Diesel oxidation catalyst and method for manufacturing 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】 This invention relates to an improved diesel oxidation catalyst (DOC), and more particularly to a method for producing DOC. The production method provides a DOC having stabilized NO to NO2 oxidation performance without impairing CO / HC oxidation performance and / or exothermic generation capacity. 【0002】 Internal combustion engines produce exhaust gases containing various pollutants, including hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides ("NOx"). Emission control systems, including catalytic converters, are widely used to reduce the amount of these pollutants released 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). DOCs typically contain palladium and / or platinum, generally supported on alumina. This catalyst converts particulate matter (PM), hydrocarbons, and carbon monoxide into carbon dioxide and water. 【0003】 In modern exhaust systems, DOC is typically used during operation to control the emission of CO and HC. The role of DOC in the passive oxidation of HC, CO, and NOx present in the exhaust gas flow is evident throughout the engine's operation and is optimized for a DOC operating window of approximately 250-300°C. DOC can also be used to facilitate the conversion of NO to NO2 for downstream passive filter regeneration (combustion of particulate matter retained on the filter in NO2 at exhaust gas temperatures lower than those in O2 in the exhaust gas, i.e., the so-called CRT® effect). 【0004】 In addition, DOC can be used as an exothermic catalyst. This is done by injecting hydrocarbon fuel into the exhaust gas. To avoid any doubt, fuel injection / exothermic events do not occur during normal operation. Normal operation is considered to be the period between fuel injection / exothermic events. The second role of exothermic generation can serve one of several purposes. For example, if an unacceptable increase in back pressure is detected, heat can be generated to burn soot on a downstream filter. Another example is for the regeneration of SCR catalysts, such as by removing sulfur from downstream CuCHA SCR catalysts. 【0005】 To generate this heat, a certain amount of hydrocarbons (HC) is injected upstream of the DOC (approximately 2000 ppm). As long as the DOC is sufficiently hot, the added HC will generate heat, heating the exhaust gases and consequently their downstream components (up to approximately 500°C). If the DOC is not sufficiently hot, it is necessary to provide hotter exhaust gases from the engine through engine management, which has an impact on energy and performance. 【0006】 Therefore, it is desirable to provide a DOC with a low heat generation temperature. The lower this temperature, the more likely the engine is to be operating above its heat generation temperature when heat generation is required, and / or less energy needs to be applied to reach a suitable operating temperature. 【0007】 It is known that the performance characteristics of catalytic articles can change over the course of their lifespan. While some of this performance can be restored through regeneration processes, other aspects are simply lost due to factors such as the sintering of platinum group metal (PGM) components. This can present challenges in providing a well-tuned exhaust system that can operate optimally over its long service life. In some cases, this performance delta can lead to difficulties such as the desirability of pre-aging components before use to reach a point where the ongoing change in performance observed by the end-user over the lifetime performance is minimized. In other words, exhaust system manufacturers may choose to sacrifice some of the initial activity to ensure more consistent performance over the lifespan of the components. 【0008】 U.S. Patent No. 8,679,434 (B1) discloses a method for preparing heat-stabilized powders. More specifically, the disclosure provides a honeycomb substrate on which a washcoat containing one or more calcined platinum group metal components is disposed 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 to provide a stable NO2 to NOx ratio when exhaust gas flows through the honeycomb substrate. These powders, whose activity is then reduced by aging to minimize performance changes during the aging process in use, are then washcoated onto the substrate to form a catalyst article. In this case, the performance of the catalyst article in use is more stable. 【0009】 U.S. Patent Application Publication No. 20160236178(A1) discloses the preparation of a chemically reduced PGM material that can be heat-treated to obtain a preferred PGM size for NO oxidation. More specifically, the disclosure provides and describes a method for preparing a catalytic composition for obtaining a stable NO2 to NO ratio in the exhaust system of a compression ignition engine. The method comprises (i) preparing a first composition comprising a platinum (Pt) compound disposed on 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 method for producing DOC to address the problems associated with the prior art and / or to provide at least a commercially viable alternative to the prior art. 【0011】 According to a first aspect, the present invention relates to a method for producing a diesel oxidation catalyst, (i) To provide a carrier substrate, (ii) To provide a first coating substrate by forming one or more platinum group metal-containing wash coat layers, each containing a refractory metal oxide carrier material, on a carrier substrate, (iii) subjecting the first coating substrate to a first heat treatment, which includes heating the first coating substrate to a first maximum temperature and holding the first coating substrate at the first maximum temperature, to form a heat-treated coating substrate. (iv) Depositing a platinum group metal-containing composition containing a refractory metal oxide support material onto at least a portion of the heat-treated coating substrate to form a second coating substrate, (v) Subjecting the second coating substrate to a second heat treatment, which includes heating the second coating substrate to a second maximum temperature and holding the second coating substrate at the second maximum temperature, in order to form a diesel oxidation catalyst. The present invention provides a method comprising a first maximum temperature of at least 600°C and a second maximum temperature of at least 25°C lower than the first maximum temperature. 【0012】 Herein, the disclosure is further described. Different aspects / embodiments of the disclosure are defined in more detail in the following sections. Each of the aspects / embodiments defined in this way may be combined with any other aspects / embodiments or more aspects / embodiments unless otherwise expressly indicated. In particular, any feature indicated as preferred or advantageous may be combined with any other or more features indicated as preferred or advantageous. 【0013】 In relation to DOC, aging tests have shown that the catalytic activity of a finished catalyst can be weakened by heat treatment. This has the effect of reducing the change between the performance when new and the performance after aging. This is particularly advantageous for systems containing SCRF components, whose low-temperature activity may be sensitive to the NO / NO2 ratio. However, heat treatment of a 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 found that by performing a heat treatment in an intermediate stage, it is possible to selectively stabilize the NO oxidation activity, and then apply an additional coating that provides most of the CO / HC and exothermic properties of a new catalyst. This additional coating is preferably applied to the inlet section, as this is the part where the CO / HC and exothermic properties are most needed. Since NO oxidation typically occurs on at least the rear section of the catalyst, this section should be treated before the stabilization heat treatment is performed. Heat treatment at temperatures above 600°C is required to stabilize the NO activity. 【0015】 More specifically, the present invention relates to a method for producing a diesel oxidation catalyst (DOC). The catalyst is generally in the form of a DOC article. A catalyst article refers to a single component for an exhaust gas treatment system. These are sometimes referred to as "bricks." 【0016】 This method includes providing a carrier substrate, which is a surface on which a catalyst layer is subsequently applied and supported. 【0017】 Preferably, the substrate is a flow-through monolith. The flow-through monolith substrate has a first surface and a second surface, with a longitudinal direction defined between them. The flow-through monolith substrate has a plurality of channels extending between the first surface and the second surface. The plurality of channels extend longitudinally and provide a plurality of inner surfaces (e.g., wall surfaces defining each channel). Each of the plurality of channels has an opening on the first surface and an opening on the second surface. The first surface is typically at the inlet end of the substrate, and the second surface is at the outlet end of the substrate. To avoid any doubt, the flow-through monolith substrate is not a wall flow filter. 【0018】 The channels may be of a certain width, and each of the multiple channels may have a uniform channel width. Preferably, in a plane perpendicular to the longitudinal direction, the monolithic substrate has 300 to 900 channels per square inch, preferably 400 to 800 channels. These channels may have cross-sections that are rectangular, square, circular, elliptical, triangular, hexagonal, or other polygonal shapes. 【0019】 The monolithic substrate acts as a carrier for holding the catalyst material. Suitable materials for forming the monolithic 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 monolithic substrates are well known in the art. 【0020】 Although the substrate described herein is a single component (i.e., a single brick), nevertheless, when forming an emissions treatment system, it should be noted that the substrate used may be formed by adhering together a plurality of channels or by adhering together a plurality of smaller substrates as described herein. 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 can 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 herein 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, one or more platinum group metal-containing wash coat layers on the carrier substrate further comprise alkaline earth metals, preferably strontium and / or barium. These materials are particularly useful for enhancing the exothermic generation characteristics of DOC. 【0025】 The formation of a washcoat layer containing PGM is known in the art. This generally involves preparing a washcoat slurry, which involves mixing a number of raw material components together. As used herein, the term “slurry” may include a liquid containing insoluble material, e.g., insoluble particles. The slurry may contain (1) a solvent, (2) soluble components, e.g., free PGM ions (i.e., outside the carrier), and (3) insoluble components, e.g., carrier particles. The slurry is particularly effective when placing the material on a substrate, especially to maximize gas diffusion and minimize pressure drop during catalytic conversion. The slurry is typically stirred, more typically for at least 10 minutes, more typically for at least 30 minutes, and even more typically for at least 1 hour. Stirring of the slurry may be performed, for example, before placing the slurry on a 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 from the group consisting of alumina, silica, and zirconia, and two or more composite oxides or mixed oxides thereof. Examples of mixed oxides or composite oxides include silica-alumina and ceria-zirconia, most preferably silica-alumina. Preferably, the refractory metal oxide carrier material does not contain ceria or 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 dopants, refractory metal oxide support materials can be stabilized or catalytic reactions of supported platinum group metals can be promoted. Typically, dopants 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. In general, dopants are distinct from refractory metal oxides (i.e., cations of refractory metal oxides). Therefore, for example, if the refractory metal oxide is titania, the dopant is neither titanium nor its oxide. 【0028】 When a 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% by weight. Preferably, the total amount of dopant is 0.25 to 7% by weight, more preferably 2.5 to 6.0% by weight. Preferably, the dopant is silica, as an oxidation catalyst containing such a support material in combination with platinum group metals and alkaline earth metals promotes oxidation reactions, such as CO and hydrocarbon oxidation. 【0029】 Preferably, the support material is selected from optionally doped alumina, silica, titania, and combinations thereof. 【0030】 Further raw material components in the washcoat are PGM components, preferably salts of PGM components. Therefore, the washcoat typically contains palladium (Pd) salts and / or platinum (Pt) salts. Preferably, these salts are readily soluble in water. Preferably, the Pd salts and Pt salts 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】 Further optional raw material components, which are standard in forming the washcoat slurry, may also be present. These include one or more binders and thickeners. Binders may include, for example, oxide materials having small particle sizes for binding individual insoluble particles together in the washcoat slurry. The use of binders in washcoats is well known in the art. Thickeners may include, for example, natural polymers having functional hydroxyl groups that interact with insoluble particles in the washcoat slurry. Thickeners serve the purpose of thickening the washcoat slurry to improve the coating profile during washcoat coating onto a substrate. Thickeners are usually burned off during washcoat or baking. Examples of specific thickeners / rheological modifiers for wash coats include glactomanna gum, guar gum, xanthan gum, curdlan schizophyllan, scleroglucan, diutan gum, huilan gum, hydroxymethylcellulose, carboxymethylcellulose, hydroxyethylcellulose, methylcellulose, methylhydroxyethylcellulose, methylhydroxypropylcellulose, and ethylhydroxycellulose. 【0032】 The slurry preferably has a solid content of 10-40%, more preferably 15-35%. Such a solid content can 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 solid content can allow for the deposition of a thin layer of washcoat onto the inner wall 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 the washcoat substrate. Placing the slurry onto the substrate can be done using techniques known in the art. Typically, the slurry is injected in a predetermined amount into the inlet of the substrate using a specific molding tool, thereby allowing the loaded carrier material to be placed on the substrate. Subsequent vacuum and / or air knife and / or drying steps may be used during the placement step, as will be discussed in more detail below. If the carrier is a filter block, the supported carrier material may be placed on the filter wall, inside the filter wall (if porous), or both. 【0034】 The pH of the slurry can be adjusted before coating to the desired pH using nitric acid or citric acid, and optionally, a base such as ammonia or barium hydroxide. The use of a base may be useful in preventing the pH from being adjusted to too low. 【0035】 Next, the method includes subjecting a first coating substrate to a first heat treatment to form a heat-treated coating substrate. The first heat treatment includes heating the first coating substrate to a first maximum temperature and holding the first coating substrate at the first maximum temperature. As can be understood, a typical heat treatment process involves passing a substrate through a furnace having a zone in which the temperature rises. The primary effect on the heated substrate is the maximum temperature reached. Therefore, the 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 held at the first maximum temperature for at least 30 minutes, preferably 1-3 hours. Shorter times may not be sufficient to achieve the desired aging, and longer times are not commercially desirable. Excessively long times may result in an undesirable high level of aging and a complete loss of the desired performance. The first heat treatment may be carried out under moisture-containing conditions, but ambient moisture is sufficient, and preferably, the aging is carried out under conditions of 5-15% by weight of H2O. 【0037】 The first heat treatment may be carried out in two steps. The first step may be a standard caking step, followed by a second aging step. However, preferably, the aging step can be performed by caking and aging simultaneously. 【0038】 When forming catalyst articles, the calcination process can be carried out at a range of temperatures, but the optimal temperature is determined by the properties of the wash coat and the application of the final catalyst. Generally, it is still preferable to use the lowest temperature at which calcination is suitable, because this results in the lowest process cost and the lowest possibility of damaging the article. Generally, the calcination temperature of DOC is in the range of about 500°C (e.g., 450-550°C), because this is sufficient to calcin the part without excessive damage or loss of function. Therefore, the first heat treatment of the present invention is carried out at a higher temperature than the normal calcination process. Furthermore, the purpose of the first heat treatment is to age the part so that the combination of the highest temperature reached and the time spent at that temperature is greater than that of the normal calcination process. 【0039】 One or more platinum group metal-containing wash coat layers formed on the substrate are preferably coated with 10-50 g / ft after the first heat treatment. 3 More comfortably 20-40g / ft 3 The PGM filling amount is provided on the coating substrate. 【0040】 One or more platinum group metal-containing wash coat layers preferably collectively 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. Alternatively, one or more platinum group metal-containing wash coat layers may collectively cover at least 40%, more preferably at least 60%, and most preferably at least 80% of the axial length of the substrate. This coverage area preferably extends from the outlet end. 【0041】 When the coating substrate has a continuous platinum group metal-containing coating that extends 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】 Following the first heat treatment, the method further comprises depositing a platinum group metal-containing composition on at least a portion of the heat-treated coating substrate to form a second coating substrate. That is, a new layer or zone of platinum group metal-containing composition is formed on the aged coating substrate. This provides a new PGM material for CO and HC oxidation, as well as potential exothermic generation properties. 【0043】 The new layer or zone may be applied by various techniques, including wash coating, as discussed above. Alternatively, the new layer or zone may be obtained by directly impregnating the aged coating 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 wash coat 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 on top of the original one or more platinum group metal-containing wash coat layers. However, in embodiments where the one or more platinum group metal-containing wash coat layers do not extend along their entire length, there may be no overlap between the new layer or zone and the aged coating on the substrate, or there may be only partial overlap. 【0045】 A second coating substrate is formed by depositing a platinum group metal-containing composition on at least a portion of a heat-treated coating substrate. The second coating substrate is then subjected to a second heat treatment, which includes 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. The second maximum temperature must 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, and more preferably 100 to 250°C lower. 【0046】 Preferably, the second heat treatment step is a standard caulking step. Preferably, the second heat treatment is performed at a second maximum temperature of 400 to 575°C, preferably 450 to 550°C. Preferably, the second heat treatment is performed by holding the second coating substrate at the second maximum temperature for at least 30 minutes, preferably 1 to 3 hours. 【0047】 As is to be understood, in a standard process where multiple layers are applied with a calcination step in between, each calcination step is carried out under the same conditions. There is no reason to switch the heat treatment temperatures, and naturally, there is no reason to make the first heat treatment hotter than the second. 【0048】 The first and second heat treatments are typically carried out in an oven or furnace, more typically in a belt-type or static oven or furnace, and typically in a specific flow of hot air from one direction. Each step may include an initial drying step. The drying and heat treatment steps may be continuous or sequential. For example, a separate wash coating may be applied after the substrate has already been wash-coated and dried together with the previous wash coating. The wash-coated substrate may also be dried and heat-treated using a single continuous heating program if the coating is complete. During heating, any complexes that may form in the solution may be decomposed at least partially, substantially, or completely. In other words, ligands of such complexes, e.g., ligands of organic compounds, may be removed or separated at least partially, substantially, or completely from the PGM ions and removed from the final catalyst article. The palladium particles thus separated may then begin to form metal-metal and metal-oxide bonds. As a result of heating (calcination), the substrate is typically substantially free of organic compounds, and more typically completely free of organic compounds. 【0049】 After each heating step, the substrate is typically cooled, more typically to room temperature. Cooling is typically carried out in air, with or without a coolant / cooling medium, typically without a coolant. 【0050】 It was found that efficient heat generation can be best achieved with a high PGM filling rate 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 lower-temperature front part, the article has improved lifespan and durability because this part does not receive the highest temperature. 【0051】 In a preferred configuration, the DOC has a front zone extending from the inlet end, where the PGM concentration is higher than in the rear zone extending from the inlet end. Preferably, the PGM concentration is at least twice, more preferably at least four times, and preferably four to ten times higher in the front zone. Typically, the rear portion, which has a lower PGM content, is more resistant to sintering due to the lower PGM content. That is, if the amount of PGM packed in the rear zone is lower, the PGMs will be more spaced apart and less likely to sinter together. Using a lower amount of PGM in the rear zone is more efficient for PGM use. Using a higher amount in the front zone allows for efficient exothermic generation without compromising performance in other respects. Passive oxidation of CO and HC occurs in the front zone, while the rear zone is sufficient to deal with the competitive NOx oxidation that subsequently needs to occur. 【0052】 Preferably, the exit zone (rear zone) has a lower g / in than the inlet zone (front zone). 3 It has a Pt filling amount in units. This is useful for an embodiment of efficient heat generation. In another embodiment, the outlet zone is greater than the inlet zone in g / in 3 The amount of Pt packed per unit is large. 【0053】 According to a preferred embodiment, a method for producing a diesel oxidation catalyst, (i) To provide a carrier substrate, (ii) To provide a first coating substrate by forming one or more platinum group metal-containing wash coat layers on a carrier substrate, the wash coat layers being located on 100% of the axial length of the carrier and containing Pt and optionally Pd, (iii) subjecting the first coating substrate to a first heat treatment, which includes heating the first coating substrate to a first maximum temperature and holding the first coating substrate at the first maximum temperature, to form a heat-treated coating substrate. (iv) Depositing a platinum group metal-containing composition on at least a portion of a heat-treated coating substrate to form a second coating substrate, wherein the composition forms an inlet zone at 10-40% of the axial length of the carrier substrate, (v) Subjecting the second coating substrate to a second heat treatment, which includes heating the second coating substrate to a second maximum temperature and holding the second coating substrate at the second maximum temperature, in order to form a diesel oxidation catalyst. A method is provided in which a first maximum temperature is 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. 【0054】 According to a preferred embodiment, a method for producing a diesel oxidation catalyst, (i) To provide a carrier substrate, (ii) To provide a first coating substrate by forming one or more platinum group metal-containing wash coat layers on a carrier substrate, the wash coat layers being located on 100% of the axial length of the carrier and containing Pt and optionally Pd, (iii) subjecting the first coating substrate to a first heat treatment, which includes heating the first coating substrate to a first maximum temperature and holding the first coating substrate at the first maximum temperature, to form a heat-treated coating substrate. (iv) Depositing a platinum group metal-containing composition on at least a portion of a heat-treated coating substrate to form a second coating substrate, wherein the composition forms an inlet zone on 10-40% of the axial length of the carrier substrate, (v) Subjecting the second coating substrate to a second heat treatment, which includes heating the second coating substrate to a second maximum temperature and holding the second coating substrate at the second maximum temperature, in order to form a diesel oxidation catalyst. A method is provided in which a first maximum temperature is 650°C to 725°C, the first heat treatment is performed for 1 to 3 hours in a moisture-containing atmosphere containing 5 to 15% by weight of H2O, and the second maximum temperature is 475 to 525°C in air for 1 to 3 hours. 【0055】 To be understood, the above method defines an intervening aging process (first heat treatment) and a final calcination process (second heat treatment) for the purpose of describing the production of a DOC having an aged platinum group metal-containing wash coat layer on top and a new platinum group metal-containing composition deposited at the inlet end. An alternative method to consider this is to consider the dispersion of PGM material in each layer. When PGM is applied to a wash coat, it is generally finely dispersed, but aging results in a sintering effect that forms larger chunks 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 has a unique structure in which it has aged larger chunks in the underlying wash coat layer, but has new, finely dispersed PGM in the upper layer (or is impregnated into the underlying wash coat layer to give a multimodal distribution). 【0056】 Whether a 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 morphology 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 CO uptake of the sample is measured using the Micromeritics Autochem 2920 instrument. The sample is pre-treated with hydrogen gas at 300°C. Carbon monoxide uptake is measured by pulsed chemiadsorption at 50°C. Then, 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. Larger particles with a lower surface area have lower dispersion. Therefore, this technique makes it possible to determine the extent to which the applied PGM has been sintered together by aging. 【0057】 Preferably, the aged platinum group metal-containing wash coat layer contains platinum group metal particles having an average particle size (D50) 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 less than 10 nm as determined by TEM. These properties of each applied layer or zone can be evaluated by TEM inspection. 【0058】 In a further embodiment, a diesel oxidation catalyst article is provided comprising a flow-through carrier substrate having an aged platinum group metal-containing wash coat layer on which a new platinum group metal-containing composition is deposited at the inlet end, wherein the aged platinum group metal-containing wash coat layer contains platinum group metal particles having an average particle size (D50) 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 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 can be obtained by the method described herein. 【0060】 According to further aspects, (A) Soot filter; (B) SCR catalyst article; (C) SCRF catalyst article; (D) Catalytic soot filter; (E) Soot filter, and then SCR catalyst article; or (F) Catalytic soot filter, and then SCR catalyst article, An exhaust gas treatment system is provided, which includes a diesel oxidation catalyst as described herein, positioned upstream of one of the following. 【0061】 These components are well known in the art. These components can be beneficial in one of two ways by providing DOC obtained by the methods disclosed herein to an upstream location. Some of these components, such as soot filters, benefit from their ability to provide heat to DOC. This additional heat acts to promote the combustion and removal of soot. Others of these components benefit particularly from their ability to produce stabilized NO2. 【0062】 This is especially 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 or SCRF catalyst can affect its performance (see Reaction 2). Generally, SCR or SCRF catalysts perform optimally when the NO2:NO ratio is approximately 1:1. This can be problematic because the exhaust gas produced by compression ignition engines during normal use typically does not contain enough NO2 for the SCR or SCRF catalyst to perform optimally (i.e., the NO2:NO ratio is much lower than 1:1). 【0064】 To compensate for such low levels of NO2, 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 NO2 generation levels are maintained throughout the entire operating life, it is possible to optimize the levels of PGM in components. 【0065】 In a further embodiment, a diesel combustion and exhaust gas treatment system is provided, which includes a diesel combustion engine and an exhaust system as described herein. 【0066】 In a further embodiment, a method for manufacturing an exhaust system as described herein is provided, comprising forming a diesel oxidation catalyst according to the method described herein and placing it upstream of any of (A) to (F). 【0067】 definition As used herein, the singular forms "a," "an," and "the" refer to multiple objects unless the context clearly indicates otherwise. 【0068】 The use of the term “comprising” is intended to be interpreted as including such features but not excluding other features, and also intended to include a selection of features that are necessarily limited to those described. In other words, the term also includes the limitations of “essentially consisting of” (intended to mean that certain further components may exist on the condition that they do not substantially affect the essential nature of the described feature) and “consisting of” (intended to mean that if the components are expressed as percentages by their proportions, these add up to 100%, while explaining any unavoidable impurities, but not including any other features). 【0069】 As used herein, the term “above” is intended to mean “directly above,” such that there is no intervening layer between one material and another. Spatially relative terms such as “below,” “beneath,” “lower,” “above,” and “upper” may be used herein to facilitate descriptions of the relationship between one element or feature and another. It will be understood that spatially relative terms are intended to encompass different orientations of the catalyst in use or operation, in addition to the orientation shown in the figures. 【0070】 The term "caking" or "caking" means heating a material in air or oxygen. This definition is consistent with the IUPAC definition of caking. (IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Created by ADMcNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). XML online revised version: http: / / goldbook.iupac.org (2006-), created by M.Nic, J.Jirat, B.Kosata; updated by A.Jenkins. ISBN 0-9678550-9-8. doi:10.1351 / goldbook). The temperature used for caking varies depending on the components in the material being caked. RMHeck et al., "Catalytic Air Pollution Control - Commercial Technology", John Wiley & Sons, Inc., 3 rd According to Chapters 2.3.3 and 2.3.4 of Edition (2009), catalyst species applied in wash coats on monolithic substrates for automotive applications, containing platinum and palladium salts, are air-dried at approximately 110°C and calcined in air to approximately 400–500°C to remove all trace amounts of the degradable salts used to prepare the catalyst. In applications involving the processes described herein (i.e., standard DOC preparation and second heat treatment), calcination is generally carried out at a temperature of approximately 400°C–600°C for approximately 1–8 hours, preferably at a temperature of approximately 400°C–550°C for approximately 1–4 hours. [Brief explanation of the drawing] 【0071】 Next, the present invention will be further described with reference to the following non-limiting figures. [Figure 1] A schematic diagram of a layer formed by the method described herein is shown. [Figure 2] A flowchart of the key steps of the method described herein is shown. [Figure 3] The NO2 / NOx performance at different temperatures for standard DOC and DOC prepared according to the present invention is shown. [Figure 4] This shows the PGM particle size in a standard DOC. [Figure 5] The PGM particle size in DOC prepared according to the present invention is shown. 【0072】 As shown in Figure 1, DOC1 is provided. DOC1 includes a substrate 5. The substrate 5 is preferably a flow-through substrate such as a porous cordierite. The substrate 5 has an inlet end 10 for receiving the exhaust gas to be treated and an outlet end 15 for releasing the treated exhaust gas. The direction of exhaust gas flow is indicated by arrow 20. 【0073】 The substrate 5 has a PGM-containing layer 25 provided along its entire length by a wash coat. This layer is generally either a layer containing only Pt, or a layer containing both Pd and Pt. The total PGM content of this layer is typically 10-50 g / ft 3 The PGM content is provided on a support material such as alumina. The PGM-containing layer 25 undergoes an aging process to stabilize the NO oxidation performance of this layer. 【0074】 An additional PGM-containing zone 30 is provided at the inlet end 10 above the PGM-containing layer 25. This can be applied by wash coating or impregnation. This zone 30 preferably contains Pt and typically an additional 10-50 g / ft 3 This provides the PGM. Since this zone 30 has not undergone an aging process, it retains its initial activity. This promotes heat generation at the DOC inlet. 【0075】 As shown in Figure 2, this method is (A) To provide a carrier substrate 5, (B) Forming a PGM-containing layer 25 on the carrier substrate 5, (C) The PGM-containing layer 25 on the carrier substrate 5 is aged at a temperature of 650°C for 1 to 3 hours in an atmosphere containing 10% by weight of moisture. (D) Forming a PGM-containing zone 30 on the aged PGM-containing layer 25 on the carrier substrate 5, Includes. 【0076】 As shown in Figure 3, conventional (standard) reference DOCs show a significant decrease in NO2 / NOx performance between their new (i.e., degreened) performance and their aged performance (140 hours at 650°C). In contrast, DOCs prepared according to the present invention show a much smaller change between their new (i.e., degreened) performance and their aged performance (140 hours at 650°C). DOCs prepared according to the present invention undergo a first heat treatment at 700°C for 3 hours. 【0077】 Figure 4 shows the significant change in PGM particle size in the reference component between an initial particle size of around 6 nm (D90 less than 10 nm) and a widely distributed particle size after aging (D50 greater than 10 nm). In contrast, Figure 5 shows that the PGM particle size of the DOC obtained according to the present invention has an initial particle size with a D50 greater than 10 nm. After aging, the particle size is smaller overall than in the comparative data. That is, the change in PGM particle size in the DOC of the present invention is smaller. 【0078】 It should be noted that Figures 4 and 5 only consider the PGM particle size in one or more platinum group metal-containing wash coat layers on the carrier substrate. Therefore, the DOC obtained according to the present invention has a further distribution of PGM (e.g., in the upper wash coat 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 embodiments. 【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, namely: (a) arranging a containing means on a substrate; (b) introducing a predetermined amount of liquid component into the containing 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 onto 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 (bare) 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 of platinum and palladium (as nitrates) and a particulate gamma alumina support material was coated onto 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 ), that is, 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 used. 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 catalyst washcoat slurry, containing aqueous platinum nitrate (the only platinum group metal present) and a particulate gamma-alumina support material, was applied to a substrate already coated with the first coating, starting from the end of the monolith substrate opposite to the end where the first coating was applied, i.e., from the exit end. The axial length of the second catalyst washcoat coating was 75% of the total substrate length, meaning that 50% of the second washcoat catalyst coating overlapped with the first washcoat catalyst coating. The concentration of the platinum salt used was 2.02 gft over the coated 75% of the axial substrate length. -3 The Pt filling amount was selected to achieve the desired result. Substrates coated with both the first and second wash coat coatings were dried in a conventional oven at 100°C for 1 hour, and then the dried parts were baked at 500°C for 1 hour to decompose the platinum and palladium salts, thereby fixing the platinum and palladium to the particulate gamma alumina support material. 【0083】 Next, an aqueous medium containing both platinum nitrate and palladium nitrate salts in a 1:1 weight ratio was impregnated onto the coating of the first catalyst wash coat to a length of 25% of the substrate's axial length, measured from the substrate's intake end. The salt concentration was 35 gft for platinum and palladium, respectively, in the impregnated length of the substrate. -3 This was chosen to achieve a weight of 35 gft in addition to the filler amount of the first catalytic wash coat undercoat. -3 The additional filling resulted in a high PGM filling rate within the inlet end zone. The impregnated parts were dried in a conventional oven at 100°C for 1 hour, and then the dried parts were baked at 500°C for 1 hour. 【0084】 All wash coats and impregnation solutions were inherently acidic, and no pH adjustment was performed. 【0085】 The final product included a monolithic substrate comprising three catalyst wash coat zones arranged in series axially, namely, a first highly packed front zone defined as approximately 25% of the axial length of the monolithic substrate measured from the inlet end and having a total platinum group metal filling amount of a first catalyst wash coat of 1Pt:1Pd underlay and a combination of impregnated 1:1 Pt:Pd; followed by a second catalyst wash coat zone, arranged in series axially and comprising approximately 50% of the axial length of the monolithic substrate, comprising a second catalyst wash coat of Pt only superimposed on the first catalyst wash coat of 1Pt:1Pd and having a lower total platinum group metal filling amount than the first catalyst wash coat zone; and finally, a third Pt only zone at the outlet end comprising approximately 25% of the axial length of the monolithic substrate, comprising a second catalyst wash coat coating and having a lower total platinum group metal filling amount than either the first or second catalyst wash coat zone. The total amount of platinum group metals packed into the monolithic substrate is 21 gft, with a total Pt:Pd weight ratio of 7:6, corresponding to 1.167:1. -3 The resulting catalyst is described herein as "new / fresh," i.e., as prepared. 【0086】 Example 2 (Comparative Example) The same product as disclosed in Reference Example 1 was prepared, except that the impregnated parts were dried in a conventional oven at 100°C for 1 hour, and then the dried parts were baked at 700°C for 3 hours. The resulting catalyst is described herein as "new / fresh," i.e., as prepared. 【0087】 Example 3 The same product as in Comparative Example 2 was prepared, except that the order of the calcination and impregnation processes was reversed. Specifically, 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, the first catalyst washcoat of this product was impregnated with an aqueous medium containing both platinum nitrate and palladium nitrate salts in a 1:1 weight ratio to 25% of the axial length of the substrate, measured from the substrate inlet end. The impregnated part was oven-dried at 100°C for 1 hour, and then the dried part was calcined at 500°C for 1 hour. The resulting catalyst is described herein as "new / as-is," i.e., as 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 performed using a diesel engine mounted on a laboratory bench. The engine was filled with EUVI B7 fuel (7% biofuel) and operated at 2200 rpm for both engine operation and exhaust gas hydrocarbon concentration (heat generation). The engine was equipped with an exhaust system including exhaust piping and a removable canning that allowed each composite oxidation catalyst to be inserted into the engine for testing, with the inlet end / high-fill washcoat zone of the first catalyst oriented upstream. The engine was a 7-liter capacity EUV 6-cylinder engine producing 235 kW at 2500 rpm, and the exhaust system included a "seventh injector" positioned to directly inject hydrocarbon fuel into the exhaust gas piping downstream from the engine manifold and upstream of the composite oxidation catalyst under test. This fuel injector is named the "seventh injector" because it is in addition to the six fuel injectors associated with the engine's cylinders. The thermocouples were positioned at the intake of 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 when new (see below) and after aging was as follows. Aging was performed as follows. Each composite oxidation 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 activity at the end of the vehicle's lifespan, and tested for average NO. Detected NO2 / total NO x A velocity / load map was created for this purpose, and the integral mean was calculated for mass flow rates of 400-1000 kg / hour versus catalyst inlet temperatures of 200-350°C in the quadrant. The results are reported in Table 1 below. 【0090】 The exothermic test was performed 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, followed by a rapid cooling process (a process known as "de-greening"). Subsequently, the exhaust gas flow rate was reduced to 720 kg / hour (for the size and volume of the tested substrate, this amounted to 120,000 hours). -1 The engine load was controlled so that the set intake exhaust gas temperature remained stable at approximately 270°C for approximately 1800 seconds, corresponding to the spatial velocity. 【0091】 Next, the ability of the composite oxidation catalyst to generate heat at stable set temperatures was tested by injecting hydrocarbon fuel through a seventh injector targeting both 600°C and stable hydrocarbon "slip" at the outlet of the composite oxidation catalyst substrate, using downstream thermocouples and hydrocarbon sensors. The test was stopped if the hydrocarbon slip measured downstream of the composite oxidation catalyst exceeded 1000 ppm C3, i.e., regardless of the length of the hydrocarbon chain in the detected hydrocarbon (in typical diesel fuel, the formal carbon chain length is C3). 16 The test was stopped when the equivalent of 1000 ppm C3 was detected. Therefore, 187.5 ppm C 16 If detected, 1000 ppm C3(C 16 (This is equivalent to 5 1 / 3 × C3 hydrocarbons.) 【0092】 Following testing at an inlet temperature set to approximately 270°C, the system was again pre-conditioned at an inlet exhaust gas temperature of 490°C for 10 minutes at a flow rate of 1000 kg / hour, then rapidly cooled, and an exothermic test was performed at a second set temperature, e.g., approximately 260°C. This cycle was repeated to test exothermic generation at set temperatures of approximately 250°C, 240°C, and 230°C. The test was stopped if the composite oxidation catalyst could not generate a stable exothermic output of 600°C at the outlet end of the composite oxidation catalyst, or if the hydrocarbon slip measured at the outlet end of the composite oxidation catalyst exceeded 1000 ppm(C3). 【0093】 [Table 1] 【0094】 The results of the 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 intake temperature that can be achieved with a stable heat-generating hydrocarbon slip, the more advantageous it is. This is because the filter regeneration event can be initiated by a lower intake exhaust gas temperature, i.e., without having to wait until the exhaust gas temperature under normal operating conditions becomes high enough to initiate filter regeneration, which is infrequent under normal operation. Furthermore, overall fuel economy is improved because 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. 【0095】 Furthermore, from the oxidation activity of each catalyst, it can be seen that the difference in NO oxidation activity from the new state to after aging is lower in 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 Reference Example 1 (54.6%-43.7%=10.9%). The smaller the "change" between the catalytic activity when new and the catalytic activity after aging, the easier it is for OEM customers to: program the engine control unit using an algorithm that allows for the degradation of NO oxidation catalytic activity over time and adjusts urea injection to the downstream SCR catalyst accordingly; or program it to shorten the interval of downstream active filter regeneration using passive soot combustion in NO2 by using the CRT® effect between active regeneration events. 【0096】 Therefore, the catalyst of Example 3 according to the present invention has a lower average NO2 / NO2 ratio between new and aged conditions than Reference Example 1. x It possesses both a change in ratio and the ability to generate heat at a lower temperature than Comparative Example 2. 【0097】 The above-mentioned detailed description is provided for illustrative and illustrative purposes only 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. Furthermore, the disclosure of the present invention may include the following embodiments. (Aspect 1) A method for manufacturing a diesel oxidation catalyst, (i) To provide a carrier substrate, (ii) To provide a first coating substrate by forming one or more platinum group metal-containing wash coat layers, each containing a refractory metal oxide carrier material, on the carrier substrate, (iii) Subjecting the first coating substrate to a first heat treatment, which includes heating the first coating substrate to a first maximum temperature and holding the first coating substrate at the first maximum temperature, to form a heat-treated coating substrate. (iv) Depositing a platinum group metal-containing composition containing a refractory metal oxide carrier material onto at least a portion of the heat-treated coating substrate to form a second coating substrate, (v) Subjecting the second coating substrate to a second heat treatment, which includes heating the second coating substrate to a second maximum temperature and holding the second coating substrate at the second maximum temperature, in order to form the diesel oxidation catalyst, 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. method. (Aspect 2) The method according to embodiment 1, wherein the first heat treatment is performed in a moisture-containing atmosphere. (Aspect 3) The first heat treatment described above is (a) at the first maximum temperature of 625 to 750°C, preferably 650 to 700°C, and / or (b) Holding the first coating substrate at the first maximum temperature for at least 30 minutes, preferably 1 to 3 hours, and / or (c) 5-15% by weight of H 2 Under the condition O, The method according to embodiment 1 or embodiment 2, which is carried out. (Aspect 4) The second heat treatment described above is (a) at the second maximum temperature of 400 to 575°C, preferably 450 to 550°C, and / or (b) The second coating substrate is held at the second highest temperature for at least 30 minutes, preferably 1 to 3 hours. The method according to any one of embodiments 1 to 3. (Appendix 5) The method according to any one of embodiments 1 to 4, wherein the carrier substrate is a flow-through substrate. (Aspect 6) The method according to any one of embodiments 1 to 5, wherein the one or more platinum group metal-containing wash coat layers on the carrier substrate contain Pt and / or Pd. (Aspect 7) The method according to any one of embodiments 1 to 6, wherein the one or more platinum group metal-containing wash coat layers on the carrier substrate further comprises an alkaline earth metal, preferably strontium and / or barium. (Pattern 8) The method according to any one of embodiments 1 to 7, wherein the coating substrate has a continuous platinum group metal-containing coating that extends from the inlet end to the outlet end of the carrier substrate. (Aspect 9) 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. (i) The exit zone has a g / in 3 Having a smaller amount of Pt filling per unit, or (ii) The exit zone has a g / in 3 Having a larger Pt filling amount per unit, The method described in aspect 8. (Aspect 10) The method according to embodiment 9, wherein the continuous platinum group metal-containing coating comprises the inlet zone and the outlet zone. (Aspect 11) Step (iv) is, (I) Applying a platinum group metal-containing wash coat to the first coating substrate, preferably forming a wash coat zone that extends from the entrance edge of the substrate; or (II) Impregnate the first coating substrate with a solution of a platinum group metal-containing salt to form a platinum group metal impregnation zone that preferably extends from the entrance end of the substrate. The method according to any one of embodiments 1 to 10, including the method described above. (Aspect 12) The method according to any one of embodiments 1 to 11, wherein the heat-treated coating 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. (Aspect 13) The method according to any one of embodiments 1 to 12, wherein the diesel oxidation catalyst includes 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. (Aspect 14) A diesel oxidation catalyst article comprising a flow-through carrier substrate having an aged platinum group metal-containing wash coat layer on which a new platinum group metal-containing composition deposited at the inlet end, wherein the aged platinum group metal-containing wash coat layer contains platinum group metal particles having an average particle size (D50) 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 less than 10 nm as determined by TEM. (Aspect 15) A diesel oxidation catalyst according to embodiment 14, obtained or obtainable by the method described in any one of embodiments 1 to 13. (Aspect 16) (A) Soot filter; (B) SCR catalyst article; (C) SCRF catalyst article; (D) Catalytic soot filter; (E) Soot filter, and then SCR catalyst article; or (F) Catalytic soot filter, and then SCR catalyst article, An exhaust gas treatment system comprising the diesel oxidation catalyst according to claim 14 or 15, positioned upstream of the exhaust gas treatment system. (Aspect 17) A diesel combustion and exhaust gas treatment system comprising a diesel combustion engine and the exhaust system described in embodiment 16. (Aspect 18) A method for manufacturing an exhaust system according to embodiment 16, comprising forming a diesel oxidation catalyst according to the method of any one of embodiments 1 to 13, and placing it upstream of any of (A) to (F). 【0098】 To avoid any doubt, the entire contents of all documents referenced herein are incorporated by reference into this application.

Claims

[Claim 1] A method for manufacturing a diesel oxidation catalyst, (i) To provide a carrier substrate, (ii) To provide a first coating substrate by forming one or more platinum group metal-containing wash coat layers on the carrier substrate, each containing Pt and Pd and a refractory metal oxide carrier material, (iii) To form a heat-treated coating substrate by subjecting the first coating substrate to a first heat treatment, which includes heating the first coating substrate to a first maximum temperature and holding the first coating substrate at the first maximum temperature, (iv) Depositing a platinum group metal-containing composition on at least a portion of the heat-treated coating substrate to form a second coating substrate, (v) Subjecting the second coating substrate to a second heat treatment, which includes heating the second coating substrate to a second maximum temperature and holding the second coating substrate at the second maximum temperature, in order to form the diesel oxidation catalyst, The first maximum temperature is 625 to 750°C, and the second maximum temperature is 400 to 575°C. method. [Claim 2] The method according to claim 1, wherein the first heat treatment is performed in a moisture-containing atmosphere. [Claim 3] The first heat treatment described above is (a) at the first maximum temperature of 650 to 700°C, and / or (b) Holding the first coating substrate at the first maximum temperature for at least 30 minutes and / or (c) 5-15% by weight of H ２ Under the condition O, The method according to claim 1 or claim 2, which is carried out. [Claim 4] The second heat treatment described above is (a) at the second maximum temperature of 450 to 550°C, and / or (b) The second coating substrate is held at the second maximum temperature for at least 30 minutes. The method according to claim 1 or claim 2, which is carried out. [Claim 5] The method according to claim 1 or claim 2, wherein the carrier substrate is a flow-through substrate. [Claim 6] The method according to claim 1 or 2, wherein the one or more platinum group metal-containing wash coat layers on the carrier substrate further contain alkaline earth metals. [Claim 7] The method according to claim 1 or claim 2, wherein the first coating substrate has a continuous platinum group metal-containing coating that extends from the inlet end to the outlet end of the carrier substrate. [Claim 8] 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. (i) The exit zone has a g / in ３ Having a smaller Pt filling amount per unit, or (ii) The exit zone has a g / in ratio greater than the inlet zone. ３ Having a larger Pt filling amount per unit, The method according to claim 7. [Claim 9] The method according to claim 8, wherein the continuous platinum group metal-containing coating comprises the inlet zone and the outlet zone. [Claim 10] Process (iv) is, (i) Applying a platinum group metal-containing wash coat to the first coating substrate, preferably forming a wash coat zone that extends from the entrance end of the substrate; or (II) Impregnating the first coating substrate with a solution of a platinum group metal-containing salt, The method according to claim 1 or claim 2, including the method described in claim 1 or claim 2. [Claim 11] The method according to claim 1 or claim 2, wherein the heat-treated coating substrate comprises platinum group metal particles having an average particle size (D50) greater than 10 nm as determined by TEM. [Claim 12] The method according to claim 1 or 2, wherein the diesel oxidation catalyst includes a layer or zone formed in step (iv) including platinum group metal particles, the particles having a D90 particle size of less than 15 nm as determined by TEM.

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