A selective catalytic reduction catalyst

Abstract

This invention relates to selective catalytic reduction (SCR) catalysts with improved tolerance towards sulfation. The invention further extends to the use of such catalysts in emission treatment systems. More specifically, the invention is related to a catalyst for treating an exhaust gas comprising NOx, said catalyst comprising a metal loaded zeolite and a silica-alumina or titania-alumina binder.

Description

[0001] P101896 1

[0002] A SELECTIVE CATALYTIC REDUCTION CATALYST

[0003] FIELD OF THE INVENTION

[0004] This invention relates to selective catalytic reduction (SCR) catalysts with improved tolerance towards sulfation. The invention further extends to the use of such catalysts in emission treatment systems.

[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 NOx into 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] 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 P101896 2 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.

[0010] There is a further problem in treating exhaust emissions. These often contain SOXand the SCR catalysts may undergo sulfation when sulfur oxides (such as SO2 and SO3) in the exhaust gas interact with active catalytic sites, especially in the presence of water. This reaction forms metal sulfates on the catalyst surface which inhibit the reduction of nitrogen oxides (NOX) by ammonia (NH3). It can also lower the catalyst's thermal stability and lead to increased SO3 formation, which can cause downstream corrosion.

[0011] It is not only SCR catalysts that suffer from sulfation. ASC catalysts can also suffer from sulfation since sulfur can react with catalytic metals or oxides, such as platinum or palladium forming sulfates. As a result, sulfation in ASC catalysts affects their ability to oxidize ammonia, leading to increased ammonia slip and reduced efficiency in converting ammonia into nitrogen and water.

[0012] An additional problem is that with washcoated catalytic formulations comprising alumina, the alumina can act as a sulfur reservoir during regeneration processes, with alumina sulfate forming on the alumina binder used in the catalytic formulation. This in turn negatively impacts metal loaded sights of zeolites, such as copper-zeolite, which in turn impact upon the activity of these sites. It is desirable to develop a catalyst that exhibits improved NOXconversion at low temperatures upon exposure to sulphur and after subsequent regeneration for facilitating sulphur removal.

[0013] The inventors have surprisingly found that the use of silica-alumina binders or titania-alumina binders can least partially, if not substantially, overcome the deleterious effects of sulfation on an SCR / ASC catalyst. P101896 3

[0014] DETAILED DESCRIPTION

[0015] According to the present invention there is provided a catalyst for treating an exhaust gas comprising one or more of SOX, NOX, and / or ammonia, said catalyst comprising (e.g. consisting essentially of or consisting of) a metal-loaded zeolite and a binder, wherein the binder comprises a silica-alumina binder or a titania-alumina binder. That is, the catalyst comprises a metal-loaded zeolite, and a silica- alumina and / or a titania-alumina binder. For example, the catalyst can comprise a metal-loaded zeolite, and either a silica-alumina or titania-alumina binder. In some preferred embodiments, the binder comprises (or consists essentially of, or consists of) a titania-alumina binder.

[0016] The metal-loaded zeolite and the silica alumina or titania alumina binder can be provided, in slurry form, on a substrate, as a washcoat, which is dried and / or calcined to affix the catalyst to the substrate. Washcoating may be carried out according to well known processes in the art.

[0017] The metal-loaded 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.

[0018] 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, LEV, LTA, KFI, MER, MON, NSI, OWE, PAU, PHI, RHO, RTH, SAT, SAV, SFW, SIV, THO, TSC, UEI, UFI, VNI, 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.

[0019] 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, P101896 4

[0020] AHT, BOF, BOZ, CGF, CGS, CHI, DAC, EUO, FER, HEU, I MF, ITH, ITR, JRY, JSR, JST, LAU, LOV, MEL, MFI,

[0021] 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, 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, MEL, MFI, and STT.

[0022] 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, M EI, 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.

[0023] In an embodiment of the invention, the metal-loaded zeolite has a framework type selected from AEI, AFX, CHA, DDR, ERI, ITE, KFI, LEV, SFW, BEA, MFI and FER, and mixtures and / or intergrowths thereof. Preferably, the zeolite has a framework type selected from CHA, AEI or AFX, or ERI, more preferably selected from CHA or AEI.

[0024] Small and medium pore zeolites, especially small pore zeolites, are preferred for use in SCR catalysts, since they may, for example, provide improved SCR performance and / or improved hydrocarbon tolerance.

[0025] In a preferred form of the invention, the zeolite of the invention is a small pore zeolite, e.g. CHA.

[0026] In a preferred form of the invention, the zeolite has a silica-to-alumina ratio of 10 to 30, preferably from 13 to 25 or from 12 to 20, e.g. from 13 to 18 or from 20 to 25.

[0027] The zeolite of the present invention is a metal loaded zeolite. Examples of metal-loaded zeolites include iron-, copper- and palladium-loaded zeolites. In a preferred embodiment, the metal loaded P101896 5 zeolite has a metal selected from the group comprising cobalt, copper, iron, cerium, manganese, nickel, palladium, platinum, ruthenium and rhenium. In a particularly preferred embodiment, the metal is selected from Cu, Fe, V, Ce and Mn. In a most preferred embodiment, the metal is copper or iron, preferably copper. An advantage of using copper loaded zeolites is that such formulations have excellent low temperature NOXreduction activity (e.g. it may be superior to the low temperature NOXreduction activity of an iron exchanged zeolite). In a metal-loaded zeolite, the loaded metal is a type of "extra-framework metal", that is, a metal that resides within the zeolite and / or on at least a portion of the zeolite surface and does not include atoms constituting the framework of the zeolite.

[0028] In an embodiment of the invention, it is preferred to have a Cu-loaded or Fe-loaded zeolite, more preferably a metal loaded zeolite which is Cu-CHA or a Cu-AEL

[0029] In an embodiment, the metal content of the metal-loaded zeolite is in the range of 1 to 10 wt%, preferably from 2 to 5wt% based on the weight of the zeolite.

[0030] The silica-alumina binder can have a silica to alumina weight ratio in a range of 1:99 to 50:50, preferably 2:98 to 45:55, suitably 2:98 to 40:60, more preferably 2:98 to 30:70. The use of a silica-alumina binder in these ranges can provide improved low temperature NOXconversion in NH3 SCR compared to catalysts comprising an alumina-only binder, in particular after sulfation and subsequent regeneration. In some embodiments, the silica to alumina weight ratio is at least 28:72, for example in a range of from 28:72 to 40:60, or 28:72 to 30:70. Silica-to-alumina ratios in these ranges can also provide improved low temperature N2O selectivity in NH3 SCR after sulfation and subsequent regeneration compared to catalysts comprising alumina-only or silica-only binders. In an embodiment of the invention the silica-alumina binder has a silica to alumina weight ratio of from about 1:99 to about 50:50, preferably from about 2:98 to about 45:55, e.g., from 3:97 to 40:60 or 4:96 to 35:65. In some embodiments, the silica-alumina binder has a silica to alumina weight ratio of from about 2:98 to about

[0031] 10:90 or from about 25:75 to about 35:65. The silica-alumina binder can have a silica to alumina P101896 6 weight ratio in a range comprising any combination of the aforementioned limits. The silica to alumina ratio can be determined by ICR

[0032] The titania-alumina binder can have a titania to alumina weight ratio in a range of 1:99 to 50:50, preferably 2:98 to 45:55, suitably 2:98 to 40:60, suitably 5:95 to 35:65, and preferably 10:90 to 30:70. The use of a titania-alumina binder in these ranges can provide improved low temperature NOXconversion in NH3 SCR compared to catalysts comprising alumina-only or silica-only binders. In some embodiments of the invention, the titania-alumina binder has a titania to alumina weight ratio of from about 2:98 to about 30:70, preferably from about 3:97 to about 20:80, preferably from about 5:95 to about 15:85, e.g. about 5:95, about 10:90, about 15:85, about 20:80 or about 30:70. The titania-alumina binder can have a titania to alumina weight ratio in a range comprising any combination of the aforementioned limits. The titania to alumina weight ratio can be determined by ICP.

[0033] In an embodiment of the invention, when a titania-alumina binder is used, titania is present in an amount from 0.1 to 2 wt%, based on the total weight of the catalyst, e.g. from 0.2 to 1.5 wt%, from 0.3 to 1.0wt%, from 0.18 to 0.9 wt%, e.g., about 0.5 wt%.

[0034] In an embodiment of the invention, the silica-alumina binder is present in an amount of 5 to 20 wt%, preferably 6 to 15 wt.%, e.g. from 8-15 wt% based on the total weight of the metal-loaded zeolite. In some embodiments, the silica-alumina binder is present in an amount of 5-15 wt%, 6-12wt% or 7- 10wt% based on the total weight of the metal-loaded zeolite. In a particularly preferred embodiment, the silica-alumina binder is present in an amount of 5-12 wt%, more preferably from 6 to 10 wt% based on the total weight of the metal-loaded zeolite.

[0035] In an embodiment of the invention, the titania-alumina binder is present in an amount of 5 to 20 wt%, preferably 6 to 15 wt.%, e.g. from 8-15 wt% based on the total weight of the metal-loaded zeolite. In some embodiments, the titania-alumina binder is present in an amount of 5-15 wt%, 6-12wt% or 7- 10wt% based on the total weight of the metal-loaded zeolite. In a particularly preferred embodiment, the titania-alumina binder is present in an amount of 5-12 wt%, more preferably from 6 to 10 wt% based on the total weight of the metal-loaded zeolite. If the amount of binder is too low (e.g. less than P101896 7

[0036] 5 wt.%), adhesion to a substrate may be impaired. If the amount of binder is too high (e.g. more than 20 wt.%), backpressure can be impacted.

[0037] The binder (i.e. the silica-alumina binder or the titania-alumina binder) can comprise particles. Preferably, the binder does not comprise fibres, e.g. silica-alumina fibres or titania-alumina fibres.

[0038] The silica-alumina binder can have a mean particle size (D5o) in a range of from 4 pm to 40 pm. In an embodiment of the invention, the silica-alumina binder has a mean particle size in the range of 20 to 40 pm, preferably from 25 to 30 pm. In an alternative embodiment, the silica-alumina binder has a mean particle size in the range of from 4 to 8 pm, preferably from 5 to 6 pm. Mean particle size (D5o) can be measured by laser diffraction particle size analysis, e.g. using a Mastersizer™ 3000 available from Malvern Panalytical Ltd, UK, which applies a mathematical Mie theory model to determine a particle size distribution. The laser diffraction system works by determining diameters for the particles based on a spherical approximation.

[0039] Without wishing to be bound by theory it is believed that the use of a silica-alumina or titania-alumina binder provides enhanced structural stability such that on sulfation at high temperatures there is less of a loss of surface area and porosity which would otherwise occur with alumina, which has more basic sites and is more reactive to SOX.

[0040] In an alternate embodiment of the invention, the catalyst may include an additional binder, e.g. metal oxides such as alumina, titania, silica, zirconia, ceria. In a preferred embodiment, the catalyst further comprises an alumina binder, e.g., a boehmite binder. In some embodiments, the catalyst includes an additional binder in an amount of from 3-15 wt%, based on the total weight of the metal-loaded zeolite. For example, the catalyst includes an additional binder in an amount from 4-10 wt% or 5- 8 wt% based on the total weight of the metal-loaded zeolite. In a particularly preferred embodiment, the catalyst includes an alumina binder in an amount from 3 to 15 wt%, e.g., from 4 to 8 wt% based on the total weight of the metal-loaded zeolite. P101896 8

[0041] In one embodiment, the invention relates to a catalyst (e.g., a catalyst for treating an exhaust gas comprising NOx) comprising, consisting essentially of or consisting of:

[0042] 1. a metal-loaded zeolite, as described herein,

[0043] 2. a silica-alumina or titania-alumina binder, as described herein, and optionally

[0044] 3. an additional binder (e.g., an additional alumina binder), as described herein.

[0045] According to a second aspect of the invention, there is provided a method of improving the durability of an SCR catalyst, said method comprising providing a catalyst as hereinbefore described.

[0046] In a third aspect of the invention, there is provided a method of improving NOXconversion activity of an SCR catalyst (e.g., by at least 10% compared to the activity of an SCR catalyst with no silica-alumina or titania-alumina binder) after aging with sulfur.

[0047] According to a fourth aspect of the invention there is provided a method for improving the durability of an SCR catalyst tolerance to sulfur, said method comprising providing a catalyst as hereinbefore described.

[0048] EXAMPLES

[0049] Example 1 - eneral preparation method

[0050] The Cu catalysts were prepared using the following methods:

[0051] • Incipient wetness impregnation (IWI) with different salts. A metal salt solution using the metal salt (e.g. copper acetate) in H2O was prepared and then added dropwise to the zeolite sample and homogenously mixed until a wet sand appearance is observed.

[0052] • Washcoat method (WC) for preparation of zeolite and binders. Water and 30% solids

[0053] (zeolite+binder) were mixed and stirred for 2 hours. The binder was 12.5% of total weight of the zeolite. P101896 9

[0054] After preparation the samples were dried for 2 hours at 1059C in static oven. Once the powders have been dried, they are activated in an oven with ramp of 109C / min up to 5009C for 2 hours in air.

[0055] The catalysts prepared are as follows:

[0056] Catalyst A is where the silica-alumina binder has a SiO2:AkO3 weight ratio of 2:98

[0057] Catalyst B is where the silica-alumina binder has a SiO2:AkO3 weight ratio of 5:95

[0058] Catalyst C is where the silica-alumina binder has a SiO2:Al2O3 weight ratio of 30:70, particle size 5.4 pm

[0059] Catalyst D is where the silica-alumina binder is SiO2:Al2O3 weight ratio of 28:72, particle size 5.7 pm

[0060] Catalyst E is where the silica-alumina binder is SiO2:Al2O3 weight ratio of 27:73, particle size 27.3 pm

[0061] Catalyst F is where the binder is pure alumina

[0062] Catalyst G is where the binder is 40% SiO2 in water

[0063] Catalyst H is where the binder is powder SiO2

[0064] Catalyst I is where the titania-alumina binder is TiO2:Al2O3 weight ratio of 10:90

[0065] Catalyst J is where the titania-alumina binder is TiC^ALOs weight ratio of 20:80

[0066] Catalyst K is where the titania-alumina binder is TiC^ALOs weight ratio of 30:70

[0067] Example 2 - SCAT data generation (powder)

[0068] The powder catalysts were pelletised and tested fresh or aged using transient testing protocol (SV=60K / h) the samples were heated to 150°C and exposed to 500 ppm NOx, 550 ppm NH3, 10 %C>2, 10 % H2O, balance N2. The temperature was ramped at 5°C / min to 550°C.

[0069] Short sulfation (SO2 and SO3)

[0070] Catalysts were Sulfur aged under several conditions in 6-way ager (flow through) (6WA) explained below: 1 g catalyst was exposed to 2L / min air, 10 ppm SO2 and 10% H2O (61 mgS / gcat) at constant P101896 10 temperature of 350°C for 72h (equal to 7gS / L). The feed consists of SO2 but Pt / alumina is used as top layer to convert SO2 to SO3 ("'60% Conversion).

[0071] Regeneration procedure

[0072] After sulfation the catalysts were exposed to thermal and reactive regeneration to facilitate S removal using SCAT rig after the transient test the catalyst was kept at 550°C in 10 % O2 for 30 min, balance N2. After the regeneration the catalyst was then brought back to 150°C and tested again with the protocol described above.

[0073] Results

[0074] Table 1 - NOx conversion of powder testing P101896 11

[0075] Table 2 - N2O selectivity of powder testing

[0076] For the fresh activity, catalysts A-E (i.e. silica-alumina binder) and catalysts l-K (titania-alumina binder) exhibited superior NOXconversion at 200°C compared to comparative catalysts F (alumina binder), G (silica binder) and H (silica binder).

[0077] After sulfation, the low temperature NOXconversion (at 200 °C) decreased for all catalysts. Catalysts A- D, F-H and K exhibited similar NOXconversion at 200 °C. Catalyst E showed the highest activity compared to all other formulations. Catalysts I and J (titania-alumina binder) exhibited significantly superior NOXconversion after sulfation.

[0078] After regeneration, the activity of all catalysts recovered to some extent. Catalyst F (alumina binder) exhibited the lowest NOXconversion at 200 °C after regeneration. Each of catalysts A-E (silica-alumina binder) and catalysts l-K (titania-alumina binder) demonstrated comparable or improved NOXconversion at 200 °C after regeneration compared to catalysts G and H (silica binder). Catalysts l-K (titania-alumina binder) exhibited significantly higher NOXconversion at 200 °C compared with the P101896 12 comparative catalysts F-H. Additionally, the NOXconversion activity for catalyst F (alumina binder) showed the lowest percentage recovery of the catalysts tested. In contrast, catalysts I and J demonstrated the highest recovery for NOXconversion at 200 °C, comparing fresh catalyst and after regeneration.

[0079] Example 3 - SCAT data generation (coated)

[0080] 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 comprised copper ion-exchanged aluminosilicate zeolite CHA and a silica-alumina binder washcoat. This washcoat was coated over the substrate so that the substrate was completely covered and was dried and calcined.

[0081] 1x3" cores were cut from the prepared catalyst of Catalyst A-E and comparative catalysts (pure silica and pure alumina) and tested fresh for NOx conversion and N2 selectivity using a series of steady state points at increasing temperatures in 750ppm NOx, 500ppm NH3, at 60k SV.

[0082] Sulfation

[0083] The catalysts were sulfur aged for 126 min at 300 °C with ~2g S / Lcat 20ppm SO2 using a SCAT rig. Following this, the catalysts were testing using the SCAT testing described above.

[0084] Regeneration

[0085] After sulfation the catalysts were exposed to thermal regeneration (450°C for 30 min) to facilitate S removal using a SCAT rig. After the regeneration the catalyst was then tested again with the SCAT testing described above. P101896 13

[0086] Results

[0087] Table 3 - NOx conversion and N2O make for coated catalysts

[0088] Table 3 shows that, when fresh, all catalysts demonstrated comparable low temperature NO;conversion; and that catalysts A, B, D and E exhibited lower N2O selectivity at 200°C compared to comparative catalysts F and G. Upon sulfation, catalysts A, B, D and E exhibited comparable or improved NOXconversion at 200°C. After regeneration, catalysts A, D and E exhibited improved N2O selectivity compared with comparative catalysts F and G. Overall, catalysts comprising silica-alumina or titania-alumina binders exhibited better sulfur tolerance than the comparative examples comprising alumina-only or silica-only binders. It is noted that Catalysts B*, D* and E* were prepared by using the silica-alumina binder blended with a pure alumina binder in a 1:1 mixture to make a total binder content of 12.5wt%.

Claims

P101896 14Claims:

1. A catalyst for treating an exhaust gas comprising NOx, said catalyst comprising a metal loaded zeolite and a binder, wherein the binder comprises a silica-alumina binder or a titania-alumina binder.

2. The catalyst of claim 1, wherein the silica-alumina binder has a silica to alumina ratio of from about 1:99 to about 50:50, preferably from about 2:98 to about 45:55, e.g., from 3:97 to 40:60 or 4:96 to 35:65.

3. The catalyst of claim 2, wherein the silica-alumina binder has a silica to alumina ratio of from about 2:98 to about 10:90 or from about 25:75 to about 35:65.

4. The catalyst of any one of claims 2-3, wherein the silica-alumina binder has a mean particle size in the range of 20 to 40 pm, preferably from 25 to 30 pm.

5. The catalyst of any one of claims 2-3, wherein the silica-alumina binder has a mean particle size in the range of from 4 to 8 pm, preferably from 5 to 6 pm.

6. The catalyst of claim 1, wherein the binder comprises the titania-alumina binder.

7. The catalyst of claim 1 or 6, wherein the titania-alumina binder has a titania to alumina ratio of from about 2:98 to about 20:80, preferably from about 5:95 to about 15:85, e.g. about 5:95, about 10:90 or about 15:85.

8. The catalyst of any preceding claim, wherein the catalyst further comprises an alumina binder.

9. The catalyst of any preceding claim, wherein the silica-alumina binder or titania-alumina binder is in an amount of 5-15 wt%, preferably from 6-12 wt% based on the total weight of the metal-loaded zeolite.

10. The catalyst of any preceding claim, wherein the metal loaded zeolite has a metal selected from the group consisting of Cu, Fe, V, Ce and Mn.

11. The catalyst of any preceding claim, wherein the metal loaded zeolite is a Cu zeolite.P101896 1512. The catalyst of any preceding claim, wherein the zeolite is a zeolite with a framework selected from AEI, AFX, CHA, DDR, ERI, ITE, KFI, LEV, SFW, BEA, MFI and FER, and mixtures and / or intergrowths thereof, preferably wherein the zeolite is a zeolite with a framework selected from AEI and CHA.

13. The catalyst of any preceding claim, wherein the zeolite has a silica-to-alumina ratio of 10 to 30, preferably from 13 to 25.

14. A method of improving the durability of an SCR catalyst, said method comprising providing a catalyst as claimed in any preceding claim.

15. A method of claim 14, wherein the NOx conversion activity of SCR is improved by at least 10% after aging with sulfur.

16. A method of claim 14 or 15, wherein the method of improving the durability of an SCR catalyst is a method of improving the tolerance of the SCR catalyst to sulfur.

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