Diesel oxidation catalyst article and method of manufacture thereof

Abstract

A diesel oxidation catalyst article comprises: a substrate; a first washcoat layer disposed on the substrate; and a second washcoat layer disposed on the substrate; wherein the first washcoat layer comprises: a first platinum group metal (PGM) component comprising Pt and / or Pd; and a support material comprising alumina; and wherein the second washcoat layer comprises: a second PGM component comprising Pd and, optionally, Pt; and a Mn-doped zirconia-based support material, which is a zirconia-based support material having a crystalline phase comprising at least 60 wt.% total of one or more cubic and / or tetragonal phases and further provided with at least one dopant, the at least one dopant comprising Mn; wherein the zirconia-based support material comprises, on an oxide basis: at least 50 wt.% Zr, based on the total weight of the zirconia-based support material; and one or more of La, Y, Nd, Ce, Pr and Hf.

Description

[0001] DIESEL OXIDATION CATALYST ARTICLE AND METHOD OF MANUFACTURE THEREOF

[0002] The present invention relates to a diesel oxidation catalyst (DOC) article, its method of manufacture 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, and a particular zirconiabased support material.

[0003] 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 are as durable as possible to meet emission standards over the main and additional lifetime of a vehicle.

[0004] 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.

[0005] Oxidation catalysts for compression ignition internal combustion engines typically contain one or more platinum group metals. The specific platinum group metal(s) 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 to platinum but has greater thermal durability.

[0006] 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.

[0007] 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).

[0008] 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.

[0009] The present invention provides a diesel oxidation catalyst article, an exhaust gas treatment system, a fuel combustion and exhaust gas treatment system, a method for the manufacture of a diesel oxidation catalyst article and a method for the treatment of an exhaust gas from a diesel engine according to the claims appended hereto.

[0010] Specifically, in a first aspect the present invention provides a diesel oxidation catalyst article comprising: a substrate; a first washcoat layer disposed on the substrate; and a second washcoat layer disposed on the substrate; wherein the first washcoat layer comprises: a first platinum group metal (PGM) component comprising Pt and / or Pd; and a support material comprising alumina; and wherein the second washcoat layer comprises: a second PGM component comprising Pd and, optionally, Pt; and a Mn-doped zirconia-based support material, which is a zirconiabased support material having a crystalline phase comprising at least 60 wt.% total of one or more cubic and / or tetragonal phases and further provided with at least one dopant, the at least one dopant comprising Mn; wherein the zirconia-based support material comprises, on an oxide basis: at least 50 wt.% Zr, based on the total weight of the zirconiabased support material; and one or more of La, Y, Nd, Ce, Pr and Hf.

[0011] 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.

[0012] The inventors have surprisingly found that when a Zr-based support material comprises additional elements such that it comprises a cubic and / or tetragonal phase, in particular a tetragonal phase (pure ZrO2 is monoclinic), the activity of the catalyst to formaldehyde oxidation, along with CO and HC oxidation, is improved. The phase of a doped zirconia has not previously been linked to a change in oxidation activity when used in a DOC. As such, in combination with the Pt- and / or Pd-based DOC layer (i.e. the first washcoat layer described herein, which may comprise a DOC), the catalyst article described herein is able to provide excellent CO, HC and formaldehyde conversion performance in use as a DOC. In this way, advantageously, by virtue of the increased DOC activity provided by the second washcoat layer described herein, it should be possible to reduce the total loading of PGM in the DOC article, reducing manufacturing costs.

[0013] Without wishing to be bound by theory, it is thought that each of the additional elements La, Y, Nd, Ce, Pr and Hf may assist in “pushing” the zirconia-based support material into a cubic and / or a tetragonal phase, in particular a tetragonal phase. It has been seen that some of the elements may be particularly effective at “pushing” the zirconia-based support material into a cubic and / or tetragonal phase, in particular a tetragonal phase, even at relatively low weight percentages of addition. The use of Nd in this regard is particularly preferred.

[0014] The DOC article is preferably a non-aged DOC article. The term “non-aged” as used herein in the context of a non-aged diesel oxidation catalyst article may encompass that the DOC article is provided in “fresh” form. In other words, the DOC article may not have yet been exposed to relatively high-temperature conditions, such as those experienced in an exhaust gas treatment system in use, such as up to about 700°C as may be seen following a filter regeneration event after, for example, about 500 to 1 ,500 km vehicular driving and / or a selective catalytic reduction (SCR) catalyst desulphation event. This is because, without wishing to be bound by theory, while it is thought that the crystalline phase of the zirconia-based support material may not change during the washcoating / manufacturing process, including drying and calcination, it is thought that the crystalline phase of the zirconia-based support material may change over time on exposure to relatively high-temperature ageing. In other words, it is thought that the non-aged phase may not be constant in use after being exposed to aging conditions. If the DOC article has been in fluid communication with a diesel engine, then the DOC article has preferably been in fluid communication with a diesel engine of a vehicle that has travelled less than 1 ,500 km, preferably less than 1000 km, more preferably less than 500 km, even more preferably 0 km. As will be appreciated, while the DOC article may continue to perform well in use after continued aging and beyond such vehicular distances, for example, the phase of the zirconia-based support material of the DOC article as defined herein may preferably be defined as in its “fresh” state. Accordingly, the non-aged DOC article may be a “fresh” DOC article.

[0015] However, the inventors have surprisingly found that, to achieve the advantages described herein, what may be important is that the invention is defined by the initial or non-aged crystalline phase of the zirconia-support material, i.e. prior to any high-temperature ageing, such as exposure to temperatures of up to about 700°C as may be seen following a filter regeneration event after, for example, about 500 to 1 ,500 km vehicular driving and / or a selective catalytic reduction (SCR) catalyst desulphation event. Accordingly, the present invention may provide a non-high-temperature-aged diesel oxidation catalyst article as described herein. Of course, the catalyst article may later become aged, through use. In other words, what is believed to be important is that the DOC article starts its life in its “fresh” state and in which the zirconia-based support material has the defined crystalline phase described herein.

[0016] Preferably, the crystalline phase of the zirconia-based support material is therefore the crystalline phase of the zirconia-based support material determined when the catalyst is fresh (or prior to exposure to relatively high-temperature conditions).

[0017] For the avoidance of doubt, it will be appreciated that the relative proportions of the components of the zirconia-based support material are defined based on the total weight of the zirconia-based support material (which in the manufacture of the Mn-doped zirconia-based support material is typically provided as a preprepared mixed oxide starting material, which is then doped with the required at least one dopant comprising Mn), whereas the relative proportions of the at least one dopant comprising Mn as defined herein are defined based on the total weight of the Mn-doped zirconia-based support material. A person skilled in the field of the present invention would be able to determine which components of the Mn-doped zirconia-based support material originated in the starting mixed oxide, such as one formed by a coprecipitation method, for example, and which components of the Mn-doped zirconia-based support material were subsequently doped into the zirconia-based support material, using known analytical techniques. Mixed (metal) oxides as defined herein comprise solid solutions, i.e. a homogeneous mixture of two different kinds of atoms in solid state and having a single crystal structure, and composite metal oxides.

[0018] In other words, the at least one dopant comprising Mn may be referred to as a “surface dopant” for example. By contrast, the one or more of La, Y, Nd, Ce, Pr and Hf may be present within the lattice structure of the zirconia-based support material, for example.

[0019] The zirconia-based support material has a crystalline phase comprising at least 60 wt.% total of one or more cubic and / or tetragonal phases. Without wishing to be bound by theory, it is thought that the DOC article described herein may exhibit the surprising effects described herein, at least to some extent, when the crystalline phase of the zirconia-based support material is majority (such as at least 60 wt.% total) cubic and / or tetragonal. The best examples have been seen for the tetragonal phase. However, due to the similar crystal structure and without wishing to be bound by theory, it is thought that the cubic phase may perform similarly. Moreover, where a zirconia-based support material has a crystalline phase that is majority tetragonal, it may be possible that at least a portion of the crystalline phase comprises a cubic phase. Accordingly, preferably, the zirconiabased support material has a crystalline phase comprising at least 60 wt.% total of one or more tetragonal phases.

[0020] Preferably, the zirconia-based support material has a crystalline phase comprising at least 70 wt.% total of one or more (cubic and / or) tetragonal phases, more preferably at least 80 wt.% total of one or more (cubic and / or) tetragonal phases, even more preferably at least 90 wt.% total of one or more (cubic and / or) tetragonal phases, even more preferably at least 95 wt.% total of one or more (cubic and / or) tetragonal phases, still more preferably at least 98 wt.% total of one or more (cubic and / or) tetragonal phases, still more preferably at least 99 wt.% total of one or more (cubic and / or) tetragonal phases, and most preferably the zirconia-based support material has a crystalline phase consisting of a (cubic and / or) tetragonal phase. Where the crystalline phase of the zirconia-based support material comprises up to 10 wt.% of a phase other than a cubic and / or tetragonal phase, for example, the phase may typically be a monoclinic phase. As such, in some embodiments, the crystalline phase of the zirconia-based support material consists of a (cubic and / or) tetragonal phase and a monoclinic phase. Preferably, the crystalline phase of the zirconia-based support material comprises less than 10 wt.% of a monoclinic phase, preferably 5 wt.% or less of a monoclinic phase. Preferably, the crystalline phase of the zirconia-based support material comprises less than 1 wt.%, preferably 0 wt.%, of a monoclinic phase. Preferably, the crystalline phase of the zirconia-based support material comprises less than 1 wt.%, preferably 0 wt.%, of a pyrochlore phase.

[0021] Preferably, the zirconia-based support material is at least 90 wt.% crystalline, preferably at least 95 wt.% crystalline, more preferably at least 99 wt.% crystalline, and most preferably, the zirconia-based support material is crystalline. In practice, it may be unavoidable that a small portion of the zirconia-based support material may be amorphous.

[0022] For the avoidance of doubt, the phase of the zirconia-based support material is the phase of the zirconia-based support material prior to doping with the at least one dopant comprising Mn.

[0023] The crystalline phase can be determined by X-Ray Diffraction analysis with reference to standard spectra or neutron diffraction, either including optional Rietveld refinement; and Raman spectroscopy (see C.G. Kontoyannis et al., Journal of Materials Science, volume 29, pages 5316-5320, (1994)), for example.

[0024] The La, Y, Nd, Ce and Pr may typically be intentionally introduced into the zirconia-based support material. Hf may also be intentionally introduced into the zirconia-based support material. However, as would be appreciated by the skilled reader, 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. For most light-duty diesel applications, the first washcoat layer preferably further comprises a zeolite. However, for heavy-duty diesel applications, the first washcoat layer may typically not comprise a zeolite. If present, the zeolite preferably comprises a beta zeolite. The first 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.

[0025] The first washcoat layer preferably further comprises a promoter, such as strontium or barium, preferably barium. The first washcoat 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.

[0026] Preferably, the first PGM component comprises a Pt:Pd ratio by weight of from 1 :0 to 1 :1 , preferably from 6:1 to 2:1 , more preferably from 5:1 to 2:1 . As will be appreciated, a Pt:Pd ratio of 1 :0 means that the first PGM component does not comprise Pd. In some embodiments, therefore, the first PGM component consists of Pt.

[0027] Preferably, the support material comprising alumina of the first washcoat layer comprises an alumina-based support material comprising, on an oxide basis, at least 85 wt.% alumina, preferably at least 90 wt.% alumina. The support material comprising alumina of the first washcoat layer comprises may comprise (gamma) alumina or a doped (gamma) alumina, such as a (gamma) alumina doped with Si and / or La. The (gamma) 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 (gamma) alumina. The first washcoat layer 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.

[0028] Preferably, the first washcoat layer at least partially overlies the second washcoat layer or the second washcoat layer at least partially overlies the first washcoat layer. When the second washcoat layer at least partially overlies the first washcoat layer, surprisingly, it has been observed that the NOx absorption of the DOC article is significantly reduced compared to the other way around, which may be advantageous for accuracy of on-board diagnostics, engine calibration and operation etc., for example. However, the CO and HC oxidation performance may be reduced in this orientation, though this can be offset to some extent by the inclusion of the Pd in the BMO layer (second washcoat layer) as described herein (i.e. the second PGM component).

[0029] In other words, in such embodiments the more-typical DOC layer (first washcoat layer) is preferably a lower washcoat layer of the DOC article and the BMO layer (second washcoat layer) is preferably an upper layer of the DOC article.

[0030] This arrangement (wherein the second washcoat layer at least partially overlies the first washcoat layer) 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 article in the particular emission treatment system in which the DOC is intended to be used.

[0031] For the purposes of designing on-board diagnostic (OBD) and on-board monitoring (OBM) systems to determine the working function of catalytic components in a vehicle exhaust system, some vehicle manufacturers are requesting that diesel oxidation catalysts adsorb as little oxides of nitrogen (NOx) as possible in order not to interfere with sensing and algorithm designs for using OBD and OBM protocols. Accordingly, Examples are included herein in which Applicant researched into ways of suppressing NOx absorption on the claimed composite diesel oxidation catalysts.

[0032] Alternatively, in some embodiments, the first washcoat layer at least partially overlies the second washcoat layer. Such an arrangement may be preferred when, in practice, on balance, HC oxidation and exotherm generation performance are preferred over reduced NOx absorption. Preferably, the DOC article consists of the substrate, the first washcoat layer and the second washcoat layer. In other words, the DOC article preferably comprises no further washcoat layers beyond the first washcoat layer and the second washcoat layer, although the first washcoat layer may itself be sub-divided into two or more zones of similar composition except for a difference in PGM concentration and / or platinum to palladium weight ratio. For example, in one embodiment, the second washcoat layer extends for 100% of an axial length of a flow-through monolith substrate and the first washcoat layer overlying the second washcoat layer is subdivided into a first inlet zone extending 64% of the substrate axial length from the inlet end and comprising platinum and palladium at 3:1 weight ratio, barium acetate at 100 g / ft3, 1 .5 g / in35wt% silica-doped alumina and 0.5 g / in3beta zeolite; and a second outlet zone extending 50% of the axial length of the substrate from the outlet end and comprising an outlet of platinum (only), 1.7 g / in3of 5wt% silica-doped alumina and 100 g / ft3manganese nitrate, wherein the outlet zone washcoat layer overlaps the inlet zone washcoat layer.

[0033] The first washcoat layer and / or the second washcoat 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.

[0034] In one preferred embodiment: the first washcoat layer at least partially overlies the second washcoat layer; the second washcoat layer preferably extends for at least 90%, preferably 100% of the axial length of the substrate; and the first washcoat layer:

[0035] (a) extends for at least 90%, preferably 100% of the axial length of the substrate; or

[0036] (b) extends from an inlet end of the substrate for from 30 to 70% of the axial length of the substrate, preferably wherein the first PGM component comprises a Pt: Pd ratio by weight of from 5:1 to 2: 1 ; and the DOC article 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 least partially overlying the second washcoat layer, wherein the third washcoat layer comprises Pt and, optionally, Pd, and Mn. The third washcoat layer may preferably further comprise an alumina-based support material and / or a zeolite (preferably being as described herein in relation to the first washcoat layer). The third washcoat layer preferably at least partially overlies the first washcoat layer. In some preferred embodiments, the first washcoat 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.

[0037] Preferably, the second 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, i.e. a ratio by weight of (1 or less):2 (i.e. , a ratio by weight of 1 :(2 or greater)). The amount of Pt may be 0 wt.%. For the avoidance of doubt, the diesel oxidation catalyst of the present invention is not intended for use as a three-way catalyst for treating gasoline engine exhaust gas. Accordingly, the diesel oxidation catalyst of the invention does not comprise the platinum group metal rhodium.

[0038] 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.

[0039] Preferably, the second washcoat layer comprises from 1 to 40 g / ft3of the second PGM component, preferably from 1 to 35 g / ft3(such as from 5 to 35 g / ft3) of the second PGM component, more preferably from 1 to 30 g / ft3(such as from 10 to 30 g / ft3) of the second PGM component, even more preferably from 5 to 25 g / ft3(such as from 15 to 25 g / ft3) of the second 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 article. Accordingly, in some embodiments the second washcoat layer comprises from 5 to 10 g / ft3of the second PGM component, which may provide acceptable performance. In particular, such lower loadings of the second PGM component may suitably balance catalytic performance and the general aim of reducing PGM loadings to minimise overall product cost.

[0040] In some embodiments, in particular where the first washcoat layer at least partially overlies the second washcoat layer, the second PGM component may be present in the second washcoat layer by virtue of “wicking” from the first washcoat layer during manufacture of the DOC article. As such, the second washcoat layer may comprise a concentration gradient of the second PGM component, typically with concentration of the second PGM component decreasing in a direction perpendicular to and in a direction towards the substrate (i.e. away from the first washcoat 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 MicroAnalysis (FE-EPMA). However, preferably, the second PGM component is intentionally introduced into the second washcoat layer, i.e. the second PGM component is added directly into the second washcoat layer during manufacture of the DOC article.

[0041] Preferably, the Mn-doped zirconia-based support material is further provided with at least one further dopant which comprises Ce and / or Zr. If present, the Ce and Mn-doped zirconia-based support material preferably comprises from 5 to 15 wt.% of the Ce dopant, preferably from 7 to 13 wt.% of the Ce dopant, more preferably from 8 to 12 wt.% of the Ce dopant, on an elemental basis, based on the total weight of the Ce and Mn-doped zirconia-based support material. If present, the Zr and Mn-doped zirconia-based support material preferably comprises from 1 to 5 wt.% of the Zr dopant, preferably from 2 to 4 wt.% of the Zr dopant, on an elemental basis, based on the total weight of the Zr and Mn-doped zirconia-based support material.

[0042] In some embodiments, the at least one dopant comprising Mn consists of Ce, Mn and Zr, together with any unavoidable impurities. In some embodiments, the at least one dopant comprising Mn consists of Ce and Mn, together with any unavoidable impurities. Preferably, the Ce and Mn-doped zirconia-based support material comprises about 10 wt.% Ce and / or about 10 wt.% Mn, on an elemental basis, based on the total weight of the Ce and Mn-doped zirconia-based support material. Without wishing to be bound by theory, it is thought that such additional dopants, in particular in such proportions of the dopants, and especially the Ce, may provide improved formaldehyde and CO abatement performance.

[0043] As noted above, a person skilled in the field of the present invention would be able to determine which components of the Mn-doped zirconia-based support material originated in the starting mixed oxide and which components of the Mn- doped zirconia-based support material were subsequently doped into the zirconia-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, for example.

[0044] In some alternative embodiments, the Mn-doped zirconia-based support material is not further doped with Ce or Zr, preferably in an embodiment in which the second washcoat layer at least partially overlies the first washcoat layer. In other words, in this embodiment, the Mn-doped zirconia-based support material is not further provided with at least one further dopant which comprises Ce and / or Zr. The presence of the Ce and Zr dopants has also been observed to increase the NOx storage of the DOC article. Thus, when low NOx storage is desired, this may be a preferred configuration. Preferably, the second washcoat layer further comprises sulphur, preferably from 0.5 to 3 g / L sulphur, based on the total volume of the substrate. Surprisingly, the further addition of S, typically provided as a (soluble) sulphate salt, such as manganese sulphate, zirconium sulphate, cerium sulphate and / or ammonium sulphate, has also been seen to reduce NOXstorage on the DOC article.

[0045] However, 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.

[0046] As an alternative - or in addition to - the addition of sulphur to suppress NOx storage, Applicant’s inventors also investigated surface doping the zirconia-based support material of the second washcoat layer with the acidic dopant tungsten in addition to the surface Mn dopant. A range of elemental tungsten loadings from 0.1 to 2.0 wt% in addition to a 10wt% Mn dopant loading on a zirconia-based support material were tested (results not exemplified). Tungsten addition was found to reduce NOx storage, but harmed formaldehyde oxidation activity by increasing the formaldehyde Tso light-off temperature. The tungsten loading with formaldehyde oxidation activity closest to the reference Mn-doped zirconia-based support material was 0.1 wt% (elemental) tungsten, but 0.2 wt% tungsten offered a compromise between retained formaldehyde oxidation performance and lower NOx storage, which tungsten loading would be preferred where NOx storage or absorption of the DOC is intended, e.g. for a 10wt% elemental manganese doped zirconia-based support material. Accordingly, where tungsten is used as a NOx storage or absorption suppressing dopant, elemental tungsten dopant loadings can be from 0.05 to 0.3 wt.%.

[0047] One advantage of the use of tungsten as a NOx storage or absorption suppressing dopant is that is can be adopted in second washcoat layer in the preferred orientation in which the first washcoat layer is coated on top of the second washcoat layer. Alternative acidic dopants to tungsten tested were silicon, molybdenum and niobium at 0.1 wt% loadings. Si and Nb helped maintain formaldehyde oxidation activity, but Mo was detrimental to the formaldehyde oxidation. In terms of the NOx storage, Si, and Nb did not offer any particular benefit, whereas Mo led to a decrease in NOx storage. Applicant’s inventors concluded that there appears to be a correlation between NOx storage sites on the one hand and formaldehyde storage sites on the other.

[0048] Preferably, the zirconia-based support material comprises, on an oxide basis, at least 5 wt.% of one or more of La, Y, Nd, Ce, Pr and Hf, based on the total weight of the zirconia-based support material, preferably at least 7 wt.%, even more preferably at least 8 wt.%.

[0049] Preferably, other than oxygen, the zirconia-based support material consists of Zr and one or more of La, Y, Nd, Ce, Pr and Hf, together with any unavoidable impurities, preferably Zr and one or more of La, Y, Nd, Ce and Hf, together with any unavoidable impurities.

[0050] In some embodiments, the zirconia-based support material may consist essentially of Zr and one or more of La, Y, Nd, Ce, Pr and Hf, together with any unavoidable impurities. As will be appreciated, the term “consists essentially of” or “consisting essentially of” as used herein may encompass that further components other than those listed may be present, provided that they do not materially affect the essential characteristics of the species or composition.

[0051] Preferably, when the zirconia-based support material comprises Y, the zirconiabased support material also comprises one or more of La, Nd, Ce, Pr and Hf. In other words, in one preferred embodiment, the zirconia-based support material comprises Y and one or more of La, Nd, Ce, Pr and Hf. In other words, the zirconia-based support material preferably does not consist of Zr and Y as the only additional element. 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 article 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 may be most prominently observed where the Y is used as a further dopant in the zirconia-based support material described herein in addition to another element, as opposed to a sole dopant of a zirconia-based support material. However, in some cases, the use of Y alone may provide acceptable performance.

[0052] Preferably, the zirconia-based support material comprises, on an oxide basis, at least 75 wt.% Zr, preferably at least 80 wt.% Zr (such as at least 81 wt.%, 82 wt.%, 83 wt.% or 84 wt.%), more preferably at least 85 wt.% Zr, based on the total weight of the zirconia-based support material.

[0053] Preferably, the zirconia-based support material comprises Nd and, optionally, one or more of La, Y, Ce, Pr and Hf. The Examples comprising Nd have been seen to provide some of the best catalytic performance as described herein.

[0054] Preferably, the zirconia-based support material comprises, on an oxide basis, at least 2 wt.% Nd, preferably at least 4 wt.% Nd, more preferably at least 5 wt.% Nd, based on the total weight of the zirconia-based support material. Preferably, 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. If the zirconia-based support material comprises, on an oxide basis, less than 5 wt.% Nd, based on the total weight of the zirconia-based support material, then preferably the zirconia-based support material further comprises one or more of La, Y, Ce, Pr and Hf.

[0055] Preferably, the zirconia-based support material comprises, on an oxide basis, one or more of: from 2 to 15 wt.% Nd; from 1 to 15 wt.% Y, preferably from 5 to 10 wt.% Y; from 1 to 45 wt.% Ce, preferably from 5 to 10 wt.% Ce; from 0.1 to 5 wt.% Hf, preferably from 0.1 to 2 wt.% Hf; from 1 to 12 wt.% La, preferably from 1 to 3 wt.% La; from 1 to 10 wt.% Pr, preferably from 5 to 9 wt.% Pr, based on the total weight of the zirconia-based support material.

[0056] 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.

[0057] 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.

[0058] In 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.

[0059] Preferably, the zirconia-based support material consists of the listed oxides (i.e. Zr and one or more of La, Y, Nd, Ce, Pr and Hf), together with any unavoidable impurities. In this embodiment, the wt.% balance will be Zr, together with any unavoidable impurities.

[0060] In some aspects, the present invention therefore provides a diesel oxidation catalyst article comprising: a substrate; a first washcoat layer disposed on the substrate; and a second washcoat layer disposed on the substrate; wherein the first washcoat layer comprises: a first platinum group metal (PGM) component comprising Pt and / or Pd; and a support material comprising alumina; and wherein the second washcoat layer comprises: a second PGM component comprising Pd and, optionally, Pt; and a Mn-doped zirconia-based support material, which is a zirconiabased support material further provided with at least one dopant, the at least one dopant comprising Mn; wherein the zirconia-based support material consists of, on an oxide basis and based on the total weight of the zirconia-based support material, (A), (B) and / or (C):

[0061] (A) from 1 to 2 wt.% La; from 12 to 14 wt.% Nd; and optionally from 0.1 to 2 wt.% Hf, and the balance Zr together with any unavoidable impurities;

[0062] (B) 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, and the balance Zr together with any unavoidable impurities;

[0063] (C) 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, and the balance Zr together with any unavoidable impurities. Preferably, the Mn-doped zirconia-based support material comprises from 4 to 30 wt.% of the Mn dopant, preferably from 5 to 15 wt.% of the Mn dopant, more preferably from 6 to 13 wt.% of the Mn dopant, on an elemental basis, based on the total weight of the Mn-doped zirconia-based support material. Such preferred amounts of the Mn dopant may improve the formaldehyde oxidation performance of the catalyst article.

[0064] Preferably, the second washcoat layer 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.

[0065] A manganese loading study vs. formaldehyde oxidation activity study is reported in Example 6 hereinbelow. Applicant’s inventors used a number of techniques to ascertain why a preferred range of 5 to 15 wt.% elemental Mn was active.

[0066] Firstly, in analysis not discussed in Example 6, X-Ray absorption spectroscopy (XAS) indicated that the active manganese oxide species in a 10wt% Mn sample had an oxidation state of Mn3+and this oxidation state appeared stable to lean hydrothermal ageing. However, at lower manganese loadings, manganese speciation became more oxidic, i.e. closer to Mn2+, which appears to correlate with reducing formaldehyde oxidation activity. Sulfation also appeared to cause a reduction of Mn3+towards Mn2+speciation, possibly as MnSC . Hence, higher loadings of manganese may help protect the more active overall Mn3+speciation as the catalyst becomes sulphated when in use.

[0067] However, Example 6 shows that formaldehyde oxidation is also impacted at higher manganese loadings, such as 25wt% Mn. Without wishing to be bound by theory, Applicant believes that the reaction mechanism for formaldehyde oxidation may include the binding of formaldehyde with hydroxide species present on the surface of the zirconia-based support. Oxidation of surface-bound formaldehyde is then promoted by adjacent “islands” of surface-supported manganese oxide species by a redox reaction. [Surface OH content of a sample can be determined by a method based on hydrogen-deuterium exchange and probed using1H NMR (see C. Penrose et al, ChemComm 2021 , 57, 12804)]. Where excess manganese is present, as in a 25wt% manganese-loaded sample, Applicant’s inventors believe that the “islands” of manganese oxide species are too large resulting in obscuration of surface hydroxide species, reducing the density of hydroxide sites for the formaldehyde to bind. This impacts reaction kinetics. That is, for desirable formaldehyde oxidation activity, there appears to be a balance between an appropriate concentration of hydroxide species to bind formaldehyde on the one hand and access to a sulphur tolerant active quantity of redox-active manganese oxide species to effect an oxidation of the hydroxide- bound formaldehyde on the other. A balance of these apparently competing parameters appears to be in the loading range of 5 to 15 wt.% manganese (as elemental manganese). There is some indication also that at higher manganese loadings, the manganese speciation includes some Mn4+character, rather than the more active Mn3+speciation.

[0068] This theory is supported by a correlation between detected hydroxide concentration (mmolOH / g) and specific surface area of the sample; and is also consistent with a detected loss of specific surface area (SSA) of the zirconiabased supported manganese oxide species composite (SSA determined using N2 absorption and BET analysis) for the 25wt% Mn sample compared with the 10wt% Mn sample, i.e. increasing manganese wt.% results in a corresponding reduction in SSA of the zirconia-based support.

[0069] Preferably, the second washcoat layer comprises from 1 .0 to 3.5 g / in3of the zirconia-based support material, preferably from 1 .2 to 3.0 g / in3of the zirconiabased support material, more preferably from 1 .5 to 2.5 g / in3of the zirconiabased support material, based on the total volume of the substrate. Without wishing to be bound by theory, it is thought that the zirconia-based support material itself may also contribute to high NOx storage of the catalyst article, which can be undesirable for some purpose as discussed herein. Accordingly, in some embodiments a relatively low loading of the zirconia-based support material may be desired. Preferably, at least some of the second PGM component is supported on the Mn- doped zirconia-based support material.

[0070] Preferably, the first washcoat layer and / or the second washcoat layer further comprises a binder. Suitable binders are known to the skilled person. The binder preferably comprises an alumina species, such as alumina, optionally derived from the addition of boehmite or an alumina sol to the washcoat. Preferably, the first washcoat layer and / or the second washcoat layer (in particular the second washcoat layer) comprises from 0.01 to 3 g / in3, more preferably from 0.02 to 2 g / 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.

[0071] Preferably, the substrate comprises a flow-through monolith substrate, sometimes referred to as a honeycomb substrate. Preferably, the substrate comprises cordierite. However, the composition of the substrate is not particularly limited. Typically, the substrate comprises or is a monolith substrate.

[0072] Alternatively, the substrate can comprise a wall flow filter substrate.

[0073] In a further aspect, the present invention provides an exhaust gas treatment system comprising the diesel oxidation catalyst article as described herein, preferably further comprising a further DOC article located downstream of the diesel oxidation catalyst article. The further DOC article may also be according to the invention and may be identical to the DOC article. In other words, the exhaust gas treatment system preferably comprises the DOC article described herein and a further, identical, but distinct, DOC article located downstream thereof. In one embodiment, the first washcoat layer at least partially overlaps the second washcoat layer in both of the DOC article and the further DOC article. In one embodiment, the second washcoat layer at least partially overlaps the first washcoat layer in the DOC article and the first washcoat layer at least partially overlaps the second washcoat layer in the further DOC article. Alternatively, the further DOC article located downstream of the diesel oxidation catalyst article may comprise a washcoat layer comprising a PGM component comprising Pt and / or Pd, a support material comprising alumina and, optionally, a zeolite. The composition of the washcoat layer may, independently, correspond to the preferred embodiments of the first washcoat layer described herein. The further DOC article may comprise such a washcoat layer as the only washcoat layer disposed thereon. Alternatively, the further DOC article may comprise a “BMO layer” (which may, independently, correspond to the (preferred) embodiments of the second washcoat 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 article may convert most of the CO and formaldehyde present in the exhaust gas, leaving the downstream DOC article to efficiently convert remaining HCs. Preferably, the further DOC article located downstream of the diesel oxidation catalyst article comprises at least 70 wt.%, preferably at least 90 wt.%, more preferably at least 95 wt.% of the total PGMs present in the DOC article and the further DOC article, based on the total weight of the PGMs in both the DOC article and the further DOC article.

[0074] The preferred embodiments and advantages of the first aspect apply equally this aspect.

[0075] 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.

[0076] 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 manufacture of a diesel oxidation catalyst article, the method comprising:

[0077] (a) providing a substrate;

[0078] (b) forming a first washcoat composition;

[0079] (c) forming a second washcoat composition;

[0080] (d) applying the first and second washcoat compositions to the substrate and calcining it, wherein the first washcoat composition comprises: a first PGM component comprising Pt and / or Pd; and a support material comprising alumina; and wherein the second washcoat composition comprises: a second PGM component comprising Pd and, optionally, Pt; and a Mn-doped zirconia-based support material, which is a zirconiabased support material having a crystalline phase comprising at least 60 wt.% total of one or more cubic and / or tetragonal phases and further provided with at least one dopant, the at least one dopant comprising Mn, and which is formed in the steps of:

[0081] (i) providing a zirconia-based support material comprising, on an oxide basis: at least 50 wt.% Zr, based on the total weight of the zirconiabased support material; and one or more of La, Y, Nd, Ce, Pr and Hf, and

[0082] (ii) doping the zirconia-based support material with: at least one dopant comprising Mn.

[0083] The preferred embodiments and advantages of the first aspect apply equally this aspect.

[0084] Preferably, the method comprises applying the first washcoat composition to the substrate prior to applying the second washcoat composition to the substrate such that the second washcoat composition at least partially overlies the first washcoat composition. 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.

[0085] Applying the washcoat composition(s) to the substrate may be carried out using techniques known in the art. Typically, the washcoat composition comprising suitable rheology modifiers may be poured onto an upper surface of the substrate using a specific moulding tool in a predetermined amount and a vacuum applied to the lower end of the substrate, thereby coating the washcoat composition along the channels of 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 the application step (d). When the substrate is a wallflow filter, the washcoat composition may be coated on the filter walls, within the filter walls (if porous) or both.

[0086] Calcining the substrate having the washcoat(s) applied thereto preferably comprises heating the substrate having the washcoat applied thereto at a temperature of 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. Calcining the substrate may occur once after both first and second washcoat compositions are applied to the substrate or may occur after each washcoat composition is applied. In the case where calcining the substrate occurs once after both first and second washcoat compositions are applied, this step may comprise allowing Pd to wick from the first washcoat composition into the second washcoat composition. The term “wick into” as used herein may encompass the migration, such as by diffusion, of the Pd in the (aqueous) washcoat compositions.

[0087] The method may comprise one or more further steps of forming one or more further washcoat layers before or after step (d). For example, preferably step (d) involves applying the first and second washcoat layers to the substrate, and the method further comprises forming one or more further washcoat layers after step (d). The one or more further steps of forming one or more further washcoat layers before or after (preferably after) step (d) may comprise applying a further washcoat composition to the substrate.

[0088] Preferably, step (ii) of doping the zirconia-based support material comprises joint (or co-) precipitation, flame spray hydrolysis, incipient wetness or wet impregnation.

[0089] In a further aspect, the present invention provides a method for the treatment of an exhaust gas from a diesel engine, the method comprising passing a diesel exhaust gas through the diesel oxidation catalyst article as described herein to obtain a treated diesel exhaust gas, preferably wherein the treated diesel exhaust gas comprises less than 1 ppm formaldehyde.

[0090] The preferred embodiments and advantages of the above aspects apply equally this aspect.

[0091] It should be noted that, as will be appreciated by those familiar with the conventions 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) zirconiabased 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 ZrC ; for Nd, the corresponding oxide is Nd20s; for Y, the corresponding oxide is Y2O3; for Ce, the corresponding oxide is CeC ; for Hf, the corresponding oxide is HfC ; for La, the corresponding oxide is La2Os; for Pr, the corresponding oxide is P^Os; and for Mn, the corresponding oxide is Mn02.

[0092] In other words, the first aspect of the present invention therefore provides a diesel oxidation catalyst article comprising: a substrate; a first washcoat layer disposed on the substrate; and a second washcoat layer disposed on the substrate; wherein the first washcoat layer comprises: a first platinum group metal (PGM) component comprising Pt and / or Pd; and a support material comprising alumina; and wherein the second washcoat layer comprises: a second PGM component comprising Pd and, optionally, Pt; and a Mn-doped zirconia-based support material, which is a zirconiabased support material having a crystalline phase comprising at least 60 wt.% total of one or more cubic and / or tetragonal phases and further provided with at least one dopant, the at least one dopant comprising Mn; wherein the zirconia-based support material comprises, on an oxide basis: at least 50 wt.% Zr (when calculated as ZrC ), based on the total weight of the zirconia-based support material; and one or more of La, Y, Nd, Ce, Pr and Hf.

[0093] In one embodiment, the second washcoat layer may extend from an inlet end of the substrate and the first washcoat layer may extend from an outlet end of the substrate, wherein the (entire) second washcoat layer is upstream of the (entire) first washcoat layer. Similarly, in an alternative aspect, provided is a diesel exhaust gas treatment system comprising: a first substrate; a second substrate; a first washcoat layer disposed on the first substrate; and a second washcoat layer disposed on the second substrate; wherein the first washcoat layer comprises: a first platinum group metal (PGM) component comprising Pt and / or Pd; and a support material comprising alumina; and wherein the second washcoat layer comprises: a second PGM component comprising Pd and, optionally, Pt; and a Mn-doped zirconia-based support material, which is a zirconiabased support material having a crystalline phase comprising at least 60 wt.% total of one or more cubic and / or tetragonal phases and further provided with at least one dopant, the at least one dopant comprising Mn; wherein the zirconia-based support material comprises, on an oxide basis: at least 50 wt.% Zr, based on the total weight of the zirconiabased support material; and one or more of La, Y, Nd, Ce, Pr and Hf.

[0094] Preferably, the second substrate is located upstream of the first substrate. The preferred embodiments and advantages of the above aspects apply equally this aspect. With this arrangement, without wishing to be bound by theory, it is thought that, in use, the upstream DOC article may convert most of the CO and formaldehyde present in the exhaust gas, leaving the downstream DOC article to efficiently convert remaining HCs. Preferably, the downstream DOC article comprises at least 70 wt.%, preferably at least 90 wt.%, more preferably at least 95 wt.% of the total PGMs present in the upstream DOC article and the downstream DOC article, based on the total weight of the PGMs in both the upstream DOC article and the downstream DOC article.

[0095] The term “substrate” as used herein may encompass, for example, a ceramic or metallic honeycomb or flow-through monolith, or a filter e.g. a wall flow filter. 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.

[0096] The term “disposed on” in the context of the present invention may encompass both having said washcoat layer directly disposed on the substrate, i.e. with no intervening material, and / or indirectly disposed on the substrate, i.e. with intervening material. If the substrate is porous, then the term “disposed on” may also encompass having said washcoat layer disposed therein, for example within the pores of the substrate, i.e. wherein the washcoat layer is disposed thereon and / 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.

[0097] 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.

[0098] As will be appreciated, the zirconia-based support material is “zirconia-based” at least because it comprises, on an oxide basis, at least 50 wt.% Zr, based on the total weight of the zirconia-based support material.

[0099] The term “doped” in the context of the “at least one dopant comprising Mn” of the invention as used herein may encompass that the dopant(s) is / are introduced into a pre-formed material and are not part of the original mixed oxide starting material.

[0100] The term “supported on” in the context of “PGM component is supported on the Mn-doped zirconia-based support material”, for example, as used herein may encompass that the relevant 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.

[0101] The invention can also be defined according to one or more of the following clauses: 1 . A diesel oxidation catalyst article comprising: a substrate; a first washcoat layer disposed on the substrate; and a second washcoat layer disposed on the substrate; wherein the first washcoat layer comprises: a first platinum group metal (PGM) component comprising Pt and / or Pd; and a support material comprising alumina; and wherein the second washcoat layer comprises: a second PGM component comprising Pd and, optionally, Pt; and a Mn-doped zirconia-based support material, which is a zirconiabased support material having a crystalline phase comprising at least 60 wt.% total of one or more cubic and / or tetragonal phases and further provided with at least one dopant, the at least one dopant comprising Mn; wherein the zirconia-based support material comprises, on an oxide basis: at least 50 wt.% Zr, based on the total weight of the zirconiabased support material; and one or more of La, Y, Nd, Ce, Pr and Hf.

[0102] 2. The diesel oxidation catalyst article of clause 1 , wherein the zirconiabased support material has a crystalline phase comprising at least 60 wt.% total of one or more tetragonal phases.

[0103] 3. The diesel oxidation catalyst article of clause 1 or clause 2, wherein the first washcoat layer further comprises a zeolite.

[0104] 4. The diesel oxidation catalyst article of any preceding clause, wherein the first PGM component comprises a Pt:Pd ratio by weight of from 1 :0 to 1 :1 , preferably from 6:1 to 2:1.

[0105] 5. The diesel oxidation catalyst article of any preceding clause, wherein the support material comprising alumina of the first washcoat layer comprises an alumina-based support material comprising, on an oxide basis, at least 85 wt.% alumina, preferably at least 90 wt.% alumina.

[0106] 6. The diesel oxidation catalyst article of any preceding clause, wherein the first washcoat layer at least partially overlies the second washcoat layer or the second washcoat layer at least partially overlies the first washcoat layer.

[0107] 7. The diesel oxidation catalyst article of any preceding clause, wherein the second 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.

[0108] 8. The diesel oxidation catalyst article of any preceding clause, wherein the second PGM component consists of Pd.

[0109] 9. The diesel oxidation catalyst article of any preceding clause, wherein the second washcoat layer comprises from 1 to 40 g / ft3of the second PGM component, preferably from 5 to 35 g / ft3of the second PGM component, based on the total volume of the substrate.

[0110] 10. The diesel oxidation catalyst article of any preceding clause, wherein the Mn-doped zirconia-based support material is further provided with at least one further dopant which comprises Ce and / or Zr.

[0111] 11 . The diesel oxidation catalyst article of any of clauses 1 to 9, wherein the Mn-doped zirconia-based support material is not doped with Ce or Zr, preferably wherein the second washcoat layer at least partially overlies the first washcoat layer.

[0112] 12. The diesel oxidation catalyst article of any preceding clause, wherein the second washcoat layer further comprises sulphur, preferably from 0.5 to 3 g / L sulphur, based on the total volume of the substrate. 13. The diesel oxidation catalyst article of any preceding clause, wherein the zirconia-based support material comprises, on an oxide basis, at least 5 wt.% of one or more of La, Y, Nd, Ce, Pr and Hf, based on the total weight of the zirconiabased support material, preferably at least 7 wt.%, even more preferably at least 8 wt.%.

[0113] 14. The diesel oxidation catalyst article of any preceding clause, wherein, other than oxygen, the zirconia-based support material consists of Zr and one or more of La, Y, Nd, Ce, Pr and Hf, together with any unavoidable impurities, preferably Zr and one or more of La, Y, Nd, Ce and Hf, together with any unavoidable impurities.

[0114] 15. The diesel oxidation catalyst article of any preceding clause, wherein, when the zirconia-based support material comprises Y, the zirconia-based support material also comprises one or more of La, Nd, Ce, Pr and Hf.

[0115] 16. The diesel oxidation catalyst article of any preceding clause, wherein the zirconia-based support material comprises, on an oxide basis, at least 75 wt.% Zr, preferably at least 80 wt.% Zr, more preferably at least 85 wt.% Zr, based on the total weight of the zirconia-based support material.

[0116] 17. The diesel oxidation catalyst article of any preceding clause, wherein the zirconia-based support material comprises, on an oxide basis, at least 2 wt.% Nd, preferably at least 4 wt.% Nd, more preferably at least 5 wt.% Nd, based on the total weight of the zirconia-based support material.

[0117] 18. The diesel oxidation catalyst article of any preceding clause, 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.

[0118] 19. The diesel oxidation catalyst article of any preceding clause, wherein the zirconia-based support material comprises, on an oxide basis, one or more of: from 2 to 15 wt.% Nd; from 1 to 15 wt.% Y, preferably from 5 to 10 wt.% Y; from 1 to 45 wt.% Ce, preferably from 5 to 10 wt.% Ce; from 0.1 to 5 wt.% Hf, preferably from 0.1 to 2 wt.% Hf; from 1 to 12 wt.% La, preferably from 1 to 3 wt.% La; from 1 to 10 wt.% Pr, preferably from 5 to 9 wt.% Pr, based on the total weight of the zirconia-based support material.

[0119] 20. The diesel oxidation catalyst article of any preceding clause, 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.

[0120] 21 . The diesel oxidation catalyst article of any of clauses 1 to 19, 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.

[0121] 22. The diesel oxidation catalyst article of any of clauses 1 to 19, 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.

[0122] 23. The diesel oxidation catalyst article of any preceding clause, wherein the zirconia-based support material consists of the listed oxides, together with any unavoidable impurities. 24. The diesel oxidation catalyst article of any preceding clause, wherein the Mn-doped zirconia-based support material comprises from 4 to 30 wt.% of the Mn dopant, preferably from 5 to 15 wt.% of the Mn dopant, more preferably from 6 to 13 wt.% of the Mn dopant, on an elemental basis, based on the total weight of the Mn-doped zirconia-based support material.

[0123] 25. The diesel oxidation catalyst of any preceding clause, wherein the second washcoat layer 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.

[0124] 26. The diesel oxidation catalyst article of any preceding clause, wherein the second washcoat layer comprises from 1.0 to 3.5 g / in3of the zirconia-based support material, preferably from 1 .2 to 3.0 g / in3of the zirconia-based support material, more preferably from 1 .5 to 2.5 g / in3of the zirconia-based support material, based on the total volume of the substrate.

[0125] 27. The diesel oxidation catalyst article of any preceding clause, wherein at least some of the second PGM component is supported on the Mn-doped zirconia-based support material.

[0126] 28. An exhaust gas treatment system comprising the diesel oxidation catalyst article of any preceding clause, preferably further comprising a further DOC article located downstream of the diesel oxidation catalyst article.

[0127] 29. A fuel combustion and exhaust gas treatment system comprising a diesel engine in fluid communication with the exhaust gas treatment system of clause 28.

[0128] 30. A method for the manufacture of a diesel oxidation catalyst article, the method comprising:

[0129] (a) providing a substrate;

[0130] (b) forming a first washcoat composition; (c) forming a second washcoat composition;

[0131] (d) applying the first and second washcoat compositions to the substrate and calcining it, wherein the first washcoat composition comprises: a first PGM component comprising Pt and / or Pd; and a support material comprising alumina; and wherein the second washcoat composition comprises: a second PGM component comprising Pd and, optionally, Pt; and a Mn-doped zirconia-based support material, which is a zirconiabased support material having a crystalline phase comprising at least 90 wt.% total of one or more cubic and / or tetragonal phases and further provided with at least one dopant, the at least one dopant comprising Mn, and which is formed in the steps of:

[0132] (i) providing a zirconia-based support material comprising, on an oxide basis: at least 50 wt.% Zr, based on the total weight of the zirconiabased support material; and one or more of La, Y, Nd, Ce, Pr and Hf, and

[0133] (ii) doping the zirconia-based support material with: at least one dopant comprising Mn.

[0134] 31 . A method for the treatment of an exhaust gas from a diesel engine, the method comprising passing a diesel exhaust gas through the diesel oxidation catalyst article according to any of clauses 1 to 27 to obtain a treated diesel exhaust gas, preferably wherein the treated diesel exhaust gas comprises less than 1 ppm formaldehyde.

[0135] The invention will now be described in relation to the following non-limiting examples.

[0136] EXAMPLES EXAMPLE 1

[0137] EXAMPLE 1.1 - ZrO2-based single oxidation catalyst layer (no PGM)

[0138] 2.0 g of a mixed oxide powder comprising 83.4 wt.% ZrO2, 13.4 wt.% Nd20s and 1 .6 wt.% La20s (the quoted content of ZrO2 includes small impurity amounts of 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 and 12.1 wt.% manganese and 3.4 wt.% zirconium, on an oxide basis, based on the total weight of the washcoat.

[0139] EXAMPLE 1 .2 - Two-layer com oxidation

[0140] To the coated substrate of Example 1 .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 2 - Effect on oxidation activity of adding palladium to lower ZrC - based layer of two-layer composite oxidation catalyst

[0141] Based on the basic two-layer composite oxidation catalyst design of Example 1 .2, a quantity of platinum group metal was introduced as aqueous palladium and / or platinum salt into the slurry of the ZrO2 lower layer corresponding to Example 1.1 was varied, as set out in Table 2 below.

[0142] EXAMPLE 2.1 - Laboratory activity testing for CO, HC and NO2 oxidation

[0143] Core 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 catalysts of Example 1 .2 but including the PGM content in the Example 1.1 layer described in Table 2. 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.

[0144] 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 1 . 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% O2 and N2 balance at 55,000 hr1space velocity followed by switching the gas mixture to the gas mixture set out in Table 1 below and conducting a “fast ramp” from 80°C to 400°C at a temperature ramp 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% O2 and N2 balance at 55,000 hr1space velocity before two successive ramp tests were conducted.

[0145] Each ramp test was done by switching the exhaust gas mixture to the mixture set out in Table 1 and conducting a temperature ramp from 80°C to 400°C at 20°C per 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.

[0146] Table 1

[0147] The hydrocarbon (HC) component was a mixture of four individual hydrocarbons: methane (CH4), the shorter chain unsaturated hydrocarbon propene (CsHe), 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.

[0148] 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 Tso, i.e. the temperature whereby 80% HC conversion is achieved. The HC Tso demonstrates the oxidation activity for propene, whereas the HC Tso demonstrates 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 2.

[0149] EXAMPLE 2.2 - Activity of Example 1.1 single layer but also now containing PGM to oxidise formaldehyde (HCHO)

[0150] Separately, the PGM-containing ZrO2-based oxidation catalyst layers set out in Table 2 otherwise having the composition of Example 1.1 were applied as a single layer only - as Example 1.1 - to separate flow-through monolith substrates. 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 1.1 but also including the described guantity of PGM shown in Table 2, and not on a powder sample of a mixture two- layer composite catalyst corresponding to Example 1 .2.

[0151] The powders recovered from the flow-through substrates were pressed and pelletised with the sieve fraction, from 355 pm to 250 pm, so the final pellet size was 250 pm. 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%, O2 10%, NO 250 ppm, N2 Balance, at 2.2 litres of gas per minute. Formaldehyde oxidation activity was determined from 80°C to 450°C at a ramp rate of 10°C / minute and the results are also presented in Table 2. Table 2

[0152] Single layer of Example 1.1 only, no PGM

[0153] It can be seen from the results shown in Table 2 that increasing the concentration of palladium up to 20 g / ft3in the lower washcoat layer improved CO oxidation, but too much palladium, i.e. 50 g / ft3, had a negative impact on CO oxidation.

[0154] 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.

[0155] Table 2 also confirms that a trend in oxidation activity of the short chain hydrocarbon (propene) follows that of CO oxidation, including the 20Pd 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 Tso HC where Pd is exchanged for Pt.

[0156] 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.

[0157] EXAMPLE 3 - Influence of zirconia phase in Base Metal Oxidation catalyst activity

[0158] A series of commercially available ZrO2-based mixed oxide powders were obtained having the compositions set out in Table 3. XRD analysis of the powders as received from the raw material supplier was conducted as follows: XRD were collected with 0.02 step size between 5-130 ° 29 on a Broker D8 Advance Davinci design diffractometer with Lynxeye-XE Detector. Analysis was performed using Broker AXS Diffrac Eva with PDF-5+ database and fitting was performed using Bruker-AXS TOPAS 5 to gain information such as ratio of crystalline phase, crystallite size (LVol-IB method) and lattice parameters using either full structural or Pawley models.

[0159] Table 3

[0160] Key: M = Monoclinic; T = Tetragonal; and A = Amorphous. Each mixed oxide powder from Table 3 was made up into a washcoat slurry similarly to Example 1.1 , including aqueous manganese nitrate, cerium nitrate and zirconium nitrate, unless otherwise indicated in Table 4 below. Each resulting washcoat was applied to two separate flow-through monolith substrates. The coated parts were dried at 110°C and then calcined to 500°C for 2 hours. The resulting catalyst is described herein as a “fresh” catalyst. The coating adhered on one substrate was recovered and collected as a powder for SCAT analysis. The powders were each tested for formaldehyde oxidation according to the methodology described in Example 2.2 hereinabove. The results are shown in Table 4 below.

[0161] The other substrate coated with the “fresh” ZrO2-based mixed oxide component of Table 3 was then coated with an upper washcoat layer to form a two-layer composite oxidation catalyst similar to Example 1 .2. The resulting two-layer composite oxidation catalyst was dried and calcined as described in Example 1 .2 and hydrothermally aged in an oven at 750°C for 16 hours in an oven box, as described above. Cores cut from the aged substrates were tested by SCAT according to the methodology described in Example 2.1 hereinabove and the results are set out in Table 4 below. Table 4

[0162] * Comparative Example 3.0 is the formulation of Example 1.1. t Manganese nitrate added only; not cerium nitrate or zirconium nitrate.

[0163] It can be seen from the results presented in Table 4 that active formaldehyde oxidation is linked to a tetragonal phase of the ZrO2 component. It would also appear to be the case that the tetragonal phase is driven by the presence of dopants, such as Nd, La, Ce and / or Y.

[0164] Additionally, it can be seen that the tetragonal ZrO2 phase is also active for CO, propene (Tso HC) and formaldehyde oxidation.

[0165] EXAMPLE 4 - Influence of additive and layer orientation on NOx adsorption in dual layer catalyst; and initial investigation of influence of sulphur on NOx adsorption in ZrO2-based catalyst layer

[0166] A series of catalysed flow-through monolith substrates similar to the two-layer composite oxidation catalyst Example 1 .2 were prepared, except that no cerium nitrate or zirconium nitrate were included in the washcoat with the mixed oxide powder described in Example 1.1 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 2.1 . Compared with the samples of Example 1 .2, the loading of the mixed oxide layer of Example 1.1 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 1 .2 were compared with a catalyst according to Example 1 .2 in which no palladium was added to the mixed oxide powder lower layer. Details of all catalysts tested is set out in Table 5. Table 5

[0167] A Comparative catalyst (Example No. 4.0) consisted of a single layer of the upper layer of Example 1 .2.

[0168] Each aged and calcined substrate shown in Table 5 was tested on a benchmounted engine in a laboratory test cell. The engine was a 4-cylinder turbocharged light-duty diesel engine of 2.0 litre capacity calibrated to the Euro 6d Final emission standard. The engine included both high- and low-pressure exhaust 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.

[0169] 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 the engine dynamometer to an ambient “cold start” temperature. Finally, two “coldstart” 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 in Table 6 below report the results of the second WLTC.

[0170] Table 6 Table 6 confirms the laboratory SCAT test data that the addition of the ZrO2- based oxidation catalyst layer improves formaldehyde oxidation relative to the catalyst of Example 4.0 (reference); and that non-sulphated samples and layered arrangements having the layer orientation of Examples 4.1 to 4.3 inclusive have improved CO oxidation over the catalyst consisting of the upper layer of Example 1.2.

[0171] It can also be seen from the results presented in Table 6 that the use of the zirconium nitrate and cerium nitrate in addition to the manganese nitrate contributes to the catalyst adsorbing significantly more NOx (compare peak NOx removal of Examples 4.1 and 4.2). Additionally, from a comparison of the peak NOx removal of Examples 4.2 and 4.3, it can be seen that the ZrO2-based mixed oxide in the lower washcoat layer contributes to NOXadsorption on the catalyst (Example 4.2 contains a loading of the ZrO2-based mixed oxide of 2.0 g / in3, whereas Example 4.3 contains a loading thereof of 1.0 g / in3).

[0172] The composition of Examples 4.3 and 4.4 are identical, the only difference being that the orientation of the lower and upper layers is reversed in Example 4.4. However, it can be seen from the results that although the peak NOx removal in Example 4.4 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 4.4.

[0173] Finally, it can be seen from a comparison of Examples 4.2 and 4.5 that the addition of sulphur to the lower ZrO2-based mixed oxide washcoat layer has the effect of lowering the peak NOx removal with moderate impact to CO, HC and formaldehyde oxidation. EXAMPLE 5 - Further investigation of influence of sulphur addition on NOx adsorption in ZrC -based catalyst layer

[0174] 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) Example 4.2 in which different levels of sulphur from soluble sulphate salt is added to the washcoat of the ZrO2-based component. As “soluble sulphate salts”, please list, e.g. manganese sulphate, zirconium sulphate, cerium sulphate and ammonium sulphate. The catalyst composites were otherwise prepared similarly to Example 1.2, aged as described in Example 2.1 and tested as described in Example 4 hereinabove. The results are presented in Table 7 below. Samples 4.2 from Table 6 - and according to the invention - is included in Table 7 as a reference Example for ease of comparison of the results of sulphated and un-sulphated Examples in the otherwise self-contained data set of Table 7. Additionally, Example 4.5 is included in Table 7 to better visualise any trends in the increasing addition of sulphur from Example 5.1 to 5.3 and finally 4.5.

[0175] Table 7

[0176] * From soluble sulphate salt. t From sulphuric acid (H2SO4).

[0177] It can be seen from the results presented in Table 7 that, broadly, the addition of increasing quantities of sulphur from ammonium sulphate has little or no effect on hydrocarbon oxidation (in this respect the %HC conversion of Example 5.1 is an outlier of the general trend). However, it can be seen that - generally - with increasing sulphur content, peak NOx removal is beneficially reduced, but at a slight 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 5.2 and 5.4, where 1 .0 g / L S derived from the use of sulphuric acid as the sulphur reagent reduces peak NOx removal compared with the equivalent sulphur loading derived from soluble sulphate salt, but at a reduced %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.

[0178] EXAMPLE 6 - further investigations into activity of various manganese loadings

[0179] Although 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.% Nd20s and 1 .6 wt.% La20s (the quoted content of ZrO2 includes small impurity amounts of Hf) and alumina binder were 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 containing 1wt%, 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 an oxide basis.

[0180] 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.

[0181] 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 1. 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% O2 and N2 balance at 55,000 hr1space velocity followed by switching the gas mixture to the gas mixture set out in Table 1 and conducting a “fast ramp” from 80°C to 400°C at a temperature ramp 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% O2 and N2 balance at 55,000 hr1space velocity before two successive ramp tests were conducted.

[0182] Each ramp test was done by switching the exhaust gas mixture to the mixture set out in Table 1 and conducting a temperature ramp from 80°C to 400°C at 20°C per 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.

[0183] The hydrocarbon (HC) component was a mixture of four individual hydrocarbons: methane (CH4), the shorter chain unsaturated hydrocarbon propene (CsHe), 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.

[0184] 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 %NO2 of 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 the second, i.e. the repeat “run”, for CO and NO oxidation are reported in Table 8.

[0185] Separately, dried and calcined (i.e. unaged) adhered washcoat was removed from each flow-through substrate and collected as a powder. The recovered powders were pressed and pelletised with the sieve fraction, from 355 pm to 250 pm, so the final pellet size was 250 pm. 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%, O2 10%, NO 250 ppm, N2 Balance, at 2.2 litres of gas per minute. Formaldehyde oxidation activity was determined from 80°C to 450°C at a ramp rate of 10°C / minute and the results are also presented in Table 8.

[0186] 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 SO2 and 10% H2O at 2 litres gas volume per minute for 1 hour. These conditions - referred to as lean hydrothermal sulphation - were selected for equivalence to a loading of 1 g / litre of elemental sulphur as if the pelleted powder had been applied as a washcoat to a substrate of that corresponding volume. 0.4 g of the resulting sulphated pellets were also tested for formaldehyde oxidation activity. The results of formaldehyde oxidation for the sulphated pellets are also set out in Table 8.

[0187] Table 8

[0188] It can be seen from the results presented in Table 8 that where no sulphur was present, 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.

[0189] In a separate analysis, to a powder of the same mixed oxide was added cerium nitrate and manganese nitrate (no zirconium nitrate) by the incipient wetness technique (see Ronald. M. Heck et al., “Catalytic Air Pollution Control - Commercial Technology”, 3rdEdition, John Wiley & Sons, Inc. (2009) at Chapter 2.3.1.1 ) in amounts to obtain 10wt% cerium (no zirconium) and either 10wt% manganese or 25wt% manganese. That is, powders were prepared instead of washcoats. The resulting powders were dried at 100°C for 1 hour and calcined in a static oven for 2 hours at 500°C. The calcined powders were then formed into pellets as described above and aged as described in Example 2.2 hereinabove. 2.5 grams of the pelleted catalyst was lean hydrothermally sulphated as described hereinabove, except in that the period of sulphation was 24 hours instead of 1 hour. These conditions were selected for equivalence to a loading of 4g / litre of elemental sulphur as if the pelleted powder had been applied as a washcoat to a substrate of that corresponding volume (a higher loading of sulphur was used to differentiate activity between the two more highly manganese loaded samples). 0.4 g of the resulting sulphated pellets were tested for formaldehyde oxidation activity as described in Example 2.2. The results of formaldehyde oxidation are set out in Table 9 and are presented as the catalyst light-off temperature, i.e. the temperature at which 50% of the formaldehyde is converted, instead of the percentage of formaldehyde converted at 140°C. The lower the light-off temperature, the better the catalytic oxidation activity. Table 9

[0190] It can be seen from Table 9 that the sample more highly loaded with manganese (25wt%) shows poorer sulphated formaldehyde oxidation activity than the 10wt% manganese-loaded sample.

[0191] 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 be apparent to one of ordinary skill in the art and remain within the scope of the appended claims and their equivalents.

[0192] 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

CLAIMS:1 . A diesel oxidation catalyst article comprising: a substrate; a first washcoat layer disposed on the substrate; and a second washcoat layer disposed on the substrate; wherein the first washcoat layer comprises: a first platinum group metal (PGM) component comprising Pt and / or Pd; and a support material comprising alumina; and wherein the second washcoat layer comprises: a second PGM component comprising Pd and, optionally, Pt; and a Mn-doped zirconia-based support material, which is a zirconiabased support material having a crystalline phase comprising at least 60 wt.% total of one or more cubic and / or tetragonal phases and further provided with at least one dopant, the at least one dopant comprising Mn; wherein the zirconia-based support material comprises, on an oxide basis: at least 50 wt.% Zr, based on the total weight of the zirconiabased support material; and one or more of La, Y, Nd, Ce, Pr and Hf.

2. The diesel oxidation catalyst article of claim 1 , wherein the zirconia-based support material has a crystalline phase comprising at least 60 wt.% total of one or more tetragonal phases.

3. The diesel oxidation catalyst article of claim 1 or claim 2, wherein the first washcoat layer further comprises a zeolite.

4. The diesel oxidation catalyst article of any preceding claim, wherein the first PGM component comprises a Pt:Pd ratio by weight of from 1 :0 to 1 :1 , preferably from 6:1 to 2:1.

5. The diesel oxidation catalyst article of any preceding claim, wherein the first washcoat layer at least partially overlies the second washcoat layer or the second washcoat layer at least partially overlies the first washcoat layer.

6. The diesel oxidation catalyst article of any preceding claim, wherein the second 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.

7. The diesel oxidation catalyst article of any preceding claim, wherein the second PGM component consists of Pd.

8. The diesel oxidation catalyst article of any preceding claim, wherein the second washcoat layer comprises from 1 to 40 g / ft3of the second PGM component, preferably from 5 to 35 g / ft3of the second PGM component, based on the total volume of the substrate.

9. The diesel oxidation catalyst article of any preceding claim, wherein the zirconia-based support material comprises, on an oxide basis, at least 5 wt.% of one or more of La, Y, Nd, Ce, Pr and Hf, based on the total weight of the zirconiabased support material, preferably at least 7 wt.%, even more preferably at least 8 wt.%.

10. The diesel oxidation catalyst of any preceding claim, wherein the second washcoat layer 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.11 . The diesel oxidation catalyst article of any preceding claim, wherein the second washcoat layer comprises from 1.0 to 3.5 g / in3of the zirconia-based support material, preferably from 1 .2 to 3.0 g / in3of the zirconia-based support material, more preferably from 1 .5 to 2.5 g / in3of the zirconia-based support material, based on the total volume of the substrate.

12. An exhaust gas treatment system comprising the diesel oxidation catalyst article of any preceding claim, preferably further comprising a further DOC article located downstream of the diesel oxidation catalyst article.

13. A fuel combustion and exhaust gas treatment system comprising a diesel engine in fluid communication with the exhaust gas treatment system of claim 12.

14. A method for the manufacture of a diesel oxidation catalyst article, the method comprising:(a) providing a substrate;(b) forming a first washcoat composition;(c) forming a second washcoat composition;(d) applying the first and second washcoat compositions to the substrate and calcining it, wherein the first washcoat composition comprises: a first PGM component comprising Pt and / or Pd; and a support material comprising alumina; and wherein the second washcoat composition comprises: a second PGM component comprising Pd and, optionally, Pt; and a Mn-doped zirconia-based support material, which is a zirconiabased support material having a crystalline phase comprising at least 90 wt.% total of one or more cubic and / or tetragonal phases and further provided with at least one dopant, the at least one dopant comprising Mn, and which is formed in the steps of:(i) providing a zirconia-based support material comprising, on an oxide basis: at least 50 wt.% Zr, based on the total weight of the zirconiabased support material; and one or more of La, Y, Nd, Ce, Pr and Hf, and(ii) doping the zirconia-based support material with: at least one dopant comprising Mn.

15. A method for the treatment of an exhaust gas from a diesel engine, the method comprising passing a diesel exhaust gas through the diesel oxidation catalyst article according to any of claims 1 to 11 to obtain a treated diesel exhaust gas, preferably wherein the treated diesel exhaust gas comprises less than 1 ppm formaldehyde.

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