Exhaust gas treatment system for a hydrogen internal combustion engine

Abstract

A fuel combustion and exhaust gas system comprises: (i) an internal combustion engine having an engine displacement and 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); (ii) a source of gaseous fuel comprising a majority fuel mass of hydrogen (H2) for supplying the internal combustion engine; and (iii) an exhaust gas treatment system for the internal combustion engine comprising means for injecting a nitrogenous reductant and a source of nitrogenous reductant and one or more filter monolith substrates comprising a mass of selective catalytic reduction (SCR) catalyst washcoat, wherein a volume ratio of the engine displacement in litres of the internal combustion engine to a total volume in litres of the one or more filter monolith substrates comprising the mass of SCR catalyst, is from 1:1 to 1:3; and, wherein a ratio of the mass in kilograms of SCR catalyst washcoat on the one or more filter monolith substrates to the engine displacement in litres of the internal combustion engine is from 1:5 to 1:110.

Description

[0001] EXHAUST GAS TREATMENT SYSTEM FOR A HYDROGEN INTERNAL COMBUSTION ENGINE

[0002] The present invention relates to the provision of an exhaust gas treatment system for exhaust from a hydrogen internal combustion engine (H2-ICE). In particular, it provides an optimal, low-complexity exhaust treatment system that can effectively reduce NOx and particulate emissions across the lifetime of the system.

[0003] The present invention has been made in the context of hydrogen-fuelled internal combustion engines, and particularly in relation to exhaust systems for heavy-duty applications of the same. Hydrogen-fuelled engines are a relatively new technology that is gaining interest. Hydrogen-fuelled engines are typically run under lean-burn conditions, such that the hydrogen fuel is fully combusted. As a consequence, the exhaust gases have NOx that needs to be treated.

[0004] The diesel exhaust technical field is an established one with a range of suitable component parts, as well as strategies for how best to run the engine and exhaust system. Since diesel and hydrogen are both lean-run systems, the initial approach of many manufacturers has been to borrow technologies and, indeed, entire conventional systems from diesel applications and apply them across to hydrogen-fuelled systems. While we note that this has met with some success, there is clearly scope remaining to optimise systems that account for the unique differences that arise from the different engine-types, based on their fuel systems. Furthermore, as hydrogen engines are employed across the range of different applications, there is a need to optimise exhaust systems, such as for heavy-duty applications.

[0005] Treatment of NOx is a well-known issue and the standard approach to treating it is to employ selective catalytic reduction (SCR) approaches. An effective method to reduce NOx from the exhaust of lean-burn engines requires reaction of NOx under lean-burn engine operating conditions with a suitable reductant such as ammonia or hydrocarbon in the presence of an SCR catalyst. Suitable SCR catalysts include metal-containing molecular sieves such as metal-containing zeolites. A useful SCR catalyst component is able to effectively catalyse the reduction of the NOx exhaust component at temperatures below 600°C, so that reduced NOx levels can be achieved even under conditions of low load which typically are associated with lower exhaust temperatures.The formation of particulate matter is a known problem for diesel systems. The problem is soot and ash. These can be prevented from atmospheric release by trapping them on a filter, such as a diesel particulate filter (DPF) or catalysed soot filter (CSF). After the soot has built up on the filter it can be destroyed through a filter regeneration process, whereby the temperature of the filter is increased, such as by an upstream exotherm generating catalyst article for a short time. The need for filter regeneration increases the cost and complexity of the system. The soot is primarily a by-product of the diesel combustion. However, current H2-ICE generates comparable #PN as Diesel engines, generally derived from engine lubricants etc.

[0006] WO2024 / 039628 relates to a catalyst body and exhaust gas aftertreatment system, with hydrogen ICEs being specifically considered. The focus of the application is an SCR catalyst article provided with an oxidation catalyst at the outlet end to treat any unburnt hydrogen emissions. The substrate is provided downstream of a filter body in the depicted embodiment. There is no indication given of the coating loadings.

[0007] EP3607183 relates to a hydrogen-assisted integrated emission control system.

[0008] EP4293207 relates to an engine system comprising a hydrogen combustion engine and an exhaust aftertreatment system.

[0009] EP4293208 relates to a hydrogen internal combustion engine and exhaust gas treatment system.

[0010] It is an object of the present invention to provide an exhaust treatment system for an H2-ICE engine, especially for heavy-duty applications, which can address the exhaust gas treatment requirements of an H2-ICE over its lifetime, with a reduced system complexity and cost. Alternatively, it is an object at least to tackle problems associated therewith in the prior art or provide a commercially viable alternative thereto.

[0011] According to a first aspect there is provided a fuel combustion and exhaust gas system comprising:

[0012] (i) an internal combustion engine having an engine displacement and 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);

[0013] (ii) a source of gaseous fuel comprising a majority fuel mass of hydrogen (H2) for supplying the internal combustion engine; and(iii) an exhaust gas treatment system for the internal combustion engine comprising means for injecting a nitrogenous reductant and a source of nitrogenous reductant and one or more filter monolith substrates comprising a mass of selective catalytic reduction (SCR) catalyst washcoat,

[0014] wherein a volume ratio of the engine displacement in litres of the internal combustion engine to a total volume in litres of the one or more filter monolith substrates comprising the mass of SCR catalyst, is from 1 : 1 to 1 :3, preferably 1 : 1.2 to 1 :2; and,

[0015] wherein a ratio of the mass in kilograms of SCR catalyst washcoat on the one or more filter monolith substrates to the engine displacement in litres of the internal combustion engine is from 1 :5 to 1:110.

[0016] 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 system may be combined with those disclosed in relation to the method and vice versa.

[0017] While it might be expected that an H2-ICE engine would have lower particulate emissions, the present inventors have now found that in practice there remains a particulate problem with H2-ICE, both from soot and ash, but also from urea particulates. Rather than coming from the combusted fuel, the soot and ash come primarily from engine oil which is required to prevent undue wear. The urea particulates come from a spray of injected nitrogenous reductant into exhaust gas and used for the SCR conversion of NOx. It has recently been observed that the use of a nitrogenous reductant (especially urea / ammonia) injection into an exhaust gas stream can lead to the production of certain aggregated compounds. 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.

[0018] A discussion of these polymerised particulate emissions may be found in Society of Automotive Engineers SAE paper no. 2017-01-0915. 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 befurther 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.

[0019] 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 thermal 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.

[0020] Accordingly, the inventors have concluded that there remains a need for a filter component in an exhaust system of an H2-ICE. The inventors have now employed an SCR-coated filter body (or several, potentially arranged in series or in parallel “legs” of the exhaust system), or so-called SCRF to address this particulate material, combining both the required SCR performance with the capture of these particulates.

[0021] SCRFs are a known catalyst component and are often employed in light-duty diesel applications where the engine conditions fluctuate more significantly than for heavy-duty. The SCRF is able to perform NOx treatment with the SCR coating and to filter out particulate matter with the filter body. The NOx can also be used to catalyse soot removal from the catalyst using the CRT® effect.

[0022] SCRFs are not conventionally used on heavy-duty diesel systems because there is competition from the SCR catalyst for the NOx, which means that there also can be not enough NOx passively to oxidise the removal of soot using the CRT® effect. Additionally, a Cu-zeolite SCR catalysed filter substrate generally requires higher temperatures to regenerate (i.e. to combust trapped particulate matter thereon) compared with a conventional platinum group metal catalysed soot filter. The inventors have now found that the SCR and particulate matter aftertreatment requirements for a H2-ICE can be handled with a single, specific SCRF.

[0023] In particular, providing an SCRF with a low SCR loading, lower than conventional in diesel applications, results in a lower back-pressure and the inventors have found that this cancope with a lifetime soot and ash build-up, e.g. over a vehicle lifetime. This means that the exhaust system does not need to have soot-specific regeneration events and there is no essential need for an upstream HOC catalyst (though one can be present). In particular, they have found that the SCRF is suitable for heavy-duty hydrogen-fuelled applications, a surprising difference from the diesel counterparts because of the lower soot content in H2-ICE such that regeneration is not required and there is no (or significantly less) competing NOx-consuming reaction.

[0024] Desirably the SCRF in the present invention can have a much higher volume than is conventional, such that the same total amount of SCR composition is present as in a diesel application or H2-ICE flow-through substrate, but the specific size will vary with the engine displacement of the specific H2-ICE. The larger catalyst article is facilitated here because of the simpler total system which takes up less total space than a corresponding diesel exhaust system because it has fewer catalyst components. It may also be possible to improve exhaust system design options by dividing a total filter substrate volume amongst two or more filter substrates in the system as a whole, e.g. by arranging two filter substrates in parallel “legs” of the system, or by disposing two filter substrates in series.

[0025] The inventors have found that the lower washcoat loading arranged over a larger filter substrate volume still provides the same catalytic benefits, but the filter performance is actually better. It is theorised that the lower space velocity associated with the larger article means that the filtered soot can be more dispersed and less likely to result in pore blocking and back-pressure issues. This can be further improved with on-wall coatings, particularly on the inlet channels, where the particulates are less likely to penetrate into the in-wall pores , which can more severely negatively impacts backpressure.

[0026] In diesel systems most of the ash is trapped by a soot-cake building up on the filter's walls during normal operation. This prevents an excessive ash build-up into the deeper sections of the filter's wall, thus limiting detrimental contribution of ash-loading to back-pressure. On the other hand, the amount of soot build-up in the filter is orders of magnitudes smaller in H2-ICEs when compared with traditional diesel systems. For this reason, potential failure modes resulting from excessive ash accumulation in deeper wall locations cannot be excluded.

[0027] With the present invention the preferred embodiment of a coating of a porous on-wall SCRF-layer in the inlet-channels of a filter behaves as a membrane with high filtration efficiency. These then have the potential to capture a significant fraction of the ash particles in theexhaust and prevent ash accumulation in deeper locations of the filter's wall. This can in principle reduce the risk of higher back-pressure failure-modes.

[0028] The present invention relates to a fuel combustion and exhaust gas system. The system comprises an internal combustion engine, a source of gaseous fuel and an exhaust gas treatment system. In use, the gaseous fuel is combusted within the internal combustion engine and the produced gases are treated within the exhaust gas treatment system to remove harmful emissions. The fuel comprises a majority fuel mass of hydrogen (H2), such that it is preferably a fuel-grade hydrogen mass and the engine is configured for combustion of the same. The nature of the hydrogen gas will be strictly defined by regulation, such that the engine is designed and configured to combust hydrogen of the available designated commercial grades. Preferably the fuel is substantially entirely hydrogen. The primary emissions of concern for H2-ICE are NOx and particulate emissions.

[0029] It should be appreciated that while there are similarities between H2-ICE and other lean-burn engines, the specific nature and configuration of H2-ICE means that the known aspects of conventional diesel exhaust systems cannot necessarily be directly applied across to H2-ICE. Nonetheless, a number of the other components employed for the present invention do have corresponding counterparts in exhaust systems of lean-burn engines as discussed below.

[0030] The focus of the present invention is on the one or more filter monolith substrates comprising a mass of selective catalytic reduction (SCR) catalyst washcoat. Filter monolith substrates are well known in the art, as are suitable SCR catalyst compositions for use in forming so-called SCRF components. The present invention is therefore distinguished by having a particularly large volume of filter substrate(s) and that they have unusually low washcoat loadings. These characteristics are defined using the parameters outlined below.

[0031] A volume ratio of the engine displacement in litres of the internal combustion engine to a total volume in litres of the one or more filter monolith substrates comprising the mass of SCR catalyst is from 1:1 to 1:3, preferably 1:1.2 to 1:2, and most preferably 1:1.4 to 1:1.8. This parameter defines the volume of the SCR-containing filter substrates compared to the engine size. It is intended to help distinguish over conventional diesel applications, reflecting that the filter can be desirably unusually large. This parameter nonetheless allows for the fact that different applications can have engines of different size and that the filter volume will scale with the corresponding size of the engines. Engines for heavy duty applications are generally at least 7 litres (preferably 11 to 15 L), such that this feature therefore requires atleast 7 litres of the filter monolith substrates. It should be appreciated that the preferred embodiment is to only have a single filter monolith substrate, or where there is more than one for the substrates to be on separate “legs” of the system, i.e. in parallel, rather than in series. This increases the total effective filtration volume of the system, while allowing a plurality of smaller filter parts to be employed.

[0032] A ratio of the mass in kilograms of SCR catalyst washcoat on the one or more filter monolith substrates to the engine displacement in litres of the internal combustion engine is from 1:5 to 1:110. Thus, in the exemplary 7L heavy duty engine, the total mass of the SCR washcoat will be from 1kg to 63g. Preferably the ratio is from 1:6 to 1:50, more preferably 1:7 to 1:10.

[0033] Preferably the total washcoat loading of SCR catalyst on the one or more filter monolith substrate is less than 2.5g / in3. Preferably the washcoat loading is less than 1.6g / in3, preferably less than 1.4g / in3, preferably less than 1.2g / in3. Preferably the loading of the SCR catalyst washcoat layer is from 0.1 to 1.5g / in3, preferably between 0.5 and 1.4g / in3, preferably between 0.8 and 1.1 g / in3. The “washcoat loading” refers to the weight of the washcoat (after calcination) per unit volume of the whole monolith filter. Preferably the SCR catalyst washcoat has a uniform loading across the layer. The volume of the whole catalytic filter is calculated based on its cross-sectional area and length, e.g. the volume of a cylinder; it does not take into account the number of channels per square inch or “open frontal area”. Conventional SCRF coatings in practical diesel applications generally have washcoat loadings of at least 1.7g / in3.

[0034] Preferably the total washcoat loading on the filter also corresponds to those ranges in the paragraph above, i.e. including the SCR washcoat layer and any other coatings or layers provided. Preferably the SCR catalyst is the only washcoat material on the filter substrates.

[0035] Preferably the volume of the one or each filter monolith substrate is at least 90in3, such as > 183in3. Preferably a total filter monolith substrate volume in the system as a whole is at least 1400in3, preferably from 1500in3to 4000in3.

[0036] Turning to the filter substrate(s) in more detail, the substrate is preferably a wall-flow filter substrate which is extremely well known in the art for diesel applications. Other filter types are known and could be applied here, but we will use the term “wall-flow” filter hereafter for simplicity - this is the most preferred embodiment. 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 andopen 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 ensures that the gases are treated while they pass through the wall.

[0037] Typical ceramic wall flow filter substrates are composed of refractory materials such as cordierite 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.

[0038] The monolithic substrates used for a wall flow filter may contain up to about 400 flow passages (or “cells”) per square inch (cpsi) (62.0 cells per cm2) of cross section, although far fewer may be used. For example, the carrier may have from 7 to 400, specifically from 150 to 400, cells per square inch (“cpsi”). The cells can have cross sections that are rectangular, square, circular, oval, triangular, hexagonal, or are of other polygonal shapes. Preferably the wall-flow filter of the SCRF catalyst article has a channel density of from 200 to 400 cpsi. Preferably the one or each filter monolith substrate has a channel wall thickness of from 0.12 to 0.4 mm.

[0039] 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. Preferably the one or each filter monolith substrate is a wall-flow filter having a porosity of from 50 to 70%, preferably from 55 to 65%.

[0040] The SCR catalyst washcoat generally comprises an oxide of a base metal, a molecular sieve, a metal-exchanged molecular sieve, or a mixture thereof. The base metal can be selected from the group consisting of cerium (Ce), chromium (Cr), cobalt (Co), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), nickel (Ni), tungsten (W), vanadium (V), yttrium (Y), neodymium (Nd), lanthanum (La), antimony (Sb), and mixtures thereof. SCR catalysts consisting of vanadium supported on a refractory metal oxide such as alumina, silica, zirconia, titania, ceria and combinations thereof are well known and widely usedcommercially in mobile applications. Typical compositions are described in US4, 010,238 and US4,085,193, of which the entire contents are incorporated herein by reference.

[0041] Compositions used commercially, especially in mobile applications, comprise TiC>2 onto which WO3 and V2O5 have been dispersed at concentrations ranging from 5 to 20 wt% and 0.5 to 6 wt%, respectively. These catalysts may contain other inorganic materials such as SiC>2 and ZrC>2 acting as binders and / or promoters.

[0042] The SCR catalyst washcoat may comprise vanadium and cerium. The molar ratio of cerium to vanadium can be from 0.3 to 0.7 or from 0.4 to 0.6. The SCR catalyst may comprise vanadium in an amount of 2-6 wt%, or 3-5 wt% on a V2O5 basis and cerium in an amount of 1-10 wt% or 2-5 wt% on a CeC>2 basis. Employing cerium and vanadium in such proportions reduces vanadium volatilisation without reducing the activity of the SCR catalyst. The reduced volatilisation permits a greater loading of vanadium, which is desirable for improving the efficiency of the SCR catalyst. In addition to vanadium and cerium, the SCR catalyst may additionally comprise antimony. Antimony may be present in an amount such that a molar ratio of antimony to vanadium is from 0.6 to 0.9, or from 0.7 to 0.8. Antimony may be present as Sb2O5. By way of example, the SCR catalyst may comprise 2-6 wt% V2O5, 2-6 wt% CeC>2, and 3-8 wt% Sb20s relative to the total weight of the SCR catalyst.

[0043] The SCR catalyst washcoat preferably comprises a molecular sieve or a metal-exchanged molecular sieve. As is used herein "molecular sieve" is understood to mean a metastable material containing tiny pores of a precise and uniform size that may be used as an adsorbent for gases or liquids. The molecular sieve can be a zeolitic molecular sieve, a non-zeolitic molecular sieve, or a mixture thereof.

[0044] A zeolitic molecular sieve 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, BEA, FAU, LTA, MFI, and 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-to-alumina molar ratio (SAR, defined as SiO2 / AhO3) from 5 to 200, from 10 to 180, or from about 20 to 150.

[0045] As used herein, the term “non-zeolitic molecular sieve” refers to corner sharing tetrahedral frameworks where at least a portion of the tetrahedral sites are occupied by an element other than silicon or aluminium. Specific non-limiting examples of non-zeolitic molecular sieves include silicoaluminophosphates such as SAPO-34, SAPO-37 and SAPO 44. Thesilicoaluminophosphates can have framework structures that contain framework elements that are found in zeolites, such as BEA, CHA, FAU, LTA, MFI, MOR and other types described below.

[0046] The SCR catalyst can comprise a small-pore, a medium-pore or a large-pore molecular sieve, or combinations thereof.

[0047] The SCR catalyst washcoat can comprise a small-pore molecular sieve selected from the group consisting of aluminosilicate molecular sieves, metal-substituted aluminosilicate molecular sieves, aluminophosphate (AIPO) molecular sieves, metal-substituted aluminophosphate (MeAIPO) molecular sieves, silico-aluminophosphate (SAPO) molecular sieves, and metal substituted silico-aluminophosphate (MeAPSO) molecular sieves, and mixtures thereof. The SCR catalyst can comprise a small-pore molecular sieve selected from the group of Framework Types consisting of ACO, AEI, AEN, AFN, AFT, AFX, ANA, APC, APD, ATT, CDO, CHA, DDR, DFT, EAB, EDI, EPI, ERI, GIS, GOO, IHW, ITE, ITW, LEV, KFI, LTA, MER, MON, NSI, OWE, PAU, PHI, RHO, RTH, SAT, SAV, SI , THO, TSC, UEI, UFI, VNI, YUG, and ZON, and mixtures and / or intergrowths thereof. The small pore molecular sieve may be selected from the group of Framework Types consisting of AEI, AFX, CHA, DDR, ERI, ITE, KFI, LTA, LEV, and SFW

[0048] The SCR catalyst washcoat can comprise a medium-pore molecular sieve selected from the group of Framework Types consisting of AEL, AFO, AHT, BOF, BOZ, CGF, CGS, CHI, DAC, EUO, FER, HEU, IMF, ITH, ITR, JRY, JSR, JST, LAU, LOV, MEL, MFI, MFS, MRE, MTT, MVY, MWW, NAB, NAT, NES, OBW, PAR, PCR, PON, PUN, RRO, RSN, SFF, SFG, STF, STI, STT, STW, -SVR, SZR, TER, TON, TUN, UOS, VSV, WEI, and WEN, and mixtures and / or intergrowths thereof. The medium pore molecular sieve may be selected from the group of Framework Types consisting of FER, MFI, and STT.

[0049] The SCR catalyst washcoat can comprise a large-pore molecular sieve selected from the group of Framework Types consisting of AFI, AFR, AFS, AFY, ASV, ATO, ATS, BEA, BEC, BOG, BPH, BSV, CAN, CON, CZP, DFO, EMT, EON, EZT, FAU, GME, GON, IFR, ISV, ITG, IWR, IWS, IWV, IWW, JSR, LTF, LTL, MAZ, MEI, MOR, MOZ, MSE, MTW, NPO, OFF, OKO, OSI, RON, RWY, SAF, SAO, SBE, SBS, SBT, SEW, SFE, SFO, SFS, SFV, SOF, SOS, STO, SSF, SSY, USI, UWY, and VET, and mixtures and / or intergrowths thereof. The large pore molecular sieve can be selected from the group of Framework Types consisting of BEA, MOR and OFF.A metal exchanged molecular sieve can have at least one metal from one of the groups VB, VI B, VI IB, VI I IB, IB, or 11 B of the periodic table deposited onto extra-framework sites on the external surface or within the channels, cavities, or cages of the molecular sieves. Metals may be in one of several forms, including, but not limited to, zerovalent metal atoms or clusters, isolated cations, mononuclear or polynuclear oxycations, or as extended metal oxides. The metals can be iron, copper, and mixtures or combinations thereof.

[0050] A metal exchanged molecular sieve can contain in the range of from about 0.10 wt% to about 10 wt% of a group VB, VI B, VII B, VII IB, IB, or I IB metal located on extra framework sites on the external surface or within the channels, cavities, or cages of the molecular sieve.

[0051] The metal exchanged molecular sieve can be a copper (Cu) supported small pore molecular sieve having from 0.1 to 20.0 wt%, 1 wt% to 6 wt%, or from 1.8 wt% to 4.2 wt% copper relative to the total weight of the metal exchanged molecular sieve.

[0052] The metal exchanged molecular sieve can be an iron (Fe) supported small pore molecular sieve having from 0.1 to 20.0 wt%, 1 wt% to 6 wt%, or from 1.8 wt% to 4.2 wt% iron relative to the total weight of the metal exchanged molecular sieve.

[0053] Preferably the washcoat layer comprises: (i) an Fe or Cu promoted zeolite; or (ii) a vanadium SCR catalyst. The Cu zeolites (such as CuCHA) have an ability to store the ammonia which allows the catalyst to cope well with the isolated spikes of NOx produced by the H2-ICE. The vanadium catalysts are capable of good NOx-conversion even when little NH3 is stored on the catalyst, then are able to cope with relatively high NOx-levels in the exhaust gas feed without the need of any preliminary “N H3-fill” stages so are also able to cope with the spikes just by being more effective. They therefore represent optimised catalyst types for this specific application.

[0054] Preferably the SCR catalyst is an Fe and / or Cu promoted zeolite. In these embodiments the total washcoat loading of SCR catalyst on the one or more filter monolith substrate is preferably less than 1.6g / in3, preferably from 0.1 to 1.5g / in3, preferably 0.9 to 1.3 g / in3. In this embodiment, preferably the ratio of a total mass in kilograms of SCR catalyst washcoat on the one or more filter monolith substrates to engine displacement in litres of the internal combustion engine is from 1:6 to 1:110.

[0055] Preferably in another embodiment the SCR catalyst is vanadium supported on a titania support; or tungsten or niobium supported on a ceria-zirconia mixed oxide or zirconiasupport, or a mixture of any two or more thereof. Preferably the tungsten or niobium is supported in combination with iron (see Applicant’s W02009 / 001131 A1). Preferably the total washcoat loading of SCR catalyst on the one or more filter monolith substrate is less than 2.5g / in3, preferably less than 2.0g / in3, such as 0.5 to 1.9 g / in3.

[0056] The SCR washcoat may contain components such as fillers, binders, stabilizers, rheology modifiers, and other additives. In certain embodiments, the washcoat comprises poreforming 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 25wt%, preferably less than 15wt%, and most preferably less than 10wt% of the layers with the balance the SCR or supported PGMs, 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 D90 of less than 7pm as measured by laser diffractometry.

[0057] The SCR catalyst can be coated onto the wall-flow filter substrate from the inlet end only, from the outlet end only, or preferably from both the inlet end and the outlet end. Preferably the SCR catalyst washcoat layer comprises a first portion on inlet channels of the wall-flow filter and a second portion on outlet channels of the wall-flow filter. Preferably no portion of an entire longitudinal length of the wall-flow filter is devoid of SCR catalyst washcoat layer.

[0058] Preferably the SCR catalyst washcoat layer is provided along an entire longitudinal length of the wall-flow filter. In some embodiments, SCR catalyst has a coating length of at least 100% of the substrate length (L) - this may be composed of partial coatings from the inlet and the outlet ends, such that preferably the coating length is in aggregate at least 105% and most preferably at least 110% of the length.

[0059] Preferably the first portion extends from the inlet end at least 30% of a longitudinal length of the wall-flow filter, more preferably at least 50%, more preferably 75 to 95%. Preferably the second portion extends from the outlet end at least 80% of the longitudinal length.

[0060] The SCR catalyst may be coated on the porous walls as “in-wall”, “on-wall”, or a combination of “in-wall” and “on-wall” coating. “In-wall” means that the catalyst is primarily present in the pores within the porous walls. For example, in an “in-wall” coating, the amount of the catalystin the pores within the porous walls is greater than 80wt%, greater than 85wt%, greater than 90wt%, or greater than 95wt% relative to the total amount of the catalyst applied to the filter substrate. “On-wall” means the catalyst is primarily present as a catalyst coating on the surface of the porous walls. For example, in an “on-wall” coating, the amount of the catalyst on the surface of the porous walls is greater than 80wt%, greater than 85wt%, greater than 90wt%, or greater than 95wt% relative to the total amount of the catalyst applied to the filter substrate. As would be appreciated by a person skilled in the art, the percentage of the coating that is present “on-wall” can be determined by known techniques, such as scanning electron microscopy (SEM) or optical microscopy.

[0061] The catalyst can be applied to the wall-flow filter substrate in the form of a washcoat. The techniques for “in-wall” or “on-wall” application can depend on the viscosity of the washcoat slurry applied, the application technique (spraying or dipping, for example) and the presence of different solvents. Such application techniques are known in the art. The viscosity of the washcoat slurry is influenced, for example, by its solids content. It is also influenced by the particle size distribution of the washcoat - a relatively flat distribution will give a different viscosity to a finely milled washcoat with a sharp peak in its particle size distribution - and rheology modifiers such as guar gums and other gums. Suitable coating methods are described in US6,599,570, US8,703,236, US9, 138,735 and US20180229228A1.

[0062] Preferably the inlet coating is an on-wall coating layer and preferably the outlet coating is inwall. This approach helps to encourage soot to deposit on the wall surfaces such that it is evenly captured across the porous on-wall membrane, which beneficially reduces the impact of the SCR catalysed filter on back pressure, compared with soot infiltrating into and clogging the substrate wall pores.

[0063] In a preferred embodiment, the SCRF further comprises an outlet ASC washcoat provided on an outlet end of the outlet channels of the wall-flow filter substrate. This further avoids the need for a separate ASC. The ASC may comprise one or more layers and preferably includes at least one PGM-containing layer. This serves to oxidise ammonia passing out of the SCRF.

[0064] For the sake of completeness, methods of applying catalyst to filters, will now be discussed further. The catalyst washcoat slurry may be applied to the substrate by known methods. There are many suitable ways to apply the catalyst washcoat slurry to the substrate. For example, coating of the substrate by the washcoat slurry can be performed by immersing the substrate vertically in the slurry such that the desired coating length is achieved. Thesubstrate can be left in the slurry for a sufficient period of time to allow the desired amount of the slurry to move into the substrate. The substrate is removed from the slurry, and excess slurry is removed from the wall-flow filter substrate first by allowing it to drain from the channels of the substrate, then by blowing on the slurry on the substrate with compressed air (against the direction of slurry penetration) and then pulling a vacuum from the direction of slurry penetration.

[0065] Another method for coating the wall-flow filter substrate includes the steps of: (a) depositing a pre-determined amount of a washcoat slurry into a containment means at an upper end of the filter substrate using a shower head, wherein the shower head comprises a plurality of apertures arranged to dispense the washcoat slurry onto an upper end face of the filter substrate; and (b) coating the channels having open ends at the upper end of the filter substrate with the pre-determined amount of washcoat slurry from the containment means by applying a vacuum to a lower end of the filter substrate to draw the liquid along the channels having open ends at the upper end of the filter substrate. See, e.g., Applicant’s US20180229228A1.

[0066] A preferred method for coating the wall-flow filter substrate is disclosed in Applicant’s EP106094B1 includes the steps of (a) locating a containment means on top of a support, (b) dosing a pre-determined quantity of a liquid component into said containment means, either in the order (a) then (b) or (b) then (a), and (c) by applying vacuum, drawing the entirety of said quantity of liquid component into at least a portion of the support, and retaining substantially all of said quantity within the support, without recycle.

[0067] The coated substrates are typically dried at about 110°C and calcined at a higher temperature, e.g., 300 to 500°C.

[0068] Preferably the exhaust gas treatment system comprises or consists, in order downstream from the internal combustion engine, of:

[0069] a) optionally, a hydrogen oxidation catalyst;

[0070] b) means for injecting a nitrogenous reductant and a source of nitrogenous reductant;

[0071] c) optionally, a urea hydrolysis catalyst;

[0072] d) optionally, a SCR catalyst coated on a flow-through monolith substrate; e) the one or more filter monolith substrates as defined in any of claims 1 to 11;

[0073] f) optionally, a SCR catalyst coated on a flow-through monolith substrate; andg) optionally, an ammonia slip catalyst.

[0074] After the final component the treated gases can be released to the atmosphere through the tail-pipe.

[0075] The means for injecting a nitrogenous reductant is typically a spray-head arranged in the channel for conducting the exhaust gases. The nitrogenous reductant provided by the means for injecting a nitrogenous reductant can be ammonia per se, 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. This means for injecting is arranged upstream of the SCRF to provide the reductant in doses suitable for carrying out the SCR reaction on the SCRF.

[0076] An HOC (optional component (a)) is a known catalyst component in the art. It typically takes the form of a washcoated flow-through monolith. The washcoat typically comprises one or more platinum group metals (PGMs) on a high surface area support, such as alumina. The HOC may be zoned or layered. The HOC is responsible for oxidising undesired species in the exhaust gases and can be used for exotherm generation, such as if additional hydrogen is mixed into the exhaust gases to be treated.

[0077] A urea hydrolysis catalyst (optional component (c)) is a known catalyst component in the art. A hydrolysis catalyst coating helps to ensure that the nitrogenous reductant, such as urea, is converted more fully into ammonia for use on the SCR component. A hydrolysis coating is a conventional washcoat and will comprise, for example, metal-exchanged zeolite and zirconia. An example of hydrolysis catalysts can be found in W02009118195A1, which is incorporated herein by reference.

[0078] An ASC (optional component (e)) is a known catalyst component in the art. They are designed to ensure that the exhaust gases do not contain unused ammonia slipped through the SCRF. The state-of-the-art ASC has a dual-layer architecture with a top layer of an SCR composition as discussed herein, preferably CuCHA, arranged on top of a layer of supported PGM, preferably Pt on alumina. This bilayer arrangement is discussed in, for example, LIS20160367973. The Pt oxidises excess ammonia.In embodiments, the HOC is not present. This is because it is not required for exotherm generation, such that the SCRF can suffice without regeneration. Avoiding the presence of an HOC component significantly reduces the system complexity, additional backpressure in the system and provides additional space for a larger SCRF component.

[0079] In the most preferred embodiments, the urea hydrolysis catalyst is not present. This is because it is not required to have urea hydrolysis, since that the SCRF can handle the formed urea particulate material given its larger volume. Avoiding the presence of a hydrolysis catalyst component can significantly reduce the system complexity and provides additional space for a larger SCRF component.

[0080] In the most preferred embodiments, the optional ASC is present to avoid unintended slip of nitrogenous reductant to atmosphere. However, where no ASC is present, the complexity of the system is reduced by having fewer parts.

[0081] Accordingly, the preferred system consists of in order downstream from the internal combustion engine: a) means for injecting a nitrogenous reductant; b) an SCRF catalyst article; and c) a ASC. After the SCRF catalyst article the treated gases can be released to the atmosphere through the tail-pipe. As discussed below, as an alternative to a separate ASC there may be provided an ASC washcoat layer on the outlet channels of the SCRF.

[0082] In embodiments comprising two or more of the filter monolith substrates as disclosed herein, the filter monolith substrates may be disposed in separate “legs” of the exhaust system. Preferably there are only two “legs” of the exhaust system and the filter substrates are divided between them.

[0083] According to a further aspect there is provided a method for the combustion of a gaseous fuel comprising a majority fuel mass of hydrogen (H2), the method comprising passing an exhaust gas from an internal combustion engine running on a mixture of air and fuel together with a nitrogenous reductant, wherein the fuel of the mixture of air and fuel is a gaseous fuel comprising a majority fuel mass of hydrogen (H2), into an exhaust gas treatment system comprising one or more filter monolith substrates comprising a mass of selective catalytic reduction (SCR) catalyst, wherein a volume ratio of an engine displacement in litres of the internal combustion engine to a total volume in litres of filter monolith substrate comprising SCR catalyst is from 1:1 to 1:3, preferably 1:1.2 to 1:2; and wherein a ratio of a total mass in kilograms of SCR catalyst on the one or more filter monolith substrate to engine displacement in litres of the internal combustion engine is from 1:5 to 1:110. Preferably thegaseous fuel consists of Hydrogen and unavoidable impurities. Preferably the method employs the exhaust system disclosed herein.

[0084] Preferably a space velocity of exhaust gas through the SCRF catalyst article is from 20k-60k v / hr, wherein v is a total volume of the one or more filter monolith substrate in the exhaust system as a whole and comprising a mass of selective catalytic reduction (SCR) catalyst. Preferably the space velocity of exhaust gas through the SCRF catalyst article is from 20k-40k v / hr, preferably 25k-35k v / hr, wherein v is the volume of the SCRF catalyst article. The inventors have found that these lower space velocities can be achieved through the provision of the larger catalyst articles and result in improved filtration properties. That is, because the particulate matter spends longer in the filter, the fine particles are more likely to settle evenly in the filter avoiding a build-up of back-pressure.

[0085] According to a preferred embodiment, the system described herein is employed in a mobile application, such as an automobile application. It is particularly preferred that the application is analogous to heavy-duty diesel. This would be the case, for example, with trucks having greater than 3,500kg axle weight and an engine size of 7.5 to 15L. In use the exhaust gas temperature at the engine outlet will predominantly be from 200-450°C, preferably 200-300°C.

[0086] BRIEF DESCRIPTION OF THE DRAWINGS

[0087] The accompanying drawings, described below, illustrate exemplary embodiments and are not to be considered limiting of the scope of the invention. The figures are not necessarily to scale, and certain features and certain view of the figures may be shown exaggerated in scale or in schematic in the interest of clarity and conciseness.

[0088] Figure 1 shows a schematic of the constituent parts in an embodiment of the present system.

[0089] Figure 2 shows a schematic of a preferred coating configuration of an SCRF as described herein.

[0090] As shown in figure 1, the system described herein comprises, in order, a hydrogen combustion engine 5 fed from a reservoir 10 containing a hydrogen fuel. The hydrogen combustion engine 5 is preferably a heavy-duty hydrogen combustion engine 5, such as onehave a displacement of at least 10L, preferably 12 to 16L, although larger engines can also be contemplated.

[0091] Exhaust gases leaving the engine 5 pass though the following catalyst components, in order, before being emitted in treated from the tail-pipe:

[0092] a) optionally, a hydrogen oxidation catalyst 15

[0093] b) means for injecting a nitrogenous reductant 20

[0094] c) optionally, a urea hydrolysis catalyst 25

[0095] d) an SCRF catalyst article 30

[0096] e) optionally, an ammonia slip catalyst 35

[0097] The treated gases can then be emitted through the tail-pipe 40.

[0098] The SCRF catalyst article 30 comprises a washcoat substrate, typically a conventional diesel SCRF substrate, coated with an SCRF washcoat. The SCRF catalyst article 30 is larger than a conventional SCRF catalyst, having a lower-than-normal washcoat loading. This prevents undue back-pressure, while providing adequate SCR washcoat for gas treatment. The SCRF catalyst article 30 is capable of providing life-time particulate removal for the system without requiring regeneration. This includes ash removal which are particulates which cannot be removed by exotherm / HOC (i.e. they are inorganic particulates). It is particularly desirable that the inlet channels have an on-wall coating as this has been found to minimise pore blocking from these impurities.

[0099] In Figure 2 the SCRF 30 is based on a wall-flow filter substrate having inlet channels 32 and outlet channels 34, and alternating end-plugs 35. An SCR coating 36a is applied on-wall on the inlet channels 32. An SCR coating 36b is applied in-wall on the outlet channels 34. The SCR coatings 36a, b extend about 80% of the length of their respective channels 32,34.

[0100] EXAMPLES

[0101] The invention will now be supported with the following non-limiting examples.

[0102] Catalyst preparation

[0103] First dose - in-wall rear:A slurry of a Cu-chabazite with SAR=19 was milled to target particle size distribution characterised by D90=3.8-3.9pm 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 La2C>3 (4.8wt%) I Nd2C>3 (4.1wt%), with particle size distribution characterised by D90=5.0-7.0pm, was added under high-speed stirring to target 11wt% with respect to the Cu-chabazite calcined weight.

[0104] 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 a suitable DPF, to a target washcoat loading of 0.9g / in3(SCRF A) or 1.43g / in3(SCRF B) with reference to the volume of the finished catalyst, thus achieving a coating depth of 80%. The brick was dried at 110°C for 20 minutes.

[0105] Second dose - in-wall front:

[0106] The Cu-chabazite washcoat described above was thickened by addition of a cellulose thickener (0.4wt% with respect to full weight of the wet washcoat) under high-speed stirring. The washcoat was stirred for 10 minutes at high speed and then coated from the front end of the mid-processed DPF containing the dry “First dose - in-wall rear” described above, to a target washcoat loading of 0.3g / in3(SCRF A) or 0.47g / in3(SCRF B) with reference to the volume of the finished catalyst, thus achieving a coating depth of 35%. The brick was dried at 110°C for 20 minutes and then calcined at 500°C for 2 hours.

[0107] SCRF A: Cu(3.3%)Chabazite washcoat - Washcoat Loading = 1.2 g / in3

[0108] SCRF B: Cu(3.3%)Chabazite washcoat - Washcoat Loading = 1.9 g / in3

[0109] Table 1 - Backpressure tested at a Space Velocity = 70 K / hr

[0110]

[0111] As can be seen from data, there is a significantly lower back pressure observed for the lower washcoat loading of SCRF A compared to the higher washcoat loading of SCRF B. Such adifference in back pressure would result in a major improvement of fuel efficiency making SCRF a credible solution for H2-ICE aftertreatments.

[0112] Synthetic Catalytic Activity Test (SCAT) apparatus testing gas feed composition and details:

[0113] NO-only = 1500ppm (high NO-concentration, worst case scenario to challenge the catalysts) ANR = 1.1 (NH3= 1650ppm)

[0114] Water = 9.5%

[0115] Oxygen = 12.5%

[0116] Hydrogen = Oppm or 1500ppm

[0117] CO, CO2, Hydrocarbons = 0%

[0118] Space Velocity = 60 K / s

[0119] Table 2 - NOx-Conversion% at different levels of NH3-stored - 200°C

[0120]

[0121] Table 3 - NOx-Conversion at NH3-saturation and different temperatures

[0122]

[0123] Table 4 - IXhO-make (ppm) at NHs-saturation and different temperatures

[0124]

[0125] Even in the challenging conditions of high NOx (1500ppm), the SCRF A showed:

[0126] - Similar NOx-conversion at 300°C, 400°C and 500°C as SCRF B;

[0127] - Similar NOx-conversion at 200°C as the SCRF B, when the catalysts operate at low / medium levels of NHs-stored;

[0128] - Lower N2O-make at 200°C, and similar N2O-make at 300°C, 400°C, 500°C vs SCRF B.

[0129] The lower washcoat loading SCRF A therefore allows for lower back-pressure, with satisfactory NOx-conversion and lower N2O-make when used in H2-ICE. H2-ICE emissions can be characterised by frequent and short bursts of high-NOx levels, spiking over a lower level of average NOx-emissions (e.g. with "low pressure, spark ignition" H2-ICE). The inventors found that SCRF A catalysts was able to successfully cope with short bursts of high NOx-emissions, even when coated at low washcoat loading (WCL).

[0130] 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.

[0131] 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.For the avoidance of doubt, the entire contents of all documents acknowledged herein are incorporated herein by reference.

Claims

CLAIMS:

1. A fuel combustion and exhaust gas system comprising:(i) an internal combustion engine having an engine displacement and 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);(ii) a source of gaseous fuel comprising a majority fuel mass of hydrogen (H2) for supplying the internal combustion engine; and(iii) an exhaust gas treatment system for the internal combustion engine comprising means for injecting a nitrogenous reductant and a source of nitrogenous reductant and one or more filter monolith substrates comprising a mass of selective catalytic reduction (SCR) catalyst washcoat,wherein a volume ratio of the engine displacement in litres of the internal combustion engine to a total volume in litres of the one or more filter monolith substrates comprising the mass of SCR catalyst, is from 1 : 1 to 1 :3, preferably 1 : 1.2 to 1 :2; and,wherein a ratio of the mass in kilograms of SCR catalyst washcoat on the one or more filter monolith substrates to the engine displacement in litres of the internal combustion engine is from 1 :5 to 1:110.

2. A fuel combustion and exhaust gas system according to claim 1 , wherein the total washcoat loading of SCR catalyst on the one or more filter monolith substrate is less than 2.5g / in3.

3. A fuel combustion and exhaust gas system according to claim 2, wherein the SCR catalyst is an Fe and / or Cu promoted zeolite and the total washcoat loading of SCR catalyst on the one or more filter monolith substrate is less than 1.6g / in3, optionally from 0.1 to 1 ,5g / in3and wherein the ratio of a total mass in kilograms of SCR catalyst washcoat on the one or more filter monolith substrates to engine displacement in litres of the internal combustion engine is from 1:6 to 1:50.

4. A fuel combustion and exhaust gas system according to claim 2, wherein the SCR catalyst is vanadium supported on a titania support; or tungsten or niobium supported on a ceria-zirconia mixed oxide or zirconia support or a mixture of any two or more thereof optionally wherein the tungsten or niobium is supported in combination with iron, and the total washcoat loading of SCR catalyst on the one or more filter monolith substrate is less than 2.5g / in3, preferably less than 2.0g / in3.

5. The fuel combustion and exhaust gas system according to any preceding claim, wherein the volume of the one or each filter monolith substrate is at least 90in3.

6. The fuel combustion and exhaust gas system according to any preceding claim, wherein a total filter monolith substrate volume in the system as a whole is at least 1400in3, preferably from 1500in3to 4000in3.

7. The fuel combustion and exhaust gas system according to any preceding claim, wherein the one or each filter monolith substrate is a wall-flow filter:(i) having a porosity of from 40 to 75%; and / or(ii) having a channel density of from 150 to 400 cpsi; and / or(iii) having a channel wall thickness of from 0.12 to 0.4mm.

8. The fuel combustion and exhaust gas system according to any preceding claim, wherein the filter monolith substrate is a wall-flow filter and the SCR catalyst is coated in a washcoat layer provided along at least a portion of an entire longitudinal length of the wallflow filter, wherein no portion of the entire longitudinal length of the wall-flow filter is devoid of SCR catalyst washcoat layer.

9. The fuel combustion and exhaust gas system according to any preceding claim, wherein the filter monolith substrate is a wall-flow filter and the SCR catalyst is coated in a washcoat layer comprising a first portion on inlet channels of the wall-flow filter and a second portion on outlet channels of the wall-flow filter, wherein no portion of an entire longitudinal length of the wall-flow filter is devoid of SCR catalyst washcoat layer.

10. The fuel combustion and exhaust gas system according to claim 8 or 9, wherein the SCR catalyst washcoat layer, at least on the inlet channels, is provided as a substantially on-wall coating.

11. The fuel combustion and exhaust gas system according to claim 9 and claim 10 when dependent on claim 9, wherein the first portion extends from the inlet end at least 30% of a longitudinal length of the wall-flow filter, and wherein the second portion extends from the outlet end at least 80% of the longitudinal length.

12. A fuel combustion and exhaust gas system according to any preceding claim, wherein the exhaust gas treatment system comprises or consists, in order downstream from the internal combustion engine, of:a) optionally, a hydrogen oxidation catalyst;b) means for injecting a nitrogenous reductant and a source of nitrogenous reductant;c) optionally, a urea hydrolysis catalyst;d) optionally, a SCR catalyst coated on a flow-through monolith substrate; e) the one or more filter monolith substrates as defined in any of claims 1 to 11;f) optionally, a SCR catalyst coated on a flow-through monolith substrate; and g) optionally, an ammonia slip catalyst.

12. The fuel combustion and exhaust gas system according to claim 11 , wherein the source of nitrogenous reductant comprises urea.

13. The fuel combustion and exhaust gas system according to any one of the preceding claims comprising two of the filter monolith substrates as defined in any of claims 1 to 11, each filter monolith substrate being disposed in a separate leg of the exhaust system.

14. A method for the combustion of a gaseous fuel comprising a majority fuel mass of hydrogen (H2), the method comprising passing an exhaust gas from an internal combustion engine running on a mixture of air and fuel together with a nitrogenous reductant, wherein the fuel of the mixture of air and fuel is a gaseous fuel comprising a majority fuel mass of hydrogen (H2), into an exhaust gas treatment system comprising one or more filter monolith substrates comprising a mass of selective catalytic reduction (SCR) catalyst, wherein a volume ratio of an engine displacement in litres of the internal combustion engine to a total volume in litres of filter monolith substrate comprising SCR catalyst is from 1:1 to 1:3, preferably 1:1.2 to 1:2; and wherein a ratio of a total mass in kilograms of SCR catalyst on the one or more filter monolith substrate to engine displacement in litres of the internal combustion engine is from 1:5 to 1:110.

15. The method according to claim 14, wherein a space velocity of exhaust gas through the SCRF catalyst article is from 20k-60k v / hr, wherein v is a total volume of the one or more filter monolith substrate in the exhaust system as a whole and comprising a mass of selective catalytic reduction (SCR) catalyst.

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