SCR catalysts

Abstract

According to the present invention, there is provided a method for reducing N2O in the selective catalytic reduction (SCR) of NOX, in a gas mixture, comprising contacting the gas mixture with a source of ammonia, over a selective catalytic reduction catalyst comprising a zeolite comprising copper (Cu), cerium (Ce) and yttrium (Y), wherein the Cu to Ce to Y weight percentage ratio, based on the weight of the zeolite, is in the range of 1.0:1.0:0.5 to 5.0:5.0:3.0. There is also provided an SCR catalyst, use of said SCR catalyst and an associated exhaust system.

Description

[0001] P101193

[0002] SCR CATALYSTS

[0003] FIELD OF THE INVENTION

[0004] This invention relates to a method for reducing N2O in the selective catalytic reduction of NOXin a gas mixture and to a selective catalytic reduction (SCR) catalyst having a reduced selectivity towards N2O in the selective reduction of NOX, wherein the gas mixture is contacted with a source of ammonia.

[0005] BACKGROUND TO THE INVENTION

[0006] Hydrocarbon combustion in diesel engines, stationary gas turbines, and other systems generates exhaust gas that must be treated to remove nitrogen oxides (NOX), which comprises NO (nitric oxide) and NO2 (nitrogen dioxide), with NO being the majority of the NOXformed. NOXis known to cause a number of health issues in people as well as causing a number of detrimental environmental effects including the formation of smog and acid rain. To mitigate both the human and environmental impact from NOx in exhaust gas, it is desirable to eliminate these undesirable components, preferably by a process that does not generate other noxious or toxic substances.

[0007] Exhaust gas generated in lean-burn and diesel engines is generally oxidative. NOXneeds to be reduced selectively with a catalyst and a reductant in a process known as selective catalytic reduction (SCR) that converts NOXinto elemental nitrogen (N2) and water.

[0008] In an SCR process, a gaseous reductant, typically anhydrous ammonia, aqueous ammonia, or urea, is added to an exhaust gas stream prior to the exhaust gas contacting the catalyst. The reductant is absorbed onto the catalyst and the NOXis reduced as the gases pass through or over the catalyzed substrate.

[0009] The reduction reaction may be described as follows:

[0010] 4NH3+4NO+O2^4N2+6H2O Formula I

[0011] In order to maximize the conversion of NOX, it is often necessary to add more than a stoichiometric amount of ammonia (NH3) to the gas stream. However, release of the excess ammonia into the atmosphere would be detrimental to the health of people and to the environment. In addition, ammonia is caustic, especially in its aqueous form. Condensation of ammonia and water in regions of the exhaust line downstream of the exhaust catalysts can result in a corrosive mixture that can damage the exhaust system. Therefore, the release of ammonia in exhaust gas should be eliminated. In many conventional exhaust systems, an ammonia oxidation catalyst (also known as an ammonia slip catalyst or "ASC") is installed downstream of the SCR catalyst to remove ammonia from the exhaust gas by P101193 converting it to nitrogen. The use of ammonia slip catalysts can allow for NOXconversions of greater than 90% over a typical diesel driving cycle.

[0012] A further problem associated with the reduction of NOXwith ammonia is that, especially with high temperature conditions, the reduction of NOXmay result in the production of nitrous oxide (N2O) such that the SCR catalyst has a higher degree of selectivity towards the production of N2O, the general reaction of which is shown in Formula II, than N2 in Formula I. It is undesirable to produce N2O because it is a greenhouse gas which may contribute to an increase in global warming. The general reaction to produce N2O may be described as:

[0013] 4NH3+4NO+3O2^4N2O+6H2O Formula II

[0014] The inventors have now found that the addition of yttrium as an additive in the production of an SCR catalyst for treating NOXin a gas mixture results in the abatement of the SCR catalyst's selectivity towards the production of N2O at high temperatures while maintaining acceptable levels of NOXconversion.

[0015] DETAILED DESCRIPTION OF THE INVENTION

[0016] According to the invention there is provided a method for reducing N2O in the selective catalytic reduction of NOX, in a gas mixture, comprising contacting the gas mixture with a source of ammonia, over a selective catalytic reduction (SCR) catalyst comprising a zeolite comprising copper (Cu), cerium (Ce) and yttrium (Y). Suitably, the Cu to Ce to Y weight percentage ratio, based on the weight of the zeolite, is in the range of 1.0:1.0:0.5 to 5.0:5.0:3.0. Cu can be present in an amount in a range of 1.0 wt.% to 5.0 wt.% (preferably 2.0 wt.% to 4.5 wt.%, preferably 2.5 wt.% to 4.25 wt.%, suitably 2.75 wt.% to 4.25 wt.%, such as 3.0 wt.% to 4.0 wt.%) based on the weight of the zeolite, Ce can be present in an amount in a range of 1.0 wt.% to 5.0 wt.% (preferably 1.0 wt.% to 4.0 wt.%, 1.0 wt.% to 3.0 wt.% and 1.4 wt.% to 2.8 wt.%) based on the weight of the zeolite, and Y can be present in an amount in a range of 0.5 wt.% to 3.0 wt.% (preferably 0.50 wt.% to 2.5 wt.%, 0.50 wt.% to 2.0 wt.%, and preferably 0.75 wt.% to 1.5 wt.%) based on the weight of the zeolite.

[0017] It will be appreciated that the SCR catalyst includes a zeolite, a promoter metal, namely Cu and two rare earth metals, being Ce and Y. These are the catalytically active components of the catalyst and may be disposed on or within a substrate as a catalyst layer. Any suitable substrate may be employed for purposes of the invention. The substrate(s) may be any of those materials typically used for preparing catalysts and will typically comprise a ceramic or metal monolith structure. Monolith substrates have a honeycomb structure, having fine, parallel gas flow passages extending therethrough P101193 from an inlet or an outlet end of the substrate, wherein the passages are open to fluid flow therethrough.

[0018] In a preferred form of the invention, the substrate is a ceramic flow through catalyst. Preferably the substrate is cordierite.

[0019] In order to provide a catalyst layer disposed on or in the substrate, a washcoat formulation comprising the catalytically active components can be prepared, disposed in or on the substrate and treated in order to affix the washcoat formulation to the substrate as a catalyst layer.

[0020] The washcoat formulation can comprise: a. a zeolite comprising Cu, Ce and Y, b. a binder, and c. a thickener.

[0021] That is, the method can comprise the additional steps of:

[0022] (i) preparing a washcoat formulation comprising: (a) a zeolite comprising Cu, Ce and Y, (b) a binder, and (c) a thickener; and

[0023] (ii) applying the washcoat formulation to a substrate to prepare the catalyst layer.

[0024] The binder can be selected from organic polymers (e.g. dextrin, sucrose, poly(vinyl alcohol)), cordierite, nitrides, carbides, borides, intermetallics, lithium aluminosilicate, a spinel, an optionally doped alumina, (e.g. boehmite), a silica source, titania, zirconia, titania-zirconia, zircon and mixtures thereof.

[0025] The thickener can be selected from polysaccharides (e.g. guar gum), cellulose (e.g. hydroxyethyl celluloses), viscoelastic surfactants, silica, organic compound thickeners and mixtures thereof.

[0026] The zeolite of the invention may comprise a small pore zeolite (e.g. a zeolite having a maximum ring size of eight tetrahedral atoms), a medium pore zeolite (e.g. a zeolite having a maximum ring size of ten tetrahedral atoms) or a large pore zeolite (e.g. a zeolite having a maximum ring size of twelve tetrahedral atoms) or a combination of two or more thereof.

[0027] When the zeolite is a small pore zeolite, then the small pore zeolite may have a framework structure represented by a Framework Type Code (FTC) selected from the group 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,

[0028] LEV, LTA, KFI, MER, MON, NSI, OWE, PAU, PHI, RHO, RTH, SAT, SAV, SFW, SIV, THO, TSC, UEI, UFI, VNI, P101193

[0029] YUG and ZON, or a mixture and / or an intergrowth of two or more thereof. Preferably, the small pore zeolite has a framework structure selected from the group consisting of CHA, LEV, AEI, AFX, ERI, LTA, SFW, KFI, DDR and ITE.

[0030] When the zeolite is a medium pore zeolite, then the medium pore zeolite may have a framework structure represented by a Framework Type Code (FTC) selected from the group consisting of AEL, AFO, AHT, BOF, BOZ, CGF, CGS, CHI, DAC, EUO, FER, HEU, IMF, ITH, ITR, JRY, JSR, JST, LAU, LOV, MEL, MFI, M FS, M RE, 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, or a mixture and / or an intergrowth of two or more thereof. Preferably, the medium pore zeolite has a framework structure selected from the group consisting of FER, M EL, M FI, and STT.

[0031] When the zeolite is a large pore zeolite, then the large pore zeolite may have a framework structure represented by a Framework Type Code (FTC) selected from the group 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, or a mixture and / or an intergrowth of two or more thereof. Preferably, the large pore zeolite has a framework structure selected from the group consisting of AFI, BEA, MAZ, MOR, and OFF.

[0032] In an embodiment of the invention, the zeolite has a framework type selected from AEI, AFX, CHA, DDR, ERI, ITE, KFI, LEV, SFW, BEA, M FI and FER. In a preferred form of the invention, the framework type is CHA.

[0033] The zeolite can comprise a silica to alumina (molar) ratio in a range of 10 to 30, preferably 11 to 27, more preferably 12 to about 26, such as 13 to 20. The SAR of zeolites may be determined by conventional analysis (e.g. by XRF). This ratio is meant to represent, as closely as possible, the ratio of the rigid atomic framework of the zeolite crystal and to exclude any silicon or aluminium, which may be present in a binder or in other form. Since it may be difficult to directly measure the silica to alumina ratio of zeolite after it has been combined with a binder material, particularly an alumina binder, these P101 193 silica-to-alumina ratios are expressed in terms of SAR of the zeolite perse, i.e., prior to the combination of the zeolite with the other catalyst components.

[0034] The catalyst comprises at least two rare earth elements, Ce and Y. In addition to Ce and Y, the catalyst may further comprise one or more additional rare earth elements, e.g., rare earth elements selected from Nd, La, Sc, Sm, Gd, Ho, Yb, Er. The rare earth elements of the catalytically active component of the catalyst may be present as extra-framework elements residing within the molecular sieve and / or on at least a portion of the molecular sieve surface, does not include aluminium, and does not include atoms constituting the framework of the molecular sieve. The rare earth element can be added to the molecular sieve via any known technique such as ion exchange, impregnation, isomorphous substitution, etc.

[0035] In a preferred form of the invention, the rare earth elements may be included in the molecular sieve through ion exchange. The ion exchange may be conducted by blending the molecular sieve into a solution containing soluble precursors of the rare earth elements. Suitable precursors of rare earth elements (e.g., Ce or Y) can be oxides, nitrates, carbonates or acetates thereof. In a preferred embodiment, the precursor of the rare earth element (e.g., Ce or Y) is an oxide or acetate. In a particularly preferred embodiment, the precursor of Ce is Ce acetate. In a particularly preferred embodiment, the precursor of Y is Y oxide.

[0036] In accordance with the method of the invention, Y is added in an amount of between 0.5wt% to 5wt%, based on the total weight of the zeolite. Preferably, Y is added in an amount from 0.5 to 4 wt%, from 0.5 to 3.0 wt%, 0.50 to 3.0 wt.%, 0.6 to 2.5 wt%, from 0.7 to 2.0 wt%, 0.8 to 1.8 wt%, or from 0.9 to 1.5 wt% based on the total weight of the zeolite. In one embodiment, Y is added in an amount of approximately 1 wt%, based on the total weight of the zeolite. In some embodiments, the Y can be added in a range of from 0.5 wt.% to 2.5 wt.%, preferably 0.50 wt.% to 2.5 wt.%, 0.50 wt.% to 2.0 wt.%, preferably 0.50 wt.% to 1.5 wt.%, preferably 0.75 wt.% to 1.5 wt.%, 0.80 wt.% to 1.2 wt.%, and suitably about 1.0 wt.%, based on the weight of the zeolite. The Y can be added in a range comprising any combination of the aforementioned limits. For example, the Y ean be added in a range of from 0.6 wt.% to 3.0 wt.%, 0.75 wt.% to 2.5 wt.%, or 0.80 wt.% to 2.0 wt.%, or 0.9 wt.% to 1.8 wt.%.

[0037] In accordance with the method of the invention, Ce is added in an amount of between 1 wt% to 7wt% based on the total weight of the zeolite. Preferably Ce is added in an amount from 1 to 6 wt%, 1 to 5 wt%, 2 to 5 wt%, 2 to 4 wt%, or 2.5 to 4.5 wt%, based on the total weight of the zeolite. In one embodiment, Ce is added in an amount of about 2 wt%, about 2.3 wt% about 2.5 wt%, about 2.8 wt%, about 3 wt%, about 3.5 wt%, about 4 wt%, about 4.3 wt% or about 4.5 wt% based on the total weight of the zeolite. In some embodiments, the Ce is added in an amount in a range of from 1.0 wt.% to P101 193

[0038] 4.0 wt.%, preferably 1.0 wt.% to 3.0 wt.%, suitably 1.4 wt.% to 2.8 wt.%, based on the weight of the zeolite. The Ce can be added in a range comprising any combination of the aforementioned limits.

[0039] In accordance with the method of the invention, additional rare earth elements (e.g., rare earth elements selected from Nd, La, Sc, Sm, Gd, Ho, Yb, Er) can be added in an amount of between 0.5 wt% to 10wt% based on the total weight of the zeolite. Preferably additional rare earth elements (e.g., rare earth elements selected from Nd, La, Sc, Sm, Gd, Ho, Yb, Er) can be added in an amount from 1 to 8 wt%, 2 to 6 wt%, or 3 to 5 wt%, based on the total weight of the zeolite. In one embodiment, additional rare earth elements (e.g., rare earth elements selected from Nd, La, Sc, Sm, Gd, Ho, Yb, Er) can be added in an amount of about 1 wt%, about 2 wt%, about 3 wt%, about 4 wt% or about 5 wt%, based on the total weight of the zeolite.

[0040] Promoter metals may be of any of the recognized catalytically active metals that are used in the catalyst industry to form metal-exchanged molecular sieves. Copper is added to the catalyst as a promoter metal. In one embodiment, at least one additional promoter metal is used in conjunction with the molecular sieve to increase the catalyst's performance. Preferred additional promoter metals are selected from the group consisting of nickel, zinc, iron, tin, tungsten, molybdenum, cobalt, bismuth, titanium, zirconium, antimony, manganese, chromium, vanadium, niobium, ruthenium, rhodium, palladium, gold, silver, indium, platinum, iridium, rhenium, and mixtures thereof. More preferred promoter metals include those selected from the group consisting of chromium, manganese, iron, cobalt, nickel, and mixtures thereof. Preferably, at least one of the additional promoter metals is manganese.

[0041] In accordance with the method of the invention, Cu is added in an amount of between 1 wt% to 10wt% based on the total weight of the zeolite. Preferably, Cu is added in an amount from 1 to 5 wt%, 1.5 to 4 wt%, 2 to 4 wt%, or 2 to 3 wt%, based on the total weight of the zeolite. In one embodiment, Cu is added in an amount of about 2 wt%, about 2.5 wt%, about 2.75 wt.%, about 3 wt%, about 3.5 wt%, about 3.75 wt%, about 4 wt.%, or about 4.25 wt%, based on the total weight of the zeolite or in a range comprising any combination of these values as upper and lower limits. For example, the Cu can be added in an amount in a range of about 1.0 wt.% to 4.5 wt.%, 2.0 wt.% to about 4.5 wt.%, 2.5 wt.% to about 4.5 wt.%, 2.50 wt.% to 4.25 wt.%, 2.75 wt.% to 4.25 wt.%, or about 3.0 wt.% to about 4.0 wt.%, based on the weight of the zeolite.

[0042] In accordance with the method of the invention, an additional promotor metal can be added in an amount of between 1 wt% to 10wt% based on the total weight of the zeolite. Preferably, an additional promotor metal can be added in an amount from 1 to 5 wt%, 1.5 to 4 wt%, or 2 to 4 wt%, based on the total weight of the zeolite. In one embodiment, an additional promotor metal can be added in an P101 193 amount of about 2 wt%, about 2.5 wt%, about 3 wt% or about 3.5 wt%, based on the total weight of the zeolite.

[0043] In an embodiment of the present invention, the Cu to Ce to Y weight percentage ratio, based on the weight of the zeolite, is in the range of 1.0:1.0:0.5 to 5.0:5.0:3.0 (e.g. Cu present in an amount of 1.0- 5.0 wt.%, Ce present in an amount of 1.0-5.0 wt.%, and Y present in an amount of 0.5-3.0 wt.%, based on the weight of the zeolite). The Cu to Ce to Y weight percentage ratio, based on the weight of the zeolite, can preferably be in the range of 2.0:1.0:0.50 to 5.0:5.0:3.0, 2.5:1.0:0.50 to 5.0:5.0:3.0, 2.5:1.0:0.6 to 5.0:5.0:3.0, 2.5:1.0:0.6 to 5.0:4.5:3.0, 2.5:1.0:0.6 to 4.5:4.5:3.0, 2.5:1.0:0.6 to 4.5:4.5:2.5, 2.5:1.0:0.6 to 4.5:4.0:2.5, 2.5:1.0:0.6 to 4.5:4.0:2.5, 2.5:1.0:0.6 to 4.5:3.5:2.5, 2.5:1.0:0.6 to 4.5:3.5:2.0, 2.5:1.0:0.6 to 4.5:3.0:2.0, preferably 2.5:1.0:0.6 to 4.5:3.0:1.5, preferably 2.5:1.0:0.60 to 4.25:3.0:1.5, 2.5:1.2:0.60 to 4.25:3.0:1.5, 2.5:1.2:0.75 to 4.25:3.0:1.5, 2.75:1.2:0.75 to 4.25:3.0:1.5, suitably 3.0:1.4:1.0 to 4.0:2.8:1.5. In some embodiments, the Cu to Ce to Y weight percentage ratio, based on the weight of the zeolite, is 2.0:1.5:0.5 to 4.5:5.0:2.5, e.g. from 3.0:2.0:0.5 to 4.5:4.5:2.0. In some preferred embodiments, the Cu to Ce to Y weight percentage ratio, based on the weight of the zeolite, is in a range of 2.5:1.0:0.50 to 4.25:3.0:1.5, such as 3.0:1.4:1 to 4.0:2.8:1. In some embodiments the Cu to Ce to Y weight percentage ratio, based on the weight of the zeolite, is about 3.0 : 2.8 : 1.0, about 3.5:2.0:1.0 or about 4.0:4.0:1.0. The Cu to Ce to Y weight percentage ratio can be in a range comprising any combination of the aforementioned limits and / or ranges.

[0044] In some embodiments, Cu is present in an amount in a range of 2.0-5.0 wt.%, 2.5-5.0 wt.%, 2.5-

[0045] 4.5 wt.%, suitably 2.5-4.25 wt.%, suitably 2.75-4.25 wt.%, and suitably 3.0-4.0 wt.%; Ce is present in an amount in a range of 1.0-5.0 wt.%, 1.0-4.5 wt.%, 1.0-4.0 wt.%, 1.0-3.5 wt.%, preferably 1.0- 3.0 wt.%, 1. -3.0 wt.%, and suitably 1.4-2.8 wt.%; and Y is present in an amount in a range of 0.5- 3.0 wt.%, 0.50-3.0 wt.%, 0.6-3.0 wt.%, 0.6-2.25 wt.%, 0.6-2.0 wt.%, 0.6-1.5 wt.%, 0.60-1.5 wt.%, 0.75-

[0046] 1.5 wt.%, 1.0-1.5 wt.%, and suitably about 1.0 wt.%. The Cu, Ce and Y can each be present in an amount in a range comprising any combination of the aforementioned limits and ranges.

[0047] In an embodiment of the present invention, a precursor of a first rare earth element (e.g. Y or Ce, preferably Y) can be added to a slurry of zeolite. The precursor of the first rare earth metal may be an oxide of the rare earth element (e.g. Y2O3) or it may be a nitrate, carbonate or acetate thereof. In a preferred embodiment, the precursor of the first rare earth element is a precursor of Y. In a particularly preferred embodiment, the precursor of the first rare earth element is Y oxide.

[0048] Following the addition of the first rare earth element (e.g. Y or Ce, preferably Y), a Cu precursor (e.g. Cu acetate) can be added to the resultant slurry. Copper is known to be a promoter metal which is used to improve the catalytic performance and / or thermal stability of a catalyst. Such promoter metal P101 193 may be added to the molecular sieve separately from the incorporation of the one or more rare earth elements.

[0049] Following the addition of Cu as the promoter metal, a second rare earth metal can be added to a slurry comprising the zeolite, first rare earth metal (e.g. Y or Ce, preferably Y) and Cu. The second rare earth metal is preferably Ce or Y, more preferably Ce, and can be added though ion exchange. Precursors of the second rare earth metal (e.g. Ce or Y, preferably Ce) may be acetates, nitrates, chlorides, oxalates, carbonates and hydroxides thereof. Preferably, the precursor for the second rare earth metal is an acetate, e.g. Ce acetate.

[0050] The resultant washcoat formulation can be applied to the substrate. The washcoat formulation can be directly dried and calcined, or can be subject to a treatment step to form a catalyst layer on the substrate. The treatment step may be carried out at room temperature or at a temperature up to about 80°C. The treatment step may be carried out for a period of about 1 to 24 hours, e.g., 2 hours. The treatment step may be carried out at a pH of about 7. The resulting catalytic molecular sieve material is preferably dried at about 100°C to 120°C. The drying step can be carried out for a period of about 1 to 24 hours, e.g., 6 to 16 hours, preferably overnight. The resulting catalyst can be calcined at a temperature of at least about 550°C.

[0051] In one embodiment of the invention, the selective catalytic reduction catalyst comprising a zeolite comprising copper (Cu), cerium (Ce) and yttrium (Y), wherein the Cu to Ce to Y weight percentage ratio, based on the weight of the zeolite, is in the range of 1.0:1.0:0.5 to 5.0:5.0:3.0, and preferably 2.0:1.0:0.50 to 5.0:5.0:3.0, 2.5:1.0:0.50 to 5.0:5.0:3.0, 2.5:1.0:0.6 to 5.0:5.0:3.0, 2.5:1.0:0.6 to 5.0:4.5:3.0, 2.5:1.0:0.6 to 4.5:4.5:3.0, 2.5:1.0:0.6 to 4.5:4.5:2.5, 2.5:1.0:0.6 to 4.5:4.0:2.5, 2.5:1.0:0.6 to 4.5:4.0:2.5, 2.5:1.0:0.6 to 4.5:3.5:2.5, 2.5:1.0:0.6 to 4.5:3.5:2.0, 2.5:1.0:0.6 to 4.5:3.0:2.0, preferably 2.5:1.0:0.6 to 4.5:3.0:1.5, preferably 2.5:1.0:0.60 to 4.25:3.0:1.5, and preferably 3.0 : 2.8 : 1.0 is prepared by:

[0052] 1. Combining a precursor of a first rare earth element (e.g. Ce or Y) with a slurry of the zeolite to form a first slurry

[0053] 2. Adding a Cu precursor to the first slurry to form a second slurry

[0054] 3. Adding a precursor of a second rare earth element (e.g. Ce or Y) to the second slurry to form a washcoat formulation, and optionally

[0055] 4. Applying the washcoat formulation to the substrate and optionally drying and calcining the formulation.

[0056] In a preferred embodiment of the invention, the selective catalytic reduction catalyst comprising a zeolite comprising copper (Cu), cerium (Ce) and yttrium (Y), wherein the Cu to Ce to Y weight P101 193 percentage ratio, based on the weight of the zeolite, is in the range of 1.0:1.0:0.5 to 5.0:5.0:3.0, and preferably 2.0:1.0:0.50 to 5.0:5.0:3.0, 2.5:1.0:0.50 to 5.0:5.0:3.0, 2.5:1.0:0.6 to 5.0:5.0:3.0, 2.5:1.0:0.6 to 5.0:4.5:3.0, 2.5:1.0:0.6 to 4.5:4.5:3.0, 2.5:1.0:0.6 to 4.5:4.5:2.5, 2.5:1.0:0.6 to 4.5:4.0:2.5, 2.5:1.0:0.6 to 4.5:4.0:2.5, 2.5:1.0:0.6 to 4.5:3.5:2.5, 2.5:1.0:0.6 to 4.5:3.5:2.0, 2.5:1.0:0.6 to 4.5:3.0:2.0, preferably 2.5:1.0:0.6 to 4.5:3.0:1.5, preferably 2.5:1.0:0.60 to 4.25:3.0:1.5, and preferably 3.0 : 2.8 : 1.0 is prepared by:

[0057] 1. Combining Y oxide with a slurry of the zeolite to form a first slurry

[0058] 2. Adding Cu acetate to the first slurry to form a second slurry

[0059] 3. Adding Ce acetate to the second slurry to form a washcoat formulation, and optionally

[0060] 4. Applying the washcoat formulation to the substrate and optionally drying and calcining the formulation

[0061] In some embodiments, the selective catalytic reduction catalyst comprising a zeolite comprising copper (Cu), cerium (Ce) and yttrium (Y), wherein the Cu to Ce to Y weight percentage ratio, based on the weight of the zeolite, is in the range of 1.0:1.0:0.5 to 5.0:5.0:3.0, and preferably 2.0:1.0:0.50 to 5.0:5.0:3.0, 2.5:1.0:0.50 to 5.0:5.0:3.0, 2.5:1.0:0.6 to 5.0:5.0:3.0, 2.5:1.0:0.6 to 5.0:4.5:3.0, 2.5:1.0:0.6 to 4.5:4.5:3.0, 2.5:1.0:0.6 to 4.5:4.5:2.5, 2.5:1.0:0.6 to 4.5:4.0:2.5, 2.5:1.0:0.6 to 4.5:4.0:2.5, 2.5:1.0:0.6 to 4.5:3.5:2.5, 2.5:1.0:0.6 to 4.5:3.5:2.0, 2.5:1.0:0.6 to 4.5:3.0:2.0, preferably 2.5:1.0:0.6 to 4.5:3.0:1.5, preferably 2.5:1.0:0.60 to 4.25:3.0:1.5, and preferably 3.0:2.8:1.0 is prepared by:

[0062] 1. providing a first slurry comprising a zeolite exchanged with a first rare earth element (e.g. Ce or Y);

[0063] 2. adding a Cu precursor to the first slurry to form a second slurry;

[0064] 3. adding a precursor of a second rare earth element (e.g. Ce or Y) to the second slurry to form a washcoat formulation, wherein the second rare earth element is different to the first rare earth element, and optionally

[0065] 4. applying the washcoat formulation to the substrate and optionally drying and calcining the formulation.

[0066] Preferably, the catalyst layer is homogeneous.

[0067] In an alternate form of the invention, the catalyst layer is disposed on the substrate in two or more distinct zones.

[0068] According to an aspect of the method of the invention, the selectivity of the selective catalytic reduction catalyst for N2O, produced from the catalytic reduction of NOX, in the presence of ammonia, P101 193 is between 5% to 8%, preferably 7%, for temperatures of between 500°C and 600°C after the catalyst has been aged for 16 hours at 850°C.

[0069] In another aspect of the invention, the conversion rate of NOXof the selective catalytic reduction catalyst is from 50% to 100% at temperatures between 150°C and 300°C after the catalyst has been aged for 16 hours at 850°C.

[0070] According to a second embodiment of the invention, there is provided the use of a selective catalytic reduction catalyst for reducing N2O in the selective catalytic reduction of NOX, in a gas mixture, the gas mixture being contacted with ammonia, the selective catalytic reduction catalyst comprising a zeolite comprising copper (Cu), cerium (Ce) and yttrium (Y), wherein the Cu to Ce to Y weight percentage ratio, based on the weight of the zeolite, is 1.0:1.0:0.5 to 5.0:5.0:3.0, and preferably 2.0:1.0:0.50 to 5.0:5.0:3.0, 2.5:1.0:0.50 to 5.0:5.0:3.0, 2.5:1.0:0.6 to 5.0:5.0:3.0, 2.5:1.0:0.6 to 5.0:4.5:3.0, 2.5:1.0:0.6 to 4.5:4.5:3.0, 2.5:1.0:0.6 to 4.5:4.5:2.5, 2.5:1.0:0.6 to 4.5:4.0:2.5, 2.5:1.0:0.6 to 4.5:4.0:2.5, 2.5:1.0:0.6 to 4.5:3.5:2.5, 2.5:1.0:0.6 to 4.5:3.5:2.0, 2.5:1.0:0.6 to 4.5:3.0:2.0, preferably 2.5:1.0:0.6 to 4.5:3.0:1.5, preferably 2.5:1.0:0.60 to 4.25:3.0:1.5, and preferably 3.0 : 2.8 : 1.0.

[0071] In accordance with a third embodiment of the invention, there is provided a selective catalytic reduction catalyst for reducing N2O in the selective catalytic reduction of NOX, in a gas mixture, wherein the gas mixture is contacted with a source of ammonia, the selective catalytic reduction catalyst comprising a zeolite comprising copper (Cu), cerium (Ce) and yttrium (Y), wherein the Cu to Ce to Y weight percentage ratio, based on the weight of the zeolite, is 1.0:1.0:0.5 to 5.0:5.0:3.0, and preferably 2.0:1.0:0.50 to 5.0:5.0:3.0, 2.5:1.0:0.50 to 5.0:5.0:3.0, 2.5:1.0:0.6 to 5.0:5.0:3.0, 2.5:1.0:0.6 to 5.0:4.5:3.0, 2.5:1.0:0.6 to 4.5:4.5:3.0, 2.5:1.0:0.6 to 4.5:4.5:2.5, 2.5:1.0:0.6 to 4.5:4.0:2.5, 2.5:1.0:0.6 to 4.5:4.0:2.5, 2.5:1.0:0.6 to 4.5:3.5:2.5, 2.5:1.0:0.6 to 4.5:3.5:2.0, 2.5:1.0:0.6 to 4.5:3.0:2.0, preferably 2.5:1.0:0.6 to 4.5:3.0:1.5, preferably 2.5:1.0:0.60 to 4.25:3.0:1.5, and preferably 3.0 : 2.8 : 1.0.

[0072] In a fourth embodiment of the invention there is provided an emission treatment system for treating a flow of a combustion exhaust gas comprising NOXand a selective catalytic reduction catalyst comprising a zeolite comprising copper (Cu), cerium (Ce) and yttrium (Y), wherein the Cu to Ce to Y weight percentage ratio, based on the weight of the zeolite, is 1.0:1.0:0.5 to 5.0:5.0:3.0, and preferably 2.0:1.0:0.50 to 5.0:5.0:3.0, 2.5:1.0:0.50 to 5.0:5.0:3.0, 2.5:1.0:0.6 to 5.0:5.0:3.0, 2.5:1.0:0.6 to 5.0:4.5:3.0, 2.5:1.0:0.6 to 4.5:4.5:3.0, 2.5:1.0:0.6 to 4.5:4.5:2.5, 2.5:1.0:0.6 to 4.5:4.0:2.5, 2.5:1.0:0.6 to 4.5:4.0:2.5, 2.5:1.0:0.6 to 4.5:3.5:2.5, 2.5:1.0:0.6 to 4.5:3.5:2.0, 2.5:1.0:0.6 to 4.5:3.0:2.0, preferably 2.5:1.0:0.6 to 4.5:3.0:1.5, preferably 2.5:1.0:0.60 to 4.25:3.0:1.5, and preferably 3.0 : 2.8 : 1.0. P101193

[0073] EXAMPLES

[0074] Example 1

[0075] A flow-through honeycomb core (4.66 inches by 3.0 inches, 400 cpsi, 4 mil wall thickness) was coated with a catalyst from both inlet and outlet and coated approximately 50% respectively. The catalyst was coated over the substrate so that the substrate was completely covered and was dried and calcined.

[0076] Catalyst A (comparative) comprised a 3 wt.% copper ion-exchanged aluminosilicate zeolite CHA (SAR = 20) and a binder.

[0077] Catalyst B (comparative) comprised a 3 wt.% copper and 2.8 wt.% cerium ion-exchanged aluminosilicate zeolite CHA (SAR = 20) and a binder.

[0078] Catalyst C (comparative) comprised a 3 wt.% copper and 0.46 wt.% yttrium ion-exchanged aluminosilicate zeolite CHA (SAR = 20) and a binder.

[0079] Catalyst D comprised a 3 wt.% copper, 2.8 wt.% cerium and 0.46 wt.% yttrium ion-exchanged aluminosilicate zeolite CHA (SAR = 20) and a binder.

[0080] Catalyst E comprised a 3 wt.% copper, 2.8 wt.% cerium and 1.0 wt.% yttrium ion-exchanged aluminosilicate zeolite CHA (SAR = 20) and a binder.

[0081] SCAT data

[0082] 1x3” cores were cut from the prepared catalyst of Catalysts A-E and aged at 850°C for 16 hours in an atmosphere of 10% water, 10% oxygen, nitrogen balance before being tested for NOXconversion and N2O selectivity using a series of steady state points at increasing temperatures in 500ppm NOxand 750ppm NH3at 60k SV (10% O2, 5% water, 350 ppm CO, and N2balance))

[0083] The results are shown in Table 1 . Table 1 shows percentage NOXconversion and N2O selectivity (%) for Catalysts A to E. The N2O selectivity is a percentage based on twice the N2O make (in ppm) divided by the amount of NOXconverted (in ppm). P101193

[0084] Table 1 :

[0085] The addition of cerium to the catalyst increased the NOx conversion and improved N2O selectivity at low temperature (up to 250°C), as shown for Catalyst B at low temperature, compared to Catalyst A with no cerium. The addition of cerium however caused an increase in N2O selectivity at high temperature . The addition of yttrium to the catalyst decreased the NOXconversion and resulted in poorer N2O selectivity at low temperatures (up to 250°C), as shown for Catalyst C compared to Catalyst A with no yttrium. The inventors have surprisingly found that the addition of cerium and yttrium provides the effect of the cerium for increasing the NOx conversion and low N2O selectivity at low temperatures (see Catalysts B, D and E showing similar NOx conversion and N2O selectivity at low temperatures, and which is improved compared to Catalysts A and C), but also exhibits lower N2O make at high temperatures compared to the catalyst comprising Cu and Ce (see Catalysts D and E being better for N2O selectivity at high temperatures than Catalyst B).

[0086] While the aforementioned effects were generally observed for catalysts comprising copper, cerium and yttrium, itwas particularly surprisingthat, on comparison of Catalysts D and E, higher yttrium loadings (e.g. loadings of above 0.50 wt.%) further improved the NOXconversion and maintained or improved N2O selectivity at all temperatures. This result was particularly unexpected given that the addition of yttrium alone generally resulted in poorer NOXconversion and poorer N2O selectivity at low temperatures, as shown by comparing Catalysts A and C. P101 193

[0087] Example 2

[0088] A flow-through honeycomb core (4.66 inches by 3.0 inches, 400 cpsi, 4 mil wall thickness) was coated with a catalyst from both inlet and outlet and coated approximately 50% respectively. The catalyst was coated over the substrate so that the substrate was completely covered and was dried and calcined.

[0089] Catalyst F (comparative) comprised a 3 wt.% copper ion-exchanged aluminosilicate zeolite CHA (SAR = 13) and a binder.

[0090] Catalyst G (comparative) comprised a 3 wt.% copper and 2.8 wt.% cerium ion-exchanged aluminosilicate zeolite CHA (SAR = 13) and a binder.

[0091] Catalyst H comprised a 3 wt.% copper, 1 .4 wt.% cerium and 1.0 wt.% yttrium ion-exchanged aluminosilicate zeolite CHA (SAR = 13) and a binder.

[0092] Catalyst I comprised a 3 wt.% copper, 2.8 wt.% cerium and 1 .0 wt.% yttrium ion-exchanged aluminosilicate zeolite CHA (SAR = 13) and a binder.

[0093] SCAT data

[0094] 1x3” cores were cut from the prepared catalyst of Catalysts F-l and aged at 800°C for 16 hours in an atmosphere of 10% water, 10% oxygen, nitrogen balance before being tested for NOXconversion and N2O selectivity using a series of steady state points at increasing temperatures in 500 ppm NOx, 750 ppm NH3at 60k SV (10% O2, 5% water, 350 ppm CO, and N2balance).

[0095] The results are shown in Table 2.

[0096] Table 2: P101193

[0097] Catalysts H and I exhibited comparable or slightly improved NOXconversion compared to Catalyst F, and provided comparable or improved N2O selectivity compared to Catalysts F and G at all temperatures. The benefit in N2O selectivity was most significant at higher temperatures.

[0098] Example 3

[0099] Catalysts were prepared, aged and tested in the same way as Example .

[0100] Catalyst J (comparative) comprised a 4 wt.% copper ion-exchanged aluminosilicate zeolite CHA (SAR = 13) and a binder.

[0101] Catalyst K (comparative) comprised a 4 wt.% copper and 2.8 wt.% cerium ion-exchanged aluminosilicate zeolite CHA (SAR = 13) and a binder.

[0102] Catalyst L (comparative) comprised a 4 wt.% copper and 1.0 wt.% yttrium ion-exchanged aluminosilicate zeolite CHA (SAR = 13) and a binder.

[0103] Catalyst M comprised a 4 wt.% copper and 2.8 wt.% cerium and 1 .0 wt.% yttrium ion-exchanged aluminosilicate zeolite CHA (SAR = 13) and a binder.

[0104] The results are shown in Table 3.

[0105] Table 3:

[0106] Catalyst M (Cu, Ce and Y) exhibited comparable or improved NOXconversion at lowtemperatures compared with Catalyst J (Cu only), Catalyst K(Cu and Ce) and Catalyst L (Cu and Y). Additionally, Catalyst M exhibited improved (lower) N2O selectivity at low temperatures and comparable or improved N2O selectivity at high temperatures compared with Catalysts J, K and L. Overall, P101193 catalyst M provided relatively good NOXconversion and low N2O selectivity across all temperatures.

Claims

P101193CLAIMS1. A method for reducing N2O in the selective catalytic reduction of NOX, in a gas mixture, comprising contacting the gas mixture with a source of ammonia, over a selective catalytic reduction catalyst comprising a zeolite comprising copper (Cu), cerium (Ce) and yttrium (Y), wherein the Cu to Ce to Y weight percentage ratio, based on the weight of the zeolite, is in the range of 1.0:1.0:0.5 to 5.0:5.0:3.0.

2. The method of claim 1, wherein the zeolite has a framework selected from the group comprising AEI, AFX, CHA, DDR, ERI, ITE, KFI, LEV, SFW, BEA, MFI and FER, preferably CHA.

3. The method of any preceding claim, wherein the selective catalytic reduction catalyst comprises a substrate having a catalyst layer applied thereon, wherein said catalyst layer comprises the zeolite comprising Cu, Ce and Y applied thereon.

4. The method of claim 3, wherein the catalyst layer is prepared from a washcoat formulation comprising i. a zeolite comprising Cu, Ce and Y, ii. a binder, and ill. a thickener.

5. The method of claim 3 or claim 4, wherein the catalyst layer is a homogeneous layer disposed on the substrate.

6. The method of any one of claims 3-5, wherein the catalyst layer is disposed on the substrate in two or more distinct zones.

7. The method of any one of claims 3-6, wherein the substrate is a ceramic material, preferably wherein the substrate is cordierite.

8. The method of any preceding claim, wherein the selectivity of the selective catalytic reduction catalyst for N2O, produced from the catalytic reduction of NOX, in the presence of ammonia, is between 5% to 8%, preferably 7%, for temperatures of between 500°C and 600°C after the catalyst has been aged for 16 hours at 850°C.P1011939. The method of any preceding claim, wherein the conversion rate of NOx of the selective catalytic reduction catalyst is from 50% to 100% at temperatures between 150°C and 300°C after the catalyst has been aged for 16 hours at 850°C.

10. The method of any preceding claim, wherein the Cu to Ce to Y weight percentage ratio, based on the weight of the zeolite, is 2.5:1.0:0.50 to 4.5:3.5:2.0, preferably 2.5:1.0:0.50 to 4.5:3.0:1.5.

11. The method of any preceding claim, wherein the zeolite has a molar ratio of silica to alumina (SAR) in a range of from 10 to 30, preferably 12 to 27.

12. Use of a selective catalytic reduction catalyst for reducing N2O in the selective catalytic reduction of NOX, in a gas mixture, the gas mixture being contacted with ammonia, the selective catalytic reduction catalyst comprising a zeolite comprising copper (Cu), cerium (Ce) and yttrium (Y), wherein the Cu to Ce to Y weight percentage ratio, based on the weight of the zeolite, is 1.0:1.0:0.5 to 5.0:5.0:3.0, preferably 1.0:2.0:0.5 to 5.0:4.0:2.0.

13. A selective catalytic reduction catalyst for reducing N2O in the selective catalytic reduction of NOX, in a gas mixture, wherein the gas mixture is contacted with a source of ammonia, the selective catalytic reduction catalyst comprising a zeolite comprising copper (Cu), cerium (Ce) and yttrium (Y), wherein the Cu to Ce to Y weight percentage ratio, based on the weight of the zeolite, is 1.0:1.0:0.5 to 5.0:5.0:3.0, preferably 1.0:2.0:0.5 to 5.0:4.0:2.0.

14. An emission treatment system for treating a flow of a combustion exhaust gas comprising NOXand a selective catalytic reduction catalyst comprising a zeolite comprising copper (Cu), cerium (Ce) and yttrium (Y), wherein the Cu to Ce to Y weight percentage ratio, based on the weight of the zeolite, is 1.0:1.0:0.5 to 5.0:5.0:3.0, preferably 1.0:2.0:0.5 to 5.0:4.0:2.0.

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