Improved Sulfur Resistance in Cu-SCR Catalysts

Abstract

The present invention relates to a catalyst composition. More specifically, the present invention relates to a catalyst composition comprising a low-SAR zeolite, copper in an amount of at least 2% by weight, and a rare earth element. The present invention also relates to a method for producing the catalyst composition. The present invention further relates to a method for treating exhaust gas, which comprises contacting the exhaust gas with a catalyst article comprising the catalyst composition and a catalyst article comprising the catalyst composition.

Description

Technical Field

[0001] The present invention relates to a catalyst composition. More specifically, the present invention relates to a catalyst composition comprising a low-SAR zeolite, copper in an amount of at least 2% by weight, and a rare earth element. The present invention also relates to a method for manufacturing a catalyst composition. The present invention further relates to a method for treating exhaust gas, which includes contacting the exhaust gas with a catalyst article containing the catalyst composition and a catalyst article containing the catalyst composition.

Background Art

[0002] Selective catalytic reduction (SCR) is the most effective technology for NOx reduction in the aftertreatment of lean-burn engine exhaust. This SCR technology refers to a technology in which a reducing agent or reducing substance in exhaust gas selectively reduces NOx to N2 under the action of a catalyst in order to avoid non-selective oxidation reactions of the reducing agent. Copper zeolite has been commercialized as an SCR catalyst because of its important advantages such as excellent catalytic performance and hydrothermal stability.

[0003] However, many SCR catalysts may be neutralized, inactivated, or experience a reduction in effectiveness when exposed to sulfur compounds over time. Deactivation by sulfur over time is a cumulative process that limits the effective life of the catalyst components or requires periodic removal of sulfur compounds.

[0004] The presence of sulfur reduces the efficiency of various components within the exhaust aftertreatment system. Currently known sulfur removal processes require exposing the SCR catalyst to very high temperatures, which can significantly affect fuel consumption and engine performance and may expose other exhaust aftertreatment system components to increased aging. Therefore, further improvements in compositions and methods for SCR aftertreatment systems are still needed.

[0005] Previous studies have also found that the stability of the catalyst is adversely affected by the use of zeolites with low SAR and / or the incorporation of copper into the catalyst.

[0006] The inventors have surprisingly found that the combination of a low SAR zeolite and a high copper loading results in significantly improved sulfur resistance compared to previous formulations without affecting the stability of the catalyst. The addition of rare earth elements can further improve sulfur resistance, hydrothermal aging, and the stability of the catalyst. SUMMARY OF THE INVENTION

[0007] One aspect of the present disclosure is a catalyst composition comprising a) a low SAR zeolite, and b) copper in an amount of at least 2 wt%, and c) a rare earth element,

[0008] Another aspect of the present disclosure is a method for preparing a catalyst composition (i.e., the catalyst composition described herein), the method comprising (i) incorporating copper into the zeolite by ion exchange to prepare a copper-substituted zeolite, and (ii) incorporating a rare earth element into the copper-substituted zeolite.

[0009] Another aspect of the present disclosure is a catalyst article for treating exhaust gas, the catalyst article comprising the catalyst composition described herein.

[0010] Another aspect of the present disclosure is an exhaust gas system comprising the catalyst article described herein.

[0011] Another aspect of the present disclosure is a method for treating exhaust gas, the method comprising contacting the exhaust gas with the catalyst article described herein. BRIEF DESCRIPTION OF THE DRAWINGS

[0012]

Figure 1

Figure 2

DETAILED DESCRIPTION OF THE INVENTION

[0013] A first aspect of the present invention is a catalyst composition comprising: a) a low SAR zeolite; and b) copper in an amount of at least 2% by weight; and c) a rare earth element, and is directed to a catalyst composition.

[0014] Here, the present disclosure is further described. In the following sections, different aspects / embodiments of the present disclosure are defined in more detail. Each aspect / embodiment so defined may be combined with any other aspect / embodiment or aspects / embodiments unless otherwise explicitly indicated. In particular, any feature shown to be preferred or advantageous may be combined with any other feature or features shown to be preferred or advantageous.

[0015] Zeolite The catalyst composition of the present invention comprises at least one zeolite. Zeolite is a structure formed from alumina and silica, and SAR determines the reaction sites within the zeolite structure. Zeolites useful in the present invention may include small pore zeolites (e.g., zeolites having a maximum ring size of 8 tetrahedral atoms), medium pore zeolites (e.g., zeolites having a maximum ring size of 10 tetrahedral atoms), large pore zeolites (e.g., zeolites having a maximum ring size of 12 tetrahedral atoms), or combinations of two or more thereof.

[0016] When the catalyst composition includes small-pore zeolite, the small-pore zeolite may have a framework structure represented by a Framework Type Code (FTC) selected from the group consisting of, for example, 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, combination, and / or intergrowth of two or more thereof. In some embodiments, the small-pore zeolite has a framework structure selected from the group consisting of, for example, consisting of CHA, LEV, AEI, AFX, ERI, LTA, SFW, KFI, DDR, and ITE. In some embodiments, the small-pore zeolite has a framework structure selected from the group consisting of, for example, consisting of CHA and AEI. The small-pore zeolite may have a CHA framework structure.

[0017] When the catalyst composition contains mesoporous zeolite, the mesoporous zeolite can have a framework structure represented by a framework type code (FTC) selected from the group consisting of, for example, consisting of AEL, AFO, AHT, BOF, BOZ, CGF, CGS, CHI, DAC, EUO, FER, HEU, IMF, ITH, ITR, JRY, JSR, JST, LAU, LOV, MEL, MFI, MFS, MRE, MTT, MVY, MWW, NAB, NAT, NES, OBW, PAR, PCR, PON, PUN, RRO, RSN, SFF, SFG, STF, STI, STT, STW, SVR, SZR, TER, TON, TUN, UOS, VSV, WEI, and WEN, or a mixture and / or intergrowth of two or more thereof. In some embodiments, the mesoporous zeolite has a framework structure selected from the group consisting of, for example, consisting of FER, MEL, MFI, and STT. In some embodiments, the mesoporous zeolite has a framework structure selected from the group consisting of, for example, consisting of FER and MFI, particularly MFI. When the mesoporous zeolite has a FER or MFI framework, the zeolite can be ferrierite, silicalite, or ZSM-5.

[0018] When the catalyst composition contains macroporous zeolite, the macroporous zeolite may have a framework structure represented by a framework type code (FTC) selected from the group consisting of (e.g., 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 of two or more thereof and / or an intergrowth. In some embodiments, the macroporous zeolite has a framework structure selected from the group consisting of (e.g., consisting of) AFI, BEA, MAZ, MOR, and OFF. In some embodiments, the macroporous zeolite has a framework structure selected from the group consisting of (e.g., consisting of) BEA, MOR, and FAU. When the macroporous zeolite has a framework structure of FTC, BEA, FAU, or MOR, the zeolite can be beta zeolite, faujasite, zeolite Y, zeolite X, or mordenite.

[0019] The catalyst composition of the present invention preferably comprises a zeolite having a low SAR. As used herein, low SAR means a zeolite having a silica to alumina molar ratio (SAR) of 10 to 30, more preferably from about 10 to about 25, such as about 10 to about 22, about 11 to 21, about 11 to about 20, about 11 to about 18, about 12 to about 15, about 10 to about 13, about 10 to about 14, and about 12 to about 14. The silica to alumina ratio of the zeolite can be determined by conventional analysis. This ratio means that it represents as close as possible the ratio in the rigid atomic framework of the zeolite crystal and excludes silicon or aluminum in the channels, in the binder, or in the cationic or other forms. Since it may be difficult to directly measure the silica to alumina ratio of the zeolite after being combined with a binder material, especially an alumina binder, these silica to alumina ratios are represented by those of the zeolite itself, i.e., the SAR before combining the zeolite with other catalyst components.

[0020] Surprisingly, it has been found that zeolites with low SAR can be used without affecting the stability of the resulting catalyst. The combination of zeolites with low SAR and the claimed copper loading and rare earth element incorporation has been found to have improved sulfur resistance and hydrothermal aging.

[0021] Copper The catalyst composition of the present invention also contains copper. In some embodiments, copper is incorporated into the zeolite, for example, by ion exchange. Copper can be present in the catalyst composition in an amount of at least 2 wt%, at least 2.5 wt%, at least 3 wt%, at least 3.5 wt%, at least 4 wt%, or at least 4.5 wt%. Copper can be present in the catalyst composition in an amount of about 1 to about 6 wt%, about 2 to about 5.5 wt%, for example, about 3 to about 5.4 wt%, about 4 to about 5.3 wt%, about 4.5 to about 5.2 wt%, about 4.6 to about 5.1 wt%, or about 4.7 to about 5 wt%. In some embodiments, copper is present in the catalyst composition in an amount of about 4 wt%, about 4.5 wt%, about 4.75 wt%, about 5 wt%, about 5.25 wt%, or about 5.5 wt%. References to wt% are based on the weight of the zeolite.

[0022] The incorporation of copper into the catalyst composition has surprisingly been found to improve the performance of the catalyst without reducing the stability of the catalyst. The use of copper in combination with rare earth elements has been found to improve sulfur resistance and hydrothermal aging while also improving the performance of the catalyst without reducing the stability of the catalyst, even when low-SAR zeolites are used.

[0023] Rare earth elements The catalyst composition of the present invention contains one or more rare earth elements. Preferably, the one or more rare earth elements are selected from the group consisting of (e.g., consisting of) La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y, and Sc, and combinations of two or more thereof. In certain embodiments, the rare earth elements include Ce, Y, Zr, Mn, Fe, La, Nd, and Nb. In some aspects, the rare earth element is Ce.

[0024] The content of the rare earth element in the catalyst composition, based on the weight of the catalyst composition, is preferably about 2 to about 5 wt%, including about 2.5 wt% to about 4.8 wt%, about 3 wt% to about 4.5 wt%, about 3.5 to 4.4 wt%, about 2.75 wt% to about 4.4 wt%. References to wt% are based on the weight of the zeolite.

[0025] The incorporation of rare earth elements has been found to improve sulfur resistance and hydrothermal aging, even when the SAR of the zeolite is low.

[0026] Preparation method A further aspect of the present invention is a method of preparing a catalyst composition (e.g., a catalyst composition as described herein), the method comprising: (i) incorporating copper into a zeolite to prepare a copper-substituted zeolite; and (ii) incorporating a rare earth element into the copper-substituted zeolite.

[0027] Copper can be incorporated into the zeolite by methods known in the art. For example, ion exchange, spray drying, and low-temperature one-pot. Ion exchange can be carried out by blending the zeolite into a solution containing a soluble precursor of copper (e.g., copper acetate or copper carbonate). The pH of the solution can be adjusted to induce precipitation of copper ions on or within the zeolite structure. For example, the zeolite is immersed in a solution containing a soluble precursor of copper (e.g., copper acetate or copper carbonate) for a time sufficient to incorporate copper ions into the molecular sieve structure by ion exchange. Un-exchanged copper ions can precipitate. Depending on the application, some of the un-exchanged copper ions can remain in the zeolite material as free metal. The copper-substituted zeolite can then be washed, dried, and calcined. In a preferred alternative embodiment, the copper-substituted zeolite is used directly in a second step to incorporate the rare earth element.

[0028] Generally, the ion exchange of copper into or onto zeolite can be carried out at room temperature or at a temperature up to about 100 °C, up to about 90 °C, up to about 80 °C, or up to about 70 °C. For example, the ion exchange can be carried out at a temperature of 15 - 90 °C, such as 18 - 80 °C, 20 - 70 °C, 21 - 50 °C, 22 - 40 °C, or 25 - 30 °C. The ion exchange can be carried out over a period of about 1 - 24 hours, such as 2 - 22 hours, 3 - 20 hours, 4 - 15 hours, or 5 - 10 hours. The ion exchange can be carried out at a pH less than 9, such as less than 8, less than 7, less than 6, less than 5, less than 4, or less than 3. In a preferred example, the pH of the ion exchange is 2 - 5 or 3 - 4. The obtained copper-substituted zeolite material is preferably directly used in a second step for incorporating rare earth elements. In an alternative embodiment, the obtained copper-substituted zeolite is dried at about 100 - 120 °C. In some embodiments, the copper-substituted zeolite material is dried for 5 - 20 hours, such as 8 - 18 hours, such as 10 - 15 hours. Then, the copper-substituted zeolite material can be calcined at a temperature of at least about 550 °C.

[0029] Copper can exist as an extra-framework metal that is present in at least a part of the interior and / or the surface of the zeolite.

[0030] Copper can exist as a counter-ion at the ion exchange sites of the framework structure.

[0031] Copper can be incorporated into the zeolite separately from the incorporation of rare earth elements. The method for preparing the catalyst composition can include incorporating rare earth elements and incorporating copper in a separate process either before or after the incorporation of rare earth elements. In an alternative embodiment, copper can be incorporated into the zeolite simultaneously with rare earth elements.

[0032] Rare earth elements (e.g., the rare earth elements described herein) can be incorporated into zeolites (e.g., the copper-substituted zeolites described herein) such that they are present as counterions at the ion exchange sites of the framework structure. The rare earth elements can be present as extra-framework elements, which are present in at least a portion of the interior of the zeolite and / or on the surface of the zeolite, do not contain aluminum, and do not contain atoms that make up the framework of the molecular sieve. The rare earth elements can be added to zeolites (e.g., the copper-substituted zeolites described herein) via any known technique such as ion exchange, spray drying, and cold one-pot. Preferably, the rare earth elements are incorporated by ion exchange.

[0033] A method of preparing a catalyst composition can include incorporating a rare earth element into a zeolite (e.g., the copper-substituted zeolites described herein) by ion exchange. Ion exchange can be performed by blending a zeolite (e.g., the copper-substituted zeolites described herein) into a solution containing a soluble precursor of the rare earth element. The pH of the solution can be adjusted to induce precipitation of catalytically active rare earth element cations onto or within the zeolite structure. For example, a zeolite (e.g., the copper-substituted zeolites described herein) is immersed in a solution containing a soluble precursor of the rare earth element for a time sufficient to allow incorporation of the active rare earth cations into the molecular sieve structure by ion exchange. Un-exchanged rare earth ions can precipitate. Depending on the application, some of the un-exchanged ions can remain in the zeolite material as free metal. The rare earth-substituted zeolite can then be washed, dried, and calcined. In a preferred alternative embodiment, the resulting rare earth-substituted zeolite is used directly to form a washcoat.

[0034] In certain embodiments, the zeolite (e.g., the copper-substituted zeolite described herein) is immersed in a solution containing cerium acetate or cerium carbonate for a time sufficient to allow incorporation of active cerium cations into the molecular sieve structure by ion exchange. Un-exchanged cerium ions may precipitate. Depending on the application, some of the un-exchanged ions may remain in the zeolite material as free cerium. The cerium-substituted zeolite can then be washed, dried, and calcined. In a preferred alternative embodiment, the resulting cerium-substituted zeolite is used directly to form a washcoat.

[0035] Generally, the incorporation of rare earth elements into or onto a zeolite to form a catalyst composition can be carried out at room temperature or at a temperature up to about 80 °C. For example, the incorporation of rare earth elements can be carried out at a temperature of 15 - 80 °C, such as 18 - 70 °C, 20 - 60 °C, 21 - 50 °C, 22 - 40 °C, or 25 - 30 °C. In a preferred embodiment, the incorporation of rare earth elements is carried out at room temperature (e.g., at a temperature of 15 - 30 °C, 16 - 28 °C, 17 - 25 °C, 18 - 23 °C, or 19 - 21 °C). The ion exchange can be carried out over a period of about 1 - 24 hours, such as 2 - 22 hours, 3 - 20 hours, 4 - 15 hours, or 5 - 10 hours. The ion exchange can be carried out at a pH of less than 9, such as less than 8, less than 7, less than 6, less than 5, less than 4, or less than 3. In a preferred example, the pH of the ion exchange is 2 - 5 or 3 - 4.

[0036] In an alternative embodiment, the zeolite is mixed with water and a copper source (e.g., copper carbonate) is added. The resulting mixture is left at room temperature (e.g., 15 - 25 °C, 16 - 24 °C, 17 - 23 °C, 18 - 22 °C or 19 - 21 °C) for a suitable period (e.g., 1 - 24 hours, such as 2 - 22 hours, 3 - 20 hours, 4 - 15 hours or 5 - 10 hours). A rare earth element can then be added (e.g., in the form of a rare earth salt such as cerium acetate or cerium carbonate) to form the catalyst composition.

[0037] Next, the catalyst composition described herein can be incorporated into a washcoat. In addition to the catalyst composition, the washcoat composition can further include (i) one or more binders selected from the group consisting of (e.g., consisting of) alumina, silica, (non-zeolite) silica-alumina, natural clay, TiO2, ZrO2, and SnO2, and / or (ii) one or more rheology modifiers (e.g., rheology modifiers such as Natrosol, TEAOH (tetraethylammonium hydroxide), Dispex, or ammonia).

[0038] The washcoat can be applied to a substrate such as a metal or ceramic flow-through monolith substrate, or a filtration substrate such as a wall-flow type filter or a sintered metal or partial filter. The resulting coated substrate is preferably dried and fired. Preferably, the coated substrate is dried at about 100 - 120 °C. Preferably, the coated substrate is dried for 15 minutes to 2 hours, e.g., 20 minutes to 1 hour. The dried coated substrate can then be fired at a temperature of 400 - 800 °C, preferably 450 - 750 °C, e.g., about 500 °C.

[0039] Catalyst article In a further aspect of the invention, there is provided herein a catalyst article for exhaust gas, comprising the catalyst composition described herein or a catalyst composition obtained by the method described herein.

[0040] The catalyst composition (e.g., the catalyst composition described herein) can be in the form of a washcoat suitable for coating a substrate such as a washcoat, preferably a metal or ceramic flow-through monolith substrate, or a filtration substrate such as a wall-flow type filter or a sintered metal or partial filter. Thus, another aspect of the present invention is a washcoat comprising the catalyst composition described herein. In addition to the catalyst composition, the washcoat composition can further comprise one or more binders selected from the group consisting of (e.g., consisting of) alumina, silica, (non-zeolite) silica-alumina, natural clay, TiO2, ZrO2, and SnO2. A further aspect of the present invention is a catalyst article comprising a substrate and the catalyst composition described herein that can be applied as a washcoat.

[0041] A preferred substrate for use is a monolith having a so-called honeycomb shape that includes a plurality of adjacent parallel channels, each channel typically having a square cross-sectional area. The honeycomb shape provides a large catalyst surface with a minimal overall size and pressure drop. The catalyst composition can be deposited on a flow-through monolith substrate (e.g., a honeycomb monolith catalyst support structure having many small, axially extending parallel channels through the entire part) or a filter monolith substrate such as a wall-flow type filter. In another embodiment, the catalyst composition is formed into an extruded catalyst. Preferably, the catalyst composition is coated on the substrate in an amount sufficient to reduce NOx contained in the exhaust gas stream flowing through the substrate. In certain embodiments, at least a portion of the substrate can also contain a platinum group metal such as platinum (Pt) for oxidizing ammonia in the exhaust gas stream or for performing other functions such as the conversion of CO to CO2.

[0042] The catalyst article described herein preferably comprises a honeycomb monolith body comprising the catalyst composition (e.g., the catalyst composition described herein), preferably as a washcoat layer thereon.

[0043] Exhaust Gas System and Method According to a further aspect, an exhaust gas system is provided that includes a catalyst article as described herein and, optionally, an internal combustion engine. The internal combustion engine can be a diesel engine, a lean burn gasoline engine, or an engine powered by liquefied petroleum gas or natural gas. Preferably, the internal combustion engine is a diesel engine. The catalyst article can be disposed downstream of the engine to treat exhaust gas discharged from the engine.

[0044] According to yet a further aspect of the present invention, a method for treating exhaust gas is provided that includes contacting the exhaust gas with a catalyst article as described herein.

[0045] Although the preferred embodiments of the present invention have been described in detail herein, it will be understood by those skilled in the art that modifications can be made without departing from the scope of the present invention or the scope of the appended claims.

[0046] Here, the present invention will be further described in connection with the following non-limiting examples and figures.

Example

[0047] General Method Zeolite, ZEO (CHA zeolite having a SAR of 14), or ZEO2 (CHA zeolite having a SAR of 23) was mixed with water, and copper acetate was added to the reaction mixture. The reaction mixture was heated at 70 °C for 4 hours. The reaction mixture was cooled, then water-dispersible boehmite alumina was added, followed by cerium acetate, and then natrosol was added, and the pH was adjusted to > 3.8 with TEAOH.

[0048] Example 1 - Varying Cu and Rare Earth Loadings The Cu-zeolite catalyst was prepared by impregnating zeolite powder with a copper(II) acetate solution to achieve a Cu loading of 2 wt% to 5.5 wt%. Ce was incorporated into the Cu-zeolite catalyst to give 25 g / ft 3 ~200 g / ft3 The achieved loading amount fluctuates. The total washcoat loading amount varied from 2.4 to 3 g / in 3 and was varied up to.

[0049] The following catalysts were prepared.

[0050]

Table 1

[0051] SCAT test The Scat test was evaluated at four temperature points (175, 200, 250, and 600 °C) with a temperature gradient of 600 °C between each stage. During the test, 500 ppm of NOx was evaluated at 1.5 ALPHA. The test was first conducted either for a set amount of time or until a 20 ppm slip of ammonia occurred. The NOx conversion rate, N2O selectivity, and NH3 storage rate were evaluated at the four temperature points. The results are shown in Figure 1.

[0052] Figure 1 shows the effects of different ceria loadings and different copper loadings for fresh and aged (50 hours at 650 °C). The optimal results for the performance delta were for Cu(4.75%)ZEO Ce(188 g / ft 3 ). Cu(4 wt%)ZEO is a reference example of ZEO having a Cu loading of 4 wt%. ZEO is a CHA zeolite framework having a SAR of 14.

[0053] Example 2 - Varying sulfur loading: The sulfur loading / doping was carried out in a synthetic gas rig, and a fully coated SCR ceramic monolith catalyst sample (1×3 inches) was subjected to a specific cycle of SO2 aging gas mixture (Table 1) a predetermined number of times. A Pt-based DOC (1×1 inch) was upstream of the SCR catalyst. Each sulfation "SOx cycle" consisted of a temperature ramp to 525°C, a cooling to 200°C, and then three cycles at 200 - 350°C. SO2 injection was performed only during the cycle steps in order to target a sulfur exposure of approximately 0.45 g / L per cycle. Before subjecting each catalyst sample to a predetermined number of "SOx cycles", the actual sulfur exposure in each "SOx cycle" was calculated by using a blank core as a sulfur inlet check against an aging gas mixture with a target of 7.5 ppm SO2.

[0054]

Table 2

[0055] SCR Performance Evaluation after Sulfation Test After a predetermined number of "SOx cycles" with a known sulfur exposure / loading via an SO2 inlet check, the SCR performance evaluation test was carried out on the SCAT using only the "loaded" SCR samples in question with an SCR evaluation gas mixture (Table 2). The SCR performance test consisted of a temperature ramp to the set temperatures (450, 500, 530°C), a cooling and hold at 200°C after each ramp, and an evaluation in the SCR evaluation gas mixture at 200°C held until 20 ppm of NH3 slip occurred. The results are shown in Figure 2. ZEO2 is a CHA zeolite with a SAR of 23.

[0056]

Table 3

[0057] Figure 2 shows Cu SCR sulfur aging using various different catalyst compositions. This figure shows that the addition of rare earth elements improves sulfur resistance and hydrothermal aging.

Claims

1. A catalyst composition comprising: a) a low SAR zeolite; b) copper in an amount of at least 2% by weight; c) a rare earth element.

2. The catalyst composition according to claim 1, wherein the zeolite has a SAR of 10 to 25, preferably 11 to 21.

3. The catalyst composition according to claim 1 or 2, wherein the copper is in an amount of at least 2.5% by weight, preferably at least 3% by weight, more preferably at least 4% by weight.

4. The rare earth element is selected from the group consisting of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y, and Sc, and preferably, the rare earth element is selected from the group consisting of Ce, Y, Zr, Mn, Fe, La, Nd, and Nb. The catalyst composition according to any one of claims 1 to 3.

5. The catalyst composition according to any one of claims 1 to 4, wherein the rare earth element is present in an amount of 2 to 5% by weight, preferably 2.75 to 4.5% by weight.

6. The catalyst composition according to any one of claims 1 to 5, wherein the zeolite is a small pore zeolite, and preferably, the small pore zeolite has a CHA or AEI framework structure type.

7. A method for preparing the catalyst composition according to any one of claims 1 to 6, the method comprising: i) incorporating copper into the zeolite to prepare a copper-substituted zeolite; ii) incorporating a rare earth element into the copper-substituted zeolite.

8. A catalyst article for an exhaust gas system, comprising the catalyst composition according to any one of claims 1 to 6 or a catalyst composition obtainable by the method according to claim 7.

9. An exhaust gas system comprising the catalyst article according to claim 8 and optionally an internal combustion engine, preferably a diesel engine.

10. A method for treating exhaust gas, the method comprising contacting the exhaust gas with the catalyst article according to claim 8.

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