Automotive Catalytic Converters: Current Status and Some Perspectives

Abstract

Automotive three-way catalysts (TWCs) have represented over the last 25 years one of the most successful stories in the development of catalysts. The aim of this paper is to illustrate the technology for abatement of exhaust emissions by analysing the current understanding of TWCs, the specific role of the various components, the achievements and the limitations. The challenges in the development of new automotive catalysts, which can meet future highly demanding pollution abatement requirements, are also discussed.

Full text

Catalysis Today 77 (2003) 419–449
Automotive catalytic converters: current status
and some perspectives
Jan Kašpar∗, Paolo Fornasiero, Neal Hickey
Dipartimento 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 the
development of catalysts. The aim of this paper is to illustrate the technology for abatement of exhaust emissions by analysing
the current understanding of TWCs, the specific role of the various components, the achievements and the limitations. The
challenges in the development of new automotive catalysts, which can meet future highly demanding pollution abatement
requirements, are also discussed.
© 2002 Elsevier Science B.V. All rights reserved.
1. Introduction
Air pollution generated from mobile sources is
a problem of general interest. In the last 60 years
the world vehicle fleet has increased from about 40
million vehicles to over 700 million; this figure is
projected 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 of
engines employ combustion of fuels derived from
crude oil as a source of energy. Burning of hydrocar-
bon (HC) ideally leads to the formation of water and
carbon dioxide, however, due to non-perfect combus-
tion control and the high temperatures reached in the
combustion chamber, the exhaust contains significant
amounts of pollutants which need to be transformed
into harmless compounds. In this paper, the control
strategies and achievements in automotive pollution
control 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: kaspar@units.it (J. Kašpar).
of catalysts, i.e. NM/CeO2–ZrO2–Al2O3contain-
ing systems; insight on the lean-DeNOxand diesel
type of catalysts is also given. The paper is focussed
essentially on the catalytic aspects of pollution abate-
ment, even though the reader should consider that
technological solutions such an electrically heated
catalysts, etc., may heavily affect the converter per-
formances [2]. A number of review papers have
described the traditional CeO2-based TWC technol-
ogy, accordingly we refer the reader to these papers
[2–13].
2. Emissions characteristics and control
strategies
Engine exhausts consist of a complex mixture, the
composition 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 or
extra-urban, vehicle speed, acceleration/deceleration,
etc. Table 1 reports typical compositions of exhaust
gases for some common engine types.
0920-5861/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S0920-5861(02)00384-X

420 J. Kašpar et al./Catalysis Today 77 (2003) 419–449
Table 1
Example of exhaust conditions for two- and four-stroke, diesel and lean-four-stroke engines [9,155,176,231]
Exhaust components
and conditionsaDiesel engine Four-stroke spark
ignited-engine Four-stroke lean-burn
spark ignited-engine Two-stroke spark
ignited-engine
NOx350–1000ppm 100–4000ppm ≈1200 ppm 100–200 ppm
HC 50–330ppm C 500–5000ppm C ≈1300 ppm C 20,000–30,000ppm C
CO 300–1200ppm 0.1–6% ≈1300ppm 1–3%
O210–15% 0.2–2% 4–12% 0.2–2%
H2O 1.4–7% 10–12% 12% 10–12%
CO27% 10–13.5% 11% 10–13%
SOx10–100ppmb15–60 ppm 20 ppm ≈20ppm
PM 65mg/m3
Temperatures (test cycle) r.t.–650◦C (r.t.–420◦C) r.t.–1100 ◦Ccr.t.–850◦C r.t.–1000◦C
GHSV (h−1) 30,000–100,000 30,000–100,000 30,000–100,000 30,000–100,000
λ(A/F)d≈1.8 (26) ≈1 (14.7) ≈1.16 (17) ≈1 (14.7)e
aN2is remainder.
bFor comparison: diesel fuels with 500ppm of sulphur produce about 20ppm of SO2[16].
cClose-coupled catalyst.
dλdefined as ratio of actual A/F to stoichiometric A/F, λ=1 at stoichiometry (A/F =14.7).
ePart 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,1unburned or partially
burned hydrocarbons (HCs), carbon monoxide (CO)
and nitrogen oxides (NOx), mostly NO, in addition to
other 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 sulphur
emissions by diminishing the sulphur content in the
fuel. Given the different nature of the three classes of
pollutants, i.e. reducing or oxidising agents, it is nec-
essary to simultaneously carry out both reduction and
oxidation reactions over the exhaust catalyst, which
can occur by a variety of reactions. Some of these are
summarised in Table 2. Importantly, this table reports
only the desirable reactions, in that many other reac-
tions could occur in the complex mixtures described
in Table 1, such as, for example, reduction of NOxto
ammonia, partial oxidation of HC to give aldehydes
and other toxic compounds, etc. Given the complexity
of the exhaust media, a high selectivity is required in
order to promote only the reactions reported in Table 2.
A perusal of the exhaust compositions reported in
Table 1 for the different type of engines reveals some
1The ability of the TWCs to simultaneously eliminate three
classes of pollutants is at the origin of their name.
significant differences: (i) even if relatively diluted,
the concentration of the various pollutants can change
even by an order of magnitude, according to the type
of engine; (ii) with the exception of the four-stroke
spark ignited-engine, which, being equipped with a
TWC, is run at stoichiometry, the other type of engines
can be run under lean conditions, i.e. in excess of
O2; (iii) extremely high temperatures are reached in
the four-stroke spark ignited-engine, particularly in the
close-coupled catalyst (CCC).
In general, the emissions depend on air-to-fuel
(A/F) ratio, as exemplified in Fig. 1. Tuning of the
engine to rich feed gives the highest power output,
which, however, occurs at expenses of high fuel con-
sumption. Under lean conditions lower combustion
Table 2
Reactions occurring on the automotive exhaust catalysts, which
may contribute to the abatement of exhaust contained pollutants
[4]
Oxidation 2CO +O2→2CO2
HC +O2→CO2+H2Oa
Reduction/three-way 2CO +2NO →2CO2+N2
HC +NO →CO2+H2O+N2a
2H2+2NO →2H2O+N2
WGS CO +H2O→CO2+H2
Steam reforming HC +H2O→CO2+H2a
aUnbalanced reaction.

J. Kašpar et al./Catalysis Today 77 (2003) 419–449 421
Fig. 1. Effect of A/F ratio (w/w) on engine emissions and engine
power (after Ref. [3]).
temperatures lead to lower NOxemissions, however,
at very high A/F engine misfire occurs, leading again
to high HC emissions. Under any A/F conditions
catalytic abatement of pollutants is needed to com-
ply with the legislation limits. Only at stoichiometric
conditions are appropriate amounts of reducing and
oxidising agents present in the exhaust to carry out
the catalytic reactions as outlined in Table 2. Un-
der such conditions TWCs effectively remove the
pollutants.
In principle, there are several advantages in re-
moving NOxfrom the automotive exhaust under
lean conditions, i.e. A/F >14.7, compared to the
stoichiometric feed (A/F =14.7) of a traditional
gasoline-fuelled engine, where the polluting compo-
nents are abated using a TWC. The most important
advantage of lean-burn engines is the significant fuel
economy. In fact, an increase of fuel consumption of
vehicles occurred in the 1990s, which was generally
attributed to the introduction of TWCs and their re-
quirement for a stoichiometric A/F ratio to achieve
best performances. However, it should be noted that
there are other factors that substantially contributed
as well, such as the increase of vehicle weight due to
implementation of security systems, the generalised
use of vehicle air conditioning, etc. There is another
advantage of lean NOxengine, which is the fact
that the highest exhaust temperatures are typically
lower (≤800–850◦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 a
disadvantage in terms of the catalyst durability.
Three types on engines can effectively run un-
der lean-burn, i.e. diesel, four-stroke/lean-burn and
two-stroke engine. The two-stroke engine is typically
employed in small motorcycles, mopeds, chain saws
and most recreational vehicles. This engine is charac-
terised by high engine power output, compact design,
and low construction costs, which makes it ideal for
the above listed applications. However, it is noisy,
it presents high fuel consumption and high levels of
emitted HCs due to partial mixing of the combustion
mixture with the exiting exhaust. It is estimated that
up to 25–30% of fuel in the feed is emitted during the
scavenging process of the exhaust mixture from the
cylinder [14]. Accordingly, the HC emitted from this
engine are predominantly C5–C6, in contrast to all
other engines where C1–C3 constitute the majority of
HC emitted. Even though various strategies of engine
management were developed for limiting such high
HC emissions, pollution control by catalytic methods
is nowadays mandatory even for these engines, which
may be achieved by introducing an oxidation catalyst
for CO and HC removal, as the legislation is generally
less demanding compared to four-wheel vehicles. Due
to the intrinsically low NOxemission levels (Table 1),
EGR (exhaust gas re-circulation) technology can be
employed for the reduction of emitted NOx. Should
legislation significantly increase the tightness of the
present and forecasted limits, it may be expected
that four-stroke engines, equipped with conventional
TWCs could gradually replace two-stroke engines, as
happened for four-wheel vehicles.
As for the exhausts originated by diesel and
lean-burn engines, let us observe that even though
both types of engines run in an excess of oxygen,
typically 5–15% of O2is present in the exhausts com-
pared to approximately 1% found in the engine fed at
stoichiometry; the precise nature of the exhaust gases
significantly differs between the two systems. In fact,
both types of engine emit HC and NOxat ppm levels
(300–800ppm) and large amounts of O2(5–15%),
water and CO2(each 10–12%). However, as far as the
HCs are concerned, there is an important difference
in that very low levels are emitted from the diesel
engine (≤NOx), which makes necessary addition of a
reducing agent to the exhaust in order to achieve ap-
preciable reduction of the emitted NOx. On the other

422 J. Kašpar et al./Catalysis Today 77 (2003) 419–449
hand, diesel emissions are characterised by a signif-
icant level of particulate matter, which itself can be
employed as a reducing agent for NOx, or, vice versa,
particulate can be abated by using NOxas oxidant as
in Johnson Matthey’s continuously regenerating trap
(CRT) [15]. In contrast, the issue of particulate mat-
ter is absent in the case of lean-burn engine powered
by gasoline; moreover, higher contents of HC are
emitted, which, in principle, allows direct selective
catalytic NOxremoval to occur. In addition to these
differences in the nature of the exhaust gases, the
range of exhaust temperatures strongly differ between
the two types of engine. For diesel exhaust, tem-
peratures are on average in the range of 80–180◦C
under the European urban driving cycle with some
maxima up to 230◦C, while in the extra-urban part of
the testing cycle a maximum temperature of 440◦C
was observed, typical temperatures being in the range
180–280◦C[16]. This represents a serious problem,
both in terms of need of activity at low temperatures
and the difficulty in de-sulphurisation of the catalysts,
which generally requires temperatures above 650◦C.
Let us now focus on the four-stroke spark ignition
engines equipped with TWCs. As above reported, the
required amounts of reducing and oxidising agents are
present in the exhaust only under stoichiometric con-
ditions. This leads to the typical dependency of the
conversion patterns of the TWCs upon the A/F ratio
(Fig. 2). Today the required conversion of pollutants
is greater than 95%, which is attained only when a
precise control of the A/F is maintained, i.e. within a
narrow 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/F
ratio as close as possible to stoichiometry (Fig. 3). To
obtain an efficient control of the A/F ratio the amount
of air is measured and the fuel injection is controlled
by a computerised system which uses an oxygen (λ)
sensor located at the inlet of the catalytic converter.
The signal from this λsensor is used as a feedback
for the fuel and air injection control loop. A second
λsensor is mounted at the outlet of the catalytic con-
verter (Fig. 3). This configuration constitutes the basis
of the so-called engine on-board diagnostics (OBD).
By comparing the oxygen concentration before and
after 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-way
catalysts.
narrow A/F window at the stoichiometric point is the
fingerprint of an effective TWC system.
The location of the catalytic converter is another
critical point which determines the conversion effi-
ciency. TWCs typically feature the so-called light-off
type conversion vs. temperature behaviour. This
curves is characterised by conversion which steadily
increases from 0 to 100% conversion, the temperature
Fig. 3. Diagram of a modern TWC/engine/oxygen sensor (λ)
control loop for engine exhaust control.

J. Kašpar et al./Catalysis Today 77 (2003) 419–449 423
Fig. 4. Cumulative HC emissions measured during the federal test
procedure (FTP cycle) on an US 1995 car: (1) tailpipe emissions
with CCC; (2) engine-out emissions (after Ref. [11]).
of 50% of conversion being indicated as the light-off
temperature. TWCs are characterised by a light-off
temperature around 250–350◦C. This means that
an under-floor catalyst is heated above the light-off
temperature within 90–120s. In contrast, when the
catalyst is closely coupled to the engine (CCC) the
heating time typically drops down to 10–20s. This
dramatically affects vehicle emissions immediately
after the start-up of the engine (Fig. 4). As shown in
this figure, the ULEV (Californian ultra-low emis-
sions vehicle) limit (0.064g HCemitted/km) is typ-
ically surpassed within 40s after the engine start-up
(Fig. 4(2)) [11]. To avoid this situation an almost
instantaneous heating of the converter is required
to achieve the required >95–98% conversion. CCCs
minimise the heating time, however, temperatures up
to 1100◦C are routinely met as a consequence of this
location 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 application
of the CCCs and OBD technologies in order to meet
the 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 the
converter will be demanded by US tier II regulations
in 2004. It should be considered that if a significant
part of the vehicle fleet fails the periodical exhaust
emission control test, converter replacement becomes
mandatory for a vehicle manufacturer. Accordingly,
an extremely efficient and robust catalyst is required
for future vehicle application. In summary, catalytic
converters suitable for 2005 and beyond must present
the 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 (<10–20s), i.e. high activity at
low temperatures.
In the case of vehicles equipped with TWCs, two
additional requirements should be considered:
•high thermal stability;
•high oxygen storage capacity.
Amazingly, problems and needs for improvements
such as those above listed have been quoted for many
years when discussing TWCs. A question arises: Why
have they not been solved as yet? The reason is that
as the performances of the TWCs improve, higher and
higher targets, e.g. decrease of emissions and increase
of durability, are pushed forward by the legislators,
asking for further improvement of the de-pollution
technology. For example, starting from 2003 the Cal-
ifornia SULEV legislation will require a 10-fold de-
crease in NOxemissions compared to the already tight
value of ULEV legislation (0.2g/km), whereas HC
emissions should drop by a factor of four, down to
0.01 g/km, with converter durability as high as 120,000
miles. Typically, cumulative tailpipe emissions exceed
such stringent values within 3–15s from the start of
the engine! This means that catalyst must be effective
a few seconds before this limit is reached and convert
nearly 100% during the remaining period of the test
procedure.
3. TWCS: principles and operation
A typical design of a modern three-way catalytic
converteris reportedin Fig. 5. Basically,it is a stainless

424 J. Kašpar et al./Catalysis Today 77 (2003) 419–449
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 honeycomb
monolith made of cordierite (2MgO·2Al2O3·5SiO2)
or metal [9]. Although this aspect is sometimes ne-
glected in the scientific literature, in must be under-
lined that the choice and geometrical characteristics
of the honeycomb monolith play a key role in deter-
mining the efficiency of the converter. In fact, high
conversion must be achieved in the converter and
therefore the catalyst works under conditions where
severe mass and heat transfer limitations apply. Typi-
cally, both metal and ceramic monoliths are employed
nowadays. The major advantage of the metallic sub-
strate is that the wall thickness is limited by the steel
rolling mill’s capabilities, not strength. In a typical
automotive 400cell/in.2application, the frontal flow
area in a ceramic monolith is 69% open (31% closed),
while the metallic version has 91% open area. This is
due to the higher wall thickness of ceramic monoliths
(0.007in. (0.178 mm)) compared to metallic ones
(0.002in. (0.050 mm)) [17,18]. However, even in this
field there has been a strong improvement of the tech-
nology, cell densities as high as 900cell/in.2or even
higher are now commonly available on the market for
both types of monoliths [19]. Traditionally, cordierite
monoliths have been employed quite extensively, pri-
marily due to their lower production cost. However,
a major advantage of the metal monoliths resides in
their high thermal conductivity and low heat capacity,
which allow very fast heating of the CCCs during
the phase-in of the engine, minimising the light-off
time.
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 deposited
material (washcoat) is then blown out with hot air and
the honeycomb is calcined to obtained the finished cat-
alyst. This is clearly a very simplified and schematic
description of the washcoating process as multiple
layer technology, or multiple catalyst-bed converters
are 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, which
contains up to 5wt.% 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 washcoating
composition:
•Alumina, which is employed as a high surface area
support.
•CeO2–ZrO2mixed oxides, principally added as
oxygen storage promoters.
•Noble metals (NM =Rh, Pt and Pd) as active
phases.
•Barium and/or lanthana oxides as stabilisers of the
alumina surface area.
3.1. Al2O3
The choice of Al2O3as carrier is dictated by the
necessity of increasing the surface area of the honey-
comb monolith which is typically below 2–4m2l−1,

J. Kašpar et al./Catalysis Today 77 (2003) 419–449 425
where the volume is that of the honeycomb [22]. This
does not allow achievement of high NM dispersion.
Alumina is chosen due to its high surface area and
relatively good thermal stability under the hydrother-
mal conditions of the exhausts. In most of the studies
␥-Al2O3is employed due to its high surface area
with respect to other transitional aluminas [23],how-
ever, also other high temperature aluminas such as ␦-
and ␪-Al2O3can be employed for high temperature
applications such as in the CCCs because of their
high thermal stability compared to ␥-Al2O3. Since
temperatures above 1000◦C can be met in the TWCs,
stabilisation of transition aluminas is necessary to pre-
vent their transformation to ␣-Al2O3, which typically
features surface areas below 10m2g−1. A number
of stabilising agents have been reported in the litera-
ture, lanthanum, barium, strontium, cerium, and more
recently, zirconium oxides or salts being the most in-
vestigated [24–31]. These additives are impregnated
onto ␥-Al2O3or, sometimes, sol–gel techniques are
employed to improve the stability of the surface area.
The exact mechanism by which these additives sta-
bilises transitional aluminas strongly depends on the
amount of the stabilising agent and the synthesis con-
ditions. This is exemplified in Fig. 6 for BaO doped
aluminas; BaO and lanthana are the most used and
effective stabilisers.
The effectiveness of each dopant on the stabilisation
of 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 stability
of BET areas of Al2O3after calcination at the indicated temper-
atures for 3h. SG: sol–gel synthesis method; C: co-precipitated
sample (after Ref. [29]).
ple, CeO2was shown to thermally stabilise Al2O3, the
maximum stabilisation effect being attained at a CeO2
levelof5%[32]. However, Morterra et al. [33,34]
found that little stabilisation of BET areas is attained
by adding CeO2to ␥-Al2O3, even though significant
modification of surface properties were detected. In
fact, using CO as a surface probe molecule of the
surface Lewis acidity of the CeO2–Al2O3mixed sys-
tems, it was revealed that CeO2accumulates prefer-
ably on the flat patches of low-index crystal planes of
the spinel structure, and that the presence of Ce cations
stabilises, also at high temperatures, the most acidic
Lewis centres. A possible rationale may be given by
the recent observation that very efficient stabilisation
of Al2O3by addition of CeO2is achieved under re-
ducing conditions compared to the oxidising ones, due
to formation of CeAlO3[31]. Apparently, the stabili-
sation effect is more pronounced as long as dispersed
Ce3+species are present at the Al2O3surface. The
presence of such species has long been detected in
CeO2–Al2O3provided that low CeO2loading is em-
ployed [35,36]. It is conceivable that CeO2stabilises
␥-Al2O3in a similar fashion to La3+, i.e. formation
of a surface perovskite-type of oxide LaAlO3[24],
which may account for the conflicting observations
reported in the literature. Under high temperature ox-
idising conditions, partial re-oxidation of Ce3+sites
may occur, with formation of CeO2particles which
tends to agglomerate and grow over the Al2O3sur-
face, making stabilisation ineffective.
Use of ZrO2has also been reported to effectively
stabilise ␥-Al2O3at high temperatures [25]. In this
case, however, the stabilisation of Al2O3seems to
be related to the ability of ZrO2to spread over the
Al2O3rather than formation of mixed oxides. Even
though formation of ZrO2–Al2O3solid solution has
been sometimes claimed, separation into ZrO2and
Al2O3occurs upon high temperature calcination, as
dictated by the phase diagram [37]. The effectiveness
of ZrO2in improving the thermal stability of Al2O3
surface area seems remarkable as surface areas as
high as 50m2g−1were observed after calcination at
1200◦C[25]. Interestingly, ZrO2appears to be more
effective than CeO2in stabilising Al2O3; consistently,
ZrO2-rich CeO2–ZrO2mixed oxides more effectively
stabilised Al2O3compared to CeO2-rich systems [30].
The overall picture concerning the development
of Al2O3-based supports is that, at present, thermal

426 J. Kašpar et al./Catalysis Today 77 (2003) 419–449
stability of the Al2O3support is not an important
issue for the next generation of TWCs in that the
progress obtained so far makes these stabilised sup-
ports suitable even for high temperature applications
such as in CCCs.
3.2. CeO2–ZrO2mixed oxides
The beneficial effects of CeO2-containing formula-
tions of the TWC performances has long been recog-
nised [38]. Many different promotional effects have
been attributed to this component, such as the ability
to:
•promote the noble metal dispersion;
•increase the thermal stability of the Al2O3support;
•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 relative
importance is beyond the scope of this work and for
this we refer the reader to earlier literature [4,12,13].
Among the different roles of CeO2in TWCs, the
OSC is certainly the most important one, at least from
the technological point of view. In fact, as above dis-
cussed, the OBD technology is based on monitoring
of the efficiency of the OSC. This is due to the fact
that 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 CeO2–ZrO2
mixed oxides, even though the reader should be aware
that a variety of complex phenomena occur under the
real exhaust conditions, originated mainly by the in-
teraction of the NM- and CeO2-based materials.
Starting from 1995, CeO2–ZrO2mixed oxides have
gradually replaced pure CeO2as OSC materials in
the TWCs [40], even though some low purity CeO2
materials may be employed for less demanding TWC
technologies [41]. The principal reason for the intro-
duction of CeO2–ZrO2mixed oxides in place of CeO2
is due to their higher thermal stability, as exemplified
in Fig. 7, which reports the OSC and BET area of
Fig. 7. Effect of CeO2content on the surface area stability and
dynamic-OSC of CeO2–ZrO2after calcination at 900◦C. OSC
measured at 400◦C by alternatively pulsing 2.5% O2in He and
5% CO in He over the catalyst (after Ref. [42]).
CeO2–ZrO2mixed oxides as a function of CeO2con-
tent [42]. Clearly, there is an important improvement
of both OSC and BET area as soon as ZrO2is in-
serted into the CeO2lattice. At first glance, there ap-
pears to be a straightforward indication in Fig. 7, that
is, CeO2-rich compositions (around 60–70mol%) are
the most effective OSC promoters for TWC applica-
tion. Unfortunately, this is a very simplified view of
the problems related to the use of CeO2–ZrO2mixed
oxides in the TWCs, the real situation being much
more complex, as described below.
3.2.1. Thermal stability of CeO2–ZrO2mixed oxides
Thermal 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 and
higher exhaust temperatures observed nowadays com-
pared to past [1], demanded for higher and higher ther-
mal stability of the washcoat and particularly of the
CeO2component. The relationship between the extent
of surface area of CeO2and the OSC property, as de-
tected by temperature programmed reduction (TPR), is
well established (see Fig. 8 as an example). As below
discussed, the ability of CeO2to undergo reduction,
i.e. release of oxygen, at low temperatures (<500◦C)
is well recognised as an immediate and useful tool to
detect deactivation of the OSC and hence of TWC ac-
tivity. Accordingly, the primary target in the develop-
ment of high temperature OSC materials was always
considered the resistance of CeO2towards sintering.
In principle, there are a number of different routes
which may lead to enhanced thermal stability of the

J. Kašpar et al./Catalysis Today 77 (2003) 419–449 427
Fig. 8. TPR profiles of CeO2((1) and (2)) and Rh/CeO2((3) and
(4)) with surface areas of, respectively, 190 m2g−1((1) and (3))
and <10m2g−1((2) and (4)).
CeO2-based materials, which may be summarised as
follows: (i) design of microstructure/textural proper-
ties by adopting an appropriate synthesis methodol-
ogy, (ii) appropriate doping of CeO2, (iii) dispersing
of CeO2on a carrier. This last aspect has already
been partially addressed since it is particularly related
to the thermal stabilisation of Al2O3(see above), as
synergic stabilisation effects have been found for the
CeO2–ZrO2–Al2O3system [30]. Of course, any com-
bination of these strategies can also be adopted, how-
ever, for sake of simplicity we prefer to discuss these
aspects ((i) and (ii)) separately.
3.2.1.1. Design of microstructure/textural properties.
The sinterability of any material is clearly related
to its textural properties and in particular to its pore
structure [43]. The pore structure, in turn, strongly
depends on the synthesis conditions. For example,
co-precipitation is typically employed to prepare
mixed oxide catalysts. It has been shown that when
the precipitated cake is treated at 80 ◦C in the presence
of surfactants, extensive mesoporous texture develops
in the CeO2–ZrO2mixed oxides, leading to remark-
ably high surface areas compared to the traditional
co-precipitation route [44]. On the other hand, the sin-
tering mechanism at high temperature was apparently
little affected, as comparable loss of surface area, in
terms of relative loss of BET area, was observed in
both samples, independently of the synthesis method
[40]. Generally speaking, as the sintering at high tem-
peratures proceeds, annihilation of small pores occurs
first leading to decreases of the cumulative pore vol-
ume and BET area. On the other hand, large pores sin-
ter with more difficulty as longer migration distances
are needed for the matter to fill the pores and sinter the
material. This concept is clearly illustrated by results
of calcination of two Ce0.2Zr0.8O2samples where
appropriate modifications of the conditions of sample
processing were applied in a controlled way to obtain
the initial pore distribution shown in Fig. 9. As a result
of this pore distribution, sample A features a surface
area of 27m2g−1after calcination at 700 ◦C, which
decreases by 85% after calcination at 1000◦C for 5 h,
giving a BET area of 4m2g−1. This is not a surpris-
ing result since BET area of few square meters per
gram are typically found after such a harsh calcination
[45,46]. Sample B features much larger pores com-
pared to sample A, leading to a BET surface area of
35m2g−1after calcination at 700 ◦C. However, when
the calcination temperature is increased to 1000◦C,
a relatively small decrease of the BET area (37%) is
observed, as the obtained product features a BET area
of 22m2g−1. This is perfectly in line with the above
reported comments on the sintering behaviour and
indicates that extreme care should be taken before the
effects of variation of CeO2–ZrO2composition on
textural stability can be assessed. Clearly, meaningful
comparison of properties of CeO2–ZrO2mixed oxides
can be obtained only when samples of comparable
textural properties are compared.
3.2.1.2. Doping of CeO2–ZrO2mixed oxides with
other elements. Even though the introduction of
CeO2–ZrO2mixed oxides into the TWCs represented
a significant breakthrough point compared to the

428 J. Kašpar et al./Catalysis Today 77 (2003) 419–449
Fig. 9. Pore distribution in two samples of Ce0.2Zr0.8O2as detected from N2desorption isotherm using the BJH method: (A) surface
area =27m2g−1; (B) surface area =35m2g−1.
CeO2-based technology, it is now recognised that un-
doped CeO2–ZrO2do not present sufficient thermal
stability for application on the 2005 type of TWC con-
verters. In fact, thermal stability in excess of 1000◦C
cannot be achieved by simple CeO2–ZrO2due to their
metastable nature. As shown in the phase diagram re-
ported in Fig. 10 [47–50], there are metastable (tand
Fig. 10. Experimental phase diagram of the CeO2–ZrO2system
(after Ref. [70]).
t) phases at intermediate CeO2compositions, which
upon heating under oxidising conditions, lead to phase
separation, CeO2-rich (cubic: c-Ce0.8Zr0.2O2) and
ZrO2-rich (tetragonal: t-Ce0.2Zr0.8O2) phases being
typically obtained [51,52]. In principle, TWCs must
show high durability; accordingly phase separation is
considered an undesirable feature of the CeO2–ZrO2
component since it may lead to unpredictable vari-
ations in the properties of the catalyst. By analogy
with ZrO2, trivalent dopants, such as yttria and lan-
thana, have been employed for the CeO2–ZrO2mixed
oxides [53–58]. However, to our knowledge no sys-
tematic study satisfying the above reported criterion
of comparable textural properties for comparison of
effects of composition has been reported so far. This
makes difficult a rationalisation of the data reported
in the literature on the effects of the doping agents.
Some qualitative and general comments can, how-
ever, be made for CeO2–ZrO2systems: (i) CeO2–ZrO2
phase 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) phase
separation is pronounced under oxidising conditions
while under reducing conditions phase homogenisa-
tion is favoured [51,52]; (iii) sintering with decrease
of surface area is very pronounced under reducing

J. Kašpar et al./Catalysis Today 77 (2003) 419–449 429
conditions, particularly when compared to oxidising
ones [59]. These general comments clearly point out
the critical need for further research aimed at a ra-
tionalisation of the role of the dopant in affecting
thermal stability of the CeO2–ZrO2mixed oxides, in
order to develop new products for high temperature
applications.
3.2.2. Oxygen storage of CeO2–ZrO2mixed oxides
A remarkable property of the CeO2–ZrO2mixed
oxides compared to CeO2is their ability to easily
remove bulk oxygen species at moderate temperature
even in highly sintered samples. Thus the reduction
peak at approximately 900◦C(Fig. 8, trace 1), which
is associated with reduction of CeO2in the bulk shifts
down to approximately 400◦C when a 40 mol% of
ZrO2was inserted into the CeO2lattice to prepare a
highly sintered Rh/Ce0.6Zr0.4O2mixed oxide catalyst
[60]. This was associated with the ability of ZrO2
to modify the oxygen sub-lattice in the CeO2–ZrO2
mixed oxides, generating defective structures and
highly mobile oxygen atoms in the lattice which can
be released even at moderate temperatures [61,62].
These early findings indicated that improved effi-
ciency of the OSC property can be achieved by using
CeO2–ZrO2mixed oxides instead of CeO2since, even
if the sample sinters under the high temperature reac-
tion conditions, it should be more effective then CeO2
due to the high oxygen mobility in the bulk; lattice
oxygen species could effectively participate in redox
processes even under fluctuating exhaust feed-stream
conditions. It is now well recognised that when the
OSC property is investigated by the TPR technique,
no appreciable distinction between the reduction in
the bulk and at the surface can be observed in the
CeO2–ZrO2mixed oxides. Both reduction at the sur-
face and in the bulk proceed with similar energetics
and occur at mild temperatures [63,64]. Typically, a
single peak reduction profile centred around is 500◦C
is obtained for a single phase CeO2–ZrO2solid solu-
tion, the presence of multiple peaks being taken as an
indication of presence of phase impurities [40].How-
ever, the changes in the TPR behaviour are even more
subtle because other factors such as textural properties
and even the pre-treatment can affect the TPR profile
[65–68]. For example, a combination of TPR followed
by mild oxidation leads to reduction phenomena oc-
curring at low temperatures [65], whereas when a
high temperature (severe) oxidation is included as a
pre-treatment, these low temperature processes re-
versibly shift to high temperatures [66,67]. There has
been some debate as to whether migration of oxygen
species in the bulk is limiting the rate of the reduction
of the CeO2–ZrO2, or whether the kinetics of redox
phenomena are rather dictated by surface properties
[45]. However, recent findings confirmed the impor-
tant role of the bulk properties for redox phenomena
occurring at low temperatures [68,69].
As above indicated, the TPR technique has been
routinely applied to investigate the redox properties
of the CeO2–ZrO2mixed oxides. It should be noted,
however, that under real exhaust conditions, the λ
value oscillates between the oxidising and reducing
conditions with a frequency of about 1Hz. In princi-
ple, this makes the so-called dynamic-OSC more use-
ful compared to the TPR technique [12,71], since this
technique allows detection of the oxygen available for
redox processes on a time scale of seconds. In fact, it
should be noted that even favourable TPR profiles, i.e.
featuring reduction peaks at low temperatures, may
not necessarily be associated with effective OSC due
to occurrence of in situ deactivation phenomena on
increasing the temperature of the measurement [72].
However, a very recent report suggested that correla-
tion between TPR profiles and effective dynamic-OSC
exists in that texturally stable samples featuring a
TPR behaviour independent of the pre-treatment, e.g.
a mild or severe oxidation, are those giving the stable
and effective dynamic-OSC, minimising the deactiva-
tion phenomena [73]. This confirms the importance
of the stabilisation of the textural properties in the
CeO2–ZrO2mixed oxides and the need for further
improvement of the thermal stability compared to
typical temperatures so far investigated (≈1000◦C).
3.3. Noble metals
Obviously, NMs represent the key component of
the TWC, as the catalytic activity occurs at the no-
ble metal centre. However, we purposely discuss the
aspects related to the NM at this point, since its inter-
action with the various components of the washcoat
critically affects the activity of the supported NM. In
principle, the first aspect to be considered is the choice
of the NM and its loading in the washcoat. Rh, Pd an
Pt have long been employed in the TWCs and there is

430 J. Kašpar et al./Catalysis Today 77 (2003) 419–449
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 pathways
for NO reduction have also been proposed [7,76,77].
Pt and Pd are considered as metal of choice to promote
the oxidation reaction, even though Rh also has a good
oxidation activity. In particular, besides some initial
use in 1975–1976, Pd has extensively been added to
TWC formulations starting from mid-1990s due to its
ability to promote HC oxidation [10,11]. In fact, bet-
ter A/F control [78] and modification of the support
provided high NOxconversion, comparable to the tra-
ditional Rh/Pt catalyst [79]. The increase of the use
of Pd in the TWC technology adversely affected Pd
market price, which is now comparable to that of Pt.
In fact, there is a large demand for Pd due to the fact
that the straightforward way to increase the efficiency
of the TWCs at low temperatures is that of increasing
the NM loading, and particularly that of Pd, which for
long 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 the
fact that cost-effective TWCs are required by the mar-
ket. In summary, the choice and loading of the NM is
a compromise between the required efficiency of the
converter and the market price of the NM; ideally a
car maker would prefer to have available a choice of
TWCs 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 fluctuations
of 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 pathway
for the deactivation of TWCs. In addition to sintering,
poisoning of the catalyst may contribute to their deac-
tivation. The latter phenomenon is essentially related
to the mileage travelled, quality of the fuel and the en-
gine lubricating oil [22]. However, there are a number
of other routes which can contribute to deactivation of
the TWCs: (i) sintering of the OSC promoter leading
to loss of OSC and, possibly, to encapsulation of the
NM [80]; (ii) sintering of Al2O3and, more important,
deactivation of Rh due to migration of Rh3+into the
alumina lattice [22]. The comprehension of the relative
importance of the different deactivation phenomena
is difficult due to the variability of the reaction con-
ditions, TWC preparation methods, etc. For example,
when NM are supported on CeO2–ZrO2mixed oxides
and aged at high temperatures under redox conditions,
encapsulation of Pd and Rh within the pores of the
support occurs, while it does not occur for Pt [81].
Although it has received relatively scarce con-
sideration [82], the issue of sulphur poisoning of
TWC needs some consideration. As above discussed,
inclusion of CeO2-based promoters into the washcoat

J. Kašpar et al./Catalysis Today 77 (2003) 419–449 431
considerably enhances the conversion efficiency of the
TWCs. On the other hand, both CeO2and ZrO2are
known to easily adsorb SOxspecies: sulphated ZrO2
is a well-known solid acid catalyst [83], whereas CeO2
is used as a DeSOxcatalyst in cracking processes [84].
Investigation of reduced and oxidised CeO2revealed
that SO2is adsorbed under various forms, with both
surface and bulk-type of sulphates being observed
[85,86], and it may even modify the microstructure of
the 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 H2
favours elimination of sulphates as H2S, which can be
easily detected as rotten-egg odour [88], particularly
in the presence of noble metal [89]. CO also promotes
the reduction of sulphates to reduced oxy-sulphur
species which unexpectedly increased the redox ca-
pability of the sulphated Pd/CeO2system compared
to the sulphur-free analogue [90,91].However,itwas
also observed that the OSC of CeO2is detrimentally
affected by the presence of SO2, while addition of
ZrO2to CeO2increases the resistance of CeO2to
sulphur poisoning, although more sulphur is adsorbed
at the surface [82]. This may be associated with the
generally higher OSC efficiency of the CeO2–ZrO2
mixed oxide compared to CeO2and the possibility
that ZrO2acts as a sulphur scavenger. Ni containing
oxides are sometimes added to the washcoat in the
USA as sulphur scavengers, while their use in Europe
is prohibited. In summary, adsorption of sulphur on
the NM/CeO2–ZrO2-containing TWCs is rather com-
plex and appears to be structure/adsorption conditions
sensitive, which readily explains some contradic-
tions in the literature. In terms of inhibition of the
three-way activity it seems, however, that the issue
of sulphur poisoning is much less stringent as com-
pared, for example, to lean-DeNOxcatalysts, due to
the high temperatures achieved in the TWCs, which
allow 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 limitations
which 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 aspect
is concerned, it should be noted that roughly 50–80%
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 been
pushed down, it appeared clearly that minimisation
of warm-up HC emissions was a major problem in
automotive pollution abatement. This issue was been
therefore addressed by introduction of the CCCs onto
the 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 be
addressed by different approaches, some of which are
listed 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 as
the temperature is increased above 250–300◦C, HC
are 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 not
the case for zeolite-based systems. We recall that tem-
peratures as high as 850–900◦C may be reached in
the under-floor catalysts. While such systems are still
under investigation, alternative approaches have been
indicated [92,93]. It must be recognised that to min-
imise the emissions, the catalysts must be heated-up
in a minimum time. This can be achieved, for exam-
ple, by electrically (Fig. 12) or combustion/chemically
heated catalysts. In the latter case hydrogen and oxy-
gen, or CO-rich feed is flowed over the catalysts [93].
Oxidation of both CO and H2are easy and exother-
mic reactions, which occur at low temperatures over
the TWCs [94], leading to rapid heating of the cata-
lyst. However, storage of H2on the vehicle or use of
rich A/F which generates high CO and H2emissions,
brings complexity to the de-pollution system. Large
amounts of HC are in fact emitted at rich A/F, which
require an additional HC trap.
Use of complex technology clearly pushes-up costs
while 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

432 J. Kašpar et al./Catalysis Today 77 (2003) 419–449
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 is
closely coupled to the exhaust manifold, became fea-
sible. This converter allows extremely rapid heating of
the catalyst, leading to enhanced conversions during
the warm-up of the engine. Metallic converter can be
easily shaped into the exhaust manifold and are very
convenient for such application also due to their low
heat capacity. In general, the composition of the CCC
is related to that of the typical TWC in that NM met-
als and particularly Pd are employed to promote HC
conversion. The OSC promoter may be omitted from
these formulations since it promotes CO conversion,
leading to local overheating because of this highly
exothermic reaction [92]. On the other hand, for the
purpose of the OBD II technology, there is a necessity
to monitor the OSC efficiency from the start-up of the
engine. Accordingly, ZrO2–rich doped CeO2promot-
ers with very high thermal stabilities [42,73,95], are
often added to this catalyst.
An alternative approach is that of developing new
catalysts showing high conversion efficiency at low,
nearly ambient, temperature [93]. A large part of these
investigations have been triggered by the observation
by Haruta et al. [96,97] that gold catalysts are able
to efficiently oxidise CO even at subambient tempera-
ture provided that nano-dispersed Au particles are pre-
pared on the support. Thus, light-off temperatures in
the conversion of the exhausts as low as 100 ◦C could
be achieved by depositing small Au particles on re-
ducible oxides such as CeO2and TiO2[98]. However,
the durability of gold catalysts under harsh conditions
is still an issue, significant deactivation of cobalt ox-
ide promoted Au catalysts was observed already after
157h of reaction at 500 ◦C under simulated exhaust
[99]. There is in fact a flourishing activity in the field of
low temperatures catalysts [97,100,101], other noble
metals, in addition to Au, being effective in low tem-
perature oxidation reactions, provided that appropri-
ate synthesis methodology is employed [100].Toour
knowledge, however, due to the nano-dispersed nature
of these catalysts, the issue of thermal stability, even at
moderately high temperatures has not been solved as
yet. Supported metal nano-particles are, in fact, quite

J. Kašpar et al./Catalysis Today 77 (2003) 419–449 433
mobile on the surface, even at ambient temperature
in the case of gold [101], which makes prevention of
sintering phenomena difficult. We believe that thermal
stabilisation of nano-dispersed metals may represent
a new breakthrough point in the development of these
environmental catalysts.
4.2. Lean DeNOxcatalysts
The recently approved California legislation on au-
tomobile fuel consumption has prompted the neces-
sity of developing new and more effective catalysts
capable of removing NOxeven in excess of O2.As
indicated in Section 2, lean-burn gasoline and diesel
engines, due to the high A/F used in the combustion
process, can achieve significant fuel savings, however,
under these conditions no TWC is effective in reduc-
ing NOxdue to the excess of O2, which is competing
for the reducing agent, in particular CO.
Studies on NOxremoval under oxidising conditions
were triggered by the discovery in 1991 that HCs could
act as selective reducing agents under excess of O2
[102]. This discovery was followed by a feverish ac-
tivity in the field of lean-DeNOxand more then 50 cat-
alysts were reported in 1991–1992 [103]. Since then
this topic has been reviewed by several researchers,
even though a comprehensive knowledge of the ex-
haust lean technology is still missing [2,5,102–116].
As outlined in a report issued by MECA (Manufac-
turers of Emissions Controls Association) [117] there
are two major strategies to control the NOxemissions
under oxidising conditions:
•DeNOx(lean-DeNOx) catalysts;
•NOxadsorbers (NOxtraps).
Fig. 13. Typical light-off behaviour for: (1) C3H6–NO–O2and (2) C3H8–NO–O2reactions over Pt/Al2O3. Pt: 1 wt.%, reactant feed:
1000ppm C3Hx, 500 ppm NO, 5% O2.W/F=4×10−4gminml−1(GHSV 72,000 h−1) (after Ref. [121]).
The former strategy employs a direct NOxreduction
catalyst, usually consisting of Pt/Al2O3and a metal-
loaded zeolite for NOxreduction at, respectively, low
and high temperature. The NOxadsorber technology
is sometimes called a NOxstorage/reduction (NSR)
catalyst. In this case typically a Pt/BaO/Al2O3cata-
lyst is used to store NOxunder oxidising conditions
as adsorbed “nitrate” species, which are then released
and reduced on a traditional TWC by temporarily
running the engine under rich conditions. Let us now
examine in some detail these systems.
4.2.1. Pt/Al2O3and derived systems
The activity of Pt/Al2O3catalysts for NOx
reduction under lean exhaust conditions has been in-
vestigated in detail by Burch et al. [118–136]. They
extensively analysed the effects of the nature of the
noble metal, reducing agent, sulphur addition, na-
ture of additives and of the support, and reaction
mechanism.
A typical reaction profile for HC reduction over
Pt/Al2O3is reported in Fig. 13. This figure sum-
marises some general features of Pt/Al2O3lean-burn
catalysts, that is: (i) a maximum of NO conversion at
relatively low temperature, NOxconversion peaking
as the HC conversion reaches 100% in the case of
C3H6; (ii) comparable starting temperatures for the
NO reduction and HC oxidation; (iii) significant NO2
formation at high temperatures when all the HC is
burned-out; (iv) strong sensitivity of the NO conver-
sion to the nature of the reducing agent (saturated
vs. unsaturated HC); (v) poor selectivity towards
di-nitrogen formation of the Pt catalyst, N2O being
the major product at low temperatures. As stated at
point (iv), this general behaviour strongly depends on

434 J. Kašpar et al./Catalysis Today 77 (2003) 419–449
the nature of the reducing agent: non-reactive (short
chain-saturated) HCs behave somewhat differently
from reactive HCs such as alkenes or long-chain HC
[127].
Let us discuss some important aspects of the above
quoted properties of the Pt lean-DeNOxcatalysts.
4.2.1.1. Light-off behaviour of the Pt catalysts. NO
conversion typically shows a volcano shaped curve ir-
respective of the nature of the HC. This is due on
one hand to the fact that NOxalways initiates to-
gether with the HC oxidation and on the other hand to
the fact that, with few exceptions, the maximum NO
conversion corresponds to 100% HC conversion. At
higher temperatures, when all the HC is consumed,
significant NO2formation occurs by reaction of NO
with excess O2, the amount of NO2formed being lim-
ited by thermodynamic constrains. Note, however, that
NO2concentration that apparently exceeded thermo-
dynamic values were also reported for an Ag/Al2O3
system [137]. This fact was attributed to the presence
organo-nitrite species as intermediates of the SCR pro-
cess, which oxidation/decomposition led to NO2pro-
duction.
The maximum of the NOxconversion is obviously
affected 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 above
light-off the reaction rate is limited either by heat or
mass transfer. As a consequence, the maximum of the
NOxconversion moves to low temperature and its in-
tensity increases as SV decreases, however, this ef-
fect is not directly proportional to the decrease of the
SV. Thus the maximum of the NO reduction peak
moved from 200◦C (conversion 70%) to 215◦C (con-
version 47%) as the SV was increased from 10,000 to
50,000h−1, while at intermediate SV =25,000h−1
a conversion of 57% was observed at approximately
210◦C[16]. From a practical point of view, exhaust
emissions of a diesel engine were characterised by
SV =28,000–88,000h−1as the temperatures varied
between 160 and 400◦C during the MVEuro2 driving
cycle [16].
A corollary of this fact is that any comparison of
activities simply based on light-off activities may be
strongly 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]:
1
T2−1
T1=R
Ealn f2(c) PtsF2
f1(c) PtsF1(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 NO
conversion, which depends on the temperature; and
f(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 of
Eq. (1) reveals that upon comparison of the activities
of two catalysts with different dispersions (i.e., there
are two different Ptsterms) under equivalent reaction
conditions (i.e., F1=F2and f1(c) =f2(c)), the
higher the number of active sites the lower the light-off
temperature. This means that the specific activity of
two catalysts may be compared using the light-off be-
haviour only when an equal number of active sites
is employed in the experiment. Unfortunately, very
few investigations concerning lean-DeNOxhave re-
ported reaction rates, light-off temperature being gen-
erally shown, which makes direct comparison difficult.
Note that when long-chain HC are investigated, which
can easily generate coke at the catalysts surface, the
light-off behaviour may not be relevant to the true cat-
alytic activity as pseudo-steady states were observed
below light-off temperature for short to medium peri-
ods (0.1–2 h) followed by a deactivation of the catalyst
[131]. It is important to keep present these limitations
when activity from different sources is compared.
Reaction rates and kinetic law for C3H6/NO/O2re-
action were reported in the literature for Pt/Al2O3
and Rh/Al2O3[140,141]. For Pt/Al2O3the following
expression were found experimentally at 230–236◦C
(NO and C3H6250–4000ppm, O20.5–12%, W/F =
0.0018gsml−1, GHSV ≈100,000 h−1, apparent ac-
tivation energy 24±3kcalmol−1):
reduction : r(NO)=kred [O2]
[C3H6]0.5
oxidation : r(C3H6)=kox [O2]
[NO]0.5[C3H6]0.5
For comparison, the kinetic expression for the com-
bustion of C3H6measured at 158◦Cwas
combustion : r(C3H6)=kcomb [O2]
[C3H6]0.5

J. Kašpar et al./Catalysis Today 77 (2003) 419–449 435
The apparent activation energies for NO reduc-
tion and C3H6oxidation were measured as 22 and
24kcal mol−1over Pt (1.5wt.%)/Al2O3[141]. Inter-
estingly, NOx, even if present at ppm level, interferes
with the HC oxidation. We recall that modification of
CO oxidation kinetics in the presence of NO is typi-
cally assumed as a direct indication that NO removal
occurs 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 Pt
lean-DeNOxcatalysts is that a narrow maximum of
activity is observed around 270–300◦C. The width
and position of this maximum of activity depends on
the reaction conditions and nature of the catalyst. As
is shown below, also the nature of the HC also plays
a fundamental role in the volcano shaped NOxcon-
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 by
researchers from Degussa (OMG) [142]. Even though
the nature of the catalyst employed was not speci-
fied, except that a 50g/ft3Pt honeycomb catalyst was
used, the finding of this paper represent typical results
observed over Pt/Al2O3in subsequent papers. These
findings can be summarised as follows:
•There is a remarkable difference in the response
of the Pt catalyst to the nature of the HC reducing
agent, e.g. the temperature of the maximum of HC
and NO conversion is nearly independent of the type
of linear alkene, while an increase of the length of
Fig. 14. Proposed reaction mechanism for lean-DeNOxreaction over Pt/Al2O3for alkane (1) and alkene (2) conversion (after Ref. [121]).
the chain of the alkane significantly shifts down this
temperature.
•At comparable HC chain length saturated HC are
much less effective compared to unsaturated ones.
Remarkably C16H34, which is a typical diesel fuel
component, featured an activity comparable to that
of 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 agent
showed a strong dependence of the conversion
efficiency on the reactivity of the molecule.
The remarkable difference of the catalytic behaviour
between the saturated and unsaturated HCs was ratio-
nalised in terms of the different reaction pathway for
the NO reduction in the presence of either saturated
or unsaturated HC (Fig. 14)[121,125]. It appears that
reduction of the unsaturated HC can be depicted as a
NOxdissociation reaction occurring the metal centre,
where the unsaturated HC is responsible for the re-
moval of the adsorbed oxygen generated by the NOx
dissociation. In the presence of weakly adsorbed re-
ducing agent, such as C3H8, adsorbed atomic oxygen
is the dominant species on the metal surface under re-
action conditions. The C3H8oxidation is inhibited by
both O2and in the facile oxidation of NO to NO2.
It is believed that the rate determining step in C3H8
oxidation by O2is the dissociative chemisorption of
C3H8involving the breaking of a C–H bond [123,125].
This is a difficult reaction and strongly depends on
the nature of the HC, which accounts for the strong

436 J. Kašpar et al./Catalysis Today 77 (2003) 419–449
effect of the carbon chain length upon the DeNOx
activity.
In addition, a comparison of different Pt/Al2O3cat-
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 NOxreaction, using C3H6as a reducing
agent, showed that even if the promoters have a signifi-
cant effect (beneficial or otherwise) on the activity and
temperature range of operation of Pt/Al2O3, they have
no significant effect on the N2/N2O selectivity. The
similarity in behaviour of the promoted catalysts and
unpromoted Pt/Al2O3suggests that the reaction mech-
anism was similar for all the catalysts tested [124]. The
ability of the Al2O3support to promote the interaction
of the adsorbed/migrated NOx/CxHyspecies gener-
ated from saturated HC, seems a particular property
of this support, since changing the support to SiO2re-
sulted in almost no reaction even with comparable Pt
loading and dispersion. Finally, a comparison of differ-
ent HCs as reducing agent revealed an unusual ability
of aromatic HC (toluene) to promote high selectivity
of the Pt catalysts towards formation of N2[122].It
should be also noted that when long-chain HCs are
employed, the light-off activity may be very different
from steady-state activity in that below or close to
light-off temperature (generally chosen as 50% of con-
version), catalysts poisoning with time-on-stream was
observed [127]. By increasing the reaction tempera-
ture conversion of adsorbed carbonaceous species oc-
curred, thus recovering the catalyst activity [130].No
deactivation was observed with small HC, formation
of carbonaceous species being minimal in this
case.
The effect of addition of SO2to the reaction feed
appears in line with the above interpretation of the
effect of the HC in that strong poisoning of activity
was observed when C3H8was employed as reducing
agent due to the formation of aluminium sulphate.
This is responsible for the poisoning of the catalytic
sites at the support [128]. In contrast the activity
was much less affected in the case of C3H6, which
is in line with the smaller sensitivity of the Pt par-
ticles towards sulphur poisoning compared to the
support.
Even though this reaction mechanism was ques-
tioned, in particular due to the lack of XPS evidence
for 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 for
development of a new generation catalyst. In fact, de-
spite the intensive research in the literature, the effect
of the nature of the support has little been investigated
[124,126,143–145], and most of the studies were car-
ried out using C3H6as reducing agent, where little or
no effect of the support should be expected. In fact, we
believe that in this case the major role of the support
is 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, effects
of support composition were detected [143], however,
there is not enough evidence on a clear effect of the
different supports in promoting Pt activity in lean-
DeNOx.
4.2.2. Other lean-DeNOxcatalysts
Numerous other catalytic systems have been investi-
gated as DeNOxcatalysts, however, they appear much
less applicability as next generation catalyst due to
generally low activity and/or stability of such systems.
For sake of convenience we will group all the catalysts
in the following categories:
•Cu-ZSM5 and related systems;
•metal oxide catalysts.
As above written, Cu-ZSM5 represented the first
major candidate for lean-DeNOxcatalysts, even
though it was quickly recognised that several prob-
lems affect the Cu-zeolite catalysts [103]. These
include: (i) poor hydrothermal stability of Cu species
and zeolite framework; (ii) appreciable activity only
at high temperature (300–400◦C), which only allows
the use of such system in conjunction with NM low
temperature catalysts; (iii) generally poor activity
under real exhaust and at high space velocities. The
higher range of reaction temperatures compared to
Pt catalysts makes these systems of potential interest
for lean-burn gasoline engine, where such high tem-
peratures are more easily met compared to the diesel
engine.
With the aim of improving the thermal stability
of the zeolite catalysts, a variety of supported and
even unsupported metal oxides have been investi-
gated. ZrO2itself was chosen as a possible candidate.
Cu/ZrO2and other supported metal oxides, both sul-
phated and sulphur-free, have been investigated to
some extent with the aim of improving the thermal

J. Kašpar et al./Catalysis Today 77 (2003) 419–449 437
stability of the catalyst compared to the zeolitic
system [146–154]. Even though some interesting
activities were claimed, to our knowledge, there is
not sufficient evidence for possible application of
such systems under real exhaust conditions. In fact,
even though some increase of thermal/hydrothermal
stability could be achieved, the activity is generally
poorer compared to the Cu-ZSM5 system. As a gen-
eral comment, it seems that despite the Edisonian
type of research on different metal oxides as DeNOx
catalysts, the “high activities” sometimes claimed
by the authors, withstand with difficulties the harsh
exhaust conditions. A relatively recent comparative
study of several types of “promising” lean-DeNOx
catalysts under diesel conditions is very illustra-
tive in this respect [155]: after testing nine different
classes of catalysts (Pt/Al2O3, Rh/Al2O3, Ag/Al2O3,
Pt-ZSM5,Cu-ZSM5, Pt/In-ZSM5, CeZSM5+Mn2O3,
Co/Al2O3, and Au/Al2O3+Mn2O3), the authors
came to the conclusion that despite the fact that some
high activities could be measured, particularly on the
NM-containing catalysts, there is no single phase cat-
alyst capable of satisfying the practical demand for
NOxremoval from diesel exhaust.
Ag catalysts are also among those extensively
studied since high activity were reported, particularly
when alcohols are employed as reducing agents (see,
for example [137,155–165]). 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/Al2O3catalysts (after Ref. [137]).
melting point of Ag, extensive sintering of the catalyst
may be expected even at relatively low temperatures.
The catalytic activity was shown to depend on the
Ag particle size [166], high particle sizes favouring
the unselective HC oxidation and N2O formation at
low temperatures (Fig. 15). On the contrary, highly
dispersed Ag particles favour formation of N2but
the reaction occurs at higher temperatures. This was
explained by the different reaction pathway according
to 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 N2selectivity
of the Ag catalysts is observed in the “high” range
of temperatures; accordingly this catalyst may be
considered as a substitute for the Cu-ZSM5 com-
ponent in a full-range of temperatures operating
lean-DeNOxcatalyst [2,169]. To promote the activity
at low temperatures, Ag could in principle be sintered,
however this promotes the unselective HC oxidation
[166]. An interesting way to promote the activity of
these catalysts is to add another NM to the system
[170,171].
Recently, we have shown that use of ZrO2or
ZrO2-rich CeO2–ZrO2mixed oxides as supports for
Ag strongly improves the activity at low tempera-
tures (Fig. 17)[172]. As shown by comparison of
activities of catalysts with different Ag dispersion

438 J. Kašpar et al./Catalysis Today 77 (2003) 419–449
Fig. 16. Reaction mechanism(s) proposed for the lean-DeNOxreaction over Ag/Al2O3catalysts (after Ref. [137]).
[172], the activity of Ag invariably occurs at lower
temperatures on ZrO2-containing supports compared
to Al2O3supports. More important is that the use
of ZrO2-containing supports remarkably facilitates
the regeneration of the catalysts from SOxpoisoning
compared to Ag/Al2O3.
4.2.3. Bi-functional lean-DeNOxcatalysts
The idea of bi-functionality to improve the activity
of the lean-DeNOxcatalysts has been pioneered by
Misono and co-workers. These studies were recently
reviewed [173], accordingly we refer the reader to this
review. Of the several systems described, it is impor-
Fig. 17. Effect of the nature of the support on lean-DeNOxactivity
of Ag catalysts: NOxreduction (empty symbols), C3H6oxidation
(filled symbols). The 1000ppm C3H6, 1000 ppm NO, 5% O2,
W/F =0.05gsml−1(after Ref. [172]).
tant to focus the mechanistic features of the authors’
work. Essentially, the authors favour the reaction path-
way which proceeds with formation of NO2as reac-
tion intermediate, which then efficiently reacts with
adsorbed HC to give surface intermediates. These in-
termediates then decompose leading to an overall re-
duction of NOx. A redox type component such as
Mn2O3or SnO2is also added, which favours oxida-
tion of NO to NO2(Fig. 18).
An interesting aspect of these systems is that the
presence of water, which normally deactivates the
DeNOxcatalysts, can even improve the catalytic
activity (Fig. 19). This was attributed to the partial
suppression of the direct HC oxidation at Mn2O3
(Fig. 18) that is responsible for non-selective HC
oxidation.
Among the bi-functional systems, the NM/zeolite
catalysts, containing particularly H-ZSM5, should
also be quoted. A number of researchers have in-
deed employed both Pt/ZSM5 and Pd/ZSM5 as
bi-functional systems for the lean-DeNOx(see, for
example [147,173–180]). Typically, such catalysts
and particularly those Pd-based were employed for
lean-DeNOxusing CH4as reducing agent. The
bi-functionality of this type of catalyst is related to
the necessity of acid sites, which apparently allow ac-
tivation of the HC at the support leading to selective
NOxreduction. In fact, using Na-ZSM5 as a support,
no NOxreduction was detected. Several zeolites were
employed for the CH4/NO/O2reaction, however,

J. Kašpar et al./Catalysis Today 77 (2003) 419–449 439
Fig. 18. Effect of addition of Mn2O3(as physical mixture) on the DeNOxactivity of Ce-ZSM5; and the proposed reaction mechanism
(after Ref. [173]).
significant deactivation of the catalyst occurred in the
presence of water and SOx.
While a somewhat extensive description of the vari-
ous attempts to develop efficient lean-DeNOxcatalysts
is reported here, it is important to outline that these
systems do not ensure sufficient activity to foresee
practical applications in a future. Development of new
breakthrough strategies is an important target for the
comming years to achieve significant environmental
benefits from the use of lean, high-efficiency engines.
Fig. 19. Effect of addition of H2O on the activity of
Mn2O3/Sn-ZSM5 bi-functional catalyst (after Ref. [173]).
4.3. Lean NOxtraps
The discovery by Toyota researchers of the
so-called NSR catalysts also triggered feverish activ-
ity in lean-DeNOxstudies [181–183]. The principle
of the reaction mechanism of these systems appears
well established [183]. Under oxidising conditions
NOxare stored at the surface of a Ba-containing cat-
alyst under various forms (surface nitrites/nitrates),
which exact nature is still matter of debate [184–187]
(Fig. 20). After a certain period, which length is an
important factor and is correlated to the specific emis-
sion/catalyst characteristics, the A/F ratio is set to
rich and the stored NOxspecies are reduced over Pt
or, more generally, TWC-type catalyst to N2[183].
The mechanism of NOxadsorption and desorp-
tion/reduction has been investigated by a number of
authors[112,184,185,187–197]. It appears now clearly
the storage/reduction is rather complex process due to
the complex nature of the exhaust mixture. For exam-
ple, model studies performed on Pt/BaO/Al2O3sug-
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 and
then oxidised to nitrates or whether both species are
formed directly by a disproportionation mechanism
[194]. However, the final species that is strongly held
on the surface and accounts for the majority of NOx

440 J. Kašpar et al./Catalysis Today 77 (2003) 419–449
Fig. 20. Principle of operation of an NSR catalyst: NOxare stored
under oxidising conditions (1) and then reduced on a TWC when
the A/F is temporarily switched to rich conditions (2).
stored appears to be a nitrate species, in particular at
high temperature due to the low thermal stability of
nitrite. Whatever is the true mechanism, it must be
underlined that the kinetics and the extent of storage
are heavily affected by the presence of water and CO2
in the exhausts: CO2slows down the NOxadsorp-
tion kinetics as the reaction can more appropriately be
seen as a transformation of surface carbonates into ni-
trates, e.g. CO2strongly competes with NOxfor the
adsorption sites [190,196]. This competition, on the
other hand, increases the rate of NOxreleases under
the rich-spike [188,189]. The effect of water is more
controversial in that promotion of NOxadsorption was
observed below 250◦C by addition of small amounts
of water (1%), whereas at higher temperature an inhi-
bition effect was observed [194]. However, such pro-
moting effect was not seen when both water CO2were
co-fed.
The NSR technology is by far the most reliable
and attractive lean-DeNOxtechnology and it has been
commercialised in Japan where low sulphur gasoline
is available. In fact, the major drawback of the NSR
catalyst is its sensitivity to SOxdue to the fact that
surface sulphates are invariably more thermally stable
compared to the nitrates [198]. The durability aspects
of the NSR catalysts were addressed, for example,
by researchers from OMG [199] and there seems to
be general agreement that poisoning of the NOxstor-
age function is directly related to the amount of SO2
passed over the catalyst. This is an important aspect
since it suggests that for application of these catalysts
to US or European markets, where higher sulphur con-
tents are present in the fuel compared to Japan, appro-
priate strategies to develop sulphur resistant NOxtrap
must be applied. In addition to the obvious require-
ment of lowering of sulphur content in the fuel, there
are strategies that can be adopted for increasing sul-
phur tolerance of the converter: (i) adoption of an SOx
adsorber that protects the NOxtrap and is periodically
regenerated; (ii) modification of the catalyst compo-
sition to promote of the removal efficiency of the ad-
sorbed SOx. An interesting example of such strategies
was recently reported by Toyota [200,201]. In their
system, TiO2was added to protect the barium-based
trap from sulphur poisoning due to its high sulphur
tolerance. LiO was added as it was observed that
Li-promoted Al2O3releases accumulated sulphur
more easily compared to pure Al2O3. Rh/ZrO2was
also employed to enhance the sulphur removal under
reducing conditions due to its effectiveness as a steam
reforming catalyst. In fact, an efficient H2generation
under the rich-spike of the cycle strongly favours the
removal of adsorbed SOx. It should be noted that
release of H2S from the catalysts is undesirable, ac-
cordingly special schedules of the modulation of the
A/F during the rich phases can be adopted, which
pump additional oxygen during the rich-de-sulphation
phase minimising H2S release [202]. Finally, thermal
deactivation due to sintering of the barium species
and formation of barium aluminates may represent
an issue in terms of durability of the catalyst [203].
Accordingly, thermally stable Ba-containing materi-
als, such as doped aluminas or perovskites, has been
investigated as NOxabsorbers [184,185,204].
It is worth of noting that the NSR strategy has also
been applied to diesel engines. In this case generation
of the rich conditions must be carefully considered
as switching A/F to rich conditions easily gener-
ates typical black-smoke-containing emissions, often
found in older vehicles. Low temperature smokeless
combustion with a massive EGR or, more frequently,
post-injection of fuel are employed to temporarily
generate rich exhaust. The interesting point is that the
NSR component may be deposited on the walls of the
porous ceramic filter so that the precious metal can

J. Kašpar et al./Catalysis Today 77 (2003) 419–449 441
Fig. 21. Engine-out and tailpipe-out emissions from a 1.9L diesel engine over an NSR catalysts deposited on a ceramic filter. Notice that
similar 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 deactivation
becomes even more stringent in the case of diesel
vehicles due to the generally higher amounts of sul-
phur in the fuel and low operating temperatures that
do not allow efficient trap regeneration. Rich condi-
tions for several minutes and temperatures as high as
650–700◦C are typically needed to achieve effective
trap de-sulphurisation [202].
4.4. Selective catalytic NOxreduction using urea
Due to the limited success of HCs as efficient re-
ducing agent under lean conditions, the use of urea
as an alternative reducing agent for NOxfrom heavy
duty diesel2vehicles has received attention. Selective
2Light-duty (LD) diesel engines are generally defined as vehicles
with an engine displacement of less than 4l and power output
of up to 100kW, and are characterised by relatively high engine
speeds. LD engines would normally be found in passenger vehicle
and light commercial vehicle applications. Heavy duty (HD) diesel
engines may be generally defined as of displacement greater than
8l and power outputs of greater than 150kW. HD engines are
found in heavy road transport, industrial and marine applications.
Medium duty engines fill the gap in the middle and are found in
medium size trucks, buses and light industrial equipment.
catalytic reduction of NOxwith NH3in the presence
of excess O2is a well implemented technology for
NOxabatement from stationary sources [206]. Typi-
cally, vanadia supported on TiO2, with different pro-
moters (WO3and MoO3) are employed in monolith
type of catalysts. A sketch of an arrangement for the
urea based NOxabatement technology is shown in
Fig. 22. Typically, the urea solution is vaporised and
injected into a pre-heated zone where hydrolysis oc-
curs according to the reaction:
H2N–CO–NH2+H2O→CO2+2NH3
Ammonia then reacts with NO and NO2on the reduc-
tion catalyst via the following reactions:
4NO +4NH3+O2→4N2+6H2O
6NO2+8NH3→7N2+12H2O
This approach has proved to be quite successful
and high NOx(up to 80%) could be achieved on
HD under driving conditions, even after reasonably
high mileages (200,000–300,000km), the activity de-
creased to about 75–80% of the initial value after over
500,000km [207,208]. A major problem of such sys-
tem is that extreme care must be exercised to develop
a suitable urea injection strategy that avoids over-
loading of the system leading to ammonia slip [209].

442 J. Kašpar et al./Catalysis Today 77 (2003) 419–449
Fig. 22. A typical arrangement for abatement of NOxfrom a heavy duty diesel engine using urea as reducing agent.
Typically, ammonia slip should not exceed 10ppm.
While the efficiency of urea-SCR technology is recog-
nised [210], there are certainly still a number of issues
concerning the catalyst efficiency at low temperatures,
the design of compact converter systems that require
higher conversion efficiency and, last but not least, the
issue of generalised urea distribution when fuelling the
vehicle.
Given the efficiency of such systems, the applica-
tion of the urea-SCR technology to LD vehicles was
also investigated [211]: while appreciable NOxcan
be 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 may
represent 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 to
diesel engines, the general interest of these sys-
tems is related to the above quoted desire to use
high-efficiency engines. Diesel particulate matter
(DPM) is the most complex of diesel emissions. Diesel
particulates, 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 fuel
sulphur. DPM contains a large portion of the polynu-
clear aromatic hydrocarbons (PAH) found in diesel
exhaust. Diesel particulates contains small nuclei
with diameters below 0.04␮m, which agglomerate
forming particles as large as 1␮m. The non-gaseous
diesel emissions are grouped into three categories:
soluble organic fraction (SOF), sulphate and soot
[212].
Removal of the liquid fraction of PM is generally
achieved by an oxidation catalyst. Oxidation catalysts
have been fitted to US medium duty diesel vehicles
since 1994 to reduce emissions of HC, the SOF con-
tent of DPM, and CO [212]. Typically, these catalysts
are composed of NM/CeO2/Al2O3(NM =Pt) sys-
tems, where porosity of the catalysts often plays a key
role since adsorption of the SOF at the support allows
its conversion at catalytic sites and hence its removal
before its desorption starts [212]. This is a critical as-
pect in the diesel exhaust removal due to the generally
low temperatures (120–350◦C) of the diesel exhaust
[213]. In fact, during the test cycle, the temperature
of the catalyst may easily fall below the light-off tem-
perature making necessary additions of an adsorbent,
typically a zeolite. Oxidation catalysts promote the
oxidation of HC and CO with oxygen in the exhaust to

J. Kašpar et al./Catalysis Today 77 (2003) 419–449 443
form CO2and H2O. Fuel sulphur levels of maximum
500ppm are required to avoid excessive production
of sulphate based PM and to minimise catalyst deacti-
vation by sulphur poisoning. Lower levels of sulphur
(50ppm) can increase the effectiveness of oxidation
catalysts by up to 50% and contribute to greater dura-
bility. Oxidation catalysts have not generally been used
in heavy vehicles with the exception of urban buses,
and are not considered necessary to meet HC and CO
requirements of future HD emission regulations.
Removal of soot may be achieved by means of fil-
tration (Fig. 23)[214]. Even though different types
of filters can be employed [215], the filtration effi-
ciency is generally high. However, the continuous use
under the driving conditions leads to filter plugging.
Regeneration of the filter is therefore a crucial step of
the 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 in
Fig. 23. However, such systems may be adopted only
in the trucks where space requirements are less strin-
gent compared to passenger cars. In addition, there are
problems arising from the high temperatures achieved
during the regeneration step when the deposited soot
is burned off. In fact, local overheating can easily oc-
cur leading to sintering with consequent permanent
plugging of the filter. To overcome these problems,
development of catalytic filters has attracted the in-
terested of many researchers (for a recent review, see
Ref. [214]).
Fig. 23. Principle of filter operation (1) and filter re-generation (2) for a soot removal system, using fuel powered burners.
Acomprehensive discussion of the diesel particulate
abatement 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 being
those of the CRT [216] and use of fuel additives that
favour combustion of the soot deposited on the filter
[217,218]. The concept of the so-called CRT has been
pioneered by researchers from Johnson Matthey [216]
and is based on the observation that NO2is a more
powerful oxidising agent towards the soot compared
to 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 second
part of CRT, DPM reacts with NO2favouring a con-
tinuous regeneration of the trap. A major drawback of
these systems is related to the capability of Pt cata-
lysts to promote SO2oxidation as well. The sulphate
thus formed is then deposited on the particulate filter
interfering with its regeneration. Moreover, the NO2
reacts with the soot to reform NO whilst reduction of
NO2to N2would be the desirable process. Accord-
ingly, it is expected that as the NOxemission limits
will be pushed down by the legislation, less NO will
be available in the exhaust for soot removal, unless
the engine is tuned for high NOxemission that are
used in the CRT and then an additional DeNOxtrap
is located after the CRT device.
Use of fuel additives, particularly those based on
CeO2, is another area of interest [219]. Rhodia has
introduced these additives on the market which now

444 J. Kašpar et al./Catalysis Today 77 (2003) 419–449
Fig. 24. The working principle of the continuously regenerating particulate trap.
have been applied in vehicles [217,218]. While there
is a significant benefit in terms of promotion of the
soot combustion from addition different additives even
at very small concentrations [218], the environmental
impact due the release of such additives into the at-
mosphere, following their widespread use, should be
considered.
5. Conclusions
The development of automotive converters has
proceeded by a continuous improvement of the cat-
alytic performances and durability of the automotive
catalysts over the past 25 years. Use of CeO2–ZrO2
technology represented a major improvement in terms
of TWC durability in the last years, however, more
and 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 effective
and that its lifetime must be equivalent to that of the
car. This demands new materials of extreme thermal
stability, exceeding 1100 ◦C, which show extremely
high conversions. The achievement of such targets
require strong research efforts; a fundamental com-
prehension of the interactions between the NM and
the other washcoat components and the deactivation
phenomena is needed.
The requirements for more and more efficient en-
gines highlights the problem of NOxabatement under
oxidising conditions. Even though huge efforts have
been dedicated to development of lean-DeNOxcata-
lysts, their durability and performances are still insuf-
ficient. Noticeably, of the different lean-DeNOxstrate-
gies for gasoline engines, the most effective and ready
to use one is the so-called NSR concept which still
uses a TWC to eliminate NOx[112].
Technologies for control of particulate emissions
from diesel engine will find increasing demand in the
next years. In particular, catalytic filters will continue
to 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 be
achieved only by adopting new strategies based more
on prevention than on control. In this respect, it is
important to highlight the great promise of hydrogen
fuel cell technology [220]. The proton exchange mem-
brane fuel cell—to make hydrogen from HCs—will be
a major focus for research in electrocatalysis and cat-
alytic fuel processing. It is worth noting that the targets
obtained in the development of materials for TWCs
constitutes an important scientific background for the
design of new catalytic system for on-board hydrogen
production.In fact, the on-board fuel reformerunit em-
ploys a number of catalytic steps involving reactions
that routinely occur under the exhaust conditions and
most of them appear to be promoted by the NM/CeO2
interactions [13]. Consistently, M/CeO2–ZrO2mate-
rials were reported to feature good activities for fuel
reforming, WGS reaction and preferential CO oxida-
tion [221–230]. However, further work is necessary
to significantly enhance their performance, in order to
obtain a miniaturisation of the system and therefore
application in automobiles.

J. Kašpar et al./Catalysis Today 77 (2003) 419–449 445
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 for
their valuable contribution to the development of the
chemistry of CeO2–ZrO2mixed oxides and helpful
discussions. MEL Chemicals, University of Trieste,
Fondo Trieste 1999, MURST-PRIN 2000 “Cataly-
sis for the reduction of the environmental impact
of mobile source emissions”, CNR Agenzia 2000,
INCA “Progetto Atmosfera Urbana” are gratefully
acknowledged for financial support.
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