Hydrogen internal combustion engine and exhaust system including catalyst article for treating ammonia slip

Abstract

A fuel combustion and exhaust-gas treatment system comprises an internal combustion engine configured to run on a mixture of air and fuel, wherein the fuel of the mixture of air and fuel is a gaseous fuel comprising a majority fuel mass of hydrogen (H2), a source of gaseous fuel comprising a majority fuel mass of H2 and an exhaust-gas treatment system comprising a catalyst article for the treatment of an exhaust gas, the catalyst article comprising a wall-flow filter substrate having inlet channels open at an inlet-end of the substrate and closed at an outlet-end of the substrate, the inlet channels adjacent outlet channels which are closed at the inlet-end of the substrate and open at the outlet-end of the substrate, the wall-flow filter comprising at least first, second and third catalytic layers, wherein: (i) the first catalytic layer extends from the inlet-end of the substrate and comprises a first SCR composition; (ii) the second catalytic layer is provided in or on the walls of the inlet channels, extending from the inlet-end of the substrate, and comprises a noble metal- containing composition, wherein: when the second catalytic layer is provided on the walls of the inlet channel, the first catalytic layer is provided on the second catalytic layer, and when the second catalytic layer is provided in the walls of the inlet channels, the first catalytic layer is provided on the walls of the inlet channels; and (iii) the third catalytic layer is provided in or on the walls of the outlet channels, extending from the outlet-end of the substrate, and comprises a second SCR composition; wherein a ratio of the noble metals in the second catalytic layer in g per ft3 to the total amount of the first and second SCR compositions in the first and third catalytic layers in g per in3 is from 1.5:8 to 2:1.

Description

[0001] HYDROGEN INTERNAL COMBUSTION ENGINE AND EXHAUST SYSTEM INCLUDING CATALYST ARTICLE FOR TREATING AMMONIA SLIP

[0002] The present invention relates to a fuel combustion and exhaust-gas treatment system comprising an internal combustion engine (ICE) configured to run on a mixture of air and fuel, wherein the fuel of the mixture of air and fuel is a gaseous fuel comprising a majority fuel mass of hydrogen (H2), a source of gaseous fuel comprising a majority fuel mass of hydrogen (H2) and an exhaust-gas treatment system comprising a catalyst article for use as an ammonia slip catalyst. In particular, the ammonia slip catalyst is provided on a wall-flow filter. The catalyst article is designed to address particulate emissions resulting from the upstream addition of reductants such as ammonia or urea, while maintaining good exhaust treatment performance.

[0003] Hydrogen has been receiving significant attention as a potential future alternative fuel for powering internal combustion engines (ICEs). It is envisioned that low-carbon hydrogen could be produced via hydrolysis of water using renewable electricity supplied by wind and solar powerplants (so-called “Green Hydrogen”). Different “types” of hydrogen are commonly referred to using a colour-scheme that reflects upstream greenhouse gas (GHG) emissions. For example, turquoise hydrogen produced from natural gas via pyrolysis (with solid carbon as a by-product); black / grey hydrogen is produced from natural gas using steam reforming of methane; blue, grey or brown hydrogen with its CO2 sequestered or repurposed and the brown hydrogen extracted from fossil fuels, usually coal, using gasification; pink / purple / red hydrogen produced by electrolysis of water using nuclear power; white hydrogen produced as a by-product of industrial processes; and yellow hydrogen produced by electrolysis of water using grid electricity.

[0004] It is known to fuel a compression ignition engine with a fuel mixture comprising diesel fuel and a minority fuel mass of hydrogen (see e.g. K.S. Varde et al, International Journal of Hydrogen Energy, 7, 549-555 (1983)). It is also known to fuel a spark-ignition engine with a fuel mixture comprising gasoline (petrol) and a minority fuel mass of hydrogen (see M. Al- Baghdadi et al, Energy Conversion and Management, 41, 77-91 (2000)). Hydrogen may be derived on the vehicle by an on-board fuel reformer (see e.g. Applicant’s WO2014 / 118574A1; and W02012 / 063082A1). For further information, Applicant refers to DieselNet Technology Guide » Alternative Fuels » “Hydrogen” particularly section “5. Hydrogen Fueled Engines”, Author: H. Jaaskelainen, Revision 2023.07 available at In contrast to prior art disclosures of a minority fuel mass of hydrogen, the present invention is directed to systems comprising internal combustion engines configured to run on a mixture of air and fuel, wherein the fuel of the mixture of air and fuel is a gaseous fuel comprising a majority fuel mass of hydrogen and an exhaust system. These engines are sometimes referred to in the art as “H2-ICE” engines. A “majority fuel mass” is >50vol% hydrogen gas and can be at least 70 vol% hydrogen gas (H2), e.g. of one of the colour-scheme sources discussed above or a mixture of two or more thereof. Exhaust gas of ICE engines powered wholly or in part by H2 include H2 itself and oxides of nitrogen (NOx). Ammonia emissions could also be possible under conditions that generate rich combustion. To compare to the redox composition of prior art diesel and gasoline exhaust gas, the lambda value of exhaust gases emitted by internal combustion engine configured to run on a mixture of air and fuel, wherein the fuel of the mixture of air and fuel is a gaseous fuel comprising a majority fuel mass of H2 can vary widely, e.g. from 1 to 30, which can make catalytic aftertreatment difficult. Lambda can be calculated using the Brettschneider Equation.

[0005] Combustion of lubricating oil can generate CO, hydrocarbon and particulate number and urea-generated particles where selective catalytic reduction is used to reduce NOx. Particulates derived from these sources may require filtration.

[0006] H2-ICE engines can be spark-ignition (SI) engines or High-Pressure Direct Injection (HPDI) engines, such as pilot injection HDPI engines, a variation on commercially available heavy- duty natural gas engine technology. For the avoidance of any doubt, the claimed invention is intended to cover both SI and HPDI engines.

[0007] Applicant’s WO2025 / 083425 discloses an apparatus comprising an internal combustion engine configured to run on a mixture of air and fuel, wherein the fuel of the mixture of air and fuel is a gaseous fuel comprising at least 95 vol% H2, a source of gaseous fuel comprising at least 95 vol% H2 and an exhaust system comprising a catalytic aftertreatment component, wherein the catalytic aftertreatment component comprises a catalyst composition comprising palladium and gold on a metal oxide support.

[0008] Although emission standards similar to e.g., the so-called Euro 7 emission standard, have yet to be set for internal combustion engines powered by H2 fuel, Applicant believes that treatment of emissions from on-road vehicles and non-road vehicles with a power rating >56kWwill require treatment of both uncombusted H2 and oxides of nitrogen (NOX), whereas non-road vehicles power rated <56kW may require treatment only of uncombusted H2. H2 emissions can be treated by oxidation to H2O, whereas NOXcan be treated by a range of possible reactions, but one preferred reaction is the selective reduction of NOxwith a nitrogenous reductant, particularly ammonia which may - according to common general knowledge - be derived from a urea precursor. NOXreduction using NH3 - and urea as a precursor thereof - has already been applied to current Diesel vehicles. Selective reduction of NOXusing a catalyst is also referred to as “selective catalytic reduction” or “SCR”.

[0009] In a SCR process, a gaseous reductant, typically anhydrous ammonia, aqueous ammonia, or urea, is added to an exhaust gas stream prior to the exhaust gas contacting the catalyst. The reductant is absorbed onto the catalyst and the NOXis reduced as the gases pass through or over the catalysed substrate.

[0010] In this regard, several chemical reactions occur in an NH3 selective catalytic reduction (SCR) process, all of which represent desirable reactions that reduce NOXto nitrogen. Specifically:

[0011] (1) 4NH3+ 4NO + O24N2+ 6H2O (i.e. 1:1 NH3:NO)

[0012] (2) 4NH3+ 2NO + 2NO24N2+ 6H2O (i.e. 1 :1 NH3:NOX)

[0013] (3) 8NH3 + 6NO2 7N2+ 12H2O (i.e. 4:3 NH3:NOX)

[0014] The dominant reaction is reaction (1). However, reaction (2) is kinetically faster than either reaction (1) and (3) and can be desirably promoted by oxidising NO in exhaust gas to NO2 upstream from the SCR catalyst so that exhaust gas entering the SCR catalyst has an approximately 1 :1 volume ratio of NO to NO2. A relevant undesirable, non-selective sidereaction is:

[0015] (4) 2NH3+ 2NO2N2O + 3H2O + N2.

[0016] Suitable catalysts active for reactions (1), (2) and (3) are known, and include aluminosilicate zeolites, particularly those known as “small pore zeolites”, i.e. containing a maximum ring size of eight tetrahedral atoms, including zeolites of the framework type code CHA and AEI promoted, e.g. ion-exchanged, with copper and / or iron; and vanadia supported on titania or titania optionally doped with silica. The titania can also be promoted with oxides of tungsten.

[0017] In order to maximize the conversion of NOX, it is often necessary to add more than a stoichiometric amount of ammonia to the gas stream. However, release of the excess ammonia into the atmosphere would be detrimental to the health of people and to the environment. In addition, ammonia is caustic, especially in its aqueous form. Condensation of ammonia and water in regions of the exhaust line downstream of the exhaust catalysts can result in a corrosive mixture that can damage the exhaust system. Therefore, the release of ammonia in exhaust gas should be eliminated, as far as possible.

[0018] Even the best SCR catalyst will not achieve maximum NOXreductions in a system with non- uniform NH3 distribution. Significant variation in load, exhaust flow rate and NOXconcentration make it difficult to deliver NH3 to the catalyst in the 1 :1 ammonia NOXratio (ANR) required by reaction stoichiometry. Non-uniform NH3 distribution can result in incomplete NOXconversion where localized ANR is low, and in NH3 slip where ANR is high.

[0019] In many conventional Diesel exhaust systems, an ammonia oxidation catalyst (also known as an ammonia slip catalyst or "ASC" (alternatively an Ammonia Oxidation or “AMOx” catalyst) is installed downstream of the SCR catalyst to remove ammonia from the exhaust gas by converting it to nitrogen. Ammonia slip catalysts are well known in the art. The purpose of an ammonia slip catalyst is to treat any ammonia passing through an upstream SCR catalyst.

[0020] To overcome the difficulty of attaining ideal stoichiometry, generally ASC technology combines an oxidation catalyst and SCR catalyst functions on a flow-through (FT) substrate to improve NOXreduction while maintaining low NH3 slip. The ASC allows continuous operation of an upstream SCR reaction at higher ANR, compensating for non-uniform NH3 distribution and boosting NOXconversion (on the upstream SCR catalyst) while maintaining low NH3 slip.

[0021] In an ASC, the oxidation catalyst and the SCR catalyst can be coated on the substrate following different design strategies, one of the most common being the so-called “Two-layer ASC” design. The two-layer ASC design is based on the sequential coating of two layers (a PGM-(bottom) layer and a SCR-(top) layer) on the walls of the flow-through channels. A functional ASC design will convert in the oxidation layer just a portion of the ammonia available in the exhaust. The remaining ammonia fraction will then be used as a reductant in the SCR layer, ideally allowing for the overall conversion of ammonia into just water, elemental nitrogen, and the smallest possible amounts of N2O and NOX. In this regard, nitrous oxide (N2O) has a global warming potential (GWP) 265-289 times that of CO2 for a 100-year timescale, i.e. , N2O emitted today remains in the atmosphere more than 100 years, on average (CO2, by definition, has a GWP of 1 regardless of the time period used, because it is the gas being used as the reference). Accordingly, it is hugely desirable to limit the quantity of N2O emitted to atmosphere by exhaust gas aftertreatment processes.

[0022] A typical ASC for use in Diesel applications is described in WO2016 / 205509. This document discloses an ASC comprising a combination of platinum on a support with low ammonia storage and a first SCR catalyst. The preferred combination is a bi-layer having a top layer comprising the first SCR catalyst and a bottom layer comprising the platinum on a support with low ammonia storage. In use, some excess ammonia can be stored in the upper SCR catalyst layer. Some of the excess ammonia can pass through to the Pt where it may be oxidised to NOX. The NOXthen passes back through the SCR layer where it is treated with the stored ammonia to provide N2. In this way the excess slipped ammonia is stored and put to use, rather than being emitted to the atmosphere.

[0023] Particulate emissions are a well-known problem for diesel engine exhaust gas treatment systems. It is known to address these emissions with the use of a filter. Known types of filter include diesel particulate filters (DPF) and catalysed soot filters (CSF). These capture soot from the exhaust and can be regenerated through an increase in exhaust temperature to burn off the accumulated soot. The conventional substrate for a DPF or a CSF is a so-called wall-flow filter.

[0024] As early development of H2-ICE and exhaust-gas treatment systems therefor progresses, there is a need to develop an exhaust-gas treatment system component that can be used alone or in a favourable combination with other system components to meet current and future emission standards to be set and met by commercialised H2-ICE engines and vehicles fitted with them.

[0025] Accordingly, it is an object of the present invention to provide an ammonia slip catalyst suitable for meeting expected future regulations on particulate emissions in the exhaust-gas treatment system of a H2-ICE, or at least to tackle problems associated therewith in the prior art or provide a commercially viable alternative thereto.

[0026] According to a first aspect there is provided a fuel combustion and exhaust-gas treatment system comprising an internal combustion engine configured to run on a mixture of air and fuel, wherein the fuel of the mixture of air and fuel is a gaseous fuel comprising a majority fuel mass of hydrogen (H2), a source of gaseous fuel comprising a majority fuel mass of hydrogen (H2) and an exhaust-gas treatment system comprising a catalyst article for the treatment of an exhaust gas, the catalyst article comprising: a wall-flow filter substrate having inlet channels open at an inlet-end of the substrate and closed at an outlet-end of the substrate, the inlet channels adjacent outlet channels which are closed at the inlet-end of the substrate and open at the outlet-end of the substrate, the wall-flow filter comprising at least first, second and third catalytic layers, wherein:

[0027] (i) the first catalytic layer extends from the inlet-end of the substrate and comprises a first SCR composition;

[0028] (ii) the second catalytic layer is provided in or on the walls of the inlet channels, extending from the inlet-end of the substrate, and comprises a noble metal-containing composition, wherein: when the second catalytic layer is provided on the walls of the inlet channel, the first catalytic layer is provided on the second catalytic layer, and when the second catalytic layer is provided in the walls of the inlet channels, the first catalytic layer is provided on the walls of the inlet channels; and

[0029] (iii) the third catalytic layer is provided in or on the walls of the outlet channels, extending from the outlet-end of the substrate, and comprises a second SCR composition; wherein a ratio of the noble metals in the second catalytic layer in g per ft3to the total amount of the first and second SCR compositions in the first and third catalytic layers in g per in3is from 1.5:8 to 2: 1.

[0030] Preferably, a maximum total loading of the first SCR composition and the third SCR composition is 2.4 g / in3.

[0031] Preferably, the noble metal-containing composition of the second catalytic layer comprises a noble metal supported on a support material. Preferably, the support material is a refractory metal oxide, preferably comprising or consisting essentially of alumina as a support for the noble metals. Preferably, a maximum total loading of the noble metal-containing composition is 0.3 g / in3.

[0032] Most preferably, to avoid excessive backpressure, a total loading of the combination of the first SCR composition, the third SCR composition and the noble metal-containing composition of the second catalytic layer is 2.0 g / in3. In a second aspect, the invention provides a vehicle comprising the fuel combustion and exhaust-gas treatment system according to the first aspect of the invention, preferably wherein at least the catalyst article is located in an underfloor location and / or encounters, in normal use, exhaust gases at a temperature of from 200 to 400°C, preferably 150-300 °C.

[0033] According to a third aspect, the invention provides a method for the treatment of an exhaust gas of an internal combustion engine configured to run on a mixture of air and fuel, wherein the fuel of the mixture of air and fuel is a gaseous fuel comprising a majority fuel mass of hydrogen (H2), the method comprising passing the exhaust gas through a catalyst article comprising: a wall-flow filter substrate having inlet channels open at an inlet-end of the substrate and closed at an outlet-end of the substrate, the inlet channels adjacent outlet channels which are closed at the inlet-end of the substrate and open at the outlet-end of the substrate, the wall-flow filter comprising at least first, second and third catalytic layers, wherein: (i) the first catalytic layer extends from the inlet-end of the substrate and comprises a first SCR composition; (ii) the second catalytic layer is provided in or on the walls of the inlet channels, extending from the inlet-end of the substrate, and comprises a PGM- containing composition, wherein: when the second catalytic layer is provided on the walls of the inlet channel, the first catalytic layer is provided on the second catalytic layer, and when the second catalytic layer is provided in the walls of the inlet channels, the first catalytic layer is provided on the walls of the inlet channels; and (iii) the third catalytic layer is provided in or on the walls of the outlet channels, extending from the outlet-end of the substrate, and comprises a second SCR composition; wherein a ratio of the PGMs in the second catalytic layer in g per ft3to the total amount of the first and second SCR compositions in the first and third catalytic layers in g per in3is from 1.5:8 to 2:1.

[0034] The present disclosure will now be described further. In the following passages different aspects / embodiments of the disclosure are defined in more detail. Each aspect / embodiment so defined may be combined with any other aspect / embodiment or aspects / embodiments unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous. It is intended that the features disclosed in relation to the product may be combined with those disclosed in relation to the method and vice versa. In the following the term “ammonia slip catalyst filter” or “ASCF” is used as a shorthand for the structure of the catalyst article as described herein.

[0035] It has recently been observed that the use of reductant (especially urea / ammonia) injection into an exhaust gas stream can lead to the production of certain aggregated compounds and ammonium salts. These are species formed by the reaction and polymerisation of the reductant before it can take part in the SCR reaction. The polymerised material then takes the form of additional fine particulate matter than can be released to the atmosphere, contrary to strict emissions requirements.

[0036] A discussion of these polymerised particulate emissions may be found in SAE 2017-01-0915 in the context of application to a Diesel exhaust system. Here it is explained that depending on the urea doser, decomposition reaction tube (DRT) design and operating conditions, incomplete decomposition of injected urea could lead to solid urea deposit formation in the diesel aftertreatment system. The formed deposits could lead to engine back pressure increase and NOx treatment performance deterioration. The formed urea deposits could be further transformed to chemically more stable substances upon exposure to hot exhaust gas, therefore it is critical to understand this transformation process. The results of the author’s experiments indicate that 1) below urea melting temperature (130°C), the formed urea deposits are still primarily urea; 2) in the temperature range 130-190°C, urea transforms to a combination of urea, biuret and CYA; and 3) above biuret melting temperature (190 °C), CYA is primarily formed with some features of ammelide. Above 200 °C exposure, urea experiences fast chemical transformation via urea decomposition, biuret formation followed by subsequent biuret decomposition, and turns into CYA in a short time.

[0037] Thus, the polymerisation of urea and ammonia components in the hot exhaust gas conditions can lead to the formation of polymeric particulate matter and this can have some thermally stability. This particulate matter is typically very fine, but it is exactly this sort of fine material that is now the subject of ever more stringent regulation. Furthermore, since the formation of the particulate matter can only occur after the reductant is dosed into the exhaust gas treatment system, it is generally formed both after any conventional particulate filter in the system (DPF or CSF) and under a lower temperature environment (i.e. , underfloor configurations) where conventional regeneration of such filters would be difficult.

[0038] Exhaust gas of H2-ICE, whether lean (HPDI) or stoichiometric (SI), particularly HPDI, has a different composition from that of Diesel exhaust gas, including substantially no soot derived from fuel combustion; a significantly higher volumetric concentration of water (as steam) than Diesel exhaust gas; and generally much lower NOXthan Diesel, although in transient operation, where an engine can exceed lambda air-fuel ratio of 2, intermittent “spikes” of NOx can occur. However, particulates can still be a problem for H2-ICE, including ash created from combustion of engine lubricant and / or the polymeric particulate matter derived from ammonia and urea reductant, mentioned hereinabove. In this regard, the volume of a filter may be selected to trap sufficient ash and / or polymeric matter, either for the lifetime of a vehicle or to last between vehicle service intervals, when a filter can be removed and cleaned of ash before reinstallation. Additionally, exhaust gas from a H2-ICE engine is generally lower than for a Diesel engine, e.g. 150-300 °C in an underfloor catalyst location,

[0039] The present inventors have now identified that it is possible to address these concerns through the provision of a catalyst article as described herein. In particular, the article is able to provide comparable ASC performance to a standard flow-through monolith, while reducing or avoiding particulate emissions. Furthermore, the inventors have found that in order to match the N2O and NOXperformance of a standard flow-through ASC it is possible and, indeed, desirable to use a reduced amount of PGMs and / or particular combinations of noble metals that have surprisingly been found to be particularly selective to NOXreduction, i.e. low N2O. This leads to a saving in cost and an optimised performance.

[0040] However, in embodiments, higher PGM loadings can be selected for assisting with any relatively high “spikes” of H2 in the system and improved selectivity to reducing NOXto N2 via the so-called “hydrogen-SCR” reaction, where hydrogen - instead of a nitrogenous reductant - is used as the reductant for NOX.

[0041] In further embodiments, catalyst components for application in the catalyst article of the invention can be selected that are particularly durable to relatively high volumetric water in H2-ICE exhaust gas.

[0042] The present invention relates to a catalyst article for use in a fuel combustion and exhaustgas treatment system comprising an internal combustion engine configured to run on a mixture of air and fuel, wherein the fuel of the mixture of air and fuel is a gaseous fuel comprising a majority fuel mass of hydrogen (H2). By catalyst article it is meant a structure as described herein having catalytic properties. The catalytic properties derive from materials included in the structure or coated thereon. The article as defined herein includes both a coated catalytic substrate as described herein and also a processed and canned ASCF unit suitable for installation on a vehicle / automobile. The catalyst article provides a catalyst that is effective at reducing ammonia slip and particulate emissions when used downstream of an SCR process.

[0043] The present invention relates to a catalyst article for the treatment of an exhaust gas of an internal combustion engine configured to run on a mixture of air and fuel, wherein the fuel of the mixture of air and fuel is a gaseous fuel comprising a majority fuel mass of hydrogen (H2). The treatment is performed to remove undesired components of the exhaust gas, such as NOx and particulate matter. Other species such as CO and uncombusted hydrocarbons (HC) may also be treated by components of an exhaust treatment system.

[0044] The catalyst article comprises a wall-flow filter substrate. These are extremely well-known in the art. A wall-flow filter has inlet channels open at an inlet-end of the substrate and closed at an outlet-end of the substrate, the inlet channels adjacent outlet channels which are closed at the inlet-end of the substrate and open at the outlet-end of the substrate. The alternate open and closed channels mean that, in use, exhaust gas entering an inlet channel is forced through the walls dividing the channels to leave via an open outlet channel. The porosity of the walls and the provision of particular coating regions and layers ensure that the gases are treated while they pass through the wall.

[0045] Typical ceramic wall flow filter substrates are composed of refractory materials such as cordierite, aluminium titanate or silicon-carbide. A common construction is a multi-passage honeycomb structure having the ends of alternate passages on the inlet and outlet sides of the honeycomb structure plugged. This construction results in a checkerboard-type pattern on either end.

[0046] The monolithic substrates used for a wall flow filter may contain up to about 62 flow passages (or “cells”) per square centimetre (400 cells per square inch (“cpsi”)) of cross section, although far fewer may be used. For example, the carrier may have from 1.1 to 62.0 cells cm2(7 to 400), specifically from 15.5 to 62.0 cells per cm2(100 to 400 cells per square inch). The cells can have cross sections that are rectangular, square, circular, oval, triangular, hexagonal, or are of other polygonal shapes.

[0047] Wall flow substrates are generally composed of ceramic-like materials such as cordierite, alpha-alumina, silicon carbide, silicon nitride, zirconia, mullite, spodumene, alumina-silica- magnesia or zirconium silicate, or of refractory metals such as stainless steel. Such materials are able to withstand the environment, particularly high temperatures, encountered in treating the exhaust streams. Ceramic wall flow substrates are typically formed of a material having a porosity of about 40 to 70%. The term “porosity” as used in this context is understood as being determined according to mercury porosity measurement according to DIN 66133. According to embodiments of the present invention, wall flow substrates are specific having a porosity in the range from 38 to 75%.

[0048] The wall-flow filter of the catalyst article for use in the invention comprises at least first, second and third catalytic layers. It is preferred that these are the only layers present in the catalyst article. Nonetheless, it should be appreciated that other conventional layers may be present or the first, second and third layers may themselves be composed of one or more sub-layers having the same or different formulations. That is, the first catalytic layer could be comprised of two layers comprising SCR catalysts, either to build up a single SCR catalyst formulation or to provide two layers of different SCR catalyst formulations. In a most preferred embodiment, the three catalytic layers are the only layers present and are each provided as a singly applied layer.

[0049] The first catalytic layer extends from the inlet-end of the substrate and comprises a first SCR composition. Preferably the first catalytic layer extends up to 100% of a longitudinal length of the substrate extending from the inlet-end to an outlet-end, preferably from 70 to 90% of the longitudinal length. Since the coating methods, especially for thin in-wall coatings, for applying such compositions typically involve dipping the substrate in a washcoat, a substantially 100% coating will extend fully along the inside of the inlet channel up to the plug blocking the outlet-end of the channel. For on-wall coatings these may be applied by drawing a washcoat into the substrate by application of a strong and short vacuum.

[0050] A preferred coating of up to 90% of the length will leave a portion of the walls at the end of the channel adjacent the plug without any coating thereon. Accordingly, the most preferred coating of 70 to 90% of the length provides an inlet region of the inlet channels provided with the composition and a region distal from the inlet end which is free from the composition. In use, the first catalytic layer will be the first catalytic element of the article contacted by the exhaust gas to be treated.

[0051] In some preferred embodiments the second and third catalytic layers do not overlap and together extend 100% of a longitudinal length of the substrate. These embodiments have been found to achieve good conversion while minimising backpressure. In these embodiments, preferably the second catalytic layer is provided in the walls of the inlet channels and the third catalytic layer is provided in the walls of the outlet channels.

[0052] Preferably the catalytically active SCR components of the first and / or second SCR compositions comprise, and preferably consist, of one or more metal-exchanged zeolites. This does not preclude the presence of additional non-catalytic binders or processing aids, as are conventionally used in washcoat formation. These are discussed further below.

[0053] Preferably the first SCR composition comprises a copper-promoted zeolite, an iron-promoted zeolite, a manganese-promoted zeolite, or a combination thereof. The total amount of Cu, Mn and Fe is preferably present in an amount of from 0.1 to 5 wt% and most preferably from 1 to 3 wt%, by weight of the promoted zeolite. The second SCR composition may be independently selected from the same materials set out herein for the first SCR composition.

[0054] A zeolite is a microporous aluminosilicate having any one of the framework structures listed in the Database of Zeolite Structures published by the International Zeolite Association (IZA). The framework structures include, but are not limited to those of the CHA, FAU, BEA, MFI, MOR types. Non-limiting examples of zeolites having these structures include chabazite, faujasite, zeolite Y, ultrastable zeolite Y, beta zeolite, mordenite, silicalite, zeolite X, and ZSM-5. Aluminosilicate zeolites can have a silica / alumina molar ratio (SAR) defined as SiO2 / AhO3) from at least about 5, preferably at least about 20, with useful ranges of from about 10 to 200. Higher SAR zeolites are preferred because the zeolites are more hydrothermally durable to the relatively high volumetric water concentrations in H2-ICE exhaust gas. However, relatively high SAR zeolites have fewer exchange sites for exchange with promoter metals, including copper and / or manganese. Accordingly, there is a balance between catalytic activity and catalyst durability. For example, it may be necessary to use higher loadings of higher SAR zeolites at lower promoter-exchanged loadings.

[0055] Preferably the metal-exchanged zeolite is a small pore zeolite, preferably having a CHA framework structure. Small-pore zeolites are framework type codes (according to the IZA) comprising a maximum pore ring size of eight tetrahedral atoms. Preferably the zeolite is exchanged with Cu and / or Mn. Preferably the zeolite is copper-promoted and has a CHA framework. Preferably the copper is present in an amount of from 0.1 to 5 wt% and most preferably from 2 to 4 wt%, by weight of the copper-promoted zeolite. The second catalytic layer is provided in or on the walls of the inlet channels, extending from the inlet-end of the substrate, and comprises a noble metal-containing composition. That is, the second catalytic layer, like the first catalytic layer, extends from the inlet-end.

[0056] In use, the second catalytic layer will be the second catalytic element of the article contacted by the exhaust gas to be treated. This is achieved by either having the second catalytic layer provided on the walls of the inlet channel and the first catalytic layer being provided on the second catalytic layer, or having the second catalytic layer provided in the walls of the inlet channels and the first catalytic layer being provided on the walls of the inlet channels.

[0057] The provision of a coating either “in-wall” or “on-wall” is well known in the art. It is achieved by careful tuning of the washcoat formulations to match the porosity of the filter walls, which techniques can include adjusting: particle size distribution, e.g. Dv50 and Dv90 (or D(v,0.50) and D(v,0.90)) as determined by Laser Diffraction Particle Size Analysis using a Malvern Mastersizer 2000, which is a volume-based technique and applies a mathematical Mie theory model to determine a particle size distribution; and / or rheology, e.g. by selection of appropriate rheology modifiers. Thus, a “thinner” composition with finer material (especially the supported noble metals) can be infiltrated more readily into the walls than a “thicker” composition containing more coarse material. Exemplary in-wall and on-wall coatings are provided in the examples. By adapting the washcoat material it is therefore possible to ensure that the coating is provided as desired in the porous walls of the article.

[0058] The second catalytic layer comprises a noble metal, optionally PGM-containing composition. Suitable noble metals include Pd, Pt and Au. Preferably the PGM-containing composition contains Pt, preferably as the only PGM, which is particularly active for H2 oxidation and potentially also for promoting the hydrogen-SCR reaction. A particularly preferred combination of noble metals with application to H2-ICE is Pd-Au, including alloys thereof. This combination has been found to be particularly active in H2-ICE exhaust gas for oxidising H2 while producing relatively minimal N2O. The noble metals are preferably provided on a support material. Suitable support materials are well known in the art. Preferably the noble metal-containing composition comprises alumina as a support for the noble metals.

[0059] Preferably the second catalytic layer extends up to 100% of a longitudinal length of the substrate extending from the inlet-end to an outlet-end, preferably from 25 to 90%, more preferably 35 to 80%, more preferably 40 to 70% and most preferably from 45 to 65% of the longitudinal length. However, it is preferred that there is no in-wall overlap of the second catalytic layer with the third catalytic layer, which is described hereinbelow. Accordingly, for embodiments comprising in-wall second and third catalytic layers, the sum of the % longitudinal length of the second catalytic layer and the % longitudinal length of the third catalytic layer is 100%. As for the first catalytic layer, a preferred coating of up to 90% of the length will leave a portion of the walls at the end of the channel adjacent the plug without any coating thereon. Accordingly, the preferred coating of 40 to 70% of the length provides an inlet region of the inlet channels provided with the composition and a region distal from the inlet end which is free from the composition.

[0060] In use, the second catalytic layer will be the second catalytic element of the article contacted by the exhaust gas to be treated. It is preferred that the first catalytic layer is provided having at least the same length as the second catalytic layer and, preferably at least 5% to 10% longer (relative to the total length of the article). This ensures that the exhaust gas contacts the first catalytic layer before the second catalytic layer.

[0061] The third catalytic layer is provided in or on the walls of the outlet channels, extending from the outlet-end of the substrate, and comprises a second SCR composition. As noted above, the second SCR composition can be selected from the same materials as the first SCR composition. Preferably the second SCR composition is a copper-promoted CHA zeolite.

[0062] The third catalytic layer extends from the outlet-end of the substrate. Preferably, where the second catalytic layer is located on-wall, the third catalytic layer extends up to 100% of a longitudinal length of the substrate extending from the outlet-end to the inlet-end. Preferably for either in-wall or on-wall embodiments of the third catalytic layer, the third catalytic layer extends from 20 to 90% of a longitudinal length of the substrate extending from the outletend to the inlet-end, more preferably 30 to 80%, preferably 30 to 70% and most preferably from 40 to 60% of the longitudinal length. In embodiments where there is no overlap with the second catalytic layer, preferably the lengths are complimentary (i.e. the second layer is 40 to 70% and the third layer is 60 to 30%). In embodiments with overlap, the third layer may be longer, such as 60 to 90% of the length). Since the coating methods for applying such compositions typically involve dipping the substrate in a washcoat, a substantially 100% coating will extend fully along the inside of the outlet channel up to the plug blocking the inlet-end of the channel. A preferred coating of up to 90% of the length will leave a portion of the walls at the end of the channel adjacent the plug without any coating thereon.

[0063] Accordingly, the most preferred coating of 70 to 90% of the length provides an outlet region of the outlet channels provided with the composition and a region distal from the outlet end which is free from the composition. In use, the third catalytic layer will be the last catalytic element of the article contacted by the exhaust gas to be treated.

[0064] However, as also explained hereinabove, it is preferred that there is no in-wall overlap of the second catalytic layer with the third catalytic layer, which is described hereinbelow.

[0065] Accordingly, for embodiments comprising in-wall second and third catalytic layers, the sum of the % longitudinal length of the second catalytic layer and the % longitudinal length of the third catalytic layer is 100%.

[0066] When applying in-wall catalyst compositions the washcoat can be coated into all of the pores of the walls. Thus, if the second and third catalyst compositions are both provided in-wall, although less preferred, there can be substantial overlap in the location of the catalyst composition. Nonetheless, it is still possible to determine from which side the compositions have been applied. This is not least because the inlet channels will include the second composition right up to the mouth of the inlet, whereas the third composition cannot be coated so far, in view of the plug blocking the inlet-end of the outlet channel. Conversely, the outlet channels will include the third composition right up to the mouth of the outlet, whereas the second composition cannot be coated so far, in view of the plug blocking the outlet-end of the inlet channel. Thus, even when both the second and third compositions are provided in-wall, there will still be an SCR-only in-wall region right at the outlet of the outlet channels.

[0067] In a preferred embodiment, the third catalyst composition is provided on-wall. Preferably the second catalytic layer is provided in the walls of the inlet channels and the third catalytic layer is provided on the walls of the outlet channels. According to another preferred embodiment, the third catalyst composition is provided in-wall, with the second catalytic layer being provided in the walls of the inlet channels. In this instance, although less preferred, there is close contact between the particles of the second and third catalyst layers if there is overlap. In some embodiments it is preferred that the second and third catalytic layers abut and do not overlap, i.e. the sum of the % longitudinal length of the second catalytic layer and the % longitudinal length of the third catalytic layer is 100%.

[0068] Preferably the composition forming the first and third layers are devoid of any noble metal component. Preferably the composition forming the second layer is devoid of any SCR component. Nonetheless, it should be appreciated that when coating a porous wall-flow filter there may be a small amount of bleeding between the layers. When describing washcoat loadings in a catalyst article, it is typical to provide noble metal loadings in g / ft3. This is an assessment of the metals per se in the applied layer. In contrast, washcoat loadings of SCR materials, which are typically much higher, are given in g / in3. Unlike for noble metals, such values for the SCR materials typically includes all elements of the washcoat, including, for example, binders. Accordingly, the SCR composition herein includes both the zeolite components and any binders. Preferably the SCR compositions are therefore synonymous with the first and third catalytic layers.

[0069] In a conventional Diesel ASC (i.e. the noble metal-containing end portion of a combined SCR / ASC) a ratio of the total loading of the noble metals in g / ft3to the total loading of SCR composition in g / in3is typically above 1 :1 and more generally above 5:4. The inventors found that when they formulated an ASCF with the same loading of catalytic compositions there was higher NOXand N2O formation. Without wishing to be bound by theory, it was considered that the exhaust gases in the new configuration were being exposed to the noble metal material to a far greater extent. That is, whereas in a flow-through ASC the exhaust gases only encounter the noble metals when they percolate through the layers of catalyst material, providing the ASCF configuration forces all of the exhaust gases directly through the noble metal-containing composition.

[0070] Therefore, in a first embodiment, the inventors sought to address these concerns and arrived at an optimised relative loading of the catalyst material in the layers. A ratio of the noble metals in the second catalytic layer in g per ft3to the total amount of the first and second SCR compositions in the first and third catalytic layers in g per in3is from 1.5:8 to 5:8, preferably 2:8 to 4:8, preferably 4.5:16 to 7:16, preferably 4.5:16 to 6:16. Preferably the ratio of the noble metals in the second catalytic layer in g per ft3to the total amount of the first and second SCR compositions in the first and third catalytic layers in g per in3is about 5:16.

[0071] However, in applications where NOXand N2O emissions are within acceptable regulations and engineering targets, in a second embodiment, the inventors sought to “build in” features to address the particular conditions encountered by catalysts in an exhaust gas aftertreatment system, i.e. unburned hydrogen and significant volumes of water as steam. In this second embodiment, N2O generation was balanced against catalyst activity, including net NOXconversion by the exhaust system, and durability. In this a ratio of the noble metals in the second catalytic layer in g per ft3to the total amount of the first and second SCR compositions in the first and third catalytic layers in g per in3is greater than 5:8 to (equal to) 2:1 , preferably 11 :16 to 3:2. The claimed invention is defined to embrace both these first and second embodiments.

[0072] Given the underlying understanding of an ASC it is surprising that the structure can also operate in a three-layer configuration as disclosed herein. In particular, the SCR layer of a conventional ASC only functions because of amounts of ammonia trapped in the SCR layer in use (see Figure 1 in US2010 / 166628A1 for proposed reaction mechanisms). It is surprising that sufficient ammonia can percolate through the first and second layers to the third catalytic layer to then perform SCR of any NOXformed on contact of the exhaust gases with the noble metals of the second catalytic layer. Nonetheless, the performance demonstrates that this does occur, and comparative examples demonstrated that in the absence of the outlet-end SCR composition the exhaust had undesirably high levels of NOXand N2O - presumably as a consequence of contacting the noble metals as the last layer before leaving the article.

[0073] Preferably a weight ratio of the catalytically active components in the first and third catalytic layers is from 2:3 to 3:2 and preferably is about 1 :1. That is, preferably the first and third catalytic layers are provided in similar amounts. Indeed, it is especially preferred from a manufacturing position that the first and third catalytic layers also have substantially the same composition.

[0074] Preferably at least two of the first, second and third catalytic layers, and preferably all three, extend 70 to 90% of the longitudinal length. As will be appreciated, when all three extend to such a length there will be overlap between all three layers. In embodiments where all three layers extend to this range of lengths there is therefore a portion of the inlet channel walls which is not provided with the first or second catalytic layers (and of course neither the third catalytic layer) and a portion of the outlet channel walls which is not provided with the third catalytic layer (and of course neither the first nor second catalytic layer), and overlap of inwall second and third catalytic layers is less preferred. However, this configuration facilitates the layer formation, avoiding the build-up at the plugged ends that might arise when attempting full channel coating, but also helps to ensure that the gases to be treated first contact the first catalytic layer and the last contact the third catalytic layer.

[0075] In embodiments where the second and third catalytic layers do not overlap, preferably the first catalytic layer extends 70 to 90% of the longitudinal length, the second catalytic layer extends 30 to 60% of the longitudinal length and the third catalytic layer extends 70 to 40% of the longitudinal length, wherein overlap of in-wall second and third catalytic layers is less preferred.

[0076] It is preferred that the second and third catalytic layers are not both in-wall coatings that overlap significantly in-wall. Instead, it is preferred that where the second and third catalytic layers are both in-wall coatings wherein the longitudinal length of the second and third coatings is selected so that the sum of their lengths is 100% of the total longitudinal length of the substrate (though in practical manufacture a small overlap is allowable). This is because, in early investigations, embodiments featuring substantial in-wall overlap between the second and third catalytic layers show less durable catalytic activity (significant NH3 slip) when aged, e.g. to mimic “near end of vehicle life” activity, compared with similar catalyst configurations with substantially no in-wall overlap between second and third catalytic layers.

[0077] The SCR and noble metal-containing compositions may comprise additional components. For example, components such as fillers, binders, stabilizers, rheology modifiers, and other additives. In certain embodiments, the washcoat comprises pore-forming agents such as graphite, cellulose, starch, polyacrylate, and polyethylene, and the like. These additional components do not necessarily catalyse the desired reaction, but instead improve the catalytic material's effectiveness, for example by increasing its operating temperature range, increasing contact surface area of the catalyst, increasing adherence of the catalyst to a substrate and the like. Typically, the only additional component will be a binder. Preferably the additional components form less than 25 wt%, preferably less than 15wt%, and most preferably less than 10 wt% of the layers with the balance the SCR or supported noble metals, respectively. The particle size of the binder particles will vary depending on whether the associated coating is in-wall or on-wall. Typical alumina binders for in-wall applications will have a Dv90 of less than 7 pm as measured by laser diffractometry.

[0078] Techniques for applying the first and second layers are well known in the art and include the application of a washcoat to the surfaces to be coated, e.g. by using Applicant’s method and apparatus disclosed in WO99 / 47260. After coating the layers onto the article, they are typically calcined to fix the layers. Calcining is well known in the art and may be carried out in air at temperatures of about 500°C.

[0079] The inventors envisioned two approaches for forming the catalyst article. According to an aspect there is provided a method for the manufacture of the catalyst article in the following steps, in order: applying a washcoat to the outlet channels to form the third coating and then calcining; applying a washcoat to the inlet channels to form the second coating and then calcining; applying a washcoat to the inlet channels to form the first coating and then calcining. In each calcining step there will generally be a preceding drying step.

[0080] According to a more preferred embodiment, since it involves less calcining, there is provided a method for the manufacture of the catalyst article in the following steps, in order: applying a washcoat to the inlet channels to form the second coating and then calcining; applying a washcoat to the outlet channels to form the third coating and then drying; applying a washcoat to the inlet channels to form the first coating and then calcining. In each calcining step there will generally be a preceding drying step. Avoiding a calcination step reduces the energy cost and the process costs.

[0081] Preferably the catalyst article comprises means for electrically heating the catalyst article. Such means are well known in the art and generally rely on resistive heating to raise the operating temperature of a catalyst article in-use. This approach may be used for in situ regeneration of the catalyst article to remove built up particulate deposits, although there are associated energy costs for achieving the temperatures required. Other approaches to regeneration are known, such as raising the temperature of the exhaust gases through engine management or hydrocarbon dosing.

[0082] According to a further aspect there is provided an exhaust-gas treatment system comprising the catalyst article described herein. Thus, the system implicitly comprises an initial manifold for receiving exhaust gases, such as from an engine, and an outlet for emissions of treated exhaust gases to the environment. In use, the catalyst article is able to treat any ammonia slipped from upstream components to produce N2, without forming and releasing excess NOXor unduly producing N2O.

[0083] Preferably the exhaust-gas treatment system comprises in order from upstream to downstream of the engine, means for injecting a nitrogenous reductant (MINR), a flow- through substrate comprising a SCR catalyst and the catalyst article described herein. Preferably the system further comprises a Hydrogen Oxidation Catalyst (HOC) upstream of the means for injecting a nitrogenous reductant, such as that disclosed in Applicant’s WO2025 / 083425, mentioned hereinabove. All these components and their formulations and composition are well known in the art. The upstream means for injecting a nitrogenous reductant and SCR catalyst article are the cause of both the potentially slipped ammonia (hence the ASCF) as well as the formation of reductant-derived particulate matter. That is, exhaust systems for use in the present invention include:

[0084] 1) Engine-out - MINR - SCR - ASCF - Tailpipe;

[0085] 2) Engine-out - HOC - MINR- SCR - ASCF - Tailpipe;

[0086] 3) Engine-out - MINR - SCR - ASCF - HOC - Tailpipe;

[0087] 4) Engine-out - MINR - ASCF - Tailpipe;

[0088] 5) Engine-out - HOC - MINR - ASCF - Tailpipe;

[0089] 6) Engine-out - MINR - ASCF - HOC - Tailpipe; or

[0090] 7) Engine-out - MINR - ASCF - Tailpipe. wherein “SCR” refers to the flow-through substrate comprising a SCR catalyst component.

[0091] In configurations 3) and 6), a hydrogen oxidation catalyst (HOC) is located downstream from the ASCF catalyst article to avoid H2 slip. However, the HOC is preferably located upstream from the SCR catalyst (see configurations 2) and 5)) to protect the SCR component of the ASCF or the SCR catalyst component from potentially reductive H2 “spikes”.

[0092] It will be recognised that where no SCR catalyst is included in the exhaust system, the means for injecting a nitrogenous reductant is still included, i.e. the ASCF catalyst article is configured for an appropriate level of NOXreduction activity.

[0093] The nitrogenous reductant provided by the means for injecting a nitrogenous reductant can be ammonia perse, hydrazine or an ammonia precursor selected from the group consisting of urea ((NH2)2CO), ammonium carbonate, ammonium carbamate, ammonium hydrogen carbonate and ammonium formate. Ammonia and urea are most preferred alternatives. Preferably the means for injecting a nitrogenous reductant further comprises a reservoir comprising urea. In embodiments, the ASCF article is the only after treatment device present in the exhaustgas treatment system. Preferably the catalyst article is, in use, the last catalyst article encountered by exhaust gas before being emitted to the atmosphere. This means that the article can suitably work as an ammonia slip catalyst and simultaneously prevent the release of urea / ammonia-derived particulate matter to the atmosphere.

[0094] According to a further aspect there is provided a vehicle comprising the fuel combustion and exhaust-gas treatment system described herein, preferably wherein at least the catalyst article described herein is located in an underfloor location and / or encounters, in normal use, exhaust gases at a temperature of from 200 to 400°C, preferably 150-300 °C. The provision of the catalyst article in an underfloor location occurs because the article is preferably at the end of the exhaust system. This gives rises to challenges where the temperature facilitates the formation of the reductant-derived particulate matter but not its destruction, and hence the ASCF is a solution to meeting emissions standards.

[0095] The invention can also be defined according to one or more of the following statements of invention:

[0096] 1 . A fuel combustion and exhaust-gas treatment system comprising an internal combustion engine configured to run on a mixture of air and fuel, wherein the fuel of the mixture of air and fuel is a gaseous fuel comprising a majority fuel mass of hydrogen (H2), a source of gaseous fuel comprising a majority fuel mass of hydrogen (H2) and an exhaust-gas treatment system comprising a catalyst article for the treatment of an exhaust gas, the catalyst article comprising: a wall-flow filter substrate having inlet channels open at an inlet-end of the substrate and closed at an outlet-end of the substrate, the inlet channels adjacent outlet channels which are closed at the inlet-end of the substrate and open at the outlet-end of the substrate, the wall-flow filter comprising at least first, second and third catalytic layers, wherein:

[0097] (i) the first catalytic layer extends from the inlet-end of the substrate and comprises a first SCR composition;

[0098] (ii) the second catalytic layer is provided in or on the walls of the inlet channels, extending from the inlet-end of the substrate, and comprises a noble metal-containing composition, wherein: when the second catalytic layer is provided on the walls of the inlet channel, the first catalytic layer is provided on the second catalytic layer, and when the second catalytic layer is provided in the walls of the inlet channels, the first catalytic layer is provided on the walls of the inlet channels; and

[0099] (iii) the third catalytic layer is provided in or on the walls of the outlet channels, extending from the outlet-end of the substrate, and comprises a second SCR composition; wherein a ratio of the noble metals in the second catalytic layer in g per ft3to the total amount of the first and second SCR compositions in the first and third catalytic layers in g per in3is from 1.5:8 to 5:8, preferably 2:8 to 4:8, and more preferably 2:8 to 3:8.

[0100] 2. The fuel combustion and exhaust-gas treatment system according to 1 , wherein, in the catalyst article, the second catalytic layer is provided in the walls of the inlet channels and / or the third catalytic layer is provided on the walls of the outlet channels.

[0101] 3. The fuel combustion and exhaust-gas treatment system according to 1 or 2, wherein, in the catalyst article:

[0102] (i) the first catalytic layer extends up to 100% of a longitudinal length of the substrate extending from the inlet-end to an outlet-end, preferably from 70 to 90% of the longitudinal length; and / or

[0103] (ii) the second catalytic layer extends up to 100% of a longitudinal length of the substrate extending from the inlet-end to an outlet-end, preferably from 25 to 90% of the longitudinal length, more preferably from 40 to 70% of the longitudinal length; and / or

[0104] (iii) the third catalytic layer extends up to 100% of a longitudinal length of the substrate extending from the outlet-end to an inlet-end, preferably from 20 to 90% of the longitudinal length, more preferably from 30 to 60% of the longitudinal length, preferably from 70 to 90% of the longitudinal length.

[0105] 4. The fuel combustion and exhaust-gas treatment system according to 3, wherein, in the catalyst article, at least two of the first, second and third catalytic layers, and preferably all three, extend 70 to 90% of the longitudinal length.

[0106] 5. The fuel combustion and exhaust-gas treatment system according to any one of 1 to 4, wherein, in the catalyst article, the second and third catalytic layers do not overlap and together extend 100% of a longitudinal length of the substrate extending from an outlet-end to an inlet end, preferably wherein the second catalytic layer is provided in the walls of the inlet channels and wherein the third catalytic layer is provided in the walls of the outlet channels. 6. The fuel combustion and exhaust-gas treatment system according to any one of 1 to

[0107] 5, wherein, in the catalyst article, the ratio of the noble metals in the second catalytic layer in g per ft3to the total amount of the first and second SCR compositions in the first and third catalytic layers in g per in3is about 5:16.

[0108] 7. The fuel combustion and exhaust-gas treatment system according to any one of 1 to

[0109] 6, wherein, in the catalyst article, the noble metal-containing composition contains Pt, preferably as the only PGM; or a combination of Pd-Au.

[0110] 8. The fuel combustion and exhaust-gas treatment system according to any one of 1 to

[0111] 7, wherein, in the catalyst article, the noble metal-containing composition comprises alumina as a support for the noble metals.

[0112] 9. The fuel combustion and exhaust-gas treatment system according to any one of 1 to

[0113] 8, wherein, in the catalyst article, a catalytically active SCR components of the first and / or second SCR compositions comprises, and preferably consists, of one or more metal- exchanged zeolites.

[0114] 10. The fuel combustion and exhaust-gas treatment system according to 9, wherein, in the catalyst article, the metal-exchanged zeolite is a small pore zeolite, preferably having a CHA framework structure, preferably wherein the zeolite is exchanged with Cu and / or Mn.

[0115] 11. The fuel combustion and exhaust-gas treatment system according to any one of 1 to

[0116] 10, wherein, in the catalyst article, a weight ratio of the first and second SCR compositions in the first and third catalytic layers is from 2:3 to 3:2 and is preferably about 1 :1.

[0117] 12. The fuel combustion and exhaust-gas treatment system according to any one of 1 to

[0118] 11, further comprising means for electrically heating the catalyst article.

[0119] 13. The fuel combustion and exhaust-gas treatment system according to any one of 1 to

[0120] 12, further comprising in order from upstream to downstream of the engine, means for injecting a nitrogenous reductant, a flow-through substrate comprising a SCR catalyst and the catalyst article, preferably wherein the system further comprises a Hydrogen Oxidation Catalyst (HOC) upstream of the means for injecting a nitrogenous reductant. 14. A vehicle comprising the fuel combustion and exhaust-gas treatment system according to any one of 1 to 13, preferably wherein at least the catalyst article is located in an underfloor location and / or encounters, in normal use, exhaust gases at a temperature of from 200 to 400°C, preferably 150-300 °C.

[0121] 15. A method for the treatment of an exhaust gas of an internal combustion engine configured to run on a mixture of air and fuel, wherein the fuel of the mixture of air and fuel is a gaseous fuel comprising a majority fuel mass of hydrogen (H2), the method comprising passing the exhaust gas through a catalyst article comprising: a wall-flow filter substrate having inlet channels open at an inlet-end of the substrate and closed at an outlet-end of the substrate, the inlet channels adjacent outlet channels which are closed at the inlet-end of the substrate and open at the outlet-end of the substrate, the wall-flow filter comprising at least first, second and third catalytic layers, wherein:

[0122] (i) the first catalytic layer extends from the inlet-end of the substrate and comprises a first SCR composition;

[0123] (ii) the second catalytic layer is provided in or on the walls of the inlet channels, extending from the inlet-end of the substrate, and comprises a noble metal-containing composition, wherein: when the second catalytic layer is provided on the walls of the inlet channel, the first catalytic layer is provided on the second catalytic layer, and when the second catalytic layer is provided in the walls of the inlet channels, the first catalytic layer is provided on the walls of the inlet channels; and

[0124] (iii) the third catalytic layer is provided in or on the walls of the outlet channels, extending from the outlet-end of the substrate, and comprises a second SCR composition; wherein a ratio of the noble metals in the second catalytic layer in g per ft3to the total amount of the first and second SCR compositions in the first and third catalytic layers in g per in3is from 1.5:8 to 5:8, preferably 2:8 to 4:8, and more preferably 2:8 to 3:8.

[0125] The invention will now be described further in the following figures, in which:

[0126] Figure 1 shows a diagrammatic cross-section of a portion of a catalyst article described herein;

[0127] Figure 2 shows a diagrammatic cross-section of a portion of a catalyst article described herein; Figure 3A and 3B show a configuration of components of an exhaust-gas treatment system downstream of an H2-lnternal Combustion engine;

[0128] Figures 4A-C show three preferred embodiments of the present invention; and

[0129] Figure 5 is a bar chart showing the results of an analysis of cold flow backpressure (CFBP).

[0130] Figure 1 shows a diagrammatic portion of a catalyst article 1 as described herein. In particular, the portion shown focuses on a single inlet channel 5 and a corresponding single outlet channel 10. The inlet channel 5 is separated from the outlet channel 10 by a porous wall 15. The inlet channel 5 is open at an inlet face 20 of the catalyst article 1 and blocked with a plug 25 at an outlet face 30 of the catalyst article 1. The outlet channel 10 is open at the outlet face 30 of the catalyst article 1 and blocked with a plug 25 at an inlet face 20 of the catalyst article 1.

[0131] There is provided a first catalytic layer 35 on a first wall surface 40 on the inlet channel 5. The first catalytic layer 35 is provided as a washcoat and comprises an SCR composition such as a Cu-CHA washcoat composition, together with conventional binders and processing aids. The first catalytic layer 35 extends from the inlet face 20 at least 70% of a total length 45 of the catalyst article 1.

[0132] There is provided a second catalytic layer 50 in the porous wall 15. The second catalytic layer 50 is provided as a washcoat and comprises a PGM-containing composition such Pt supported on alumina, together with conventional binders and processing aids. The second catalytic layer 50 extends from the inlet face 20 at least 70% of a total length 45 of the catalyst article 1. The second catalytic layer 50 is preferably a little shorter than the first catalytic layer 35.

[0133] There is provided a third catalytic layer 55 on a second wall surface 60 on the outlet channel 10. The third catalytic layer 55 is provided as a washcoat and comprises an SCR composition such as a Cu-CHA washcoat composition, together with conventional binders and processing aids. The third catalytic layer 55 extends from the outlet face 30 at least 70% of a total length 45 of the catalyst article 1. In use, exhaust gases entering the catalyst article 1 through the inlet face 20 pass into the inlet channel 5, through the porous wall 15, into the outlet channel 10 and out of the article 1. This route is shown by the large arrows. The gases pass through the first, second and third catalytic layers (35, 50, 55) in order. Ammonia is stored on the first and third catalytic layers (35, 55) until it can be used to perform an SCR reaction to decompose NOXproduced on the second catalytic layer 50. Particulate matter from the ammonia / urea accumulates in the inlet channel 5 rather than being released to the atmosphere. The particulate matter can be combusted, if necessary, to reduce undue build-up and this can be assisted, if necessary, but localised resistive heating using an electrical element (not shown).

[0134] Figure 2 shows an alternative configuration of the first, second and third catalytic layers (35, 50, 55) in the catalyst article 1. All reference numerals refer to the same components and all the compositional details remain the same.

[0135] The second catalytic layer 50 is provided on the wall 40 of the inlet channel 5. The first catalytic layer 35 is provided on the second catalytic layer 50 and, since it is a little longer, provided overlapping a small portion of the wall 40 of the inlet channel 5. The third catalytic layer 55 is provided in the porous wall 10, although it could instead be provided on the surface 60 of the outlet channel 10.

[0136] Figure 3A shows an exhaust gas treatment system comprising, downstream of an engine 65, a hydrogen oxidation catalyst (HOC) 70, then a means for the injection of a nitrogenous reductant (MINR) 80 and finally a catalyst article 1 as described herein, i.e. , an ASCF according to the claimed invention. The difficulty in regenerating the catalyst article 1 were soot to build up is exacerbated by the end-of-system location (such as underfloor) where the lower temperatures do not suffice.

[0137] Figure 3B shows an alternative exhaust gas treatment system to Figure 3A in which like- numbered components are also present and wherein a flow-through substrate coated with a SCR catalyst 85 is disposed between MINR 80 and ASCF 1.

[0138] EXAMPLES

[0139] The invention will now be described further in relation to the following non-limiting examples.

[0140] Exemplary catalyst articles were produced and tested as explained below. EXAMPLE 1

[0141] Sample Preparation

[0142] Inventive examples corresponding to Figures 4A-4C were prepared using the following compositions. Figures 4A-4C use the same reference numerals as Figures 1 and 2 for the same component parts. The inventive examples used a wall-flow filter substrate of 10.5” x 6.0”. A core was cut from the larger coated block of 1” x 6”.

[0143] Figure 4A resembles Figure 2, but this is an embodiment where the second catalytic layer 50 has a shorter length, extending about 50% of the total length 45 of the catalyst article 1. The first catalytic layer 35 extends about 80% of the total length 45 of the catalyst article 1. Coating the PGM layer as a porous coating on-wall in Figure 4A was an attempt to prevent / minimise close contact with the SCR catalyst particles (which are dispersed within the wall volume and then fundamentally segregated from the PGM on-wall).

[0144] Figures 4B and 4C are embodiments where the second catalytic layer 50 does not overlap the third catalytic layer 55. Both layers (50, 55) are provided as in-wall washcoats and together coat the total length 45 of the catalyst article 1. In Figure 4B the second catalytic layer 50 is about 25% of the total length 45 and the third catalytic layer 55 is about 75% of the total length 45. In Figure 4C the second catalytic layer 50 is about 50% of the total length 45 and the third catalytic layer 55 is about 50% of the total length 45.

[0145] As supported in the embodiments 4B and 4C, avoiding overlap between these layers avoids close contact between the platinum group metals and the SCR catalysts.

[0146] SCR “in-wall” coating - “Coating 1” for reference numeral “55”:

[0147] A slurry of spray-dried Cu(3.3wt%)Chabazite (SAR 20) was milled to target particle size distribution characterised by Dv90=3.8-3.9 pm and stirred overnight. The milled slurry was adjusted to target pH 9.9-10.2 by addition of tetraethylammonium hydroxide aqueous solution and stirred for 5 minutes. Finally, a low particle size mixed oxide containing AI2O3 (91.2wt%) I La2Os (4.8wt%) I Nd2Os (4.1wt%), with particle size distribution characterised by Dv90=3.0-5.0 pm, was added under high-speed stirring to target 11 wt% with respect to CuChabazite calcined weight. The washcoat with the composition above was stirred overnight, then re-adjusted to pH 9.9-10.2 by addition of tetraethylammonium hydroxide aqueous solution. The pH-adjusted washcoat was stirred for 30 minutes and coated from the rear end of the wall-flow filter to a target washcoat loading of 0.8 g / in3with reference to the volume of the ASCF brick.

[0148] The coated block was dried at 110°C for 30 minutes and calcined at 500°C for 2 hours.

[0149] PGM in-wall coating - “Coating 2” for reference numeral “50”:

[0150] Succinic acid was dissolved in demineralised water to target 40 g / ft3and the solution was stirred for 5 minutes. To this solution, platinum(IV)nitrate was added slowly to target 0.5 g / ft3of platinum, and the mixture was stirred for 5 minutes. Finally, low particle size alumina (previously milled to target particle size distribution characterised by Dv90=4.8-5.0 pm) was added as a slurry, to target 0.06 g / in3. The washcoat with the composition above was stirred for 3 hours and coated from the front end of the mid-processed wall-flow filter containing the calcined SCR “Coating 1” (described above), then dried at 115°C for 30 minutes and calcined at 500°C for 2 hours.

[0151] All the targets above refer to final loadings with reference to the volume of the ASCF brick.

[0152] SCR porous on-wall coating - “Coating 3” for reference numeral “35”:

[0153] A slurry of spray dried Cu(3.3wt%)Chabazite (SAR 20) was milled to target particle size distribution characterised by Dv90=3.8-3.9 pm and stirred overnight. The slurry was transferred in a chiller tank, and colloidal aluminium oxide hydroxide (boehmite) was added under high-speed stirring to target 18wt% with respect to CuChabazite calcined weight. The mixture was stirred for 30 minutes, then a cellulose pore former (Arbocel LIFC100) was added under high-speed stirring to target 54 wt% with respect to CuChabazite calcined weight. The mixture was stirred for 30 minutes, then a cellulose thickener was added under high-speed stirring to target 0.2 wt% with respect to full weight of the wet washcoat. The mixture was stirred for 30 minutes at high speed, then the washcoat was stored in a sealed container for 2 days. The washcoat was then stirred for 3 minutes at high speed and coated from the front end of the mid-processed DPF containing the calcined SCR “Coating 1” and the calcined PGM “Coating 2” (described above), to a target washcoat loading of 0.8 g / in3with reference to the volume of the finished catalyst. The brick was dried at 110°C for 45 minutes and then calcined at 500°C for 2 hours.

[0154] That is, in the finished product the combined loading of Coatings 1 and 3 was 1.6 g / in3and the platinum loading was 0.5 g / ft3, i.e. a ratio of the noble metals in the second catalytic layer in g per ft3to the total amount of the first and second SCR compositions in the first and third catalytic layers in g per in3is 0.3125.

[0155] EXAMPLE 2

[0156] H2-ICE SCAT testing

[0157] “Fresh”, i.e. unaged, catalyst activity was tested on substrates coated as described below that had been dried at 110°C for 30 minutes and calcined at 500°C for 2 hours. Testing was performed in a laboratory synthetic catalytic activity testing (SCAT) apparatus. This type of testing does not include any other catalysts upstream or downstream but artificially provides the core with a gas composition that is representative of a H2-ICE engine had the following composition: 500 ppm NO, 500 ppm NH3, 125,000 ppm (12.5 vol%) O2, 200,000 ppm (20 vol%) H2O, 1,500 ppm H2, balance N2. A gas flow rate of 80L / min was used for all experiments, which - for the filter samples (see dimensions below) - corresponded to a Space Velocity of 90,000 hr1. As the volume of the “Comparative (flow-through ASC)” sample is smaller than that of the filter (see dimensions below), the Space Velocity of the 80L / min gas flow across the “Comparative (flow-through ASC)” sample was higher than for the filter samples. Inlet gas temperature was adjusted by ramping the temperature from a steady state temperature of 150°C at a ramp rate of 10°C per minute.

[0158] All the filter samples in this Example 2 were based on a wall-flow filter substrate of 1” x 5.53” (4.34 cubic inches).

[0159] Example 2.1 had a similar construction to that of Figure 4A incorporating the indicated elements from Figure 2, except in that “Coating 2” (labelled “50”) targeted a 1.5 g / ft3loading of platinum and 0.18 g / in3of low particle size alumina but was otherwise as described with reference to Figure 4A. That is, in the finished product the combined loading of Coatings 1 and 3 was 1.6 g / in3and the platinum loading of Coating 2 was 1.5 g / ft3, i.e. a ratio of the noble metals in the second catalytic layer in g per ft3to the total amount of the first and second SCR compositions in the first and third catalytic layers in g per in3is 0.9375.

[0160] Example 2.2 had a similar construction to that of Figure 4C, except in that “Coating 2” (labelled “50”) targeted a 1.5 g / ft3loading of platinum and 0.18 g / in3of low particle size alumina but was otherwise as described with reference to Figure 4C. That is in-wall “Coating 2” was coated to an axial length of 50% of the total axial length 45 of the wall-flow filter substrate from the inlet end thereof; and the third catalytic layer 55 (Coating 3) was coated in-wall to an axial length of 50% of the total axial length 45 from the outlet end thereof.

[0161] Example 2.3 had a similar construction to that of Example 2.2, except in that “Coating 2”, i.e. the second catalytic layer 50, was coated in-wall along an axial length of about 70% of the total axial length 45 of the wall-flow filter substrate (in-wall) from the inlet end thereof; and the third catalytic layer 55 was coated to an axial length of about 80% of the total axial length 45 (in-wall) from the outlet end thereof. In this regard, the mass of PGM and the loading of alumina in Coating 2 was the same as in Examples 2.1 and 2.2, except that these components were “stretched” over a longer longitudinal length. So, the substrate wall comprised three “zones”, an inlet zone of about 20% comprising “Coating 2”; a central zone of an axial length of about 50% comprising a combination of “Coating 2” and the third catalytic layer; and an outlet zone comprising the third catalytic layer of an axial length of about 30%.

[0162] The “Comparative (flow-through ASC)” example was a conventional ASC-product coated on a flow-through monolith substrate having a lower layer corresponding to Coating 2 described in Example 1 above extending over an entire length of the substrate and a platinum loading of 3 g / ft3; and an upper layer, completely overlying the lower layer and corresponding to Coating 3 except at a 2.4 g / ft3loading and at a 4.0 wt% copper loading and no cellulose pore former, also extending over an entire length of the substrate. The dimensions of the flow through substrate are 1-inch diameter x 3-inch length (2.36 cubic inches volume). Taking into account the core dimensions of the inventive filter substrate Examples and the Comparative flow-through substrate Examples the copper mass in the inventive and comparative samples was identical, i.e., 0.2 g copper (Cu), and the mass of platinum was substantially the same (4.1 mg Pt in the Comparative Example; and 3.8 mg Pt in the inventive filter Example).

[0163] The results for the “fresh” and unaged samples described hereinabove in this Example 2 are presented in Tables 1 to 3 hereinbelow. Table 1

[0164] Table 2 Table 3 The results presented in Tables 1 to 3 hereinabove indicate that the performance of the filter substrates as ammonia slip catalysts was surprisingly close to that of a “specialist” ASC, i.e. the Comparative (flow-through ASC). However, of course, the Comparative (flow-through ASC) does not control particulate matter emissions. Of the filter samples, sample 2.3 had higher N2O and NOXgeneration at 250°C than samples 2.1 and 2.2, which is a less preferred property.

[0165] EXAMPLE 3

[0166] Cold Flow back Pressure testing

[0167] Figure 5 shows the results of an analysis of cold flow backpressure (CFBP) at 21°C ambient temperature and at a flow rate of 600 m3 / hr of “fresh” coated filters using a Superflow SF1020 apparatus (available commercially at https: / / superflow.com / products / flowbenches / ), comparing the backpressure of embodiments 4A-4C and sample 2.3 prepared identically to the corresponding samples in Example 2, except the loading of Coating 2 is 0.5 g / ft3of platinum supported on 0.06 g / in3low particle size alumina and are therefore referred to in this Example 3 as “adapted sample 2.3” etc. In the sample of Figure 4B, the second catalytic layer 50 is about 25% of the total length 45 and the third catalytic layer 55 is about 75% of the total length 45.

[0168] The bars in the bar chart of Figure 5 from left to right are: the adapted sample 2.3; adapted sample 2.2 (Figure 4C); a sample corresponding to Figure 4B; and adapted sample 2.1 (Figure 4A), respectively. The CFBP for an uncoated filter is indicated as a horizontal line for comparison. Figure 5 shows that the lowest back pressure is obtained for the embodiment of Figures 4C (62 mBar) and 4B (63 mBar). That is, in-wall location of Coating 2 is beneficial to back pressure.

[0169] The term “comprising” as used herein can be exchanged for the definitions “consisting essentially of’ or “consisting of”. The term “comprising” is intended to mean that the named elements are essential, but other elements may be added and still form a construct within the scope of the claim. The term “consisting essentially of’ limits the scope of a claim to the specified materials or steps and those that do not materially affect the basic and novel characteristic(s) of the claimed invention. The term “consisting of’ closes the claim to the inclusion of materials other than those recited except for impurities ordinarily associated therewith. The foregoing detailed description has been provided by way of explanation and illustration and is not intended to limit the scope of the appended claims. Many variations in the presently preferred embodiments illustrated herein will be apparent to one of ordinary skill in the art and remain within the scope of the appended claims and their equivalents.

[0170] For the avoidance of doubt, the entire contents of all documents acknowledged herein are incorporated herein by reference.

Claims

1. CLAIMS:1 . A fuel combustion and exhaust-gas treatment system comprising an internal combustion engine configured to run on a mixture of air and fuel, wherein the fuel of the mixture of air and fuel is a gaseous fuel comprising a majority fuel mass of hydrogen (H2), a source of gaseous fuel comprising a majority fuel mass of hydrogen (H2) and an exhaust-gas treatment system comprising a catalyst article for the treatment of an exhaust gas, the catalyst article comprising: a wall-flow filter substrate having inlet channels open at an inlet-end of the substrate and closed at an outlet-end of the substrate, the inlet channels adjacent outlet channels which are closed at the inlet-end of the substrate and open at the outlet-end of the substrate, the wall-flow filter comprising at least first, second and third catalytic layers, wherein:(i) the first catalytic layer extends from the inlet-end of the substrate and comprises a first SCR composition;(ii) the second catalytic layer is provided in or on the walls of the inlet channels, extending from the inlet-end of the substrate, and comprises a noble metal-containing composition, wherein: when the second catalytic layer is provided on the walls of the inlet channel, the first catalytic layer is provided on the second catalytic layer, and when the second catalytic layer is provided in the walls of the inlet channels, the first catalytic layer is provided on the walls of the inlet channels; and(iii) the third catalytic layer is provided in or on the walls of the outlet channels, extending from the outlet-end of the substrate, and comprises a second SCR composition; wherein a ratio of the noble metals in the second catalytic layer in g per ft3to the total amount of the first and second SCR compositions in the first and third catalytic layers in g per in3is from 1.5:8 to 2: 1.

2. The fuel combustion and exhaust-gas treatment system according to claim 1 , wherein, in the catalyst article, the second catalytic layer is provided in the walls of the inlet channels and / or the third catalytic layer is provided on the walls of the outlet channels.

3. The fuel combustion and exhaust-gas treatment system according to claim 1 or claim 2, wherein, in the catalyst article:(i) the first catalytic layer extends up to 100% of a longitudinal length of the substrate extending from the inlet-end to an outlet-end, preferably from 70 to 90% of the longitudinal length; and / or(ii) the second catalytic layer extends up to 100% of a longitudinal length of the substrate extending from the inlet-end to an outlet-end, preferably from 25 to 90% of the longitudinal length, more preferably from 40 to 70% of the longitudinal length; and / or(iii) the third catalytic layer extends up to 100% of a longitudinal length of the substrate extending from the outlet-end to an inlet-end, preferably from 20 to 90% of the longitudinal length, more preferably from 30 to 60% of the longitudinal length, preferably from 70 to 90% of the longitudinal length.

4. The fuel combustion and exhaust-gas treatment system according to claim 3, wherein, in the catalyst article, at least two of the first, second and third catalytic layers, and preferably all three, extend 70 to 90% of the longitudinal length.

5. The fuel combustion and exhaust-gas treatment system according to any preceding claim, wherein, in the catalyst article, the second and third catalytic layers do not overlap and together extend 100% of a longitudinal length of the substrate extending from an outlet-end to an inlet end, preferably wherein the second catalytic layer is provided in the walls of the inlet channels and wherein the third catalytic layer is provided in the walls of the outlet channels.

6. The fuel combustion and exhaust-gas treatment system according to any preceding claim, wherein, in the catalyst article, the ratio of the noble metals in the second catalytic layer in g per ft3to the total amount of the first and second SCR compositions in the first and third catalytic layers in g per in3is about 1.5:16.

7. The fuel combustion and exhaust-gas treatment system according to any preceding claim, wherein, in the catalyst article, the noble metal-containing composition contains Pt, preferably as the only PGM; or a combination of Pd-Au.

8. The fuel combustion and exhaust-gas treatment system according to any preceding claim, wherein, in the catalyst article, the noble metal-containing composition comprises alumina as a support for the noble metals.

9. The fuel combustion and exhaust-gas treatment system according to any preceding claim, wherein, in the catalyst article, a catalytically active SCR components of the first and / or second SCR compositions comprises, and preferably consists, of one or more metal- exchanged zeolites.

10. The fuel combustion and exhaust-gas treatment system according to claim 9, wherein, in the catalyst article, the metal-exchanged zeolite is a small pore zeolite, preferably having a CHA framework structure, preferably wherein the zeolite is exchanged with Cu and / or Mn.

11. The fuel combustion and exhaust-gas treatment system according to any preceding claim, wherein, in the catalyst article, a weight ratio of the first and second SCR compositions in the first and third catalytic layers is from 2:3 to 3:2 and is preferably about 1 :1.

12. The fuel combustion and exhaust-gas treatment system according to any of the preceding claims, further comprising means for electrically heating the catalyst article.

13. The fuel combustion and exhaust-gas treatment system according to any preceding claim, further comprising in order from upstream to downstream of the engine, means for injecting a nitrogenous reductant, a flow-through substrate comprising a SCR catalyst and the catalyst article, preferably wherein the system further comprises a Hydrogen Oxidation Catalyst (HOC) upstream of the means for injecting a nitrogenous reductant.

14. A vehicle comprising the fuel combustion and exhaust-gas treatment system according to any preceding claim, preferably wherein at least the catalyst article is located in an underfloor location and / or encounters, in normal use, exhaust gases at a temperature of from 200 to 400°C, preferably 150-300 °C.

15. A method for the treatment of an exhaust gas of an internal combustion engine configured to run on a mixture of air and fuel, wherein the fuel of the mixture of air and fuel is a gaseous fuel comprising a majority fuel mass of hydrogen (H2), the method comprising passing the exhaust gas through a catalyst article comprising:a wall-flow filter substrate having inlet channels open at an inlet-end of the substrate and closed at an outlet-end of the substrate, the inlet channels adjacent outlet channels which are closed at the inlet-end of the substrate and open at the outlet-end of the substrate, the wall-flow filter comprising at least first, second and third catalytic layers, wherein: (i) the first catalytic layer extends from the inlet-end of the substrate and comprises a firstSCR composition;(ii) the second catalytic layer is provided in or on the walls of the inlet channels, extending from the inlet-end of the substrate, and comprises a noble metal-containing composition, wherein: when the second catalytic layer is provided on the walls of the inlet channel, the first catalytic layer is provided on the second catalytic layer, and when the second catalytic layer is provided in the walls of the inlet channels, the first catalytic layer is provided on the walls of the inlet channels; and(iii) the third catalytic layer is provided in or on the walls of the outlet channels, extending from the outlet-end of the substrate, and comprises a second SCR composition; wherein a ratio of the noble metals in the second catalytic layer in g per ft3to the total amount of the first and second SCR compositions in the first and third catalytic layers in g per in3is from 1.5:8 to 2:1.

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