SCR catalyst article and system for reducing n2o in exhaust gas

Abstract

An exhaust gas treatment system includes a first catalyst article comprising a first catalyst composition and a second catalyst article downstream of the first catalyst article, the second catalyst article comprising a second catalyst composition. The first catalyst composition and the second catalyst composition are different. The first catalyst composition is an SCR catalyst composition, and the second catalyst composition includes an Fe-supported small or mesoporous molecular sieve having a silica to alumina ratio (SAR) of from 6 to 19. An SCR catalyst article comprising a substrate having a second catalyst composition on the substrate downstream of a first catalyst composition wherein the first catalyst composition and the second catalyst composition are different, and wherein the first catalyst composition is an SCR catalyst composition, and the second catalyst composition comprises a Fe-supported small or mesoporous molecular sieve having a silica to alumina ratio (SAR) of from 6 to 19.

Description

[0001] The present invention relates to an SCR catalyst article and system in which N2O emissions ultimately released from an exhaust gas treatment system can be reduced.

[0002] Exhaust gases from internal combustion engines and the like typically contain pollutants such as NO X (NO and NO2), which have well-known negative impacts on the environment and health. Accordingly, systems for treating such exhaust gases typically include a catalyst for NO X abatement, such as a selective catalytic reduction (SCR) catalyst. SCR catalysts and their mechanism of action are well-known.

[0003] Such exhaust gases may also contain nitrous oxide (N2O), typically at relatively low concentrations. However, the use of SCR catalysts and other known catalysts upstream may also produce additional N2O as a by-product. N2O also contributes to air pollution, to a lesser extent than NO X but still significantly. However, conventional exhaust gas treatment systems generally do not include a catalyst for the primary purpose of N2O abatement, and little development has been done to produce catalysts that effectively reduce N2O in exhaust gases.

[0004] Accordingly, there is a need to develop exhaust gas catalysts and exhaust gas treatment systems that can effectively reduce the amount of N2O ultimately released into the atmosphere while still effectively reducing more significant pollutants such as NO X .

[0005] The present invention seeks to address at least some of the problems associated with the prior art or at least provide a commercially acceptable alternative solution thereto.

[0006] The present invention provides an SCR catalyst article, an exhaust gas treatment system, a fuel combustion and exhaust gas treatment system, a method of treating exhaust gas, and use of a catalyst as described herein according to the appended claims.

[0007] Specifically, in a first aspect, the present invention provides an SCR catalyst article comprising a substrate having a second catalyst composition downstream of a first catalyst composition thereon, wherein the first catalyst composition and the second catalyst composition are different, and wherein the first catalyst composition is an SCR catalyst composition and the second catalyst composition comprises an Fe-loaded small pore molecular sieve or mesoporous molecular sieve having a silica to alumina ratio (SAR) of from 6 to 19.

[0008] Unless expressly indicated to the contrary, each aspect or embodiment as defined herein may be combined with any other aspect or embodiment. Specifically, any feature indicated as preferred or advantageous may be combined with any other feature indicated as preferred or advantageous.

[0009] The present inventors have surprisingly found that a catalyst comprising Fe-loaded small and mesoporous molecular sieves can exhibit particularly high N2O conversion when treating exhaust gas, and further, reducing the SAR of said molecular sieve can even further increase the N2O conversion of the catalyst. This is unexpected.

[0010] Accordingly, such catalysts can be particularly effectively used in exhaust gas systems for treating exhaust gas in which there is a relatively high N2O concentration, for example, such as downstream of an SCR catalyst. In other words, the second catalyst composition of the present invention can be used to reduce N2O in exhaust gas in a manner similar to an ammonia slip catalyst (ASC) for treating any "escaped" ammonia. That is, the second catalyst composition of the present invention can be used to reduce "N2O slip" passing through and / or generated by the first catalyst composition. This is advantageous and such uses have not been previously contemplated.

[0011] "N2O conversion" is intended to refer to the typical chemical reaction promoted by such catalysts, which is: 2N2O → 2N2 + O2.

[0012] The present inventors have also surprisingly found that the Fe-loaded molecular sieves of the present invention exhibit increased N2O conversion compared to Fe-loaded macroporous molecular sieves and small and mesoporous molecular sieves having a higher SAR. This has not been previously studied.

[0013] These results are also particularly surprising because it is not conventional to aim to reduce the SAR in molecular sieves for treating exhaust gas from internal combustion engines. This is because it is known that generally reducing the SAR of such molecular sieves (or particularly zeolites) can reduce the hydrothermal stability of the molecular sieve-based catalyst. For such applications where the typical operating environment can involve temperatures of several hundred degrees Celsius, a reduction in hydrothermal stability is clearly undesirable. However, for the present invention, the advantages obtained in N2O conversion can at least to some extent offset the disadvantages of reducing the SAR of the molecular sieve.

[0014] As used herein, the terms "first", "second", etc. are intended as labels and do not limit the relative arrangement or position of the corresponding features (here, the catalyst composition) within the exhaust gas treatment system, unless otherwise specified, such as by using the terms "upstream" or "downstream" to define the relative positions in the catalyst article or the exhaust gas treatment system.

[0015] As used herein, the term "upstream" indicates the direction in the system towards the exhaust gas source. In contrast, the term "downstream" as used herein indicates the direction in the system away from the exhaust gas source. For an SCR catalyst article, these directions of course refer to the intended use direction of the SCR catalyst article when present in the exhaust gas treatment system.

[0016] In this aspect, the second catalyst composition is downstream of the first catalyst composition. In this context, the phrase is intended to mean that at least a portion of the second catalyst composition is downstream of the first catalyst composition. In other words, a portion of the first catalyst composition and the second catalyst composition may overlap such that a portion of the first catalyst composition and the second catalyst composition are substantially in the same position in the (literal) upstream-to-downstream direction, or even a portion of the second catalyst composition may be upstream of the first catalyst composition.

[0017] The phrase may also cover a situation where the first catalyst composition is present in an upper layer, zone, region, or washcoat disposed on a substrate, and the second catalyst composition is present in a lower layer, zone, region, or washcoat disposed on the substrate, such that the SCR catalyst article is configured such that in use the exhaust gas will contact the first catalyst composition before contacting the second catalyst composition. In such embodiments, the upper layer, zone, region, or washcoat including the first catalyst composition may thus completely cover the lower layer, zone, region, or washcoat including the second catalyst composition.

[0018] However, preferably, the phrase is intended to cover that the second catalyst composition (at least a portion thereof) is downstream of the first catalyst composition in the (literal) upstream-to-downstream direction (such as in a linear upstream-to-downstream direction). More preferably, based on the total weight of the second catalyst composition, at least 70 wt%, even more preferably at least 80 wt%, still more preferably at least 90 wt%, yet more preferably at least 95 wt%, still more preferably at least 99 wt%, and most preferably 100 wt% of the second catalyst composition is downstream of the first catalyst composition (in the (literal) upstream-to-downstream direction, such as in a linear upstream-to-downstream direction).

[0019] As used herein, the term "article" or "catalyst article" may cover an article on or in which a catalyst is carried. The article may take the form of, for example, a honeycomb monolith such as a flow-through monolith or a filter (e.g., a wall-flow filter). The catalyst article may also be in the form of pellets.

[0020] Thus, as used herein, the term "SCR catalyst article" may cover a catalyst article suitable for catalyzing selective catalytic reduction reactions (such as the selective catalytic reduction of NO X ), preferably in the presence of a reducing agent such as a nitrogen-containing reducing agent or methane. In some preferred embodiments, the SCR catalyst article may be a selective catalytic reduction filter (SCRF). That is, the article / substrate may include a filter. In other words, the substrate may be a filter substrate.

[0021] As used herein, the term "catalyst composition" generally can encompass compositions that are catalytically active for pollutants present in exhaust gases.

[0022] As used herein, the term "substrate" can encompass, for example, ceramic or metallic flow-through honeycombs or filter monoliths (such as wall-flow filters). The substrate can include ceramic or metallic monolithic substrates. The substrate can vary in its material composition, size and configuration, pore shape and density, and wall thickness. Suitable substrate materials are known in the art, such as cordierite.

[0023] As used herein, the term "having thereon" can encompass the first catalyst composition and / or the second catalyst composition being disposed on the substrate and / or the first catalyst composition and / or the second catalyst composition being present in the substrate, such as in the form of an extruded substrate made from an extrudate comprising the first catalyst composition and / or the second catalyst composition.

[0024] As used herein, the term "disposed on" can encompass the catalytic composition being disposed directly on the substrate (i.e., without an intervening material), and / or indirectly on the substrate (i.e., with an intervening material). If the substrate is porous, the term "disposed on" can also encompass the catalytic composition being disposed therein, for example, in the pores of the substrate, i.e., where the catalytic composition is disposed on and / or within it. The catalytic composition is typically disposed on the substrate in the form of a washcoat. As used herein, the term "washcoat" is well known in the art and refers to an adherent coating that is typically applied to the substrate during catalyst production. How the molecular sieve is disposed on the substrate is not particularly important for the purposes of the present invention.

[0025] As used herein in the context of the first catalyst composition and the second catalyst composition, the term "different" can encompass the catalyst compositions being completely different, for example, if the first catalyst composition does not include a molecular sieve or is not based on a molecular sieve. Alternatively, the term "different" can only encompass the framework of the molecular sieve (i.e., the framework type code) or another property of the molecular sieve (such as SAR) being different, or even, for example, a metal other than Fe being present in the first catalyst composition (such as being loaded on the molecular sieve).

[0026] Preferably, the first catalyst composition and the second catalyst composition are also separated. As used herein, the term "separated" in the context of "separated first catalyst composition and second catalyst composition" can encompass the first catalyst composition and the second catalyst composition being disposed at different discrete locations on or in a substrate. In other words, the first catalyst composition and the second catalyst composition can be non-mixed such that they do not exist in the same catalytic zone, layer, region, or washcoat. Of course, it should be understood that a small amount of mixing may occur at any boundary between the first catalyst composition and the second catalyst composition, for example, within practical limits. In some embodiments, it may be possible that mixing of the first catalyst composition and the second catalyst composition may result in the deactivation of one or both of the catalyst compositions, such that it may be preferred that the first catalyst composition and the second catalyst composition remain separated.

[0027] Accordingly, as used herein, the term "SCR catalyst composition" can encompass a catalyst composition suitable for catalyzing a selective catalytic reduction reaction (such as the selective catalytic reduction of NO X ), preferably in the presence of a reducing agent such as a nitrogen-containing reducing agent or methane. However, it should be understood that the SCR catalyst composition is not limited to such uses. For example, the SCR catalyst composition may also exhibit further catalytic activity for use as a diesel oxidation catalyst (DOC) and / or passive NO X adsorbent (PNA) and / or ASC and / or another type of known exhaust gas catalyst. In some embodiments, the SCR catalyst article may also include other layers, zones, regions, or washcoats containing a catalyst that exhibits such catalytic activity.

[0028] In an alternative aspect of the present invention, the first catalyst composition is a DOC and / or PNA and / or ASC catalyst composition (i.e., and not necessarily an SCR catalyst composition). The DOC, PNA, and / or ASC may also generate additional N2O into the exhaust gas. Accordingly, the same advantages described with respect to the first aspect still apply herein.

[0029] As used herein, the term "Fe loading" can encompass the metal promotion of a molecular sieve with Fe, wherein the Fe can be loaded into the molecular sieve. In a metal-loaded molecular sieve, the loaded metal is a type of "extra-framework metal", i.e., a metal that resides within and / or on at least a portion of the surface of the molecular sieve. This definition does not include the atoms that make up the molecular sieve framework.

[0030] Generally, metal-loaded molecular sieves and methods for manufacturing such metal-loaded molecular sieves are known to those skilled in the art. For example, several methods for preparing metal-loaded molecular sieves, particularly metal-loaded zeolites, have been mentioned in the literature. The direct synthesis of metal-loaded zeolites is a complex process and depends on the synthesis conditions (see M. Moliner, ISRN Materials Science, 2012, Article ID 789525). An alternative is to use a commercial zeolite support and subsequently add the metal by post-synthetic treatment of the zeolite, such as wet impregnation, wet ion exchange, or solid-state ion exchange.

[0031] Known wet ion exchange methods for adding metals to molecular sieves (such as zeolites) typically employ soluble metal salts, such as metal acetates, metal sulfates, or metal chlorides, as active metal precursors, where the active metal precursors react with the molecular sieve in an aqueous solution. To accelerate the ion exchange, such methods typically require a heating step, where the mixture can be heated to a temperature in the range of 70 °C to 80 °C for up to several hours.

[0032] The term "molecular sieve" as used herein is well known to those skilled in the art and can encompass crystalline or quasi-crystalline materials, which can be, for example, aluminosilicates (zeolites) or silicoaluminophosphates (SAPOs). Such molecular sieves are composed of repeating SiO4, AlO4, and optionally PO4 tetrahedral units, which are connected together, for example, in the form of rings to form a framework with regular intracrystalline cavities and molecular-sized channels. The specific arrangement of the tetrahedral units (ring members) gives rise to the framework of the molecular sieve, and by convention, the International Zeolite Association (IZA) assigns a unique three-letter code (e.g., "CHA") to each unique framework.

[0033] Molecular sieves can exist, for example, in the "H + form" or "NH4 + form" molecular sieves. The term "H + form" with respect to molecular sieves refers to a molecular sieve having an anionic framework, where the charge of the framework is counterbalanced by protons (i.e., H + cations). The term "NH4 + form" with respect to molecular sieves refers to a molecular sieve having an anionic framework, where the charge of the framework is counterbalanced by ammonium cations (i.e., NH4 + cations). When loaded with metals (such as Fe), these molecular sieves are no longer considered to be in their "H + form" or "NH4 + form".

[0034] Molecular sieves (such as zeolites) can also be classified according to pore size, for example, the maximum number of tetrahedral atoms present in the framework of the molecular sieve. As defined herein, a "small pore" molecular sieve (such as CHA) has a maximum ring size of eight tetrahedral atoms, while a "medium pore" molecular sieve such as MFI has a maximum ring size of ten tetrahedral atoms; and a "large pore" molecular sieve (such as BEA) has a maximum ring size of twelve tetrahedral atoms. In the present invention, small pore molecular sieves or medium pore molecular sieves are defined according to these commonly understood definitions.

[0035] This aspect requires that the second catalyst composition includes an Fe-loaded small pore molecular sieve or medium pore molecular sieve. Thus, the present invention can of course cover embodiments in which the second catalyst composition includes Fe supported on two or more different small pore molecular sieves or medium pore molecular sieves. The term "different" small pore molecular sieves or medium pore molecular sieves can include different frameworks of the molecular sieve (i.e., framework type codes) or another property of the molecular sieve (such as SAR).

[0036] The term "silica to alumina ratio" or "SAR" as used herein can cover the molar ratio of silicon to aluminum present in the molecular sieve framework, which is calculated based on silica (SiO2) and alumina (AI2O3) rather than silicon and aluminum atoms. Of course, in the chemical structure of the molecular sieve framework, silicon and aluminum generally do not exist in the form of discrete silica and alumina. SAR is a commonly used term in the art and will be understood by those skilled in the art.

[0037] In the case where the molecular sieve is a small pore molecular sieve, the small pore molecular sieve can have a framework defined by a framework type code selected from ACO, AEI, AEN, AFN, AFT, AFX, ANA, APC, APD, ATT, CDO, CHA, DDR, DFT, EAB, EDI, EPI, ERI, GIS, GOO, IHW, ITE, ITW, KFI, LEV, LTA, MER, MON, NSI, OWE, PAU, PHI, RHO, RTH, SAT, SAV, SFW, SIV, THO, TSC, UEI, UFI, VNI, YUG, and ZON and mixtures and / or co-products thereof.

[0038] In the case where the molecular sieve is a mesoporous molecular sieve, the mesoporous molecular sieve may have a framework defined by a framework type code selected from 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 copolymers thereof.

[0039] Preferably, the molecular sieve of the second catalyst composition is a small pore molecular sieve. However, it is also preferred to have a small pore molecular sieve or a mesoporous molecular sieve with a framework defined by a framework type code selected from CHA, FER, MFI, AEI, and AEI-CHA copolymer, preferably selected from CHA, FER, AEI, and AEI-CHA copolymer. Most preferably, the small pore framework type codes are CHA and AEI. That is, the molecular sieve preferably has a framework defined by a framework type code selected from CHA and AEI, and most preferably the molecular sieve has a framework defined by the framework type code CHA.

[0040] Fe-loaded molecular sieves having a framework defined by these preferred specific framework type codes have shown higher N2O conversion rates than small pore molecular sieves with a higher SAR (such as Fe / CHA with an SAR of 25) and Fe-loaded macroporous molecular sieves (such as Fe / BEA).

[0041] Preferably, the small pore molecular sieve or mesoporous molecular sieve of the second catalyst composition is a small pore or mesoporous zeolite.

[0042] The small pore molecular sieve or mesoporous molecular sieve is preferably a powdered crystalline molecular sieve (i.e., in the form of particles), where these particles include individual crystals, aggregates of crystals, or a combination of both. As measured by scanning electron microscopy (SEM), the average crystal size of the crystalline molecular sieve can be ≥0.5 μm, preferably between about 0.5 μm and about 15 μm, such as about 0.5 μm to 10 μm, about 0.5 μm to about 5 μm, about 1 μm to about 5 μm, or about 2 μm to about 5 μm.

[0043] The SCR catalyst composition may include a metal oxide-based SCR catalyst composition, preferably comprising vanadium or tungsten or a mixture thereof supported on a refractory oxide, which is preferably selected from alumina, silica, titania, zirconia, ceria, and combinations thereof. The metal oxide-based SCR catalyst composition may include vanadium oxide (e.g., V2O5) and / or tungsten oxide (e.g., WO3) supported on a refractory oxide selected from titania (e.g., TiO2), ceria (e.g., CeC2), and a mixed oxide or composite oxide of cerium and zirconium (e.g., Ce x Zr (1-x) O2, where x = 0.1 to 0.9, preferably x = 0.2 to 0.5). When the refractory oxide is titania (e.g., TiO2), the concentration of vanadium oxide is preferably 0.5 wt% to 6 wt% (e.g., based on the metal oxide-based SCR catalyst composition), and / or the concentration of tungsten oxide (e.g., WO3) is 5 wt% to 20 wt%. More preferably, vanadium oxide (e.g., V2O5) and tungsten oxide (e.g., WO3) are supported on titania (e.g., TiO2). When the refractory oxide is ceria (e.g., CeO2), the concentration of vanadium oxide is preferably 0.1 wt% to 9 wt% (e.g., based on the metal oxide-based SCR catalyst composition), and / or the concentration of tungsten oxide (e.g., WO3) is 0.1 wt% to 9 wt%. The metal oxide-based SCR catalyst composition may include vanadium oxide (e.g., V2O5) supported on titania (e.g., TiO2) and optionally tungsten oxide (e.g., WO3).

[0044] Alternatively, the SCR catalyst composition may comprise a zeolite-based SCR catalyst composition. The zeolite-based SCR catalyst composition comprises a zeolite, which is optionally a transition metal-exchanged zeolite. Preferably, the SCR catalyst composition comprises a transition metal-exchanged zeolite. The zeolite may be a small-pore zeolite or a mesopore zeolite as described herein. The zeolite may be a large-pore zeolite. When the zeolite is a large-pore zeolite, the large-pore zeolite may have a framework defined by a framework type code selected from: 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, or a mixture and / or coprecipitate of two or more of them. Preferably, the large-pore zeolite has a framework defined by a framework type code selected from AFI, BEA, MAZ, MOR, and OFF. More preferably, the large-pore zeolite has a framework defined by a framework type code selected from BEA, MOR, and MFI. When the large-pore zeolite is a zeolite and has a framework defined by the framework type code BEA, FAU, or MOR, the zeolite may be beta zeolite, faujasite, zeolite Y, zeolite X, or mordenite.

[0045] Preferably, the zeolite is a zeolite.

[0046] The transition metal may be selected from cobalt, copper, iron, manganese, nickel, palladium, platinum, ruthenium, and rhenium. Due to the known low-temperature NO x reduction activity of copper, copper may be particularly preferred.

[0047] Preferably, the first catalyst composition comprises Cu / CHA or a V-based SCR catalyst.

[0048] As used herein, the term "Cu / CHA" and the like may encompass a zeolite having a framework defined by the CHA framework type code and comprising Cu supported thereon, as conventionally used in the art. The zeolite is preferably a zeolite. Preferably, Cu / CHA is loaded with 0.1 wt% to 20 wt% of Cu, more preferably 0.5 wt% to 10 wt% of Cu, even more preferably 1 wt% to 5 wt% of Cu, based on the total weight of Cu / CHA.

[0049] As used herein, the term "V-based catalyst" can encompass SCR catalyst compositions that include vanadium, such as those described above.

[0050] Such catalysts can typically produce particularly high concentrations of N2O as a byproduct of the SCR reaction, such as at operating temperatures of 300 °C to 500 °C, and thus it may be particularly advantageous to have a second catalyst composition as defined herein downstream thereof.

[0051] Preferably, the small-pore or mesoporous molecular sieve of the second catalyst composition has a SAR of 6 to 18, more preferably 6 to 17, more preferably 6 to 15, more preferably 6 to 13, more preferably 7 to 12, more preferably 9 to 11, and even more preferably about 10. As described above, it has surprisingly been found that decreasing the SAR of a small-pore or mesoporous molecular sieve with a reduced Fe loading can increase the N2O conversion at which the Fe-loaded molecular sieve can act as a catalyst. Thus, the above ranges can be particularly preferred. Additionally, the inventors have unexpectedly found that when decreasing the SAR of the molecular sieve, the N2O conversion performance appears to plateau at a SAR of about 10. Thus, although all preferred Fe-loaded molecular sieves provide a significant improvement over small-pore and mesoporous molecular sieves and macroporous molecular sieves with higher SARs, a SAR of about 10 may be most preferred. This is because decreasing the SAR below this value may return minimal benefits in terms of N2O conversion performance while risking decreasing the hydrothermal stability of the molecular sieve. In other words, a SAR of about 10 provides an unexpected balance between the stability of the molecular sieve and the N2O conversion performance of the Fe-loaded molecular sieve as a catalyst.

[0052] Accordingly, in some preferred embodiments, the present invention provides an SCR catalyst article comprising a substrate having on it a second catalyst composition downstream of a first catalyst composition, wherein the first catalyst composition and the second catalyst composition are different, and wherein the first catalyst composition is an SCR catalyst composition and the second catalyst composition comprises an Fe-loaded small-pore or mesoporous zeolite having a silica to alumina ratio (SAR) of 6 to 13, preferably wherein the small-pore or mesoporous zeolite has a framework defined by a framework type code selected from CHA, FER, MFI, AEI, and AEI-CHA co-crystals, and more preferably CHA or AEI.

[0053] Preferably, the small-pore or mesoporous molecular sieve of the second catalyst composition is loaded with at least 0.5 wt% Fe, more preferably at least 1.5 wt% Fe, preferably 1.5 wt% to 4 wt% Fe, and more preferably about 3 wt% Fe, based on the total weight of the Fe-loaded small-pore or mesoporous molecular sieve.

[0054] Catalysts having an Fe loading of about 3 wt% have been shown to exhibit the favorable N2O conversion described herein.

[0055] Preferably, the SCR catalyst article is a flow-through monolith or a wall-flow filter. In other words, the substrate preferably takes the form of a flow-through monolith or a wall-flow filter. Of course, the substrate may also include a "fully active extrudate" in which the substrate comprises an extrudate of an Fe-loaded small-pore molecular sieve or mesopore molecular sieve. If the SCR catalyst article is a wall-flow filter, the first catalyst composition is preferably coated on the inlet end of the wall-flow filter, and the second catalyst composition is preferably coated on the outlet end of the wall-flow filter. In other words, the inlet channels of the wall-flow filter preferably comprise the first catalyst composition, and the outlet channels of the wall-flow filter preferably comprise the second catalyst composition.

[0056] Preferably, the first catalyst composition is present in a first zone and the second catalyst composition is present in a second zone, and wherein the first zone forms 40% to 90% of the axial length of the SCR catalyst article, and the second zone forms 60% to 10% of the axial length of the SCR catalyst article, preferably wherein the first zone forms 75% to 90% of the axial length of the SCR catalyst article, and the second zone forms 25% to 10% of the axial length of the SCR catalyst article. For example, the axial length of the SCR catalyst article may correspond to the upstream to downstream direction of the SCR catalyst article. As used herein, the term "zone" may encompass a discrete region of the SCR catalyst article in which the catalyst composition is present and in which substantially no other catalyst composition is present, such as in an amount less than 0.1 wt% based on the total weight of the zone.

[0057] Preferably, the first catalyst composition is present in a first zone and the second catalyst composition is present in a second zone, and wherein the first zone extends from the inlet end of the SCR catalyst article and the second zone extends from the outlet end of the SCR catalyst article.

[0058] As used herein, the term "inlet end" may encompass the end of the SCR catalyst article in the upstream direction. As used herein, the term "outlet end" may encompass the end of the SCR catalyst article in the downstream direction.

[0059] Preferably, the small-pore molecular sieve or medium-pore molecular sieve of the second catalyst composition is substantially free of base metals other than Fe. As used herein, the term "substantially free of" may cover that the small-pore molecular sieve or medium-pore molecular sieve of the second catalyst composition comprises less than 0.1 wt%, more preferably less than 0.05 wt%, even more preferably less than 0.01 wt%, and still more preferably less than 0.001 wt% of a component (herein a base metal other than Fe) based on the total weight of the Fe-loaded small-pore molecular sieve or medium-pore molecular sieve. More preferably, the small-pore or medium-pore molecular sieve of the second catalyst composition does not include base metals other than Fe loaded thereon. More preferably, the small-pore molecular sieve or medium-pore molecular sieve of the second catalyst composition is substantially free of transition metals other than Fe. Most preferably, the small-pore molecular sieve or medium-pore molecular sieve of the second catalyst composition does not include transition metals other than Fe loaded thereon.

[0060] In other words, Fe is preferably the only base metal loaded on the small-pore molecular sieve or medium-pore molecular sieve, and more preferably the only transition metal loaded on the small-pore molecular sieve or medium-pore molecular sieve. Of course, if the catalyst article has other layers or regions provided thereon, there is a possibility that other metals may be present. For example, the other layers or regions are for different purposes and may thus include different components which may contain metals other than Fe. That is, other metals may be transferred due to physical contact. This is not intended to exclude such embodiments.

[0061] On the other hand, the present invention provides an exhaust gas treatment system comprising an SCR catalyst article according to the first aspect. All the preferred embodiments and features described herein with respect to the first aspect equally apply to this aspect.

[0062] On the other hand, the present invention provides an exhaust gas treatment system comprising a first catalyst article as defined herein and a second catalyst article downstream of the first catalyst article as defined herein, the first catalyst article comprising a first catalyst composition and the second catalyst article comprising a second catalyst composition. All the preferred embodiments and features described herein with respect to the first aspect equally apply to this aspect where appropriate.

[0063] Preferably, the exhaust gas treatment system of this aspect further includes an ASC article located between the first catalyst article and the second catalyst article. The ASC article can exist as an article or brick separated from the first catalyst article and / or the second catalyst article. Alternatively, the ASC article can be closely coupled with the first catalyst article and / or the second catalyst article, or be a part of the article or brick that is the same as the first catalyst article and / or the second catalyst article, such as in a separate area on the catalyst article. The ASC has the function of converting residual ammonia present in the exhaust gas into N2 and H2O. The ASC can also convert NO x into N2 and H2O. Catalysts suitable for such functions are well known to those skilled in the art, and the specific form of the ASC article is not particularly limited.

[0064] Suitable ASC articles can include PGM-loaded support materials, such as for example Pt-loaded zeolite or Pt-loaded alumina. Suitable ASC compositions are also described, for example, in WO2012138405A1, WO2017134454A1, WO2018178627A1, WO2019186121A1, and EP2885514A1.

[0065] If there is a component for injecting a nitrogen-containing reducing agent upstream of the exhaust gas, or if the exhaust gas comes from, for example, an ammonia internal combustion engine, ammonia may be present in the exhaust gas.

[0066] Similar to the SCR catalyst article, the ASC article can also generate additional N2O in the exhaust gas. Therefore, the same advantages associated with having the second catalyst composition of the present invention downstream of the first / SCR catalyst composition apply to this embodiment of the ASC article. Thus, such a system also helps to reduce ammonia and N2O emissions. If the exhaust gas comes from an ammonia internal combustion engine, such an arrangement may also be particularly advantageous. This is because when the fuel includes ammonia, the ammonia concentration in the exhaust gas may be particularly high. Therefore, such an exhaust gas treatment system can reduce both ammonia emissions and N2O emissions simultaneously.

[0067] Preferably, the exhaust gas treatment system of this aspect or the above aspect further includes a component for injecting a nitrogen-containing reducing agent upstream of the second catalyst composition, preferably upstream of the first catalyst composition. The nitrogen-containing reducing agent preferably includes ammonia and / or urea, and more preferably is ammonia and / or urea. Other known ammonia precursors may also be suitable.

[0068] The effect of the presence of such a reducing agent upstream of the SCR catalyst is well known. However, the presence of ammonia can also enable the Fe-based molecular sieve of the present invention to convert N2O at a lower temperature than in the absence of ammonia. The additional ammonia can actually also help in the reduction of N2O emissions. This mechanism provides another unexpected benefit associated with treating exhaust gas using the Fe-loaded molecular sieve of the present invention in the embodiments described herein.

[0069] Thus, in some embodiments, it may be beneficial to introduce ammonia artificially into the exhaust gas upstream of the second catalyst composition and downstream of the first catalyst composition, i.e., between the first catalyst article and the second catalyst article, with or without an additional component for injecting a nitrogen-containing reducing agent upstream of the first / SCR catalyst composition.

[0070] For embodiments in which the exhaust gas is from an ammonia internal combustion engine (i.e., where the engine is an ammonia internal combustion engine), it may not be necessary to include a component for injecting a nitrogen-containing reducing agent upstream of the SCR catalyst, since the exhaust gas may already contain an acceptable concentration of ammonia. However, for the reasons described herein, it may be beneficial to even further increase the ammonia concentration in the exhaust gas upstream of the SCR catalyst article, e.g., to a level greater than that required for the SCR reaction, such that some ammonia intentionally passes through the unreacted SCR catalyst. This means that some of the ammonia (or nitrogen-containing reducing agent) introduced upstream of the SCR catalyst article can still reach the second catalyst composition of the present invention.

[0071] Alternatively, for a particular application, if the ammonia concentration at the SCR catalyst composition is considered acceptable, the component for injecting a nitrogen-containing reducing agent can be (only) provided between the SCR catalyst composition and the second catalyst composition of the present invention (i.e., between the first catalyst article and the second catalyst article), e.g., such that it is determined that some ammonia will reach the second catalyst composition to achieve the benefits described herein.

[0072] Furthermore, since the second catalyst article (including the second catalyst composition) may preferably be at least the second catalyst article or the third catalyst article present in the exhaust gas treatment system in the upstream to downstream direction, the exhaust gas temperature at the location of the second catalyst article of the present invention can be relatively low. This means that the temperature of the second catalyst article itself can be relatively low.

[0073] Thus, ensuring the presence of ammonia in the exhaust gas contacting the second catalyst article provides a further advantage, since as described herein, the presence of ammonia can enable the Fe-based molecular sieve of the present invention to convert N2O at a lower temperature than in the absence of ammonia.

[0074] Preferably, the exhaust gas treatment system further includes a first member for injecting a nitrogen-containing reducing agent upstream of the first catalyst article and a second member for injecting a nitrogen-containing reducing agent between the first catalyst and the second catalyst article. Having members for injecting a nitrogen-containing reducing agent at both of these locations can achieve all of the associated advantages described above. In addition, such an arrangement can enable adjustment and / or variation of the specific concentration of ammonia in the exhaust gas at different locations in the exhaust gas treatment system, for example, according to a specific use or specific conditions. For example, at low temperatures near the start of the engine cycle, it may be beneficial to introduce a relatively high concentration of the nitrogen-containing reducing agent immediately upstream of the second catalyst article of the present invention, i.e., when the catalyst may be cold, and then introduce a relatively low concentration of the nitrogen-containing reducing agent as the engine and thus the second catalyst article of the present invention begin to warm up to a higher operating temperature. When the exhaust gas treatment system further includes an ASC article upstream of the second catalyst article, the second member for injecting the nitrogen-containing reducing agent is preferably downstream of the ASC article. Preferably, the second catalyst article is the last catalytic component of the exhaust gas system before the treated exhaust gas is released into the atmosphere.

[0075] The second catalyst article can be configured to be electrically heated. In other words, the second catalyst article is preferably an electrically heated catalyst (EHC). As described above, since the second catalyst article can preferably be at least the second catalyst article or the third catalyst article present in the exhaust gas treatment system in the upstream to downstream direction, the exhaust gas temperature at the location of the second catalyst article of the present invention can be relatively low. This also means that the activity of the catalyst may be lower than its potential. This problem can be solved by actively heating the second catalyst article with an electric heater. EHCs are generally known to those skilled in the art. This embodiment is not limited to a specific type of EHC. If an ASC is present, an EHC may not be required (or may only require less heating), since the ASC may generate exotherm during use that can heat the second catalyst article, especially if the ASC and the second catalyst article of the present invention are closely coupled.

[0076] In another aspect, the present invention provides a fuel combustion and exhaust gas system that includes an engine and the exhaust gas treatment system described herein. It should be understood that the engine is in fluid communication with the exhaust gas treatment system. Preferably, the engine is a diesel, hydrogen, methanol, or nitrogen-containing fuel internal combustion engine.

[0077] All of the preferred embodiments and features described herein with respect to other aspects equally apply to this aspect.

[0078] As used herein, the term "diesel, hydrogen, methanol or nitrogen-containing fuel internal combustion engine" can encompass engines designed to combust fuels that are respectively or include diesel, hydrogen, methanol or nitrogen-containing molecules. Preferably, more than 30 mol%, more preferably more than 50 mol%, even more preferably more than 70 mol%, still more preferably more than 90 mol% of the molecules in the nitrogen-containing fuel contain nitrogen. Preferably, the nitrogen-containing fuel includes one or more of ammonia, ammonia precursors (such as amines and / or urea), and organic waste. Preferably, the nitrogen-containing fuel includes ammonia. Of course, terms such as "nitrogen-containing fuel or hydrogen internal combustion engine" can thus encompass engines designed to combust dual fuels or blended fuels that include a nitrogen-containing fuel (preferably ammonia) or hydrogen, such as fuels blended with diesel, marine diesel, heavy fuel oil, and / or natural gas. In some alternative embodiments, the term "nitrogen-containing fuel or hydrogen internal combustion engine" can encompass systems that combust such fuels, such as in industrial processes that include ammonia cracking and (organic) waste incineration processes, rather than, for example, engines for automobiles and the like. However, preferably, the engine is an engine in the conventional sense, preferably an engine for automobiles and the like.

[0079] In another aspect, the present invention provides a method for treating exhaust gas, the method comprising passing the exhaust gas through the exhaust gas treatment system described herein, preferably wherein the exhaust gas is provided by a diesel, hydrogen, methanol or nitrogen-containing fuel internal combustion engine. Preferably, the temperature of the exhaust gas and / or the catalyst article is 350 °C to 500 °C.

[0080] All of the preferred embodiments and features described herein with respect to other aspects are equally applicable to this aspect.

[0081] In another aspect, the present invention also provides the use of an Fe-loaded small pore molecular sieve or mesoporous molecular sieve with an SAR of 6 to 19 in reducing the concentration of N2O in exhaust gas.

[0082] All of the preferred embodiments and features described herein with respect to other aspects are equally applicable to this aspect. As described herein, such uses have not been envisioned before. The advantages of such uses are discussed herein.

[0083] The present invention will now be described in connection with the following non-limiting drawings, wherein:

[0084] Figure 1 Shows the N2O conversion of five catalyst compositions at 400 °C compared to four comparative catalyst compositions, the five catalyst compositions corresponding to Fe-loaded small pore or mesoporous zeolites suitable for the second catalyst composition of the present invention.

[0085] The present invention will now be described in connection with the following non-limiting examples. Examples

[0086] Example 1:

[0087] Nine catalyst compositions were prepared by incipient wetness impregnation of FeCl2 salt onto preformed zeolites having a specific framework and SAR. Each catalyst composition was loaded with 3 wt% Fe based on the total weight of Fe / zeolite. Specifically, a metal salt solution using FeCl2 (alpha-ethylferric chloride, anhydrous, 99.5% (metal basis)) and double-distilled H2O was prepared and then dropped onto the relevant zeolite samples. The mixture was thoroughly mixed until a wet sand appearance was observed. After preparation, the samples were dried in a static oven at 105 °C for 2 h. Once dried, the powder was heated to and held at 500 °C for 2 h in a tube furnace under N2 atmosphere at a heating rate of 10 °C / min for activation.

[0088] The zeolite composition of each catalyst composition and the N2O performance of each catalyst composition at each temperature are shown in Table 1 below.

[0089] Catalyst compositions 1 to 5 correspond to small-pore or medium-pore zeolites with Fe loading suitable for the second catalyst composition of the present invention. Catalyst compositions C1 to C4 are comparative examples outside the scope of Fe-loaded small-pore or medium-pore molecular sieves of the present invention.

[0090]

[0091] Table 1

[0092] Table 1 shows the results of N2O performance tests at four different temperatures: 300 °C, 350 °C, 375 °C, and 400 °C. Specifically, 0.2 g of granulated sample was tested at a total flow rate of 100 ml / min, where 58% was He, 40% Ar, 1% N2O, and 1% O2. The temperature range explored was from 300 °C to 400 °C. For each temperature, the heating rate was set at 10 °C / min and then held for 45 min. The data were analyzed using a mass spectrometer. "N2O performance" refers to the N2O conversion at that specific temperature, i.e., the percentage of N2O consumed from the sample gas after passing through the exemplary catalyst, in ppm. For the avoidance of doubt, CHA and AEI are small-pore zeolites, FER and MFI are medium-pore zeolites, and BEA is a large-pore zeolite.

[0093] As described herein, it can be seen that the N2O performance of the catalyst compositions for the SCR catalyst articles and the exhaust gas treatment systems of the present invention is significantly higher than that of the comparative catalyst compositions, where the zeolite has a higher SAR and / or is a large-pore zeolite. Additionally, surprisingly, the N2O performance of the catalyst compositions increases as the SAR decreases. The N2O performance of the CHA zeolite with an SAR of 7 and the CHA zeolite with an SAR of 10 (catalyst compositions 1 and 2, respectively) is comparable.

[0094] Figure 1 shows the N2O conversion at 400 °C for five catalyst compositions compared to four comparative catalyst compositions, the five catalyst compositions corresponding to Fe-loaded small- or medium-pore zeolites suitable for the second catalyst composition of the present invention. In other words, Figure 1 is a visual representation of the rightmost column of Table 1. From left to right, Figure 1 the bars in [ ] sequentially relate to catalyst composition 1, catalyst composition 2, catalyst composition 3, catalyst composition 4, catalyst composition 5, catalyst composition C1, catalyst composition C2, catalyst composition C3, and catalyst composition C4.

[0095] The foregoing detailed description has been presented by way of illustration and example, and is not intended to limit the scope of the appended claims. Many variations of the presently preferred embodiments illustrated herein will be apparent to those of ordinary skill in the art and remain within the scope of the appended claims and their equivalents.

Claims

1. An exhaust gas treatment system, the exhaust gas treatment system comprising a first catalyst article and a second catalyst article downstream of the first catalyst article, the first catalyst article comprising a first catalyst composition, the second catalyst article comprising a second catalyst composition, wherein the first catalyst composition and the second catalyst composition are different, and wherein the first catalyst composition is an SCR catalyst composition, and the second catalyst composition comprises an Fe-loaded small pore molecular sieve or mesoporous molecular sieve having a silica to alumina ratio (SAR) of 6 to 19.

2. An SCR catalyst article, the SCR catalyst article comprising a substrate having a second catalyst composition downstream of a first catalyst composition on the substrate, wherein the first catalyst composition and the second catalyst composition are different, and wherein the first catalyst composition is an SCR catalyst composition, and the second catalyst composition comprises an Fe-loaded small pore molecular sieve or mesoporous molecular sieve having a silica to alumina ratio (SAR) of 6 to 19.

3. The system according to claim 1 or the SCR catalyst article according to claim 2, wherein the first catalyst composition comprises a Cu / CHA or V-based SCR catalyst.

4. The system according to claim 1 or the SCR catalyst article according to claim 2 or claim 3, wherein the small pore molecular sieve or mesoporous molecular sieve of the second catalyst composition has a framework defined by a framework type code selected from CHA, FER, MFI, AEI, and AEI-CHA co-crystals, preferably CHA and AEI, more preferably CHA.

5. The system according to claim 1 or the SCR catalyst article according to any one of claims 2 to 4, wherein the small pore molecular sieve or mesoporous molecular sieve of the second catalyst composition has an SAR of 6 to 13, preferably 7 to 12, more preferably 9 to 11, even more preferably about 10.

6. The system according to claim 1 or the SCR catalyst article according to any one of claims 2 to 5, wherein the small pore molecular sieve or mesoporous molecular sieve of the second catalyst composition is loaded with at least 0.5 wt% Fe, preferably 1.5 wt% to 4 wt% Fe, more preferably about 3 wt% Fe, based on the total weight of the Fe-loaded small pore molecular sieve or mesoporous molecular sieve.

7. The SCR catalyst article according to any one of claims 2 to 6, wherein the SCR catalyst article is a flow-through monolith or a wall-flow filter.

8. The SCR catalyst article according to any one of claims 2 to 7, wherein the first catalyst composition is present in a first zone and the second catalyst composition is present in a second zone, and wherein the first zone forms 40% to 90% of the axial length of the SCR catalyst article, and the second zone forms 60% to 10% of the axial length of the SCR catalyst article, preferably wherein the first zone forms 75% to 90% of the axial length of the SCR catalyst article, and the second zone forms 25% to 10% of the axial length of the SCR catalyst article.

9. The catalyst article according to any one of claims 2 to 8, wherein the first catalyst composition is present in a first zone and the second catalyst composition is present in a second zone, and wherein the first zone extends from the inlet end of the SCR catalyst article, and the second zone extends from the outlet end of the SCR catalyst article.

10. An exhaust gas treatment system comprising the SCR catalyst article according to any one of claims 2 to 9.

11. The exhaust gas treatment system according to any one of claims 1 and 3 to 6, further comprising an ammonia slip catalyst (ASC) article located between the first catalyst article and the second catalyst article.

12. The exhaust gas treatment system according to any one of claims 1, 3 to 6 and 10 to 11, further comprising means for injecting a nitrogen-containing reducing agent upstream of the second catalyst composition, preferably upstream of the first catalyst composition, preferably wherein the nitrogen-containing reducing agent is ammonia and / or urea.

13. A fuel combustion and exhaust gas system comprising an engine and the exhaust gas treatment system according to any one of claims 1, 3 to 6 and 10 to 12, preferably wherein the engine is a diesel, hydrogen, methanol or nitrogen-containing fuel internal combustion engine.

14. A method of treating exhaust gas, the method comprising passing the exhaust gas through the exhaust gas treatment system according to any one of claims 1, 3 to 6 and 10 to 12, preferably wherein the exhaust gas is provided by a diesel, hydrogen, methanol or nitrogen-containing fuel internal combustion engine.

15. Use of an Fe-loaded small pore or mesoporous molecular sieve with an SAR of 6 to 19 for reducing the concentration of N2O in exhaust gas.

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