Diesel oxidation catalyst and method of manufacture thereof

Abstract

A diesel oxidation catalyst comprises one or more washcoat layers disposed on a substrate, wherein at least one of the one or more washcoat layers comprises a doped zirconia-based support material which is a zirconia-based support material provided with two or more dopants, wherein the zirconia-based support material comprises, on an oxide basis: at least 80 wt.% Zr, based on the total weight of the zirconia-based support material; at least 2 wt.% Nd, based on the total weight of the zirconia-based support material; and, optionally, one or more of Y, Ce, Hf and La, and wherein the two or more dopants comprise: from 5 to 15 wt.% Ce and from 5 to 30 wt.% Mn, on an oxide basis, based on the total weight of the doped zirconia-based support material.

Description

[0001]P102393 DIESEL OXIDATION CATALYST AND METHOD OF MANUFACTURE THEREOFThe present invention relates to a diesel oxidation catalyst (DOC), its method ofmanufacture and its use in a method for the treatment of an exhaust gas from a diesel engine. The invention relates in particular to a DOC comprising a base metal oxide (BMO), such as manganese oxide. Combustion engines produce exhaust gases that contain pollutants, such as carbon monoxide (CO), unburned hydrocarbons (HCs), oxides of nitrogen (NOx) and particulate matter (PM). Emissions standards for pollutants in an exhaust gas produced by a combustion engine, particularly for vehicular engines, are becoming increasingly stringent. There is a need to provide improved catalysts and exhaust systems for treating and removing the pollutants in such exhaust gases that can meet these standards and which are cost-effective. It is particularly desirable that these catalysts show consistent performance over their lifetime. The exhaust gas from gasoline and diesel engines is commonly treated with a catalyst that can oxidise (i) carbon monoxide (CO) to carbon dioxide (CO2); and (ii) hydrocarbons (HCs) to water (H2O) and carbon dioxide (CO2). The exhaust gas from a compression ignition engine, such as a diesel engine, is typically treated with an oxidation catalyst (commonly called a diesel oxidation catalyst (DOC)) that performs oxidation reactions (i) and (ii). Some diesel oxidation catalysts are also designed to oxidise nitrogen monoxide (NO) to nitrogen dioxide (NO2), which can aid removal of NOxusing an additional, downstream emissions control device, typically a selective catalytic reduction (SCR) catalyst; and / or oxidise particulate matter trapped on a downstream filter substrate at lower temperatures than in O2; the so-called CRT®effect. Oxidation catalysts for compression ignition internal combustion engines typically contain one or more platinum group metals. The specific platinum group metal(s) P102393 selected for inclusion in an oxidation catalyst will depend on a variety of factors, such as reactivity toward specific pollutants and under differing exhaust gas conditions, cost, durability at high temperatures, chemical compatibility with the support material and any other components of the catalyst, and susceptibility to poisoning by impurities. For example, platinum group metals (PGMs) such as platinum (Pt) and palladium (Pd) are each able to oxidise carbon monoxide (CO) and hydrocarbons (HCs) in an exhaust gas from a compression ignition engine.Palladium is more susceptible to poisoning by sulphur in fuel compared toplatinum but has greater thermal durability. PGMs are (increasingly) expensive and there is therefore a desire to provide suitable DOCs, preferably with improved catalytic activity for the above pollutants, having reduced PGM loadings or even without PGMs. In some jurisdictions, formaldehyde is also a regulated pollutant and there is a desire to provide DOCs which also demonstrate improved formaldehyde abatement, at minimal cost (such as with reduced PGM loadings). WO2022 / 047132A1 discloses oxidation catalyst compositions comprising a platinum group metal (PGM) component comprising palladium, platinum, or a combination thereof; a manganese component; and a first refractory metal oxide support material comprising zirconia; catalytic articles; and exhaust gas treatment systems, as well as methods of making and using such oxidation catalyst compositions, for example, to reduce formaldehyde levels in engine exhaust emissions. WO2022 / 047134A1 discloses oxidation catalyst compositions comprising a platinum group metal (PGM) component comprising palladium, platinum, or a combination thereof; a first oxide chosen from oxides of cerium, silicon, iron, cobalt, zinc, chromium, molybdenum, nickel, tungsten, magnesium, antimony, tin, lead, yttrium, and combinations thereof; and a first refractory metal oxide support P102393 material; catalytic articles; and exhaust gas treatment systems, as well as methods of making and using such oxidation catalyst compositions. WO 2020 / 046266 A1 discloses a catalytic article including a catalytic material on a substrate, wherein the catalytic material has a first layer and a second layer. The first layer includes a platinum group metal (PGM) component impregnated on a porous support material; and the second layer includes a rhodium component impregnated on a support material, wherein the support material is a composite material including zirconia doped with baria, alumina, or combinations thereof, wherein the zirconia-based support material includes zirconia in an amount from about 80 to about 99 wt.%. The present invention seeks to tackle at least some of the problems associated with the prior art or at least to provide a commercially acceptable alternative solution thereto. The present invention provides a diesel oxidation catalyst, a method for themanufacture of a diesel oxidation catalyst and a method for the treatment of anexhaust gas from a diesel engine according to the claims appended hereto.Specifically, in a first aspect the present invention provides a diesel oxidation catalyst comprising one or more washcoat layers disposed on a substrate, wherein at least one of the one or more washcoat layers comprises a doped zirconia-based support material which is a zirconia-based support materialprovided with two or more dopants,wherein the zirconia-based support material comprises, on an oxide basis: at least 80 wt.% Zr, based on the total weight of the zirconia-based support material; at least 2 wt.% Nd, based on the total weight of the zirconia-basedsupport material; and, optionally, one or more of Y, Ce, Hf and La, and wherein the two or more dopants comprise: P102393 from 5 to 15 wt.% Ce and from 5 to 30 wt.% Mn, on an oxide basis, based on the total weight of the doped zirconia-based support material. Each aspect or embodiment as defined herein may be combined with any other aspect(s) or embodiment(s) unless clearly indicated to the contrary. In particular, any features indicated as being preferred or advantageous may be combined with any other feature indicated as being preferred or advantageous. While investigating suitable catalyst compositions and support materials for the abatement of formaldehyde, in particular those comprising BMOs over PGMs, the inventors have surprisingly found that there is an improvement in HC light-off performance when the support material comprises Nd. The performance may be improved or comparable relative to the examples shown in WO2022 / 047132, particularly Example 4. That is, while the use of La and Nd as dopants in the zirconia-based support material may provide comparable performance to eachother for CO and NO, these two options may each result in improvedperformance relative to the use of other possible dopants; surprisingly, the use ofNd in this way can provide improved or comparable performance relative to the use of La, where the selection of an alternative dopant element, such as Y or Si, may not. The content and disclosure of WO2022 / 047132 is incorporated herein by reference. This aspect of the invention may therefore enable a DOC to be provided having reduced PGM loading, while still providing suitable or improved HC (and CO and formaldehyde) abatement performance. For example, while PGMs may still be present in the same washcoat layer or a further washcoat layer of the DOC, the support material of this aspect alone provides good catalytic performance as a DOC. It has also been found that, surprisingly, particularly when the doped zirconia-based support material of the invention is combined with a PGM-based catalyst(optionally in a different layer of the DOC), the DOC of the invention may provide P102393 comparable CO abatement performance to a DOC comprising known doped zirconia-based support materials wherein the zirconia-based support material consists of La and Zr, while providing improved CO abatement performance compared to alternative zirconia-based support materials comprising alternative dopants without the presence of Nd. It has also been found that, surprisingly, particularly when the doped zirconia-based support material of the invention is combined with a PGM-based catalyst(optionally in a different layer of the DOC), the DOC of the invention may provide comparable NO2“make” performance, i.e. the activity to oxidise NO to NO2, to a DOC comprising known doped zirconia-based support materials wherein the zirconia-based support material consists of La and Zr, while providing improved NO2make performance compared to alternative zirconia-based support materials comprising alternative dopants without the presence of Nd. NO2 make corresponds to the ratio of NO2 to NOx in the exhaust gas after treatment with the DOC. For improved performance of further downstream catalysts, such as selective catalytic reduction (SCR) catalysts, the ratio of NO2 to NOx is preferably close to 50%. Such a DOC may therefore improve the performance of an exhaust gas emissions treatment system as a whole. For the avoidance of doubt, it will be appreciated that the relative proportions ofthe components of the zirconia-based support material are defined based on thetotal weight of the zirconia-based support material (which in the manufacture ofthe doped zirconia-based support material is typically provided as a pre-preparedmixed oxide starting material, which is then doped with the required dopants), whereas the relative proportions of the two or more dopants as defined herein aredefined based on the total weight of the doped zirconia-based support material. Aperson skilled in the field of the present invention would be able to determinewhich components of the doped zirconia-based support material originated in thestarting mixed oxide, such as one formed by a coprecipitation method, for example, and which components of the doped zirconia-based support material were subsequently doped into the zirconia-based support material, using known P102393 analytical techniques. Mixed (metal) oxides as defined herein comprise solid solutions, i.e. a homogeneous mixture of two different kinds of atoms in solidstate and having a single crystal structure, and composite metal oxides. The useof such a solid-solution-formed zirconia-based support material comprising Nd may have a greater catalytic performance as described herein in comparison to a zirconia-based support material in which the Nd is included by subsequent doping, for example. Preferably, the at least one of the one or more washcoat layers comprises from 0.125 to 0.35 g / in3Mn, preferably from 0.15 to 0.30 g / in3Mn, based on the total volume of the substrate. Here, the loading in g / in3is determined based on the weight of elemental Mn, not on an oxide basis. Preferably, the two or more dopants comprise from 5 to 15 wt.% Mn, on anelemental basis, based on the total weight of the doped zirconia-based supportmaterial. That is, the two or more dopants preferably comprise from 5 to 15 wt.%Ce and from 5 to 15 wt.% Mn, on an elemental basis, based on the total weight ofthe doped zirconia-based support material. It has surprisingly been found thatsuch an Mn content, in combination with the remaining components of the doped zirconia-based support material, may exhibit improved catalytic activity for the abatement of formaldehyde. Preferably, the two or more dopants comprise from 6to 14 wt.%, more preferably from 7 to 13 wt.%, even more preferably from 8 to 12wt.%, still more preferably from 9 to 11 wt.% Mn, on an oxide basis, based on thetotal weight of the doped zirconia-based support material. Preferably, the zirconia-based support material comprises at least 3 wt.% Nd,more preferably at least 4 wt.% Nd, on an oxide basis, based on the total weightof the zirconia-based support material. Preferably, the zirconia-based supportmaterial comprises from 5 to 15 wt.% Nd, more preferably from 10 to 15 wt.% Nd,on an oxide basis, based on the total weight of the zirconia-based supportmaterial. If the zirconia-based support material comprises, on an oxide basis, lessthan 5 wt.% Nd, based on the total weight of the zirconia-based support material, P102393 then preferably the zirconia-based support material further comprises one or more of La, Y, Ce and Hf. Preferably, the zirconia-based support material comprises, on an oxide basis, at least 81 wt.%, more preferably at least 82 wt.%, even more preferably at least 83 wt.%, still more preferably at least 84 wt.%, still more preferably at least 85 wt.% Zr, based on the total weight of the zirconia-based support material. In some preferred embodiments, the zirconia-based support material comprises, on an oxide basis, one or more of: from 1 to 7 wt.% Y; from 1 to 7 wt.% Ce;from 0.1 to 5 wt.% Hf; from 1 to 3 wt.% La, based on the total weight of the zirconia-basedsupport material. The Y, the Ce and the La may typically be intentionally introduced into the zirconia-based support material. Hf may also be intentionally introduced into the zirconia-based support material. However, Hf may typically be present as an impurity in zirconia starting materials (such as its ore) and may therefore inevitably be present in the zirconia-based support material. The zirconia-based support material may preferably further comprise Y. The presence of the further Y may be preferred if, for example, improved formaldehyde abatement performance is a desired property of the DOC in the particular emission treatment system in which the DOC is intended to be used. Moreover, such zirconia-based support materials may be less affected by sulphur poisoning. However, it is noted that this effect has only been observed where the Y is used as a further dopant in the zirconia-based support material described herein, as opposed to a sole dopant of a zirconia-based support material. If the zirconia-based support material comprises Y, then the zirconia-based support P102393material preferably comprises, on an oxide basis, from 3 to 5 wt.% Y, based onthe total weight of the zirconia-based support material. If the zirconia-based support material comprises Ce, then the zirconia-basedsupport material preferably comprises, on an oxide basis, from 3 to 5 wt.% Ce,based on the total weight of the zirconia-based support material. If the zirconia-based support material comprises La, then the zirconia-basedsupport material preferably comprises, on an oxide basis, from 2 to 3 wt.% La,based on the total weight of the zirconia-based support material. If the zirconia-based support material comprises Hf, then the zirconia-basedsupport material may typically comprise, on an oxide basis, from 0.2 to 3 wt.% orfrom 0.3 to 2 wt.% Hf, based on the total weight of the zirconia-based support material. In some preferred embodiments, the zirconia-based support material comprises, on an oxide basis, from 1 to 2 wt.% La; from 12 to 14 wt.% Nd; and optionally from 0.1 to 2 wt.% Hf, based on the total weight of the zirconia-based support material.In some preferred embodiments, the zirconia-based support material comprises,on an oxide basis, from 1 to 2 wt.% La; from 4 to 6 wt.% Nd; from 4 to 6 wt.% Ce; and optionally from 0.1 to 2 wt.% Hf, based on the total weight of the zirconia-based support material. P102393In some preferred embodiments, the zirconia-based support material comprises,on an oxide basis, from 4 to 6 wt.% Nd; from 4 to 6 wt.% Ce; from 4 to 6 wt.% Y; and optionally from 0.1 to 2 wt.% Hf, based on the total weight of the zirconia-based support material. Preferably, the zirconia-based support material consists of the listed oxides,together with any unavoidable impurities. In this embodiment, the wt.% balancewill be Zr, together with any unavoidable impurities. Preferably, the zirconia- based support material consists of the Zr, the Nd, and optionally the Hf and the La in the recited amounts, together with any unavoidable impurities. If there are no Hf impurities, then even more preferably, the zirconia-based support material consists of the Zr, the Nd and optionally the La in the recited amounts, together with any unavoidable impurities. Most preferably, the zirconia-based support material consists of the Zr, the Nd, the La and optionally the Hf in the recited amounts, together with any unavoidable impurities.Preferably, the two or more dopants further comprise from 1 to 5 wt.% Zr, on anoxide basis, based on the total weight of the doped zirconia-based supportmaterial. As noted above, a person skilled in the field of the present inventionwould be able to determine which components of the doped zirconia-basedsupport material originated in the starting mixed oxide and which components ofthe doped zirconia-based support material were subsequently doped into thezirconia-based support material, using known analytical techniques. In other words, a person skilled in the field would be able to identify which Zr is present as part of the zirconia-based support material and which is present as a dopant, i.e. an additional dopant.Preferably, the two or more dopants consist of Ce and Mn, and optionally Zr,together with any unavoidable impurities. In other words, in one preferred P102393 embodiment, the two or more dopants consist of Ce, Mn and Zr, together withany unavoidable impurities. In an alternative preferred embodiment, the two ormore dopants consist of Ce and Mn, together with any unavoidable impurities.Accordingly, preferably, the doped zirconia-based support material consists of thezirconia-based support material, which preferably consists of the listed oxides, together with any unavoidable impurities, and two or three dopants consisting of Ce and Mn, and optionally Zr, together with any unavoidable impurities.Preferably, the two or more dopants comprise from 8 to 12 wt.% Ce and / or from 8to 12 wt.% Mn, more preferably the two or more dopants comprise about 10 wt.% Ce and / or about 10 wt.% Mn, on an oxide basis, based on the total weight of thedoped zirconia-based support material. Without wishing to be bound by theory, itis thought that such proportions of the dopants, in particular the Ce, may provide improved formaldehyde and CO abatement performance. Preferably, at least one of the one or more washcoat layers comprises platinum and / or palladium, preferably in a weight ratio of 3:1 to 1:1 and / or in a total loadingof from 0.2 to 5.0 wt.%, based on the total weight of the platinum- and / orpalladium-containing washcoat layer. The presence of the Pt and / or the Pd mayimprove the catalytic activity of the DOC against the pollutants discussed herein. Without wishing to be bound by theory, it is thought that the CO and HC oxidation performance of the washcoat layer comprising the doped zirconia-based supportmaterial described herein may be improved if the washcoat layer that comprisesthe Pt and / or Pd is the at least one of the one or more washcoat layers that comprises the doped zirconia-based support material. However, in preferred embodiments of the invention the washcoat layer that comprises the Pt and / or Pd is a different washcoat layer to the at least one of the one or more washcoat layers that comprises the doped zirconia-based support material. Without wishing to be bound by theory, it is thought that if the Pt and / or Pd is present in a different washcoat layer to the at least one of the one or more washcoat layers that P102393 comprises the doped zirconia-based support material, then the catalytic activity of the Pt and / or Pd may be higher. This may be because, for example, the Pt and / or Pd may be supported on a different support material in the different washcoat layer, such as alumina. Accordingly, in some preferred embodiments, the platinum and / or palladium is supported (i.e. directly supported) on the doped zirconia-based support material. More preferably, the platinum and / or palladium is supported (i.e. directlysupported) on alumina. That is, the platinum- and / or palladium-containingwashcoat layer preferably comprises alumina and the platinum and / or palladiumis preferably supported (i.e. directly supported) on the alumina. The platinumand / or palladium may be supported (i.e. directly supported) on both of the doped zirconia-based support material and alumina. The alumina is preferably doped with silica, such as from 1 to 10 wt.% silica, preferably from 4 to 6 wt.% silica, based on the weight of the alumina. The washcoat layer comprising the alumina preferably comprises from 0.1 to 3 g / in3of the alumina, more preferably from 1 to 2 g / in3of the alumina, based on the total volume of the substrate.Preferably, the platinum- and / or palladium-containing washcoat layer comprises100 g / ft3or less Pt and / or Pd, more preferably 80 g / ft3or less Pt and / or Pd, even more preferably 60 g / ft3or less Pt and / or Pd, still more preferably 50 g / ft3or less Pt and / or Pd, based on the total volume of the substrate. As described herein, the present invention may enable the PGM loading to be reduced, while still achieving suitable or improved HC (and CO and formaldehyde) abatementperformance, as well as suitable or improved NO2 make. Preferably, theplatinum- and / or palladium-containing washcoat layer comprises a Pt:Pd ratio byweight of from 1:0 to 1:1, preferably from 6:1 to 2:1, more preferably from 5:1 to2:1. As will be appreciated, a Pt:Pd ratio of 1:0 means that the platinum- and / orpalladium-containing washcoat layer does not comprise Pd. In someembodiments, therefore, the platinum- and / or palladium-containing washcoatlayer consists of Pt. P102393In some embodiments, preferably the platinum and / or palladium supported onalumina is provided in a separate washcoat layer which at least partially overliesthe washcoat layer comprising the doped zirconia-based support material. Inother words, the at least one of the one or more washcoat layers that comprisesthe doped zirconia-based support material is a lower washcoat layer of the DOCand the platinum- and / or palladium-containing washcoat layer is an upper layer ofthe DOC. Most preferably, the one or more washcoat layers of the DOC consistsof two washcoat layers, the two washcoat layers being (i) the doped-zirconia- based-support-material-containing washcoat layer as a bottom layer and (ii) theplatinum- and / or palladium-containing washcoat layer described herein as a toplayer. This arrangement may be preferred where, for example, low or reduced absorption of NOxby the DOC is not considered to be a desired property of the DOC in the particular emission treatment system in which the DOC is intended to be used. Preferably, the at least one washcoat layer comprising platinum and / or palladium further comprises a zeolite. This embodiment may be particularly preferred where the platinum and / or palladium supported on alumina is provided in a separatewashcoat layer to the at least one of the one or more washcoat layers thatcomprises the doped zirconia-based support material. The zeolite preferably comprises a beta zeolite. The washcoat layer comprising the zeolite preferably comprises from 0.1 to 1 g / in3of the zeolite, more preferably from 0.4 to 0.8 g / in3of the zeolite, based on the total volume of the substrate. Preferably, the at least one of the one or more washcoat layers that comprises the doped zirconia-based support material (hereinafter also referred to as “BMO layer”) further comprises a PGM component comprising Pd and, optionally, Pt. Preferably, the PGM component comprises a ratio by weight of Pt to Pd of 1:2 or less, preferably 1:4 or less, more preferably 1:5 or less, even more preferably 1:10 or less (including 0:1). By “a ratio by weight of Pt to Pd of 1:2 or less” it is meant that there is at least double the amount of Pd compared to Pt by weight, P102393 i.e. a ratio by weight of (1 or less):2 (i.e., a ratio by weight of 1:(2 or greater)). Theamount of Pt may be 0 wt.%. Most preferably, the PGM component consists ofPd. The PGM component is preferably supported on the doped zirconia-based support material. The addition of Pd to the BMO layer has been found to improve the CO and (shorter chain) HC oxidation activity of the layer. Surprisingly, replacing (some of) the Pd with Pt does not provide the same effect, either showing worse oxidation performance or similar oxidation performance in some cases. Fully replacing the Pd with Pt may noticeably lower the formaldehyde oxidation performance of the catalyst. Accordingly, it is most preferred that the PGM component consists of Pd. However, at least some Pt may be present depending on the purpose (and / or costs), to provide acceptable activity. The BMO layer preferably comprises from 1 to 40 g / ft3of the PGM component, more preferably from 1 to 35 g / ft3(such as from 5 to 35 g / ft3) of the PGM component, more preferably from 1 to 30 g / ft3(such as from 10 to 30 g / ft3) of the PGM component, even more preferably from 5 to 25 g / ft3(such as from 15 to 25 g / ft3) of the PGM component, based on the total volume of the substrate. Surprisingly, approaching 20 g / ft3of the PGM component has been observed to have an optimised CO oxidation activity, i.e. increasing or decreasing away from this loading may lower activity, though may still provide an acceptable level of activity. However, commercially, costs may factor into the final DOC. Accordingly,in some embodiments the BMO layer comprises from 5 to 10 g / ft3 of the PGMcomponent, which may provide acceptable performance. In particular, such lower loadings of the PGM component may suitably balance catalytic performance and the general aim of reducing PGM loadings to lower costs. In some embodiments, in particular where a washcoat layer of the at least one washcoat layer comprising platinum and / or palladium (hereinafter also referred to as “PGM-containing layer”) at least partially overlies a BMO layer, the PGM component may be present in the BMO layer by virtue of “wicking” from the PGM- containing layer during manufacture of the DOC. As such, the BMO layer may comprise a concentration gradient of the PGM component, typically with concentration of the PGM component decreasing in a direction perpendicular to P102393 and in a direction towards the substrate (i.e. away from the PGM-containing layer). The concentration gradient may be a continuous, but not necessarily linear, concentration gradient. This phenomenon may be observed by, for example, by Field Emission-Electron Probe Micro-Analysis (FE-EPMA). However, preferably, the PGM component is intentionally introduced into the BMO layer, i.e. the PGM component is added directly into the BMO layer during manufacture of the DOC. In some embodiments, the BMO layer preferably at least partially overlies the PGM-containing layer (preferably wherein the BMO layer is different from the PGM-containing layer, i.e. wherein the PGM-containing layer is (substantially) free of zirconia-based support material; the BMO layer may still comprise PGM as described herein). In other words, the PGM-containing layer is preferably a lower washcoat layer of the DOC and the BMO layer is preferably an upper layer of the DOC. Most preferably, the one or more washcoat layers of the DOC consists of two washcoat layers, the two washcoat layers being (i) the PGM- containing layer described herein as a bottom layer and (ii) the BMO layer described herein as a top layer. Surprisingly, it has been observed that orienting the washcoat layers in this arrangement may significantly reduce the NOxabsorption of the DOC article compared to alternative layer arrangements, such as those in which the BMO layer is the lower layer. Low NOxabsorption may be advantageous in applications where accuracy of on-board diagnostics, engine calibration and operation, for example, is important. However, it has also been observed that the CO and HC oxidation performance may be reduced when arranging the washcoat layers in this orientation, though this can be offset by the inclusion of the PGM component in the BMO layer as described herein. It may therefore be particularly preferred that the BMO layer at least partially overlies the PGM-containing layer, and the BMO layer further comprises a PGM component comprising Pd and, optionally, Pt. P102393 This arrangement may therefore be preferred where, for example, low or reduced absorption of NOxby the DOC is considered to be a desired property of the DOC in the particular emission treatment system in which the DOC is intended to be used. Preferably, the BMO layer further comprises sulphur, more preferably from 0.5 to 3 g / L sulphur, based on the total volume of the substrate. Surprisingly, the furtheraddition of S, typically provided as a (soluble) sulphate salt, such as manganesesulphate, zirconium sulphate, cerium sulphate and / or ammonium sulphate, hasalso been seen to reduce NOxstorage on the DOC article. However, similarly, this may also slightly reduce the CO and HC oxidation activity. Whether or not this feature is preferred may therefore depend on the desired properties of the DOC in the particular emission treatment system in which the DOC is intended to be used. As described herein, the PGM-containing layer preferably further comprises a support material comprising alumina and / or a zeolite. In other words, the PGM- containing layer may comprise a DOC. The PGM-containing layer preferably further comprises a promoter, such as strontium or barium, preferably barium. The PGM-containing layer preferably comprises from 50 to 200 g / ft3of the promoter (preferably of barium), more preferably from 100 to 150 g / ft3, based on the total volume of the substrate. Preferably, the at least one of the one or more washcoat layers that comprisesthe doped zirconia-based support material comprises from 0.5 to 5 g / in3 of thedoped zirconia-based support material, more preferably from 1 to 3 g / in3of the doped zirconia-based support material, based on the total volume of the substrate. In some preferred embodiments, the “BMO layer” comprises from 0.1 to 3 g / in3of the doped zirconia-based support material, based on the total volume of the substrate, preferably from 0.5 to 2.5 g / in3of the doped zirconia-basedsupport material, more preferably from 1.5 to 2.5 g / in3 of the doped zirconia-based support material. The doped zirconia-based support material itself may P102393 also contribute to increased NOx storage of the DOC, which can be undesirable in some applications as discussed above. Accordingly, in some embodiments a lower loading of the doped zirconia-based support material may be desired. Preferably, the at least one of the one or more washcoat layers that comprisesthe doped zirconia-based support material further comprises a binder. Suitablebinders are known to the skilled person. The binder preferably comprises an alumina species, such as alumina or an alumina sol. Preferably, the at least one of the one or more washcoat layers that comprises the doped zirconia-basedsupport material comprises from 0.01 to 2 g / in3, more preferably from 0.02 to 1.7g / in3, even more preferably from 0.03 to 1.5 g / in3, still more preferably from 0.04 to 1.3 g / in3, still more preferably from 0.05 to 1.2 g / in3, still more preferably from 0.06 to 1.0 g / in3of the binder, based on the total volume of the substrate. Preferably, the substrate comprises a wall flow filter substrate. In an alternative preferred embodiment, the substrate comprises a flow-through substrate. Preferably, the substrate comprises cordierite. However, the composition of the substrate is not particularly limited. The PGM-containing layer and / or the BMO layer may extend for up to 100% of the axial length of the substrate, from an inlet end or an outlet end of the substrate, such as for from 20 to 80% of the axial length of the substrate. In one preferred embodiment: the PGM-containing layer at least partially overlies the BMO layer; the BMO layer preferably extends for at least 90%, preferably 100% of the axial length of the substrate; and the PGM-containing layer: (a) extends for at least 90%, preferably 100% of the axial length of the substrate; or (b) extends from an inlet end of the substrate for from 30 to 70% of the axial length of the substrate, preferably wherein the PGM-containing layer P102393 comprises a Pt:Pd ratio by weight of from 5:1 to 2:1; and the DOC further comprises a third washcoat layer extending from an outlet end of the substrate for from 30 to 70% of the axial length of the substrate and preferably at leastpartially overlying the BMO layer, wherein the third washcoat layer comprises Ptand, optionally, Pd, and Mn. The third washcoat layer may preferably further comprise alumina and / or a zeolite (preferably being as described herein in relation to the PGM-containing layer). The third washcoat layer preferably at least partially overlies the PGM-containing layer. In some preferred embodiments, the PGM-containing layer extends from an inlet end of the substrate for about 70% of the axial length of the substrate and the third washcoat layer extends from an outlet end of the substrate for about 50% of the axial length of the substrate. In a further aspect, the present invention provides an exhaust gas treatment system comprising the diesel oxidation catalyst as described herein, preferably further comprising a further DOC located downstream of the diesel oxidation catalyst. The further DOC may also be according to the invention, and may beidentical to the DOC (in particular where the DOC comprises the PGM-containinglayer and the BMO layer). In other words, the exhaust gas treatment system preferably comprises the DOC described herein and a further, identical, but distinct, DOC located downstream thereof. In one embodiment, the PGM- containing layer at least partially overlaps the BMO layer in both of the DOC and the further DOC. In one embodiment, the BMO layer at least partially overlaps the PGM-containing layer in the DOC and the PGM-containing layer at least partially overlaps the BMO layer in the further DOC. Alternatively, the further DOC located downstream of the diesel oxidation catalyst may comprise a washcoat layer comprising a PGM component comprising Pt and / or Pd, alumina and, optionally, a zeolite. The composition of the washcoat layer may, independently, correspond to the preferred embodiments of the PGM- containing layer described herein. The further DOC may comprise such a washcoat layer as the only washcoat layer disposed thereon. Alternatively, the further DOC may comprise a “BMO layer”, which may, independently, correspond P102393to the (preferred) embodiments of the BMO layer described herein, extending(preferably for from 10 to 60% of the axial length of the substrate) from an inlet end of the substrate and the above-described washcoat layer at least partially overlying the “BMO layer”, such as to extend from an outlet end of the substrate for up to 100% of the axial length of the substrate. With this arrangement, without wishing to be bound by theory, it is thought that, in use, the upstream DOC (i.e. the DOC of the invention, which may be on a separate upstream substrate) may convert most of the CO and formaldehyde present in the exhaust gas, leaving the downstream DOC to efficiently convert remaining HCs. Preferably, the further DOC located downstream of the diesel oxidation catalyst comprises at least 70 wt.%, preferably at least 90 wt.%, more preferably at least 95 wt.% of the total PGMs present in the DOC and the further DOC, based on the total weight of the PGMs in both the DOC and the further DOC. The preferred embodiments and advantages of the first aspect apply equally this aspect. In a further aspect, the present invention provides a fuel combustion and exhaust gas treatment system comprising a diesel engine in fluid communication with the exhaust gas treatment system of the above aspect. The preferred embodiments and advantages of the first aspect apply equally this aspect.In a further aspect, the present invention provides a method for the manufactureof a diesel oxidation catalyst, the method comprising: (a) providing a substrate; (b) forming a washcoat composition; (c) applying the washcoat composition to the substrate and calcining it, wherein the washcoat composition comprises a doped zirconia-based support material which is a zirconia-based support material provided with two or more dopants and which is formed in the steps of: P102393 (i) providing a zirconia-based support material comprising, on an oxide basis: at least 80 wt.% Zr, based on the total weight of the zirconia- based support material; at least 2 wt.% Nd, based on the total weight of the zirconia-based support material; and, optionally, one or more of Y, Ce, Hf and La, and (ii) doping the zirconia-based support material with: from 5 to 15 wt.% Ce and from 5 to 30 wt.% Mn, on an oxide basis, based on the total weight of the doped zirconia-based support material. The preferred embodiments and advantages of the first aspect apply equally this aspect. Forming a washcoat composition typically comprises providing an aqueous slurry comprising the components of the washcoat, using techniques known to those skilled in the field. Applying the washcoat composition to the substrate may be carried out usingtechniques known in the art. Typically, the washcoat composition may be pouredinto the inlet of the substrate using a specific moulding tool in a predeterminedamount, thereby coating the washcoat composition on the substrate.Alternatively, applying the washcoat composition to the substrate may be carried out by immersing the substrate in the washcoat composition. Subsequent vacuum and / or air knife and / or drying steps may be employed during theapplication step (c). When the substrate is a wall flow filter, the washcoatcomposition may be coated on the filter walls, within the filter walls (if porous) orboth. Calcining the substrate having the washcoat applied thereto preferably comprises heating the substrate having the washcoat applied thereto at a temperature of P102393 from 400 to 600°C for from 5 to 60 minutes, preferably in a static oven. The heating may be carried out in air or in an inert atmosphere, typically in air. The method may comprise one or more further steps of forming one or more further washcoat layers before or after step (c). For example, preferably step (c) is the first washcoat layer applied to the substrate such that the washcoat layerthat comprises the doped zirconia-based support material is directly in contactwith the substrate, and the method further comprises forming one or more further washcoat layers after step (c), to provide the preferred DOCs as described herein. The one or more further steps of forming one or more further washcoat layers before or after (preferably after) step (c) may comprise applying a further washcoat composition to the substrate, the further washcoat compositioncomprising Pd, and allowing the Pd to wick into the washcoat compositioncomprising the doped zirconia-based support material. The term “wick into” as used herein may encompass the migration, such as by diffusion, of the Pd in the (aqueous) washcoat compositions. Thus, in such a step, the washcoat composition comprising the doped zirconia-based support material and the further washcoat composition are preferably each applied prior to calcining the entire substrate. In other words, the washcoat composition comprising the doped zirconia-based support material is preferably not calcined prior to application of the further washcoat composition. Preferably, step (ii) of doping the zirconia-based support material comprises joint (or co-) precipitation, flame spray hydrolysis, incipient wetness or wet impregnation.In a further aspect, the present invention provides a method for the treatment ofan exhaust gas from a diesel engine, the method comprising passing a dieselexhaust gas through the diesel oxidation catalyst as described herein to obtain a P102393 treated diesel exhaust gas, preferably wherein the treated diesel exhaust gas comprises less than 1 ppm formaldehyde.The preferred embodiments and advantages of the above aspects apply equallythis aspect.It should be noted that, as will be appreciated by those familiar with theconventions in the field of the present invention, the term “on an oxide basis” as used herein with respect to the wt.% of components of the (doped) zirconia- based support material means that the wt.% of the component is calculated as if the component is present as its oxide, i.e. based on the weight of the respective oxide. For example, for the purpose of the wt.% calculation, for Zr, the corresponding oxide is ZrO2; for Nd, the corresponding oxide is Nd2O3; for Y, the corresponding oxide is Y2O3; for Ce, the corresponding oxide is CeO2; for Hf, the corresponding oxide is HfO2; for La, the corresponding oxide is La2O3; and for Mn, the corresponding oxide is MnO2.In other words, the first aspect of the present invention therefore provides adiesel oxidation catalyst comprising one or more washcoat layers disposed on a substrate, wherein at least one of the one or more washcoat layers comprises a doped zirconia-based support material which is a zirconia-based support material provided with two or more dopants, wherein the zirconia-based support material comprises, on an oxide basis: at least 80 wt.% Zr (when calculated as ZrO2), based on the totalweight of the zirconia-based support material; at least 2 wt.% Nd (when calculated as Nd2O3), based on the totalweight of the zirconia-based support material; and, optionally, one or more of Y (when calculated as Y2O3), Ce (when calculatedas CeO2), Hf (when calculated as HfO2) and La (when calculated asLa2O3), and wherein the two or more dopants comprise: P102393 from 5 to 15 wt.% Ce (when calculated as CeO2) and from 5 to 30wt.% Mn (when calculated as MnO2), on an oxide basis, based on the totalweight of the doped zirconia-based support material. The term “substrate” as used herein may encompass, for example, a ceramic ormetallic honeycomb or flow through monolith, or a filter block, e.g. a wall flowfilter. The substrate may comprise a ceramic monolithic substrate. The substrate may vary in its material composition, size and configuration, cell shape and density, and wall thickness. Suitable substrates are known in the art.The term “disposed on” in the context of the present invention may encompassboth having said washcoat layer directly disposed on the substrate, i.e. with nointervening material, and / or indirectly disposed on the substrate, i.e. with intervening material. If the substrate is porous, then the term “disposed on” mayalso encompass having said washcoat layer disposed therein, for example withinthe pores of the substrate, i.e. wherein the washcoat layer is disposed thereonand / or therein. The term “washcoat” as used herein is well-known in the field and refers to an adherent coating that is applied to a substrate usually during the production of a catalyst. The term “support material”, in general, as used herein may encompass any known support material that may be used to support PGMs in the field of the present invention, typically in powder form. However, since, as demonstrated herein, the support material itself has catalytic activity, the term “support material” does not necessitate that the support material described herein must be present to support a further component, such as a PGM. It is optional that the support material supports a further component. As will be appreciated, the term “support material” is being used herein as it is typically used in the field of the present invention. P102393 As will be appreciated, the zirconia-based support material is “zirconia-based” at least because it comprises, on an oxide basis, at least 80 wt.% Zr, based on the total weight of the zirconia-based support material. The term “doped” in the context of the “two or more dopants” of the invention as used herein may encompass that the dopants are introduced into a pre-formed material and are not part of the original mixed oxide starting material. The term “supported on” in the context of “the platinum and / or palladium issupported on alumina”, for example, as used herein may encompass that therelevant PGM, typically in the form of nanoparticles, is directly in contact, and physical and / or chemically bound to the surface of the support material. The term “surface of the support material” may encompass the surface of the pores within a porous support material, for example. The invention can also be defined according to one or more of the following definitions:1. A diesel oxidation catalyst comprising one or more washcoat layersdisposed on a substrate, wherein at least one of the one or more washcoat layers comprises a doped zirconia-based support material which is a zirconia-based support material provided with two or more dopants, wherein the zirconia-based support material comprises, on an oxide basis: at least 80 wt.% Zr, based on the total weight of the zirconia-based support material; at least 2 wt.% Nd, based on the total weight of the zirconia-based support material; and, optionally, one or more of Y, Ce, Hf and La, and wherein the two or more dopants comprise: from 5 to 15 wt.% Ce and from 5 to 30 wt.% Mn, on an oxide basis, based on the total weight of the doped zirconia-based support material. P102393 2. The diesel oxidation catalysts according to 1, wherein the two or moredopants comprise from 5 to 15 wt.% Mn, on an elemental basis, based on thetotal weight of the doped zirconia-based support material.3. The diesel oxidation catalyst according to 1 or 2, wherein the zirconia-based support material comprises from 5 to 15 wt.% Nd, on an oxide basis, based on the total weight of the zirconia-based support material.4. The diesel oxidation catalyst according to any of 1 to 3, wherein thezirconia-based support material comprises, on an oxide basis, one or more of: from 1 to 7 wt.% Y; from 1 to 7 wt.% Ce; from 0.1 to 5 wt.% Hf; from 1 to 3 wt.% La, based on the total weight of the zirconia-based support material. 5. The diesel oxidation catalyst according to any one of 1 to 4, wherein the zirconia-based support material comprises, on an oxide basis, from 1 to 2 wt.% La; from 12 to 14 wt.% Nd; and optionally from 0.1 to 2 wt.% Hf, based on the total weight of the zirconia-based support material. 6. The diesel oxidation catalyst according to any one of 1 to 4, wherein the zirconia-based support material comprises, on an oxide basis, from 1 to 2 wt.% La; from 4 to 6 wt.% Nd; from 4 to 6 wt.% Ce; and optionally from 0.1 to 2 wt.% Hf, based on the total weight of the zirconia-based support material. P102393 7. The diesel oxidation catalyst according to any of 1 to 4, wherein the zirconia-based support material comprises, on an oxide basis, from 4 to 6 wt.% Nd; from 4 to 6 wt.% Ce; from 4 to 6 wt.% Y; and optionally from 0.1 to 2 wt.% Hf, based on the total weight of the zirconia-based support material.8. The diesel oxidation catalyst according to any one of 1 to 7, wherein thezirconia-based support material consists of the listed oxides, together with any unavoidable impurities.9. The diesel oxidation catalyst according to any one of 1 to 8, wherein thetwo or more dopants further comprise from 1 to 5 wt.% Zr, on an oxide basis, based on the total weight of the doped zirconia-based support material.10. The diesel oxidation catalyst according to 9, wherein the two or moredopants consist of Ce, Mn and Zr, together with any unavoidable impurities.11. The diesel oxidation catalyst according to any one of 1 to 8, wherein thetwo or more dopants consist of Ce and Mn, together with any unavoidable impurities.12. The diesel oxidation catalyst according to any one of 1 to 11, wherein thetwo or more dopants comprise about 10 wt.% Ce and / or about 10 wt.% Mn, on an oxide basis, based on the total weight of the doped zirconia-based support material.13. The diesel oxidation catalyst according to any one of 1 to 12, wherein atleast one of the one or more washcoat layers comprises platinum and / or palladium, preferably in a weight ratio of 3:1 to 1:1 and / or in a total loading of P102393from 0.2 to 5.0 wt.%, based on the total weight of the platinum- and / or palladium-containing washcoat layer.14. The diesel oxidation catalyst according to 13, wherein the platinum and / orpalladium is supported on the doped zirconia-based support material, or wherein the platinum and / or palladium is supported on alumina.15. The diesel oxidation catalyst according to 14, wherein the platinum and / orpalladium supported on alumina is provided in a separate washcoat layer which at least partially overlies the washcoat layer comprising the doped zirconia-based support material.16. The diesel oxidation catalyst according to any one of 1 to 15, wherein theat least one washcoat layer comprising platinum and / or palladium further comprises a zeolite. 17. The diesel oxidation catalyst according to any one of 1 to 16, wherein the at least one of the one or more washcoat layers that comprises the doped zirconia-based support material (hereinafter also referred to as “BMO layer”) further comprises a PGM component comprising Pd and, optionally, Pt. 18. The diesel oxidation catalyst according to 17, wherein the PGM component consists of Pd. 19. The diesel oxidation catalyst according to 17 or 18, wherein the BMO layer comprises from 1 to 40 g / ft3of the PGM component, based on the total volume of the substrate. 20. The diesel oxidation catalyst according to any one of 1 to 14 and 16 to 19, wherein the BMO layer at least partially overlies a washcoat layer of the at least one washcoat layer comprising platinum and / or palladium (hereinafter also referred to as “PGM-containing layer”). P102393 21. The diesel oxidation catalyst according to any one of 1 to 20, wherein the BMO layer further comprises sulphur.22. A method for the manufacture of a diesel oxidation catalyst, the methodcomprising: (a) providing a substrate; (b) forming a washcoat composition; (c) applying the washcoat composition to the substrate and calcining it, wherein the washcoat composition comprises a doped zirconia-based support material which is a zirconia-based support material provided with two or more dopants and which is formed in the steps of: (i) providing a zirconia-based support material comprising, on an oxide basis: at least 80 wt.% Zr, based on the total weight of the zirconia-based support material; at least 2 wt.% Nd, based on the total weight of the zirconia-based support material; and, optionally, one or more of Y, Ce, Hf and La, and (ii) doping the zirconia-based support material with: from 5 to 15 wt.% Ce and from 5 to 30 wt.% Mn, on an oxide basis, based on the total weight of the doped zirconia-based support material.23. A method for the treatment of an exhaust gas from a diesel engine, themethod comprising passing a diesel exhaust gas through the diesel oxidation catalyst according to any of 1 to 16 to obtain a treated diesel exhaust gas, preferably wherein the treated diesel exhaust gas comprises less than 1 ppm formaldehyde. The invention will now be described in relation to the following non-limiting examples. P102393 Zirconium and hafnium occur most commonly in nature as the mineral zircon (ZrSiO4) and less commonly as baddeleyite (ZrO2). Separating hafnium from zirconium is a challenging problem for manufacturers of raw materials because of their great chemical similarities and the use of high temperature and aggressive chemicals applied to the zircon sand zirconia source. Zirconium and hafnium have similar atomic radii (1.45 Å and 1.44 Å, respectively), and similar valence electron configurations of 4d25s2and 5d26s2, respectively. Accordingly, it is typical in the technical field for a total quoted quantity of zirconia in a sample to include small or trace quantities of hafnia (HfO2), i.e., the quoted wt.% of ZrO2in the Examples below is in fact a combination of ZrO2and HfO2. The order of trace HfO2 content in a commercial ZrO2 raw material typically used in the present technical field is insignificant by comparison with the content of the ZrO2. For example, the quoted 85wt% quantity of ZrO2in the mixed oxide powder inExample 6.1 hereinbelow includes approximately 1.6 wt.% HfO2.EXAMPLE 1 – preparation of catalyst compositionsReference catalyst compositions and catalyst compositions according to the present invention were prepared as follows. Reference Catalyst Composition 1:2.0 g of a mixed oxide powder comprising 91 wt.% ZrO2 and 9 wt.% La2O3 wasimpregnated with cerium nitrate (dried and calcined at 500°C) then impregnated with manganese nitrate (dried and calcined 500°C). The final powder compositioncomprised 10 wt.% cerium and 25 wt.% manganese, on an oxide basis, based onthe total weight of the final powder composition. This final powder composition is referred to as Reference Catalyst Composition 1 herein. P102393 Catalyst Composition 2:2.0 g of a mixed oxide powder comprising 85 wt.% ZrO2, 13 wt.% Nd2O3 and 2wt.% La2O3 (and small impurity amounts of Hf) was impregnated with cerium nitrate (dried and calcined at 500°C) then impregnated with manganese nitrate(dried and calcined 500°C). The final powder composition comprised 10 wt.%cerium and 25 wt.% manganese, on an oxide basis, based on the total weight ofthe final powder composition. This final powder composition is referred to as Catalyst Composition 2 herein. Catalyst Composition 3:2.0 g of a mixed oxide powder comprising 85 wt.% ZrO2, 13 wt.% Nd2O3 and 2wt.% La2O3 (and small impurity amounts of Hf) was impregnated with cerium nitrate (dried and calcined at 500°C) then impregnated with manganese nitrate(dried and calcined 500°C). The final powder composition comprised 10 wt.%cerium and 10 wt.% manganese, on an oxide basis, based on the total weight of the final powder composition. This final powder composition is referred to as Catalyst Composition 3 herein. Each of the above catalyst compositions were hydrothermally aged at 750°C for15 hours to provide hydrothermally aged compositions.EXAMPLE 2 – catalytic performance testing of catalyst compositionsEach of the above catalyst compositions were tested in the fresh andhydrothermally aged conditions using a powder synthetic gas (synthetic catalyticactivity test (SCAT)) apparatus with a gas mix of 1500 ppm CO, 100 ppmpropene, 20 ppm formaldehyde, 250 ppm NO, 5.5% water, 10% CO2, 14% O2, N2balance at a space velocity of 55,000 / hour. The temperature was ramped from 80to 350°C at a rate of 20°C per minute with post-catalyst emissions measuredthroughout. P102393 CO and propene (comparing Nd vs. no Nd in catalyst composition): The CO and propene emissions were recorded and compared for Reference Catalyst Composition 1 and Catalyst Composition 2 in the aged condition. The results are shown in Table 1. Table 1 Catalyst Composition T50 (°C) CO T50 (°C) PropeneReference Catalyst 282 302Composition 1 Catalyst Composition 2 282 291The T50 (otherwise known as the light-off temperature) is the temperature at which 50% conversion of the particular pollutant is reached. As can be seen in Table 1, Catalyst Composition 2 exhibits a comparable CO light-off temperature to Reference Catalyst Composition 1, but surprisingly exhibits an improved light-off temperature for propene, a HC. Formaldehyde (comparing different Mn wt.%): The formaldehyde emissions were recorded for Catalyst Composition 2 in the fresh and aged conditions. The results are shown in Table 2. Table 2 Catalyst Composition 2T80 (°C) T90 (°C)condition Fresh 201 -Aged - 82 P102393 The T80 is the temperature at which 80% conversion of the particular pollutant is reached. The T90is the temperature at which 90% conversion of the particular pollutant is reached. The T80and the T90were measured rather than the T50because the activity was so high. Surprisingly, it has been found that for the catalysts of the invention, the light-off temperature improves with ageing. It should be noted that, of course, the T80is necessarily the same or a lower temperature than the T90. The effect of Mn loading in the particular catalyst composition identified as having improved propene (i.e. HC) abatement performance was also investigated. In particular, the formaldehyde emissions were recorded and compared for Catalyst Composition 2 and Catalyst Composition 3 in the fresh condition. The results are shown in Table 3. Table 3 Catalyst Composition T80 (°C)Catalyst Composition 2 201Catalyst Composition 3 <80As can be seen in Table 3, the formaldehyde light-off temperature improves significantly when the Mn loading is reduced from 25 wt.% to 10 wt.%.EXAMPLE 3 – further investigations into activity of various manganese loadingsAlthough manganese was found to be a promoter of formaldehyde oxidation, in practice Applicant was aware that manganese-containing catalysts can be poisoned by gaseous sulphur species in the exhaust gas derived from sulphur species present in engine fuel. In order to test for the impact of sulphation on the inventive catalysts, a series of washcoats based on a mixed oxide powder comprising 83.4 wt.% ZrO2, 13.4 wt.% Nd2O3 and 1.6 wt.% La2O3 (the quoted content of ZrO2includes small impurity amounts of Hf) and alumina binder were P102393 added to a solution of cerium nitrate, zirconium nitrate and different concentrations of manganese nitrate. The resulting slurry washcoat was applied as a single layer only to separate flow-through monolith substrates. The coated parts were dried at 100°C and then dynamically calcined to 500°C for 45 minutes. The quantity of mixed oxide powder, alumina binder, cerium nitrate and zirconium nitrate were each selected to obtain a post-calcined loading of 2 g / in3mixed oxide, 0.2 g / in3alumina powder, 0.25 g / in3cerium and 0.1 g / in3zirconium. The manganese nitrate concentration was adjusted to obtain samples containing1wt%, 2wt%, 5wt% and 10wt% (calculated on an elemental basis of) manganese,corresponding to 1.9 wt.%, 3.8 wt.%, 9.0 wt.% and 16.5 wt.% manganese on anoxide basis. Post-calcination, the CO oxidation and NO oxidation of these samples were tested according to the following methodology. Core samples of 1-inch diameter x 3.82 inches in length were cut from a dried and calcined monolith substrate catalysts. The cores were hydrothermally aged at 750°C for 16 hours in 10% water / air in an oven box. A constantly replenished gas mixture of compressed air / 10% water (equivalent to 20% O2) controlled by mass flow and liquid flow controllers respectively was pumped into the oven box at a flow rate of 6 litres per minute. The honeycomb monolith substrate cores were arranged within the oven box so that the channels extended vertically and the flowing gas mixture was forced to enter the channels of each core disposed within the oven box from a lower end thereof. The catalytic oxidation activity of each aged core was determined using a synthetic gas laboratory catalytic activity test (SCAT) using the simulated exhaust gas mixture shown in Table 4 Prior to each activity assessment, each aged catalyst core was first “pre-conditioned” in the SCAT apparatus by “soaking” to equilibrate each core sample at a 80°C catalyst inlet temperature in a gas mixture consisting of 4% CO2, 4% H2O, 14% O2and N2balance at 55,000 hr-1space velocity followed by switching the gas mixture to the gas mixture set out in Table4 below and conducting a “fast ramp” from 80°C to 400°C at a temperature ramp P102393 rate of 100°C per minute. Following the “fast ramp” pre-conditioning, the catalyst core sample was cooled to the 80°C “soak” temperature in the gas mixture consisting of 4% CO2, 4% H2O, 14% O2and N2balance at 55,000 hr-1space velocity before two successive ramp tests were conducted. Each ramp test was done by switching the exhaust gas mixture to the mixture setout in Table 4 and conducting a temperature ramp from 80°C to 400°C at 20°Cper minute. Between each ramp test, the catalyst core sample was cooled and “soaked” at 80°C using the “soak gas” composition described above before the ramp test was repeated a second time. Table 4 Gas Mixture Component Constituent AmountCO 1500 ppm (0.15%)HC (as C1) 430 ppm (0.043%)NO 100 ppm (0.01%)CO2 4% H2O 4%O214% N2Balance Space velocity 55,000 / hourThe hydrocarbon (HC) component was a mixture of four individual hydrocarbons: methane (CH4), the shorter chain unsaturated hydrocarbon propene (C3H6), the longer chain saturated hydrocarbon decane (C10H22) and the aromatic hydrocarbon toluene (C6H5CH3). Diesel fuel is a mixture of thousands of hydrocarbon compounds, most of which have carbon numbers between 10 and 22. Most of the compounds in diesel fuel are hydrocarbons of the paraffinic (e.g. decane), naphthenic, or aromatic class. In the test, toluene is used as a representative aromatic, although it is not generally a component of diesel fuel. P102393 SCAT oxidation activity for CO is reported by the light-off temperature whereby 50% conversion is achieved (T50) (the lower temperature, the more active the catalyst). Additionally, the %NO2of total oxides of nitrogen (NOx) at 250°C is reported (the higher the % value, the more active the catalyst at that temperature for oxidising NO in the feed gas to NO2). SCAT oxidation activity results for thesecond, i.e. the repeat “run”, for CO and NO oxidation are reported in Table 5.Separately, dried and calcined (i.e. unaged) adhered washcoat was removedfrom each flow-through substrate and collected as a powder. The recovered powders were pressed and pelletised with the sieve fraction, from 355 μm to 250 μm, so the final pellet size was 250 μm. The pellets were hydrothermally aged in an oven at 750°C in 10% water / air for 15 hours in a static ageing oven to produce aged pellet samples. 0.4g of the pelletised catalyst was used in each SCAT test. A catalyst reactor bed of about 14mm was used in a reactor having a length of 70 mm and diameter 8 mm. The following synthetic gas mixture was used for the SCAT test: CO 1500 ppm (0.15%), 100 ppm propene, 50 ppm formaldehyde, CO2 10%, H2O 5%, O210%, NO 250 ppm, N2 Balance, at 2.2 litres of gas per minute. Formaldehyde oxidation activity was determined from 80oC to 450oC at a ramp rate of 10oC / minute and the results are also presented in Table 5. As an additional step, a 0.46 gram sample of the aged pellets was sulphated in a dedicated sulphation apparatus, similar to a SCAT apparatus, by contacting the pellets at 350°C in a gas mixture consisting of air, 20 ppm SO2and 10% H2O at 2litres gas volume per minute for 1 hour. These conditions – referred to as leanhydrothermal sulphation – were selected for equivalence to a loading of 1g / litre ofelemental sulphur as if the pelleted powder had been applied as a washcoat to asubstrate of that corresponding volume. 0.4 g of the resulting sulphated pelletswere also tested for formaldehyde oxidation activity. The results of formaldehydeoxidation for the sulphated pellets are also set out in Table 5. P102393 Table 5 Aged, T50CO (°C) % NO2 / NOx at Aged, Sulphated Manganese (unsulphated) 250°C Unsulphated HCHO (elemental) on (unsulphated) HCHO Conversion Example wt.% washcoated on Conversion at 140°C No. monolith washcoated at 140°C (%) (%) cores monolith (Pelletised (Pelletised cores Powder, Powder, SCAT only) SCAT only)6.1 1 163.5 19.2 84 226.2 2 163.0 18.3 90 236.3 5 153.5 27.7 77 696.4 10 156.0 28.4 69 60It can be seen from the results presented in Table 5 that where no sulphur waspresent, manganese is more active for formaldehyde oxidation at relatively lower manganese elemental weight percentages, although manganese does contribute to a small improvement in CO and NO oxidation activity. However, in a more representative exhaust gas environment, where sulphur contacts a catalyst disposed in an exhaust system, when in use, the trend is reversed, and higher weight percentages of manganese are beneficial. For this reason, the higher weight percentages of manganese are preferred for commercial industrial application. EXAMPLE 4 – preparation of catalyst articlesCatalyst articles were prepared having a bottom washcoat layer applied directly to the substrate and a top washcoat layer applied directly to the bottom washcoat layer. P102393 The bottom washcoat layer contained 2 g / in3of a zirconia-based support material as defined in Table 6, further doped with 0.25 g / in3of Mn, 0.25 g / in3of Ce and 0.05 g / in3of Zr in a similar manner to the preparation methods of Example 1. The bottom washcoat layer further contained 0.2 g / in3of a water dispersible boehmite binder. The top washcoat layer contained 50 g / ft3of Pt and Pd in a 2:1 ratio by weight, 1.4 g / in3of 5% silica stabilised alumina powder, 100 g / ft3of Ba, 50 g / ft3of citric acid and 0.6 g / in3of beta zeolite. The “g / ft3” and “g / in3” units are defined based on the total volume of the substrate, i.e. assuming that the substrate is a solid geometric shape and ignoring open volume comprising channels in the substrate for gas flow. The catalyst articles were labelled as CA1-CA8. A reference catalyst article, labelled CAR was also manufactured having only the top layer described above, and no bottom layer. The catalyst articles were each hydrothermally aged at 750°C for 16 hours at 10% relative humidity. Table 6 Zr La Nd Ce Hf Y Si (wt.%) (wt.%) (wt.%) (wt.%) (wt.%) (wt.%) (wt.%) CA1 100 - - - - - -CA2 85 1.5 13.5 - - - -CA3 91 9 - - - - -CA4 88 1.7 5.3 5 - - -CA5 81.85 - 5 5 1.65 5 -CA6 86.3 - - - - 13.7 -CA7 80 10 - - - 10 -CA8 90 - - - - - 10 P102393EXAMPLE 5 – catalytic performance testing of catalyst compositionsThe aged catalyst articles of Table 6 as well as the aged CAR catalyst articlewere tested using a synthetic gas rig with a gas mix of 1500 ppm CO, 430 ppmhydrocarbons, 100 ppm NO, 4% water, 4% CO2, 14% O2, N2 balance at a spacevelocity of 55,000 / hour. The temperature was ramped from 80 to 400°C at a rate of 20°C per minute with emissions measured throughout. CO The CO T50 was measured for each aged catalyst article and the results are shown in Table 7. Table 7 Catalyst article T50 (°C) COCAR 167.9CA1 168.6CA2 157.8CA3 155.1CA4 154.9CA5 156.9CA6 168.9CA7 162.6CA8 178.1As can be seen in Table 7, surprisingly, the catalyst articles containing Nd- containing zirconia-based support materials (CA2, CA4 and CA5) show comparable performance to the catalyst article containing a zirconia-basedsupport material further containing only La (CA3), while catalyst articles CA2-CA5each perform significantly better than the other catalyst articles containing P102393 zirconia-based support materials with different further additive elements as well as the reference catalyst article CAR. NO2 “make”, i.e. conversion of NO to NO2 The NO2make, which is the ratio of NO2to total NOxin the treated gas mixture,was also measured for each aged catalyst article at 300°C and the results areshown in Table 8. Table 8 Catalyst article NO2 make (mol.%)CAR 27.1CA1 37.8CA2 56.3CA3 56.6CA4 51.7CA5 48.2CA6 39.7CA7 51.6CA8 35.6As can be seen in Table 8, similar results as for CO T50 are observed for the NO2make for catalyst articles containing the La- and Nd-containing zirconia-basedsupport materials compared to the other zirconia-based support materials and the reference catalyst article CAR. EXAMPLE 6Example 6.1 – ZrO2-based single oxidation catalyst layer (no PGM)2.0 g of a mixed oxide powder comprising 83.4 wt.% ZrO2, 13.4 wt.% Nd2O3 and1.6 wt.% La2O3(the quoted content of ZrO2includes small impurity amounts of P102393 Hf) was added to a solution of cerium nitrate, manganese nitrate and zirconium nitrate. The resulting slurry washcoat was applied to an axial length of 100% of the channels of a 1.4 litre volume cordierite flow-through (honeycomb) monolith substrate with 62 cells per square centimetre (400 cells per square inch) using established coating techniques disclosed in WO1999 / 047260 A1. The coated part was dried at 110°C and then calcined to 500°C for 2 hours. The weight percentages of the cerium nitrate, magnesium nitrate and zirconium nitrate were selected to achieve a calcined loading (as their oxides) of 10.3 wt.% cerium and12.1 wt.% manganese and 3.4 wt.% zirconium, on an oxide basis, based on thetotal weight of the washcoat.Example 6.2 – Two-layer composite oxidation catalystTo the coated substrate of Example 6.1, an aqueous slurry washcoat of silica- alumina powder (alumina doped with 5wt% silica), beta (BEA*, i.e. a BEA intergrowth) zeolite, barium acetate and soluble platinum and palladium salts was applied as an upper washcoat layer to an axial length of 100% using the identified established coating techniques. The resulting two-layer composite oxidation catalyst was dried at 110°C and then calcined to 500°C for 2 hours. The composition of the aqueous slurry washcoat was selected so that the final oxidation catalyst layer comprised 1.4 g / in35wt% silica-doped alumina, 0.6 g / in3Beta zeolite, a 2:1 weight ratio of Pt:Pd at a loading of 50 g / ft3and barium at a loading of 125 g / ft3.Example 7 – Effect on oxidation activity of adding palladium to lower ZrO2-based layer of two-layer composite oxidation catalyst Based on the basic two-layer composite oxidation catalyst design of Example 6.2, a quantity of platinum group metal was introduced as aqueous palladium and or platinum salt into the slurry of the ZrO2lower layer corresponding to Example 6.1was varied, as set out in Table 10 below. P102393Example 7.1 – Laboratory activity testing for CO, HC and NO2 oxidationCore samples of 1-inch diameter x 3.82 inches in length were cut from a dried and calcined two-layer composite oxidation catalyst monolith substrate catalystsof Example 6.2 but including the PGM content in the Example 6.1 layer describedin Table 10. The cores were hydrothermally aged in an oven at 750°C for 16 hours in 10% water / air in an oven box. A constantly replenished gas mixture of compressed air / 10% water (equivalent to 20% O2) controlled by mass flow and liquid flow controllers respectively was pumped into the oven box at a flow rate of 6 litres per minute. The honeycomb monolith substrate cores were arranged within the oven box so that the channels extended vertically and the flowing gas mixture was forced to enter the channels of each core disposed within the oven box from a lower end thereof. The catalytic oxidation activity of each aged core was determined using a synthetic gas laboratory catalytic activity test (SCAT) using the simulated exhaust gas mixture shown in Table 9. Prior to each activity assessment, each aged catalyst core was first “pre-conditioned” in the SCAT apparatus by “soaking” to equilibrate each core sample at a 80°C catalyst inlet temperature in a gas mixture consisting of 4% CO2, 4% H2O, 14% O2and N2balance at 55,000 hr-1space velocity followed by switching the gas mixture to the gas mixture set out in Table9 below and conducting a “fast ramp” from 80°C to 400°C at a temperature ramprate of 100°C per minute. Following the “fast ramp” pre-conditioning, the catalyst core sample was cooled to the 80°C “soak” temperature in the gas mixture consisting of 4% CO2, 4% H2O, 14% O2 and N2 balance at 55,000 hr-1space velocity before two successive ramp tests were conducted. Each ramp test was done by switching the exhaust gas mixture to the mixture setout in Table 9 and conducting a temperature ramp from 80°C to 400°C at 20°Cper minute. Between each ramp test, the catalyst core sample was cooled and “soaked” at 80°C using the “soak gas” composition described above before the ramp test was repeated a second time. P102393 Table 9 Gas Mixture Component Constituent AmountCO 1500 ppm (0.15%)HC (as C1) 430 ppm (0.043%)NO 100 ppm (0.01%)CO2 4% H2O 4%O214% N2Balance Space velocity 55,000 / hourThe hydrocarbon (HC) component was a mixture of four individual hydrocarbons: methane (CH4), the shorter chain unsaturated hydrocarbon propene (C3H6), the longer chain saturated hydrocarbon decane (C10H22) and the aromatic hydrocarbon toluene (C6H5CH3). Diesel fuel is a mixture of thousands of hydrocarbon compounds, most of which have carbon numbers between 10 and 22. Most of the compounds in diesel fuel are hydrocarbons of the paraffinic (e.g. decane), naphthenic, or aromatic class. In the test, toluene is used as a representative aromatic, although it is not generally a component of diesel fuel. SCAT oxidation activity for CO is reported by the light-off temperature whereby 50% conversion is achieved (T50) (the lower temperature, the more active the catalyst). HC oxidation is demonstrated by reporting both the T50 and T80, i.e. the temperature whereby 80% HC conversion is achieved. The HC T50 demonstrates the oxidation activity for propene, whereas the HC T80demonstrates the oxidation activity for the longer chain decane hydrocarbon and the aromatic toluene hydrocarbon. Additionally, the %NO2 of total oxides of nitrogen (NOx) at 250°C is reported (the higher the % value, the more active the catalyst at that temperature). SCAT oxidation activity results are reported in Table 10. P102393Example 7.2 – Activity of Example 6.1 single layer but also now containingPGM to oxidise formaldehyde (HCHO) Separately, the PGM-containing ZrO2-based oxidation catalyst layers set out inTable 10 otherwise having the composition of Example 6.1 were applied as asingle layer only – as Example 6.1 – to separate flow-through monolithsubstrates. The coated parts were dried at 110°C and then calcined to 500°C for 2 hours. The adhered coating was then removed and collected as a powder for formaldehyde oxidation activity analysis in a SCAT apparatus. That is, formaldehyde oxidation activity was tested on a powder sample of a single washcoat layer corresponding to Example 6.1 but also including the describedquantity of PGM shown in Table 10, and not on a powder sample of a mixturetwo-layer composite catalyst corresponding to Example 6.2. The powders recovered from the flow-through substrates were pressed and pelletised with the sieve fraction, from 355 μm to 250 μm, so the final pellet size was 250 μm. The pellets were hydrothermally aged in an oven at 750°C in 10% water / air for 15 hours in a static ageing oven to produce aged pellet samples. 0.4g of the pelletised catalyst was used in each SCAT test. A catalyst reactor bed of about 14mm was used in a reactor having a length of 70 mm and diameter 8 mm. The following synthetic gas mixture was used for the SCAT test: CO 1500 ppm (0.15%), 100 ppm propene, 50 ppm formaldehyde, CO210%, H2O 5%, O210%, NO 250 ppm, N2Balance, at 2.2 litres of gas per minute. Formaldehyde oxidation activity was determined from 80oC to 450oC at a ramp rate of 10oC / minute and the results are also presented in Table 2A. P102393 Table 10 PGM loading % conversion (g / ft3) in HCHO at 140°C lower layer (Powder SCAT T50CO T50HC T80HC % NO2 / NOx(Two-layeronly of single(°C) (°C) (°C) at 250°C sampleslayer based onaged at Example 6.1; 750°C for aged as pellets) 16 hours) Example 264 248 301 12 986.1* 0156 164 211 28 985Pd 149 160 221 23 9910 Pd 137 157 222 21 9810 @ Pt:Pd 137 157 217 28 981:5 20 Pd 117 152 216 25 9850 Pd 128 155 218 27 9910 Pt 156 164 214 28 90* Single layer of Example 6.1 only, no PGMIt can be seen from the results shown in Table 10 that increasing theconcentration of palladium up to 20g / ft3in the lower washcoat layer improved CO oxidation, but too much palladium, i.e.50 g / ft3, had a negative impact on CO oxidation. Substituting Pd for Pt in the lower washcoat layer resulted in poor CO oxidation activity. That is the CO oxidation activity trend can be represented as 20Pd>50Pd>>10Pd = 1:5 Pt:Pd>>5Pd>0 Pd = 10Pt.Table 10 also confirms that a trend in oxidation activity of the short chainhydrocarbon (propene) follows that of CO oxidation, including the 20Pd P102393 embodiment performing better than the 50Pd embodiment. It can also be seen that propene oxidation is not improved by exchanging Pd for Pt in the lower layer. Additionally, oxidation activity for longer chain (decane) and aromatic (toluene) hydrocarbons for all embodiments is broadly the same. This is not particularly surprising since the formulation of the upper washcoat layer remains unchanged. NO oxidation activity is also broadly similar across all embodiments. There is some small improvement in T80 HC where Pd is exchanged for Pt. Finally, the formaldehyde oxidation is broadly the same across all Pd-containing samples. However, where Pt only is added to the lower washcoat layer, the formaldehyde oxidation activity is noticeably lower.EXAMPLE 8 – Influence of additive and layer orientation on NOx adsorptionin dual layer catalyst; and initial investigation of influence of sulphur onNOx adsorption in ZrO2-based catalyst layer A series of catalysed flow-through monolith substrates similar to the two-layer composite oxidation catalyst Example 6.2 were prepared, except that no cerium nitrate or zirconium nitrate were included in the washcoat with the mixed oxide powder described in Example 6.1 hereinabove. This change was chosen in an effort to reduce the quantity of NOxadsorbed on the final catalyst. All substrates were hydrothermally aged at 750°C for 16 hours in 10% water / air using the test methodology of Example 7.1. Compared with the samples of Example 6.2, the loading of the mixed oxide layer of Example 6.1 was varied; and in one embodiment the orientation of the lower and upper layer was switched, i.e. the mixed oxide layer was coated on top of the PGM-containing layer. Both of these measures were intended to reduce the quantity of NOxadsorbed on the final catalyst. These modified samples of Example 6.2 were compared with a catalyst according to Example 6.2 in which P102393 no palladium was added to the mixed oxide powder lower layer. Details of all catalysts tested is set out in Table 11. Table 11 Layered catalyst Example No. component8.1 8.2 8.3 8.4 8.5composition PtPd 2:1 (50 g / ft3),Upper layer UpperUpper Lower Upper 5wt% silica-doped layer layer layer layer alumina (1.4 g / in3), beta zeolite (0.6 g / in3), barium (100 g / ft3) Mixed oxide ofLower layer N / A N / A N / A N / AExample 6.1 (2.0 g / in3), manganese (0.25 g / ft3), cerium (0.25 g / ft3), zirconium (0.75 g / ft3) Mixed oxide ofN / A Lower N / A N / A N / AExample 6.1 (2.0 layer g / in3), manganese (0.25 g / ft3), 5 g / ft3palladium Mixed oxide ofN / A N / A LowerUpper N / A Example 6.1 (1.0 layer layer g / in3), manganese (0.25 g / ft3), 5 g / ft3palladium Mixed oxide ofN / A N / A N / A N / A LowerExample 6.1 (2.0 layer P102393 Layered catalyst Example No. component8.1 8.2 8.3 8.4 8.5composition g / in3), manganese (0.25 g / ft3), 5g / ft3palladium, 2.0 g / L S from soluble sulphate* * Based on substrate volume and calculated as elemental sulphur. A Comparative catalyst (Example No.8.0) consisted of a single layer of the upper layer of Example 6.2.Each aged and calcined substrate shown in Table 11 was tested on a bench-mounted engine in a laboratory test cell. The engine was a 4-cylinder turbo- charged light-duty diesel engine of 2.0 litre capacity calibrated to the Euro 6dFinal emission standard. The engine included both high- and low-pressureexhaust gas recirculation (EGR) loops and a catalysed filter substrate was installed downstream of the diesel oxidation catalyst substrate samples to be tested to ensure that the backpressure in the system was the same as in use and that the EGR loops remained free of soot / particulate matter. The exhaust system included demountable canning to enable diesel oxidation catalyst substrate samples to be switched between “runs” for evaluation. Sensors located upstream and downstream of the DOC substrate enabled the determination of %CO, %hydrocarbon and %formaldehyde conversion across the whole WLTC. Reported peak NOx removal by the catalyst during the WLTC is the maximum difference (or “delta”) between the detected NOx in exhaust gas entering the catalyst substrate and the detected NOx in exhaust gas exiting the catalyst substrate at any point in the WLTC. P102393 Each sample was run over a standard cycle as follows: the engine was warmed up over about 6 minutes to an exhaust gas inlet temperature to the DOC substrate of 275°C. A filter regeneration cycle of 10 minutes was then commenced to an exhaust gas temperature at the DOC substrate inlet of 525°C, followed by a “motored” engine cool-down using the engine dynamometer to an ambient “cold-start” temperature. The DOC substrate was then pre-conditioned (de-greened) from cold-start for 15 minutes at an inlet temperature to the DOC substrate of 400°C, followed by a further a “motored” engine cool-down using theengine dynamometer to an ambient “cold start” temperature. Finally, two “cold-start” Worldwide Harmonized Light Vehicles Test Cycle (WLTC) were conducted “back-to-back”, each separated by a “motored” engine cool-down using engine dynamometer to an ambient “cold start” temperature. The results presented inTable 12 below report the results of the second WLTC.Table 12 % % % Peak conversion conversion conversion Example NOx CO during HC during HCHO No. removal WLTC WLTC during (g) WLTC 8.0 88 82 82 0.08(reference) 8.1 95 88 97 0.908.2 96 89 98 0.668.3 95 87 98 0.318.4 85 79 99 0.068.5 89 87 95 0.12Table 12 confirms the laboratory SCAT test data that the addition of the ZrO2-based oxidation catalyst layer improves formaldehyde oxidation relative to thecatalyst of Example 8.0 (reference); and that non-sulphated samples and layeredarrangements having the layer orientation of Examples 8.1 to 8.3 inclusive have P102393improved CO oxidation over the catalyst consisting of the upper layer of Example6.2.It can also be seen from the results presented in Table 12 that the use of thezirconium nitrate and cerium nitrate in addition to the manganese nitrate contributes to the catalyst adsorbing significantly more NOx (compare peak NOx removal of Examples 8.1 and 8.2). Additionally, from a comparison of the peak NOx removal of Examples 8.2 and 8.3, it can be seen that the ZrO2-based mixed oxide in the lower washcoat layer contributes to NOxadsorption on the catalyst (Example 8.2 contains a loading of the ZrO2-based mixed oxide of 2.0 g / in3, whereas Example 8.3 contains a loading thereof of 1.0 g / in3). The composition of Examples 8.3 and 8.4 are identical, the only difference being that the orientation of the lower and upper layers is reversed in Example 8.4. However, it can be seen from the results that although the peak NOx removal in Example 8.4 has been substantially reduced. This is at the expense of a significant loss of CO and HC oxidation activity, as the Pt and Pd are less accessible (mass transfer limited) to the gas in the lower layer position and possibly also because some additional palladium is not able to migrate “upwardly” from the lower layer to the upper layer in Example 8.4. Finally, it can be seen from a comparison of Examples 8.2 and 8.5 that theaddition of sulphur to the lower ZrO2-based mixed oxide washcoat layer has theeffect of lowering the peak NOx removal with moderate impact to CO, HC and formaldehyde oxidation.EXAMPLE 9 – Further investigation of influence of sulphur addition on NOxadsorption in ZrO2-based catalyst layer To build on the applicant’s understanding of the effect of sulphating ZrO2-based component on NOx adsorption on the diesel oxidation catalyst, the applicant prepared a series of additional samples based on the sample of (non-sulphated) P102393Example 8.2 in which different levels of sulphur from soluble sulphate salt isadded to the washcoat of the ZrO2-based component. The catalyst composites were otherwise prepared similarly to Example 6.2, aged as described in Example7.1 and tested as described in Example 8 hereinabove. The results arepresented in Table 13 below. Samples 8.2 from Table 12 – and according to theinvention – is included in Table 13 as a reference Example for ease ofcomparison of the results of sulphated and un-sulphated Examples in the otherwise self-contained data set of Table 13. Additionally, Example 8.5 isincluded in Table 13 to better visualise any trends in the increasing addition ofsulphur from Example 9.1 to 9.3 and finally 8.5.Table 13 Example No. %% % Peak NOx conversion conversion conversion removal (g) CO during HC during HCHO WLTC WLTC during WLTC 8.0 (reference) 88 82 82 0.088.2 (reference, 96 89 98 0.66not sulphated) 9.1 (0.5 g / L S)* 96 85 97 0.669.2 (1.0 g / L S)* 95 88 97 0.399.3 (1.5 g / L S)* 93 88 96 0.248.5 (2.0 g / L S)* 89 87 95 0.129.4 (1.0 g / L S)† 91 87 95 0.11* From soluble sulphate salt.† From sulphuric acid (H2SO4).It can be seen from the results presented in Table 13 that, broadly, the addition ofincreasing quantities of sulphur from ammonium sulphate has little or no effect onhydrocarbon oxidation (in this respect the %HC conversion of Example 9.1 is anoutlier of the general trend). However, it can be seen that – generally – with P102393increasing sulphur content, peak NOx removal is beneficially reduced, but at aslight negative impact of reduced %CO oxidation and, to a lesser extent, formaldehyde oxidation. This compromise can be seen in particular in the comparison of Examples 9.2 and 9.4, where 1.0 g / L S derived from the use ofsulphuric acid as the sulphur reagent reduces peak NOx removal compared withthe equivalent sulphur loading derived from soluble sulphate salt, but at areduced %CO conversion. Therefore, if it is desired to suppress peak NOx removal, the catalyst developer should consider what level of reduced CO and formaldehyde oxidation is an acceptable compromise for this goal. The foregoing detailed description has been provided by way of explanation and illustration and is not intended to limit the scope of the appended claims. Many variations in the presently preferred embodiments illustrated herein will beapparent to one of ordinary skill in the art and remain within the scope of theappended claims and their equivalents. For the avoidance of any doubt, the entire content of any and all documents cited herein is incorporated by reference into the present application.

Claims

P102393 CLAIMS:

1. A diesel oxidation catalyst comprising one or more washcoat layersdisposed on a substrate, wherein at least one of the one or more washcoat layers comprises a doped zirconia-based support material which is a zirconia-based support material provided with two or more dopants, wherein the zirconia-based support material comprises, on an oxide basis: at least 80 wt.% Zr, based on the total weight of the zirconia-basedsupport material; at least 2 wt.% Nd, based on the total weight of the zirconia-based support material; and, optionally, one or more of Y, Ce, Hf and La, and wherein the two or more dopants comprise: from 5 to 15 wt.% Ce and from 5 to 30 wt.% Mn, on an oxide basis,based on the total weight of the doped zirconia-based support material.

2. The diesel oxidation catalysts according to claim 1, wherein the two ormore dopants comprise from 5 to 15 wt.% Mn, on an elemental basis, based onthe total weight of the doped zirconia-based support material.

3. The diesel oxidation catalyst according to claim 1 or claim 2, wherein thezirconia-based support material comprises from 5 to 15 wt.% Nd, on an oxidebasis, based on the total weight of the zirconia-based support material.

4. The diesel oxidation catalyst according to any preceding claim, whereinthe zirconia-based support material comprises, on an oxide basis, one or more of: from 1 to 7 wt.% Y; from 1 to 7 wt.% Ce; from 0.1 to 5 wt.% Hf;from 1 to 3 wt.% La, based on the total weight of the zirconia-based support material.P102393 5. The diesel oxidation catalyst according to any preceding claim, wherein the zirconia-based support material comprises, on an oxide basis, (A), (B) or (C), wherein (A) comprises from 1 to 2 wt.% La; from 12 to 14 wt.% Nd; and optionally from 0.1 to 2 wt.% Hf, based on the total weight of the zirconia-based support material; wherein (B) comprises from 1 to 2 wt.% La; from 4 to 6 wt.% Nd; from 4 to 6 wt.% Ce; and optionally from 0.1 to 2 wt.% Hf, based on the total weight of the zirconia-based support material; and wherein (C) comprises from 4 to 6 wt.% Nd; from 4 to 6 wt.% Ce; from 4 to 6 wt.% Y; and optionally from 0.1 to 2 wt.% Hf, based on the total weight of the zirconia-based support material.

6. The diesel oxidation catalyst according to any preceding claim, whereinthe zirconia-based support material consists of the listed oxides, together with any unavoidable impurities.

7. The diesel oxidation catalyst according to any preceding claim, whereinthe two or more dopants further comprise from 1 to 5 wt.% Zr, on an oxide basis,based on the total weight of the doped zirconia-based support material.

8. The diesel oxidation catalyst according to claim 7, wherein the two or moredopants consist of Ce, Mn and Zr, together with any unavoidable impurities.P1023939. The diesel oxidation catalyst according to any of claims 1 to 6, wherein thetwo or more dopants consist of Ce and Mn, together with any unavoidable impurities.

10. The diesel oxidation catalyst according to any preceding claim, whereinthe two or more dopants comprise about 10 wt.% Ce and / or about 10 wt.% Mn,on an oxide basis, based on the total weight of the doped zirconia-based support material.

11. The diesel oxidation catalyst according to any preceding claim, wherein atleast one of the one or more washcoat layers comprises platinum and / or palladium, preferably in a weight ratio of 3:1 to 1:1 and / or in a total loading offrom 0.2 to 5.0 wt.%, based on the total weight of the platinum- and / or palladium-containing washcoat layer and optionally further comprises a zeolite.

12. The diesel oxidation catalyst according to claim 11, wherein the platinumand / or palladium is supported on the doped zirconia-based support material, or wherein the platinum and / or palladium is supported on alumina.

13. The diesel oxidation catalyst according to claim 12, wherein the platinumand / or palladium supported on alumina is provided in a separate washcoat layer which at least partially overlies the washcoat layer comprising the doped zirconia- based support material.

14. A method for the manufacture of a diesel oxidation catalyst, the methodcomprising: (a) providing a substrate; (b) forming a washcoat composition; (c) applying the washcoat composition to the substrate and calcining it, wherein the washcoat composition comprises a doped zirconia-basedsupport material which is a zirconia-based support material provided with two ormore dopants and which is formed in the steps of:P102393 (i) providing a zirconia-based support material comprising, on an oxide basis: at least 80 wt.% Zr, based on the total weight of the zirconia-based support material; at least 2 wt.% Nd, based on the total weight of the zirconia- based support material; and, optionally, one or more of Y, Ce, Hf and La, and (ii) doping the zirconia-based support material with: from 5 to 15 wt.% Ce and from 5 to 30 wt.% Mn, on an oxidebasis, based on the total weight of the doped zirconia-based support material.

15. A method for the treatment of an exhaust gas from a diesel engine, themethod comprising passing a diesel exhaust gas through the diesel oxidationcatalyst according to any of claims 1 to 13 to obtain a treated diesel exhaust gas,preferably wherein the treated diesel exhaust gas comprises less than 1 ppmformaldehyde.