction between a solid fractal soot particle and a solid catalyst interface is schematically drawn. It is clear that the number of contact points between the fractal soot and the catalytic surface are small, compared to the size of the soot particle.
Figure 1.
Simplified view of the contact between a fractal soot particle and the catalyst, a) normal solid catalyst, b) liquid catalyst
The contact between a liquid catalyst and a soot particle is schematically drawn in Figure lb. Because of the wetting capacity of the liquid catalyst, the contact area between soot and catalyst can be significantly higher than in the case with solid catalysts. It is possible that the wetting of the soot particle by the molten salt imposes a limitation on the transport of oxygen to the soot particle, as suggested by McKee [3]. On the other hand, an oxidation cycle with the catalyst as an intermediate, can be the dominating mechanism. This would imply that the complete coverage of the soot with a film of the liquid salt is perhaps favourable. 2.2. Particle capture The technology used for the application of a molten salt catalySt is a dominant issue. The soot particle has to be separated from the gas stream and brought into contact with the liquid catalytic phase, whereas the liquid phase should not be blown out or evaporated to the gas phase. The surface structure that is in contact with the gas stream should be large to increase
669 the contact area between particulates and catalyst and the structure should be porous to retain the catalyst. It is clear that a normally used filter, such as a wall flow monolith, is not preferred for the application with a molten salt, because the particles are collected on the filter in stead of in the filter. An interesting material can be a ceramic foam. Their normally lower filter efficiency can possibly be compensated by the wetting of the soot particles by the molten salt, resulting in high sticking efficiency of the soot onto the filter surface.
2.3. Catalyst selection The temperatures, where the system should operate range from 425 K to 1250 K. The water partial pressure in the exhaust gas (up to 10 %) and the gas velocity in the exhaust system (-~2.5 m/s) are high. This imposes a number of requirements that the molten salts have to fulfil, apart from intrinsic catalytic activity. The number of possible salt mixtures are of course large and rational design procedures are called for. The number of possibilities can be reduced as follows. Several molten salts are volatile under the applied conditions. Chlorides for example are known for their high vapour pressure. Nitrates, usually forming low melting point eutectic mixtures, are unstable and will decompose. Another point of interest is the reaction of the original salts into, for example, carbonates and sulfates with components in the exhaust gas, e.g. carbon dioxide or sulfur dioxide, respectively. This could influence the composition of the melt and, thereby, increase the melting point and subsequently reduce or even destroy the activity. The intrinsic activities of the catalytic material are also of importance. From earlier studies and literature, several components can be selected as promising candidate, for example vanadium, cobalt, and molybdenum [1].
2.4. Catalyst testing The soot oxidation activity of a molten salt can be studied using for example a thermobalance (TGA) or a micro-flow reactor. The long term stability of the salt cannot be tested straightforward using soot oxidation with this kind of equipment. Long period tests using a diesel engine have some drawbacks. Engine experiments are complicated and not always reproducible. Further, separate parameters, such as soot production, cannot be set without changing other parameters, such as exhaust temperature and the gas-phase composition of the exhaust stream, such as NOx, H20, and SO2 concentration. Therefore, a dedicated experimental set-up has been developed and is currently being evaluated. It consists of a feeding system, with which a constant amount of model soot can be introduced into the gas stream. The soot loaded gas stream is led towards a catalyst system. The pressure drop over the catalyst filter and the production of CO and CO2 are measured. In this way, the performance of the catalyst can be determined quantitatively. The feed gas composition can be varied and components such as water, SO2, and NOx, can be added without changing the amount of soot.
670 3. E X P E R I M E N T A L
3.1. Catalyst preparation The eutectic mixtures discussed here were CsVO3 - MoO3 (75.5 mol% - 24.5 mol%) (A) and CSEMoO4 - V205 (49 mol% - 51 mol%) (B), with a eutectic melting point of 635 K and 625 K, respectively. C s V O 3 was prepared by sintering of stoichiometric quantities of Cs2CO3 and V205 at 725 K overnight, followed by melting at 925 K and cooling down during several hours. The obtained crystals were white. Cs2MoO4 w a s prepared by sintering of stoichiometric quantities of Cs2CO3 and MoO3 at 725 K overnight, followed by melting at 1200 K and cooling down in several hours. The obtained crystals were white. The eutectic mixtures were prepared by heating of physical mixtures of stoichiometric quantities of the composing salts at 100 K above the eutectic melting point for 1 hour, followed by cooling down. Differential thermal analysis, (DTA) performed with a STA 1500 from Polymer Laboratories confirmed the presence of a eutectic composition. The eutectics were milled and sieved.
3.2. Flow reactor experiments The flow reactor equipment used is described in detail by Neeft [5]. As a model soot, Printex-U, a gift from Degussa, was used. Details about the properties of Printex-U are given by Neeft [5]. Catalyst samples, sieve fraction 50 - 215 p m , and Printex-U were mixed with a mass ratio of 4:1 using a spatula. From these samples, 100 mg were used in the flow reactor. The activity of the eutectic mixtures A (CsVO3 - MoO3) and B (Cs2MoO4 - V205) and of copper oxide, and molybdenum oxide, all in loose contact with the soot, was measured at a temperature of 650 K. The gas composition was 10% oxygen in argon. The influence of NO on the oxidation rate with catalyst B was investigated at a temperature of 675 K, with 10 % oxygen and 1000 ppm NO in argon.
3.3. TGA experiments A STA 1500 thermobalance from Polymer Laboratories was used to investigate the activity of the eutectic mixtures. The ratio catalyst : Printex-U used was 4 : 1. The samples were diluted with silicon carbide. A heating rate of 5 K/min was applied in synthetic air. In a Cahn TG 131 thermobalance, two samples were analysed. One sample was blank cordierite EX47 (Coming) and one sample was cordierite impregnated with CsVO3 - MoO3. The load of salt on the cordierite was 1 g/g. From both samples, a segment of lxl cm and 1 channel (-~0.5 cm) thick was cut. Both segments were then covered with about 40 mg PrintexU. The soot covered segments were placed in the thermobalance. The sample was flushed with air. The temperature was increased from room temperature to 625 K with 5 Fdmin and was sequentially heated up to 675 K with 0.5 K/min, kept there for 6 hours, and heated with 1 FUmin up to 875 K. The mass signal and the temperature signal were simultaneously recorded.
671 4. RESULTS AND DISCUSSION
4.1. Flow reactor experiments
4.1.1. Comparison with single oxide catalysts In Figure 2, the oxidation rate of Printex-U at 650 K is plotted as a function of conversion for the eutectic mixtures CSEMoO4- 7 2 0 5 and C S 7 0 3 - MoO3 and for the single oxides MoO3 and CuO, all in loose contact. The experiments were performed under identical conditions. It is clear that over the whole conversion range the oxidation rate is 2-3 times higher for the eutectic mixtures than for the single oxides. The difference in oxidation rate increases with higher conversion. In contrast to copper and molybdenum, the rate of oxidation in the eutectic mixtures remains more or less constant up to nearly complete conversion. This means that the contact between the soot and the catalyst is maintained during oxidation.
8.0.E-05
, i
.L,t"
E
6.0.E-05
i
4.0.E.05
~
g
!
",
", Cs2MoO4.V205 ",
i 2.0.E-05
"--
- ~ ~ . . ~ . ~ C
uO
" * " ~ .....
MoO3 O.O.E+O0
. 0.1
.
. 0.3
. 0.5 conversion
0.7
0.9
(-)
Figure 2. Oxidation rate (in mg C combusted / mg C initially present/s) of Printex-U at 650 K. The gas phase was 10 % 02 in argon. The experimental time on stream was the same for each experiment. The ratio soot : catalyst was 1:4 (g/g).
4.1.2. The influence of NO In Figure 3, the influence of NO on the oxidation rate of Printex-U is shown. The eutectic mixture used for this experiment is CsVO3 - MOO3. The influence of NO on the oxidation rate is significant, although the shape of the oxidation curve is not changed.
672
1.5E-04
...........................................................................................................................................................................................................
1.0E-04
o
~
o
,,~
"- o o
E
=,
~
with NO ",..
O~ v
E .~
~
5.0E-05
o o
,,o
without
0.0E+00 0.20
NO
I
i
i
0.40
0.60
0.80
1.00
c o n v e r s i o n (-)
Figure 3. The influence of the NO on the oxidation rate (in mg C combusted / mg C initially present / s) of Printex-U at 675 K catalysed by CsVO3 MoO3. The gas phase was 10 % O2 in argon. The NO is probably enhancing the oxidation of soot by an oxidation cycle. This cycle is discussed by Mul [2] and Hawker [6]. Mul proposed a cycle as shown in figure 4. The NO is oxidised catalytically into NO2, whereas the soot is oxidised by NO2 uncatalysed. The oxidation by NO2 can also be catalysed. This could be a possible application of two different catalytic systems. One metal then catalyses the oxidation of NO to NO2 and the other metal catalyses the soot oxidation. Recent work with fuel additives supports the existence of such a mechanism [7].
02
I catalyst
c NO
NO2
I Icatalyst I ] Figure 4. Catalytic cycle with NO as intermediate [2]
co + co2
673 4.2. T G A
experiments
In Figure 5, the TGA-DTA curves for the eutectic mixture Cs2M004 - V205 are shown.
1.2 1.0 "7", m 0.8
15 "--"
i. . . . . .
:= _ - ~ N ~ \
.....
-
, relative mass --heatflow
o.6 4.a
r
.....
0.4
/!i___
0.0 -0.2 400
................. mittin eutect of c i 500
1
600
! 700 temperature
~r'
g
o
0
a)
0.2
10
--,,.,,..' i
1
800
900
-10 1000
(K)
Figure 5. TGA-DTA curves for the eutectic catalyst mass ratio catalyst" soot was 4 19 (g/g).
Cs2M004
- V205
9The
Around the melting point of the eutectic, 620 - 640 K, the DTA signal shows a small dip for all mixtures, representing the fusion enthalpy. From this point on, the catalyst immediately shows combustion activity, as can be seen from the rising DTA signal. In Figure 6, the mass loss and temperature are plotted against the time for the two cordierite samples that were analysed with the Cahn TG 131 thermobalance. It is clear that in the isothermal part at 675 K, the mass loss of the impregnated cordierite sample occurs significantly faster than that of the blank cordierite sample. The majority of the soot is burnt at this temperature. For the blank, not impregnated, cordierite sample, the major part of the soot only bums at higher temperature. It should be noted that the circumstances for contact between soot and catalyst were not optimal under these conditions. The soot was deposited on the segments when they were at ambient conditions. Therefore, the catalyst was in the solid state and no wetting could have occurred. This means that significantly higher oxidation rates could be observed under realistic conditions. Although detailed studies are called for, these results show the potential of eutectic catalysts. The difference between the rates for the blank and the impregnated samples is not very large although the molten salts clearly show a higher oxidation rate.
674
1000
~-'- -'-~
-10
~__,_.
..~.. _~_. - 9~
.
v ry~ ~O -20
.
.
.
.
,~'~'-"~---~-" / /
.
.
.
.
.
.
.
.
.
.
.
.
.
.
~.~,,~ s
.st ~t
. ~ J"
_ . . . . . . . . .
800
.~
e"
impregnated ~.-- cordierite ~
\~ \ blank ~-cordierite
600
400 -30
-
~ - ~
~1\ 200
-40
I ..................................
0
200
I .......
400
I
600
800
time (minutes) Figure 6.
TGA-curves for CsVO3 - MoO3 impregnated and blank cordierite segments, covered with Printex-U.
5. CONCLUSIONS The use of a Supported Liquid Phase Catalyst offers an opportunity for continuous removal of soot from diesel exhaust gas. Molten salt mixtures show a higher activity compared to solid metal oxides. This high activity can be ascribed to a better contact between the liquid catalyst and the soot. The contact between soot and catalyst remains intact during oxidation. The oxidation rate of soot is further enhanced by NO, which is present in diesel exhaust gas and this effect is, therefore, favourable. For successful application, a technology has to be developed to bring the soot into contact with the catalyst without losing the catalyst during operation. Ceramic foams impregnated with a molten salt or dedicated catalyst/filter systems are thought to be promising candidates for this application.
REFERENCES 1. 2. 3. 4. 5. 6. 7.
J.P.A. Neeft, M. Makkee and J.A. Moulijn, Chem. Eng. J. 64 (1996) 295 G. Mul, Catalytic diesel exhaust purification, PhD thesis (1997) D.W. McKee, C.L. Spiro, P.G. Kosky, and E.J. Lamby, FUEL 64 (1985) 805 D.W. McKee, Carbon, 25 (1986) 587 J.P.A. Neeft, Catalytic oxidation of soot, PhD thesis, 1995 P.N. Hawker, Platinum Metals Rev. 39 (1995) 2 S.J. Jelles, M. Makkee, J.A. Moulijn, unpublished results
AUTHOR INDEX
675
Abi-Aad E.
625
Bowker M.
Abouka'is A.
625
Brungard N.L.
165
Acke F.
285
Burch R.
199
Andersson B.
317
Calis H.P.A.
357
Angelidis T.N.
155
Capelle S.
625
Arendarskii D.A.
507
Cataluna R.
591
Avila P.
233
Cauqui M.I.
611
Azelee W.
495
Cerrato G.
601
BNagy J.
365
Chajar Z.
335
Baiker A.
61
Chevrier M.
135,335
Bailey C.
495
Ciambelli P.
307, 635
Bainier- Davias N.
335
Cochardo P.
611
93
Conesa J.C.
591
Bazin P.
571
Connerton J.
327
Bennett R.
431
Corbo P.
307
Bensaddik A.
265
Corro G.
93
Bernal S.
611
Cort6s Corberan V.
591
Blanchard G.
571
Courcot D.
625
Blanco G.
611
Cunningham J.
409
Blanco J.
233
Daniell W.
495
Boix A.
175
Dathy C.
419
Borovinskaya I.P.
477
Day J.Paul
353
Botas T.A.
73
Deeba M.
529
Bourges P.
213
Denton P.
335
t
Barbier J.
409,431
676
Gauthier C.
Descorme C.
275
Di Monte R.
185,569
Gelin P.
275
Donnerstag A.
125
Gilot P.
297
Drewsen A.
113
Goguet A.
275
Dubien C.
399
Gonzalez-Velasco
Duprez D.
549
Granger P.
Eckhoff S.
223
Graziani M.
Eigenberger G.
125
Grigoryan E.H.
477
Emons H.
155
Gubitosa
569
Eriksson L.
285
Guczi L.
347
Essayem N.
137
Guilhaume N.
581
Farrauto R. J.
529
Guti6rrez-Ortiz
Farrell F.
409
Guyon M.
297
Feeley J.
529
Halfisz J.
367
Ferrero A.
569
Harrison P. G.
495
Ferret R.
73
137, 335,549
73 419 185,569
73
Hecq W.
Filkin N.C.
255
Herzog P.
35
Fonda E.
185
Hesse D.
223
Hickey N. J.
409
Hoebink J.H.B.J.
389
Irusta S.
175 507
Fornasiero P. Fr6ty R.
Fridell E.
185,569 137 113,285,537
Fritz A.
243
Ismagilov Z. R.
Frost J.C.
379
Isomura N.
83
Gambino M.
307
J~irSs S. G.
465
Garin F.
27, 265
Jelles S.J.
621,655,667
677
Jobsson E. Johansson S.
113 285,537
Le Govic A.M.
571
Leason P.
513
Jones I.
431
Leclercq G.
419
Joyner R.W.
327
Leclercq L.
419
Kapteijn F.
645
Lecomte J.J.
419
L~cuyer C.
275
Leyrer J.
223
Kaspar J.
185,569
Kessler H.
297
Kirchner T. Kiricsi I.
25 347, 367
Lindner D. Lioutas Ch.B.
155 495
Kisenyi J.M.
513
Lloyd N.C.
Knapp C.B.
233
Lox E.S.
Kiinig A.
125
Lunati S.
Kiippel R.A.
61
51
Mabilon G.
51,223 213 213,419
Koryabkina N.A.
507
Magnacac G.
Koutlemani M.M.
a55
Maire G.
Krawczyk M.
265
Makkee M.
621,645,655,667
Kreijveld R.J.M.
389
Marecot P.
93
51
Marin G.B.
389
Kreuzer T.
601 147, 243,265
Kucherov A.V.
441
Marques Da Silva A.
Kiinkel C.
317
Martinez-Arias A.
591
Massardier J.
103
93
Lafyatis D.S.
79
Lakeev S. G.
441
Mathis F.
137
Lambert R.M.
255
Merzhanov A.G.
477
Lavalley J-C.
571
'Migliardini F.
307
Le Chanu V.
297
Mir6 E.
175
678
Moraweck B.
103
Primer M.
Morgan G.
513
Pritchett D.
513
Morterra C.
601
Qian M.
485
Mouaddib N.
265
Rajaram R.R.
379
Mouaddib-Moral N.
549
Renouprez A.
103
Rodriguez-Izquierdo
611
Rogemond E.
137
51
Rohe R.
147
645
Russo P.
635
621,645,655
Salasc S.
137
Moulijn J. A. Mubarak A.T. Muf~mann L. Mul G. Neeft J. P.A.
621,645,655,667 495
137,275,335,581
Nibbelke R.H.
389
Saur O.
571
Odenbrand I.
317
Schay Z.
347
Palermo A.
225
Schiibel G.
367
Palma V.
635
Schweich D.
399
Pang X.Y.
485
Searles R.A.
23
Papadakis V.G.
155
Shelef M.
441
Perrichon V.
a37
Shikina N.V.
a07
Pettersson L.J.
465
Shinjoh H.
Petunchi J.
175
Shkrabina R.A.
507
Pintado J.M.
611
Sklavounos S.A.
155
Pitchon V.
83
147,243,265
Skoglundh M.
113,285,537
Poulston S.
431
Smaling R. M.
165
Prado G.
297
Smelder G.
285
Praliaud H. Prigent M.
103,335 419
Sobukawa H. Soe K.
83 513
679
Soria J. Sugiura M. Sung S. Tagliaferri S.
591 83
165 61
Taha R.
549
Thorm~ihlen P.
113
Tikhov M.S.
255
Tooby C.
513
Tiirncrona A.
113,537
Trillat J.F.
103
Vaccaro S.
635
van den Bleek C.M.
357
van den Tillaart
223
van Kooten W.E.J.
357
van Setten B.A.A.L. van Yperen R..
621,667 51
Varga J.
367
Vassallo J.
175
Vlaic G.
185
Voulgaropoulos A.
155
Wahlberg A.M.
465
Walker A.P
379
Walsh M. Watling T.C.
199
Weeks C.
409
Westerberg B. Yates M.
Yentekakis l.V.
285, 317, 537 233 255
Zeng H.C.
485
Zillikens M.
513
This Page Intentionally Left Blank
681
LIST OF PARTICIPANTS
ACKE
Filip
Chalmers University of Technology
Sweden
ADELMAN
Bradley Jay
Universit6 Pierre et Marie Curie
France
ANDERSSON
Lennart
Volvo Car Corporation
Sweden
ANDORF
Renato
Daimler- Benz A.G.
Germany
ANDRE
Didier
Total Raffinage Distribution
France
ANGELIDIS
Thomas
Aristotle University
Greece
ASUQUO
Raymond
Chemopetrol A.S.
Czech Republic
AUERNHAMMER
Markus
Swiss Federal Institute of Technology Switzerland
BAIKER
Alfons
Swiss Federal Institute of Technology Switzerland
BAILEY
Craig
Nottingham University
United Kingdom
BARBIER
Jacques
Universit6 de Poitiers
France
BASHFORD- ROGERS Graham
Delphi Automotive Systems
Luxemburg
BASTIN
Jean-Marie
Universit6 Libre de Bruxelles
Belgium
BAZIN
Philippe
ISMRA
France
BELOT
G.
PSA Peugeot Citroen
France
BENNETT
Paul
BP Oil
United Kingdom
BERGER
Marc
Universit6 Pierre et Marie Curie
France
BERNAL
Serafin
University of Cadiz
Spain
682 BERTILSSON
Tommy
Scania
Sweden
BONNEFOY
Frederic
Bosal
Belgium
BORDET
Antoine
Johnson Matthey Ltd
United Kingdom
BORNER
Robert
Universit6 libre de Bruxelles
Belgium
BOURGES
Patrick
Institut Frangais du P6trole
France
Royal Institute of Technology
Sweden
BOUTONNET KIZLING Magali BOWKER
Michael
University of Reading Whiteknights United Kingdom
BRINKMEIER
Clemens
Universit~itStuttgart
Germany
BROECKX
Willy
Texaco
Belgium
BROWN
David
Zeton Altamira
BRUSSELAERS
Union Mini6re Hoboken
Belgium
BUESS- HERMAN
Claudine
Universit6libre de Bruxelles
Belgium
BUGLASS
John
Shell International Chemicals BV
The Netherlands
BURCH
Robbie
University of Reading, Whiteknights United Kingdom
BURGER
Beate
University of Stuttgart
Germany
Delft University
The Netherlands
CALLS - VAN KOOTEN CAPANNELL1
Gustavo
Universita Genova
Italy
CAUVEL
Anne
Katholieke Universiteit van Leuven
Belgium
CERRATO
Guiseppina
University of Turin
Italy
CHANDES
Karine
E.C.I.A.
France
CIAMBELLI
Paolo
University of Salerno
Italy
683 CONESA
Jos~ C.
Consejo Superior de Investigaciones Cientificas
Spain
CONNERTON
Jan
Nottingham Trent University
United Kingdom
CORBO
Pasquale
Instituto Motori CNR
Italy
Combo
Griselda
Universit6 de Poitiers
France
COURCOT
Dominique
Universit6 du Littoral
France
COVEY
David
Shell Additives International LTD
United Kingdom
Cox
Julian
Johnson Matthey Technology Centre
United Kingdom
DATH
Jean-Pierre
Fina Research S.A.
Belgium
DAVIAS
Nathalie
Renault S.A.
France
DAVIES
Michael J.
Rover Group Ltd
United Kingdom
DAY
J. Paul
Coming Incorporated
NY, USA
DE DEKEN
Jacques
Catalytica Inc.
CA, USA
DECKER
S6bastien
Universit6 libre de Bruxelles
Belgium
DEMEL
Yvonne
Degussa AG
Germany
DEMIDDELEER
Jean-Pierre
Universit6 libre de Bruxelles
Belgium
DREWSEN
Astrid
Chalmers University of Technology
Sweden
DUBIEN
Cecile
CNRS
France
DUESTERDIEK
Thorsten
Volkswagen AG
Germany
DUPREZ
Daniel
Universit6 de Poitiers
France
DURAND
Daniel
Institut Fran~ais du P6trole
France
ECKHOFF
Stephan
University Hannover
Germany
684 EL OUADI
Brahim
Universit6 de La Rochelle
France
ERIKSSON
Lars
Chalmers University of Technology
Sweden
ESPRIT
Marleen
Union Mini~re
Belgium
EUSDEN
Alan T.
Coming GmbH
Germany
EVANS
Julia
Johnson Matthey Technology Center United Kingdom
FARRAUTO
Bob
Engelhard Corp.
Iselin NJ, U.S.A
FAVENNEC
Jean
Rosi
France
FONTAINE
Jean-Luc
Universit6 de Liege
Belgium
FORNASIERO
Paolo
University of Reading, Whiteknights
United Kingdom
FRANCIS
Agna
Katholieke Universiteit van Leuven
Belgium
FRENNET
Alfred
Universit6 Libre de Bruxelles
Belgium
FRENNET
Elsie
Universit6 Libre de Bruxelles
Belgium
FRIDELL
Erik
Chalmers University of Technology
Sweden
FUCALE
Michele
Fiat Auto
Italy
GABRIELSSON
Par
Haldor Topsoe A/S
Denmark
GAGNERET
Philippe
Allied Signal
France
GALTAYRIES
Anouk
Facult6s Universitaires Notre Dame de la Paix
Belgium
GARIN
Francois
Institut Le Bel
France
GELIN
Patrick
Universit6 Claude Bemard Lyon I
France
GLOCKER
Reiner
Condea Chemie Gmbh
Germany
GOERIGK
Christian
Porsche AG
Germany
685 GONZALEZVELASCO Juan R.
Universitad del Pais Vasco
Spain
GRAFFAGNO
Giovanni
Johnson Matthey Ltd
Italy
GRANGE
Paul
Universit6 Catholique de Louvain
Belgium
GRANGER
Pascal
France
GRAZIANI
Mavro
Universit6 des Sciences et Technologies de Lille University of Trieste
GRIGORYAN
Eduard
Inst. of Structural Macrokinetics RAS Russia
GUBITOSA
Giuseppe
Magneti Marelli DSS
Italy
GUILHAUME
Nolven
Universit6 Claude Bemard Lyon I
France
GUYON
Marc
Renault S.A.
France
HALASZ
Janos
Joseph Attila University
Hungary
HALPIN
Eibhlin
University of Reading, Whiteknights
United Kingdom
I-IAN
HYUN-Sik
Heesung Engelhard
Korea
HANNA
Josh
Coming Incorporate
NY, USA
HANSEN
Poul Lenvig
Haldor Topsoe A/S
Denmark
HANSSON
Maria
Volvo Car Components Corporation
Sweden
HARMSEN
Italy
Eindhoven University of Technology The Netherlands
HARRISON
Philip G.
Nottingham University
United Kingdom
HARTOFELIS
Christopher
Coming GmbH
Germany
HARTWEG
Martin
Daimler-Benz AG
Germany
HAWKER
Pelham
Johnson Matthey Technology Center United Kingdom
HECQ
Walter
Universit6 Libre de Bruxelles
Belgium
686 HENN
Jtirgen
Coming GmbH
Germany
HERKT-BRUNS
Christian
ULB
Belgium
HERZOG
Peter
Gesellschaft fiir Verbrennungskraftmaschinen und Messtechnik m.b.H.
Austria
HICKEY
Neal
University College Cork
Ireland
HJORTSBERG
Ove
Volvo Car Corporation
Sweden
HODJATI
Shahin
ECPM- LERCSI
France
HOEBINK
J.H.B.J.
Eindhoven University of Technology The Netherlands
HOLMGREN
Anna
Chalmers University of Technology
Sweden
HOLY
Gerhard
Gesellschaft ftir Verbrennungskraft maschinen und Messtechnik m.b.H.
Austria
HUBERT
Claude
Universit6 libre de Bruxelles
Belgium
IRUSTA
Silvia
Fac d Ingenierio Qca UNL Argentino Argentina
ISMAGILOV
Zinfer
Boreskov Institute of Catalysis
Russia
JANNES
Georges
Institut Meurice
Belgium
JAPENGA
Marten
Zeton Altamira
The Netherlands
JARAS
Sven
Royal Institute of Technology
Sweden
JAYAT
Francois
Katholieke Universiteit van Leuven
Belgium
JELLES
Sytse
Delft University
The Netherlands
Volvo Car Corporation
Sweden
JOBSON JOHANSEN
Keld
Haldor Topsoe A/S
Denmark
JOHANSSON
Stefan
Chalmers University of Technology
Sweden
JONES
Hannah
Johnson Matthey Technology Center United Kingdom
687 Friedrich
Emitech GmbH.
Germany
KASPAR
Jan
University of Trieste
Italy
KEENAN
Matthew
Rover Group Ltd/Warwick University
Untied Kingdom
KESSLER
Henri
Ecole Nationale Sup6rieure de Chimie France de Mulhouse
KEUNG
Michael
Condea Chemie GmBH
Germany
KHARAS
Karl C.C.
ASEC Manufactoring
Tulsa Oklahoma, U.S.A.
KIM
[3Seok Jae
Hyundai Motors co
Korea
KIRCHNER
Thomas
Bayer AG
Germany
KISENYI
Jonathan
Ford Motor Co
United Kingdom
KNAPP
Carlos
Inst. de Catalisis CSIC
Spain
KONRAD
Brigitte
Daimler-Benz AG
Germany
KONSTANDOPOULOS Athanasios
CPERI
Greece
KOPPEL
Ren6
Swiss Federal Institute of Technology Switzerland
KOSTERS
Martina
Universitiit Hannover
Germany
KREUZER
Thomas
Degussa AG
Germany
KRIJNSEN
Henrike
Delft University
The Netherlands
I~USE
Norbert
Universit6 Libre de Bruxelles
Belgium
KRUTZSCH
bernd
Daimler Benz A.G.
Germany
KUMBERGER
Otto
BASF Aktiengesellschaft A.G.
Germany
KWON
Yeong
Esso Petroleum Co
United Kingdom
LAURELL
Mats
Volvo Car Corporation
Sweden
KAISER
688 LAVALLEY
Jean-Claude
ISMRA
France
LECLERCQ
Lucien
Universit6 des Sciences et Technologies de Lille
France
LECLERCQ
Ginette
Universit6 des Sciences et Technologies de Lille
France
LEDUC
Bemard
Universit6 Libre de Bruxelles
Belgium
LEHMANN
Ulrich
Condea Chemie GmBH
Germany
LEMAIRE
Jacques
Rh6ne-Poulenc Chimie S.A.
France
LI
Xinsheng
Universit6 libre de Bruxelles
Belgium
LICKES
Jean-Paul
ULB
Belgium
LJUNGSTROM
Sten
Chalmers University of Technology
Sweden
LOENDERS
Raf
Katholieke Universiteit van Leuven
Belgium
LOFBERG
Axel
Universit6 des Sciences et Technologies de Lille
France
Lox
Egbert
Degussa AG
Germany
MABILON
Gil
Institut Fran~ais du Pdtrole
France
MAKKEE
Michiel
Delft University of Technology
The Netherlands
MARECOT
Patrice
Universit6 de Poitiers
France
MARET
Dominique
E.C.I.A.
France
MARTENS
Johan
Katholieke Universiteit van Leuven
Belgium
MARTIN
Brigitte
Institut Frangais du P6trole
France
MARTIN
Ashley
Johnson Matthey Ltd
United Kingdom
MASSARDIER
Jean
Inst. de Recherches sur la Catalyse CNRS
France
MASUDA
Masaaki
NGK Europe GmbH
Germany
689 MAUNULA
Teuvo
Kemira Metalkat
Finland
MEDVEDYEV
Valentyn
Universit6 Libre de Bruxelles
Belgium
MERKLE
Friedebald
Degussa AG
Germany
Mel Chemicals
United Kingdom
MOLES MOLLER
Thomas
Condea Chemie Gmbh
Germany
MONTICELLI
Orietta
Katholieke Universiteit van Leuven
Belgium
MONTIERTH
Max R.
Coming GmbH
Germany
MOSCHOUDIS
Nikos
Aristotle University Thessaloniki
Greece
MURTAGH
Martin
Coming Incorporate
NY, USA
MUSSMANN
Lothar
Degussa AG
Germany
NIBBELKE
R.
Eindhoven University of Technology The Netherlands
NIEUWENHUYS
Bemard
Leiden University
The Netherlands
NOBILE
Cosimo
Politecnico di Bari
Italy
O'MEARA
Rainaldo
Johnson Matthey Ltd
United Kingdom
PAOLO
Ingemar
Chemical Engineering II
Sweden
PARMENTIER
B6atrice
Universit6 Libre de Bruxelles
Belgium
PENTENERO
Andr6
Universit6 Henri Poincar6
France
PERRICHON
Vincent
Universit6 Claude Bernard Lyon I
France
PETTERSSON
Lars J.
Royal Institute of Technology
Sweden
PHILLIPS
Paul
Johnson Matthey Ltd
United Kingdom
PITCHON
V6ronique
LERCSI
France
690 PONCELET
Georges
Universit6 Catholique de Louvain
Belgium
PRIGENT
Michel
Institut Fran~ais du P6trole
France
PRIMET
Michel
Universit6 Claude Bernard Lyon I
France
PRIN
Marie-Agnes Pechiney
France
RICHTER
Thomas
VolkswagenAG
Germany
RIED
Thomas
TH- Darmstadt
Germany
RINGQVIST
GOran
Degussa Norden AB
Sweden
ROBOTA
Heinz J.
Allied Signal
USA
ROHE
Renaud
LERCSI
France
ROUSSEAU
Paul
Touring Secours, FEDERAUTO, Moniteur Automobile
Belgium
Russ
Gerald
Adam Opel AG
Germany
RYOO
Wan-Hyang
SABATINO
Luigina
Eniricerche S.p.A.
Italy
SALAMATI
Hassan
Delphi Automotive Systems
Grand Duch6 de Luxembourg
SALIN
Laurence
PSA- Universit6 de Paris 6
France
SCHARR
Detlef
Daimler Benz AG
Germany
SCHAY
Zolt~n
Institute of Isotopes
Hungary
SCHIMMER
Peter
Degussa AG
Germany
SCHMITT
Dietmar
TH- Darmstadt
Germany
SCHMITZ
Guy
Universit6 libre de Bruxelles
Belgium
SCHMITZ
Eric
ULB
Belgium
S-Korea
691 SCHNEIDER
Stephanie
Automobiles Peugeot
France
SCHULZ
Philippe
Elf Antar France
France
SCOTT
Stephen
NGK Europe GmbH
United Kingdom
SEARLES
Robert
Automobile Emissions Control by Catalyst
Belgium
SEGUELONG
Thierry
Rh6ne-Poulenc Recherches S.A.
France
SEXTON
Brian
Delphi Automotive Systems
Luxemburg
SHELEF
M.
Ford Motor Co.
MI, USA
SHINJOH
Hirofumi
Toyota Central
Japan
SIEMUND
Stephan
Engelhard Technologies G m b H
Germany
SKOGLUNDH
Magnus
ChalmersUniversity of Technology
Sweden
SUNG
Shiang
Engelhard Corporation
Iselin NJ, USA
TAHIR
Saad
King's College London
United Kingdom
TANAKA
Hirohisa
Daihatsu Motor Co. Ltd.
Japan
TERWAGNE
Albert
Universit6libre de Bruxelles
Belgium
THORM,i~HLEN
Peter
Chalmers University of Technology
Sweden
TORNCRONA
Anders
Chalmers University of Technology
Sweden
TOURET
Olivier
Rh6ne - Poulenc Chimie S.A.
France
VAN DEN TILLAART
Johan
Degussa AG
Germany
VAN GEMERT
R.
Eindhoven University of Technology The Netherlands
VAN KOOTEN
Wijnand
Delft University
The Netherlands
VAN SETTEN
Barry
Delft University of Technology
The Netherlands
692 VAN YPEREN
Rene
Degussa AG
Germany
VARGA
Judith
Joseph Attila University
Hungary
VISART
Thierry
Universit6libre de Bruxelles
Belgium
W.A. BAKAR
W. Azelee
Universityof Technology Malaysia
Malaysia
WAHLBERG
Annika
Royal Institute of Technology
Sweden
Johnson Matthey Technology Center United Kingdom
WALKER WALSH
Michael
Arlington VA USA
WARREN
James
Johnson Matthey Ltd
WATLING
Timothy
University of Reading, Whiteknights UnitedKingdom
WEIBEL
Michel
Daimler Benz AG
Germany
WESTERBERG
Bjorn
Chemical Engineering II
Sweden
ZAKUMBAEVA
Gaoukhar
ZANDIRI
Stefania
Centre Ricerche Fiat
Republic of Kazakhstan Italy
ZENG
Hua Chun
NationalUniversity of Singapore
Singapore
United Kingdom
693 STUDIES IN SURFACE SCIENCE A N D CATALYSIS Advisory Editors: B. Delmon, Universit6 Catholique de Louvain, Louvain-la-Neuve, Belgium J.T. Yates, University of Pittsburgh, Pittsburgh, PA, U.S.A. Volume 1
Volume 2
Volume 3
Volume 4
Volume 5
Volume 6 Volume 7 Volume 8 Volume 9 Volume 10 Volume 11
Volume 12 Volume 13 Volume 14 Volume 15
Preparation of Catalysts I.Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the First International Symposium, Brussels, October 14-17,1975 edited by B. Delmon, P.A.Jacobs and G. Poncelet 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 ofthe Chemical Processes in Relation to Practical Applications by V.V. Boldyrev, M. Bulens and B. Delmon Preparation of Catalysts I1. 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 Growth and Properties of Metal Clusters. Applications to Catalysis and the Photographic Process. Proceedings of the 32nd International Meeting of the Soci~t~ de Chimie Physique, Villeurbanne, September 24-28, 1979 edited by J. Bourdon Catalysis by Zeolites. Proceedings of an International Symposium, Ecully (Lyon), September 9-11, 1980 edited by B. Imelik, C. Naccache, u Ben Taarit, J.C. Vedrine, G. Coudurier and H. Praliaud Catalyst Deactivation. Proceedings of an International Symposium, Antwerp, October 13-15,1980 edited by B. Delmon and G.E Froment New Horizons in Catalysis. Proceedings of the 7th International Congress on Catalysis, Tokyo, June 30-July4, 1980. Parts A and B edited by T. Seiyama and K. Tanabe Catalysis by Supported Complexes by u u B.N. Kuznetsov and V.A. Zakharov Physics of Solid Surfaces. Proceedings of a Symposium, Bechyhe, September 29-October 3,1980 edited by M. L&zni~:ka Adsorption at the Gas-Solid and Liquid-Solid Interface. Proceedings of an International Symposium, Aix-en-Provence, September 21-23, 1981 edited by J. Rouquerol and K.S.W. Sing Metal-Support and Metal-Additive Effects in Catalysis. Proceedings of an International Symposium, Ecully (Lyon), September 14-16, 1982 edited by B. Imelik, C. Naccache, G. Coudurier, H. Praliaud, P. Meriaudeau, P. Gallezot, G.A. Martin and J.C. Vedrine Metal Microstructures in Zeolites. Preparation - Properties - Applications. Proceedings of a Workshop, Bremen, September 22-24, 1982 edited by P.A. Jacobs, N.I. Jaeger, P.Jin3 and G. Schulz-Ekloff Adsorption on Metal Surfaces. An Integrated Approach edited by J. Bdnard Vibrations at Surfaces. Proceedings of the Third International Conference, Asilomar, CA, September 1-4, 1982 edited by C.R. Brundle and H. Morawitz Heterogeneous Catalytic Reactions Involving Molecular Oxygen by G.I. Golodets
694 Volume 16
Volume 17 Volume 18 Volume 19 Volume 20 Volume 21 Volume 22 Volume 23 Volume 24 Volume 25 Volume 26 Volume 27 Volume 28 Volume 29 Volume 30 Volume 31
Volume 32 Volume 33 Volume 34 Volume 35
Preparation of Catalysts i11.Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Third International Symposium, Louvain-la-Neuve, September 6-9, 1982 edited by G. Poncelet, P. Grange and P.A.Jacobs Spillover of Adsorbed Species. Proceedings of an International Symposium, Lyon-Villeurbanne, September 12-16, 1983 edited by G.M. Pajonk, S.J. Teichner and J.E. Germain Structure and Reactivity of Modified Zeolites. Proceedings of an International Conference, Prague, July 9-13, 1984 edited by P.A.Jacobs, N.I. Jaeger, P.JidJ, V.B. Kazansky and G. Schulz-Ekloff Catalysis on the Energy Scene. Proceedings of the 9th Canadian Symposium on Catalysis, Quebec, P.Q.,September 30-October 3, 1984 edited by S. Kaliaguine and A. Mahay Catalysis by Acids and Bases. Proceedings of an International Symposium, Villeurbanne (Lyon), September 25-27, 1984 edited by B. Imelik, C. Naccache, G. Coudurier, Y. Ben Taarit and J.C. Vedrine Adsorption and Catalysis on Oxide Surfaces. Proceedings of a Symposium, Uxbridge, June 28-29, 1984 edited by M. Che and G.C. Bond Unsteady Processes in Catalytic Reactors by Yu.Sh. Matros Physics of Solid Surfaces 1984 edited by J. Koukal Zeolites: Synthesis, Structure, Technology and Application. Proceedings of an International Symposium, Portoro~-Portorose, September 3-8, 1984 edited by B. Dr;~aj,S. HoEevar and S. Pejovnik Catalytic Polymerization of Olefins. Proceedings of the International Symposium on Future Aspects of Olefin Polymerization, Tokyo, July 4-6, 1985 edited by T. Keii and K. Soga Vibrations at Surfaces 1985. Proceedings of the Fourth International Conference, Bowness-on-Windermere, September 15-19, 1985 edited by D.A. King, N.V. Richardson and S. Holloway Catalytic Hydrogenation edited by L. Cerven~ New Developments in Zeolite Science and Technology. Proceedings of the 7th International Zeolite Conference, Tokyo, August 17-22, 1986 edited by Y. Murakami, A. lijima and J.W. Ward Metal Clusters in Catalysis edited by B.C. Gates, L. Guczi and H. Kn6zinger Catalysis and Automotive Pollution Control. Proceedings of the First International Symposium, Brussels, September 8-11, 1986 edited by A. Crucq and A. Frennet Preparation of Catalysts IV. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Fourth International Symposium, Louvain-la-Neuve, September 1-4, 1986 edited by B. Delmon, P.Grange, P.A. Jacobs and G. Poncelet Thin Metal Films and Gas Chemisorption edited by R Wissmann Synthesis of High-silica Aluminosilicate Zeolites edited by P.A.Jacobs and J.A. Martens Catalyst Deactivation 1987. Proceedings of the 4th International Symposium, Antwerp, September 29-October 1, 1987 edited by B. Delmon and G.F. Froment Keynotes in Energy-Related Catalysis edited by S. Kaliaguine
695 Volume 36 Volume 37 Volume 38 Volume 39 Volume 40 Volume 41
volume 42 Volume 43 Volume 44
Volume 45 Volume 46
Volume 47 Volume 48 Volume 49 Volume 50
Volume 51 Volume 52 Volume 53 Volume 54
Methane Conversion. Proceedings of a Symposium on the Production of Fuels and Chemicals from Natural Gas, Auckland, April 27-30, 1987 edited by D.M. Bibby, C.D. Chang, R.F. Howe and S. Yurchak Innovation in Zeolite Materials Science. Proceedings of an International Symposium, Nieuwpoort, September 13-17, 1987 edited by P.J. Grobet, W.J. Mortier, E.F. Vansant and G. Schulz-Ekloff Catalysis 1987. Proceedings of the 10th North American Meeting of the Catalysis Society, San Diego, CA, May 17-22, 1987 edited by J.W. Ward Characterization of Porous Solids. Proceedings of the IUPAC Symposium (COPS I), Bad Soden a. Ts., April 26-29,1987 edited by K.K. Unger, J. Rouquerol, K.S.W. Sing and H. Kral Physics of Solid Surfaces 1987. Proceedings of the Fourth Symposium on Surface Physics, Bechyne Castle, September 7-11, 1987 edited by J. Koukal Heterogeneous Catalysis and Fine Chemicals. Proceedings of an International Symposium, Poitiers, March 15-17, 1988 edited by M. Guisnet, J. Barrault, C. Bouchoule, D. Duprez, C. Montassier and G. Perot Laboratory Studies of Heterogeneous Catalytic Processes by E.G. Christoffel, revised and edited by Z. Pa&l Catalytic Processes under Unsteady-State Conditions by Yu. Sh. Matros Successful Design of Catalysts. Future Requirements and Development. Proceedings ofthe Worldwide Catalysis Seminars, July, 1988, on the Occasion of the 30th Anniversary of the Catalysis Society of Japan edited by T. Inui Transition Metal Oxides. Surface Chemistry and Catalysis by H.H. Kung Zeolites as Catalysts, Sorbents and Detergent Builders. Applications and Innovations. Proceedings of an International Symposium, W~rzburg, September 4-8,1988 edited by H.G. Karge and J. Weitkamp Photochemistry on Solid Surfaces edited by M. Anpo and T. Matsuura Structure and Reactivity of Surfaces. Proceedings of a European Conference, Trieste, September 13-16, 1988 edited by C. Morterra, A. Zecchina and G. Costa Zeolites: Facts, Figures, Future. Proceedings of the 8th International Zeolite Conference, Amsterdam, July 10-14, 1989. Parts A and B edited by P.A. Jacobs and R.A. van Santen Hydrotreating Catalysts. Preparation, Characterization and Performance. Proceedings of the Annual International AIChE Meeting, Washington, DC, November 27-December 2, 1988 edited by M.L. Occelli and R.G. Anthony New Solid Acids and Bases. Their Catalytic Properties by K. Tanabe, M. Misono, Y. Ono and H. Hattori Recent Advances in Zeolite Science. Proceedings of the 1989 Meeting of the British Zeolite Association, Cambridge, April 17-19, 1989 edited by J. Klinowsky and P.J. Barrie Catalyst in Petroleum Refining 1989. Proceedings of the First International Conference on Catalysts in Petroleum Refining, Kuwait, March 5-8, 1989 edited by D.L. Trimm, S. Akashah, M. Absi-Halabi and A. Bishara Future Opportunities in Catalytic and Separation Technology edited by M. Misono, Y. Moro-oka and S. Kimura
696 Volume 55
New Developments in Selective Oxidation. Proceedings of an International Symposium, Rimini, Italy, September 18-22, 1989 edited by G. Centi and F.Trifiro Volume 56 Olefin Polymerization Catalysts. Proceedings of the International Symposium on Recent Developments in Olefin Polymerization Catalysts, Tokyo, October 23-25,1989 edited by T. Keii and K. Soga Volume 57A Spectroscopic Analysis of Heterogeneous Catalysts. Part A: Methods of Surface Analysis edited by J.L.G. Fierro Volume 57B Spectroscopic Analysis of Heterogeneous Catalysts. Part B: Chemisorption of Probe Molecules edited by J.L.G. Fierro Volume 58 Introduction to Zeolite Science and Practice edited by H. van Bekkum, E.M. Flanigen and J.C. Jansen Volume 59 Heterogeneous Catalysis and Fine Chemicals II. Proceedings of the 2nd International Symposium, Poitiers, October 2-6, 1990 edited by M. Guisnet, J. Barrault, C. Bouchoule, D. Duprez, G. Pdrot, R. Maurel and C. Montassier Volume 60 Chemistry of Microporous Crystals. Proceedings of the International Symposium on Chemistry of Microporous Crystals, Tokyo, June 26-29, 1990 edited by T. Inui, S. Namba and T. Tatsumi Volume 61 Natural Gas Conversion. Proceedings of the Symposium on Natural Gas Conversion, Oslo, August 12-17, 1990 edited by A. Holmen, K.-J. Jens and S. Kolboe Volume 62 Characterization of Porous Solids II. Proceedings of the IUPAC Symposium (COPS II), Alicante, May 6-9, 1990 edited by E Rodriguez-Reinoso, J. Rouquerol, K.S.W. Sing and K.K. Unger Preparation of Catalysts V. Scientific Bases for the Preparation of Heterogeneous Volume 63 Catalysts. Proceedings of the Fifth International Symposium, Louvain-la-Neuve, September 3-6, 1990 edited by G. Poncelet, P.A.Jacobs, P.Grange and B. Delmon New Trends in CO Activation Volume 64 edited by L. Guczi Catalysis and Adsorption by Zeolites. Proceedings of ZEOCAT90, Leipzig, Volume 65 August 20-23, 1990 edited by G. (~hlmann, H. Pfeifer and R. Fricke Dioxygen Activation and Homogeneous Catalytic Oxidation. Proceedings of the Volume 66 Fourth International Symposium on Dioxygen Activation and Homogeneous Catalytic Oxidation, Balatonffired, September 10-14, 1990 edited by L.I. Simfindi Structure-Activity and Selectivity Relationships in Heterogeneous Catalysis. Volume 67 Proceedings ofthe ACS Symposium on Structu re-Activity Relationships in Heterogeneous Catalysis, Boston, MA, April 22-27, 1990 edited by R.K. Grasselli and A.W. Sleight Catalyst Deactivation 1991. Proceedings of the Fifth International Symposium, Volume 68 Evanston, IL, June 24-26, 1991 edited by C.H. Bartholomew and J.B. Butt Zeolite Chemistry and Catalysis. Proceedings of an International Symposium, Volume 69 Prague, Czechoslovakia, September 8-13, 1991 edited by P.A.Jacobs, N.I. Jaeger, L. Kubelkovfi and B. Wichterlovfi Poisoning and Promotion in Catalysis based on Surface Science Concepts and Volume 70 Experiments by M. Kiskinova
697 Volume 71 Volume 72
Volume 73 Volume 74 Volume 75 Volume 76 Volume 77
Volume78
Volume79 Volume 80 Volume 81 Volume82
Volume 83 Volume84
Volume 85 Volume 86 Volume 87
Catalysis and Automotive Pollution Control II. Proceedings of the 2nd International Symposium (CAPoC 2), Brussels, Belgium, September 10-13, 1990 edited by A. Crucq New Developments in Selective Oxidation by Heterogeneous Catalysis. Proceedings of the 3rd European Workshop Meeting on New Developments in Selective Oxidation by Heterogeneous Catalysis, Louvain-la-Neuve, Belgium, April 8-10, 1991 edited by R Ruiz and B. Delmon Progress in Catalysis. Proceedings of the 12th Canadian Symposium on Catalysis, Banff, Alberta, Canada, May 25-28, 1992 edited by K.J. Smith and E.C. Sanford Angle-Resolved Photoemission. Theory and Current Applications edited by S.D. Kevan New Frontiers in Catalysis, Parts A-C. Proceedings of the 10th International Congress on Catalysis, Budapest, Hungary, 19-24 July, 1992 edited by L. Guczi, F. Solymosi and R T6t6nyi Fluid Catalytic Cracking: Science and Technology edited by J.S. Magee and M.M. Mitchell, Jr. New Aspects of Spillover Effect in Catalysis. For Development of Highly Active Catalysts. Proceedings of the Third International Conference on Spillover, Kyoto, Japan, August 17-20, 1993 edited by T. Inui, K. Fujimoto, T. Uchijima and M. Masai Heterogeneous Catalysis and Fine Chemicals III. Proceedings of the 3rd International Symposium, Poitiers, April 5 - 8, 1993 edited by M. Guisnet, J. Barbier, J. Barrault, C. Bouchoule, D. Duprez, G. P6rot and C. Montassier Catalysis: An Integrated Approach to Homogeneous, Heterogeneous and Industrial Catalysis edited by J.A. Moulijn, RW.N.Mo van Leeuwen and R.A. van Santen Fundamentals of Adsorption. Proceedings of the Fourth International Conference on Fundamentals of Adsorption, Kyoto, Japan, May 17-22, 1992 edited by M. Suzuki Natural Gas Conversion II. Proceedings ofthe Third Natural Gas Conversion Symposium, Sydney, July 4-9, 1993 edited by H.E. Curry-Hyde and R.F. Howe New Developments in Selective Oxidation II. Proceedings of the Second World Congress and Fourth European Workshop Meeting, Benalm~dena, Spain, September 20-24, 1993 edited by V. Cortes Corberdn and S. Vic Bell6n Zeolites and Microporous Crystals. Proceedings of the International Symposium on Zeolites and Microporous Crystals, Nagoya, Japan, August 22-25, 1993 edited by T. Hattori and T. Yashima Zeolites and Related Microporous Materials: State of the Art 1994. Proceedings ofthe 10th International Zeolite Conference, Garmisch-Partenkirchen, Germany, July 17-22, 1994 edited by J. Weitkamp, H.G. Karge, H. Pfeifer and W. H61derich Advanced Zeolite Science and Applications edited by J.C. Jansen, M. St6cker, H.G. Karge and J.Weitkamp Oscillating Heterogeneous Catalytic Systems by M.M. Slin'ko and N.I. Jaeger Characterization of Porous Solids III. Proceedings of the IUPAC Symposium (COPS III), Marseille, France, May 9-12, 1993 edited by J.Rouquerol, F. Rodriguez-Reinoso, K.S.W. Sing and K.K. Unger
698 Volume 88 Volume89
Volume90 Volume91
Volume 92
Volume 93 Volume94 Volume 95 Volume 96
Volume 97 Volume98
Volume 99 Volume 100
Volume 101 Volume 102 Volume 103 Volume 104 Volume 105
Catalyst Deactivation 1994. Proceedings ofthe 6th International Symposium, Ostend, Belgium, October 3-5, 1994 edited by B. Delmon and G.F. Froment Catalyst Design for Tailor-made Polyolefins. Proceedings of the International Symposium on Catalyst Design for Tailor-made Polyolefins, Kanazawa, Japan, March 10-12, 1994 edited by K. Soga and M. Terano Acid-Base Catalysis II. Proceedings of the International Symposium on Acid-Base Catalysis II, Sapporo, Japan, December 2-4, 1993 edited by H. Hattori, M. Misono and Y. Ono Preparation of Catalysts VI. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Sixth International Symposium, Louvain-La-Neuve, September 5-8, 1994 edited by G. Poncelet, J. Martens, B. Delmon, P.A. Jacobs and P.Grange Science and Technology in Catalysis 1994. Proceedings of the Second Tokyo Conference on Advanced Catalytic Science and Technology, Tokyo, August 21-26, 1994 edited by Y. Izumi, H. Arai and M. Iwamoto Characterization and Chemical Modification of the Silica Surface by E.F.Vansant, P.Van Der Voort and K.C. Vrancken Catalysis by Microporous Materials. Proceedings of ZEOCAT'95, Szombathely, Hungary, July 9-13,1995 edited by H.K. Beyer, H.G.Karge, I. Kiricsi and J.B. Nagy Catalysis by Metals and Alloys by V. Ponec and G.C. Bond Catalysis and Automotive Pollution Control II1. Proceedings of the Third International Symposium (CAPoC3), Brussels, Belgium, April 20-22, 1994 edited by A. Frennet and J.-M. Bastin Zeolites: A Refined Tool for Designing Catalytic Sites. Proceedings of the International Symposium, Qu6bec, Canada, October 15-20, 1995 edited by L. Bonneviot and S. Kaliaguine Zeolite Science 1994: Recent Progress and Discussions. Supplementary Materials to the 10th International Zeolite Conference, Garmisch-Partenkirchen, Germany, July 17-22, 1994 edited by H.G. Karge and J. Weitkamp Adsorption on New and Modified Inorganic Sorbents edited by A. Dqbrowski and V.A. Tertykh Catalysts in Petroleum Refining and Petrochemical Industries 1995. Proceedings of the 2nd International Conference on Catalysts in Petroleum Refining and Petrochemical Industries, Kuwait, April 22-26, 1995 edited by M. Absi-Halabi, J. Beshara, H. Qabazard and A. Stanislaus 1lth International Congress on Catalysis - 40th Anniversary. Proceedings ofthe 1lth ICC, Baltimore, MD, USA, June 30-July 5, 1996 edited by J. W. Hightower, W.N. Delgass, E. Iglesia and A.T. Bell Recent Advances and New Horizons in Zeolite Science and Technology edited by H. Chon, S.I. Woo and S. -E. Park Semiconductor Nanoclusters - Physical, Chemical, and Catalytic Aspects edited by P.V. Kamat and D. Meisel Equilibria and Dynamics of Gas Adsorption on Heterogeneous Solid Surfaces edited by W. Rudzifiski, W.A. Steele and G. Zgrablich Progress in Zeolite and Microporous Materials Proceedings ofthe 1lth International Zeolite Conference, Seoul, Korea, August 12-17,1996 edited by H. Chon, S.-K. Ihm and Y.S. Uh
699 Volume 106
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Volume 111 Volume 112
Volume 113
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Volume 115 Volume 116
Hydrotreatment and Hydrocracking of Oil Fractions Proceedings ofthe 1st International Symposium / 6th European Workshop, Oostende, Belgium, February 17-19, 1997 edited by G.F. Froment, B. Delmon and R Grange Natural Gas Conversion IV Proceedings ofthe 4th International Natural Gas Conversion Symposium, Kruger Park, South Africa, November 19-23, 1995 edited by M. de Pontes, R.L. Espinoza, C.P. Nicolaides, J.H. Scholtz and M.S. Scurrell Heterogeneous Catalysis and Fine Chemicals IV Proceedings of the 4th International Symposium on Heterogeneous Catalysis and Fine Chemicals, Basel, Switzerland, September 8-12, 1996 edited by H.U. Blaser, A. Baiker and R. Prins Dynamics of Surfaces and Reaction Kinetics in Heterogeneous Catalysis. Proceedings ofthe International Symposium, Antwerp, Belgium, September 15-17,1997 edited by G.F. Froment and K.C. Waugh Third World Congress on Oxidation Catalysis. Proceedings of the Third World Congress on Oxidation Catalysis, San Diego, CA, U.S.A., 21-26 September 1997 edited by R.K. Grasselli, S.T. Oyama, A.M. Gaffney and J.E. Lyons Catalyst Deactivation 1997. Proceedings of the 7th International Symposium, Cancun, Mexico, October 5-8, 1997 edited by C.H. Bartholomew and G.A. Fuentes Spillover and Migration of Surface Species on Catalysts. Proceedings ofthe 4th International Conference on Spillover, Dalian, China, September 15-18, 1997 edited by Can Li and Qin Xin Recent Advances in Basic and Applied Aspects of Industrial Catalysis. Proceedings ofthe 13th National Symposium and Silver Jubilee Symposium of Catalysis of India, Dehradun, India, April 2-4,1997 edited by T.S.R. Prasada Rao and G. Murali Dhar Advances in Chemical Conversions for Mitigating Carbon Dioxide. Proceedings of the 4th International Conference on Carbon Dioxide Utilization, Kyoto, Japan, September 7-11, 1997 edited by T. Inui, M. Anpo, K. Izui, S. Yanagida and T. Yamaguchi Methods for Monitoring and Diagnosing the Efficiency of Catalytic Converters. A patent-oriented survey by M. Sideris Catalysis and Automotive Pollution Control IV. Proceedings ofthe 4th International Symposium (CAPoC4), Brussels, Belgium, April 9-11, 1997 edited by N. Kruse, A. Frennet and J.-M. Bastin
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