BASF-2037.001
`
`

`
`m
`
`BASF-2037.002
`
`
`

`
`%CA’E‘AE,YEi€
`Am PGEE.,EJ”fE@N
`CGNTRGL
`
`Second Edition
`
`BA
`
`2037.003
`
`BASF-2037.003
`
`
`

`
`
`
`;F.»
`
` E“e£Em@E®gy
`
`I
`
`E5
`.. =.,_.;?—.,,,..4—. -.—,..«_ _...,r,.,.__:,,.....,_ v..._.___— _..‘.,.. ....,_,.....V . .... .;._.,.._....,.._.g...,k:,....‘,..,..,
`
`Sewnd Edifima,
`
`Rmnald M. Heck and Robert J. Panama
`with Suresh 1“.'Gu1au
`
`"
`“ WELEY?’
`
`ENTERSCEENCE
`A I-H‘? WHEY & SONS, fl\TC., PU
`
`SLECATEON
`
`
` 37.004
`
`BASF-2037.004
`
`

`
`This book is printed on acid—free paper.
`
`Copyright © 2002 by John Wiley & Sons, Inc., New York All rights reserved.
`
`Published simultaneously in Canada.
`
`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, scanning or otherwise,
`except as permitted under Sections 107 or 108 of the 1976 United States Copyright Act, without
`either the prior Written permission of the Publisher, or authorization through payment of the
`appropriate per—copy fee to the Copyright Clearance Center, 222 Rosewood Drive, Danvers, MA
`01923, (978) 750-8400, fax (978) 750-4744. Requests to the Publisher for permission should be
`addressed to the Peirnissions Department, John Wiley &*Sons, Inc., 605 Third Avenue, New
`York, NY 10158-0012, (212) 85016011, fax (212) 850-6908, E-Mail: PERMREQ@WlLEY,COM.
`
`' For ordering and cutomer service information please call l~800—CALL~W1LEY.
`
`Library of Congress Cz1tal0ging—in—Pub[ication Data is available.
`
`ISBN 0—47143624—O
`
`Printed in the United States of America.
`
`109876
`
`
`
`
`
`/\-I..,...-..;-:._
`
`
`
`BASF-2037.005
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`BASF-2037.005
`
`

`
`To my Wife, Barbara, whose friendship, support, understanding (especially on
`lost weekends), humor and selflessness made this endeavor much easier; Mer-
`cedes and Unk for always being there for support; and to Dutch who was
`overseeing it all.
`
`Ronald M. Heck
`
`To my wife Olga (Olechka) who has given me love, understanding; focus and a
`new vision of the wonders of life; my loving daughters Jill Marie and Maryellen
`and their husbands Glenn and Tom; special dedication to my beautiful grand-
`. children Nicky, Matthew, Kevin and Jillian and; God willing,
`their future
`brotliers and sisters.
`
`Robert J. Farrauto
`
`To my Wife Teresa Whose encouragement and support helped make this possi-
`ble; my sons Raj and Prem for their “you can do it, dad!” attitude; and my
`darling daughter Sonya for her “how can I help you, dad?” attitude throughout
`this project.
`
`Sures/1 T. Gulati
`
`BASF-037.006
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`BASF-2037.006
`
`

`
` ooataura
`
`.
`PREFACE
`ACKNOWLEDGMENTS
`
`_’
`
`»
`
`'
`
`-
`
`’
`
`ACKNOWLEDGMENTS, FIRST EDITION
`
`’
`
`I FUNDAIVMNTALS
`
`1 Catalyst Fundamentals
`
`1.1 The Basics: Activity and Selectivity 3
`1.2 Dispersed Catalyst Model
`5
`1.3 The Steps in Heterogeneous Catalysis
`1.4 The Arrhenius Equation 9
`1.5 Significance of the Rate—Lirniting Step
`
`6
`
`10
`
`2 The Preparation of Catalytic Materials: Carriers, Active
`Components, and Monolithic Substrates
`2.1
`Introduction
`11
`2.2 Carriers
`ll
`
`16
`2.3 Making the Finished Catalyst
`l8 _
`2.4 Nomenclature for Dispersed Catalysts
`2.5 Monolithic Materials as Catalyst Substrates
`2.6 Prepaiiug Monolithic Catalysts
`22
`2.7 Catalytic Monoliths
`23
`23
`2.8 Catalyzed Monolithic Nomenclature
`2.9
`Precious—Metal Recovery from Monolithic
`Catalysts
`23
`
`18
`
`3 Catalyst Characterization
`3.1
`Introduction 25
`
`3.2 Physical Properties of Catalysts
`3.3 Chemical Properties
`34
`3.4 EX Situ Techniques
`43
`
`26
`
`xiii
`xvii
`
`xix
`
`1
`
`3
`
`11
`
`25
`
`vii
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`
`viii
`
`CONTENTS
`
`4 Monolithic Reactors for Environmental Catalysis
`4.1
`Introduction 45
`4.2 Chemical Kinetic Control
`4.3 Bulk Mass Transfer
`47
`
`45 I
`
`4.4 Reactor Bed Pressure Drop
`4.5 Summary
`55
`
`53
`
`5 Catalyst Jeactivation
`
`5.1
`
`Introduction
`
`56
`
`5,2 Thermally Induced Deactivation
`5.3 Poisoning
`61
`5.4 Washcoat Loss
`
`63
`
`56
`
`H IVIOBILJE SOURCES
`
`6 Automotive Catalyst
`
`45
`
`56
`
`67
`
`69
`
`69
`6.1 Emissions and Regulations
`6.2 The Catalytic Reactions for Pollution
`Abatement
`72
`
`6.3 The Physical Structure of the Catalytic
`Converter
`73
`
`6.4 First—Generation Converter: Oxidation Catalyst
`(1976-1979)
`79
`6.5 NO” CO; and HC Reduction: The Second
`Generation (1979~1986)
`83
`6.6 Vehicle Test Procedure (US, Europe, and
`Japan)
`88
`6.7 NO,,, CO, and HC Reduction: The Third
`Generation (1986-1992)
`92
`6.8 Palladium TVVC Catalyst: The Fourth Generation
`(Mid—90s)
`100
`v
`'
`6.9 Low—Emission Catalyst Technologies 103
`110
`6.10 Modern TWC Technologies for the 20005
`6.11 Toward a Zero-Emission Stoichiometric Spark-
`Ignited Vehicle
`112
`A
`6.12 Lean Burn Sparlelgnited Gasoline Engine
`
`116
`
`7 Automotive Substrates
`
`.
`
`I 130
`
`Introduction to Ceramic Substrates
`7.1
`7.2 Requirements for Substrates
`132
`7.3 Design and Sizing of Substrates
`
`134
`
`130
`’
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`‘ BASF-2037.008
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`CONTENTS
`
`ix
`
`7.4
`
`p 7.5
`7.6
`7.7
`73
`7A
`
`139
`
`Physical Properties of Substrates
`Physical Durability
`147
`Advances in Substrate Development
`Commercial Applications
`171
`Summary
`178
`Appendix
`179
`
`160
`
`8 Diesel Engine _Ernissions'
`8.1
`8.2
`8.3
`8.4
`8.5
`
`~
`186
`Introduction.
`Worldwide Diesel Emission Standards
`NOx—Particulate Tradeoff
`191
`Analytic Procedures
`192
`
`186
`
`_‘
`188
`
`Diesel Oxidation Catalyst for Treating SOF
`Portion of Paiticulates
`192
`'
`
`Catalytic Reduction of Emissions from Diesel
`Passenger Cars
`196
`
`Catalyst Deactivation of the Diesel Oxidation
`Catalyst (DOC)
`198
`Treating Soot Using Diesel Particulate Filters
`(DPFS)
`200
`Dry Carbon Oxidation: Technologies under
`Development
`202
`NOX Reduction Technologies under
`Development
`204
`Natural—Gas Engines
`
`208
`
`8.6
`
`8.7
`
`8.8
`
`8.9
`
`8.10
`
`8.11
`
`Introduction 212
`
`9 Diesel Catalyst Supports
`9.1
`9.2
`9.3
`9.4
`9.5
`9.6
`9.7
`9.8
`
`212
`216
`
`Diesel Oxidation Catalyst Supports
`Design and Sizing of Diesel Filters
`Regeneration Techniques
`229
`Physical Properties and Durability 235
`Advances in Diesel Filters
`240
`Applications
`249
`Summary 259
`
`110 Ozone Abatement within Jet Aircraft
`10.1
`10.2
`10.3
`10.4
`10.5
`
`Introduction 263
`Ozone Abatement
`Deactivation
`266
`
`263
`
`Analysis of In~fiight Samples
`New Technology
`276
`
`269
`
`212
`
`263
`
`BASF-2037.009
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`mu...»-"2-A...:_._.'.....
`
`__
`
`X
`
`m _
`
`CONTENTS
`
`STATIONARY SOURCES
`
`11 Volatile Organic Compounds
`
`Introduction 281
`1 1.1
`11.2 Catalytic Incineration 282
`11.3 Halogenated Hydrocarbons
`11.4 Food Processing 291
`11.5 Wood Stoves
`295
`11.6 Small Engines
`295
`11.7 Process Design 298
`1 1.8 Deactivation 298
`11.9 Regeneration of Poisoned Catalysts
`
`285
`
`298
`
`12 Reduction of NO;
`
`Introduction 306
`121
`12.2 Nonselective Catalytic Reduction
`NO,
`306
`Selective Catalytic Reduction (SCR) of NOA.
`12.3
`12.4 Commercial Experience
`318
`12.5 Nitrous Oxide (N20)
`324
`12.6 Catalytically Supported Thermal Combustion 324
`
`(NSCR) of
`
`310
`
`13 Carbon Monoxide and Hydrocarbon Abatement from Gas
`Turbines
`A
`
`Introduction 334
`13.1
`13.2 Catalyst for Carbon Monoxide Abatement
`13.3 Nonmethane Hydrocarbon (NMHC)
`Removal
`336
`338
`13.4 Oxidation of Reactive Hydrocarbons
`13.5 Oxidation of Unreactive, Light Paraifins
`13.6 Catalyst Deactivation 341
`
`p
`339
`
`334
`
`IV El\/EERGJENG TECHNOLOGIES '
`
`14 Fuel Cells
`
`Introduction 347
`14.1
`14.2 Background 348
`14.3 The Proton Exchange Membrane (PEM) Fuel
`Cell
`351
`
`14.4 ‘Hydrogen Generation 355
`14.5 Alkaline Fuel Cell
`365
`14.6 Phosphoric Acid Fuel Cell
`
`366
`
`279
`
`281
`
`334
`
`345
`
`347
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`14.7 Molten Carbonate Fuel Cell
`14.8
`Solid Oxide Fuel Cell
`369
`14.9 Direct Methanol Fuel Cell
`14.10 Commentary
`371
`
`‘
`
`367
`
`370
`
`15 Ambient Air Cleanup
`
`Introduction
`376 ‘
`151
`Pren1Air_® Catalyst Systems
`15,2
`.153 Other Approaches
`384
`’
`
`‘376
`
`INDEX
`
`CONTENTS
`
`Xi
`
`3'76
`
`337
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`-2037.011
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`BASF-2037.011
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` Antainotive Qataiyst
`
`6.11
`
`SIVJUISSTONSA AND JREGULATTTONS
`
`The development of the spark—ignited combustion engine permitted the con-
`trolled combustion of gasoline that provides the power to operate the auto-
`mobile. Gasoline, which contains a mixture of paraflins and aromatic hydro-
`carbons,
`is combusted with controlled amounts of air producing complete
`combustion products of CO2 and H30
`
`
`
`Hydrocarbons in gasoline + O; ———> CD2 + H20 +heat
`
`(6.1)
`
`and also some incomplete combustion products of CO and unburned hydro-
`carbons (UI-ICs). The CO levels ranges from 1 to 2 vol%, while the unburned
`hydrocarbons range from 500 to 1000 vppm. During the combustion process
`Very high temperatures are reached due to difiusion burning of the gasoline
`droplets, resulting in thermal fixation of the nitrogen in the air to form NOX
`(Zeldovich 1946). Levels of NOX are in the 100-3000 vppm ranges. The ex
`haust also contains approximately 0.3 moles of H2 per mole of CO. The quan-
`tity of pollutants varies With many of the operating conditions of the engine but
`is influenced predominantly by the air:fuel ratio in the combustion cylinder.
`Figure 6.1 shows the engine emissions from a spark—ignited. gasoline engine as a
`function of the airzfuel ratio (Kummer 1980).
`When the engine is operated rich of stoichiometric, the CO and HC emis-
`sions are highest while the NOX emissions are depressed. This is because com—
`Dlete burning of the gasoline is prevented by the deficiency in 02. The level of
`NOX is reduced because the adiabatic flame temperature is reduced. On the
`162111 side of stoichiometiic, the CO and HC are reduced since nearly complete
`C0IJ1bustion dominates. Again, the NOX is reduced since the operating temper—
`ature is decreased. Just lean of stoichiometiic operation, the NO; is a maxi-
`mllm, since the adiabatic flame temperature is the highest. At stoichiometiic,
`the adiabatic flame temperature is lowered because of the heat of vaporization
`Of the liquid fuel gasoline. The actual operating region of combustion for the
`5P&rk~ignited engine is defined by the lean and rich flame stability, beyond
`Which the combustion is too unstable (Searles 1989).
`Within the region of operation of the spark~ignited engine, a significant
`amount of CO, HC, and NOX is emitted to the atmosphere. The consequences
`Of these emissions has been well documented (Viala 1993) but, briefly, CO is a
`69
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`BASF-2037.012
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`AUTOMOTIVE CATALYST
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`C0, CO2, 02 (Volume "/a)
`
`NO (103 vppm),l-1C as C6 (102 vppm)
`16
`
`14 '
`-1%-C02
` 12 ~
`10 [-
`‘
`
`Stoichometric
`4" Air/Fuel
`
`‘[2
`
`
`
`
`
`17
`
`18
`
`19
`
`20
`
`10
`
`11
`
`12
`
`13
`
`16
`15
`14
`<=-Rich Lean%
`
`Air to Fuel Ratio (wgt./wgt.)
`
`Sparl<—ignited gasoline engine emissions as a function of air:fuel 1'ati«
`Figure 6.1
`[Copyright © 1980,
`reproduced with lcind permission from Elsevier Sciences Lt:
`(Kummer 1980).]
`
`direct poison to humans, while HC and NO; undergo photochemical reaction
`in the sunlight leading to the generation of smog and ozone.
`The need to control engine emissions was recognized as early as 190‘.
`(Frankel 1909). The necessity to control automobile emissions in the Unite<
`States came in 1970 when the U.S. Congress passed the Clean Air Act. The re
`quirements under the Clean Air Act were changing as the technology was being
`evaluated. As a point of reference, the 1975/76 federal (49 states) requirement;
`were 1.5 g/mi ofHC, 15.0 g/n1i of CO, and 3.1 g/mi of NO, (Hightower 1974)
`The Environmental Protection Agency (EPA) established the Federal Test Pro
`cedure (FTP) simulating the average driving conditions in the United States it
`which CO, HC, and NO; would be measured. The FTP cycle was conducted
`on a vehicle dynarnorneter and included measurernents from the automobile
`during three conditions: (1) cold start, after the engine was idle for 8 h; (2) hot
`start, and (3) acombination of urban and highway driving conditions. Separate
`bags would collect the emissions from all three modes, and a weighing factor
`applied for calculating the total emissions. Complete details on the FTP test
`procedure are discussed later in this chapter. Typical precontrolled vehicle
`emissions in the total FTP cycle were 83-90 g/mi of CO, 13-16 g/mi of HC,
`and 3.5~7.0 g/mi of NOX (Hydrocarbon Processing 1971). A number of changes
`in engine design and control technology were implemented to lower the engine-
`out emissions; however, the catalyst was still required to obtain >90”/o conver~
`sion of CO and HC by 1976 and to maintain performance for 50,000 mi.
`Amendments in the early 1990s to the C1ean‘Air Act have set up more
`stnngent requirements for automotive emissions (Calvert et al. 1993). The cat-
`
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`BASF-2037.013
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`BASF-2037.013
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`6.1 EMISSIONS AND REGULATIONS
`
`71
`
`alysts will be required to last 100,000 mi for new automobiles after 1996. Fur-
`ther, these amendments (Which were contingent on tier 2 standards to be set by
`EPAgency), reduce nonmethane hydrocarbon (NMHC) emissions to a maxi-
`mum of 0.125 lg/mi by 2004 (dovm from 0.41 g/mi in 1991), carbon monoxide
`to l.7 g/mi (down from 3.4 g in 1991), and nitrogen oxides to 0.2 g/mi (down
`from 1.0. g). California in this same timeframe continued to set even more
`stringent regulations: NMHC emissions must be reduced to 0.075 g/mi by 2000
`for 96% of all passenger cars. By 2003, 10% of these must have emissions no
`greater than 0.04 g/mi, and 10% -must emitino NMHCS at all.
`The current summaiy of the California emission standards for passenger
`cars is given below. LEV is the abbreviation for l0w—e1mfssi071 vehicle, While the
`T is transitional, U is ultra, and S is super; ZEV means zero—emission vehicle.
`The NMOG is nonmethane organics. As of 2000, the regulations are as fol-
`lows:
`
`Category
`TLEV
`
`LEV
`
`ULEV
`
`SULEV
`ZEV
`
`Durability
`Basis (miles)
`50,000
`120,000
`50,000
`120,000
`50,000
`120,000
`120,000
`—0—
`
`NMOG
`(g/mile)
`0.125
`0.156
`0.075
`0.09
`0.04
`0.055
`0,010
`-0-
`
`CO
`(g/mile)
`3.4
`4.2
`3.4
`4.2
`l.7
`2.1
`1.0
`—0~«
`
`NO;
`(g/mile)
`0.4
`0.6
`0.05
`0.07
`0.05
`0.07
`0.02
`-0-
`
`These are the most stiingent Worldwide vehicle emission regulations and will be
`the targets for all the worldwide manufacturers.
`In comparison, the European standards for light duty gasoline engine pas—
`senger is as follows
`
`Category
`
`CO
`UHC
`NOX
`
`Stage 3
`(2000)
`
`2.3 g/km
`0.2 g/km
`0.15 g/km
`
`Stage 4
`(2005)
`
`l.0 g/km
`0.l g/km
`0.08 g/km
`
`’
`
`The conversion factor from g/km to g/mile is 062.
`Engine manufacturers have explored a wide variety of technologies to meet
`the requirements of the Clean Air Act. Catalysis has proved to be the most ef—
`fective passive system. Presently the major Worldwide suppliers of automotive
`Catalysts are Engelhard, Johnson Matthey, DMC2, and Delphi. As the auto-
`mobile engine has became more sophisticated, the control devices and com-
`bustion modifications have proved to be very compatible with catalyst tech-
`
`) 0 -2037.014
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`BASF-2037.014
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`72
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`AUTOMOTIVE CATALYST
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`nology, to the point Where today engineering design incorporates the emission
`control unit and strategy for each vehicle. The ZEV vehicles will probably be
`battery—operated. California Air Resources Board (CARB) is also implement-
`ing the PZEV vehicle, which is basically a SULEV vehicle with zero evapo-
`rative emissions.
`
`The basic operation of the catalyst is to perform the following reactions in the
`exhaust of the automobile:
`‘
`
`Oxidation of CO and HC to CO2 and H20:
`
`1
`co + 502 »~+ co;
`C0 + H20 gs C0; + Hz
`’ Reduction of NO/N02 to N2:
`
`(6.3)
`(6.4)
`
`1
`(6.5)
`NO(or N02) + co «A. EN; + CO2
`(6.6)
`NO(or N02) + H2 —» %N2 + H20
`(6.7)
`(2 + NO(or N02) + C}/I—In
`(_1+§)N2 +‘yC(§2 +EH20
`The underbody location of the catalytic converter in" the automobile is
`shown pictorially in Figure 6.2 (Mooney 1994). When a driver first starts the
`
`BASF-2037.015
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`6.3 THE PHYSICAL STRUCTURE OF THE CATALYTIC CONVERTER
`
`73
`
`Electronic
`controller
`
`Fuel injection
`
`4
`C? Exhaust gas
`-
`recirculation
`
`i
`L‘
`(L I
`l
`l
`r,
`=
`J
`v
`'
`
`®
`
`@
`Rf
`\
`
`_.
`
`H
`
`o
`
`.
`
`.
`
`.
`
`\\
`
`-
`
`»
`
`Dual—element
`
`r
`
`Tailpipe
`
`\
`\
`Acoustic
`muffler
`
`Exhaust Oxygen
`SQHSOT
`
`monolith converter
`
`Closed~loop dual—catalyst system for emissions control using dual element monolith
`converter, which is three~way and oxidizing.
`
`Figure 6.2 Location of a catalyst in the underbody of an automobile. [Reprinted by
`permission of John Wiley & Sons, copyright © 1994 (Mooney 1994),]
`
`followed by the HC and NOX reactions. When the vehicle exhaust is hot, the
`chemical reaction rates are fast, and pore diffusion and/or bulk mass transfer
`controls the overall conversion of the exhaust pollutants.
`
`6.3 ’ TEEPHYSTCAJL STRUCTURE OF THE CATALYTIC
`CONVERTER
`
`Both beaded (or particulate) and monolithic catalyst have been used for pas-
`Senger Vehicles from the onset of automotive emissions controls. In the United
`States, GM (General Motors) was the major company using spherical beads,
`While Ford and others used monoliths.
`<
`In parallel with all the studies related to catalyst screening, deactivation, and
`durability, there were many engineering issues that needed to be addressed in
`the early 19703. How much backpressure would the presence of a catalytic re-
`actor in the exhaust manifold contribute (increased backpressure translates to a
`loss in power a11d fuel economy)? Would the catalyst be able to maintain its
`Physical integrity and shape in the extreme temperature and corrosive environ-
`ment "of the exhaust? How much weight would be added to the automobile, and
`What Would be the effect on fuel economy? Another complicating problem was
`that the exhaust catalyst operation is in a continuously transient mode, in
`Contrast to normal catalyst operation. These problems were critical because
`the <§0I1surner needed a cost—effective, highly reliable, trouble—free vehicle with
`readily delivered performance.
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`BASF-2037.016
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`74
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`AUTOMOTIVE CATALYST
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`
`
`Figure 6.3 Bead bed reactor design. (Courtesy WR Grace & Co.)
`
`6.3.1 The Beaded Catalyst
`
`A quite important question that the engine manufacturers had to address is
`how to house the catalyst in the exhaust. The most traditional way was to use
`spherical particulate y—A12O3 particles, anywhere from § to i in. in diameter,
`into which the stabilizers and active catalytic components (i.e.., precious metals)
`would be incorporated’. These “beads” would be mounted in a spring—loaded
`reactor bed downstream, just before the muflier. Since the engine exhaust gas
`was deficient in oxygen, air was added into the exhaust using an air pump. The
`rationale was sirnple—catalysts had been made on these types of supports for
`many years, and manufacturing facilities to mass produce them were already in
`place. There were known reactor designs and flow’ models that would make
`scaleup easy and reliable. One major concern was the attritionresistance of the
`y—Al2O3 particles, since they would experience many mechanical stresses during
`the lifetime of the converter. A typical bead bed reactor design for the early
`oxidation catalysts is shown in Figure 6.3.
`‘
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`BASF-2037.017
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`BASF-2037.017
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`6.3 THE PHYSICAL STRUCTURE OF THE CATALYTIC CONVERTER
`
`75
`
`The beads are manufactured with the stabilizers (discussed later) incorpo-
`rated into the structure. The precious metal salts are impregnated into the bead
`and, using proprietary methods, fixed in particular locations to insure adequate
`performance and durability for 50,000 mi. They are then dried at typically
`120°C, and calcined to about 500°C to their finished state. The finished catalyst
`usually had about 0.05 wt% precious metal with a Pt:Pd weight ratio of 2.5: 1.
`After 1979 the need for NOX reduction required the introduction of small
`amounts of Rh into the second—generation catalysts. To control the levels of
`deactivation and performanee”of the bead catalysts, many studies were con-
`ducted oi varying the location of the active catalysts within the bead structure
`(Hegedus and Gumbleton 1980).
`
`6.3.2 Tie Honeycomb Catalyst
`
`An alter ative approach for supporting the catalytic components was that of
`a ceramic honeycomb monolith with parallel, open channels (see Figure 2.2,
`Chapter 2). In the mid-19603, Engelhard began investigating the use of mono-
`lithic structures for reducing emissions from forklift trncks, mining vehicles,
`stationary engines, and so on (Cohn 1975). Catalyst preparation studies on
`these PTX purifiers formed the basis for washcoating technology for the auto-
`motive applications. The effects of operating temperature and feed impurities
`on catalyst durability were also determined. Some PTX converters had opera-
`tional life of 10,000 h. This background experience showed that the monolithic
`support was a viable material for automotive applications. The precious metal
`)I—Al2O3 catalyst was Washcoated or deposited onto the walls of the honeycomb
`chamiels. A Washcoated honeycomb typical of that used in the early catalytic
`converters is shown in Figure 2.2 in Chapter 2. One major advantage would be
`low-pressure drop, since the honeycomb structure had a very high open frontal
`area (~70%) and parallel channels. Furthermore, given their monolithic struc-
`ture, they could be oriented in a number of ways to fit in the exhaust manifold.
`Also, the monoliths were available in different cell densities or cells per square
`inch (cpsi). From the experience in forklift trucks, there was a small database
`from which to design catalytic reactors. They offered potential flexibility, but
`naturally, the materials and geometries had to be optimized and designed for
`this new and very demanding application.
`The ceramic companies continued to modify the materials and structures
`to provide sufficient strength and resistance to cracking under thermal shock
`Conditions experienced during rapid acceleration and deceleration, The thermal
`Shock condition was eventually satisfied by mechanical design coupled with the
`use of a low—thern1a1-expansion ceramic material called cordierite (synthetic
`Cordierite has‘ a composition approximating 2MgO, 5SiO2, and 2Al2O3). In
`Preparing the catalyst, this desirable property has to be matched by the thermal
`Cxpansion properties of the washcoat to prevent a mismatch in themial prop-
`erties. Monolithic structures were ultimately produced by a novel extrusion
`technique, which allowed mass production to be cost-effective. The first honey-
`
`i 1.IirI
`
`
`
`BASF-2037.018
`
`

`
`76 - AUTOMOTIVE CATALYST
`
`comb catalysts of large quantity to be used in automobile exhaust had 300 cells
`per square inch (cpsi), with wall thickness of about 0.012 in., and open frontal
`area ofabout 63%. These dimensions were finalized, on the basis of mechanical
`specifications and activity performance requirements, to ensure a high degree of
`contact between the reactants and the catalyst washcoat (high mass transfer)
`and the lowest possible lightoff temperature. Later developments in extrusion
`technology resulted in a 400—cpsi honeycomb with a wall thickness of 0.006 in.
`and open frontal area of 71%. This increased the geometric surface area for the
`mass-transfer—controlled reactions.
`
`Catalyst companies began to explore these new structures as catalyst sup—
`ports. They developed slurrles of the catalytic coating that could be deposited
`onto the walls of the honeycomb, producing adherent “washcoats.” The wash~
`coat thickness could be kept at a minimum to decrease pore diffusion effects
`while allowing suflicient thickness for anticipated aging due to deposition of
`contaminants. The washcoat is about 20 and 60 um on the walls and corners
`(fillets), respectively.‘ One method of preparing a washcoated honeycomb is to
`submerge it in a slightly acidified slurry (slip) containing the y-A1303 already
`impregnated with stabilizers and precious metals. The washcoat bonds chemi-
`cally and physically to the honeycomb surface, where some of the washcoat fills
`the large pores of the ceramic. The slurry must have the proper particle size
`distribution to be compatible with the pores of the ceramic wall. Another
`method involves first washcoating the honeycomb with the alumina slurry,
`drying and calcining it, and then dipping it into the impregnating solutions.
`The coated honeycomb is air—dried, and calcined to about 450—500°C to ensure
`good adhesion. Typically,
`the catalyst contains about 0.l—0.l5% precious
`metals. For the oxidation catalysts of the first generation, the weight ratio of
`Pt to Pd was 2.5: 1, whereas the second generation contained a weight ratio of
`5:1 Pt:Rh.
`
`The honeycomb catalyst is mountedin a steel container with a resilient
`matting material wrapped around it to ensure vibration resistance and reten-
`tion (Keith et ‘al. 1969). Positive experience with honeycomb technologies has
`resulted in increased use of these structures over that of the beads, due to size
`and weight benefits. Today almost all automobiles are equipped with hone-
`ycomb~supported catalysts similar to that shown in Figure 6.4.
`Although the early honeycombs were ceramic, recently metal substrates
`have been finding use because they can be made with thinner walls and have
`open frontal areas of close to 90%, allowing lower pressure drop. Cell densities
`greater than 400 cpsi can be used, which permits smaller catalyst volumes when
`higher cell densities are used. With some metal substrate suppliers, the catalyst
`is first coated onto the sheet metal and then fabricated into the honeycomb
`structure. This has the advantage of producing uniform coating thickness, thus
`eliminating the fillets. Because of expense and temperature limitations, these
`catalysts are not preferred. However, they are finding some markets because of
`their loW—pressure—drop characteristics.
`
`01 BASF-2037.019
`
`BASF-2037.019
`
`

`
`
`
`
`
`6.3 THE PHYSICAL STRUCTURE OF THE CATALYTIC CONVERTER
`
`77
`
`Figure 6.4 Monolithic reactor design. (Courtesy Engelhard Corp.)
`
`By the year 2000, over 30 years of catalyst technology development had been
`devoted to the automotive exhaust catalyst. Figure 6.5 shows a typical automo—
`tive catalyst design.
`These technology advances have been driven by the quest for a zero emis-
`sion vehicle using the sparlc-ignited engine as the powertrain. Along with the
`advances in catalyst technology, the automotive engineers were developing new
`Engine platforms and new sensor and control technology. This has resulted in
`the full integration of the catalyst into the eniission control system. The catalyst
`has become integral in the design strategy for vehicle operation. During this
`time period the automotive catalyst has progressed through the following de-
`V€10pI"nent phases.
`
`-2037.020-
`
`BASF-2037.020
`
`

`
`78
`
`AUTOMOTIVE CATALYST
`
`THREE WAY CATALVST itweioesaem
`
`CATALYST INSULATION
`
`CEHAMVC
`HONEYCOME
`
`INSULATION
`
` SHIELD
`
`
`
` CATALYST
`
` COATING
`HALFSHELL
`
`HOUSING
`(AL UMINA)
`+ Pt/Pd/Flh
`
`SUBSTRATE
`
`
`
`INTUMESCENT
`MAT
`
`Figure 6.5 Schematic of cutaway of typical auto catalyst design. (Courtesy Engelhard
`Corp-)
`
`!
`
`' Oxidation catalyst
`
`Bead and monolith support
`HC and CO emissions only
`Pt—based catalyst
`Stabilized alumina
`
`i
`- Three—way catalyst
`
`V
`
`HC, CO, and NOx emissions
`Pt/Rh-based catalyst
`Ce oxygen storage
`”
`l
`' Hig4h—t‘en1peVratu-re three-way catalyst
`Approaching 950°C
`Stabilized Ce with Zr
`
`Pt/Rh, Pd/Rh, and Pt/Rh/Pd
`L
`* All~palladium thIee—way catalyst
`Layered eoating
`Stabilized Ce with Zr
`
`
`
`BASF-2037.021
`
`BASF-2037.021
`
`

`
`
`
`6.4 F1RST—GENERATION CONVERTERS: OXIDATION CATALYST (1976-1979)
`
`79
`
`° Low—emission vehicles
`
`i
`
`High temperature, with/without Ce, close—coupled
`Catalyst, approaching 1050°C
`
`Wifli underfloor catalyst
`l
`
`- Ultra—low—ernission vehicles
`
`High temperature, with/Without Ce, close-coupled catalyst, approaching l050°C
`‘Increased volume underfloor, higher preciousimetal loading
`Optional trap
`I
`
`6.4 F ?lST—GENE'RA.TION CONVERTERS: OXIDATION CATALYST
`(1976— 979)
`
`During the early implementation of the Clean Air Act, the catalyst was only
`required to abate CO and HC. The NOX standard was relaxed so engine man-
`ufacturers used exhaust gas recycle (EGR) to meet the NOX standards. EGR
`dilutes the combustion gas and lowers the combustion flame temperature,
`which results in less thermal N01. formation as predicted by the Zeldovich
`mechanism (Zeldovich 1946). The engine was operated just rich of stoichio~
`metric to further reduce the formation of NOI, and secondary air was pumped
`into the exhaust gas to provide sufficient 02 for the catalytic oxidation of CO
`and HC on the catalyst.
`—
`Dining this period, many catalytic materials were studied and the area of
`high—temperature stabilization of alumina was explored. It was known that the
`precious metals, Pt and Pd, were excellent oxidation catalysts; however,
`the
`cost and supply of these materials was bothersome. Therefore, many base metal
`Candidates were investigated, such as Cu, Cr, Ni, and Mn. They were less active
`than the precious metals but substantially cheaper and more readily available
`(Yao 1975; Kuminer 1975).
`Table 6.1 shows the relative activities of Pt and Pd versus non~precious— ‘
`metal oxides (base metal oxides) for oxidizing simulated exhaust pollutants at
`300°C (Kunimer 1975). From the relative activities, it is clear the precious metals
`are considerably more active than the base metals. Also, the activity depends
`on the species to be catalyzed. Palladium is the most active for CO and eth-
`ylene oxidation, whereas Pt is equally active for ethane oxidation. Precious
`metals would, therefore, be preferred over base metals if not for the expense
`and limited availability. The base metal oxides could be viable, but their lower
`activity would require larger reactor volumes (lower space velocities). This
`Would be a problem in the engine exhaust underfloor piping where space is at a
`P1‘6II1iuln. Studies also showed that the base metal oxides were very susceptible
`to Sulfur poisoning (Farrauto and Wedding 1974; Fishel et al. 1974; Taylor
`
`-2037.022
`
`BASF-2037.022
`
`

`
`
`
`F.
`.1,.1..
`
`80
`
`AUTOMOTIVE CATALYST
`
`TABLE 6.1 Comparison between Relative Activities of Precious—l\/letal and Base Metal
`Catalysts for Different Reactants“
`
`A
`
`Reactant
`
`Pd‘
`Ft
`C0303
`CEO/CIZO3
`Au
`MnO2
`CuO
`LaCoO3
`F6203
`Cl‘zO3
`N10
`
`1
`
`1% co
`
`500
`100
`so
`“40
`15
`4.4
`45
`35
`0.4
`0.03
`0013
`
`'
`
`0.1% C2H5
`
`01% C211,
`
`.
`
`»
`
`.
`
`100
`12
`0.6
`0.8 1
`0.3
`0.04
`0.6
`0.03
`0.006
`0.004 .
`00007
`
`'
`
`1
`1
`0.05
`0.02
`<02
`4 A
`—
`—
`—4
`0003
`0.0008
`
`'
`“Reaction in oxidizing atmosphere at. 300°C.
`Source: Kummer (1975); reprinted with permission, Copyright© 1975, American Chemical Society.
`
`1990). Interest still exists for Cu—based systems, as shown by ongoing studies
`(Kapteijn et al. 1993; Theis and LaBarge 1992).
`Therefore,
`t11e first~generatio11 oxidation catalysts were a combination of
`Pt and Pd and operated in the temperature range of 250~600°C, With space
`velocities varying during vehicle operation from 10,000 to 100,000 l/h, de-
`pending on the engine size and mode of the driving cycle (i.e., idle, cruise, or
`acceleration). Typical catalyst compositions were Pt and Pd in a 2.5 :1 or 5:1
`ratio ranging from 0.05 to 0.1 troy oz/car (a troy oz is ~31 g).
`
`6.4.11 Deactivatjon
`
`The oxidation catalyst was negatively af:"ected by the exhaust impurities of sul-
`fur oxides and tetraethyl lead from the octane booster, both present in the gas-
`oline, and phosphorus and zinc from engine lubricating oil (Doelph et al. 1975;
`Acres et al. 1975). An example of one of these studies (see Figures 6.6 and 6.7)
`shows the elfect Pb, S, and thermal aging have on Pd versus Pt catalysts for the
`temperature at Which 90% of the pollutant ispconverted (Doelph et al. 1975).
`The catalyst was formulated to have a-constant 0.05 wt% total precious metal.
`Clearly, the addition of Pt improved the resistance to Pb poisoning by show-
`ing a continuous decrease in the 90% conversion temperature. This study also
`noted an improvement in sulfur resistance for increases in Pd content. When
`the catalysts were aged in an oxidizing atmosphere at 982°C, the Pd catalyst
`retained more activity relative to Pt. Pt catalysts do sinter in oxidizing envi-
`ronments to a much greater degree than do Pd catalysts (Klimisch et al. 1975),
`so Pt loses activity relative to Pd after thermal aging at 982°C, as shown.
`As the research was ongoing for improved catalyst compositions, the Pb
`present as an octane booster continued to deactivate most severely all the cat-
`
`
`
`BASF-2037.023
`
`BASF-2037.023
`
`

`
`__ 2037.024
`
`
`
`6.4 FlRST—GENERATlON CONVERTERS: OXIDATION CATALYST (l976—l979)_
`
`81
`
`Temperature for 90% conversion (°C)
`
`400
`
`350
`
`300 ~
`
`250
`
`Thermal aged at 982°C\
`
`200
`
`150
`
`o
`
`I
`0.01
`
`I
`
`'
`*
`0,03
`0.02
`Platin um content (weight %)
`Pre5sure= 1 atm., VHSV =15,nU0 1/hr
`
`'
`0.04
`
`0.05
`
`Figure 6.6 Effect of lead, sulfur, and thermal aging on carbon monoxide activity for
`Pt+Pd combined oxidation catalyst. [Reprinted with permission, © 1975 American
`Chemical Society (Doelph et al. l975).]
`
`400
`
`’ Temperature for 90% conversion (°C)
`
`Lead
`
`300 ~
`
`Thermal aged at 982°C
`
`i//,§£”/Ur////”
`200 ~
`Fresh
`
`O
`
`I
`0.01
`
`I
`0.02
`
`'
`
`'7 ‘
`0.03
`
`l
`0.04
`
`‘Platinum content (weight %)
`Pressure :1 atm, VHSV =15,0D0 1lhr
`
`0.05
`
`'
`
`Figure 6.7 Elfect of lead, sulfur, and thermal aging on propylene activity for Pt+ Pd
`