Patent Publication Number: US-2018043342-A1

Title: Exhaust system for a compression ignition engine having a capture region for volatilised platinum

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority benefit to Great Britain Patent Application No. 1613849.7 filed on Aug. 12, 2016, which is incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The invention relates to an oxidation catalyst and an exhaust system for treating an exhaust gas produced by a compression ignition engine. The invention further relates to methods and uses of the oxidation catalyst, the exhaust system and the region for capturing volatilised platinum (Pt). The invention also relates to a vehicle comprising the oxidation catalyst or the exhaust system. 
     BACKGROUND TO THE INVENTION 
     Compression ignition engines produce an exhaust gas that contains a variety of pollutants that are the subject of environmental legislation around the world. These pollutants include carbon monoxide (CO), unburned hydrocarbons (HCs), oxides of nitrogen (NO x ) and particulate matter (PM). 
     To meet permissible levels of pollutants that may be emitted into the atmosphere set by legislation, exhaust systems for treating the exhaust gas produced by a compression ignition generally contain several emissions control devices. In such exhaust systems, the exhaust gas is usually conducted to a first emissions control device that is able to oxidise carbon monoxide (CO) and the unburned hydrocarbons (HCs) that are present in the gas. The first emissions control device may, for example, be a diesel oxidation catalyst (DOC), a catalysed soot filter (CSF), a NO x  storage catalyst (NSC), a passive NO x  adsorber (PNA), a diesel exotherm catalyst (DEC), or a cold start concept (CSC™) catalyst. 
     For NO x  emissions, exhaust systems for compression ignition engines may contain a catalyst for the selective catalytic reduction of NO x , such as selective catalytic reduction (SCR) catalyst or selective catalytic reduction filter (SCRF™) catalyst. The selective catalytic reduction (SCR) of NO x  primarily occurs by the following three reactions: 
       4NH 3 +4NO+O 2 →4N 2 +6H 2 O;  (1)
 
       4NH 3 +2NO+2NO 2 →4N 2 +6H 2 O; and  (2)
 
       8NH 3 +6NO→7N 2 +12H 2 O.  (3)
 
     The ratio of NO 2 :NO in the exhaust gas that enters an SCR catalyst or SCRF™ catalyst can affect its performance. In general, SCR catalysts or SCRF™ catalysts show optimum performance when the ratio of NO 2 :NO is about 1:1. This can be problematic because the exhaust gas produced by a compression ignition engine during normal use typically contains insufficient NO 2  (i.e. the ratio of NO 2 :NO is much lower than 1:1) for optimal performance of the SCR catalyst or the SCRF™ catalyst. To compensate for such low levels of NO 2 , the first emissions control device often contains a catalytic material that has been formulated to oxidise nitrogen monoxide (NO) to nitrogen dioxide (NO 2 ), thereby increasing the ratio of NO 2 :NO in the exhaust gas. The SCR catalyst or SCRF™ catalyst is usually arranged downstream of the first emissions control device in an exhaust system, so that the exhaust gas will pass through the first emissions control device before passing through the SCR catalyst or SCRF™ catalyst. 
     The catalytic material for oxidising NO to NO 2  typically comprises platinum (Pt). When the first emissions control device is exposed to relatively high temperatures for a sufficient period of time, low levels of platinum (Pt) may volatilise from the catalytic material and can become trapped on the SCR/SCRF™ catalyst. Such relatively high temperatures may occur during normal use, especially in heavy duty diesel applications, or during filter regeneration, such as when the first emissions control device is a CSF or when there is an upstream diesel particulate filter (DPF). Pt trapped on the SCR catalyst or the SCRF™ catalyst can have a highly detrimental effect on the catalyst&#39;s performance because it can oxidise ammonia (NH 3 ). The trapped Pt can consume the NH 3  that is intended for the selective catalytic reduction of NO x  (thereby decreasing NO x  conversion) and undesirable, secondary emissions may be produced. 
     The problem of Pt volatilisation is discussed in our publications WO 2013/088133, WO 2013/088132, WO 2013/088128, WO 2013/050784 and International patent application no. PCT/GB2016/050285. 
     SUMMARY OF THE INVENTION 
     The invention provides an oxidation catalyst for treating an exhaust gas produced by a compression ignition engine comprising: a substrate; a catalytic material disposed on the substrate, wherein the catalytic material comprises platinum (Pt); and a region comprising a capture material, wherein the capture material comprises a Pt-alloying metal disposed or supported on a refractory oxide, and the refractory oxide comprises at least 65% by weight of zirconia, wherein the region is arranged to contact the exhaust gas after the exhaust gas has contacted and/or passed through the catalytic material. 
     The region comprising a capture material is a region for capturing volatilised platinum (Pt). After exhaust gas has been contacted with and/or passed through the catalytic material, the exhaust gas may contain volatilised Pt, especially when the exhaust gas is relatively hot, such as when the engine has been operated under a heavy load for a prolonged period of time or when the engine is a heavy duty engine. The region is arranged to contact the exhaust gas after the exhaust gas has contacted or passed through the catalytic material. 
     Materials in the art that have been found to be effective at capturing volatilised Pt can lower the amount of NO 2  (and also the ratio of NO 2 :NO), particularly in the temperature region of the downstream emissions control device that is sensitive to the ratio of NO 2 :NO, resulting in a potential reduction in the performance of the downstream device, especially when it is a SCR catalyst or a SCRF™ catalyst. Existing capture materials may negate the benefit of any Pt that is included in the catalytic material of an emissions control device for the generation of NO 2 . 
     The inventors have developed a capture material for trapping volatilised Pt that does not affect (i.e. decrease) the amount of NO 2  (e.g. the ratio of NO 2 :NO) in an exhaust gas. The capture material will not reduce the amount of any NO 2  produced from the oxidation of NO by Pt in the catalytic material of the oxidation catalyst. Exhaust gas that has passed though both the catalytic material of the oxidation catalyst and the capture material will consequently contain a higher ratio of NO 2 :NO compared to the exhaust gas that was initially produced by the compression ignition engine, so that optimal performance of a downstream SCR/SCRF™ catalyst can be obtained. 
     Some capture materials for trapping volatilised Pt are described in our International patent application no. PCT/GB2016/050285. These capture materials contain at least one of (a) relatively large particles of a Pt-alloying metal (e.g. having a mean particle size ≧about 10 nm and/or a dispersion of ≦about 10%) and (b) particles of a refractory oxide having a low surface area (e.g. a mean specific surface area ≦about 50 m 2 /g). To manufacture a capture material having the desired properties, it is convenient to thermally treat the refractory oxide and/or the Pt-alloying metal and to then isolate the resulting material as a powder. A region comprising the capture material can then be formed on the oxidation catalyst using conventional washcoat techniques, where the washcoat is formed by dissolving or dispersing the powder in a solution. A disadvantage of this manufacturing method is that it is time consuming and costly. 
     The capture material of the invention can be manufactured in a simple, cost-effective process. The capture material is typically formed in situ from one or more salts of the Pt-alloying metal and the refractory oxide during the preparation of a washcoat. The refractory oxide and the Pt-alloying metal salts are typically dispersed in a washcoat, which is then coated onto and adheres to a surface of the substrate. The coated substrate is usually then dried and calcined, which converts the Pt-alloying metal salts into a Pt-alloying metal or an oxide thereof and fixes it onto a surface of the refractory oxide. 
     The invention also provides a capture brick. The capture brick is suitable for capturing volatilised platinum (Pt) from a catalytic material comprising platinum (Pt) in an exhaust system for a compression ignition engine. The capture brick comprises: a substrate and a capture material disposed on the substrate, wherein the capture material comprises a Pt-alloying metal disposed or supported on a refractory oxide, wherein the refractory oxide comprises at least 65% by weight of zirconia. 
     The invention further provides an exhaust system for treating an exhaust gas produced by a compression ignition engine. The exhaust system comprises:
     (i) an oxidation catalyst for treating the exhaust gas, wherein the oxidation catalyst comprises a substrate and a catalytic material disposed on the substrate, wherein the catalytic material comprises platinum (Pt); and   (ii) a region comprising a capture material, wherein the capture material comprises a Pt-alloying metal disposed or supported on a refractory oxide, and the refractory oxide comprises at least 65% by weight of zirconia;
       wherein the region is arranged to contact exhaust gas after the exhaust gas has contacted and/or passed through the catalytic material.   
       

     The region comprising the capture material is arranged to contact exhaust gas after the exhaust gas has contacted and/or passed through the catalytic material. The region for capturing volatilised platinum (Pt) may be an integral part of the oxidation catalyst. 
     Additionally or alternatively, the region for capturing volatilised platinum (Pt) may be part of a capture brick. The region comprising the capture material is provided by the capture brick, which is arranged to contact exhaust gas after the exhaust gas has contacted and/or passed through the catalytic material when the exhaust gas has passed through the oxidation catalyst. 
     The exhaust system of the invention may comprise:
     (i) an oxidation catalyst for treating the exhaust gas, wherein the oxidation catalyst comprises a first substrate and a catalytic material disposed on the first substrate, wherein the catalytic material comprises platinum (Pt); and   (ii) a capture brick for capturing volatilised platinum (Pt) from the catalytic material, wherein the capture brick comprises a second substrate and a capture material disposed on the second substrate, wherein the capture material comprises a Pt-alloying metal disposed or supported on a refractory oxide, and the refractory oxide comprises at least 65% by weight of zirconia;
       wherein the capture brick is arranged to contact exhaust gas after the exhaust gas has passed through the oxidation catalyst.   
       

     The “first” substrate is separate to the “second” substrate (e.g. the “first” substrate is spatially separated from the “second” substrate). The terms “first” and “second” in this context are merely labels for identifying each substrate and they do not limit the scope of the invention. 
     In the exhaust system above, the oxidation catalyst may or may not be an oxidation catalyst of the invention. 
     The invention further provides a vehicle. The vehicle comprises a compression ignition engine and either an oxidation catalyst, an exhaust system or a capture brick of the invention. 
     Another aspect of the invention relates to the use of a region for capturing volatilised platinum (Pt) in an exhaust system for treating an exhaust gas produced by a compression ignition engine. The exhaust system comprises an oxidation catalyst for treating the exhaust gas, the oxidation catalyst comprising a substrate and a catalytic material disposed on the substrate, wherein the catalytic material comprises platinum (Pt), and wherein the region for capturing volatilised platinum (Pt) comprises a capture material comprising a Pt-alloying metal disposed or supported on a refractory oxide, wherein the refractory oxide comprises at least 65% by weight of zirconia, and wherein the region is arranged to contact the exhaust gas after the exhaust gas has contacted or passed through the catalytic material and/or the oxidation catalyst. 
     A further aspect of the invention relates to a method of capturing volatilised platinum (Pt) from a catalytic material in an exhaust system for a compression ignition engine. 
     The invention also relates to a method of treating an exhaust gas produced by a compression ignition engine. 
     Each of the above methods of the invention comprise the step of passing an exhaust gas produced by a compression ignition engine through an exhaust system comprising:
     (i) an oxidation catalyst for treating the exhaust gas, wherein the oxidation catalyst comprises a substrate and a catalytic material disposed on the substrate, wherein the catalytic material comprises platinum (Pt); and   (ii) a region comprising a capture material, wherein the capture material comprises a Pt-alloying metal disposed or supported on a refractory oxide, and the refractory oxide comprises at least 65% by weight of zirconia;
       wherein the region is arranged to contact exhaust gas after the exhaust gas has contacted and/or passed through the catalytic material.   
       

     Generally, volatilised platinum may be present in an exhaust gas (i.e. in an exhaust system) when the temperature of the exhaust gas is ≧700° C., such as ≧800° C., preferably ≧900° C. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic drawing of a laboratory reactor used for testing platinum contamination on a Cu/CHA zeolite SCR catalyst. 
         FIG. 2  is a schematic drawing showing an exhaust system embodiment of the invention. Exhaust gas ( 20 ) passes through an oxidation catalyst ( 1 ) of the invention, which has a capture zone ( 3 ). After exhaust gas ( 20 ) has passed through the oxidation catalyst ( 1 ), it passes through a second emissions control device ( 10 ), such as a selective catalytic reduction (SCR) catalyst or a selective catalytic reduction filter (SCRF™) catalyst. If SCR is to be performed actively, then a source of ammonia ( 30 ) may be introduced into the exhaust gas. For passive SCR, the source of ammonia ( 30 ) may not be present. 
         FIG. 3  is a schematic drawing of an oxidation catalyst of the invention. The oxidation catalyst has a region or zone ( 2 ) disposed on a substrate ( 1 ), which comprises a catalytic material containing Pt. There is capture zone ( 3 ) at or near the outlet end of the oxidation catalyst, which capture zone comprises a capture material for capturing volatilised Pt. 
         FIG. 4  is a schematic drawing of an oxidation catalyst of the invention. The oxidation catalyst has a layer ( 2 ) disposed on a substrate ( 1 ), which layer comprises a catalytic material containing Pt. There is capture zone ( 3 ) at or near the outlet end of the oxidation catalyst that is disposed on the layer ( 2 ) of catalytic material. The capture zone ( 3 ) comprises a capture material for capturing volatilised Pt. 
         FIG. 5  is a schematic drawing of an oxidation catalyst of the invention. The oxidation catalyst has a region or zone ( 2 ) disposed on a substrate, which region or zone comprises a catalytic material containing Pt. There is capture region ( 3 ) at or near the outlet end of the oxidation catalyst. The capture region ( 3 ) overlaps the region or zone ( 2 ) containing the catalytic material. The capture region ( 3 ) comprises a capture material for capturing volatilised Pt. 
         FIG. 6  is a schematic drawing showing an exhaust system embodiment of the invention. Exhaust gas ( 20 ) passes through an oxidation catalyst ( 1 ) of the invention, which has a capture material ( 3 ) disposed at an outlet end surface of the substrate. After exhaust gas ( 20 ) has passed through the oxidation catalyst ( 1 ), it passes through a second emissions control device ( 10 ), such as a selective catalytic reduction (SCR) catalyst or a selective catalytic reduction filter (SCRF™) catalyst. If SCR is to be performed actively, then a source of ammonia ( 30 ) may be introduced into the exhaust gas. For passive SCR, the source of ammonia ( 30 ) may not be present. 
         FIG. 7  is a schematic drawing of an oxidation catalyst of the invention. The oxidation catalyst has a catalytic material ( 2 ) disposed on a substrate ( 1 ). There is a capture material ( 3 ) disposed or supported at an outlet end surface of the substrate. 
         FIG. 8  is a schematic drawing of an oxidation catalyst of the invention. The oxidation catalyst has a catalytic material ( 2 ) disposed on a substrate ( 1 ). There is a capture material ( 3 ) disposed or supported at an outlet end surface of the substrate, which partially overlies the catalytic material ( 2 ). 
         FIG. 9  is a schematic drawing showing an exhaust system embodiment of the invention. Exhaust gas ( 20 ) passes through an oxidation catalyst ( 1 ), which may or may not be an oxidation catalyst of the invention. After exhaust gas ( 20 ) has passed through the oxidation catalyst ( 1 ), it passes through a capture brick ( 4 ) comprising a substrate and a region ( 3 ) for capturing volatilised Pt. The exhaust gas ( 20 ) then flows onto a second emissions control device ( 10 ), such as a selective catalytic reduction (SCR) catalyst or a selective catalytic reduction filter (SCRF™) catalyst. If SCR is to be performed actively, then a source of ammonia ( 30 ) may be introduced into the exhaust gas after it has passed through the capture brick ( 4 ). For passive SCR, the source of ammonia ( 30 ) may not be present. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention relates to a region for capturing volatilised platinum (Pt), which comprises, or consists essentially of, a capture material. The region can be arranged in a variety of ways to trap or capture volatilised Pt in an exhaust gas that has passed through an upstream catalytic material, typically of an oxidation catalyst. The region is arranged to prevent the volatilised Pt from condensing on a second, downstream emissions control device, such as a SCR catalyst or SCRF™ catalyst. 
     The capture material comprises, or consists essentially of, a Pt-alloying metal disposed or supported on a refractory oxide, wherein the refractory oxide comprises at least 65% by weight of zirconia. 
     The inventors have found that the use of a capture material in accordance with the invention can reduce or prevent volatilised Pt from becoming deposited on a downstream SCR catalyst or SCRF™ catalyst when the capture material is arranged within the exhaust system in an appropriate manner. The capture material of the invention has low catalytic activity, particularly toward the oxidation of CO and/or unburned HCs, and especially toward the oxidation of NO x . In general, the capture material is also substantially catalytically inactive toward the degradation or reduction of NO 2 , particularly under the conditions at which Pt volatilisation occurs and/or in the temperature region at which a downstream SCR catalyst or SCRF™ catalyst is sensitive to the ratio of NO 2 :NO. 
     Generally, it is preferable that the capture material (i.e. when new or unused) or the capture region is substantially free of platinum and/or rhodium. More preferably, the capture material does not comprise platinum and/or rhodium. 
     It may also be preferable that the capture material or the capture region does not, in general, comprise a base metal, such as barium or vanadium. 
     Typically, the region for capturing volatilised platinum (Pt) or the capture material thereof comprises a loading of the refractory oxide of 0.1 to 3.5 g in −3 , preferably 0.2 to 2.5 g in −3 , still more preferably 0.3 to 2.0 g in −3 , and even more preferably 0.5 to 1.75 g in −3  (e.g. 0.75 to 1.5 g in −3 ). 
     The capture material comprises particles of a refractory oxide typically having a mean specific surface area ≧about 50 m 2 /g (&gt;about 50 m 2 /g), such as ≧about 60 m 2 /g (&gt;about 60 m 2 /g), preferably ≧about 75 m 2 /g (&gt;about 75 m 2 /g), more preferably ≧about 90 m 2 /g (&gt;about 90 m 2 /g), even more preferably ≧about 100 m 2 /g (&gt;about 100 m 2 /g). 
     The mean specific surface area (SSA) of the particles of the refractory oxide can be determined by nitrogen physisorption at −196° C. using the volumetric method. The mean SSA is determined using the BET adsorption isotherm equation. 
     The refractory oxide may have a d90 of &lt;100 micron. The refractory oxide may preferably have a d90 of &lt;75 micron, such as &lt;50 micron (e.g. &lt;30 micron), and more preferably &lt;20 micron. When the refractory oxide has a smaller d90, better packing and adhesion can be obtained. For the avoidance of doubt, the d90 measurements may be obtained by Laser Diffraction Particle Size Analysis using a Malvern Mastersizer 2000, which is a volume-based technique (i.e. D90 may also be referred to as D v 90 (or D(v,0.90)) and applying a mathematical Mie theory model to determine a particle size distribution. 
     Typically, the refractory oxide has a d90 of &gt;0.1 micron. It is preferred that the refractory oxide has a d90 of &gt;1.0 micron, such as &gt;5.0 micron. 
     The refractory oxide comprises at least 65% by weight of zirconia (ZrO 2 ), preferably at least 70% by weight, and more preferably at least 80% by weight of zirconia (e.g. at least 90% by weight of zirconia). 
     It is preferred that the refractory oxide comprises less than 20% by weight of ceria (CeO 2 ), more preferably less than 15% by weight of ceria, and even more preferably less than 10% by weight of ceria. 
     Typically, the refractory oxide is substantially free of ceria. More preferably, the refractory oxide does not comprise ceria. 
     The refractory oxide may further comprise an oxide of neodymium (e.g. Nd 2 O 3 ), an oxide of lanthanum (e.g. La 2 O 3 ), an oxide of hafnium (e.g. HfO 2 ), an oxide of yttrium (e.g. Y 2 O 3 ) and/or an oxide of praseodymium (e.g. Pr 2 O 3 , PrO 2 , and/or Pr 6 O 11 ). These oxides may provide a stabilising function to the refractory oxide. Some these oxides may also be present as impurities in the refractory oxide. 
     It may be preferable that the refractory oxide further comprises an oxide of neodymium (e.g. Nd 2 O 3 ). The refractory oxide may further comprise 1 to 20% by weight of an oxide of neodymium (e.g. Nd 2 O 3 ), preferably 2.5 to 15% by weight (e.g. 5 to 15% by weight), such as 5 to 10% by weight. 
     The refractory oxide may consist essentially of zirconia. 
     Typically, the Pt-alloying metal comprises, or consists essentially of, a metal and/or an oxide thereof. The metal is preferably selected from the group consisting of palladium (Pd); gold (Au); copper (Cu); a mixture of Pd and Au; a mixture of Pd and Cu; a mixture of Au and Cu; a mixture of Pd, Au and Cu; a bimetallic alloy of Pd and Au; a bimetallic alloy of Pd and Cu; a bimetallic alloy of Au and Cu; and a trimetallic alloy of Pd, Au and Cu. It is preferred that the metal is selected from the group consisting of palladium (Pd), a mixture of Pd and Au, and a bimetallic alloy of Pd and Au. More preferably, the metal is palladium (Pd). 
     For the avoidance of doubt, the Pt-alloying metal does not comprise platinum (e.g. when new or unused). 
     It is preferred that the particles of the Pt-alloying metal have a mean particle size ≦about 20 nm (e.g. &lt;about 20 nm), such as a mean particle size &lt;15 nm (e.g. &lt;about 15 nm), more preferably ≦about 10 nm (e.g. &lt;about 10 nm), especially ≦8 nm (e.g. &lt;about 8 nm). 
     The particles of the Pt-alloying metal particles typically have a dispersion of &gt;about 10%, preferably ≧15% (e.g. 15 to 35%), such as ≧20% (e.g. 20 to 30%). The measurement of the dispersion refers to unused Pt-alloying metal particles (i.e. fresh particles, which have not been subjected to repeated or prolonged use). 
     The “mean particle size” and the “dispersion” as used herein with reference to the Pt-alloying metal, particularly when the Pt-alloying metal is palladium, can be determined by CO chemisorption, as follows. The Pt-alloying metal content can be measured by Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). The CO uptake of the sample can be measured using a Micromeritics Autochem 2920 instrument. The sample is pre-treated with hydrogen gas at 300° C. Carbon monoxide uptake is measured by pulse chemisorption at 50° C. The Pt-alloying metal particle size can then be calculated using Autochem 2920 software based on the CO uptake and S-trapping metal content for the sample. A chemisorption ratio for CO:Pd of 1:1 is used in the calculation. 
     The dispersion of the Pt-alloying metal is a measurement of the particle size of the Pt-alloying metal. Large particles with a low surface area have a low dispersion. 
     Generally, the capture material or capture region has a total loading of Pt-alloying metal (e.g. the metal content of the Pt-alloying metal) of 1 g ft −3  to 50 g ft −3 , preferably 4 g ft −3  to 40 g ft −3 , even more preferably 8 g ft −3  to 30 g ft −3 . 
     The Pt-alloying metal is disposed or supported on the refractory oxide. The Pt-alloying metal may be disposed directly onto or is directly supported by the refractory oxide (e.g. there is no intervening support material between the Pt-alloying metal and the refractory oxide). For example, the Pt-alloying metal, such as palladium, can be dispersed on a surface of and/or impregnated into the refractory oxide. 
     Typically, the refractory oxide is not a material that promotes the catalytic activity of the Pt-alloying metal, particularly when the Pt-alloying metal comprises palladium (e.g. the catalytic activity of the palladium in the oxidation of CO and/or unburned HCs). 
     At least one particle of the Pt-alloying metal may be disposed or supported on at least one particle of the refractory oxide. Preferably, a plurality of particles of Pt-alloying metal are disposed or supported on at least one particle of the refractory oxide. More preferably, there is a plurality of particles of the refractory oxide, wherein a plurality of particles of Pt-alloying metal are disposed or supported on each particle of the refractory oxide. 
     The invention includes various arrangements of the capture material in relation to the catalytic material comprising (Pt), which is part of an oxidation catalyst. In principle, the capture material could be used with an oxidation catalyst comprising such a catalytic material. 
     When the oxidation catalyst itself comprises the capture material (i.e. the capture material is disposed or supported on the same substrate as the catalytic material), then the oxidation catalyst is an oxidation catalyst of the invention. 
     When the capture material is part of a capture brick, then the oxidation catalyst may be an oxidation catalyst of the invention or an oxidation catalyst without a region comprising a capture material. 
     When the oxidation catalyst is an oxidation catalyst of the invention, then it may comprise a region comprising a first capture material and the capture brick may comprise a second capture material. The first capture material may have a composition that is the same or different to the composition of the second capture material. When the compositions of the first capture material and the second capture material are different, then each capture material may independently have a composition as defined above. 
     When the oxidation catalyst is an oxidation catalyst without a region comprising a capture material, then the oxidation catalyst comprises, or consists essentially of, a catalytic material disposed on the substrate, wherein the catalytic material comprises platinum (Pt). 
     Features below described in relation to the oxidation catalyst relate to the oxidation catalyst of the invention and/or the oxidation catalyst without a region comprising a capture material, unless the context indicates otherwise. 
     The oxidation catalyst may be a diesel oxidation catalyst (DOC), a catalysed soot filter (CSF), a NO x  storage catalyst (NSC), a passive NO x  adsorber (PNA), a diesel exotherm catalyst (DEC), a cold start concept (CSC™) catalyst [see WO 2012/166868 and International patent application no. PCT/US14/69079, which are each incorporated herein by reference] or an ammonia slip catalyst (ASC). It is preferred that the oxidation catalyst is a DOC, a CSF, a NSC, a PNA or a DEC. More preferably, the oxidation catalyst is a DOC or a CSF. 
     For the avoidance of doubt, the capture material and the catalytic material have different compositions. 
     Generally, the catalytic material comprises platinum (Pt) disposed or supported on a support material (referred to herein as the support material of the catalytic material or “CM support material”). The platinum may be disposed directly onto or is directly supported by the support material (e.g. there is no intervening support material between the platinum and the support material). For example, platinum can be dispersed over a surface of and/or impregnated within the support material. 
     The CM support material comprises, or consists essentially of, a refractory oxide (referred to herein as the refractory oxide of the catalytic material). Particles of the refractory oxide typically have a mean specific surface area ≧75 m 2 /g, such as ≧85 m 2 /g, and preferably ≧100 m 2 /g. 
     The refractory oxide of the CM support material is typically selected from the group consisting of alumina, silica, titania, ceria and a mixed or composite oxide thereof. For example, the refractory oxide may be selected from the group consisting of alumina, silica, titania, ceria, silica-alumina, titania-alumina, zirconia-alumina, ceria-alumina, titania-silica, zirconia-silica, zirconia-titania, ceria-zirconia and alumina-magnesium oxide. 
     When the CM support material or the refractory oxide thereof, comprises or consists essentially of a mixed or composite oxide of alumina (e.g. silica-alumina, alumina-magnesium oxide or a mixture of alumina and ceria), then preferably the mixed or composite oxide of alumina comprises at least 50 to 99% by weight of alumina, more preferably 70 to 95% by weight of alumina, even more preferably 75 to 90% by weight of alumina. 
     When the CM support material or the refractory oxide thereof, comprises or consists essentially of ceria-zirconia, then the ceria-zirconia may consist essentially of 20 to 95% by weight of ceria and 5 to 80% by weight of zirconia (e.g. 50 to 95% by weight ceria and 5 to 50% by weight zirconia), preferably 35 to 80% by weight of ceria and 20 to 65% by weight zirconia (e.g. 55 to 80% by weight ceria and 20 to 45% by weight zirconia), even more preferably 45 to 75% by weight of ceria and 25 to 55% by weight zirconia. 
     The CM support material or the refractory oxide thereof may optionally be doped (e.g. with a dopant). The dopant may be selected from the group consisting of zirconium (Zr), titanium (Ti), silicon (Si), yttrium (Y), lanthanum (La), praseodymium (Pr), samarium (Sm), neodymium (Nd) and an oxide thereof. 
     When the CM support material or the refractory oxide thereof is doped, the total amount of dopant is 0.25 to 5% by weight, preferably 0.5 to 3% by weight (e.g. about 1% by weight). 
     The CM support material or the refractory oxide thereof may comprise or consist essentially of alumina doped with a dopant. It is particularly preferred that the CM support material or the refractory oxide thereof comprises, or consists essentially of, alumina doped with a dopant when the catalytic material comprises an alkaline earth metal, preferably when the oxidation catalyst is a diesel oxidation catalyst (DOC) or a catalysed soot filter (CSF). 
     The alumina may be doped with a dopant comprising silicon (Si), magnesium (Mg), barium (Ba), lanthanum (La), cerium (Ce), titanium (Ti), or zirconium (Zr) or a combination of two or more thereof. The dopant may comprise, or consist essentially of, an oxide of silicon, an oxide of magnesium, an oxide of barium, an oxide of lanthanum, an oxide of cerium, an oxide of titanium or an oxide of zirconium. Preferably, the dopant comprises, or consists essentially of, silicon, magnesium, barium, cerium, or an oxide thereof, particularly silicon, or cerium, or an oxide thereof. More preferably, the dopant comprises, or consists essentially of, silicon, magnesium, barium, or an oxide thereof; particularly silicon, magnesium, or an oxide thereof; especially silicon or an oxide thereof. 
     Examples of alumina doped with a dopant include alumina doped with silica, alumina doped with magnesium oxide, alumina doped with barium or barium oxide, alumina doped with lanthanum oxide, or alumina doped with ceria, particularly alumina doped with silica, alumina doped with lanthanum oxide, or alumina doped with ceria. It is preferred that the alumina doped with a dopant is alumina doped with silica, alumina doped with barium or barium oxide, or alumina doped with magnesium oxide. More preferably, the alumina doped with a dopant is alumina doped with silica or alumina doped with magnesium oxide. Even more preferably, the alumina doped with a dopant is alumina doped with silica. 
     When the alumina is alumina doped with silica, then the alumina is doped with silica in a total amount of 0.5 to 45% by weight (i.e. % by weight of the alumina), preferably 1 to 40% by weight, more preferably 1.5 to 30% by weight (e.g. 1.5 to 10% by weight), particularly 2.5 to 25% by weight, more particularly 3.5 to 20% by weight (e.g. 5 to 20% by weight), even more preferably 4.5 to 15% by weight. 
     When the alumina is alumina doped with magnesium oxide, then the alumina is doped with magnesium oxide in an amount as defined above or an amount of 1 to 40% by weight (i.e. % by weight of the alumina), such as 5 to 28% by weight. More preferably, the alumina is doped with magnesium oxide in amount of 10 to 25% by weight. 
     Alternatively or additionally, the CM support material or refractory oxide thereof may comprise, or consist essentially of, an alkaline earth metal aluminate. The term “alkaline earth metal aluminate” generally refers to a compound of the formula MAl 2 O 4  where “M” represents the alkaline earth metal, such as Mg, Ca, Sr or Ba. Such compounds may comprise a spinel structure. 
     Typically, the alkaline earth metal aluminate is magnesium aluminate (MgAl 2 O 4 ), calcium aluminate (CaAl 2 O 4 ), strontium aluminate (SrAl 2 O 4 ), barium aluminate (BaAl 2 O 4 ), or a mixture of two or more thereof. Preferably, the alkaline earth metal aluminate is magnesium aluminate (MgAl 2 O 4 ). 
     In the oxidation catalyst, the catalytic material may comprise a single platinum group metal (PGM), which is platinum (e.g. the catalytic material comprises platinum as the only platinum group metal). 
     Alternatively, depending on the application of the oxidation catalyst, the catalytic material may comprise (i) platinum (Pt), and (ii) palladium (Pd) and/or rhodium (Rh). 
     In general, when the catalytic region or the catalytic material thereof comprises Pt and Pd (and optionally Rh), then typically the ratio by mass of Pt to Pd is ≧1:1. The catalytic material may comprise Pt and optionally Pd, such that the ratio by mass of Pt to Pd is from 1:0 to 1:1. It has been found that volatilisation of platinum occurs when the catalytic material is relatively Pt rich. 
     It is preferred that when the catalytic material comprises Pt and Pd (and optionally Rh), then the ratio by mass of Pt to Pd is ≧1.5:1, more preferably ≧2:1 (e.g. ≧3:1), even more preferably ≧4:1, such as ≧10:1. The ratio by mass (i.e. mass ratio) of Pt to Pd is preferably 50:1 to 1:1, more preferably 30:1 to 2:1 (e.g. 25:1 to 4:1), even more preferably 20:1 to 5:1, such as 15:1 to 7.5:1. 
     Generally, when the catalytic material comprises Pt and Rh (and optionally Pd), then typically the ratio by mass of Pt to Rh is ≧1:1. The catalytic material may comprise Pt and optionally Rh, such that the ratio by mass of Pt to Rh is from 1:0 to 1:1. When the catalytic material comprises Pt and Rh (and optionally Pd), then preferably the ratio by mass of Pt to Rh is ≧1.5:1, more preferably ≧2:1 (e.g. ≧3:1), even more preferably ≧4:1, such as ≧10:1. The ratio by mass (i.e. mass ratio) of Pt to Rh is preferably 50:1 to 1:1, more preferably 30:1 to 2:1 (e.g. 25:1 to 4:1), even more preferably 20:1 to 5:1, such as 15:1 to 7.5:1. 
     If the catalytic material comprises Pd (and optionally Rh), then the catalytic material may comprise Pd disposed or supported on the CM support material. If Rh is also present, then the catalytic material may comprise Pd and Rh disposed or supported on the CM support material. 
     Typically, the oxidation catalyst has a total loading of PGM of 5 to 500 g ft −3 . Preferably, the total loading of PGM is 10 to 400 g ft −3 , more preferably 20 to 300 g ft −3 , still more preferably, 25 to 250 g ft −3 , and even more preferably 30 to 200 g ft −3 . 
     In a first oxidation catalyst embodiment (with or without the capture material), the oxidation catalyst is a diesel oxidation catalyst (DOC), a diesel exotherm catalyst (DEC) or a passive NO x  adsorber (PNA). 
     When the oxidation catalyst is a diesel oxidation catalyst (DOC), a diesel exotherm catalyst (DEC), a passive NO x  adsorber (PNA), a cold start concept (CSC™) catalyst or an ammonia slip catalyst (ASC), then typically the oxidation catalyst or the catalytic material thereof has a total loading of PGM is 20 to 200 g ft −3 , more preferably 40 to 160 g ft −3 . 
     In a second oxidation catalyst embodiment (with or without the capture material), the oxidation catalyst is a catalysed soot filter (CSF). 
     When the oxidation catalyst is a catalysed soot filter (CSF), then preferably the oxidation catalyst or the catalytic material thereof has a total loading of PGM is 1 to 100 g ft −3 , more preferably 5 to 50 g ft −3 . 
     When the oxidation catalyst is a diesel oxidation catalyst (DOC), a diesel exotherm catalyst (DEC), a passive NO x  adsorber (PNA), a cold start concept (CSC™) catalyst, an ammonia slip catalyst (ASC) or a catalysed soot filter (CSF), then preferably the oxidation catalyst or the catalytic material thereof does not comprise rhodium (Rh). The catalytic material may comprise platinum (Pt) or platinum (Pt) and palladium (Pd), typically as the only platinum group metals (PGMs). 
     When the oxidation catalyst is a diesel oxidation catalyst (DOC), diesel exotherm catalyst (DEC) or a catalysed soot filter (CSF), it is preferred that the refractory oxide comprises alumina, such as alumina optionally doped with a dopant (e.g. where the dopant comprises silicon or an oxide thereof, or the dopant is silica) or a mixed or composite oxide of alumina (e.g. silica-alumina). Alternatively, the refractory oxide may consist essentially of alumina. 
     The catalytic material in the first and second oxidation catalyst embodiments may further comprise a catalyst promoter. The catalyst promoter may comprise, or consist essentially of, an alkaline earth metal. The alkaline earth metal may be selected from the group consisting of magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba) and a combination of two or more thereof. It is preferred that the alkaline earth metal is calcium (Ca), strontium (Sr), or barium (Ba), more preferably strontium (Sr) or barium (Ba), and most preferably the alkaline earth metal is barium (Ba). 
     Generally, the catalytic material comprises a total amount of the alkaline earth metal of 10 to 500 g ft −3  (e.g. 60 to 400 g ft −3  or 10 to 450 g ft −3 ), particularly 20 to 400 g ft −3 , more particularly 35 to 350 g ft −3 , such as 50 to 300 g ft −3 , especially 75 to 250 g ft −3 . 
     Typically, the catalyst promoter (e.g. alkaline earth metal) and platinum (and optionally palladium) are supported on the CM support material. 
     In a third oxidation catalyst embodiment (with or without the capture material), the oxidation catalyst is a NO x  storage catalyst (NSC). 
     When the oxidation catalyst is a NO x  storage catalyst (NSC), then preferably the oxidation catalyst or the catalytic material thereof may comprise:
     (a) platinum (Pt) and palladium (Pd), preferably Pt and Pd as the only PGMs; or   (b) platinum (Pt) and rhodium (Rh), preferably Pt and Rh as the only PGMs; or   (c) platinum (Pt), palladium (Pd) and rhodium (Rh), preferably Pt, Pd and Rh as the only PGMs.   

     When the oxidation catalyst is a NO x  storage catalyst (NSC), then preferably the oxidation catalyst or the catalytic material thereof has a total loading of PGM is 20 to 200 g ft −3 , more preferably 40 to 160 g ft −3 . 
     If the catalytic material comprises Pd, then the Pd may be disposed or supported on the CM support material. 
     If the catalytic material comprises Pd, then the Pd may be disposed or supported on the CM support material. 
     In the third oxidation catalyst embodiment, it is preferred that the CM support comprises, or consists essentially of, a refractory oxide selected from the group consisting of alumina-magnesium oxide (e.g. a mixed or composite oxide thereof), alumina doped with magnesium oxide and magnesium aluminate (MgAl 2 O 4 ). More preferably, the refractory oxide is selected from the group consisting of alumina-magnesium oxide (e.g. a mixed or composite oxide thereof) and alumina doped with magnesium oxide. The alumina-magnesium oxide or the alumina doped with magnesium oxide comprise magnesium oxide in an amount of 1 to 40% by weight (i.e. % by weight of the alumina), such as 5 to 28% by weight. More preferably, the alumina is doped with magnesium oxide in amount of 10 to 25% by weight. 
     When the oxidation catalyst is a NO x  storage catalyst (NSC), then typically the oxidation catalyst or the catalytic material thereof comprises a NO x  storage component. 
     The NO x  storage component comprises an alkaline earth metal selected from the group consisting of magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba) and a combination of two or more thereof. It is preferred that the alkaline earth metal is calcium (Ca), strontium (Sr), or barium (Ba), more preferably strontium (Sr) or barium (Ba), and most preferably the alkaline earth metal is barium (Ba). 
     Typically, the NO x  storage component consists of an oxide, a carbonate or a hydroxide of the alkaline earth metal. 
     When the oxidation catalyst is a NO x  storage catalyst (NSC), then typically the oxidation catalyst or the catalytic material thereof comprises a total amount of NO x  storage component (e.g. alkaline earth metal) of 100 to 6000 g ft −3 , preferably 250 to 900 g ft −3  (e.g. 250 to 500 g ft −3 ), particularly 300 to 850 g ft −3  (e.g. 300 to 450 g ft −3 ), more particularly 400 to 800 g ft −3 , such as 450 to 600 g ft −3 . In some circumstances, the CM support material and/or the NSC support material may provide some NO x  storage activity, such as when the support material comprises ceria. For the avoidance of doubt, the total amount of NO x  storage component typically does not include the amount of the CM support material and/or the amount of the NSC support material. 
     The NO x  storage component may be disposed or supported on the CM support material. 
     The oxidation catalyst or the catalytic material thereof may further comprise a NO x  storage component support material (referred to herein as “NSC support material”). In addition or as an alternative to disposing or supporting the NO x  storage component on the CM support material, the NO x  storage component may be disposed or supported on the NSC support material. 
     The NSC support material comprises, or consists essentially of, refractory oxide, such as a refractory oxide selected from the group consisting of ceria and a mixed or composite oxide thereof. The mixed or composite oxide of ceria may be selected from the group consisting of ceria-alumina and ceria-zirconia. It is preferred that the refractory oxide is selected from the group consisting of ceria and ceria-zirconia. 
     When the NSC support material or the refractory oxide thereof, comprises or consists essentially of ceria-zirconia, then the ceria-zirconia may consist essentially of 20 to 95% by weight of ceria and 5 to 80% by weight of zirconia (e.g. 50 to 95% by weight ceria and 5 to 50% by weight zirconia), preferably 35 to 80% by weight of ceria and 20 to 65% by weight zirconia (e.g. 55 to 80% by weight ceria and 20 to 45% by weight zirconia), even more preferably 45 to 75% by weight of ceria and 25 to 55% by weight zirconia. 
     The oxidation catalyst (including the first to third oxidation catalyst embodiments) or the catalytic material thereof may further comprise a zeolite. It is preferred that the zeolite is a medium pore zeolite (e.g. a zeolite having a maximum ring size of ten tetrahedral atoms) or a large pore zeolite (e.g. a zeolite having a maximum ring size of twelve tetrahedral atoms). Examples of suitable zeolites or types of zeolite include faujasite, clinoptilolite, mordenite, silicalite, ferrierite, zeolite X, zeolite Y, ultrastable zeolite Y, AEI zeolite, ZSM-5 zeolite, ZSM-12 zeolite, ZSM-20 zeolite, ZSM-34 zeolite, CHA zeolite, SSZ-3 zeolite, SAPO-5 zeolite, offretite, a beta zeolite or a copper CHA zeolite. The zeolite is preferably ZSM-5, a beta zeolite or a Y zeolite. 
     Typically, the zeolite has a silica to alumina molar ratio of at least 25:11, preferably at least 25:1, with useful ranges of from 25:1 to 1000:1, 50:1 to 500:1 as well as 25:1 to 100:1, 25:1 to 300:1, from 100:1 to 250:1. 
     When the oxidation catalyst or catalytic material thereof comprises a zeolite, then typically the total loading of zeolite is 0.05 to 3.00 g in −3 , particularly 0.10 to 2.00 g in −3 , more particularly 0.2 to 0.8 g in −3 . 
     In general, the oxidation catalyst comprises a region comprising the catalytic material. The region comprising the catalytic material is referred to herein as the “catalytic region”. The catalytic region is typically disposed or supported on the substrate. The catalytic region may be disposed directly on to the substrate (i.e. the catalytic region is in contact with a surface of the substrate). 
     The capture region may be:
     (a) disposed or supported on the catalytic region; and/or   (b) disposed directly on to the substrate [i.e. the capture region is in contact with a surface of the substrate]; and/or   (c) in contact with the catalytic region [i.e. the capture region is adjacent to, or abuts, the catalytic region].   

     In general, the capture region is disposed or supported on a plurality of channel walls of the substrate (i.e. each channel wall within the substrate). 
     The oxidation catalyst of the invention also comprises a region comprising the capture material, which is arranged to contact the exhaust gas after the exhaust gas has contacted and/or passed through the catalytic material. The region comprising the capture material is referred to herein as the “capture region”. 
     Generally, the capture region is arranged to contact the exhaust gas as it leaves the oxidation catalyst. The catalytic region may be arranged or oriented to contact exhaust gas before the capture region. In the first to third arrangements of the oxidation catalyst of the invention, the capture region is arranged to contact the exhaust gas as it leaves the oxidation catalyst and optionally the catalytic region is arranged or oriented to contact exhaust gas before the capture region. 
     It is preferred that the capture region is a capture zone. More preferably, the capture zone is disposed or supported at or near an outlet end of the substrate. 
     In general, the capture zone has a length of ≧0.5 inch (≧12.7 mm), preferably ≧1 inch (≧25.4 mm). This length of the capture zone is independent of the length of the substrate. The capture zone typically has a length of 2.5 to 80% of the length of the substrate (e.g. 5 to 60%), preferably 10 to 50% of the length of the substrate (e.g. 15 to 35%), more preferably 15 to 30% of the length of the substrate. 
     In a first arrangement of the oxidation catalyst of the invention (including the first to third oxidation catalyst embodiment), the catalytic region is disposed or supported upstream of the capture zone. Preferably, the catalytic region is a catalytic zone. More preferably, the catalytic zone is disposed or supported at or near an inlet end of the substrate. 
     Typically, the catalytic zone has a length of 10 to 90% of the length of the substrate (e.g. 50 to 90%), preferably 15 to 80% of the length of the substrate (e.g. 55 to 80%), more preferably 20 to 75% (e.g. 30 to 65%) of the length of the substrate, still more preferably 30 to 65%. 
     The catalytic zone may adjoin the capture zone. Preferably, the catalytic zone is in contact with the capture zone. When the catalytic zone adjoins the capture zone or the catalytic zone is in contact with the capture zone, then the catalytic zone and the capture zone may be disposed or supported on the substrate as a layer (e.g. a single layer). Thus, a layer (e.g. a single) may be formed on the substrate when the catalytic and capture zones adjoin or are in contact with one another. Such an arrangement may avoid problems with back pressure. 
     The catalytic zone may be separate from the capture zone. There may be a gap (e.g. a space) between the catalytic zone and the capture zone. 
     The capture zone may overlap the catalytic zone. An end portion or part of the capture zone may be disposed or supported on the catalytic zone. The capture zone generally only partly overlaps the catalytic zone. 
     In a second oxidation catalyst arrangement of the invention (including the first to third oxidation catalyst embodiment), the catalytic region is a catalytic layer. It is preferred that the catalytic layer extends for substantially an entire length of the substrate, particularly the entire length of the channels of a monolith substrate. 
     The capture zone is typically disposed or supported on the catalytic layer. Preferably the capture zone is disposed directly on to the catalytic layer (i.e. the capture zone is in contact with a surface of the catalytic layer). 
     When the capture zone is disposed or supported on the catalytic layer, it is preferred that the entire length of the capture zone is disposed or supported on the catalytic layer. The length of the capture zone is less than the length of the catalytic layer. 
     In a third oxidation catalyst arrangement (including the first to third oxidation catalyst embodiment), the capture region arranged to contact the exhaust gas after the exhaust gas has contacted and/or passed through the catalytic material is a capture material disposed or supported on the outlet end surface (i.e. of the substrate). Thus, the oxidation catalyst comprises: a substrate having an inlet end surface and an outlet end surface; the catalytic material disposed on the substrate; and a capture material, wherein the capture material is disposed or supported on the outlet end surface (i.e. of the substrate). 
     The third oxidation catalyst arrangement provides a cost effective solution of reducing or preventing volatilised platinum from escaping a platinum-containing oxidation catalyst (which can also avoid decreasing the amount of NO 2  that has been generated by the catalyst) because it does not require the use of large quantities of expensive materials, such as noble metals or rare earth metals. 
     The capture material may be directly disposed or supported onto the outlet end surface of the substrate (e.g. the capture material is in contact with the outlet end surface of the substrate). 
     In the third oxidation catalyst arrangement (including the first to third oxidation catalyst embodiment), then preferably the oxidation catalyst has a total loading of Pt-alloying metal (e.g. the metal content of the Pt-alloying metal) of 1 g ft −3  to 500 g ft −3  (e.g. 50 to 400 g ft −3 ), preferably 4 g ft −3  to 250 g ft −3  (e.g. 75 to 250 g ft −3 ), even more preferably 8 g ft −3  to 150 g ft −3  (e.g. 100 to 150 g ft −3 ). The capture material can occupy a relatively small volume of the substrate and it may be necessary for a high loading of the Pt-alloying metal to be present. The Pt-alloying metal, such as palladium, may be disposed or supported on an outlet end surface of the substrate (e.g. the Pt-alloying metal is directly coated onto the outlet end surface of the substrate). 
     The capture material is disposed or supported on an outlet end surface of the substrate (e.g. the downstream, end face of the substrate). The outlet end surface of a substrate typically comprises a plurality of channel wall edges. 
     The outlet end surface of the substrate may be planar (e.g. as in conventional honeycomb substrates) or non-planar. When the outlet end surface of the substrate is non-planar, then the outlet end surface may have a three-dimensional topographical configuration. Examples of substrates having a non-planar end surface are described in U.S. Pat. No. 8,257,659. Substrates having non-planar end surfaces may provide a larger surface area for the capture material to trap volatilised platinum than substrates having planar end surfaces. 
     In general, it is preferred that the outlet end surface of the substrate is planar. 
     In addition to being disposed or supported on an outlet end surface of the substrate, the capture material may be disposed or supported on a plurality of channel walls within the substrate. During application of the capture material, some of the capture material may enter the channels of the substrate thereby partially coating the channel walls within the substrate. 
     When the capture material is disposed or supported on a plurality of channel walls within the substrate, then the oxidation catalyst comprises a capture zone, wherein the capture zone comprises, or consists essentially of, the capture material. 
     The capture zone typically has a mean length (e.g. from the outlet end surface of the substrate) of ≦25 mm, preferably ≦20 mm, such as ≦15 mm, more preferably ≦10 mm (e.g. ≦5 mm), and even more preferably ≦3 mm (e.g. &lt;3 mm). For the avoidance of doubt, the mean length refers to a length in the axial direction of the substrate. 
     In general, the oxidation catalyst comprises a catalytic material disposed on the substrate. The catalytic material is disposed or supported on a plurality of channel walls within the substrate. 
     In the third oxidation catalyst arrangement (including the first to third oxidation catalyst embodiment), when the substrate is a filtering monolith substrate, the catalytic material may be disposed or supported on a plugged or sealed end of an inlet channel. It is preferred that the catalytic material is disposed or supported on the plugged or sealed ends of a plurality of inlet channels. Each plugged or sealed end of an inlet channel is at a downstream end (i.e. exhaust gas outlet side) of the substrate. 
     When the oxidation catalyst of the invention is a diesel oxidation catalyst (DOC), a diesel exotherm catalyst (DEC), passive NO x  adsorber (PNA), a NO x  storage catalyst (NSC), a CSC™ catalyst, an ASC or a catalysed soot filter (CSF), then the oxidation catalyst may have the first, second or third oxidation catalyst arrangement above. 
     When the oxidation catalyst of the invention is a catalysed soot filter (CSF), then both the catalytic region (or catalytic layer or catalytic zone) and the capture region (or capture zone) may be disposed or supported on (i) a plurality of inlet channel walls of the substrate, and/or (ii) a plurality of outlet channel walls of the substrate. 
     Alternatively, it preferred that when the oxidation catalyst of the invention is a catalysed soot filter (CSF), then the catalytic region (or catalytic layer or catalytic zone) is disposed or supported on a plurality of inlet channel walls of the substrate, and the capture region (or capture zone) is disposed or supported on a plurality of outlet channel walls of the substrate. 
     Substrates for supporting oxidation catalysts are well known in the art. Methods for making washcoats to apply the catalytic material or capture material onto a substrate and methods for applying washcoats onto a substrate are also known in the art (see, for example, our WO 99/47260, WO 2007/077462 and WO 2011/080525). 
     The substrate typically has a plurality of channels (e.g. for the exhaust gas to flow through). Generally, the substrate is a ceramic material or a metallic material. 
     It is preferred that the substrate is made or composed of cordierite (SiO 2 —Al 2 O 3 —MgO), silicon carbide (SiC), Fe—Cr—Al alloy, Ni—Cr—Al alloy, or a stainless steel alloy. 
     Typically, the substrate is a monolith (also referred to herein as a monolith substrate). Such monolith substrates are well-known in the art. 
     The monolith substrate may be a flow-through monolith substrate. Alternatively, the monolith substrate may be a filtering monolith substrate. 
     A flow-through monolith substrate typically comprises a honeycomb monolith (e.g. a metal or ceramic honeycomb monolith) having a plurality of channels extending therethrough, which channels are open at both ends. When the substrate is a flow-through monolith substrate, then the oxidation catalyst of the invention is typically a diesel oxidation catalyst (DOC), a NO x  storage catalyst (NSC), a passive NO x  adsorber (PNA), a diesel exotherm catalyst (DEC), a cold start concept (CSC™) catalyst or an ammonia slip catalyst (ASC). 
     A filtering monolith substrate generally comprises a plurality of inlet channels and a plurality of outlet channels, wherein the inlet channels are open at an upstream end (i.e. exhaust gas inlet side) and are plugged or sealed at a downstream end (i.e. exhaust gas outlet side), the outlet channels are plugged or sealed at an upstream end and are open at a downstream end, and wherein each inlet channel is separated from an outlet channel by a porous structure. When the substrate is a filtering monolith substrate, then the oxidation catalyst of the invention is typically a catalysed soot filter (CSF) or a NO x  storage catalyst (NSC) on a filter, preferably a catalysed soot filter (CSF). 
     When the monolith substrate is a filtering monolith substrate, it is preferred that the filtering monolith substrate is a wall-flow filter. In a wall-flow filter, each inlet channel is alternately separated from an outlet channel by a wall of the porous structure and vice versa. It is preferred that the inlet channels and the outlet channels are arranged in a honeycomb arrangement. When there is a honeycomb arrangement, it is preferred that the channels vertically and laterally adjacent to an inlet channel are plugged at an upstream end and vice versa (i.e. the channels vertically and laterally adjacent to an outlet channel are plugged at a downstream end). When viewed from either end, the alternately plugged and open ends of the channels take on the appearance of a chessboard. 
     In principle, the substrate may be of any shape or size. However, the shape and size of the substrate is usually selected to optimise exposure of the catalytically active materials in the catalyst to the exhaust gas. The substrate may, for example, have a tubular, fibrous or particulate form. Examples of suitable supporting substrates include a substrate of the monolithic honeycomb cordierite type, a substrate of the monolithic honeycomb SiC type, a substrate of the layered fibre or knitted fabric type, a substrate of the foam type, a substrate of the crossflow type, a substrate of the metal wire mesh type, a substrate of the metal porous body type and a substrate of the ceramic particle type. 
     The invention also provides a capture brick comprising, or consisting essentially of, a substrate and a capture material disposed on the substrate. The capture material may be a capture material as defined above. 
     Typically, the capture brick comprises a layer comprising, or consisting essentially of, the capture material. The layer is disposed or supported on the substrate. It is preferred that the layer extends for substantially an entire length of the substrate, particularly the entire length of the channels of the substrate. 
     The capture brick may consist essentially of a substrate and a layer comprising, or consisting essentially of, the capture material. 
     The capture brick may have a loading of the refractory oxide of 0.1 to 3.5 g in −3 , preferably 0.2 to 2.5 g in −3 , still more preferably 0.3 to 2.0 g in −3 , and even more preferably 0.5 to 1.75 g in −3  (e.g. 0.75 to 1.5 g in −3 ). 
     The substrate of the capture brick is generally a monolith (also referred to herein as a monolith substrate), such as defined above. The monolith substrate is preferably a flow-through monolith substrate. 
     Typically, the substrate of the capture brick has an axial length of from 30 mm to 300 mm (e.g. 30 mm to 100 mm), preferably 40 mm to 200 mm, such as 50 mm to 150 mm. 
     The invention further provides an exhaust system for treating an exhaust gas produced by a compression ignition engine. Typically, the exhaust system comprises (i) an oxidation catalyst of the invention and/or a capture brick of the invention, and (ii) an emissions control device. 
     Examples of an emissions control device include a diesel particulate filter (DPF), a NO x  storage catalyst (NSC), a lean NO x  catalyst (LNC), a selective catalytic reduction (SCR) catalyst, a diesel oxidation catalyst (DOC), a catalysed soot filter (CSF), a selective catalytic reduction filter (SCRF™) catalyst, an ammonia slip catalyst (ASC) and combinations of two or more thereof. Such emissions control devices are all well known in the art. 
     It is preferred that the exhaust system comprises an emissions control device selected from the group consisting of a NO x  storage catalyst (NSC), an ammonia slip catalyst (ASC), diesel particulate filter (DPF), a selective catalytic reduction (SCR) catalyst, a catalysed soot filter (CSF), a selective catalytic reduction filter (SCRF™) catalyst, and combinations of two or more thereof. More preferably, the emissions control device is selected from the group consisting of a diesel particulate filter (DPF), a selective catalytic reduction (SCR) catalyst, a catalysed soot filter (CSF), a selective catalytic reduction filter (SCRF™) catalyst, and combinations of two or more thereof. Even more preferably, the emissions control device is a selective catalytic reduction (SCR) catalyst or a selective catalytic reduction filter (SCRF™) catalyst. 
     When the exhaust system of the invention comprises an SCR catalyst or an SCRF™ catalyst, then the exhaust system may further comprise an injector for injecting a nitrogenous reductant, such as ammonia, or an ammonia precursor, such as urea or ammonium formate, preferably urea, into exhaust gas upstream of the SCR catalyst or the SCRF™ catalyst. Typically, the injector is downstream of the oxidation catalyst and/or the capture brick. Such an injector may be fluidly linked to a source (e.g. a tank) of a nitrogenous reductant precursor. Valve-controlled dosing of the precursor into the exhaust gas may be regulated by suitably programmed engine management means and closed loop or open loop feedback provided by sensors monitoring the composition of the exhaust gas. Ammonia can also be generated by heating ammonium carbamate (a solid) and the ammonia generated can be injected into the exhaust gas. 
     Alternatively or in addition to the injector, ammonia can be generated in situ (e.g. during rich regeneration of a NSC disposed upstream of the SCR catalyst or the SCRF™ catalyst). Thus, the exhaust system may further comprise an engine management means for enriching the exhaust gas with hydrocarbons. 
     The SCR catalyst or the SCRF™ catalyst may comprise a metal selected from the group consisting of at least one of Cu, Hf, La, Au, In, V, lanthanides and Group VIII transition metals (e.g. Fe), wherein the metal is supported on a refractory oxide or molecular sieve. The metal is preferably selected from Ce, Fe, Cu and combinations of any two or more thereof, more preferably the metal is Fe or Cu. 
     The refractory oxide for the SCR catalyst or the SCRF™ catalyst may be selected from the group consisting of Al 2 O 3 , TiO 2 , CeO 2 , SiO 2 , ZrO 2  and mixed oxides containing two or more thereof. The non-zeolite catalyst can also include tungsten oxide (e.g. V 2 O 5 /WO 3 /TiO 2 , WO x /CeZrO 2 , WO x /ZrO 2  or Fe/WO x /ZrO 2 ). 
     It is particularly preferred when an SCR catalyst, an SCRF™ catalyst or a washcoat thereof comprises at least one molecular sieve, such as an aluminosilicate zeolite or a SAPO. The at least one molecular sieve can be a small, a medium or a large pore molecular sieve. By “small pore molecular sieve” herein we mean molecular sieves containing a maximum ring size of 8, such as CHA; by “medium pore molecular sieve” herein we mean a molecular sieve containing a maximum ring size of 10, such as ZSM-5; and by “large pore molecular sieve” herein we mean a molecular sieve having a maximum ring size of 12, such as beta. Small pore molecular sieves are potentially advantageous for use in SCR catalysts. 
     In the exhaust system of the invention, preferred molecular sieves for an SCR catalyst or an SCRF™ catalyst are synthetic aluminosilicate zeolite molecular sieves selected from the group consisting of AEI, ZSM-5, ZSM-20, ERI including ZSM-34, mordenite, ferrierite, BEA including Beta, Y, CHA, LEV including Nu-3, MCM-22 and EU-1, preferably AEI or CHA, and having a silica-to-alumina ratio of about 10 to about 50, such as about 15 to about 40. 
     In a first exhaust system arrangement, the exhaust system comprises an oxidation catalyst of the invention (e.g. the oxidation catalyst comprises a region comprising a capture material). 
     In a first embodiment of the first exhaust system arrangement, the exhaust system comprises the oxidation catalyst of the invention (e.g. as a DOC, a DEC, a NSC, a PNA, a CSC™ catalyst or an ASC) and a selective catalytic reduction filter (SCRF™) catalyst. The oxidation catalyst of the invention is typically followed by (e.g. is upstream of) the selective catalytic reduction filter (SCRF™) catalyst. A nitrogenous reductant injector may be arranged between the oxidation catalyst and the selective catalytic reduction filter (SCRF™) catalyst. Thus, the oxidation catalyst may be followed by (e.g. is upstream of) a nitrogenous reductant injector, and the nitrogenous reductant injector may be followed by (e.g. is upstream of) the selective catalytic reduction filter (SCRF™) catalyst. 
     In a second embodiment of the first exhaust system arrangement, the exhaust system comprises the oxidation catalyst of the invention (e.g. as a CSF, DOC, a DEC, a NSC, a PNA, a CSC™ catalyst or an ASC) and a selective catalytic reduction (SCR) catalyst and optionally either a catalysed soot filter (CSF) or a diesel particulate filter (DPF). 
     In the second embodiment of the first exhaust system arrangement, the oxidation catalyst of the invention is typically followed by (e.g. is upstream of) the selective catalytic reduction (SCR) catalyst. A nitrogenous reductant injector may be arranged between the oxidation catalyst and the selective catalytic reduction (SCR) catalyst. Thus, the oxidation catalyst may be followed by (e.g. is upstream of) a nitrogenous reductant injector, and the nitrogenous reductant injector may be followed by (e.g. is upstream of) the selective catalytic reduction (SCR) catalyst. The selective catalytic reduction (SCR) catalyst may be followed by (e.g. are upstream of) the catalysed soot filter (CSF) or the diesel particulate filter (DPF). 
     A third embodiment of the first exhaust arrangement relates to an exhaust system comprising a diesel oxidation catalyst (DOC), the oxidation catalyst of the invention, preferably as a catalysed soot filter (CSF), and a selective catalytic reduction (SCR) catalyst. The diesel oxidation catalyst (DOC) is typically followed by (e.g. is upstream of) the oxidation catalyst of the invention. The oxidation catalyst of the invention is typically followed by (e.g. is upstream of) the selective catalytic reduction (SCR) catalyst. A nitrogenous reductant injector may be arranged between the oxidation catalyst and the selective catalytic reduction (SCR) catalyst. Thus, the oxidation catalyst may be followed by (e.g. is upstream of) a nitrogenous reductant injector, and the nitrogenous reductant injector may be followed by (e.g. is upstream of) the selective catalytic reduction (SCR) catalyst. 
     In a second exhaust system arrangement, the exhaust system comprises (i) an oxidation catalyst for treating the exhaust gas, wherein the oxidation catalyst comprises a first substrate and a catalytic material disposed on the first substrate, wherein the catalytic material comprises platinum (Pt); and (ii) a capture brick of the invention; wherein the capture brick is arranged to contact exhaust gas after the exhaust gas has passed through the oxidation catalyst. The oxidation catalyst may or may not be an oxidation catalyst of the invention. 
     In a first embodiment of the second exhaust system arrangement, the exhaust system comprises an oxidation catalyst (e.g. as a DOC, a DEC, a NSC, a PNA, a CSC™ catalyst or an ASC), such as an oxidation catalyst as defined above or an oxidation catalyst of the invention, the capture brick of the invention and a selective catalytic reduction filter (SCRF™) catalyst. The oxidation catalyst of the invention is typically followed by (e.g. is upstream of) the capture brick. The capture brick is typically followed by the selective catalytic reduction filter (SCRF™) catalyst. A nitrogenous reductant injector may be arranged between the oxidation catalyst and the selective catalytic reduction filter (SCRF™) catalyst, preferably between the capture brick and the selective catalytic reduction filter (SCRF™) catalyst. Thus, the oxidation catalyst may be followed by (e.g. is upstream of) a capture brick, the capture brick may be followed by (e.g. is upstream of) a nitrogenous reductant injector, and the nitrogenous reductant injector may be followed by (e.g. is upstream of) the selective catalytic reduction filter (SCRF™) catalyst. 
     In a second embodiment of the second exhaust system arrangement, the exhaust system comprises an oxidation catalyst (e.g. as a CSF, DOC, a DEC, a NSC, a PNA, a CSC™ catalyst or an ASC), such as an oxidation catalyst as described above or an oxidation catalyst of the invention, the capture brick of the invention and a selective catalytic reduction (SCR) catalyst and optionally either a catalysed soot filter (CSF) or a diesel particulate filter (DPF). 
     In the second embodiment of the second exhaust system arrangement, the oxidation catalyst is typically followed by (e.g. is upstream of) the capture brick, and the capture brick is typically followed by (e.g. is upstream of) the selective catalytic reduction (SCR) catalyst. A nitrogenous reductant injector may be arranged between the oxidation catalyst and the selective catalytic reduction (SCR) catalyst, preferably the nitrogenous reductant injector is arranged between the capture brick and the selective catalytic reduction (SCR) catalyst. Thus, the oxidation catalyst may be followed by (e.g. is upstream of) the capture brick, and the capture brick may be followed by (e.g. is upstream of) a nitrogenous reductant injector, and the nitrogenous reductant injector may be followed by (e.g. is upstream of) the selective catalytic reduction (SCR) catalyst. The selective catalytic reduction (SCR) catalyst may be followed by (e.g. are upstream of) the catalysed soot filter (CSF) or the diesel particulate filter (DPF). 
     A third embodiment of the second exhaust system arrangement relates to an exhaust system comprising a diesel oxidation catalyst (DOC), an oxidation catalyst, such as described above or an oxidation catalyst of the invention, as a catalysed soot filter (CSF), a capture brick and a selective catalytic reduction (SCR) catalyst. The diesel oxidation catalyst (DOC) is typically followed by (e.g. is upstream of) the oxidation catalyst (e.g. CSF). The oxidation catalyst of the invention is typically followed by (e.g. is upstream of) the capture brick, and the capture brick is typically followed by (e.g. is upstream of) the selective catalytic reduction (SCR) catalyst. A nitrogenous reductant injector may be arranged between the oxidation catalyst and the selective catalytic reduction (SCR) catalyst, preferably the nitrogenous reductant injector is arranged between the capture brick and the selective catalytic reduction (SCR) catalyst. Thus, the oxidation catalyst may be followed by (e.g. is upstream of) the capture brick, and the capture brick may be followed by (e.g. is upstream of) a nitrogenous reductant injector, and the nitrogenous reductant injector may be followed by (e.g. is upstream of) the selective catalytic reduction (SCR) catalyst. 
     In any of the embodiments of the first or second exhaust system arrangements described hereinabove, an ASC catalyst can be disposed downstream from the SCR catalyst or the SCRF™ catalyst (i.e. as a separate substrate monolith), or more preferably as a zone on a downstream or trailing end of the substrate monolith comprising the SCR catalyst can be used as a support for the ASC. 
     The invention further provides a vehicle. The vehicle comprises a compression ignition engine and either an oxidation catalyst, an exhaust system or a capture brick of the invention. The compression ignition engine is preferably a diesel engine. The diesel engine may be a homogeneous charge compression ignition (HCCI) engine, a pre-mixed charge compression ignition (PCCI) engine or a low temperature combustion (LTC) engine. It is preferred that the diesel engine is a conventional (i.e. traditional) diesel engine. 
     The vehicle may be a light-duty diesel vehicle (LDV), such as defined in US or European legislation. A light-duty diesel vehicle typically has a weight of &lt;2840 kg, more preferably a weight of &lt;2610 kg. 
     In the US, a light-duty diesel vehicle (LDV) refers to a diesel vehicle having a gross weight of ≦8,500 pounds (US lbs). In Europe, the term light-duty diesel vehicle (LDV) refers to (i) passenger vehicles comprising no more than eight seats in addition to the driver&#39;s seat and having a maximum mass not exceeding 5 tonnes, and (ii) vehicles for the carriage of goods having a maximum mass not exceeding 12 tonnes. 
     Alternatively, the vehicle may be a heavy-duty diesel vehicle (HDV), such as a diesel vehicle having a gross weight of &gt;8,500 pounds (US lbs), as defined in US legislation. 
     The emissions control device having a filtering substrate may be selected from the group consisting of a diesel particulate filter (DPF), a catalysed soot filter (CSF), a selective catalytic reduction filter (SCRF™) catalyst and a combination of two or more thereof. 
     Definitions 
     The term “mixed oxide” as used herein generally refers to a mixture of oxides in a single phase, as is conventionally known in the art. The term “composite oxide” as used herein generally refers to a composition of oxides having more than one phase, as is conventionally known in the art. 
     The acronym “PGM” as used herein refers to “platinum group metal”. The term “platinum group metal” generally refers to a metal selected from the group consisting of Ru, Rh, Pd, Os, Ir and Pt, preferably a metal selected from the group consisting of Ru, Rh, Pd, Ir and Pt. In general, the term “PGM” preferably refers to a metal selected from the group consisting of Rh, Pt and Pd. 
     The term “capture region” as used herein is a synonym for the “region for capturing volatilised platinum (Pt)”. 
     The expression “end surface” as used herein, particularly with reference to an “inlet end surface” or an “outlet end surface”, is synonymous with the expression “end face”. The end surface or end face of a substrate is typically formed by the wall edges (e.g. at an exterior surface of the substrate) that define or bound the channels through the substrate. 
     The expression “Pt-alloying metal” as used herein refers to a material capable of forming an alloy with platinum (i.e. platinum metal), preferably when the temperature of the exhaust gas is &lt;900° C., particularly &lt;800° C., such as &lt;700° C. 
     The expression “consist essentially” as used herein limits the scope of a feature to include the specified materials, and any other materials or steps that do not materially affect the basic characteristics of that feature, such as for example minor impurities. The expression “consist essentially of” embraces the expression “consisting of”. 
     The expression “substantially free of” as used herein with reference to a material, typically in the context of the content of a washcoat region, a washcoat layer or a washcoat zone, means that the material in a minor amount, such as ≦5% by weight, preferably ≦2% by weight, more preferably ≦1% by weight. The expression “substantially free of” embraces the expression “does not comprise”. 
     The expression “about” as used herein with reference to an end point of a numerical range includes the exact end point of the specified numerical range. Thus, for example, an expression defining a parameter as being up to “about 0.2” includes the parameter being up to and including 0.2. 
     The term “selective catalytic reduction filter catalyst” as used herein includes a selective catalytic reduction formulation that has been coated onto a diesel particulate filter (SCR-DPF), which is known in the art. 
     EXAMPLES 
     The invention will now be illustrated by the following non-limiting examples. 
     Example 1 (Reference SCR Catalyst) 
     Preparation of Substrate Coated with 3 wt % Cu/CHA Zeolite 
     Commercially available aluminosilicate CHA zeolite was added to an aqueous solution of Cu(NO 3 ) 2  with stirring. The slurry was filtered, then washed and dried. The procedure can be repeated to achieve a desired metal loading. The final product was calcined. After mixing, binders and rheology modifiers were added to form a washcoat composition. 
     A 400 cpsi cordierite flow-through substrate monolith was coated with an aqueous slurry of the 3 wt % Cu/CHA zeolite sample using the method disclosed in WO 99/47260. This coated product (coated from one end only) is dried and then calcined and this process is repeated from the other end so that substantially the entire substrate monolith is coated, with a minor overlap in the axial direction at the join between the two coatings. The coated substrate monolith was aged in a furnace in air at 500° C. for 5 hours. A core of 1 inch (2.54 cm) diameter×3 inches long (7.62 cm) was cut from the finished article. 
     Example 2 (Comparative) 
     Silica-alumina powder was slurried in water and milled to a d 90 &lt;20 micron. Barium acetate was added to the slurry followed by appropriate amounts of soluble platinum and palladium salts. Beta zeolite was added such that the slurry comprised 77% silica-alumina and 23% zeolite by mass. The slurry was then stirred to homogenise. The resulting washcoat was applied to the inlet channels of a cordierite flow through monolith having 400 cells per square inch using established coating techniques. The part was then dried. 
     A second slurry of silica-alumina was milled to a d 90 &lt;20 micron. Platinum nitrate solution was added followed by manganese nitrate solution. The mixture was stirred to homogenise. The slurry was applied to the outlet end of the substrate using established coating techniques. The part was then dried and calcined at 500° C. The resulting catalyst had a total PGM loading of 150 g ft −3  and a Pt:Pd weight ratio of 3:1. The Mn loading was 130 g ft −3 . 
     Example 3 
     Silica-alumina powder was slurried in water and milled to a d 90 &lt;20 micron. Barium acetate was added to the slurry followed by appropriate amounts of soluble platinum and palladium salts. Beta zeolite was added such that the slurry comprised 77% silica-alumina and 23% zeolite by mass. The slurry was then stirred to homogenise. The resulting washcoat was applied to the inlet channels of a cordierite flow through monolith having 400 cells per square inch using established coating techniques. The part was then dried. 
     A second slurry of silica-alumina was milled to a d 90 &lt;20 micron. Platinum nitrate solution was added followed by manganese nitrate solution. The mixture was stirred to homogenise. The slurry was applied to the outlet end of the substrate using established coating techniques. The part was then dried and calcined at 500° C. The resulting catalyst had a total PGM loading of 150 g ft −3  and a Pt:Pd weight ratio of 3:1. The Mn loading was 130 g ft −3 . 
     A third slurry was prepared by adding stabilised-zirconia (13.5% Nd 2 O 3  by weight) to water. Pd nitrate was added and the mixture stirred to homogenise. 10% alumina binder was added and the slurry was applied to the outlet channels of the substrate using established coating techniques to a coating depth of 1 inch. The part was dried and calcined at 500° C. The stabilised-zirconia loading was 2.0 g in −3  and the Pd loading of this third coating was 20 g ft −3 . 
     Example 4 
     Silica-alumina powder was slurried in water and milled to a d 90 &lt;20 micron. Barium acetate was added to the slurry followed by appropriate amounts of soluble platinum and palladium salts. Beta zeolite was added such that the slurry comprised 77% silica-alumina and 23% zeolite by mass. The slurry was then stirred to homogenise. The resulting washcoat was applied to the inlet channels of a cordierite flow through monolith having 400 cells per square inch using established coating techniques. The part was then dried. 
     A second slurry of silica-alumina was milled to a d 90 &lt;20 micron. Platinum nitrate solution was added followed by manganese nitrate solution. The mixture was stirred to homogenise. The slurry was applied to the outlet end of the substrate using established coating techniques. The part was then dried and calcined at 500° C. The resulting catalyst had a total PGM loading of 150 g ft −3  and a Pt:Pd weight ratio of 3:1. The Mn loading was 130 g ft −3 . 
     A third slurry was prepared by adding stabilised-zirconia (13.5% Nd 2 O 3  by weight) to water. Pd nitrate was added and the mixture stirred to homogenise. Formic acid was added at 0.7% of the slurry by mass and the mixture stirred for 1 hour. During this time the slurry turned from yellow to grey as the Pd salt was reduced. 10% alumina binder was added and slurry was applied to the outlet channels of the substrate using established coating techniques to a coating depth of 1 inch. The part was dried and calcined at 500° C. The stabilised-zirconia loading was 2.0 g in −3  and the Pd loading of this third coating was 20 g ft −3 . 
     Experimental Results 
     System Tests 
     Tests were performed on a first synthetic catalytic activity test (SCAT) laboratory reactor illustrated in  FIG. 1 , in which an aged core of the coated Cu/CHA zeolite SCR catalyst of Example 1 was disposed in a conduit downstream of a core of the catalyst of Example 2, 3 or 4. A synthetic gas mixture was passed through the conduit at a rate of 6 litres per minute. A furnace was used to heat (or “age”) the oxidation catalyst samples at steady-state temperature at a catalyst outlet temperature of 950° C. for 2 hours. The SCR catalyst was disposed downstream of the oxidation catalyst sample and was held at a catalyst temperature of 300° C. during the ageing process by adjusting the length of tube between the furnace outlet and the SCR inlet, although a water cooled heat exchanger jacket could be used as appropriate. Temperatures were determined using appropriately positioned thermocouples (Ti and T 2 ). The gas mixture used during the ageing was 40% air, 50% N 2 , 10% H 2 O. 
     A reference “blank ageing” sample was tested where there was no platinum containing oxidation catalyst placed upstream of the SCR core in the ageing apparatus. That is, the blank ageing was carried out in the absence of a platinum containing catalyst and hence platinum volatilisation could not occur. 
     Following the oxidation catalyst ageing, the SCR catalysts were removed from the first SCAT reactor and inserted into a second SCAT reactor specifically to test the NH 3 —SCR activity of the aged samples. The SCR catalysts were then tested for SCR activity at 500° C. using a synthetic gas mixture (O 2 =10%; H 2 O=5%, CO 2 =330 ppm; NH 3 =400 ppm; NO=500 ppm; NO 2 =0 ppm; N 2 =balance, i.e. an alpha value of 0.8 was used (ratio of NH 3 :NO x ). The NO x  conversion results are normalised and reported relative the SCR core from the blank ageing test. Normalised NO x  conversion results are shown in Table 1. 
                         TABLE 1               Catalyst upstream of SCR core   Normalised NO x  conversion of       during lab ageing at 950° C.   SCR core at 500° C. (%)                                        blank   100       2   49       3   88       4   79                    
Table 1 shows the NO x  conversion activity of aged SCR catalyst cores taken from Example 1 after ageing with upstream oxidation catalyst cores at 950° C. for 2 hours. The NO x  conversion at 500° C. for the blank ageing run represents the baseline conversion that is achieved after ageing without platinum volatilisation.
 
     The SCR sample that was aged with a core from Example 2 in the upstream location shows a significant reduction on NO x  conversion. The catalyst of Example 2 has no platinum capture material. The SCR sample that was aged with a core from Example 3 in the upstream location shows only a slight reduction in NO x  conversion performance compared to blank ageing run. The oxidation catalyst of Example 3 comprises a capture material that comprises zirconia. The capture material coating was prepared without the need to isolate material as a powder. The SCR sample that was aged with a core from Example 4 in the upstream location shows a similar NO x  conversion performance to the SCR sample that was aged behind Example 3. Example 4 comprises a capture material that comprises zirconia and Pd that has been reduced using formic acid. The capture material coating was prepared without the need to isolate material as a powder. 
     Measurement of Catalytic Activity 
     Core samples were taken from the catalysts of Examples 2, 3 and 4. The cores were hydrothermally aged at 790° C. for 16 hours using 10% water. 
     The catalytic activity for all cores was determined using a synthetic gas bench catalytic activity test (SCAT). The aged cores were tested in the simulated exhaust gas mixture shown in Table 2. In each case the balance is nitrogen. The oxidation activity for NO is determined as the percentage conversion at 250° C. The oxidation activity for CO and HC is determined by the light off temperature whereby 50% conversion is achieved (T50). 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
             
            
               
                   
                 CO 
                 1500 ppm  
               
               
                   
                 HC (as C 1 ) 
                 430 ppm 
               
               
                   
                 NO 
                 100 ppm 
               
               
                   
                 CO 2   
                 4% 
               
               
                   
                 H 2 O 
                 4% 
               
               
                   
                 O 2   
                 14%  
               
               
                   
                 Space velocity 
                 55000/hour 
               
               
                   
                   
               
            
           
         
       
     
     Results 
     The results from the SCATs are shown in Tables 3 and 4. 
     
       
         
           
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                 Example No. 
                 T50 CO (° C.) 
                 T50 HC (° C.) 
               
               
                   
               
             
            
               
                 2 
                 148 
                 153 
               
               
                 3 
                 149 
                 153 
               
               
                 4 
                 150 
                 155 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
             
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                 Example No. 
                 NO oxidation at 250° C. (%) 
               
               
                   
                   
               
             
            
               
                   
                 2 
                 83 
               
               
                   
                 3 
                 77 
               
               
                   
                 4 
                 80 
               
               
                   
                   
               
            
           
         
       
     
     The results shown in Table 3 show the CO and HC light off temperatures for the catalysts of Examples 2, 3 and 4. All of the catalysts in the Examples have similar CO and HC light off activity. The catalysts of Examples 3 and 4 comprise a capture material that comprises zirconia. The inclusion of the capture material has no negative impact on CO and HC light off. 
     Table 4 shows the NO oxidation activity at 250° C. All of the catalysts of the Examples show a similar NO oxidation activity. Examples 3 and 4 comprise a capture material that comprises zirconia. The inclusion of the capture material has no negative impact on NO oxidation activity. 
     For the avoidance of any doubt, the entire content of any and all documents cited herein is incorporated by reference into the present application.