catalytic converter

Catalysis Today 77 (2003) 419449

Automotive catalytic converters: current statusand some perspectives

Jan Kapar, Paolo Fornasiero, Neal HickeyDipartimento di Scienze Chimiche, University of Trieste, via L. Giorgieri 1, I-34127 Trieste, Italy

Abstract

Automotive three-way catalysts (TWCs) have represented over the last 25 years one of the most successful stories in thedevelopment of catalysts. The aim of this paper is to illustrate the technology for abatement of exhaust emissions by analysingthe current understanding of TWCs, the specific role of the various components, the achievements and the limitations. Thechallenges in the development of new automotive catalysts, which can meet future highly demanding pollution abatementrequirements, are also discussed. 2002 Elsevier Science B.V. All rights reserved.

1. Introduction

Air pollution generated from mobile sources isa problem of general interest. In the last 60 yearsthe world vehicle fleet has increased from about 40million vehicles to over 700 million; this figure isprojected to increase to 920 million by the year 2010[1]. The environmental concern originated by mo-bile sources is due to the fact that the majority ofengines employ combustion of fuels derived fromcrude oil as a source of energy. Burning of hydrocar-bon (HC) ideally leads to the formation of water andcarbon dioxide, however, due to non-perfect combus-tion control and the high temperatures reached in thecombustion chamber, the exhaust contains significantamounts of pollutants which need to be transformedinto harmless compounds. In this paper, the controlstrategies and achievements in automotive pollutioncontrol are discussed. Attention is focussed on re-cent developments in the field of the three-way type

Corresponding author. Fax: +39-40-5583903.E-mail address: [email protected] (J. Kapar).

of catalysts, i.e. NM/CeO2ZrO2Al2O3 contain-ing systems; insight on the lean-DeNOx and dieseltype of catalysts is also given. The paper is focussedessentially on the catalytic aspects of pollution abate-ment, even though the reader should consider thattechnological solutions such an electrically heatedcatalysts, etc., may heavily affect the converter per-formances [2]. A number of review papers havedescribed the traditional CeO2-based TWC technol-ogy, accordingly we refer the reader to these papers[213].

2. Emissions characteristics and controlstrategies

Engine exhausts consist of a complex mixture, thecomposition depending on a variety of factors such as:type of engine (two- or four-stroke, spark- or compres-sion (diesel)-ignited), driving conditions, e.g. urban orextra-urban, vehicle speed, acceleration/deceleration,etc. Table 1 reports typical compositions of exhaustgases for some common engine types.

0920-5861/02/$ see front matter 2002 Elsevier Science B.V. All rights reserved.PII: S0 9 2 0 -5861 (02 )00384 -X

420 J. Kapar et al. / Catalysis Today 77 (2003) 419449

Table 1Example of exhaust conditions for two- and four-stroke, diesel and lean-four-stroke engines [9,155,176,231]Exhaust componentsand conditionsa

Diesel engine Four-stroke sparkignited-engine

Four-stroke lean-burnspark ignited-engine

Two-stroke sparkignited-engine

NOx 3501000 ppm 1004000 ppm 1200 ppm 100200 ppmHC 50330 ppm C 5005000 ppm C 1300 ppm C 20,00030,000 ppm CCO 3001200 ppm 0.16% 1300 ppm 13%O2 1015% 0.22% 412% 0.22%H2O 1.47% 1012% 12% 1012%CO2 7% 1013.5% 11% 1013%SOx 10100 ppmb 1560 ppm 20 ppm 20 ppmPM 65 mg/m3Temperatures (test cycle) r.t.650 C (r.t.420 C) r.t.1100 Cc r.t.850 C r.t.1000 CGHSV (h1) 30,000100,000 30,000100,000 30,000100,000 30,000100,000 (A/F)d 1.8 (26) 1 (14.7) 1.16 (17) 1 (14.7)e

a N2 is remainder.b For comparison: diesel fuels with 500 ppm of sulphur produce about 20 ppm of SO2 [16].c Close-coupled catalyst.d defined as ratio of actual A/F to stoichiometric A/F, = 1 at stoichiometry (A/F = 14.7).e Part of the fuel is employed for scavenging of the exhaust, which does not allow to define a precise definition of the A/F.

As shown in Table 1, the exhaust contains princi-pally three primary pollutants,1 unburned or partiallyburned hydrocarbons (HCs), carbon monoxide (CO)and nitrogen oxides (NOx), mostly NO, in addition toother compounds such as water, hydrogen, nitrogen,oxygen, etc. Sulphur oxides, though polluting, are nor-mally not removed by the post-combustion treatments,since the only effective way is to reduce them to el-emental sulphur, which would accumulate in the sys-tem. Accordingly, it is preferred to minimise sulphuremissions by diminishing the sulphur content in thefuel. Given the different nature of the three classes ofpollutants, i.e. reducing or oxidising agents, it is nec-essary to simultaneously carry out both reduction andoxidation reactions over the exhaust catalyst, whichcan occur by a variety of reactions. Some of these aresummarised in Table 2. Importantly, this table reportsonly the desirable reactions, in that many other reac-tions could occur in the complex mixtures describedin Table 1, such as, for example, reduction of NOx toammonia, partial oxidation of HC to give aldehydesand other toxic compounds, etc. Given the complexityof the exhaust media, a high selectivity is required inorder to promote only the reactions reported in Table 2.

A perusal of the exhaust compositions reported inTable 1 for the different type of engines reveals some

1 The ability of the TWCs to simultaneously eliminate threeclasses of pollutants is at the origin of their name.

significant differences: (i) even if relatively diluted,the concentration of the various pollutants can changeeven by an order of magnitude, according to the typeof engine; (ii) with the exception of the four-strokespark ignited-engine, which, being equipped with aTWC, is run at stoichiometry, the other type of enginescan be run under lean conditions, i.e. in excess ofO2; (iii) extremely high temperatures are reached inthe four-stroke spark ignited-engine, particularly in theclose-coupled catalyst (CCC).

In general, the emissions depend on air-to-fuel(A/F) ratio, as exemplified in Fig. 1. Tuning of theengine to rich feed gives the highest power output,which, however, occurs at expenses of high fuel con-sumption. Under lean conditions lower combustion

Table 2Reactions occurring on the automotive exhaust catalysts, whichmay contribute to the abatement of exhaust contained pollutants[4]Oxidation 2CO+ O2 2CO2

HC+ O2 CO2 + H2Oa

Reduction/three-way 2CO+ 2NO 2CO2 + N2HC+ NO CO2 + H2O+ N2a2H2 + 2NO 2H2O+ N2

WGS CO+ H2O CO2 + H2Steam reforming HC+ H2O CO2 + H2a

a Unbalanced reaction.

J. Kapar et al. / Catalysis Today 77 (2003) 419449 421

Fig. 1. Effect of A/F ratio (w/w) on engine emissions and enginepower (after Ref. [3]).

temperatures lead to lower NOx emissions, however,at very high A/F engine misfire occurs, leading againto high HC emissions. Under any A/F conditionscatalytic abatement of pollutants is needed to com-ply with the legislation limits. Only at stoichiometricconditions are appropriate amounts of reducing andoxidising agents present in the exhaust to carry outthe catalytic reactions as outlined in Table 2. Un-der such conditions TWCs effectively remove thepollutants.

In principle, there are several advantages in re-moving NOx from the automotive exhaust underlean conditions, i.e. A/F > 14.7, compared to thestoichiometric feed (A/F = 14.7) of a traditionalgasoline-fuelled engine, where the polluting compo-nents are abated using a TWC. The most importantadvantage of lean-burn engines is the significant fueleconomy. In fact, an increase of fuel consumption ofvehicles occurred in the 1990s, which was generallyattributed to the introduction of TWCs and their re-quirement for a stoichiometric A/F ratio to achievebest performances. However, it should be noted thatthere are other factors that substantially contributedas well, such as the increase of vehicle weight due toimplementation of security systems, the generaliseduse of vehicle air conditioning, etc. There is anotheradvantage of lean NOx engine, which is the factthat the highest exhaust temperatures are typicallylower (800850 C) compared to the stoichiomet-ric engines. In the latter engine, temperatures up to

1100 C are met by the CCC, which may result in adisadvantage in terms of the catalyst durability.

Three types on engines can effectively run un-der lean-burn, i.e. diesel, four-stroke/lean-burn andtwo-stroke engine. The two-stroke engine is typicallyemployed in small motorcycles, mopeds, chain sawsand most recreational vehicles. This engine is charac-terised by high engine power output, compact design,and low construction costs, which makes it ideal forthe above listed applications. However, it is noisy,it presents high fuel consumption and high levels ofemitted HCs due to partial mixing of the combustionmixture with the exiting exhaust. It is estimated thatup to 2530% of fuel in the feed is emitted during thescavenging process of the exhaust mixture from thecylinder [14]. Accordingly, the HC emitted from thisengine are predominantly C5C6, in contrast to allother engines where C1C3 constitute the majority ofHC emitted. Even though various strategies of enginemanagement were developed for limiting such highHC emissions, pollution control by catalytic methodsis nowadays mandatory even for these engines, whichmay be achieved by introducing an oxidation catalystfor CO and HC removal, as the legislation is generallyless demanding compared to four-wheel vehicles. Dueto the intrinsically low NOx emission levels (Table 1),EGR (exhaust gas re-circulation) technology can beemployed for the reduction of emitted NOx . Shouldlegislation significantly increase the tightness of thepresent and forecasted limits, it may be expectedthat four-stroke engines, equipped with conventionalTWCs could gradually replace two-stroke engines, ashappened for four-wheel vehicles.

As for the exhausts originated by diesel andlean-burn engines, let us observe that even thoughboth types of engines run in an excess of oxygen,typically 515% of O2 is present in the exhausts com-pared to approximately 1% found in the engine fed atstoichiometry; the precise nature of the exhaust gasessignificantly differs between the two systems. In fact,both types of engine emit HC and NOx at ppm levels(300800 ppm) and large amounts of O2 (515%),water and CO2 (each 1012%). However, as far as theHCs are concerned, there is an important differencein that very low levels are emitted from the dieselengine (NOx), which makes necessary addition of areducing agent to the exhaust in order to achieve ap-preciable reduction of the emitted NOx . On the other


hand, diesel emissions are characterised by a signif-icant level of particulate matter, which itself can beemployed as a reducing agent for NOx , or, vice versa,particulate can be abated by using NOx as oxidant asin Johnson Mattheys continuously regenerating trap(CRT) [15]. In contrast, the issue of particulate mat-ter is absent in the case of lean-burn engine poweredby gasoline; moreover, higher contents of HC areemitted, which, in principle, allows direct selectivecatalytic NOx removal to occur. In addition to thesedifferences in the nature of the exhaust gases, therange of exhaust temperatures strongly differ betweenthe two types of engine. For diesel exhaust, tem-peratures are on average in the range of 80180 Cunder the European urban driving cycle with somemaxima up to 230 C, while in the extra-urban part ofthe testing cycle a maximum temperature of 440 Cwas observed, typical temperatures being in the range180280 C [16]. This represents a serious problem,both in terms of need of activity at low temperaturesand the difficulty in de-sulphurisation of the catalysts,which generally requires temperatures above 650 C.

Let us now focus on the four-stroke spark ignitionengines equipped with TWCs. As above reported, therequired amounts of reducing and oxidising agents arepresent in the exhaust only under stoichiometric con-ditions. This leads to the typical dependency of theconversion patterns of the TWCs upon the A/F ratio(Fig. 2). Today the required conversion of pollutantsis greater than 95%, which is attained only when aprecise control of the A/F is maintained, i.e. within anarrow operating window. Accordingly, a complex in-tegrated system is employed for the control of the ex-haust emissions, which is aimed at maintaining the A/Fratio as close as possible to stoichiometry (Fig. 3). Toobtain an efficient control of the A/F ratio the amountof air is measured and the fuel injection is controlledby a computerised system which uses an oxygen ()sensor located at the inlet of the catalytic converter.The signal from this sensor is used as a feedbackfor the fuel and air injection control loop. A second sensor is mounted at the outlet of the catalytic con-verter (Fig. 3). This configuration constitutes the basisof the so-called engine on-board diagnostics (OBD).By comparing the oxygen concentration before andafter the catalyst, A/F fluctuations are detected. Exten-sive fluctuations of A/F at the outlet signal system fail-ure. This OBD arrangement implicitly assumes that a

Fig. 2. Effect of A/F ratio on the conversion efficiency of three-waycatalysts.

narrow A/F window at the stoichiometric point is thefingerprint of an effective TWC system.

The location of the catalytic converter is anothercritical point which determines the conversion effi-ciency. TWCs typically feature the so-called light-offtype conversion vs. temperature behaviour. Thiscurves is characterised by conversion which steadilyincreases from 0 to 100% conversion, the temperature

Fig. 3. Diagram of a modern TWC/engine/oxygen sensor ()control loop for engine exhaust control.


Fig. 4. Cumulative HC emissions measured during the federal testprocedure (FTP cycle) on an US 1995 car: (1) tailpipe emissionswith CCC; (2) engine-out emissions (after Ref. [11]).

of 50% of conversion being indicated as the light-offtemperature. TWCs are characterised by a light-offtemperature around 250350 C. This means thatan under-floor catalyst is heated above the light-offtemperature within 90120 s. In contrast, when thecatalyst is closely coupled to the engine (CCC) theheating time typically drops down to 1020 s. Thisdramatically affects vehicle emissions immediatelyafter the start-up of the engine (Fig. 4). As shown inthis figure, the ULEV (Californian ultra-low emis-sions vehicle) limit (0.064 g HC emitted/km) is typ-ically surpassed within 40 s after the engine start-up(Fig. 4(2)) [11]. To avoid this situation an almostinstantaneous heating of the converter is requiredto achieve the required >9598% conversion. CCCsminimise the heating time, however, temperatures upto 1100 C are routinely met as a consequence of thislocation of the catalyst.

It must be realised that the latest US and Euro-pean legislation (EURO phase V and US TIER II)limits for automotive emissions require applicationof the CCCs and OBD technologies in order to meetthe emission standards. A high durability is also an

important requirement for present and future TWCs,for example a durability up to 120,000 miles of theconverter will be demanded by US tier II regulationsin 2004. It should be considered that if a significantpart of the vehicle fleet fails the periodical exhaustemission control test, converter replacement becomesmandatory for a vehicle manufacturer. Accordingly,an extremely efficient and robust catalyst is requiredfor future vehicle application. In summary, catalyticconverters suitable for 2005 and beyond must presentthe following characteristics:

High activity and selectivity (conversions >98%)which increases up to 99% for Californian SULEV(super ultra-low emission vehicle).

Very fast light-off (


Fig. 5. Diagram of a typical catalytic converter (1) and a metallic honeycomb (a monolith from Emitec GmbH; adapted with permission) (2).

steel container which incorporates a honeycombmonolith made of cordierite (2MgO2Al2O35SiO2)or metal [9]. Although this aspect is sometimes ne-glected in the scientific literature, in must be under-lined that the choice and geometrical characteristicsof the honeycomb monolith play a key role in deter-mining the efficiency of the converter. In fact, highconversion must be achieved in the converter andtherefore the catalyst works under conditions wheresevere mass and heat transfer limitations apply. Typi-cally, both metal and ceramic monoliths are employednowadays. The major advantage of the metallic sub-strate is that the wall thickness is limited by the steelrolling mills capabilities, not strength. In a typicalautomotive 400 cell/in.2 application, the frontal flowarea in a ceramic monolith is 69% open (31% closed),while the metallic version has 91% open area. This isdue to the higher wall thickness of ceramic monoliths(0.007 in. (0.178 mm)) compared to metallic ones(0.002 in. (0.050 mm)) [17,18]. However, even in thisfield there has been a strong improvement of the tech-nology, cell densities as high as 900 cell/in.2 or evenhigher are now commonly available on the market forboth types of monoliths [19]. Traditionally, cordieritemonoliths have been employed quite extensively, pri-marily due to their lower production cost. However,a major advantage of the metal monoliths resides intheir high thermal conductivity and low heat capacity,which allow very fast heating of the CCCs duringthe phase-in of the engine, minimising the light-offtime.

The monolith is mounted in the container with a re-silient matting material to ensure vibration resistance[10,20]. The active catalysts is supported (washcoated)

onto the monolith by dipping it into a slurry contain-ing the catalyst precursors. The excess of the depositedmaterial (washcoat) is then blown out with hot air andthe honeycomb is calcined to obtained the finished cat-alyst. This is clearly a very simplified and schematicdescription of the washcoating process as multiplelayer technology, or multiple catalyst-bed convertersare also employed [10,21]. The exact method of depo-sition and catalyst composition is therefore highly pro-prietary and specific for every washcoating company.For example, the metallic honeycombs are non-porous,which makes adhesion of the washcoat difficult. Ac-cordingly a FeCrAl based alloy is employed, whichcontains up to 5 wt.% of aluminium; after an appro-priate pre-treatment this element then acts as an an-choring centre for adhesion of the washcoat [19].

However, there are some common components,which represent the state-of-art of the washcoatingcomposition:

Alumina, which is employed as a high surface areasupport.

CeO2ZrO2 mixed oxides, principally added asoxygen storage promoters.

Noble metals (NM = Rh, Pt and Pd) as activephases.

Barium and/or lanthana oxides as stabilisers of thealumina surface area.

3.1. Al2O3

The choice of Al2O3 as carrier is dictated by thenecessity of increasing the surface area of the honey-comb monolith which is typically below 24 m2 l1,


where the volume is that of the honeycomb [22]. Thisdoes not allow achievement of high NM dispersion.Alumina is chosen due to its high surface area andrelatively good thermal stability under the hydrother-mal conditions of the exhausts. In most of the studies-Al2O3 is employed due to its high surface areawith respect to other transitional aluminas [23], how-ever, also other high temperature aluminas such as -and -Al2O3 can be employed for high temperatureapplications such as in the CCCs because of theirhigh thermal stability compared to -Al2O3. Sincetemperatures above 1000 C can be met in the TWCs,stabilisation of transition aluminas is necessary to pre-vent their transformation to -Al2O3, which typicallyfeatures surface areas below 10 m2 g1. A numberof stabilising agents have been reported in the litera-ture, lanthanum, barium, strontium, cerium, and morerecently, zirconium oxides or salts being the most in-vestigated [2431]. These additives are impregnatedonto -Al2O3 or, sometimes, solgel techniques areemployed to improve the stability of the surface area.The exact mechanism by which these additives sta-bilises transitional aluminas strongly depends on theamount of the stabilising agent and the synthesis con-ditions. This is exemplified in Fig. 6 for BaO dopedaluminas; BaO and lanthana are the most used andeffective stabilisers.

The effectiveness of each dopant on the stabilisationof alumina is difficult to predict, due to the variabil-ity of the factors involved in the synthesis. For exam-

Fig. 6. Effect of synthesis method and BaO content of the stabilityof BET areas of Al2O3 after calcination at the indicated temper-atures for 3 h. SG: solgel synthesis method; C: co-precipitatedsample (after Ref. [29]).

ple, CeO2 was shown to thermally stabilise Al2O3, themaximum stabilisation effect being attained at a CeO2level of 5% [32]. However, Morterra et al. [33,34]found that little stabilisation of BET areas is attainedby adding CeO2 to -Al2O3, even though significantmodification of surface properties were detected. Infact, using CO as a surface probe molecule of thesurface Lewis acidity of the CeO2Al2O3 mixed sys-tems, it was revealed that CeO2 accumulates prefer-ably on the flat patches of low-index crystal planes ofthe spinel structure, and that the presence of Ce cationsstabilises, also at high temperatures, the most acidicLewis centres. A possible rationale may be given bythe recent observation that very efficient stabilisationof Al2O3 by addition of CeO2 is achieved under re-ducing conditions compared to the oxidising ones, dueto formation of CeAlO3 [31]. Apparently, the stabili-sation effect is more pronounced as long as dispersedCe3+ species are present at the Al2O3 surface. Thepresence of such species has long been detected inCeO2Al2O3 provided that low CeO2 loading is em-ployed [35,36]. It is conceivable that CeO2 stabilises-Al2O3 in a similar fashion to La3+, i.e. formationof a surface perovskite-type of oxide LaAlO3 [24],which may account for the conflicting observationsreported in the literature. Under high temperature ox-idising conditions, partial re-oxidation of Ce3+ sitesmay occur, with formation of CeO2 particles whichtends to agglomerate and grow over the Al2O3 sur-face, making stabilisation ineffective.

Use of ZrO2 has also been reported to effectivelystabilise -Al2O3 at high temperatures [25]. In thiscase, however, the stabilisation of Al2O3 seems tobe related to the ability of ZrO2 to spread over theAl2O3 rather than formation of mixed oxides. Eventhough formation of ZrO2Al2O3 solid solution hasbeen sometimes claimed, separation into ZrO2 andAl2O3 occurs upon high temperature calcination, asdictated by the phase diagram [37]. The effectivenessof ZrO2 in improving the thermal stability of Al2O3surface area seems remarkable as surface areas ashigh as 50 m2 g1 were observed after calcination at1200 C [25]. Interestingly, ZrO2 appears to be moreeffective than CeO2 in stabilising Al2O3; consistently,ZrO2-rich CeO2ZrO2 mixed oxides more effectivelystabilised Al2O3 compared to CeO2-rich systems [30].

The overall picture concerning the developmentof Al2O3-based supports is that, at present, thermal


stability of the Al2O3 support is not an importantissue for the next generation of TWCs in that theprogress obtained so far makes these stabilised sup-ports suitable even for high temperature applicationssuch as in CCCs.

3.2. CeO2ZrO2 mixed oxides

The beneficial effects of CeO2-containing formula-tions of the TWC performances has long been recog-nised [38]. Many different promotional effects havebeen attributed to this component, such as the abilityto:

promote the noble metal dispersion; increase the thermal stability of the Al2O3 support; promote the water gas shift (WGS) and steam re-

forming reactions; favour catalytic activity at the interfacial metal-

support sites; promote CO removal trough oxidation employing a

lattice oxygen; store and release oxygen under, respectively, lean

and rich conditions.

A detailed discussion of these roles and their relativeimportance is beyond the scope of this work and forthis we refer the reader to earlier literature [4,12,13].

Among the different roles of CeO2 in TWCs, theOSC is certainly the most important one, at least fromthe technological point of view. In fact, as above dis-cussed, the OBD technology is based on monitoringof the efficiency of the OSC. This is due to the factthat unambiguous relationships between the TWC ac-tivity and OSC performances have been established[39]. For this reason, we will principally discuss ther-mal stability and the OSC property of the CeO2ZrO2mixed oxides, even though the reader should be awarethat a variety of complex phenomena occur under thereal exhaust conditions, originated mainly by the in-teraction of the NM- and CeO2-based materials.

Starting from 1995, CeO2ZrO2 mixed oxides havegradually replaced pure CeO2 as OSC materials inthe TWCs [40], even though some low purity CeO2materials may be employed for less demanding TWCtechnologies [41]. The principal reason for the intro-duction of CeO2ZrO2 mixed oxides in place of CeO2is due to their higher thermal stability, as exemplifiedin Fig. 7, which reports the OSC and BET area of

Fig. 7. Effect of CeO2 content on the surface area stability anddynamic-OSC of CeO2ZrO2 after calcination at 900 C. OSCmeasured at 400 C by alternatively pulsing 2.5% O2 in He and5% CO in He over the catalyst (after Ref. [42]).

CeO2ZrO2 mixed oxides as a function of CeO2 con-tent [42]. Clearly, there is an important improvementof both OSC and BET area as soon as ZrO2 is in-serted into the CeO2 lattice. At first glance, there ap-pears to be a straightforward indication in Fig. 7, thatis, CeO2-rich compositions (around 6070 mol%) arethe most effective OSC promoters for TWC applica-tion. Unfortunately, this is a very simplified view ofthe problems related to the use of CeO2ZrO2 mixedoxides in the TWCs, the real situation being muchmore complex, as described below.

3.2.1. Thermal stability of CeO2ZrO2 mixed oxidesThermal stability of the TWCs has always been a

major issue in the development of the TWCs. The in-crease of the cruised mileage of passenger cars andhigher exhaust temperatures observed nowadays com-pared to past [1], demanded for higher and higher ther-mal stability of the washcoat and particularly of theCeO2 component. The relationship between the extentof surface area of CeO2 and the OSC property, as de-tected by temperature programmed reduction (TPR), iswell established (see Fig. 8 as an example). As belowdiscussed, the ability of CeO2 to undergo reduction,i.e. release of oxygen, at low temperatures (


Fig. 8. TPR profiles of CeO2 ((1) and (2)) and Rh/CeO2 ((3) and(4)) with surface areas of, respectively, 190 m2 g1 ((1) and (3))and


Fig. 9. Pore distribution in two samples of Ce0.2Zr0.8O2 as detected from N2 desorption isotherm using the BJH method: (A) surfacearea = 27 m2 g1; (B) surface area = 35 m2 g1.

CeO2-based technology, it is now recognised that un-doped CeO2ZrO2 do not present sufficient thermalstability for application on the 2005 type of TWC con-verters. In fact, thermal stability in excess of 1000 Ccannot be achieved by simple CeO2ZrO2 due to theirmetastable nature. As shown in the phase diagram re-ported in Fig. 10 [4750], there are metastable (t and

Fig. 10. Experimental phase diagram of the CeO2ZrO2 system(after Ref. [70]).

t) phases at intermediate CeO2 compositions, whichupon heating under oxidising conditions, lead to phaseseparation, CeO2-rich (cubic: c-Ce0.8Zr0.2O2) andZrO2-rich (tetragonal: t-Ce0.2Zr0.8O2) phases beingtypically obtained [51,52]. In principle, TWCs mustshow high durability; accordingly phase separation isconsidered an undesirable feature of the CeO2ZrO2component since it may lead to unpredictable vari-ations in the properties of the catalyst. By analogywith ZrO2, trivalent dopants, such as yttria and lan-thana, have been employed for the CeO2ZrO2 mixedoxides [5358]. However, to our knowledge no sys-tematic study satisfying the above reported criterionof comparable textural properties for comparison ofeffects of composition has been reported so far. Thismakes difficult a rationalisation of the data reportedin the literature on the effects of the doping agents.

Some qualitative and general comments can, how-ever, be made for CeO2ZrO2 systems: (i) CeO2ZrO2phase separation is favoured at the intermediate com-positions and it is retarded or even prevented by ad-dition of an appropriate low-valent dopant; (ii) phaseseparation is pronounced under oxidising conditionswhile under reducing conditions phase homogenisa-tion is favoured [51,52]; (iii) sintering with decreaseof surface area is very pronounced under reducing


conditions, particularly when compared to oxidisingones [59]. These general comments clearly point outthe critical need for further research aimed at a ra-tionalisation of the role of the dopant in affectingthermal stability of the CeO2ZrO2 mixed oxides, inorder to develop new products for high temperatureapplications.

3.2.2. Oxygen storage of CeO2ZrO2 mixed oxidesA remarkable property of the CeO2ZrO2 mixed

oxides compared to CeO2 is their ability to easilyremove bulk oxygen species at moderate temperatureeven in highly sintered samples. Thus the reductionpeak at approximately 900 C (Fig. 8, trace 1), whichis associated with reduction of CeO2 in the bulk shiftsdown to approximately 400 C when a 40 mol% ofZrO2 was inserted into the CeO2 lattice to prepare ahighly sintered Rh/Ce0.6Zr0.4O2 mixed oxide catalyst[60]. This was associated with the ability of ZrO2to modify the oxygen sub-lattice in the CeO2ZrO2mixed oxides, generating defective structures andhighly mobile oxygen atoms in the lattice which canbe released even at moderate temperatures [61,62].These early findings indicated that improved effi-ciency of the OSC property can be achieved by usingCeO2ZrO2 mixed oxides instead of CeO2 since, evenif the sample sinters under the high temperature reac-tion conditions, it should be more effective then CeO2due to the high oxygen mobility in the bulk; latticeoxygen species could effectively participate in redoxprocesses even under fluctuating exhaust feed-streamconditions. It is now well recognised that when theOSC property is investigated by the TPR technique,no appreciable distinction between the reduction inthe bulk and at the surface can be observed in theCeO2ZrO2 mixed oxides. Both reduction at the sur-face and in the bulk proceed with similar energeticsand occur at mild temperatures [63,64]. Typically, asingle peak reduction profile centred around is 500 Cis obtained for a single phase CeO2ZrO2 solid solu-tion, the presence of multiple peaks being taken as anindication of presence of phase impurities [40]. How-ever, the changes in the TPR behaviour are even moresubtle because other factors such as textural propertiesand even the pre-treatment can affect the TPR profile[6568]. For example, a combination of TPR followedby mild oxidation leads to reduction phenomena oc-curring at low temperatures [65], whereas when a

high temperature (severe) oxidation is included as apre-treatment, these low temperature processes re-versibly shift to high temperatures [66,67]. There hasbeen some debate as to whether migration of oxygenspecies in the bulk is limiting the rate of the reductionof the CeO2ZrO2, or whether the kinetics of redoxphenomena are rather dictated by surface properties[45]. However, recent findings confirmed the impor-tant role of the bulk properties for redox phenomenaoccurring at low temperatures [68,69].

As above indicated, the TPR technique has beenroutinely applied to investigate the redox propertiesof the CeO2ZrO2 mixed oxides. It should be noted,however, that under real exhaust conditions, the value oscillates between the oxidising and reducingconditions with a frequency of about 1 Hz. In princi-ple, this makes the so-called dynamic-OSC more use-ful compared to the TPR technique [12,71], since thistechnique allows detection of the oxygen available forredox processes on a time scale of seconds. In fact, itshould be noted that even favourable TPR profiles, i.e.featuring reduction peaks at low temperatures, maynot necessarily be associated with effective OSC dueto occurrence of in situ deactivation phenomena onincreasing the temperature of the measurement [72].However, a very recent report suggested that correla-tion between TPR profiles and effective dynamic-OSCexists in that texturally stable samples featuring aTPR behaviour independent of the pre-treatment, e.g.a mild or severe oxidation, are those giving the stableand effective dynamic-OSC, minimising the deactiva-tion phenomena [73]. This confirms the importanceof the stabilisation of the textural properties in theCeO2ZrO2 mixed oxides and the need for furtherimprovement of the thermal stability compared totypical temperatures so far investigated (1000 C).

3.3. Noble metals

Obviously, NMs represent the key component ofthe TWC, as the catalytic activity occurs at the no-ble metal centre. However, we purposely discuss theaspects related to the NM at this point, since its inter-action with the various components of the washcoatcritically affects the activity of the supported NM. Inprinciple, the first aspect to be considered is the choiceof the NM and its loading in the washcoat. Rh, Pd anPt have long been employed in the TWCs and there is


a general agreement about the specificity of Rh to pro-mote NO dissociation, thus enhancing the NO removal[4,6,74,75], even if alternative mechanistic pathwaysfor NO reduction have also been proposed [7,76,77].Pt and Pd are considered as metal of choice to promotethe oxidation reaction, even though Rh also has a goodoxidation activity. In particular, besides some initialuse in 19751976, Pd has extensively been added toTWC formulations starting from mid-1990s due to itsability to promote HC oxidation [10,11]. In fact, bet-ter A/F control [78] and modification of the supportprovided high NOx conversion, comparable to the tra-ditional Rh/Pt catalyst [79]. The increase of the useof Pd in the TWC technology adversely affected Pdmarket price, which is now comparable to that of Pt.

In fact, there is a large demand for Pd due to the factthat the straightforward way to increase the efficiencyof the TWCs at low temperatures is that of increasingthe NM loading, and particularly that of Pd, which forlong was the cheapest NM among the three employed(Fig. 11). On the other hand, use of high NM load-ing may favour sintering at high temperatures, lead-ing to deactivation of the TWCs, in addition to thefact that cost-effective TWCs are required by the mar-ket. In summary, the choice and loading of the NM isa compromise between the required efficiency of theconverter and the market price of the NM; ideally acar maker would prefer to have available a choice ofTWCs with different formulations, which would allow

Fig. 11. Effect of NM loading on the light-off temperature in CCCs.

a selection to be made according to price fluctuationsof the NMs.

3.4. Deactivation of the TWCs

Generally speaking, sintering of NM, leading to de-crease of the number of active sites, is a major pathwayfor the deactivation of TWCs. In addition to sintering,poisoning of the catalyst may contribute to their deac-tivation. The latter phenomenon is essentially relatedto the mileage travelled, quality of the fuel and the en-gine lubricating oil [22]. However, there are a numberof other routes which can contribute to deactivation ofthe TWCs: (i) sintering of the OSC promoter leadingto loss of OSC and, possibly, to encapsulation of theNM [80]; (ii) sintering of Al2O3 and, more important,deactivation of Rh due to migration of Rh3+ into thealumina lattice [22]. The comprehension of the relativeimportance of the different deactivation phenomenais difficult due to the variability of the reaction con-ditions, TWC preparation methods, etc. For example,when NM are supported on CeO2ZrO2 mixed oxidesand aged at high temperatures under redox conditions,encapsulation of Pd and Rh within the pores of thesupport occurs, while it does not occur for Pt [81].

Although it has received relatively scarce con-sideration [82], the issue of sulphur poisoning ofTWC needs some consideration. As above discussed,inclusion of CeO2-based promoters into the washcoat


considerably enhances the conversion efficiency of theTWCs. On the other hand, both CeO2 and ZrO2 areknown to easily adsorb SOx species: sulphated ZrO2is a well-known solid acid catalyst [83], whereas CeO2is used as a DeSOx catalyst in cracking processes [84].Investigation of reduced and oxidised CeO2 revealedthat SO2 is adsorbed under various forms, with bothsurface and bulk-type of sulphates being observed[85,86], and it may even modify the microstructure ofthe CeO2-based oxide [87]. Curiously, under oxidis-ing conditions bulk sulphates decomposed by 600 C,whereas surface sulphates persisted up to 700 C [85].Use of reducing conditions in the presence of H2favours elimination of sulphates as H2S, which can beeasily detected as rotten-egg odour [88], particularlyin the presence of noble metal [89]. CO also promotesthe reduction of sulphates to reduced oxy-sulphurspecies which unexpectedly increased the redox ca-pability of the sulphated Pd/CeO2 system comparedto the sulphur-free analogue [90,91]. However, it wasalso observed that the OSC of CeO2 is detrimentallyaffected by the presence of SO2, while addition ofZrO2 to CeO2 increases the resistance of CeO2 tosulphur poisoning, although more sulphur is adsorbedat the surface [82]. This may be associated with thegenerally higher OSC efficiency of the CeO2ZrO2mixed oxide compared to CeO2 and the possibilitythat ZrO2 acts as a sulphur scavenger. Ni containingoxides are sometimes added to the washcoat in theUSA as sulphur scavengers, while their use in Europeis prohibited. In summary, adsorption of sulphur onthe NM/CeO2ZrO2-containing TWCs is rather com-plex and appears to be structure/adsorption conditionssensitive, which readily explains some contradic-tions in the literature. In terms of inhibition of thethree-way activity it seems, however, that the issueof sulphur poisoning is much less stringent as com-pared, for example, to lean-DeNOx catalysts, due tothe high temperatures achieved in the TWCs, whichallow release of sulphur under driving conditions.

4. Future trends

4.1. Engine start-up emissions

As above discussed, TWCs represent a quite ma-ture, highly effective technology for pollution abate-

ment which, however, has some inherent limitationswhich need further improvement and development.These aspects are essentially related to: (i) low activ-ity at low temperatures (start-up of the engine) and(ii) use of stoichiometric A/F. As far as the first aspectis concerned, it should be noted that roughly 5080%of HC emissions during the test procedures are emit-ted before the TWC reaches the light-off temperature.When, in recent years, the emissions limits have beenpushed down, it appeared clearly that minimisationof warm-up HC emissions was a major problem inautomotive pollution abatement. This issue was beentherefore addressed by introduction of the CCCs ontothe market. This required development of TWCs fea-turing thermal stabilities well above 1000 C [92].

In reality, the issue of the start-up emissions can beaddressed by different approaches, some of which arelisted in Fig. 12.

A first possibility is that of collecting the HC emit-ted during the warm-up of the converter in a HC trap,typically composed of hydrophobic zeolite. In an op-timal trap, HC are trapped at low temperatures and asthe temperature is increased above 250300 C, HCare released and converted on the TWCs [92]. A suit-able trap must also feature very high thermal stabil-ity under hydrothermal conditions, which often is notthe case for zeolite-based systems. We recall that tem-peratures as high as 850900 C may be reached inthe under-floor catalysts. While such systems are stillunder investigation, alternative approaches have beenindicated [92,93]. It must be recognised that to min-imise the emissions, the catalysts must be heated-upin a minimum time. This can be achieved, for exam-ple, by electrically (Fig. 12) or combustion/chemicallyheated catalysts. In the latter case hydrogen and oxy-gen, or CO-rich feed is flowed over the catalysts [93].Oxidation of both CO and H2 are easy and exother-mic reactions, which occur at low temperatures overthe TWCs [94], leading to rapid heating of the cata-lyst. However, storage of H2 on the vehicle or use ofrich A/F which generates high CO and H2 emissions,brings complexity to the de-pollution system. Largeamounts of HC are in fact emitted at rich A/F, whichrequire an additional HC trap.

Use of complex technology clearly pushes-up costswhile the simplest technology is desirable. Accord-ingly, there has been a strong effort aimed at improv-ing the thermal stability of the washcoat [42]. With


Fig. 12. Some strategies for the abatement of engine start-up emissions.

the availability of thermally stable washcoats, appli-cation of a start-up converter, i.e. converter that isclosely coupled to the exhaust manifold, became fea-sible. This converter allows extremely rapid heating ofthe catalyst, leading to enhanced conversions duringthe warm-up of the engine. Metallic converter can beeasily shaped into the exhaust manifold and are veryconvenient for such application also due to their lowheat capacity. In general, the composition of the CCCis related to that of the typical TWC in that NM met-als and particularly Pd are employed to promote HCconversion. The OSC promoter may be omitted fromthese formulations since it promotes CO conversion,leading to local overheating because of this highlyexothermic reaction [92]. On the other hand, for thepurpose of the OBD II technology, there is a necessityto monitor the OSC efficiency from the start-up of theengine. Accordingly, ZrO2rich doped CeO2 promot-ers with very high thermal stabilities [42,73,95], areoften added to this catalyst.

An alternative approach is that of developing newcatalysts showing high conversion efficiency at low,

nearly ambient, temperature [93]. A large part of theseinvestigations have been triggered by the observationby Haruta et al. [96,97] that gold catalysts are ableto efficiently oxidise CO even at subambient tempera-ture provided that nano-dispersed Au particles are pre-pared on the support. Thus, light-off temperatures inthe conversion of the exhausts as low as 100 C couldbe achieved by depositing small Au particles on re-ducible oxides such as CeO2 and TiO2 [98]. However,the durability of gold catalysts under harsh conditionsis still an issue, significant deactivation of cobalt ox-ide promoted Au catalysts was observed already after157 h of reaction at 500 C under simulated exhaust[99]. There is in fact a flourishing activity in the field oflow temperatures catalysts [97,100,101], other noblemetals, in addition to Au, being effective in low tem-perature oxidation reactions, provided that appropri-ate synthesis methodology is employed [100]. To ourknowledge, however, due to the nano-dispersed natureof these catalysts, the issue of thermal stability, even atmoderately high temperatures has not been solved asyet. Supported metal nano-particles are, in fact, quite


mobile on the surface, even at ambient temperaturein the case of gold [101], which makes prevention ofsintering phenomena difficult. We believe that thermalstabilisation of nano-dispersed metals may representa new breakthrough point in the development of theseenvironmental catalysts.

4.2. Lean DeNOx catalysts

The recently approved California legislation on au-tomobile fuel consumption has prompted the neces-sity of developing new and more effective catalystscapable of removing NOx even in excess of O2. Asindicated in Section 2, lean-burn gasoline and dieselengines, due to the high A/F used in the combustionprocess, can achieve significant fuel savings, however,under these conditions no TWC is effective in reduc-ing NOx due to the excess of O2, which is competingfor the reducing agent, in particular CO.

Studies on NOx removal under oxidising conditionswere triggered by the discovery in 1991 that HCs couldact as selective reducing agents under excess of O2[102]. This discovery was followed by a feverish ac-tivity in the field of lean-DeNOx and more then 50 cat-alysts were reported in 19911992 [103]. Since thenthis topic has been reviewed by several researchers,even though a comprehensive knowledge of the ex-haust lean technology is still missing [2,5,102116].As outlined in a report issued by MECA (Manufac-turers of Emissions Controls Association) [117] thereare two major strategies to control the NOx emissionsunder oxidising conditions:

DeNOx (lean-DeNOx) catalysts; NOx adsorbers (NOx traps).

Fig. 13. Typical light-off behaviour for: (1) C3H6NOO2 and (2) C3H8NOO2 reactions over Pt/Al2O3. Pt: 1 wt.%, reactant feed:1000 ppm C3Hx , 500 ppm NO, 5% O2. W/F = 4 104 g min ml1 (GHSV 72,000 h1) (after Ref. [121]).

The former strategy employs a direct NOx reductioncatalyst, usually consisting of Pt/Al2O3 and a metal-loaded zeolite for NOx reduction at, respectively, lowand high temperature. The NOx adsorber technologyis sometimes called a NOx storage/reduction (NSR)catalyst. In this case typically a Pt/BaO/Al2O3 cata-lyst is used to store NOx under oxidising conditionsas adsorbed nitrate species, which are then releasedand reduced on a traditional TWC by temporarilyrunning the engine under rich conditions. Let us nowexamine in some detail these systems.

4.2.1. Pt/Al2O3 and derived systemsThe activity of Pt/Al2O3 catalysts for NOx

reduction under lean exhaust conditions has been in-vestigated in detail by Burch et al. [118136]. Theyextensively analysed the effects of the nature of thenoble metal, reducing agent, sulphur addition, na-ture of additives and of the support, and reactionmechanism.

A typical reaction profile for HC reduction overPt/Al2O3 is reported in Fig. 13. This figure sum-marises some general features of Pt/Al2O3 lean-burncatalysts, that is: (i) a maximum of NO conversion atrelatively low temperature, NOx conversion peakingas the HC conversion reaches 100% in the case ofC3H6; (ii) comparable starting temperatures for theNO reduction and HC oxidation; (iii) significant NO2formation at high temperatures when all the HC isburned-out; (iv) strong sensitivity of the NO conver-sion to the nature of the reducing agent (saturatedvs. unsaturated HC); (v) poor selectivity towardsdi-nitrogen formation of the Pt catalyst, N2O beingthe major product at low temperatures. As stated atpoint (iv), this general behaviour strongly depends on


the nature of the reducing agent: non-reactive (shortchain-saturated) HCs behave somewhat differentlyfrom reactive HCs such as alkenes or long-chain HC[127].

Let us discuss some important aspects of the abovequoted properties of the Pt lean-DeNOx catalysts.

4.2.1.1. Light-off behaviour of the Pt catalysts. NOconversion typically shows a volcano shaped curve ir-respective of the nature of the HC. This is due onone hand to the fact that NOx always initiates to-gether with the HC oxidation and on the other hand tothe fact that, with few exceptions, the maximum NOconversion corresponds to 100% HC conversion. Athigher temperatures, when all the HC is consumed,significant NO2 formation occurs by reaction of NOwith excess O2, the amount of NO2 formed being lim-ited by thermodynamic constrains. Note, however, thatNO2 concentration that apparently exceeded thermo-dynamic values were also reported for an Ag/Al2O3system [137]. This fact was attributed to the presenceorgano-nitrite species as intermediates of the SCR pro-cess, which oxidation/decomposition led to NO2 pro-duction.

The maximum of the NOx conversion is obviouslyaffected by the space velocity (SV/GHSV). For a typ-ical light-off curve, the reaction rate is kinetically lim-ited only below the light-off temperature, while abovelight-off the reaction rate is limited either by heat ormass transfer. As a consequence, the maximum of theNOx conversion moves to low temperature and its in-tensity increases as SV decreases, however, this ef-fect is not directly proportional to the decrease of theSV. Thus the maximum of the NO reduction peakmoved from 200 C (conversion 70%) to 215 C (con-version 47%) as the SV was increased from 10,000 to50,000 h1, while at intermediate SV = 25,000 h1a conversion of 57% was observed at approximately210 C [16]. From a practical point of view, exhaustemissions of a diesel engine were characterised bySV = 28,00088,000 h1 as the temperatures variedbetween 160 and 400 C during the MVEuro2 drivingcycle [16].

A corollary of this fact is that any comparison ofactivities simply based on light-off activities may bestrongly misleading. Differences in light-off tempera-tures are in fact related to the number of active sites(Pts), total flow-rate (F) and kinetic law for the reac-

tion by the following equation [138,139]:1T2 1

T1=(R

Ea

)ln

f2(c)Pts F2f1(c)Pts F1

(1)

For Eq. (1), a rate equation of the type r = k(T )f (c)is assumed, where k(T) is the rate constant for NOconversion, which depends on the temperature; andf(c) represents the remaining factor of the rate equa-tion, which depends on surface coverage and is inde-pendent of temperature [138,139]. An inspection ofEq. (1) reveals that upon comparison of the activitiesof two catalysts with different dispersions (i.e., thereare two different Pts terms) under equivalent reactionconditions (i.e., F1 = F2 and f1(c) = f2(c)), thehigher the number of active sites the lower the light-offtemperature. This means that the specific activity oftwo catalysts may be compared using the light-off be-haviour only when an equal number of active sitesis employed in the experiment. Unfortunately, veryfew investigations concerning lean-DeNOx have re-ported reaction rates, light-off temperature being gen-erally shown, which makes direct comparison difficult.Note that when long-chain HC are investigated, whichcan easily generate coke at the catalysts surface, thelight-off behaviour may not be relevant to the true cat-alytic activity as pseudo-steady states were observedbelow light-off temperature for short to medium peri-ods (0.12 h) followed by a deactivation of the catalyst[131]. It is important to keep present these limitationswhen activity from different sources is compared.

Reaction rates and kinetic law for C3H6/NO/O2 re-action were reported in the literature for Pt/Al2O3and Rh/Al2O3 [140,141]. For Pt/Al2O3 the followingexpression were found experimentally at 230236 C(NO and C3H6 2504000 ppm, O2 0.512%, W/F =0.0018 g s ml1, GHSV 100,000 h1, apparent ac-tivation energy 243 kcal mol1):

reduction : r(NO) = kred [O2][C3H6]0.5

oxidation : r(C3H6) = kox [O2][NO]0.5[C3H6]0.5For comparison, the kinetic expression for the com-bustion of C3H6 measured at 158 C was

combustion : r(C3H6) = kcomb [O2][C3H6]0.5


The apparent activation energies for NO reduc-tion and C3H6 oxidation were measured as 22 and24 kcal mol1 over Pt (1.5 wt.%)/Al2O3 [141]. Inter-estingly, NOx , even if present at ppm level, interfereswith the HC oxidation. We recall that modification ofCO oxidation kinetics in the presence of NO is typi-cally assumed as a direct indication that NO removaloccurs through the NO/CO reaction over TWCs [4].In summary, independently of the reaction mecha-nism, the key factors in the light-off behaviour of Ptlean-DeNOx catalysts is that a narrow maximum ofactivity is observed around 270300 C. The widthand position of this maximum of activity depends onthe reaction conditions and nature of the catalyst. Asis shown below, also the nature of the HC also playsa fundamental role in the volcano shaped NOx con-version curve because of the different HC reactivity.

4.2.1.2. The effect of the reducing agent and promot-ers on the reaction mechanism. The effect of the na-ture of the reducing agent was investigated in detail byresearchers from Degussa (OMG) [142]. Even thoughthe nature of the catalyst employed was not speci-fied, except that a 50 g/ft3 Pt honeycomb catalyst wasused, the finding of this paper represent typical resultsobserved over Pt/Al2O3 in subsequent papers. Thesefindings can be summarised as follows:

There is a remarkable difference in the responseof the Pt catalyst to the nature of the HC reducingagent, e.g. the temperature of the maximum of HCand NO conversion is nearly independent of the typeof linear alkene, while an increase of the length of

Fig. 14. Proposed reaction mechanism for lean-DeNOx reaction over Pt/Al2O3 for alkane (1) and alkene (2) conversion (after Ref. [121]).

the chain of the alkane significantly shifts down thistemperature.

At comparable HC chain length saturated HC aremuch less effective compared to unsaturated ones.Remarkably C16H34, which is a typical diesel fuelcomponent, featured an activity comparable to thatof C3H6.

Use of alcohols resulted in high activity at low tem-peratures, the nature of the alcohol affecting the ac-tivity to a lesser extent compared to alkanes.

Use of aromatic compounds as reducing agentshowed a strong dependence of the conversionefficiency on the reactivity of the molecule.

The remarkable difference of the catalytic behaviourbetween the saturated and unsaturated HCs was ratio-nalised in terms of the different reaction pathway forthe NO reduction in the presence of either saturatedor unsaturated HC (Fig. 14) [121,125]. It appears thatreduction of the unsaturated HC can be depicted as aNOx dissociation reaction occurring the metal centre,where the unsaturated HC is responsible for the re-moval of the adsorbed oxygen generated by the NOxdissociation. In the presence of weakly adsorbed re-ducing agent, such as C3H8, adsorbed atomic oxygenis the dominant species on the metal surface under re-action conditions. The C3H8 oxidation is inhibited byboth O2 and in the facile oxidation of NO to NO2.It is believed that the rate determining step in C3H8oxidation by O2 is the dissociative chemisorption ofC3H8 involving the breaking of a CH bond [123,125].This is a difficult reaction and strongly depends onthe nature of the HC, which accounts for the strong


effect of the carbon chain length upon the DeNOxactivity.

In addition, a comparison of different Pt/Al2O3 cat-alysts promoted by metal oxides (Ba, Ce, Co, Cs, Cu,K, La, Mg, Mo, Ti) or noble metals (Ag, Au, Pd, Rh)in the lean NOx reaction, using C3H6 as a reducingagent, showed that even if the promoters have a signifi-cant effect (beneficial or otherwise) on the activity andtemperature range of operation of Pt/Al2O3, they haveno significant effect on the N2/N2O selectivity. Thesimilarity in behaviour of the promoted catalysts andunpromoted Pt/Al2O3 suggests that the reaction mech-anism was similar for all the catalysts tested [124]. Theability of the Al2O3 support to promote the interactionof the adsorbed/migrated NOx /CxHy species gener-ated from saturated HC, seems a particular propertyof this support, since changing the support to SiO2 re-sulted in almost no reaction even with comparable Ptloading and dispersion. Finally, a comparison of differ-ent HCs as reducing agent revealed an unusual abilityof aromatic HC (toluene) to promote high selectivityof the Pt catalysts towards formation of N2 [122]. Itshould be also noted that when long-chain HCs areemployed, the light-off activity may be very differentfrom steady-state activity in that below or close tolight-off temperature (generally chosen as 50% of con-version), catalysts poisoning with time-on-stream wasobserved [127]. By increasing the reaction tempera-ture conversion of adsorbed carbonaceous species oc-curred, thus recovering the catalyst activity [130]. Nodeactivation was observed with small HC, formationof carbonaceous species being minimal in thiscase.

The effect of addition of SO2 to the reaction feedappears in line with the above interpretation of theeffect of the HC in that strong poisoning of activitywas observed when C3H8 was employed as reducingagent due to the formation of aluminium sulphate.This is responsible for the poisoning of the catalyticsites at the support [128]. In contrast the activitywas much less affected in the case of C3H6, whichis in line with the smaller sensitivity of the Pt par-ticles towards sulphur poisoning compared to thesupport.

Even though this reaction mechanism was ques-tioned, in particular due to the lack of XPS evidencefor reduced Pt sites [143], we believe that the ma-jor findings and suggestions reported by Burch and

co-workers still provide an important guide-line fordevelopment of a new generation catalyst. In fact, de-spite the intensive research in the literature, the effectof the nature of the support has little been investigated[124,126,143145], and most of the studies were car-ried out using C3H6 as reducing agent, where little orno effect of the support should be expected. In fact, webelieve that in this case the major role of the supportis to affect the Pt dispersion rather then other effects,thus modifying the activity of the supported metal.When saturated HCs are included in the feed, effectsof support composition were detected [143], however,there is not enough evidence on a clear effect of thedifferent supports in promoting Pt activity in lean-DeNOx .

4.2.2. Other lean-DeNOx catalystsNumerous other catalytic systems have been investi-

gated as DeNOx catalysts, however, they appear muchless applicability as next generation catalyst due togenerally low activity and/or stability of such systems.For sake of convenience we will group all the catalystsin the following categories:

Cu-ZSM5 and related systems; metal oxide catalysts.

As above written, Cu-ZSM5 represented the firstmajor candidate for lean-DeNOx catalysts, eventhough it was quickly recognised that several prob-lems affect the Cu-zeolite catalysts [103]. Theseinclude: (i) poor hydrothermal stability of Cu speciesand zeolite framework; (ii) appreciable activity onlyat high temperature (300400 C), which only allowsthe use of such system in conjunction with NM lowtemperature catalysts; (iii) generally poor activityunder real exhaust and at high space velocities. Thehigher range of reaction temperatures compared toPt catalysts makes these systems of potential interestfor lean-burn gasoline engine, where such high tem-peratures are more easily met compared to the dieselengine.

With the aim of improving the thermal stabilityof the zeolite catalysts, a variety of supported andeven unsupported metal oxides have been investi-gated. ZrO2 itself was chosen as a possible candidate.Cu/ZrO2 and other supported metal oxides, both sul-phated and sulphur-free, have been investigated tosome extent with the aim of improving the thermal


stability of the catalyst compared to the zeoliticsystem [146154]. Even though some interestingactivities were claimed, to our knowledge, there isnot sufficient evidence for possible application ofsuch systems under real exhaust conditions. In fact,even though some increase of thermal/hydrothermalstability could be achieved, the activity is generallypoorer compared to the Cu-ZSM5 system. As a gen-eral comment, it seems that despite the Edisoniantype of research on different metal oxides as DeNOxcatalysts, the high activities sometimes claimedby the authors, withstand with difficulties the harshexhaust conditions. A relatively recent comparativestudy of several types of promising lean-DeNOxcatalysts under diesel conditions is very illustra-tive in this respect [155]: after testing nine differentclasses of catalysts (Pt/Al2O3, Rh/Al2O3, Ag/Al2O3,Pt-ZSM5, Cu-ZSM5, Pt/In-ZSM5, CeZSM5+Mn2O3,Co/Al2O3, and Au/Al2O3 + Mn2O3), the authorscame to the conclusion that despite the fact that somehigh activities could be measured, particularly on theNM-containing catalysts, there is no single phase cat-alyst capable of satisfying the practical demand forNOx removal from diesel exhaust.

Ag catalysts are also among those extensivelystudied since high activity were reported, particularlywhen alcohols are employed as reducing agents (see,for example [137,155165]). A general comment con-cerning the Ag based catalysts is that, due to the low

Fig. 15. Effect of Ag loading on the activity and product selectivity in Ag/Al2O3 catalysts (after Ref. [137]).

melting point of Ag, extensive sintering of the catalystmay be expected even at relatively low temperatures.The catalytic activity was shown to depend on theAg particle size [166], high particle sizes favouringthe unselective HC oxidation and N2O formation atlow temperatures (Fig. 15). On the contrary, highlydispersed Ag particles favour formation of N2 butthe reaction occurs at higher temperatures. This wasexplained by the different reaction pathway accordingto the nature of the supported Ag phase (Fig. 16).

The high sinterability of Ag may represent an im-portant drawback for practical application unless par-ticular synthesis methodology is employed [167,168].As shown in Fig. 15, high activity and N2 selectivityof the Ag catalysts is observed in the high rangeof temperatures; accordingly this catalyst may beconsidered as a substitute for the Cu-ZSM5 com-ponent in a full-range of temperatures operatinglean-DeNOx catalyst [2,169]. To promote the activityat low temperatures, Ag could in principle be sintered,however this promotes the unselective HC oxidation[166]. An interesting way to promote the activity ofthese catalysts is to add another NM to the system[170,171].

Recently, we have shown that use of ZrO2 orZrO2-rich CeO2ZrO2 mixed oxides as supports forAg strongly improves the activity at low tempera-tures (Fig. 17) [172]. As shown by comparison ofactivities of catalysts with different Ag dispersion


Fig. 16. Reaction mechanism(s) proposed for the lean-DeNOx reaction over Ag/Al2O3 catalysts (after Ref. [137]).

[172], the activity of Ag invariably occurs at lowertemperatures on ZrO2-containing supports comparedto Al2O3 supports. More important is that the useof ZrO2-containing supports remarkably facilitatesthe regeneration of the catalysts from SOx poisoningcompared to Ag/Al2O3.

4.2.3. Bi-functional lean-DeNOx catalystsThe idea of bi-functionality to improve the activity

of the lean-DeNOx catalysts has been pioneered byMisono and co-workers. These studies were recentlyreviewed [173], accordingly we refer the reader to thisreview. Of the several systems described, it is impor-

Fig. 17. Effect of the nature of the support on lean-DeNOx activityof Ag catalysts: NOx reduction (empty symbols), C3H6 oxidation(filled symbols). The 1000 ppm C3H6, 1000 ppm NO, 5% O2,W/F = 0.05 g s ml1 (after Ref. [172]).

tant to focus the mechanistic features of the authorswork. Essentially, the authors favour the reaction path-way which proceeds with formation of NO2 as reac-tion intermediate, which then efficiently reacts withadsorbed HC to give surface intermediates. These in-termediates then decompose leading to an overall re-duction of NOx . A redox type component such asMn2O3 or SnO2 is also added, which favours oxida-tion of NO to NO2 (Fig. 18).

An interesting aspect of these systems is that thepresence of water, which normally deactivates theDeNOx catalysts, can even improve the catalyticactivity (Fig. 19). This was attributed to the partialsuppression of the direct HC oxidation at Mn2O3(Fig. 18) that is responsible for non-selective HCoxidation.

Among the bi-functional systems, the NM/zeolitecatalysts, containing particularly H-ZSM5, shouldalso be quoted. A number of researchers have in-deed employed both Pt/ZSM5 and Pd/ZSM5 asbi-functional systems for the lean-DeNOx (see, forexample [147,173180]). Typically, such catalystsand particularly those Pd-based were employed forlean-DeNOx using CH4 as reducing agent. Thebi-functionality of this type of catalyst is related tothe necessity of acid sites, which apparently allow ac-tivation of the HC at the support leading to selectiveNOx reduction. In fact, using Na-ZSM5 as a support,no NOx reduction was detected. Several zeolites wereemployed for the CH4/NO/O2 reaction, however,


Fig. 18. Effect of addition of Mn2O3 (as physical mixture) on the DeNOx activity of Ce-ZSM5; and the proposed reaction mechanism(after Ref. [173]).

significant deactivation of the catalyst occurred in thepresence of water and SOx .

While a somewhat extensive description of the vari-ous attempts to develop efficient lean-DeNOx catalystsis reported here, it is important to outline that thesesystems do not ensure sufficient activity to foreseepractical applications in a future. Development of newbreakthrough strategies is an important target for thecomming years to achieve significant environmentalbenefits from the use of lean, high-efficiency engines.

Fig. 19. Effect of addition of H2O on the activity ofMn2O3/Sn-ZSM5 bi-functional catalyst (after Ref. [173]).

4.3. Lean NOx traps

The discovery by Toyota researchers of theso-called NSR catalysts also triggered feverish activ-ity in lean-DeNOx studies [181183]. The principleof the reaction mechanism of these systems appearswell established [183]. Under oxidising conditionsNOx are stored at the surface of a Ba-containing cat-alyst under various forms (surface nitrites/nitrates),which exact nature is still matter of debate [184187](Fig. 20). After a certain period, which length is animportant factor and is correlated to the specific emis-sion/catalyst characteristics, the A/F ratio is set torich and the stored NOx species are reduced over Ptor, more generally, TWC-type catalyst to N2 [183].

The mechanism of NOx adsorption and desorp-tion/reduction has been investigated by a number ofauthors [112,184,185,187197]. It appears now clearlythe storage/reduction is rather complex process due tothe complex nature of the exhaust mixture. For exam-ple, model studies performed on Pt/BaO/Al2O3 sug-gested that the first step is the oxidation of NO to NO2,which is active species being adsorbed on the surface[190,191], even though kinetic studies could not dis-tinguish whether surfaces nitrites are formed first andthen oxidised to nitrates or whether both species areformed directly by a disproportionation mechanism[194]. However, the final species that is strongly heldon the surface and accounts for the majority of NOx


Fig. 20. Principle of operation of an NSR catalyst: NOx are storedunder oxidising conditions (1) and then reduced on a TWC whenthe A/F is temporarily switched to rich conditions (2).

stored appears to be a nitrate species, in particular athigh temperature due to the low thermal stability ofnitrite. Whatever is the true mechanism, it must beunderlined that the kinetics and the extent of storageare heavily affected by the presence of water and CO2in the exhausts: CO2 slows down the NOx adsorp-tion kinetics as the reaction can more appropriately beseen as a transformation of surface carbonates into ni-trates, e.g. CO2 strongly competes with NOx for theadsorption sites [190,196]. This competition, on theother hand, increases the rate of NOx releases underthe rich-spike [188,189]. The effect of water is morecontroversial in that promotion of NOx adsorption wasobserved below 250 C by addition of small amountsof water (1%), whereas at higher temperature an inhi-bition effect was observed [194]. However, such pro-moting effect was not seen when both water CO2 wereco-fed.

The NSR technology is by far the most reliableand attractive lean-DeNOx technology and it has beencommercialised in Japan where low sulphur gasolineis available. In fact, the major drawback of the NSRcatalyst is its sensitivity to SOx due to the fact thatsurface sulphates are invariably more thermally stablecompared to the nitrates [198]. The durability aspectsof the NSR catalysts were addressed, for example,by researchers from OMG [199] and there seems to

be general agreement that poisoning of the NOx stor-age function is directly related to the amount of SO2passed over the catalyst. This is an important aspectsince it suggests that for application of these catalyststo US or European markets, where higher sulphur con-tents are present in the fuel compared to Japan, appro-priate strategies to develop sulphur resistant NOx trapmust be applied. In addition to the obvious require-ment of lowering of sulphur content in the fuel, thereare strategies that can be adopted for increasing sul-phur tolerance of the converter: (i) adoption of an SOxadsorber that protects the NOx trap and is periodicallyregenerated; (ii) modification of the catalyst compo-sition to promote of the removal efficiency of the ad-sorbed SOx . An interesting example of such strategieswas recently reported by Toyota [200,201]. In theirsystem, TiO2 was added to protect the barium-basedtrap from sulphur poisoning due to its high sulphurtolerance. LiO was added as it was observed thatLi-promoted Al2O3 releases accumulated sulphurmore easily compared to pure Al2O3. Rh/ZrO2 wasalso employed to enhance the sulphur removal underreducing conditions due to its effectiveness as a steamreforming catalyst. In fact, an efficient H2 generationunder the rich-spike of the cycle strongly favours theremoval of adsorbed SOx . It should be noted thatrelease of H2S from the catalysts is undesirable, ac-cordingly special schedules of the modulation of theA/F during the rich phases can be adopted, whichpump additional oxygen during the rich-de-sulphationphase minimising H2S release [202]. Finally, thermaldeactivation due to sintering of the barium speciesand formation of barium aluminates may representan issue in terms of durability of the catalyst [203].Accordingly, thermally stable Ba-containing materi-als, such as doped aluminas or perovskites, has beeninvestigated as NOx absorbers [184,185,204].

It is worth of noting that the NSR strategy has alsobeen applied to diesel engines. In this case generationof the rich conditions must be carefully consideredas switching A/F to rich conditions easily gener-ates typical black-smoke-containing emissions, oftenfound in older vehicles. Low temperature smokelesscombustion with a massive EGR or, more frequently,post-injection of fuel are employed to temporarilygenerate rich exhaust. The interesting point is that theNSR component may be deposited on the walls of theporous ceramic filter so that the precious metal can


Fig. 21. Engine-out and tailpipe-out emissions from a 1.9 L diesel engine over an NSR catalysts deposited on a ceramic filter. Notice thatsimilar efficiencies may be achieved over the NSR catalysts in the lean gasoline engines (after Ref. [205]).

contemporarily promote removal of the particulate[205]. Thus, very high pollutant conversion efficiency(ca. 80%) could be obtained in the engine test proce-dure (Fig. 21). Clearly, the issue of trap deactivationbecomes even more stringent in the case of dieselvehicles due to the generally higher amounts of sul-phur in the fuel and low operating temperatures thatdo not allow efficient trap regeneration. Rich condi-tions for several minutes and temperatures as high as650700 C are typically needed to achieve effectivetrap de-sulphurisation [202].

4.4. Selective catalytic NOx reduction using urea

Due to the limited success of HCs as efficient re-ducing agent under lean conditions, the use of ureaas an alternative reducing agent for NOx from heavyduty diesel2 vehicles has received attention. Selective

2 Light-duty (LD) diesel engines are generally defined as vehicleswith an engine displacement of less than 4 l and power outputof up to 100 kW, and are characterised by relatively high enginespeeds. LD engines would normally be found in passenger vehicleand light commercial vehicle applications. Heavy duty (HD) dieselengines may be generally defined as of displacement greater than8 l and power outputs of greater than 150 kW. HD engines arefound in heavy road transport, industrial and marine applications.Medium duty engines fill the gap in the middle and are found inmedium size trucks, buses and light industrial equipment.

catalytic reduction of NOx with NH3 in the presenceof excess O2 is a well implemented technology forNOx abatement from stationary sources [206]. Typi-cally, vanadia supported on TiO2, with different pro-moters (WO3 and MoO3) are employed in monolithtype of catalysts. A sketch of an arrangement for theurea based NOx abatement technology is shown inFig. 22. Typically, the urea solution is vaporised andinjected into a pre-heated zone where hydrolysis oc-curs according to the reaction:

H2NCONH2 + H2O CO2 + 2NH3Ammonia then reacts with NO and NO2 on the reduc-tion catalyst via the following reactions:

4NO+ 4NH3 + O2 4N2 + 6H2O6NO2 + 8NH3 7N2 + 12H2O

This approach has proved to be quite successfuland high NOx (up to 80%) could be achieved onHD under driving conditions, even after reasonablyhigh mileages (200,000300,000 km), the activity de-creased to about 7580% of the initial value after over500,000 km [207,208]. A major problem of such sys-tem is that extreme care must be exercised to developa suitable urea injection strategy that avoids over-loading of the system leading to ammonia slip [209].


Fig. 22. A typical arrangement for abatement of NOx from a heavy duty diesel engine using urea as reducing agent.

Typically, ammonia slip should not exceed 10 ppm.While the efficiency of urea-SCR technology is recog-nised [210], there are certainly still a number of issuesconcerning the catalyst efficiency at low temperatures,the design of compact converter systems that requirehigher conversion efficiency and, last but not least, theissue of generalised urea distribution when fuelling thevehicle.

Given the efficiency of such systems, the applica-tion of the urea-SCR technology to LD vehicles wasalso investigated [211]: while appreciable NOx canbe achieved, it must be recognised that a ratio of en-gine displacement-to-catalyst volume of 1:3 is typi-cally employed for the urea-SCR systems that mayrepresent a serious problem in compact LD vehicles.Clearly, an important improvement of the catalytic per-formances is needed before such systems can be ef-fectively considered for LDV application.

4.5. Particulate matter removal

Even though this topic is specifically related todiesel engines, the general interest of these sys-tems is related to the above quoted desire to usehigh-efficiency engines. Diesel particulate matter(DPM) is the most complex of diesel emissions. Dieselparticulates, as defined by most emission standards,are sampled from diluted and cooled exhaust gases.This definition includes both solids, as well as liquid

material which condenses during the dilution process.The basic fractions of DPM are elemental carbon,heavy HCs derived from the fuel and lubricating oil,and hydrated sulphuric acid derived from the fuelsulphur. DPM contains a large portion of the polynu-clear aromatic hydrocarbons (PAH) found in dieselexhaust. Diesel particulates contains small nucleiwith diameters below 0.04m, which agglomerateforming particles as large as 1m. The non-gaseousdiesel emissions are grouped into three categories:soluble organic fraction (SOF), sulphate and soot[212].

Removal of the liquid fraction of PM is generallyachieved by an oxidation catalyst. Oxidation catalystshave been fitted to US medium duty diesel vehiclessince 1994 to reduce emissions of HC, the SOF con-tent of DPM, and CO [212]. Typically, these catalystsare composed of NM/CeO2/Al2O3 (NM = Pt) sys-tems, where porosity of the catalysts often plays a keyrole since adsorption of the SOF at the support allowsits conversion at catalytic sites and hence its removalbefore its desorption starts [212]. This is a critical as-pect in the diesel exhaust removal due to the generallylow temperatures (120350 C) of the diesel exhaust[213]. In fact, during the test cycle, the temperatureof the catalyst may easily fall below the light-off tem-perature making necessary additions of an adsorbent,typically a zeolite. Oxidation catalysts promote theoxidation of HC and CO with oxygen in the exhaust to


form CO2 and H2O. Fuel sulphur levels of maximum500 ppm are required to avoid excessive productionof sulphate based PM and to minimise catalyst deacti-vation by sulphur poisoning. Lower levels of sulphur(50 ppm) can increase the effectiveness of oxidationcatalysts by up to 50% and contribute to greater dura-bility. Oxidation catalysts have not generally been usedin heavy vehicles with the exception of urban buses,and are not considered necessary to meet HC and COrequirements of future HD emission regulations.

Removal of soot may be achieved by means of fil-tration (Fig. 23) [214]. Even though different typesof filters can be employed [215], the filtration effi-ciency is generally high. However, the continuous useunder the driving conditions leads to filter plugging.Regeneration of the filter is therefore a crucial step ofthe soot removal systems. This can be achieved ther-mally, by burning the soot deposits on the filter, using,for example a dual filter systems such as depicted inFig. 23. However, such systems may be adopted onlyin the trucks where space requirements are less strin-gent compared to passenger cars. In addition, there areproblems arising from the high temperatures achievedduring the regeneration step when the deposited sootis burned off. In fact, local overheating can easily oc-cur leading to sintering with consequent permanentplugging of the filter. To overcome these problems,development of catalytic filters has attracted the in-terested of many researchers (for a recent review, seeRef. [214]).

Fig. 23. Principle of filter operation (1) and filter re-generation (2) for a soot removal system, using fuel powered burners.

A comprehensive discussion of the diesel particulateabatement systems is beyond the scope of this paper,however, it is important to quote some emerging tech-nologies in this field, one of the most important beingthose of the CRT [216] and use of fuel additives thatfavour combustion of the soot deposited on the filter[217,218]. The concept of the so-called CRT has beenpioneered by researchers from Johnson Matthey [216]and is based on the observation that NO2 is a morepowerful oxidising agent towards the soot comparedto O2. The concept of CRT is illustrated in Fig. 24:a Pt catalysts is employed in front of the filtering de-vice in order to promote NO oxidation; in the secondpart of CRT, DPM reacts with NO2 favouring a con-tinuous regeneration of the trap. A major drawback ofthese systems is related to the capability of Pt cata-lysts to promote SO2 oxidation as well. The sulphatethus formed is then deposited on the particulate filterinterfering with its regeneration. Moreover, the NO2reacts with the soot to reform NO whilst reduction ofNO2 to N2 would be the desirable process. Accord-ingly, it is expected that as the NOx emission limitswill be pushed down by the legislation, less NO willbe available in the exhaust for soot removal, unlessthe engine is tuned for high NOx emission that areused in the CRT and then an additional DeNOx trapis located after the CRT device.

Use of fuel additives, particularly those based onCeO2, is another area of interest [219]. Rhodia hasintroduced these additives on the market which now


Fig. 24. The working principle of the continuously regenerating particulate trap.

have been applied in vehicles [217,218]. While thereis a significant benefit in terms of promotion of thesoot combustion from addition different additives evenat very small concentrations [218], the environmentalimpact due the release of such additives into the at-mosphere, following their widespread use, should beconsidered.

5. Conclusions

The development of automotive converters hasproceeded by a continuous improvement of the cat-alytic performances and durability of the automotivecatalysts over the past 25 years. Use of CeO2ZrO2technology represented a major improvement in termsof TWC durability in the last years, however, moreand more demanding regulations are on the horizon.It is now clear that a TWC is a complex and in-tegrated system that must be immediately effectiveand that its lifetime must be equivalent to that of thecar. This demands new materials of extreme thermalstability, exceeding 1100 C, which show extremelyhigh conversions. The achievement of such targetsrequire strong research efforts; a fundamental com-prehension of the interactions between the NM andthe other washcoat components and the deactivationphenomena is needed.

The requirements for more and more efficient en-gines highlights the problem of NOx abatement underoxidising conditions. Even though huge efforts havebeen dedicated to development of lean-DeNOx cata-lysts, their durability and performances are still insuf-

ficient. Noticeably, of the different lean-DeNOx strate-gies for gasoline engines, the most effective and readyto use one is the so-called NSR concept which stilluses a TWC to eliminate NOx [112].

Technologies for control of particulate emissionsfrom diesel engine will find increasing demand in thenext years. In particular, catalytic filters will continueto be the subject of intense research.

Summarising, the end-of-pipe technologies for au-tomotive pollution control, and in particular the TWC,have, and are, playing a key role in reducing air pol-lution. However, a new breakthrough point can beachieved only by adopting new strategies based moreon prevention than on control. In this respect, it isimportant to highlight the great promise of hydrogenfuel cell technology [220]. The proton exchange mem-brane fuel cellto make hydrogen from HCswill bea major focus for research in electrocatalysis and cat-alytic fuel processing. It is worth noting that the targetsobtained in the development of materials for TWCsconstitutes an important scientific background for thedesign of new catalytic system for on-board hydrogenproduction. In fact, the on-board fuel reformer unit em-ploys a number of catalytic steps involving reactionsthat routinely occur under the exhaust conditions andmost of them appear to be promoted by the NM/CeO2interactions [13]. Consistently, M/CeO2ZrO2 mate-rials were reported to feature good activities for fuelreforming, WGS reaction and preferential CO oxida-tion [221230]. However, further work is necessaryto significantly enhance their performance, in order toobtain a miniaturisation of the system and thereforeapplication in automobiles.


Acknowledgements

The authors gratefully thanks Dr. R. Di Monte,Prof. M. Graziani, University of Trieste, and Drs.P. Moles and C. Norman from MEL Chemicals fortheir valuable contribution to the development of thechemistry of CeO2ZrO2 mixed oxides and helpfuldiscussions. MEL Chemicals, University of Trieste,Fondo Trieste 1999, MURST-PRIN 2000 Cataly-sis for the reduction of the environmental impactof mobile source emissions, CNR Agenzia 2000,INCA Progetto Atmosfera Urbana are gratefullyacknowledged for financial support.

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