CATALYTIC
`AIR POLLUTION
`CONTROL
`Commercial Technology
`
`Second Edition
`
`Ronald M. Heck and Robert J. Farrauto
`with Suresh T. Gulati
`
`i.1
`
`ffiWILEY-
`~ INTERSCIENCE
`A JOHN WILEY & SONS, INC., PUBLICATION
`
`1
`
`JM 1011
`
`

`

`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 Permissions Department, John Wiley & Sons, Inc., 605 Third Avenue, New
`York, NY 10158-0012, (212) 850-6011, fax (212) 850-6008, E-Mail: PERMREQ@WILEY.COM.
`
`For ordering and cutomer service information please call F800-CALL-WILEY.
`
`Library of Congress Cataloging-in-Publication Data is available.
`
`ISBN 0-471-43624-0
`
`Printed in the United States of America.
`
`10987654
`
`2
`
`

`

`8 Diesel Engine Emissions
`
`8.1
`
`INTRODUCTION
`
`Driving behind a diesel bus or truck, whether on a highway or in a city, often is
`an unpleasant experience because of its emissions. The black smoke, or soot, is
`the most visible, but other, less visible pollutants are also present. The emis(cid:173)
`sions from a diesel engine are composed of three phases: solids, liquids, and
`gases. The combined solids and liquids are called particulates, or total particu(cid:173)
`late matter (PM), and are composed of dry carbon (soot), inorganic oxides
`primaiily as sulfates, and liquids. When diesel fuel (Song et al. 2000) is burned,
`a portion of the sulfur is oxidized to sulfate that, upon reaction with the mois(cid:173)
`ture in the exhaust, becomes H2S04. The liquids are a combination of un(cid:173)
`burned diesel fuel and lubricating oils, called the soluble organic fi'actions
`(SOFs) or volatile organic fractions (VOFs), which form discrete aerosols and/
`or are adsorbed within the dry carbon particles (Zelenka et al. 1990). Gaseous
`hydrocarbons, carbon monoxide, nitrogen oxides, and sulfur dioxide are the
`constituents that make up the third phase. Figure 8.1 gives a breakdown of
`diesel particulates from what would be considered a wet exhaust, specifically, a
`high liquid or SOF content.
`The SOF shown is 35% lube and 20% fuel for a total of 55%. Other engines
`may generate a dry exhaust in which the SOF is lower, where the balance is
`primarily dry carbon. Diesel emissions are clearly more complex than those from
`a gasoline engine, and hence their catalytic treatment is more complicated and
`requires new technology.
`The popularity of diesel engines is derived primarily from their fuel effi(cid:173)
`ciency relative to the gasoline spark-ignited engine. Diesels operate very lean of
`stoichiometric, with air: fuel ratios greater than about 22. They have good fuel
`economy, producing less C02. It is not uncommon for a diesel engine to have a
`life of l 111.lillon IT11ies, or about 10 times that of the gasoline e1lgi1le. The diesel
`fuel and air mixture is highly compressed raising the gas temperature to the
`point of combustion. Its lean nature results in a cooler combustion with less
`gaseous NOx, CO, and HC emissions than its gasoline counterpart. The design
`of the combustion process, however, result~ in high particulate emission levels.
`Engine manufacturers have made great progress in redesigning the engine (fuel
`injectors, timing, and so on) to minimize particulate emissions (Farrauto et al.
`1992a). The units of measurement for particulates are grams per brake-horse(cid:173)
`power generated in an hour, or g/(bhp·h). For example, in 1986, particulate
`emissions from a typical diesel truck engine were 0.6 g/(bhp·h). By 1990 man-
`
`186
`
`3
`
`

`

`8.1
`
`INTROD\_JCTION
`
`187
`
`Lube SOF
`35%
`
`Dry carbon or soot
`43%
`
`Fuel SOF
`20%
`
`Figure 8.1 Diesel particulate emission breakdown: pie chart showing components of
`diesel particulates.
`
`NewYork
`Non-freeway
`297 sec.
`
`[
`
`Los Angeles t
`
`Non-freeway
`300 sec.
`
`Normalized torque (%)
`
`Los Angeles
`Freeway
`305 sec.
`
`t
`
`New York
`Freeway
`297 sec.
`
`120
`
`100
`
`80
`
`60
`
`40
`
`20
`
`0
`0
`
`'i
`
`11 I
`
`~I
`
`I '
`
`Ill
`
`200
`
`800
`
`1000
`
`1200
`
`~I I' i
`
`I
`
`600
`400
`Time (seconds)
`Figure 8.2 U.S. heavy-duty transient cycle for diesel trucks.
`
`ufacturers had redesigned the engine to reduce the particulates to 0.25 g/bhp·h.
`In the Uiiited States~ einissions are n1easiired over a siai1daidized Federal Test
`Procedure (FTP), a trace of which is shown in Figure 8.2.
`The FTP simulates U.S. truck driving conditions. The Clean Air Amend(cid:173)
`ment of 1990 required that by 1994 particulate emissions be reduced to 0.1 g/
`(bhp·h) for trucks and no greater than 0.07 g/(bhp·h) for buses. Catalysts were
`effectively used to bring engines into compliance with the standards (Neitz
`1991). During the late 1990s, the use of catalyst was curtailed since engine
`manufacturers made substantial progress in producing cleaner engines to meet
`the particulate emissions. However, the newer NOx and particulate regulations
`will need catalysts in 2002 to meet these requirements.
`
`lS
`lS
`lS(cid:173)
`nd
`
`~u­
`les
`~d,
`tlS-
`111-
`ms
`id/
`ms
`the
`of
`r, a
`
`nes
`;: lS
`om
`md
`
`::ffi-
`1 of
`fuel
`1e a
`es el
`the
`less
`sign
`rels.
`fuel
`t al.
`1rse(cid:173)
`Llate
`1an-
`
`4
`
`

`

`188
`
`DIESEL ENGINE EMISSIONS
`
`TABLE 8.1 Emission Regulations for U.S. Trucks
`
`HC
`NOx
`PM
`
`1994
`
`1.3
`5
`0.1
`
`1998
`
`1.3
`4
`0.1
`
`2003
`
`0.5
`2
`0.1
`
`2007
`
`0.5
`2
`0.01
`
`2009
`
`0.14
`0.2
`0.01
`
`2010
`
`0.14
`0.2
`0.01
`
`8.2 WORLDWIDE DIESEL EMISSION STANDARDS
`
`Diesel emission control is being addressed worldwide. In the United States,
`Europe, and Japan essentially all of the trucks and buses operate with diesel(cid:173)
`fueled engines. In Europe, the favorable price of diesel fuel relative to gasoline
`has resulted in a large number of diesel fueled passenger cars. Trucks are used
`to ship goods; buses to transport groups of people (usually within urban areas),
`and passenger cars are utilized for individual or small groups in both urban and
`rural locations. Consequently, the allowable emission standards differ for each
`application. Furthermore, the terrain in the United States, Japan, and Europe
`differ sufficiently that the test conditions must reflect local driving habits. Each
`vehicle must meet specific emission standards as measured in standardized
`driving cycles that reflect the duty cycle anticipated for the particular engine.
`Within the United States, a significant reduction in particulates for heavy(cid:173)
`duty trucks came in 1994: from 0.25 to 0.10 g/(bhp·h) (grams per brake horse(cid:173)
`power per hour), as measured during the U.S. heavy-duty (HD) FTP transient
`cycle. This test reflects a continuous measure of emissions during various
`speeds and loads in different U.S. cities, as shown in Figure 8.2. Each task re(cid:173)
`sults in a different exhaust temperature and flow condition. Thus the catalyst
`must convert the particulates over a broad range of conditions. The U.S. stan(cid:173)
`dards for HD trucks for HC, NOx, and PM remained unchanged up until 1998,
`after wlu~h a decrease in NOx from 5 to 4 g/(bhp·h) was required. Tlus is a
`significant challenge for which a catalyst solution is still currently under intense
`investigation. The catalyst must decompose or reduce NOx in diesel exhausts
`and other lean environments. Presently the engine control strategy of exhaust
`gas recycle (EGR) is being used to reduce the engine out NOx emissions. The
`emissions regulations for HD trucks out to the year 2010 in the units of
`g/(bhp:fif are- smnmarized in Table 8-.1. The years Inentioned show the k:ey
`changes in emission regulations for HC, NOx, and PM. PM reductions are ex(cid:173)
`pected to change dramatically in the year 2007, and this will require new tech(cid:173)
`nologies.
`Japanese standards for 1994, for trucks in a weight class above 1265 lb, re(cid:173)
`quired particulate emissions < 0.2 g/mi, which were be met by engine mod(cid:173)
`ifications alone and thus will not require catalysts. Proposed new standards are
`for both diesel passenger cars and trucks. The standards for passenger cars and
`trucks for the 2002/03 timeframe are shown in Table 8.2.
`
`5
`
`

`

`8.2 WORLDWIDE DIESEL EMISSION STANDARDS
`
`189
`
`TABLE8.2
`
`Emission Standards (2002/03) for Japanese Passenger Cars and Trucks
`
`Small Cars
`( < 1.25 tons)
`
`Medium Cars
`(> 1.25 tons)
`
`Small Trucks
`( < 1.7 tons)
`
`Medium Trucks
`(1. 7-2.5 tons)
`
`HC
`co
`NOx
`PM
`
`0.12
`0.63
`0.28
`0.052
`
`0.12
`0.63
`0.3
`0.056
`
`0.12
`0.63
`0.28
`0.052
`
`0.12
`0.63
`0.49
`0.06
`
`TABLE 8.3
`
`Emissions Standards (2005) for Japanese Passenger Cars
`
`Small Cars
`(<1.25 tons),
`2002
`
`Small Cars
`(> 1.25 tons),
`2005
`
`Medium Car
`(<l.25 tons),
`2002
`
`Medium Cars
`(>l.25 tons),
`2005
`
`HC
`co
`NOx
`PM
`
`0.12
`0.63
`0.28
`0.052
`
`0.06
`0.315
`0.14
`0.026
`
`0.12
`0.63
`0.3
`0.056
`
`0.06
`0.315
`0.15
`0.028
`
`TABLE 8.4 Japanese Heavy Duty Truck Standards for
`Various Years
`
`HC
`co
`NOx
`PM
`
`2000
`
`2.9
`7.4
`0.45
`0.25
`
`2003/04
`
`0.87
`2.22
`0.338
`0.18
`
`Standards for the year 2005 for passenger cars are shown in Table 8.3.
`The units for-Japai1ese passenger car a11d truck tegulatioi1S are given in
`g/lun. The HD truck regulations in Japan are shown in Table 8.4.
`In Europe, passenger cars must meet standards reflecting both urban (BCE)
`conditions and high-speed extra urban driving (EUDC). These cycles are shown
`as Figure 8.3a, b. The cycle A test combines the emissions from both the BCE
`and EUDC modes. The catalyst inlet temperatures are cool (200-250°C) for
`the BCE mode but increase to 300-550°C, depending on the engine, for the
`EUDC mode; therefore, the catalyst must function over a broad temperature
`range. For European heavy-duty truck applications, a 13-mode steady-state
`
`10
`
`L4
`2
`)1
`
`~s,
`~1-
`ne
`ed
`s),
`1d
`ch
`pe
`ch
`ed
`
`'Y-
`
`:nt
`us
`~e-
`1st
`.n(cid:173)
`)8,
`:a
`tse
`sts
`ist
`he
`of
`:ey
`~x-
`~h-
`
`re(cid:173)
`>d(cid:173)
`i.re
`nd
`
`6
`
`

`

`190
`
`DIESEL ENGINE EMISSIONS
`
`Speed (Km/hr)
`
`50
`
`40
`
`30
`
`20
`
`10
`
`0+--'-~-'-~-',-~~~~-'---r-~"--~~~~~~--1--1
`50
`150
`200
`0
`100
`Time (seconds)
`(a)
`
`Speed (Km/hr)
`
`140.----~~~~~~~~~~~~~~~~~~~~~~
`
`120
`
`100
`
`80
`
`60
`
`40
`
`20
`
`0+-~~.-~----.-~~~~~~~~~~~~~~-'-~~
`250
`200
`100
`50
`300
`0
`400
`150
`350
`
`Time (seconds)
`(b)
`Figure 8.3 European cycle A: (a) ECE 15 urban cycle (city driving); (b) EUDC extra
`urban driving cycle (highway driving).
`
`test with various loads/torques and speeds is utilized. This test puts emphasis
`on the high-temperature performance of the catalyst, reflecting high-load con(cid:173)
`ditions. At such high temperatures, minimum generation of sulfate by catalytic
`oxidation of gaseous S02 becomes the most demanding requirement. The Eu(cid:173)
`ropean emission regulations for passenger cars are shown in Table 8.5.
`The information in Table 8.5 reflects the years where there are changes in
`the emission regulations. The European regulations for HD trucks are shown in
`Table 8.6.
`
`7
`
`

`

`TABLE 8.5 Emission Standards to 2010 for European Passenger Cars
`
`8.3 NOx-PARTICULATE TRADEOFF
`
`191
`
`co
`HC+NOx
`PM
`
`1994
`
`2.88
`0.87
`0.14
`
`1996
`
`1.06
`0.63
`0.08
`
`2000
`
`0.64
`0.5
`0.05
`
`2005
`
`0.5
`0.25
`0.025
`
`TABLE 8.6 Emission Standards to 2010 for European Trucks
`
`HC
`co
`NOx
`PM
`
`1994
`
`1.1
`8
`4.5
`0.36
`
`1996
`
`1.1
`7
`4
`0.15
`
`2000
`
`0.66
`5
`2.1
`0.1
`
`2005
`
`0.46
`3.5
`1.5
`0.02
`
`2008
`
`0.5
`0.25
`0.025
`
`2008
`
`0.46
`2
`1.5
`0.02
`
`2010
`
`0.25
`0.125
`0.0125
`
`2010
`
`0.46
`2
`1.5
`0.02
`
`Emission units are given in g/kWh. Again, the information in Table 8.6 re(cid:173)
`flects the years where there are changes in the emission regulations.
`
`8.3 NOx-PARTICULATE TRADEOFF
`
`The control of particulate emissions and NOx represent significant challenges
`to the diesel engine manufacturer because they are coupled inversely. When the
`engine operates cooler, it produces less NOx but more particulate. At higher
`temperatures combustion is more complete, generating less particulate but
`more NOx. This is referred to as the NOx-particulate tradeoff; when one is high
`the other is low.
`A commonly engine management strategy to control NOx is exhaust gas re(cid:173)
`circulation (EGR). Part of the exhaust of 0 2 and enriched in H20 and C02, is
`recycled back to the combustion chamber. The presence of the H20 and C0 2
`reduces the average combustion temperature, limiting NOx production. This
`leads to greater particulate and especially dry soot formation and lower fuel
`economy.
`If a downstream NOx reduction system were discovered the engine could be
`operated without EGR and consequently much hotter minimizing particulates
`with the additional NOx reduced downstream. The other possibility is to use
`EGR, which decreases NOx formation, and treat the dry soot downstream
`provided a reduction technology were available.
`For this reason there has been an enormous amount of research and devel(cid:173)
`opment with the goal of finding catalyst technology, which, when located down(cid:173)
`stream from the combustion chamber, will reduce NOx and/or particulates.
`
`:tra
`
`LSlS
`)n(cid:173)
`rtic
`~u-
`
`' 111
`l 111
`
`·,.;-.
`
`'!:
`
`'f
`
`' . ~
`
`8
`
`

`

`f
`
`192
`
`DIESEL ENGINE EMISSIONS
`
`Since particulates are composed primarily of dry soot and SOF, the search for
`reduction technologies is threefold: (1) SOF, (2) dry soot, and (3) NOx.
`This chapter discusses progress in treating all three pollutants.
`
`8.4 ANALYTIC PROCEDURES
`
`Analysis of the diesel exhaust is much more complicated than that of the
`internal-combustion (IC) engine because of the three phases of pollutants pres(cid:173)
`ent (Cuthbertson and Shore 1988). The Federal Test Procedure defines "par(cid:173)
`ticulates" as matter that is collected on a filter at 52°C to condense SOF. The
`particulates are collected in a dilution tunnel, which cools the exhaust to the
`required temperature. Because of the hygroscopic nature of the sulfates, the filter
`must be further conditio'ned in a controlled atmosphere to equilibrate the water
`content. The unburned fuel and lubricating oil is extracted from the filter with
`methylene chloride, making it the soluble organic fraction (SOF). It is then in(cid:173)
`jected into a gas chromatograph (GC), where a capillary column separates the
`fuel from the lube fraction. In this manner, the effectiveness of the catalyst to(cid:173)
`wards converting each component of the SOF can be assessed. Some labo(cid:173)
`ratories volatilize the unburned fuel and lube directly from the filter into the
`GC. This gives a volatile organic fraction or VOF. The results are essentially
`equivalent to the SOF ni.ethod.
`Another portion of the conditioned filter is then extracted with water to dis(cid:173)
`solve the sulfate. The aqueous solution is then injected into an ion chromato(cid:173)
`graph for sulfate analysis. The dry carbon is obtained by difference, or in some
`cases can be subjected to the1111al analysis for a burnoff after the extraction
`steps, to complete the material balance.
`
`8.5 DIESEL OXIDATION CATALYST FOR TREATING SOF
`PORTION OF PARTICULATES
`
`Unlike those of gasoline engines, diesel gaseous HC and CO emissions are rel(cid:173)
`atively low, so they did not need to be reduced to meet the 1994 U.S. truck
`standards. Thus, the problem was to reduce the particulates. The dry carbon
`engine_ out emissions can be reduced by engjne modification, but this causes an
`increase in SOF. One approach to meet emission requirements was to have the
`catalyst oxidize the SOF (which represents up to about 65% of the particulate),
`thereby greatly reducing the total particulates emitted (Porter et al. 1991).
`The engine-out particulate sulfate emissions have been reduced by decreas(cid:173)
`ing the fuel sulfur content from the pre-1994 value of 0.2-0.3% to the 1994
`value of 0.05%. These alone account for a drop of from 0.005 to 0.01 g/bhp·h
`of particulates as sulphate.
`A combination of reduced fuel sulfur, the presence of a catalyst to reduce
`the SOF, and engine modifications to decrease the dry carbon emitted would
`
`9
`
`

`

`8.5 DIESEL OXIDATION CATALYST FOR TREATING SOF PORTION
`
`193
`
`bring U.S. trucks into compliance with standards for 1994 (Farrauto et al.
`1992a), provided suitable catalysts could be developed.
`
`8.5.1 Catalytic Oxidation of the SOF
`
`The manufacturers of trucks sold in the United States preferred a flowthrough
`monolithic honeycomb catalyst structurally similar to that used in the gasoline
`engine. Given the composition of the diesel exhaust and the particulate reduc(cid:173)
`tions required, new and different catalyst formulations would be necessary. The
`general idea was for the catalyst to oxidize the SOF without oxidizing the S0 2
`to sulfate. For such a system to function, the dry soot had to be further reduced
`to eliminate plugging of the channels of the monolith. The engine design pa(cid:173)
`rameters that had to be modified to allow for reduction in the dry soot resulted
`in an increase in the SOF, which could then be oxidized by the catalyst.
`The cool operating condition of the engine requires that the catalyst first
`adsorb and retain the SOF at low temperatures (i.e., idle), followed by its
`combustion as the exhaust temperature reaches lightoff (i.e., 200-250°C):
`
`(8.1)
`
`Catalyst design required an organophilic surface to enhance SOF adsorption
`at low temperatures. The surface area and pore size also had to be optimized
`to enhance condensation and maintain a high adsorption capacity for the low
`temperature idle conditions. The SOF would then volatilize and catalytically
`oxidize as the exhaust temperature heated up (Voss et al. 1994b).
`An extremely important catalytic property was low activity towards the ox(cid:173)
`idation of the gaseous S02 to S03, the latter that quickly forms sulfate partic:.
`ulates after reaction with H20:
`
`,.J:
`~'.
`
`(8.2)
`
`This was quite challenging because catalysts with high activity for oxidizing the
`SOF would also enhance S0 2 conversion. Thus, a highly selective catalyst was
`required.
`Catalysts containing precious metal such as Pt and Pd were considered the
`most likely candidates because they do have good low-temperature activity for
`hydrocarbon conversions. However, they are also very active for the S0 2 oxi(cid:173)
`dafiOn reaction. In oi1e catalyst the additioi1 ofV20 5 was fouiid fo suppre-ss the
`activity of the Pt for the S0 2 conversion reaction without diminishing the rate
`of the SOF oxidation reaction (Beckmann et al. 1992; Domesle et al. 1992;
`Wyatt et al. 1993).
`Another approach utilized Pd as the catalytic metal because its intrinsic S02
`activity is generally lower than that of Pt but still has reasonable SOF activity
`(Horiuchi et al. 1991). Still another approach was to completely eliminate the
`use of precious metals and to rely on base metal oxides to catalytically oxidize
`the SOF (Farrauto et al. 1993) without oxidizing the S02 .
`
`or
`
`he
`~s-
`
`lr(cid:173)
`he
`he
`:er
`:er
`;th
`m(cid:173)
`he
`:o(cid:173)
`)0-
`he
`lly
`
`.lS(cid:173)
`to-
`11e
`on
`
`·el(cid:173)
`LCk
`ion
`an
`! the
`. :e),
`
`as(cid:173)
`>94
`p·h
`
`LlCe
`uld
`
`10
`
`

`

`194
`
`DIESEL ENGINE EMISSIONS
`
`Mass emissions (g/BHP-hr)
`I rncarbon m!SOF DS03+H20 I
`
`0.14
`
`0.12
`
`0.1
`
`0.08
`
`0.06
`
`0.04
`
`0.02
`
`0
`
`5.9 BASE 5 Pt
`
`40 Pt
`
`10.3 BASE 5 Pt
`
`40 Pt
`
`5.9 L Engine, Maximum T = 300°c
`10.3 L Engine, Maximum T= 475°C
`
`Figure 8.4 Particulate emissions exiting from two different engines and two different
`catalysts.
`
`Although catalysts with reduced activity for the S02 ---+ S03 reaction were
`found, catalytic generation of sulfate was still a major contributor to partic(cid:173)
`ulates at temperatures in excess of ""400°C. Since some S03 was inevitable and
`Al203 washcoated catalysts deactivate due to formation of Ah(S04)3, alterna(cid:173)
`tive carrier materials had to be used. Titanium oxide (Beckmann et al. 1992;
`Domesle et al. 1992), silicon dioxide (Ball and Stack 1990), and zirconium ox(cid:173)
`ide (Horiuchi et al. 1991) are sufficiently inactive towards reaction with S03
`and thus could be used as carriers for the Pt and/ or Pd metals. They could also
`be prepared with the proper pore size to adsorb the SOF at low temperatures.
`The U.S. heavy-duty FTP test simulates the driving cycle of a diesel truck,
`including idle/low-load to high-speed/high-load conditions. The composition
`and temperature of the exhaust depend on the type of engine. This, in turn,
`influences the function of the catalyst. For example, a 10.3-liter engine has a
`particulate composition composed of 30% SOF, whereas the SOF represents
`40% of the particulates in a 5.9-liter engine. The temperature histograms ge~1-:­
`erated during the U.S. FTP test show the exhaust from the 10.3-liter engine
`to operate up to 475°C, whereas the smaller (5.9-liter) engine never exceeds
`300°C. The performance of the catalyst would have to be much different for
`treating the exhausts from these two engines. To illustrate the importance of
`matching the catalyst performance with the exhaust characteristics, Figure 8.4
`compares two platinum containing catalysts installed in the exhausts of the
`10.3- and 5.9-liter engines.
`A completely alternative approach was taken by (Farrauto et al. 1993; Far(cid:173)
`rauto and Voss 1996) when they discovered that a specially activated Ce02
`
`11
`
`

`

`8.5 DIESEL OXIDATION CATALYST FOR TREATING SOF PORTION
`
`195
`
`when physically blended with y-Alz03 (50: 50 wt%) produced a highly active
`catalyst for the SOF conversion. The mechanism proposed by the authors was
`that the SOF was adsorbed into the pore structure of the catalyst during cold
`( <200°C) operation. When the engine exhaust exceeded 200°C, the SOP volat(cid:173)
`ized and catalytically converted on the Ce0 2 component to C02 and H20.
`Greater detail is available in the literature (Farrauto and Voss 1996). Traces of
`CO were produced so a small amount of Pt (0.5 g/ft3) was added to complete
`the oxidation of the CO and any residual hydrocarbon. Surprisingly, when the
`Pt was deposited on the Ce0 2 directly, it further decreased the oxidation of
`S0 2 (Farrauto et al. 1993). A version of this catalyst on a monolith is still used
`today on medium-duty trucks and buses throughout the world.
`
`8.5.2 Commercial Diesel Oxidation Catalysts for Trucks
`
`The catalysts used on U.S. trucks are monolithic honeycomb structures of cor(cid:173)
`dierite with 200-400 cells per square inch (cpsi). The catalyzed washcoat is de(cid:173)
`posited onto the walls of each channel at a loading of about 2 g/in 3 and is
`typically no more than ""40-80 µm in the thickest locations (corners or fillets)
`on the monolith. The first-generation Engelhard catalyst is composed of ""50%
`catalytically active Ce0 2 in combination with an equal amount of y-Alz03 with
`small amounts of Pt (0.5-2 g/ft3) for truck and bus applications in the United
`States and Europe (Farrauto et al. 1993; Farrauto et al. 1995) and (Farrauto
`and Voss 1996). A joint venture between Nippon Shokubai and Degussa (now
`DMC2) promoted the use of a combination of Pd (40 g/ft3) supported on zir(cid:173)
`conia with various promoter oxides such as the rare earths for trucks (Horiuchi
`et al. 1991).
`The catalyst volume as a general rule equals the displacement volume of the
`engine. Therefore, a 6-liter medium-duty truck engine has about 6 liters of cat(cid:173)
`alyst. The catalyst is contained in a steel can with a mounting material made of
`a ceramic wrapped around its outside diameter to ensure mechanical integrity
`and resistance to vibration (Clerc et al. 1993). Space velocities vary between
`""20,000 and ""250,000 h- 1, depending on the duty cycle of the vehicle. Cata(cid:173)
`lyst diameters are 7-10 in. (17.78-25.40 cm), with lengths of 5-7 in. (12.7-
`17.18 cm) for trucks.
`Two studies by the Manufacturers of Emission Controls Association
`_(MECA) sh,o\V the performa11ce of tl1e curren_t c;qn1111t::r~ially availe.ble tech(cid:173)
`nologies for DOC application for trucks. The importance of fuel sulfur was
`also mentioned in these studies. Although this study did not reveal the compo(cid:173)
`sition or the suppliers of the DOCs evaluated, the perfom1ance can be viewed
`in reference to the new emission regulations (MECA 1999a,b). Following are
`the conclusions from these studies:
`
`· The test program demonstrated that advanced exhaust emission control
`technology can be used to meet the program targets of a 0.03 g/(bhp·h)
`PM emission level combined with a 1.5 NOx + HC emission level for stan-
`
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`12
`
`

`

`196
`
`DIESEL ENGINE EMISSIONS
`
`dard no. 2 diesel fuel (368 ppm) and a 0.01-g/(bhp·h) PM emission level
`combined with a 1.5 NOx + HC emission level for lower sulfur no. 2 diesel
`fuel (54 ppm)
`• Commercially available diesel oxidation catalysts significantly reduced hy(cid:173)
`drocarbon, carbon monoxide, and particulate emissions from a diesel en(cid:173)
`gine over the heavy-duty engine FTP and off-cycle test points. Optimized
`catalyst systems can achieve emission reductions in excess of 35% for PM
`and 70% for HC and CO.
`· Diesel oxidation catalysts enable a current heavy-duty engine using stan(cid:173)
`dard no. 2 diesel fuel (368 ppm) to meet a particulate emission level of
`0.05 g/(bhp·h).
`• Diesel oxidation catalysts enable a current heavy-duty engine using 54
`ppm sulfur no. 2 diesel fuel to meet a particulate emission level of 0.046
`g/(bhp·h).
`• Diesel oxidation catalysts reduce toxic compounds in diesel exhaust, such
`as polyaromatic hydrocarbons, by an average of approximately 55% using
`standard no. 2 diesel fuel (368 ppm sulfur).
`• Reducing the sulfur content in the fuel from. 54 ppm to zero improved the
`performance of the catalyst.
`· Fuel sulfur restricts catalyst technology, due to the conversion of sulfur
`dioxide to sulfate at high exhaust temperatures. Gas-phase and SOF emis(cid:173)
`sion reductions are directly related to the catalyst activity. Highly active
`gas-phase catalysts will make a significant amount of sulfate over the FTP.
`While catalyst formulations can be designed to provide significant HC,
`CO, and PM control, utilizing low sulfur fuel increases the opportunity to
`use formulations which can achieve even greater HC, CO, and PM con(cid:173)
`trol.
`
`In 1999 most truck manufactures in the United States engineered the diesel
`oxidation catalyst off the exhaust system, with the exceptional of some fleets in
`California. However, it is expected that they will be reinstalled in 2002 because
`of more stringent emissions. Compositions and designs of the catalysts are con(cid:173)
`tilmously being modified as the emission standards become more severe and
`engine control technology more advanced.
`
`8.6 CATALYTIC REDUCTION OF EMISSIONS FROM DIESEL
`PASSENGER CARS
`
`. 8.6.l . The European Cycle
`
`An added benefit of the catalytic. treatment for reduction of SOF is the con(cid:173)
`version of the small quantities of unbu111ed gaseous HC and CO. Although the
`truck engine-out gaseous emissions are well within the standards in the United
`
`13
`
`

`

`8.6 CATALYTIC REDUCTION OF EMISSIONS
`
`197
`
`Mass Emissions (g/km)
`..................................... 1 .. E?.l!~ .... ~g.?. .... ~.~.~.~i.?.~.1~~.~1 .............................................................. .
`
`/ /
`
`Base
`
`2 Pt
`
`20 Pt
`European Cycle A
`2 Litre Engine, Max. T = 42o0c
`
`20 Pt+ I
`
`0.4
`
`0.3
`
`0.3
`
`0.2
`
`0.2
`
`0.1
`
`0.1
`
`0
`
`Figure 8.5 Passenger car particulates and gas-phase emissions using Pt-containing cat(cid:173)
`alysts for European cycle A. Catalyst 20 Pt+ I (Inhibitor) generates less particulate
`(sulfate) than does 20 Pt without inhibitor.
`
`States, their removal further cleans the exhaust. Furthermore, unburned or(cid:173)
`ganics are often odor-bearing aldehydes, benzopyrenes, ketones, butadienes,
`and so on. Hence their removal improves the quality of the exhaust. This is es(cid:173)
`pecially important for European diesel passenger car applications where stan(cid:173)
`dards require some reduction of the gaseous emissions by the catalyst. For
`passenger cars the standards are expressed in g/km.
`The European cycle A, shown in Figure 8.3, embraces the cool urban driv(cid:173)
`ing portion, called ECE, where the catalyst inlet temperature typically is below
`200°C, reflecting slow-moving traffic. During these cool conditions odors
`caused by partially oxidized hydrocarbons are generated. Odor reduction can
`be achieved by increasing the concentration of Pt in the catalyst as well as by
`incorporating hydrocarbon trapping materials in the formulation (see Section
`8.6.2). In the second portion of the test, reflecting high-speed highway driving
`and called EUDC, the maximum inlet temperature to the catalyst typically
`rises above 350°C.
`
`------ 8.6.2 Catalysts for Passenger Cars -
`
`The effectiveness of increasing amounts of Pt for converting HC and CO is
`clear from Figure 8.5.
`The catalyst designated 2 Pt contains 2 g/ft 3 of Pt supported on high-sur(cid:173)
`face-area Si02 . The 20 Pt catalyst contains 20 g/ft3 of Pt on the same carrier
`and shows the largest reduction in HC and CO emissions. The engine-out par(cid:173)
`ticulates of 0.15 g/km are composed of rv40% SOF; and the balance, dry car(cid:173)
`bon and traces of sulfate. A conversion of 100% of the SOF results in a 40%
`conversion of the particulates provided no additional sulfate is formed. The
`
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`
`14
`
`

`

`198
`
`DIESEL ENGINE EMISSIONS
`
`particulate emissions exiting from the 20 Pt catalyst are actually higher than the
`base emissions with no catalyst, due to formation of sulfate. This illustrates the
`issue of selectivity. The approach initially taken was to use high concentrations
`of Pt or Pd with added promoters, designated as I, to inhibit sulfate formation.
`The effectiveness of this approach is shown in the final data set in Figure 8.5, in
`which 20 Pt+ I is shown to control particulate make (i.e., sulfate make, relative
`to 20 Pt) while still reducing HC and CO. Inhibitors such as vanadium (Beck(cid:173)
`mann et al. 1992; Domesle et al. 1992) and rhodium (Fukano et al. 1993) have
`been shown to be effective in controlling sulfate make at high temperatures.
`The literature reports that the early Degussa (now DMC2
`) catalyst was com(cid:173)
`posed of combinations of Pt (20-60 g/ft 3), 3-10% V2 0 3 , and the balance Ti0 2
`(Beckmann et al. 1992; Domesle et al. 1992), Johnson-Matthey is believed to
`be using a variation of Pt and vanadia for selected applications (Wyatt et al.
`1993).
`Sulfate make was not a major problem during the ECE or cooler urban
`driving cycle, but the CO and hydrocarbon emissions were. In particular, the
`odors generated from the partially oxidized hydrocarbons were unacceptable,
`especially in an urban environment. To address this issue, Yavuz and his asso(cid:173)
`ciates in the 1990s (Yavuz et al. 2001) added specially treated zeolites to the
`catalyst that acted as traps adsorbing the hydrocarbons at low temperatures.
`As the exhaust temperature increased, the hydrocarbons desorbed and were
`oxidized on the Pt function of the catalyst. One typical catalyst was composed
`of Pt on Ce0 2 admixed with a zeolite such as beta, mordenite, and ZSM-5 all
`deposited on a monolith. The Ce0 2 functioned to oxidize the SOF while the Pt
`oxidized the CO and hydrocarbons released from the zeolite. A fom1 of this
`technology is still in use today, but improvements are always being made to
`improve the low-temperature performance (or lightoff) of the catalyst for CO
`and hydrocarbon oxidation, using a zeolite function for HC trapping (Standt
`and Konig 199 5).
`Typically the diesel oxidation catalyst is 3 x 5 in. or 4.66 x 3-6 in. long.
`They are housed in metallic exhaust systems similar to the gasoline catalyst.
`As we move into the new millennium, the movement to low-sulfur fuel will
`continue to pr