Progress and future challenges in controlling automotive exhaust gas emissions

Applied Catalysis B: Environmental 70 (2007) 2–15 www.elsevier.com/locate/apcatb

Progress and future challenges in controlling automotive exhaust gas emissions Martyn V. Twigg * Johnson Matthey Catalysts, Royston, Herts SG8 5HE, United Kingdom Available online 30 June 2006

Abstract By the early 1970s increased use of cars in some major cities had resulted in serious concerns about urban air quality caused by engine exhaust gas emissions themselves, and by the more harmful species derived from them via photochemical reactions. The three main exhaust gas pollutants are hydrocarbons (including partially oxidised organic compounds), carbon monoxide and nitrogen oxides. Engine modifications alone were not sufficient to control them, and catalytic systems were introduced to do this. This catalytic chemistry involves activation of small pollutant molecules that is achieved particularly effectively over platinum group metal catalysts. Catalytic emissions control was introduced first in the form of platinum-based oxidation catalysts that lowered hydrocarbon and carbon monoxide emissions. Reduction of nitrogen oxides to nitrogen was initially done over a platinum/rhodium catalyst prior to oxidation, and subsequently simultaneous conversion of all three pollutants over a single three-way catalyst to harmless products became possible when the composition of the exhaust gas could be maintained close to the stoichiometric point. Today modern cars with three-way catalysts can achieve almost complete removal of all three exhaust pollutants over the life of the vehicle. There is now a high level of interest, especially in Europe, in improved fuel-efficient vehicles with reduced carbon dioxide emissions, and ‘‘leanburn’’ engines, particularly diesels that can provide better fuel economy. Here oxidation of hydrocarbons and carbon monoxide is fairly straightforward, but direct reduction of NOx under lean conditions is practically impossible. Two very different approaches are being developed for lean-NOx control; these are NOx-trapping with periodic reductive regeneration, and selective catalytic reduction (SCR) with ammonia or hydrocarbon. Good progress has been made in developing these technologies and they are gradually being introduced into production. Because of the nature of the diesel engine combustion process they produce more particulate matter (PM) or soot than gasoline engines, and this gives rise to health concerns. The exhaust temperature of heavy-duty diesels is high enough (250–400 8C) for nitric oxide to be converted to nitrogen dioxide over an upstream platinum catalyst, and this smoothly oxidises retained soot in the filter. The exhaust temperature of passenger car diesels is too low for this to take place all the time, so trapped soot is periodically burnt in oxygen above 550 8C. Here a platinum catalyst is used to oxidise higher than normal amounts of hydrocarbon and carbon monoxide upstream of the filter to give sufficient temperature for soot combustion to take place with oxygen. Diesel PM control is discussed in terms of a range of vehicle applications, including very recent results from actual on-road measurements involving a mobile laboratory, and the technical challenges associated with developing ultra-clean diesel-powered cars are discussed. # 2006 Elsevier B.V. All rights reserved. Keywords: Vehicle emissions; Autocatalysts; Three-way catalysts (TWCs); NOx-traps; Selective catalytic reduction (SCR); Diesel particulate filters

1. Introduction Even by the 1940 and 1950s air quality problems were experienced in some urban cities because of the increasing number of cars [1–6]. This was especially noticeable in locations such as the Los Angeles’ basin where temperature inversions trap and recycle polluted air [7]. By the 1960s cars had been in large-

* Tel.: +44 1763 253 141; fax: +44 1763 253 815. E-mail address: [email protected]. 0926-3373/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2006.02.029

scale mass production for many years, and they gave personal mobility to an increasing range of people. But, oxidation of gasoline in the engine to CO2 and H2O was far from completely efficient, Scheme 1, so the exhaust gas contained significant amounts of unburned hydrocarbons and lower levels of partially combusted products like aldehydes, ketones, and carboxylic acids, together with large amounts of CO. Unburned fuel and other hydrocarbons formed by pyrolysis, and various oxygenated species are referred to as ‘‘hydrocarbons’’ and designated HC. At the high temperature during the explosive combustion in the cylinder N2 and O2 react to establish the endothermic

M.V. Twigg / Applied Catalysis B: Environmental 70 (2007) 2–15

3

Scheme 2. Scheme 1.

equilibrium with nitric oxide (NO). This equilibrium is then frozen as the hot product gases are rapidly cooled and ejected into the exhaust manifold. The combination of NO and any of its oxidised form nitrogen dioxide (NO2) is referred to as NOx and more than a thousand ppm can be present in exhaust of a gasoline engine. The three major primary pollutants in the exhaust gases from cars are therefore NOx, HC and CO. In some American cities irritating photochemical smogs became so frequent that air quality was a major health concern. The origin of these photochemical smogs was two of the primary pollutants from cars. They were of concern in their own right, but they underwent photochemical reactions to generate ozone, a strong irritant, as well as low levels of other even more noxious compounds [8]. Fig. 1 shows the increase in atmospheric oxidant levels during a day in summer in Los Angeles during the 1970s; peak levels were reached during the early afternoon. This trend followed the sunlight intensity, and it was established ozone was the main ‘‘oxidant’’ that was produced via the photochemical

Scheme 3.

dissociation of NO2 followed by the reaction of the atomic oxygen formed with O2, as shown in Scheme 2 in which ‘‘M’’ is a ‘‘third body’’ that removes energy that would otherwise cause the dissociation of O3. However, it is mainly NO that is formed in engines, and not NO2, and the rate of NO oxidation to NO2 in air is extremely slow. The oxidation of NO to NO2 is a third-order reaction the rate of which depends on the square of the very low NO concentration [9], as illustrated in Scheme 3, so the formation of NO2 could not have resulted from the direct oxidation of NO. The actual formation of NO2 in air, the ozone precursor, actually involves free radical oxidation of HC (or more slowly with CO), and one of the more important series of free radical reactions leading to NO2 is shown in Scheme 4. Overall the process corresponds to the oxidation of hydrocarbon in the presence of NO to give NO2, an aldehyde, and water. The reactive aldehyde can undergo further reactions with NO2 to give, for example, peroxyacetylnitrate (PAN) according to Scheme 5. PAN is a very strong lachrymator [10,11], and even minute traces of it cause serious eye irritation and painful breathing. Levels of tailpipe pollutants from American cars in the mid1960s were typically HC 15 g/mile; CO 90 g/mile; NOx 6 g/ mile [12]. Engine modifications could not alone meet the

Scheme 4.

Fig. 1. Variation of ambient atmospheric ‘‘oxidant’’ levels in a California City during a Summer day in the 1970s. The ‘‘oxidant’’ is mainly ozone, and peaked in early afternoon.

Scheme 5.

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demands of the 1970 Clean Air Act [13] that was designed to make significant improvements in air quality over a reasonable period of time. As a result, catalytic systems were introduced to control exhaust emissions. 2. Early autocatalysts

Scheme 6.

2.1. Choice of catalyst types The exhaust of an internal combustion engine is a demanding environment, and very unlike the steady-state operation of most chemical plant catalytic processes [14]. The catalyst must function at low temperature, resist effects of thermal excursions up to about 1000 8C, tolerate the presence of poisons (especially sulphur species) and not be affected by gas flow pulsations and severe mechanical vibrations. At first it was necessary to oxidise HC and CO, and base metal catalysts, for example those containing copper and nickel were tested. It was established they were sensitive to poisoning (initially by lead and halide from antiknock additives in the fuel, and sulphur dioxide derived from sulphur compounds originally present in the fuel and lubrication oils), and furthermore they did not have good thermal durability [15,16]. Platinum group metal catalysts were found to be very active, and a huge amount of work was done with ruthenium, but its oxides are so volatile it was not possible to prepare a catalyst that did not lose ruthenium during use [17]. Even iridium oxides are too volatile at high temperatures, so this metal could not be used in practical catalysts [18]. However, especially platinum, as well as palladium and rhodium met the requirements of having the nobility to remain metallic under most operating conditions, and not have volatile oxides that led to metal loss, and these metals have been used in autocatalysts since their introduction [19]. Of these platinum is the most noble, but catalysts containing it when very hot and exposed to oxygen for long periods can sinter through a process involving migration of oxide species. Palladium forms a more stable oxide, and this is catalytically active in oxidation reactions. Rhodium oxide (Rh2O3) is quite readily formed from the metal under hot oxidising conditions, and this can readily undergo reactions with catalyst support compounds such as alumina as shown in Scheme 6. The main role of rhodium is in NOx reduction, and

since it is metallic rhodium that is active in this reaction, it is important this can be made available rapidly when any oxidising conditions return to being slightly reducing. However, this can be a relatively slow process, because for example, there are no active centres for the dissociation of reductant species, especially hydrogen. This re-reduction process can itself be catalysed by the presence of a small amount of other metallic species that provide, for example, a mechanism for hydrogen dissociation and accelerated reduction of rhodium oxide. Table 1 lists some of the physical properties of platinum group metals together with those for base metals that are used in industrial catalysts. Frequently two or more metals are used in combination in autocatalysts. Platinum/palladium was used in some of the early oxidation catalysts, as was platinum/rhodium that was also used under rich conditions for NOx reduction. Today three-way catalysts (see below) most commonly contain palladium/rhodium although platinum/rhodium is still used on some cars. 2.2. Early oxidation catalysts The first cars with oxidation catalysts injected air into the rich (excess fuel and therefore reducing) exhaust gas to provide oxygen for oxidation of HCs and CO. Some traditional pelleted platinum catalysts, from the chemical process industry were used, in a flat radial flow-like reactor. This configuration was not ideal because of gas-by-pass, but at that time the conversions required were not as high as today so sufficient conversions could be achieved. However, attrition of the pellets caused by their movement against each other under the influence of the pulsating gas flow and vibration of the vehicle was a major concern. An alternative catalyst structure made use of a ceramic monolithic honeycomb. For strength reasons they had relatively low porosity that made them unsuitable as a

Table 1 Physical properties of some selected metals and their oxides relevant to their behaviour in autocatalysts Metal

Atomic number

Atomic weight

Density (g cm3)

mp (K)

Reduction potential (V) Mn+ ! M0 (n)

Oxide stability under operating conditions

Platinum Iridium Palladium Rhodium Osmium Ruthenium Copper Cobalt Nickel Iron

77 46 45 76 44 29 27 28 26 78

195.08 192.22 106.42 102.91 190.2 101.07 63.33 58.93 58.69 55.85

21.45 22.56 12.02 12.41 22.59 12.37 8.96 8.90 8.90 7.87

2045 2683 1825 2239 3327 2583 1357 1768 1726 1808

1.19 (2) 1.16 (3) 0.92 (2) 0.76 (3) N/A (2) N/A (2) 0.34 (2) 0.28 (2) 0.30 (2) 0.44 (2)

Unstable oxides Moderately stable oxides Stable oxides Stable oxides Very volatile oxides Very volatile oxides Stable oxides Stable oxides Stable oxides Stable oxides

Numerical data from: J. Emsley, The Elements, 2nd ed., Clarendon Press, Oxford, 1993. mp = melting point.

M.V. Twigg / Applied Catalysis B: Environmental 70 (2007) 2–15

Fig. 2. An example of how a cordierite monolith is retained in a stainless steel mantle with an intumescent mat. Vermiculite in the mat exfoliates when heated and permanently retains the monolith in place (Courtesy Corning Inc.).

catalyst support [20]. This was overcome by applying a thin layer of high surface area catalytically active material to the channel walls [21]. This layer, typically 20–150 mm thick depending on the nature of the honeycomb and the application concerned, is referred to as a washcoat. The process of applying it is called washcoating and the washcoat surface area is typically about 100 m2/g. The monoliths are made from cordierite that has an exceptionally low coefficient of thermal expansion that is needed to prevent them from cracking when thermally stressed during use. Monoliths are manufactured by extruding [22] a suitable mixture of clay, talc, alumina and water with various organic additives, that is dried and fired at high temperature. During this process cordierite, 2MgO2Al2O35SiO2, is formed [23]. Fig. 2 shows one way a ceramic monolith can be retained in a stainless steel mantle that is welded into the exhaust system. It is wrapped in an intumescent mat typically containing inorganic fibres (such as rock wool), vermiculite and an organic binder. When first used the converter experiences temperatures that decompose the organic binder and cause the vermiculite to exfoliate. The force of this expansion is considerable, and it exerts a pressure on the monolith that keeps it firmly in place for the life of the vehicle. The long-term retaining force provided by the exfoliation of vermiculite is amazing, as was the impact of fitting oxidation catalysts in the exhaust systems of cars. There was a very considerable reduction in HC and CO emissions, but there was little or no effect on the NOx emissions. 2.3. Early Nox emissions control Nitric oxide is a thermodynamically unstable compound (enthalphy of formation DHf = +90 kJ/mol), it is a free radical yet under practical conditions in the presence of oxygen dissociation does not take place [24], and it can only be converted to nitrogen via a reductive process. The first approach for controlling NOx from car engines was to reduce it over a

5

Fig. 3. Schematic arrangement of oxidation catalyst and air injection point used initially to lower HC and CO emissions. The lower later modification used air injection after a platinum/rhodium catalyst operating under rich conditions to reduce NOx, then HC and CO were oxidised in a second stage after air injection. In this way all three pollutants were controlled in a two stage process.

platinum/rhodium catalyst in rich exhaust gas before air was added to permit oxidation of HC and CO over an oxidation catalyst [25]. This arrangement, and the earlier oxidation catalyst only system are illustrated schematically in Fig. 3. The selectivity of the catalyst used and the conditions employed for the NOx reduction had to ensure a high degree of selectivity so as not to reduce NOx to NH3 or SO2 to H2S. These reactions shown in Scheme 7 are undesirable because of the smell and toxicity of the products. Good selectivity was achieved and this system enabled markedly lower emissions of HC, CO and NOx to be achieved in a reliable way. 3. Three-way gasoline catalysts (TWCs) The engines with the earliest catalytic emissions control systems were fuelled via carburettors that could not precisely control the amount of fuel that was mixed with the intake air. Often the air/fuel ratio moved randomly either side of the stoichiometric point, and it was observed a platinum/rhodium catalyst could, under appropriate conditions, simultaneously convert CO and HC (oxidations) and reduce NOx with high efficiency [26]. This catalyst concept became known as a threeway catalyst (TWC), because all three pollutants are removed from the exhaust gas simultaneously. Application of the TWC required three elements: 1. Electronic fuel injection (EFI) so precise amounts of fuel could be metered to provide a stoichiometric air/fuel mixture.

Scheme 7.

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2. An oxygen sensor in the exhaust to provide an electrical signal indicating if the engine is running rich or lean. 3. A microprocessor to control a feedback-loop using oxygen sensor signals to determine the amount of fuel to be injected under specific conditions to maintain the exhaust gas close to the stoichiometric point. Scheme 9.

By the early 1980s all of the elements necessary for the operation of TWCs were available. This became a more efficient means of controlling HC, CO and NOx emissions than the earlier two catalyst systems, and it was also more cost effective. Soon TWCs were universally adopted. 3.1. Oxygen storage components During the development of TWC formulations cerium compounds were incorporated that are redox active: under lean conditions (oxidising) they absorb oxygen, and under rich (reducing) conditions oxygen is released from them [27]. In this way the composition of the exhaust is buffered around the stoichiometric point, and this enhances conversion of all three pollutants, and especially NOx. The reactions involved in oxygen storage are illustrated in Scheme 8, and make use of the two easily accessible oxidation states of cerium at exhaust gas temperatures. The total oxygen storage capacity (OSC) is directly related to the amount of cerium oxide present, although not all of this is available during short engine transients for kinetic reasons. Since the introduction of oxygen storage components into TWCs there was a move to using increasingly more thermally stable components, and minimising negative interactions between them. For example, it is possible to optimise the environment around platinum, and if this is different from that which is optimal for rhodium it is advantageous to physically divide the catalyst into two (or more) layers containing wellseparated different active metal dispersions with their specific promoter packages [28]. Usually platinum and palladium function best in oxidation roˆles, and they are often located in the bottom part of a two-layer TWC. Rhodium in the top layer is then exposed to all of the reductant species that reduce NOx before the exhaust gases diffuse to the lower layer where they are oxidised. Physical separation into layers enhances overall catalyst performance and life by preventing alloy formation, separating otherwise incompatible promoters, and encouraging desired reactivity by matching catalytic functionality by imposing appropriate diffusing conditions on reactants. By the correct use of promoters, particularly alkaline earth and lanthanide oxides, it was possible to modify the catalytic properties of palladium so it can function as a TWC and catalyse reduction of NOx as well as oxidation of CO and HC [29]. This entails interplay between catalysis by palladium

Scheme 8.

metal and its oxide, the presence of which can be controlled by close contact with cations that stabilise surface oxygen. Again separating the catalyst coating into two layers can minimise cross-contamination, and help obtain long lasting high activity. Perhaps the alkaline promoted NOx reduction reaction with palladium-only TWCs involves the water gas shift reaction that produces hydrogen which very efficiently reduces NOx as shown in Scheme 9. Here it is postulated surface formate intermediates may be involved in converting CO to H2 as in some copper catalysed synthesis gas reactions [30], although other mechanisms involving reduced cerium species are also possible. 3.2. TWC substrate types Extruded ceramic monoliths are widely used for TWC production, but in some situations monoliths made by rolling metal foils are used. For example, the use of thin foil can provide, when appropriately coated, a catalyst with low backpressure characteristics that can be advantageous on high performance cars. These metal-based catalysts can be welded directly into the exhaust system [31]. More recently there have been advances in extruding thin wall ceramic monoliths, and these have been widely used. They have relatively low thermal mass and high geometric surface area that facilitate fast catalyst light-off after the engine has started. The decision about which type of substrate is used, metallic or ceramic, depends on a balance between these properties and the overall system cost. 3.3. Control of H2S emissions TWCs can be exposed to periods of lean driving, for example during decelerations when there may be fuel cut-off to help improve fuel economy. Then sulphur dioxide present in the fuel as organosulphur compounds can be oxidised to sulphur trioxide which in turn can be captured by basic catalyst components that are converted to sulphates such as aluminium and cerium sulphates. Later under slightly rich conditions, for example during hard accelerations stored sulphate can be reduced to hydrogen sulphide, that has a particularly objectionable odour. Careful control of the engine operating parameters (calibration), and modified catalyst formulations have enabled good control of these undesirable emissions [32], but of course the presence of high levels of sulphur compounds in gasoline is itself undesireable because SO2 is an unwanted pollutant, and today sulphur fuel levels are very much less than they were. To moderate H2S emissions catalyst formulations often contained components such as iron, nickel, and

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manganese compounds that acted as getters to capture reduced sulphur species and prevent them from being released into the exhaust gas. Later when conditions were slightly oxidising the trapped sulphur compounds were released as much less odourous SO2.

characterising them in detail [34] it is possible to use these data to simulate the overall behaviour in a computer model. Such models are very useful in designing systems for new engine/ vehicle combinations by predicting tailpipe emissions for different possible layouts. A recently reported [35] model incorporated reactions (1)–(9):

3.4. On-board diagnostics (OBD)

H2 þ 0:5O2 ! H2 O

(1)

Legislation now demands the functioning of TWCs is periodically interrogated during use, and if performance is lower than a predetermined level the incident is reported and stored in the on-board computer [33]. If poor performance persists a malfunction indicator lamp (MIL) is turned on, so the driver can have the fault corrected. The OBD system makes use of two oxygen sensors, one upstream and one downstream of the catalyst. By running slightly lean for a short period the oxygen storage component in the catalyst is converted into its fully oxidised form, at which point the engine is run slightly rich and the time taken for the gas exiting the catalyst to become slightly rich, as detected by the second oxygen sensor, is a direct measure of the oxygen storage capacity. This measurement is related to catalytic performance, and so it can be used as a criterion for the OBD requirement. In practice this approach, or a modified alternative form, works very well, and Fig. 4 illustrates the fundamentals of monitoring OSC using two oxygen sensors.

CO þ 0:5O2 ! CO2

(2)

C3 H6 þ 4:5O2 ! 3CO2 þ 3H2 O

(3)

C3 H8 þ 5O2 ! 3CO2 þ 4H2 O

(4)

H2 þ NO ! H2 O þ 0:5N2

(5)

CO þ NO ! CO2 þ 0:5N2

(6)

C3 H6 þ 9NO ! 3CO2 þ 3H2 O þ 4:5N2

(7)

C3 H8 þ 10NO ! 3CO2 þ 4H2 O þ 5N2

(8)

Ce2 O3 þ 0:5O2 Ð 2CeO2

(9)

3.5. Computer modelling TWCs Much is now known about the individual reactions and processes involved in the operation of TWCs, and by carefully

The oxidation reactions (1)–(4), the reduction reactions (5)– (8), and the catalyst’s reversible uptake and release of oxygen reaction (9) describe the overall chemistry taking place. The model developed had reactions involved in oxygen storage and release combined in a single rate expression that gave an acceptable mathematical description when tested against emissions from a SULEV car. The validating car had a rich start-up strategy with air injected into the exhaust gas to ensure very rapid heating of the underfloor catalyst following a cold

Fig. 4. Arrangement of two oxygen sensors upstream and downstream of a three-way catalyst for monitoring catalyst characteristics during driving. When the catalyst is active the oxygen level oscillations are damped by the oxygen storage components in the catalyst, should deactivation take place the oscillations break through the catalyst as illustrated by the dashed traces.

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M.V. Twigg / Applied Catalysis B: Environmental 70 (2007) 2–15

Fig. 5. Decrease in emissions from American cars over the period 1970–1990. The introduction of oxidation catalysts markedly lowered emissions of HC (left graph, upper curve) and CO (right graph) before NOx control was introduced (left graph, lower curve). Since 1990 emissions continued to decrease as increasingly stringent legislation was introduced.

start. There was good agreement between measured and simulated temperatures in the catalyst, and small differences were attributed to difficulties of temperature measurement. There was also good agreement between simulated and measured cold start emissions. Special attention was given to transient behaviour, and in general good agreement with the simulation was obtained. The model helped in understanding the observed overall behaviour of the TWC on the vehicle. Table 2 California (CARB) Emissions standards post-1994 Year

Emissions (g/mile, FTP test) Category

PM

CO

NOx

a

Tier 1 Tier 1

0.25 0.25b 0.25c

3.40 3.40 3.40

0.40 0.40 0.40

2004

TLEV1 d LEV2e,f

0.125 0.075

3.40 3.40

0.40 0.05

0.08 0.01

2005

LEV1 d ULEV2e,f

0.075 0.040

3.40 1.70

0.40 0.05

0.08 0.01

2006

ULEV1 d SULEV2e,f,g

0.040 0.010

1.70 1.0

0.20 0.02

0.04 0.01

2007

ZEV1 ZEV2

0 0

0 0

0 0

0 0

1993 1994 2003

HC

Note: LEV = low emission vehicles, SULEV = super low emission vehicles, ZEV = zero emission vehicles. a NMHC = non-methane hydrocarbons, i.e., all hydrocarbons excluding methane. b NMOG = non-methane organic gases, i.e., all hydrocarbons and reactive oxygenated hydrocarbon species such as aldehydes, but excluding methane. Formaldehyde limits (not shown) are legislated separately. c FAN MOG = fleet average NMOG reduced progressively from 1994 to 2003. d LEV1 type emissions categories phasing out 2004–2007. e LEV2 type emissions limits phasing in 2004 onwards. f LEV2 standards have same emission limits for passenger cars and trucks <8500 lb gross weight. g SULEV2 onwards 120,000 miles durability mandated.

4. Gasoline car emissions legislation The progress made in reducing exhaust emissions from traditional gasoline cars during the first decade following the introduction of legislation in America can be judged from the decrease in the amount of HC, CO and NOx emitted annually that is shown in Fig. 5. There were around 10 million tonnes of HC and seven times this amount of CO, and about 5 million tonnes of NOx emitted each year. The amount of NOx was significant when compared with the nitrogen ‘‘fixed’’ in the Haber–Bosch process as ammonia mainly for fertilizer use. The way the legislation was tightened over the years since catalysts were fitted to cars to control emissions is also a measure of progress, and recent California legislation trends are shown in Table 2. The improvements are such, that although the number of cars dramatically increased, the total emissions decreased (Fig. 6), and, for example, the ‘‘alert days’’ in Los Angeles have been effectively eliminated. In fact, the most demanding legislation in the world today, California’s SULEV HC limit (Table 2) is in some cases lower than ambient air. So, although

Fig. 6. Decrease in Stage 1 ‘‘Alert Days’’ in Los Angeles (A) compared with the number of cars on the road (B). The decreasing peak ozone levels are shown in (C) line. Total emissions dramatically decreased in spite of the increased number of cars.

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these emissions are not zero, they are extremely low, and the improved air quality reflects this. 5. Control of emissions from diesel engines In traditional stoichiometric gasoline engines the combusting mixture always contains sufficient oxygen to just combine with the fuel. In contrast, in a diesel engine oxygen is always in excess, since only sufficient fuel is injected into compressed hot air in the cylinder to produce the power required at a particular instant [36]. The consequence of this mode of combustion is diesel exhaust always contains an excess of oxygen, and while this should be advantageous for the oxidation of HCs and CO, it makes controlling NOx emissions extremely difficult because under practical conditions NOx can only be converted to N2 by reaction with a reductant. So far diesel car legislative NOx emissions requirements have been met by engine control measures alone. But, this may not be possible in the future with lower NOx emissions limits, so lean-NOx control will then be necessary. Because of the nature of the combustion process some carbonaceous particulate material (PM or ‘‘soot’’), is formed by diesel engines. Over recent years engine modifications reduced the amount of PM formed, and reliable means of controlling the remaining PM have been devised and successfully introduced. This section is concerned with the control of these three classes of emissions associated with diesel engines: oxidation of HC and CO, lean-NOx control system, and the two most important technologies for lean-NOx control, and PM control. 5.1. Diesel hydrocarbon and carbon monoxide oxidation The catalytic oxidation of HC and CO under the lean conditions in a diesel exhaust should be straightforward. However, the fuel-efficient characteristics of diesel engines results in their exhaust gas temperature being very low, especially during low-speed driving. This, together with SO2 in the exhaust gas (derived from sulphur compounds in the fuel) that is a catalyst poison, means achieving and maintaining very good low temperature catalytic performance is very challenging. A decade or so ago European diesel fuel sulphur levels were as high as 2000 ppm (or even higher), more recently 350 ppm and currently below 50 ppm with 10 ppm being introduced. These improvements were key in maintaining high catalytic performance in use as increasingly demanding emissions legislation was introduced. Platinum-based catalysts are used to oxidise CO and HC, and to achieve the performance and durability required catalyst formulations have the platinum in a very highly dispersed form, that is well stabilised against thermal sintering. When the engine is started the catalyst is insufficiently warm to oxidise the hydrocarbons initially present in the exhaust gas, and incorporating zeolites into the catalyst significantly improved the performance during the so-called ‘‘cold start’’. The zeolites function by adsorbing HC so preventing them inhibiting the active platinum sites. This improves the low temperature CO and apparent HC (because they are absorbed) oxidation performance [37]. At higher

Fig. 7. The effect of incorporating zeolite into a platinum diesel oxidation catalyst. The control of hydrocarbon emissions at low temperature is improved by their retention in the zeolite. At higher temperatures released HC is oxidised over the catalyst. The amount of zeolite increased from loadings 1 to 3.

temperature the HC is desorbed from the zeolite and oxidised over the platinum catalyst sites. Fig. 7 shows the effect of zeolite addition to a platinum catalyst on HC oxidation performance. 5.2. Control of NOx under lean conditions Although nitric oxide is thermodynamically unstable (DHf = +90 kJ/mol) and a free radical, under practical lean conditions it is not possible to achieve its catalytic dissociation to O2 and N2. This is because of the high affinity of metallic catalyst surfaces for O2 compared to that for N2 that leads to ‘‘oxygen poisoning’’ of the metal surface (especially with rhodium that is one of the best metals for NO dissociation). The surface becomes covered with strongly adsorbed oxygen so preventing NO adsorption, and a reducing species is required to remove oxygen from the surface to allow further adsorption and dissociation of NO [38]. By periodic exposure of a rhodium catalyst to reductant, accumulating oxygen on the surface can be removed and NO dissociation under lean conditions sustained [39]. Addition of zeolite [40] or an oxygen storage component [41] to the catalyst formulation can enhance the dissociation process. However, quite high amounts of reductant are required to reduce NOx under lean conditions in this way. This is what takes place smoothly on a three-way catalyst when operating around the stoichiometric point where the overall amount of free oxygen is much smaller (e.g. 2%). Under lean conditions the only easy reaction of NO is its oxidation to NO2, and while this is of value in the context of controlling diesel PM emissions and the functioning of NOx-traps (vide infra), it is in itself not directly helpful in the conversion of NOx to N2. 5.2.1. Lean NOx-trapping catalysts There are two practical means for converting NOx to N2 under lean conditions. The first involves storage of NOx as a nitrate phase during lean driving, then periodically, when the NOx-trapping material is becoming saturated, the exhaust gas composition is made slightly reducing for a short period. As shown in Scheme 10 oxidation of NO to NO2 is the key first stage. This is catalysed by platinum and determines the low temperature performance of a NOx-trapping catalyst [42]. At

10

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Scheme 11.

Scheme 10.

the operating temperature this destabilises the NO3 and releases the stored NOx, which is reduced over a rhodiumcontaining component in the catalyst to N2 [43]. Evidence for the presence of the NO3 phase has been obtained from X-ray diffraction and infrared experiments. Fig. 8 shows a series of Xray diffraction patterns during the growth of NO3 at the expense of CO32 during the storage phase, which is followed by loss of NO3 during reductive regeneration. These sequential X-ray diffraction patterns were obtained using a barium containing NOx-trap under a controlled atmosphere in an environmental cell at 350 8C. In effect, the last part of the process is like that with a three-way gasoline catalyst operating around the stoichiometric point. The NOx storing and the NOx release/reduction processes are shown in Scheme 10. Here M represents a suitably basic element, typically an alkaline earth, or an alkali metal cation. NOx-trapping catalysts commonly have two layers with the bottom one containing Pt for oxidation of NO to NO2 together with the storage component that can be Ba, Sr or far higher temperature operation K (M in Scheme 10). Rhodium is incorporated into the top layer so during the periodic enrichments of the exhaust gas the NOx released from the bottom has to pass through the Rh-containing layer in the presence of reductant so it is reduced to N2. The basic component will also react with other acidic gases, especially SOx. Although at low levels a proportion of the SO2 in the

exhaust is oxidised over Pt to form SO3 that reacts to give MSO4. Since sulphates are thermodynamically more stable than nitrates MSO4 accumulates in the NOx-trap causing its NOx storage capacity to decrease during use [44]. Fortunately it is possible to decompose the sulphate under slightly rich conditions at temperatures higher than that used for decomposing the corresponding nitrate. In this way the full storage capacity of the NOx-trap can be recovered [45]. 5.2.2. Selective catalytic reduction of NOx The second lean NOx control method is selective catalytic reduction (SCR) where reduction of NOx successfully competes with the reduction of oxygen, even though the latter is present in a large excess. This is illustrated in Scheme 11 where the reductant is a hydrocarbon. Under actual diesel exhaust conditions on a car, with a platinum catalyst only moderate NOx conversions are obtained unless high ratios of HC to NOx are used. Then the process becomes uneconomical because of the amount of HC consumed. Catalysts explored for HC lean-NOx control include those containing platinum [46], copper [47] and iridium [48], and recently there has been considerable interest in the behaviour of silver catalysts [49]. Here it appears the nature of the support (various modified aluminas) can have a profound effect on the catalytic performance, as can the presence of zeolite that traps hydrocarbons within the catalyst and effectively increases the local HC concentration. As previously mentioned, in Section 5.2, periodic enrichment of lean exhaust gas over a rhodium catalyst can result in significant NOx

Fig. 8. X-ray diffraction patterns showing the growth of nitrate phase and decrease of carbonate during NOx absorption in a NOx absorbing catalyst, followed by reductive regeneration that converts the nitrate back to carbonate. The NOx liberated during regeneration is reduced to N2 under the rich conditions over a rhodium component.

M.V. Twigg / Applied Catalysis B: Environmental 70 (2007) 2–15

11

Scheme 12.

conversion, but this is not very efficient in terms of the amount of reductant (fuel) used. The reactivity of HCs in lean-NOx conversion depends on their nature, the catalyst and the temperature; different HCs can behave slightly differently. At higher temperatures competitive oxidation of HC becomes increasingly important, and then most of the HC reductant is oxidised giving little opportunity for NOx reduction. This is a consequence of the activation enthalpy of HC oxidation being larger than that for NOx reduction, and results in a restricted temperature ‘‘window’’ in which significant NOx reduction can be achieved. The maximum conversion within this temperature ‘‘window’’ can be increased by having more HC present, but this has an economic penalty. Catalyst formulations containing zeolite, can provide enhanced NOx reduction due to their ability of maintaining a high concentration of hydrocarbon in the catalyst. A feature of many lean-NOx reduction reactions is there is insufficient reduction capability on the surface to reduce NOx completely to N2, and a significant amount of N2O can be formed according to Scheme 12 [50]. The relative importance of this depends on the nature of the catalyst surface concerned, the nature and concentration of reductant, and the temperature as well as exhaust gas flow rates, etc. Hydrogen also participates in lean-NOx reduction, and because hydrogen is very reactive it reduces NOx at a relatively low temperature, so its operating window is centred at a low temperature compared to that for most HCs. Fig. 9 shows the behaviour of a range of HCs in lean-NOx reduction in a series of laboratory experiments in which very high NOx conversions were possible. With an appropriate catalyst NH3 can function as a very good selective NOx reductant, and as shown in Fig. 10, platinum

Fig. 10. Selective catalytic reduction of NOx (200 ppm) by NH3 (200 ppm) in the presence of excess oxygen (12%) over platinum catalysts as a function of temperature. Catalyst B contains twice the amount of Pt than catalyst A. Negative conversions result from oxidation of NH3 to NO at higher temperatures. Vanadium-based catalysts are used at higher temperatures.

catalysts can function at relatively quite low temperatures, but vanadium-based catalysts are commonly used at temperatures typical of heavy-duty diesel engine exhaust gas. Traditionally V2O5 supported on TiO2 has been used because under operating conditions TiO2 does not form a sulphate. The V2O5 spreads over the surface driven by the difference in surface and interface free energy [51]. In practice stabilised TiO2 is used such as that containing a small amount of WO3, but now the quest is for V-free catalysts, and there has been much work reported on metal exchanged zeolite systems; especially those containing Fe, Co, Cu, and Ce [52]. Iron/zeolite catalysts have been commercialised, and have been used in several NH3/SCR applications. A large body of knowledge now exists about the nature of these systems and the mechanisms of the NH3/SCR reactions over metal exchanged zeolite catalysts [53]. High NOx conversions are possible, but oxidation of NH3 affords NO at high temperatures so the apparent conversion of NO decreases as increasing amounts of NO are formed from NH3. Careful control of operating conditions is therefore necessary. The NH3/SCR process over vanadium catalysts has good selectivity for conversion of NO to N2 with only moderate formation of undesirable N2O, and it is interesting oxygen participates in the overall reduction process. Ammonia SCR has been used extensively for NOx removal from power generation and chemical plant exhaust gases [54]. The ammonia SCR reactions are summarised in Scheme 13. It may well be expected that this technology will be used more widely in vehicle applications in the future. 5.3. Diesel exhaust particulate control A characteristic of old diesel engines was ‘‘black soot’’ in their exhausts caused by the combustion process itself in which

Fig. 9. Effect of different hydrocarbons in the reduction of NOx over a platinum catalyst under lean conditions. A wide range of reactivities are observed, and methane (not shown) is unreactive except at high temperatures. In each case the C/NOx ratio was 14; A = n-octane; B = methylcyclopentane; C = toluene; D = propene, E = iso-octane. Adapted from Ref. [66].

Scheme 13.

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M.V. Twigg / Applied Catalysis B: Environmental 70 (2007) 2–15

Scheme 14.

Fig. 11. A schematic representation of a ceramic wall-flow filter. The arrows indicate the gas flow through the walls. Particulate matter is retained in the upstream side of the filter, and this has to be removed to prevent unacceptable pressure-drop across the filter.

very small ‘‘atomised’’ droplets of fuel burning in hot compressed air left an unburnt core of fine carbon particles onto which other species in the exhaust gas, including HCs, sulphur compounds NOx and water adsorbed. Recently tremendous advances were made in the fuelling and combustion processes of modern high-speed diesel engines used in passenger cars. This involved very high pressure pumps, an increased number of smaller injector nozzles, and multiple injections. As a result soot or particulate matter (PM), emissions have been reduced to low levels. Nevertheless, there are still concerns about the possible health effects of diesel PM and there is a move to eliminate this by filtration. A variety of ceramic and sintered metal-based filters have been developed, and the most successful are the so-called wallflow filters illustrated in Fig. 11. Here a honeycomb monolithic structure made from porous material has channels that are plugged alternately at the ends so exhaust gas is forced through the channel walls. PM is too large to pass through the walls, so it is retained in the upstream side of the filter. If too much PM accumulates backpressure across the filter will increase and degrade engine performance, and ultimately the engine will cease to function. It is essential the backpressure is not allowed to rise above a predetermined limit. The most satisfactory means of removing trapped PM is to oxidise it to CO2 and water. On heavy-duty diesel vehicles, such as trucks and buses, the engine is often working at high load and the exhaust temperature is in the range 250–400 8C. Under these conditions it is possible to use the already present NO in the exhaust gas in a process that continually oxidises trapped PM. An oxidation catalyst upstream of the filter oxidises HCs and CO to CO2 and water, and also converts NO to NO2 that is a very powerful oxidant, and this continually removes PM, as shown in Scheme 14 in which PM is represented chemically as ‘‘CH’’. The advantage of this system is it requires no attention, but the NO

oxidation is strongly inhibited by the presence of SO2, so this technology could not be introduced until low sulphur diesel fuel became available. Because the process of NO2 formation and its smooth oxidation of retained soot takes place continuously in many heavy-duty applications, it is called a Continuously Regenerating Trap (CRT1) [55]. Now many tens of thousands of these filter units are in service around the world on buses, trucks, and larger delivery vehicles [56,57]. By catalysing the filter in addition to having the upstream oxidation catalyst it is possible to reoxidise the NO to NO2 and so improve the overall efficiency of soot removal. This so-called Catalysed Continuously Regenerating Trap (CCRT1) this is being used [58] on selected heavy-duty diesel applications where, for example, the duty cycle does not always provide sufficient temperature for proper functioning of the CRT1. Normally the conversion of NOx over a CRT1 is somewhat less than 10% that results from the HC lean-NOx reaction discussed in Section 5.2.2. In the presence of some catalysts it appears PM can reduce NO to N2. For example, catalysts containing modified iron oxides when intimately mixed with diesel PM reacts with NOx at higher temperatures to give N2 and CO2. This is a complex reaction [59–61] that so far has not been commercialised because of the difficulty of maintaining intimate contact between PM and catalyst. This problem is overcome by incorporating the catalyst precursor in the fuel so PM and catalyst are formed together in systems that can enhance PM combustion in oxygen. Catalysts based on Cu, Fe, Sr, and Ce have been used in this way [62]. The exhaust temperatures of diesel passenger cars rarely exceed 250 8C in town driving, so use of NO2 to combust PM is inappropriate except when driving at higher speeds when this reaction, in some circumstances, can keep the filter clean. However, the key to employing filters on diesel cars is to use ‘active’ approaches to cleaning PM from the filter. These boost exhaust gas temperatures at intervals to that at which the soot burns. The three different system architectures for car PM filter systems are shown schematically in Fig. 12. The first utilises a platinum oxidation catalyst in front of a filter to control HC and CO emissions, and also to oxidise NO to NO2 for low temperature combustion of PM in the downstream filter when driving conditions are appropriate for this to take place. This catalyst is also used to burn partially combusted and extra fuel injected into the engine to raise the exhaust gas temperature high enough to promote PM combustion with oxygen (usually above 550 8C). Variations of this system are already in production in Europe, where a base metal fuel additive is used to help lower the temperature required to combust PM with oxygen. The second generation has a platinum catalyst on the filter that promotes the rate of soot combustion at higher temperatures. The benefit of this over the first generation is that it removes the need for a fuel additive and a means of

M.V. Twigg / Applied Catalysis B: Environmental 70 (2007) 2–15

Fig. 12. Three filter systems used on diesel cars. The first has an oxidation catalyst before the filter to burn partially combusted fuel to achieve high temperatures, and a fuel additive is used to lower the PM combustion temperature. No additive is employed in the second generation system, the filter contains catalyst to accelerate PM combustion. In the third generation system all of the required catalyst functionality is incorporated in a single filter.

dispensing it periodically into the fuel tank. The presence of platinum on the filter also removes HC and CO during times when the filter is regenerating. The third generation does not have a separate oxidation catalyst, but comprises only a single catalysed filter. This has all of the necessary oxidation catalyst functionality included in it to oxidise HC and CO during normal driving. In addition, the catalyst oxidises NO to NO2 to provide some passive PM removal when this is possible, as well as periodically oxidising extra HCs/CO to give sufficient temperature to burn PM with oxygen when it is necessary to clean the filter. This system is the most thermally efficient of the

13

three because there is only one substrate to heat that is close to the engine so heat losses are minimised, and the reactions on the filter surface create heat in the direct vicinity of the PM. There are a significant number of first generation filter systems on cars on the road in Europe. Second generation technology began to appear a few months ago, and the latest third generation technology has just been introduced into mass production. Future legislation standards are likely to demand PM emissions levels that will force the use of filters on all diesel cars, and already there is public demand for their fitment. Given this progress, the diesel car will soon be ‘‘seen’’ as a much more environmentally friendly vehicle than it was previously, and platinum catalysts have key roles in this. 5.3.1. Recent advances in diesel particulate control Fitment of suitable particulate filters has been shown to effectively eliminate the larger coarse and accumulation modes of PM from the exhaust gas of diesel engines (Fig. 13). These particles, and especially the accumulation mode typically of about 100 nm diameter, comprises almost all of the mass of the PM. They are derived from the burnt droplets of fuel, and are in the form of agglomerates with adsorbed hydrocarbons etc. referred to previously. Although they have very little mass, attention has recently been directed towards nanoparticles with diameters less than 50 nm, and typically about 10 nm, that may have serious health implications. Recent research has demonstrated these nuclei mode particles form from volatile precursors as the exhaust gas cools. The results obtained using a mobile laboratory as well as laboratory studies indicate that

Fig. 13. Classification of diesel engine PM according to size. Most of the PM mass (dashed line) is associated with the accumulation mode (100 nm), but there are many more nanoparticles (solid line) in the nuclei mode (10 nm) that are so small they readily penetrate the human bronchial system (Courtesy Professor D.B. Kittelson).

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M.V. Twigg / Applied Catalysis B: Environmental 70 (2007) 2–15

Fig. 14. A compact emissions control design for heavy-duty diesel vehicles that includes oxidation catalyst, SCR ammonia NOx control, and PM filter. The first catalyst is a platinum oxidation catalyst to remove CO/HC and oxidise NO to NO2, the final annular platinum oxidation catalyst is present to remove any adventitious ammonia that may slip from the vanadium-based SCR catalyst.

when a good oxidation catalyst is present in the exhaust system that removes effectively all of the hydrocarbons, the nuclei mode is made up of ‘‘sulphate’’, probably sulphuric acid or ammonium sulphate. It has been demonstrated [63] the lifetime of these nanoparticles is short under normal ambient conditions, but there is concern about their potential health effects because they are so small they can readily penetrate the human bronchial system. However, very recently it was shown that careful design of the catalyst/filter can control the emissions of nanoparticles as well as the coarser PM [64]. 5.3.2. Combined diesel emissions control systems In the future several diesel emissions control systems will be combined into a single unit to minimise space requirements, and for cost and efficiency considerations. Examples of this include oxidation functions in particulate filters already mentioned in the previous sections, and in the future NOx control will also be included. Already oxidation catalyst, PM filtration and ammonia SCR for NOx control on heavy-duty diesels have been ingeniously combined in a single compact container [65], and this is illustrated schematically in Fig. 14. The exhaust gas first passes through a platinum oxidation catalyst that oxidises CO and HCs, as well as converting NO to NO2 that continuously oxidises PM in the filter. The exiting NOx is then reduced to N2 over two SCR catalysts. The ammonia here is obtained from the decomposition of urea that is sprayed into the system as an aqueous solution, and any adventitious ammonia is prevented from passing into the environment by a final platinum catalyst that would oxidise it to NO. 6. Conclusions Over the last three decades since the introduction of the first catalysts on gasoline cars there has been a huge reduction in exhaust gas pollutant emissions from them. Today three-way catalysts are exceptionally efficient, and because of this millions of tonnes of pollutants have not been released into the atmosphere. As a result urban air quality has been significantly

improved with many associated environmental benefits. Now new emissions control systems are being developed for the more fuel-efficient (lower CO2) lean-burn engines, especially for the increasingly popular modern high-speed diesel engines. Here catalysts are used to oxidise CO and HCs, and in the future some form of NOx control will also be required. To meet these demands two technologies have been developed, NOx-trapping and selective catalytic reduction (SCR). Both are being introduced, and refinements may be expected in the future. Filter systems are being introduced to effectively eliminate particulate emissions that were formerly such a characteristic feature of diesel engines. Here catalysts are used to produce NO2 as an intermediate for low temperature soot combustion, and for oxidising high levels of HC/CO to achieve temperatures needed to burn trapped soot in a filter. All of these improvements in controlling automotive exhaust emissions depend on catalysis, and the most recent on-road measurements show that appropriately designed catalytic systems are capable of effectively eliminating not only the larger so-called coarse mode and accumulation mode of particulate matter from diesel engine exhaust, but also the extremely fine nanoparticles in the nucleation mode. Thus the application of catalytic systems to the control emissions from internal combustion engines of all types continues to improve urban air quality.

References [1] A.J. Haagen-Smit, Ind. Eng. Chem. 44 (1952) 1342. [2] A.J. Haagen-Smit, M.M. Fox, J. Air Pollut. Contr. Assoc. 4 (105) (1954) 136. [3] A.J. Haagen-Smit, M.M. Fox, Ind. Eng. Chem. 48 (1956) 1484. [4] A.J. Haagen-Smit, E.F. Darley, M. Zaitlin, H. Hull, W. Noble, Plant Physiol. 27 (1952) 18. [5] A.J. Haagen-Smit, C.E. Bradley, M.M. Fox, Ind. Eng. Chem. 45 (1953) 2086. [6] A.J. Haagen-Smit, M.M. Fox, SAE Trans. 63 (1955) 575. [7] J.T. Middleton, J.B. Kendrick, H.W. Schwalm, U.S.D.A. Plant Dis., Rep. 34 (1950) 245. [8] R.P. Wayne, Chemistry of Atmospheres: An Introduction to the Chemistry of the Atmospheres of Earth the Plants and Their Satellites, 2nd ed., Oxford University Press, 1993, pp. 221–225, 252–263. [9] W.A. Glasson, C.S. Tuesday, J. Am. Chem. Soc. 85 (1963) 2901. [10] B.J. Finlayson-Pitts, J.N. Pitts, Ads. Environ. Sci. Technol. 7 (1977) 75. [11] B.J. Finlayson-Pitts, J.N. Pitts (Eds.), Chemistry of the Upper and Lower Atmosphere—Theory, Experiments and Applications, Academic Press, New York, 2000, pp. 4–8, 86–126, 265–287. [12] Hydrocarbon Process. (1971) 85. [13] The 1970 US National Ambient Air Quality Standards (NAAQS) required the Environmental Protection Agency (EPA) to identify and set standards for pollutants identified as harmful to human health and the environment. [14] M.V. Twigg (Ed.), The Catalyst Handbook, Manson, London, 1996, pp. 85–139. [15] G.J. Barnes, Am. Chem. Soc. Ads. Chem. Ser. (1975) 143. [16] Y. Yao, J. Catal. 36 (1975) 226. [17] M. Shelef, H.S. Gandhi, Platinum Met. Rev. 18 (1974) 2. [18] A.G. Graham, S.E. Wanke, J. Catal. 68 (1981) 1. [19] M.V. Twigg, Platinum Met. Rev. 44 (1999) 168; G.J.K. Acres, B. Harrison, Top. Catal. 28 (2004) 3. [20] S.T. Gulati, in: A. Cybulski, J.A. Moulijn (Eds.), Structured Catalysts and Reactors, 2nd ed., CRC Press, Boca Raton, 2006, pp. 21–70. [21] M.V. Twigg, A.J.J. Wilkins, in: A. Cybulski, J.A. Moulijn (Eds.), Structured Catalysts and Reactors, Dekker, New York, 1999, pp. 91–120.

M.V. Twigg / Applied Catalysis B: Environmental 70 (2007) 2–15 [22] J. Benbow, J. Bridgewater (Eds.), Paste Flow and Extrusion, Oxford University Press, Oxford, 1993, pp. 7–24. [23] W.A. Deer, R.A. Howie, J. Zussman (Eds.), An Introduction to the Rock Forming Minerals, Longmans, London, 1966, pp. 84–89. [24] E.L. Yuan, J.I. Slaughter, W.E. Ko¨rner, F. Daniels, J. Phys. Chem. 63 (1959) 952. [25] G.J.K. Acres, B.J. Cooper, Platinum Met. Rev. 16 (1973) 74. [26] B.J. Cooper, W.D.J. Evans, B. Harrison, Catalysis and Automotive Pollution Control, Elsevier, Amsterdam, 1987, p. 117. [27] M. Shelef, G.W. Graham, R.W. McCabe, in: A. Trovarelli (Ed.), Catalysis by Ceria and Related Materials, Imperial College Press, London, 2002, pp. 343–389. [28] E. Jobson, O. Hjortsberg, S.L. Andersson, I. Gottberg, SAE Technical Paper 960801, 1996. [29] J.C. Summers, J.J. White, W.B. Williamson, SAE Technical Paper 890794, 1989; R.J. Brisley, G.R. Chandler, H.R. Jones, P.J. Anderson, P.J. Shady, SAE Technical Paper 950259, 1995. [30] M.V. Twigg, M.S. Spencer, Top. Catal. 22 (2003) 191; M.S. Spencer, M.V. Twigg, Appl. Catal. A: Gen. 212 (2001) 161, and references therein. [31] M.V. Twigg, D.E. Webster, in: A. Cybulski, J.A. Moulijn (Eds.), Structured Catalysts and Reactors, Dekker, New York, 1999, pp. 59–90. [32] R.G. Bending, M.R. Albert, M.J. D’Aniello, S.E. Golunski, A.F. Diwell, T.J. Truex, SAE Technical Paper 930777, 1993 H. Windawi, T.J. Truex, Stud. Surf. Sci. Catal. 38 (1987) 601; A.F. Diwell, S.E. Golunski, J.R. Taylor, T.J. Truex, Stud. Surf. Sci. Catal. 71 (1991) 417. [33] Automotive Handbook, 6th ed., Bosch, Stuttgart, 2004, pp. 584–591; J. Rieck, N.R. Collins, J. Moore, SAE Technical Paper 980665, 1998. [34] D. Chatterjee, O. Deutschmann, J. Warnatz, Faraday Discuss. 119 (2001) 371. [35] C. Brinkmeier, G. Eigenberger, S. Bu¨chner, A. Donnerstag, SAE Technical Paper 2003-01-1001, 2003; For another example see: J. Braun, T. Hauber, H. To¨bben, J. Windmann, P. Zacke, D. Chatterjee, C. Correa, O. Deutschmann, L. Maier, S. Tischer, J. Warnatz, SAE Technical Paper 2002-01-0065, 2002. [36] Diesel Engine Management Handbook, 3rd ed., Bosch, Stuttgart, 2004, pp. 16–33. [37] J.M. Fisher, P.G. Gray, R.R. Rajaram, H.G.C. Hamilton, P.G. Ansell, Worldwide Patent 96/39244 (1996); P.R. Phillips, G.R. Chandler, D.M. Jollie, A.J.J. Wilkins, M.V. Twigg, SAE Technical Paper 1999-01-3075, 1999. [38] F.J. Williams, A. Palermo, M.S. Tikhov, R.M. Lambert, J. Phys. Chem. 104 (2000) 11883. [39] T. Nakatsuji, R. Yasukawa, K. Tabata, K. Ueda, M. Niwa, Appl. Catal. B: Environ. 21 (1999) 121. [40] T. Nakatsuji, V. Komppa, Appl. Catal. B: Environ. 30 (2001) 209. [41] T. Nakatsuji, J. Ruotoistenma¨ki, V. Komppa, Y. Tanaka, T. Uekusa, Appl. Catal. B: Environ. 38 (2002) 101.

15

[42] W. Bo¨gner, M. Kra¨mer, B. Krutsch, S. Pischinger, D. Voigtla¨nder, G. Wenninger, F. Wirbeleit, M.S. Brogan, R.J. Brisley, D.E. Webster, Appl. Catal. B: Environ. 7 (1995) 153. [43] L.J. Gill, P.G. Blakeman, M.V. Twigg, A.P. Walker, Top. Catal. 28 (2004) 157. [44] W.S. Epling, L.E. Campbell, A. Yezerets, N.W. Currier, J.E. Parks, Catal. Rev. 46 (2004) 163. [45] S. Poulston, R.R. Rajaram, Catal. Today 81 (2003) 603. [46] R. Burch, P.J. Millington, A.P. Walker, Appl. Catal. B: Environ. 4 (1994) 65. [47] W. Held, A. Ko¨nig, German Patent (1987), 3642018; W. Held, A. Ko¨nig, SAE Technical Paper 900496, 1990 A. Ko¨nig, W. Held, T. Richter, Top. Catal. 28 (2004) 99. [48] K.C. Taylor, J.C. Schlatter, J. Catal. 63 (1980) 53. [49] J. Shibata, Y. Takada, A. Shichi, S. Satakawa, A. Satsuma, T. Hattori, Appl. Catal. B: Environ. 54 (2004) 137, and references therein. [50] G.P. Ansell, A.F. Diwell, S.E. Golunski, J.W. Hayes, R.R. Rajaram, T.J. Truex, A.P. Walker, Appl. Catal. B: Environ. 2 (1993) 81. [51] H. Kno¨zinger, E. Taglauer, in: J.J. Spivey, S.K. Agarwal (Eds.), Catalysis, vol. 10, Royal Society of Chemistry, London, 1993, p. 1. [52] D. Berthomieu, N. Jardillier, G. Delahay, B. Coq, A. Goursot, Catal. Today 110 (2005) 294. [53] H.-Y. Chen, X. Wang, W.M.H. Sachtler, Appl. Catal. A: Gen. 194/195 (2000) 159. [54] I. Nova, A. Beretta, G. Groppi, L. Lietti, E. Tronconi, P. Forzatti, in: A. Cybulski, J.A. Moulijn (Eds.), Structured Catalysts and Reactors, Dekker, New York, 2006, pp. 171–214. [55] B.J. Cooper, J. Thoss, SAE Technical Paper 890404, 1989. [56] P.N. Hawker, Platinum Met. Rev. 39 (1995) 1. [57] A.P.E. York, J.P. Cox, T.C Watling, A.P. Walker, D. Bergeal, R. Allanson, M. Lavenius, SAE Technical Paper 2005-01-0954, 2005. [58] R. Allansson, C. Go¨rsmann, M. Lavenius, P.R. Phillips, A.J. Uusimaki, A.P. Walker, SAE Technical Paper 2004-01-0072, 2004. [59] W.F. Shangguan, Y. Teraoka, S. Kagawa, Appl. Catal. B: Environ. 12 (1997) 237. [60] W. Weisweiler, K. Hizbullah, S. Kureti, Chem. Eng. Technol. 25 (2002) 140. [61] S. Kureti, W. Weisweiler, K. Hizbullah, Chem. Eng. Technol. 26 (2003) 1003. [62] F. Novel-Cattin, F. Rincon, O. Trohel, M. Leflamand, F. Dionnet, SAE Technical Paper 2001-01-1914, 2000. [63] D.B. Kittelson, W.F. Watts, M.V. Twigg, in preparation. [64] D.B. Kittelson, W.F. Watts, J.P. Johnson, C. Rowntree, M. Payne, S. Goodier, C. Warrens, H. Preston, U. Zink, M. Ortiz, C. Go¨rsmann, M.V. Twigg, A.P. Walker, R. Caldow, J. Aerosol Sci., submitted for publication. [65] A.P. Walker, R. Allansson, P.G. Blakeman, M. Lavenius, S. Erkfeld, H. Landalv, B. Ball, P. Harrod, D. Manning, L. Bernegger, SAE Technical Paper 2003-01-0778, 2003. [66] R. Burch, D. Ottery, Appl. Catal. B: Environ. 13 (1997) 105.