Diesel oxidation catalyst and method for producing the same

Abstract

A method for manufacturing a diesel oxidation catalyst comprises: (i) providing a carrier substrate; (ii) forming on the carrier substrate one or more platinum group metal-containing washcoat layers each containing a refractory metal oxide carrier material to provide a first coated substrate; (iii) subjecting the first coated substrate to a first heat treatment comprising heating the first coated substrate to a first maximum temperature and holding the first coated substrate at the first maximum temperature to form a heat-treated coated substrate; (iv) depositing a platinum group metal-containing composition containing a refractory metal oxide carrier material on at least a portion of the heat-treated coated substrate to form a second coated substrate; and (v) subjecting the second coated substrate to a second heat treatment comprising heating the second coated substrate to a second maximum temperature and holding the second coated substrate at the second maximum temperature to form a diesel oxidation catalyst, wherein the first maximum temperature is at least 600 °C and the second maximum temperature is at least 25 °C lower than the first maximum temperature.

Description

Technical Field

[0001] The present invention relates to an improved diesel oxidation catalyst (DOC), and more particularly to a method for manufacturing a DOC. The manufacturing method provides a DOC having a stable oxidation performance from NO to NO2 without impairing the CO / HC oxidation performance and / or the heat generation ability.

[0002] In internal combustion engines, exhaust gases containing various pollutants, including hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides ("NOx"), are generated. Emission control systems including exhaust gas catalytic converters are widely used to reduce the amounts of these pollutants emitted into the atmosphere. In the case of compression ignition (i.e., diesel) engines, the most commonly used catalytic converter is the diesel oxidation catalyst (DOC). The DOC typically contains palladium and / or platinum, generally supported on alumina. This catalyst converts particulate matter (PM), hydrocarbons, and carbon monoxide to carbon dioxide and water.

[0003] In modern exhaust systems, the DOC is used during normal operation to control the emissions of these CO and HC. The role of the DOC in the passive oxidation of HC, CO, and NOx present in the exhaust gas stream appears throughout the operation of the engine, and is optimized for the DOC operating window of about 250 - 300°C. The DOC can also be used to promote the conversion of NO to NO2 for downstream passive filter regeneration (combustion of particulate matter retained on the filter in NO2 in the exhaust gas at an exhaust gas temperature lower than that in O2 in the exhaust gas, i.e., the so-called CRT (registered trademark) effect).

[0004] In addition, the DOC can be used as a heat - generating catalyst. This is done by injecting hydrocarbon fuel into the exhaust gas. To avoid doubts, the fuel injection / heat - generating event does not occur during normal operation. Normal operation is considered to be the period between fuel injection / heat - generating events. The second role of heat generation can serve one of several purposes. For example, when an unacceptable increase in back - pressure is detected, heat can be generated to burn soot on the downstream filter. Another example is for the regeneration of the SCR catalyst, such as by removing sulfur from the downstream CuCHA SCR catalyst.

[0005] To cause these heat generations, an amount of hydrocarbon (HC) is injected upstream of the DOC (about 2000 ppm). As long as the DOC is hot enough, the added HC results in heat generation, heats the exhaust gas, and as a result, heats its downstream components (up to a temperature of about 500 °C). If the DOC is not hot enough, it is necessary to provide hotter exhaust from the engine through engine management, which involves impacts on energy and performance.

[0006] Therefore, it is desirable to provide a DOC with a low heat - generating temperature. The lower this temperature is, the higher the likelihood that the engine is already operating above the heat - generating temperature when heat is required, and / or the less energy that needs to be added to reach the suitable operating temperature.

[0007] It is known that the performance characteristics of a catalyst article can change over the life of the catalyst article. Some of this performance can be restored by a regeneration process, but some of the performance is simply lost, such as by sintering of the platinum group metal (PGM) components. This can pose a difficulty when attempting to provide a well-regulated exhaust system that can operate optimally over the length of its service life. In some instances, this performance delta can result in difficulties such as the fact that it may actually be desirable to pre-age the components prior to use in order to reach a point where the in-service change in performance observed by the end user is minimized within the lifetime performance. That is, the exhaust system manufacturer may choose to sacrifice some of the as-new activity in order to ensure more consistent performance over the life of the part.

[0008] U.S. Patent No. 8,679,434 (B1) discloses a method for preparing thermally stabilized powders. Specifically, this disclosure provides a honeycomb substrate having a washcoat disposed thereon that contains one or more calcined platinum group metal components dispersed on a refractory metal oxide support disposed on the honeycomb substrate, wherein the platinum group metal components have an average crystallite size in the range of about 10 to about 25 nm in order to provide a stable ratio of NO2 to NOx as exhaust gas flows through the honeycomb substrate. These powders, which are aged to reduce activity and minimize performance changes during in-service aging, are then washcoated onto the substrate to form a catalyst article. In this case, the performance of the catalyst article during use is more stable.

[0009] U.S. Patent Application Publication No. 2016 / 0236178 (A1) discloses the preparation of a chemically reduced PGM material that can be heat-treated to obtain a preferred PGM size for NO oxidation. Specifically, this disclosure provides a method for preparing a catalyst composition for obtaining a stable ratio of NO2 to NO in the exhaust system of a compression ignition engine, which is described. This method includes: (i) preparing a first composition containing a platinum (Pt) compound disposed or supported on a carrier material; (ii) preparing a second composition by reducing the platinum (Pt) compound to platinum (Pt) using a reducing agent; and (iii) heating the second composition to at least 650 °C.

[0010] The object of the present invention is to provide an improved manufacturing method for DOC, to address the problems associated with the prior art, and / or to at least provide an alternative to the commercially viable prior art.

[0011] According to a first aspect, the present invention is a method for manufacturing a diesel oxidation catalyst, comprising: (i) providing a carrier substrate; (ii) forming one or more platinum group metal-containing washcoat layers each containing a refractory metal oxide carrier material on the carrier substrate to provide a first coated substrate; (iii) subjecting the first coated substrate to a first heat treatment including heating the first coated substrate to a first maximum temperature and holding the first coated substrate at the first maximum temperature to form a heat-treated coated substrate; (iv) depositing a platinum group metal-containing composition containing a refractory metal oxide carrier material on at least a portion of the heat-treated coated substrate to form a second coated substrate; (v) subjecting the second coated substrate to a second heat treatment including heating the second coated substrate to a second maximum temperature and holding the second coated substrate at the second maximum temperature to form a diesel oxidation catalyst. Provided is a method that includes a first maximum temperature of at least 600° C. and a second maximum temperature that is at least 25° C. lower than the first maximum temperature.

[0012] Here, the present disclosure is further described. In the following sections, different aspects / embodiments of the present disclosure are defined in more detail. Each aspect / embodiment so defined can be combined with any other aspect / embodiment or aspects / embodiments unless otherwise explicitly indicated. In particular, any feature indicated as being preferred or advantageous can be combined with any other feature or features indicated as being preferred or advantageous.

[0013] In relation to DOC, it is known from aging tests that the heat treatment of the finished catalyst can be used to weaken the catalyst activity at the time of new product. This has the effect of reducing the change between the performance at the time of new product and the performance after aging. This is particularly advantageous for systems containing SCRF components where the low temperature activity can be sensitive to the NO / NO2 ratio. However, performing heat treatment on the finished catalyst also weakens the treatment of CO and HC, as well as the exothermic activity of the catalyst, which is undesirable.

[0014] The inventors have now found that by performing heat treatment at an intermediate stage, it is possible to selectively stabilize the NO oxidation activity and then apply a further coating that provides most of the CO / HC characteristics and exothermic characteristics at the time of new product. This additional coating is desirably applied to the inlet section because this is the part where the CO / HC characteristics and exothermic characteristics are most needed. Since NO oxidation typically occurs on at least the rear section of the catalyst, this section should be applied before the stabilization heat treatment is performed. Heat treatment at a temperature above 600° C. is required to stabilize the NO activity.

[0015] More particularly, the present invention relates to a method for manufacturing a diesel oxidation catalyst (DOC). The catalyst is generally in the form of a DOC article. A catalyst article means a single component for an exhaust gas treatment system. These may also be referred to as "bricks".

[0016] The method includes providing a carrier substrate. This is a surface onto which a catalyst layer is subsequently applied and supported.

[0017] Preferably, the substrate is a flow-through monolith. A flow-through monolith substrate has a first face and a second face, and defines a longitudinal direction therebetween. The flow-through monolith substrate has a plurality of channels extending between the first face and the second face. The plurality of channels extend in the longitudinal direction and provide a plurality of inner surfaces (e.g., the surfaces of the walls defining each channel). Each of the plurality of channels has an opening at the first face and an opening at the second face. The first face is typically at the inlet end of the substrate, and the second face is at the outlet end of the substrate. To avoid doubt, the flow-through monolith substrate is not a wall-flow filter.

[0018] The channels can be of a constant width, and each of the plurality of channels can have a uniform channel width. Preferably, in a plane orthogonal to the longitudinal direction, the monolith substrate has 300 to 900 channels per square inch, preferably 400 to 800 channels. These channels can have a cross-section that is rectangular, square, circular, elliptical, triangular, hexagonal, or other polygonal shape.

[0019] The monolith substrate acts as a carrier for holding the catalyst material. Suitable materials for forming the monolith substrate include ceramic-like materials such as cordierite, silicon carbide, silicon nitride, zirconia, mullite, spodumene, alumina-silica-magnesia or zirconium silicate, or porous refractory metals. Such materials and their use in the manufacture of porous monolith substrates are well known in the art.

[0020] Although the substrate described in this specification is a single component (i.e., a single brick), it should be noted that when forming an emissions treatment system, the substrate used may be formed by adhering a plurality of channels together or by adhering a plurality of smaller substrates as described in this specification together. Such techniques are well known in the art, along with suitable casings and configurations for emissions treatment systems.

[0021] In embodiments where the catalyst article of the present invention includes a ceramic substrate, the ceramic substrate may be composed of any suitable refractory material, and the refractory material may be, for example, alumina, silica, ceria, zirconia, magnesia, zeolite, silicon nitride, silicon carbide, zirconium silicate, magnesium silicate, aluminosilicate, and metalloaluminosilicate (such as cordierite and spodumene), or a mixture or mixed oxide of any two or more of these. Cordierite, magnesium aluminosilicate, and silicon carbide are particularly preferred.

[0022] In embodiments where the catalyst article of the present invention includes a metal substrate, the metal substrate may be made of any suitable metal, particularly heat-resistant metals and metal alloys such as titanium and stainless steel, and ferritic alloys containing iron, nickel, chromium, and / or aluminum in addition to other trace metals.

[0023] The method includes forming one or more platinum group metal-containing washcoat layers on a carrier substrate to provide a first coated substrate. The platinum group metals, i.e., PGMs, discussed in this specification are selected from a list including or consisting of ruthenium, rhodium, palladium, osmium, iridium, and platinum. However, in practice, these preferably include or consist of platinum and palladium.

[0024] Preferably, the one or more platinum group metal-containing washcoat layers on the carrier substrate further contain an alkaline earth metal, preferably strontium and / or barium. These materials are particularly useful for enhancing the exothermic generation characteristics of the DOC.

[0025] The formation of a washcoat layer containing PGM is known in the art. This generally involves preparing a washcoat slurry. This involves mixing a number of raw material components together. As used herein, the term "slurry" may encompass a liquid containing insoluble materials, such as insoluble particles. The slurry can include (1) a solvent, (2) soluble components, such as free PGM ions (i.e., outside the carrier), and (3) insoluble components, such as carrier particles. The slurry is particularly effective in maximizing gas diffusion and minimizing pressure drop during catalytic conversion when disposing the material on the substrate. The slurry is typically agitated, more typically agitated for at least 10 minutes, more typically agitated for at least 30 minutes, and even more typically agitated for at least 1 hour. The agitation of the slurry can be carried out, for example, before disposing the slurry on the substrate.

[0026] The first preferred raw material component in the washcoat slurry is a carrier material. The carrier material is generally a refractory metal oxide powder. Preferably, the refractory metal oxide carrier material is selected from the group consisting of alumina, silica, zirconia, ceria, and two or more composite oxides or mixed oxides thereof, and most preferably is selected from the group consisting of alumina, silica, and zirconia, and two or more composite oxides or mixed oxides thereof. Examples of the mixed oxide or composite oxide include silica-alumina and ceria-zirconia, and most preferably silica-alumina. Preferably, the refractory metal oxide carrier material does not include ceria nor a mixed oxide or composite oxide containing ceria. More preferably, the refractory oxide is selected from the group consisting of alumina, silica, and silica-alumina. The refractory oxide may be alumina. The refractory oxide may be silica. The refractory oxide may be silica-alumina.

[0027] By including a dopant, the refractory metal oxide support material can be stabilized or the catalytic reaction of the supported platinum group metal can be promoted. Typically, the dopant can be selected from the group consisting of zirconium (Zr), titanium (Ti), silicon (Si), yttrium (Y), lanthanum (La), praseodymium (Pr), samarium (Sm), neodymium (Nd), barium (Ba), and their oxides. Generally, the dopant is different from the refractory metal oxide (i.e., the cation of the refractory metal oxide). Thus, for example, when the refractory metal oxide is titania, the dopant is neither titanium nor its oxide.

[0028] When the refractory metal oxide support material is doped with a dopant, typically, the refractory metal oxide support material contains a total amount of dopant of 0.1 to 10 wt%. Preferably, the total amount of the dopant is 0.25 to 7 wt%, more preferably 2.5 to 6.0 wt%. Preferably, since an oxidation catalyst containing such a support material in combination with a platinum group metal and an alkaline earth metal promotes an oxidation reaction, such as CO and hydrocarbon oxidation, the dopant is silica.

[0029] Preferably, the support material is selected from optionally doped alumina, silica, titania, and combinations thereof.

[0030] A further raw material component in the washcoat is a PGM component, preferably a salt of the PGM component. Thus, the washcoat typically contains a palladium (Pd) salt and / or a platinum (Pt) salt. Preferably, these salts are readily soluble in water. Preferably, the Pd salt and the Pt salt are independently selected from nitrates, chlorides, and bromides. Preferably, the washcoat slurry does not contain Rh. Preferably, the platinum group metals present in the washcoat slurry consist of Pt and Pd.

[0031] Optional additional raw material components that are standard when forming the washcoat slurry may also be present. These include one or more of a binder and a thickener. The binder may include, for example, an oxide material having a small particle size for binding individual insoluble particles together in the washcoat slurry. The use of a binder in a washcoat is well known in the art. The thickener may include, for example, a natural polymer having functional hydroxyl groups that interacts with the insoluble particles in the washcoat slurry. The thickener serves the purpose of thickening the washcoat slurry for improving the coating profile during the washcoat coating onto the substrate. The thickener is usually burned off during the calcination of the washcoat. Examples of specific thickeners / rheology modifiers for washcoats include glactomanna gum, guar gum, xanthan gum, curdlan sizophyllan, scleroglucan, diutan gum, wheylan gum, hydroxymethylcellulose, carboxymethylcellulose, hydroxyethylcellulose, methylcellulose, methylhydroxyethylcellulose, methylhydroxypropylcellulose, and ethylhydroxycellulose.

[0032] The slurry preferably has a solids content of 10 to 40%, preferably 15 to 35%. Such a solids content may enable a slurry rheology suitable for placing the supported carrier material onto the substrate. For example, if the substrate is a honeycomb monolith, such a solids content may enable the deposition of a thin layer of washcoat onto the inner walls of the substrate.

[0033] Forming a washcoat layer to obtain a coated substrate involves the step of applying a washcoat slurry to at least a portion of the substrate to form a washcoat substrate. Placing the slurry on the substrate can be done using techniques known in the art. Typically, the slurry is injected into the inlet of the substrate using a specific forming tool in a predetermined amount, whereby the loaded carrier material can be placed on the substrate. As will be discussed in more detail below, subsequent vacuum and / or air knife and / or drying steps can be used during the placement step. If the carrier is a filter block, the supported carrier material can be placed on the filter wall, within the filter wall (in the case of porosity), or both.

[0034] The pH of the slurry can be adjusted prior to coating using nitric or citric acid and optionally a base such as ammonia or barium hydroxide to obtain the desired pH. The use of a base can be useful to prevent the pH from being adjusted to too low a pH.

[0035] The method then includes subjecting the first coated substrate to a first heat treatment to form a heat-treated coated substrate. The first heat treatment includes heating the first coated substrate to a first maximum temperature and holding the first coated substrate at the first maximum temperature. As will be understood, a typical heat treatment process involves passing the substrate through a furnace having a zone where the temperature increases. The main influence on the heated substrate is the maximum temperature reached. Thus, important parameters of the heat treatment process are the maximum temperature reached and the time spent at that temperature.

[0036] The first maximum temperature is at least 600 °C. Preferably, the first maximum temperature is 625 - 750 °C, preferably 650 - 700 °C. Preferably, the first coating substrate is maintained at the first maximum temperature for at least 30 minutes, preferably for 1 hour to 3 hours. Shorter times may not be sufficient to achieve the desired aging, and longer times are less commercially desirable. Excessively long times can result in an undesirably high level of aging and complete loss of the desired performance. The first heat treatment may be carried out under moisture-containing conditions, but environmental moisture is sufficient, and preferably, the aging is carried out under conditions of 5 - 15 wt% H2O.

[0037] The first heat treatment may be carried out in two steps. The first step is a standard calcination step, and then a second aging step may be carried out. However, preferably, the aging step is sufficient by carrying out calcination and aging simultaneously.

[0038] When forming a catalyst article, the calcination step can be carried out at a range of temperatures, but the optimum temperature depends on the nature of the washcoat and the intended use of the final catalyst. Generally, it is desirable to use the lowest temperature at which calcination is still preferably suitable, because this results in the lowest production cost and the lowest possibility of damaging the article. Generally, the calcination temperature of the DOC is in the range of about 500 °C (such as 450 - 550 °C), because this is sufficient to calcine the parts without excessive damage or loss of function. Therefore, the first heat treatment of the present invention is carried out at a temperature higher than that of a normal calcination step. Further, the purpose of the first heat treatment is to age the parts such that the combination of the reached maximum temperature and the time spent at that temperature is greater than that of a normal calcination step.

[0039] One or more platinum group metal-containing washcoat layers formed on the substrate preferably provide a PGM loading of 10 - 50 g / ft 3 , more preferably 20 - 40 g / ft 3 on the coating substrate.

[0040] The one or more platinum group metal-containing washcoat layers preferably together cover substantially the entire length of the substrate. That is, preferably, the coated substrate has a continuous platinum group metal-containing coating extending from the inlet end to the outlet end of the carrier substrate. As another option, the one or more platinum group metal-containing washcoat layers may together cover at least 40%, more preferably at least 60%, and most preferably at least 80% of the axial length of the substrate. This coverage preferably extends from the outlet end.

[0041] When the coated substrate has a continuous platinum group metal-containing coating extending from the inlet end to the outlet end of the carrier substrate, preferably, the continuous platinum group metal-containing coating is zoned, with the inlet zone containing Pt and Pd and the outlet zone containing Pt and optionally Pd. Preferably, the continuous platinum group metal-containing coating consists of an inlet zone and an outlet zone.

[0042] After the first heat treatment, the method further includes depositing a platinum group metal-containing composition on at least a portion of the heat-treated coated substrate to form a second coated substrate. That is, a new layer or zone of the platinum group metal-containing composition is formed on the aged coated substrate. This provides a new PGM material for CO and HC oxidation and further potential exothermic generation characteristics.

[0043] The new layer or zone may be applied by various techniques including washcoating as discussed above. As another option, the new layer or zone may be obtained by directly impregnating the aged coated substrate (or a portion thereof) with a salt of PGM.

[0044] Preferably, the new layer or zone is provided only on the upstream portion of the substrate extending from the inlet end of the substrate. Preferably, the washcoat is provided over less than 40% of the axial length of the substrate extending from the inlet end of the substrate, preferably over 10-30% of the axial length. Generally, the new layer or zone is entirely present on top of the original one or more platinum group metal-containing washcoat layers. However, in embodiments where the one or more platinum group metal-containing washcoat layers do not extend over the entire length, there may be no overlap or only partial overlap between the new layer or zone and the aged coating on the substrate.

[0045] After depositing a platinum group metal-containing composition on at least a portion of the heat-treated coated substrate to form a second coated substrate, the second coated substrate is subjected to a second heat treatment comprising heating the second coated substrate to a second maximum temperature and holding the second coated substrate at the second maximum temperature to form a diesel oxidation catalyst. The second maximum temperature needs to be at least 25 °C lower than the first maximum temperature. Preferably, the second maximum temperature is at least 50 °C lower than the first maximum temperature, more preferably 100-250 °C lower.

[0046] Desirably, the second heat treatment step is a standard calcination step. Preferably, the second heat treatment is carried out at a second maximum temperature of 400-575 °C, preferably 450-550 °C. Preferably, the second heat treatment is carried out by holding the second coated substrate at the second maximum temperature for at least 30 minutes, preferably 1 to 3 hours.

[0047] As will be appreciated, in a standard process where multiple layers are applied with an intervening calcination step, each calcination step is carried out under the same conditions. There is no reason to switch the heat treatment temperature, and of course, there is no reason to make the first heat treatment at a higher temperature than the second heat treatment.

[0048] The first and second heat treatments are typically performed in an oven or furnace, more typically in a belt or static oven or furnace, and typically in a specific flow of hot air from one direction. Any of the processes may include an initial drying process. The drying and heat treatment processes may be continuous or sequential. For example, after the substrate has already been wash-coated and dried together with the previous wash-coating, a separate wash-coating may be applied. The wash-coated substrate may also be dried and heat-treated using one continuous heating program if the coating is complete. During heating, any complexes that may form in the solution may decompose at least partially, substantially, or completely. In other words, the ligands of such complexes, for example the ligands of organic compounds, may be removed or separated from the PGM ions at least partially, substantially, or completely and may be removed from the final catalyst article. The thus-separated palladium particles may then begin to form metal-metal and metal-oxide bonds. As a result of heating (calcination), the substrate typically contains substantially no organic compounds and more typically contains no organic compounds at all.

[0049] After each heating step, the substrate is typically cooled, more typically cooled to room temperature. Cooling is typically performed in air, typically without using a coolant / cooling medium or typically without using a coolant.

[0050] It has been found that efficient heat generation can be best achieved with a high PGM loading in the front zone of the DOC. This means that heat generation occurs in the front part of the DOC, but the strong heating effect is received by the rear part of the DOC. By increasing the PGM concentration in the cooler front part, the article has improved life and durability since this part does not receive the highest temperature.

[0051] According to a preferred configuration, the DOC has a front zone that extends from the inlet end, and in this front zone, the concentration of PGM is higher than that of the rear zone that extends from the inlet end. Preferably, the PGM concentration is at least 2 times, more preferably at least 4 times, preferably 4 to 10 times higher in the front zone. Typically, the rear part having a lower PGM content has higher resistance to sintering due to the lower PGM content. That is, when the filling amount of PGM in the rear zone is lower, the PGM is arranged at a greater interval and is less likely to sinter together. Using a lower amount of PGM in the rear zone is more efficient with respect to the use of PGM. By using a higher amount in the front zone, efficient heat generation is possible without sacrificing performance in other respects. In the front zone, passive oxidation of CO and HC is carried out, while the rear zone is then sufficient to handle the competing NOx oxidation that still needs to occur.

[0052] Preferably, the outlet zone (rear zone) has a lower Pt filling amount in g / in 3 units than the inlet zone (front zone). This is useful for embodiments of efficient heat generation. In another embodiment, the outlet zone has a higher Pt filling amount in g / in 3 units than the inlet zone.

[0053] According to a preferred embodiment, a method for manufacturing a diesel oxidation catalyst, (i) providing a carrier substrate, (ii) forming, on the carrier substrate, one or more platinum group metal-containing washcoat layers that are on 100% of the axial length of the carrier and contain Pt and optionally Pd to provide a first coated substrate, (iii) subjecting the first coated substrate to a first heat treatment that includes heating the first coated substrate to a first maximum temperature and holding the first coated substrate at the first maximum temperature to form a heat-treated coated substrate. (iv) depositing a platinum group metal-containing composition on at least a portion of the heat-treated coated substrate to form a second coated substrate, wherein the composition forms an inlet zone over 10-40% of the axial length of the carrier substrate, forming the second coated substrate; (v) subjecting the second coated substrate to a second heat treatment, the second heat treatment including heating the second coated substrate to a second maximum temperature and holding the second coated substrate at the second maximum temperature, to form a diesel oxidation catalyst; comprising, wherein the first maximum temperature is a temperature of 600 °C to 750 °C, the first heat treatment is carried out for 1 to 3 hours in a moisture-containing atmosphere containing 5 to 15% by weight of H2O, and the second maximum temperature is 450 to 550 °C in air, a method is provided.

[0054] According to a preferred embodiment, a method for manufacturing a diesel oxidation catalyst, (i) providing a carrier substrate; (ii) forming, on the carrier substrate, one or more platinum group metal-containing washcoat layers, which are over 100% of the axial length of the carrier and contain Pt and optionally Pd, to provide a first coated substrate; (iii) subjecting the first coated substrate to a first heat treatment, the first heat treatment including heating the first coated substrate to a first maximum temperature and holding the first coated substrate at the first maximum temperature, to form a heat-treated coated substrate; (iv) depositing a platinum group metal-containing composition on at least a portion of the heat-treated coated substrate to form a second coated substrate, wherein the composition forms an inlet zone over 10-40% of the axial length of the carrier substrate, forming the second coated substrate; (v) subjecting the second coating substrate to a second heat treatment, the second heat treatment including heating the second coating substrate to a second maximum temperature and holding the second coating substrate at the second maximum temperature, to form a diesel oxidation catalyst; A method is provided that includes, the first maximum temperature being a temperature of 650 °C to 725 °C, the first heat treatment being carried out over 1 to 3 hours in a moisture-containing atmosphere containing 5 to 15 wt% H2O, and the second maximum temperature being 475 to 525 °C in air over 1 to 3 hours.

[0055] As will be appreciated, the above method defines an intermediate aging treatment (first heat treatment) and a final calcination step (second heat treatment) for the purpose of describing the manufacture of a DOC having an aged platinum group metal-containing washcoat layer thereon and a new platinum group metal-containing composition deposited at the inlet end. An alternative approach, taking this into account, is to consider the PGM material dispersion in each layer. When PGMs are applied to the washcoat, they are generally finely dispersed, but aging results in a sintering effect that forms larger agglomerates of PGM. This means that the degree of aging can be determined by inspection of the PGM dispersion. The product of the method described herein provides a unique structure having larger agglomerates aged in the underlying washcoat layer but new finely dispersed PGM in the upper layer (or impregnated in the underlying washcoat layer to give a multimodal distribution).

[0056] Whether the PGM-containing layer has been aged can be determined using known techniques. Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) can be used to evaluate the form and state of the PGM components in the catalyst. In this way, the degree of aging can be confirmed, thereby demonstrating which parts of the catalyst have been aged and which parts remain as new. To perform the measurement, the Micromeritics Autochem 2920 apparatus is used to measure the CO uptake of the sample. The sample is pretreated with hydrogen gas at 300 °C. The carbon monoxide uptake is measured by pulse chemisorption at 50 °C. Subsequently, using the Autochem 2920 software, the dispersion and particle size of the PGM material can be calculated based on the CO uptake and PGM material content of the sample. The dispersion of the PGM material is a measure of the particle size of the PGM material. Large particles with a low surface area have a low dispersion. Thus, this technique enables determination of the extent to which the applied PGM has been sintered together by aging.

[0057] Preferably, the aged platinum group metal-containing washcoat layer contains platinum group metal particles having an average particle size (D50) of greater than 10 nm as determined by TEM, and the new platinum group metal-containing composition contains platinum group metal particles having a D90 particle size of less than 10 nm as determined by TEM. These properties of each applied layer or zone can be evaluated by TEM inspection.

[0058] According to a further aspect, there is provided a diesel oxidation catalyst article comprising a flow-through carrier substrate having an aged platinum group metal-containing washcoat layer thereon and a fresh platinum group metal-containing composition deposited at an inlet end, wherein the aged platinum group metal-containing washcoat layer comprises platinum group metal particles having an average particle size (D50) of greater than 10 nm as determined by TEM, and the fresh platinum group metal-containing composition comprises platinum group metal particles having a D90 particle size of less than 10 nm as determined by TEM. Advantageously, this configuration provides stabilized NO oxidation without significantly affecting CO / HC and exothermic activity.

[0059] Preferably, the diesel oxidation catalyst is obtained or obtainable by the method described herein.

[0060] According to a further aspect, (A) a soot filter; (B) an SCR catalyst article; (C) an SCRF catalyst article; (D) a catalyzed soot filter; (E) a soot filter and then an SCR catalyst article; or (F) a catalyzed soot filter and then an SCR catalyst article, there is provided an exhaust gas treatment system comprising the diesel oxidation catalyst described herein disposed upstream of one of them.

[0061] These components are well known in the art. These components can obtain benefits in one of two ways by providing a DOC obtained by the method disclosed herein at an upstream position. Some of these components, such as soot filters, obtain benefits from the exothermic providing ability of the DOC. This additional heat acts to promote soot combustion and removal. Others of these components obtain particular benefits from the stabilized NO2 generating ability.

[0062] This is particularly true for SCR and SCRF components. Selective catalytic reduction (SCR) of NOx occurs mainly through the following three reactions. (1) 4 NH3 + 4 NO + O2 → 4 N2 + 6 H2O; (2) 4 NH3 + 2 NO + 2 NO2 → 4 N2 + 6 H2O; and (3) 8 NH3 + 6 NO → 7 N2 + 12 H2O

[0063] Therefore, the NO2:NO ratio in the exhaust gas entering the SCR catalyst or SCRF catalyst can affect its performance (see Reaction 2). Generally, the SCR catalyst or SCRF catalyst exhibits optimal performance when the NO2:NO ratio is approximately 1:1. This can be a problem since the exhaust gas typically generated by a compression ignition engine during normal use does not contain sufficient NO2 for the optimal performance of the SCR catalyst or SCRF catalyst (i.e., the NO2:NO ratio is much lower than 1:1).

[0064] To compensate for such low levels of NO2, the DOC is formulated to oxidize nitric oxide (NO) to nitrogen dioxide (NO2), thereby increasing the NO2:NO ratio in the exhaust gas. By providing an improved DOC in which the NO2 production level is maintained throughout the operating life, it is possible to optimize the level of PGM in the component.

[0065] According to a further aspect, a diesel combustion and exhaust gas treatment system is provided, including a diesel combustion engine and the exhaust system described herein.

[0066] According to a further aspect, a method of manufacturing the exhaust system described herein is provided, including forming a diesel oxidation catalyst according to the method described herein and disposing it upstream of any of (A)-(F).

[0067] Definitions As used herein, the singular forms "a", "an", and "the" include plural referents unless the context clearly dictates otherwise.

[0068] The use of the term "comprising" is intended to be interpreted as including such features but not excluding others, and is also intended to include options for features that are not necessarily limited to those described. In other words, this term also includes, unless the context clearly dictates otherwise, "consisting essentially of" (which is intended to mean that certain additional components may exist provided that they do not substantially affect the essential characteristics of the described features) and "consisting of" (which is intended to mean that when the components are expressed as percentages by their proportions, they total 100%, while accounting for any inevitable impurities but precluding the inclusion of other features).

[0069] As used herein, the term "on" is intended to mean "directly on" such that there is no intervening layer between a material that is said to be "on" another material. Spatially relative terms such as "below", "beneath", "lower", "above", "upper", etc. may be used herein to facilitate description of the relationship of one element or feature to another element or feature. It will be understood that spatially relative terms are intended to encompass different orientations of the catalyst during use or operation in addition to the orientation shown in the figures.

[0070] The term "calcine" or "calcination" means heating a material in air or oxygen. This definition is consistent with the IUPAC definition of calcination. (IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Prepared by A.D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). XML online corrected version: http: / / goldbook.iupac.org (2006-), created by M. Nic, J. Jirat, B. Kosata; update by A. Jenkins. ISBN 0-9678550-9-8. doi:10.1351 / goldbook). The temperature used for calcination varies depending on the components in the material being calcined. According to Chapter 2.3.3 and 2.3.4 of R.M. Heck et al., "Catalytic Air Pollution Control - Commercial Technology", John Wiley & Sons, Inc., 3 rd Edition (2009), the catalyst species applied in the washcoat on a monolith substrate for automotive applications, which contains salts of platinum and palladium, is dried with blowing air at about 110 °C and calcined in blowing air up to about 400 - 500 °C to remove all trace amounts of the decomposable salts used to prepare the catalyst. In an application involving the process described herein (i.e., standard DOC preparation and the second heat treatment), calcination is generally carried out at a temperature of about 400 °C to about 600 °C for approximately 1 - 8 hours, preferably at a temperature of about 400 °C to about 550 °C for approximately 1 - 4 hours.

Brief Description of the Drawings

[0071] Next, the present invention will be further described with reference to the following non-limiting figures.

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

[0072] As shown in FIG. 1, a DOC1 is provided. The DOC1 includes a substrate 5. The substrate 5 is preferably a flow-through substrate such as in the form of porous cordierite. The substrate 5 has an inlet end 10 for receiving the exhaust gas to be treated and an outlet end 15 for discharging the treated exhaust gas. The flow direction of the exhaust gas is indicated by arrow 20.

[0073] The substrate 5 has a PGM-containing layer 25 provided along the entire length of the substrate 5 by a washcoat. This is generally a layer containing only Pt, or a layer containing Pd and Pt. The total PGM content of this layer is typically 10 - 50 g / ft 3 . The PGM content is provided on a carrier material such as alumina. The PGM-containing layer 25 has undergone an aging process to stabilize the NO oxidation performance of this layer.

[0074] On top of the PGM-containing layer 25, an additional PGM-containing zone 30 is provided at the inlet end 10. This can be applied by, for example, washcoating or impregnation. This zone 30 preferably contains Pt and typically provides an additional 10 - 50 g / ft 3 of PGM. Since this zone 30 has not undergone an aging process, it has fresh activity. This promotes heat generation at the inlet of the DOC.

[0075] As shown in FIG. 2, this method involves (A) providing a carrier substrate 5, (B) forming a PGM-containing layer 25 on the carrier substrate 5, Subjecting the PGM-containing layer 25 on the carrier substrate 5 to aging at a temperature of 650° C. for 1 to 3 hours in an atmosphere containing 10% by weight of moisture, forming a PGM-containing zone 30 on the aged PGM-containing layer 25 on the carrier substrate 5, and comprise.

[0076] As shown in FIG. 3, a conventional (standard) reference DOC shows a significant decrease in NO2 / NOx performance between the performance at the time of new product (i.e., degreasing) and the performance after aging (140 hours at 650° C.). In contrast, the DOC produced according to the present invention shows a much smaller change between the performance at the time of new product (i.e., degreasing) and the performance after aging (140 hours at 650° C.). The DOC produced according to the present invention has undergone a first heat treatment at 700° C. for 3 hours.

[0077] FIG. 4 shows a significant change in the PGM particle size in the reference component between the starting particle size of around 6 nm (D90 less than 10 nm) and the widely distributed particle size after aging (D50 greater than 10 nm). In contrast, FIG. 5 shows the state in which the DOC obtained according to the present invention has a starting particle size having a D50 greater than 10 nm for the PGM particle size. After aging, the particle size is smaller as a whole than in the case of the comparative data. That is, the change in the PGM particle size in the DOC of the present invention is smaller.

[0078] Note that FIGS. 4 and 5 consider only the PGM particle size in one or more platinum group metal-containing washcoat layers on the carrier substrate. Therefore, the DOC obtained according to the present invention has a further distribution of PGM (such as in the upper washcoat layer) having a D90 of less than 15 nm, preferably less than 10 nm.

Examples

[0079] Next, the present invention will be further described with respect to the following non-limiting examples.

[0080] The present invention will now be illustrated by the following non-limiting examples. To avoid doubt, all coating steps were carried out using the methods and apparatus disclosed in International Publication No. 99 / 47260 by the applicant, i.e., (a) arranging a receiving means on a substrate; (b) introducing a predetermined amount of a liquid component into the receiving means in either order of (b) after (a) or (a) after (b); and (c) applying a vacuum to draw the entire amount of the liquid component into at least a portion of the substrate and retain substantially all of the amount within the substrate without recycling.

[0081] Example 1 (Reference Example) An uncoated cordierite honeycomb flow-through monolith substrate having a length of 13 inches and a diameter of 5 inches was coated by a catalytic washcoat in a zoned arrangement as follows. A first catalytic washcoat slurry containing an aqueous salt (as nitrate) of platinum and palladium and a particulate gamma-alumina support material was coated on the monolith substrate from one end as the inlet end up to an axial length of 80% of the total length of the monolith substrate. The concentrations of the platinum salt and the palladium salt were selected to achieve a coating loading of 6.65Pt:6.65Pd (gft -3 ), i.e., with a weight ratio of platinum to palladium of 1:1 during the first catalytic washcoat coating and a total PGM loading of 13.3 gft -3 was obtained. Then, this inlet coating was dried in a conventional oven at 100 °C for 1 hour to remove excess water and other volatile species.

[0082] A second catalytic washcoat slurry containing aqueous platinum nitrate as the only platinum group metal present and a particulate gamma-alumina support material was coated onto a substrate already coated with a first coating, from the end of the monolith substrate opposite the end to which the first coating was applied, i.e., from the outlet end. The axial length of the coating of the second catalytic washcoat was 75% of the total length of the substrate, i.e., 50% of the second washcoat catalyst coating overlapped with the first washcoat catalyst coating. The concentration of the platinum salt used was selected to achieve a Pt loading of 2.02 gft -3 in the 75% axial substrate length that was coated. The substrate coated with both the first and second washcoat coatings was dried in a conventional oven at 100 °C for 1 hour, and then the dried part was calcined at 500 °C for 1 hour to decompose the platinum salt and palladium salt and fix platinum and palladium to the particulate gamma-alumina support material.

[0083] Next, an aqueous medium containing both salts of platinum nitrate and palladium nitrate in a 1:1 weight ratio was impregnated onto the coating of the first catalytic washcoat in the 25% axial length of the substrate measured from the suction end of the substrate. The concentration of the salts was selected to achieve a weight of 35 gft -3 for each of platinum and palladium in the impregnated length of the substrate. This resulted in a high PGM loading in the zone of the inlet end with an additional loading of 35 gft -3 in addition to the loading of the underlying first catalytic washcoat coating. The impregnated part was dried in a conventional oven at 100 °C for 1 hour, and then the dried part was calcined at 500 °C for 1 hour.

[0084] All washcoats and impregnation solutions were inherently acidic and no pH adjustment was performed.

[0085] The final product comprises a monolith substrate containing three catalyst washcoat zones arranged in series axially, i.e., defined as approximately 25% of the axial length of the monolith substrate measured from the inlet end, a first high loading front zone having a total platinum group metal loading which is a combination of a 1Pt:1Pd first catalyst washcoat of the substrate and an impregnated 1:1 Pt:Pd with a total platinum group metal loading; subsequently, axially in series, a second catalyst washcoat zone comprising a second catalyst washcoat of only Pt overlaid on the 1Pt:1Pd first catalyst washcoat, having a total platinum group metal loading lower than that of the first catalyst washcoat zone and being approximately 50% of the axial length of the monolith substrate; and finally, a third zone of only Pt at the outlet end comprising a second catalyst washcoat coating, having a total platinum group metal loading lower than either the first or the second catalyst washcoat zone and being approximately 25% of the axial length of the monolith substrate. The total platinum group metal loading overall on the monolith substrate was a total Pt:Pd weight ratio of 7:6 corresponding to 1.167:1 and 21 gft -3 It was. The resulting catalyst is described herein as "as new / fresh", i.e., as-made.

[0086] Example 2 (Comparative Example) An impregnated component was dried in a conventional oven at 100 °C for 1 hour and then the dried component was calcined at 700 °C for 3 hours to prepare the same product as disclosed in Reference Example 1. The resulting catalyst is described herein as "as new / fresh", i.e., as-made.

[0087] Example 3 A product similar to Comparative Example 2 was prepared, except that the order of the calcination step and the impregnation step was reversed. That is, the substrate coated with the first and second overlapping coatings was calcined at 500 °C for 1 hour, and then the product was further aged by calcining in air up to 700 °C for 3 hours. Next, an aqueous medium containing both salts of platinum nitrate and palladium nitrate in a weight ratio of 1:1 was impregnated into the first catalyst washcoat of this product up to 25% of the axial length of the substrate as measured from the substrate inlet end. The impregnated component was oven-dried at 100 °C for 1 hour, and then the dried component was calcined at 500 °C for 1 hour. The resulting catalyst is described herein as "as new / freshly made", i.e., as it was prepared.

[0088] Example 4: Test method Thermal analysis of each aged composite oxidation catalyst prepared according to Reference Example 1, Comparative Example 2, and Example 3 (according to the present invention) was carried out using a diesel engine mounted on a test bench. The engine was injected with EUVI B7 fuel (7% biofuel) for both engine operation and exhaust gas hydrocarbon enrichment (heat generation), operated at 2200 rpm, and each composite oxidation catalyst was oriented with the inlet end / high loading washcoat zone of the first catalyst upstream and equipped with an exhaust system including an exhaust pipe and a removable canning that could be inserted internally for testing. The engine was a 7-liter capacity EUV 6-cylinder engine that produced 235 kW at 2500 rpm, and the exhaust system included a "seventh injector" placed in the exhaust gas pipe downstream from the engine manifold and upstream of the composite oxidation catalyst being tested for direct injection of hydrocarbon fuel. Since this fuel injection device was added to the six fuel injection devices associated with the cylinders of the engine, it was named the "seventh injector". Thermocouples were located at the inlet to the composite oxidation catalyst and inserted at various axial positions along the centerline of the substrate monolith of each composite oxidation catalyst.

[0089] The NO oxidation activity of each catalyst at the time of being degreened and new (see below) and after aging worked as follows. Aging was carried out as follows. Each composite oxide catalyst prepared according to Reference Example 1, Comparative Example 2, and Example 3 (according to the present invention) was oven-aged in air at 650°C for 140 hours corresponding to the end-of-vehicle-life activity and tested for average NO. The detected NO2 / total NO x A speed / load map for was created, the integrated average in the quadrant of mass flow rate from 400 to 1000 kg / hour versus catalyst inlet temperature from 200 to 350°C was calculated, and reported in Table 1 below.

[0090] The exothermic test was carried out as follows. Each aged catalyst was conditioned for 10 minutes at an inlet exhaust gas temperature of 490°C with an exhaust gas flow rate of 1000 kg / hour, and then a rapid cooling process was performed (a process known as "degreening"). Next, the exhaust gas flow rate was set to 720 kg / hour (corresponding to a space velocity of 120,000 hours for the size and volume of the tested substrate), and the engine load was controlled so that the set inlet exhaust gas temperature reached about 270°C and stabilized for about 1800 seconds. -1 Then, by injecting hydrocarbon fuel through a seventh injector targeting both a hydrocarbon "slip" stabilized at 600°C and the exit of the composite oxide catalyst substrate by means of a downstream thermocouple and a hydrocarbon sensor, the ability of the composite oxide catalyst to generate heat at a stable set temperature was tested. The test was stopped when the hydrocarbon slip measured downstream of the composite oxide catalyst exceeded 1000 ppm C3, that is, regardless of the length of the hydrocarbon chain in the detected hydrocarbons (the formal carbon chain length in typical diesel fuel is C

[0091] ), and the test was stopped when 1000 ppm C3 equivalent was detected. Therefore, when 187.5 ppm C 16 was detected, it was regarded as equivalent to 1000 ppm C3 (C 16 is equivalent to 5 1 / 3×C3 hydrocarbon). 16 is equivalent to 5 1 / 3×C3 hydrocarbon).

[0092] Following the test at the inlet temperature set at about 270°C, the system was pre-conditioned again at a flow rate of 1000 kg / h for 10 minutes at an inlet exhaust gas temperature of 490°C, and then rapidly cooled, and a heat generation test was conducted at a second set temperature, for example, about 260°C. This cycle was repeated to test heat generation at set temperatures of about 250°C, 240°C, and 230°C. The test was stopped if the composite oxidation catalyst could not generate stable heat generation of 600°C at the composite oxidation catalyst outlet end, or if the hydrocarbon slip measured at the composite oxidation catalyst outlet exceeded 1000 ppm (C3).

[0093]

Table 1

[0094] The results of these tests conducted in Reference Example 1, Comparative Example 2, and Example 3 (according to the present invention) are shown in Table 1 above. It will be understood that the lower the inlet temperature at which stable heat generation can be achieved with an acceptable hydrocarbon slip, the more advantageous it is. This is because the filter regeneration event can start without having to wait until the exhaust gas temperature under normal operating conditions, which has a lower occurrence frequency in normal operation, becomes high enough to start filter regeneration due to a lower inlet exhaust gas temperature, thereby increasing the design flexibility of the system. Also, since it is not necessary to inject a large amount of hydrocarbons to achieve the desired exhaust gas temperature at the outlet of the composite oxidation catalyst, the overall fuel economy is improved.

[0095] Furthermore, from the oxidation activity of each catalyst, the difference in NO oxidation activity from the state at the time of new product to after aging is lower in the case of Comparative Example 2 (37.8% - 33.1% = 4.7%) and Example 3 (according to the present invention) (41.0% - 35.2% = 5.8%) than in the case of Reference Example 1 (54.6% - 43.7% = 10.9%). The lower the "change" between the catalyst activity at the time of new product and the catalyst activity after aging, the easier it is for the following for OEM customers. Programming the engine control unit using an algorithm that allows for deterioration of the NO oxidation catalyst activity over time and adjusts urea injection into the downstream SCR catalyst accordingly; or programming to shorten the interval of downstream active filter regeneration using passive soot combustion in NO2 using the CRT (registered trademark) effect between active regeneration events.

[0096] Therefore, the catalyst of Example 3 according to the present invention has a lower change in the average NO2 / NO ratio between the case at the time of new product and after aging than Reference Example 1, and the ability to generate heat at a temperature lower than that of Comparative Example 2. x And have the ability to generate heat at a temperature lower than that of Comparative Example 2.

[0097] The foregoing detailed description is provided for purposes of explanation and illustration and is not intended to limit the scope of the appended claims. Many variations of the preferred embodiments of the invention illustrated herein will be apparent to those skilled in the art and are included within the scope of the appended claims and their equivalents.

[0098] To avoid doubt, the entire contents of all documents cited herein are incorporated by reference into this application.

Claims

1. A method for manufacturing a diesel oxidation catalyst, comprising: (i) providing a carrier substrate; (ii) forming one or more platinum group metal-containing washcoat layers, each containing a refractory metal oxide carrier material, on the carrier substrate to provide a first coated substrate; (iii) subjecting the first coated substrate to a first heat treatment, the first heat treatment including heating the first coated substrate to a first maximum temperature and holding the first coated substrate at the first maximum temperature, to form a heat-treated coated substrate; (iv) depositing a platinum group metal-containing composition containing a refractory metal oxide carrier material on at least a part of the heat-treated coated substrate to form a second coated substrate; (v) subjecting the second coated substrate to a second heat treatment, the second heat treatment including heating the second coated substrate to a second maximum temperature and holding the second coated substrate at the second maximum temperature, to form the diesel oxidation catalyst; wherein the first maximum temperature is at least 600 °C, and the second maximum temperature is at least 25 °C lower than the first maximum temperature. A method.

2. The method according to claim 1, wherein the first heat treatment is carried out in a moisture-containing atmosphere.

3. The first heat treatment is (a) at the first maximum temperature of 625 to 750 °C, preferably 650 to 700 °C, and / or (b) holding the first coated substrate at the first maximum temperature for at least 30 minutes, preferably for 1 to 3 hours, and / or (c) 5 to 15% by weight of H ２ under the condition of O, The method according to claim 1 or claim 2.

4. The second heat treatment is (a) at the second maximum temperature of 400 to 575 °C, preferably 450 to 550 °C, and / or (b) holding the second coated substrate at the second maximum temperature for at least 30 minutes, preferably for 1 to 3 hours, The method according to any one of claims 1 to 3.

5. The method according to any one of claims 1 to 4, wherein the carrier substrate is a flow-through substrate.

6. The method according to any one of claims 1 to 5, wherein the one or more platinum group metal-containing washcoat layers on the carrier substrate contain Pt and / or Pd.

7. The method according to any one of claims 1 to 6, wherein the one or more platinum group metal-containing washcoat layers on the carrier substrate further contain an alkaline earth metal, preferably strontium and / or barium.

8. The method according to any one of claims 1 to 7, wherein the coated substrate has a continuous platinum group metal-containing coating extending from the inlet end to the outlet end of the carrier substrate.

9. The continuous platinum group metal-containing coating is zoned, the inlet zone contains Pt and Pd, and the outlet zone contains Pt and optionally Pd. (i) The outlet zone has a lower Pt loading in g / in than the inlet zone, or ３ has a lower Pt loading per unit in g / in, or (ii) The outlet zone has a higher Pt loading in g / in than the inlet zone. ３ having, The method according to claim 8.

10. The method according to claim 9, wherein the continuous platinum group metal-containing coating consists of the inlet zone and the outlet zone.

11. Step (iv) is (I) applying a platinum group metal-containing washcoat to the first coated substrate, preferably forming a washcoat zone extending from the inlet end of the substrate; or (II) impregnating the first coated substrate with a solution of a platinum group metal-containing salt, preferably forming a platinum group metal impregnation zone extending from the inlet end of the substrate. The method according to any one of claims 1 to 10, comprising.

12. The method according to any one of claims 1 to 11, wherein the heat-treated coated substrate contains platinum group metal particles having an average particle size (D50) of more than 10 nm, preferably more than 20 nm, as determined by TEM.

13. The diesel oxidation catalyst contains a layer or zone formed in step (iv) containing platinum group metal particles, and the particles have a D90 particle size of less than 15 nm, preferably less than 10 nm, as determined by TEM. The method according to any one of claims 1 to 12.

14. A diesel oxidation catalyst article comprising a flow-through carrier substrate having an aged platinum group metal-containing washcoat layer thereon and a new platinum group metal-containing composition deposited at the inlet end, wherein the aged platinum group metal-containing washcoat layer contains platinum group metal particles having an average particle size (D50) of more than 10 nm as determined by TEM, and the new platinum group metal-containing composition contains platinum group metal particles having a D90 particle size of less than 10 nm as determined by TEM.

15. The diesel oxidation catalyst according to claim 14, obtained or obtainable by the method according to any one of claims 1 to 13.

16. (A) A soot filter; (B) An SCR catalyst article; (C) An SCRF catalyst article; (D) A catalyzed soot filter; (E) A soot filter, and then an SCR catalyst article; or (F) A catalyzed soot filter, and then an SCR catalyst article, An exhaust gas treatment system comprising the diesel oxidation catalyst according to claim 14 or 15 disposed upstream of any of (A) to (F).

17. A diesel combustion and exhaust gas treatment system comprising a diesel combustion engine and the exhaust system according to claim 16.

18. A method for manufacturing the exhaust system according to claim 16, comprising forming a diesel oxidation catalyst according to the method according to any one of claims 1 to 13 and disposing the same upstream of any of (A) to (F).

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