Particle filter having SCR-active coating

Abstract

The invention relates to a particle filter, which comprises a wall flow filter and SCR-active material, wherein the wall flow filter comprises ducts which extend in parallel between the first and the second end of the wall flow filter and which are alternately closed in a gas-tight manner either at the first or the second end and which are separated by porous walls, the pores of which have inner surfaces, and the SCR-active material is located in the form of a coating on the inner surfaces of the pores of the porous walls, characterized in that the coating has a gradient, such that the side of the coating facing the exhaust gas has a higher selectivity in the SCR reaction than the side of the coating that faces the inner surfaces of the pores. The SCR-active material is preferably a small-pore zeolite, which has a maximum ring size of eight tetrahedral atoms and is exchanged with copper and/or iron.

Claims

1. A particle filter comprising: a wall flow filter and SCR-active material, wherein the wall flow filter comprises channels which extend parallelly between a first and a second end of the wall flow filter, which are alternatingly sealed in a gas-tight manner either at the first or at the second end, and which are separated by porous walls whose pores have inner surfaces, and the SCR-active material is located in the form of a coating on the inner surfaces of the pores of the porous walls, wherein the coating has a gradient, and wherein a side of the coating facing the exhaust gas has a higher selectivity in the SCR reaction than a side of the coating facing the inner surface of the pores.

2. The particle filter according to claim 1, wherein the wall flow filter consists of silicon carbide, aluminum titanate, or cordierite.

3. The particle filter according to claim 1, wherein the wall flow filter in the uncoated state has a porosity of 30 to 80%.

4. The particle filter according to claim 1, wherein the wall flow filter has an average pore size in the uncoated state of 5 to 30 micrometers.

5. The particle filter according to claim 1, wherein the SCR-active material comprises a small-pore zeolite that is exchanged with copper and/or iron.

6. The particle filter according to claim 5, wherein the small-pore zeolite belongs to the AEI, CHA (chabazite), ERI (erionite), LEV (levyne), AFX, DDR, or KFI structure type.

7. The particle filter according to claim 5, wherein the small-pore zeolite is an aluminum silicate, silicoaluminophosphate, or aluminophosphate.

8. The particle filter according to claim 5, wherein the amounts of iron or copper are 0.2 to 6 wt. %, calculated as Fe.sub.2O.sub.3 or CuO and based on the total weight of the exchanged small-pore zeolite.

9. The particle filter according to claim 1, wherein the coating consists of two or a plurality of layers which differ with respect to the selectivity of the SCR-active material, wherein the outermost layer facing the exhaust gas has the highest selectivity and the innermost layer has the lowest selectivity.

10. The particle filter according to claim 1, wherein the coating consists of two layers, wherein the layer facing the exhaust gas has a higher selectivity in the SCR reaction than the layer facing the inner surface of the pores.

11. The particle filter according to claim 1, wherein the term “selectivity” is defined as the quotient from the conversion of NOx and the conversion of NH.sub.3, that is,
S=X.sub.(NOx)/X.sub.(NH3) applies, where S is the selectivity, X.sub.(NOx) the conversion of NO.sub.x in %, and X.sub.(NH3) the conversion of NH.sub.3 in %.

12. The particle filter according to claim 11, wherein in order to determine X.sub.(NOx) and X.sub.(NH3), a wall flow filter is coated with the SCR-active material of each coating, then hydrothermally aged for 16 hours at 800° C., and then the conversion of NO.sub.x or NH.sub.3 is determined at 500° C. in a test gas with the following composition: TABLE-US-00005 N.sub.2 Balance O.sub.2 10 percent by volume NOx 500 ppm NO.sub.2  0 ppm NH.sub.3 750 ppm CO 350 ppm C.sub.3H.sub.6 100 ppm H.sub.2O 5 percent by volume GHSV/h.sup.−1 60,000 and the quotient X.sub.(NOx)/X.sub.(NH3) is then calculated.

13. The particle filter according to claim 1, wherein the amount of coating with SCR-catalytically active material amounts to 70 to 150 g/L, in relation to the volume of the wall flow filter.

14. The particle filter according to claim 1, wherein the coating with SCR-active material consists of two layers each containing a small-pore zeolite exchanged with copper, wherein the small-pore zeolite of the layer facing the exhaust gas contains 0.3 to 3 wt. % Cu, calculated as CuO and based on the exchanged zeolite of this layer, and the small-pore zeolite of the layer facing the inner pore surface contains 0.5 to 5 wt. % Cu, calculated as CuO and based on the exchanged zeolite of this layer.

15. The particle filter according to claim 1, wherein the coating with SCR-active material consists of two layers each containing a copper-exchanged zeolite, wherein the zeolite contained in the layer facing the exhaust gas contains less CU, calculated as CuO and based on the exchanged zeolite of this layer, than the zeolite contained in the layer facing the inner pore surface.

16. The particle filter according to claim 1, wherein the coating with SCR-active material consists of two layers each containing a copper-exchanged zeolite of the chabazite (CHA) structure type, wherein the zeolite of the layer facing the exhaust gas contains less than 3 wt. % Cu, calculated as CuO and based on the exchanged zeolite of this layer, and the zeolite of the layer facing the inner pore surface contains more than 3 wt. % Cu, calculated as CuO and based on the exchanged zeolite of this layer.

17. The particle filter according to claim 1, wherein the coating with SCR-active material consists of two layers each containing a copper-exchanged zeolite of the levyne (LEV) structure type, wherein the zeolite of the layer facing the exhaust gas contains less than 3 wt. % Cu, calculated as CuO and based on the exchanged zeolite of this layer, and the zeolite of the layer facing the inner pore surface contains more than 3 wt. % Cu, calculated as CuO and based on the exchanged zeolite of this layer.

18. A method for purifying exhaust gas of a lean-operated combustion engine, which comprises passing the exhaust gas over a particle filter according to claim 1.

19. A device for purifying exhaust gas of lean-operated combustion engines, wherein it comprises a particle filter according to claim 1 and an injector for a reducing agent.

20. The device according to claim 19, wherein the reducing agent is an aqueous urea solution.

21. The device according to claim 19, wherein it comprises an oxidation catalyst.

22. A device for purifying exhaust gas of lean-operated combustion engines, which comprises a particle filter according to claim 1 and a nitrogen oxide storage catalyst.

Description

(1) The invention is explained in more detail in the following examples and figures.

(2) FIG. 1 shows a cross-section through the porous wall of a particle filter according to the invention, comprising two layers with SCR-catalytically active material, wherein (1) the pores of the porous wall (2) the porous wall (3) the layer with the SCR-catalytically active material which has the lower selectivity and (4) the layer with the SCR-catalytically active material which has the higher selectivity and The arrows show the direction of the exhaust gas

(3) FIG. 2 shows the NO.sub.x conversion of catalysts K1, VK1 and VK2 (example 1 and comparative examples 1 and 2)

(4) FIG. 3 shows the NO.sub.x conversion of K2 and VK3 (example 2 and comparative example 3) as well as catalysts VKB and VKC from example 2.

EXAMPLE 1

(5) a) A commercially available wall flow filter made of silicon carbide having a porosity of 65% and an average pore size of 23 μm was coated with 60 g/L of a washcoat by means of a conventional dipping method, which washcoat contained a copper-exchanged chabazite, having an SiO.sub.2/Al.sub.2O.sub.3 ratio (SAR) of 30, having an amount of copper of 3 wt. % (calculated as CuO and based on the exchanged chabazite). The average particle size of the copper-exchanged chabazite was 1.16 μm. The coated wall flow filter was then dried and calcined.
b) In a second step, the wall flow filter coated according to a) was provided with a second coating. To this end, coating with 60 g/L of a washcoat was effected by means of a conventional dipping method, which washcoat contained a copper-exchanged chabazite (SAR=30) having an amount of copper of 1 wt. % (calculated as CuO and based on the exchanged chabazite). The average particle size of the copper-exchanged chabazite was 1.05 μm. The coated wall flow filter was then dried and calcined. The amount of copper calculated over the entire wall flow filter was 2 wt. % (calculated as CuO and based on the exchanged chabazite).

(6) The catalyst is referred to below as K1.

COMPARATIVE EXAMPLE 1

(7) A commercially available wall flow filter made of silicon carbide having a porosity of 65% and an average pore size of 23 μm was coated with 120 g/L of a washcoat by means of a conventional dipping method, which washcoat contained a copper-exchanged chabazite (SAR=30) having an amount of copper of 2 wt. % (calculated as CuO and based on the exchanged chabazite). The average particle size of the copper-exchanged chabazite was 1.06 μm. The coated wall flow filter was then dried and calcined.

(8) The amount of copper calculated over the entire wall flow filter was 2 wt. % (calculated as CuO and based on the exchanged chabazite).

(9) The catalyst is referred to below as VK1.

COMPARATIVE EXAMPLE 2

(10) a) A commercially available wall flow filter made of silicon carbide having a porosity of 65% and an average pore size of 23 μm was coated on 50% of its length on the inlet side with 60 g/L of a washcoat by means of a conventional dipping method, which washcoat contained a copper-exchanged chabazite (SAR=30) having an amount of copper of 1 wt. % (calculated as CuO and based on the exchanged chabazite). The average particle size of the copper-exchanged chabazite was 1.05 μm. The coated wall flow filter was then dried and calcined.
b) In a second step, the wall flow filter coated in accordance with a) was also provided with a second coating on the still uncoated section of its length (50%) on the outlet side. To this end, coating with 60 g/L of a washcoat was effected by means of a conventional dipping method, which washcoat contained a copper-exchanged chabazite (SAR=30) having an amount of copper of 3 wt. % (calculated as CuO and based on the exchanged chabazite). The average particle size of the copper-exchanged chabazite was 1.16 μm. The coated wall flow filter was then dried and calcined.

(11) The amount of copper calculated over the entire wall flow filter was 2 wt. % (calculated as CuO and based on the exchanged chabazite).

(12) The catalyst is referred to below as VK2.

(13) Determining the NO.sub.x conversion of K1, VK1, and VK2

(14) a) K1, VK1 and VK2 were first aged for 16 h at 800° C. in a hydrothermal atmosphere (10% water, 10% oxygen, remaining part nitrogen).

(15) b) The NO.sub.x conversion of the particle filter K1 according to the invention and the comparative particle filters VK1 and VK1 depending on the temperature upstream of the catalyst was determined in a model gas reactor in the so-called NO.sub.x conversion test.

(16) The NO.sub.x conversion test consists of a test procedure that comprises a pretreatment and a test cycle that is run through for various target temperatures. The applied gas mixtures are noted in Table 1.

(17) Test Procedure:

(18) 1. Preconditioning at 600° C. in nitrogen for 10 min 2. Test cycle repeated for the target temperatures a. Approaching the target temperature in gas mixture 1 b. Addition of NO.sub.x (gas mixture 2) c. Addition of NH.sub.3 (gas mixture 3), wait until NH.sub.3 breakthrough>20 ppm, or a maximum of 30 min. in duration d. Temperature-programmed desorption up to 500° C. (gas mixture 3)

(19) The maximum conversion for the test procedure range 2c is determined for each temperature point. A plot as shown in FIG. 2 results from plotting the maximum NO.sub.x conversion for the different temperature points.

(20) TABLE-US-00002 Gas mixture 1 2 3 N.sub.2 Balance Balance Balance O.sub.2 10 percent by 10 percent by 10 percent by volume volume volume NOx 0 ppm 500 ppm 500 ppm NO.sub.2 0 ppm  0 ppm  0 ppm NH.sub.3 0 ppm  0 ppm 750 ppm CO 350 ppm  350 ppm 350 ppm C.sub.3H.sub.6 100 ppm  100 ppm 100 ppm H.sub.2O  5 percent by 5 percent by 5 percent by volume volume volume GHSV/h−1 60,000 60,000 60,000

(21) As can be seen from FIG. 2, K1 demonstrates a significantly better NO.sub.x conversion compared with VK1 and VK2.

EXAMPLE 2

(22) I) Determining the selectivity in the SCR reaction

(23) a) A commercially available wall flow filter made of silicon carbide having a porosity of 63% and an average pore size of 20 μm was coated with 80 g/L of a washcoat by means of a conventional dipping method, which washcoat contained a copper-exchanged chabazite (SAR=30) having an amount of copper of 4.5 wt. % (calculated as CuO and based on the exchanged chabazite). The average particle size of the copper-exchanged chabazite was 1.43 μm. The coated wall flow filter was then dried, calcined at 350° C. and annealed at 550° C.

(24) The catalyst is referred to below as VKB.

(25) b) A commercially available wall flow filter made of silicon carbide having a porosity of 63% and an average pore size of 20 μm was coated with 30 g/L of a washcoat by means of a conventional dipping method, which washcoat contained a copper-exchanged chabazite (SAR=30) having an amount of copper of 2 wt. % (calculated as CuO and based on the exchanged chabazite). The average particle size of the copper-exchanged chabazite was 1.93 μm. The coated wall flow filter was then dried, calcined at 350° C. and annealed at 550° C.

(26) The catalyst is referred to below as VKC.

(27) c) The catalysts VKB and VKC were hydrothermally aged for 16 hours at 800° C. and then their conversions of NO.sub.x and NH.sub.3 were determined under the test conditions specified in the following table.

(28) TABLE-US-00003 Temperature 500° C. N.sub.2 Balance O.sub.2 10 percent by volume NOx 500 ppm NO.sub.2  0 ppm NH.sub.3 750 ppm CO 350 ppm C.sub.3H.sub.6 100 ppm H.sub.2O 5 percent by volume GHSV/h−1 60,000

(29) Afterwards, the quotient for VKB and VKC was determined from the conversion of NO.sub.x and the conversion of NH.sub.3. The following results were obtained:

(30) TABLE-US-00004 VKB VKC X(NO.sub.x)/% 66.20 24.74 X(NH.sub.3)/% 96.18 30.06 X(NO.sub.x)/x(NH.sub.3) 0.69 0.82

(31) It was shown that VKC has a higher quotient from the conversions of NO.sub.x and NH.sub.3 than VKB. Therefore, VKC is to be coated on the side facing the gas and VKB on the side facing the wall flow filter.

(32) II) In accordance with the findings in the preceding paragraph Ic), a particle filter according to the invention was obtained as follows:

(33) A commercially available wall flow filter made of silicon carbide having a porosity of 63% and an average pore size of 20 μm was coated with 80 g/L of a washcoat by means of a conventional dipping method, which washcoat contained a copper-exchanged chabazite (SAR=30) having an amount of copper of 4.5 wt. % (calculated as CuO and based on the exchanged chabazite). The average particle size of the copper-exchanged chabazite was 1.43 μm. The coated wall flow filter was then dried and calcined at 350° C.

(34) In a second step, the wall flow filter coated according to a) was provided with a second coating. To this end, coating with 30 g/L of a washcoat was effected by means of a conventional dipping method, which washcoat contained a copper-exchanged chabazite (SAR=30) having an amount of copper of 2 wt. % (calculated as CuO and based on the exchanged chabazite). The average particle size of the copper-exchanged chabazite was 1.93 μm. The coated wall flow filter was then dried, calcined at 350° C. and annealed at 550° C.

(35) The amount of copper calculated over the entire wall flow filter was 3.8 wt. % (calculated as CuO and based on the exchanged chabazite).

(36) The catalyst is referred to below as K2.

COMPARATIVE EXAMPLE 3

(37) A commercially available wall flow filter made of silicon carbide having a porosity of 63% and an average pore size of 20 μm was coated with 110 g/L of a washcoat by means of a conventional dipping method, which washcoat contained a copper-exchanged chabazite (SAR=30) having an amount of copper of 3.8 wt. % (calculated as CuO and based on the exchanged chabazite). The average particle size of the copper-exchanged chabazite was 1.61 μm.) The coated wall flow filter was then dried and calcined. The catalyst is referred to below as VK3.

(38) The NO.sub.x conversion of K2 and VK3 (and of VKB and VKC) was determined as described in example 1. The results are shown in FIG. 3.