Catalysis and Automotive Pollution Control II

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Studies in Surface Science and Catalysis 71 CATALYSIS AND AUTOMOTIVE POLLUTION CONTROL II

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Studies in Surface Science and Catalysis Advisory Editors: 6.Delmon and J.T. Yates Vol. 71

CATALYSIS AND AUTOMOTIVE POLLUTION CONTROL II Proceedingsof the Second InternationalSymposium (CAPoC 2). Brussels, Belgium, September 10-13,1990

Editor

A. Crucq Unite de Recherche sur la Catalyse, Universite Libre de Bruxelles, Brussels, Belgium

ELSEVIER

Amsterdam

- Oxford - New York - Tokyo

1991

ELSEVIER SCIENCE PUBLISHERS B.V. Sara Burgerhartstraat 25 P.O. Box211,1000AEAmsterdam,The Netherlands Distributors forthe Unitedstates and Canada: ELSEVIER SCIENCE PUBLISHING COMPANY, INC. 655 Avenue of the Americas NewYork, NY 10010, USA

Library of C o n g r e s s Cataloging-in-Publication

Data

1990 Brussels, Belgium) C A P O C (2nd C a t a l y s i s a n d a u t o m o t i v e p o l l u t i o n c o n r r o l I1 proceedings 3f t h e S e c o n d I n t e r n a t i o n a l S y m p o s i u m ( C A P o C 2). B r u s s e l s . B e l g i u m . S e p t e m b e r 10-13. 1990 / e d i t o r , A. C r u c q . p. c m . -- ( S t u d i e s in s u r f a c e s c i e n c e a n d c a t a l y s i s 71) I n c l u d e s b i b l i o g r a p h i c a l r e f e r e n c e s a n d index. I S B N 0-444-88787-3 1. Automobiles--Catalytic converters--Congresses. 2. C a t a l y s i s - 2 o n g r e s s e s . 3. A u t o m o b i l e s - - M o t o r s - - E x h a u s t gas--Congresses. 4. C e r i u m o x i d e s - - C o n g r e s s e s . I. C r u c q . A . (And?&,. 193311. T i t l e . 111. S e r i e s . TL214.P6C3E8 1990 629.25'28--dc20 9 1-30962

.

CIP

ISBN: 0-444-88787-3

0 Elsevier Science Publishers B.V., 1991 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 Publishers B.V./ Academic Publishing Division, P.O. Box 330,1000 AH Amsterdam,The Netherlands. Special regulations for readers in the USA - This publication has been registered with the Copyright Clearance Center Inc. (CCC), Salem, Massachusetts. Information can be obtained from theCCCaboutconditions underwhich photocopiesof partsofthis publication may be made in the USA. All other copyright questions, including photocopying outside of the USA, should be referred to the Publisher. No responsibility is assumed by the Publisher for any injury and/or damage t o 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. Although all advertising material is expected to conform to ethical (medical) standards, inclusion in this publication does not constitute a guarantee or endorsement of the quality or value of such product or of the claims made of it by its manufacturer. This book is printed on acid-free paper. Printed in The Netherlands

V

CONTENTS

- Foreword - Acknowledgments - Financial Support - Organizing and Scientific Committee

X XI1 XI11 XIV

- Scientific Papers GENERALINTRODUCTION TO THE PROBLEM OF EXHAUST GAS POLLUTION

Opening address E. Deworme

1

Energy and Air Pollutants in Belgium. The Contribution of Automotive Trafic since 1980 W.J. Hecq (General lecture)

5

Environmentalist Views on Automotive Exhaust Gases K. Taschner (General lecture)

17

The Evolution of the EEC Regulations on Vehicle Emissions H. Henssler (General lecture)

35

The Technological Development of the European Vehicles and the Environment C. Cucchi and M. Hublin (General lecture)

41

Industrial Application of Catalysts for Offgas Treatment T. Schmidt (General lecture)

55

Characterization of Exhaust Emissions from Two Heavy Duty Vehicles Fueled with Eight Different Diesel Fuels K.E. Egeback, G. Mason, U. Rannug and R. Westerholm

-

75

Fuels Evolution in Europe and Consequences on Exhaust Catalysis J.C. Guibet (General lecture)

93

Supply and Demand of Precious Metals for Automotive and Other Uses M.C.F. Steel (General lecture)

105

The Value of Spent Exhaust Catalysts W. Fierain (General lecture)

115

VJ TECHNICAL PAPERS

Role of CeOz in automotive exhaust catalysts

Cerium Oxide Stabilization :Physical Property and Three-way Activity Considerations J.E. Kubsh, J.S. Rieck and N.D. Spencer

125

The Role of Ceria in Three-way Catalysts A.F. Diwell, R.R. Rajaram, H.A. Shaw andT.J. Truex

-

139

Characterization of Oxidation Catalysts by CO-TPR and Selective Carbon-Carbon Bond Rupture Ch. Serre, F. Garin, G . Belot, G. Maire and R. Roche

-

153

Design and Performance Evaluation of Automotive Emission Control Catalysts R.G. Silver, J.C. Summers and W.B. Williamson

167

Influence of Rh and Ce02 Addition on the Activity and Selectivity of a PtIA1203 Catalyst in the CO+NO and CO+NO+O2 Reactions G. Leclercq, C. Dathy, G. Mabilon and L. Leclercq

181

Influence of Water in the Activity of Catalytic Converters M. Weibel, F. Garin, P. Bernhardt, G. Maire and M. Prigent

195

The Role of the Oxygen Vacancies at the Support in the CO Oxidation on RhlCe02 and RhlTi02 Autocatalysts G. Munuera, A. Fernandez and A.R. Gonzalez-Elipe

207

Physico-Chemical Properties of Ce-Containing Three-Way Catalysts and the Effect of Ce on Catalyst Activity J.G. Nunan, H.J. Robota, M. J. Cohn and S.A.Bradley

22 1

Studies to the Functioning of Automotive Exhaust Catalysts Using In-situ Positron Emission Tomography K.A. Vonkeman, G. Jonkers and R.A. van Santen

239

TPD and XPS Studies of CO and NO on Highly Dispersed Pt+Rh Automotive Exhaust Catalysts : Evidence for Noble Metal-Ceria Interaction P. Loof, B. Kasemo, L. Bjomkvist, S. Andersson and A. Frestad

253

VII

Laser Raman Characterizationof Sulface Phase Precious Metal Oxides Formed on Ce02 L.L. Murrell, S.J. Tauster and D.R. Anderson

275

Depollution of Diesel engines Diesel Emission Control E.S. Lox, B.H. Engler and E. Koberstein (General lecture) - 291 Kinetics of Soot Oxidation on Potassium-Copper-Vanadium Catalyst P. Ciambelli, P. Parrella and S. Vaccaro

323

Catalystsfor Diesel Powered Vehicles D. J. Ball and R.G. Stack

337

Use of base metals in exhaust catalysis Catalytic Automotive Pollution Control without Noble Metals S. Stegenga, N. Dekker, J. Bijsterbosch, F. Kapteijn, J. Moulijn, G. Belot and R. Roche

353

Three-way Activity and Sulfur Tolerance of Single Phase Perovskites D. Jovanovic, V. Dondur, A. Terlecki-Baricevic and B. Grbic

371

Fundamental studies on model catalysts NO Reduction and Adsorption Intermediates on Pt-Rh Alloy Catalysts L. Heezen, V.N. Kilian,R.F. van Slooten, R.M. Wolf and B.E. Nieuwenhuys

381

A Comparative Kinetic Study of the CO-02 Reaction over Pt-Rh ( I l l ) , (100),(410) and (210) Single Crystal Sulfaces J. Siera, R. van Silfhout, F. Rutten and B.E. Nieuwenhuys -

395

Molecular Beam Studies of CO Oxidation on Rh(l10) M. Bowker, Q. Guo, P.D.A. Pudney and R.W. Joyner

409

-

VIII

Unregulated emissions Role of Sulphate Decomposition in the Emission and Control of Hydrogen Sulphide from Autocatalysts A.F. Diwell, S.E. Golunski, J.R. Taylor and T.J. Truex

-

The Effect of Aging on Nitrous Oxide N 2 0 Formation by Automotive Three-way Catalysts M. Prigent, G. de Soete and R. Dozikre

417

425

Theoretical approach to the catalytic converter Flow, Heat and Mass Transfer in a Monolithic Catalytic Converter D. Schweich (General lecture)

437

A New Theoretical Approach to Catalytic Converters J.P. Leclerc. D. Schweich and J. Villermaux

465

Supports for Automotive Catalysts New developments in Catalytic Converter Durability S.T. Gulati (General lecture) Study of Platinum Deposit on Ferritic Stainless Steel Conversion Coatings L. Aries, P. de Veyrac, M. Mantel and J.P. Traverse

48 1

~

509

Monolith Substrate Efsects on Catalyst Light-Off T.S. Jasper, K. Robinson, D. Anderton and D.H. Cuttler -

523

Comparison of Metal Foil and Ceramic Monolithic Automotive Catalytic Converters G. L. Vaneman

537

Sols as Precursors to Transitional Aluminas and these Aluminas as Host Supports for Ce02 and Zr02 Micro Domains L. L. Murrell and S.J. Tauster

547

Preparation of Thermostable High-Sugace-Area Aluminas and Properties of the Alumina-Supported Pt Catalysts F. Mizukami, K. Maeda, M. Watanabe, K. Masuda, T. Sano and K. Kuno

557

IX

Ageing of Automotive Catalysts Kinetics of the Physico-Chemical and Catalytic Activity Evolution of a Pt-Rh Catalyst in Automotive Exhaust Gas G. Mabilon, D. Durand and M. Prigent

569

Characterization of Bimetallic Su$aces by 1801160 Isotopic Exchange. Application to the Study of the Sintering of PtRhlA1203 Catalysts S. Kacimi and D. Duprez

581

The Effect of Sulfur on Three-way Catalysts D.R. Monroe, M.H. Krueger, D.D. Beck and M.J. D'Aniello, Jr.

593

Effect of Severe Thermal Aging on Noble Metal Catalysts H. Shinjoh, H. Muraki and Y. Fujitani

617

Morphological Transformations in Reducing and Steam Atmospheres of Alumina-Supported Rhodium and Platinum Catalysts D. Duprez, F. Sadi, A. Miloudi and A. Percheron-Guegan

629

Miscellaneous The Influence of Three-way Catalyst Parameters on Secondaly Emission -B. Engler, E. Koberstein, D. Lindner and E. Lox

64 1

Improvements in Techniques for Reducing Emissions by Using Computer Simulation M. Hashimoto, R. Matsumura, F. Yukawa, M. Saitoh and M. Matsumoto

657

PdIA1203 Catalysts for the NO-CO-02 Reaction :"In Situ" Determination of the Palladium State under the Reactant Mixture 667 J.L. Duplan and H. Praliaud AUTHOR INDEX

679

STUDIES IN SURFACE SCIENCE AND CATALYSIS (other volumes in the series)

681

X

FOREWORD

In September 1986, we organized in Brussels the first International Symposium on “Catalysis and Automotive Pollution Control” known under the acronym CAPoC 1. At the end of this symposium, most participants expressed their hope that it would be repeated after a not too long interval of 3 to 4 years. A CAPoC 2 was thus organized in September 1990 and this volume constitutes the proceedings of the symposium. CAPoC 2 has been a great success from the point of view of its scientific interest, as evidenced by the content of this book, and also from the point of view of the participation, as 260 scientists, as compared with 177 in 1986, attended the symposium. This is the proof that exhaust catalysis remains a major topic of interest. As in 1986, most participants came from EEC countries, with large delegations from Belgium (48), France ( 5 9 , Germany (36), and the United Kingdom (31), but we again note the importance of the U.S. (23) and Swedish (17) delegations and the interest of people coming from all over the world, including Japan, P.R.China, Venezuela, South Africa and Algeria. About two-thirds of the participants came from the industrial world, mainly the car and oil industries and catalyst manufacturers. As in 1986, the Organizers choose to devote the first day of this fourday symposium to non technical, non catalytic problems, related either to pollution in general or specifically to automotive pollution. Nine general lectures on these subjects were presented and are published in this book. Concerning the strictly catalytic part of the Symposium, the Organizers received 66 papers for consideration, of which 42 were chosen for presentation and of which 34 are published here. This book also contains the text of three general lectures out of the four that were orally presented. In order to accomodate 42 papers and 4 general lectures within 3 days, a poster presentation of some of the accepted papers was necessary. The choice made by the Scientific Committee between oral or poster presentation was based not on the quality of the papers but on the topics. In this way, and with the aim of stimulating the discussion, it was possible to organize half-day sessions with oral presentations on the same topics.

XI

In these proceedings, the lectures and papers are grouped into the following headings: 1- The role of Ce02 in exhaust catalysts (1 1 papers) 2- Supports for automotive catalysts (6 papers) 3- Ageing of automotive catalysts (5 papers) 4- Depollution of Diesel engines (3 papers) 5- Fundamental studies (3 papers) 6- Base metals in exhaust catalysis (2 papers) 5- Fundamental studies (3 papers) 7- Unregulated emissions (2 papers) 8- Theoretical approach of the catalytic converter (2 papers) 9- Miscelleaneous It is worth well to mention that in 1986 not a single paper was presented concerning the role of Ce02 in exhaust catalysis. The Organizers thank all the people that have submitted papers for their interest in CAPoC 2, and all authors of presented papers for the high quality of their contributions. Thanks are also due to all delegates for their participation in the very fruitful discussions both inside and outside the conference room.

Dr. A.CRUCQ

XI1

ACKNOWLEDGMENTS

The CAPoC 2 conference has been held in the “Salle Dupreel” of the “Institut de Sociologie” of the University of Brussels. The Organizers thank the authorities of the University for their hospitality and for the welcome address given by the representative of the Rector of the University. The Organizers express their gratitude to E.Deworme, State Secretary to the Energy, for his interest for the symposium and for the welcome address given at the opening session. Special thanks are due to all lecturers of the introductory session: W.Hecq, K.Taschner, H.Henssler, C.Cucchi, T.Schmidt, K.E.Egeback, J.C.Guibet, M.C.F.Stee1 and W.Fierain - for the high quality of their contribution. The Organizers are pleased to thank Drs S.T.Gulati, E.Lox, D.Schweich and M.Shelef for the outstanding general lectures they have given as an introduction to the topics discussed in the papers presented during the following scientific sessions. Finally, the Organizers want to recognize the important but sometimes hidden contribution of all members of the ”Unit6 de Recherches sur la Catalyse” to the success of the symposium.

XI11

FINANCIAL SUPPORT

The Organizers of this Symposium gratefully acknowledge the financial support sponsorship from the following organizations: A.C.Rochester (Division of General Motors) Commission of the European Community Degussa AG Fabrimetal FEBIAC Ford Motor Cy Johnson Matthey plc Strategic Analysis Europe

XIV

ORGANIZING COMMITTEE : Frennet A. (Chairman) Crucq A. (Secretary) Campinne M. Derouane E. Gerryn C1. Koberstein E. Poncelet G. Schmitz J.C. Evans W.D.J. Germain A. Hecq W. Leduc B. Prigent M. -

UniversitC Libre de Bruxelles UniversitC Libre de Bruxelles Ecole Royale Militaire FacultC Notre Dame de la Paix Ford Europe Inc. Degussa AG UniversitC Catholique de Louvain General Motors Eur. Tech. Center Johnson Matthey plc UniversitC de Likge UniversitC Libre de Bruxelles UniversitC Libre de Bruxelles Institut Francais du PCtrole

SCIENTIFIC COMMITTEE : All members of the organizing committee, plus : Belot G. Peugeot S.A. Brunelle J.P. Rhone Poulenc Garin F. UniversitC de Strasbourg UniversitC de Lille Leclercq L. Universitk de Nancy Pentenero A. UniversitC de Leiden Ponec V. Johnson Matthey plc Webster D.E. -

1

OPENING ADDRESS by Elie Deworme

Belgian State Secretary for Energy Mr President, Mrs. Rector, Ladies and Gentlemen, The State Secretary of Energy, Elie DEWORME insists on thanking the organizers of this symposium for having invited him to take part in their activities and having offered him the occasion to give a speech. Unfortunately, the current international situation requires his full attention and this is the reason why he has asked me to represent him. However, he wishes to stress the great interest he takes in your activities. Indeed, the State Secretary is convinced that the subject of this meeting is important and he notes with satisfaction that an impressive number of experts from all over the world are taking part in this meeting, which will last for four days. Environmental protection constitutes a problem that knows no frontiers; this is why international collaboration is essential. When we evoke international collaboration, it means that we have to promote a harmonization of the policies and the regulations both on a European and a world-wide level. Furthermore, and this is undoubtedly the meaning of your approach, it seems equally important to combine our efforts on a scientific level, and to bring together regularly the knowledge in this field. However, we would like to draw your attention to another aspect of this problem, which brings us closer to the current events: saving energy. The recent events in the Gulf, and the international tension resulting from it, calls back the ghost of the oil crisis. It is obvious that we are not in the same situation as in 1973-1974 and 1979-1980, when the industrialized countries were subjected to an "embargo" of the oil producing countries. Today, the consuming countries supported by certain producers are organizing the embargo.

2

That does not prevent a large number of OECD-countries, including Belgium, from preparing themselves to deal with a supply deficit. Last week, we had a reunion with several regional ministers in order to discuss a number of measures to increase the consumer’s awareness of energy savings. In the course of the 198O’s, our country has conducted, both at its own initiative and by means of a concerted action within international organizations such as the EEC or the International Energy Agency, a series of actions on a national level, aiming at energy savings; those actions included information campaigns (distribution of brochures, spots on television, participation in fairs and exhibitions, etc.), financially encouraging measures (grants or fiscal advantages for private individuals or firms achieving energy saving investments) and even restrictive measures (for example to insure that the equipment put on the market possesses sufficient energy efficiency). Those measures have allowed us to obtain significant results, although it is not always easy to identify all the factors underlying the decrease of energy consumption. The evolution curves of energy intensity, that is the relationship between the energy consumption and the gross domestic profit, show a general decline that leaves no doubt, at least during the first half of the 1980’s. We are inclined to think that this period was favourable for the environment. One of the sectors in which energy saving is desirable and directly affects the public is obviously the transport sector, and more specifically the automobile sector. In this connection, we should salute the remarkable efforts of the researchers of the firms to reduce the consumption and the pollution due to automobiles; I often state that the only energy source that does not pollute is that which we do not use. It is true that for the last one or two years, the automobile constructors have realized enormous investments to reduce both the consumption of the cars and their pollution. On the other hand, the number of cars in circulation has considerably increased, as well as the yearly distances covered by each car. Consequently, we have cars that pollute less but in a larger number. This indicates the size of the efforts we still have to make together. You, the scientists, by finding new techniques to limit the pollution, and we, the politicians, by inciting the population to adopt a “nature-friendly’’ behaviour. The automobile constructors have introduced catalysts in a large number of models. These are solutions to promote and to encourage. In turn, the Belgian Government has taken measures to encourage use of catalysts by offering a premium to the owners of old cars who equip their car with a catalyst.

3

It is necessary that the public understands the value of its environment and the importance of the heritage we give to our future generations. This is the message that the State Secretary for Energy, Elie DEWORME, would like to give for the week ahead: to combine transport security with energy savings but also with struggle against pollution. I wish you fruitful activities during these coming days and I thank you.


A. Crucq (Editor), Catalysis and Automotive Pollution Control ZI 0 199 1 Elsevier Science Publishers B.V., Amsterdam

5

ENERGY AND AIR POLLUTANTS IN BELGIUM The Contribution of Automotive Traffic since 1980 Walter J. HECQ.

Political Economy Centre, Universite' Libre de Bruxelles During the seventies, three important events: the publication of the Rome Club's Meadow Report, the United Nations Conference in Stockholm, and the oil shocks revealed the dangers threatening the Western economies, 1.e. : - the long-term depletion of natural resources; - the atmospheric pollution produced by combustion gases, which is showing its international scale; - an over-dependence on imported oil fuels. This latter has been brought out by recent events. The measures taken in the meantime have made it possible to slow down the quasi continuous increase in energy consumption and the pollution associated with it. A number of indicators show this in various countries including Belgium. Firstly, primary energy consumption, which had been on the increase since the end of the Second World War rose from 19.1 Mtoe in 1950 to 45.3 Mtoe in 1973, slowed down and then reached 48.5 Mtoe in 1979. Secondly, since 1970, the daily average ambient concentrations of SO2 and smoke in the air started to decline in Brussels and elsewhere, as indicated by the monitoring station network. The decline in the use of coal and its progressive replacement by natural gas, and also reductions in the sulfur content of oil products has made it possible to make a reduction in the larger number of local pollution sources. However, these measures revealed themselves to be insufficient with a long distance pollution still adding to the latter and remaining. This long distance pollution has led to the establishment of a European level Policy. This took the shape of EEC Directives on the progressive restriction of exhaust emissions from vehicles and major combustion plants. For the latter, 1980 levels are used as reference levels which must not be exceeded. 1980 is also the first year in which energy consumption started to diminish in Belgium and some other Western countries. So, this year will be the starting point of the comparisons that I shall make later. If energy consumption - like emissions of certain pollutants - has become more or less stabilised since then, five forms of atmospheric energy-

Table 1. Five Exemples of Environmental Concerns. CONFORMITY WlTH

LEVEL

POLLUTION COMMON NAME

POLLUTANTS

CONTRIBUTION TO ADVERSE EFFECTS

ATMOSPHERIC TRENDS

ATMOSPHERIC

WHO STANDARDS IN

LIFE TIME

BELGIUM

~

PARTICULATE MAmAND

LOCAL URBAN

W O R

CARBON MONOXIDE EFFECT

co

CONSTANT OR LOW

TENS OF DAYS

DECREASE

SQ?

TOTAL

CONSTANT OR LOW GROWTH

YES EXCEPT EPISODES IN URBANIZEDAREAS

SEVERAL

ID.

MONTHS LOCAL AND REGIONAL

ACIDRAIN SOZ, N&, HCL

sqr

60%

LOW DECREASE

2 - 5 DAYS

NO

Nox

30%

GROWTH

1 - 2 DAYS

NO

03

N.D.

M E D m

--

YES, EXCEPT EPISODES IN SUBURBS AREAS

co"u0us INCREASE

a?

GLOBAL

W N E HOLE

CFCs, NO, CO; ....

50%

MEDIUM INCREASE

8 YEARS

N20

=lo%

ID.

170 YEARS

03

40%

ID.

--

NO

?

GROWTH

?

7

related pollution continue to cause concern. They have approximately three geographical ranges: local, local and regional (continental) and global. (i) Local (urban) atmosDheric Dollution:: Two kinds of pollution are observed in urban areas (table 1). The g e c t of particulate matter : particulates from mineral residue and unburned organic matter in fuels is associated with, inter alia, the use of heavy fuels (diesel and fuel oil) and coal. Their effects on building materials (appearance and accentuation of corrosion phenomena) are well known. In epidemiological and toxicological studies, these particulates are also held to be a factor leading to a growth in respiratory illness (chronic bronchitis) and mortality. The effect of particulates is reinforced by the presence of SO2 in the air (synergy effect). It is for this reason that the guide values counselled by the W.H.O. (short term, 24 hours : black smoke < 125 pg/m3, SO2 < 125 pg/m3; long term, 1 year : black smoke < 50 pg/m3, SO2 < 50 pg/m3) (W.H.O., 1987) and the limit values set in the council directives (Official Journal) link these pollutants. These established limit values are still exceeded during the winter periods. Domestic heating and diesel vehicles are the principle sources. The effect of carbon monoxide (CO) : carbon monoxide is an intermediary combustion product which is very common in the urban atmosphere. It is emitted principally by two types of source, involving incomplete combustion i.e. small coal fires and gasoline motors. With the disappearance of the former and the increasing number of motor vehicle, carbon monoxide is produced in great quantities by automobile traffic. Also, in unfavourable weather conditions (poor dispersion) and in the big cities, it is not uncommon to see mean concentrations rise above the limits counselled by the W.H.O. (11.5 mg/m3 over 8 hours, 30 mg/m3 per hour) (W.H.O., 1987). Given its well known affinity with haemoglobin in the blood, during such periodes, carbon monoxide is a danger (hypoxia) to at risk groups i.e. angina pectoris patients, bronchitics, young children and pregnant women.... (ii) Atmospheric pollution at local and regional level : Two types of pollution cause concern (table 1) on a larger geographic scale. Acid rain : this phenomenon, widely described in the press, takes the form of wet and dry deposits. These deposits attack building materials and health; they make the soil and surface waters acid at great distances from their origin. Two pollutants are the main contributors of acid deposits, namely sulphur dioxide ( S O 9 and nitrogen oxides (NO, : NO + NO2). Both are produced by burning fossil fuels. Sulphur oxides are formed from residual sulphur in petroleum fuels and coal. Nitrogen oxides are formed by

8

two mecha-nisms: either by the oxidation of nitrogen compounds contained in fuels, or by oxidation of atmospheric nitrogen. If gasoline contains reduced amounts of sulphur and nitrogen, taking account of combustion conditions, it is the nitrogen oxides from the air which form the family of polluting acids produced in significant quantities by gasoline vehicles; Photochemical pollution : this pollution is the result of secondary pollutants and among them, tropospheric ozone ( 0 3 ) is an aggressive oxidant. Briefly, ozone is formed by a process catalyzed by sunshine and NO,. The NO, are recycled thanks to volatile hydrocarbons (VOC)' . If the NO produced is greater than the NO --> NO2 photochemical conversion rate, the NO reacts with the ozone and the 0 3 concentration is reduced. This is a common phenomenon in urban areas. During daylight periods, when the NO produced is insufficient, NO2 - VOC photochemical conversion produces 0 3 . Concentrations of this gas in the atmosphere are therefore rising. The average annual concentration of 0 3 in the troposphere rose almost continuously from 30 to 50 ppb in 16 years. The W.H.O. guide values -150-200 pg/m3 for one hour and 100-120 pg/m3 for 8 hours - are exceeded from time to time and alterations to both animal and vegetable tissues result. (iii)AtmosDherk pollution at global level: While acid rain and photo-chemical pollution have spread at continental level, the "greenhouse effect" is found worldwide (table 1). This phenomenon, the effects of which are still poorly understood although without doubt of great significance, is closely associated with the use of fossil fuels. Four energy related gases, including the principal product of combustion C02, and also nitrous oxide (N20), methane (CH4) and ozone ( 0 3 ) are involved in the process to differing degrees (Table 1). These gases have the property of absorbing the IR re-radiated back by the earth, thus heating the troposphere. Given the quantities emitted and the time factor involved in the process, models predict that an increase of 1.5 to 5.5 C" in the temperature of the troposphere is likely by 2030-2050. Consequently, a general modification of the climate is expected, accompanied by a rise in the level of the sea (thermal expansion and the melting of polar ice). Changes in crop yields and the composition of flora, the disrippearance of coastal areas and the salination of estuaries are among the repercussions of the greenhouse effect most often mentioned. 1 - CO also play a role in this process. It controls the level of the OH radical which in turn, determines the lifetime of hydrocarbons

9

Seven air pollutants (particulates, CO,S02,NO,, VOC,C02,N20)are thus implicated in the five major current forms of pollution associated with the combustion of fossil fuels. Emissions of these pollutants are a function of five parameters : - the quantity of fuel consumed; - the systems of combustion used; - the conditions under which combustion takes place; - fuel composition; - any abatment technologies applied. The last four parameters are, to variable degrees, currently subject to regulations and can define the emission factors. As for the other parameter, the amount of fuel consumed, this also plays a significant role in pollutant emissions. It is also an important factor from the point of view of the energy dependence of our economies and the rational use of non-renewable resources - which include fossil fuels.

T

I

e

30

W

I 25 Y

Dlesel Gasollne

e 20 a

Transponatlon

1 I I 1

0 Resld.Agr.Com.PubL

15

Industry

10

1980

1981

1982

1983

1984

1985

1986

1987

1988

Figure I . Inland primary energy consumption in Belgium.

Developments in energy consumption in Belgium since 1980. As in other industrialised countries (France, the Netherlands, ...), Belgian energy consumption, which peaked in the seventies, started to decline in 1980 (figure 1). It reached its lowest level in 1983, by which time consumption had declined by 12% compared with 1980. Then, with the economic recovery and the reduction in fuel prices, energy consumption started to rise slowly once more. In 1988, the gross inland primary energy

10

consumption was very little higher than in 1980 (46Mtoe). During this period, gasoline consumption followed a similar curve, but with a time lapse of two years : a 15% decrease between 1980 and 1985 followed by a 17% increase between 1985 and 1988. Thus, gasoline consumption in 1988 returned to the same level as in 1980 (3.1 Mtoe), which represents 6.7-6.8% of gross inland primary energy consumption. In fact, during this same period the number of gasoline driven cars on the road had followed the same pattern. It decreased from 2.8 million units in 1980 to 2.6 million in 1985, increasing again to 2.7 million units in 1988. Studies (L. DE BORGER, 1987; T. MOROVIC et al., 1987; TestAchats, 1983-1989) also show that specific vehicle consumption decreased by more than 10% during this period (vehicle improvement, energy saving, etc ...). So it is overall annual vehicle kilometrage which grew between 1980 and 1988 (FEBIAC INFO, 1990). The diesel situation is different, consumption of this fuel increased steadily in 1980 (2.1 Mtoe) and 1988 (3.5 Mtoe) and numbers of diesel vehicles have also increased greatly: from 240,000 in 1980 to 810,000 in 1988. It should be noted that in Belgium, there is less tax on diesel oil than on gasoline, and that relatively speaking, "diesel" cars consume less than gasoline cars. Furthermore, they are reputed to be more robust. These observations explain the success of diesel cars in this country.

Combustion pollutants emissions between 1980 and 1988 and the role of traffic. As stated pertinently in the last W.E.C. report (W.E.C., 1989), "Breakdown of manmade pollutant emission by sector is an essentialstarting point for policy decisions concerning energy system structure and pollutant control". According to the figures 2, 3, 4, 5, there has been a general decrease' in combustion pollutants in Belgium since 1980. This decrease is explained h the first place by the significant increase during the 1980s in the numbers of nuclear power plants, which do not produce chemical pollution. The second explanation is the progressive implementation of regulations restricting emission of pollutants from fossil fuels (a reduction in the sulphur content of oil products and of CO, VOC, NO, emissions from gasoline vehicles, inter alia). A third explanation has to do with structural and economic'changes after the oil shocks which have influenced the development and distribution of the fuels used. This has resulted in overall reductions in: - particulates, SO2 and CO emissions whose effects are felt locally; 1

- Volatile hydrocarbon are an exception, their emissions have increased

11

1800000 1600000 1400000 ~

4

1000000

r

600000

;

Q Electricity 6 Heat

D Diesel

t 1200000

El Gasoline

800000

Transportation

0 Rwid.Agr.Corn.Pub1.

8 Industry

400000 200000 0 1980 1981 1982 1983 1984 1985 1986 1987 1988

Figure 2. CO emissionsfromfossil fuel combustion in Belgium. 450000

-I

400000

t

350000

0 Electricity & Heat

300000

0 Diesel

4 250000

N Gasoline

; 200000

E! Transportation

r

0 Resid.Agr.Corn.Pub1. W Industry

100000 50000 0

1980 1981 1982 1983 1984 1985 1986 1987 1988

Figure 3 N O x emissions from fossil; fuel; combu tion in Belgium (expressed in NO2 ) T

140000 120000 t

I

100000

Electricity h Heat

Dleset

Q Gasoline

80000 a

r

~

I

150000

160000

I

Transportation

60000

0 Resid.Agr.Com.Pub1.

40000

8 Industry

20000 0

Figure 4. VOC emissions from fossil fuel combustion in Belgium.

~

I I

PARTICULATESEMISSIONS

80000 70000

Irn

SO2 EMISSONS 700000

MWOO

OEMMh&h.(

50000 I

1

I 40000 I I

In

soowo I 400000

:

30000

r

0Iway.COnplaL

J00000

r

20000

200000

10000

lwOW

0

0

1980 1981 1982 1983 1984 1985 1986 1987 1988

1980191t 1982 19131911 19151916 19871918

Nx) EUSSIONS

C02 EMISSIONS 14mo

120000 100000

40000

JSOOO

rnEMH1yLH.M

I 30000

U

;

4sm

in {

80000

25000

:

I

'

60000

r

20000 11000

40000

1oooo

P0000

KO00

0nnuqI.corrrpubL

0

0 I980 1081 1982 1983 1984 1985 I986 I987 1988

1910 1911 1982 1985 1984 1986 1931 1087 1918

Figure 5 :Particulate, SO2 CO;!and N20 emissions from fossil fuel combustion in Belgium.

13

- emissions of acid pollutants (S02, NO,) and of one ozone precursor (NO,) which have effects at local and regional level1 ; - and, finally, emissions of greenhouse gases (C02, N20) which have worldwide implications. In this concept involving seven forms of pollutants, gasoline vehicle exhaust has a significant influence in three forms: - at local level: here we are talking about carbon monoxide (CO) emissions, 80% of which are caused by motor vehicles in Belgium (figure 2). If global CO emissions in Belgium have been reduced by 2.9%, the emissions from gasoline vehicles fell by 6.8% between 1980 and 1988 (from 1.394.106 t to 1.299.106 t ) thanks to the progressive adaptation of vehicles to EEC standards (R 15.00, 15.01, 15.03, 15.04). However, after decreasing by 16.6% between 1980 and 1985, CO emissions rose again by 11.6% between 1985 and 1988. - at local and regional level : this concerns one of the "acid' and ozone precursor pollutant, oxides of nitrogen (NO,). Total emissions of this pollutant dropped by 11.3% in Belgium between 1980 and 1988 (figure 3). The contribution of gasoline vehicles represents more than 30% of these total emissions in Belgium. On the other hand and despite the adaptation of vehicles to EEC R 15.02, 15.03, 15.04 standards, NO, emissions by gasoline vehicles increased slightly, by 0.7% between 1980 and 1988. From 142,600 t of NO2 in 1980 and 143,600 t of NO2 in 1988. In this case, increased vehicle kilometrage limited the effect of implementing the regulations. However, these emissions fell continuously until 1985 (- 14.3%), and then the trend was reversed from (+ 17.6%). - at local and regional level: this third form has to do with both the NO, emissions mentioned previously and emissions of volatile hydrocarbons (VOC), both precursors of photochemical pollution. In total, VOC emissions increased in Belgium by 1.6% between 1980 and 1988, largely due to the increased contribution of diesel engines (figure 4). The proportion due to gasoline engines in 1988 (97,650 t of VOC) is lower by 7.2% than that for 1980 (105,300 t of VOC). Such values represent about 64%-70% of total hydrocarbon emissions due to combustion in Belgium. As for CO and NO, emissions, VOC emissions fell between 1980 and 1985 (- 14%) then rose (+ 7.7%) from 1985 to 1988. As for other pollutant emissions (figure 5): particulates, S02, C02, N20, gasoline vehicles make relatively low contributions; < 10% of total emissions from other sources. The same is true for the proportion of diesel vehicle emissions in total emissions due to combustion, the role of these emissions are limited with the exception of particulates, and, to a lesser 1-

And even at global level since 0 3 is a greenhouse gas

14

extent, of NO, and CO. To summarize, the role of gasoline motors in pollutant emissions from combustion essentially concerns three pollutants -- CO, NO, and VOC -- the negative effects of which are mainly local (urban areas) and regional. If a significant reduction in CO and VOC emissions from gasoline engines is observable for the 1980-1988 period, this is mainly the result of a net decrease observed for the period between 1980 and 1985. This decrease was followed by an increase in these emissions from 1985 on, at which point, the price of petroleum products started to fall (a fall in the price of the $ and of crude oil). As for NO, emissions, these follow a similar "v" configuration, but were higher in 1988 than in 1980.

300,

NO7 (ug/m31

0KRO0I

03-24

DECEMBER 1986

CO ( p p m l

0KRO81

83-24

OECPlBER 1986

NUMB. CRRSJHR

0KR08l

83-24

OECM B m 1986

-1

Figure 6 . Relationship between trafic density (number - cars I hr.) and NO, N02. CO concentrations ( semi - hourly ) in Brussels.

15

When we know the relationship between traffic density and the concentration of pollutants in the ambient air as shown in the figure 6, it is not surprising that measures intended to reduce emissions of CO, VOC and NO, were adopted by both the Belgium and the Community authorities with the coming into force of improved standards provided for in the Luxemburg Agreement (Directive 88/76/EEC) and the signing, in June 1989 of the decrees applicable to vehicles with an engine capacity below 1400 cc. In order to conform these new standards, the solution chosen is the extensive use of the catalytic converter : one of the technologies which appears better suited at present to wiping out increases in emissions of these three pollutants (CO, NO,, VOC) which have been observed since 1985; this technology also has the indirect merit of causing a drop in the emission of lead particles into the atmosphere. In this context, we should emphasize the initiative of the Belgian authorities who, like several other European governments (Germany, Holland), have decided to grant premiums to private individuals buying vehicles with an engine capacity of two liters or less and equipped with a catalytic converter. This measure, which has been effective from July 1st this year, will remain in force for two years. Given the encouraging prospect of such measures and also the prospect which justifies our presence here i.e. the improvement of catalytic processes, we must keep in mind not only the diesel vehicles whose number is growing but also another goal in preparing for the future : namely that we must make ongoing efforts to economize oil, a non-renewable resource located in large amounts in politically instable states; a resource on which our country and many others are still very dependent. REFERENCES.

- DE BORGER, L. (1987): "Modelvonning van personenvervoer (vervoer en energie) op basis van een personenenquEte" - Ministerie van Economische Zaken, Programma Energie, 55 p., December 1987. - FEBIAC INFO (1990): "Dossier mobilis" - FEBIAC INFO, no 8.8 p., juin 1990. - Journal Officiel (1988): "Proposition de directive du Conseil modifiant la directive 80/779/CEE du Conseil concernant des valeurs limites et des valeurs guides de qualit6 atmosphkique pour l'anhydride sulfureux et les particules en suspension" - Journal Officiel des Communautt?s Europcknnes, no C 254/6,30/09/1988. - MOROVIC T.. GRUNDING F.-J., JAGER F.. JOCHEM E.. MANNSBART W., POPPKE H., SCHON M., TOTSCH I. (1987): "Energy Conservation.Indicators",SpringerVerlag CEE, FhG, 333 p.. 1987. - Test-Achats (1985): "Dossier voitures ? essence" i (1) et (2) - Test-Achats Magazine, no 273,... DD. 617. 1985 et no 274. DD. 4-18. 1986. - Test-Achats (1989): '*Dos&rvoitures no 1 et 2: L'expkriencedes automobilistes" - TestAchats Magazine, no 317, pp. 3645,1989 et no 318, pp. 29-37.1990. - W.E.C. (1989): "An Assessment of Worldwide Energy-Related Atmospheric Pollution" - World Energy Conference, Report 1989,224 p., London, August 1989. - W.H.O. (1987): "Air Quality Guidelines for Europe" - W.H.O., Regional Publication, European Series, no 23, Copenhagen, 1987.


A. Crucq (Editor), Catalysis and Automotive Pollution Control II 1991 Elsevier Science Publishers B.V., Amsterdam

17

ENVIRONMENTALISTS' VIEWS ON AUTOMOTIVE EXHAUST GASES

Karola Taschner European Environmental Bureau , Bruxelles, Belgium. INTRODUCTION

Automotive transport is very popular. It gives the possibility to move fast, far and in the first place self-determined and independent. The car means freedom for the driver, in the first years of the automobile for some " happy few", and nowadays, almost for everybody. The car was a success. It is only over the last decade that the disavantages emerged, due to the continuously growing car park. The all too numerous cars met physical barriers: lack of space, air pollution and noise were increasing. About 80 % of the EC population live in urban areas and so are concerned. But according to estimates of Commission, the situation will even aggravate, because car ownership rises continually The same is expected for vehicles for freight transport which will increase in number, weight and size due to the Single Market, which will result in a considerable rise in ton kilometers, between 30-50 % by 2000, according to the Task Force Report contracted by the Commission and up to 80 % according to the Institut fur Landes und Stadtentwicklung, Dortmund. Figure 1 indicates the evolution of goods and passenger traffic in the EC. Among noise, degradation of the urban and rural environment, consumption of land - which is a limited resource - and congestion, exhaust gases are the problem. Figure 2 shows the evolution and forecast of NOx and C02 emissions by sector. Clearly these emissions will not decrease in the next two decades. COMPOSITION AND EFFECTS OF NOXIOUS EXHAUST EMISSIONS

A number of damages are attributed to tailpipe emissions. the toxic pollutants are causing health impairments, plant damage and damages to material. The first pollutant to raise concern was carbon monoxide because it was emitted in so high quantities that ambient air concentrations were acutely toxic. Through concentrations have decrease meanwhile, carbon monoxide is still a problem because not much is known about chronic effects and because it

18

Car ownership

EUROPE 12

600 500 400 300

200 100 n "

I

1965

1970

1975

1980

1985

1990

1995

2000

2005

Evolution of goods traffic

180 160 140 120 100 80 60 1985

1990

1995

2000

2005

2010

Evolution of passenger traffic

1985

1990

1995

2000

2005

2010

Figure 1 :Transport indicators (ref 2 )

2010

2015

61 OLOZ OLOZ

oooz oooz

066 L 066 L

086 L 086 L

Figure 2: NOx and C02 emissions for Europe 12 by sector (ref 2)

20

contributes to the climate effect as its greenhouse potential is five times that of C 0 2 . In addition, it binds the "scrubbers" in the air thus reducing the selfpurification of the troposphere. Black fumes from cars disturb visibility and are also responsible for soot formation. But particulates which have a smaller diameter - white smoke- have risen concern to cause cancer because they contain on their very porous surface all kinds of carcinogenic compounds such as heterocyclic hydrocarbons. Particulates have such a small diameter that they pass right into the lung alveoles. An investigation into cancer statistics and occupation in London found that drivers of taxis and others diesel-powered vehicles had an increased incidence of bladder cancer. One pilot study among railway workers in the US has suggested that individuals exposed to diesel fumes stand a 42 % greater chance of developing cancer compared to individuals not exposed. In another study, of retired railway workers, an association has been found between lung cancer and exposure to diesel fumes. Automotive transport is responsible for more than 50 % of nitrogen oxides. This is demonstrated by the NOx emission map of the Federal Republic of Germany, which repeats accurately the grid of the most busy motorways. This is evident as long as only traffic emissions are reported, but still when total emissions are presented (Ref. 3). Nitrogen oxides are affecting in particular elderly people, asthmatics and people suffering from heart and lung disease. The limit value for air quality was fixed at 200 pg /m3, but that seems to be too high, especially for this group and in particular when NOx occur together with other air pollutants. The damages to vegetation due to NOx are manifold. In the acid rain discussion, nitrogen oxides were late comers, but they have not stopped ever since to raise concern. Acidification is nowadays caused to an increasing proportion by NOx, as sulphur dioxide emissions decreased over the last couple of years. Forest die-back is seen in connection with nitrogen oxides. The balance of sensitive ecosystems like moor, heather, is easily disturbed by excess input of acidifying substances, but also by the fact that nitrogen oxides act as fertilizer so bringing competitional disadvantages for plants which are sustained by extremely poor nutritional conditions. It is those plants which we find on the Red Lists of extinct or endangered species. Nitrogen oxides, together with hydrocarbons, play a role as precursors for tropospheric ozone and photochemical smog. - Ozone is a toxic gas for both man and animals. It reduces lung function. - Ozone affects plants, in particular in combinaison with other air pollutants, like NOx and SO2 - synergistic effects can be observed, entailing higher damages than one pollutant alone, e.g. reduction of harvest. -Tropospheric ozone affects the climate by contributing to the greenhouse effect.

21

Ozone data recorded between 1876 and 1910 near Paris were at a level of about 10 ppb all over the year. Nowadays, they show seasonal variations reaching a peak of 40 to 50 ppb in spring and summer. The differences in concentration and their variation are obviously due to human activities. Since 1970, the mean annual increase in ozone abundance in the Northern hemisphere has been between 0.5 and 1% per year, and about 2% in heavily polluted regions. High ozone values were reported from all countries of the European Community this summer, also from the North. Values exceeding 300 pg/m3 occurred in many Member States. WHO regards 120 pg/m3 as the threshold value where health effects occur. The Dutch Institute of public health has studied health effects at ozone concentrations higher than 240 pglrn3 and found them to provoke headache, cough, nausea, vertigo, irritation of eyes and nose, and a decrease of lung function. The importance of these effects varies with the exposition period and the quantity of ozone inhaled. The persons at risk are those working outside, people practicing sport and allergics. Frequent expositions might cause irreversible damages The Swiss, Austrian, and German ministers of the environment have agreed in August on a limit of 180-200 pg/m3 where the public should be warned to refrain from excessive outdoor physical activity. In Germany, a controversy has arisen if prohibition of automotive circulation during photochemical smog periods would reduce the risk. The Federal Minister of the Environment thought it to be useless, the Hamburg Senator was in favour of taking such a measure. In Belgium, in the region of Gent, ozone values higher than 300pg have occurred. The mayor of Gent demanded that industrial entreprises reduce their emission of air pollutants. So it can be concluded that the emissions from automotive transport have not improved: acid rain is still an unsolved problem.particulates. NO, and 0 3 continue to cause air pollution. REDUCTION STRATEGIES FOR EXHAUST GAS EMISSIONS

I In principle, it was evident since a long time that vehicles emit noxious exhaust gases and that it was necessary to reduce them by applying the best available technolow. I1 Environmentalists used to be critical with individual passenger transport. They have since long favoured public transDort for several reasons. Nowadays in the discussion, the concern of increasing air pollution and of the climate change prevails.

22

Another efficient measure to combat air pollution consists in speed limits. Reductions by 10 % could be reached by imposing speed limits which really should be applied, i.e. controlled effectively.

I11

I

-

Vehicle exhaust gas reduction by technical means.

There are two goals to be met: one consists in reducing C02 emissions by encouraging the development and the use of energy-efficient cars. The other aim is the reduction of pollutants from car exhausts to US 83 limit values by 1993 and US 93 standards by 1995 taking into consideration that only conformity of production is relevant for the environment and not type approval. This point of view is held by consumers as well and the European Parliament.(Table 1) Was there another convincing technology besides the 3-way catalyst? There were motoric measures, exhaust gas recirculation, etc. There were expectations in the lean burn engine. But nothing had ever been as efficient. The 3-way catalyst improved in performance and still does. Since the early eighties, European environmentalists have pleased for the introduction of US 83 limit values. I do not want to repeat the sad story of deceptions, when equivalence between US and EC was defined by the total amount of emissions over all the territory and not by what came out of the single tailpipe, and what was called the "Luxembourg Agreement", which fixed limit values for the nineties which were outdated already in the eigthies.

Table 1 : Limit values of exhaust emissions for cars (in g/km)

23

The proportion of cars applying state of the art pollution control has increased only slowly in Europe though faster in the contries of the Stockholm Group (EFTA countries) (fig. 3, 4 & 5). But the car market of the E R A countries is ten percent the EC's (fig. 6). Even measures taken now in adopting stricter limit values will be mitigated by the continous increase in the European vehicle fleet. Carbon monoxide and hydrocarbon emissions will rise by 2005 (fig 7 & 8) and nitrogen oxides by the turn of century already (fig. 9). 100% 90% 80% 70%

60% -

50%

-

40% -

30% 20% 10% -

0%

1985

1988

I

I

1991

1994

MODEL YEAR

S t d h l m Group

+

EmopeanCwmnnity

0

West Elrope

Figure 3 :Pollution controls on cars in Europe (state of the art controls) (Ref 1). Driving cycle The current driving cycle simulates urban driving conditions with a maximum speed of 50 km/h. Now an extra - urban part will be added, which goes up to 120 k d h . Both driving cycles will be combined and receive a common limit value (see table I), though two separate limit values would allow for a more exact determination of the pollutants, already two limit values for the urban and extra urban test cycle for CO would be an improvement. It would not come as a surprise if car types appeared on the market which could meet the commission's limit-values with an open loop three-way catalyst because they can compensate high NOx emissions in extra-urban driving conditions by low ones in urban conditions. This means compromissing at the expense of the environment. Here two separate limit values for NOx and HC would be preferable.

1: 1:

11

ia 9 8

7 8 i

5 4

3 2

f

1

0

1985

1

1

1

1988

1991

1994

MODEL YEAR Totnl New Car Sales

I

Slate of Art Cors

Figure 4 :Pollution controls on in Europe (European Community) (Ref 1) 1 .)

1 .:

1.; 1.1 1

'

0.7

0.0 0.5 0.4 0.3

0.2 0.1

0

'1985

1988

1991

1994

M W E L YEAR Tolnl New Car Sales

i

Stnte of Art Cars

Figure 5 :Pollution Controls on cars in Europe (Stockholm group ) (Ref 1)

25 14 13

12 11 10 9 -

!

7

.-

8

-

8

i

5 4

-.

3 2 1 -

0 1985

1988

1991

1994

MODEL YEAR

+

Total New Car Sales

Slate of Art Cars

Figure 6 :Pollution Controls on cars in Europe (Total Western Europe) (Ref 1) 3!

15

0 0

10

5

0

I

1985

1

I

1995

NOGROWTH

2005

+

1%

I

2015

0

2%

A

2025

3%

Figure 7 :EEC vehicle exhaust emissions trends :CO (alternative VMT growth rates) (Ref 1)

26 2.e 2.4 2 1 2

P

1 4

I? 1 2

Y

o.: 0.6

0 4 0.2

0

1985

1995

8

NOGROWTH

2005

I

1%

2015

0

2%

A

2025

31

Figure 8 :EEC vehicle exhaust emissions trends :HC (alternative VMT growth rates) (Ref 1) 4.5

4

3.5

3 2.5 2 1.5

1

0.5

0

I

1

I

1

I

1985

1995

2005

2015

2025

8

NOGROWTH

+

1%

0

2%

A

3%

Figure 9 :EEC vehicle exhaust emissions trends :NOx (alternative VMT growth rates) (Ref 1)

27

Durability testing. Durability testing is envisaged to guarantee the efficiency of pollution abatement systems. The US durability test which goes over 80.000 km will be valid also in the E.C., but provisions are made also for an alternative European test which requires testing only over 30.000 km though under particular stress, even deterioration factors would suffice in case an automobile producer does not want to carry out any durability testing. Testing after 80.000 km would be the most effective, the alternatives are compromises for economic reasons. However, these provisions only concern new cars. To guarantee the reduction of exhaust gases during the vehicle's whole lifetime, a testing requirement must be provided for in-use vehicles. Here the BEUC and the EEB would like to see the Commission coming up with a proposal for the testing of in use cars to make inspection and maintenance mandatory. US driving cvcle and limit values The recognition of US driving cycle and limit values is laid down in Annex 3A of the current Directive on car exhaust: This practice is to be phased out according to the proposal. Annex 3A was the guarantee that US limit values for car exhausts could be applied in the EC and make Member States and their authorities aware of the difference in standards. In suppressing Annex 3A we will lose the possibility to compare and an incentive to conform to stricter US limit values which should remain the goal. Otherwise we will widen the technical gap between the US and the EC market. Small commercial vehicles. As most of the small utility vehicles derive from passenger car concepts there is no reason why they should not meet the same limit values as passenger cars. Concerning energy efficiency, the EEB suggests to contemplate tax incentives for cars consuming less than 4 1 per 100 km.The use of natural gas would be useful as its combustion emits less C02 per energy unit. Natural gas had a good performance in city buses concerning the reduction of exhaust gas pollutants. Smaller, lighter and better designed cars will help to improve energy efficiency. It is not appropriate to have a vehicle weight of more than one ton to transport one person! Electric cars best fed by solar energy would be an ideal solution for some uses, but technically, they have not reached maturity in particular because storage facility and weight of batteries are still unsatisfactory.

28

Given the climate effect, diesel and lean bum rise in the appreciation of environmentalists and they regret the incompatibility of these engine concepts with the catalytic converter technology. Critical evaluation of the Catalytic technology to reduce air pollution from automotive vehicles. The 3-way catalyst has been a success. It has been the precondition for a considerable amelioration of car emissions. Platinum is a precious metal and so is not chemically reactive. Nevertheless, there have been questions about how it behaves in the environment, what kind of reactions it might catalyze. But a Swiss study gave evidence on its harmlessness. It has been tried to use oxidation catalysts to get rid of the particulates. But this was less successful as the use of an oxydation catalyst results in sulfate formation, which cloggs up the catalyst. Catalysts were poisened and had a short lifetime. In addition, there is the problem that the working temperature is too low: 140 "C instead of 300 "C. The discussion on trap oxidizers using copper and vanadium as catalysers have, not yet the expected performance, besides the fact that the use of copper might have led to copper pollution in the environment. Copper is toxic for plants and animals. Also considerations to use iron salts as catalytic additive for diesel fuel meets scepticism because under these circumstances, very small needle shaped iron salt cristals are formed. They would have the capacity to penetrate lung alveoles like asbestos fibers and it is not advisable to wait until the bad experiences repeat before iron salt additives are banned. To combat diesel particultes with the help of catalysts does not seem to be the adequate technical solution. Engine modifications for cars but also for trucks have proved to be more reliable and burner controlled particulate traps got good notes in the German field test on buses and lorries.

I1

-

Public transport.

Given the hopeless situation at the time of the Luxembourg agreement, it seemed impossible to obtain the strict limit values for car exhausts which were needed. Environmentalists looked for a resort to reduce automotive emissions and found it: public transport for passengers and rail transport instead of road haulage. This simple concept found much sympathy. Comparisons made between the subsidies received as state aid for road and rail revealed that the road user does not pay the full social and environmental cost he is generating. The amount he is paying as excise duty does not cover health effects and accidents (paid by the forest owners, farmers etc and the public at large).

29

Facts such as accidents and casualties on the road, hours spent in congestions, and the rising amount of the time which had to be dedicated to automotive transport are not to the advantage neither of the car, nor of the lorry. Transport is responsible for 58 percent of nitrogen oxide emissions and 22 percent of carbon dioxide (table 2).

Table 2 : Atmospheric emissions (9%)

-

1987

-

EEC

Source: Energy 2010 (1989). Comments to Table 2 : Transports contribute considerably to NOx emissions accounting for 60% of total emissions in 1987 in the Community. Its contribution to C02 emissions and thus to the green-house effect is also important and due to the burning of fossil fuels. The energy production sector accounts however for almost 40% of total C02 emissions. No technology exists at present capable of filtering C02. The only possibilities are less energy consumption, energy efficiency and cleaner fuels.

Of the different transport modes, road transport represents the most considerable environmental impact concerning air pollution and surface use (table 3). Air pollution from road transport is 13 fold higher concerning NOx and 20 to 60 fold more energy is needed. Only SO2 emissions are comparable (table 4). The surface covered by road infrastructure are 4,7 % in Belgium and 3 % in The Netherlands, 1,3 % in the average of the EC (table 5). The social and environmental costs of road transport -land use cost and long term cost along with the consequences of an intensification of the greenhouse effect not included - are running up to five percent of the GDP (table 6). Nonetheless, Member States of the EC continue to invest preferentially in road infrastructure instead of railways . In a market economy, state aids have to be balanced among competitors. Up to now the road has received a preferential treatement at the expense of the public transport. Environmentalists demand to put them on an equal footing in raising the excise duty on fuel by at least doubling the fuel price; the German Greens even demand an increase by the factor five.

30

Table 3 : Hierarchy of impacts per component of environment and of transport mode.

*: Small impact; **: Significant impact; ***: Severe impact; --:very small impact. (a) (b) (c)

Plus danger to transfer the problem to the electricity supply. Impact may become severe in case of mishap. Environmental effects of mobile sources at high altitude are still not well known

Comments on Table 3 : These rankings are based on conventional knowledge and experts' consensus. For a given transport mode, horizontal comparison between impacts is hampered by value judgements about the relative importance of different components of the environment (water, air, land, health). Road transport has the most significative overall impact on the environment. This is not only due to the fact that road modes are the most widely used but also to the more intense unitary impacts (per tonne-km or passenger-km) they provoke.

Table 4 : Emission factors and energy per mode of transportation

(a) Unit emissions only concern lomes : Charge factor for energy consumption equal to 50 % . With a charge factor close to 100 % , high capacity road transports should be almost as energy saving as rail.

Source : Strategic plan for rail freight. This study was ordered by the Netherlands Ministery of Transports and Waterways. (Rotterdam, July 1989 ) Comments to table 4 : By many standards, rail is more environmental-friendly than road transport. Data are not available for passenger traffic and C 0 2 emissions but it is currently suggested that the picture concerning emissions and energy consumption should be broadly similar to that of the table. This evidence provides the primary argument for a policy aimed at the encouragement of transport modes alternative to road transport (modal shift).

31

Table 5 : Road substructure (all roads), density of the road system and coverage of the ground (1985) (a). ~~

Length

Surface covered

1000 km

km2

I L NL P UK

127.9 70.1 491.2 34.5 151.8 804.7 92.3 303.0 5.2 96.3 51.9 348.3

3UR (b)

km of roads per 1000 km2

1445 792 5800 390 1600 8740 1043 3424 59 1110 586 3936

Surface covered in % of the total surf. 4.7 1.8 2.3 .3 .3 1.6 1.5 1.1 2.3 3 .O .6 1.6

4197 1627 1975 26 1 301 1466 1313 1006 2008 2582 564 1427

km of roads per inhabitant (EUR=100: 162 171 101 43 49 182 326 66 177 83 64 77

2577.2

28925

1.3

1143

100 (c)

USA

6600.0

87000

.9

704

345

JAPON

1127.5

9900

2.7

3028

117

B DK D ELL E F IRL

(a>

Colums 2 and 3 correspond to estimations made at the begining of the 8 0 s

(b)

Growth of the road system from 1975 to 1985 was of 3,7%, or 0,36%per year (0,47% from 75 to 80 and 0,25% from 80 to 85)

(c>

8 km per lo00 inhabitants

Origin: OCDE (1988) and CDP elaboration

32

Table 6 : The environmental costs of road transport (as % of GDP) Source Community of

I

Pollution (a) Congestion (b) Accidents (c) Total (d)

5

Environmental Costs in 1989 in ECU (e)

220

0.7 - 1.5 2.6 - 3.1 2.5 5.8 - 7.1

I

255 - 312

Socio-economic costs of pollution due to exhaust gases from road vehicles estimated for the Federal Republic of Germany. These costs are mainly health related. Time and energy use in the large european comdors : towns and links between towns (Bouladon report)). Socio-economiccosts of road accidents (France, Luxembourg, Belgium and the Federal Republic of Germany). The probability of accidents, in the EEC, is 125 times greater for private car users than for train passengers. Mostly non intemalised socio-economic costs. Costs related to land use are not available.

EEC's GDP in 1989 is estimated at ECU bn 4391 (EUROSTAT). Sources: OCDE (1988) (page 11) and Community of European Railways (1989) (page 24). Comments to table 6 : These data should be interpreted with caution for they come from various sources. The OCDE estimate of 5% of GDP for non-internalised costs of transports is perhaps the most solid. The other source helps to form an idea about the likely breakdown of the aggregate figure. Given that land-use costs and long-term costs associated to the consequences of an intensification of the green-house effect (C02 emissions) are not included, it is legitimate to interpret the 5% figure as a lower limit. According to all experts, these costs should be fully paid by the users of the different transport modes (via road pricing, vehicle mileage and fuel taxes, etc.). Today they are paid by tax-payers and victims and this suggests that the services of transport modes, and in particular road transport, are simply too cheap to the users.

The calculation finally on what and how much individual transport emits and how much could be avoided by using preferably public transport clearly stroke the balance into the direction of public transport In all countries, environmental organisations nowadays mobilize against the preferential treatment of automotive transport. An additional driving force is the concern raised by the greenhouse effect. Automotive transport is less energy efficient than public transport.

33

Quite new coalitions are formed and what is more important, people realize that there have been developments in transport policy of wich they are the victims and now they pronounce their disagreement. I only want to remind you of this summer, when alpine transit almost came to a standstill because one bridge collapsed. Inhabitants neighbouring the bypass routes were unwilling as an emergency to receive the extra burden of lorries. Heavy lorries are in the first place the target of displeasure. The car still maintains its popularity as can be read from the increase of cars sold over the past years. But also here changes are evident. Some urban municipalities take measures to reduce the number of vehicles in the city or ban them altogether. Nine renown Swiss holiday resorts advertised their having banned internal combustion vehicles. And these measures are by no means acompanied by a storm of indignation, on the contrary, people applaud. Automotive transport passes a period of criticism. Characteristic for the reflections was the title of a book: "Rethinking the automobile". All the different measures proposed are aimed to redress the balance which has certainly been too much into the direction of automotive transport. There are people who have never gone by train. This is an obvious sign that the possibilities are not well used. It is not to sweep the balance to public transport exclusively, but to create conditions without congestions, undue air pollution and noise, accidents and roads consuming the surface. Who wants to save automotive transport has to be for the proportionate development of public transport. That is common sense.

References 1

2 3

M.P.Walsh, "NOx emissions from road traffic in Europe" Workshop on projections of NOx emissions, Oslo, December 1989 "Major themes in energy", EC-Community Daten zur Umwelt 1988/89,pp 284-288, Umweltbundesamt Erich Schmidt Verlag


A. Crucq (Editor), Catalysis and Automotive Pollution Control ZI 0 199 1 Elsevier Science Publishers B.V., Amsterdam

35

THE EVOLUTION OF THE EEC REGULATIONS ON VEHICLE EMISSIONS H. Henssler, Commission of the European Communities

1 - INTRODUCTION This paper deals mainly with the evolution of the regulations of the European Communities relating to the control of exhaust emissions from motor cars since the first CAPOC-seminar in 1986. As an introduction the main principles of the so called "Luxemburg Compromise" of June 1985, are briefly recalled as they represent the key for understanding the further evolution of the European emission standards. These principles were : (a) to set up European standards which in their effect on the environment were equivalent to the emission standards of the USA, however, taking into account the specific conditions of Europe concerning its car fleet and traffic patterns. (b) to allow for a choice of technologies to comply with the future European standards, these technologies being essentially the closed loop three-way catalyst and, as an alternative supposed to be cheaper, lean burn engine concepts, if necessary, fitted with simple oxidation catalysts. The realisation of these principles have led to a regulation containing 3 distinct sets of limit values according to 3 categories of engine capacities : 2 litres and more, 1.4 to 2 litres and less than 1.4 litres. In correspondance with the assumed availability of the concerned technologies also distinct implementation dates have been fixed : - For the category above 2 litres : 1 October 1988 for new car types, 1 October 1989 for the registration of new cars, - For the category between 2 litres and 1.4 litres : 1 October 1991 for new types and 1 October 1993 for all new cars and - For the category below 1.4 litres : 1 October 1990 for new types and 1 October 1991 for all new cars. The standards of the Luxemburg compromise are based on the existing European test procedure, the so-called urban driving cycle, but allowing, as a transitional measure, the alternative use of the US - FTP 75 driving cycle. These standards also require all spark ignition engines to be adapted to the use of lead-free petrol of 95 RON. They are equally applicable to diesel engines, however, engines with more than 2 litres being submitted to the standards of

36

the medium category. The Luxembourg compromise had to wait until 3 December 1987 to become an EEC Directive thanks to the European Single Act of February 1986, which introduced the possibility of decisions by a qualified majority, thus overcoming the Danish and Greek vetos. This Directive with the reference 88/76/EEC represents the 5th amendement to Directive 70/220/EEC which had established in 1970 the basic emission standards of the European Community. For the sake of completeness it may be added that the United Nations' Economic Commission for Europe translated in 1988 this latest EEC Directive into a new ECE Regulation no 83 intended to replace their traditional regulation no 15 covering motor car emissions.

-

2 FURTHER EVOLUTION The further evolution of the European emission standards since 1985 has been largely determined by a number of commitments which were part of the Luxemburg compromise or have been taken in connection with the adoption of Directive 88/76 : These commitments concerned : - the substitution of the interim standards for the car category below 1.4 litres by more severe European standards - the completion of the existing urban test procedure by sequences representing driving conditions on highways and motorways - the setting of standards for the emission of particulate matter by diesel powered cars - the introduction of requirements concerning the durability of emission control devices and the evaporation of fuel.

2.1 - The Extra-Urban Driving Cycle Immediately after the Luxemburg Council the Commission charged its well established expert group on vehicle emissions known under the abbreviation MVEG with the technical work resulting from these commitments. This group is composed of the experts of the interested national administrations, of the motor and petrol industry and of consumer and environmental organizations. Its work in relation to the future European extra-urban driving cycle started on the basis of studies of typical highway and motorway driving patterns in Germany and the United Kingdom and by the Committee of Common Market Constructors. The results of these studies were put together and resulted in a synthesized driving cycle which was presented in March 1988 as the European Extra Urban Driving Cycle "EUDC", caracterized by a relatively simple driving pattern and presenting a peak speed of 120 km/h and an average speed of 62.6 kmh. MVEG then examined the possible combination of this cycle with the

31

existing urban driving cycle with the help of tests carried out on a representative sample of 133 cars of different types and featuring different emission control systems. Eventually, the group agreed on a combination where the urban cycle precedes the extra-urban cycle, a combination which became known as "MVEGA" and which preserves the potential of the urban cycle to measure emissions at a cold start.

2.2

-

Diesel Particulate Standards

In parallel, MVEG worked on the future European standards for the emissions of particulate matter of cars powered by diesel engines on the basis of an investigation of the actual emissions from present European diesel car models. As sampling and analyzing method that of the present US regulation was retained and efforts were made to adapt it to the European test procedure. In June 1988 this work resulted in the adoption of Directive 88/436/EEC. Because of the lack of precision of the sampling method and the resulting relatively high limit values (1.1 dtest for type approval and 1.4 g/test for the control of production conformity), it was understood that this Directive could only represent a first stage of the particulate control in the EEC and the European Parliament fixed as the objective for a further more stringent stage with 0.8 g/test for type approval and 1.0 g/test for the control of production conformity. 2.3

- Emission Standards for Small Cars

Finally, MVEG examined the question of definitive European emission standards for the car category below 1.4 litres. The result of the technical work suggested that if the principles of "technical choice" and "reasonable costs" were to be retained, the limit values for the gaseous emissions of the small car category should be equal to those of the medium category i.e. 30gtest for CO and 8gtest for the combined mass of HC and NOx. These limit values were proposed by the Commission in 1987 and were approved in November 1988 as the common position of the Council and the Commission in accordance with the Single Act procedure. The European Parliament, however, with which Council and Commission have to co-operate under this new procedure, was of the opinion that the environmentai situation had becoms dramatic in the EEC, on the one hand, and that the closed loop three-way catalyst technology was now available to the whole European car industry, on the other hand. Consequently, the Parliament requested limit values which are at least as severe as those of the USA for the small car category which represents roughly 60% of the present European car fleet. After some discussions, the Commission followed the Parliament and

38

presented in May 1989 a revised proposal. The majority of the Member States agreed in June 1989 and adopted directive 89/458/EEC with the limit values of 19 gjtest for CO and 5 g/test for HC+NOx (type approval limits) to be applied on 1 July 1992 to new car types and on 31 December 1992 to all new cars. This decision represents another milestone of the motor vehicle emission regulations of the European Community : The principle of the technological choice has been abandonned and replaced by the principle of the best available technology on which the future European standards have to be based. Consistantly, the Council committed the Commission to present a proposal which aligns the limit values of all car categories to those of the small car category and, at the same time, to adapt these standards to the improved test procedure including the extra-urban driving sequences.

2.4

-

The Consolidated Directive

In compliance with these commitments, the previous ones from June and November 1985, and the recent ones from June 1989, the Commission decided to comprehend in one directive the different requirements relating to gaseous and particulate emissions of passenger cars and thus to consolidate the EEC emission standards for this vehicle category. Of course, the proposal is based on the results of the MVEG work described above : the driving cycle completed by the extra urban sequences, the improved particulate sampling method and the specifications for testing the durability of emission control devices and the evaporation of petrol from the fuel system of the cars. The limit values of this proposal for the gaseous emissions constitute a purely technical transposition of the small car standards into the new complete test procedure. The proposed particulate standards correspond to the objective fixed by the Parliament (0.8/1 .O gjtest). For this transposition the appropriate results of the 133 car sample established for the choice of the driving cycle combination has been used i.e. only those cars, both petrol and diesel, whose gaseous emissions were below the limit values of 19 g/test CO and 5 @test HC+NOx. The limit values which resulted from this operation are for the approval of new car types : 2.72 g/km C0.0.97 g/km HC+NOx and 0.19 g/km particles.The corresponding values for the control of production conformity are 3.16g/km CO, 1.13g/km HC+NOx and 0.24g/km particles. It is worth to note that these standards represent approximately 10% of the limit values of the first EEC directive of 1970 (70/220/EEC) relating to emission control. According to international practice, these standards are now expressed in "grams per kilometer" of the new European test procedure combining the urban and the extra-urban driving cycle representing a test distance of 1lkm. In accordance with the directive on small cars these limits are proposed to apply from 1 July 1992 to new car types and from 1 January 1993 to all new cars.

39

The Commission wanted of course to be sure that these limit values comply in fact with the postulate to be at least as severe as the existing US standards. Hence, a study was commissioned to 2 national testing laboratories in order to compare the actual emissions measured with the proposed European test cycle to those measured with the US test procedure FTP'75. The study was carried out with 44 representative European car models- 31 with petrol engines and equipped with closed loop three-way catalysts and 13 with diesel engines - complying with the CO standard of 19g/test and the HC+NOx standard of 5g/test of directive 89/458. This study showed despite an important scatter of the results, a clear trend of the European test procedure to yield generally higher emissions than the US procedure. This trend was most obvious for CO where the European test resulted in average in 45% higher values than the FTP'75 test. For HC+NOx and particulate emissions the average of the results were 15% higher than in FTP'75 test. In terms of limit values, this means, that the proposed European standards for gaseous emissions are indeed more severe - about 12% for CO and 4% for HC+NOx than the US standards. However, for particulate emissions the study showed that the objective fixed by the European Parliament and followed by the Commission has been too lenient : a value corresponding to the present US standard of 0.20gImile would be 0.14gkm In the European test and the corresponding standard for the control of production would be 0.18g/km. The Commission is prepared to correct its proposal in this respect.

3

-

ASSESSMENT OF THE PROPOSAL FOR A CONSOLIDATED EMISSION DIRECTIVE AND ITS POSSIBLE INDUSTRIAL EFFECTS

The main characteristics of the proposal are, on the one hand, the strict respect, by the Commission, of the previous agreements and commitments which are mentionned in the first part of this paper. The Commission believes in the continuity of the EC policy on vehicle emission standards because of its economic importance, in general, and of its bearing on the European automotive industry, in particular. On the other hand, this proposal is caracterized by the determination of the Commission to find a genuine European solution to the environmental problems caused by road traffic, a solution which is most adapted to the European conditions in respect of both its car fleet and its driving patterns. It needs to be said that the proposal which was presented to the Council of Ministers on 5 January 1990 and to the European Parliament on 22 February 1990 has not received a unanimous approval from these institutions. The majority of the European Ministers who discussed it at their meetings of 22 March and of 7 June 1990 declared it nevertheless to be a good basis for a rapid decision and confirmed their interest in a genuine European solution. The problems which have been identified so far appear to be more of a technical nature than fundamental. The European Parliament, at least its

40

Environmental Committee, appears, however, convinced that, in principle, the best for Europe would be to copy the American emission regulations. In the forthcoming dicussions, the Commission has decided to stand firm concerning its proposals and the following months will show what eventually, will be the solution which rallys the majorities of the European institutions. In any case, whatever the solution will be, from an industrial point of view, it appears clearly that from January 1993 on all cars produced for the market of the EEC in big series will be, independent of their engine capacity equipped with closed loop three-way catalysts if they are powered by petrol engines. As far as diesel powered cars are concerned, the proposed standards both for gaseous and particulate emissions, do not assume the use of catalysts or trap oxydizers. The reason for this is that expert consultations did not allow the Commission to assess neither the potential of these techniques, nor the time of their availability, with sufficient accuracy. The Commission will, of course, follow their development and, if possible, adapt the standards to their potential, in order to generalize their fitting to the concerned vehicles. 4

- OUTLOOK INTO THE NEXT FUTURE

The Commission is aware that catalysts deteriorate within a certain period of usage. The regulatory durability tests available now including the US - 50 000 miles- test which has been also included into the directive proposal do not offer a safe means to determine the moment when catalysts are no longer efficient and have to be replaced. The Commission believes that only a Community-wide system of periodic inspections of the cars in use is appropriate for this purpose and allows eventually to take the full environmental benefit of this technology. Consequently, the Commission is actively working on a directive proposal in this respect which it deems necessary to complete its present proposal relating to vehicle construction. Another complementary measure which the Commission will present in a few months, concerns the emission standards for all vehicles covered by the scope of this directive and not being passenger cars. Such vehicles, generally being light commercial vehicles of category N1 but also heavy passenger cars having more than six seats and/or a total mass exceeding 2.5 tomes, are presently governed by transitional provisions which submit them to more lenient standards than passenger cars. The reason for this is essentially that such vehicles have a power-to-weight ratio considerably inferior to that of typical passenger cars. Consequently, these vehicles have higher emissions than passenger cars under identical test conditions. At present, we are studying, a system of standards which is adapted to the technical conditions of these vehicles but represents the same level of severity as the proposed passenger car standards. Consequently, it can be foreseen that after the mid-nineties all motor vehicles driven by petrol engines will have to be equipped with closed loop three-way catalysts.

A. Crucq (Editor), Catalysis and Automotive Pollution Control II 0 199 1 Elsevier Science Publishers B.V., Amsterdam

41

THETECHNOLOGICAL DEVELOPMENT OF THE EUROPEAN VEHICLES AND THE ENVIRONMENT

C. Cucchi and M. Hublin

CCMC 1. INTRODUCTION

On June '85, the Council of Environment reached an agreement on a plan for further tightening of the emission standards for passenger cars. This plan, known as the "Luxembourg Agreement", was motivated by the need to contain the impact of the road traffic on the environment and was intended to establish an equivalency between the total amount of pollutants emitted by the the European and the US car fleets, taking into account the difference in the composition and the patterns of use of the two vehicle parks. Table 1 summarizes the decision reached on the emission limit values and the corresponding dates of implementation. The basis for the following further actions were also layed down at that time: - Step I1 emission standards for small cars with an engine displacement below 1.4 litres - limit values for particulate matter emissions from passenger cars equipped with a diesel engine to be implemented in two steps. - introduction of an added driving cycle representative of extra-urban driving conditions. - limit values for vehicle evaporative emissions - durability requirements The Luxembourg Agreement was transposed into a Directive (Dir. 88/76/EEC dated 3 December '87), with a two-year-delay as a consequence of the late coming into force of the Single European Act Since '87, the Commission, with the support of the MVEG, has defined the issues that were left open. In particular: - a limit value of PM = l.lg/test was selected for diesel vehicles to be implemented on 1/10/90 for all vehicle types. - Step II emission standards for passenger cars below 1.4 It. was defined as: CO = 19g/test; HC + NOx = 5g/test and the corresponding dates of implementation put forward to 1/7/92 - 31/12/92 - the driving sequence shown in Fig. I was developed to reproduce the extra-urban driving conditions, and between the possible combinations

42

of the urban and extra-urban driving sequences Fig. 2, configuration A was retained as the new European driving cycle. - a test procedure to measure vehicle evaporative emissions within a European context was developed.

TABLE 1 Luxembourg Agreement Exhaust Emission Limit Values

I

CAR

NOx GfIEST

CATEGORY

Gasoline Engines >2000 cc

I

DATES IMPLETATION

I 3.6

NM 1/10/91 AT 1/10/93

1400-2000 cc

t

NM* 1/10/89 AT 1/10/89

I

el400 cc STEP 1

45

15

5

TO BE DEFINED

NM 1/10/92 AT 1/10/93

Diesel Engines** >1400 cc

c 1400 cc STEP 1

45

NM 1/10/90 AT 1/10/91

NM 1/10/90 AT 1/10/91

1

15

6

NM 1/10/90 AT 1/10/91

STEP 2

* **

NM-NEWMOD FOR DIRECTINJECTION DIESEL ENGINES. 2000 c c , THE DATES OF IMPLANTATION HAVE BEEN FIXED AT 1/10/96 FOR ALL TYPES.

While particulate emission limit values and Step I1 standards for small cars have already been finalized in Directives (Dir. 88/436/EEC of 16 June 1988 and Dir. 89/458EEC of 18 July 1989 respectively), the new driving cycle, the evaporative emission requirements and the durability requirements are planned to be introduced in a New Consolidated Directive which will impose a set of standards common for all car categories as from 31/12/92 (1/7/92 for new models). The Commission has recently published a proposal on this issue (COM (89) 662 dated 2 February 1990) as discussed in the paper “Evolution of the EEC Regulations on Vehicles Emissions” presented by Mr. Henssler

43

during this meeting. This fast evolution of the emission requirements has brought about an intensive engineering activity for the development of emission systems optimized within a European context. The auto industry has always felt that the technical solutions defined for the US market may not always be the most suited for a vehicle park which differs from the point of view of vehicle size, engine displacement/types and patterns of use. The economic aspects of new legislative requirements in terms of vehicle purchase prices and operating costs cannot be overlooked. 120

100

-

STATISTICS: LENGTHS :

-

VMAX.:

i5

i

6955 rn 400 S 120Kmh 62 Km/h

80-

s E3w

60-

n 40-

20

-

00

I

I

I

I

I

I

I

I

50

100

150

200

250

300

350

400

I

I

I

TIME (Sec.)

Fig. 1 EUDC Driving Cycle 2. EXHAUST EMISSION TECHNOLOGIES

The evolution of the car park in the US has been characterized by the following trends as a consequence of the emission requirements that are being enforced in this country: - exclusive use of electronically controlled fuel metering systems for gasoline engines - exclusive use of the three-way catalyst concept with or without an added oxidation catalyst - disappearance of the diesel engines despite the interest shown even by local manufacturers as a consequence of the corporate average fuel economy requirements and the large investments made. - very large percentage of vehicles equipped with fuel injection systems. Practically all US and European models are fuel injected.

44

120 -100 c 2 180 A;;W

40 v)

20 0

0

100 200

300 400

500

600 700 800

900 lo00 1100 1200

Time (s)

120

..-. 100 280

B Z W $40 a 20 0

0

100

200

300 400 500

600 700 m 900 lo00 1100 1200

Time (s)

1

120 -f

.

I I

80 60 40 20 0

0

m 40.

a

(I)

20.

100 200

n

300 400 500

600 700 800 900 loo0 1100 1200

I

nI7 I Time (s)

120 100 80

60 40 20 0

0

100 200

300 400

500 600 700 Time (s)

800 900 loo0 1100 1200

Fig 2 :Driving Cycle Configurations

45

The final decision of the European legislators concerning the emission standards in terms of driving cycle and limit values will determine the future situation in Europe.

2.1 Gasoline engines Various levels of severity, in terms of driving cycle and limit values, have been brought forward since the time of the Luxembourg Agreement. Each of them has a different impact on the technological choices available to the manufacturers and on the final price of the vehicles. The following paragraphs review the main proposals.

2.1 .I Urban Cycle - CO = 30gltest ;HC

+ NO* = 8gltest

These are the standards chosen for medium cars at the time of the Luxembourg Agreement and, at an early stage, also proposed by the Commission for small cars as Step II. They offer the choice between different technolgies such as: - lean-bum engine with an oxidation catalyst - three-way catalyst in an open-loop configuration - engine modifications + fuel injection system - engine modifications + EGR + oxidation catalyst As a consequence, the car manufacturer remains free to select the optimum solution on the basis of the following criteria: - emission reduction efficiency - engine performances including fuel consumption - driveability - system costs Of particular interest for this level of seventy is the lean-bum technology.On the basis of the response of gasoline engines to the aidfuel ratio Fig. 3, it is theoretically possible to reach optimum emission and fuel consumption levels in the lean region. However, under these conditions, engine operations become critical due to an increase in the combustion duration and in cycle to cycle scatter. To push forward the lean missfiring area, it is necessary to create high turbulance in the combustion chamber by resorting to high swirl intake ducts, air jets at the valve or butterflies valves in the intake ducts Fig. 4. The definition of a lean burn engine requires long development times. The optimum performances that can be achieved in terms of fuel consumption are counter-balanced by the following issues when compared with a conventional engine: - increase in HC emissions - higher NOx emissions during the cold phase

46

- higher dispersion of the emission levels as a function of the tolerances of the fuel metering system - driveability.

It

T

"1-

30

10

m ;lrH

N E L ~ Y s T I o N

mo

m$sa

M 1

14

1

1

18

1

1

18

1

1

20

1

8

1

p

iuno

1

24

Fig. 3 Engine Emissions and Specific Fuel Consumption vs AIF Ratio

Fig. 4 a :High swirl intake ducts

Fig 4b :Buttelflies valves at the intake ducts

Fig 4c :Air jet at the valve

47

As a consequence of the above, an oxidation catalyst and an improved carburetor or an injection system are needed to meet emission limit values as stringent as HC + NOx =8g/test. Further reduction in the emission levels are difficult if not impossible to achieve as NOx emissions can only be controlled through an optimization of the air/fuel ration setting. No EGR or three-way catalyst can be used in a true lean bum engine for obvious reasons. 2.1.2. Urban cycle - CO = 25gltest ;HC

+ NOX = 6.5 gitest

These are the standards that are in force in Europe since 10/89 for passenger cars with an engine displacement above 2.0 It.. This level of severity can only be met by the technolgy of the three-way catalyst in a closed-loop configuration. This technology is based on the property of Platinum and Rhodium to act as catalyst for both the oxidation of CO and HC and the reduction of NO,. Clearly, the highest efficiencies for the oxidation reactions are achieved when the exhaust gases flowing through the catalyst have an excess of O~(1ean mixture) and the highest efficiency for the reduction reaction when they are poor in 0 2 (rich mixture). Nevertheless, there is a window around the stoichiometry where the conversion efficiencies of the catalyst for both the oxidation and the reduction reactions are satisfactory (80-90%). An electronically controlled fuel metering system is required to ensure the correct composition of the exhaust gases flowing through the catalyst under all the engine running conditions. An oxygen sensor detects any deviation from stoichiometry , sends a signal to a control unit that corrects accordingly the amount of fuel delivered (closed-loop systems). In practical terms, the aidfuel ratio achieved with such systems is never set on a constant value but oscillates around a constant value (- stoichiometry). The frequency and amplitude of these oscillations are a function of system transport time (delay between the information given by the sensor and the corresponding modification of the A/F ratio at the inlet valve) and the control strategies. The overall efficiency of the three-way catalyst results from see Fig.5: - the base calibration of the fuel metering system - the frequency and amplitude of the aidfuel ratio oscillations around the above value. Therefore, the selection of the fuel metering system and the control strategies play a key role in the achievement of low vehicle emission levels. Electronically controlled carburetors, throttle body fuel injection systems and multipoint fuel injection systems are today available for use in combination with a three-way catalyst. In the category of passenger cars above 2.0 It, fuel injection systems are almost exclusively utilized.

48

1m

-

-

*

;E

t,

zp w U

aa-

)(01

m-

Hc

2

P

40-

v)

a

z8

10-

m

o

,

,

,

,

u

,

,

,

,

I

Fig. 5 (a) Catalyst Eficiency om

om

U11

1m

14.60

1.aY

1.m

IS09

14-

AIRIFUEL RAno AMPLITUDE 0.3 M FFIEWENCY 1.8 HZ

-

i RICH

om

I

0.1 I l

1

I

LEIN

1.w

1.02

I

A

RICH

A L E A N

too

I

I

NOx

HC

8,

NOx

10

WPUNDE

AIR'FUEL wno 1.0 M . FRCOUENCY 1 Hz

AYPUNDE

URlRlELRATIO 1.0 M - FREQUENCY 0.7 +k

+_

Fig. 5 (b) Three-Way Catalyst Efficiency

49

2.1.3 Urban cycle - CO = I9gltest ;HC

+ NOx = Sgltest

These are the emission standards chosen as Step II for small cars below 1.4 litres. They can be met only by the technology of the three-way catalyst in a closed-loop configuration. In practical terms, they correspond to a level of severity more stringent than that of the US '83 requirements as a consequence of the specific configuration of the European urban driving cycle. Their approval by the Council of the EC will have an evident impact on the vehicle prices (see Table 2) and their operating costs. It will also require large investments, especially for the auto industry suppliers, as a consequence of the widespread use of the electronics for the engine management and the phase-out of the production of carburetors. Electronically controlled carburetors are not always the most economical technology due to their intrinsic characteristics (poor control of the A/F ratio if compared with fuel injection systems). 2.1.4 New European cycle - CO = 2.72glkm ;HC

+ NO, 0.97glKm

The above limit values have been proposed by the Commission for all categories of passenger cars in connection with the introduction of the new European driving cycle. They are intended to meet the following prerequisites: - to be at least as stringent as the US '83 standards - to align the requirements for passenger cars with an engine displacement above 1.4 It. to the standards already approved for small cars (i.e. Step I1 - urban cycle - CO = 19dtest HC + NOx = 5g/test). The new European driving cycle is unique as it introduces driving modes at vehicle speeds higher than the ones covered by other test cycles including the US highway cycle. Under heavy load conditions, CO and NOx engine emissions rise sharply and the means to control them are limited without penalty on driveability and/or fuel consumption. As a consequence, the technologies today available to abate vehicle exhaust emissions must be reconsidered in function of the new cycle and the corresponding limit values. A comprehensive study that was recently carried out by CCMC shows that the three-way catalysts in a closed-loop configuration technology is already required if the limit values for the new European driving cycle are set at: CO = 5-

HC + NOx = 1.25 g/km see Fig. 6

50

TABLE 2 Emission Technologies and their Impact on Emission, Reduction, Price, Increase and Fuel Consumption Variation Consumer Fuel Price Consumption Increase Increase

Technologies Conventional Engine for "Lux. Agreement"(Step 1) Lean Bum Engine with Carburetor and Conventional Ignition Conventional Engine for Tax Incentives in

(%I

(%)

0.5

2

*

Overall Emission Increase

Limit Values

0 22 **

1.o

-2

22

1.5

3.5

34

Luxemburg Agreement step1 cQ=45g/t HC+NOx = 15 g/t

FRG-NL Pulsair and EGR LeanBum Engine with Carburetor and Recalibrated Conventional engine with EFI Lean-Bum Engine and EFI Lean-burn Engine

4.5

Davignon 1

I

Open loop-3Way-Cat Carburettor Lean-burn Engine Closed Loop - EFI Var. Intake System 3 Way-Cat

** ***

34

I

9*

34**

-15%

CO = 38 g/t HC+NOx = 12.8 g/t

2

34

-7

34**

-3

53***

Commission Proposal

+2

53***

CO = 30 g/t HC + NOx = 8 g/t

-7

approx. 60

+3

73

US '83 or equivalent

Fuel Consumption change is the average of 90/120/Urban consumptions Some of the technologies described above are only at an early stage of development. This varies from manufacturer to manufacturer

BASELINE = small vehicle, 1.4 L conventional carburetted engine meeting ECE 15/04

51

4

HC

+ NQx

3

2 1,25 1

0

CQ

0 0

2

4

5

6

8

10

Fig. 6. CCMC Study on the Impact of the New European Driving Cycle on Emissions- Lowest Levels Achieved by Vehicles not equipped with a three-Way Catalyst and Closed-Loop System. Other technologies such as a three-way catalyst in an open-loop configuration or an oxidation catalyst + E.G.R., which can still achieve very low emission levels in limited cases on the urban cycle, cannot control at the same time CO and HC + NO, emissions below the above figures when tested according to the new driving cycle . The Commission proposal, being even more stringent than the above values, will require added optimization of the present state of the art technology (i.e. three-way catalyst in a closed-loop configuration). Control strategies of the engine management system, catalyst composition and design, auxiliary devices such as E.G.R. must be reengineered on the basis of the constraints posed by these new European standards in order to achieve the desired result. Models meeting the US '83 requirements do not necessarily have emission levels below CO = 2.72g/km when tested according to the urban + extra-urban driving cycles as shown in Fig. 7. 2.2 Diesel engines

At present, the emission standards for pasenger cars equipped with diesel engines are set at CO = 30g/test, HC + NOx= 8g/test, PM = 1.1 g/test (but for engines below 1.4 It for which CO = 45g/test, HC + NOx= 15g/test). The only constraint is represented by the trade-off between particulate matter and HC + NO, emissions. The optimization of the engine calibration is sufficient to meet todays requirements. The Commission proposal concerning the introduction of the new European driving cycle provides also for a

52

further tightening of the particulate emission limit values. This means that diesel engines will have to meet: HC + NO, = 0.97g/km,

CO = 2.72g/km,

PM = 0.19 g/km.

Under these conditions, the trade-off PM/HC + NO, becomes critical within the technologies presently available to abate diesel exhaust emissions, i.e.: - combustion chamber design - fuel injection system design - EGR - improved diesel fuels. ---. .. -- .. . - M TEST I I t S U L l S IN t-i"/ K 5.0

-EEC PROC.

SLOPE

CO (With DF)

=

1.452

3.5 3.0 -

4.5

4.0

2.5

-

1.5 -

2.0

1.0

-

0.5

-

*/

US PROC.

0.0 0.0

I

I

I

I

I

I

I

I

I

1

0.5

10

1.5

2.0

2.5

3.0

3.5

40

4.5

5.0

0 GASOLINE VEHICLE

DIESEL VEHICLE

Fig. 7 Comparison between Emision Levels Achieved on the New European Test Procedure by Vehicles Designed for the US '83 Standards. It should be noted that NO, emissions cannot be dealt with an after-treatment system such as a three-way catalyst. At the same time, particulate filters for automotive applications are being investigated with still poor results as far as the filter life is concerned. Only oxidation catalysts have proved to be useful in reducing HC levels and, to a somewhat lesser extent, also particulate matter levels. The need of using oxidation catalysts as a possible essential mean to meet the future legislative requirements add to the urgency that the car manufacturers feel concerning the establishment of appropriate specifications for the European diesel fuels. Diesel fuel quality has an important impact on the exhaust emissions of diesel engines. In addition, the widespread use of catalysts requires the availability of sulphur-free diesel fuels.

53 3. EVAPORATIVE EMISSIONS

Passenger cars contribute to the emissions of Volatile Organic Compounds due to the evaporation of the gasoline during prolonged parking as a consequence of the fluctuations of the ambient temperature, when parked after use and, in a lesser amount, when used. In line with the mandate of the Luxembourg Agreement, the recent Commission proposal also introduces requirements in this area. A test procedure has been defined to measure the vehicle V.O.C. emissions during prolonged parking (diurnal losses) and when parked after use. A 2gltest limit value was proposed. The technology presently available to reduce this type of emissions and attain the above goal is based on the following principle: - the fuel system (fuel tank, carburetor bowl, etc ...) is vented exclusively through an activated carbon canister - the carbon canister is purged by fresh air drawn through the canister into the intake manifold during engine operations. This technology also allows the control of any evaporative loss when the vehicle is running. The constraints added by the evaporative emission requirements have an indirect bearing on the selection of the technology to abate exhaust emissions as: - the fuel metering system must be able to deal also with the canister purge flow which contains an unmeasurable amount of fuel - some fuel metering systems generate higher amount of fuel vapour than others (for example: carburetors versus fuel injection sytems). 4. CONCLUSIONS

The increased concern of the public opinion about the environment and the need to protect it has spurred the Commission towards the implementation of emission standards of increased severity. With the new Consolidated Directive recenlty proposed by the Commission, the legislative requirements concerning exhaust emissions will become more stringent than the corresponding requirements presently in force in the US. This will mean that all gasoline cars will be soon equipped with 3-waycatalysts in a closed-loop configuration while diesel engines will incorporate the latest development in terms of engine and fuel system design. This evolution is putting a strain on the industry that in a short period of time has to adapt the technology of the 3-way catalyst, known through the US experience, to the European requirements in terms of test procedures and specific patterns of use of the vehicles. There will be added costs due to the complexity of the new systems and the need for large investments in the component sectors, in order to meet the demand for new sophisticated products.


A. Crucq (Editor), Catalysis and Automotive Pollution Control I1 199 1 Elsevier Science Publishers B.V., Amsterdam

55

INDUSTRIAL APPLICATION OF CATALYSTS FOR OFFGAS TREATMENT Dr.-Ing. Torsten Schmidt,

Kali-Chemie AG, Hannover, West Germany

Table of Content. 1 1.1 1.2 1.3 1.3.1 1.3.2 1.4 2 2.1 2.2 2.2.1 2.2.2 2.2.3 2.3 2.3.1 2.3.2 2.4 2.4.1 2.4.2 3

Fundamentals Air pollution control: legislation Air Dollution control: process selection Principles of catalytic reactor desia Chemical reactions, heat and mass transfer Catalyst deactivation Plant design Examples of.apdication Selectivecatalytic nitric oxide reduction 0ffg.a~ treatment ef stationam internal combustion eneines or turbines Exhaust gas emission control for spark ignition engines according to the three way system Diesel engines Gas engines (two stroke), gas turbines Claus plant o f f a SULFREEN process Catalytic afterburning Catalytic $terburning of VOC for industrial production processes Survey Example: Laminating of paper Literature

SUMMARY Catalytic offgas treatment requires adaptation of catalysts to the existing production pmess and its emissions. Industrial catalytic offgas treatment offers opportunities in conversion of noxious emissions into environmentally compatible reaction products at low temperatures without formation of noxious by-products or residues. Due to the versatility of catalysts and offgas treatment processes their application yields various economies. Limitations include catalyst deactivation due to presence of poisons or the Occurence of concentrationpeaks. Those are tackled by chemical and engineering research and development of catalyst and plant manufacturers. Kali Chemie AG, Hannover @). offers optimized catalysts as a result of long term experience in cracking, isomerization, reforming, chemical, automobile exhaust and industrial offgas treatment catalysts.

56

-

1 FUNDAMENTALS 1.1 Air pollution control: legislation Technological measures for reduction of gaseous emissions to the atmosphere can be divided into three categories and priorities. 1. Avoidance of emissions by process optimization (primary measure) 2. Recycling of emissions (secondary measure) 3. Conversion of emissions (tertiary measure) [2] Catalytic processes for air pollution control are employed for the latter purpose. In 1987 the catalyst market consisted of three major fields of application: - refineries 950 Mio $ - chemical industries 1.496 Mio $ - air pollution control 568 Mio $ Market volume of air pollution control catalysts is divided into automobile exhaust purification (90%) and industrial offgas treatment (10 %)'. Performance of offgas treatment processes must comply with statutory regulations. Thus, design of plants and catalytic reactors must yield conversions of pollutants which ensure concentrations below the threshold levels specified for the particular pollutant and emission process. In West Germany threshold levels are mandatory, and are based on the Federal Emission Control Law (Bundesimmissionsschutzgesetz)and particular instructions (e.g. TALuft; GroPfeuerungsanlagenverordnung). 113 Some examples of emission threshold levels are given in table 1.

1.2 Air pollution control: process selection The primary goal of catalytic processes in air pollution control is the conversion of noxious emissions (e.g. hydrocarbons) into environmentally compatible reaction products (e.g. carbon dioxide and water) at minimum energy consumption and without formation of either noxious residual substances or noxious by products. Unlike many catalytic processes in refining or in petrochemicals, the basic principle in air pollution control is that the catalyst has to fit to the (existing) production process and its operational parameters.

1 - Source: Prof. J.T. Richardson: Introduction on Catalysis: Application of heterogeneous catalysts, Loughborough, 31.07.88-06.08.88

57

Table 1 - Emission Limit Values According to German Air Pollution Control Legislation POLLUTANT

THRE~HoLDLIMIT

THERMAL POWERPLANTS

NOx

0,2 G/M3

TWO STROKE GASENGINES

NOx

0,80 G/M3 0,65 G/M3

co

NM-HC ALDEHYDES SOLVENT APPLICATION

(E.G.P R I ” G , LAMINATING) CHEMICAL SYNTHESIS PLANT

0,13 G/M3 20 MG/M3

VOC CLASS III (E,G,ACETONE) CLASS II (E,G, TOLUENE) CLASS I (E,G, FORMALDEHYDE)

0,15G/M3 0,l G W 3

20 MGM3

The basic offgas parameters are: (i) atmospheric pressure (ii) low temperatures (iii) low concentration of pollutants (below 1 vol.%) (iv) wide variety of substances with several changes during catalyst service time (v) non stationary operation. [ 10,1] Catalytic processes are evaluated according to the following selection criteria: (i) efficiency (compliance with threshold levels) (ii) effort (energy consumption; demand for maintenance and supervision) (iii) reliability (iv) compatibility with production plant and energy supplies (v) avoid residues or by products (e.g. spent catalysts, filters, products of incomplete oxidation) Competing processes many include: condensation, adsorption, absorption, reverse osmosis, biological treatment, thermal oxidation. Recently catalyst application is state-of-the-art in the following fields of application: [ 123 - flue gas treatment (NOx control by selective catalytic reduction; SCR) - stationary internal combustion engine offgas treatment (oxidation of CO and HC; non selective NOx reduction) - treatment of Claus plant offgases (oxidation of H2S, CS2, COS) - complete oxidation of volatile organic compounds (so called catalytic af terburning)

Table 2. Chemical reactions in Catalytic Air Pollution Control

OXIDATION EXAMPLE

COMPLETEOXIDATION OF HYDROCARBONS CmHnOp+ (m+n/4-p/2) 0 2 -+ mC02 + n/2 H20

REDUCTION

DECOMPOSITION

SCR 4 NH3+ 4 NO + 0 2 + 6 H 2 0 + 4N2

DECOMPOSITIONOF OZONE 203 -+ 302

NSCR CO +NO + 1/2 N2 +C02 CmHn +2 (m +n/4) NO (m + n/4) N2 +n/2 H20 + mC02 TEMPERATURE RANGE

200 - 600 "C

RESIDENCE TIME

0.42 - 0.02 s

GHSV

5000-50000 h-

APPLICATION

CATALYTICAFI'ERBURNING; ENGINEOFFGAS

SCR NSCR 200 - 400 "C 2 - 0.3 s

0.42 - 0.02 s

1000-5000 h-1 5000-50000 h-1

DENOx

ENGINE OFFGAS

20 - 100 "C

0,7

- 0,3 S

5000-10000 h-l STERILISATION

59

1.3 Principles of catalytic reactor design

1.3.1 Chemical reactions, heat and mass transfer Chemical conversions arising in connection with chemical and biological exhaust gas treatment processes can be subdivided into - oxidation reactions - reduction reactions - decomposition reactions. Catalvtic exhaust gas purification refers to processes in which the polluting gaseous and vaporous substances (pollutants) are converted into harmless substances by means of chemical reactions on the surface of a solid, inorganic auxiliary material (catalyst). [ 5 ] For that reason, technical catalysts consist mainly of highly porous materials which provide a large surface area for the chemical reaction. Due to the fact that the reactants (pollutants and oxidation or reducing agents) are present in a different physical state than the catalyst, there are not only chemical reactions at the catalyst but also physical transport phenomena (table 2). [3] The essential mass transfer steps of such heterogeneously catalyzed reactions are (figure 1): -1. mass transfer between the offgas flow and the boundary layer, -2. diffusion of reactants through the boundary layer to the outer (geometric) catalyst surface and from there through pores to active sites on the internal surface, -3. adsorption at active sites, -4. chemical reaction, -5. desorption of reaction products from active sites, -6. diffusion of products through pores and boundary layer, -7. mass transfer into the offgas flow. The application of solid catalysts in gas reactions (heterogenous catalysis) increases the velocity with which the reactions in question are occurring through - compression of the reactant gases at the catalyst surface, which results in an increased number of collisions, - decrease of the activation energy. As a result of the increased velocity of reaction, the throughput of air polluting substances achieved by the chemical reaction is reached (i) at lower temperature (ii) at smaller reactor dimensions (shorter residence times) compared with thermal processes. High reaction rates over highly active catalysts lead to a decrease of the effectiveness factor. Thus, after light-off only active sites on the external surface are used for conversion, because the catalyst bed is heated up due to exothermal reactions. Effectiveness factor is high in low temperature

60

applications. Heat transfer between catalyst surface and the offgas flow preheat the catalyst surface to the ignition temperature of the gas purification reaction and removes the heat of reaction from the surface. During reactor design stationary and non stationary transfer phenomena have to be considered to ensure adequate contact time of pollutants with the catalyst and to prevent the catalyst overheating. [6,7, 81 Figure I :Mass Transfer in Heterogeneous Catalysis

1. Transport of Reactants from Bulk of Gas Flow to Boundary Layers 2. Diffusion of Reactants through Boundary Layer to External Catalyst Surface

3. Diffusion of Reactants through Pores to Internal Catalyst Surface 4. Adsorption of Reactants

at Active Sites

5. Chemical Reaction

6. Desorption of Reaction Products

11

7. Diffusion of Products through Pores

8. Diffusion of Products through Boundary Layer 9. Transport of Products into Bulk of Gas Flow Generally chemical engineering principles (mass and heat balance, plug-flow tubular reactor) can be applied to catalytic reactor design, if rate equations are known. This requires detailed investigation to determine activation energies and frequency factors for each substance. In practice, catalyst sizing is carried out empirically according to pilot plant information. These yield conversion versus either temperature or gas hourly space velocity (sv) curves. From these curves catalyst volume (v) and operating temperatures are chosen for the required conversion at a given flow rate (4)v). S v '4 )yV

61

1.3.2 Catalyst selection Catalysts for air pollution control are delivered as bulk material (beads, pellets) or honeycomb carriers. Carrier materials mainly consist of porous ceramic material (y-Al2O3 alumosilicate, silica). Dense materials such as cordierite or mullite or steel are covered with active layers (wash-coats) before impregnation with active materials. Active materials may be selected from the noble metals (e.g. platinum, palladium) or metal oxides (e.g.oxides of manganese, copper, cerium, titanium, vanadium, chromium). Metal oxide catalysts often are produced as mixed catalysts (prepared by precipitation methods). Noble metal catalysts are always supported catalysts (prepared by impregnation methods). [ 161 Catalyst for offgas treatment have to conform to the following technical and commercial selection criteria [9]: -high activity: high reaction rate of desired reactions at low temperatures and low catalyst volumes, -high selectivity: formation of noxious by-products must be avoided (e.g.: CO, N20, SO3) -high stability against chemical, thermal and mechanical impacts -low pressure drop to minimize electrical energy consumption for blowers 1 3.3 Catalyst deactivation

The economics of catalytic offgas purification require long service times (e.g. beyond two years). Catalyst lifetime can be reduced due to catalyst deactivation. The following factors can cause catalyst deactivation: [4]: - Chemical factors (so-called poisoning) Substances which are not desorbed or react irreversibly with catalyst carrier (e.g. acids) or active material (e.g. sulphur, arsenic, heavy metals) lead to changes in chemical composition, loss of active sites, reduction of accessible surface or covering of active sites (so-called fouling). - Thermal factors (so-called aging) Rates of change in structure of solid catalysts (e.g. crystallite growth, spinells, sintering) increase with increase of temperature. Structural changes can lead to decrease of surface area or reduction of dispersion. For each catalyst maximum operating temperatures must be used. - Mechanical factors: (i) attrition (e.g. by vibration, abrasive dusts), (ii) crushing (e.g. external stresses of reactor shell) Risks of catalyst deactivation must be considered during process and plant design. Minimization of risks start with process and catalyst selection. Pilot plants or trial tests can be camed out to check offgas composition for catalyst poisons or peak concentration, which could lead to catalyst

62

overheating. If catalyst deactivation occurs due to unforeseen reasons, analysis of phenomena can give hints, whether or not chances for reactivation exist. Reversible mechanisms like: - coking, - covering with aerosol or salt, - condensation of organics, - strongly adsorbed species (e.g. HCl, P2O5) may be removable by heating or washing. Otherwise (irreversible deactivation, e.g. sintering) the catalyst charge has to be replaced completely or partially or fresh catalyst has to be added.

1.4 Plant design considerations The equipment and instrumentation of the projected plant is determined by the following factors: [13] a) crude gas composition (average, peak) b) crude gas temperature c) crude gas pressure d) necessary conversion efficiencies (admissable residual emission) e) type of catalyst f ) possibilities of energy supply g) possibilities of heat recovery. With regard to the technical realization of catalytic exhaust gas purification processes it is necessary to adapt plant concepts to specific exhaust gas problems. A basic flow sheet for catalytic exhaust gas purification plants is represented in fig. 2. Figure 2 :Unit operations in catalytic off-gas treatment

Q I

I

1 - TAKE-OVER OF THE OFFGAS

2 - MECHANICAL CONVEYING 3 - SEPARATION OF DISTURBING COMPONENTS 4 - PREHEATING BY HEAT EXCHANGE 5 - EXTERNAL HEAT-SUPPLY 6 - MIXTURE W OXIDIZING OR REDUCING SPECIES 7 - CATALYTIC REACI'ION

8 - HEAT-RECOVERY 9 - DISCHARGE OF THE PURIFIED GAS

63

The following processing stages can be considered as unit operations of catalytic exhaust gas purification plants: 1.- Take-over of the offgas (e.g. by suction) 2.- Mechanical conveying of the offgas e.g. by ventilators, compressors) 3.- Separation of disturbing components for prevention of catalyst deactivation (e.g. filters, adsorbers, absorbers) 4.- Heating of the offgas by heat exchange (e.g. recuperative tubular heat exchangers or regenerative heat exchangers) [17 I] 5.- External energy supplies (e.g. burners, preferably using natural gas as additional fuel, electrical resistance heating) 6.- Mixture with oxidizing or reducing agents (e.g. oxygen or ammonia) 7.- Catalytic reactors. Commonly fixed bed reactors, honeycomb or bulk material type catalysts. Flow direction can be chosen arbitrarily if honeycombs are employed. Bulk material catalysts require flow direction top to bottom for prevention of bed movement (attrition) or channelling. Very rarely fluidized bed reactors are reported. [14] 8.- Heat recovery (e.g. steam or thermal oil heating) 9.- Discharge of the purified gases (e.g. stacks) Fig. 3 shows the simplified thermal energy balance for an afterburning plant. In the case where the energy demand of a process is designed to involve heat recovery by pollutant oxidation (table 3), the catalyst ignition temperature is reached by heat exchange, without the consumption of external fuel. Table 3 - Stoichiometry and heat of reaction for some complete oxidation reactions

Compound

I

Stoi chiometry

C7Hg + 902+ 7C02 + 4H20 CH30H + 3/2 0 2 + C02 + 2H20 C3H60 + 402+ 3C02 + 3H20 co co + 1/202+ c02 NH3 NH3 + 3/4 02- N + 3/2 H20 H2S H2S + 3/2 0 2 + SO2 + H20 Ethylacetate C4Hg02 + 5 0 2 + 4C02 + 4H20 Toluene Methanol Acetone

Heat of Reaction

IT pro 1g/m:

40.935 kJkg 21.119 H/kg 29.065 kJ/kg 10.423 H/kg 18.424 H/kg 15.355 kJ/kg 23.530 H/kg

30,6 K 15,8 K 21,9 K 7,5 K 13,9 K 11 K 18 K

64

Figure 3 :Thermal Energy Balance

T'WTa

QU

Catalytic Oxidation

P,CJ

External Energy Supply

W A

Thermal Energy Balance, Stationary

2. Convective Heat Transport

0 = OE,K+OE,F+Q&,S+OE,WR-~E,V

OE,K= @v,L. P O L . CL(T'WT,~ - T'wT,~:

3. External Energy Supply

@E,F = W,EG .A~U,EG

4.Combustion

QE,S =

5. Heat Losses (negligible due to thermal insulation)

@E,V

A ~ u , s. PS . @V,L

-)

0

Such a process is self supporting. These conditions (pollutant concentration and rates of heat transfer in the crude gadpurified gas heat exchanger) allow significant reduction in operating costs. Use of separation units (e.g. filter, adsorption, absorption, thermal afterburning) can reduce the risk of deactivation by poisons, dust or highpeak concentrations. Temperature measurement in front, behind and inside the catalyst bed and control (e.g. by additional fuel supply, heat exchange rate, fresh air dilution) reduces the risk of overheating. Economics of plants treating low concentration offgas can be improved by the incorporation of adsorptive concentration units. [19, 201

65

2 EXAMPLES OF APPLICATIONS

2.1

- Selective

catalytic nitric oxide reduction [21] (SCR, DeNOx, "denitrification")

Field of jxmlication Removal of nitric oxides from flue gases or process exhaust gases, e.g. boilers, nitric acid plants, glass melts, municipal waste incineration. PrinciDal reactions After admixture of ammonia, nitric oxides are converted selectively (also in the presence of oxygen) into molecular nitrogen. 4 NH3 + 4 NO + 0 2 4 6 H20 + 4 N2 Process description - SCR for Dower station flue gass Two process variants must be distinguished: (a) High-Dust process (Fig. 4) The flue gases are directly withdrawn from the steam generator (behind the economizer and before the air preheater) and are passed over the catalyst after the admixture of the reducing agents.

Bolier

sCR

D e Dusting

De-Sulphuritation

Fig. 4 :SCR process - crude gas operation

66

The largely nitric oxide free flue gas is fed to the air preheater for the purpose of preheating the combustion air (heat recovery). Depending on the fuels used, additional exhaust gas purification stages are installed downstream (dedusting, desulphurization). Crude gas preheating or fuel supply before the catalyst is not necessary, since the crude gases (= flue gases) are withdrawn at a place in the steam generator where the latter possess a sufficient temperature (350-450"C). Honeycomb mixed contacts are used as catalvsts. Material: Ti, V, Fe, W, Ni, Co, Cu, Cr, U, Mo, Sn as oxides Ag, Be, Mg, Zn, B, Al, etc. as metals [lo] Selectivity: SO2 + SO3 - conversion must be avoided Activity: as low NH3, consumption as possible Stability: high demands because of the abrasion resulting from the dust load of the flue gas oDeration (Fig. 5) dust and sulphur oxides are (b) In case of pure removed by electrostatic filters or flue gas scrubbers, before the crude gas is mixed with reducing agents and reaches the catalyst. The adjustment of the necessary reaction temperature (300-400"C) requires preheating of the crude gas and external energy supplies. In principle, similar, but more active honeycomb catalysts are used. Because of the preseparation of the disturbing components, lower demands are made on selectivity. Furthermore, a higher life time is expected.

Fue 1

Boiler

Dedusting

De-Sulphurization

Fig. 5 :SCR process - purified gas operation

67

2.2

Offgas treatment of stationary internal combustion engines or turbines

2.2.1 Exhaust gas emission control for spark ignition engines according to the firee-wav system 122.231 Exhaust gases from combustion engines contain the following classes of p011u tants - hydrocarbons (CnHm) - carbon monoxide (CO) - nitrogen oxides (NO4 By means of special catalysts and an appropriate carburetion, the three pollutant classes can be jointly removed for spark-ignition, four-stroke engines through one catalytic converter (three way system). catalytic converter

b

lambda-probe control-unit control

IL----------------Jl

temperature

I1

t-I

Throttle

I

t-I

IL-

OXIDATION

CmHn + (m+n/4)02 +mC02 +n/2H20 c0+1/202 + c02 H2 + 1/2 0 2 + H2O

Speed

REDUCI'ION

CmHn + (m+n/4) NO + (m+n/4) + C02 +n/2H20 CO + NO+ 1/2 N2 + C02 H2 + NO + 1/2 N2 + H20

Fig. 6 :Three-way catalyst system

68

As shown in Fig. 6 [ l 11, the catalyst is integrated into the exhaust system of the engine. Preheating or external energy supply is thus not necessary because of the given exhaust gas temperature. The necessary concentrations of oxidation agents (02, NO) and reducing agents (CnHm, CO, H2) are adjusted by the electronic oxygen sensor control (lambda probe) through a reactive effect on the carburation of the engine. The purified exhaust gases are discharged via the remaining exhaust system. In the case of gas engine driven block heating stations, the thermal energy contained in the pure gases is recovered. Precious metal impregnated honeycomb catalysts are used as catalysts. The precious metals used in this connection are mainly platinum and rhodium. The coatings and the supports are optimized with regard to a high temperature stability. 2.2.2 Diesel ennines Hydrocarbon and CO emission can be reduced by catalytic oxidation with noble metal supported catalysts (bulkmaterial or honeycomb). Catalysts must be optimized with respect to selectivity to avoid NO/N02 and Sods03 conversion.(Fig. 7)

catalytic afterburning

J i l l

Fig. 7 :Diesel engine ofSgas treatment

69

Particles can be removed by filtering. Some filters are catalytically impregnated to reduce temperature of coke/soot burning during regeneration. Due to high oxygen concentrations NOx emission control is not possible with TWC system (see 2.2.1). Feasible solutions are using offgas recycling or SCR system (see 2.1). 2.2.3 Gas emines (two stroke). gas turbines Offgases sometimes contain concentrations of hydrocarbons (alkanes, aldehydes) or carbon monoxide which exceeds the threshold limits. Noble metal or honeycomb catalysts are employed for oxidation of these pollutants. Catalysts contain high noble metal concentrations if temperatures are low (300 "C) and concentrations of ethane are high. Future tasks are to be seen in the removal of methane emission and other gases which until recently were not regarded as pollutants (non methanehydrocarbons, NMHC).

2.3 CLAUS plant offgases Typical Claus plant exhaust gases contain 1 % of H2S, COS, CS2 and 0.5 % S02. 2.3.1 SULFREEN process 1241 The flow sheet and the basic reactions of the SULFREEN process are presented in fig. 8. The reactions of the Claus process occur at lower temperatures (120140 "C). Modified pelleted alumina catalysts are used. The elementary sulphur arising as a product at the reaction of H2S with SO2 adsorbs on the inner surface of the aluminium oxides (pores). As a result, the catalyst is reversibly contaminated. Regeneration of the catalyst and recovery of sulphur is achieved by cyclical purging with hot inert gas. 2.3.2 Catalytic afterburning [15] Before released to the atmosphere residual sulphur compounds (H2S; COS, CS2) have to be converted to S02. Selectivity of the catalyst must avoid SO3 formation which could cause severe corrosion problems in afterbuming plants. Application of modified y-alumina catalysts allow lower energy consumption at high flow rates compared to thermal afterbuming. Process Darameters

temperature: space velocity:

350 "C 1500-2000 h-1

Table 4 Complete oxidation of volatile organic compounds catalytic afterburning : fields of application

INDUSTRIES

POLLUTANTS

EXAMPLES

FOODSTUFF

ODOURS

SMOKING; ROASTING ETHYLENEOXIDE; OZONE WIRE -ENAMELLING ARTIFICIAL LEATHER POLYESTER PHENOLIC-RESIN; PVC PROCESSING, LAMINATING PRINTING

STERILIZERS

MEDICAL

SOLVENTS

SURFACE COATING

SOLVENTS

PLASTIC PROCESSING

MONOMERS

GRAPHICAL PROCESSES CHEMICAL AND REFINERIES

SOLVENTS

SYNTHESIS

MONOMERS,BY-PRODUCTS, RESIDUAL REACTANTS

SOLVENTS

VAPOURS

DEPOTS SEWAGE WATER TREATMENT

CARBON-MONOXIDE

I OXYGENATED-HYDROCARBONS I

Parameters : Flowrate : 102-105m3/h ;

Temp.: 200 "C- 600 "C ;

FORMALDEHYDE; MALEIC ACID; PHTALIC ACID; VINYLCHLORIDE; FIBRE-PRODUCTION BITUMENE; FUEL, BENZENE; SOLVENTS OFFGASES OF W E T OXIDATION

Pollutant C o n t e n t :102-104ppm

71

CLAUS PROCESS : CONVERSION OF H2S INTO S

3 H2S + 3/202 + 2 H2S + SO2 + H 2 0 2 H2S + SO2 + 3/2 S2 + 2 H 2 0 3 H2S + 3/202

+ 3/2 S2 + 3 H 2 0 Afterburning

I'

c1

Fig. 8 :SULFREEN process 2.4

Catalytic afterburning of volatile organic compounds for industrial production processes

These processes are applied to reduce emission of volatile organic compounds by means of complete catalytic oxidation. They are tailor made to a variety of production processes. 2.4.1 Survev A survey on fields of application for complete oxidation of volatile organic compounds is shown in table 4.

72

2.4.2 Laminating of Daper [lo] During laminating of printed paper with polypropylene foils, solvents containing offgas are emitted. Due to flexibility of the production, concentrations and flow rates differ temporarily. Typical solvent mixtures contain ethanol, ethylacetate, methylethylketone, toluene. Metal oxide catalysts yield sufficient activity at ignition temperatures of about 250 "C. A catalyst volume of 600 1 is implemented to purify an offgas flow of 8.000 m3h and 8 g/m3 at maximum conditions. Table 5 shows purified gas composition and some process parameter for a service time of more than 8.000 h. Activity of the catalyst keeps concentrations of hydrocarbons within threshold limits (0,l g/m3). Due to selfsupporting plant operation, C02 emissions are equivalent to the carbon content in offgas. CO and NOx emissions are quite low. Gas chromatographic investigations (table 6) show that during catalytic complete oxidation the number of hydrocarbon compounds is reduced. Residual hydrocarbon contents in purified gas consist of unconverted parts of the feed components. Thus it can be concluded that no other hydrocarbon compounds are formed during contact with catalyst. Table 5 Purified gas composition after certain catalyst service periods Flow rate : 8000 m3 h-1 Concentration : 2- 8 g m-3 Solvents : Ethylacetate; MEK, Toluene; I-Propylacetate Catalyst : Mixed metal-oxides (KCO SG 21 16) Operating Hours

Operating Mode

Ignition Temp "C

Purified Gas Composition TOC CO NOx mg/m3 mgjm3 mg/m3

1275 3000 8000

SelfSupporting SelfSupporting Selfsupporting

238

2

250

10

240

18

30

1

C02 %

0.5

Implementation of plant ensures flexible adaptation of preheating (burner. aidair heat exchanger) and prevents catalyst overheating (450 "C). Catalyst temperature control is accomplished by thermocouples in the catalyst bed.

73

Dilution of offgas with fresh air allows operating, even if design concentrations are exceeded. Heat recovery (hot water, hot air) improves the economy of the offgas treatment plant (fig. 9). Table 6

-

Gas chromatographic investigation of reaction products of complete oxidation of organic solvents Solvents : Ethanol, Ethyl acetate, MEK, Toluene Catalyst : Mixed Metal Oxides KCO SG 2216 Ignition Temperature : 220 "C GHSV: 70o0h-1 Toc

OF DETECTED

'

~

OFFGAS

I

OTHERS

ETHANOL

ACETATE

8

4090 mg/m3

585 mg/m3

I

466 ppm C3

3

2,55 mg/m3

100%

I

99,9%

PEAKS

~

99,9%

mn-

Fig. 9 :Catalytic dterburning plant for laminating processes

74

3. LITERATURE 1 2 3 4 5

6 7 8 9 10 11 13 14 15 16 17 18 19 20 21 22 23 24

Janee, M,: Auswirkungen von gesetzlichen Vorschriften und technischen Regelwerken auf die Abgasreinigung. in: VDI-Bericht Nr. 730, VDI-Verlag, Dusseldorf, 1989. Zlokamik. Umweltschutz - eine standige Herausforderung. Chem.-Ing. Technik Nr. 5 (1989) S. 378-385. Schlosser. E. G;Heterogene Katalyse. Verlag Chemie Weinheim (1972). &L B;Reaction Kinetics on Reactor Design. Prentice Hall Inc. Englewood Cliffs, New Jersey (1980). firchner, K, Kaiczik, A.: Grundlagen der katalytischen Nachverbrennung. VDI-Berichte 525 S. 119-145, VDI-Verlag Dusseldorf (1985). Eigenberger. Q Stabilitat und Dynamik heterogenkatalytischer Reaktionssysteme. Chem.-Ing. Tech. 50 (1978) S. 924-933. Wicke. E.: Grundlagen der katalytischen Nachverbrennung. Chem.-Ing. Tech. 37 (1965) S. 892-904 Kanzler, W.; Schedler, J.; Thalhammer, H.: Theoretische und experimentelle Untersuchungen bei katalytischen Nachverbrennungsanlagen. Chem.-Ing. Tech. 59 (1987) S. 582-585 Szepe. S.;Levenspiel, k Proc.4th European Symp. on React. Eng., Brussel (1968) Pergamon Press, London (197 1) Schmidt. T.: Katalytische Verbrennung chlorierter Kohlenwasserstoffe. VDI-Berichte 730 S. 201-238, VDI-Verlag, Dusseldorf (1989) SuiveY, J. Complete Catalytic Oxidation of Volatile Organics. Ind. Eng. Chem. Res. 1987,26, S. 2165-218012 VDI-Richtlinie 3476 Sattler. K.; Umweltschutz, Entsorgungstechnik Vogel Verlag, Wiirzburg 1982 Hardison. L. C.: Dowd. E. J.: Emission Control via Fluidized Bed Oxidation CEP August, 1977, S. 31-35 Ketmer. R.: Lubke. T.: Betrieb von KNV-Anlagen nach Clausanlagen VDI-Bericht 730, S. 255-274, VDI-Verlag Diisseldorf (1989) Eneler. B.; Koberstein, E.:Katalysatoren in der Abgasreinigung. VDI-Bericht 730 S .97-120, VDI-Verlag Diisseldorf 1989 Taneer.Y; Energiesparendes KNV-Verfahren durch neuartige ProzePfuhrung mit regenerativen Warmeaustauschem. VDI-Bericht 730 S . 189-200, VDI-Verlag Diisseldorf 1989 . Martin. fL;Wirmeiibertrager. G. Thieme Verlag Stuttgart 1988 Brauer.fl& Adsorptive Reinigung schwachund mittelbelasteter Abluftstrome Vortrag auf dem 2. Fachsymposium Umweltschutz in Mainz, Mai 19 8 8 . Walter. B;Anwendungsbeispiele fur die katalytische Nachverbrennung. VDI-Berichte 525 (1985) S. 347-365 Richter.E;Die katalytische NOx-Reduktion - eine Ubersicht in: VDI-Bericht Nr. 730 S . 121-156, VDI-Verlag Diisseld. 1989 . Koberstein, Katalytische Motorabgasreinigung. in: VDI-Bericht 525 S. 217-246, VDI-Verlag Diisseldorf (1985). Publikation: Engelhard Kali-Chemie AutoCat GmbH, Hannover. Ruhl. E.: Katalytische Reinigung von Claus-Anlagen-Abgasen. VDI-Berichte 730 S. 255-274, VDI-Verlag (1989), Dusseldorf.

A. Crucq (Editor), Catalysis andAutomotive Pollution Control 11 0 199 1 Elsevier Science Publishers B.V., Amsterdam

75

CHARACTERIZATION OF EXHAUST EMISSIONS FROM TWO HEAVY DUTY VEHICLES FUELED WITH EIGHT DIFFERENT DIESEL FUELS Egeback K.E. (1) Mason G. (21, Rannug U. (3) and Westerholm R. (4) (1)-Environmental Section, The Swedish Motor Vehicle Inspection Company, Box 508, S-162 I5 Vallingby, SWEDEN (2)-Department of Medical Nutrition, Karolinska Institute, Huddinge University Hospital F60, Novum, S-141 86 Huddinge, SWEDEN (3)-Department of Genetics, Stockholm University, S-I06 91 Stockholm, SWEDEN (4)-Department of Analytical Chemistry, Arrhenius Laboratory, Stockholm University, S-106 91 Stockholm, SWEDEN

Abstract The impact on exhaust emissions of diesel fuel composition was studied by a comprehensive program compiled after discussions with the oil industry, the motor vehicle manufacturers and the Swedish Environmental Protection Agency. In order to reduce the emissions from diesel fueled vehicles different possibilities have to be taken into consideration such as improvement of the engine, use of exhaust purification devices and use of diesel fuels of improved quality. The aim of this investigation was to study the connection between certain fuel parameters and exhaust emissions.Eight different diesel fuels were used in the investigation and the emission tests were carried out using a city bus and a truck, driven on a chassis dynamometer. Both regulated andunregulated emissions were measured. In addition to the emission measurements samples were taken and prepared for biological testing.The investigation showed that there is a clear correlation between certain fuel parameters and the emissions and biological activity of the exhaust. Multivariate analysis was undertaken on groups of data derived from the eight blends of fuels in order to study the correlation between the fuel and emission parameters. A least square regression analysis of the correlation between 23 fuel parameters and 43 exhaust emission parameters resulted in correlation coefficients, which were higher than 0.95 for the two vehicles tested. It was confirmed that the most important fuel parameters to be controlled are the density, boiling point range, the content of aromatics and cetane index. In addition to these parameters the content of polycyclic aromatic hydrocarbons should be controlled and limited. Sulfur is also an important parameter to be controlled especially when the fuel is going to be used for vehicles equipped with catalysts or catalytically activated particulate filters.

76

INTRODUCTION

In many countries where actions have been taken to reduce the emissions from light duty vehicles there is now a growing concern regarding the emission from heavy duty vehicles. To reduce the emissions from such vehicles operating in city areas is the most urgent action to be taken. As work was going on in Sweden to prepare a definition of low emitting heavy duty vehicles operating in city areas there was a need to look into the area of diesel fuels and their impact on exhaust emissions. The Swedish Environmental Protection Agency which was authorized to prepare proposals for the Swedish Government concerning vehicle emissions and automotive fuels asked the Automotive Emissions Research Laboratory, organized within the Swedish Motor Vehicle Inspection Company, to define a program and to carry out an investigation in order to chemically and with bioassy tests characterize the emissions produced when changing some of the diesel fuel properties. The program was prepared in collaboration with the Swedish Urban Air Project and especially with the Emission Group Branch, who have also been carrying out analysis of unregulated emissions in addition to carrying out biological testing. The results in this article should be seen as an extended abstract of a report covering the whole investigation which will be published during the autumn 1990 (Westerholm and Egeback, 1990). EXPERIMENTALS

The objectives of the investigation were fourfold: - How do the aromatics in the fuel influence the emissions? - Does the sulfur content of the fuel influence the emissions? - Does the use of light diesel fuel influence the emissions? - How does the ignition improver used in diesel fuel influence the emissions ? In order to answer these questions the program was composed of a literature study, experimentals and analysis of the data generated by the experimentals.

Fuels Eight fuels were included in the investigation, see Table 1. The fuels were selected after discussions with the Swedish Environmental Protection Agency, different oil companies, diesel engine manufacturers and other experts. In order to answer the objectives of the investigation it was quite clear that at least for some of the fuels only the content of aromatics and sulfur should be changed so as to study their effect on the exhaust fuels should show the impact on the emission of sulfur and three of the fuels to be used should determine the impact of aromatics. Two light diesel fuels (kerosene) were to be

77

used as well as one with approximately the same composition as the light diesel fuels but somewhat heavier. One additional fuel would be regarded as the worst case. When the fuels were manufactured it appeared that the outcome was not as expected especially for the fuels which were meant to be used to determine the impact of aromatics. When changing the aromatic and sulfur content other properties in the fuel were also changed. However, the unintended changes were not too large and, therefore, there appeared to be a good possibility to study the effect on the exhaust emissions of fuel aromatics and sulfur, respectively. Of the eight fuels D1, D2 and D4 are from the same base fuel, table 1. The only main difference between these three fuels are that D2 has a higher content of aromatics compared to D1 and that D4 has a higher content of both sulfur and aromatics than D1. There is also a difference of fuel density. D1 has the lowest and D4 the highest density of the three fuels. The fuel D5 is a blend of different kerosenes. Its density is nearly as high as for a common commercial diesel fuel in Sweden. The contents of sulfur and aromatics are on the same level as for a low sulfur light diesel fuel in Sweden. The fuel D6 is a common commercial (summer) fuel and was regarded to be a reference for this type of commercial fuel in Sweden. The fuels D7 and D8 are the same fuels except that 2000 ppm ignition improver, ethyl hexyl nitrate (EHN) was added to D8. The fuel D9 is a blend of cracked gasoils with a low content of sulfur. Table1.-Fuel characteristics D6

D7

D8

D9

52.8 50.0 81 1.7 821.3

47.2 47.0 832.0 831.3

48.3 836.8

44.7 808.3

55.7 808.7

52.8 813.2

231 252 260 43.10

220 233 289 252 323 261 42.88 42.98

205 329 364 42.87

190 245 300 43.24

187 243 289 43.24

205 282 301 43.19

26.1 20.2 4.8 1.1 1.0 110 0.16

20.0 17.2 2.7

20.5 17.2 2.7

0.1

0.6

0.9 14.3 0.02 0.0

0.2 207 0.01 0.2

17.3 14.5 2.2 0.6 0.7 11.0 CO.01

D1 Cetan Nr Density (1) Distillation (2) 10%

90%

FBP Energy (3) Aromatics (4) -Total -Mono -Di -Tri

Olefms (4) Nitrogen (5) Sulfur (6) EHN(7)

238 251 26 1 43.20 1.8 1.8

D2

16.6 16.2 ~ 0 . 0 5 0.4

D4

23.0 18.1 4.9

D5

25.1 21.1 3.8

~ 0 . 0 5 ~ 0 . 0 5 ~ 0 . 0 5 0.2

1.4 2.0 2.2 0.2 12 0.34 3.9 ......

?me.,.

Figure 3. Emissions of HC, NOx, CO, particles and aldehydes, vehicle 1 , mean values

Particles General: There is a clear relationship between the fuel properties and the particulate emissions. Heavier fuels emit more particles than lighter fuels in most cases. A vehicle dependency can also be seen. The relationship between the content of sulfur and aromatics respectively, and the emissions based on multivariate analysis are discussed below. Vehicle I: In line with the general conclusions there is a clear relationship between the fuel density and the emissions. Vehicle 2: Apart from the general conclusions, fuel D9 emitted more particles than D7 despite these fuels being of similar density. When comparing D9 to D7 no significant differences in their properties can be seen except for their cetane numbers, 52.8 for D9 and 44.7 for D7.Considering the influence of the cetane number on the combustion process one possible conclusion to be

83

drawn (and confirmed by the engine manufacturers) could be that the engine in vehicle 2 reacts to differences in cetane number in almost the opposite way with regard to particulate emissions than the engine in vehicle 1.

.".

S"CI.

0".

="ox.

0".

CYSI.

Figure 4 . Emissions of HC, NOx, CO, particles and aldehydes, vehicle 2 mean values

Aldehydes General: The emission of aldehydes is not only sensitive to the fuel composition but also to the type of engine. There is a large variation from fuel to fuel and a great difference in the emission pattern when comparing vehicle 1 to vehicle 2. Of all eight fuels only fuel D7 shows the same pattern for both vehicles. The high rate of aldehyde emission for D7 may be caused by the less complete combustion as a result of its low cetane number, 44.7. To fully understand the nature of the emission of aldehydes a more thorough investigation should be undertaken.

84

Vehicle I : There is more than a three fold difference between the lowest value of the sum of formaldehyde plus acetaldehyde (D6 and D7) and the highest value of the sum for D7. The ratio between the emission of formaldehyde and acetaldehyde is not the same for all fuels. The acetaldehyde part of the aldehydes is much smaller (16 %) for D7 than for all the other fuels. For these, D4, D6 and D7 it is still small (22 %) while the acetaldehyde part is 48% for D9. Vehicle 2: There is almost a three fold difference between the lowest value of the sum of formaldehyde plus acetaldehyde @2) and the highest value (D7). By examining the results presented in figure 5 it can be seen that the sum of aldehyde emissions is lower for fuels with a short distillation curve like Dl and D2. In addition to the general conclusion drawn about fuel D7, the tendency is that the lighter fuels emit less formaldehyde than the heavier. The acetaldehyde part of the sum of aldehydes ranges from 23 % to 64 %. The fuels which gave the highest proportion of acetaldehyde are D8 and D9, i.e. fuels with a high cetane number and a low initial boiling point. This may be an indication of the risk that the front end hydrocarbons contribute to the formation of aldehydes. PAH levels in fuels The polycyclic aromatic hydrocarbon (PAH) levels in the eight diesel fuels tested are presented in figure 5. The 28 PAH's analyzed ranged from three ringed (phenethrene) to six ringed (coronene). All the individual PAH analyzed in the fuels were added together and termed sum of PAH when presented in the figures. All figures represented by blocks are mean values (N=3), the standard deviation being represented by the overlying block on top of the mean value. The factor on the top of block is the multification factor by which the bar has been multiplied. From the figure it can be seen that fuels D1, D2 and D4 contain less than 5 mg PAHL, fuels D5, D7, D8 and D9 contain in the range of 150 to 350 mg PAHL and fuel D6 contains approximately 1000 mg PAH/L. Major contributors to fuel PAH contents are methylphenanthrenes and methyl anthracenes. PAH emissions

All PAH measured in the exhaust emission samples are the same as quantified in the fuel samples. All the individual PAH analyzed in the fuel have been added together and termed sum of PAH when presented in the figures. Particulate associated PAH was sampled on Pallflex T60A20 filters and the semivolatile phase PAH was sampled down stream of the filter on an absorbent material, poly-urethane foam (PUF). The particulate and the semivolatile phase associated PAH were chemically analyzed separately. The sampling and the

85

chemical analysis procedures are described in detail elsewhere (Westerholm and Egeback,l990). FUEL

m g / l

D1

D4

D2

m

M

V

D5

D6

D7

DB

D9

LLJSTD

Figure 5. Polycyclic aromatic hydrocarbon contents in fuels, mean value (N=3) and standard deviation, mg PAHIL. General: From the results of the emission measurements on both vehicles it can be concluded that the emission of both particulate , PAH and semivolatile phase associated to PAH are fuel dependent. In the most favourable case the lowest emission (considering both particulate and semivolatile associated) is approximately 35 pg/km and in the worst case the sum of PAH is approximately 450 pg/km i.e. by the selection of a better fuel the PAH emission is lowered by a factor of approximately 10. Vehicle I Particulate associated PAH emissions are presented in figure 6. The emissions from fuels D1, D2, D4 are in the range of 25 to 45 pg/km, and from fuels D5, D7, D8 and D9 in the range of 75 to 120 pg/km. The corresponding value from fuel D6 is approximately 220 mg/km. Regarding the semivolatile phase sum of PAH emission, displayed in figure 6, it can be seen that for the fuels D2, D4 and D7 emission levels are less than 35 pg/km. Fuels D l , D5, D8 and D9 were emitting in the range of 75 to 150 ug/krn and fuel D6 approximately 230 p g k m . Considering both the particulate and the semivolatile associated PAH emissions: Fuels D1, D2, D4, D7 and D8 were emitting less than 150 pg/km, fuels D5 and D9 in the range 150 to 300 pg

86

P A H b and finally D6 more than 300 pLg/km. The semivolatile phase contribution to the total measured PAH emissions variates between 8% and 70% (Dl). SUM o f P A H u g / k m

V e h i c l e

140

1

300 280

-

260

-

220

-

200

-

180

-

160

-

140

-

240

i

I I I

II

120 100

I

80 60 40

20 0

T D1 D1

D2DZ

D4 D4 I

m

M

V

___

D5D5 f

D6D6

D7D7

'liter

11---PUF

m

0

S

T

D8 08

D9 D9

Figure 6.-Polycyclic aromatic hydrocarbon emissions; mean value and standard deviation, pglkm. Vehicle 2 : The particulate associated PAH emissions are presented in figure 7. The PAH emissions from fuels D1,D2, D4, D7 are lowest (approx. 50-80 pg/km), fuels D5 and D8 intermediary (90-120 pg/km) and largest for D6 and D9 which are in the range of 160 to170 pLg/km. Also the semivolatile phase associated PAH emissions are presented in figure 6. From the figure it can be seen that the lowest emissions are obtained from fuels D1, D2, D5, D7 and D8 which are in the range of 10 to 40 pg/km. Intermediary are fuels D4 and D9 in the range of 70 to 120 pg/km. The highest emissions was detected in the exhaust originating from fuel D6, approximately 200 pgkm. When comparing the total emission of PAH i.e. both particulate and semivolatile phase associated emissions it is seen that the lowest emissions originate from fuels D1, D2, D4, D5, D7 and D8, less than 150 pg/km, intermediate D9 less than 300 pg/km and finally D6 more than 300 p g k m . These results imply that fuel D6 is the most unfavourable fuel with respect to PAH emissions and that fuels D1 and D7 best. The per cent contribution from the semivolatile phase varies between 12% (D7) to 63% (D4) which underlines the importance of sampling and analysing the semivolatile phase PAH emissions.

87 SUM o f P A H

u g i / k m

V e h i c l e

230

240 260

N o 2

1

I

I

fl I1

220 200 1.30 160

140

120 100

80

60 40

20 0

D1 D1

DZD2

D4D4

D5D5

D6D6

I----Filter m

M

V

a

S

11--

T

D7D7

DBDB

D9D9

P U F

D

Figure 7-Polycyclic aromatic hydrocarbon emissions; mean value and standard deviation, pglkm. Mutagenicity tests

General The results from the Ames tests show a fuel dependent mutagenicity. Both Salmonella tester strains TA98 and TAlOO gave in most cases a similar fuel dependent mutagenicity profile. The mutagenicity level also showed a vehicle dependence. However, in this summary of the investigation only the results from tester strain TA100tS59 are shown in the figures 8 and 9. Using multivariate analysis a good correlation between PAH emission and mutagenicity in Salmonella was found, especially in the presence of a metabolizing system. Particulate phase For both tester strains the particulate extracts showed a similar mutagenicity with and without S9 or, as in most cases, a higher mutagenicity in the absence of S9. Based on the mutagenicity of the particulate extracts the fuels could be classified in low (Dl, D4, D7, DS), medium (D5, D9) and high mutagenicy (D6) fuels. This classification was more clear out with vehicle 2. Semivolatile phase In general the semivolatile phase gave lower mutagenic effects compared to the particulate phase. The fuel dependent mutagenicity profiles of the semivolatile phase followed the profiles of the particulate phase. Also in this case fuel D1 showed the lowest mutagenicity.

88

T A 100 + S 9

r a v e r t o n t s / r n

V e h i ~ l c N O

1

220 200 180 160 140 120 1 00

80 60 40

20 0

D1 D1

DZDZ

D4 D4

D5D5

D7D7

D6D6

DBDB

D9D9

--- PUF

I --- Filter I1 S T D

Figure 8.-Mutagenic effects,vehicle 1 ; mean value and standard deviation, revertantslm.

,

T A l[DD+SS

r e v e r t a n t a / r n

6oo

N o 2

V e h i c l e

I

I

I

I

7

D1 D1

DZDZ

D4D4 I

B

M

V

D5D5

D6D6

--- Filter

--

I1 S T D

T

D7 D7

D8 D8

D9 D9

PUF

Figure 9.-Mutagenic effects, vehicle 2 ; mean value and standard deviation, revertantslm.

89

TCDD-receptor affinity tests For each fuel sample extracts of both the particulate and the semivolatile phase collected using the PUF technique associated emission products were tested for TCDD receptor binding activity. The binding affinities were expressed as EC5o values (Toftgard et a1.,1955) and are shown in figures.10-11 Vehicle 1 Particulate phase The ECso values for binding to the dioxin receptor of the particulate phase exhaust emissions are given in figure 10. Dioxin receptor binding activity was observed in all the emission samples tested. The EC50 values for binding to the dioxin receptor of the samples varied over an approximately 8-fold arange. Fuel D 1 displayed the lowest binding affinity. Fuel D6 produced the emission sample with the highest affinity for the dioxin receptor with the lowest EC5o value. This EC50 was significantly lower than those for fuels D1, D2, D5 and D8 but not significantly different from those of the remaining fuels.

TCDD

rn

V e h i c l e

0 6

No.1

I II

>0.5

0 5

0 4

03

0 2

01

0

. .

. . . . .

7

P2 DZ

D1 D1

D4 D4 I

r

n

M

V

. . . . .

7

D5 D5

--- F i I t c ~

r S

D7D7

D6D6 I1 T

,

,

.

DB Da

D9 49

--- P U F D

Figure 10 - EC50 values, vehicle one; particulate phase and semivolatile phase, mean values and standard deviation.

90

Semivolatile phase The ECso values for binding to the dioxin receptor of the semivolatile phase exhaust emissions are shown in figure 10. Dioxin receptor binding activity was observed in all the samples except that from fuel D7. Fuel D6 gave rise to the semivolatile phase emission with the highest affinity for the dioxin receptor with the lowest EC50 With the exception of the sample from fuel D1, the dioxin receptor binding activities of the semivolatile phase emissions were similar or lower than those of the corresponding particulate phase emissions. Vehicle 2:

Particulate phase Figure 11 shows the EC5o values for dioxin receptor binding of the particulate phase exhaust emissions. Dioxin receptor binding activity was again observed in all the samples. Due to technical difficulties an ECso value for fuel D4 could not be estimated and insuf'ficient sample remained to repeat the analysis. On examination of the fuel dependency of the ECso values obtained it was observed that fuel D6 once again produced the emission with the highest receptor binding affinity with an EC50. However, there were no significant differences between any of the fuels with regard to the ECso's of their emissions. Only a 2-fold variation in EC50 values was observed in the samples from vehicle 2. TCDD

rn

N o 2

V e h i c l e

0 6

I m

M

V

--- Filter ~

S

II

--- PUF

T

D

Figure 11.- EC50 values, vehicle two; particulate and semivolatile phase,mean values and standard deviation.

91

Semivolatile phase Figure 1 1 shows the EC50 values for dioxin receptor binding of the semivolatile phase exhaust emissions. No detectable dioxin receptor binding activity was seen in the samples arising from fuels D1,D4 and D8.Receptor binding activity was seen in the semivolatile phase emissions from fuels D2 and D9. CONCLUSIONS

Answers to the main objectives of this study can be summarized as: 1.-Aromaticity shows a clear influence on the emission, those fuels with high aromatic contents have high levels of PAH in their emissions measured. 2.-The sulfur content of the fuel does not seem to have a marked influence on the emission, but for particulate and SOXemissions. 3.-Light density diesel fuels are favourable in terms of emissions compared to high density fuels. 4.The ignition improver did not have a major influence on the exhaust emissions. Refemng to the outcome of this study, improvements of the diesel fuel quality will have a clear beneficial effect on the air quality in city areas.

Acknowledgments We would like to thank all participants involved in this investigation. The project was financed by the Swedish Environmental Agency.

References Impact of Fuels On Diesel Exhaust Emissions: A chemical and biological charactenzation. Westerholm R. and Egeback K-E Swedish Environmental Protection Agency Report Series. Stockholm, 1990 Codes of Federal Register 40 Parts 81 to 99 Protection of Environment. Revised as of July 1, 1987 Characterization of TCDD-receptor ligands present in extracts of urban particulate matter. Toftgard R., FranzCn B., Gustafsson J.A. and Lofroth G. (1985) Environment International, Vol. 1 1, pp 369-374.


A. Crucq (Editor), Catalysis and Automotive Pollution Control 11 0 199 1 Elsevier Science Publishers B.V., Amsterdam

93

FUELS EVOLUTION IN EUROPE AND CONSEQUENCES ON EXHAUST CATALYSIS J.C. Guibet

Institut Franqais du Pbrole, Techniques &Application Energktiques 1-4, avenue du Bois-Prkau, F - 92506 Rueil Malmaison France SUMMARY

1 - The market for unleaded gasolines will certainly grow rapidly in Europe in the early 1990 s (use with and without catalysts) 2 - The Eurosuper fuel (RON 95) is the most widespread product in Europe with relatively low characteristics variation from one country to another. West Germany (regular-grade) and France (superpremium) represent today extreme situations in the quality of unleaded fuels. There is no evolution of characteristics of unleaded fuels likely to alter the efficiency of catalysts. Lead traces are very low; sulfur contents are relatively low and can be further improved Interest in the future will be focused on : - the share of superpremium in the overall unleaded gasoline pool - the relations between the chemical composition of unleaded fuels and exhaust emissions in relation with catalyst performance INTRODUCTION

The major event in the European motor fuel distribution context is the recent appearance and penetration, at different rates in the various countries, of unleaded fuels. This development logically comes before the generalized spread of vehicles equipped with catalytic converters. These are still relatively few in number. Their population in 1990 is estimated at about two million vehicles out of a total of 140 million units. This means that, today, the majority of european unleaded fuels are used in vehicles without catalyst. This situation is bound to change because, from 1992, the future emissions standards will require the use of catalytic converters on all new vehicles. We intend to examine here the main properties of unleaded fuels distributed in Europe in 1989. Some more recent data on the first half of 1990 are provided for certain countries, including France. Among the quality criteria considered, we will particularly describe those likely to affect the efficiency of the catalytic converter. Our aim is to see how, for Europe as a

94

whole, the mutual adaptation of unleaded fuels and the new vehicles in circulation is being prepared, in anticipation of the generalized use of exhaust catalysts. 1. THE SHARE OF UNLEADED MOTOR FUELS IN THE EUROPEAN DISTRIBUTION NETWORK IN 1989/1990.

The different types of unleaded motor fuel currently distributed in Europe are divided into three grades : - regular-grade, mainly used in West Germany : its RON is close to 92 to 93 and its MON between 82 and 83. - standard motor fuel corresponding to the European specification (minimum RON 95, minimum MON 85) : it is found in all the EEC countries, as well as Austria, Finland, Norway, Sweden and Switzerland, Table 1 :European and world market for unleaded motor fuels Situation between end 1989 and June 1990 Country Japan Canada U.S.A. Austria Belgium Denmark Finland France West Germany Greece Ireland Italy Luxembourg Netherlands Norway Portugal Spain Sweden Switzerland United Kingdom

Share of Unleaded Fuels

Distri ution by PI duct Type tegular Eurosuper Superpremium 0 (%) A%L 14 100 86 23 4 65 92 90 53 30 50 50 10.5 67

x 60 30

28

6 23 28 50 50 2 33

0 0.2 4 21 40 45 0.2 0.5 42 58

0. 4 19 33 45 0.2 0.5 41.5 58

25

21

24 2

8.5 6

2 7

0.5 4

95

- high-octane premium grade (RON 98, MON 88) : France, the Netherlands and, to a lesser degree, West Germany and the United Kingdom currently consume this type of fuel. For each country, Table 1 gives an estimate of the share of unleaded motor fuel distribution in the total gasoline pool. The situation of nonEuropean countries, such as the United States, Canada and Japan, is given for reference, and as an example of geographic areas where the use of catalytic converters has become widespread. The figures given must be considered as approximate on account of the fairly rapid growth of the unleaded fuel market in many countries. Thus Table 2 shows that, in France, the situation has been changing from month to month since the beginning of 1990. Table 2 : Penetration of unleaded motor fuels in France in the first half of 1990 Relative Market Share Product Type Regular-grade Gasoline Conventional Leaded Premium Unleaded Premium* TOTAL

(%)

January 4.6 89.1 6.3 100

March

5.o

86.6 8.4 100

April 4.5 85.4 10.1 100

June 4.3 80.5 15.2 100

For Europe as a whole, the unleaded motor fuel market is estimated in June 1990 at about 31 Mt/year for a total of 118 Mdyear. Thus, the share of unleaded fuels would be approximately 26 % of the total gasoline european market. 2. 2. GENERAL PROPERTIES OF DIFFERENT GRADES OF MOTOR FUELS IN EUROPE

The data given below are taken from recent surveys published by OCTEL and analyses made by Institut Frangais du Petrole on samples taken at service stations. The information is merely indicative, and does not claim to describe accurately the present situation of the gasoline pool which, as we have already pointed out, is changing fast.

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2.1. Leaded motor fuels These products, which are still largely in the majority, are only examined briefly here because they are incompatible with catalytic converters. However, it may be interesting to recall their general properties, which could have repercussions on refining and therefore on the quality of unleaded fuels. Today, the maximum lead content in conventional motor fuels is 0.15 g/l in nearly all European countries except France, where it is still 0.25 g/l, and Spain and Portugal, where it is 0.40 g/l. Table 3 : Average and extreme properties of leaded premium-grade gasolines in some European Countries.

I Country France R.F.A. Italy Benelux United Kingdom

Lead I Content g/l

0.25 0.15 0.15 0.15 0.15

~

MON

RON I

Min 96.8 97.9 94.9 97.9

Average 97.6 98.9 97.0 98.1

Max 98.3 99.9 98.1 98.3

Min 85.6 87.9 86.2 86.8

Average 87.0 88.8 88.1 88.4

Max 90,l 90.2 90,4 89,3

97.5

98.2

99.6

86.4

87.2

88,O

In most cases (Table 3), the octane numbers of leaded premium- grade gasoline average about 98 RON and 88 MON. Assuming that a lead content of 0.15 gll achieves a gain of about 3 points in the RON and MON, it is clear that the development of Eurosuper type unleaded gasoline (RON 95, MON SS), to the detriment of conventional leaded premium-grade gasolines, could take place without any important change in the refining process. On the other hand, the production of large amounts of superpremium unleaded gasoline (98 RON) instead of leaded gasoline would require a deep adaptation of refining process. Concerning other leaded gasolines it must be noted that regular grade was recently eliminated in West Germany. It is still distributed in very limited quantities in France (5 % of the market). 2.2. Unleaded regular-grade gasoline

This type of product is essentially found in West Germany and Austria, where it accounts for 42 and 57 % respectively of all unleaded fuels. Table 4 shows the average and extreme octane numbers of unleaded regular-grade gasolines in.West Germany. We point out that the RON varies

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over a fairly wide range between 90.5 and 94.1, whereas the MON levels are all in the 82 to 83 zone. The average RON (92.8) and MON (83.0) values are only 2 points under the Eurosuper type unleaded premium-grade gasoline specifications.[SP] Table 4 : Octane numbers of unleaded regular-grade gasolines in West Germany RON MON

Min 90.5 82.3

Average 92.8 83.0

Max 94.1 83.5

2.3. Standard unleaded gasoline, Eurosuper type. This is the most widespread product in Europe today in the class of unleaded gasolines. Table 5 shows that its octane numbers only differ slightly from one country to another, and meet the requisite specifications. The MON is generally very close to 85, while the RON is well above 95, as much as 96 or even 97. This shows that, for the refiner, the strongest constraint in terms of fuel blending concerns the MON. Very often, to obtain the minimum value required (85 for MON), the RON has to be let far above 95 (give away). This situation occurs particularly if significant amounts of olefin-rich, low MON catcracker gasolines are blended in the fuel. Table 5 : Average and extreme properties of eurosuper type unleaded premium-grade gasolines Country West Germany Italy Benelux UnitedKingdom Scandinavia

Min 94.9 94.7 94.7 96.2 94.4

RON Average 96.1 95.6 95.1 97.0 95.4

Max 98.6 96.4 95.4 97.7 97.4

Min 85.0 85.1 85.2 85.5 84.7

MON Average 85.4 87.0 85.8 86.0 85.3

Max 85.8 87.0 86.4 87.6 85.8

2.4. High-octane unleaded gasoline For this type of product, the minimum values are 98 for RON and 88 for MON respectively. This is the most widespread unleaded fuel in France today. The octane numbers found in the distribution network (Table 6) show a very slight dispersion around 88 for MON. The RON very often exceeds 98, and could reach or even go over 100 (exceptionally up to 102). As in the

98

case of Eurosuper, this reflects a more severe constraint for MON than for RON. Table 6 : Average and extreme octane numbers of superpremium type unleaded gasolines distributed in France.(Spring 1990) RON MON

Min 98.4 88.1

Average 99.7 88.4

Max 101.1 88.8

3. EXAMINATION OF SOME PROPERTIES LIKELY TO AFFECT AUTOMOBILE CATALYSIS

Catalysts run satisfactorily with a wide variety of motor fuels, provided they do not contain lead or other impurities liable to poison the catalyst. Hence the influence of the fuel is not decisive. However, it may be important in terms of light-off temperatures, composition of unconverted hydrocarbons, emissions of by-products such as H2S, and the endurance behavior of the catalyst. We examine a number of points that appear interesting to consider.

3.1. Lead content The maximum permissible content in Europe is 13 mg/l. Although complete analytical results for all the countries are not available, it is very likely that this specification is easily obtained. Thus, in a survey conducted in France in summer of 1989 on 22 samples, the traces of lead observed were always less than 2.5 mg/l, except in one case, where they reached 3.2 mg/l.

3.2. Chemical composition We are going to examine the distribution of components by hydrocarbon family (paraffins, olefins, aromatics) and its possible influence on catalyst effectiveness. This subject is still not perfectly understood, and merits a further investigation. However, we can recall the main conclusions of a study carried out recently at Institut FranGais du PCtrole1 in an engine bench test, equipped with a three-way catalyst and tested with 12 motor fuels covering a very wide range of compositions. 1- M.F. PRIGENT, B.C. MARTIN and J.C. GUIBET. Engine Bench Evaluation of gasoline composition effect on pollutants conversion rate by a three-way catalyst. SAE 900153. Detroit, 1990.

99

It was demonstrated that the light-off temperature (corresponding to 50 % conversion) tends to decrease with an olefin-rich fuel, such as a catcracker

gasoline, and to increase with an aromatic fuel, such a reformate. The differences of 20 to 25°C shown in Table 7 are significant. The tendencies observed have been confirmed on commercial fuels which differ by their olefins and aromatics content (Table 8). Moreover, the presence of oxygenated components does not appear to have any clear effect on the activation temperature of the catalytic converter. Table 7 : Light-off temperatures for CO, HC and NO, on a three-way catalyst. Comparison of behaviour of a reformate with a catcracker gasoline.

Table 8 : Light-off temperatures for CO, HC and NO, on a three-way catalyst. Comparison of behavior of two unleaded gasolines of different chemical compositions.

Given the tendencies shown here, it may be interesting to examine the ranges of variation in the olefins and aromatics contents of European unleaded gasolines. Table 9, related to products of the Eurosuper type (RON 95, MON 85), shows relatively narrow dispersions. On the average, the olefins content ranges between 6 and 9 % in the different countries, and the aromatics content between 35 and 39 % (42 % in Scandinavia).

100

Table 9 : Olefins and aromatics contents of European unleaded motor fuels. (Eurosuper grade, RON 95, MON 85)

If we compare, for Europe, the chemical composition of unleaded fuels as a function of their classification (regular, Eurosuper, Superpremium), a very logical increase in the aromatics content is observed and a reduction in the olefins content, when going from regular grade to Superpremium (Table 10). The higher aromatics content is explained by the need to meet an increasingly higher RON requirement (93 + 95 + 98) and the drop in the olefins content results from growing constraints on the MON (83 + 85 + 88). The variable addition of oxygenated products may slightly disturb these tendencies, but without significantly affecting them. Table 10 : Comparaison of European unleaded motorfuels as a function of classification.

3.3. Sulfur content

The small amounts of sulfur present in the gasoline can be stored in the form of sulfates on the three-way catalyst, in normal operation, and then discharged in the form of hydrogen sulfide (H2S) when vehicle runs on a rich mixture, as in starting up period. The unpleasant odor of the exhaust gases is a significant drawback, and the car manufacturers wish the sulfur content of the fuels to be as low as possible. For Eurosuper, the maximum permissible content is 1000 ppm today. However, French manufacturers, for example, in their quality label,

101

recommend average and maximum values, not exceeding 200 and 300 ppm respectively. Table 11 : Sulfur content of unleaded motor fuels (superpremium) distributed in France in 1989.

A survey conducted in France in the summer of 1989 on Superpremium type fuels revealed very low sulfur contents, as shown in Table 11. Generally, much lower sulfur contents are found in Superpremium gasolines (RON 98) than in Eurosuper (RON 95). This is because the Superpremium products usually contain a high proportion of reforming gasoline, which is practically sulfur-free, whereas the Eurosuper products contain a certain proportion of cracking gasoline, which is richer in sulfur. Table 12, which compares the sulfur contents of several samples of Eurosuper and Superpremium type products, clearly illustrates this tendency.

Table 1 2 : Comparison of sulfur contents of two types of unleaded gasoline pool. (France Spring 1990)

-

3.4. Oxygenated compounds The main oxygenated component of gasoline in Europe is MTBE. Table 13 shows the average percentage of unleaded fuels which contained it in 1989, as well as the average and extreme contents found. Some samples of Superpremium contained up to 11 % in France, and 15 to 16 % in West Germany.

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Table 13 : Estimation of MTBE contents in European unleaded gasolines. Country

Type of Gasoline

Benelux Eurosuper France Superpremium* West Germany Superpremium** West Germany Eurosuper** West Germany Regular United Kingdom Eurosuper Italy Eurosuper

* : Summer 1989

Share of Fuels containing MTBE (%)

100 64 100 42 42 25 50

Contents Observed (%weight) Min Average Max 0.8 2.1 4 .O 1.0 6.8 11.4 1.3 9.6 15.9 0.7 1.1 1.6 0.1 0.5 1.1 4.4 2.2 4.0 6.2

** : December 1988

It is important to note that MTBE may be found in all types of product conventional leaded premium-grade gasolines, unleaded regular-grade (in West Germany), Eurosuper (many countries) and Superpremium (France). Methanol and tert butanol were still found in 1989 in West German fuels, leaded and unleaded. The average contents were about 1.2 % for methanol and 1.4 % for tert-butanol. These compounds could be present in the fuels together with MTBE. In the operation of catalytic converters, the presence of oxygenated components does not appear to have any substantial effect. However, car manufacturers do not wish an excessively wide range of concentrations of oxygenated products. This is because, in developing the inlet system, they have to consider the possible extreme compositions for motor fuels, and obtain satisfactory combustion in all circumstances. For example, the discrepancy of equivalence-ratio due to the variation of MTBE contents could modify the emission of unburnt products (carbon monoxyde and hydrocarbons), in the starting up period where the catalyst is not completely effective. CONCLUSION.

The market for unleaded gasolines will certainly grow rapidly in Europe in the early 1990's. These products will continue their penetration, first in a large proportion of present vehicles in circulation not yet fitted with catalytic converters, and then on new vehicles equipped with catalysts to meet future European pollution standards. Around these two extreme situations, illustrated by unleaded regulargrade gasoline (West Germany) and Superpremium (RON 98) which is very

103

widespread in France, the conventional Eurosuper type fuel (RON 95) is the most widespread product in Europe, with relatively minimal variations in grades and characteristics from one country to another. An examination of the main properties of motor fuels likely to alter the operation of the catalyst fails to reveal any disturbing situation or development. The residual lead contents are very low. The sulfur contents are relatively low and can be further improved. Interest in the future will be focused on the growing share of Superpremium in the overall unleaded gasoline pool, and on relationship between the chemical composition of the products (content of aromatics, olefins, oxygenated components) and exhaust emissions, incorporating the operation of catalytic converters in this examination. Institut FranGais du PCtrole, which is engaged in this type of study, hopes to contribute in its way to further progress in motor fuels, vehicles and high-performance catalysts in order to reach a better protection of the environment.


A. Crucq (Editor), Catalysis and Automotive Pollution Control II 199 1 Elsevier Science Publishers B.V., Amsterdam

105

SUPPLY AND DEMAND OF PRECIOUS METALS FOR AUTOMOTIVE AND OTHER USES Dr M.C.F. Steel

Johnson Matthey PLC, London, United Kingdom

Introduction The three precious metals used in automotive pollution control are platinum, palladium and rhodium. These three metals form part of the platinum group, are closely related in terms of chemical and physical properties, and occur together in nature. Their geological abundance, detailed applications and supply/demand fundamentals are, however, significantly different and this paper will outline and discuss these differences.

Platinum G r o w Metals (PGM1 The platinum group of metals consists of six elements: ruthenium, rhodium, palladium, osmium, iridium and platinum. The metals are all quite rare and occur only in a few comparatively concentrated deposits.

G r o w Ib

Groun VIII Ru 0s

Rh 1

I

I

Au

I

These invariably contain all six metals but in varying quantities depending upon the mineral source. Separation of the platinum metals is a long and difficult process owing to the marked chemical similarities between both the metals and their compounds. As table 1 shows, the platinum metals occur in the same group of the periodic table as the ferrous metals. Although chemically the resemblances between them are slight they do tend to occur together in nature. The platinum metals are often referred to as precious metals along with gold and

106

silver. Although they share some properties, such as the nobility of gold and PGM, and applications, for example as jewellery and investment media, the PGM are much more important as industrial metals.

The sources of platinum group metals in the world that are suitable for economic exploitation are few. Figure 1 shows where these resources are located. The principal source is the Bushveld Igneous Complex in South Africa. Other significant mining for PGM occurs in the USSR, Canada and the USA. China is believed to produce same PGM but is still a net importer of the metals. Small amounts of PGM are also recovered elsewhere in the world either as placer deposits or as by-products of base-metal mining, but the contribution of these to world supplies is insignificant.

USSR

Figure I :World sources of platinum group metals

Although there has been much exploration in recent years for PGM in such countries as Australia and Canada no significant economically viable deposits have been found. In Zimbabwe, however, a well established but relatively low grade PGM deposit known as the Great Dyke is once again receiving attention as a possible source of platinum metals.

107

Table 2 : Reserves of platinum, palladium and rhodium (million 0 2 )

I I

Pt

Pd

Rh

333 437 160

141 365 175

17 83 12

CANADA, Sudbury

3

4

1

URSS, Noril'sk,

50

142

6

USA, Stillwater,

7 ------

------

------

850

120

Origin SOUTHAFRICA Merensky UG2 Platreef

23

3

As table 2 indicates, the proven reserves of platinum, palladium and rhodium in South Africa, the USSR, Canada and the USA are considerable and sufficient to last for between 250 and 400 years at current rates of consumption. The Bushveld Igneous Complex contains overwhelmingly the largest reserve of PGM (Refs 1,2). Although adequate reserves of PGM exist, the mining of these metals will always be expensive because of the enormous mechanical and human resources needed to mine the ore (it takes approximately 10 tonnes of ore to yield 1 oz of platinum in South Africa) and the complexity of chemically separating and refining the six platinum group metals to the high purity required in the market place. The relative mix of the six PGM - the 'mine mix' - is important, with the deposits in the northern hemisphere showing a preponderance of palladium whereas in South Africa platinum is the principal metal. Although the deposits in South Africa are mined principally for PGM, those in Canada and the USSR are essentially nickel and copper deposits with PGM being produced as by-products. This means that the output of PGM from Canada and the USSR is inelastic as far as PGM supply/demand fundamentals are concerned.

1 i (Ref 3) Automotive pollution control is a major application for PGM but the relative importance for the three autocatalyst metals differs considerably as Table 3 shows. In 1989 same 42% of the western world demand for platinum was for the manufacture of autocatalyst, whereas for palladium the

108

corresponding figure was only 8%. For rhodium, the market for which is only one-tenth the size of those for platinum and palladium, autocatalyst usage accounted for a massive 81% of total demand.

Table 3. Western World Demand for PGM in 1989.

In 1989 demand for platinum reached 3.425 million 0 2 . Figure 2 shows that autocatalyst accounted for 37% of demand (after allowing for recovery of spent autocatalyst), with jewellery taking 38%, investment 5% and a number of important industrial applications the remaining 20%. Industrial uses for platinum include petroleum reforming catalysts for gasoline production, ammonia oxidation catalysts for nitric acid production, bushings for glass fibre and high purity glass production, and thick film pastes and sputtering targets for the electronics industry. Supplies of platinum in 1989 are shown in Figure 3 and came from South Africa (77%), the USSR (16%), Canada (4%) and other miscellaneous producers (3%) . The relatively small shortfall of newly mined supply compared with demand was met from industry stocks held around the world.

Figure 2 :Platinum demand in 1989. Total demand = 3,425,000 oz

109

SOUTH AFRICA 77%

USSR SALES 16%

OTHERS 3% CANADA 4%

Figure 3 :Platinum supplies in 1989. Total supplies = 3375,OO oz

Demand for palladium of 3.31 million oz also exceeded newly-mined supplies by a similar amount. Figure 4 shows that for this metal the USSR was the principal supplier (51 %) with South Africa contributing 35%, Canada 6% and others 7%. Half the western worlds palladium was used for the electronics industry in 1989, with 30% going into dental alloys, 6% into autocatalyst (net), 5% jewellery and the remaining 9% a variety of industrial applications mainly in the chemical industry. Figure 5 shows the breakdown of palladium uses.

SOUTH AFRICA 35%

CANADA 6%

OTHERS 7%

Figure 4 :Palladium supplies in 1989. Total supplies = 3,205,000 oz

The market for rhodium is approximately one tenth that of platinum and palladium and in 1989 supply and demand were in balance at 0.33 million oz. As stated earlier, same fourth-fifths of demand was for autocatalyst, with other chemical catalyst uses accounting for a further 9%, electrical uses 4%, and miscellaneous applications the residual 8%. Supplies were from South

110

Africa (56%), the USSR (39%) and Canada (5%). Figures 6 and 7 show, respectively, demand and supply of rhodium in 1989. ELECTRICAL 50%

DENTAL 30%

AUTOCATALYST 6%

Figure 5 :Palladium demand in 1989. Total demand = 3,310,000 oz AUTOCATALYST(net) 79%

CHEMICAL 9%

GLASS 1% OTHER 7% ELECTRICAL 4%

Figure 6 :Rhodium demand in 1989. Total demand = 330,000 oz

CANADA 5%

Figure 7: Rhodium supplies in 1989. Total supplies = 330,000 oz

111

A significant difference between the markets for platinum and palladium and that for rhodium, in addition to their absolute size, is the relative liquidity of these markets. Platinum and palladium are widely traded around the world with futures markets existing in both New York and Tokyo and a well established spot market in London and Zurich. This and the wide spread of applications for each metal has led to the accumulation of stocks of metal around the world that can be used either to satisfy supply shortfalls or to absorb occasional surpluses. For rhodium, however, there are no formal spot or futures markets, the total market is much smaller and dominated by a single application, and industry stocks are therefore significantly lower than for platinum and palladium. In addition, supply of rhodium is rather inflexible due to the fact that rhodium is essentially a byproduct of platinum mining and that it takes six months or more from the mining of ore to the emergence of pure rhodium from the refinery.

PGM in Autocata lVsl When considering the outlook for the use of platinum, palladium and rhodium in automotive pollution control, it is important to consider the ratios of the metals in exploitable ore deposits and in autocatalyst formulations. The requirement for platinum, palladium and rhodium in autocatalyst is determined principally by the reactions that the metals are present to catalyse (Ref 4). These are shown in Figure 8.

Fig 8 Autocatalyst Reactions

HC

co

oxidation

H20 + CO2 + oxidation ---

---+

co*

In the early days of autocatalyst the principal requirement was to oxidise unburned hydrocarbons (HC) and carbon monoxide (CO). To do this catalysts of platinum, palladium, or a mixture of these two elements, were chosen. As legislation has become stricter and has focussed on additionally removing nitrogen oxides (NOx) the role of rhodium to catalyse the reduction of NO, to nitrogen and water has become critical. In the early nineteen-eighties dual bed systems were used containing a reduction catalyst (usually Pt/Rh) followed by an oxidation catalyst (usually PtPd). Modern engine technology employs sophisticated control of air-fuel ratios by using electronic fuel injection and sensors and has enabled the auto industry to converge on the use of a single catalyst to remove all three pollutants in one unit. Such 'three-

112

way' catalysts almost invariably contain platinum and rhodium, in a ratio of 5:1, as the active metals. N AMERICA 50%

JAPAN 24%

1- 1 W EUROPE 21%

Figure 9 :Platinum demand for autocatalyst by region in 1989 Total demand (gross) = 1,450,000 oz Figure 9 shows that the demand for platinum in autocatalyst in 1989 was 1.45 million oz and this is expected to grow substantially through the current decade. Initially the focus is on Western Europe, where by 1993 all new cars are expected to be fitted with catalysts. The use of catalytic converters in several Eastern European countries now also looks probable. In Brazil and Mexico legislation is in place that will result in catalysts being used in the next five years, and other countries with severe pollution problems may be expected to follow suit. In addition, the US Congress is currently progressing a new Clean Air Act which will contain tighter emission standards that will set a new world standard that other countries can be expected to adopt in years to come. This progressive tightening and geographical widening of automotive pollution control will lead to a growth in PGM consumption for autocatalyst. The question that follows is whether the necessary PGM will be available and from which sources.

Given the reserve base existing in the Bushveld Igneous Complex (Table 2) and the flexibility there of mining principally for platinum metals it is clear that South Africa must remain the prime source of PGM. Considerable expansion is already underway to satisfy the expected demand for autocatalyst production. A feature of the Bushveld Complex is that the PGM are contained in three different mineral formations. These are the Merensky Reef, the UG2 reef and the Platreef. As table 4 shows, the mine mix of PGM in these formations is significantly different, with the UG2

113

providing a mix that is closest to the current favoured autocatalyst ratio of Pt:Rh at 5:1. Table 4 : South African Ore Ratios

UG2

I Platreef I

14.0

I

15.3

I

1

I

Given the certainty of the growth of the market for autocatalyst during the remainder of this decade, it is not surprising to find that the main thrust of current expansion plans in South Africa is for greater exploitation of the UG2 reef (table 5). In 1988 some 92% of South Africa's platinum production was from the Merensky Reef and only 8% from UG2. Table 6 indicates that by 1996 it is expected that 70% will come from Merensky. 27% from UG2 and 3% from the Platreef.

Table 5 : PGM Production in South Africa Present Mining

Expansion

Ru stenburg Lebowa Potgietersrust

MNG2 M

UG2>M M P

Impala Messina

M / UG2

UG2 / M UG2>M

Western Eastern Karee

M / UG2 UG2 (new) M / UG2 (new)

UG2 UG2 M / UG2

Crocodile River Kennedy's Vale

UG2 (new)

UG2 UG2

Northam

Merensky UG2 Platreef

M

1988 92% 8% 0%

1992 80% 20% 0%

1996 70% 27 % 3%

114

Conclusion The market for platinum metals in automotive pollution control is expected to grow substantially over the next ten years, particulary for platinum and rhodium. To meet this growth mining companies are investing in new mines and in the expansion of existing mines to provide extra PGM. In making these investments attention is being focussed on the particular requirement for extra rhodium. This should ensure that the demands of the auto industry for PGM are satisfied as the market for automotive pollution control using PGM catalysts grows.

References 1

D L Buchanan, "Platinum Group Metal Production from the Bushveld Complex", (1979)

2

J R Loebenstein, "Mineral Facts and Problems", US Bureau of Mines, Bulletin 675, (1985)

3

J S Coombes, "Platinum 1990", Johnson Matthey, (1990)

4

M C F Steel, "Changing Patterns of Platinum Group Metals Use in Autocatalyst" SAE International Congress Paper 880127, (1988)

A. Crucq (Editor), Catalysis and Automotive Pollution Control I1 Q 199 1 Elsevier Science Publishers B.V., Amsterdam

115

THE VALUE OF SPENT EXHAUST CATALYSTS

W. Fierain MHO, BU HOBOKEN, BELGIUM

Abstract There is some logic in scheduling this paper at the end of the session since it deals with spent catalysts which, by definition, have reached the end of their useful life. Yet, the ultimate stage in life for a spent catalyst is a very valuable one since it becomes then part of the largest secondary source of platinum group metals worldwide. This paper will explain how and why and will indicate the obvious relationship between the development of this source and all the key elements governing the use of autocatalysts, meaning: - the environmental legislation planned or in place - the currently available technology - future new car sales - supply, demand and prices of the platinum group metals. INTRODUCTION.

Before going any further, a rapid sketch of MHO's activity might help understanding why we do consider spent exhaust catalysts as a "rising star" material. MHO, a Division of ACEC-UNION MINIERE, has two production sites in Belgium, one at Olen and one at Hoboken near Antwerp. Highly specialized in the treatment of complex precious metals bearing raw materials, the Hoboken Plant uses hydro and pyrometallurgical processes for the refining of - precious metals: Ag - Au - Pt - Pd - Rh - base metals: Pb - Cu - Sb - Sn - special metals: Se - Te - In - Bi On a yearly basis, the Hoboken Plant receives from all over the world more than 300,000 tonnes of various types of non-ferrous and precious metals bearing raw materials in the form of concentrates, metallurgical and chemical byproducts, spent petrochemical catalysts or industrial discards from the electronic industry. In addition to that and since the early 803, it has refined several thousand tons of spent catalysts from the automotive industry, mainly

116

of North American origin. With almost a decade of experience in the field, MHO is still actively pursuing its marketing and metallurgical research efforts with the aim of extensively implementing the optimal recycling scheme for a device which is or will be part of day-to-day's life for a vast majority of individuals in the industrialized world. THE VALUES

Different values can be associated with the spent exhaust catalysts market: strategic - ecological - industrial -economical. Let us examine each of these.

The strategic value ("putting something by for a rainy day): Biginning in the mid-70s, it has become a general concern to reduce the level of exhaust emissions from motor vehicles. Both Japan and the USA were the first to adopt tight standards for carbon monoxide, unburnt hydrocarbon and nitrous oxide emissions. The fitting of platinum group metals catalytic converters became the only efficient tool to achieve these standards. A similar package of standards applies or will soon apply in Europe and the installation of the so-called 3-way catalyst, which is mainly a platinum rhodium catalyst with a ratio 5 to 1 P m h , seems unavoidable. Because of public pressure and tax incentives, some European countries have been ahead of the EEC legislation: this is the case of Austria, Switzerland, Norway and Sweden. Likewise early this year, 80 % of the motor cars sold in Germany were already fitted with catalysts. This growing use of automotive catalytic converters has had a tremendous impact on the demand for PGM's. Starting from nil in the early ~ O ' SPt , demand from the automotive industry reached 40 % of total Western world platinum demand in 1989, as shown by Metals and Minerals Research Services. This is very much in line with the figures issued by Johnson Matthey in the 1989 Platinum review. For the same year 1989, platinum recovered from autocatalyst recycling was estimated at about 5.5 tonnes, equivalent to approximately 5 % of the total demand for platinum in the Western World. This is far from being an insignificant figure: indeed this secondary supply is, for instance, three times the estimated platinum output of the Stillwater Mine in Montana, the sole primary producer of platinum group metals in the United States Those 5.5 tonnes Pt are equal to the production of Canada, third largest supplier of platinum in the world, with Inco and Falconbridge producing

117

platinum as a byproduct of nickel smelting and refining. As a third comparison, this secondary supply is no less than the total US platinum demand from the chemical, electrical and glass industries all together. And last but not least, these 5.5 tonnes represent 50 % of the 1989 European platinum demand from the autocatalyst industry. Japan and United States together represent 75 % of the gross demand for platinum from the automotive industry i.e. about 34 tonnes; by comparison recycled platinum worldwide represents 16 % of their combined consumption in autocatalyst during 1989. The strategic value becomes obvious when remembering that more than 70 % of the western world supply originates from South Africa and that Soviet Union supplies more than 10 % of the Western market requirements. No doubt all producers have substantial quantities of PGM in the pipeline; no doubt PGM consumers own security stocks against possible disruption at mines or refineries due to political events or technical failures: pipeline and stocks altogether represent about one year of demand. In this respect, there is no doubt either that recycled PGM is definitely an hedge against any impediment in primary shipments or temporary tight supply. This has not escaped to Japan which is gearing up to import more platinum group metals derived from spent converters from the United States and has established its own collection network locally. In a similar approach, some German car producers are already today organized to have the spent catalysts collected by their dealers, decanned and the PGMs recycled and returned to the fresh catalyst producers. Palladium is showing a lesser strategic sensitivity in this context because regulations on emission standards are leading to the preferred use of platinum rhodium catalysts. By contrast, rhodium undoubtedly exceeds by far the strategic value of platinum because with a platinum/rhodium consumption ratio of 5 to 1 and a mine production ratio of 15 to 1, rhodium stocks are gradually vanishing in an already tight market.

The ecological value (Recycling the product): An article issued by John Griffiths in the Financial Times of May 22, 1990 announced that a major German car manufacturer was designing a pilot plant to search for and experiment the best way to achieve complete car recycling; the pilot facility will be used to disassemble and process a total of 1,500 cars over the next 12 months. One of the most important objectives of this project is to apply knowledge learned from the disassembly process to the design of future cars so that they can be specifically developed to be much more easily recycled than it is the case today.

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After the pilot phase, a permanent recycling plant capable of processing

250,000 cars per year could be build within the next two years. The hope is that such plants would become a norm for the motor industry with the farreaching idea of recycling 100 % of every car built. Recycling of spent catalysts perfectly matches this valuable objective and a close cooperation should exist between collectors/refiners and catalysts producers/car manufacturers. Indeed, optimizing the complete recycling circuit still presents a lot of handicaps and challenging aspects. The next 2 topics will clarify what we are speaking about but before proceeding let me point out that the recycling industry is presently deeply concerned with two enhancing trends in environmental regulations: first, to classify all recyclable and secondary material as waste and second to prohibit the international transport of waste. We are very much worried by such a possibility which ignores the fact that the inherent value of precious metals bearing secondary raw materials already commands such careful management that it is inevitably protective of the environment.

The industrial value: The extensive process of recovering autocatalysts involves many different steps and players: - pipes and canisters removal: - car dismantling yards - car shredders - muffler shops - car dealers - garages - pipes

and canisters collection - sorting:

- scrap dealers - specialized collectors

- opening of the canisters (decanning): sorting of the catalysts lot preparation (possible crushing and pre-sampling)

- specialized collectors and consolidators

- sampling :

- refiners

- smelting and refining :

- refiners

The first part of the chain: the scrap collection network includes: - pipes and canisters collection - catalysts unloading

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- catalysts sorting - catalysts defining - catalysts decanning - catalysts packinglstoring - catalysts empty shells The second part of the chain starts after receipt of the material at the refining plant; at MHO the sampling procedure for spent auto catalysts can be shortly described as follows, auto catalysts being delivered in the form of honeycombs (crushed, partly broken or fresh) or beads. Pretreatment Uncrushed or partly broken honeycombs are crushed with a hammer-mill to obtain a fine product that can be sampled in the sweeps automatic installation. Primary sampling The precrushed honeycombs or the beads are sampled with the automatic sweeps sampler. This unit has been especially designed to weigh, empty and sample fine materials packed in drums or big bags. The drums or big bags are unloaded in 2500 1. hoppers. Two of these hoppers are emptied in a 8000 1 biconical blender. After 15 min blending, the blender is unloaded over a rotary divider, which takes a 10 % sample. This 10 % sample goes in a 500 1. hopper, which is emptied in a 1000 1. biconical blender. After 15 min blending, the second blender is unloaded over another rotary divider, which takes a 1 or 2 % sample (i.e. 0.1 or 0.2 % of the original input). Final sample preparation The samples from the successive blending operations are transferred to the final sampling area where they are recomposed into a 3 kg sample with due proportion taken of their individual weight. This 3 kg sample is blended by means of a small biconical blender. After blending, it is crushed in a vibrating ring mill and screened on a 130 mesh screen. The fine material is then blended in a turbula blender and divided into analytical samples by means of a rotary splitter. The analytical sample-bags are sealed and sent to the analytical laboratories of the supplier and of MHO. Sealed bags are kept for possible umpire analysis. The final assay results will be determined by exchange of the figures determined on the official samples by the supplier and by MHO.

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The so-determined final assay results will govern the settlement of the delivered material either by payment or by return of refined metals to the suppliers. Refining

The third part of the chain concerns the refining. Several flowsheets have been developed. These include: -hydrometallurgical technologies like: substrate dissolution with sulphuric acid or sodium hydroxide resulting in a PGM residue suitable for conventional refining -or selective dissolution of precious metals in a hydrochloric acid solution containing an organic oxidant plus proprietary chemicals -dry chlorination at 400/1000 "C followed by recuperation and refining of the volatile PGM chlorides: this process is not yet commercially proven. -pyrometallurgical technologies like: melting of spent autocatalysts in a plasma arc furnace or an electrical furnace using iron or copper as collector: the iron alloy contains 2 to 4 % PGM and can either be granulated and leached to produce PGM concentrate suitable for conventional refining or the iron or copper alloy at 2 to 4 % PGM can be directly sent to a refiner for further processing. or direct melting and refining of spent catalysts through the conventional nickel, copper or lead refining circuits. Each process has its own economical, ecological and metallurgical constraints. However, up to now, the pyro-metallurgical circuits seem to be the most efficient processes as they are the final outlet for more than 75 % of the spent automotive catalysts available on the market. Why is it so ? We all know that autocatalysts consist of a refractory oxide carrier such as alumina or cordierite (Al2O3, Si02, MgO) on which two or three PGM are spread in low concentration. The monolith type catalyst (honeycomb) is typically made of cordierite and the bead type (pellets) is made of gamma alumina. Average assays of the catalysts received by MHO from the USA during the past 10 years show the precious metals concentrations (drnetric ton) given in the following table and thus, as you can notice, very low PGM concentrations.:

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In addition, the spent catalysts also pick up other metals from gazoline or motor additives like Pb, Mn, S, P or carbon and eventually stainless steel (nickel) or even dirt during the decanning. Furthermore, catalysts batches do not reach the refiner on a regular basis throughout the year. In order to cope with these unstable parameters we have chosen, at Hoboken, to load the spent catalysts after sampling at the sintering plant, together with a large variety of other raw materials such as concentrates, lead sulphate, dust or slimes for instance. The aim is to achieve a stable composition of the mix for sintering, therefore optimizing the recovery rates of the non ferrous metals including the PGMs. Another advantage of the Hoboken operations is to avoid the generation of by-products which would be economically or environmentally burdensome. Alumina, silica and magnesia are tapped from the blast furnace as a neutral slag while precious metals including the PGMs are collected in the lead bullion, the copper matte or the nickel speiss, which intermediate products are then further refined.

The economical value: With such strategic, ecological and industrial values associated to it as major driving forces autocatalyst recycling should have reached the "cash cow" stage. This is not yet the case, as shown hereunder. Justifications for low recycling rate in the USA are: - the anti-converter attitude on the part of many vehicle owners who remove and simply discard the converters in order to save money by using leaded rather than unleaded fuel and to possibly gain engine performance; the Environmental Protection Agency (EPA) did estimate that in 1984, 7 % of cars were running without converters. - the reconditioning of old converters as replacement equipment after quick cleaning and painting: this accounted for 10 to 20 % of the catalyst market in the.early 80s - the lack of an organized scrap collection network - the loss of PGMs due to mechanical attrition during use coupled with loss in the collecting and decanning process - finally, the difficulty for consolidators to maintain a regular feed flow because of price fluctuations arising from competition or PGM's price volatility. Nevertheless, there is now a definite trend for improvement in the reclamation rate thanks to: - the phasing out of leaded fuel and better campaigns promoting the merits of converters.

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- the enforcement of EPA performance standards since November 1986 limiting the sale of reconditioned converters - the EPA legislation stipulating that dismantling yards must remove spent catalysts before scrapping vehicles because of the residual lead contained in some of them. Consequently, quantities of spent catalysts made available in the USA have developed as follows: 1985 1986 1987 1988 1989

3,600 4,500 5,000 6,000 7,500

tons tons tons tons tons

In Europe our forecast of theoretical availability of PGM in spent autocatalysts is as follows, based on: - trend in new petrol powered car sales - existing car exhaust emission legislation - an actual average of 1.8 g of PGM to be recovered per vehicle - disposal of spent catalyst 8 years after the first registration of the vehicle What does it mean in view of economies ? The total recovery of PGM from spent autocatalysts will reasonably reach 20 tonnes at the end of the decade as we have some room for improvement in the USA and as the collecting system in Europe is more developed and probably more cost efficient than it is in the United States. At the level of 20 tonnes, the primary producers will see a substantial part of the market requirements already filled up and this might possibly delay or cancel the development of the most expensive mines for which cost of production could reach US $ 350/oz. Such a scenario means that spent automotive catalyst recovery would be a determinant factor for the floor price of platinum. On the contrary, should the market be oversupplied, the platinum price would fall preventing recovery from being profitable; indeed at prices below US $ 450/oz, recovery becomes quickly unprofitable: in the US collectors buy for cash at prices between US $10 and 20 per unit depending on the type of catalyst, the PGM prices and the overall availability of scrap. However, in such a case, and contrary to what happened in the early 80s when the prices of platinum and palladium fell to levels which were so low that they discouraged any risky position, we are convinced that salvage companies will not throw catalysts away but will simply stockpile them for a time.

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The result will be that at some future time the so-freezed PGMs will flow back onto the market anyway. This is commonly observed in the copper, lead, zinc, silver and gold market and it will apply to PGM as well. At MHO we have learned to live and operate with these realities and not to believe that the world is other than it is.

SOURCES Automobile catalyst and the PGM market by Jeffrey M. Christian, CMP Group Metal Bulletin 3rd Platinum Seminar 1987. Platinum 1990, Johnson Matthey PLC May 1990 Financial Times, Tuesday May 22, 1990 Charles E. Cunningham, A1 Specialized Services and Suppliers Inc One decade of commercial recovery of platinum group metals from autocatalyst: some change and implication New York Copper Club, March 1990. Francis Van Bellen - Sampling strategy for precious metals bearing materials at MHO IPMI, San Diego 1990 Platinum Lake Technology process for recovery of precious metals from automotive catalyst Recycling of Metalliferous Materials Platinum the way ahead - Metals and Minerals Research Services Ltd July 1990



125

CERIUM OXIDE STABILIZATION: PHYSICAL PROPERTY AND THREE-WAY ACTIVITY CONSIDERATIONS J. E. Kubsh, J. S. Rieck and N. D. Spencer W. R. Grace & Co.-Conn. Research Division Columbia, Maryland U.S.A.

Abstract Improved cerium oxide surface area stability in high temperature oxidizing environments has been demonstrated for stabilized cerias containing La, Nd or Y dopants. BET areas in excess of 30 m2/g can be achieved after a one hour, 980 O C calcination for a ceria stabilized with 10 mole % La. Three-way activity comparisons between catalysts promoted by stabilized and unstabilized cerias show significant activity penalties for La and Nd-stabilized ceria-containing catalysts after air oxidations for four hours at 980 "C. X P S characterizationof La- and Nd- stabilized cerias has revealed significant surface enrichment of ceria by La and Nd under high temperature oxidizing conditions. This ceria encapsulation is believed responsible for the observed loss in activity. INTRODUCTION

During the last five years, much of the focus of automotive exhaust catalysis research has been on catalyst thermal stability. This emphasis stems in part from the increasing number of vehicle applications that utilize closecoupled catalyst locations, in order to minimize vehicle emissions during cold starts. At high speeds and high loads, these close-coupled locations subject catalysts to temperature extremes that can often be in excess of 900 "C. Operation of three-way catalysts at these high temperatures can result in significant activity losses due to the large scale sintering of the alumina support, sintering of the noble metals, and sintering of other catalyst components including the cerium oxide promoters. Numerous recent studies have characterized this thermal deactivation process in terms of support surface area loss, or noble metal and cerium oxide crystallite growth [I-41. Improvements in three-way catalyst thermal stability have resulted from the incorporation of stabilized aluminas and high levels of cerium oxide promoters. Alumina stabilization has been the subject of several studies with La203 stabilization being generally preferred [5-81.

126

The increased levels of ceria present in three way catalysts have focused some attention on ceria physical properties and thermal stability in these systems. Both the patent literature [9-121 and the scientific literature [13,14] contain references to the synthesis of high surface area Ce02 or the stabilization of ceria for auto catalyst applications. In one recent study Miyoshi et al. [ 131 provided evidence for improved three-way catalyst thermal stability by incorporating lanthanum oxide in both the alumina support and in the ceria promoter The high ceria loadings currently used in three-way catalyst formulations are most readily achieved by physical mixing of bulk cerium oxide with alumina. In order to maximize the catalytic properties of the ceria additive, a high surface area is clearly desirable. On the other hand, the ceria should be resistant to sintering under high temperature conditions. In this paper we discuss the sintering characteristics and surface chemistry of stabilized and unstabilized ceria, and the performance of stabilized ceria in three-way catalysts.

Experimental Details Stabilized cerium oxides were prepared by co-precipitating aqueous solutions of cerium nitrate and the corresponding nitrate of the stabilization agent with a dilute NH40H solution. Precipitations were performed at 85 "C and at a constant pH=9. Stabilization agents investigated included La, Y, Nd and Al. After precipitation, the resulting mixed hydroxides were washed with deionizing water, dried at 120 "C and then calcined for one hour at 540 "C to yield a cerium oxide composite. Stabilized cerium oxides were also prepared by an incipient wetness impregnation of a commercially available high surface area ceria obtained from Novamet Specialty Products Corp. [Wyckoff, NJ U.S.A.] with nitrate solutions of either lanthanum or neodymium. After impregnation, the powder was dried at 120 "C and then calcined for one hour at 540 "C to yield the mixed oxide. XPS characterization of materials was carried out using a Perkin-Elmer 5400 Series ESCA instrument, operating with 300 W of Mg K a radiation. A hemispherical electron energy analyzer was used, giving an energy resolution of 1 eV [on Ag 3d5/2] at a pass energy of 18 eV. The spectrometer energy scale was calibrated with respect to the Cu 2 ~ 3 ~Au 2 , 4f712, and Ag 3d5/2 core levels at 932.7 eV, 84.0 eV, and 368.3 eV, respectively. Powder samples were examined by dusting them onto double-sided adhesive tape, mounted into a stainless steel sample stub. Charge correction was performed using the (CH), peak at 284.6 eV. Surface concentrations were calculated using PHI sensitivity factors.

127

Three-way activity measurements were made using a simulated exhaust gas reactor system described elsewhere [ 151. This simulated exhaust stream contained 20 vppm S2, 400 vppm hydrocarbons (3/1 molar ratio of propene/propane) , 1850 vvpm NO, 10 ;ol.% H20, 14,5 vol.% CO2, and appropriate levels of CO and 0 2 to cover the stoichiometric range of interest. The reactor system included provisions for oscillating the feed gas stoichiometry at 1 Hz to simulate closed loop operations typical of vehicle conditions. Conversion levels of hydrocarbons, CO and NO, were measured at 480 "C over a range of air/fuel ratios using a gas hourly space velocity of ca. 40,000 hr-1 (at STP). In these activity studies three-way catalyst washcoats were prepared by adding bulk cerium oxide or bulk stabilized cerium oxide to a La203-stabilized alumina that contained Pt and Rh (introduced by impregnation). This mixture was wet ball milled and then coated onto a corrugated 0.05 mm metal foil (Allegheny Ludlum, Alfa IV alloy). The coated foil was then spirally wound and inserted into a thinwalled stainless steel tubular case to form a sample 2.54 cm dia. x 2.54 cm long (total catalyst volume of 12.9 cm3). Noble metal concentrations on the washcoat used for these samples were chosen to yield 0.71 g/liter loadings with Pt/Rh=5/1 by weight. CERIASTABILISATION Stabilized cerias made by the standard co-precipitation process described in brief above were subjected to a high temperature calcination (one hour in air at 980 "C) to determine the surface area stability of these materials relative to a precipitated ceria containing no additives. Table 1 compares BET surface areas of these oxides as prepared (with 10 mole % stabilizer, calcined to 540 "C) and after the high temperature calcination. Included in this table are also surface area results for a commercial, high purity ceria. As indicated by the data in Table 1, the precipitation process used here was successful in producing ceria and mixed oxides with surface areas in excess of 70 m2/g after a 540 "C calcination. The second important result is that the incorporation of La, Nd and Y into the ceria matrix significantly improved the high temperature stability of the mixed oxides in comparison to the pure ceria made by precipitation or the commercial ceria. In all three cases La-, Nd- and Y-doped cerias exhibited surface areas in excess of 25 m2/g after this severe calcination. A1 incorporation into the ceria matrix resulted in some improved thermal stability relative to pure ceria but not to the same extent as La. Nd or Y stabilization.

128

TABLE 1 - BET Surface Area Data for Stabilized Cerias

* - including La and Nd Since the L a 2 0 3 / C e 0 2 composite provided the highest degree of surface area stability for the dopants examined here, a more detailed study was made of the effect of lanthana content on the thermal stability of this mixed rare earth oxide. Figure 1 compares BET surface areas for La203/Ce02 made by this constant pH precipitation as a function of the La content of the mixed rare earth oxide. The 10 mole % La level used in the initial investigations is close to the optimal stabilizer level for maximizing thermally aged surface area. This maximum, however, does not appear Mole 56 La sharp, with La levels between 5 and Figurel: BET areas for La203-Ce02 20 mole % yielding BET areas in co-precipitated mixed oxides excess of 25 m2/g after calcination at 980 "C. As-prepared surface areas also appear to track with thermally aged areas, with the highest surface area after 540 "C corresponding to about 10 mole % La. In the composition region between 0 and 20 mole % La, the two rare earth oxides form a solid solution, with Ce02 being the only crystalline phase present in the X-ray diffractograms of these solids, even after air calcinations to 980 "C. Both pure ceria and pure lanthana made by precipitation exhibited poor thermal stability relative to the stabilized La203/Ce02 composite.

129

THREE-WAY ACTIVITY Washcoats were prepared using the stabilized La203/Ce02 material described above in order to determine if the improved sintering characteristics of this material resulted in improved catalyst thermal stability as measured by three-way conversion levels after high temperature aging. As described previously, a standard washcoat formulation was used in these studies. In comparative evaluations, all washcoat ingredients were kept constant except the nature of the bulk ceria additive. The amount of ceriabased additive was also kept constant at 25 wt% on a washcoat-only basis. Before proceeding with - any high temperature treatments, the activities IConversion at Stoich. Conditions of all samples were evaluated at our 100 standard three-way conditions in the as-prepared state which included a Activity measured at 48OOC 80 mild calcination in air at 500 "C. All samples gave equivalent fresh conversion levels with respect to 60 hydrocarbons, CO and NOx. In the first thermal stability comparison, catalyst activities were 40 compared for a washcoat containing a relatively pure commercial ceria and 20 a coating containing a lanthanastabilized ceria (10 mole % La) made by co-precipitation. Catalyst samples 0 CO NOx Hydrocarbons were aged for four hours at 980 "C in air to simulate a severe high Unstabilized ceria La-stabilized ceria (,o La, co-precip.) temperature oxidation in a lean exhaust. Figure 2 compares threeFigure 2: Three-way activity way conversion levels for these two comparisonfor PtlRh washcoats after samples in a stoichiometric simulated 980 "C calcination ( 4h ). exhaust after this high temperature calcination. In all cases the catalyst prepared with the stabilized ceria showed lower conversion levels. Activity comparisons of these two catalysts were also made over a wide range of aidfuel stoichiometries typical of the range found under normal vehicle operations and again in all cases the stabilized ceria washcoat exhibited performance below that observed for the unstabilized ceria washcoat. Based on this result a more detailed study was undertaken to investigate stabilized ceria performance in high temperature environments after both lean and stoichiometric aging schedules. In these experiments both lanthana~

130

stabilized ceria and neodymia-stabilized ceria additives were compared against the commercial, high purity ceria-containing standard catalyst. In the case of stabilized ceria-containing catalysts, several levels of lanthana and neodymia stabilizers were evaluated. In each case the stabilizer was added to the high purity ceria by impregnation using an appropriate aqueous nitrate solution. Although not detailed here, this stabilizer impregnation scheme resulted in similar improvements in ceria surface area stability in high temperature calcinations as reported for the co-precipitation scheme. Figure 3 compares CO conversion levels in a stoichiometric simulated exhaust of the stabilized and unstabilized ceria-containing% CO Conversion at Stoich. Conditions catalysts after a four hour calcination ' O 8 1 at 980 "C. Both the lanthanastabilized and neodymia-stabilized ceria catalysts show performance Activity measured at 480% drop-offs with increasing levels of 60 stabilizer, confirming the poor Nd-Ceria Promoter performance observed for the coprecipitated L a 2 0 3/CeO2. CO 50 conversion levels for catalysts with about 10 mole % ceria stabilizers 40 were only about one-half those of the standard catalyst with unstabilized La-Ceria Promoter ceria. Conversion levels for hydrocarbons and NO, showed similar trends to the CO data of 20 8 IO 12 Figure 3. Mole % La or Nd A similar comparison was made using lanthana and neodymiaFigure 3 : co activity ComParisonfor stabilized cerias after heat treatments PtIRH washcoats after 980°C in a stoichiometric gas mixture calcination ( 4 h . ) . containing CO, 02, N2 and H20.

In these experiments, catalyst samples were treated for 95 hours at 960 "C in this flowing gas mixture and then evaluated for three-way activity. Figure 4 compares stoichiometric conversion levels for stabilized and unstabilized ceria-containing catalysts after this stoichiometric high temperature treatment. Unlike the air calcined samples, stoichiometrically treated, stabilized ceria-containing catalysts performed on a par with their unstabilized counterparts.

131

YO Conversion at Stoich. Conditions

80 I

Activity measured at 480°C

60

40

20

0

co

Hydrocarbons

=

Unstabilized c e r i a

: ,L

1 La-stabilized

ceria ( 6 mole % L a )

N Ox N d -s t a b i Ii ze d c e r Ia (5 mole Yo N d )

Fig.4: Three-way activity comparisonfor PtlRh washcoats afer 960 “C stoichiometric aging ( 9Sh ).

980 960 940 920 900 880 860 840 820 800 780 Binding Energy (eV)

Fig 5: Ce 3d and La 3d XPS signals for La-stabilized ceria (ca.10 mole% La)

132

XPS CHARACTERIS ATION In an effort to explain the poor three-way performance of the lanthanaand neodymia-stabilized ceria-containing catalysts after high temperature calcinations, these additives were characterized by XPS for relative surface compositions. The stabilized cerias containing approximately 10 mole % La or Nd (made by impregnation of the commercial ceria) and the high purity, commercial ceria were analyzed as-prepared (following calcination for 1 hr at 540 "C) and after a four hour calcination at 980 "C. Cer ia- 10% - Neod y mia h

v) c .-

C

3

2

v

x c .-

cn

K a, C

c

K

.-0 v) v)

._

5

0 0

Ce 3d

c

r

a

hv

1253.6 eV

= I

,

I

,

I

,

I

,

I

.

I

,

I

,

I

~

I

.


Fig. 6: Ce 3d and Nd 3d XPS signals for Nd-stabilized ceria (ca.lOmole% Nd) Figures 5 and 6 compare signals in the 3d region for the lanthana- and neodymia-stabilized cerias, respectively. Both the stabilized cerias show significant surface enrichment of the stabilizer oxide relative to Ce02 after the 980 "C calcination. In both cases the Ce 3d signals have decreased and the La or Nd 3d signals have increased relative to the as-prepared oxides after the severe thermal treatment in air. An even more striking indication of this surface enrichment is shown in Figure 7a. In this figure the 0 1s signal for the lanthana-stabilized ceria is compared for the as-prepared oxide and after calcination at 980 "C. After the high temperature calcination the oxide exhibits a significant enlargement of the higher binding energy oxygen signal centered around 531.5 eV. A similar behavior was observed for the neodymia-stabilized ceria.

133

Ceria-10%-Lanthana

0 Is hv = 1253.6 eV


Fig. 7a: 0 I s XPS signals for La-stabilized ceria (ca. 10 mole% La ).

Ceria

0 1s hv = 1253.6 eV

I

,

I

,

I

,

I

,

I

.

I

,

I

,

I

.

I

,

0 538 536 534 532 530 528 526 524 522 5: 0

Binding Energy (eV)

Fig. 7b: 0 I s XPS signals for a commercial, high purity ceria.

134

Figure 7b shows 0 1s signals for the commercial ceria after the same two calcination treatments. In contrast to the stabilized ceria data, these spectra do not show an increase in the higher binding energy oxygen signal. This higher binding energy oxygen signal is consistent with the more reactive nature of lanthana and neodymia than ceria towards C02. Both lanthana and neodymia readily form surface carbonates in the presence of C 0 2 181 and the higher binding energy oxygen signal is indicative of a surface carbonate species. As the ceria surface becomes enriched with either lanthana or neodymia, these carbonates become more abundant in the presence of C02, and the higher binding energy oxygen signal increases.

0

Binding Energy (eV)

Fig. 8: 0 I s X P S signals for La203 and h2(co3)3reagents. Confirmation of this carbonate assignment comes from XPS scans of reagent La2O3, reagent La2(C03)3, and La203 heated to 900 "C in low-CO2 air and transferred to the spectrometer under nitrogen. 0 1s signals for these materials are summarized in Figure 8. The oxygen signals of the two reagents are nearly identical with a peak at about 531 eV (similar location as the higher binding energy oxygen signal in Figure 7). This suggests that the lanthana is covered with a carbonate overlayer. The heated lanthana, on the other hand, shows a reduced carbonate band at 531.5 eV due to the decomposition of the carbonate overlayer and a lower binding energy oxygen signal that can be assigned to oxidic surface oxygen.

135

D IS c US SIO N The results of this investigation indicate that additives such as La, Nd, Y and to some extent A1 are capable of retarding ceria surface area loss after high temperature oxidations. The mechanism of this stabilization was not specifically addressed by this study but some inferences can be made about the ceria stabilization process. In the case of co-precipitated, lanthana-stabilized ceria, surface enrichment of the lanthana was observed by XPS even after drying and calcining the mixed hydroxides at mild temperatures (540 "C). La/Ce surface atomic ratios obtained from XPS for Ce02 containing 10 mole % La were about 70% higher than the levels associated with the bulk composition of this material. The La/Ce surface ratios were even larger for samples made by impregnation. TABLE 2 - Bulk and Surface Atomic Ratios for La-Stabilized Ceria

Co-precipitated La203/Ce02 Impregnated La203/Ce02

La/Ce, Surface

La/Ce, Surface (after 980 "C)

0.1 1

0.19

0.25

0.12

0.25

0.71

Table 2 summarizes La/Ce ratios for these materials. This lanthanarich surface phase may be associated with ceria grain boundaries or reactive ceria surface sites that initiate the sintering process. The lack of any discernable La-containing phases by X-ray diffraction before or after calcination to 980 "C is also consistent with a highly dispersed stabilizer phase. The presence of the lanthana-rich surface phase is then postulated to impede the growth of ceria particles at high temperatures. Similar surfacestabilizing phases such as LaA103 have been characterized as being responsible for the stabilizing effect of La on aluminum oxide [7,8]. Although effective at retarding ceria surface area loss, the rare earth stabilizers appear to drastically alter the promoter action of ceria in threeway catalysts following severe oxidations such as the 980 "C calcinations used in this study. The poor activity observed for the stabilized ceria-containing catalysts after high temperature calcinations appears directly related to the significant enrichment of non-cerium oxides on the surface of the bulk ceria additives that also occurs under these conditions. As shown by the XPS results, the resulting stabilized ceria surface becomes more like neodymia or lanthana in composition and reactivity. For comparison, a catalyst sample was prepared with a pure, low surface area lanthana in place of the ceria or stabilized ceria (all other catalyst details kept constant, including noble metal

136

content and total rare earth loadings). After a similar four hour calcination at 980 "C the lanthana-only promoted catalyst had almost no measurable conversion (99.995%) are used without any further purification.

155

* Light-off - TPO - TPR. These tests are performed under helium at a flow rate of 100 cc/min (VVH=2350 h-1). First of all, the fresh catalyst is heated in He up to 700°C (10"C/min) for l h in order to dehydroxylate [9] the alumina support (H2O is trapped in TR1) and to avoid formate formation during the subsequent CO treatment [lo], then it is cooled at room temperature (R.T.), reduced in l.S%CO/He from R.T. up to 700°C (lO°C/min) plus l h isothermal at 700"C, purged in He and cooled back to R.T..

T W

FI

Figure 1: experimental apparatus. FC: mass flowcontroller TRi,TRz : traps TCD: thermalconductivity detector c:catalyst F furnace S : sampler I.R.: CO analyser F1: flow meter

After this pretreatment procedure, the catalyst surface is expected to be "clean and in a reproducible reduced state". The catalyst is then subjected to a sequential temperature programmed oxidation/reduction procedure, as shown in figure 2.

Two types of oxidation are performed, depending on the oxidizing atmosphere composition. The first one, referred as "light-off', is carried out

156

under a gas mixture containing 1.5%CO-1.5%02/He, i.e. in excess of oxygen according to the stoichiometry of CO oxidation. The second type of oxidation (noted "TPO") is simply conducted under 02/He (under various Po2). The thermal treatment is the same for both oxidations: after getting, at room temperature, a stationary outgas composition, the temperature is rised up to 700°C at a constant rate of 10"C/min, and kept l h isothermal at 700°C. The oxidative atmosphere is then sweeped out by pure He before catalyst's cooling. At the end of the oxidation step, the catalyst surface is more or less covered by different kinds of adsorbed oxygen atoms, which are characterized by a subsequent thermo-programmed reduction (see fig. 2). The TPR is performed under a flow of lS%CO/He, by the same procedure as for the previous oxidation: (i> stabilisation of the CO level at R.T., (ii) heating (10"C/min) until 7OO"C,(iii) l h isothermal at 700"C, (iv) purge with He, and (v) cooling back to R.T.. After this first oxidation /reduction cycle, the catalyst can be subjected to another cycle, similar to the leadrich atmospheres in the exhaust pipes, either to test the reproducibility, or to study the influence of the oxidative atmosphere ( Po2, presence of CO ) on the TPR results. Considering the widely used catalytic test of light off, let us stress the fact that, in our particular conditions, it is not possible to take into account this activity characterisation. Indeed, because of our very low space velocity, the temperature at which 50% of CO is oxidized into C 0 2 is in the range 100140°C for all catalysts, i.e. almost within the experimental error of the temperature determination. Furthermore, our apparatus is not equipped either with C02 nor with 0 2 detectors, and the advancement of the reaction is only followed by the desapearence of CO in the effluent gaz. Consequently, our light off curves consist of the superposition of two phenomena: (i) CO oxidation and (ii) CO adsorption on the catalyst surface. For these two reasons, the oxidation step of the procedure is not studied by itself, but our attention is focussed on its influence on the TPR results.

* Hydrogenolysis of Methylcyclopentane. This molecule is used as an "in situ" chemical test as this hydrocarbon reaction is structure sensitive. The aim of this study will be to point out the possible surface modifications that may occur during high temperature treatment (700°C) under various atmospheres: He, CO/He or 02/He. Therefore, the hydrogenolysis investigations are undertaken by reproducing the successive steps of our oxidation/reduction cycle i.e.: 1. fresh catalyst 2. neutral treatment: in He, RT+70OoC, 10"C/min + l h isothermal at 700°C " 3. reducing treatment: in 1.5%CO/He 4. oxidative treatment: in 5%02 /He '6

'6

157

After each step, the catalyst is reduced overnight with H2 at 300”C, and then tested using the methylcyclopentane (MCP) hydrogenolysis reaction. This catalytic test is also investigated with the apparatus described in figure 1,but with pure H2 as carrier gas and a flow rate of 71 ml/min. 10 pl MCP (puriss: >99.5%) are injected in TR2 which is maintained at -38°C in order to get a constant vapor pressure of MCP in the H2 flow. The reaction temperature is fixed at 200°C. After the passage of MCP on the catalyst (detected with the 2nd TCD), 5ml of the products are sampled in S and analysed on an independant FID chromatograph apparatus (DC 200 column).

Physical characterization. - The Pt catalysts’s dispersion is determined by H2 chemisorption at R.T. using a pulsed technique on prereduced catalysts (450°C under 10%H2 /Ar). - The mean Pt particle sizes are measured by the Pt line broadening of X-ray diffraction spectra. RESULTS AND DISCUSSION. TPR results. As mentioned previously, the temperature-programmed reductions are performed under a mixture of lS%CO/He, therefore they reflect the ability of the different type of surface oxygen to be reduced by CO. Generally speaking, the TPR spectra are reproducible and do not evolve with the number of the oxidatiordreduction cycle (for some catalysts, up to 10 successive cycles have been performed). This means that after our pretreatment procedure, the catalyst has reached a stable state. The effluent composition is analysed by two methods: (i) the %CO is measured by the IR detector and (ii) the second TCD gives a global composition of the gas. These data are entered in a computer and converted to plots versus temperature. Moreover, the TCD scale is recalculated knowing that after the stabilization of the effluent composition at R.T., both detectors are measuring 1.5%CO. As a result of this computer treatment we get two curves noted respectively “CO by IR” and “TCD” (for example on Fig.3). Assuming that the only reaction occuring during the TPR is the oxidation of CO in C02 , the difference between the two curves corresponds to the level of CO2 in the effluent. This calculated signal (noted “TCD-CO”) is represented on the graphs (see Fig.3 for example) but one must keep in mind the assumption that no parasite reaction will occur, and that this curve is drawn in arbitrary units.

158

Catalvsts without ceria. *0.2%PtIA1203. Platinum oxide is known to be unstable at 700°C even under an oxidative atmosphere [ 111, therefore under our experimental conditions this catalyst does not exhibit any reduction peak in the range 25400°C after the oxidation step either under 0 2 /He or under C 0 + 0 2 /He. The only CO consumption is very small and occurs after 600°C. Therefore we can exclude CO disproportionation as an important reaction below this temperature, probably because of the low CO% used. *1.06%Pd/A1203.In contrast with platinum, PdO is stable at 700°C in an oxidative environment, and the TPR spectra presents many peaks depending on the previous oxidative treatment as shown in Fig.3(a and b). At present, we do not have a proven assignment for the oxygen species associated with each one of the peaks, but it is worth noting that the relative peak intensities are affected by the presence (Fig.3.a) or the absence (Fig.3.b) of CO in the oxidative atmosphere. On the basis of this observation, adsorbed CO is thought to induce a surface restructuration of palladium particles [ 121 leading to a modification of the oxygen species distribution. Fig.3.b TPR of 1.062Pd/AlZOS (aft- dW0n 15202)

Jooad 2JQM)

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8

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- CO bylR

21%

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-- TCD -.‘“m-co

st&

so0

7bo

lc3 ‘V .(.=\cpCO

*0.2%Pt-l.7%PdIA1203. T h e data are presented in Fig.4 which have to be compared to the equivalent Pt-free spectra of Fig.3.a. One can notice that the peak positions in Fig.3.a remain almost unchanged by the addition of 0.2%Pt but that their intensity decreases markely. This effect is attributed to the existence of an interaction between the two precious metals and points out the real bimetallic character of the catalyst.

159

8

B

Catalysts containing ceria. *14S%CeIA1203. The shape of the spectrum (Fig.5) indicates a wide peak between 300 and 550°C accompained by an unresolved peak on the high temperature side. These results are very close to those obtained by using the classical TPR performed under hydrogen [4,12-131. *0.52%Pt-13.3%CelA1203. As previously mentioned, platinum reduction does not occur during our TPR experiments. The spectra in Fig.6 is thus only representative of ceria reduction. By comparing Fig.5 with Fig.6, the presence of platinum appears to promote the reduction of “capping oxygen” of ceria [12] (shift of the first peak from 500°C down to 350°C).In the case of TPR under H2, the same effect is observed [4,12-131 and could be attributed to H2 spillover on Pt particles, as proposed by Boudart [14] for the reduction by H2 of W 0 3 catalysts containing Pt. Taking into account this explanation, one might expect Pt to have no effect on ceria reduction by CO, which is not the case. Therefore, and in agreement with Yao [13], the results reported here are explained in terms of an interaction between Pt and ceria leading to the destabilisation of the Ce-0 bond. *I .51 %Pd-9.8%CelA1203. This W.7 TPR Of 1.51md-Q.BzCe/U203 catalyst exhibits many unresolved (it* an) TPR peaks as sketched in Fig.7. From 50000Fig.3.a (1%Pd/A1203) and Fig.5 2soOo. (14.5%Ce/A1203), one can deduce that the low temperature side of the spectra (T773K.

INTRODUCTION

In a series of papers (1-4) we have previously examined the RhRi02 system from the point of view of the interaction of the noble metal with the reducible Ti02 support after reduction with hydrogen at ca. 773K what leads to a "Strong Metal Support Interaction" state (i.e. SMSI), a phenomenon first described by Vannice et al. ( 5 ) more than a decade ago. In principle, this SMSI-state greatly enhances the selectivity of Rh/Ti02 catalysts to give oxygenates in Fisher-Tropsch synthesis (i.e.COLH2 reaction) (6,7) as well as their catalytic activity for NOx reduction (i.e. NOx/CO reaction) (8). From this previous work on Rh/Ti02, we have concluded that hydrogen is incorporated into the Ti02 support during the reduction step in the preparation of the catalysts (2,4) producing, in a first stage, a partially reversible (upon outgassing) SMSI-state, which prevent H2 and CO adsorption on the metallic rhodium. The transfer of such hydrogen involves "spillover" from the metal to fill the oxygen vacancies mainly generated at the metal-support interface during reduction, thus leading to the formation of hydride-like species (i.e. TiH3+). In a second stage at higher temperatures, these species also promote a great mobility of the support favouring the decoration of the metal particles with TiOx(H) moieties which strongly

208

interact with the rhodium probably through the same hydride species (9). In principle, Ce02 is a much easily reducible oxide than Ti02 and it is well known (10) that when heated in hydrogen at T800K) while H2 (m/e=2) and traces of CH4 (m/e=15) cou1.d be detected in the gas phase during all the TPD runs.

212

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400

600

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Fig.4.-(A) T P Dfrom RhlCe02-H and HV samples crfter interaction f o r 30 min. at 400K with C0+02 -(B) The same f o r Ce02HV.

213

However, the previous removal of hydrogen greatly enhances the amount of CO desorbed from sample HV, which gives now a well defined peak at 525K and a very strong increase in the CO pressure from T>800K. The new peak at 525K should be ascribed to CO adsorption on the metallic rhodium, which appears now enhanced by the removal of hydrogen from the support (loss of the SMSI state (7)) However, the other form of adsorbed CO in this Rh/Ce02-HV sample is very different from that recorded for a similarly reduced Ce02 support, shown in Fig.3(B), where only a sharp single peak at 800K occurs. When the above TPD-profiles for CO are compared with those obtained after interaction with the C0/02 mixture in Fig.4(A), several new features appear. First at all, TPD desorption profiles of CO are now very similar for both samples, indicating that the presence of 0 2 (0.1 ton-) in the gas mixture has removed the SMSI-state induced by the hydrogen incorporated in sample Rh/Ce02-H. Secondly, besides the CO, a considerable amount of C02 is now also desorbed from both samples. However while for sample Rh/Ce02-H this C02 appears as two well defined maxima at ca. 525K and >900K, for sample HV a third broad peak centered at ca. 730K is observed. The position of this peak coincides with that of the single one recorded in the TPD spectrum of the Ce02 support which had been under CO/O2 under similar conditions, as shown in Fig. 4(B). Finally, the comparison of the intensities of the TPD-profiles after adsorption of CO and C0/02 indicates that, as a whole (i.e. evolved as CO+CO2), the presence of a small pressure of oxygen has greatly enhanced the adsorption of CO, which is now being desorbed either as such or as C02 during the TPD experiments. XPS spectra for sample HV, corresponding to the two set of TPD experiments described above, are shown in Fig.5 and were very similar to those recorded for sample H. In this figure we can observe that adsorption of CO does not modify the Rh(3d) and Ce(3d) peaks. However, when the samples have been in contact with the C0/02 mixture a total oxidation of Ce3+ readily occurs while the rhodium is almost completely oxidized to Rh+/Rh3+, the amount of Rh3+ being somewhat higher for sample HV than for sample H. By comparing data in Fig. 5 and Fig. 2, we can conclude that, in the case of Rh/Ce02-HV, a deeper oxidation of the metal to Rh3+ occurs when CO is present together with 0 2 , in spite of the much less oxidizing conditions (i.e.C0/02 vs 02). This suggests that CO plays a certain role in the stabilization of this oxidation state of the rhodium in our Rh/Ce02 catalysts.

214

315 311 307 B.E. /eV

913 901 889 R . E . /eV

315 311 307 B.E./eV

913 901 B . E . /eV

FigS.-Rh(3d) and Ce(3d) XPS-spectra of RhICe02-HV: -Right: after interaction with C0+02.-43:with CO (see text).

Rh/Ti02 interaction with 0 2 and C 0 + 0 2 In order to try to go further in the comparison of these RhKe02 catalysts with the R W i 0 2 samples, previously studied by us (1-4), a similar set of experiments with 0 2 and C0/02 were carried out using the same Rh/TiO2 catalyst (either in its H and HV state). TPD experiments show in this case a single broad peak with maxima at ca. 625 and 675K for CO and C02 respectively for both samples. Meanwhile, as shown in Fig. 6, reduction of the Ti02 support after heating with hydrogen at 773K cannot be detected by XPS (though Ti3+ ions can be recorded by EPR (3)).This fact indicates the lack of a massive reduction at the surface layers of the Ti02 support in our conditions, in contrast with the observed reduction of the ceria support. On the other hand, the Rh(3d) XPS-spectra in the same figure show that, while for sample Rh/TiOz-H oxidation of the metal does not occur either with 0 2 or with C0/02, a small amount is oxidized by the C0/02 mixture in sample Rh/Ti02-HV, thus suggesting again that removal of hydrogen from the Ti02 support leads to a somewhat easier oxidation of the metallic phase. To check further the role played by oxygen vacancies in this process one experiment was made using sample Rh/TiOa-H and doing a very soft

215

sputtering with Ar+ ions (30 s.,0.5 Kv) to generate a small amount of oxygen vacancies at the Ti02 surface by differential sputtering of oxygen (9). As we can see in Fig. 6 , this very soft etching produces Ti3+ (i.e. TiVo3+ sites) at the uppermost layers of the Ti02 surface. When 0 2 was introduced at 300K in contact with this sample, a broad shoulder corresponding to Rh+/Rh3+, could be clearly observed in the Rh (3d) XPS spectrum, while the small shoulder due to Ti3+ disappeared from the Ti(2p) spectrum. Furthermore, when the same experiment was made using the C0/02 mixture a much deeper oxidation of the metal into Rh3+ occurs, as shown in the same figure thus clearly indicating the promoting effect of the CO in the oxidation of the rhodium up to Rh3+. _ Rh (3d) Rh (-1 Ti ( 2 p ) a) m:02

1 b) ADS:CO+02 /I

I!

c) REDUCED

(hr+-aichd)

312

304 B . E . /eV

316

312

308

304

B . E . /ev

B . E . /eV

Fig.4.-Rh(3d) and Ti(2p)XPS-spectra of RhlTi02: -(a)after interaction with 0 2 ; - ( b )after interaction with CO +02; -(c) effect of Ar+ etching on the Ti(2p) spectrum. DISCUSSION The comparative assessment of the results for Rh/Ce02 above with those obtained for RhfTi02 in our previous papers indicate differences in the behaviour of the two systems which can be ascribed to the degree of reducibility of both supports and their further interactions with 0 2 and CO. One of the most important differences observed is in fact the partial

216

reduction of the surface of the Ce02 support at 773K in H2 as compared with the lack of such deep reduction in the Ti02 (i.e. compare XPS in Figs. 1 and 6). We had previously found (9) that hydrogen reduction of Rh/TiOz is accompanied by an enhanced ionic mobility of the oxide lattice leading to Ti3+ migration toward the bulk while oxide ions diffuse backward to the surface, thus reaching a rather oxidized steady-state at the surface. Since, in principle, oxide mobility in Ce02 should be faster than in Ti02 its rate of reduction with H2 should be much higher to exhaust the surface ,thus leading to a deeper reduced steady-state within a thick layer at the surface of the oxide. As it is well known, TPR of ceria (12) gives two reduction peaks, one at ca. 773K ascribed to surface reduction while the other at 1073K is due to reduction of the bulk with formation of lower oxides of cerium. The addition of rhodium seems particularly effective in promoting the surface reduction of Ce02 down to ca. 625K (12). So, in our experimental conditions a deep surface reduction should be expected as found by XPS. Meantime, in our reduction conditions hydrogen must be incorporated to the reduced oxide as previously found by Fierro et al. (10). Part of this hydrogen is probably incorporated by spillover to oxygen vacancies (Vo) near Ce3+, as we have previously found in Rh/Ti02 (4), according to: (CeVo)3++ Rh-H

**

(CeH)3+ + Rh

(1)

many of them remaining close to the interface with the support. XPS spectra in Fig. I show that removal of this hydrogen does not produce any noticeable change in the Rh(3d) or Ce(3d) peaks except for a small broadening of the former what is also observed for the Rh/Ti02 catalyst in Fig. 6(C). Nevertheless, as we had previously found for R m i 0 2 (7) its loss leads to an increase in the capacity for H2 and CO adsorption on the metal due to a partial recovery of the non-SMSI state. This fact is actually observed in Figs.1 and 3 where TPD profiles indicate the existence now of peaks at ca. 410K and 500K for H2 and CO, respectively, due to the adsorption of these two species of the rhodium particles. The reduced state of the support in RWCe02 (H and HV) is completely removed in contact with a small amount of 0 2 at 300K as shown in Fig. 2 for the sample HV. It is worthy of note that even at 300K the Ce3+ becomes completely oxidized while simultaneously an important amount of the rhodium is oxidized to Rh+ what contrast with the stability of this metal under similar conditions in Rh/Ti02 (14). This fact suggests that oxygen should be incorporated to the reduced ceria according to:

217

ce4+-0;-m+

(3)

in fact, formation of 0;and 0; has been observed on reduced Ce02 by EPR (10) and m - I R (15) after 0 2 adsorption showing a great reactivity even at T800K. This new form of adsorption does not occur in the Ce02 support where only a single sharp peak at 800K is observed (see Fig.3B) which is less intense in sample H where oxygen vacancies should be filled by hydrogen. These two facts suggest that the presence of rhodium generates a new type of very stable adsorbed CO which, in principle, might be tentatively ascribed to CO interacting with both Rh and CeVo3+ sites at the metal support-interface, as previously suggested by Burch et a1 (16) for NiRiOz catalysts and recently by Lavalley et al. (17), to explain the broad i.r.-band centered at ca. 1725 cm-1 which appears when Rh/Ce02 catalysts are contacted with CO. XPS spectra in Fig.5 indicate that the interaction of CO with these two types of sites occurs without redox exchange since both Rh(3d) and Ce(3d) peaks remain unchanged in these spectra. The absence of the TPD-peak of CO characteristic of the CeO2 support at ca. 800K in the two Rh/Ce02 samples deserves some further comment. Since, in principle, a deeper reduction should be expected in the later (12) it is likely that oxygen vacancies far from the metallic particles would rearrange either forming "shear planes" or migration toward the metallic particles thus reducing their number in sample HV, while they will remains filled with hydrogen in sample H. So, they will not be ready to adsorb CO in our deeply reduced RhKe02 Catalysts. However, when the two Rh/Ce02 samples have been in contact with the C 0 / 0 2 mixture , TPD profiles in Fig.4B first indicate that oxygen is able to remove the SMSI-state of the metal in sample H, which now adsorbs CO on the rhodium (TPD-peak at 500K) as well as on the sites at the metal support interface as also does sample HV. In addition a certain amount of this CO is now oxidized to C02 in these two types of sites giving TPD-peaks at 525 and 900K. However, it is worthy of note that a third TPD-peak of C02 appears at ca. 730K only for the sample Rh/Ce02-HV in these conditions. This peak would correspond to the one observed for the oxidation of CO on the reduced Ce02 support. This fact suggest that in sample H CeH3+ sites far from the metallic particles cannot be activated by C 0 / 0 2 in our experimental

218

conditions while sites CeVo3+ in sample HV are activated by the 0 2 . Looking at the XPS spectra in Fig.5 we can see that the COIO2 mixture reoxidizes all the CeVo3+ at the support while part of the rhodium is also oxidized mainly to Rh3+. A comparison of the XPS spectra in this figure with those included in Fig2 indicates that, after interaction with 0 2 at 400K,oxidation of the metal to Rh3+ is much more important when CO is also present in spite of the much less oxidizing atmosphere in this case. This suggests a promoting effect of the CO in the stabilization of this oxidation state, though, as shown by the XPS spectra recorded after the TPD runs, the initial reduced state of the Rh/Ce02 surface is completely restored as could be expected in the cyclic catalytic process. This fact can be explained assuming that, in the presence of CO, the reaction (3) above progress according to:

the initial state of the catalysts being restored, at high temperatures, according to: A Ce4+ O= O= Rh3+(C0)2 (CeVo)3+ + Rh + 2C02 (5) TPD The importance of oxygen vacancies at the support (i.e.CeVo3+ sites) close to the rhodium particles in the redox processes involved in the oxidation of CO with 0 2 at high temperatures can be better observed on Rh/TiO2 due to the greater stability of Ti02 for deep hydrogen reduction. In fact, the results in Fig.6 confirm that 0 2 alone is much less efficient than the C0/02 mixture to partially oxidize the rhodium particles and that the presence of CO stabilizes mainly Rh3+. Moreover, the presence of Ti3+ in the Ti(2p) spectrum of the Ar+ sputtered sample, which indicates the formation of oxygen vacancies ( i.e. TiVo3+ sites), enhances considerably the oxidation of the rhodium to Rh3+ particularly in the presence of CO. In our view these results gives a clear evidence of the important role played by oxygen vacancies at the metal-support interface in the redox chemistry of the rhodium, and probably other noble metals, in these type of catalysts under CO oxidation conditions.

...

*

Acknowledgements Authors thank the CICYT project nr.MAT88-223 for financial support.

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REFERENCES 1 -G.Munuera, J.C.Conesa, A.Muiioz, V.Rives, JSanz and J.Soria, Stud.Surf.Science and Catalysis.,l7 (1983) 149 2 -G.Munuera, J.C.Conesa, P.Malet, A.Muiioz, M.T.Sainz andJ.Soria, Proc. 8th Intern. Congress on Catalysis 5 (1984) 217,Elsevier,Amsterdam 3 -G.Munuera,J.C.Conesa, P.Malet, J.Sanz and J.Soria, J.Phys.Chem.,SS (1984) 2986 4 -J.Sanz, J.M.Rojo, P.Malet, G.Munuera, M.T.Blasco, J.C.Conesa and J.Soria, J.Phys.Chem.89 (1985) 5427 5 -M.A.Vannice and R.L.Garten, J.Catal.,SO (1977) 228 6 -A.F.Solymosi, A.Erdohelyi and T.Bansangi, J.Catal.,68 (1981) 371 7 -A.Muiioz, A.R.GonzBlez-Elipe, G.Munuera, J.P.Espinos andV.Rives-Amau, Spectrochimica Acta (Part A),43 (1987) 1599. 8 -G.Munuera and V.Rives-Amau, Appl.Surf.Science., 6 (1980) 122 9 -G.Munuera, A.R.Gonz6lez-Elipe, J.P.Espinos, J.C.Conesa, J.Soria and J.Sanz, J.Phys.Chem.,91 (1987) 6625. 10 -J.L.G.Fierro, J.Soria, J.Sanz and J.M.Rojo, J. Solid State Chem., 66 (1987) 154 11 -J.Cunningam, S.OBnen, J.Sanz, J.M.Rojo, J.A.Soria andJ.L.G.Fierro, J. Mol. Catalysis.,57 (1990) 379 12 -B.Harrison, A.F.Diwel1 and C.Hallett, Platinum Metals Rev.,32 (1988) 73 13 -F.Le Normand, L.Hilaire, K.Kili, G.Kril1 and G.Maire, J.Phys. Chem.,92 (1988) 2561 14 -A.Muiioz, G.Munuera, P.Malet, A.R.GonzBlez-Elipe andJ.P.Espinos, Surf. Int. Anal.,l2 (1988) 247. 15 -C.Li, K.Domen, K-1.Maruya and T.Onishi, J.Cata1.,123 (1990) 436. 16 -R. Burch and A. Flambard, J. Catalysis 78 (1982) 389 17 -J.L. Lavalley, J. Saussey, J. Lamotte, R. Breault, J.P.Hindermann and A. Kiennemann. J. Phys. Chem.94 (1990)5941.


A. Crucq (Editor), Catalysis and Automotive Pollution Control II 0 199 1 Elsevier Science Publishers B.V., Amsterdam

22 1

PHYSICO-CHEMICAL PROPERTIES OF Ce-CONTAINING THREE-WAYCATALYSTS AND THE EFFECT OF Ce ON CATALYST ACTIVITY

John G. Nunan, Heinz J. Robota, Michelle J. Cohn? and Steven A. Bradley?

Allied-Signal Research and Technology t UOP Research Center 50 East Algonquin Road, P. 0.Box 5016 Des Plaines, Ill., 60017-5016 ABSTRACT The activity of Pt,Rh,Ce/y-A1203 and Pt,Rh/CeO2 catalysts has been studied in a full complement synthetic exhaust gas mixture consisting of H2, CO, C3Hg. C3H8, NO, 0 2 , N2, C02, H20 and S02. Direct interaction between Pt and Ce02 was shown to lead to large improvements in catalyst performance after activation of the catalyst in the synthetic exhaust gas. Catalyst activation was shown to be due to reduction of the noble metals and surface Ce and this state of the catalyst was found to be particularly effective for CO oxidation. The key component responsible for activation was shown to be H2, even at the low level present in the exhaust gas (0.26 vol.%), reduction is shown to be very facile. The degree of Pt/Ce interaction and thus activity after catalyst activation could be controlled by controlling the Ce02 crystallite size. Decreasing the Ce02 crystallite size led to greater Pt/Ce interaction as shown by TPR and STEM analysis and resulted in greater activity for both fresh and laboratory aged catalysts. Direct Pt/Ce interaction was also shown to lead to a synergistic reduction of Pt and surface Ce andthis feature of the catalyst was shown to qualitatively correlates with catalyst performance after activation.

INTRODUCTION

The effectiveness of Ce02 in three-way-conversion catalysts is well established. However the detailed role of Ce02 in activity enhancement is still a subject of competing hypotheses. Ce02 has been reported to act as an oxygen storage component [ l , 21, to stabilize "/-A1203 [3], to promote water gas shift activity [3,4], and to stabilize noble metal (NM) dispersion [5].Ce02 has also been proposed to promote CO oxidation activity. Several mechanisms have been proposed to account for this selective promotion. These include retardation of CO inhibition during oxidation and a reduced activation energy f o r CO oxidation [6,7]. Ce02 has also been proposed as a direct source of oxygen during CO oxidation [ 7 ] , and reduced Ce has been linked to the formation of highly reactive 0 2 - radical anions[8,9] that are very effective in oxidizing CO.

222

Several authors have also described synergistic interactions between NM and CeO2. Some investigators have observed that reduction of Pt, Pd and Rh is more facile when in contact with Ce02 [3,10]. Also coupling of NM and CeO2 reduction was observed with reduction of both components at lower temperature than when they are not in contact. Yao et al [lo] proposed that such synergism enhances oxygen storage by the catalyst. Previous work has focussed primarily on model reactions involving limited reactants using carefully activated catalysts. The present work addresses the performance of Pt,Rh,Ce/y-A1203 catalysts in a full complement synthetic exhaust gas. Further the catalysts studied were not initially activated by reduction before testing. We attempted to correlate some of the physicochemical properties of our catalysts with conversion performance in the synthetic exhaust gas. We describe the impact of exhaust gas composition and the degree of NM/Ce interaction on the light-off activity of fresh, calcined catalysts and laboratory aged catalysts. Attempts were made to systematically vary the extent of NM/Ce interaction during catalyst preparation by using catalysts of varying CeO2 loading and Ce02 crystallite size. We show that direct interaction between CeO2 and Pt leads to dramatic improvements in catalyst light-off activity after activation in the exhaust gas.

EXPERIMENTAL A wide range of catalysts was prepared where the Ce loading, its method of introduction and the CeO2 crystallite size were varied. Table 1 describes the catalysts studied, including Ce loading, NM content and the Ce02 crystallite size as measured by x-ray diffraction (XRD). The NM and Ce contents were measured by inductively coupled plasma - atomic emission analysis. Different routes were followed for different Ce02 sizes. Fresh catalysts having Ce02 sizes in the range 60 - 110 A were prepared by impregnating a y-Al2O3 support with various Ce salts and by using colloidal Ce02. Samples with Ce02 crystallite sizes of 110 -350 A were prepared by air calcining Ce(N03)3 and Ce(OAc)3 before milling the resulting Ce02 together with y-Al203. A highly sintered reagent Ce02 powder was used to prepare samples 10 and 12. Samples 12-16 were laboratory aged before testing in 10%H20/90%air at 900°C for 4 hours and the CeO2 crystallite sizes are given after aging. Following Ce02 introduction, supports were sized to 425-850 pm with wire screens. Pt and Rh were introduced by impregnation with aqueous solutiuons of H2PtC16 and RhC13, the catalysts were oven dried and finally calcined in air at 600°C for 6 hours. Pure Ce02 supports were prepared by precipitation of Ce(N03)3 using

223

(NH3)2C03 followed by filtration, drying and calcination at 600°C for 6 hours. NMs were added as described. TABLE 1 Summary of catalyst compositions used in testing.

iample

Catalyst Description

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Pt ,Rh/y-A 1203 6% Ce/y-A1203 Pt,Rh,6% Ce/y-A1203 Pt,Rh/y-A1203 Pt,Rh,6% Ce/y-A1203 Pt,Rh,6% Ce/y-A1203 Pt,Rh,6% Ce/y-A1203 Pt,Rh,6%Ce/y-A1203 Pt,Rh,6% Cely-Al203 Pt,Rh,6% Ce/y-A1203 Pt,Rh,24% Ce/y-A1203 Pt,Rh,25% Ce/y-A1203 Pt,Rh,25% Ce/y-A1203 Pt,Rh,25% Ce/y-A1203 Pt,Rh,25% Ce/y-A1203 Pt,Rh/y-A1203

Metal content

Ce02 Crystallite Size(l11) A

Pt wt % Rh wt% Ce wt % 0.045 0.745 61 k 2 5.52 61 zk 2 5.52 0.044 0.7 1 0.80 0.033 72f 2 5.60 0.76 0.040 113 f 3 5.67 0.76 0.04 1 145 f 4 5.67 0.76 0.040 213 k 7 5.70 0.79 0.04 1 332 f 16 5.62 0.78 0.040 =1000 0.79 4.99 0.048 24.1 0.066 65 f 5 0.343 22.6 =1000 0.82 0.043 22.2 270 f 5 0.034 0.50 130 k 5 21.5 0.042 0.77 120 f 5 21.2 0.83 0.048 0.80 0.033

Finished catalysts were evaluated in a full complement synthetic exhaust gas consisting of H2, CO, C3H6, C3H8, NO, 0 2 , N2, CO2, H20 and S02. Catalysts were tested in a tubular reactor using l g of catalyst and a flow rate of 5 l/min. Catalysts could be exposed to rich (equivalence ratio (ER) 2.0), lean (ER=0.5) or stoichiometric mixtures. Different testing cycles were used with or without S02. Without S02, catalysts were heated at 5"C/min in the stoichiometric mixture to 450°C (Rise-1), held at 450°C for 0.5 hour, cooled to 5O"C, then reheated to 450°C in the stoichiometric mixture (Rise-2). Testing with SO2 was similar except that the maximum temperature for Rise-1 and Rise-2 was 600°C which was held for 1 hour. During the hold and drop between Rise-1 and Rise-2, catalysts could be exposed to various combinations of rich, lean, and stoichiometric exhaust gas mixtures.

224

The catalysts were characterized using XRD, x-ray photoelectron spectroscopy (XPS), scanning transmission electron microscopy (STEM) and temperature programmed reduction (TPR). Both TPR and XPS measurements were carried out on fresh samples and on samples that had been treated in the synthetic exhaust gas. Samples analyzed after pretreatments in the exhaust gas were removed from the reactor in N2 (02 content = 50 ppm) and the TPR was carried out without further pretreatment. RESULTS

Testing of the Pt,Rh,Ce/y-A1203 catalysts showed that they were subject to large activity changes following exposure to exhaust gas, especially when Pt, Rh, and Ce were all present. Some activity change was expected as the catalysts were not initially activated by reduction. However, the considerable variation in catalyst activity related to exhaust gas composition was unexpected. The nature of the activation and the experimental factors that control its onset were studied in detail over sample 11. This was done by exposing the catalyst to varying combinations of rich, lean and stoichiometric gas after Rise-1 and studying the effect on the Rise-2 light-off. The effect on CO oxidation during Rise-2 light-off is shown in Figure 1. It is evident that Rise-2 light-off activity is very sensitive to the exhaust gas composition seen by the catalyst prior to Rise-2. The lowest Rise-2 activity occurs after a lean hold/lean drop whereas the highest activity occurs after a rich hold/rich drop and, after a lean hold/rich drop. Clearly, catalyst activation is associated with exposure to rich exhaust gas. Activation of the catalyst after a lean hold by the rich drop further showed that the drop is critical in the activation. Further, a rich hold followed by a drop in N2 resulted in activation equivalent to a rich hold/rich drop. Thus, the rich drop can either activate the catalyst or maintain the catalyst in the activated state. The drop between Rise-1 and Rise-2 takes about 10 minutes indicating that activation is accomplished rapidly. The gas components contributing to activation were determined by removing various components one at a time from the rich drop, followed by another ramp in the full complement gas. Removal of CO, hydrocarbon (HC) and 0 2 had no effect on catalyst activation. However, subsequent removal of H2 was found to have a dramatic effect. Figure 2 compares Rise-2 light-off for CO oxidation with C02, NO, H 2 0 and H2 present during the drop to the case where only C02, NO, and H20 remain. Clearly, removal of H2 leads to complete loss of activation. Similar changes in catalyst performance were observed for HC and NO conversion. Apparently, catalyst activity can be dramatically altered by a low concentration of H2 (0.26 ~01%)in a relatively short time period.

225

0

Leon Hold / Leon Drop

0 Rich Hold / Lean Drop

lean @'dJ_Ric_h W E .

0

A Rich Hold / Stoich. Drop

RichHold(-~~~~~?~--.

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70

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$

u8

50 C

1

I

I

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40

30 20 D

0

100

I50

200

250

300

350

400

450

Inlet Temperature,"C

Figure 1 :Effect of varied in-situ conditioning in the SOz-free synthetic exhaust gas mixture on the Rise-2 light-off activity for CO. Catalyst: PtRh,24wt.%Cel yAl2O3; Pt = 0.343wt.%; Rh = 0.066wt.% Stoich.Hold / Rich Drop 0

GO5 N_Oo,&O,H~

is d

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0 Con,

NO, H.0 in drop

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100 90 80

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Inlet Temperature,' C

Figure 2 :Removal of H2 during the drop results in degraded Rise-2 CO conversion activity. Catalyst: PtRh,24wt.%Cel yAl2O3; Pt = 0.343wt.%; Rh = 0.066wt.%

226

The effect of Ce02 crystallite size on catalyst performance was studied both with and without S02. Fresh catalysts 4-10 were tested without S02. Rise-1 and Rise-2 activities are compared in Figures 3 and 4 . The catalysts were exposed to a stoichiometric mixture after Rise-1. Relative activities are compared using the temperatures at 25% CO, HC and NO conversion plotted vs CeO2 crystallite size. Ce02 crystallite sizes shown were determined by XRD line broadening of the (111) reflection. As evident in Figure 3, Ce02 size has no effect on Rise-1 activity. However, this is clearly not the case for Rise-2. Figure 4 reveals dramatic improvement in light-off activity for all three exhaust gas components with diminishing Ce02 crystallite size, especially for sizes less than 250 A. It is also observed that the activity of the Pt,Rh/y-A1203 sample is less than that for the Ce containing samples. Further a comparison of Figures 3 and 4 shows that performance improvement is not the same for all components. The promotion of CO conversion (AT =: l5O0C) is appreciably greater than that for HC (AT = 100°C) or NO (AT = 100°C>. Since Ce02 size has essentially no effect on Rise-1 activity, it is also clear that Ce02 size is affecting the degree of catalyst activation. TABLE 2

Rise-2 light-off activity over a series of aged Pt,Rh, 25wt. % Ce/y-A1203 samples having varying CeO2 crystallite sizes. Activity expressed as the temperature for 25% conversion. Pt nominal loading = 0.77wt.%; Rh nominal loading = 0.04wt.%

Laboratory aged samples were tested with S02. Catalysts were exposed to stoichiometric exhaust during the hold and drop following Rise-1. Rise-2 light-off activities are summarized in Table 2. Also shown are the CeO2 crystallite sizes after aging and testing. Clearly, large improvements in Rise-2 activity are observed for all three componets with decreasing Ce02 size. An approximately 80°C reduction in CO, HC and NO 25% conversion temperature accompanies the reduction of CeO2 size from 270 A to 120 A.

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500

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700

CeO, Crystdlite Size

800

(A)

900

ma

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Figure 3 :Rise-1 activity as afinction of Ce02 crystallite size tested in a stoichioinetric S02-fiee exhaust gas mixture. Activiry is expressed as the temperature needed for 25% conversion. Catalyst: Pt,Rh,6wt. %Ce/ y-Al2O3; Pt = 0.77wt. %; Rh loading = 0.04wt. %

0

100

200

300

400

SO0

600

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CeO, crystallite Size

800

$00

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Figure 4 :Rise-2 activity as aJicnction of CeOz crystallite size tested in a stoichiometric SO2-free exhaust gas mixture. Activity is expressed as the temperature needed f o r 25% conversiorr. Catalyst: Pt,Rh,6wt. %Ce/ y-Al203; Pt = 0.77wt. %; Rh loading = 0.04wt. %

228

Several fresh samples with varying CeO2 size were analyzed by scanning transmission electron microscopy (STEM). It was observed that a larger fraction of the Pt was associated with Ce02 than expected based on the relative surface area exposed by Ce02 compared with y-Al2O3. Further, the fraction of Pt associated with CeO2 increased with decresing Ce02 size. Thus, for maximum Pt/Ce02 interaction, we need supports with the smallest Ce02 size. This observation suggests that the effect of Ce02 crystallite size is related to increased Pt/Ce interaction. In order to test this hypothesis, Pt was supported separately on Ce02 and y-Al2O3. The temperatures required to achieve 25% conversion are reported in Table 3. Unlike the previous tests, these catalysts were exposed to the rich exhaust during the hold and drop following Rise-1. It is evident that Pt alone supported on pure Ce02 undergoes large activations when exposed to the rich exhaust gas following Rise-1. It is further evident that the Rise-2 activities of the Pt/Ce02 samples are much greater than those for the Ptly-Al203 samples. Thus, the results shown in Table 3 and Figures 3 and 4 clearly demonstrate the beneficial effect of Ce02 in promoting catalyst activation in stoichiometric or rich exhaust gas. Further work showed that activations in the stoichimetric/rich gas mixtures were reversible upon exposure to lean exhaust gas conditions and could result in a net deactivation. TABLE 3

Effect of varying the support composition on Rise-1 & 2 light-off. Activity expressed as the temperature needed for 25% conversion. Testing was done in a stoichiometric exhaust gas mixture with a rich hold/rich drop following Rise-1. Pt = 0.6wt.%

Since activation of the Ce02 containing samples was clearly associated with exposure to H2 in the exhaust gas, several of the above catalysts were characterized using TPR. These included samples 1-3, 5, 8 and 10. The purpose of the analysis was to determine the effect of both Ce02 introduction and CeO2 crystallite size on catalyst reduction, Figure 5 shows the TPR spectra of the y-Al2O3 support, Pt,Rh/y-A1203, Cely-Al203, and Pt,Rh,Ce/yA1203 (samples 1-3). Quantitative H2 uptake values are tabulated in Table 4 .

229

0

100

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300

400

500

600

700

800

Temperature, OC Figure 5 :Comparison of the TPR spectra for: (a) 3cAl203; ( b ) Pt,Rhl yAl2O3; (c) 6wt.%Cel y-Al20.3; (d) Pt,Rh,6wt.%Cel yAi2O3. Pt = 0.77wt%; Rh = 0.04wt.%. TABLE 4

Summary of quantitative H2 uptake as determined by TPR for: (a) y-Al2O3; Pt,Rh/y-A1203; (b) 6wt.%Ce/y-A1203; and (c) Pt,Rh,6wt.%Ce/y-A1203 Sample Description y - A1203 Support Pt, Rh/y - A1203 6 wt % Ce/ y - A1203 Pt, Rh, 6 wt % Ce/ y - A1203

Temperature Range 37 - 797 29 - 481 481 - 794 33 -439 439 - 560 560 - 736 736 - 795 35 -235 235 - 407 407 -790

Temperature Maximum 710 260 672 36 1 490 690 795 175 280 720

H2 Uptake m mole/g 0.064 0.085 0.069 0.06 1 0.034 0.070 0.090 0.099 0.030 0.184

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The ?-A1203 substrate takes up some H2 at about 700°C. This feature is present in Pt,Rh/y-A1203 and Ce/y-A1203 over a similar temperature range. One broad, low temperature reduction, centered at 260"C, is assigned to reduction of Pt and Rh in the Pt,Rh/y-A1203 catalyst. Since the Rh content of these samples is very low, distinct features of Rh reduction were not distinguishable. The 0.085 mmoles of H2 consumed is consistent with the amount required for complete Pt + Rh reduction (0.083 mmoles). H2 uptake by the Ce/y-A1203 sample is shown in Figure Sc. Distinct peaks centered at 360°C and 490°C and a broad feature above 600°C are observed. The low temperature peaks have been previously assigned to surface Ce reduction[lO,ll] and the feature above 6000 is probably due to both H2 uptake by y-AI203 and some bulk Ce reduction. The TPR spectrum of Pt,Rh,Ce/y-A1203 is shown in Figure 5d. The spectrum is clearly not a superposition of thc previous two, but instead, has completely new features consisting af a sharp low temperature peak centered at 175"C, a shoulder at 280°C and a broad peak centered at 720°C. The shoulder at 280°C is likely due to reduction of the Pt,Rh on the y-Al203 component of the catalyst. We see no evidence for the surface Ce reduction peaks below 500°C and the Pt,Rh/y-A1203 peak is clearly attenuated, as shown by comparing H2 uptakes in Table 4. Further, the H2 uptake in the low temperature peak is greater than expected for exclusively Pt,Rh reduction. The absence of a distinct surface Ce reduction feature suggests that the extra reduction at low temperature is associated with surface Ce. Thus, this low temperature reduction feature appears to be a synergistic reduction of Pt,Rh + some surface Ce. Such a synergistic reduction in NM/Ce containing catalysts has been observed previously [3,10]. Again, we attribute the high temperature uptake to reduction of bulk Ce and uptake by y-Al2O3 The influence of Ce02 crystallite size on catalyst reducibility was also examined. Results for samples 5 , 8 and 10 are shown in Figure 6. Sample 1 is also shown for comparison. Changing the Ce02 crystallite size has a large impact on the TPR spectra, especially at lower temperatures. Decreasing the CeO2 size leads to a systematic increase of the low temperature peak at 175"C, which has been assigned to the synergistic reduction of Pt,Rh + surface Ce. The appearance of this peak and its growth with decreasing Ce02 size, follows the previously observed trend of improved light-off for smaller Ce02 size. The effect of catalyst preconditioning on the TPR spectra was studied in detail for a Pt,Rh/Ce02 sample. TPR was run on a fresh sample and after conditioning in rich and lean exhaust at 450°C for 0.5 hours. Results are summarized in Figure 7 and Table 5. Clearly, conditioning in rich or lean exhaust has a large impact on catalyst reduction. For the fresh sample we see a single sharp H2 uptake at 180°C which is about 4.5 times greater than that

23 1

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Figure 6 :Effect of Ce02 crystallite size on the TPR spectra for a series of Pt, Rh, 6wt.%Cel y-Al20.3 samples. Pt = 0.77wt.%; Rh = 0.04wt.%. 7.5

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Figure 7 :Comparison TPR spectra for fresh, rich, and lean conditioned (450°C for 0.5 hours) Pt, Rhl Ce02. Pt = 0.6wt.%; Rh = O.lwt.%.

232

needed for Pt,Rh reduction. This suggests that most of the H2 uptake is associated with the CeO2 support. There is no evidence for distinct surface Ce reduction between 300-500°C as reported by other authors [3,10,11]. However, above 500"C, there is additional gradual H2 uptake. The H2 uptake in the temperature region between 500-650°C may be associated with some high temperature surface Ce reduction and uptake above 700°C with bulk Ce reduction. In the lean and rich treated samples, H2 uptake above 500°C is very similar to that of the fresh sample as seen by comparing the spectra in Figure 7 with the H2 uptake values in the temperature ranges 250-700°C and 700-800°C in Table 5 . Lean conditioning results in a moderate decrease in low temperature reduction with a distinct shift to 100°C. In contrast, rich conditioning causes a net decrease in uptake centered at 80°C. Thus, under rich gas conditions, extensive catalyst reduction occurs. As for Pt,Rh,Ce/yA 1 2 0 3 samples, we see no evidence of separate Pt and Rh reduction, suggesting that reduction of both noble metals occurs under the low temperature peak. TABLE 5 H 2 uptake determined by TPR for a fresh, rich, and lean

conditioned Pt,Rh/CeOz catalyst. Conditioning was done in the rich or lean synthetic exhaust gas at 450°C for 0.5 hours. Pt = 0.6wt.%; Rh = O.lwt.%

Pt loading : 3.08 10-2 mmole/g ; Rh loading : 1.00

mmole/g

Pt,Rh/Ce02 and Pt,Rh,Ce/y-A12Og samples were characterized using XPS after rich and lean conditioning in the exhaust gas at 450°C for 0.5 hours. The samples were removed under N2 and transferred to the XPS instrument. During the XPS measurements sample charging was compensated by using a low energy electron flood gun. The binding energies were then

233

internally referenced to the 01s peak located at 528.84 eV. Some of the XPS peak intensities were weak due to the low levels of NM present and only the peak positions for more intense peaks are reported The reported peak positions are accurate & 0.2 eV Peak positions and their assignments are summarized in Table 6. The lean conditioned Pt,Rh/Ce02 sample has peaks located at 75.79 eV (Pt4fsp) and 72.52 eV (Pt4f712) indicating the presence of oxidixed Pt [12] and at 74.53 eV (Pt4f5/2) and 71.26 eV (Pt4f712) following rich conditioning, indicating the presence of Pto [ 121. Also shoulders at 76.03 and 72.6 eV following rich conditioning indicate the presence of some oxidized Pt. The observed binding energies for the Pt4d peaks are also given in Table 6. The peak locations are consistent with oxidized Pt following lean conditioning and Pto following rich treatment. Pto peaks are expected at 314315.5eV whereas for oxidized Pt, the Pt4d5/2 peak occurs above 315.5 eV 1131. Rh was not detected by XPS in these samples. This may be due to coverage of highly dispersed Rh by Ce02. Both Pt/Ce02 [14] and Rh/CeO;! [ 151 have been reported to exhibit strong metal support interaction whereby the dispersed metal becomes covered by the support. TABLE 6

XPS analysis of Pt and Rh oxidation states for fresh, rich, and lean conditioned Pt,Rh,6wt.%Ce/y-A1203 and Pt,Rh/CeOz samples. For Pt,Rh,6%Ce/y-A1203; Pt = 0.6wt.%, Rh = 0.2wt.%.

x

SAMPLE PRETREATMENT

Pt,Rh 6 wt % Ce/ ')-A1203

Calcined

PEAK RATIOS

I

-

-

I -

Lean Rich

I 316.8 I 310.3

I

I

I

I

I 317.0 I 309.9

-

315.5

Pt,Rh/Ce02

Lean

72.5

75.8

315.7

Pt=0.60wt% Rh=O.lOwt%

Rich

71.3 72.6

74.5 76.0

314.2 316.5

308.0

I 0.074 0.053

+:I: 0.037 0.062

Results for the fresh Pt,Rh,Ce/ y-Al2O3 sample and after exposure to the rich and lean gas are also summarized in Table 6. For this sample the Pt loading was 0.6wt.% and the Rh loading was 0.2wt.%. The approximate Ce02

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crystallite size was 60 A. Again evidence for oxidized Pt in the fresh and lean conditioned samples is provided by Pt4dgl2 peaks located at 316.84 and 3 17.01 eV respectively. Following rich conditioning, evidence for Pto is shown by a Pt4dgl2 peak at 315.5 eV. In this sample, Rh was also detected. Like Pt, it is oxidized in the fresh and lean treated samples. This is supported by peaks at 310.25 eV (Rh3d512) and 315.0 eV (Rh3dy2) for the fresh sample and at 309.9eV and 314.7eV for the lean treated sample [13]. Following rich treatment, Rh was found as reduced Rh with peaks at 308.0 eV and 312.8 eV [13]. The Pt/Ce and Rh/Ce peak ratios were also measured, and reveal a decrease in the Pt/Ce ratio but no change in the Rh/Ce ratio following gas treatments. Assuming no change in Ce02 dispersion which is supported by extensive STEM analysis, this indicates partial Pt sintering during the rich and lean treatments while Rh remains highly dispersed.

DISCUSSION We have found that catalysts formulated with CeO2 are very sensitive to their exhaust gas environmant and can undergo facile activation in a rich gas. The key components responsible for activation are Pt and Ce02. This is shown clearly by comparing Rise-1 and Rise-2 activities in Table 3 for Pt/Ce02 and Pt/y-A1203. We see that the Pt/Ce02 catalyst is less active during Rise-1 than Pt/y-A1203, whereas after activation the opposite is true. The light-off temperature for CO oxidation decreases 37°C over Pt/y-A1203 between Rise-1 and Rise-2, while over Pt/Ce02, the change is 250°C. Similar results were observed with S02, where, after activation, the Pt/Ce02 catalyst shows light-off advantages of 267°C for CO, 170°C for HC and 178°C for NO conversion to Pt/y-A1203. This demonstrates that Ce02 can be a far more effective support for Pt than ?-A1203 after exposure to rich or stoichiometric conditions. Activation of the catalyst is also very facile. This is shown clearly in Figure I where full activation of the catalysts was shown to occur during the rich drop after a lean hold at 450°C for 0.5 hours. During the rich drop the catalyst is exposed to 0.26vol.% H2 for a very short period of time. Figure I also shows that these catalysts can exhibit varying activity depending on their immediate exhaust gas history. Both the effect of Ce02 crystallite size on activity and the physicochemical properties of the catalysts suggest that Pt selectively migrates to CeO2 during catalyst preparation. Thus the Ce02 crystallite size effect reflects increasing Pt/Ce interaction with its subsequent improvement in catalyst performance. Direct evidence for this conclusion comes from STEM studies where Pt was found selectively associated with the CeO2, even though most of the catalyst surface area is y-Al2O3, Indirect evidence comes from

235 noting that performance features of the Pt/CeOz catalyst are most similar to the Pt,Rh,Ce/y-A1203 samples having the smallest Ce02 crystallite sizes. This is clearly seen by comparing performance results for Pt/Ce02 in Table 3 with the activity results in Figures 3 and 4. We see the same activation features in catalyst performance as observed for Pt/Ce02. Further evidence for extensive Pt/Ce interaction comes from the TPR studies outlined in Figures 5 and 6. Figure 5 indicates that direct Pt/Ce interaction leads to a synergistic reduction of Pt and surface Ce. This synergistic reduction was shown to be a function of CeO2 crystallite size in Figure 6 and to be very extensive for samples having small Ce02 sizes as shown in Figures 5 and 6. The results from the present work suggest that Ce02 is not an inert support but instead undergoes physico-chemical changes similar to the NM and that these changes relate to the catalyst achieving its optimum active state. These changes include reduction of the NM as observed by TPR and XPS and surface Ce reduction as suggested by TPR. NM and Ce reduction is coupled and appears related to the light-off performance of the catalyst after activation. This is supported by a comparison of Figures 4 and 6. The exhaust gas component responsible for bringing about catalyst activation is H2, which is present in low levels in the rich exhaust gas. These correlations suggest that NM in contact with reduced Ce plays a key role in catalyst performance and may represent the most active state of the catalyst. Direct evidence for the nature of the catalyst's active state comes from XPS and TPR. The XPS results in Table 6 indicate that the NM in fresh and lean conditioned catalysts are in oxidized states. Rich conditioning clearly leads to reduction of the NM. Since both fresh and lean conditioned catalysts exhibit low activity while rich conditioned catalysts have high activity, it appears that fully reduced NM result in the most active state of the catalyst. Evidence for reduced Ce was not found by XPS of the rich treated samples. This may have been due to partial reoxidation of the catalyst during sample handling or that insufficient reduced Ce was present for detection by XPS. The XPS peaks of Ce3+ tend to overlap with those of Ce4+ [ 13,161. Evidence for facile re-oxidation of Ce3+ in the N2 used for sample handling was obtained from in-situ reduction experiments in the XPS instrument. Clear evidence for Ce3+ could be obtained in the Pt,Rh/Ce02 sample following reduction at 300°C in 1 torr of H2. However, exposure of the reduced sample to the glove box N2 for 2 days at room temperature led to complete loss of Ce3+ features.(O2 concentration was found to be much greater than the 50 ppm values present during sample handling in the TPR). Since the XPS sample handling was done in N2 having higher 0 2 contents than for the TPR experiment, reoxidation of the reduced Ce could easily occur in preparation for the XPS measurement.

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The TPR results provide further evidence for the nature of the catalyst's active state. TPR results in Figure 5 show that Ce not only promotes reduction of the NM but also undergoes a synergistic reduction with the NM. The fraction of Ce02 that couples with the NM reduction seems to be selectively the surface Ce component. This is supported by: (i) the dissapearance of the peaks assigned to surface Ce reduction when the NM are present; (ii) direct evidence for reduced Ce was observed by XPS following reduction at 300°C; and (iii) bulk Ce reduction is thermodynamically unfavorable at the low temperatures observed for synergistic reductions [ 171 A number of authors have reported H2 uptake by CeO2 [18] where H2 consumption was not necessarily associated with Ce reduction. However, the evidence here favors surface Ce reduction as the source of H2 uptake in TPR. It was further found that decreasing the Ce02 size led to greater coupling of NM/Ce reduction as shown in Figure 6, The TPR studies of the Pt,Rh/Ce02 before and after treatment in the exhaust gas give further insight into the nature of the NM/Ce interaction and its relation to the active state of the catalyst. For the fresh and lean conditioned catalyst we again clearly see evidence for synergistic reduction of the Pt and Ce. This is supported by the volume of hydrogen uptake in the low temperature reduction peak, as shown in Table 5, and the absence of surface Ce reduction peaks below 5OO0C, as reported by other authors [3,11]. After lean conditioning, less H2 is consumed by the fresh sample, along with a shift to lower temperature. A similar temperature shift is observed for the rich treated sample. The shift to lower temperature indicates more facile reduction after exhaust gas treatment. This may be explained by partial noble metal sintering in addition to removal of residual chloride. Sintering is supported by lower Pt/Ce XPS ratios following rich and lean conditing than observed in fresh sample in the case of the Pt,Rh,6%Ce/y-A1203 sample. It has been reported that reduction of partially sintered Pt is more facile than highly dispersed Pt [19]. The lower H2 uptake following lean gas treatment may be due to a combination of less Pt reduction and poorer coupling with CeO2 resulting from partial sintering. In contrast, the rich conditioned sample showed much lower H2 uptake indicating that considerable reduction of the catalyst had occurred. The small H2 uptake by the rich conditioned sample may have been due to reduction of surface 'PtO' species or incompletely reduced surface Ce. These observations suggest that the most active state of the catalyst after rich conditioning is reduced noble metal in contact with partially reduced Ce. This study shows that direct interaction between Ce02 and Pt leads to dramatic improvements in catalyst light-off activity. Since catalyst performance was measured under stoichiometric non-oscillating conditions, the impact of Ce02 cannot be explained by oxygen storage. Further

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explanations such as promotion of the water gas shift reaction, promotion of NM dispersion, stabilization of the support cannot explain the large beneficial effect of Ce02 under the present testing conditions. An interesting feature of Ce02 promotion is the enhanced conversion of all the exhaust gas components. Also, fresh catalysts show exceptional CO light-off activity after activation. This is clearly seen by comparing Figures 3 and 4 and from the data presented in Table 3. Further, CO light-off is promoted to a greater extent than HC or NO light-off, especially in tests without S02, as seen by comparing light-off temperatures in Figures 3 and 4 and Table 3. Since CO can inhibit oxidation reactions over reduced Pt and Rh, the removal of CO via its effective oxidation at low temperatures may lead to more effective HC and NO conversion. In summary, our results indicate that the most active state of the catalyst is reduced surface Ce in contact with reduced NM which is particularly effective in CO oxidation. These conclusions are in general agreement with the proposals of Sass et a1 [8] and Oh et a1 [7] where the impact of CeO2 was on CO oxidation. For testing under more severe conditions, such as high temperature and in oscillating richDean exhaust gas mixtures, other factors, such as water-gas shift promotion or oxygen storage, may become important in catalyst performance. ACKNOWLEDGEMENT

The authors are indebted to their respective organizations for financial support, encouragement and provision of facilities. We acknowledge the following colleagues for their assistance at both Allied-Signal and UOP: Mark T. West for catalyst testing; David E. Mackowiak for catalyst preparation; Jeffry T. Donner and Sharon G. Varga for XPS analysis; Mary A. Vanek and Judy A.Triphahn for XRD analysis; and Tomatsu Imai for helpful discussions and encouragement. REFERENCES 12345-

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Gandhi, H. S., Piken, A. G., Shelef, M., and Delosh, R. G., SAE Paper No. 760201, 1976 Su, E. C., Montreuil, C. N., and Rothchild, W. G., Appl. Catal., 17, 75 (1985) Hamson, B., Diwell, A. F. and Hallett, C., Platinum Metals Rev., 32, 73, (1988) Kim, G., Ind. Eng. Chem. Prod. Res. and Dev., 21, 267 (1982) Sergeys, F., J., Masellei, J. M., and Ernest, M. V., W. R. Grace Co., U.S.Patent 3,903,020 (1974).; Hindin S. G., Engelhard Mineral and Chemical Co., U.S.Patent 3,870,455 (1973) Yao, Y. Y.-F., J. Catal., 87, 152 (1984) Oh, S. H., and Eickel, C. C., J. Catal., 112, 543 (1988) Sass, A. S., Shvets, V. A., Savel'era, G. A., Povova, N. M., and Kazanskii, V. B., Kinet. Katal., 27, 894 (1986)

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1516171819-

Tarasov, A. L., Przheval'skaya, Shvets V. A., and Kazanskii, V. B., Kinet. Katal., 29, 1181 (1988) Yao, H. C. and Yao, Y. Y.-F., J. Catal., 86, 254 (1984) Johnson, M. F. L. and Mooi, J., J. Catal., 103, 502 (1987) Kim, K. S., Wingrad, N., and Davis, R. E., J. Amer. Chem. Soc.,93, 6296 (1971) Handbook of X-Ray Photoelectron Spectroscopy, Muilenberg, ed., Perkin-Elmer Corp., 1979 Meriaudeau, P., Dutel, J. F., Dufaux, M., and Naccache, C., in "Metal-Support and Metal Additive Effects in Catalysis", B. Imelik et al., Eds. Vol. 11, p.95, Elsevier, Amsterdam (1982) Cunningham, J., OBrien, S., Sanz, J., Rojo, J., M., Soriao, J., A., and Fierro, J. L. G., J. Mol. Catal., 57, 379 (1990) Shyu, J. Z., Weber, W. H., and Gandhi, H. S., J. Phys. Chem., 92, 4964 (1988) Bevan, D. J. M., J. Inorg. Nucl. Chem., 1, 49, (1955) Fierro, J. L. G., Soria, J., Sanz, J., and Rojo, J. M., J. Catal., 66, 154 (1987) McCabe, R. W., Wong, C., and Woo, H. S., J. Catal., 114, 354 (1988)

A. Crucq (Editor), Catalysis and Automotive Pollution Control 11 199 I Elsevier Science Publishers B.V., Amsterdam

239

STUDIES TO THE FUNCTIONING OF AUTOMOTIVE EXHAUST CATALYSTS USING IN-SITU POSITRON EMISSIONTOMOGRAPHY

K.A. Vonkeman, G. Jonkers and R.A. van Santen",

KoninklijkelShell-Laboratorium,Amsterdam, ABSTRACT Studying the reactions in an automotive exhaust catalyst is complex as the exhaust gases contain many components of which the composition constantly varies and the catalyst contains different active components. To elucidate reaction kinetics in exhaust catalysis, the application of a technique stemming from nuclear medicine, i.e. Positron Emission Tomography, has been developed. Short lived positron emitting nuclides 11C (t1E = 20 min), 13N (tin = 10 min) and l5O (tin = 2 min) have been used to synthesise actual reactant molecules such as 11C0, llC@., llCHq, 13N2, 1500,C'5O and C1500. The labelled reactants could be monitored in a catalyst bed right through the reactor wall using a Positron Emission Tomograph, with a resolution of 1 cm in place and 1 s in time. Hereby, a large amount of information on the residence time of the reactants is obtained directly from the catalyst surface under actual reaction conditions. The mechanism and the rate limiting step in the carbon monoxide oxidation on platinum under cold start conditions of a car was studied. A strong interaction was found between gas phase C 0 2 and CeO2 at the catalyst surface, whereby the oxygen atoms contained in lamce Ce02 exchange rapidly with the oxygen atoms of the adsorbed C02

INTRODUCTION Automotive exhaust gases contain carbon monoxide, nitrogen oxides, hydrogen, various hydrocarbons, sulphur oxides and a number of components in trace amounts, such as lead oxide and phosphorus oxides [ l ] . The composition of engine exhaust gases depends on many factors, such as type of engine in the car, lubrication of the engine, state of service of the engine, driving behaviour of the driver and, to a lesser extent, the type of fuel used. The main ingredients of most commercial automotive exhaust catalysts are platinum and/or palladium, rhodium and ceria [e.g. 2,3]. For these reasons the study of the functioning of automotive exhaust catalysis is complex. Here we report studies on the kinetics of CO oxidation catalysed by ceriumpromoted platinum.

240

Positron Emission computed Tomography (PET) is a relatively new 3D imaging technique emerging from nuclear medicine and is capable of mapping quantitatively the concentration of a positron-emitting tracer [4,5]. The application of positron-emitting nuclides in reactant molecules is a useful technique to study automotive exhaust catalyst kinetics both in-situ and noninvasively. The Positron Emission Tomography technique is in essence an extension of the transient techniques as it does yield direct information on the degree of occupation of the surface sites by reacting components. This information can be used to obtain the reaction rate parameters of the individual adsorption, desorption and reaction steps. The reactants or reaction products are labelled with positron emitting nuclides, in particular 11C, 13N and 1 5 0 . With the advent of Positron Emission computed Tomography as a research tool in nuclear medicine, the utilisation of short-lived positron emitting nuclides like 11C (ti12 = 20.39 min; 100% positron yield; Emax = 1.0 MeV), 13N (9.965 min; 100%; 1.2 MeV) and 1 5 0 (2.037 min; 100%; 1.7 MeV) has increased considerably. The positron is the anti-matter counterpart of the electron, which implies that upon their encounter the positron and the electron annihilate, i.e. they are converted into electromagnetic radiation. To conserve momentum two y-photons are emitted in opposite directions in this conversion process.(see Fig.1). The energy of the y-photons is dictated by the energy conservation law and is almost completely determined by the conversion of the mass (m) of the positron/electron pair into electromagnetic radiation according to E = mc2 = 51 1 keV.

INCIDENT POSITRON

Figure I :Schematic drawing of a positronlelectron annihilation event.

Owing to this annihilation process the distance over which a positron travels in the catalyst matter (p = 1 g/cm3) is very limited (ca. 1.6 mm for positrons with Emax = 1 MeV). However, the resulting y-photons may travel through a catalyst over elongated distances (half thickness value in catalyst

24 1

matter approximately 7.5 cm). The detection of positron emitters occurs indirectly by the registration of the 51 1 keV annihilation photons. This can be carried out using 3"*3" NaI(T1) scintillation detectors and with a positron emission tomograph equipped with Bi4(Ge04)3 (BGO) detectors. The use of positron emitters such as 11C in kinetic studies offers the following attractive aspects: - (1). Nuclides such as 11C, 13N and 150 can be incorporated into the actual reactant molecules and the resulting positron-labelled molecules are chemically not distinguishable from unlabelled ones. Isotope effects in the pre-exponential factors of adsorption and desorption rates are very small and will be neglected in this study. - (2). The state of the catalyst surface can be studied both in-situ and noninvasively, since the indirectly emitted radiation can be easily detected through a reactor wall (half thickness value in steel ca. 11 mm), thus yielding direct information on reaction kinetics. With another radioisotope of carbon, such as 14C, the product stream can be monitored, as the P--radiation originating from 14C cannot penetrate reactor walls. On the other hand, many surface science techniques can only yield direct information about the status of the catalyst surface at very low pressures. - (3). The utilisation of scintillation detectors as detection system for the annihilation photons offers the possibility of detecting coincidences and hence acquiring absolute positron emitter concentrations at a well-defined time and location.[4,6]. - (4). An advantage related to the use of short-lived radionuclides is their high specific activity, resulting in a high sensitivity of measurement with very small quantities of the labelled molecules (order of magnitude picomoles) in comparison with the amount of reacting molecules. Because of the short halflife of these radionuclides, there are relatively few problems with waste, product or environmental contamination. For more general information on the properties of positron-emitting nuclides we refer to Knoll [7]. EXPERIMENTAL

The Platinum-Ceria-Alumina Catalyst A standard highly dispersed platinum catalyst was synthesised on a y-alumina carrier containing highly dispersed ceria at the surface. This catalyst formulation is a simplification of a commercial automotive exhaust catalyst as it only contains one type of noble metal. The preparation and the characterisation of the catalyst is reported elsewhere.[8]. In Table 1 the characteristics of the platinum catalyst are summarised.

242

T

m

: Catalyst characterisation parameters. 3.9 g cat. in cat. bed.

Parameter support Particle size BET surface area Pore volume Ceria loading Ceria surface Noble metal loading Noble metal dispersion Maximum CO/Me ratio at surface CO ads. cap. of noble met. Noble met. surf. in cat. bed C02 ads. cap. of Ce02 of carrier C02 ads. cap. of CeO2 of cat.

A1203 / Ce02 30 - 80 mesh 111 m2/g 56 ml/g 3 0.6 %w highly dispersed on A1203 surface 12 %w 84% 1 2.4.10-5 mol/m2 1 m2 5.6.10-5 mol 3.1.10-5 mol

Positron-emitter-labelled Molecules The experiments have been carried out at the Institute for Nuclear Sciences (INW) of the State University of Ghent, Belgium.[9]. At the INW a cyclotron was present with which positron emitting nuclides could be produced. Expertise and equipment were available to synthesise gas molecules containing these positron-emitting nuclides, with a radiochemical purity greater than 99.9%. These purities were confirmed by means of (radio)gas chromatography. At the INW a NeuroECAT positronemission tomograph (PET-camera) was available, which was used as detection system in the experiments described in this paper. At the INW the application of PET to the visualisation of industrial processes has successfully been explored for oil displacement in model reservoir rock and dehydration of water-in-oilemulsions using the NeuroECAT tornograph.[ 101. In this study the positron emitters 11C, 13N and 1 5 0 were used. With these nuclides the molecules 11C0, ClSO, 11C02, ClsOO, 1 5 0 0 and 13" synthesised, which were used in the pulse experiments. Details on the production of positron emitters and the synthesis of the positron-emitter labelled molecules has been reported elsewhere. The Experimental Set-up with the Reactor Oven Svstem Fig. 2 shows a schematic representation of the equipment in which the transient radioisotope experiments were carried out. The experimental set-up was identical to that described in our previous paper in this series.[8], apart

243

from the fact that an aluminium oven block was used and that the complete reactor oven system was now placed in the field of detection of a positronemission tomograph.(Fig. 2). INLET PULSE DETECTOR

I

;I

I

I '

I

I '

I

u

I

o

m

PULSE DETECTOR

II I/

r-/

xi

L RCC

i

]

I 11-Bank T O M O C W E

Fipure 2.: Schematic drawing of the experimental set-up.

The main features of the set-up and the experiments carried out are summarised below: - (1). Amounts of less than 0.1 nanomol of CO containing ca. 0.15 picomol (or 50 MBq) of 11CO or l l C O 2 in argon, were extracted from a leadshielded container. - (2). These amounts were taken up into a gas mixture, which was continuously flowing through the catalyst bed in the reactor, without disturbing the pressure or the chemical composition of this gas flow (ca. 0.3 pmol CO/s). - (3). The total gas flow of 40 mVmin STP, containing 1.0% CO, 0.5% 0 2 and 10%C02 was led over 3.9 g crushed catalyst, sieve fraction 30-80 mesh. This catalyst sample was placed in a 3/8-inch reactor tube (Hoke stainless steel, internal diameter 7 mm; catalyst bed length 14 cm). Under these conditions (100°C c T < 170°C; ambient pressure) the reaction rates were limited by kinetics and not by transport phenomena. 10% C02 was present in the gas mixture to suppress the effect of the strong adsorption of C02 to the ceria present at the catalyst surface, as described in the previous paper in this series [81.

244

- (4). The chemical composition of both reactant and product stream was determined using gas chromatography and mass spectroscopy. - (5). At the outlet, samples taken from the effluent stream were sent to a CO/C02-separating gas chromatograph equipped with a proportional counter for the detection of radiation. This counter was placed behind the Thermal Conductivity Detector (TCD). After separation of CO and C02 the amounts of 11CO and 11CO2 could be quantified so that the distribution of labelled l 1 C 0 and " C 0 2 in the effluent stream from the reactor outlet could quantitatively be determined in the course of an experiment. - (6). To ensure that complete steady state was reached at the moment the pulse experiments were carried out, the catalysts were submitted to the gas flow for at least 12 hours in advance. The Positron Emission TomograDh Positron emitters are detected indirectly by the recording of the 511 keV annihilation photons. For the photon detection Bi4(Ge04)3 (BGO) scintillation detectors are utilised, which are placed in detector banks in a positron emission tomograph. This tomograph was used for detection of radioactivity directly from the catalyst bed, while norma1.(3"*3") scintillation detectors were used for an accurate characterisation of the labelled reactor inlet pulse and reactor outlet signal.(Fig.2). For product identification of the labelled compounds a gas chromatograph equipped with a proportional radioactivity counter was employed. At the INW a NeuroECAT positron emission tomograph was available, which was used in this study. The utilisation of a NeuroECAT positron emission tomograph has the following advantages: - (1). A temporal resolution of about 1.2 s can be achieved in a detection mode, in which the scanning gantry is kept in a fixed position. - (2). A spatial resolution of about 8 mm Full Width at Half Maximum (FWHM) along the flow tube axis can be achieved, if the scanning gantry is kept in a fixed position. - (3). Twenty one measuring locations are equally spaced over the total length of the reactor tube. Furthermore, there is no detection sensitivity profile over the catalyst bed. Both characteristics of a positron tomograph are in contrast with those of the experimental method in which NaI (Tl) scintillation detectors were utilised, as reported in a former paper.[ 131. - (4).The pair of detectors, which records a coincidence event, determines the location of the centre of the volume from which the signal originates. This sensitive volume is determined by the spatial resolution and the radial cross-sectional area of the flow tube. - (5). A line source calibration of the tomograph determines a quantitative relation between the radioactivity in the sensitive volume (Bq/cm3) and the

245

response of the tomograph (counts/s). This allows us to obtain quantitative information on the concentration of labelled compounds in the catalyst bed. - (6). The presentation of the data as a 'reaction image' provides direct information on the distribution as a function of time and place of the labelled compound on the catalyst bed in the reactor. From this information, kinetic parameters can be derived with the help of simulations of the experiments by a mathematical model of the catalyst kinetics.

MODEL OF THE REACTION MECHANISM Reaction Mechanism of CO Oxidation bv 0 2 over a Pt/CeO~/Y-A17Q3 - - Catalvst

To simulate the reaction of CO oxidation by 0 2 over noble metal catalysts, a mathematical model of the reaction mechanism was constructed. Details on this mathematical model were reported in previous publications .[ 8,141. The model contains three reaction steps for CO oxidation over noble metal surfaces and one for C02 interaction with the ceria: CO adsorption at the noble metal surface (Me) CO desorption from the noble metal surface Irreversible dissociative 0 2 adsorption at noble metal surface Irreversible surface CO/O conversion and C02 desorption C 0 2 adsorption at the carrier (A) C 0 2 desorption from the carrier CO 02

+ +

MeCO+ A +

Me 2Me Me0 CO;!

f~

+

+

f~

MeCO 2Me0 2Me+C02 AC02

(1.1) and (1.2) (2) (3) (4.1) and (4.2)

The reaction rate constants for the adsorption steps (l.l), (2) and (4.1) are based on the sticking probabilities of the molecules to the surfaces. The desorption step (1.2) and (4.2) and the surface reaction step (3) are taken activated, using Arrhenius equations. As was reported by Yates et al. [15] and Oh et al. [ 161 the dissociative adsorption of oxygen can be modelled best to fit the data exploiting a first-order dependence of the fraction of free surface sites. The second-order dependence, which is expected from step (2) in the model was found not to agree in their experiments. Previous studies have shown that the oxidation of carbon monoxide by oxygen was promoted by the presence of ceria on the catalyst surface [8]. This is caused by an enhancement by ceria of the sticking coefficient for oxygen on the platinum surface.

246

RESULTS & DISCUSSION Reaction Images The data recorded by the tomograph can be displayed in the form of an "image" [13,17] in very much the same way as graphical representation is carried out in medical experiments with a tomograph, e.g. brain scans [11,12]. A "reaction image" is a 1D distribution of the labelled compounds along the axis of the tubular reactor. The place coordinate is plotted along the horizontal axis in the reaction image, with position 0 cm in the centre of the catalyst bed (Fig. 3, catalyst bed -7 to +7 cm).

Figure 3 . : A reaction image of the 1 5 0 0 pulse experiment over the PtlCe02IyAl203 catalyst under steady-state conditions. The intensity of the labelled compounds in the reaction image is indicated in the linearly scaled grey-bar; the lighter grey-shades represents the higher concentrations. The PRF curve represents the distribution of the labelled compounds over the catalyst bed along the axial position, during the 200th second after injection of the pulse into the system. The RTD curve shows the residence time distribution at position -3.2 cm in the catalyst bed.

247

Because of the high length (L)/diameter (D) ratio of the reactor tube ( L D = 20), gradients in the radial direction are neglected compared to those in the axial direction. The second dimension in the reaction image is formed by the time, which is plotted along the vertical axis. All data displayed in the reaction images are corrected for radioactive decay and are smoothed moderately [ 131. The lighter grey shades in the reaction image represent the locations with the higher concentrations of the radioactive label. The reaction image presented in Fig. 3 is the result of a 1 5 0 0 pulse experiment over the platinum catalyst in the standard CO, 02, C02 gas flow. The gas flow was from the left-hand side to the right-hand side in the reaction image and the pulse of 1 5 0 0 reached the entrance of the catalyst bed 14 seconds after the tomograph was started. Two cross-sections of the reaction image are given. A profile (PRF) curve is shown, which represents the distribution of the 1 5 0 label over the catalyst bed during the 200th second. An RTD curve is also given, which indicates the Residence Time Distribution at position 3.3 cm. This experiment will be discussed in more detail below. Dispersion in the catalvst bed

Fig. 4 shows the results of six pulse experiments. All six experiments were carried out under the same conditions of ambient pressure, 140°C and applying the Pt/Ce02/y-A32(33 catalyst, except for the experiment shown in Fig. 4a, in which the reactor tube was filled with Sic. During the pulse experiments shown, the standard CO, 02, CO2 gas mixture was continuously flowing through the catalyst bed. In the first experiment (Fig. 4a ) 13" was pulsed over a bed of inert Sic. As no interaction between 13" and the catalyst bed took place, Fig. 4a shows the behaviour of inert gas molecules. The gas phase flowed through the catalyst bed in 4 seconds. The intensity of the displayed signal was identical at all positions along the reactor axis in the catalyst bed due to the calibration of the detector pairs. This experiment indicates that no significant dispersion takes place in the catalyst bed so that the reactor can be described as a plugflow reactor. Simulations of the other experiments (which had a longer time scale than the experiment in Fig 4a) with the computer program confirmed that dispersion is not a significant effect in the reactor system. Irreversible Adsomtion of Oxvgen at Noble Metal Surfaces From Fig. 3 (Pt catalyst; 140°C; 76% conversion; pulse of 1 5 0 0 ) it can be derived that part of the labelled compounds reaches the reactor exit without delay and another part remains in the catalyst bed for a relatively long time (up to 700 s). The molecules that reach the reactor exit within 4 s flow with the gas-phase through the catalyst bed and do not adsorb. This is

248

confirmed by the results of an experiment with an inert catalyst, S i c Fig. 4a . Comparison of Fig. 3 with Fig. 4a reveals that the unconverted oxygen molecules flow undisturbed through the catalyst bed. The RTD curve given in Fig. 3 shows clearly the peak of 1 5 0 0 molecules flowing without delay through the catalyst bed, followed by the long tail representing the l5O species that have been adsorbed in the catalyst bed.

FiPure 4 : A set of six reaction images of pulse experiments over the PtlCe02/yA1203 catalyst. Under identical conditions (standard gas mixture and jlow rate; T = 140°C; 76% conversion), six diserent labelled compounds was pulsed over the were pulsed over the catalyst. In experiment a 13” catalyst, showing the behaviour of inert gas molecules. In b an 11CO pulse experiment is shown, in c ‘ICO2, in d 1 5 0 0 , in e Cl5O and in f C1500. Product identification of the labelled compounds at the reactor exit revealed that 24% of the 1 5 0 0 molecules flow without interaction with the catalyst bed through the reactor and leave the reactor unchanged, as 1 5 0 0 . 76% of the 0 2 is converted to C02 via reaction with CO. Fig. 3 indicates that this part of the 1 5 0 label remains for an extended period of time in the

249

catalyst bed. Product identification showed that the labelled compounds leaving the catalyst bed after the initial 1 5 0 0 peak (at times > 20 s after start of detection) are all in the chemical form of C15OO. This clearly indicates that indeed the oxygen adsorption at the catalyst surface is irreversible; after desorption the only possibility for an oxygen atom to leave the platinum surface again is via the CO-scavenging reaction, forming C02. When the oxygen adsorption would have been reversible, at least some 1 5 0 0 would have been found in the product gas at times >20 s. The proportional radioactivity counter used for product identification was very sensitive and allows detection of less than pro-mille quantities of the total amount of labelled compounds injected. The Exchange of C02 Oxvgen Atoms with Ceria Lattice Oxvgen The time scale of the reaction image of Fig. 3 indicates that the oxygen molecules, which adsorb at the platinum surface, remain for a relatively long time in the catalyst bed. After 500 seconds still significant amounts of labelled compounds are recorded at the catalyst surface. In Fig. 4 the reaction images of six pulse experiments are shown. In Fig. 4a the reaction image of inert gas molecules (13") is given, while the other five reaction images represent pulse experiments carried out over the platinum catalyst at 140 "C, at which 76% conversion was reached Fig. 4h - 4f. Identical conditions with respect to gas flow, composition of the gas mixture and catalyst were applied in all five experiments, but different labelled components were pulsed into the reacting system. In Fig. 4b the reaction image of an 11CO pulse experiment is given, while in Fig. 4e a pulse of C l s O was applied. Surprisingly, there is a considerable difference between both reaction images. The oxygen atoms of the CO molecules remain much longer in the catalyst bed than the carbon atoms of these molecules. This clearly points to a dissociation between carbon and oxygen atoms somewhere in the reaction mechanism of the catalytic process. Similar observations are made for the labelled C02 molecules shown in Fig. 4c (an 11C02 pulse) and Fig. 4f(a ClsOO pulse). Again it is found that the oxygen atoms remain much longer in the catalyst bed than the carbon atoms. In these C02 pulse experiments, the effect is even more pronounced than that observed for CO. The carbon atoms of the initial C 0 2 molecules all have left the reactor even before the first oxygen atoms of these C 0 2 molecules reach the reactor exit in this experiment. It was found that the difference between the residence time distribution of the oxygen and carbon atoms in the CO pulse experiments was dependent on the conversion, whereas it was independent of conversion in the labelled C 0 2 pulse experiments. In experiments where low CO conversions were reached in the catalyst bed, the overlap between the reaction images for 11CO

250

and ClSO was greater than at higher conversions. The importance of the ceria lattice oxygen atoms in CO oxidation reactions was reported by Jin et al. [18] and Daniel [19]. The interaction between C02 molecules and ceria present at the catalyst surface was addressed in previous publications [8,13]. This interaction follows an adsorption and desorption mechanism of the C 02 molecules at the ceria surface. From the experiments shown in Fig. 4 it was concluded that the adsorbed C 0 2 molecules dissociate at the ceria surface and associate to CO2 again before desorption. The dissociation of the C02 molecules can very well take place via a carbonate-formation mechanism. It was reported by Daniel [19] and Li et a1 [20] that carbonate groups are present at the ceria surface in catalysts. The following mechanism is proposed: - C02 molecules adsorb onto the ceria present at the catalyst surface; - the C02 molecules form carbonate groups at the Ce02 surface; - the ceria carbonate groups can dissociate to form C02 molecules and Ce02 again; the oxygen atoms incorporated in the C 0 2 molecule need not necessarily be the same as those initially adsorbed onto the ceria surface. A probability of 1 to 7 or 8 for an oxygen atom to form a C02 molecule with the same carbon atom that adsorbed initially was established from these experiments. As reported in a previous paper in this series [8], it was found that 34%mol of the ceria present in the catalyst formulation forms an adsorption site for CO2. It is therefore likely that all oxygen atoms of the ceria participate in the exchange mechanism with the carbonate groups at the surface. Barrault et a1 [2l]reported that ceria significantly enhances the rates of exchange reactions between lattice oxygen and the oxygen atoms of gasphase molecules. The 0 2 molecules which dissociatively adsorb to the platinum surface react with CO molecules and form C02, which desorbs from the platinum surface. When these C02 molecules are moving with the gas-phase through the catalyst bed, they get adsorbed to the ceria surface with formation of carbonate groups. These carbonate groups can dissociate again, to form C02 molecules, which desorb from the surface. The oxygen atoms exchange in this way with the ceria at the catalyst surface, and remain in the catalyst bed for an extended period of time. This mechanism is visible in the reaction image of the experiment in which 1 5 0 0 was pulsed over the catalyst, shown in Figs. 3 and 4d. The experiments were simulated with the mathematical model of the reaction kinetics. These simulations, together with those of experiments at different temperatures made it possible to establish the kinetic reaction parameters, such as sticking coefficients in the adsorption steps and activation energies and pre-exponential factors in the desorption and reaction steps. The results of these simulations are summarized and compared to literature values [22] in Table 2.

25 1

DULL

Values used for the kinetic reaction parameters in the model. Kinetic Reaction Parameters CO stick. prob. at metal Act. Ener. CO desorption CO desorp. pre-exp. fac. 0 2 stick. prob. at met. Act. Ener. CO/O reaction Pre-exp. factor reaction C 0 2 stick. prob. at Ce02 Des. const. C02 from Ce02

I

Values used in this stud

I

Values used by Lynch et al..[22]*) 2.10-5 9.103 K 1.8.10'O mol/m3/s 4.1O9 mol/m3/s 9.10-9 8.103 K 8.10'K Eam 5.8.1013 s-lm-' 5.8.1013 s-lm-l ko * 6.6.1 0-5 sco2 -* 7.9 s-1 Kdes

(*) No ceria present in this catalys

CONCLUSIONS

The use of positron emitters in catalytic studies makes it possible to establish kinetic parameters. A positron emission tomograph supplies a large amount of information about the catalyst surface. The concept of a 'reaction image' is a valuable tool for the display and analysis of the recorded data in this type of experiments. A clear relation between gas phase conditions and surface component concentrations can be established directly from the results obtained with the tomograph. Also the dynamics of the processes of adsorption, desorption and surface reaction under steady-state reaction conditions can be directly derived from the 'reaction images'. The reaction mechanism for CO oxidation by 0 2 on supported noble metal catalysts was studied and oxygen adsorption at the noble metal surface was found to be rate limiting. Also the irreversible dissociative character of the oxygen adsorption was demonstrated. In the CO oxidation by 0 2 under the reaction conditions prevailing during the cold start of a car, the surface of noble metals is predominantly covered by adsorbed CO molecules. This is an important observation for automotive exhaust catalysis as it inhibits the reaction to take place at lower temperatures. A strong adsorption of C02 to the ceria surface was found, which most probably leads to cerium carbonate formation. The oxygen atoms from the carbonates exchange with the ceria surface. The carbonates can dissociate again to form C02 molecules which desorb from the catalyst surface. The use of positron emitters and a PET camera in catalysis studies has been demonstrated. In principle there is no constraint in placing a complete automotive exhaust converter in a PET camera, passing real exhaust gases through it and pulsing positron-emitter labelled molecules into this gas

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stream. In this way complicated aspects of the functioning of exhaust catalysts occurring under realistic driving conditions (such as the processes taking place at a cold start) can be studied in detail. ACKNOWLEDGEMENT We gratefully acknowledge the expertise and cooperation from Dr. K. Strijckmans and Dr. P. Goethals of the "Laboratory of Analytical Chemistry", Institute for Nuclear Sciences, Rijksuniversiteit Ghent, Belgium for the production of the radioisotopes, the synthesis of the labelled reactants and the help received in experimenting. We also gratefully acknowledge Dr. I. Lemahieu and Ing. K. de Kesel of the "Laboratory for Electronics and Metrology" of the same Institute for the help received in experimenting with the NeuroECAT PET camera.

REFERENCES 1. Williamson, W.B., Perry, J., Gandi, H.S. and Bomback, J.L., Appl. Catal. 15,277 (1985). 2. Taylor, K.C., Catalysis, Science and Technology, Anderson, J.R.and Boudart, M. Ed. jQ),119 (1984). 3. Walsh, M.P., Platinum Met. Rev. 106 (1986). 4. Phelps, M.E., Mazziotta, J.C. and Schelbert, H.R., Positron Emission computed Tomography and Autoradiography, Raven Press (1986) New York. 5. Webb, S., Medical Science Series, The Physics of Medical Imaging, Adam Hilger, Bristol(l988) 6. Hilger, A., The Physics of Medical Imaging, (S. Webb, Ed.), Bristol, U.K. (1988). 7. Knoll, G.T., Radiation Detection and Measurement, 2nd edition, John Wiley & Sons, New York (1989). 8. Vonkeman, K.A., Jonkers, G., Van der Wal, S.W.A., Oosterbeek, H., de Jong, J. and Van Santen, R.A., Part I, submitted to J. Catal. 9. Strijckmans, K., Annual Report 1987, Rijksuniversiteit Ghent, Lab. for Analytical Chemistry, Ed. Dams, R., p 99 (1988). 10. Van den Bergen, E.A., Jonkers, G., Strijckmans, K., Goethals, P., 407 (1989). Nucl. Geophys. 11. Williams, C.W., Crabtree, M.C., Burke, M.R., Keyser, R.M., Burgiss, S.G., Hoffman, E.J. and Phelps, M.E., IEEE Trans. on Nucl. Sci. NS-28, 1736 (1981). 12. Hoffman, E.J., Phelps, M.E. and Huang, S.C., J. Nucl. Med.24.245 (1983). 13. Vonkeman, K.A., Jonkers, G., Van der Wal, S.W.A., Oosterbeek, H., De Jong, J. and Van Santen, R.A., Part 11, submitted to J. Catal. 14. Vonkeman, K.A., Van der Wal, S.W.A., Jonkers, G. and Van Santen, R.A., submitted to Chem. Eng. Sci. 461 (1980). 15. Yates, J.T. Jr., Thiel, P.A. and Memll, R.P., J. Catal. 16. Oh, S.H., Fisher, G.B., Carpenter, J.E. and Goodman, D.W., J. Catal. 100,360 (1986). 17. Vonkeman, K.A., Van der Wal, S.W.A., Jonkers, G. and Van Santen, R.A., submitted to Chem. Eng. Sci. 18. Jin, T., Okuhara, T., Mains, G.J. and White, J.M., J. Phys. Chem.91, 3310 (1987). 19. Daniel, D.W., J. Phys. C h e m . 2 , 3891 (1988). 20. Li, C., Sakata, Y., Arai, T., Domen, K. Maruya, K. and Onishi, T. J. Chem. SOC.Faraday Trans. 1 8341,929 and m,1451 (1989). 21. Barrault, J. and Alouche, A., Appl. Catal. 58,255 (1990). 22. Lynch, D.T., Emig, G. and Wanke, S.E., J. Catal.97.456

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A. Crucq (Editor), Catalysis and Automotive Pollution Control II 0 199 1 Elsevier Science Publishers B.V.. Amsterdam

253

TPD AND XPS STUDIES OF co AND NO ON HIGHLY DISPERSED PT+RHAUTOMOTIVE EXHAUST CATALYSTS: EVIDENCE FOR NOBLE METAL-CERIA INTERACTION

P. Loof (la), B. Kasemo (la), L. Bjornkvist(Ib), S . Andersson(2) and A. Frestad(2)

( I ) Chalmers University of Technology, S-41296 Gothenburg, Sweden (a)Physics Department (b) Chemical Department (2) Svenska Emissionsteknik AB, S-44501 Bohus, Sweden Abstract Temperature programmed desorption(TPD) measurements of CO and NO, flow reactor measurements, and XPS analysis were performed on Pt,Rh and Pt+Rh supported on alumina, alumina+ceria and ceria. Ceria induces significant modifications in the CO, NO TPD spectra and in the activity of the catalysts, indicating noble metal-ceria interaction but no evidence for charge transfer between Pt and ceria was found in the XPS spectra. Successive TPD runs of NO showed that ceria-containing samples maintained their capacity to dissociate NO for longer time periods than catalysts without ceria and also that there was a gradual decrease in the low temperature N2 desorption peak associated with noble metal-ceria interaction.The observed influence of ceria on the CO TPD spectra and on the steady-state CO oxidation activity vanishes for samples exposed to SO2 demonstrating that SO2 poisons the noble metal-ceria interaction. This is probably associated with the formation of Ce3-sulphate, as demonstrated by XPS spectra, which blocks double bonded CO and/or dicarbonyl sites. Under oscillating conditions the SO2 poisoning is less severe. The noble metal ceria interaction may be associated with oxygen vacancies in ceria adjacent to the noble metal particles. 1. INTRODUCTION

Metal-support interaction and the effect of SO2 in catalytic reactions over supported metal catalysts has received considerable attention over the past years (1-12). In this work we have addressed these questions for the specific case of Pt,Rh and Pt+Rh supported on alumina, ceria and alumina+ceria. For earlier results on the role of ceria see e.g. (3,6). The main methods of characterization were temperature programmed desorption (TPD) of CO and NO and XPS (X-ray photon spectroscopy) of highly

254

dispersed platinum and rhodium. This is an extension of previous work (17). Kinetic measurements were made on the activity of the catalysts with and without SO2 in the feedgases. The results reveal significant metal-support interaction. 2. EXPERIMENTAL

2.1 Sample preparation Three different types of supports were used in this study: A1203 which contained the gamma and delta phases with a BET-area of 140 m2/g. High surface area Ce02 with a BET-area of 107 m2/g. (b) Ce02 on A1203 with a CeO2 content of 20 wt% and a total BET-area (c) of 120 m2/g. The A1203 was impregnated with an aqueous solution of Ce(N03)3, dried and calcined (5OO0C, 2 hr, air). Coated monoliths, ceria and alumina powder were impregnated with an aqueous solution of H2PtC16 and/or RhC13, dried and calcined at 500°C in air for two hours. The noble metal loading of the monolithic samples was 1.5 wt% Pt and/or.0.3 wt% Rh based on the weight of the washcoat support. The noble metal loading of the ceria and alumina powder was 1 or 3 wt% Pt or 1 wt% Rh. In the flow-reactor and TPD measurements monolithic samples (400 cells per square inch) were used. TPD measurements were also conducted with ceria powder samples. In the XPS measurements disc shaped pellets, obtained from powdered alumina and ceria, were used. (a)

2.2 TPD The TPD experiments were performed in a quartz flow reactor cell described previously (13). The reactor with the sample is connected on-line to a mass spectrometer which continously analyzes the composition of the gas sampled by a quartz capillary-type probe (14) positioned about 4 mm down stream the catalyst. The temperature of the reactor can be raised linearly up to 1000 K by an external heating coil. An inert carrier gas (Ar) flows continously over the sample at atmospheric pressure. The selected gas flow rates of the carrier gas were 60 ml/min and 30 mVmin for 1 gram and 0.5 gram samples, respectively. The catalyst temperature is measured by a O.lmm diam. chromel-alumel thermocouple located in one of the channels of the monolithic sample, or in the center of the powder in the case of ceria powder samples. Quantitative measurements were obtained by calibration of the mass spectrometer with known mixtures of CO,NO, etc in Ar. Control runs with

255

the reaction cell empty were performed at regular intervals to check that wall reactions did not influence the measurements. The gas mixtures used were ultra high purity 4% CO in Ar, 4% H2 in Ar, 4% NO in Ar and 2% 0 2 in Ar and 100% Ar, respectively (99.9997% purity). All measurements were performed at 1 atm. The samples (except for the samples exposed to S 0 2 ) were always pretreated in the following way, prior to experiments. First the sample was oxidized in 2% 0 2 in Ar for 5 minutes at 600°C to remove carbon contamination. It was then reduced in 4% H2 in Ar for the same time and temperature. The reaction cell was thereafter flushed with pure Ar for 1 minute and then cooled down to room temperature over a period of 7 minutes. CO was then adsorbed, by continous flow of the gas over the catalyst for 2 min. and NO by pulse injection into the carrier gas. After gas adsorption TPD runs were performed by heating the reaction cell by an external heating coil at a rate of 2"C/s. This rate was found an optimum compromise to avoid temperature gradients, to minimize readsorption effects, and to maximize the sensitivity. After CO adsorption the CO and C02-signals were continuously recorded during the TPD run, and after NO adsorption NO, N 2 0 and N2-signals were recorded. 2.3 XPS A Ce2(S04)3.xH20 standard was prepared by precipitation, mixing hot aqueous solutions of Ce(N03)3 and Na2S04. The precipitate was thoroughly washed with hot distilled water and subsequently pressed into disc-shaped pellets ( 13 mm diam.) in the wet condition. The pellets were dried at 120°C in air over night. The samples were characterized with X-ray powder diffraction and BET surface area measurements. For the XPS-analyses the powder was compressed in a die to disc shaped pellets. The Hewlett Packard ESCA instrument (5950A) used is equipped with an auxillary vacuum system. This includes a furnace (20+11OO0C) , in which the samples can be heat treated in a desired atmosphere. The oxidation of catalyst samples was made at 500°C and 200 mbar oxygen pressure and reduction was performed at 500°C and 40 mbar of hydrogen in nitrogen (250 mbar total pressure), for 15 min. Calibration The spectrometer was calibrated linearly against the Au 4f7/2 and Cu 2 ~ 3 1 2peaks at 84.0 and 932.7 eV respectively. The static charging of nonconducting samples was compensated for by the use of an electron flood gun, emitting low energy electrons to the sample surface (typically 4 eV and

256

0.2 mA). A small spot of gold was sputtered onto the reference samples to obtain an absolute binding energy reference (15). The accuracy of the calibration method used was checked by the reproducibility of repeated measurements and by correlating the measured binding energies (B.E.) of support signals, such as A12p and 0 1 s of y-Al2O3, with literature values. The accuracy has, in this way, been determined to better than f 0.2 eV.

2.4 Flow reactor measurements. This reactor consisted of a quartz tube placed in a furnace and a sample holder made of quartz (this is a different reactor than the one used for TPD). The gas mixture at the reactor inlet consisted of 0 2 , C02, H20 and N2. CO, C3H6, NO, SO2 and 0 2 were added at a mixing zone in the sample holder. All the gas flows were controlled by mass flow-meters. The water was pumped through a vaporizer. To simulate the lambda-oscillations part of CO and 0 2 were alternately added through two magnetic valves. They were controlled in such a way that the oscillation frequency and amplitude could be adjusted to predetermined values. The concentrations of the different gases at the reactor outlet were determined using conventional gas analysis equipment. In the beginning of a run the sample inlet temperature was kept at 150°C until the gas composition was stable. During a run, data were sampled with a sampling frequency of 1 Hz. Two types of tests were made:

(a)Steady-state conditions. The activity of the catalyst sample was measured as a function of reactor inlet temperature between 150°C and 450°C. The space velocity was 65.000 hr-1.Table 1 shows the gas composition. A first run was made to condition the sample which was kept at 450°C for 30 min. before cooling the sample to 150°C. In the second and third runs the activity was measured with 0 vppm and 20 vppm SO2 respectively. Table 1 Gas comDosition (steadv-state conditions). Nature of gas Concentration

CO 1.8%

02 0.24%

H20 10%

co2 10.9%

so2 10000) (2) some of which (Polycyclic Aromatic Hydrocarbons) are suspected to be cancerous (4). In the last years the growing concern of worldwide countries toward environmental problems triggered the development of new technologies for pollutant removal from engine exhausts. In particular, ceramic honeycomb wall-flow filters and wire mesh traps, employed for Diesel exhaust filtering,

*

Present address: Dipartimento di Ingegneria Chimica e Alimentare Universith di Salerno 84081 Baronissi (SA).

324

showed a high soot removal efficiency (5). In these systems periodic soot bum out is required to allow for filter regeneration. However, spontaneous thermal ignition of captured soot is difficult to obtain needing high temperature or high oxygen partial pressure in the gas exhausted by the engine. This leads to soot accumulation in the trap that causes high engine backpressure and filter overheating during regeneration, with consequent trap failure (6). The use of a catalyst to decrease ignition temperature allows for more frequent combustions of captured soot so keeping low both filter pressure drop and operating temperature. Research work in this field proceeded through two different lines. Car (7) and filter manufacturers (5) operated on full scale systems building catalytic traps and coupling them to the exhaust manifolds of Diesel engines. However, they used metal based catalysts often borrowed from spark ignition engine applications. The main role of the catalyst in this case is to catalyze the combustion of unburned hydrocarbons present in the gas, slightly increasing exhaust temperature and indirectly enhancing soot oxidation. On the contrary, academic researchers operated on a laboratory scale developing catalysts specific for soot combustion and testing their activity with samples of carbonaceous materials. The catalyst, generally, contains transition metal oxides or alkaline metal compounds and is derived from those employed for coal oxidation and gasification (8-1 1). This type of catalyst plays a direct role in the reaction of carbon combustion as it has been shown by studies carried out on graphite oxidation (12). A supported catalyst (1 37AA), containing vanadium, copper and potassium compounds and active already at 300-350°C in soot oxidation, has been developed (13) and patented (14) by the present authors. Its catalytic activity in the oxidation of Diesel soot has been investigated. The results of catalytic combustion experiments, carried out with a differential reactor, are discussed in order to derive a reaction kinetic equation. EXPERIMENTAL

Catalyst Catalyst 137AA was prepared by wet impregnation of a-A1203 powder with aqueous solutions of ammonium vanadate N H 4 V 0 3 , copper chloride CuC12*2H20 and potassium chloride KC1 (BAKER Chemicals). After drying at 120°C the solid sample was calcinated at 700°C overnight (14). Chemical analysis of the catalyst was performed by coulorimetry and atomic absorption spectrophotometry. Catalyst characterization was carried out by different techniques such as X-ray diffraction, thermal analysis and nitrogen adsorption for BET surface area measurement.

325

Carbonaceous materials Soot particulates were generated by a direct injection single cylinder Diesel engine (Ruggerini RP 170), naturally aspirated, aircooled, with a compression ratio of 18 and displacement of 746 cm 3 . Commercially available Diesel fuel, with 0.3 wt % of sulfur and cetane number = 50, was fed to the engine. Soot samples were collected during steady state operating conditions of the engine : 2000 r.p.m., ratio between air and fuel mass flow rates (a)= 17, 25 and 51, start of combustion -5" crank angle before top dead center. From now on soot samples obtained at different (a) will be recalled as DS17, DS25 and DS51, respectively. Their volatile content was determined by thermal analysis up to 800°C in nitrogen flow. Commercial carbon blacks (CBN110 and CBN330 DEGUSSA) and pure graphite were employed as reference materials. Reactivitv tests The performance of catalyst 137AA in the combustion of such different carbonaceous materials was preliminarly determined by simultaneous TG-DTG-DTA analysis carried out with a NETZSCH STA 409 analyzer. Samples, handly mixed with catalyst without pressing, were loaded in an alumina crucible and heated in flowing high purity cylinder air (200 cm3/min) at 20"C/min rate from room temperature. Soot-kaolin mixtures at the same weight ratio as for soot-catalyst mixtures were loaded to the analyzer when performing thermal combustion tests without catalyst. REACTOR

TC

OVEN

CO2 ANALYZER

I I

I

I k L J COMPUTER

b'ig. I :Scheme of the experimental apparatus for kinetic runs. (mfc, mass flow controllers; cw, cooling water).

Catalvtic kinetic runs Catalytic combustion tests were determined by the experimental apparatus shown in Fig. 1. Cylinder air and nitrogen (99.999 % purity) were mixed downstream the mass flow controllers (HI-TECH) for

326

obtaining different oxygen partial pressures. A 20mm I.D. glass tubular flow reactor, heated by an electrical furnace driven by a temperature programmercontroller, was loaded with 100 mg of catalyst 137AA mixed to different amounts of carbonaceous material with a mean size of 10 pm. Reactor temperature was measured by a Ni-Cr thermocouple which can be moved inside an axial sheat along the reactor. Gas outlet from the reactor was cooled down by water in a glass heat exchanger and fed to a continuous C 0 2 NDIR analyzer (URAS 3K, HARTMANN & BRAUN). The signal from the analyzer was sent to a personal computer for the calculation of carbon conversion and combustion rate. All kinetic runs were carried out under differential conditions and constant temperature. RESULTS AND DISCUSSION

The chemical content of the main components of catalyst 137AA is K 6.9, Cu 6.2 and V 3.4 wt %. Therefore copper content was partially lowered whereas metal amount and ratio of V and K resulted similar to those employed for catalyst preparation, indicating that calcination at 700°C caused only a small loss of active components. Results of the thermal analysis of the catalyst precursor are shown in Fig. 2a and 2b. From the first figure, where the TG curve (weight loss % of impregnated a-A1203) is reported together with its derivative (DTG curve, arbitrary scale), three main weight loss steps are evident. Up to about 150°C a 2% loss is associated to dehydration of copper chloride also corresponding in Fig. 2b to the endotherm peak of the DTA curve (differential thermal analysis, arbitrary scale) centered at about 130°C. The following TG step in Fig.2a (from 180°C to 350°C) corresponds to a complex set of DTA signals (Fig. 2b) which should be attributed to deammoniation with consequent formation of vanadates. In the range 350450°C Cu-K and Cu-V interactions occur leading to binary phases as effect of the stabilizing role of potassium (15). Indeed, in the absence of potassium complete decomposition of copper chloride has been observed (15). Partial decomposition of copper chloride should be responsible of the small weight loss up to 700°C (Fig. 2a) and of the corresponding exothermic signals (Fig. 2b). The surface area of the catalyst after calcination (2.7 m2/g) was comparable with A1203 surface area, showing that agglomeration of catalyst particles due to calcination was not significant. Prolonged calcination up to 20 hours did not result in any modification of the catalyst. X-ray diffraction data of calcined catalyst are reported in TABLE 1. Preliminary analysis of these results allows to associate the main signals to crystalline phases of the catalyst. Therefore, together with the diffraction peaks

321

of a-A1203, the signals of KC1 and K2CuC14 are present. The remaining peaks can be associated to the formation of mixed phases K V O,, whose X Y identification needs deeper investigation. Nevertheless, the presence of all these phases is compatible with the findings of thermal analysis discussed above. T ("C)

weight

I D S5 (%I

a

1

400

30

\

6001+

DTG

T

800

1000

0.0

*0° , 0 0 0 1

0.5

1.o

1.5

1"

u

t 0.0

1 ' O

20

10

T

a5

1.o

1.5 2 t (hr)

Fig. 2 :Thermal analysis of the catalyst precursor (50 mg samples; 20 "Clmin heating rate). a: TG and DTG curves; b: DTA curve. Performances of catalyst 137AA in the combustion of Diesel engine soot were at first investigated by simultaneous TG-DTG-DTA analysis. Soot samples, generated under the engine operating conditions specified in EXPERIMENTAL, were expected to have different content of volatile matter.

328

TABLE 1 :X-ray diffraction data of catalyst 137AA d(A) 1 5.48 3.96 3.48 3.22 3.19 3.14 2.97

w w m w w s w

(c) (c) (a) (d) (d) (b) (d)

d(A) 1

d(A) 1

2.80 w (d) 2.71 w (c) 2.55 s (a) 2.38 w (a) 2.22 w (b) 2.09 s (a) 1.96 w (a)

1.87 1.81 1.74 1.60 1.57 1.51 1.40

w w m s w w m

(c) (b)

(a) (a) (a,b) (a) (a,b)

(a) a-A1203, (b) KCl, (c) K2CuC14, (d) K,VyO, eight loss

I%( -.-'

if

i!

200

;L-.....

----_

+....-.- ........._....... ..........

400

600

800

T IT1

Fig.3 : Thermogravimetric curves of carbonaceous materials in flowing nitrogen. - _ _ _D S l 7 ; -DS2.5; .*--*-CBNllO; 50 mg samples; 20 "Clmin heating rate) This is confirmed by thermogravimetric analysis of soot in nitrogen flow, shown in Fig. 3 for DS17 and DS25. The former loses only 3% weight up to 800°C whereas the latter shows a higher volatile content reaching 15% weight loss. Carbon black CBNl 10 behaves like a dry soot (DS17) (Fig. 3). When the soot sample is heated in air flow the weight loss curve, corrected for the loss in inert gas, indicates that the combustion starts at about 500°C (curve T in Fig. 4). After 600°C T curve becomes steeper as ignition occurs and at 820°C

329

soot is burnt out. The effect of catalyst on soot combustion is shown in the same Figure (curve C). Temperatures of soot combustion are so much lowered than complete conversion is observed already at about 400°C. Moreover, the temperature at which the DTG curve displays a maximum decreases from 650°C down to 336°C. The comparison between TG analysis of CBNllO with and without catalyst, reported in Fig. 5 , indicates that catalyst activity does not change significantly, ignition and DTG maximum temperatures being comparable to those of soot. In contrast, the range of temperatures in which catalytic combustion of graphite occurs is larger resulting in a higher bum out temperature, especially for crystalline graphite (Fig. 6). Nevertheless, it must be remarked that also in these cases the catalyst allows the combustion temperatures to be lowered by about 300°C. After the preliminary investigations on the activity of catalyst 137AA in the soot combustion process, the study was aimed to derive a kinetic equation for the reaction. In fact, deriving such an equation is an essential step for designing a catalytic trap. Making use of the differential reactor described above, catalytic reaction was studied under different experimental conditions in order to find the dependence of reaction rate on temperature and on reactant concentrations. In Fig. 7 the times necessary to obtain 20% and 40% of soot bum off are reported as a function of the initial soot/catalyst weight ratio in the mixture. Being the weight of catalyst constant (100 mg) for all the runs , the abscissa in Fig. 7 is equivalent to the initial weight of soot loaded in the reactor. Changing that weight by about 5 times does not influence the time of burn off, suggesting that the rate of soot mass change by catalytic oxidation (-dm/dt) is linearly dependent on the mass m. Indeed, only in this case the time to reach a given conversion is independent from the initial amount of the reactant. Even in the case of carbon black the time of burn off is not affected by the soot/catalyst ratio as shown in Fig. 8 for CBNl 10. The influence of oxygen partial pressure on the reaction rate has been also investigated. In Fig. 9 the amount -dm/mdt, as evaluated at 20% soot conversion, is reported as a function of oxygen partial pressure in the range 0.03-0.21 atm. The rate of catalytic combustion is strongly dependent on gas oxygen concentration and the order of reaction is 1. The same order of reaction was found for carbon black catalytic combustion as evidenced by the linear plots of Fig. 10 and 11. After finding the dependence of reaction rate on sample mass and oxygen partial pressure we were able to evaluate the apparent kinetic constant, k,from the equation: dm/dt = - k m Po2

(1)

330

0

25 5075 -

100

C

I1 ___

1 .

Fig. 4 :Thermogravimetric curves of DSI 7 in flowing air. T, no catalyst; C, sootlcatalyst mixture, 15% weight of soot.

0

Fig. 5 :Thermogravimetric curves of CBNllO inflowing air. T, no catalyst; C, CBlcatalyst mixture, 15% weight of CB.

0

Fig. 6 :Thermogravimetric curves of graphite (a=amorphous, c=crystalline) in flowing air. T, no catalyst; C, graphitelcatalyst mixture, 15% weight of graphite,

33 1

3

1-

0

0

2-

-

0

0

r.-1

Fig. 7 :( o )20% and ( )40% burn off times in air of DSI 7 as afunction of the initial weight of soot (T=360"C,gasjlow rate= 500 cclmin).

-

0

0 m

0

"

0

Y

"

9

0

1

Fig. 8 :( o ) 20% and ( ) 40% burn offtimes in air of CBNIIO as a function of the initial weight of CB (T=340°C, gasjlow rate= 500 cclmin).

332

04

dmirndt h ~ n - ~ l

03-

02-

000

005

010

015

PO2 latml

020

025

Fig. 9 : Effect of oxygen partial pressure on catalytic combustion of D S l 7 (T= 340"C, 10% wt of soot, gasflow rate=500 cclmin).

0.0 000

0.05

0.10

0 15

PO2 latml

0.20

0.25

Fig. 10 :Effect of oxygen partial pressure on catalytic combustion of CBNlIO (T=330"C,5% wt of CB, gasflow rate=.500 cclmin).

Fig. I 1 :Effect of oxygen partial pressure on catalytic combustion of CBN330 (T=340"C,5% wt of CB, gasflow rate=500 cclmin).

333

and the apparent energy of activation from the Arrhenius type equation: k = ko exp [-Em (T + 273.1) ]

(2)

In Fig. 12 values of log (- dm/m dt) against the reverse absolute temperature of the catalytic combustion of DS17 and DS51 are reported. For both samples the apparent activation energy is 100 kJ/mol, markedly lower than the values reported for soot thermal combustion (16). The independence of the activation energy on the volatile content of soot indicates that catalyst 137AA produces a direct effect on the oxidation of the carbonaceous matrix of soot. This finding seems to be confirmed by the data obtained from the catalytic combustion of carbon blacks. The apparent energy of activation evaluated from data reported in Fig. 13 and 14 is 84 kJ/mol for CBNllO and 90 kJ/mol for CBN330. It must be pointed out that the values of -dm/dt in Figs 12-14 refer to 20% soot or carbon black conversion. The apparent energy of activation evaluated for higher conversions changes less than 15 kJ/mole up to about 50% bum off. At higher conversions a progressive reduction of the apparent kinetic constant k seems to occur, suggesting that at these values of conversion the process proceedes slower. This could be attributed to the progressively less efficient contact between the two solid phases with the advance of the reaction. Actually, due to the nature of compounds detected by X-Ray diffraction analysis, it should be possible the change to liquid phases. Nevertheless, thermoanalysis results do not give any evidence of thermal effects connected to those transformations. CONCLUSIONS

Catalyst 137AA is able to reduce ignition and combustion temperatures of all tested carbonaceous materials by about 300°C. In the presence of the catalyst the rate of combustion is enhanced by two or three order of magnitude. These are essential features for potential application in soot filtering traps for automotive engines. Given the complexity of the reacting system constituted by two solid phases and one gas phase, a rigorous approach to the reaction kinetics is not simple to be performed. Nevertheless, this work has shown that at least in the range of conversion up to 50% a simple kinetic equation can be used to describe the experimental results. In such an equation the catalytic combustion rate of soot is linearly dependent on both soot mass and oxygen partial pressure. The apparent activation energy is decreased by the catalyst to about 100 kJ/mol. The catalyst exhibits comparable performances in the combustion of carbon black.

334

Fig. 12 :Effect of temperature on the catalytic combustion of DSI 7 (0)and DS51 (0)in air at 20% conversion (5% wt of soot, gas flow rate = 500 cclmin). t

0.1 :

Fig. 13 :EfSect of temperature on the catalytic combustion of CBNllO in air at 20% conversion (5% wt of CB, gasflow rate = 500 cc/min).4 dm/rndt Irnin-'l

1,

t

0.OlL 1.5

1.6

lOOO/r IK-'I

17

1 1.8

Fig. 14 :Effect of temperature on the catalytic combustion of CBN330 in air at 20% conversion (5% wt of CB, gas flow rate = 500 cclmin).

335 LIST OF SYMBOLS

a E

k m PO2

R t T

ratio between air and fuel mass flow rates fed to the engine apparent activation energy, kJ/mol apparent kinetic constant, atm-1 min-1 mass of carbon oxygen concentration, atm gas law constant, kJ K/mol time temperature, "C ACKNOWLEDGEMENTS

The authors gratefully thank Dr. P. Corbo of Istituto Motori C.N.R. for providing Diesel engine soot samples. One of us (P.P.) acknowledges the C.N.R. of Italy for a post-graduate scholarship. REFERENCES M. Chinon Studies, Surf. Sci. Catal. 30 (1987) 1-10 M.P. Walsh Studies, Surf. Sci. Catal. 30 (1987) 51-67 C.P. Fang and D.B. Kittelson, SAE Paper No 840362, (1984) C.F. Funkebush, D.G. Leddy and J.H. Johnson, SAE Paper No 790418 (1979) B. Engler, E. Roberstein and H. Volker, S A E Paper No860007 (1986) S.T. Gulati, SAE Paper No 860008 (1986) 11-18 Y. Niura, K. Ohkubo and K. Yagi, SAE Paper No 860290 (1986) 163-172 T. Inui and T. Otowa, Appl. Catal. 14 (1985) 83-93 P.J. Goethel and T.Y. Yang, J. Catal. 119 (1989) 201-214 T. Inui, T. Otowa and Y. Takegami, J. Catal. 76 (1982) 84-92 C.A. Minus and J.R. Pabst, Fuel 62 (1983) 176-179 K. Hashimoto, K. Miura, JJ. Xu, A. Watanabe and H.Masukami, Fuel 65 (1986) 489-494 P. Ciambelli, P. Corbo, P. Parrella, M. Scialo' and S.Vaccaro , Thermochimica Acta 162 (1990) 83-89 P. Ciambelli, P. Corbo, M. Scialo' and S. Vaccaro, Italian Patent No A40421/88 (1988) P. Ciambelli, P. Parrella, and S. Vaccaro, Submitted for publication (1990) K.B. Lee, M.W. Thring and J.M. Beer, Comb. Flame 6 (1962) 137-145.


A. Crucq (Editor), Catalysis and Automotive Pollution Control II 0 199 1 Elsevier Science Publishers B.V.. Amsterdam

337

CATALYSTS FOR DIESEL POWERED VEHICLES Douglas J. Ball and Robert G. Stack

A.C.Rochester Division of G.M, Flint, MI-48556,USA

ABSTRACT A theory of how diesel oxidation flow-through type converters reduce gaseous and particulate emissions is explained. The effects of substrate, catalyst support and application of noble metals are discussed and investigated. Experiments were performed to determine, (1) the sulfur storage characteristic of silica and alumina catalyst supports, (2) the SO2 oxidation abilities of platinum and palladium and (3) the effects of ceramic and metal monolithic substrates on converter emission performance. Light duty FTP results are also presented.

INTRODUCTION

Diesel powered vehicles are of concern to many world governments. Some governments have chosen to legislate significant reductions of vehicle emissions, while others have them under consideration. Table 1 shows the current and proposed diesel emission laws for several major world markets. The most significant particulate requirements are: light duty US Federal 0.20 g/mile, light duty California 0.08 g/mile, and heavy duty US truck 0.10 ghhp-h. Several European countries have adopted the US Federal light duty requirements. Manufacturers of diesel engines are addressing these emission requirements through the modernization of engines. Changes such as: higher injection pressures, optimization of injection pressure, turbocharging and combustion chamber optimization to name 'a few, have shown potential for reducing particulate mass while maintaining vehicle performance (2). Additional methods such as; fuel additives (3) and exhaust aftertreatment have shown potential for reducing particulates (1,4,5,6,7). The use of diesel exhaust traps (or filters) that collect and oxidize particulates have demonstrated good performance at relatively low mileage (8,9), but have not proven to be effective with respect to high mileage durability. Other drawbacks are the initial costs (lo), vehicle performance and the difficulties the vehicle has in producing adequate trap regeneration conditions at appropriate driving intervals (8).

338

Table 1 - Current and Proposed Worldwide Diesel Emissions Laws Market

Test

Class

HC

CO

US 87

FTP g/mile

LD

0.4 1

3.4

1.0

-

0.20

Calif. 1989

FTP g/mile

LD

0.39*

7.0

0.4

-

0.08

us 91 Truck

HD Cycle gbhp-h

HD

1.3

15.5

5.0

-

0.25

us 94 Truck

HDCycle gbhp-h

HD

1.3

15.5

5.0

-

0.10

Japan 1990

10 Mode ~1,265kg 0.62 gkm >1,265kg 0.62

2.7 2.7

0.72 0.84

-

None None

Japan 1994

10 Mode ~1,265kg 0.62 g/km >1,265kg 0.62

2.7 2.7

0.5 0.6

-

0.2 0.2

EEC 1990

EEC g/test

19 30 25

3.5

5 8 6.5

1.10 1.10 1.10

2.72

-

0.97

0.19

EEC 1993

EEC +EUDC g/km

4.4L 1.4-2.OL >2.0L

-

ALL

-

NOx HC+NOx

Part

*- non-methane As observed by others, a diesel catalytic converter of a flow-through type design can be quite effective in lowering both particulate and gaseous tailpipe emissions (1,2,7,11). Additionally, such a design requires no regeneration and imposes virtually a negligible effect on exhaust system backpressure. To achieve maximum organic fraction and gaseous emissions oxidation, a diesel catalytic converter requires an understanding of the effects certain critical parameters such as; noble metals, substrate design, catalyst support materials and application environment. To obtain the lowest vehicle tailpipe emissions, a total systems approach is required where; the design of the engine, the engine control system and the catalytic converter is balanced.

339

This paper presents a theory on how diesel catalytic converters work. It also includes results of investigations designed to evaluate the effects of the before mentioned critical parameters on emissions. THEORY OF OPERATION

The combustion products of spark ignited gasoline and diesel engines vary considerably. Modern gasoline engines with closed-loop emission controls produce exhaust that is near chemical stoichiometry and at temperatures of 375 to 850°C. The exhaust contains significant amounts of hydrocarbons (HC), carbon monoxide (CO) and oxides of nitrogen (NOx) that must be reduced using a catalytic converter in order to meet the current US light duty requirements. Gasoline engine particulate emissions are within the US light duty and heavy duty requirements. Diesel engines, on the other hand, combust lean airfuel mixtures of >20/1 that produce exhaust temperatures of 100-450°C. This exhaust contains significant amounts of particulates that may require an aftertreatment device such as a catalytic converter in order to meet current and pending requirments. However, the concentrations of HC, CO, and NOx in diesel exhaust are typically below the current US light duty and heavy duty requirements. Gasoline converters using straight flow-through monoliths are very effective at reacting the gaseous constituents in the exhaust such as HC, CO and NOx to water vapor, C02, and nitrogen. These gases must diffuse and adsorb onto the catalyst surface before a reaction can take place. The driving force of diffusion, is the boundary layer concentration gradient of the gaseous species between the bulk gas phase and the catalyst surface. Once at the catalyst surface, the gases are chemically adsorbed. Chemical adsorption, or chemisorption, is a process that describes the adsorption of reactive gases onto an active catalyst surface. Chemisorptions generally occur at high temperatures and the rate of adsorption increases with temperature. This process is highly exothermic and the gases adsorb onto the catalyst surface in a monolayer. In order to properly describe how a diesel converter works it is necessary to first classify the constituents in the exhaust. See Table 2. The particulates are defined, as the species that collect on the filter during performance testing. The ash consists of inorganic compounds containing elements like zinc and phosphorous that originate from fuel and engine oil. The sulfate particulate and the associated waters of hydration originate from the combustion of sulfur containing fuels and the oxidation of SO2 by the catalyst. The organic particulate is a collection of relatively high boiling point compounds originating from the fuel, engine oil and their combustion products.

340

Table 2 Diesel Exhaust pecies

Other particulates, include catalyst and iron oxide from the exhaust system. As for the gaseous species, the Particulates Gases amounts of HC, CO, and NOx in the exhaust are not of primary concern, since Ash, Carbon HC Sulfate & H20 co they are typically below regulated requirements. SO2 however, can be NOx Organic Particulates Other so2 oxidized to sulfate particulate by the catalyst. SO2 is formed in the combustion chamber by the combustion of fuc s containing sulfur. Roughly, 99% of sulfur in the fuel is converted to SO2 in the engine. Since the exhaust temperatures of diesel exhaust are below the oxidation temperature of carbon, a diesel converter achieves particulate reduction by maximizing the oxidation of the organic particulate. In general, the organic particulate can exist in three states in the exhaust system (1); (i)- as a gas in the exhaust system, which will condense out in the dilution tunnel onto the particulate filter, (ii)- as a mist or aerosol, and (iii)- absorbed onto the carbon particulate. The ratio of aerosol versus gaseous organic particulate increases with decreasing temperature. In states 2 and 3 the organic particulates behave more like a particulate than a gas, having mass and momentum relative to the gas stream. A large portion of the organic particulates do not chemisorb onto the catalyst surface like HC, CO, and NOx do in gasoline converters, because of the relatively low exhaust temperatures and the high boiling points of the organic particulates. Some of the organic particulate that remains as a gas in the exhaust stream prior to the converter physically adsorbs. The physical adsorption of a species onto a surface occurs when it is near it's condensation temperature. The rate of physical adsorption increases with decreasing temperature and the species can be adsorbed in multilayer coverage. This statement implies that the effectiveness of a converter may decrease as exhaust temperatures rise. Studies performed by Horiuchi et al. (7) also suggests that the organic particulate is being physically adsorbed at lower temperatures and oxidized before they desorb. It is important to operate a diesel converter at temperatures where the physical adsorption of the organic particulate can occur and where the catalyst is kinetically active. In addition, a diesel converter can be designed to improve the collision of organic particulates onto the catalyst surface. A converter with a tortuous flow path (the exhaust through the converter changes direction) would aid in the collision of the organic particulates that have mass and momentum with the catalyst surface. Straight flow-through monoliths then should not perform as well as some metal monolith and pelleted converter designs that have tortuous flow paths.

34 1

Not only must a diesel converter be designed to maximize the oxidation of the organic particulate, it also must not create an sulfate particulate. Sulfate particulate can be derived from three sources. (i) - Sulfur trioxide (SO3) is formed in the combustion chamber and combines with water vapor in the exhaust to form sulfuric acid. The sulfuric acid and waters of hydration condense on the particulate filter during performance testing. (ii) - SO2 is oxidized by the catalytic converter to SO3 and combines with water vapor to form sulfuric acid. (iii) - Stored sulfur can be released from the catalyst support during high temperature transients to form sulfate particulate. Typically, SO3 can react with the alumina catalyst supports to form aluminum sulfate which will later decompose at elevated exhaust temperatures to alumina and SO3. Aluminum sulfate decomposes at 770°C,however experience with alumina supports has shown releases of sulfate particulate at lower temperatures (1). This suggests that the sulfur released from the alumina may be stored in a less fixed manner. Steps can be taken to minimize these sources of sulfate particulates. The use of lower sulfur fuels will directly lower engine out sulfate particulate values. A proper choice of noble metal catalyst can minimize the oxidation of SO2 to SO3. Catalyst support materials can be used that minimize the storage and release of sulfate particulate during exhaust temperature transients. In summary, for a diesel converter to maximize particulate efficiencies, it must operate within a temperature window that allows the physical adsorption and reaction the organic particulate without chemically adsorbing and reacting S02. In addition, the converter must be designed to maximize the contact of the organic particulate with the catalyst surface. However, it must be designed to pass the carbon, ash and other particulates through the converter. The storage of these particulates, can increase converter back pressure, cause a release of a black cloud during transient high flow conditions and if stored carbon were to ignite, it could cause irreparable damage to the catalyst. EXPERIMENTAL

Sulfir Storage Experiment It is important that a diesel converter is an emission stable device. More specifically, the converter must not release significant amounts of carbon or sulfate particulate during transient flow conditions. Release of black smoke or carbon can be minimized by manufacturing engines to produce less black smoke or by designing converters that do not store or release significant amounts of carbon. Transient sulfate emissions can be minimized by using lower sulfur containing fuels, or by operating the catalyst at temperatures that

342

do not oxidize SO2 to SO3 and/or by using a catalyst support that does not store or release sulfur during temperature transients. Experiments were performed to determine the sulfur storage characteristics of pelleted alumina and silica catalyst supports. Pelleted converters containing 2.6 liters of catalyst were used. The alumina supported catalyst had a BET surface area of 125 m2/g and a pellet diameter of 4-6mm. The silica supported catalyst had a BET of 275 m2/g and a pellet diameter of 4-6mm. Each converter contained 0.043 troy oz. of palladium. The converter were aged together on a 6.2L DDA V-8 engine, one converter on each exhaust manifold. The fuel contained 0.28 wt% sulfur. The aging schedule consisted of five, 50 hour aging intervals at temperatures of 200, 300, 400, 300, and 200°C, respectively. After each interval, a small portion of the catalyst was analyzed for sulfur using x-ray fluorescence. The results are shown in

Alumina

Silica

Fresh

200

300

400

300

200

Time at Temperature, 50 Hours @ each Temp. (Celsius)

Figure 1:Sulfur Storage on Silica and Alumina Catalyst Supports (I)

343

Figure 1 shows that the silica supported catalyst has an initial storage of sulfur to about 0.3% after the first aging interval. Then after the following four aging intervals, the amount of stored sulfur in the silica appears to be insensitive to changes in exhaust temperature. This implies that this catalyst support will not release any significant amounts of sulfur from the support during high temperature transients. The alumina catalyst support stored roughly 0.45% sulfur after the first aging interval at 200°C and continued to store sulfur to 0.83% after the second aging interval at 300°C. This implies that the alumina supported catalyst did not store sulfur to its equilibrium value after the first aging interval. The alumina support appeared not to store any additional sulfur after the third aging interval at 400°C. This would imply that an alumina support is at it's equilibrium storage value at 400°C. However, if all the alumina were to form aluminum sulfate, the weight percent of sulfur would be 28 wt%. The alumina support stored sulfur to 0.96% after the fourth aging interval and showed no further increase in sulfur storage after the fifth aging interval at 200°C. This experiment did not adequately show alumina's ability to release sulfur during high temperature transients. In conclusion, Figure 1 shows that the silica support stores less sulfur that the alumina support and that the silica does not store or release sulfur during temperature transients. SULFUR DIOXIDE OXIDATION

It is important to minimize the oxidation of SO2 to sulfate particulate, since oxidizing one gram of SO2 will produce 3.7 grams of sulfuric acid particulate, including the waters of hydration. SO2 oxidation can be minimized, by operating the converter at low temperatures where either oxidation of SO2 is minimal, adding base metals, and by chemically and thermally treating the catalyst. Noble metals also have different abilities to oxidize S02. Platinum is very effective at oxidizing SO2 in a lean environment. Adams et al. (12) showed that platinum oxidized three times the SO2 than palladium. Experiments were run in an effort to understand the ability of platinum and palladium to oxidize SO2 in diesel exhaust as a function of temperature. Three pelleted converters having 2.6 liters of catalyst volume were used. Silica catalyst support of 4-6mm pellet diameter were impregnated with platinum and palladium. Silica catalyst support was used based on previous experiments showing that it does not store or release any significant amounts of sulfur during temperature transients. The noble metal loadings of the three converters were 0.030 troy oz. platinum, 0.043 troy oz. palladium and 0.180 troy oz. palladium. Each converter was aged for 100 hours on a water brake dynamometer with a 6.2L DDA V-8 engine. The fuel contained 0.28 wt%

344

sulfur. See the appendix for a description of the aging schedule. After aging, the steady state performance of the converters were evaluated on a 6.2 liter DDA engine with an electric dynamometer at exhaust temperatures of, 500, 400, 300, 200, 150, 200, 300, 400, and 500"C,respectively. During each evaluation point the engine ran at 2000 rpm. The exhaust temperatures were achieved by varying the engine load with the dynamometer. Mini-dilution tunnels were used to collect the particulates before and after the converter.

FILTER APPARATUS

DRY TEST METER SAMPLE PUMP

~

Figure 2. Diesel Converter Performance Test Facility (1)

Figure 2 is a schematic of the converter performance test facility. Pallflex 47mm diameter filters were used. The particulate filters were analyzed for sulfate using a wet chemistry technique. It is assumed the SO2 oxidized by the converter is directly proportional to the sulfate particulate found on the filters. The performance evaluations at the same temperatures were averaged. Figure 3 shows that the platinum catalyst oxidized 3-5 times more SO2 than the palladium catalysts at temperatures above 200°C. The two palladium catalysts have similar SO2 oxidation profiles. However, the higher loaded palladium catalyst with 0.18 troy oz. of metal appears to oxidize less SO2 than the lower loaded catalyst at 0.042 troy oz. when evaluated at 400 and 500°C. This difference may be experimental error. If this is so, then it would appear that the oxidation of SO2 is not sensitive to palladium loading.

345 11 10 9

a Sulfate out Sulfate in

7

6 5 4

3 2 1

0

150

200

300 400 Temperature (Celsius)

500

Figure 3. Sulfate Particulate vs. Temperature (1) .28% wt Fuel Sulfur, Eval. on 6.2L DDA

Information from Figure 3 can be helpful in choosing a noble metal catalyst for an application. For light duty applications when exhaust temperatures do not exceed 250-300°C on the light duty US FTP cycle, palladium or platinum may be used. However, if off cycle temperatures exceed 30O-35O0C, platinum containing catalysts can produce significant amounts of sulfate particulate. For heavy duty applications where exhaust temperatures exceed 350-400°C on the heavy duty transient cycle, palladium should be used. It is estimated that if an engine manufacturer is to meet the US 1994 heavy duty particulate emission requirements, a catalytic converter can only a oxidize a.maximum 1% of SO2 in the exhaust to sulfate particulate. RESULTS

Evaluation of Fresh and Aged Alumina and Silica Supported Catalysts

Two metal monolith converters having 1.8 liters of catalyst volume were coated with alumina and silica washcoat technologies. The BET surface area of the alumina and silica washcoats were 125 and 200 m2/g, respectively. The metal monolith substrate was of a herringbone design at 35 cells/cm2. Each converter was impregnated with 0.05 troy oz. of palladium. Palladium was used to minimize SO2 oxidation at high exhaust temperatures so the effects of

346

sulfur storage and release could be seen. The converters were aged for 300 hours on the durability schedule described in the appendix. The particulate and sulfate efficiencies were determined at 400, 300, and 200°C before and after aging on a 6.2 liter DDA engine described in Figure 2. Sulfate efficiencies were measured at 400°C. Table 3 presents the results.

Table 3

-

Performance of Silica and Alumina Supported Catalysts

Converter support

Aging (hours

Alumina

0

400 300 200

-10 24 40

6

300

400 300 200 400 300 200

-120 -10 5 -33 56 81

16

400 300 200

-40 42 67

Silica

0

300

Particulate Evaluation Temperature ("C) (% Efficiency)

Sulfate Out Sulfate In

Both converters reduced particulate emissions during the fresh evaluation at 300 and 200°C. However, the silica supported catalyst had twice the particulate efficiencies at these temperatures. At the 400°C evaluation, both converters produced particulates. This is seen by the negative particulate efficiencies of -10 and -33 for the alumina and silica catalyst supports, respectively. The generated particulate is due to the formation of sulfate particulate from the catalyst oxidizing S02. After the 300 hours of aging the alumina supported catalyst performed poorer. The particulate efficiencies fell to -120, -10 and 5% for temperatures of 400, 300, and 200"C, respectively. This aged converter is generating significant amounts of sulfate particulate at 400°C and probably at 300°C also. The converter increased engine out sulfate emissions by 16 times at 400°C. The 400°C fresh evaluation of the alumina supported catalyst only increased sulfate emissions by six times. The difference in sulfate emissions of the fresh and aged alumina converter appears to be from the release of sulfur

347

(sulfate) from the catalyst support. The silica supported catalyst, on the other hand, did not exhibit a drastic change in emission performance after aging. After aging, the silica catalyst particulate efficiencies dropped about 15% at the three evaluation temperatures and the converter created about the same amount of sulfate particulate at 400°C. It is evident from Table 3 that the silica supported catalyst would be an emission stable device. The actual performance of alumina supported catalyst would probably be difficult to quantify as the converter is exposed to diesel exhaust for significant periods of time. The data shows that the performance of the alumina containing converter can change drastically with aging. If alumina is used, it should be stabilized in such a manner that it does not release significant amounts of sulfate particulate during high temperature operation. DIESEL CONVERTER PARAMETER STUDY

An experiment was performed to determine the effects of catalyst volume, substrate, and noble metal loading on fresh converter performance. The experiment compared the performance of converters having 1.8 and 1.4 liters of catalyst volume using either a 62 cell/cm2 ceramic monolith or a 35 cell/cm2 metal monolith substrate. The metal monolith substrate used a stacked herringbone design in which the flow of exhaust through the substrate follows a tortuous path. The substrates were coated with the same silica washcoat technology and impregnated with various combinations of noble metals. The converters were evaluated on a 1.9 liter turbocharged Lancia Dedra using the US light duty FI'P cycle. The average and peak temperatures during the FTP were 150 and 300°C, respectively. European specified fuel was used containing 0.20 wt.% sulfur. 30% of the particulate during the US FTP cycle was organic. Table 4 describes the converters that were built and tested. Prior to evaluation, each converter was aged for 24 hours on the dynamometer schedule described in the appendix. Each converter was evaluated three times on the vehicle. The averages of the FTP evaluations are shown in Table 5 in % FTP efficiency. The individual FTP results were regressed to determine the significance of the converter design parameters. Table 6 summarizes the analysis of the data. Keep in mind that the results of this study pertain to the silica washcoat technology investigated. The effects of different washcoat technologies, base metals additives, and other noble metals, such as rhodium, were not investigated.

348

Table 4

-

Diesel Converter Descriptions

I Substrate

No

Noble Metals (Troy oz per converter) I Pd Pt 0.00 0.15 0.00 0.09 0.15 0.09 0.15 0.03 0.15 0.03 0.15 0.09 0.15 0.00 0.09 0.00

Volume (liters) 1.4 1.4 1.8 1.8 1.4 1.4 1.8 1.8

Ceramic Ceramic Ceramic Ceramic Metallic Metallic Metallic Metallic

Table 5 - US Light Duty FTP Performance 1.9L Turbocharged Lancia Dedra ( % FTP Efficiency ) Converter No 1 2 3 4

co

HC 33 58 9 3 25 -5 21 31

5 6 7 8

Emissions Best Volume Best (liters) Substrate HC

NOx 4 2

2 87 1 3 12 2 7 64

co

1.4 1.4"

Ceramic

NOx Part.

1.4

Metallic*

5 0 9 1 8 -1

Noble Metal Pt Pd PtPd

+* -* +* -* -

Particulate 22 1 1 -8 14 8 23 19

Legend for Table 6.

* Parameter is significant to 90% level.

-* + Parameter is good for reducing emissions. +* - Parameter is poor for

+

reducing emissions.

All other listed parameters have F-Cal values of greater than 1.0. Unlisted parameters have F-Cal values less than 1.0. (F-Cai is defined as; the variance attributed to the parameter divided by the variance of the model)

349

From the results presented in Tables 5 and 6 the following statements are made. 1.- The smaller converters having 1.4 liters of volume appear to work best for HC, CO, and particulate emissions. This may be due to the higher concentration of noble metals in the smaller converters. 2.- The ceramic substrate may work best for CO emissions. However, the metal monolith worked best at reducing particulate emissions. This result appears to support statements make in the theory of operation, that straight flow through ceramic monoliths are effective at reducing gaseous emissions and that a substrate like the metal monolith with a tortuous flow path may be more effective at reducing particulates. In addition to the metal monolith having a tortuous flow path, it also has lower thermal inertia than the ceramic substrate. This unique thermal property may also improve particulate performance. Since the organic particulate physically adsorbs onto the catalyst surface at low temperatures, and the rate of physical adsorption increases with decreasing temperature, a substrate that gets cooler faster will physically adsorb more of the organic particulate. Also, if a portion of the organic particulate adsorbs at low temperature and is oxidized at higher exhaust temperatures, a substrate that heats up faster will oxidize a larger portion of the organic particulate before it desorbs. 3.- Platinum works best for reducing the HC and CO emissions, but is poor for NOx and particulate emissions. The particulate result is supported by Figure 3 which shows the ability of platinum to oxidize S02. It appears that platinum is producing considerable amounts of sulfate particulate. 4.-Palladium is poor for reducing HC and CO emissions. However, it appears to work well at reducing particulate and NOx emissions. The contrasting effects of palladium and platinum to reduce particulates is supported by Figure 3. Palladium is not producing any significant amounts of sulfate particulate during the evaluation. 5.- Do not mix platinum and palladium in a single converter. These converters performed poorly. SUMMARY

A diesel converter can be designed to specifically reduce engine out emissions. To maximize the reduction of particulates, the data and theory presented suggests that; 1 - A tortuous exhaust flow path would improve the contact of a portion of the organic particulate that has mass and momentum with the catalyst surface. 2 - A catalyst substrate with low thermal inertia may improve the adsorption and reaction of the organic particulate.

350

3 - Use a noble metal that does not oxidize S02. The choice of noble metal is application dependent. - For hot applications where exhaust temperatures exceed 300"C, palladium should be used. - For cooler applications, platinum or palladium may be used. However, when exhaust temperatures exceed 350°C significant sulfate emissions may be produced with platinum. 4 - Use a catalyst substrate material that does not store or release sulfur during temperature transients. To maximize the reduction of the gaseous species, HC and CO; 1 - Use platinum rather than palladium. 2 - Ceramic monoliths may work better than metal monoliths.

Acknowledgments The authors wish to thank the following groups and individuals; - Fiat Auto Spa DPSI for providing the vehicle for The Converter Parameter Study. - J. C. Schmitz and N. Meyrer who performed the FTP testing at The European Technical Center, in Luxembourg. - R. Beckmeyer, M. Tsang, W. Symons, S . Shaffer, S. Majkowski, J. Marklan, C. Metcalf, S. Barnhart, T. Kudza, and J. Finlayson of AC Rochester's Research and Test Lab Organization. APPENDIX

Diesel Dynamometer Aging Schedule Description Engine: 6.2L V-8 Cylinder w/split Exhaust Pipes Fuel: 0.28 wt.% Sulfur Capabilities: Ages two converters per engine. Step

Converter Inlet Temperature

1

2 3 4

5 6 7 8 9 10

1500 2500 1500 1000 1500 Repeat Steps 1-6 2500 1000 Repeat Steps 2-9

3.3 0.6 1.8 2.4 4.3

3.3 0.3

240 365

35 1 REFERENCES 1 - D. Ball, R. Stack, "Catalysts for Diesel Powered Vehicles", SAE Paper No. 9021 10, To be published October 1990. 2 - A. Gill, "Design Choices for 1990s Low Emission Diesel Engines", SAE Paper No. 880350, February 1988. 3 - T. Truex, W. Pierson, D. McKee, L. Casi and R. Baker, "Effects of Barium Fuel Additive and Fuel Sulfur Level on Diesel Particulate Emissions", Environmental Science and Technology, Volume 14, No. 9, September 1980. 4 - M. Arai, S. Miyashita and K. Sato, "Development and Selection of Diesel Particulate Trap Regeneration System", SAE Paper No. 870012, 1987. 5 - G. Lepperhoff and G. Kron, "Impact of Particulate Traps on the Hydrocarbon Fraction of Diesel Particulates", SAE Paper No. 859913, 1985. 6 - Y. Kiyota, K. Tsuji, S. Kume and 0. Nakayama, "Development of Diesel Particulate Trap Oxidizer System", SAE Paper No. 860294, 1986. 7 - M. Horiuchi, K. Saito and S. Ichihara, "The Effects of Flow-Through Type Oxidation Catalysts on the Particulate Reduction of 1990s Diesel Engines", SAE Paper No. 900600, February 1990. 8 - F. Indra, "Diesel Particulate Filters and Their Regeneration", SAE Paper No. 885151, 1988. 9 - M. Barris, "Durability Studies of the Trap Oxidizer Systems", SAE Paper No. 900108, February 1990. 10 - J. Wall, S. Shimpi and M. Yu, "Fuel Sulfur Reductions for Control of Diesel Particulate Emissions", SAE Paper No. 872139, November 1987. 11 - G. Andrews, I. Iheozor-Ejiofor and S . Pang, "Diesel Particulate SOF Emissions Reduction Using an Exhaust Catalyst", SAE Paper no. 870251, 1987. 12 - K. Adams and H. Gandhi, "Palladium-Tungsten Catalysts For Automotive Exhaust Treatment", Ind. Eng. Chem., Prod. Res. Dev. 198-3, 22, 207-212.



353

CATALYTIC AUTOMOTIVEPOLLUTION C O N T R O L WITHOUT NOBLE METALS

Sander Stegenga, Nico Dekker, Jowi Bijsterbosch, Freek Kapteijn, Jacob Moulijn, Gerard Belot* and RenC Roche* Department of Chemical Engineering, University of Amsterdam Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands * P.S.A. Etudes et Recherches, 18 Rue des Fauvelles 922.50 La Garenne-Colornbes, France ABSTRACT The catalytic activity for the NO reduction and the CO oxidation, and the influence of several parameters on this activity, has been studied for Cu and Cu-Cr catalysts on pelleted and monolithic supports. It has been shown that 10 wt% Cu-Cr/A1203 catalysts are very effective for NO reduction, CO and HC oxidation. The highest activity is obtained at a Cu/Cr ratio of 2. Monolithic-type catalysts exhibit a three-way catalyst behaviour, comparable to noble metal catalysts under the same conditions. The conversion performance under oscillating compositions is even better than under stationary composition conditions. The presence of 0 2 leads to an inhibition of the NO reduction by CO. NO does not affect the CO oxidation. In the NO reduction with CO at low NO conversions predominantly N 2 0 is formed, whereas at higher conversion levels the N 2 0 formation passes through a maximum and at higher temperatures (T > 523 K) only N2 is observed. The catalyst system deactivates at higher temperatures (> 775 K) in stoichiometric CO/NO/02 mixtures, but the original activity returns in an overall oxidizing environment at 623 K. The CO oxidation activity level of a freshly prepared catalyst under reducing conditions is higher than under oxidizing conditions, after thermal treatment in a stoichiomemc mixture this is the reversed case. A tentative explanation is given. Cu catalysts prepared from EDTA complexes and a La-stabilized alumina-support appear to have a high thermal stability and a final activity comparable to that of the Cu-Cr catalysts.

INTRODUCTION

The activity of the catalysts used for automotive pollution control is based on that of the noble metals Pt, Rh and Pd [l].These noble metal catalysts are very active and fairly resistant to the sulphur present in the exhaust gas. There are, however, also some obvious disadvantages in the use of noble metal catalysts; they are relatively scarce and therefore expensive, they require a strict aidfuel ratio control, and because of the low amounts of metal present they are relatively sensitive to impurities. Mainly because of these disadvantages, our research was focused on the development of non-noblemetal alternatives. In the early years of exhaust gas catalysis research, base metals were recognized as promising active components for catalysts,

354

especially, combinations of Cu and Cr were found to be very active [2,3,4]. Forced by the pace of the evolution of the legislation, the attention was focussed on the noble metals, leaving the base metal catalysts underdeveloped. The last decade shows a renewed interest in the possibilities of base metal catalysts [5,6,7]. Previous research at our laboratory on carbon-supported catalysts showed that catalysts based on the combination Cu and Cr have an activity comparable to that of noble metal catalysts, both for the oxidation and the reduction reactions required [ 5 ] . The performance of alumina-supported catalysts has been tested in two, for the catalytic purification of exhaust gases, important reactions: NO reduction: CO oxidation:

2 N 0 + 2CO 2CO + 0 2

=3

2C02+N2 2c02

(1)

(2)

These reactions have been carried out both separately and simultaneously. A range of catalysts with varying Cu/Cr-ratio has also been tested in the reaction system containing NO, CO, and 0 2 . Special attention has been given to the thermal stability of differently pretreated catalysts. On the basis of the results with alumina-supported catalysts, also a monolith- supported catalyst has been prepared. These monoliths contained a Zr-stabilized alumina washcoat and here loaded with active phase resulting in a 10 wt% loading of the washcoat. These monoliths have also been tested on "bench-scale'' at the research laboratory of P.S.A. France for simultaneous NO reduction, CO and HC oxidation, both under stationary and oscillating feed conditions.

EXPERIMENTAL Catalysts As catalyst support a standard y-Al203 (Ketjen 000-1.5E, CK 300) was used (dp = 105 to 145 pm; Sa = 195 m2/g, BET-Nz ). The catalysts were prepared by either wet or pore-volume impregnation of the alumina support with a solution of the metal nitrates to result in a 10 wt% Cu-Cr/Alz03 catalyst. Catalysts with a fractional Cu loading of Xcu = 0, 0.33, 0.5, and 0.66 have been prepared. After overnight drying at about 353 K, the sample was heated in a 100 ml/min air flow. The heating rate was 2-5 K/min. First the temperature was increased to 353 K, and subsequently to 673 and 773 K, each of these temperatures was maintained for about 30 min. For one batch the maximum calcination temperature was increased to 1073 K. As monolitic support, batches drilled from a Zr stabilized Degussa monolith (OM 724 MLKV), containing a 10 wt% A1203 washcoat, were used (cut to dimensions of 1 cm diameter and 3 cm length). The preparation method was the same as for the particle catalysts. The monolith was impregnated to yield a 10 wt% Cu-

355

Cr loading of the A1203 washcoat. Also a La-stabilized alumina support, containing 2.5 wt% La, was used. This support was impregnated with CuEDTA and calcined at 725 K 181.

Apparatus For the activity measurements the experimental set-up [5] consists of a gas mix section enabling us to use a wide range of reactant mixtures, followed by a reactor section and an analysis section. Except for the U-shaped quartz reactor, the complete set-up is made of stainless steel tubing. The reactor is placed in a water-cooled, temperature controlled furnace. The combination of a continuous NO/NOx-analyser and a dual-column gas chromatograph enables us to quantitatively analyse the following compounds NO, NOx, N20, N2, C02, CO, and 02. Experimental procedures The catalytic activity was measured in temperature programmed experiments. Standard conditions for these measurements are given in table 1. For screening the monolith catalyst, a 1 g sample was used to make the absolute amount of Cu-Cr comparable with the experiments using particulate catalysts. For the separate and combined performance in NO reduction and CO oxidation, a set of four gas compositions was used (table 2). TABLE 1.- Standard conditions for the activity measurements Temperature range Pressure Catalyst: particles monolith Gas flow rate VHSV: particles monolith

293 - 1073 0.15 100 1.o

98 72 3.9

K MPa mg g pmolls 10 /h 1O h

The catalysts were screened by heating the sample at 2 K/min from 293 to 675-775 K and subsequently cooling at 2 K/min to 293 K. The thermal stability of the catalysts under these reaction conditions (gas comp. 1) was investigated by increasing the maximum screening temperature with 100 K in four consecutive experiments (Tmax = 773, 873, 973, and 1073 K), followed by an experiment up to 773 K to measure the final activity after these treatments.

356

1 2 3 4

1 % CO 1 %CO

1%co

0.2 % co

0.4 % 0 2 0.4%02 0.8%02

0.2% NO 0.2% NO

NO reduction and CO oxidation CO oxidation (reducing) CO oxidation (oxidizing) NO reduction

Apparatus and Experimental Procedures (at P.S.A France) The apparatus used to investigate the catalyst behaviour in closed-loop emission control systems is developed according to the guidelines given by Schlatter et a1 [9.] The relatively inert compounds, CO and N2 , are mixed and heated up to the reaction temperature. The reactants NO, CO, 0 2 , and HC are added to the heated gas. This reactant gas is led over the catalyst, either a packed bed or a monolith, at a flow rate corresponding to a VHSV of 75.000 h-1 . Both reactant and product gas can be analyzed, CO by I.R., HC by F.I.D., and NO by chemiluminescence. Rich and lean gas compositions are produced by either adding CO or 0 2 to a standard composition. For this purpose two other inlets are present, one for 0 2 and one for CO, constructed to open and close at a frequency of 1 Hz, to induce an oscillation in the gas mixture composition. In table 3 a characteristic rich and lean gas composition are given. h is used as a measure for the stoichiometry of the gas composition, and is defined as the ratio between the actual air/fuel ratio and the stoichiometric aidfuel ratio.

Lean ( h =1.02) 0.4*104 0.8* 1 04

02

Rich ( h =0.98) 1.4*104 0.4*104

HC (C3H6 &z C3H8) NO H20

500

500

1000 1.0*105

1000 1.0*105 1.4*105

Gas

co

co2

i4*in5

RESULTS In figure 1 the NO reduction activity (gas comp. 4) is shown for a standard catalyst, the concentration curves are plotted as function of temperature. It can be seen from this figure that all NO and CO is converted below 625 K. In the temperature range 420 - 520 K some N 2 0 is produced. Below 450 K the NO is predominantly converted to N20, above 520 K N2 is

357

the only reaction product. In figure 2 a comparison is made for the activities of NO reduction and CO oxidation, both separately and simultaneously. This figure shows that there is no influence of the addition of NO on the CO oxidation and also that there is only a small influence of the addition of 0 2 on the NO reduction. The NO concentration curve is clearly shifted to higher temperatures, while no N 2 0 formation was observed and the N2 formation is unchanged.

---I

f

P

P \

C

0 .c

I

+ I

C Q)

0 C

0

0

300

400

600

500

700

800

T/K

Figure 1.

NO reduction with CO. Course of the concentrations of reactants and products as function o j the temperature for 10 wt% Cu-Cr on Al2O3. Pco = PNO = 0.3 kPa.

Combined NO, CO, and 0 2 experiments have also been conducted with catalysts with varying Xcu,viz. 0.66(A), 0.5(B), 0.33(C) and O(D). Figure 3 shows the 0 2 and N2 concentrations as a function of the temperature, as a measure of the activity for CO oxidation and NO reduction, respectively. Clearly, N2 is only produced when all Xcu has been consumed. All three Cu containing catalysts show a similar reaction behaviour, a relatively steep 0 2 consumption curve and a rather smooth N2 production curve. A catalyst with Xcu = 0.66 seems to be the most active, full conversion of CO is achieved at 450 K and of N O below 600 K. The chromium catalyst (Xcu= 0) shows a very low CO oxidation activity and a very steep N2 production curve, full CO conversion is achieved at 610 K and full NO conversion at 650 K.

358

E

10

P P

8

0

0 F

6

\

C

.-40-

2

4

w

C Q1

0

2

c

0

0

0

300

400

500

600

700

800

T/K

Figure 2.

E

a

P 0

Comparison between NO+C0+02 , C 0 + 0 2 and NO+CO. Course of the concentrations as function of the temperature for 10 wt% Cu-Cr on A1203

-1

4 -

0,

0 7

\

c

0 .4-

F

c

c Q)

0 C

0

0

300

400

500

600

700

800

T/K

Figure 3.

NO reduction and CO oxidation f o r catalysts with varying X c u in Cu-Cr on A1203 , all 10 wt%. N2 (--- : NO reduction) and 0 2 (: CO oxidation) as function of the temperature . Xcu: 0.66(A), 0.5(B), 0.33(C) and O(D)

359

In figure 4 the results of thermal stability experiments are shown for a catalyst (Xc, = 0.5) freshly calcined at 1073 K. The 0 2 consumption curves became less steep with higher pretreatment temperature, while the N2 production curves are parallel shifted to higher temperatures. A pretreatment temperature of 873 K had a small influence on the CO oxidation and NO reduction activity of the catalyst. Pretreatment at 973 K resulted in a larger deactivation, while a consecutive pretreatment at 1073 K has only a small additional effect on the at 1073 K calcined catalyst.

300

400

500

600

700

800

T/K

Figure 4. Influence of pretreatment temperature on the NO reduction and CO oxidation. N2 production and 0 2 consumption as function of the temperature for 10 wt% Cu-Cr on A1203 , calcined at 1073 K. Similar experiments have been conducted with the Cu catalyst on Lastabilized A1203 (figure 5). This figure shows that, in contrast to the Cu-Cr catalyst, there is a slight delay after full 0 2 consumption before the N2 production starts. The activity of the Cu catalyst after thermal treatment at 773 K is lower than of any of the catalysts based on Cu-Cr. However, the thermal stability of this catalyst is much better, even after pretreatment at 973 K the activity of the catalyst has hardly changed. Only after pretreatment at 1073 K, the activity shows a remarkable decrease. In figure 6 the influence of the addition of NO on the CO oxidation is measured for a Cu-Cr/monolith catalyst (Xc, = 0.5; 1 wt% Cu-Cr). The concentrations of the gases has been plotted as a function of the reaction temperature. This figure clearly shows that there is no influence of NO on the CO oxidation, for both gas compositions the curves completely coincide. In agreement with the result described before NO reacts with the excess amount of CO as soon as all 0 2 has been consumed.

360

kP (1

0 r \

C

0 .w

z

w

C

a, 0 C

0

0

300

400

600

500

700

800

T/K

Figure 5.

Influence of pretreatment temperature on the NO reduction and CO oxidation. N2 production and 0 2 consumption as function of the temperature for 10 wt% Cu on La-stabilized A1203 .

ka

l2

I

0

0 r \

s

.-

.I-

z

.I-

C

a, 0 C

0

0 300

400

600

500

700

800

T/K

Figure 6.

The influence of NO on the CO oxidation : NO+C0+02 and C0+02. Course of the gas phase concentrations as function of the temperature over I wt% Cu-Cr on a monolith.

Figure 7 shows the influence of 0 2 addition on the NO reduction over the same monolith catalyst. The N2 production is plotted as measure for the NO reduction. The NO reduction is shifted to 100 K higher temperatures in the presence of 0 2 , and only starts when all 0 2 has reacted.

36 1

The thermal stability of the monolith catalyst has also been investigated. Figure 8 shows that pretreatment at 873 K resulted in a large shift of both the 0 2 consumption and N2 production curves to higher temperatures. Higher pretreatment temperatures, 973 and 1073 K, resulted in only a small additional shift. E

5

Q P

C

C

0

0

0-

300

400

600

500

700

800

T/K

The influence of 0 2 on the NO reduction : NO+CO+O2 and NO+CO. Course of the gas phase concentrations as functionof the temperature over I wt% Cu-Cr on a monolith.

Figure 7.

EP 0

0 l-

\

C

.-0 4-

z

c C

a, 0 C

0

0

300

400

500

600

700

aoo

T/K

Figure 8.

Influence of pretreatment temperature on the NO reduction and CO oxidation. N2 production and 0 2 consumption as function of the temperature for I wt% Cu-Cr on a monolith.

362

Directly after these thermal stability tests, CO oxidation experiments have been conducted with the thermally treated catalysts calcined at 1073 K and the supported monolith calcined at 773 K, in an overall oxidizing environment. The results are plotted as CO concentration as function of the temperature (figure 9), for the catalyst calcined at 1073 K and for the Cu/Crmonolith . For both catalysts the same pattern is observed. At the increasing temperature branch the activity is the same as shown by the most deactivated curve in figure 4 and 8. At 100% CO conversion the overall environment is still oxidizing, which apparently leads to a reactivation of the catalyst. The final activity is even slightly higher as compared to the initial activity before the thermal stability experiments. Cu-Cr based catalysts are very sensitive to their pretreatment or reaction conditions [ 101. Application of the catalyst in reducing environment increased the activity of both a catalyst freshly calcined at 773 K and a catalyst freshly calcined at 1073 K, the conversion level reached does not change during measurements in reducing environment. In oxidizing environment the activity is decreased to a lower, although stable, level compared to the level in reducing environment. Figure 10 and 11 systematically compare the CO oxidation activity in overall oxidizing or reducing environment before and after treatment in reactants at 1073 K. The activity of the catalyst freshly calcined at 1073 K is lower, both in oxidizing and reducing environment, than that of the catalyst calcined at 773 K. Treatment of these catalysts in a stoichiometric reaction mixture at 1073 K has a large influence on the catalyst performance.

300

400

500

600

700

800

T/K

Figure 9.

CO oxidation activity in overall oxidizing environment (gas comp. 3 ) after reaction in NO+C0+02 at 1073 K . CO2 production as firnction of temperature, for I0 wt% Cu-Cr on A1203 and on a monolith .

363

12

E

n n " 0 l-

\ N

0 0

3 00

500

700

T/K

Figure 10. CO oxidation activity in overall oxidizing or reducing environment before and after treatment in reactants at 1073 K. 10 wt% Cu-Cr on Al2O3, calcined at 773 K. Before treatment (---) ;reducing ( 0 ) oxidizing ; After treatment (-) ;reducing (0); oxidizing (D 12 10

8

6 4

2 0

3 00

500

700

T/K

Figure 11. CO oxidation activity in overall oxidizing or reducing environment before and aper treatment in reactants at 1073 K. 10 wt% Cu-Cr on A1203 , calcined at 1073 K. Before treatment (----); reducing (0); oxidizing 0) After treatment (--); reducing (0);oxidizing U,

364

Again stable activity levels of the catalysts are obtained in oxidizing and reducing environments, but now the use in a reducing environment results in the lower activity as compared to that in an oxidizing environment. After the thermal treatment the activity of the catalyst freshly calcined at 1073 K is higher than that of the catalyst calcined at 773 K. In figures 12 and 13 the results are shown of a standard three-way activity test of a Cu-Cr/monolith catalyst. The conversion of NO, CO and HC is plotted as function of lambda, for stationary conditions (figure 12) and for oscillating conditions (figure 13). These figures show that under stationary conditions in an oxidizing environment ( h > 1) the NO conversion level is low, while the HC and CO conversions are very high. In a reducing environment ( h < 1) the NO conversion level is relatively high, while the HC and CO conversions decrease with decreasing h. In the situation of oscillating gas composition the conversion curves are broadened. The HC, NO and CO conversion levels are higher at h < 1 and h > 1, only at around 1 the NO and CO conversions are at an intermediate level. Especially the overall conversion of NO and HC has increased. 1 .o

0.8 0.6

0.4 0.2

0 0.94

0.96

0.98

1.00

1.02

1.04

1.06

Lambda

Figure 12. Conversion of NO (A), CO ( 0 )and CHx ( 0 )as function of lambda under stationary Conditions jbr 1 wt% Cu-Cr on a monolith.

DISCUSSION Although slightly less active than carbon-supported Cu-Cr catalysts [11,12] , alumina- supported Cu-Cr catalysts are also shown to be very active for both CO oxidation and NO reduction. Full conversion of a stoichiometric mixture of 0 2 , CO and NO is reached below 625 K. In all situations, the CO oxidation occurs at lower temperatures than the NO reduction.

365

The production of N 2 0 is often observed in catalytic NO reduction [13-151. For alumina-supported Cu-Cr catalysts it is shown that, in a stoichiometric NO-CO mixture, a small amount of N 2 0 is formed between 420 and 520 K (figure 1). This is in agreement with previous results on carbonsupported catalysts [ l l ] , and also with results of Bauerle et a1.[15], who report the production of N 2 0 during the catalytic reduction of NO by CO in the presence of 0 2 , for both base- and noble- metal catalysts at temperatures below 650 K.

1 .o 0.8 0.6 0.4

0.2

0

0.94

0.96

0.98

1.00

1.02

1.04

1.06

Lambda

Figure 13. Conversion of NO (A), CO (0)and CHx ( 0 )asfunction of lambda under oscillating conditionsfor 1 wt% Cu-Cr on a monolith. The mechanistic aspects of the formation and reaction of N 2 0 over alumina-supported Cu-Cr catalysts is subject of a further study. The addition of NO to a mixture of CO and 0 2 has no influence on the CO oxidation activity of both the particle and monolith catalyst. The absence of an effect of NO is in contrast with many noble-metal catalysts, where the presence of low concentrations NO can severely decrease the CO oxidation activity, which is a result of strong chemisorption of NO on Pt above 473 K [16;19]. On the other hand, the addition of 0 2 to a NO, CO mixture results in a strong inhibition of the NO reduction, certainly for the monolith catalyst. For all NO reduction experiments in the presence of 0 2 , the conversion of NO is inhibited until all 0 2 is removed by reaction with CO. With the monolith catalyst this results in the NO reduction to occur at temperatures 100 K higher as compared to the situation without 0 2 .

366

For both the NO reduction and the CO oxidation over copper-based catalysts a redox mechanism is assumed [20,21]. In this redox cycle the partially reduced sites are vacant for oxidation by either NO or 0 2 . Apparently 0 2 reoxidizes these sites much faster than NO, which explains the inhibition of the NO reduction by 0 2 and the absence of any influence of NO on the CO oxidation with 0 2 . This conclusion is supported by results obtained from a kinetic modelling study of the CO oxidation, which shows that during CO oxidation, all the active sites are completely oxidized as long is oxygen is present in the gas phase [22]. Similar to the carbon-supported catalysts, a composition with Xcu = 0.66 results in the most active catalyst. This fraction does not correspond to any Cu-Cr compound as reported by Laine et a1.[6], but will more likely correlate to a compound in which the Cu-Cr compound might be present to stabilize the CuO. Previous results [ 111, which are in agreement with results reported by Shelef et a1.[4] and Gassan-Zade et a1.[23], show that supported Cr catalysts have a good NO reduction activity and a low CO oxidation activity, which would suggest a preference for the NO reduction during simultaneously performance of NO reduction and CO oxidation. As already found for carbonsupported catalysts, the preference for the NO reduction is not observed. The NO reduction is completely inhibited by the presence of 0 2 , and only when all 0 2 has reacted, NO reduction will occur, shown by a very steep N2 production curve. By comparing the results of the La-stabilized Cu catalyst [8] in figure 5 with figure 3 and 8 it can be seen that the initial activity of the Cu catalyst is lower than that of any of the Cu-Cr catalysts, either supported on A1203 or on the monolith, both for the CO oxidation and for the NO reduction. Compared to the most active Cu-Cr combination (figure 3) the CO oxidation is completed at a 50 K higher temperature, while the NO reduction sets on at a 90 K higher temperature. This lower initial activity seems to be compensated by the high thermal stability. After pretreatments above 973 K, in a stoichiometric NO, CO and 0 2 reaction mixture, the Cu-based catalyst shows a better CO oxidation performance and a comparable NO reduction performance as compared to the alumina-supported Cu-Cr catalyst. However, after treatment at 1073 K the Cu based catalyst exhibits a lower activity than Cu-Cr/A1203 . The Cu-Cr based catalysts show a decrease in activity as result of an increased pretreatment temperature (figure 4 and 8). Initially the activity of the catalyst calcined at 1073 K is lower than the activity of a catalyst calcined at 773 K. However, pretreatment of these catalysts in a stoichiometric reaction mixture up to 1073 K results in a comparable CO oxidation activity and a higher NO reduction activity of the catalyst calcined at 1073 K. These thermally treated catalysts can be reactivated by use in oxidizing environment at 623 K, the resulting activity is comparable to the initial activity of the

367

catalysts. The stable activities before and after thermal treatment at 1073 K in reactants show for the catalyst calcined at 773 K a decrease in the overall performance in oxidizing and reducing environment, whereas the catalyst calcined at 1073 K even shows a slight increase in overall performance. Both a catalyst freshly calcined at 773 K and a catalyst freshly calcined at 1073 K, exhibit a higher activity in reducing environment and a lower activity in oxidizing environment. After pretreatment of these catalysts at 1073 K in reactants this is reversed: a higher activity in oxidizing environment and a lower activity in reducing environment. For the understanding of these observations the following tentative explanations can be taken into account. In a fresh catalyst Cu(I1) is stabilized in or by the CuCr204 spinel structure [24]. In reducing environment at 100 9% 0 2 conversion Cu cations can migrate from the bulk to the surface, resulting in an increased surface Cu concentration [lo], and hence more active sites. Several authors report such a migration for spinel type structures, where Cu migrates to either bulk or surface in oxidizing or reducing conditions, respectively [10,25,26]. At least a part of this migration is considered being a fast, reversible process [24], for some Cu containing spinel type compounds even under surprisingly mild conditions [27]. The increased activity in reducing environment can also be a result of the reduction of Cu +2 to Cuo . The oxygen metal bond strength, which is the rate determining parameter in the CO oxidation is lower for Cu as compared to Cu +2 [ 5 ] . For thermal treatment in a stoichiometrical reactant mixture two phenomena are observed, the activity is decreased due to the high temperature, and a change in catalyst takes place, which causes activation to occur in oxidizing instead of reducing environment. For the catalyst calcined at 1073 K the activity does not decrease because it has already been subjected to a high temperature, but the change, causing the switch in activation-deactivation behaviour, is also observed. Calcination in air at 1073 K does not induce this change in behaviour, so this phenomenon has to be the result of the combined effect of high temperature and gas phase composition. The decrease in activity is probably due to the formation of Cu(I1) aluminate spinel of a part of the Cu present [28]. The mentioned change in the catalyst can be speculated to be a result of a thermally induced reduction of Cu(I1) to Cu(1) in the spinel structure, which would not occur during high temperature calcination in air, because of the stabilized Cu(I1) oxidation state. Similar activation behaviour in oxidizing environment has been observed for CuO/A1203 catalysts after treatment in N2 at 1073 K [8], but the explanation given by the author was not satisfactory. However, the present results can also not conclusively elucidate the mechanism for the reversed activation-deactivation behaviour in oxidizing and reducing environment. A extensive investigation in to this behaviour is presently executed.

368 The Cu-Cr/monolith catalyst shows a typical three-way behaviour. In overall oxidizing environment the conversion of CO and HC is high and of N O is very low. In overall reducing environment the conversion of NO is high and of CO and HC is fairly low. This behaviour fully corresponds to the three-way behaviour of noble-metal catalysts [9]. Only around stoichiometric conditions, all components show a high level of conversion. In order to provide the proper stoichiometrically balanced exhaust gas composition, a closed-loop electronic air-fuel ratio control is required. This continuous adjusting of h results in a perturbation of the exhaust gas composition. The introduction of this oscillation results in two changes in the conversion curves. First, at h< 1 the CO and HC conversions and at h > 1 the HC and N O conversions are improved and second, for NO and CO the maximum conversion at h= 1 is somewhat lowered. However, the conversion of HC is higher over the whole h range. As an overall result the NO and HC conversions are improved by an oscillating feed.

CONCLUSIONS It has been shown that 10 wt% Cu-Cr/A1203 catalysts are very effective for NO reduction, CO and HC oxidation. Monolithic type catalysts exhibit a three-way catalyst behaviour, comparable to noble metal catalysts under the same conditions. Bench scale tests under oscillating composition conditions show an even better conversion performance than under stationary conditions. A Cu/Cr ratio of 2 seems to result in the highest catalytic activity. The affinity of 0 2 for the catalyst is much higher than of NO, which leads to an inhibition by 0 2 of the NO reduction by CO. Consequently NO does not affect the CO oxidation. In the NO reduction with CO predominantly N 2 0 is formed at low N O conversions. At higher conversion levels the N 2 0 formation passes through a maximum and at higher temperatures only N2 is observed. The catalyst system seems to deactivate at higher temperatures (up to 1073 K) in stoichiometric CO/NO/O2 mixtures, but the original activity returns in an overall oxidizing environment. Before the thermal treatment the final CO oxidation activity level under reducing conditions is higher than under oxidizing conditions, after the thermal treatment this was the reversed case. Cu catalysts prepared from EDTA complexes and La-stabilized alumina supports appear to have a high thermal stability and final activities comparable to that of the Cu-Cr catalysts.

369

References 1. 2. 3. 4.

5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28.

J.T. Kummer, J. Phys. Chem., 90, 4747 (1986) J.T. Kummer, Adv. Chem. Series, 143, 178 (1975) T. Ohara, in "The Catalytic Chemistry of Nitrogen Oxides", R.L. Klimisch and J.G. Larson, eds., p. 191, Plenum Press, New York 1975 M. Shelef, K. Otto and H. Gandhi, J. Catal, 12, 361 (1968) S. Stegenga, A.J.C Mierop, F. Kapteijn and J.A. Moulijn, to be published J. Laine, A. Albornoz, J. Brito, 0. Carias, G. Castro, F. Severino and D. Valera, in "Studies in Surface Science and Catalysis", Vol. 30, A. Crucq and A. Frennet, eds., Elsevier, Amsterdam, 1987 p. 387 P. Gagneret, presentation at "Catalysts for the future", VDMA-Haus Frankfurt, 1989 1.I.M Tijburg, Ph-D Thesis, University of Utrecht, The Netherlands 1989 J.C. Schlatter, R.M. Sinkevitch and P.J. Mitchell, I. & E.C., Prod.Res.Dev., 22, 51 (1983) J. Laine, J. Brito, F. Severino, G. Castro, P. Tacconi, S. Yunes and J. Cruz, Catalysis Letters, 5 4 5 (1990) S. Stegenga, R. van Soest, F. Kapteijn and J.A. Moulijn, to be published S. Stegenga, R. van Soest, F. Kapteijn and J.A. Moulijn, Recl. Trav. Chim. Pays-Bas 109, 112 (1989) C.M. Fu, V.N. Korchak and W.K. Hall, J. Catal., 68, 166 (1981) J.O. Petunchi and W.K. Hall, J. Catal. 78, 327 (1982) G.L. Bauerle, G.R. Service and K. Nobe, I. & E.C., Prod.Res.Dev., 11(I), 54 (1972) J. Wei, Advances in Catalysis, Vol. 24, 110.57, Academic Press, New York (1975) S.E. Voltz, C.R. Morgan, D. Liedernian and S.M. Jacob, I. & E.C., Prod.Res. Dev., 12(4), 294 (1973) S.H. Oh and J.E. Carpenter, J. Catnl., 101, 114 (1986) E. Koberstein and G. Wannemacher, in "Studies in Surface Science and Catalysis", Vol. 30, A. Crucq and A. Frennet, eds., Elsevier, Amsterdam, 1987 p.155 J.W. London and A.T. Bell, J. Catal, 31, 96 (1973) R.T. Rewick and H. Wise, J. Catal, 40, 301 (1975) S. Stegenga, N.J.J. Dekker, F. Kapteijn and J.A. Moulijn, to be published G.Z. Gassan-Zade, M.Y. Woode and T.G. Alkhazov, React. Kin. Catal. Lett., 34( I), 225 (1987) A, Iimura, Y. Inoue and I. Yasumori, Bull. Chem. Soc. Jpn., 56, 2203 (1983) K.P. de Jong, J.W. Geus and J. Joziasse, J. Catal., 65, 437 (1980) J.O. Petunchi and W.K. Hall, J. Catal., 80, 403 (1983) R. Hierl, H. Knozinger and H-P. Urbach, J. Catal., 69, 475 (1981) M. Lojacono and M. Schiavello, in "Preparation of Catalyst", B. Delmon, P.A. Jacobs and G. Poncelet, eds., Elsevier, Amsterdam, 1976


A. Crucq (Editor), Catalysis and Automotive Pollution Control 11 0 1991 Elsevier Science Publishers B.V., Amsterdam

37 1

THREE-WAY ACTIVITY AND SULFUR TOLERANCE OF SINGLE PHASE PEROVSKITES D.Jovanovic(1) , V.Dondur(2) , A.Terlecki-Baricevic(1) and B.Grbic(1) (1)Institute

of Chemistry, Technology and Metallurgy, Njegoseva 12,11000 Belgrade, Yugoslavia

(2) Institute of Physical Chemistry, Faculty of Science, University of Belgrade, P.O.Box 550, I I000 Belgrade,Yugoslavia

ABSTRACT Single phase perovskites, as LaCo03, LaCr03, La Cog 5 Cro 503, Lao.7Sro gCrO3 and Lag 7Sro 3Cro 95Ruo 0503,were synthesized and three-way activity and sulfur tolerance were measured in a pulse-flame catalyst testing system in a wide range of redox-potential (R). Comparison of activities indicates that perovskite with ruthenium ion has the best redox behaviour in the redox potential interval 0.75 R> 0.4. The sulfur effect on the three-way catalysts activity was investigated in the redox potential range R < 0.8 by aging the catalysts with fuel containing 0.03 wt.% of sulfur, which is an average sulfur concentration corresponding to European gasolines. Sulfur wax added in the form of ethanethiol (C2H5SH, analytic grade) and the sulfur quantity in the isooctane was checked by a standard method for sulfur determining in fuels [41,42]. RESULTS AND DISCUSSIONS

The X-ray diffraction patterns of the synthesized catalysts showed that they were all of perovskite-type structure and no second components were detected. The amount of active transition metals in the B position of perovskite lattices, i.e. Co, Cr and Ru, in the catalysts, determined by X-fluorescence analysis, corresponds to the stoichiometric values of the proposed chemical compositions.

374

The pulse-flame catalysts efficiency testing runs were performed in a wide range of redox potential (R) by pure isooctane fuel combustion. All catalysts activity testing runs were done in the temperature range 100°C 700°C and 550°C was chosen as an optimal average three-way catalysts testing temperature . Effect of redox potential (R) on CO conversion over synthesized catalysts is presented in F i g . ] .

__-___-----

0

redox potential

(R)

Fig.] Percentage carbon monoxide conversion as a finction of redox potential (R) for synthesized perovskite-type catalysts.

All investigated catalysts for CO oxidation can be divided in two groups: very active catalysts (LaCoo.5Cr0.503 and La0.7Sr0.3Cr0.95Ru0.0~03) with an oxidizing efficiency over 80% for R < 1.5, and low activity catalysts (LaCrOg and La0.7Sro.3Cr03.) Unsaturated hydrocarbons (HC,,.) simultaneously testing runs (Fig.2) indicates that the LaCoo.5Cr0.503 sample presents similar catalytic properties as so called "low activity" catalysts. Meanwhile, samples like La0.7Sr0.3Cr0.95Ru0.0~03 and Lac003 have very high oxidation level over 80% for unsaturated HC at R c 1.5. In the same experimental conditions, influence of redox potential (R) on nitrogen oxides (NO,) reduction, is shown in Fig.3. Ruthenium perovskite catalyst and LaCoOg have satisfactory NO, reduction efficiency, as well as the LaCo0.5Cr0.503 sample. As it was expected, the ruthenium containing sample has a R shift towards lean mixture (R < 1).

375

From Figs.1-3, it is obvious that all perovskites with solitary chromium ion in B lattice position, have extremely low three-way activity for all pollutants. Only two catalysts, Lac003 and Lao.7Sro.3Cro.g5Ruo.0503,show high three-way conversion (over 80%) on the lean side of the redox potential scale, close to the stoichiometric value (R=l) and at R = 0.75, respectively.

redox potential

(R)

Fig.2 Percentage of unsaturated hydrocarbons conversion as a function of redox potential (R)for synthesized perovskite-type catalysts (Symbols as in Fig.1).

Fig.3. Percentage of nitrogen oxides conversion as a function of a redox potential (R) for synthesized perovskite-type catalysts (Symbols as in Fig.1).

376

A three-way activity "window" of a catalyst is defined arbitrarily as the width of the R scale over which a catalyst converts 80% or more of CO, NOx and HCun. Perovskite LaCo0.5Cr0.503 is less active than Lac003 The efficiency of LaCrOg and La0.7Sr0.3Cr03 in reducing CO, NO, and HCun is extremely low in the whole investigated range of R. Activity of the perovskite with a strontium ion incorporated into A site does not show any improvement in redox activity in relation to the base matrix LaCr03. Total HC conversion for all five perovskites is apparently low (less than 40%) as a results of high saturated hydrocarbons fraction (about 62 vol.%) in the total HC; these results are not represented in three-way activity curves (Figs. 1-3 ). The proposed "window" for all three pollutants are satisfactory only for LaCr03 and La0.7Sr0.3Cr0.95Ru0.0503 .

Sulfur poisoning resistant is one of the important catalyst characteristics, as well as a high three-way activity. In the combustion process, fuel sulfur is oxidized to sulfur dioxide (S02) [42]. The exhaust gases then proceed via the exhaust manifold to the oxidation catalyst. In the catalyst, excess CO and HC are oxidized to water vapor and C02, while some of the SO2 is oxidized to sulfur trioxide (SO3). Gaseous sulfur oxides can react with alumina carrier in catalysts, or with catalysts base metal components, to form sulfate or sulfite salts which lead to catalysts degradation [44-461. Because of that reason very carefully sulfur aging tests on the serial catalysts, with differerit transition metals in B site, were made. Catalysts were aged by combustion of isooctane containing ethanethiol. Change of the degree of CO, total HC and mixture obtained by C2HsSH combustion in isooctane is shown in Fig.4. All tests were performed in oxidizing atmosphere (R < 0.8) for six hours at 550°C and at velocity of 32,500h-1. Dynamics of SO, poisoning is different for all investigated samples. Lanthanum chromite (LaCrOg), La0.7Sr0.3Cr03 and Lao.7Sro.3Cro.95 RU0.0503 maintained almost initial activity during six hours of aging. Activity of Lac103 suddenly decreased during the first hour, whereas LaCo0.5Cr0.503 activity monotonously decreased within the first three hours and remained at that value during further aging. After three hours of aging, activity level of this sample was comparable to that of LaCrOg. Activity of the sample Lao.7 Sro.3 Cro.95 Ru0.05 0 3 , during the aging stage, remained practically unchanged. Small decrease of the total HC conversion was due to the slight decrease of ethane (C2H6) conversion. Taking into account very low initial activities of NOx reduction for all samples under oxidizing conditions (except for the ruthenium containing perovskite) it can be said that outlet NOx concentration practically did not change during SOx poisoning.

377

LaCo03 0

- co

0

- HC (tot.)

A

-

NOX

La0.7 sr0.3 Cro 3

aging time (h)

Fig.4 Catalysts eficiency for CO, total HC and NOx during aging with fuel containing sulfur ( R < 0.8).

378 CONCLUSION

In order to prevent possible degradation of perovskite structure due to metal sulfate formation, almost inactive (Figs 1-3), sulfur tolerant, matrix of LaCr03 perovskite was chosen for substitution of ruthenium ion. Substitution of Ru+4 for Cr3+ in sulfur resistant perovskite LaCr03 (Fig. 4 ) requires simultaneous partial substitution of La+3 for bivalent ion strontium (Sr). Preliminary investigation by AUGER method of SrO surface, aged under similar conditions as other perovskites, shows no trace of chemically bonded sulfur on the oxide surface [39]. According to the literature data [47] in this kind of substitution the charge would be compensated by transition of part of Cr+3 to Cr+4 and/or formation of anion vacancies. In order to examine only the effect of strontium substitution in LaCrOg lattice on the sulfur poisoning +3

+2

+3

+4

resistance the structure La0.7 Sr0.3 Cr0.7 Cr0.3 0 3 was synthesized. In this perovskite struture only a small fraction of chromium ion was substituted by iuthenium and the following investigated composition was obtained: +3

+2

+3

+4

+4

La0.7 Sr0.3 Cr0.7 Cr0.25 Ru0.05 0 3 In a redox potential range 1.40 - 0.75, at 550°C, and in the presence of sulfur oxides, the ruthenium perovskite reduced CO, NOx and unsaturated HC by over 80%. It can be concluded that the new synthesized ruthenium perovskite can be successfully used for the removal of carbon monoxide, unsaturated hydrocarbons and nitrogen oxides from exhaust gases of IC engines, within a wide range of temperature and oxido-reducing operating conditions. ACKNOWLEDGMENT

This work was supported by the US-Yugoslav Joint Board on Scientific and Technological Cooperation (Grants EPA 513 and 885).

REFERENCES 1. R.J.H.Voorhoeve, Perovskite-Related Oxides as Oxidation-Reduction Catalysts, Advanced Materials, Academic Press, Inc., New York,(1977) 2. F.Galasso, Structure and Properties of Inorganic Solids, Pergamon, New York, (1970). 3. O.Prakash, P.Ganguly, G.Rama Rao, C.N.R.Rao, D.S.Rajcria, V.G.Bhide, Mater.Res.Bull., 9,1173 (1974). 4. R.J.H.Voorhoeve, D.W.Johson Jr., J.P.Remeike, P.K.Gallaghar, Science, 195,827 (1977). 5 . R.J.H.Voorhoeve, J.P.Remeike, D.W.Johnson, Science, 180,62 (1973). 6. P.K.Gallagher, D.W.Johnson, F.Schrey, Mater.Res.Bul1.. 9, 134(1974).

379 7. R.J.H.Voorhoeve, J.P.Remeike, P.E.Freeland, B.T.Matthias, Science, 177,353 (1972). 8 R.J.H.Voorhoeve, J.P.Remeike, L.E.Trimble, Mater.Res. Bull., 9-1393( 1974). 9. S,Angelov, E.Zhecheva, LDirnitrova, A.Terlecki-Baricevic, D.Jovanovic, Proc. 2 nd Czeschoslovak Conference on Preparation and Properties of Heterogeneous Catalysts, Bechyne, CSSR, (1985). 10. S.Angelov, D.Mehandjiev, V.Zharkov, A.Terlecki-Baricevic, D.Jovanovic. Z.Jovanovic, Appl.Catal., 16,431 (1985). 11. A.Terlewcki-Baricevic, B.Grbic, D.Jovanovic, S.Angelov, D.Mehandjiev, C.Marinova, P.Kirilov-Stefanov, Appl.Catal.47, 145 (1989). 12. A.Lauder, US Patent 4,126,580 (1979). 13. A.Lauder, US Patent 3,897,367 (1975). 14. G.Mai, RSepmann, F.Kummer, US Patent 3,900,428 (1975). 15. A.Lauder, US Patent 4,182,694 (1980). 16. G.Mai, RSepmann, US Patent 3,905,918 (1975'. 17. T.E.Volin, US Patent 4,134,852 (1979). 18. E.L.McCann, US Patent 4,151,123 (1979). 19. A.Lauder, US Patent 4,110,254 (1978). 20. A.Lauder, US Patent 4,049,583 (1977). 21. S.Matsumoto, JP Patent 62,053,737 (1987). 22. M.Kawabata, S.Matsumoto, JP Patent 62,065,746 (1987). 23. K.Yamamura, K.Tachibana, S.Kondo, JP Patent 63,158,130 (1988). 24. K.Tabata, I.Matsumoto, JP Patent 61,149,245 (1986). 25. K.Tachibana, K.Yamamura, S.Sekido, JP Patent 61,082,843 (1986). 26. C.Koch, DE Patent 3,504,556 (1986). 27. T.Kanbara, K.Yamarnura, K.Tachibana, SSekido, JP Patent 61,097,032 (1986). 28. K.C.Taylor, The Catalytic Chemistry of Nitrogen Oxides, R.L.Klimisch and J.G.Larson Eds., Plenum, New York, (1975). 29. H.Remy, M.Kohn, Z.anorg.allgem.Chem.,m,365 (1924). 30. W.Bel1, M.Tagami, J.Phys.Chem., 67,2432 (1963) 31. R.A.Da1la Betta, H.S.Gandhi, J.T.Kummer, M.Shelef, US Patent 3,8 19,536 (1974). 32. H.S.Gandhi, J.T.Kummer, M.Shelef, US Patent 3,835,069 (1974). 33. T.P.Kobylinsky, B.W.Taylor, J-E.Young, SAE Paper 740250 (1975). 34. M.Shelef, H.S.Gandhi, Platinum Met. Rev., 18, 2 (1974). 35. H.S.Gandhi, H.K.Stepien, MShelef, SAE Paper 750177 (1975). 36. Yugoslav Patent Appl. P-1211/90, 2l.jun (1990) 37. D.Jovanovic, A.Terlecki-Baricevic, K.Petrovic, Gonva rnaziva, 23, 33 (1984). 38. A.Terlecki-Baricevic, D.Jovanovic, K.Petrovic, Motori motoma vozila, X-56/57, 310 (1984). 39. D.Jovanovic, PhD Thesis, PMF Belgrade, (1989). 40. HSGandhi, A.G.Piken, M.Shelef, R.G.Delosh, SAE Paper 760201 (1976). 41. ASTM - D 1266-80. 42. I S 0 - 2192. 43. J.M.KaweGki, EPA Report 600/9-78-028 (1978). 44. J.N.Braddock, EPA Report 600/2-77-237 (1977). 45. R.E.Gibbs, G.P.Wotzak, S.M.Byer, N.P.Kolak, EPA Report 600/9-79-047 (1979). 46. T.J.Truex, H.Windawi, P.C.Ellgen, SAE Paper 872162 (1987). 47. N.Iehisa, K.Fukaya, K.Matsuo, N.Horiuchi, N.Karube, J.Appl.Phys., 59, 317 (1986).


A. Crucq (Editor), Catalysis and Automotive Pollution Control I1 0 199 1 Elsevier Science Publishers B.V., Amsterdam

38 1

NO REDUCTION AND A1)SORPTION INTERMEDIATES O N Pt-Rh AI,I,OY CATALYSTS L. Heezen, V.N. Kilian, R.F. van Slooten, R.M. Wolf and B.E. Nieuwenhuys Leiden University, Gorlaeus hboratories, P. 0. Box 9502, 2300 RA Leiden, The Netherlands

Abstract The reduction of NO by H2 was studied over a pure Rh, a pure Pt and several Pt-Rh alloy catalysts supported on silica in a fixed-bed plug-flow reactor operating at atmospheric pressure. The results were compared with results obtained earlier for the reduction of NO with CO using the same catalysts. The results can be interpreted on the basis of earlier Pt-Rh single crystal studies. The catalytic behaviour of the Pt-Rh catalysts is determined by factors such as gas phase composition,bulk composition and reaction temperature. No synergism between Pt and Rh was observed and the differences in activity can be explained in terms of the surface composition and the behaviour of the constituent metals towards the reactants. INTRODUCTION

The automotive three-way catalyst is an excellent catalyst for purification of automotive exhaust gas. Its high activity and selectivity, combined with its outstanding thermal stability and resistance to poisoning make it a very effective catalyst. The active components include the precious metals platinum or palladium and rhodium. In 1989 the automotive catalysts already accounted for 40 % of the world total Pt consumption and for 80 % of the Rh consumption [l]. R is an effective catalyst for the oxidation of CO and hydrocarbons : 2c0+02 'HC' + x 0 2

+ -+

2c02 yH20+zC02

Nitrogen oxides are reduced over the three-way catalyst to nitrogen according to the following overall reactions : 2NO+2CO 2 NO + 2 H2

+

+

N2+2C02 N2+2H20

382

Unfortunately, N O and hydrogen can also react to the undesired N 2 0 and NH3. The overall reactions are:

The selectivity of Pt to promote the NO reduction to N2 rather than to NH3 is poor, especially at low temperatures. Therefore, rhodium is the essential ingredient in the three-way catalyst for the selective conversion of N O to N2. Recently, it has been found that the precious metals form alloy particles in the three-way catalyst [2]. Alloy particles often exhibit catalytic properties (e.g. selectivity, activity, stability and poison resistance) different from those of the constituent metals. Despite the widespread use of bimetallic Pt-Rh catalysts, only a few studies have been published concerning the performance of Pt-Rh alloy catalysts in the relevant reactions. This contrasts with the monometallic Pt and Rh catalysts, about which several investigations have been reported. In view of the enormous importance of bimetallic Pt-Rh catalysts we have investigated in our laboratory the properties of various Pt-Rh alloy single crystal surfaces and supported Pt-Rh alloy catalysts [3]. In this paper we describe comparative studies of NO reduction with hydrogen over several Pt-Rh alloy catalysts, a pure Pt catalyst and a pure Rh catalyst. These novel results will be compared with those previously obtained for the NO reduction with CO over the same catalysts [4].The adsorption of CO, NO and their interaction were also studied on the same catalysts using infrared spectroscopy [5].In addition, some preliminary results are shown concerning the effect of the metal particle size on the NO-H2 reaction in order to distinguish effects of alloy formation from particle size effects.

EXPERIMENTAL ADDaratUS : The experiments were performed in a single-pass fixed-bed flow reactor operating at atmospheric pressure. The gases used were research grade and contained a mixture of 5% of the reactive gases in helium. The gas flow in the system was 20 cm3/min. The gas stream passed the powdered catalyst which was supported by a porous glass filter in a Pyrex reactor. Temperatures were measured using a chromel-alumel thermocouple attached to the glass filter. Analysis of the gas stream containing the products was accomplished using a computer controlled quadrupole mass spectrometer, which was differentially pumped by a turbomolecular pump group. During the measurements the catalysts temperature was increased with a linear heating rate of 4.5 K.min-1. This heating rate was sufficiently slow to maintain steady-state reaction rate conditions at each temperature. Reaction

383

rates were calculated from measurements of the total flow rate and the composition of the gas stream leaving the reactor. For more detailed information see Ref. [4]. The IR study was performed using a Perkin-Elmer 580B doublebeam grating spectrometer (resolution 5 cm-1) equipped with a stainless-steel cell in which a self-supporting sample disk was placed. The cell had silicon windows and was part of a vacuum chamber (base pressure 1 x 10-4 mbar) with a gas introduction system. Because of the temperature dependence of the IR spectra, background spectra were collected at five selected temperatures : 300, 350, 425, 525 and 625 K. After dosing 2 mbar of the gases to the cell, spectra were recorded at those five temperatures. The proper background spectra were subtracted and the resulting spectra smoothed using a 25-point smoothing function. Materials : In order to avoid possible support effects, Si02 was used as the support for the noble metal particles. Pt and Rh catalysts were prepared by homogeneous precipitation of Aerosil 200 (200 m2/g) with aqueous solutions of either Rh(N03)2 or H2Pt(OH)6. The alloy catalysts were also prepared by homogeneous precipitation of Aerosil 200, but with both precursors in aqueous solution. The metal loading of all catalysts was 5 wt%. The resulting catalyst was calcined overnight at 383 K and, subsequently, reduced in flowing H2 at 673 K for 16 h. One extra reduction step was used for the IR experiments; after the reduction a self supporting disk was pressed and this was reduced in situ at 623 K for 16 h. All catalysts were examined using X-ray diffraction to determine the average particle size, and, in the case of the alloy catalysts, to ensure that alloy particles had been formed. Finally, metal surface areas were determined by CO adsorption assuming a 1:l stoichiometry between noble metal atoms on the surface and adsorbed CO molecules. These measurements showed that the difference in specific surface areas of the catalysts was less than 25 %. RESULTS

1

-

NO-CO reaction

In order to compare the present results for the NO+H2 reaction, the most relevant results which were obtained earlier for the adsorption of NO, CO, NO+CO and for the NO+CO reaction on the same catalysts, will be briefly presented. In table 1 the results of the adsorption of CO, NO and a COP40 mixture on a pto.5Rho5/Si02 alloy catalyst are summarized.

384

Table 1

-

Location of the main IR absorbance hands (in cm-1) for difTerent gases adsorbed on a Pt0.5Rh0.5 alloy catalyst at different temperatures

Adsorbate

co NO

co/No

I

I

625 300 350 2030 2070 2065 1870 1860 1855 1845 1830 1790 1795 1790 1645 1645 1665

2180 2180 2075 2070 1875 1860 1645 1645

-

Assignement Pt/Rh-CO (linear) (Pt-Rh)-CO (bridged) Rh-NO (linear) Rh -NO (bent)

Rh-NCO 2195 Pt-CO (linear) 2055 2045 2040 (Pt-Rh)-CO (bridged) 1850 1845 Rh-NO (bent) 1635 1645

The results can be summed up as follows. The absorption band at lo75 cm-1 found on the alloy catalyst after CO+NO adsorption is also observed when CO is solely adsorbed on the alloy catalyst. The band is stable up to at least 625 K just like the corresponding band on pure Pt. On Rh the linear band disappears already around 425 K in the presence of the CO-NO mixture. Furthermore, its shift with increasing temperature is similar to that found on Pt and different from that observed on Rh. Hence, it is likely that this band is largely due to CO bonded to a Pt atom. The band around 1875 cm-1 is associated with multiply coordinated CO. Its relative intensity, its location and its shift with increasing temperature and, hence, decreasing coverage, suggests that at least a part of this multiply bonded CO is bonded to both a Pt and a Rh atom. The spectra of NO on the Pt-Rh alloy catalyst are similar to those found on the pure Rh catalyst, suggesting that most of the NO is adsorbed on Rh sites. The most relevant conclusions are summarized as follows : -Both Pt and Rh sites are present at the surface of the Pt-Rh alloy catalyst, as is shown by the adsorption of CO and NO on both Pt and Rh. -The CO molecules have a tendency to adsorb on Pt atoms, while NO molecules are primarily adsorbed on Rh atoms. This is consistent with the relative heats of adsorption of CO and NO on pure Pt and Rh. -For multiply coordinated CO a mixed Pt-Rh adsorption site is proposed. The steady-state reaction rate was measured on five catalysts, viz. 100% Pt, 75% Pt-25% Rh, 50% Pt-50% Rh, 25%Pt-75% Rh and 100%Rh. The gas phase composition was varied from CO rich (NO/CO=1/4), stoichiometric (NO/CO=l/l) to CO lean (NO/CO=4/1). For illustration

385

figure 1 shows the temperature required to achieve a constant turnover frequency of 0.05 s-1 versus the catalyst composition. The turnover frequency is defined as the number of NO molecules reacting per second per metal (Pt and Rh) atom on the catalyst surface. The number of metal atoms on the surface was estimated on the basis of CO adsorption (see experimental) 680

1

660

s

640

2 620 2

c

m

600

a

$ 580

I-

560

0

25

75

50

100

-%Rhodium NO/CO=1/1 NO/CO=1/4 NO/CO=4/1

+

.--A--

0

Figure I . Temperature f o r a constant turnoverfrequency of 0.05 sec-1 for the NO + CO reaction over Pt - Rh alloys vs bulk compostion. The main reaction product found was N2 with only traces of N 2 0 . The activities of the alloy catalysts were between those of the pure components, synergetic effects were not observed. Rh and Rh-rich alloys are much better catalysts for the reduction of NO with CO than R and R-rich alloys, both under CO rich and CO lean conditions. The activities of Rh and the Rh-rich alloys vary only slightly with the flow composition, whereas for the Pt and Pt-rich alloys the influence of the flow composition is larger. On the pure Pt and Pt-rich alloys catalysts, a high conversion is obtained at lower temperatures under CO lean than under CO rich conditions. The activity of the m.25-Rh0.75 catalyst is almost equal to that of the pure Rh catalyst, independent of the flow composition. Under stoichiometric and CO lean conditions, the Pb.5-Rho5 catalyst has an activity almost equal to that of the pure Rh catalyst. Under reducing conditions its activity is between those of R and Rh. The Pt0.75-Rho25 catalyst shows an activity equal to that of pure Pt under net-reducing conditions. Under stoichiometric and net-oxidizing conditions its activity is intermediate to those of pure Pt and Rh.

386

2- NO-H2 reaction

To assess the effect of catalyst aging, three series of measurements on each catalyst were performed. First, the conversion and the product distribution were followed on the freshly reduced catalyst using the T-t program described in the experimental. The maximum temperature used was 673 K. These experiments were repeated twice. The results of the second series of experiments were similar to those of the first series within experimental accuracy.

300

', 0

I

25

-

I

,

I

50

75

100

% Rhodium

NO/HP=l/I NO/H2=1/3 +

Figure 2. Temperaturefor a constant turnover frequency of 0.005 sec -1 for the NO + H2 reaction over Pt - Rh alloys vs bulk composition.

In figure 2 results are shown for Pt, Rh and the three Pt-Rh alloy catalysts, using two NO/H2 ratios, viz. 1/1 and 1/3. The freshly reduced catalysts were slightly more active than the aged catalysts. This effect was completely reversible after reduction. XRD showed that the average particle size of the aged catalysts was equal to that of the fresh catalysts. The effect of aging is significantly larger for the alloy catalysts than for the pure Pt and Rh catalysts, especially for the NO/H2=1/1 ratio. The activity of the alloy catalysts was between those of pure Pt and pure Rh. However, after aging its activity resembles that of the pure Rh catalyst for the NOD32 ratio is 1/1. The results shown are those found for the aged catalysts. However, this is not relevant for the discussion, since plots for the fresh catalyst show qualitatively the same differences in the behaviour of the various catalysts. The figure illustrates the different behaviour of the five different catalysts for the two flow compositions.

387

275

325

375

425

475

525

575

625

--> Temperature (K)

Figure 3. Selectivity vs reaction temperature for the NOH2 = I / / reaction over Pt. 1

=! Y

Q

0

0 275

325

375

425

475

525

575

625

--> Temperature (K)

Figure 4. Selectivity vs reaction temperature for the NO/H2 I / ] reaction over Rh. The reaction products found are N2, N 2 0 and NH3. The temperature dependence of the product distribution is shown in figures 3 to 5 for pure Pt, Pto.sRho5 and pure Rh using a NO/H2 ratio equal to unity (stoichiometric composition for the formation of N2). For a NO/H2 ratio of 1/3 all the catalysts produce large amounts of NH3 at T below 525 K. The differences in selectivity are smaller than for the NOM2 ratio of 1/1 and are, therefore, not shown in this paper.

388

275

325

375

425

475

525

575

625

--> Temperature (K)

Figure 5. Selectivity vs reaction temperature for the NO/H2 reaction over Pto.5 Rho.5. The relevant observations can be summed up as follows : No synergetic effects, like enhancement of the reaction rate, for the Pt-Rh alloy catalysts are observed when compared with the pure metal catalysts. The activity of the alloy catalysts is always between those of the pure Pt and Rh catalysts. - Whereas the activities of Rh and the alloy catalyst for the NO+CO reaction vary only slightly with changing gas phase composition, this effect is much larger for the NO+H2 reaction. However, for the pure Pt catalyst the effect of the flow composition is smaller for the NO+H2 than for the NO+CO reaction. - When the relative activities are considered, Rh and Rh-rich alloys are the better catalysts for the NO+CO reaction. On the other hand Pt is the most active catalyst for the NO+H2 reaction, while all Rh containing catalysts are less active. - Rh has a better selectivity towards N2 formation, while Pt produces relatively large amounts of N 2 0 and NH3 at temperatures below 525 K. The selectivity of the alloy catalyst is different from both Pt and Rh. It resembles that of Rh but its N 2 0 production is larger in a broad temperature range. -

111 Effects of particle size

In order to distinguish effects resulting from alloy formation from particle-size effects, a series of WSiO2 catalysts was prepared with average particle-sizes ranging from 20 to 300 A. These were used as catalysts for the

389

NO+H2 reaction. No significant differences were found regarding the selectivities and activity, although catalysts with large particles tended to have a somewhat larger activity than those with small particles. These effects, however, were much smaller than the effects of composition of the catalysts (Pt,Rh and alloy catalysts). DISCUSSION

The reduction of NO to N2 with either CO or H2 is one of the important reactions taking place over the three-way catalyst. Both activity, viz. a high conversion at low temperatures, and selectivity, high to N2, are dependent on the composition of both catalyst and gas phase. For the reduction of NO with CO both platinum and rhodium have a good selectivity to N2, while rhodium has the better activity. This last observation is consistent with literature data [lo]. The mechanism of the NO + CO reduction proceeds via the following steps [3]:

NO NOa

co

N a + Na NOa+Na CO,+O, NOa+Oa

---* +

+

--+

+

+ --+

NO, Na+Oa COa N2 N2+Oa

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

C@

(5) (6)

N20

(7)

It is often assumed that the dissociation of NO is the rate determining step. The lower activity of Pt compared with that of Rh can be understood on the basis of the activity of both metals for the dissociation of NO. The dissociation of NO is more difficult on platinum than on rhodium [ 3 ] . The IR experiments show that both CO and NO are adsorbed on the alloy surface as would be expected from the relative heats of adsorption. Indications are found of the existence of mixed Pt-Rhadsorption sites for CO. However, CO on mixed Pt-Rh sites has no apparent effect on the overall catalytic activity. Most likely the concentration of CO on such sites is too small to influence the catalytic properties of the alloy catalyst noticeably. When H2 is used to reduce NO, platinum has the better activity. When selectivity is considered it is rhodium that is the better catalyst, because NH3 is a main product on the platinum catalyst. Again this is in accordance with literature [7]. As is the case with the NO + CO reaction, all three alloys behave either like the pure metals or show an intermediate activity with no indications of a synergetic effect.

390

The most likely mechanisms for N2. N 2 0 and NH3 formation are dissociation of N O followed by reaction :

In these mechanisms products are formed via an atomic N species. This mechanism was proposed by Siera et a1.[8] who found that NH3 can be formed on Pt-Rh( 100) and Rh( 100) via nitrogen adatoms. The better activity of Rh for the NO+CO reaction, may be attributed to the better activity of Rh in the N O dissociation which is often assumed to be the rate determining step. However, this model fails to explain the better activity of Pt for the NO+H2 reaction, because N O dissociation proceeds more rapidly on Rh than on Pt. Therefore, a different rate determining step should be considered. One possibility can be found by looking at the temperature at which N O starts to desorb from Pt and Rh. Table 2 shows that this temperature is lower for N O desorption from Pt than from Rh. NO desorption is prerequisite to create free sites adjacent to NO molecules required for N O dissociation.

, Td

Pt (111) Rh( 111)

270

350

350

Ref.

El

~131

Pt(100) Rh( 100)

400

r141

Pt(l10)

270 350

~151

[61

[GI

Onset temperatures (in K) for the NO desorptiun from three Pt and Rh single crystal surfaces under the conditions of TDS ex eriments.

Since the light-off temperatures on the Pt catalyst for the NO+H2 reaction are not far above the temperatures at which N O dissociates under UHV conditions, it is not certain whether N O dissociation or N O desorption initiates the reaction on Pt. The observation that the reaction on Pt is not very sensitive to the partial hydrogen pressure may point to NO dissociation as the rate determining step on Pt. On Rh the reaction takes place at temperatures that are much higher than the N O dissociation temperature [9]. Apparently, the reaction rate is limited by the presence of NO on the surface.

39 1 The observation that the reaction rate on Rh is very sensitive to the partial pressure of hydrogen is consistent with this model. Introduction of CO instead of hydrogen changes the situation considerably. For Pt an increase of the reaction rate was observed when the NO/CO ratio was increased. On Rh the effect was much smaller. A study by Kobylinski et al. [lo] showed also that the reduction of NO over Pt is strongly inhibited by CO. Therefore, it is likely that CO inhibition is responsible for the high temperatures required for the NO+CO reaction, especially on Pt. The heats of adsorption of NO and CO are rather similar. Therefore, site competition may be expected. IR experiments [l 11 show that CO is able to displace preadsorbed NO on Pt at room temperature. CO inhibition is responsible for the high reaction temperature required for the NO reduction by CO over Pt catalysts. Rh seems to have some preference for NO. Hecker and Bell [12] found that a Rh catalyst is nearly saturated with adsorbed NO during the NO+CO reaction for NO conversions below 50%. Hence, during reaction NO is predominantly present on Rh and it inhibits the reaction. The selectivity for the reduction of NO with hydrogen towards N2 rather than NH3 or N 2 0 is the second factor determining the overall performance of the catalysts. Pure platinum catalysts produce much NH3, while pure rhodium catalysts produce mainly N2. This difference can be explained by looking at the relative concentrations of atomic hydrogen and atomic nitrogen on the surface of the catalyst [8] When the concentration of atomic hydrogen is relatively high, as it is on a platinum catalyst, the NH3 production will be high. On rhodium the NO dissociation proceeds more easily, the Rh-N bond strength is higher than the Pt-N bond strength, and thus the atomic nitrogen concentration will be higher, therefore producing more N2 [8]. The selectivity of the alloys is always between that of pure rhodium and pure platinum. A factor that could influence the behaviour of the alloy catalysts, is the effect of the particle size on both selectivity and activity. Our experiments with WSi02 catalysts showed only negligible particle size effects. However, unfortunately, no information is available about the influence of the particle size on the alloy catalysts. We cannot exclude that, for example, surface segregation of one of the components can occur preferentially on the edges. This could result in a different behaviour when the particle size changes, compared to a monometallic catalyst. There are, however, no indications that the contribution of this possible effect might have a larger contribution to the catalyst performance than that of alloying alone. Moreover, the difference in particle sizes was not more than 10%(around 170 A). Both the activity and selectivity of the Pt-Rh alloy catalysts are between those of the constituent metal catalysts. We have found previously

392 that the surface composition of the Pt-Rh alloys varies strongly with the experimental conditions such as the gas phase composition [3]. Clean Pt-Rh alloy surfaces are enriched with Pt. Adsorbates can easily induce segregation of Pt or Rh to the surface. Oxygen in the gas phase induces a Rh surface segregation because of the high Rh-0 bond strength relative to that of Pt-0. For the NO+CO and NO+H;! reactions the activity of the Pt0.5Rh0.5 alloy catalyst is like pure Rh under net-oxidizing conditions, whereas under netreducing conditions its activity is between those of Pt and Rh. This may suggest that the surface composition varies with the experimental conditions from almost pure Rh under net-oxidizing conditions to, perhaps, a bulk-like surface composition under net-reducing conditions. However, the selectivity of this catalyst for the NO+H2 reactions is different from both Pt and Rh. This indicates that this catalyst contains Pt atoms in the surface that influence the selectivity. The activity of the Pt075Rhfj25 alloy catalyst is almost equal to that of pure Pt under net-reducing conditions whereas its activity is between those of Pt and Rh under stoichiometric and net-oxidizing conditions. Again, this suggests that the surface composition changes with the feed composition. The properties of the Pt0.25Rh0.75 alloy catalysts are noticeable. Both for the NO+CO and NO+H2 reactions its activity is like that of pure Rh under net-oxidizing, stoichiometric and net-reducing conditions. Under netoxidizing conditions a behaviour like that of pure Rh may indeed be expected. However, under net-reducing conditions the presence of both pt an Rh atoms should be expected on the surface. The relatively high activity of this catalyst for the NO+CO reaction might be caused by a beneficial effect of the presence of both Rh and Pt : the low CO inhibition on Rh sites and a beneficial effect of Pt on, for example, the amount of N or NO on the surface. For the NO+H2, however, the presence of Pt atoms does not seem to diminish the inhibition effect of NO. In conclusion, it was shown that the catalytic properties of silica supported Pt-Rh alloy catalysts are strongly dependent on the gas phase composition, the bulk composition of the alloy particles and the reaction temperature. A complex of factors determine the activity of the different catalysts. Under oxidizing conditions at temperatures higher than 600 K, Rh segregation to the surface takes place [3]. This is reflected in the catalytic activity of the Pt-Rh alloys which can vary between that of pure Pt and pure Rh. No synergism, caused by alloying of Pt with Rh, was observed. The differences in activity, observed for the different catalysts, can be explained in terms of the specific properties of the pure metals towards the adsorbates.

393

Acknowledgment The authors acknowledge the Johnson Matthey Technology Centre (Reading, U.K.) for the loan of the Pt and Rh salts. This research was supported in part by NATO via grant = 86.352.

References 1. 2.

3. 4.

5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

Platinum 1989, Johnson Matthey, 24 (1989). B. R. Powell, Appl. Catal. 53, 53, (1989). S . Kim, M. J. d'Aniello, Appl. Catal., 56, 23, (1989). S . Kim, M.J. dAniello, Appl. Catal., 56, 45 , (1989). R.M. Wolf, J. Siera, F.C.M.J.M. van Delft and B.E. Nieuwenhuis Faraday Discuss. Chem SOC.,8 2 , 2 7 5 , (1989). A.G. v.d. Bosch-Driebergen, M.N.H. Kieboom, A.v. Dreumel, R.M. Wolf, F.C.M.J.M. v.Delft and B.E. Nieuwenhuis, Catal. Lett., 2, 235, (1989). R.F. v.Slooten and B.E. Nieuwehuis, J.Catal. ,229 ,429 , (1990). R.J. Gorte, L/D. Schmidt and J.L. Gland, Surf. Sci., 109 ,367 , (1981). T.B. Kobylinski and B.W. Taylor, J. Catal. , 33 , 150 , (1974). J. Siera, B.E. Nieuwenhuis, H. Hirano, T. Yamada and K.I. Tanaka, Cata. Lett. , 3 , 179 , (1 989), and to be published. R.M. Wolf, J.W. Baker and B.E. Nieuwenhuis, Surf. Sci., accepted for publication T.B. Kobylinski and B.W. Taylor, J. Catal. , 377 , (1974). G.M. Alilina, A.A. Davidov, I.S. Sazanova and V.V. Popovskii, React. Kinet. Catal. Lett. , 27 (2) ,279 , (1985). W.C. Hecker and A.T. Bell, J. Catal. , 289 , (1984). T.W. Root, L.D. Schmidt and G.B. Fisher.,Surf. Sci., 134,30, (1983). P. Ho and J.M. White, J. Chem. Phys. , fl , 103 , (1987)/ R.J. Baird, R.C. Ku and P. Wynblatt, Surf. Sci. 97,346 , (1980)

,a

,a


A. Crucq (Editor), Catalysis andAutomotive Pollution Control II 0 1991 Elsevier Science Publishers B.V., Amsterdam

395

A COMPARATIVE KINETIC STUDY OF THE C O - 0 2 REACTION OVER Pt-Rh (lll), (loo), (410) AND (210) SINGLE CRYSTAL SURFACES. J. Siera, R. van Silfhout, F. Rutten and B.E. Nieuwenhuys Gorlaeus labaratories, Leiden University, P.O. BOX 9502, The Netherlands ABSTRACT The CO-02 reaction has been studied as a function of temperature and partial pressure of CO and 0 2 over the (1 1l), (loo), (210) and (410) surfaces of a Pt025-Rho.75single crystal. The effects of alloying and surface structure are discussed. It is found that the reaction rates on the alloy surfaces show a positive order in oxygen at high temperature (%OK), whereas under these conditions the reported order in oxygen is negative for Rh(ll1). Furthermore, considerable differences are found in the steady-state rate of C02 formation for the four surfaces. On Pt-Rh(210)the CO oxidation starts around 500K, while over Pt-Rh(ll1) the rate has already reached its maximum value at this temperature. Comparison of the four surfaces shows that CO oxidation over terraces can proceed at lower temperature than over step sites. Hence, CO oxidation over stepped surfaces can proceed in two different stages. The first one occurs at relatively low temperature on terrace sites, and the second one on step sites at higher temperature. The possible origin of the controversy in the literature concerning surface structure sensitivity or insensitivity in the CO oxidation over metals is briefly discussed.

INTRODUCTION

Supported Pt-Rh catalysts are used as active components in the automotive three-way catalyst for controlling pollution from combustion products such as nitric oxide (NO), carbon monoxide (CO) and hydrocarbonsl. The reaction mechanism and kinetics of the CO-02 reaction over noble metals have been extensive y studied in the past [2-111. The formation of C 0 2 is usualTABLE 1 ly thought to proceed through a Langmuir-Hinshelwood mechanism between adsorbed CO and adsorbed oxygen atoms, as confirmed by molecular beam experiments [ 101.The reaction steps are summarised in table I . In this study we present results or the CO-02 reaction over Pt-Rh(ll1 Pt-Rh( loo), Pt-Rh(410) and Pt-Rh(210) under low pressure conditions. It will 9

396

be shown that there exists large differences in the behaviour of the four surfaces studied. At low temperature the inhibition of the steady state rate of CO2formation by CO is very high on the (210) surface. This effect is less pronounced for the two flat surfaces (1 11) and (loo), while the (410) surface shows an intermediate behaviour. The C 0 2 production found for the four surfaces differs also at high temperatures where CO inhibition does not take place. Under these conditions the (111) surface shows only low activity, probably caused by a low sticking probability of oxygen. EXPERIMENTAL

All experiments were carried out in a standard UHV system with a base pressure better than 2x10-10 mbar. It was equipped for quadrupole mass spectrometry (QMS), Auger electron spectroscopy (AES) and low energy electron diffraction (LEED). The single crystal was oriented by means of the Laue back reflection method to within 0.50 of the desired direction. The samples were cut by spark erosion and mechanically polished down to 0.25 pm grain size using standard techniques. The samples were cleaned by cycles of Ar-ion bombardment, oxidising treatments and flashing to high temperature (1400K) in vacuum. The crystals were heated by passing current through tantalum leads spotwelded to the edges of the crystals. The temperature of the crystals was measured by means of a Pt/Pto. 10-Rho.90 thermocouple spotwelded to the edge of the crystal. The system is pumped by an ion pump, a turbo-molecular pump and a titanium sublimation pump. During reactions the valve separating the ion pump from the main chamber was closed and the system was only pumped by the turbo-molecular pump. In this way, reaction rates were measured by using the main chamber of the UHV system as a continuous flow reactor. Gases were dosed to the main chamber through variable valves. It was assured that the background pressure contributed to less than 1% of the total reactant pressure. RESULTS

A

-

Surface composition

In the case of an alloy sample knowledge of the surface composition is required to understand the obtained kinetic results. The surface composition of Pt-Rh alloys has been studied extensively in the past few years [8,12-171. In general, a large surface Pt enrichment is found. In table 2 the platinum surface concentrations for the four surfaces are given as observed by AES after annealing the samples at 1300K in vacuum. The AES measurements on (210), (100) and (111) were done by directly measuring the signal intensities of the Rh222, Rh256, Rh302, Pt64 and Pt168 eV Auger transitions. In the evaluation of

397

the data the Gallon model was applied [18]. Calibration with pure Pt and Rh samples is needed to determine quantitatively the surface composition by means of this model. Furthermore, the assumption is made that the second layer has the same Pt concentration as the bulk. Tsong et a1 1141 found that the second layer is enriched in Rh atoms. However, the deviation from the bulk concentration is small and our assum tion will not lead to large errors. In the case of Pt-Rh(21O) it was Table 2. also assumed that the backscattering Composition of various factors and escape depths of the surfaces of a Pt0.25-Rh0.75 generated Auger electrons are the same as for the (100) surface. The AES alloy single crystal (at 9%) following annealing at 1300K. determination of the surface composition of Pt-Rh(410) has been done by Surface Pt concentration(%) oxygen titration, a method described by van Delft et a1 [ 121. Information conPt-Rh( 11 1) cerning the distribution of Pt and Rh 32f5 Pt-Rh( 100) atoms along the steps and terraces is, 40 f 5 P t - R h ( 2 10) unfortunately, not available.The open 55 k 5 Pt-Rh(4 10) (210) surface shows the largest Pt 4 0 k 10 enrichment, implying that on stepped I surfaces the Pt segregation may be irger at steps than on the terraces. Van Delft et a1 [17] that oxygen induces Rh surface segregation in the temperature range from 600 to 1000K. All experiments described in the rest of this paper were done after a standard annealing procedure in order to produce a well known surface concentration. However, it can not be excluded that the surface concentration changes during the course of the reaction, especially under oxidizing conditions at temperatures higher than 600K.

B - Temperature dependence of the reaction rate under steady state conditions. The formation of C02 was followed by monitoring the intensity of the amu=44 mass signal. It was assured that the reaction rate reached its steadystate value. In figures I to 4 the results are shown for the CO-02 reaction carried out over Pt-Rh(l1 l), Pt-Rh( loo), Pt-Rh(410) and Pt-Rh(210), respectively. Under stoichiometric reaction conditions the total reactant pressure was 3x 10-7 Torr. Partial pressures were also varied from oxidizing to reducing conditions in order to study the order of the reaction in CO or 0 2 at different temperatures. The first observation to notice is that there exists striking differences in the rate of COT formation as a function of increasing temperature for the four single crystal surfaces. For all the surfaces studied the reaction rate increases with increasing temperature until a maximum rate is

398

0

m

-0

0

.O b

0

m 0

0

x

m

Y

I-

-t -

t Y

I-

-

-0 0

0

.O PI

0 0 I n

0

0

Figures I to 4 ; Steady state rate of C02 formation versus temperature over : ffig.1) :Pt-Rh(ll1) ;(fig.2) :Pt-Rh(100) ;cfig.3) :Pt-Rh(4lO) ;cfig.4) :Pt-Rh(210)

399

observed at a temperature Tm. This value of Tm is one parameter which strongly depends on the surface structure. For Pt-Rh(ll1) the value of Tm is only 490K, the lowest Tm measured in this study. Pt-Rh(210) and Pt-Rh(410) have Tm 's of 615K and 650K, respectively. The Pt-Rh(100) shows a different behaviour, for this surface two local maxima are observed in the rate of C02 formation. The first maximum lies around 500K and the second one around 580K. A similar behaviour has also been observed for pure Rh(100) [7]. The positions of the local maxima in the case of pure Rh(100) are almost similar to those on Pt-Rh( 100). The relative contribution of the low temperature maximum is larger for Pt-Rh(100) than for Rh(100). In addition to the variation of the value of Tm for the four surfaces, also the increase in the rate of C 0 2 formation with increasing temperature is different in the low temperature range. These differences are reflected by the values of the activation energy of the process. At low temperature ( 1, one finds:

@s,lim+ 2 Ln(K C,)

(28c)

The latter threshold can be much larger than unity suggesting that the Weisz modulus given in the first column of Table 3 does not necessarily imply an internal diffusion limitation. Let us finally mention that when the reaction is strongly inhibited by the products, then @s,limcan be much lower than unity. Further details on this subject can be found in Froment and Bischoff (1979). When the reaction rate obeys equation (28a), internal diffusion limitations becomes beneficial at high reactant concentration because of the negative apparent order. Any diffusion limitation wili reduce the reactant concentration and thus favors the reaction.

4.1.2 Convective transport in the wash-coat Because of the finite permeability of the wash-coat, the bulk gas flow can induce convection inside the porous material. In standard catalyst pellets (a few mm or cm in diameter) this effect is negligible except for large pore catalysts (Rodrigues et al., 1982; Cresswell, 1985). With a very thin catalytic layer, internal convection cannot be excluded even if the pores are narrow. Moreover, the roughness of the wash-coat surface can enhance the internal convection.

45 1

Although there is no available literature concerning convection in thin catalytic layers, it is obvious that the convection process would reduce the mass transfer resistance. This brief analysis shows that the internal mass transfer problem remains unsolved. Only experiments with wash-coats of various thickness would help to get further insight in the problem. 4.2 Heat transfer In the previous section T, = T(ts

g 1

j k m s

Gas Species Reactions Cells Averagevalue Solid

479

Literature cited: Chen D.K.S, S.E. Oh, E.J. Bisett, and D.L. Van Ostrom: A three dimensionam model for the analysis of transient thermal and conversion charateristics of monolithic catalytic converter. SAE paper 880282, International Congress and Exposition, Detroit, Mich., (1988). Howitt J.S., and T.C. Sekella: Flow effects in monolitic honeycomb automotive catalytic converters. SAE paper 740244, Automotive Engineering Congress, Detroit, Mich., (1974). Leclerc J.P., D. Schweich, and J. Villermaux: Transfert de chaleur en rCgime transitoire dans un monolithe destinC i l'kpuration de gaz dkchappement automobile. SFT 89 Congress, "Thermique et GCnie des PrwCdCs", Nancy, (1989a). Leclerc J.P., D. Schweich, and J. Villermaux: Hydrodynamique et transfert de chaleur dans un monolithe destine i 1'Cpuration de gaz dkchappement automobile. Symposium "GCnie des procCdCs", Toulouse, (1989). RCcents progrks en gCnie des procCdCs, Lavoisier Ed., 3(8a), 518-524, (1989b). Lemme C.D., and W.R. Givens: Flow through catalytic converters - An analytical and experimental treatement. SAE paper 740243, Automotive Engineering Congress, Detroit, Mich., (1974). Oh S.H., and J.C. Cavendish: Transients of monolithic catalytic converters; Response to step changes in feedstream temperature as related to controling automobile emissions. Ind. Eng. Chem. Prod. Res. Dev., 21, 29, (1982). Schweich D., and J.P. Leclerc: Flow, heat and mass transfer in a monolithic catalytic converter. 2nd International Congress on Catalysis and Automotive Pollution Control, Brussels, 10-13 Sept., (1990). Shah R.K., and T.C London: Flow forced convection in ducts. Advances in heat transfer Laminar., Academic Press, New-York, (1978). Voltz S.E., C.R. Morgan, D. Liederman, and S.M. Jacob: Kinetic study of carbon monoxide oxidation on platinium catalysts. Ind. Eng. Chem. Prod. Res. Devel., 12, 295, (1973). Villermaux. J.: Gtnie de la rkaction chimique. Conception et fonctionnement des rCacteurs. Tec & Doc, Lavoisier, Paris, (1985). Wendland D.W.: The segmented oxiding monolith catalytic converter, theory and performance. J. Heat Transfer, 102, 194, (1980). Wenland D.W., and W.R. Matthes: Visualization of automotive catalytic converter internal flows. SAE paper 861554, International Fuel and Lubricants Meeting and Exposition, Philadephia, Pens., (1986). Young L.C., and B.A. Finlayson: Mathematical models of the monolith catalytic converter, part 11, Application to automobile exhaust. AIChE J., 22, 343, (1976).


A. Crucq (Editor), Catalysis andAutomotive Pollution Control 11 0 199 1 Elsevier Science Publishers B.V., Amsterdam

48 1

NEW DEVELOPMENTS IN CATALYTIC CONVERTER DURABILITY Suresh T. Gulati

R , D & E Laboratory, Corning Incorporated, Corning, New York 14831 USA ABSTRACT

A number of new developments in the areas of material composition, cell geometry, wall porosity, substrate contour, washcoat formulation, thermal and acoustic insulation, and clamshell and heatshield designs have occurred in recent years to meet the more demanding performance and durability requirements of automotive catalytic converters. This paper reviews the major developments in ceramic and metal substrates, stable washcoat systems, nonintumescent mounting materials, edge-shielded intumescent ceramic mats, ceramic insulation rings, dual can and dual cone packaging designs, and perforated heatshields with convection cooling. These improvements offer higher conversion efficiency, lower pressure drop and extended durability. A brief comparison of ceramic and metal substrate durability, based on laboratory tests, is also included. INTRODUCTION

This paper reviews some of the new developments in catalytic converter durability. In particular, it focuses on the ceramic converter system due to its proven success over the last decade and a half in the automotive industry [l]. A brief comparison of ceramic and metal converter systems, based on laboratory testing, is also included. The need for a durable converter system continues to grow worldwide due to highter air quality legislation, namely faster light-off and higher conversion efficiency, lower pressure drop, and longer product life requirements without compromising vehicle performance. To meet these requirements simultaneously, it is imperative to take the systems approach [2-4] wherein each component of the converter assembly is carefully designed, tested and optimized. Starting with

482

the automakers' specifications with respect to converter size and location, available space, fuel management, engine emissions and acceptable power loss penalty, the converter designer is challenged with selecting appropriate materials, substrate contours, catalyst volume and washcoat formulation, and assembling them in a durable package. In view of the variety of materials, configurations and microstructures employed in various converter components, the task of designing the total system becomes even more complex and requires cooperative effort by all of the component suppliers to meet the automakers' space, performance and cost specifications. The durability requirements begin with the substrate design both in terms of material and geometry. The initial design of the substrate is generally dictated by performance requirements [5-71. To meet the durability requirements, however, certain trade-offs must be considered. On the one hand, the mechanical and thermal durabilities call for high strength, high deformation resistance, and excellent thermal shock resistance of washcoated monoliths which, in turn, require low porosity, good cell definition and low coefficient of thermal expansion. On the other hand, faster light-off, higher conversion efficiency, and lower pressure drop call for high cell density, low thermal inertia, and thinner cell walls which are contrary to durability requirements. However, certain composition and process modifications can help optimize ceramic monoliths to meet both the performance and durability requirements. Recent developments in the design and properties of ceramic substrates of dense cordierite and mullite aluminum titanate compositions are reviewed. In addition, the high temperature properties of ceramic and metal substrates are compared [8]. The role of high surface area washcoats is also important in this context because their interaction with the substrate has a major impact on physical properties which ultimately control its durability [ 9 ] . For example, the formulation (composition, particle size, and rheology), the loading (96 weight) and the processing (number of calcining cycles, peak temperature and total cycle time) of washcoat and catalyst affect the strength, thermal expansion coefficient, and structural modulus of coated monolith which, in turn, determine its long-term durability. Both the microstructure and substrate/washcoat interaction have strong influence on these properties. The next critical element in the converter system is the ceramic/wiremesh mat which provides mechanical and thermal insulation against vibrational and thermal loads [ 10, 113. The compression, shear, erosion and frictional properties of these materials, notably at high temperature, control the stress level in the monolith and hence its durability. These properties, together with the criteria for selecting the optimum mat, are reviewed. New developments in ceramic mats of special composition, designed for metal monoliths, are also discussed briefly. In certain dual-bed converters, a ceramic spacer ring is employed between the monoliths to shield the mat and

483

insulate the can against high temperature gases thereby preserving the can's rigidity and minimizing its deformation [12]. The critical data for ensuring the durability of these rings is highlighted. Finally, the pertinent design and physical properties of stainless steel can and heatshield are addressed [13] The key role of the can, besides housing the catalyst, is to ensure a positive mounting pressure on the monolith, under all driving conditions, without excessive deformation in the operating temperature range. The latter is best achieved by stiffening the can in critical areas via ribs, shoulders and end cones and by designing effective cooling measures to minimize its operating temperature. It is here that the heatshield design commands meticulous attention to meet the dual requirement of shielding the floor pan against the intense heat of the converter and keeping the can temperature as low as possible via effective convection cooling. However, the use of acoustical pads between the can and heatshield to minimize vibrational noise in certain applications negates some of these benefits and must be readdressed from durability point of view. Alternative can designs that do not require a heatshield and yet meet the insulation, cooling, and acoustical requirements are also reviewed [ 14,151. The paper concludes with a brief review of special packaging requirements for metal monoliths, with and without the outer mantle, to compensate for their low rigidity at high operating temperatures, e.g. in close-coupled applications. NEW DEVELOPMENTS IN SUBSTRATES

With stricter emission standards and low back pressure requirement, both ceramic and metal substrates have undergone significant developments over the past few years. The thrust in the ceramics area has centered on thinwall structures to minimize pressure drop and on high temperature refractoriness to permit higher use temperature in close-coupled, light-off, and heavy-duty applications [ 16 -191. The thin-wall structures are extruded from dense cordierite to provide strength and thermal shock resistance equivalent to those of standard cordierite substrate to meet the physical durability requirements; see Fig. 1. Known as Celcor XT, they are available in two different cell sizes: (i) 35"/5-5 for low back pressure and (ii) 47015 for higher conversion efficiency and lower back pressure than the standard 400/6.5substrate. The pertinent geometric and physical properties of four different cordierite substrates are summarized in Table 1.

484 TABLE 1

Nominal Properties of Standard and Thin-Wall Cordierite Substrates

I

COMPOSITION EX-20 (Stdl

EX-32 (St4

EX-22 EX-22 (Thin-Wall: :Thin-Wall)

Cell Structure Cell Shape Wall Thickness, mm Open Porosity Mean Pore Size, pm Wall Strength, MPa (Psi) Wall Modulus, GPa MIF Substrate Strength* TIF TSR OFA, % ( I ) GSA, cm2/cm3(2) Avg. CTE 8OO0C,(3) 10-7pC Dh, mm Ap(4)

40016.5 0 0.188 35% 3 .O 20.3 (2950) 26.1 0.025 73k 6.9 llOK 73 26.4

23611 1 A 0.279 42%

350/5.5 0 0.140 20%

7.0 14.3 (2080) 20.9 0.031 64k 7.4 105K 65 22.2

2.0 43.1 (6250) 42.9 0.012 75k 9.7 130K 80 26.4

470/5 0 0.127 20% 2.0 43.1 (6250) 42.9 0.013 81k 9.7 140K 80 30.4

6.0 1.106 172C

5.0 1.171 191C

4.5 1.218 129C

4.5 1.045 154C

Wall Density, dcm3

1.61

1.42

2.0

2.0

Substrate Density, g/cm3 Heat Capacity of Substrate, cal/cm3'C

0.43

0.49 1

0.39

0.41

0.11

0.12

0.10

0.10

(1) OFA: Open Field Aperture. (2) GSA: Gross Surface Area. (3) CTE: Coefficient of Thermal Expansion. (4) The constants k and C are normalization constants to help compare strength and pressure drop characteristics of cordierite substrates with different porosity and cell structures.

485

EX- 22 ,350/ 5.5

EX- 20 ,400/6.5

EX - 3 2 , 2 3 6 / 11

Figure I - Comparison of standard and thin-wall cordierite substrates

486

The first two are standard commercial products of EX-20 and EX-32 compositions while the last two are newly developed from the dense cordierite composition EX-22. These properties help compare light-off and steady state conversion activity through heat capacity and GSA values, engine performance through Ap, and physical durability through substrate strength, TIF and CTE [6]. The pressure drop values in Table 1 are based on constant flow rate i.e. given engine conditions) and constant total GSA (i.e. equivalent conversion activity). They are given by equations (1) and (2) for square and triangular cell respectively:

where

cs =

- 4.flow-rate.(substrate length) total surface area

4.4.1 4A(L-t)

2

L2 Ct = 443

+

2

- 4.flow-rate.(substrate length) total surface area (L-td?)

4.Q.1

2

L

in which Q denotes flow rate, A and 1 are cross-sectional area and length of substrate respectively, and L and t are side length and wall thickness of the unit cell. The constancy of total GSA requires that

which relates the cell dimensions and volume of the four substrates being compared in Table 1. Similarly, the strength of the substrate is proportional to the product of mechanical integrity factor (MIF) and wall strength with k as the proportionality constant [3] strength = kow (MIF)

(4)

Its values are shown in Table 1. The thermal shock resistance (TSR) of the substrate is related to its strength, CTE, thermal integrity factor (TIF) and wall modulus [5,6]:

487

TSR =

k cw(MIF) (TIF)

2E w a w k

TSR =

(TIF) 3Ewaw

for square cell

Gw(MIF)

for triangular cell

(6)

We may summarize the key points in Table 1 as follows: (i) thin-wall substrates enjoy similar or higher GSA than standard substrates thereby providing equivalent or improved catalyst activity; in particular, the 470/5 substrate offers 15% higher geometric surface area than the 400/6.5 standard substrate; (ii) thin-wall substrates result in lower pressure drop than standard substrates at constant flow rate and total surface area; in particular, the 350/5.5 substrate experiences 25% lower pressure drop than the 400/6.5 standard substrate; see Figs. 2 and 3; (iii) thin-wall substrates offer similar or higher physical durability as standard substrates; in particular, their thermal expansion is lower than that of 400/6.5 standard substrates; see Fig. 4, implying higher thermal shock resistance. Table 2 compares the crush strength data along A, B and C axes for standard and thin-wall substrates. In addition it lists the axial MOR and E-modulus data for these substrates. It is clear from Table 2 that thin-wall substrates are just as strong as the standard substrates and possibly stronger. The differential expansion strain values (DES) over the two different temperature ranges"(450" to 825°C and 600" to 9OO"C), are taken from Fig. 4 to compute the thermal shock parameter using eqn 7 : TSP = MOR E x DES

(7)

The higher the TSP value is, the more thermal shock resistant the substrate will be. It is clear from Table 2 that the thin-wall substrate has a higher thermal shock resistance than the standard substrate under both North American and European driving conditions.This is also borne out by the laboratory thermal cycling tests on full monoliths. *The 450" to 825°C range represents the nominal catalyst skin and center temperatures under North American driving conditions, and 600" to 900°C range represents the same temperatures under European driving conditions.

488

50

-

1

1

200

1

1

1

1

1

1

1

1

/

,

1

1

280 320 Flow Rote (SCFM)

240

1

,

1

,

400

360

Figure 2.- Pressure Drop vs Flow-rate through Uncoated Standard and Thinwall Ceramic Substrate. - 81mmx170mmxll7mm long 70 -

350/55

t h i n wall

-

20

200

'

/

"

'

240

/

'

I

"

280

'

'

'

I

320

'

I

'

/

360

'

400

Flow Rate (SCFM)

Figure 3.- Pressure Drop vs Flow-ratet hrough Coated Standard and Thin-wall Substrates.

-

1

5 - -Thin '600- ---

'

1

Wall Substrate

'

1

'

1

'

,

Figure 4.- Thermal Expansion Curvesfor Standard and Thin-Wall Ceramic Substrates.

489 TABLE 2

Comparison of Strength and Thermal Shock Parameter of Standard and Thin-Wall Substrates EX-20 400/6 5 (Stdl Axis Crush Strength MPa B Axis Crush Strength MPa C Axis Crush Strength MPa Axial MOR, MPa Axial E Modulus, GPa

EX-22 350/5 5 (Thin-Wall)

20

47

3.6

45

0.3 22 6.5

0.3 3 .O 8.4

DES

45OoC-825"C TSP 450°C-825"C

DES 600"C-900"C

TSP 600"C-900"C

525 x 10-6 0.64 485 x 10-6 0.70

430 x 10-6 0.83 433 x 10-6

0.82

A more refractory ceramic material, mullite aluminum titanate is under development for high temperature applications, e.g heavy-duty gasoline truck engine. Its composition has been optimized to eliminate high-temperature decomposition and minimize thermal cycling growth, both being undesirable from physical durability point of view [18,19]. Its density is 20% greater and the use temperature 200°C higher than the corresponding values for standard cordierite. The initial testing is centered on physical durability of 400/6.5 square cell structure with geometric properties (GSA, OFA, Dh) and performance parameters (TIF, Ap) identical to those of standard cordierite substrate.

490

Both the high temperature vibration test and exhaust gas simulation test were carried out for a heavy-duty truck converter comprising of three 6" diameter x 3.2" long MAT substrates wrapped in Interamtm IV ceramic mat, 8800 g/m2, with a mount density of 1.1 g/cm3 see Fig. 5. All three substrates survived the 20 hour vibration test at 1OOO"C under an acceleration of 33 g's and a frequency of 100 Hz there was no detectable movement of either the substrates or the mat. The EGS test was performed using the propane gas burner/Roots air blower at an inlet gas temperature of 950°C. The typical cycle consisted of 135 sec.heat-up, 600 sec. hold, and rapid Figure 5. MAT Substrate Assembly for cool-down. The typical cycle consisted of 135 sec heat-up, 600 sec High Temperature Vibration Test. hold, and rapid cooldown. A steady state radial gradient of 150°C was measured at midbed of the substrate and a temperature drop of 400°C was recorded across the insulating mat. All three of the MAT substrates survived five such cycles with no visible or audible damage, testifying to the excellent thermal-shock resistance of these substrates. Similar tests with coated MAT monoliths are in progress and will be reported in the future. The metal foil monoliths of Fe-Cr-A1 and Fe-Cr-Al-Ni compositions and 400/21 cell structure offer larger open frontal area, higher geometric surface area and bigger hydraulic diameter with potential light-off, pressure drop and conversion efficiency advantages relative to the standard cordierite substrates [20]. However, the field data though sparse, do not confirm these advantages in a consistent manner due to minimal differences in heat capacity and hydraulic diameter of ceramic and metal cell structures; there are also certain durability issues with the washcoat/metal adhesion. Furthermore, the metal monoliths have poor physical durability above 800"C[21]. They oxidize and become brittle, and/or they deform permanently under sustained operating stresses at high temperature [22]. While the foil thickness is 0.002", the average wall thickness of metal substrates as determinrd by image analysis can be twice as large due to overlaying of foil in certain image regionsduring manufacturing. 1

49 1

n"

-I

15

CelcorR Ceramic EX-20, 400/6 5

L

o

-

0 Lo

)

-

Emicat-S. 400/2

400

600

Temperature,

000

1000

1200

O C

Figure 6.- Measured Strength vs Test Temperature for Standard Celcor, Emicat-S and Metalit SQ Discs.

Time ( m i n )

Figure'/.- Percent Creep Strain in Emicat-S, 40012 Specimens at Various Temperatures under a Static Compressive Stress of 0.09 MPa. Figure 6 compares the high temperature biaxial strength of standard cordierite and commercial metal foil discs in the concentricring flexure test [22,23]. Not only are metal discs weaker, their strength decays rapidly above 400°C. The typical failure mode consists of either "peeling" or "telescoping" due to weakness of joints between crimped and flat layers which are unable to transmit the operational loads exerted by inlet gas pressure and the engine and chassis vibrations.

492

-s

I -10 -

I

I

I -

T= 600°C

P = 0 0 9 MPa-

c

z -70

-

-"O

! 20

1200°C

40

60

80

I00

120

Time (min)

Figure 8.- Creep Strain in Metalit SQ, 15012 Specimens at Various Temperatures under a Static Compressive Stress of 0.09 MPa. Similarly, Figs. 7 and 8 show the high temperature creep deformation experienced by 1 cm x 1 cm x 0.5 cm thick specimens of two different metal monoliths under a modest pressure of 0.09 MPa representative of canning load in service. As the creep sets in and the cells deform, the potential advantages of large frontal area geometric surface area and hydraulic diameter are steadily lost! It should also be pointed out that the differential expansion strain between the foil core and solid mantle also contributes to radial pressure responsible for high temperature creep. Identical tests on cordierite Ceramic specimens of 400/6.5 cell structure yielded no permanent deformation up to a test temperature of 1200°C. SUBSTRATE/WASHCOAT INTERACTION

When viewed individually, both the substrate and alumina washcoat offer optimum set of properties for their prescribed function. However, when processed together to form a catalytically active support, their composite properties may be altered due to an order of magnitude higher thermal expansion of alumina washcoat. Since the overall catalyst durability, i.e. mechanical, thermal and catalytic durability, depends on composite properties the fundamental expansion incompatibility between cordierite ceramic and y-alumina must be managed properly through judicious control of raw materials, substrate microstructure and washcoat processing. For prolonged durability, the coated substrate should have high strength, low E-modulus, and minimal increase in thermal expansion relative to uncoated substrate

P31.

493

I

Porous Alumina Washcoat

-t - - -

--

Figure 9.- Schematic of Washcoated Substrate Wall. It is only then that the catalyst possesses sufficient mechanical and thermal integrity to withstand vibrational and thermal shock stresses encountered in service. Figure 9 is a schematic of typical washcoated substrate wall which may be treated as a 3-layer symmetric composite with mechanical adhesion at alumina interface. The expansion coefficient of such a composite which controls the thermal shock resistance, is given by [25]:

where Es, a, and Ec, ac denote E-modulus and expansion coefficient of substrate and washcoat respectively, and ts and tc denote the thicknesses of substrate wall and washcoat layer respectively. Using the nominal properties of these two materials listed in Table 3, we compute a composite expansion coefficient of 29 x lO-7/"C - an increase of 400% over that of cordierite substrate! Obviously, such a high value of composite expansion coefficient is unacceptable from a thermal shock point of view. Moreover, the measured value of composite expansion coefficient, as shown in Figure 10, generally falls between 6 x lO-7/"C and 15 x 10 -7PC for different washcoats indicating that either the washcoat modulus Ec is not as high as shown in Table 3 or the interfacial adhesion between substrate and washcoat is less than perfect. Both of these scenarios are controlled by washcoat formulation and processing. It is here that the coaters can play a key role in ensuring the catalyst durability without compromising its activity and engine performance. On the one hand, higher and prolonged catalyst activity calls for either an increase in washcoat loading or the use of denser washcoat; on the other hand, these very elements can have an adverse impact on E-modulus and composite expansion coefficient both of which affect the thermal shock resistance.

494 TABLE 3

Nominal Properties of Standard Cordierite /Alumina Composite Cordierite/ Alumina Comuosite 30 1.60 27.8 0.25 0.225 29

35 1.61 26.1 0.25 0.165 6

Porosity (%) p, dcm3 E,GPa V

t, mm a,10-7/oc (Avg. Value: 25-80OOC)

TABLE 4

Effect of Washcoat Modulus on Composite Expansion Coefficient Ec GPa

32.3 30.0 25.O 20.0 15.0 10.0 7.5 5.0 2.5 0.0

a (10-7pc)

29.0 27.8 25.1 21.o 18.8 15.0 13.0 10.8 8.5 6.0

1400

,

,

,

,

,

Temperature,

I

'

.I

("C)

Figure 10. Thermal Expansion Curves for Three Standard Ceramic Catalysts with Different Washcoat Systems Through judicious formulation and process control, the coater can modify the washcoat micro-structure and its E-modulus. The latter can range from a high value of 32.3 GPa, as shown in Table 3, to as low as zero! The

495

impact of such a range of washcoat moduli on composite expansion coefficient, as given by eqn. 8, is shown in Table 4. It is clear from these values that a low modulus washcoat helps preserve the low expansion properties of cordierite substrate and is, therefore, highly desirable. Since the measured values of a range from 6 x lO-7/"C to 15 x 10-7 /"C, the practical range of Ec according to Table 4 is between 0 and 10 GPa, i.e. the bond between cordierite substrate and alumina washcoat is not a theoreticallyperfect-bond which is good news for physical durability. In view of its high expansion, the alumina washcoat can also induce significant tensile stresses in the substrate wall during heating. The magnitude and impact of such stresses on the mechanical integrity of catalyst depend on interfacial bond quality, microscopic distribution of alumina particles in the open pores of substrate wall, and the morphology of these pores. The maximum tension induced in the substrate wall due to a differential expansion strain of (AaAT) is given by [26]

while that due to constrained e: pansion of spherical alumina grains in filled pores is given by [27,28]

&+Y 2tc

2 E s m

I

where A a = ac - as and AT denotes the temperature difference between alumina and cordierite. Using the nominal properties of Table 3 and assuming A a = 80 x 10-7 /"C and AT = 300°C during washcoat processing, eqns. 9 and 10 yield osw = 21 MPa and o ~ =p 61 MPa stresses which exceed the wall strength shown in Table 1. Even with a more realistic value of washcoat modulus of 10 GPa, o ~ =p 32 MPa which exceeds the fatigue threshold of cordierite wall and can lead to strength degradation via slow growth of microcracks either during processing or in service. Figure 11 is a Weibull plot of tangential modulus of rupture of three catalysts with different washcoat formulations and thermal history; it demonstrates clearly that while most of the test specimens yielded a mean strength of 1.5 to 2 MPa, regardless of coating differences, washcoat C3 resulted in the largest variation with minimum strength approaching 0.25 MPa! Such a variability is detrimental to the long-term reliability of ceramic monoliths. It is also clear from eqn. 10 that OSP can be kept near zero by limiting the washcoat modulus, Ec, to very low values through process control. With Ec near zero,

496

the effective differential expansion strain, AaAT , also approaches zero with the result that the induced tension in substrate wall approaches zero, thereby preserving the mechanical integrity of the total catalyst. 0 999 0990 0900

0 800

0600 t 0400

a 0

e

a 2

0200

= 9

0100 0050

0 020 0010

0

Tang. Modulus o f Rupture (MPa)

Figure I 1 .- Weibull Distribution of Tangential MOR of Three Standard Ceramic Catalysts with Different Washcoat Systems. DEVELOPMENTS IN CERAMIC MATS AND INSULATORS

As the converter location moves closer to exhaust manifold to help meet light-off requirements, for both medium-and high performance engines, the need for thermal insulation of stainless steel can and end cones increases to minimize their high temperature deformation and potential loss of holding pressure [29]. Several approaches are being considered by the automotive companies to improve thermal insulation; e.g. dual cone and/or dual can designs with some form of insulation in the annulus; hybrid mat system comprised of wiremesh, ceramic fiber, and intumescent ceramic mat; and ceramic insulation rings between the catalysts. The introduction of metal monoliths for certain strategic applications has also led to the development of special ceramic mats to meet both thermal and acoustic insulation requirements simultaneously. And finally, to minimize thermal erosion of ceramic mat at high inlet gas temperature, the standard intumescent mat with edges shielded by wiremesh screen is now a commercial product. Thus, the major developments in insulation hardware are primarily those associated with the ceramic materials.

497

The standard intumescent ceramic mat is available in a wide range of free densities and initial thickness so that the desired mount density can be obtained by using one or more of these to meet mechanical, thermal and acoustical requirements; see Table 5. TABLE 5

Standard Ceramic Mat and Nominal Compressed Thickness Mat Weight g/m2

Initial Thickness (mm)

1050 2100 2600 3100 3662 4070 6200

1.7 3.3 4. I 4.9 5.4 6.0 9.8

pm=O.gg/cd 1.2 2.3 2.9 3.4 4.1 4.5 6.9

pm=I .Og/cm2 1.05 2.10 2.60 3.10 3.70 4.10 6.20

The room temperature holding pressure depends on mount density, Pm and can be estimated (in bars) from p = 2760 exp (-6.7/pm)

(11)

This pressure will relax somewhat following the welding of clamshells and end cones. An initial pressure of 1.6-3.4 bars, corresponding to a mount density of 0.9 to 1.0 g/cm3- is highly beneficial to monolith durability. At higher temperatures, as the mat expands, this initial pressure may increase by 50 to 100% depending on heat-up rate. The maximum pressure could approach 7 to 10 bars which is well below the isostatic strength of the monolith. An improvement over the standard mat, in terms of thermal erosion resistance, is the Series WS mat with stainless steel wire screen wrapped around the edges; see Fig. 12. This special mat is designed for high inlet/exit gas temperature in that the wire screen shields the edges from thermal erosion. In view of its higher mass density and elastic stiffness in the edge region. Series WS mat offers higher mount density and exerts larger holding pressure in that region. Consequently, it should be positioned carefully during the canning operation so as not to.line up with an inner stiffener rib and cause shear cracks in the monolith due to a localized pressure band. And yet, Series WS mats are easy to assemble; they provide acoustic damping against gas

498

pulsations; they help contain frayed vermiculite and fiber segments; and they are available in the same wide range of free densities as the standard mat (See Table 5).

Figure 12.- Series WS Ceramic Mat with Screen-Shielded Edges Special ceramic mats have also been developed to meet the packaging requirements of metal monoliths which differ from those of ceramic monoliths. These special mats, Series MM for example, are made from ultrathin “sol-gel’’ fiber of alumina-silica-boria composition stitch-bonded to polyethylene cloth; they are available with or without an intumescent mat layer for additional holding pressure. The special construction of Series MM mat provides excellent thermal and acoustic insulation, critical for metal monoliths, with significantly lower holding pressure so as to minimize hightemperature deformation of 0.05 mm thick cell walls. This mat is very effective for metal monoliths with or without the outer mantle. As the metal core expands against the mantle or outer can, it compresses the mat appreciably with modest increase in holding pressure. The special, high strength, high alumina, sol-gel fibers behave elastically even at high operating temperatures and provide the needed holding pressure in a reliable manner. The flexural stress in these fibers, induced by mat compression, is relatively small in view of their ultrathin size and does not produce fracture; thus the mat retains its resiliency and continues to operate reliably throughout its lifetime. The typical Series MM mat is 10.5 mm thick and weighs 1500 dm2.

499

Table 6 shows the room temperature holding pressure as function of compressed thickness and mount density. TABLE 6

Compressed Thickness (mm) 12.7

7.6 6.3 5.1 3.8 2.5

Mount Density (g/cm3> 0.12 0.20 0.24 0.30 0.39 0.59

Pressure at 25°C (bar) 0 0.28 0.62 1.52 3.45 9.93

The nominal mount density for assembling metal monoliths is 0.25 g/cm3 which translates into a holding pressure of 0.7 bars. Upon heating to 800°C followed by a 15 minute soak time, the mount density and holding pressure increase to 0.3 g/cm3 and 1.5 bars respectively. The experimental data show that the metal monolith assembled with this mat can survive 20 cycles of 900°C vibration test at 40g acceleration and lOOHz frequency; the same monolith when assembled with wiremesh or intumescent, or fiberfrax mats did not survive even the first cycle. The aluminum titanate spacer rings are employed in certain multibed converters to shield the insulating mat between two catalysts from direct impingement of hot gases [12]. These rings are exposed to 900°C gas on the inner surface and often develop both through-wall and peripheral temperature gradients due to nonuniform flow profile. In addition to thermal stresses arising from these gradients, the ring is also subjected to mechanical loads due to elastic deformation of clamshells during canning. To ensure the durability of insulation rings, the combined thermal and mechanical stresses must not exceed the material strength. The room temperature MOR data of approximately 20 slip-cast aluminum titanate rings were obtained in the diametral compression test. In addition, the fractured ring, sections were retested in 4-point bending to characterize the material strength more completely; both of these tests are shown schematically in Fig. 13. The MOR data are plotted on Weibull paper in Fig. 14 to demonstrate strength variability. Both tests yielded similar strength values which ranged from 9 MPa to 58 MPa! The failure origin in both types of specimens was a spherical void buried just below the surface;

500

such voids are generally associated with raw material impurities and the firing process. The thermal stresses in the ring due to through-wall and peripheral gradients are given by [30]:

where Er, Vr and Ctr denote E-modulus, Poisson's ratio, and expansion coefficient of ring material and AT denotes the temperature gradient. Figure 15 plots these stresses as function of temperature gradient using the following properties2 Er = 29.6 GPa ;

Vr = 0.25 ;

It shows that the thermal stress due to each of these gradients can approach a value of 55 MPa which can lead to fracture.Conversely, to limit the thermal stress to below the minimum ring strength the maximum AT value must be kept below 100°C by controlling the flow distribution. Alternatively, the strength of ring material must be improved significantly and/or its expansion coefficient must be reduced by a factor of 8 through compositional and process modifications. New ceramic materials with good refractoriness and low expansion coefficient are currently being developed for more durable rings.

a r 2 28 x lO-7/"C ;

1

t

Figure 13 Diametral Compression and 4-Point Bending of Aluminum titanate Insulation Rings

2 The elastic properties were measured by sonic resonance; the low value of Er is due to 13% open porosity measured by mercury porosimeter.

50 1

0 99

zI

080 060 -

0

2 a

020-

0 0

010-

.

a

9

I

005 -

002 0 01

I

I

i

0.

8.

0 Bar Data

t = 040n 0 n

-3-

I

I

I 0 Ring Data

O0

0.

. .

8"1'

O

-

:

-

-

-

I

I

10

20

I 30

I 40

I 50

I 60 70

Figure 14.- Weibull Distribution of MOR Data for Aluminum Titanate Insulation Rings.

-

50

0

P

5

Figure IS. Dependence of Thermal Sh-ess on Temperature Gradient.

f $!

40

301

in

g

20

L

al

t' lot

0'

'

400

I

I

I

I

300

200

I00

Temp. Gradient AT ( " C )

I

502

DEVELOPMENTS IN PACKAGING DESIGN

In view of the more demanding driving conditions in Europe, increased concern over warranty costs and future trends toward lOOK vehicle mile durability, the total canning system has undergone a number of design improvements, notably [4,12,14,15] (i) (ii) (iii) (iv) (v) (vi)

dual can system with multiple types of insulation; dual entry and exit cones with ceramic fiber insulation; more rigid stiffener ribs; ceramic insulation ring in transition zone; screen-shielded intumescent mat; heatshields with air scoop effect and special stiffeners to minimize resonant vibrations.

A successful package for catalytic converters is one that ensures positive holding pressure on ceramic catalysts, promotes symmetric entry of inlet gases, minimizes radial temperature gradient in the catalyst, and provides good vibrational dampinq under all operating conditions [31]. The above design improvements certainly help meet these requirements. Both a stiffer clamshell design and an effective thermal insulation system are key to good packaging design. Should the catalyst crack due to operating stresses, the successful package will help hold it together without jeopardizing its catalytic activity. The upper limit on holding pressure is dictated by the isostatic strength of washcoated substrates3 which, as mentioned before, depends on substrate/washcoat interaction. Table 7 summarizes the isostatic strength data for four different substrates coated with three different washcoats. It shows that the coated substrate is at least an order of magnitude stronger (>lo MPa or 100 bars) than the maximum holding pressure exerted by the mat at high temperature (10 bars). Moreover, a thick mat with high mount density which ensures positive pressure and excellent insulation will experience minimal erosion and continue to perform reliably; the can will also experience lower temperatures and undergo minimal permanent deformation. The use of internal stiffener ribs was originally intended for wiremesh mounted systems to ensure positive gripping at high temperature. Such internal ribs should be minimized or placed in the midbed region for matmounted systems to minimize frictional strain transfer from can to the monolith. The external ribs are equally effective as stiffeners although they require additional space which is at premium both in the engine compartment and under the chassis. Fortunately, with extra insulation, the can temperature is lower and requires shallower stiffeners thereby alleviating space 3

Or by crush strength along B and C axes.

503

constraints. Another option available to the canners is to use a thicker sheet of stainless steel for higher rigidity which will also enhance its durability.

x.AWL7

Isostatic Strength of Coated and Uncoated Cordierite Substrates (MPa)* --~ --

-

--

With With With Coating C: Coating C2 Coating C3 -~

EX-20 400/6.5 Sq. Cell 3.2" x 5" Oval

9.3

10.0

11.0

EX-20 400/6.5 Sq. Cell 3.18" x 6.68" Wide Oval

8.9

10.9

--

3x-20 300/11 Sq. Cell 4.16" Round

18.9

--

--

11.9

--

18.9

EX-32 236/11 Tri. Cell 3.4" x 5.0" Oval ____---

*

Uncoated

---

-

Above data are based on a sample of 15 substrates per test. NOTE:lMPa = 10 bars

In view of the local gap changes due to internal or external ribs, the mount density and the corresponding holding pressure on ceramic monolith will also experience step changes. Such changes result in localized strain discontinuities in the monolith skin and produce tensile stress in the axial direction. Figure 16 shows the results of finite element analysis of axial stress distribution in the vicinity of the stiffener [31] For maximum durability, this stress should be taken into account along with the thermal stress, and the combined stress should be limited to less than 50% of the monolith strength to prevent potential fatigue degradation [32]. One way to minimize the combined stress is to position the stiffener rib away from maximum thermal stress location, i.e. away from midbed region.

504

monolith

1 I mner con

fiber insulotm

A = 1.38MPo B = 1.05 MPo C=0.71 MPo D = 0.38MPa E = 0.04MPo

outer can

MIN = 0.29 MPo MAX = 1.72 MPc

Fig. 16. - Axial Stress a, in Ceramic Monolith Near the Stiffener Rib.

Fig. 17. - Close-Up View of Dual-Can and Dual-Cone Converter Assembly.

The thermal stresses can be minimized by reducing the radial temperature gradient in the monolith. Figure 17 shows a close-up view of dual-can and dual-cone converter assembly [12,14]. The former system uses a wiremesh mat next to the monolith and fiber insulation in the annular space between the two cans. Contrary to initial expectations, the monolith of this system experiences a lower temperature gradient due to (i) heat conduction from inlet gases to exposed wiremesh and from wiremesh to monolith skin and (ii) minimal heat loss through well-insulated dual can. The heat loss through the single mat layer and can in the latter system is higher and results in a larger temperature gradient; only in the dual-cone region is the heat loss minimized. Consequently, the thermal stresses in the monolith will be larger in the latter system despite the use of ceramic insulation mat In addition, the radial expansion of the inner cone at high inlet gas temperature could increase both the mat density and pressure loading causing deformatlon of inner and outer cones and exposing the mat to gas impingement. This should, of course, be avoided to minimize thermal erosion of the mat lest the support system disintegrate and expose the monolith to vibrational and impact damage. It

505

appears, therefore, that even the dual cone system may require stiffeners to resist deformation at high temperature. Alternatively, the thermal insulation used between the cones should be nonintumescent, e.g. Series MM mat, to limit the pressure build-up at high temperature. Returning to dual-can assembly, while the wiremesh mat plays a beneficial role of minimizing the radial temperature gradient, it does so at the risk of annealing and potential loss of mat pressure which is again undesirable. The use of high weavedensity austenitic steel wiremesh would reduce the risk of annealing. In certain converter assemblies, notably those employing a single can, one or two heatshields are used to protect the adjacent components, floor pan, and ground vegetation from thermalradiation [4]; see Fig. 18. However, their structural design and relative positioning are critical to converter durability. The annular space between the heatshield and can should be maximized to promote both the air scoop effect and convection cooling of the can; the latter can also be achieved by perforating the heatshields. Should the perforations reduce the flexural rigidity and produce unacceptable vibrational noise, the heatshield design and anchoring system may need to be stiffened. The use of acoustical pads to reduce the vibrational noise would defeat systems durability by raising the can temperature and causing excessive deformation; hence the acoustical pads between the heatshield and can should be avoided, if possible [ 141.

MONOLITH MOUNTING MATERIAL I . ,’

n

fi

HEATSHIELD

Figure 18. - Schematic of Converter Assembly Showing Heatshields

506 SUMMARY

A number of new developments in the areas of material composition, cell geometry, wall porosity, substrate contour, washcoat formulation, thermal and acoustical insulation, and clamshell and heatshield design have occurred in recent years to meet the simultaneous requirements of faster light-off, higher conversion efficiency, lower pressure drop, longer durability, lower warranty cost and affordable system cost. Noteworthy among these are (i) thin-wall ceramic substrates with equivalent or improved performance and durability, MAT ceramic substrates for 200°C higher use temperature, and metal foil substrates with equivalent performance and durability at lower use temperature than those of standard cordierite ceramic substrates; (ii) compatible washcoat systems with beneficial interaction with ceramic substrates; (iii) thick and monolithic intumescent ceramic mats for improved insulation; wire screen shielded ceramic mats for minimal thermal erosion; high alumina sol-gel fiber mat for low mounting pressure for metal substrates; and slip-cast ceramic spacer rings for shielding the mat and can in transition region; (iv) dual can and dual cone clamshell designs with multiple insulation for prolonged durability; (v) perforated and stiffened heatshields with air scoop effect for convection cooling of the can. Undoubtably, as we gain new field data and apply this technology to motor cycles, diesel buses and trucks, alternate fuel vehicles, and industrial emissions, additional developments will be needed to combat higher NOx and HC emissions, ash contamination, aldehyde formation and high amplitude/frequency vibrations. ACKNOWLEDGEMENTS

The author is grateful to many of his colleagues at CORNING, 3M Co., Carborundum, Johnson Matthey, AC Rochester, Mercedes Benz, Ford and Leistritz for helpful discussions and to Virginia Doud, Nancy Foster and Irene Tone of R,D and E Labs at CORNING for their assistance in the preparation of this paper.

507

REFERENCES 1. M.R. Montierth and J. P. Day; 7th CIMTEC Conference, Montecatini Terme, Italy; June, 1990. 2. S.T. Gulati; 5e SIMEA SIMPOSIO DE ENGEHARIA AUTOMOTIVA; Sao Paulo, Brazil; September, 1989. 3. S.T. Gulati; SAE Paper No. 850130; February, 1985. 4. P.D. Stroom, R.P. Meny and S.T. Gulati; SAE Paper No. 900500, Detroit, MI; Feburary, 1990. 5. S.T. Gulati; SAE Paper No. 750171; February, 1975. 6. S.T. Gulati; SAE Paper No. 881685; Portland, OR; October, 1988. 7. J. P. Day; SAE Int'l Fuels & Lubricants Meeting, Tulsa, OK,USA; October 22-25, 1990. 8. J.P. Day and D.F. Thompson; 7th ClMTEC World Ceramics Congress; Montecatini Terme, Italy; June, 1990. 9. S.T. Gulati, J.C. Summer et.al.; SAE Paper No. 890796; February, 1989. 10. "3M Interam Heat Expandable Ceramic Mat," 3M Co., St.Paul, MN. 11. S.T.Gulati and R.P. Merry; SAE Paper No. 840074; February, 1984. 12. D. Kattge; SAE Paper No. 880284; February, 1988. 13. ARMCO Praduct Bulletin, 1984. 14. J. Abthoff. H.D. Schuster, F. Nunnemann and W. Zahn; SAE Paper 900267; Detroit, MI; February, 1990. 15. H. Weltens, H. Bressler, and M. Doll; SAE Paper No. 880318; February, 1988. 16. L.M. Sheppard; Ceramic Bull; Vol. 69, No. 6, 1990. 17. H. Yamamoto, H. Hone, J. Kitagawa and M. Machida, S A E Paper No. 900614; Detroit, MI; February, 1990. 18. J.P. Day and R.L. Locker, 1st Int'l Symp. and Exposition Ceramics for Environmental Protection; Koln, W. Germany; December, 1988. 19. J.P. Day and R.L. Locker; U.S. Patent No. 4,855,265; August 8, 1989. 20. M. Nonnenmann; SAE Paper No. 900270; Detroit, MI; February, 1990. 21. M. Maattanen and R. Lylykangas; SAE paper No. 900505; February, 1990. 22. S.T.Gulati; SAE Int'l Fuels & Lubricants Meeting, Tulsa, OK, USA; October 22-25, 1990. 23. S.T.Gulati and R.D. Sweet; SAE Paper No. 900268; February, 1990. 24. S.T.Gulati et. al., SAE Paper No. 880101, Detroit, MI; February, 1990. 25. S.T.Gulati and W.A. Plummer; Thermal Expansion-1973; AIP Proc,No. 17; Am. Inst. Physica, New York, 1974. 26. S.T.Gulati and H.E. Hagy; J. Am. Ceram. Soc., Vol. 61, No. 5-6, 1978. 27. S.T. Gulati; Coming Incorporated Internal Report R-4545; 1971 . 28. R.J. Lee and R.A. Westmann; J. Comp. Mat; Vol. 4, 1970. 29. A. Verma, D.K.S. Chen and D.L. VanOstrom; Mechanical Engineering; Vol. 110, No. 11, 1988, pp 56-61. 30. S.P. Timoshenko and S.Woinowsky-Rrieger, "Theory of Plates and Shells," Sec. Ed., McGraw-Hill, NY, 1959. 3 1. D.K.S. Chen and A. Verma; ASME Winter Annual Meeting; Dallas, TX, November, 1990. 32. J.D. Helfinstine and S.T. Gulati; SAE Paper No. 852100, Tulsa, OK;October, 1985.


A. Crucq (Editor), Catalysis and Automotive Pollution Control I1 0 199 1 Elsevier Science Publishers B.V., Amsterdam

509

STUDY OF PLATINUM DEPOSIT ON FERRITIC STAINLESS STEEL CONVERSION COATINGS L. ARIES1, P. DE VEYRAC2, M. MANTEL3 and J.P. T R A V E R S E 1 1 - Laboratoire de Recherche sur I'Energie, Universite' Paul Sabatier - 31 062 TO ULOUSE CEDEX 2 - Service De'veloppement UGINE A.C.G., PARIS LA DEFENSE 3 - Centre de Recherche d'UGINE , 73403 UGINE CEDEX ABSTRACT An alternative solution to the alumina washcoat layer on a metal support is investigated. Chemical treatment of the refractory steel surface allows to form a conversion coating. The layers which are about 400 nm thick and present a fractal character, a large specific area and a high porosity, are perfectly apt for impregnation with platinum by electrolysis of hexachloroplatinic acid and for codeposition of platinum with lanthanum oxide. Homogeneous deposit of fine particules of platinum -a few hundred to a few thousand of Angstrom - is obtained. Codeposition with lanthanum oxide allows to reduce coalescence and diffusion of platinum during ageing. Problem of the formation of a superficial probably alumina film appears after heat treatment in the air at 1000 "C for a long period.

INTRODUCTION

We €ooked for an alternative solution to the alumina washcoat layer on a metal support [l]. We used conversion coatings on refractory ferritic steels. They were ferritic stainless steels with seventeen percent chromium stabilized with zirconium, titanium or niobium in order to preserve the ferritic structure at all temperatures. Table 1 Chemical Composition of F18 A1 Steel

Fe 78.30

I

I

Cr 17.43

I I

A1 1.77

I I

Weight per cent Si I c u I Ti 0.75 I 0.38 I 0.36

I I

Ni 0.18

I I

Mn 0.13

I

I

Mo 0.05

They also contained about 2 % aluminium to resist oxidation at high temperatures. They were about nought point one millimeters in thickness. Their aluminium content was lower than that of the steels usually used as a support for the alumina washcoat (4 %). We are aware of the problems that this can bring about for thin foils since the substrate plays the role of

510

aluminium reserve susceptible of being oxidized. Levels of about 2 % could possibly be insufficient to prevent aluminium exhaustion. However, these steels present economic advantages over the steels with 4 % aluminium. A conversion coating is obtained by the chemical tranformation of the surface of the steel. At the last congress at Louvain [2], we presented a study of the influence of the preparation parameters on the characteristics of the layers. It is a microporous medium with a catalytic activity of its own. We have developed a technique to impregnate the layer with platinum and the layer plays the role of support although it can also contribute to the catalysis by its composition and texture. PRESENTATION OF THE CONVERSION LAYER

An original method for catalyst preparation has been developped [3] [4]. The process involves either anodic oxidation or chemical treatment of ironchromium based alloys. Supported catalyst can be prepared in one main step from the substrate which furnishes constitutive elements of the coating.

/

Fig. 1 Typical anodic polarization curve of stainless steels in sulfuric acid solutions. Eoc :natural corrosion potential, Ep :passivity potential, Er :rupture potential. In the chemical treatment the surfaces were prepared by dipping of the steel into a bath. One of the main conditions of the treatment is the fitting of the electrode potential of the sample to the value of the natural corrosion potential of the steel (Eoc) in the active state Fig.]. It is then necessary to

511

control the surface potential during the treatment having previously determined the electrochemical characteristics of the interface metal solution, by means of polarization curves. For some alloys, this condition of potential is naturally fulfilled for the treatment baths used. Generally, the potential can be adjusted to the required value by cathodic activation of the surface in the treatment bath with the help of a current generator and counter electrode playing the part of anode. The coatings were prepared in an acid bath with suitable additives, particulary substances containing chalcogenides : Sulphur seems to give the best results, and it is preferable to put sodium sulphide or sodium thiosulphate in the bath. It can be profitable to add a corrosion inhibitor specific to the alloy and the treatment bath to further control the thickness of the coating. The presence of propargyl alcohol reduces the aggressiveness of the bath and leads to a decrease in the coat thickness. The compositions of baths used in this part of the study are given in table 11.

Sulfuric Acid (vol%)

Sulfured Species

(all)

Propargyl Alcohol (ml/l>

Temperature

("C)

Treatment Duration (min)

10

60

45

Na2S203.5H20 5

1

After the preparation of the conversion coating, the samples were washed with water. They were then dried in an oven at 90 "C or dried in ambient air for about 10 minutes. After rinsing, in some cases, the coatings were subjected to chemical oxidation treatment, in an aqueous bath or to heat oxidation treatment in air. This type of treatment has proved to be suitable for making coatings on substrates of very diverse types. It has been adapted, without any major difficulties, for the use on refractory steels. The conditions of pretreatment of the steel before immersion in the bath (degreasing and also possibly harsher cleaning) and the rinsing after pretreatment play a significant role. The optimal conditions determined are rinsing in the presence of agents to improve wettability before drying. The bath composition, temperature and duration of treatment used lead to coating thicknesses of about 400 nm with a high fractal dimension: 2.43 as opposed to 2.27 for the thinner coating studied elsewere [2][5]. The coatings are sufficiently porous for impregnation with platinum. Their composition is similar to that of the ferritic steel coatings as shown in Fig 2 [4]. Here, the main oxide component is a magnetite substituted with chromium three (Cr 111) and aluminium three (A1 111).

512

ME

STEEL ATOMIC PROPORTION

Fig. 2 : Representation of the composition of a typical ferritic steel coating : the scheme gives the atomic proportion against the depth. Atomic proportion is the ratio between the number of metal atoms in the different compounds to the total number of metal atoms in the diflerent compounds. (A) :the superficial film, ( B ) :the external zone (deep zone), (C) :the internal zone (deep zone), I :domain of Fe3+ and Cr3+ oxide and hydroxide, II :domain of Cr3+ substituted magnetite, III :domain of metallic iron and chromium (alloy), N :domain of metal sulphate (s), V :domain of metal sulphide (s). DEPOSITION OF PLATINUM

For impregnation with platinum alone and subsequent study of the deposit, the electrical conductivity of the coating was used to deposit the platinum by electrolysis. Aqueous solutions of hexachloroplatinic acid were used for cathode deposition. Typical conditions were : 10 grams per litre of hexachloroplatinic acid An electrolysis current of ten milliamps per square centimeter for an electrolysis duration of ten seconds. This gave about 1.5 grams of platinum deposited per square meter of steel sheet. The resulting material, examined by scanning electron microscopy, is shown in Fig. 3 , (top left) where the general appearance of the coating is seen to be hardly modified by the presence of platinum. EDAX analysis - on Fig. 3 (top right) - shows the contribution of platinum which is only very weak here owing to the depth of penetration of the probe which was about 1

513

Fig. 3 S.E.M. micrographs and microprobe analysis of conversion coating with non uniform platinum distribution.

514

pm. The figures in the centre and at the bottom illustrate a problem typical of this type of deposition. The platinum can have a non-uniform distribution. This was later overcome as shown in Fig. 4 where the morphology of the platinum grains is shown. Their size ranges from a few hundred to a few thousand angstroms. The large white particle is a grain of titanium nitride (covered with titanium carbite). The platinum is also deposited on its surface. The composition of the coating was analysed by SIMS. The profiles obtained are shown in Fig. 5. Oxidation is progressive from the substrate up to the surface. The ionic intensities of chromium, iron and aluminium show a corresponding decrease. There is, however, an enrichment of titanium. These analyses confirm the presence of platinum at the surface. The influence of qgeing was studied on samples which had been subjected to heat treatment in air for 24h at temperatures ranging from 400 to 1000 "C. Scanning electron microscope observations of samples heated in the air at 1000 "C for 24h are showp Irr Fig. 5 along-side the corresponding EDAX analyses. Coalescence of the grains of platinum occurs and they are roughly five times larger than before. Simultaneously, analysis shows an increase in the level of A1 related to oxidation of the substrate.Corresponding SIMS analysis of oxygen, aluminium aqd platinum indicates diffusion of platinum towards the substrate which, at 1000 "C, becomes quite extensive, Fig. 7. Diffusion of aluminium towards the surface from 800 "C was also confirmed. The oxidation state of platinum was shown by XPS to remain at zero, the metallic form persisting even after heating at 1000 "C for 24h. CODEPOSITION OF PLATINUM AND LANTHANUM OXIDE

In order to resolve the problems encountered - that is to say diffusion and dispersion of fie platinum and coalescence of the grains - we modified the treatment and electrolysed the platinum in the presence of lanthanum oxide dispersed in the medium ( 6 grams per litre) while the other conditions remained unchanged. At the same time the dispersion of the platinum is improved. The shape of the grains appears particularly clearly on the scanning electron micrograph, Fig. 8 , where the deposit can be seen on grain of titanium nitride. In fact, platinum distribution can be made more even in different ways : electrolysis conditions clearly play an important role. In any case the coatings obtained always show perfect adherence. SIMS analysis of the coating shows that a codeposit of platinum and lanthanum is obtained Fig. 9, top. On ageing testing Fig. 9, bottom SIMS analysis of a sample aged in air for 24 hours at 1000 "C shows a considerable reduction of the diffusion of codeposited platinum towards the substrate. Also the coexistence of platinum and lanthanum is maintained in a similar way to that observed before treatment. Aluminum is still seen to diffuse towards the surface but less than through layers coated with platinum alone. The shift observed along the

515

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m:

52 1

erosion time scale is attributed to the formation of very thin insulating layer but it was not possible to determine either its thickness, since it was extremelly thin, or composition. We can however suggest that it was mainly superficial alumina. We studied the behaviour of uncoated steel and of coated steel, with and without the layer of platinum, on oxidation at 1000 "C.The results are shown in Fig. 10 where the quantity of oxide formed is shown against time. From results given by thermogravimetry over a shorter time scale, oxidation is seen to be slowed down in the presence of platinum codeposited with lanthanum oxide

P

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50

24

0

18 Al

+

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Fig. 10 Weight of oxide formed in the air at 1000 "C against treatment time for alloys with and without coating. CONCLUSION Conversion coatings optimized for the purpose are perfectly apt for impregnation with platinum and for codeposition of platinum with lanthanum oxide. No degradation of the intrinsic properties of the coating was observed. It remains microporous and still presents a fractal character both after preparation of the layer and after heat treatment. Secondly, we have demonstrated the feasability of making a homogeneous deposit of fine particules of platinum. The material obtained

522

with codeposition conditions shows a relatively high chemical stability. The coalescence of the platinum particles was reduced as was the diffusion of platinum toward the substrate. Lastly a problem remains : that of the formation of what is probably a superficial film of alumina after heating in air at 1000 "C for long periods. We have not yet tested the catalytic performance of the coating after this type of treatment but it is possible that the film creates a barrier to the diffusion of gas molecules and thus to catalysis. Further studies will be performed on this material which seems to have retained its potential but heat treatment has to be restricted to shorter time at 1000 "C or to lower temperature.

REFERENCES 1

2

3 4 5

PRATT A.S. and CAIRNS J.A. Platinum Metals Rev., 2,74-83.( 1977) L. ARIES, A. KOMLA and J.P. TRAVERSE Roc. Vth Int. Symp. "Scientific bases for the preparation of heterogeneous catalysts". Louvain La Neuve. 3-6 sept. 1990. L. ARIES and J.P. TRAVERSE Brevet Fr no 86.18124 (1986) L. ARIES and J.P. TRAVERSE Brevet Fr no 88.08402 (1988) PCT/FR no 89.00295 L. ARIES, P. FORT, J.A. FLORES and J.P. TRAVERSE Sol. Energy Materials, l4,143-159.(1986)


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MONOLITH SUBSTRATE EFFECTS ON CATALYST LIGHT-OFF T. S. Jasper (11, K. Robinson (2) , D. Anderton (2) and D. H. Cuttler (3) (1) Coventry Polytechnic (2) The Institute of Sound and Vibration Research, Soushampton University (3) Jaguar Cars Ltd ABSTRACT A test rig has been designed and built to examine the effects of catalyst construction in isolation. The test method utilises a fully stabilised engine with constant manifold downpipe temperature and mass flowrate. The catalysts examined were chosen to emphasize the thermal aspects of catalyst design. The light-off performance of four different catalysts was investigated to determine the effects of cell density (for three metallic catalysts) and monolith material (ceramic versus metallic catalysts). The cell density determines the thermal inertia and, for the metallics, it was found that the thermal inertia was directly related to the light-off performance - low cell density had low thermal inertia and produced faster light-off. Direct comparison between 400 cells/in2 ceramic and metallic monoliths showed the ceramic to be superior on light-off, despite a higher thermal inertia. The low thermal conductivity of the ceramic monolith kept the centre of the monolith at a higher temperature than the metallic and produced a better light-off performance.

INTRODUCTION In general, a large proportion (80% or more) of regulated emissions are emitted during the early period of the Federal US emissions test while the catalysts are not hot enough to work effectively. One of the objectives of a designer, therefore, is to minimise the time taken for the catalysts to attain their working temperature and thereby achieve significant reductions in toxic emissions. In the automotive industry, the transition from a non-working catalyst to a working catalyst is loosely termed "light-off '. During the design of new systems, choices have to be made in respect of substrate material, construction, cell density, volume, aspect ratio and precious metal coating. These choices will be influenced by factors such as mass flowrates (space velocities), gas temperatures, gas composition, total catalyst system configuration and catalyst durability (both chemical and physical), as well as cost, weight and size. Recent advances in substrate materials, such as thin-walled ceramics and more specifically metallic monoliths have further complicated the designer's choice. Claimed advantages of metallic over ceramic monoliths are lower

524

pressure drops for the same cell density, greater surface to volume ratio offering more effective conversion (or the same conversion for a smaller catalyst volume) and greater thermal and mechanical shock resistance. The high thermal conductivity of the metallic substrate is claimed to enhance lightoff and improve resistance to high temperature ageing [l]. Improvements in manufacturing techniques have opened up many more design options with an increased flexibility in construction. Balancing these advantages have been the drawbacks of higher cost, doubts about washcoat integrity at high temperatures under vibration and counter-claims about the light-off performance. Other work [2,3,4,5,6] has been conducted on parametric comparisons between ceramic and metallic substrates. These studies mainly address results obtained when using the 1975 Federal Test Procedure (FTP75) rather than focusing on the early stages and hence the mechanisms of light-off. Pelters et a1 [S] describe a development and application programme for a particular vehicle in which the balance of design considerations favoured the selection of a metallic catalyst. Faster catalyst light-off was listed as one of the advantages of this choice. Nashizawa et a1 [4] compared a conventional ceramic catalyst with a metallic catalyst of the same cell density but 24% less volume. Over the FTP cycle, little difference was observed in light-off performance. The work by Hawker et a1 [ 6 ] makes a similar comparison, and noted that there was a tendency for the metallic to achieve 70% conversion efficiency faster than the ceramic, but longer to achieve 90% efficiency. Jaguar Cars Ltd, in conjunction with Bosal Research Ltd and Johnson Matthey PLC sponsored a research project with the Automotive Group at the Institute of Sound and Vibration Research, Southampton University [7,8]. This research was designed to investigate the effects of cell density on the light-off performance of three metallic monolith catalysts and compare their behaviour with that of a conventional ceramic monolith catalyst. The catalyst volumes, cell densities, chemical and physical properties are shown in Table 1. In order to examine the effects of substrate material and construction on light-off, the complex engine warm-up and catalyst light-off processes were simplified. The experimental technique developed for this project used a fully warm engine producing a known gas temperature, flowrate, and composition. The test conditions were chosen so that the light-off performance of the catalyst was dominated by the response of the monolith to the changing inlet gas temperature. TEST EQUIPMENT AND PROCEDURE

The test apparatus consisted of a Jaguar 3.6 litre six cylinder in-line engine, coupled to a water-brake dynamometer. The exhaust system had two separate branches with manually operated ball valves to control the flow path. The test facility is shown schematically in Figure 1.

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Prior to the start of each test, the exhaust gas was directed down the bypass section whilst the engine and coolant reached normal operating temperatures. The engine, now under closed-loop fuelling control ( h = l), was then set at the required speed and load for the test condition. A stabilisation period of fifteen minutes ensured that the exhaust gas temperature achieved a steady state condition. At the start of each test, the exhaust gas was directed from the bypass section to the test section while the time histories of the following were recorded on a data logger: (i) the %CO concentration of the gas exiting the catalyst; (ii) the gas temperature into the catalyst; (iii) the gas temperature exiting the catalyst; (iv) the monolith temperature, mid-length, centre axis; (v) the monolith temperature, mid-length, mid radial point; (vi) the exterior surface temperature of the converter case. The thermocouple positions are shown schematically in Figure 2. The length of each test was 200 seconds, by which time the CO conversion efficiencies were around 75% for all test samples. The use of steady state engine operation with a stabilised exhaust gas virtually eliminated variations in gas composition, flow rate and temperature. Two series of tests were conducted, each having a different gas temperature and mass flowrate (see Table 2). Cenaebed Thermocouple I

Skin Thermocouple

Gas flow

Half-Wd y-ill-bed -lkmKmuplC

catalyst

Monolith

Figure 2 . A Schematic Representation of the Thermocouples in the Catalysts.

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TABLE 2 Engine Speed BMEP Downpipe Exhaust Mass Space Velocity Flow(g/s) (hr-1) (bar) Temp("C) (rpm) Series One 1500 1.8 465 18.0 175000 Series Two 1500 3 .O 510 23.4 2 loo00 Figure 3 shows the catalyst inlet gas temperatures for the four catalysts at the Series One and Series Two test conditions. The repeatability of the test procedure vindicated direct comparisons between the monoliths, as each was exposed to the same mass flowrate and inlet temperature - time history. Also shown on Figure 3 is the inlet gas temperature profile for the downpipe catalyst of a Jaguar vehicle over the first part of a Federal test cycle. RESULTS AND DISCUSSION

The temperature - time histories of the four catalysts at the Series One test conditions are shown in Figures 4-7. The Series Two test results, which are not shown, exhibit similar trends to the Series One results but the differences are much less marked. The post-catalyst %CO emissions for all the tests are shown in Figures 8 and 10. Figures 9 and ZZ show the cumulative %CO emissions. Cell Densitv Effects Close examination of the temperature - time histories for the metallic catalysts Figures 4 , 5 & 6 along with the CO conversion curves Figures 8 & 1 0 reveals that the 200 cell metallic catalyst has the fastest light-off performance. In terms of CO conversion, the 300 and 400 cell catalysts are very similar in the first series, but the 300 cell is much closer to the 200 cell in the second series. As most of the exothermic chemical heat that is released by the catalyst comes from the conversion of CO into C02, the quicker the reduction in CO, the quicker the rise in catalyst temperatures, both centre-bed and half-bed (and, consequently, gas-out). The 200 cell metallic shows the fastest rise in centre-bed and half-bed temperatures and the best light-off performance. The 400 cell has much slower and more rounded rises in bed temperatures and consequently a poorer lightoff performance. The 300 cell metallic has a faster rise in centre-bed temperature (though not as fast as the 200 cell) but a slower rise in half-bed temperature than the 400 cell. From the %CO emission results of the first series, it appears that the increased conversion through the centre of the 300 cell metallic monolith (owing to the higher temperatures present) balances the poorer conversion through the outer regions. The result is a light-off

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performance very similar to the 400 cell. In the second series, there was much less difference between all the half-bed and centre-bed temperatures with the result that the 300 cell metallic performed more like the 200 cell than the 400 cell. The results clearly indicate that the light-off performance of metallic monoliths under these test conditions was related to their cell density. As the cell density decreased, the light-off improved. Table 1 shows that the heat capacity (i.e. the thermal inertia) of the monoliths increased with cell density. The differences in light-off performance were attributed to this change in the thermal inertia. Active surface areas and precious metal loadings have been closely linked to light-off performance [9,10]. Although lower cell density monoliths have lower active surface areas upon which catalytic activity may take place, the precious metal loadings were the same for all the catalysts. On a lower cell density monolith, the precious metal was concentrated over a smaller area so the effect of active surface area upon light-off was minimal. Total active surface area only became important as the steady state was approached. This was particularly evident in the Series Two tests, when the CO conversion became limited by the mean residence time of the gas over the surface see Figures 10 and 1 I. Substrate Material Effects Comparison of the temperature profiles Figures 6 & 7 and also the CO conversion curves Figures 8 & 10 between the 400 cell ceramic and the 400 cell metallic clearly show that the 400 cell ceramic monolith has a superior light-off performance when compared with the equivalent 400 cell metallic monolith. Table 1 shows that the two catalysts have similar washcoat loadings, catalyst active surface areas and volumes, but rather different masses, thermal conductivities and heat capacities. The metallic had more mass but had a lower heat capacity owing to the much higher specific heat of the ceramic substrate. The ceramic centre-bed and half-bed temperatures rose significantly faster than those of the metallic, particularly from the point when they exceeded the inlet gas temperature. Over the latter portion of the test (160-200 secs), the 400 cell ceramic had a higher centre-bed but lower half-bed temperature than the 400 cell metallic. The ceramic thus had a larger radial temperature gradient than the metallic (c$ Figures 6 and 7 ) in spite of the matting wrapped around the ceramic monolith for packaging purposes. Vehicle packaging constraints usually force short entry cones with large included cone angles (70 or more), which are less than ideal. The gas flowrates encountered in automotive exhaust systems, coupled with these large cone angles, mean that the gas cannot flow smoothly around the sharp angles and flow separation results [11,12,13]. A plume of gas is directed at the centre of the monolith with recirculation zones in the outer parts of the cone. The result

534

is a significant flow maldistribution with high flow rates through a central 'core' of the monolith and lower flowrates in the outer regions. Most of the gas, and therefore most of the heating, is directed at the centre of the monolith. The higher thermal conductivity of the metallic substrate allows this heat to be conducted radially outwards, preventing any heat build-up. The ceramic substrate behaves in the opposite manner by effectively insulating the central core and allowing the temperature to rise rapidly. The higher core temperatures facilitate earlier exothermic chemical reactions, producing more heat and accelerating the light-off process. The results in Figures 6 and 7 are consistent with the ceramic catalyst rapidly generating a hot core which promoted early chemical heat release and resulted in a faster light-off. The metallic monolith, in contrast, did not produce a hot core but rather, it spread the heat through the whole monolith with a consequently slower light-off. Design Implications On a large vehicle such as the Jaguar XJ6, the catalysts used in these tests would make up approximately 20% of the total catalyst volume. The choice of a suitable starter catalyst will be determined by the position of the main catalyst. With the main catalyst positioned close to the starter (and both close to the manifold), the main catalyst will light-off in a reasonably short time (roughly 30 seconds). The starter catalyst should therefore be small with a fast light-off. The 200 cell metallic has these desirable light-off characteristics. Conversely, if the main catalyst is remote from the starter, the light-off of the main unit will take considerably longer (100 seconds or more). The starter catalyst must be able to achieve a rapid light-off and also be capable of dealing with the engine-out emissions (without breakthrough) until the main unit is working. For such a system, the 400 cell ceramic may be a better compromise. Catalyst performance deteriorates with ageing, and the various substrates will age differently. Any flow maldistribution will concentrate the ageing (both thermal and poisoning) to the central core of the monolith. The metallic, with its high thermal conductivity, will suffer less from the thermal ageing effects in the core than the ceramic. Thus, the characteristics of lightoff will change in different ways for the two types of monolith material during their working life, influencing a designer's final choice.

535

CONCLUSIONS A test method which can be used to rank the light-off performance of exhaust gas catalysts has been successfully developed. The physical design of the monolith substrate has a significant effect on the light-off performance. For a metallic substrate, it was found that the changes in cell density greatly affected the light-off performance. The cell density determines the mass of metallic substrate and therefore, all other things being equal, the thermal inertia. The light-off performance improved as the thermal inertia was reduced, i.e. as the cell density was lowered. Direct comparison between 400 cellsfin2 ceramic and metallic monoliths of similar dimensions showed the light-off performance of the ceramic to be significantly better. This difference was attributed to the poor thermal conductivity of the ceramic substrate effectively insulating the central core of the monolith. The resulting higher temperatures produced a faster light-off.

References 1 Nonneman, M., 'New High-Performance Gas Flow Equalizing Metal Supports For Automotive Exhaust Gas Catalysts', SAE 900274 2 Nonneman, M., 'Metal Supports for Exhaust Gas Catalysts', SAE 850131 3 Dulieu, C. et al, 'Metal Supported Catalysts for Automotive Applications', SAE 770299 4 Nishizawa, K. et al, 'Development of Improved Metal Supported Catalyst', SAE 890188 5 Pelters, S. et al, 'The Development and Application of a Metal Supported Catalyst for Porsche 91 1 Carrera 4, SAE 890488 6 Hawker, P. et al, 'Metal Supported Catalysts for Use In Europe', SAE 880317 7 Jasper, T. S., 'An Investigation Into The Parameters Affecting Catalyst Light-Off, M.Sc. Thesis, The ISVR, Southampton University, (1988) 8 Robinson, K., 'An Investigation Into Parameters Affecting Light-Off Performance of Exhaust Gas Catalysts' M.Sc. Thesis, The ISVR, Southampton University, (1989) Zygourakis, K. and Becker, E. Robert, 'Design Options for Rapid Light-Off of 9 Monolithic Catalytic Converters' submitted to Ind. Eng. Chem. Research Journal (1987) 10 Summers, J. et al, 'Improvements in Converter Durability and Activity via Catalyst Formulation' SAE 890796 1 1 Howitt, J. and Sekella T., 'Flow Effects In Monolithic Honeycomb Automotive Catalytic Converters', SAE 740244 12 Lemme, C. and Givens, W., 'Flow Through Catalytic Converters - An Analytical and Experimental Treatment', S A E 740243 13 Wendland, D. and Matthes, W., 'Visualisation of Automotive Catalytic Converter Internal Flows', SAE 861554



537

COMPARISON OF METAL FOIL AND CERAMIC MONOLITH AUTOMOTIVE CATALYTIC CONVERTERS G.L. Vaneman AC Rochester Division - General Motors Corporation ABSTRACT

Metal foil substrates are being promoted as the substrates of choice for automotive catalytic converters in the European market. Thinner walls are seen to offer both lower flow restrictions and improved catalytic performance, while metal's thermal characteristics are seen to benefit both performance and durability. AC Rochester has been investigating the advantages and disadvantages of metal foil monoliths for some time and recently developed a unique low cost design that is now in low volume production for @el. An extensive test was recently conducted to compare the aged performance of various metal foil and ceramic monolith converter designs. The herringbone-corrugated ACR metal units outperformed equivalent plate-fin metal and conventional ceramic units and approached the performance of 25% larger ceramic converters. However, metal's thermal characteristics may require the addition of insulation to compensate for rapid cooling during the ten minute shut-down and hot restart portion of the 23 cycle US FTP test. It was also found that tailpipe emission numbers and converter efficiencies may not accurately judge converter performance when the pollutant levels from the test engine are not constant, but vary with converter type.

BACKGROUND

Ceramic pellets, and more particularly ceramic monoliths, have been the substrates of choice for automotive catalytic converters over the past fifteen years. Recently however, many papers [ 1-41 have been published promoting certain advantages of metal foil substrates over conventional ceramic monolith substrates. Some of these advantages were seen to be especially important to the new European catalytic converter market. In particular, the advantage of thinner walls and the resulting lower solids fraction of the foil substrates should provide reduced flow restriction, thus promoting better engine breathing and increased power output. Alternatively, the cell density of foil units could be increased while holding the solids fraction and flow restriction constant, thus increasing the geometric surface area and thereby improving catalytic performance and reducing emissions. The significantly higher thermal conductivity of the metal foil should improve converter durability by quickly dissipating thermal "hot spots" produced by engine control problems, thereby decreasing both catalytic and

538

structural thermal damage. It has also been suggested that the higher thermal conductivity and lower heat capacity of the metal foil units should decrease emissions by reducing the time required to heat a cold converter to its lightoff temperature. However, ceramic monolith proponents have argued that the lower thermal conductivity of ceramic monoliths will help heat from the initial exhaust gas remain in the catalyzed washcoat layer, thus producing faster light-offs for ceramic substrates. Metal monoliths are also promoted as being more robust and less fragile than ceramic monoliths, and as being a better match for the thermal expansion of the converter shell. This eliminates the need for a mat support system, greatly simplifying the packaging of the substrate in the shell, thus offering both lower cost and improved converter durability. However, metal foil monoliths have also demonstrated several disadvantages that have limited their use in primary underfloor applications. Foil units inherently have substantially higher material costs and they also typically weigh more than their ceramic monolith counterparts. The nonporous nature of metal foil, coupled with the mismatch in thermal expansion between the foil and the ceramic washcoat, contributes to a washcoat adhesion problem during thermal cycling. In addition, the low strength of the foil at high operating temperatures has led to structural deformation problems in certain applications. DESIGN

AC Rochester, the automotive catalytic converter manufacturing division of General Motors, has been actively investigating metal foil substrates for more than ten years. The goal of this activity was to fully evaluate the manufacturability and costs of metal units while extensively investigating the performance and possible size reduction of foil substrates. After exploring many alternatives, the ACR design for metal monoliths took a different direction than that followed by most other foil substrate manufacturers. Following upon earlier work conducted at General Motors Research Laboratories, ACR adopted a whisker-producing oxidation treatment of the iron-chromium-aluminum foil that dramatically improved washcoat adhesion. In order to incorporate this treatment, and to avoid the high costs of brazing used by most manufacturers to rigidize their foil substrates, a design was adopted that involved the cutting, stacking, and interlayer welding of the foil sheets. This is then followed by the addition of retainer end rings that can subsequently be welded into pockets in the converter shell, as shown in Figure 1. Another very significant design change occurred when the conventional plate-fin (PF) foil corrugation pattern was replaced with the unique herringbone (HB) pattern, also shown in Figure 1. Unlike the PF design, in which alternate foil layers are uncorrugated and simply serve to separate the

539

corrugated "fin" layers, all foil in a HB unit is corrugated. "Nesting" between HB layers is prevented by reversing the HB pattern on alternate layers of the foil. By thus eliminating the need for separator sheets, the HB design uses approximately 30% less foil, thus offering a substantially lower solids fraction substrate that is therefore much less restrictive and significantly lower in weight. It also offers faster light-off and is significantly lower in cost.

AC ROCHESTER METAL MONOLITH CATALYTIC CONVERTER

OUTLET

HERRINGBONE SUBSTRATE

v-

ANGLED

CORRUGATIONS

Figure I , - Converter design using stack of H B foil layers captured in end rings welded into shell pockets.

Table I is a physical comparison of typical ceramic, PF, and HB monolith substrates and it reflects many of the differences described above. It is of interest to note that the bulk density of the HB design is quite low, indicating that such a converter would weigh approximately the same as a typical ceramic unit. And because of their differences in specific heat, the heat capacity of the HB design is approximately half that of the ceramic design. It should also be noted that the geometric surface area of all three designs is

540

approximately the same after washcoating. This is due to the significant losses suffered by the closed cell ceramic and PF designs when the washcoat slurry fills in the corners of the cells due to surface tension effects.

Table 1 PHYSICAL COMPARISON (Metal Foil vs. Ceramic Substrates)

Cell Density (#/cm2) Wall Thickness (mm) Solid Fraction Material Density (g/cm3) Bulk Density (g/cm3) Material Porosity (%)

STD. Ceramic 62.0 0.16 0.24 1.70 0.41 35

Plate Fin Metal Foil 62.0 0.05 0.097.10 0.63 0

Herringbone Metal-Foil 43.4 0.05 0.06+ 7.10 0.43

Geom. S.A. (cm2/cm3) - Washcoated

28 22

33 23

25 23

Specific Ht (KJKGOC) Heat Capacity (KJLOC)

1.05 0.43

0.5 0.32

0.5 0.22

Thermal Exp. (Rel.) Thermal Cond. (Re].)

1 1

17 20

17

0

The lack of discrete closed cells in the HB pattern, coupled with the stacked layer design, allows lateral internal flow of exhaust gases that contributes to the more complete utilization of the entire substrate, as well as to a more uniform distribution of temperature and flow. This contributes to both lower flow restriction and to reduced thermal stresses during thermal cycling. The unique HB pattern also offers better gas mixing and the disruption of gaseous boundary layers, thus contributing to both improved heat transfer and to better catalytic activity and reduced emissions. EXPERIMENTAL

Even with this lower cost herringbone-plus-retainer ring design, it became clear that only a smaller foil substrate with reduced noble metal content could successfully compete against ceramic monoliths for the primary underfloor catalytic converter market. Although some promising results had been seen in certain performance tests, it was difficult to consistently quantify a size/performance advantage for metal foil units, even for the new HB design. It was therefore decided to conduct a. comprehensive comparison test between various metal foil and ceramic monolith converters. This statistically designed experiment, described in a recent FISITA

54 1

paper [ 5 ] ,had three primary goals: first, to compare a standard ACR 1.4L PF metal foil unit to both an equivalent ceramic unit and to a larger 1.8L ceramic unit containing proportionately increased amounts of noble metals; second, to compare the HB design to the PF and the ceramic units; and third, to compare a new ACR washcoat technology to a conventional state-of-the-art washcoat applied by a commercial coater using PF substrates. A summary of the physical characteristics of the five converter types involved in this test is shown in Table 11. In all cases, noble metal loading was held constant at 3Og/ft3 of Pt:Rh at a ratio of 7:l. Note that flow restrictions, as reflected by pressure drops measured at approximately 480OC and a flow rate of approximately 23 s/sec, are directly related to the solids fraction values. To provide statistical validity, five randomly selected samples of each of the five different converter substrate types were carefully built, aged, and tested. Aging was done in random order on a group of 4.3L engine dynamometers running a severe accelerated schedule designed to simulate 80,000+ kms of customer driving in 100 hours. Performance was then measured on several different tests, often repeated to yield more accurate average values. Overall converter performance is probably best measured on an actual driving test such as the USA 23 cycle FTP schedule. That composite transient schedule includes a cold start, various urban-suburban driving cycles, and a shut-downhot restart. On that test, converter performance is determined by some complex combination of chemical kinetics and mass transfer effects. The mass transfer effects will in turn reflect the thermal characteristics of the engine, the exhaust system, and the converter itself. RESULTS

Table I11 summarizes the results from the FTP chassis dynamometer performance tests conducted on all 25 aged test converters. Each value in the Table is the average of 15 FTP tests (3 on each of 5 converters) run on the same 1.8L 'J' car by the same three drivers. Comparing the resulting tailpipe "bag" numbers for HC, CO, and NOx, the following conclusions were drawn: (1) the ACR washcoated PF unit was essentially equivalent to the PF unit coated by the commercial coater, thus proving the quality of the ACR washcoat technology; (2) both PF units had performances that were marginally worse than the equivalent 1.4L ceramic units, showing higher HC and NOx emissions and somewhat lower CO emissions; and (3) the HB units were judged better than the PF units and, due to their lower CO emissions, were also judged marginally better than the equivalent ceramic units. However, none of the smaller 1.4L metal converters approached the overall performance of the larger, more heavily loaded 1.8L ceramic units, although the metal units did approach the CO level of the larger ceramics.

TABLE I1 Substrate Summary From ACR Metal-Ceramic Monolith Comparison Test (1)(3)

Vol (1)

Frontal Cell Area Length Type Density Cell Coater (c/cm2) Shape (cm2) (mm) ~

(2)

Final Solid Fraction

Geometrical Surface

(2)

(2)

1.8 100 90(x2) Cer 92 150 Cer 1.4 1.4(1.57) 92(103) 150 Metal 1.4(1.57) 92(103) 150 Metal 1.4(1.57) 92(103) 150 Metal Notes:

wt. (8)

62.0 62.0 73.6 73.6 43.4

SQ SQ PF PF HB

-

A A

B ACR ACR

0.37 1050 0.37 810 1160(1390) 0.21(0.33) 1160(1390) 0.21(0.33) 810( 1000) 0.16(0.28)

(1) Noble metal loading 30 g/ft3 (1060 m a ) (2) MFM values in parentheses include rings and foil behind rings. (3) 7 : O : 1 = P t : P d : R h f o r a l l .

(2 1

23ds 480°C

3.90 3 .OO 3.67(4.17) 3.67(4.17) 3.31(3.76)

1.74 1.80 1.64 1.60 1.26

(m2)(Final)

543

Table I11 SUMMARYOF FTP RESULTS (Average of 15 runs ; 1.8L 'J' car) Engine Outs

Type 1.8LC

Coater A

HC 0.16

CO 2.17

Total g/mi Converted NOx Total Total (g/mi) (E.O. - T.P) 0.46 2.79 13.07 10.28

1.4LC 1.4LPF 1.4LPF 1ALHB

A B ACR ACR

0.19 0.26 0.24 0.20

2.52 2.13 2.32 2.30

0.57 0.60 0.61 0.58

Bag Emissions (g/mi)

3.28 2.99 3.17 3.08

12.97 13.15 13.18 13.58

9.69 10.16 10.01 10.50

However, in analyzing all of the FTP data, it was noted that, contrary to expectations, the total "engine out" emissions (HC -t CO + NOx) were not constant for all twenty-five converters, but in fact varied inversely with converter flow restriction. This variation was apparently caused either by the back-pressure-regulated exhaust gas recirculation system used on this 1985 test vehicle or by some sensitivity of its engine control scheme to exhaust backpressure. To correct for any adverse effect these differences in "engine outs" may have had on the tailpipe bag numbers, converter performance was instead measured on the basis of the total amount of pollutants eliminated by the converters, that is, by subtracting the tailpipe emissions from the "engine out" emissions. As shown in Table IV, the "amount converted" is a more valid reflection of converter performance than "tailpipe" or "efficiency" numbers when the engine out values are not held constant. Notice that when using that measure of performance, the PF units were superior to the equivalent ceramic units and the HB units outperformed all other converters, even the much larger and more heavily loaded ceramic units.

Tailpipe CO (g/mi) Efficiency EO - TP EO Engine Out CO (g/mi) CO Converted (EO - TP)

Conv. A Conv. B 3 .O 2.5 75% 75% 10.0 7.5

12.0 9.0

"A" Better "A" and "B" Equal

(11.0 +_ 10%) "B" Better

Further analysis of the FTP data revealed that the HB units had their poorest relative performance on the hot restart portion of the FTP schedule, that is, on cycles 19 and 20. Thermocoupled converters revealed that during

5 44

the ten-minute shutdown prior to the hot restart, the high conductivity, low heat capacity HB units were cooling below their light-off temperature of approximately 300OC and consequently they had poor efficiencies. An example is shown in Figure 2. However, the ceramic units were remaining well above 300OC and they therefore maintained their high efficiencies. Adding a wrap of insulation around such a metal converter was found to prevent this excessive cooling, and the converter performance was measurably improved.

FTP EMISSIONS AND TEMPERATURE ON COLD STARTlHOT RESTART

8gl o 95

285

475

665

855

1W5

1235

1425

1615

1805

TIME (SEC 1

Figure 2. - HC efficiency and bed temperature during 23 cycle FTP test for fast-cooling metal monolith converter.

Subsequent work on insulated HB converters revealed that although improved, such units continued to have relatively poor performance on the hot restart. Although the insulation kept the substrates above 300OC during the ten-minute shutdown, the HB units were quickly cooled below the lightoff temperature by the initially cool exhaust gases from early in cycle 19. The high heat transfer rates and high thermal conductivity of the HB units, advantages under warm gas/cool converter conditions, are thus seen as disadvantages under cool gas/warm converter situations. Work with an insulated downpipe prevented excessive cooling of exhaust gases early in cycle 19 and resulted in significant performance

545

improvements for the HB design. Since ceramic units never cooled below 300OC before or after the hot restart, they did not experience the same performance improvements as the HB units. However, ceramic units did show some improvement with the insulated downpipes, apparently because average converter temperatures were moderately increased throughout much of the FTP test. CONCLUSIONS

Based on earlier work, as well as on extensive metal/ceramic comparison tests described herein, it has been concluded that standard PF metal foil monoliths are unlikely to compete successfully with ceramic monoliths for the underfloor catalytic converter market because their costs are too high and their benefits too small. On the other hand, the HB design does compare well with ceramic monoliths. Performance advantages for the HB units should allow sufficient downsizing (approximately 20%) to make its costs very competitive. Other benefits of the HB design, such as low restriction, high thermal conductivity, and low thermal capacity, then become significant advantages. Accurate comparisons of converter performance are difficult to obtain. When such comparisons are needed, multiple samples and numerous replications should be incorporated into a statistically valid experiment. In addition, it should be noted that variations in converter and exhaust system restrictions can affect engine breathing and hence influence engine controls and "engine out" emissions. As a consequence, tailpipe emissions can also be affected and thereby confound any conclusions made about converter performance. In such circumstances, the concept of the "amount of pollutants converted" should be considered when comparing catalytic converter performance. REFERENCES 1. P. N. Hawker, C. Jaffray, A. J. J. Wilkins, and J. Alker, "Metal Supported Automotive Catalysts for Use in Europe," SAE paper 880317, International Congress, Detroit, Michigan, March 1988. 2 . P. Oser, "Novel Autocatalyst Concepts and Strategies for the Future with Emphasis on Metal Supports," SAE paper 880319, International Congress, Detroit, Michigan, March 1988. 3. K. Nishigawa, K. Matsuda, H. Horie, and J. Hirohashi, "Development of Improved Metal-Supported Catalyst," SAE paper 890188, International Congress, Detroit, Michigan, February-March 1989. 4. S. Pelters, F. Kaiser, and W. Maus, "The Development and Application of a Metal Supported Catalyst for Porsche's 91 1 Cdrrera 4," SAE paper 890488, International Congress, Detroit, Michigan, February-March 1989. 5 . G. L. Vaneman, "Performance Comparisons of Automotive Catalytic Converters: Metdl vs. Ceramic Substrates," FISITA paper 9051 15, XXIII International Congress, Torino, Italy, May 1990.


A. Crucq (Editor), Catalysis and Automotive Pollution Control 11 199 1 Elsevier Science Publishers B.V., Amsterdam

547

SOLS AS PRECURSORS TO TRANSITIONAL ALUMINAS AND THESE ALUMINAS AS HOST SUPPORTS FOR Ce02 AND ZrO2 MICRO DOMAINS L. L. Murrell, S. J. Tauster Engelhard Corporation, Menlo Park, CN 40 Edison, NJ 08818 ABSTRACT The possible relationship of alumina sols to the final three dimensional structure of calcined aluminas prepared by commercial procedures has been referred to only briefly in the literature. We have established that the pore sizes and pore volumes of transitional aluminas prepared by sol precipitation using ammonium hydroxide are strongly dependent on the size of the alumina sol precursor. Preparation of alumina from gelation of a 2 nm diameter alumina sol produces a relatively dense phase with an average pore diameter of 5.5 nm. Aluminas made by gelation of a 20 nm average diameter size alumina sol have three-fold the pore volume of the alumina produced by gelation of the 2 nm alumina sol. In the case of the 20 nm size sol the average pore size is 7 nm in diameter which is very close to that of aluminas produced by conventional aqueous procedures. The high temperature stability of the alumina formed from sols argues that very little amorphous alumina is formed as a separate phase. Ceria sols from 1 to 20 nm size were co-precipitated with the 20 nm alumina sol such that the Ce@ component is the "guest phase" within the three-dimensional alumina phase acting as the "host phase" of Ce02 micro domains. By varying the ceria content, highly dispersed Ce02 with rod-shaped morphology could be prepared within the relatively stable alumina host structure. It was established that these rod-shaped Ce02 structures would transform at increasing temperatures to cylinder morphologies which were isolated from each other within the alumina host structure. Studies with a Ce02 sol size of 12 nm size revealed that the micro domains formed within the alumina were very similar to the starting sol size. However, in the case of a Ce& sol of 1 nm size agglomeration to produce larger domains than the starting sol only occur at CeO2 contents of 5% or greater. The high temperature stability and characterization results for Ce-A1 and temperature stability and characterization results for Ce-A1 and Ce-Zr-A1 systems will be described. INTRODUCTION

Aluminas play a vital role as supports in a broad class of catalyst applications.[ 11. It is not generally appreciated that extruded aluminas owe their strength to dissolved and/or alumina sol species which crystallize to yield the "binder phase" of the non-soluble alumina phase. Since the "binder phase" of alumina can potentially produce a pore structure and surface chemistry which is distinct from the "non-binder" alumina phase it becomes critical to understand better the aluminas derived from sol precursors. In cases where a

548

slurry mixture is coated onto a ceramic or metal substrate, it becomes especially critical that the properties of the binder phase be well understood. Automotive catalysts rely on the alumina acting as a binder to be an integral support for the precious metals within the three-dimensional washcoat structure. This application is also the one which requires the alumina to exhibit excellent high temperature stability. In this paper we have studied the high temperature stability of sol-derived aluminas. The alumina made from the 20 nm sol precursor was also investigated as the "host phase" of Ce02 and Zr02 micro crystalline or micro domain structures. These micro domain structures were formed as the "guest phase" by co-gelation ceria and zirconia sols with the alumina sol. Characterization work established that the sol phases form micro domains which have thin rodshaped to cylinder-shaped morphologies depending on the starting sol size, concentration within the alumina host, and temperature of heat treatment. These "host" encapsulated CeO2 and Zr02 phase are much more stable than the bulk Ce02 and Zr02 phases. It appears that mixed sol preparation procedures may be a useful and general approach [2] to improve the stability of crystalline phases to severe conditions. EXPERIMENTAL

The sols used in this work were obtained from Nalco Chemical and Nyacol. The 2 nm size alumina sol (2A1) was a Nalco product and the average 20 nm size alumina sol (20A1) was a Nyacol product. The ceria and zirconia sols were obtained from Nyacol, and the sol sizes were provided by Nyacol, and were obtained from Coulter particle size analysis. The alumina sols were both precipitated by addition of ammonium hydroxide with rapid agitation until a constant pH of 9 was obtained. The 20 A1 sol was easily precipitated by addition of dilute nitric acid. The 2 A1 sol could not be gelled by addition of dilute nitric acid. In the case of deposition of the 20 A1 sol onto a flow-through monolith substrate by step-by-step build-up of ca 20 micron thick layers (measured from the comer of the channel to the top of the coating), it was found essential to calcine the sample after two layers were deposited by slow heating to 400°C in ca. 2 hrs. with an hour treatment at 400°C. The coating procedure was straight forward. The monolith was dipped into the sol slurry and then removed and the excess slurry in the channels was allowed to drain. The excess slurry was removed from the channels by gentle air knife pressure with the channels held in a horizontal posiposition. The air knife was used alternately in both directions to give a uniform coating on the monolith substrate.

549 RESULTS AND DISCUSSION

In a recent review article on producing aluminas with controlled pore structure, Trimm and Stanislaus state: "Improvements in pore size control can be expected, and studies of colloidal chemistry are the most fertile field from which improvements should originate." Despite this clear implication that alumina sols may be useful to control pore size, little information is available on the relationship between sol size and pore size in aluminas made by sol precipitation. Since sols play an important role as binders of washcoats in automotive application we investigated the properties of aluminas made from commercially available sol precursors. Two alumina sols were investigated, one having 2 nm size units and another having 20-30 nm size units, hereafter referred to as 2 A1 and 20 Al. Both sols were gelled with ammonium hydroxide until a pH of 9 was obtained. Both aluminas showed the characteristic broad lines in the x-ray diffraction of boehmite after calcining in air at 200°C. After calcination at 500°C both aluminas showed the characteristic x-ray pattern of delta-alumina, which also persisted for both samples when calcined in air at 900°C for two hours. The most important feature of these aluminas when calcined at 900°C was the of lines in the x-ray diffraction of alpha-alumina. Any amorphous alumina in these samples would have been converted to alphaalumina at this calcination temperature.[3]. It can be concluded that alumina sols are practical precursors to prepare high temperature stable aluminas with stabilities rivaling high purity aluminas prepared by conventional aqueous methodology.[ 1,4,5] When the physical properties of the 2 A1 and 20 A1 sols are compared it is quite clear that the starting sol has a remarkable degree of influence on the final alumina formed upon gelation. Firstly, the 2 A1 sol produces a much more dense alumina than the 20 A1 sol. The pore volume of the former is only 0.18 cc/g and the latter 0.44 cc/g. This physical characteristic difference alone argues that the 2 A1 sol is very reactive in a relative sense and must polymerize during the gelation step to produce a compact alumina structure. This hypothesis is supported by the fact that the average pore diameter of the 500°C calcined alumina from the 2 A1 sol has an average pore diameter of 3.6 nm. The size of the pores of this alumina argues that during crystallization, pores are formed which are larger than the size of the starting sol. Recent work [3] has also established that re-crystallized amorphous aluminas can have a wide range of pore sizes depending on the re-crystallization conditions of the amorphous phase. Therefore, it seems reasonable that the small 2 A1 sol can undergo polymerization and crystallization to form boehmite crystals where the final structure has relatively narrow pores compared to most aqueouslyprepared aluminas, and a dense three-dimensional structure. The surface area of this relatively dense alumina was 171 m2/g.

.

550

In the case of the 20 A1 sol-derived alumina, the average pore diameter was 7.5 nm with a surface area of 219 m2/g. This combination of pore volume, pore size, and surface area is almost identical to many aqueouslyprepared aluminas.[l,3]. These physical properties make a case that the larger A1 sol of 20 nm size is almost unreactive to re-crystallization during gelation. The structure of this transitional alumina is apparently made up of boehmite precursor particles which are of similar size to the alumina sol from which they are formed. This would make an overview, perhaps deceptively, straightforward: the link between alumina's pore structure and alumina sols in solution is simply that sols of size of ca. 20 nm are intrinsically stable and become" unreactive" because they can not "de-polymerize" to produce narrow pore size phases. Smaller size alumina sols, in contrast, are highly reactive, and undergo polymerization to produce aluminas with relatively narrow pores and in some cases low pore volume. If this is correct then it is no coincidence that the physical properties of the alumina made from the 20 A1 sol, and those made from conventional aqueous procedures are so similar. The surface area Stability aluminas at 900°C is informative because this temperature is sufficient to convert any amorphous alumina to the alphaphase. In earlier work [2,3] amorphous alumina would convert completely to alpha-alumina at 800°C. However, if this amorphous phase was contacted with water which allowed formation of crystalline boehmite particles, then an alumina was produced with good high temperature stability, i.e., 120 m2/g when calcined at 900°C. When calcined at 900°C the 2 A1 and 20 A1 sols gelled with ammonium hydroxide had surface areas of 114 and 142 m2/g, respectively. The difference in these aluminas' surface area is due to the difference in surface area of the starting aluminas, 171 and 219 m2/g, respectively. Both of these aluminas showed the characteristic shift to larger size pores as observed for calcined aqueous-prepared aluminas.[ 1,431. Both aluminas when calcined at 900°C showed no evidence of an alpha-alumina phase as noted previously. A sample of the 20 A1 sol gelled with nitric acid was also calcined for 2 hrs. at 900°C. In this case the surface area of the calcined sample was 100 m2/g with the pore distribution similar to the same sol precursor gelled under basic pH conditions. There was no alpha-alumina phase present for the 900°C calcined alumina precipitated under acidic conditions. This result shows that 20 A1 sol does not require basic conditions in order to form an alumina with good high temperature stability. This result is especially relevant to the preparation of coating slurries for automotive catalyst application where the desired pH of the slurry is usually between 3-4.5. Multi-coating of the 20 A1 sol onto a monolith honeycomb produced a washcoat of ca. 200 micron thickness. The surface area and pore volume of the alumina were 220 m2/g and 0.43 cc/g. This shows that a uniform coating of alumina can be obtained even after 8 sequential deposits of a sol precursor and that the alumina is essentially identical to the precipitated sol using ammonium hydroxide.

55 1

In summary, alumina sols can transform to boehmite phases even when formed in conditions not considered ideal for alumina re-crystallization [4]. Since it was established that the pore volume of the alumina made from the 20 A1 sol was 3-fold higher than that for the alumina made from the 2 A1 sol, then the 20 A1 sol was the clear choice as the "host" support for other oxide components. The concept of preparing mixed oxides from sol precursors has been disclosed previously in independent work.[2] We were interested in preparing ceria structures with bulk CeO2 surface redox properties but which were stable at temperatures of 900"C, or higher for automotive application. In our initial studies, a Ce02 sol of 12 nm size was mixed at three compositions with the 20 A1 sol and gelled with ammonium hydroxide. This Ce02 sol will be referred to as 12 Ce. The three compositions of interest were: 10 wt% Ce0290 wt% Al2O3, 20 wt% Ce02-80 wt% Al2O3, and 30 wt% Ce02-70 wt% Al2O3. The surface areas of these three compositions designated 10 Ce-90 Al, 20 Ce-80 Al, 30 Ce-70 A1 were 203, 195, and 150 m2/g when calcined at 500"C, respectively. The surface area decrease of the 30-70 composition compared to the other two is due to a real densification at the highest CeO2 level in this series, and not due to the density differences between Ce02 and Al2O3. We will see in subsequent work with similar Ce02 compositions containing low levels of a ZrO2 sol that only small differences in surface area are obtained over this same composition range. The pore volumes of these three compositions were ca. 0.44 cc/g, very similar to the sol-derived alumina "host." The surface area of these three compositions following 900°C calcination are 126, 116, and 103 m2/g, respectively. It is very important to note that the pore volumes and surface area stabilities of these three compositions were analogous to the alumina formed without the Ce02 sol present. The 20 nm size A1 sol appears to be acting as an ideal "host" for the C e 0 2 sol phase. The CeO2 domain size was obtained by x-ray diffraction (XRD) line broadening for these three compositions dried at 120"C, calcined at 500 and at 900°C. For all three compositions the estimated Ce02 domain size was estimated to be ca. 5.9 nm. This was somewhat of a surprise since the starting CeO2 sol size was almost double that estimated by x-ray diffraction. There are three pieces of evidence which point convincingly that XRD is significantly underestimating the size of the CeO2 domains. - Firstly, high resolution transmission electron microscopy (TEM) of the 30 Ce-70 A1 composition shows from the hundreds of Ce02 domains visible in the micrographs that CeO2 domains are composed of cylinders from 3-6 nm in width to 10-16 nm in length. The structure of these CeO2 domains are clearly consistent with the Ce02 sol being "locked" into the alumina acting as a host

552

where Ce02 domains are formed which are crystallizing as small cylinders which appear as "rice grain" structures within the host. - Secondly, when the three Ce-A1 samples were calcined at 900°C the XRD domain size for the three compositions: 10-90, 20-80, 30-70 were 12, 9.4, and 15 nm, respectively. Apparently, the 900°C calcination removes domains within the crystalline domains so that the line broadening is now more in accord with the starting sol size of 12 nm. - Thirdly, when 12 Ce was gelled with NH40H without A1 sol present, the bulk Ce02 obtained gave an XRD derived particle size of 5.9 nm. The physical properties of this CeO2 sample were very much in accord however, with significant re-crystallization of the CeO2 phase. The pore volume was only 0.2 cc/g with a surface area of 100 m2/g. The surface area appears quite high but all of the pores were of narrow distribution centered at 3 nm diameter. Formation of such a narrow pore diameter material with low pore volume argues that the Ce02 sol undergoes massive recrystallization to form a relatively dense bulk Ce02 phase. The XRD particle size of 5.9 nm for this bulk Ce02 phase again reflects domains within the bulk Ce02 particles. Calcination of this sol-derived Ce02 sample at 900°C showed a major collapse of the porous structure to give a material with a surface area of 3 m2/g. The XRD of this sample was consistent with formation of very large Ce02 primary domains of >60 nm diameter. The contrast in the domain size for the CeO2 within the host alumina structure to the bulk Ce02 phase serves to prove the remarkable enhancement in stability for Ce02 structures when encapsulated within an alumina host structure. We were fortunate that a Ce02 sol of only 1 nm size was available for preparation as a mixed Ce-A1 composite. This smaller size Ce02 sol could lead to smaller domains of Ce02 within the alumina host structure. Smaller Ce02 domains would lead to a much greater number of exposed Ce02 surface groups at a given CeO2 composition than for a sol size of ca. 10 nm. A Ce02 crystallite of ca. 10 nm size would only have about 1-in-10 CeO2 groups as a surface group whereas a 1 nm Ce02 crystallite would have every Ce02 group a surface group. The 1 nm Ce02 system would be referred to in catalysis terms as being highly-dispersed. For the 1 nm Ce02 sol-20 nm A1203 sol compositions a very wide range was chosen since it was expected that at some critical point the high dispersion of the Ce02 domains or crystals due to the starting sol would be lost. This loss of high dispersion would be associated with polymerization of the very "numerous" Ce02 sol units within the much larger A1203 host particles. These systems were prepared over a composition range from 2.5 Ce - 97.5 A1 to 45 Ce - 55 Al. The surface areas for 5 Ce - 95 Al, 15 Ce - 85 Al, 30 Ce - 70 Al, and 45 Ce - 55 A1 were: 196, 178, 180 and 181 m2/g, respectively. After 900°C calcination of these four compositions the surface areas were: 123, 127, 115, 90 m2/g respectively. The domain size

553

measured by XRD for these samples calcined at 500°C were 5.4, 4.6, 6.2 and 5.2 nm, respectively. This information makes it clear that little advantage has been realized in using the 1 nm CeO2 sol to form small CeO2 particles within the alumina host structure compared to the starting sol size of 12 nm described previously. The domain size for this series of samples calcined at 900°C even makes this point more clearly. The XRD domain sizes were 8.3, 9.5, 10.6 and 13.1 nm, respectively, for this series of samples. Note that these domain sizes are really very close to those described previously using the Ce02 sol of 12 nm size. Since the domain size only changes from 8.3 to 13.1 nm for samples calcined at 900°C over the composition range from 5 to 45% Ce02 content, then it is apparent that for high temperature application having a Ce02 sol of 1 nm in size is entirely irrelevant to obtain improved Ce02 dispersion. What is clear from this work is that ceria domain size is remarkably stable over a very wide composition range when co-gelled with an appropriate-sized alumina sol. In a final attempt to obtain evidence for small crystalline particles of Ce02 within an alumina host phase the 1 nm Ce02 sol was co-gelled with the 20 A1 sol. The compositions investigated were 2.5 Ce - 97.5 A1 and 5 Ce - 95 Al. TEM analysis clearly showed, for both of these compositions calcined at 500"C, that the CeO2 domains were thin rods of very high dispersion, or a high ratio of surface ceria groups to bulk ceria groups. The 2.5 Ce - 97.5 A1 composition showed that the rods were 2 nm in diameter and 45 nm in length. For the 5 Ce - 95 A1 composition the rods became thicker and much shorter, 4 nm in diameter and 20 nm in length. The TEM of the 5 Ce - 95 A1 composition calcined at 900°C gave very clear evidence for sintering of the Ce02 domains. Cylinder or "rice grain" structures were observed of similar size, 8 nm by 13 nm cylinders, to those observed for the 30 Ce - 70 A1 composition using the starting Ce02 sol of much larger size discussed previously. The XRD determined CeO2 domain size of 8.3 nm is in reasonable agreement to the 8 by 13 nm domains established by TEM for the 900°C calcined 5 Ce - 95 A1 sample. It is clear that high dispersion of CeO2 can be achieved in the form of thin rods within the alumina host structure which are stable to 500°C calcination temperatures. Calcination conditions of 900"C, however, result in transformation of these small micro domains to particles of size similar to those achieved by starting with a Ce02 sol precursor over an order of magnitude larger in size. In one series of samples a tri-gel of Ce-Zr-A1 was prepared using Ce, Zr, and A1 sols of 20, 10, and 20 nm respectively, gelled with ammonium hydroxide. In these systems three compositions were investigated: 10 Ce - 7 Zr - 83 Al, 20 Ce - 7 Zr - 73 Al, and 30 Ce - 7 Zr - 63 Al. All of these three compositions showed Ce02 and ZrO2 domain sizes obtained by TEM consistent with the starting Ce and Zr sol sizes. After 900°C calcination, XRD showed clear evidence that the CeO2 and Zr02 micro domains were of a size

554

similar to the starting sol size and that Ce02 and ZrO2 were similar to the starting sol and that CeO2 and Zr02 were intact as separate domains. The surface areas of these three compositions showed enhanced stability compared to the corresponding Ce-A1 systems when calcined at both 500 and 900"C, see Table 1. Apparently, the low level of the Zr02 phases serves to enhance significantly the stability of the composite which probably acts to retard the C e 0 2 particle sintering during the gelation step of the preparation. A stabilizing effect of the Zr02 particles on the alumina surface area can not be ruled out as is well established, at least for highly-dispersed ZrO2 phases on alumina. Table 1 Surface Areas for Ceria-Alumina and Ceria-Zirconia-Alumina Mixed Composites Calcined at Intermediate and High Temperature Sample (wt % as oxide)

Calcination Temperature "C

Surface Area m2/g

10 Ce - 90 A1 20 Ce - 80 A1 30 Ce - 70 A1 10 Ce - 90 A1 20 Ce - 80 A1 30 Ce - 70 A1

500 500 500 900 900 900

203 195 150 126 116 103

500 500 500 900 900 900

196 185 186 148 141 132

10 Ce -7 Zr 20 Ce -7 Zr 30 Ce -7 Zr 10 Ce -7 Zr 20 Ce -7 Zr 30 Ce -7 Zr

- 83 A1 - 73 A1 - 63 A1 - 83 A1 - 73 A1 - 63 A1

CONCLUSIONS

The size of alumina sols employed as precursors of aluminas plays a major role in the final pore structure of the calcined alumina phase. A 2 nm size alumina sol is apparently easily polymerized to give an unexpectedly dense alumina phase. In the case of a 20 nm alumina sol the final alumina is remarkably similar to that expected from little reactivity of the alumina sol during gelation.. The surface area, pore size, and pore volume of the alumina made from the 20 nm size alumina sol is very similar to that of many

555

aqueously-prepared aluminas. Perhaps the aqueous preparation methods simply produce a sol of size which is stable over a wide range of conditions of ca. 20 nm diameter which is then precipitated to give the aluminas which we are most familiar. In the case of co-gelation or tri-gelation of Ce-A1 and of Ce-Zr-A1 mixed composites it was established that the alumina host structure leads to micro domain structures of CeO2 or Zr02 within the alumina phase which were quite stable to high temperature conditions, This work would suggest that mixed sol procedures can be a new method to obtain a range of dispersions of bulk phases which are not presently accessible by non-sol routes. These encapsulated micro domain structures can be of unusual morphology as indicated by the thin rod-shaped structures observed in the case of the 1 nm Ce02 at low Ce02 content in the composite Ce-A1 phase. It has been shown that the size of the "guest phase" or micro domain formed within the alumina host is strongly dependent on the starting sol precursor of the host phase. Domains of Ce02 and ZrO2 were found to be isolated in the alumina host, so it appears that multi-component oxide systems can be obtained by gelation of mixed sol slurries. These micro domain systems will have the properties of their bulk counterparts, but will have the advantage of improved stability as demonstrated in the case of the stability of the alumina host in this work. REFERENCES

1. 2.

3. 4. 5.

D. L. Trimm and A. Stanislaus, Applied Catalysis, 21, 215-238, (1986) Baiher, A., Dollenmeier, P., Glinski, M., Reller, A. and Sharma, V. K., J. Catal. 111, 273 (1988). L. L. Murrell, N. C. Dispenziere, Jr., K. S. Kim, in "Novel Materials in Heterogeneous Catalysis," Chapter 9, p 97 Eds. R. T. K. Baker and L. L. Murrell, ACS Symp. Series 437, (1990), Washington, D.C., T. Ono, Y. Ohgachi, 0. Togari, in "Preparation of Catalysts 111", Eds. G. Poncelet, P. Grange, P. A. Jacobs, Elsevier, Amsterdam, p. 63 1,.( 1983) M. F. L. Johnson, J. Catal. 123, 245 (1990).



557

PREPARATION OF THERMOSTABLE HIGH-SURFACE-AREA ALUMINAS AND PROPERTIES OF THE ALUMINASUPPORTED Pt CATALYSTS

F. Mizukami*, K. Maeda*, M. Watanabe*, K. Masuda**, T. Sano* and K. Kuno**

*National Chemical Laboratory for Industry, -I Higashi, Tsukuba,Ibaraki 305, Japan **CentralIEngineering Laboratories, Nissan Motor Co.,Ltd., I Natsushima-cho, Yokosuka,Kanagawa 23 7,Japan ABSTRACT Thermostable high-surface-area aluminas were obtained by investigating the influence of preparation procedures, raw materials and additives on sintering of alumina. The complexing agent-assisted sol-gel method in which a complexing agent such as hexylene glycol was used as a solvent gave aluminas with large surface area and high durability at around 1OOO"C as compared with those prepared by conventional methods. Upon addition of BaO, La203 and SrO to alumina using the sol-gel method, the sintering of alumina was furthermore retarded and the effect was maximum when 10wt% BaO was incorporated to alumina. Alkoxide was the most suitable raw material for alumina, and alkoxide and acetylacetonate were suitable for BaO. Three lwt% Pt catalysts were prepared from a commercial alumina (A) derived from alkoxide, the sol-gel alumina (B) and the sol-gel BaOA1203 (C), and their catalytic properties were compared. The activity order of the three catalysts for the CO oxidation was (A) < (B) < (C). This sequence could be explained by the differences in the Pt crystal growth and a-Al203 appearance rates in the three catalysts.

INTRODUCTION

Alumina has such a superior thermal stability as compared with other high surface area materials that it is used as support to automobile exhaust gas catalysts and its demand is increasing from year to year. However, the thermal stability of the alumina is not always sufficient to satisfy the present requirements, and attempts at preparing thermostable aluminas with high surface area are still under way. There are two ways to obtain such aluminas. One is to retard the sintering of active aluminas with high surface area or prevent the transformation of the active aluminas into a-alumina with low surface area by adding certain additives. The other is to prepare an alumina showing high surface area and thermal stability without additives by investigating the preparation proeedure. For the former, although there are few systematic researches about the effect of preparation procedure on the structure and surface area of mixed metal oxide-aluminas, the addition of

558

yttrium, lanthanum, cerium, barium etc into alumina is known to be extremely effective, namely, the addition of yttrium or lanthanum by coprecipitation 11-21 and the addition of baryum by kneading or coprecipitation [3-51lead to the formation of thermostable p-alumina and magnetoplumbite (Ba0.6A1203) structures, respectively, and the addition of a rare earth by impregnation [6] inhibits the grain growth of transition aluminas and the formation of a-alumina. But for the latter, an efficient method has so far not yet been found . For the past several years, we have been studying the control of the particle size and structure of metal oxides with the sol-gel procedure (complexing agent-assisted sol-gel method) [7- 111 which is intended to regulate the hydrolysis and dehydration-condensation-polymerizationof metal ions or complexes with polyfunctional organic compounds (polydentate and bridging ligands). We applied the sol-gel procedure to the synthesis of aluminas or aluminas containing an additive, and tried to obtain thermostable aluminas with high surface area as well as highly active Pt catalysts supported on these aluminas. EXPERIMENTAL

Preparation of catalysts (i) Aluminas. Aluminium isopropoxide (120 g) as source of aluminum and hexylene glycol (2-methyl-2,4-pentanediol, 280 g) as solvent were heated at 120°C for 4h,a viscous homogeneous solution being obtained with ligand exchange and evolution of 2-propanol. To this solution 9 equiv. water was added to the alkoxide at 100°C. After the wet gel was allowed to stand for 1012h at 80°C, alumina (a) was obtained by drying in a rotary evaporator under reduced pressure. For comparison, aluminas were also prepared by two conventional methods which are known to give aluminas with high surface area; one method is the hydrolysis of aluminium isopropoxide in 2-propanol at 70°C (b), and the other is double decomposition in which 8.4wt% aluminium sulfate aqueous solution (830g) and 8wt% sodium aluminate aqueous solution (8298) were mixed at room temperature in the presence of gluconic acid (2.lg) to obtain alumina gel precipitate (c) [12]. These aluminas and a commercial alumina (Ketjen, CK-300 high purity) (d), as a reference, were calcined at various temperatures under the same conditions. (ii) Mixed aluminas. Aluminium isopropoxide (50 g) was dissolved in an ethanol solution (150 g) containing hexylene glycol (50 g) and the solution was heated in an oil bath at 120°C for 0.5h. A given amount of a raw material of a metal oxide as additive was added to the solution and heated for 3h. To the solution, 6 equivalent water to total metal ions were supplied. The remainder of the procedure used to obtain mixed metal oxide-alumina is identical to the method for the alumina described above. For comparison,

559

mixed metal oxide-aluminas were also prepared by the following coprecipitation method. A solution of aluminium sec-butoxide (87.6 g) dissolved in 2-propanol (50 g) was heated in an oil bath at 120°C for 0.5h. A given amount of a raw material of an additive component was added to the solution, heated for l h and the oil bath was cooled to 80°C. Then, water (320 g) at 80°C was added to the solution in the oil bath. After the precipitate occurred, it was filtered and treated in the same manner as above. (iii) lwt% Pt catalvsts Platinum catalysts were prepared by a conventional impregnation technique.with an aqueous solution of dinitro diammine platinum complex and the above aluminas previously calcined at 1000°C for 4h. They were calcined at 400°C for 3h, then at 1000°C for 20h, and activated in a hydrogen stream of 100 cm3/min at 400°C for lh.

CO oxidation Activities of the Pt catalysts for the CO oxidation were measured using a fixed-bed continuous flow microreactor with 0.1 g of catalyst. A gas mixture containing CO (0.47 mol%), 0 2 (0.25 mol%) and helium was flowed through the reactor at the rate of 100 cm3/min and analyzed by gas chromatography. The CO conversions and C 0 2 selectivities were calculated on the basis of the concentration of helium used as internal standard.

Characterization Specific surface areas of the aluminas were obtained from the nitrogen adsorption and desorption isotherms at 77 K, using a micro-BET apparatus (an AccuSorb 2100-01 of Micromentics). X-ray diffraction patterns of the aluminas were recorded on a Philips PW 1700 diffractometer equipped with a curved graphite monochrometer. With CuKa radiation, the aluminas were scanned over the range 28=5 - 60". Differential thermal analyses were carried out at 10"C/min in air with a Rigaku TG-DTA using a-Al203 as a standard. The Pt catalyst surfaces were examined with a Hitachi S-4000 scanning electron microscope. RESULTS AND DISCUSSION

Aluminas Figure 1 shows the variation of the specific surface area with calcination temperature of the sol-gel alumina (a) in comparison with the conventional ones, (b) and (c). The aluminas prepared from alkoxide, (a) and (b), show higher surface areas than (c). It should be emphasized that (a) keeps the highest surface area in the range of 600 to 1000°C and holds high values

560

(136 m2g-1) even after calcination at 1000°C for Soh, whereas the surface areas of (c) and of the commercial alumina (d), not originated from alkoxide, deteriorate remarkably on calcination at the same temperature (Fig. 2). Such a deterioration tendency was also observed in X-ray diffraction analyses, namely, (c) and (d) transformed into a-alumina at faster rates and lower temperatures than (a) and (b).

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Figure 1 Effect of the calcination temperature on the sugace area of the aluminas. Calcination time: 3h. (a) Sol-gel method (a); (0)Conventional alkoxide method (b); ( A ) Double decomposition method (c). Mixed aluminas First of all, metal oxides such as MgO, CaO, SrO, BaO, La2O3, Ce02 and Zr02 which have high melting points [9] were selected as the additives to alumina, and the effects on the specific surface areas of the metal oxide-mixed aluminas were examined. With the 10wt% mixed metal oxide-aluminas which were prepared from alkoxides by the sol-gel procedure and calcined at 1OOO"C for 3h, the aluminas containing SrO, BaO, La203 and ZrO2 showed specific surface areas of more than 100 m2g-1, while values lower than 40 m2g-1 were obtained for the others.

56 1

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Figure 3 Specific surface areas of IOwt% MxOylA1203 prepared by complexing-agent-assistedsol-gel procedures and coprecipitation. (m) Sol-gel method (0) Coprecipitaton method. MxOy :(a) La203, (b) SrO, (c) BaO, (d) Zr02

562

Figure 3 shows the effect of the preparation procedures on the specific surface areas of mixed La203-, SrO-,BaO- and ZrO2- aluminas prepared from alkoxides and calcined at the temperatures of 550 to 1200°C for 3h. It should be emphasized that the sol-gel aluminas show higher surface areas between 550 and 1000°C than the corresponding coprecipitated ones, although the differences between the two procedures are almost unexisting at calcination temperatures higher than 1000°C. The efficiency of the four additives for keeping high surface area is BaO > La203 > SrO > Zr02 at calcination temperatures of 1000°C or above. The effect of starting compounds was also examined with different mixed metal oxide-aluminas prepared by the sol-gel method. As a result, it was found that, in order to obtain mixed metal oxide-aluminas with large surface area and high thermal stability, simple alkoxides such as propoxide, butoxide etc. are suitable raw materials for alumina and zirconia, whereas alkoxides and acetylacetonates are preferred as sources of BaO, La203 and SrO. Figure 4 shows the X-ray diffraction patterns of the mixed aluminas prepared from aluminium isopropoxide and metal nitrates or metal alkoxides. The combination of alkoxide and nitrate yields more easily crystalline phases, which result in a lowering of the surface area, than the use of two alkoxides. In the four mixed aluminas prepared from alkoxides, the order in which crystalline phases are produced is Zr02 > SrO > L a 2 0 3 > BaO. This is consistent with the above order in which high surface areas are kept at high temperatures. The above results cannot be clearly explained at present, but they may be related to the mobility of additives. Generally, alkoxides and acetylacetonates can produce metal oxides interacting with aluminas through oxygen atoms by simple hydrolysis-dehydration reaction, but nitrates used here are hydrates; in this case, the dehydration is usually difficult unless the pH of the reaction mixture.is changed. Thus nitrates tend to produce metal oxides with high mobility. The mobility of a metal ion or oxide is also inversely proportional to its size, namely, the ionic size increases in the order: Zr+4<Sr+2=La+3 BaOaA1203 > a-Al2O3 > BaO [4,5]. Furthermore, an amorphous metal oxide generally has a higher specific surface area than the corresponding crystalline metal oxide . Thus, the results of table 2 are consistent with the values of the specific surface areas given in table 1.

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Figure 4 X-ray diffraction patterns of I Owt% MxOylA120.3 prepared by the sol-gel procedures. MXOY :(a) SrO, (b) BaO, (c) LazOj, (d) Zr02 Raw materials :A1203 Al(0Pr-i)~ ;( a ] ) ,(bl),( c l ) metal nitrates ;(a2) Sr(OPri)2 ;(b2) Ba(OBuS)2 ;(c2) La(OPrn)3 ;(d) Zr(OPrn)3 Assignment : (0)SrOeA1203, @) a-Al203, (0) BaO*A1203, (0)ZrO2

564

Table 1

Influence of BaO content on specific surface areas of BaO/A1203 prepared by the sol-gel procedure Calcination Temp. ("C)+ Calcination Time.(h)-+ BaO wt % 1 5 10 15 20 25

1450 800 lo00 1200 5 3 3 3 Specific Surface Area (m2/g) 1 162 111 17 4 219 171 96 7 227 176 32 8 202 141 13 11 111 75 10 6

550 3 444 375 488 340 364

Table 2

Calcination Temp. ("C)+ Calcination Time.(h)+ BaO wt % 5 5 10 15 20 25

lo00 3 a>>c a a a a>>s

1200 3 Structure c>>m>s a c Pdsol-gel alumina> Wcommercial alumina. The aluminas in the three catalysts sinter in the order: commercial alumina > the sol-gel alumina > the mixed BaO-alumina which is in good agreement with the order on the Pt crystal growth rates of the three catalysts. REFERENCES 1. S. Matsuda, Bull. Ceram. SOC.Jpn., 20, 189(1985). 2. S.Matsuda, A.Kato, M.Mizumoto and H.Yamashita, 8th Int.Congress on Catalysis, Proceedings, Berlin (1984),Vol4, p 879 3. J.K. Kesselring, Proceeding 3rd Workshop on Catalytic Combustion (1978). 4. M. Machida, K. Eguchi and H. Arai, Chem. Lett., 151(1986). 5. M. Machida, K. Eguchi and H. Arai, Bull. Chem. SOC. Jpn., 61, 3659(1988). 6. M.Ozawa, M.Kimura and A.Isaogai J.Mater.Sci;Lett, 9,709 (1990); J.Less-Common Metals ,162,297 (1990) 7. F. Mizukami, S.Y.Matsuzaki, K. Funrkori, S. Niwa, M. Toba and J. Imamura, J. Chem. Soc., Chem Commun., 678(1986). 8. S. Niwa, F. Mizukami, S. Isoyama, T. Tsuchiya, K. Shimizu, S. Imai and 1. Imamura, J. Chem. Tech. Biotechnol., 36, 236(1986). 9. F. Mizukami, S. Niwa, M. Toba, T. Tsuchiya, K. Shimizu, S. Imai and J. Imamura, In Studies in Surface Science and Catalysis Vol. 31, Preparation of Catalysts IV (B. Delmon, P. Grange, P.A. Jacobs, and G. Poncelet, Eds.), Elsevier (1987), p.45. 10. M.Toba, F.Mizukami, S.Niwa and K.Maeda, J.Chem.Soc.Commun.,l212 (1990) 11. K.Maeda, F.Mizukami, S. Miyashita, S.Niwa and M.Toba, J.Chem.Soc.Commun.,l268 (1990) 12. Jpn. Pat. 1978-19000 (1978) 13. I.E. Campbell, “High Temperature Technology”, John Wiley & Sons, (1956), p.31 14. F.A.Cotton and G.Wilkinson, “Advanced Inorganic Chemistry, A Comprehensive Text“, 2nd Ed., John Wiley & Sons (1962) 15. F.Baso1o and R.G.Pearson,”Mechanisms of Inorganic Reactions, A Study of Metal Complexes in Solution“, 2nd Ed., John Wiley & Sons (1968), Table 2.10. 16. E M Levin, C R Robbins and H F. McMurdie, “Phase Diagrams for Ceramists”, American Ceramic Society (1964) Fig. 206.


569

KINETICS OF THE PHYSIC0 CHEMICAL AND CATALYTIC ACTIVITY EVOLUTION OF A Pt-Rh CATALYST IN AUTOMOTIVE EXHAUST GAS G . Mabilon, D. Durand and M. Prigent Institut Francais du Pitrole, BP 31 I , F-92506 Rueil-Malmaison Cedex, France

ABSTRACT A Pt-RWAl2Og catalyst on cordierite monolith has been submitted to severe condition engine bench aging over extended periods. Sintering of alumina and precious metals was approximately proportional to the logarithm of the aging time. For aged catalysts, the ratio Pt/Rh in the metallic particles increased with increasing size. Phosphorus, zinc and lead coming from lubricating oil and gasoline accumulated on the catalyst at a steady rate. Sulfur concentration remained very low, probably due to high temperature inhibition of sulfate deposition. Testing of the catalyst under a constant composition stoichiomemc gas mixture, shows that the light-off temperature increased nearly proportionally to the logarithm of the aging time. Sintering of the precious metals explain the activity decrease for aging times up to 6 hours. For longer aging times both sintering and poisoning are responsible for the activity decrease.

INTRODUCTION Automotive post-combustion catalysis is characterized by severe conditions leading to a strong decrease in catalytic performances. Temperatures generally vary between ambient up to 1073-1173 K, and can even exceed with a poorly tuned ignition system. The oxide and metallic components of the catalyst sinter and interact under these conditions leading to a decrease and modifications of the surface of the support and of the active metallic surface [1,2]. Combustion of the gasoline and of part of the lubricating oil leads to the formation of water, carbon and nitrogen oxides, and also of compounds of phosphorus, sulfur, zinc, lead. These compounds can deposit on the active species of the catalyst, leading to either physical or chemical deactivation (enhancement of the diffusion limitation or poisoning by chemical interaction) [3]. These phenomena have been recognized for a long time, but have essentially been described by comparing the physico-chemical and catalytic properties of fresh and aged samples [4,5]. The purpose of this study is then to model the evolution of the physico-chemical properties and activity of a PtRh/A1203 catalyst as a function of aging time. Catalyst formulation without

5 70

ceria was chosen in order to avoid its adverse effects on precious metal characterisation. As sintering phenomena are frequently modeled by power laws versus time, aging time was varied exponentially between 0.17 and 200 h. A better knowledge of the important parameters of the aging kinetics under real exhaust gas could enable better vehicle aging prediction and better aging simulation under laboratory conditions.

EXPERIMENTAL The catalyst was made on a round cordierite monolith support (diameter 118.4 mm, height 76.2 mm) having 62 cells per cm2. It was coated with an alumina wash-coat and impregnated with chloroplatinic acid and rhodium trichloride at a concentration of 1.06 g/l of precious metals (about 1 5% wt with respect to the wash-coat) with a Pt/Rh mass ratio of 5. After impregnation the catalyst was calcined during 2 hours at 773 K. A four cylinder engine of 2.2 litre displacement with electronic fuel injection was used for catalyst aging. The accelerated aging cycle was composed of three modes [6]. Mode 1 simulates driving a car up-hill at 140150 km/h. It lasts 60 s and brings the exhaust gas temperature to about 1053 K. Mode 2 (3 s) simulates a sudden deceleration with suppression of 20 % of spark ignition. In mode 3 (3 s) lubricating oil (40 ml/h) and an excess of 30 % air is admitted in the engine. Temperature in the catalyst is of about 1043 K in mode 1, and rises up to nearly 1273 K in modes 2 and 3. The BET surface area was determined on a GIRA x SORB apparatus by nitrogen adsorption at 77 K. The surface area of the wash-coat was derived from that of the catalyst by dividing by the wash-coat mass fraction (cordierite has a negligible surface area of about 0.2 m2/g). Porous volume of the catalyst was measured on a Micromeritics 9200 mercury porosimeter. Cristallographic analysis of the wash-coat was made on a Siemens diffractometer. Metal dispersion was determined by hydrogen-oxygen titration. Catalyst samples were reduced in hydrogen at 723 K, cooled to room temperature in hydrogen; after having purged the reactor with helium, hydrogen adsorbed on the metals was titrated by oxygen. A second reduction in hydrogen at room temperature was done, followed by a second titration with oxygen. Only the second titration results were used. The first titrations were slightly modified by oxygen consumption by non precious metal compounds. The dispersion was calculated assuming that Pt and Rh had the same dispersion and that the titration equations were : PtH + 3/4 0 2

+

PtO + 1/2 H20

RhH + 02

+

1/2 Rh2O3 + 1/2 H20

57 1

Electron microscopy was performed on a JEOL 120 CX for the conventional mode and on a VG HB5 for the analytical scanning mode. For these studies catalysts were analyzed directly after engine bench aging without further treatment. Particle size distribution was determined from the measurement of about 200-300 particles. The mean diameter has been determined from the following equation :

Precious metal and poison distribution in the wash-coat was determined by microprobe analysis on a Camebax apparatus. The catalytic activity was determined for cylindrical samples (diameter 30 mm, height 76.2 mm), directly cut in the aged catalysts, on a laboratory test working at stoichiometry and under programmed temperature increase. Gas flowmeters were used to prepare a mixture containing : CO 1.5 %, NO 0.2 %, C3Hs 0.04 %, 0 2 0.85 %, C02 10 %, H20 10 %, the make-up being nitrogen. On-line analyzers allowed conversions of CO (IR detection), C3Hg (FID detector) and NO (chemiluminescence detection) to be followed as a function of temperature. Results have been expressed by the half-conversion temperatures for the three pollutants and by the apparent activation energy for CO oxidation. Half-conversion, or light-off, temperatures were measured at the inlet of the catalyst, as is usually made in engine bench or vehicle tests. For the determination of the apparent activation energy, catalyst temperature was chosen as the mean value between inlet and outlet, for conversions of no more than 10 %. As NO and C3Hs conversions generally took place after that of CO, their activation energies were not determined because of large thermal gradients in the reactor.

RESULTS Five identical catalysts were aged on the engine bench for increasing times of 0.17, 1, 6 , 40 and 200 h (plus respectively 4 and 2 min idle at start and before stopping the engine). At the end of the preparation step the colour of the catalysts is nearly white. The treatment in the exhaust gas at high temperature causes the reduction of the precious metals : the colour turns from white to grey. When comparing the catalysts aged after increasing times the colour lightens increasingly, probably due to the decrease of the exposed metallic surface. The following results will first concern the characterization of the aged catalysts, then their catalytic performances.

572 EVOLUTION OF THE PHYSICO-CHEMICALCHARACTERISTICS

Carrier Automotive post-combustion catalysts require a large porous volume with bimodal distribution in order to allow both the reactants and the products to diffuse and the precious metals to be well dispersed [7]. In the case of catalysts made on ceramic monoliths the porous volume is the sum of the contributions of the monolith and of the wash-coat. The porous volume contribution of the monolith is of the order of 0.18 cm3/g, and is only associated with the presence of macropores. As the manufacture of the monolith includes a calcination step at around 1673 K, its textural properties are assumed to remain unmodified during exhaust gas aging treatment. The porous volume of the catalysts is about 0.35 cm3g and appears to be unsensitive to aging time between 0.17 and 200 h (Fig. I). The macropore diameter remains nearly constant at about 3 microns. Nevertheless the micropore diameter increases from 20 to 40 nm. This increase is associated with a decrease of the surface area of the wash-coat with the aging time (Fig. I). This decrease is nearly proportionnal to the logarithm of the aging time. Various laws are used to model sintering. Those expressing surface area versus aging time are Figure l generally empirical and do not alEvolution of wash-coat sugace area and low the determination of a sintering catalyst porous volume of Pt-Rhl A1203 mechanism. For supported metals versus the logarithm of the time the rate of decrease of accessible area versus accessible area to the nth power (eq 2) is often used [8]. The value of the exponent has been correlated with the sintering mechanism. This expression has already been applied for alumina sintering [9].

These two kinds of law have been used. Considering that the initial wash-coat surface area is 160 m2/g it is possible to model the evolution of the surface area versus time according to the expression:

573

From the logarithmic representation of So - St versus t , it is possible to determine the following expression of S versus t :

If this expression is valid for aging times longer than 200 h, then it would take 1000 h to decrease the wash-coat surface area down to 50 m2/g. The integration of equation (2) leads to equation (5) :

For kt>>l, the slope of the straight line representing Ln (So / Sr ) versus Ln t is equals to l/(n-1) : n is found equal to 14. Values between 8 and 15 have been determined by JOHNSON [9] although no physical significance is given for these high n values. They are interpreted by a model of dehydroxylation at contact points of particles with subsequent formation of new Al-0-A1 bonds. The diffractogram of the original wash-coat shows only the peaks of gamma alumina with that of delta alumina. As the aging time increases the peaks due to theta alumina appear with increasing importance and eventually become the major diffraction lines.

Precious metals

Figure 2 Evolution of metal dispersion of Pt-Rhl A1203 catalysts versus the logarithm of aging time

The dispersion of the precious metals decreases monotonously with the logarithm of the aging time as shown in Fig. 2. Assuming that the dispersion of a catalyst undergoing only the heating and cooling phases of the engine bench aging (aging time equals zero) is 50 %, it is possible to calculate the sintering rate versus the dispersion using equation (2). The calculated exponent value (sintering order) is 4, which is not characteristic of a particular sintering process : it can either be related to sintering by particle (4 .3 z 0

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TEMPERATURE, C Fiaure 12 :The effect of 5 and 20 ppm of SO2 as a function of temperature for a thermally aged monolithic catalyst in cycled stoichiometric exhaust. The test after SO2 is the first test following the removal of SO2 . Propane was used as the hydrocarbon.

613

Warmed-Up Cycled Pelformance The impact of sulfur on the warmed-up cycled performance of a monolithic catalyst was investigated by operating a sample for 1 h in a sulfur free feed, then for 3 h with either 0, 5, 20 or 90 pprn of SO2 in the feed, then for two hours with no sulfur in the feed. This is the same schedule as was used with the pelleted catalyst. These results are shown in Figure 13. The HC, CO, and NO, conversions all fell rapidly when the sulfur was added to the feed, then rose rapidly once the sulfur was removed from the feed. The conversions from the catalysts which had been exposed to 5 and 20 ppm of SO2 rose to their pre-SO2 level, while the conversions from the catalyst which had been exposed to 90 ppm of SO2 rose to close to its pre-SO2 level. The rate of loss of activity, the rate of recovery of activity and the the extent of the recovery of activity were all greater on the monolithic catalyst than on the pelleted catalyst. The slow response of the pelleted catalysts made it difficult to determine whether the steady state conversions were a function SO2 concentration or whether it only affected the rate at which they approached the steady state conversion. With the monolithic catalysts it is very clear that the stabilized conversions of HC, CO, and NO, all decrease as the SO2 concentration increases. All of the cycled results presented to this point were performed using a cycling frequency of 0.5 Hz. This is the fastest cycling rate at which the reactor could operate without attenuating the cycle amplitude because of mixing before the catalyst bed, It should be noted that cars operate over a range of cycling frequencies, with the cycle frequency frequently greater than 1 Hz. With this in mind, the monolithic catalyst was operated at both 0.5 and 1 Hz with 0, 5, and 20 ppm of SO2 in the feed to determine if there was a difference in the impact of the SO2 as the cycling frequency was increased. (It should be remembered that as the cycling frequency was increased in this reactor it was accompanied by a reduced cycling amplitude.) The results are shown in Figure 1 4 . All of the conversions were similar at the two frequencies in the absence of S02. The loss of conversion efficiency was much greater, though, as the sulfur was increased at the 0.5 Hz cycling than at the 1.0 Hz cycling, indicating that cars with poor A/F control would be more susceptible to sulfur poisoning. Impact of Pd A sample of the Pt/Rh monolith was impregnated with 0.4 mg Pd/cm3 (0.012 ozt/l). This sample was then thermally aged and tested using the 1 h of no sulfur, 3 h of 20 ppm S02, and 2 h of no SO2 schedule. The HC, CO, and NOx conversions after 1 h were 82.3, 76.9, and 77.1%, respectively, fell to

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Catalysis and Automotive Pollution Control II

Catalysis and Automotive Pollution Control (Studies in Surface Science and Catalysis)

Catalysis and Automotive Pollution Control III (Studies in Surface Science and Catalysis, Volume 96)

Environmental Pollution and Control

Industrial Combustion Pollution and Control

Integrated Pollution Prevention and Control

Industrial Combustion Pollution and Control

Pollution: Causes, Effects and Control

Pollution Control and Energy Needs

industrial water pollution control

Environmental Pollution Control Microbiology

Homogeneous Catalysis II

Catalysis and automotive pollution control IV: proceedings of the Fourth International Symposium (CAPoC4), Brussels, Belgium, April 9-11, 1997

Industrial Combustion Pollution and Control (Environmental Science & Pollution)

Automotive Air Conditioning and Climate Control Systems

Pollution Control in Fertilizer Production

Environmental Pollution and Control, Fourth Edition

Monitoring, Control and Effects of Air Pollution

Air Pollution Control Technology Handbook

Air pollution control equipment calculations

Costs of Air Pollution Control

Handbook of Chemical Technology and Pollution Control

Handbook Of Pollution Control And Waste Minimization

Environmental Pollution and Control, Fourth Edition

Integrated Pollution Prevention and Control (IPPC)

Advanced Air and Noise Pollution Control:

Handbook of Air Pollution Prevention and Control

Handbook of Pollution Control and Waste Minimization

Pollution Control in Fertilizer Production (Environmental Science and Pollution Control Series)

Auto - Automotive Engines - Control, Estimation, Statistical Detection

Automotive Engines: Control, Estimation, Statistical Detection

Catalysis and Automotive Pollution Control II

Catalysis and Automotive Pollution Control (Studies in Surface Science and Catalysis)