Manufacturers are increasingly interested in engines which operate under lean-running conditions to power their vehicles. One reason for this is because lean-burn engines produce less COz. This is advantageous because future emission legislation aims to reduce CO2, but the consumer also benefits from the increased fuel economy. Using engine management techniques and/or employing one or more catalytic converter in a vehicle's exhaust system can control the gaseous composition of the exhaust so that the vehicle meets the relevant emission legislation.

One form of lean-burn engine is a gasoline direct injection engine, which is designed to operate mostly under stoichiometric and lean conditions. When running lean, relatively low levels of NOx are formed that cannot be reduced (removed) in the presence of the relatively high levels of oxygen in the exhaust gas. Using conventional 3-way catalyst technology, reducing species, e. g. unburnt hydrocarbons (HC) and CO, can reduce NOx to N2 during stoichiometric-or rich-running conditions, as comparatively less oxygen, and more reducing species, is present than during lean-running conditions. In order to control NOx in lean-burn engines, there has been devised a NOx-absorber/catalyst which can store NOx, e. g. as nitrate, when an engine is running lean. In a stoichiometric or rich environment, the nitrate is understood to be thermodynamically unstable, and the stored NOx is released and is catalytically reduced by the reducing species present in the exhaust gas. This NOx-absorber/catalyst is commonly called a NOx-trap. By periodically controlling a lean-burn engine to run stoichiometrically or rich, stored NOx is released/reduced and the NOx-trap regenerated.

A typical NOx-trap composition comprises a catalytic oxidation component, platinum, a NOx-storage component, barium, and a NOx-reduction catalyst, rhodium.

See for example www. dieselnet. com/tech/cat nox. trap. html, Revision 2000.07. One mechanism commonly given for NOx-storage during lean engine operation for this composition is: (i) NO + 1/202-- N02 ; and (ii) BaO + 2N02 + 1/202 < Ba (NO3) 2.

In the first step, the nitric oxide reacts with oxygen on active oxidation sites on the

platinum to form NO2. The second step involves absorption of the NO2 by the storage material in the form of an inorganic nitrate.

When the engine runs under rich conditions or at elevated temperatures, the nitrate species become thermodynamically unstable, producing NO or N02 according to equation (iii) below. Under rich conditions, these nitrogen oxides are subsequently reduced by carbon monoxide, hydrogen and HC to N2, which can take place over the reduction catalyst. (iii) Ba (N03) 2 o BaO + 2NO + 3/202 or Ba (NO3) 2-"BaO + 2NO2 + 1/202 ; and (iv) NO + CO o 1/2N2 + COa (and other reactions). (In the reactions of (i)- (iv) above the reactive barium species is given as the oxide. However, it is understood that in the presence of exhaust gases most of the barium is in the form of the carbonate, the nitrate or possibly the hydroxide. The above reaction schemes can be adapted accordingly for species of barium other than the oxide.) The typical platinum/barium/rhodium NOx-trap composition has a window of temperature activity. At the lower end of this temperature window, activity is limited by the ability of platinum to catalyse the oxidation of NO- N02. Catalysis of this reaction is poor below about 150°C (see Fig. 1). At the upper end of the temperature window, activity is limited by the thermal stability of barium nitrate in the lean-burn exhaust gas environment. The more C02 is present in an exhaust gas, the more conditions favour barium carbonate formation over barium nitrate at a given temperature. Of course, in lean-burn conditions excess oxygen is present so that combustion of the hydrocarbon is substantially complete. The more lean the mixture, the higher the expected CO2 concentration of an exhaust gas. Generally, in lean-burn conditions barium nitrate begins to form the carbonate above about 350°C and this occurs more rapidly above about 400°C (see Fig. 2). However, in laboratory conditions barium nitrate exposed to a synthetic lean-burn exhaust gas except with the COa removed would be expected to remain thermally stable up to 450°C or above. Barium nitrate is more thermally unstable at lower temperatures in stoichiometric-or rich-running conditions to enable the NOx-trap composition to be regenerated, but this is because of the presence of reductant.

However, in many situations, particularly with GDI engines or at high load and/or speed, lean-burn exhaust gas temperatures can exceed 350°C. We have investigated elements the nitrate form of which are generally more thermally stable than

barium nitrate with a view to developing NOx-trap compositions that are active at temperatures above which barium carbonate formation is favoured in lean-burn exhaust gases. We have now found that, very surprisingly, the typical reduction catalyst rhodium can be used as an oxidation catalyst component in a relatively high temperature NOx-trap composition.

According to one aspect, the invention provides a NOx-trap composition comprising a platinum group metal (PGM), at least one NOx storage component and a first support for supporting the PGM and the at least one NOx storage component, which at least one NOx storage component comprises at least one of an alkali metal and a rare earth metal or a mixture of any two or more thereof, characterised in that the PGM consists of rhodium.

It is believed, although we do not wish to be bound by such belief, that although rhodium is a relatively poor NO oxidation catalyst compared with platinum, the rhodium oxidation mechanism is being driven by NO2 removal as it is converted to the nitrate.

An advantage of the present invention is that it is possible to make a NOx-trap composition that is free of platinum and/or palladium. This is because we have discovered that it is possible for rhodium to provide both the oxidative and reductive catalytic functions required in a NOx-trap composition. Accordingly, it is not necessary to include further platinum group metals in the composition for oxidising NO.

Furthermore, it is expected that the NOx-trap composition of the present invention has a higher tolerance to sulphur than those comprising platinum-based components. Presently, fuels and engine lubricants comprise a relatively high level of sulphur e. g. up to 350 ppm. Whilst a platinum oxidation catalyst would be expected to oxidise NO to NO2, it is also proficient at oxidising available SO2 derived from the fuel and/or lubricant to S03 at or above 300°C. The S03 forms sulphates of the NOx-storage material. These sulphates are more stable than the corresponding nitrate and their presence leads to reduction of the NOx-storage capacity that must be reversed by a high temperature rich treatment which reduces fuel economy. Rhodium however requires much higher temperatures (~500°C) for lean S02 oxidation. By replacing platinum with

rhodium as the oxidation catalyst in a NOx-trap composition, it would be expected that sulphation of the catalyst would be reduced.

Whilst the invention has application in general to lean-burn internal combustion engines, particularly gasoline engines such as gasoline direct injection engines, it can also be used in connection with other lean-burn engines such as diesel engines.

The at least one NOx-storage component of the present invention is usually present in the form of one or more of its oxides, but it is known that in the presence of typical engine exhaust gases these materials can also be present in the form of carbonates, nitrates and hydroxides.

The first support can comprise at least one oxide selected from ceria, zirconia, alumina or titania or a mixed oxide of any two or more thereof, or a mixture of any two or more of ceria, zirconia, alumina or titania. In one illustrative embodiment, the first support is gamma alumina. Current indications are that the NOx-storage efficiency of the NOx-trap composition according to the invention is reduced where the support comprises ceria. Accordingly, in an illustrative embodiment the first support comprises a minority of, or no, ceria.

A further illustrative embodiment is a cordierite honeycomb flow-through monolith comprising a NOx-trap composition consisting of rhodium and potassium supported on a gamma alumina.

According to a further aspect, the invention provides a metal or ceramic substrate comprising a NOx-trap composition according to the invention.

In a further aspect, the invention provides a shell or can comprising a substrate according to the present invention.

According to a further aspect, the invention provides an exhaust system for a lean-burn engine comprising a NOx-trap composition according to the present invention.

By"lean-burn engine"herein, we mean an engine which is controlled so that during at

least part of its normal operation it runs on a lean of stoichiometric air-to-fuel ratio, i. e. where k > 1. Lean-burn engines as defined herein comprise partial lean-burn gasoline engines using a variety of injectors comprising those with air assisted direct injection and high-pressure direct injection, diesel engines or engines which run on alternative fuels such as compressed natural gas or liquid petroleum gas.

In a further aspect, the invention provides a vehicle comprising a lean-burn engine and an exhaust system according to the invention. The vehicle can comprise an engine management means for imposing a lean/rich cycle on the engine for regenerating the NOx-trap composition.

In a further aspect, the invention provides the use of a NOx-trap composition according to the invention or of a substrate according to the invention to absorb NOx from exhaust gases of a lean-burn engine during lean-running conditions.

The substrate can have any arrangement commonly used in the art, such as a honeycomb flow-through monolith. However, foam or bead forms of a substrate can be used in the alternative.

Methods of making a NOx-trap composition for use in the exhaust systems are well known and will not be explained in detail here. The supports of the composition can be obtained using solid/solid reaction of the oxides or any other precursor such as carbonates. They may also be prepared by a wet route, i. e. by precipitation with a base of the salts of the support components, then calcining. Alternatively, materials to be supported can be impregnated onto the supports utilising the incipient wetness technique and calcining.

In order that the invention may be more fully understood, the following Examples are provided by way of illustration only and with reference to the accompanying drawings in which: Figure 1 is a graph showing the NO oxidation activity of a fresh, i. e. not aged,

catalyst as a plot of the amount of NO2 product obtained against temperature for a synthetic gas mixture comprising NO; Figure 2 is a graph showing the NOx-storage efficiency of various fresh, i. e. not aged, NOx-trap compositions comprising platinum and a NOx-storage component against temperature; Figure 3 is a graph comparing the NOx-storage efficiency of a platinum- containing NOx-trap composition and a rhodium-containing NOx-trap composition according to the invention both fresh and an aged and having potassium as the NOx-storage component. The support in both cases is a ceria-zirconia-alumina mixed oxide; Figure 4 is a graph comparing the NOx-storage efficiency of a fresh platinum- containing NOx-trap composition and fresh and aged NOx-trap composition according to the invention both having potassium as the NOx-storage component. The support in both cases is gamma-alumina. The graph also comprises results of an aged NOx-trap composition according to the invention wherein the potassium NOx-storage component illustrated is exchanged for a caesium NOx-storage component for the purposes of comparison ; Figure 5 is a graph of NOx concentration against time to compare the reduction of NOx by rhodium in a NOx-storage composition according to the invention with a NOx-storage composition containing platinum (and no rhodium); Figure 6 is a graph comparing the effect on NOx-storage efficiency of the support material in various NOx-trap compositions according to the invention comprising potassium as the NOx-storage component; and Figure 7 is a graph comparing the NOx-storage efficiency of a NOx-trap composition consisting of rhodium and potassium supported on gamma-alumina according to the invention with a NOx-trap composition consisting of rhodium and

caesium supported on gamma-alumina according to the invention for both aged and fresh catalyst.

Key: in the Figures, a single oblique line ("/") between two components in a composition represents that the component before the oblique line was impregnated on the support and the impregnated support was calcined in a separate step from the component appearing after the oblique line, whereas a dash between components ("-") indicates that the components were co-impregnated on the support before calcination.

The values given in the legend for each component are in wt. % of the support. F500 indicates results for a NOx-trap composition fired in air at 500°C for 2 hours; and F800 indicates results for an F500 NOx-trap composition fired further at 800°C for 4 hours, as explained in greater detail in the Examples below.

EXAMPLE 1 This Example is designed to investigate the NO oxidation activity of a fresh, i. e. not aged, catalyst by measuring the ability of the catalyst to convert NO in a synthetic gas mixture to NO2 as measured by mass spectrometry. The synthetic gas mixture comprised 200 ppm NO, 200 ppm CO, 4.5% C02, 12% O2, 5% H20, 600 ppm Cl hydrocarbon, balance N2 and was designed to simulate a diesel, i. e. lean, exhaust gas.

The catalyst comprised lwt% M supported on a gamma alumina support, wherein M was platinum or rhodium. The catalyst was prepared by incipient wetness impregnation of a fine powder of the support, and the resulting support was fired in air at 500°C for 2 hours. The impregnated support was then pressed into a tablet and the tablet was then crushed. The crushed, pressed support was then sieved and the 25011m to 355Rm fraction was placed in a test rig supplied with a synthetic gas mixture. The temperature was set to increase at a rate of 5°C per minute and the rate of supply of the synthetic gas stream was 40,000hr~1 GHSV.

Whilst the synthetic gas mixture comprises Cl hydrocarbon, it would be expected that substantially all hydrocarbon would be oxidised above about 250°C. Thus results for oxidation of NO above about 250°C would be expected to be unaffected by the presence of hydrocarbon.

As can be seen in Figure 1, platinum is a more effective NO oxidation catalyst in lean-running conditions above the temperature at which barium nitrate decomposes, i. e. about 350°C, compared with rhodium. At about 475°C, platinum and rhodium have similar NO oxidation activities because N02 decomposes to NO at this temperature.

EXAMPLE 2 The NOx-storage efficiency of fresh NOx-trap compositions consisting of platinum and one of barium, caesium and potassium on an alumina support were tested to show how the thermal stability of the nitrate form of the NOx-storage component affects the ability of the NOx-trap component to store NOx at varying temperatures.

The catalysts were each prepared in the manner described in Example 1 above, except as explained below. 0.6 g of each support was used. A potassium on alumina support composition was also tested to show the effect on the NOx-storage efficiency of a NOx-trap component which does not comprise an associated oxidation catalyst component.

A synthetic gas mixture designed to approximate key features of exhaust gas from a lean-burn gasoline engine was used to test the above compositions on a laboratory test (SCAT) unit. More particularly, the composition of the synthetic gas mixture was periodically switched from a composition typical of lean-running conditions (lambda 1.4), to one found in rich-running conditions (lambda 0.8). This regime was designed to mimic a so-called rich/lean cycle of management of a vehicle with a lean-burn engine comprising an exhaust system fitted with a NOx-trap. Lean exhaust gases were produced by adding oxygen and simultaneously reducing carbon monoxide concentrations. Rich or stoichiometric gas compositions were produced by the reverse procedure. In the lean phase, the NOx was stored by the composition under test. During the rich phase desorption occurred regenerating the NOx-storage material.

In practice, generally a NOx-trap composition comprises a reduction catalyst, e. g. rhodium, to catalyse the reduction of NOx released by reducing species present in the rich or stoichiometric exhaust gas, such as carbon monoxide and unburnt HCs.

However, in this experiment no reduction catalyst is comprised in the test NOx-trap compositions, and so the majority of the released NOx is exhausted per se. A mass spectrometer was used to determine and quantify the composition of gas exiting the catalyst.

Test conditions applied comprising the composition of synthetic gases entering the catalyst are as follows: (lean, 94 seconds) 12% 02, 15% CO2, 4.5 % H20, 400ppm propene, 500ppm NO, 0.5% CO, balance N2 (rich, 3 seconds) 0. 1% O2, 15% CO2, 4.5 % H20, 400ppm propene, 500ppm NO, 12% CO, balance Nz. Gas hourly space velocity (GHSV) = 40,000 ho''.

The results of the NOx-storage efficiency of the various compositions as determined by the gas composition of gases exiting the catalysts at various temperatures are shown graphically in Figure 2. As can be seen, the NOx-storage efficiency of potassium alone is relatively poor, but that its performance is improved when the composition comprises a platinum oxidation catalyst component. It can also be seen that where the NOx-storage component is barium, its NOx storage efficiency tails off significantly above about 350°C. A reason for this is because barium carbonate is favoured over barium nitrate above about 350°C in the synthetic lean-burn exhaust gas conditions of the test. It is for this reason that we can refer to barium and other alkaline earths as relatively low temperature NOx-storage components.

By contrast, the NOx-storage efficiency of fresh compositions comprising an alkali metal, such as potassium and caesium, with the platinum remains at or near its maximum up to about 550°C, the performance of potassium being slightly better than caesium. This shows that the nitrate of potassium or caesium is more thermally stable than that of barium and for this reason we can refer to potassium and caesium as relatively high temperature NOx-storage components.

EXAMPLE 3 This Example is designed to show how a NOx-trap composition according to the

invention comprising a ceria-zirconia-alumina mixed oxide support performed when compared with an identical NOx-trap composition except that the PGM is platinum instead of rhodium. Catalyst was prepared as described in Example 2, and tests were performed on the"fresh"catalyst which had been fired in air at 500°C for 2 hours and on aged catalyst; fresh catalyst by firing in air at 800°C for 4 hours. Test conditions were identical to those in Example 2 above.

As can be seen from Figure 3, the NOx-storage efficiency of the NOx-trap composition according to the invention is similar compared with the platinum-containing composition. The trend of NOx-storage efficiency between"fresh"and aged composition is similar for both compositions.

EXAMPLE 4 Fresh NOx-trap compositions consisting of platinum or rhodium and potassium supported on gamma alumina were prepared and tested according to the method set out in Example 2 above. The results are shown in Figure 4. The results for the aged rhodium- containing embodiment are also included. Results for an F800 embodiment wherein caesium instead of potassium is used as the NOx-storage component is included for the purposes of comparison.

It can be seen that the rhodium-containing NOx-trap composition performs similarly to the platinum-containing composition according to the invention at temperatures above 400°C. In view of the trend observed between fresh and aged catalyst in Example 3, it would be expected that an aged platinum-containing NOx-trap composition would perform similarly to the aged rhodium/potassium/gamma-alumina NOx-trap composition. The NOx-storage efficiency of the caesium embodiment is not as good as the potassium embodiment, but, as can be seen in Examples 5 and 6 below, activity can be dependent on the nature of the support.

In order to show that the reductive catalytic qualities of rhodium in a NOx-trap composition from the present Example are unaffected by the presence of potassium compared with the platinum-containing NOx-trap composition (ceria-zirconia-alumina

support, from Example 3), Figure 5 plots the NOx concentration of the exhaust gas leaving the sample during rich-lean cycling against time for aged catalyst at 450°C at which the NOx storage efficiency of the rhodium embodiment is 63% and the platinum- containing composition is 65%. It can be seen that NOx concentration leaving the rhodium-containing NOx-trap composition is less than that leaving the platinum- containing composition. This is indicative of the NOx being reduced by inter alia CO catalysed by the rhodium during the"rich"pulses for regenerating the NOx-storage component.

EXAMPLE 5 To test what effect, if any, the support had on the NOx-trap composition according to the invention, various NOx-trap compositions (0.5 Rh/lOK) were prepared each comprising a different support material. Support materials tested were gamma alumina (Condia), a ceria-zirconia-alumina mixed oxide and a ceria-zirconia mixed oxide wherein the majority of the mixed oxide is derived from ceria. In all other respects the methods of this Example were as set out in Example 2 above. The results are set out in Figure 6.

It can be seen that the trend between"fresh" (fired in air at 500°C) and aged catalyst (fresh catalyst fired at 800°C for a further 4 hours) first observed in Example 2 is repeated. Furthermore, NOx-trap composition comprising the gamma-alumina support performed best of all those tested, then the ceria-zirconia-alumina mixed oxide support and then the ceria-zirconia mixed oxide support. From these results, we surmise that the presence of ceria in the support material may have a limited but negative affect on the NOx-storage efficiency of the NOx-trap composition according to the invention. To test this theory, we prepared a NOx-trap composition wherein the support was a high surface area ceria and the results for an aged sample is shown in Figure 6. As can be seen, the presence of ceria in the support tends to lower the NOx-storage efficiency of the NOx- trap composition of the invention.

EXAMPLE 6 In order to compare the NOx-storage efficiency of NOx-trap compositions comprising caesium or potassium according to the invention, NOx-trap compositions according to the invention (ceria-zirconia-alumina support) comprising one of these NOx-storage compositions were prepared and tested according to the methods set out in Example 2. The results are set out in Figure 7 and show that each composition had similar activity.