Method for treating exhaust gas using a zoned ammonia slip catalyst for improved selectivity, activity and poison tolerance

Abstract

The invention relates to a method for reducing N2O formation in the catalytic treatment of an exhaust gas comprising NOx and ammonia. The catalytic article comprises a substrate; a first catalytic region, deposited on the substrate comprising an ammonia slip catalyst (ASC) which comprises a first catalytic zone with a first PGM component and second catalytic zone with a second PGM component; and a second catalytic region deposited on the first catalytic region, comprising a selective catalytic reduction (SCR) catalyst; and wherein the loading of the second PGM component is greater than the loading of the first PGM component.

Description

[0001] ZONED AMMONIA SLIP CATALYST FOR IMPROVED SELECTIVITY, ACTIVITY AND POISON TOLERANCE

[0002] FIELD OF THE INVENTION

[0003] The invention relates to a method for the abatement of N2O from an exhaust gas comprising NOXand ammonia and to a catalyst article for use in such method.

[0004] BACKGROUND OF THE INVENTION

[0005] 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 NOXin exhaust gas, it is desirable to eliminate these undesirable components, preferably by a process that does not generate other noxious or toxic substances.

[0006] 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.

[0007] 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.

[0008] The reduction reaction may be described as follows:

[0009] 4NH3+4NO+O2— >4N2+6H2O

[0010] 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 converting it to nitrogen. The use of ammonia slip catalysts can allow for NOXconversions of greater than 90% over a typical diesel driving cycle.

[0011] A further problem associated with the reduction of NOXwith ammonia is that the reduction of NOXmay result in the production of nitrous oxide (N2O). The general reaction may be described as:

[0012] 4NH3+4NO+3O2— >4N2O+6H2O

[0013] It is undesirable to produce N2O because it is a greenhouse gas which may contribute to an increase in global warming.

[0014] Despite various efforts in the field, there is still a need for better ASC catalyst with improved technical benefits wherein the amount of ammonia in an exhaust gas is minimised and wherein the production of N2O is abated.

[0015] SUMMARY OF THE INVENTION

[0016] For purposes of this specification the singular forms "a", "an" and "the" include plural referents unless the context clearly indicates otherwise. Thus, for example, reference to "a catalyst" includes a mixture of two or more catalysts, and the like.

[0017] The term “about” means approximately and refers to a range that is optionally ± 25%, preferably ± 10%, more preferably, ± 5%, or most preferably ± 1 % of the value with which the term is associated.

[0018] As is used herein "molecular sieve" is understood to mean a metastable material containing tiny pores of a precise and uniform size that may be used as an adsorbent for gases or liquids. The molecules which are small enough to pass through the pores are adsorbed while the larger molecules are not. The molecular sieve can be a zeolitic molecular sieve, a non-zeolitic molecular sieve, or a mixture thereof.

[0019] The term “N2selectivity” means the percent conversion of ammonia into nitrogen.

[0020] The term “platinum group metal” or “PGM” refers to platinum (Pt), palladium (Pd), ruthenium (Ru), rhodium (Rh), osmium (Os) and iridium (lr). The term “transition metal” refers to any element that may be in group III to group XII of the periodic table.

[0021] The term “substrate” means the surface on which catalytic reactions occur.

[0022] The term “support” means the material to which a catalyst is fixed.

[0023] Finally, when a range, or ranges, for various numerical elements are provided, the range, or ranges, can include the values, unless otherwise specified.

[0024] According to an aspect of the present invention there is provided a method for reducing N2O formation in the catalytic treatment of an exhaust gas comprising NOXand ammonia, wherein the exhaust gas is contacted with a catalytic article, wherein said catalytic article comprises:

[0025] (a) a substrate comprising an inlet end and an outlet end with an axial length of L;

[0026] (b) a first catalytic region, deposited on the substrate comprising an ammonia slip catalyst (ASC) which comprises:

[0027] (i) a first catalytic zone, extending from the inlet end of the substrate, having a first platinum group metal (PGM) component and a support, and

[0028] (ii) a second catalytic zone, extending from the outlet end of the substrate, having a second PGM component and a support;

[0029] (c) a second catalytic region deposited on the first catalytic region and covering the entire axial length L, comprising a selective catalytic reduction (SCR) catalyst; and wherein the loading of the second PGM component is greater than the loading of the first PGM component.

[0030] The method is a method of catalytically treating an exhaust gas comprising NOXand ammonia. Preferably, the exhaust gas to be treated comprises ammonia in an amount of not more than 500 ppm, and suitably not more than 450 ppm. For example, the exhaust gas to be treated can comprise ammonia in an amount in a range of from >0 ppm to 500 ppm, suitably 1 ppm to 500 ppm, 1 ppm to 450 ppm, or 1 ppm to 300 ppm, or 1 ppm to 250 ppm, 10 ppm to 200 ppm, 25 ppm to 150 ppm, 50 to 100 ppm, or suitably about 75 ppm. The amount of ammonia in the exhaust gas can comprise any combination of the aforementioned upper and lower limits. At such low levels of ammonia, it has been found that the method of the invention results in the minimisation of the production of N2O.

[0031] In accordance with the invention, each of the first catalytic region, the second catalytic region, the first catalytic zone and the second catalytic zone are substantially discrete relative to each other.

[0032] The substrate

[0033] The substrate for the catalytic article of the present invention may be any material typically used for preparing automotive catalysts that comprises a flow-through or filter structure, such as a honeycomb structure, an extruded substrate, a metallic substrate, or a selective catalytic reduction filter (SCRF).

[0034] The invention is not limited to a particular substrate type, material, or geometry.

[0035] In an embodiment of the invention, a ceramic substrate may be used which may be made of any suitable refractory material, such as cordierite, cordierite-a alumina, a- alumina, silicon carbide, silicon nitride, zirconia, mullite, spodumene, alumina-silica magnesia, zirconium silicate, sillimanite, magnesium silicates, zircon, petalite, aluminosilicates and mixtures thereof.

[0036] The substrate can be a high porosity substrate. The term “high porosity substrate” refers to a substrate having a porosity of between about 40% and about 80%. The high porosity substrate can have a porosity preferably of at least about 45%, more preferably of at least about 50%. The high porosity substrate can have a porosity preferably of less than about 75%, more preferably of less than about 70%. The term porosity, as used herein, refers to the total porosity, preferably as measured with mercury porosimetry.

[0037] The first catalytic region

[0038] The first catalytic zone and the second catalytic zone of the first catalytic region, oxidise NH3 into NOXand water. The first catalytic region essentially comprises an ASC catalyst. At low levels of ammonia, it has been found that the catalytic article, and in particular the features of the first catalytic region, used in the method of the invention, results in the minimisation of the production of N2O.

[0039] The first catalytic zone extends from the inlet end of the substrate over an axial length Li and the second catalytic zone extends from the outlet end of the substrate over an axial length L2, and wherein the ratio of Li to L2 is from 10:90 to 90:10, preferably from 25:50 to 50:25, such as about 25:50, about 50:50 or about 50:25.

[0040] The first PGM component in the first catalytic zone and the second PGM component in the second catalytic zone are preferably independently selected from the group consisting of platinum, palladium, ruthenium or rhodium, more preferably platinum, palladium and rhodium, most preferably platinum.

[0041] The loading of the first PGM component, on the support, from the inlet end of the substrate, is between >0 g / ft3and 6 g / ft3, preferably from 0.1 g / ft3to 5.5 g / ft3, more preferably from 0.5 g / ft3to 4 g / ft3, e.g. from 0.5 g / ft3to 3 g / ft3, preferably between 0.5 g / ft3to 2 g / ft3, more preferably between 1 g / ft3to 1 .5 g / ft3.

[0042] The loading of the second PGM component, on the support, from the outlet end of the substrate, is between 1.5 g / ft3to 20 g / ft3, preferably from 2 g / ft3to 19 g / ft3, or from 3 g / ft3to 20 g / ft3, more preferably 3 g / ft3, 5 g / ft3, 16.5 g / ft3or 18.5 g / ft3. The loading of the second PGM component, on the support, from the outlet end of the substrate can be in a range of from 1 .5 g / ft3to 9 g / ft3, suitably 2 g / ft3to 4 g / ft3.

[0043] The first PGM component and its support, and the second PGM component and its support, may be layered on the substrate such that the layers are arranged side by side in the same plane relative to the substrate. In an alternate embodiment of the invention the first PGM component and its support, and the second PGM component and its support, may be layered on the substrate as a stack, one on top of the other.

[0044] The support for the first PGM component and the support for the second PGM component may be the same or may be different.

[0045] The support for the first PGM component and the second PGM component may be a refractory metal oxide such as alumina, silica, zirconia, titania, ceria, and physical mixtures or composites thereof. The refractory metal oxide of alumina is particularly preferred. In an alternate form of the invention the first and / or second catalytic zone can further comprise a molecular sieve. For example, the first and / or second catalytic zone can further comprise a bare molecular sieve. When the first and second catalytic zone comprise a molecular sieve, the molecular sieve can be the same or different. The molecular sieve may be a zeolitic molecular sieve or a non-zeolitic molecular sieve or may be a mixture of both and is hereinafter referred to as the ASC molecular sieve.

[0046] The ASC molecular sieve may preferably be a zeolitic molecular sieve being a microporous aluminosilicate having any one of the framework structures listed in the Database of Zeolite Structures published by the International Zeolite Association (IZA). Preferably the zeolite has a framework structure selected from AEI, BEA, CHA, or FER or a combination or blend of one or more of these frameworks. Preferably the framework structure is BEA or FER, or a combination of both.

[0047] In a preferred form of the invention, the ASC molecular sieve has a silica-to-alumina ratio (SAR) of 10 to 30, preferably 20 to 25. In a further embodiment, the ASC molecular sieve has a SAR of from 12 to 28, e.g. from 13 to 25, from 11 to 18, from 18 to 22, from 20 to 25 or from 24 to 28. In an alternative embodiment, the ASC molecular sieve has a SAR of about 12, about 13, about 15, about 20, about 23 or about 25.

[0048] In an alternative embodiment, the first catalytic zone comprises: 1 ) a first ASC molecular sieve (e.g. AEI, BEA, CHA or FER, preferably FER or BEA) and 2) a first PGM component on a support, and the second catalytic zone comprises: 1 ) a second ASC molecular sieve (e.g. AEI, BEA, CHA or FER, preferably FER or BEA) and 2) a second PGM component on a support.

[0049] In preferred embodiments, the first catalytic zone does not comprise a transition metal exchanged molecular sieve. Preferably the first catalytic zone does not comprise a metal exchanged molecular sieve.

[0050] In preferred embodiments, the second catalytic zone does not comprise a transition metal exchanged molecular sieve. Preferably the second catalytic zone does not comprise a metal exchanged molecular sieve.

[0051] In preferred embodiments, the first catalytic region does not comprise a transition metal exchanged molecular sieve. Preferably, the first catalytic region does not comprise a metal exchanged molecular sieve. According to an alternate embodiment of the invention the first PGM component is supported by a first ASC support and the second PGM component is supported by a second ASC support. These supports may be the same or different.

[0052] In a particularly preferred embodiment of the invention, the first ASC support is alumina and the second ASC support is alumina.

[0053] The second catalytic region

[0054] The second catalytic region is primarily responsible for the selective reduction of NOX, in an exhaust gas, to nitrogen and water.

[0055] The SCR catalyst of the second catalytic region may comprise a transition metal and a support, in the form of a molecular sieve, hereinafter referred to as the SCR molecular sieve. The transition metal can be copper (Cu), iron (Fe), manganese (Mn), vanadium (V), or a combination thereof. The SCR catalyst is preferably a Cu-SCR catalyst or a Fe-SCR catalyst, more preferably a Cu-SCR catalyst. The Cu-SCR catalyst comprises copper and the SCR molecular sieve. The Fe-SCR catalyst comprises iron and the SCR molecular sieve.

[0056] Typically, the SCR catalyst comprises the transition metal (preferably iron or copper, most preferably copper) in an amount of 0.10 to 10 wt% of the transition metal exchanged molecular sieve, preferably an amount of from 0.5 to 7 wt%, from 1 to 6 wt%, or from 2 to 5 wt%, e.g. from 3 to 5 wt% of the transition metal exchanged molecular sieve. In a particularly preferred embodiment, the SCR catalyst comprises the transition metal (preferably iron or copper, most preferably copper) in an amount of 3 to 4.5 wt% of the transition metal exchanged molecular sieve.

[0057] The SCR molecular sieve may comprise a small pore, a medium pore or a large pore molecular sieve, or a mixture thereof. A "small pore molecular sieve" is a molecular sieve containing a maximum ring size of 8 tetrahedral atoms. A "medium pore molecular sieve" is a molecular sieve containing a maximum ring size of 10 tetrahedral atoms. A "large pore molecular sieve" is a molecular sieve having a maximum ring size of 12 tetrahedral atoms.

[0058] The SCR molecular sieve can be a zeolitic molecular sieve, a non-zeolitic molecular sieve, or a mixture thereof. Preferably it is a zeolitic molecular sieve having a framework structure listed in the IZA. The SCR zeolitic molecular sieve is preferably a metal exchanged zeolitic molecular sieve.

[0059] The SCR molecular sieves (e.g. molecular sieves of Cu-SCR and Fe-SCR) are preferably selected from the framework structures 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, LEV, KFI, MER, MON, NSI, OWE, PAU, PHI, RHO, RTH, SAT, SAV, SIV, THO, TSC, UEI, UFI, VNI, YUG, ZON, BEA, MFI and FER and mixtures and / or intergrowths thereof. More preferably, the SCR molecular sieves (e.g., the Cu-SCR or Fe-SCR molecular sieves) are selected from framework structures selected from the group consisting of AEI, AFX, CHA, DDR, ERI, ITE, KFI, LEV, SFW, BEA, MFI and FER, and mixtures and / or intergrowths thereof. Most preferably the framework structures are AEI and CHA.

[0060] In a preferred form of the invention, the SCR molecular sieve has a silica-to-alumina ratio (SAR) of 10 to 30, preferably 20 to 25. In a further embodiment, the SCR molecular sieve has a SAR of from 12 to 28, e.g. from 13 to 25, from 11 to 18, from 18 to 22, from 20 to 25 or from 24 to 28. In an alternative embodiment, the SCR molecular sieve has a SAR of about 12, about 13, about 15, about 20, about 23 or about 25.

[0061] The loading of the SCR molecular sieve in the SCR catalyst is between 1 and 3 g / in3, preferably between 1 .8 to 2.5 g / in3.

[0062] In an alternate form of the invention, the second catalytic region may comprise third catalytic zone comprising a first SCR catalyst, which extends from the inlet end of the substrate. A fourth catalytic zone is also provided, the fourth catalytic zone comprising a second SCR catalyst, which extends from the outlet end of the substrate.

[0063] The third catalytic zone can extend from the inlet end of the substrate over an axial length L3 and the fourth catalytic zone can extend from the outlet end of the substrate over an axial length L4, and wherein the ratio of L3 to L4 is from 10:90 to 90:10, preferably from 25:50 to 50:25, such as about 25:50, about 50:50 or about 50:25.

[0064] The first SCR catalyst and the second SCR catalyst may be the same or different. For example, the first SCR catalyst can be different to the second SCR catalyst.

[0065] Both the first SCR catalyst and the second SCR catalyst can comprise a transition metal, and a first SCR molecular sieve and a second SCR molecular sieve respectively. The transition metal for the first SCR catalyst may be the same or different to the transition metal for the second SCR catalyst.

[0066] The first SCR molecular sieve and the second SCR molecular sieve are generally as hereinbefore described with reference to the SCR molecular sieve. In some preferred embodiments, the first SCR (zeolitic) molecular sieve has an AEI framework and the second SCR (zeolitic) molecular sieve has a CHA framework.

[0067] The total loading of the first SCR molecular sieve and the second SCR molecular sieve in the first SCR catalyst and the second SCR catalyst, respectively, may be from 1 to 3 g / in3, preferably 1 .8 to 2.5 g / in3.

[0068] In a preferred form of the invention the first SCR molecular sieve is different to the second SCR molecular sieve, preferably wherein the first SCR molecular sieve has an AEI framework, and the second SCR molecular sieve has a CHA framework.

[0069] Emissions Treatment System

[0070] In one embodiment, the invention relates to an emission treatment system for treating a flow of a combustion exhaust gas. The emission treatment system can comprise the catalytic article as herein described. The emission treatment system preferably further comprises a DOC, a DPF, at least one additional SCR, at least one additional ASC, or a combination thereof.

[0071] The emissions treatment system can comprise a means for introducing NH3 into the exhaust system. The NH3 can be added to the flowing exhaust gas by any suitable means for introducing NH3 into the exhaust gas. Suitable means include an injector, sprayer, or feeder. Such means are well known in the art.

[0072] The NH3 for use in the system can be ammonia per se, hydrazine, or an ammonia precursor selected from the group consisting of urea, ammonium carbonate, ammonium carbamate, ammonium hydrogen carbonate, and ammonium formate. Urea is particularly preferred.

[0073] The emission treatment system may also comprise a means for controlling the introduction of NH3 into the exhaust gas in order to reduce NOx therein. Preferred control means may include an electronic control unit, optionally an engine control unit, and may additionally comprise a NOx sensor located downstream of the NO reduction catalyst.

[0074] The following examples merely illustrate the invention; the skilled person will recognize many variations that are within the spirit of the invention and scope of the claims.

[0075] DESCRIPTION OF THE FIGURES

[0076] Figure 1 : is a graphic representation of the ammonia conversion activity of catalyst A, B and C, prepared in accordance with Example 1 hereunder.

[0077] Figure 2: is a graphic representation of the ammonia conversion activity of catalyst D and E being prepared in accordance with Example 2 hereunder.

[0078] Figure 3: is a graphic representation of the ammonia conversion activity of catalyst F and G being prepared in accordance with Example 3 hereunder.

[0079] Figure 4: is a graphic representation of NOx conversion vs average N2O emission of catalyst F and G during engine bench testing being prepared in accordance with Example 3 hereunder.

[0080] Figure 5: is a graphic representation of NOx conversion vs peak N2O emission of catalyst F and G during engine bench testing being prepared in accordance with Example 3 hereunder.

[0081] EXAMPLES

[0082] Example 1 :

[0083] Catalyst Preparation

[0084] A flow-through honeycomb core (4.66 inches by 3.0 inches, 400 cpsi, 4 mil wall thickness) was coated with a first oxidation catalysts from both inlet and outlet and coated approximately 50% respectively, to form a bottom layer and then coated with a second SCR catalyst (front and rear) to form a top layer over the bottom layer to give Catalyst A.

[0085] The catalyst of the bottom layer was a standard ASC bottom layer washcoat slurry containing aqueous salts (as nitrates) of platinum, an alumina support and a binder with a homogeneous platinum loading of 3g / ft3. The washcoat slurry was applied to the honeycomb core as a bottom catalyst layer and the coated core dried and calcined.

[0086] The catalyst of the top layers comprised copper ion-exchanged aluminosilicate zeolite CHA and binder washcoat. This washcoat was coated over the bottom catalyst layer so that the bottom layer is completely covered, then were dried and calcined.

[0087] Additional catalysts (Catalysts B&C) were prepared. Catalyst B differs from Catalyst A in that the inlet was coated with an oxidation catalyst loaded at 1 g / ft3and outlet coated at 5g / ft3. Catalyst C differs from Catalyst A in that the inlet was coated with an oxidation catalyst loaded at 5g / ft3and outlet coated at 1 g / ft3.

[0088] SCAT Data

[0089] 1x3” cores were cut from the prepared catalyst of Catalyst A, Catalyst B and Catalyst C and aged for 580°C for 100hours in an atmosphere of 10% water in air before being tested for NH3 conversion and N2 selectivity using a series of steady state points at increasing temperatures in 75ppm NH3 at 120k SV (10% O2, 5% water and N2 balance).

[0090] Catalyst A, B and C exhibit similar NH3 conversion activity (Figure 1 ) but crucially sample B results in a lower N2O make. This is summarised in Table 1 .

[0091] Table 1 Summary of catalytic activity and N2O make of catalyst A, B and C

[0092] Example 2:

[0093] Catalyst Preparation

[0094] A flow-through honeycomb core (4.66 inches by 3.0 inches, 400 cpsi, 4 mil wall thickness) was coated with a first oxidation catalysts from both inlet and outlet and coated approximately 50% respectively, to form a bottom layer and then coated with a second SCR catalyst (front and rear) to form a top layer over the bottom layer to give Catalyst D.

[0095] The catalyst of the bottom layer was a standard ASC bottom layer washcoat slurry containing aqueous salts (as nitrates) of platinum, an alumina support, a bare molecular sieve and a binder as described in WO2019116268A1 . The washcoat slurry was applied to the honeycomb core as a bottom catalyst layer and the coated core dried and calcined.

[0096] The catalyst of the top layers comprised copper ion-exchanged aluminosilicate zeolite CHA and binder washcoat. This washcoat was coated over the bottom catalyst layer so that the bottom layer is completely covered, then were dried and calcined.

[0097] Alternative catalyst (Catalyst E) was prepared. Catalyst E differs from Catalyst D in that the inlet was coated with an oxidation catalyst loaded at 1 g / ft3and outlet coated at 5g / ft3.

[0098] SCAT Data

[0099] 1x3” cores were cut from the prepared catalyst of Catalyst D and Catalyst E and aged for 650°C for 100hours in an atmosphere of 10% water in air under forceflow conditions before being tested for NH3 conversion and N2 selectivity using a temperature ramp in 75ppm NH3 at 120k SV (10% O2, 5% water and N2 balance).

[0100] Catalyst D and E exhibit similar NH3 conversion activity but crucially sample E results in a lower N2O make (Figure 2). This is summarised in Table 2.

[0101] Table 2 Peak N2O make of catalyst D and E

[0102] Example 3:

[0103] Catalyst Preparation Flow-through honeycombs (4.66 inches by 3.0 inches, 400 cpsi, 4 mil wall thickness and 12.5 inches by 3.0 inches, 400 cpsi, 4 mil wall thickness respectively) were coated with a first oxidation catalysts from both inlet and outlet and coated approximately 50% respectively, to form a bottom layer and then coated with a second SCR catalysts (front and rear) to form a top layer over the bottom layer to give Catalyst F.

[0104] The catalyst of the bottom layer was a standard ASC bottom layer washcoat slurry containing aqueous salts (as nitrates) of platinum, an alumina support, a bare molecular sieve and a binder as described in WO2019116268A1 . The washcoat slurry was applied to the honeycomb core as a bottom catalyst layer and the coated core dried and calcined.

[0105] The catalyst of the top layers comprised copper ion-exchanged aluminosilicate zeolite CHA and binder washcoat. This washcoat was coated over the bottom catalyst layer so that the bottom layer is completely covered, then were dried and calcined.

[0106] Alternative catalyst (Catalyst G) was prepared. Catalyst G differs from Catalyst F in that the inlet was coated with an oxidation catalyst loaded at 1 g / ft3and outlet coated at 5g / ft3.

[0107] SCAT Data:

[0108] 1x3” cores were cut from the prepared catalyst of Catalyst F and Catalyst G and aged for 650°C for 100 hours in an atmosphere of 10% water in air under forceflow conditions before being tested for NH3 conversion and N2 selectivity using a temperature ramp in 75ppm NH3 at 120k SV (10% O2, 5% water and N2 balance).

[0109] Catalyst F and G exhibit similar NH3 conversion activity but crucially sample G results in a lower N2O make (Figure 3). This is summarised in Table 3.

[0110] Table 3 Peak N2O make of catalyst F and G Engine test details

[0111] 12.5x3” samples were tested on engine.

[0112] Reference System

[0113] V-SCR followed by Catalyst F

[0114] System 1

[0115] V-SCR followed by Catalyst G

[0116] The tailpipe emissions measured for Reference System and System 1 on an engine test bed during a set of six World Harmonised Transient Cycle (WHTC) when dosed at an Ammonia to NOx ratio (ANR) of 0, 0.6, 0.8, 0.95, 1.1 , 1 .25 respectively were compared.

[0117] ASC were tested after simulated lifetime ageing of 650°C / 100hrs / 10% Water. The same V-SCR was used in both systems.

[0118] As shown in Figure 4, Figure 5 and summarised in Table 4. System 1 containing catalyst G showed lower average and peak N2O slip during comparative cycles whilst achieving equivalent NOXconversion when compared to Reference System 1 .

[0119] Table 4: Summary of catalytic activity and N2O make of catalyst F and G

[0120] The inventors have surprisingly discovered that catalysts according to the invention (e.g. catalysts where the PGM loading is higher at the outlet end than the inlet end), exhibit a lower N2O make than the comparative examples, whilst still maintaining the same or similar NH3 conversion to the comparative examples.

Claims

CLAIMS1 . A method for reducing N2O formation in the catalytic treatment of an exhaust gas comprising NOXand ammonia, wherein the exhaust gas is contacted with a catalytic article, wherein said catalytic article comprises:(a) a substrate comprising an inlet end and an outlet end with an axial length of L;(b) a first catalytic region, deposited on the substrate comprising an ammonia slip catalyst (ASC) which comprises:(i) a first catalytic zone, extending from the inlet end of the substrate, having a first platinum group metal (PGM) component and a support, wherein the first catalytic zone does not comprise a transition metal exchanged molecular sieve, and(ii) a second catalytic zone, extending from the outlet end of the substrate, having a second PGM component and a support;(c) a second catalytic region deposited on the first catalytic region and covering the entire axial length L, comprising a selective catalytic reduction (SCR) catalyst; and wherein the loading of the second PGM component is greater than the loading of the first PGM component.

2. The method of claim 1 , wherein the first catalytic zone extends from the inlet end of the substrate over an axial length Li and the second catalytic zone extends from the outlet end of the substrate over an axial length L2, and wherein the ratio of Li to L2 is from 25:50 to 50:25, e.g., about 25:50, about 50:50 or about 50:25.

3. The method of claim 1 or 2, wherein the loading of the second PGM component on the support from the outlet end of the substrate is between 1.5 g / ft3to 20 g / ft3, preferably from 2 g / ft3to 19 g / ft3, more preferably 3 g / ft3, 5 g / ft3, 16.5 g / ft3or 18.5 g / ft3.

4. The method of any one of claims 1 to 3, wherein the loading of the first PGM component on the support from the inlet end of the substrate is between 1 .0 g / ft3to 1 .5 g / ft3.

5. The method of any one of claims 1 to 4, wherein the first PGM component and / or the second PGM component is selected from the group consisting of Pt, Rh and Pd or any combination thereof, preferably wherein both the first PGM component and second PGM component are Pt.

6. The method of any one of claims 1 to 5, wherein the SCR catalyst comprises a transition metal and a support in the form of an SCR zeolitic molecular sieve.

7. The method of claim 6, wherein the SCR zeolitic molecular sieve has a CHAorAEI framework or a blend of CHA and AEI frameworks.

8. The method of claim 6 or claim 7, wherein the SCR zeolitic molecular sieve has a silica-to-alumina ratio of 10 to 30, preferably 20 to 25.

9. The method of any one of claims 6 to 8, wherein the loading of the SCR zeolitic molecular sieve in the SCR catalyst is between 1 and 3 g / in3, preferably between 1 .8 to 2.5 g / in3.

10. The method of any one of claims 6 to 9, wherein the transition metal is Cu or Fe, preferably Cu.

11. The method of any one of claims 1 to 10, wherein the second catalytic region comprises a first SCR catalyst having a transition metal supported on a first SCR zeolitic molecular sieve and a second SCR catalyst having a transition metal supported on a second SCR zeolitic molecular sieve and preferably wherein the first SCR catalyst and the second SCR catalyst are the same or different.

12. The method of claim 11 , wherein the first SCR zeolitic molecular sieve is different to the second SCR zeolitic molecular sieve, preferably wherein the first SCR zeolitic molecular sieve has an AEI framework and the second SCR zeolitic molecular sieve has a CHA framework.

13. The method of any one of claims 1 to 12, wherein the first catalytic zone comprises a bare molecular sieve, preferably having a framework structure selected from AEI, BEA, CHA or FER or a combination or blend of one or more of these frameworks, and more preferably wherein the framework structure is BEA or FER, or a combination of both.

14. The method of any one of claims 1 to 13, wherein the second catalytic zone comprises a bare molecular sieve, preferably having a framework structure selected from AEI, BEA, CHA or FER or a combination or blend of one or more of these frameworks, and more preferably wherein the framework structure is BEA or FER, or a combination of both.

15. The method of any one of claims 1 to 14, wherein the exhaust gas comprises ammonia in an amount in a range of 1 to 500 ppm.

16. An emission treatment system for treating a flow of a combustion exhaust gas comprising the catalytic article as described in any one of claims 1 to 15, preferably further comprising a DOC, a DPF, at least one additional SCR, at least one additional ASC, or a combination thereof.

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