CATALYST SYSTEM FOR REDUCING NITROGEN OXIDES

Abstract

The invention relates to a catalyst system for reducing nitrogen oxides, which comprises a nitrogen oxide storage catalyst and an SCR catalyst, wherein the nitrogen oxide storage catalyst consists of at least two catalytically active washcoat layers on a supporting body, wherein a lower washcoat layer A contains cerium oxide, an alkaline earth compound and/or alkali compound, as well as platinum and palladium, and an upper washcoat layer B, which is arranged over the washcoat layer A, contains cerium oxide, platinum and palladium, and no alkali compound and no alkaline earth compound. The invention also relates to a method for converting NOx in exhaust gases of motor vehicles that are operated by means of engines that are operated in a lean manner.

Claims

1. Catalyst system for reducing nitrogen oxides, which comprises a nitrogen oxide storage catalyst and an SCR catalyst, wherein the SCR catalyst contains a small-pore zeolite with a maximum ring size of eight tetrahedral atoms and a transition metal, and wherein the nitrogen oxide storage catalyst consists of at least two catalytically active washcoat layers A and B on a supporting body, and the washcoat layer A is arranged directly on the supporting body and contains cerium oxide, an alkaline earth compound and/or an alkali compound, as well as platinum and palladium; and the washcoat layer B is arranged over the washcoat layer A and contains cerium oxide, as well as platinum and palladium, and is free of alkali and alkaline earth compounds; characterized in that the ratio of cerium oxide in washcoat layer B to cerium oxide in washcoat layer A, calculated in kg/m.sup.3 (g/L) and in relation to the volume of the supporting body, is 1:2 to 3:1, wherein the sum of cerium oxide in washcoat layer A and washcoat layer B, calculated in kg/m.sup.3 (g/L) and in relation to the volume of the supporting body, is 100 to 240 kg/m.sup.3 (100 to 240 g/L).

2. Catalyst system according to claim 1, characterized in that the washcoat layer B contains cerium oxide in a quantity of 46 to 180 kg/m.sup.3 (from 46 to 180 g/L).

3. Catalyst system according to claim 1, characterized in that the washcoat layer A contains cerium oxide in a quantity of 14 to 95 kg/m.sup.3 (from 14 to 95 g/L).

4. Catalyst according to claim 1, characterized in that the total washcoat loading of the supporting body is 300 to 600 kg/m.sup.3 (300 to 600 g/L), in relation to the volume of the supporting body.

5. Catalyst system according to claim 4, characterized in that the loading with washcoat layer A amounts to 150 to 500 kg/m.sup.3 (150 to 500 g/L), and the loading with washcoat layer B amounts to 50 to 300 kg/m.sup.3 (50 to 300 g/L), in relation to the volume of the supporting body in each case.

6. Catalyst system according to claim 4, characterized in that the loading with washcoat layer A amounts to 250 to 300 kg/m.sup.3 (250 to 300 g/L), and, with washcoat layer B, amounts to 100 to 200 kg/m.sup.3 (100 to 200 g/L), in relation to the volume of the supporting body in each case.

7. Catalyst system according to claim 1, characterized in that the ratio of platinum to palladium in the washcoat layers A and B is 2:1 to 18:1.

8. The catalyst according to claim 1, characterized in that the alkaline earth compound in washcoat layer A is magnesium oxide, barium oxide, and/or strontium oxide.

9. Catalyst system according to claim 1, characterized in that the SCR catalyst contains a zeolite which belongs to the AEI, CHA, KFI, ERI, LEV, MER, or DDR structure type and is exchanged with cobalt, iron, copper, or mixtures of two or three of these metals.

10. Catalyst according to claim 1, characterized in that the SCR catalyst contains a zeolite of a chabazite type, which is exchanged with copper, iron, or copper and iron.

11. Catalyst according to claim 1, characterized in that the nitrogen oxide storage catalyst contains a bottom washcoat layer A cerium oxide in a quantity of 14 to 95 kg/m.sup.3 (from 14 to 95 g/L), platinum and palladium in a mass ratio of 8:1 to 10:1, as well as magnesium oxide and/or barium oxide; and a top washcoat layer B is arranged over the bottom washcoat layer A and contains no alkaline earth compound and no alkali compound, platinum and palladium in a mass ratio of 8:1 to 10:1, as well as cerium oxide in a quantity of 46 to 180 kg/m.sup.3 (from 46 to 180 g/L), and wherein the SCR catalyst comprises a zeolite or a molecular sieve with a chabazite structure, which contains copper in a quantity of 1 to 10 wt %, calculated as CuO and in relation to the SCR catalyst.

12. Catalyst according to claim 1, characterized in that the nitrogen oxide storage catalyst contains at least two catalytically active washcoat layers on a supporting body, wherein a bottom washcoat layer A contains cerium oxide in a quantity of 25 to 120 kg/m.sup.3 (from 25 to 120 g/L), platinum and palladium in a mass ratio of 8:1 to 10:1, as well as magnesium oxide and/or barium oxide; and a top washcoat layer B is arranged over the bottom washcoat layer A and contains no alkaline earth compound and no alkali compound, platinum and palladium in a mass ratio of 8:1 to 10:1, as well as cerium oxide in a quantity of 50 to 180 kg/m.sup.3 (from 50 to 180 g/L), and wherein the SCR catalyst comprises a zeolite or a molecular sieve with a chabazite structure, which contains copper in a quantity of 1 to 10 wt %, calculated as CuO and in relation to the SCR catalyst.

13. Catalyst system according to claim 1, characterized in that nitrogen oxide storage catalysts and SCR catalysts are arranged on different supporting bodies or that nitrogen oxide storage catalysts and SCR catalysts are arranged on the same supporting body.

14. Method for converting NO.sub.x in exhaust gases of motor vehicles that are operated by means of engines that are operated in a lean manner, characterized in that the exhaust gas is fed over a catalyst system for reducing nitrogen oxides, which comprises a nitrogen oxide storage catalyst and an SCR catalyst, wherein the SCR catalyst contains a small-pore zeolite with a maximum ring size of eight tetrahedral atoms and a transition metal, and wherein the nitrogen oxide storage catalyst consists of at least two catalytically active washcoat layers A and B on a supporting body, and the washcoat layer A is arranged directly on the supporting body and contains cerium oxide, an alkaline earth compound and/or an alkali compound, as well as platinum and palladium; and the washcoat layer B is arranged over the washcoat layer A and contains cerium oxide, as well as platinum and palladium, and is free of alkali compounds and alkaline earth compounds; and characterized in that the ratio of cerium oxide in washcoat layer B to cerium oxide in washcoat layer A, calculated in kg/m.sup.3 (g/L) and in relation to the volume of the supporting body, is 1:2 to 3:1, wherein the sum of cerium oxide in washcoat layer A and washcoat layer B, calculated in kg/m.sup.3 (g/L) and in relation to the volume of the supporting body, is 100 to 240 kg/m.sup.3 (100 to 240 g/L); and wherein the exhaust gas is fed through the catalyst system such that it flows through first the nitrogen oxide storage catalyst and then the SCR catalyst.

Description

[0102] The invention is explained in more detail in the following examples and figures.

[0103] FIG. 1: NOx conversion of the catalyst systems KS1, KS2, KS3, and VKS1 as a function of the temperature.

EXAMPLE 1

[0104] a) To produce a catalyst system according to the invention, a ceramic carrier with a honeycomb structure is coated with a first washcoat layer A, which contains Pt and Pd supported on a lanthanum-stabilized alumina, cerium oxide in a quantity of 47 kg/m.sup.3 (47 g/L), as well as 17 kg/m.sup.3 (17 g/L) barium oxide and 15 kg/m.sup.3 (15 g/L) magnesium oxide. Neither barium oxide nor magnesium oxide are supported on the cerium oxide. In the process, the loading of Pt and Pd amounts to 1.77 kg/m.sup.3 (1.77 g/L) or 0.177 kg/m.sup.3 (0.177 g/L), and the total loading of the washcoat layer amounts to 181 kg/m.sup.3 (181 g/L) in relation to the volume of the ceramic carrier.

[0105] b) An additional washcoat layer B, which contains Pt, Pd, and Rh supported on a lanthanum-stabilized alumina, is applied to the first washcoat layer. The loading of Pt, Pd, and Rh in this washcoat layer amounts to 1.77 kg/m.sup.3 (1.77 g/L), 0.177 kg/m.sup.3 (0.177 g/L), and 0.177 kg/m.sup.3 (0.177 g/L), respectively. The washcoat layer B also contains 94 kg/m.sup.3 (94 g/L) of cerium oxide for a washcoat loading for layer B of 181 kg/m.sup.3 (181 g/L).

[0106] The catalyst thus obtained is referred to below as SPK1.

[0107] c) To produce the SCR catalyst, a ceramic carrier with a honeycomb structure is coated with a zeolite of a chabazite type with an SAR of 28 and exchanged with copper. The washcoat comprises 85 wt % of zeolite, 3 wt % of CuO, and 12 wt % of aluminum oxide. The catalyst thus obtained is referred to below as SCRK1.

[0108] d) The catalysts SPK1 and SCRK1 are combined to form a catalyst system, which is referred to as KS1 below.

EXAMPLE 2

[0109] The steps a) through d) of example 1 are repeated, with the difference that, in step a), cerium oxide is used in a quantity of 70 kg/m.sup.3 (70 g/L), and, in step b), cerium oxide is used in a quantity of 70 kg/m.sup.3 (70 g/L).

[0110] The catalyst system thus obtained is referred to below as KS2.

EXAMPLE 3

[0111] The steps a) through d) of example 1 are repeated, with the difference that, in step a), cerium oxide is used in a quantity of 93 kg/m.sup.3 (93 g/L), and, in step b), cerium oxide is used in a quantity of 47 kg/m.sup.3 (47 g/L).

[0112] The catalyst system thus obtained is referred to below as KS3.

COMPARATIVE EXAMPLE 1

[0113] The steps a) through d) of example 1 are repeated, with the difference that, in step a), cerium oxide is used in a quantity of 116 kg/m.sup.3 (116 g/L) instead of 47 kg/m.sup.3 (47 g/L), and, in step b), cerium oxide is used in a quantity of 24 kg/m.sup.3 (24 g/L) instead of 94 kg/m.sup.3 (94 g/L).

[0114] The catalyst system thus obtained is referred to below as VKS1.

EXAMPLES 4 THROUGH 6

[0115] To produce further catalyst systems according to the invention, the nitrogen oxide storage catalysts specified in table 1 below or the SCR catalysts specified in table 2 were manufactured analogously to example 1 a) or b) and combined into the catalyst systems specified in table 3.

TABLE-US-00001 TABLE 1 Cerium oxide Cerium oxide washcoat A washcoat B Pt:Pd ratio Pt:Pd ratio Catalyst [kg/m.sup.3 (g/L)] [kg/m.sup.3 (g/L)] layer A layer B SPK4 100 70 10:1 2:1 SPK5 100 100  5:1 5:1 SPK6 70 100 10:1 5:1

TABLE-US-00002 TABLE 2 Metal quantity in wt %, calculated as oxide Catalyst Zeolite Metal (CuO or Fe.sub.2O.sub.3) SCRK2 SAPO-34 Cu 2.5 SCRK3 LEV Cu 4.0 SCRK4 KFI Fe 3.5

TABLE-US-00003 TABLE 3 Nitrogen oxide Catalyst storage catalyst SCR catalyst KS4 SPK4 SCRK4 KS5 SPK5 SCRK2 KS6 SPK6 SCRK1

[0116] Determining the NOx conversion of KS1, KS2, KS3, and VKS1

[0117] a) KS1, KS2, KS3, and VKS1 were first aged for 16 h at 800° C. in a hydrothermal atmosphere.

[0118] b) The NOx conversion of the catalyst systems KS1, KS2, KS3 according to the invention and of the comparison catalyst system VKS1 as a function of the temperature upstream of the catalyst was determined in a model gas reactor in the so-called NOx conversion test. In this test, synthetic exhaust gas with a nitrogen monoxide concentration of 500 ppm, 10 vol % of carbon dioxide and water respectively, a concentration of 50 ppm of a short-chain hydrocarbon mixture (consisting of 33 ppm of propene and 17 ppm of propane), as well as a residual oxygen content of 7 vol %, is fed over the respective catalyst sample in a model gas reactor at a space velocity of 50 k/h, wherein the gas mixture alternately contains excess oxygen for 80 s (“lean” gas mixture with air/fuel ratio A of 1.47) while nitrogen oxides are stored, and has an oxygen deficit for 10 s to regenerate the catalyst sample (“rich” gas mixture with air/fuel ratio A of 0.92; by adding 5.5 vol % of carbon monoxide with simultaneous reduction of the residual oxygen content to 1 vol %). In so doing, the temperature is reduced from 600° C. to 150° C. by 7.5° C./min, and the conversion over each 90-second-long lean-rich cycle is determined.

[0119] The NOx regenerative capacity at 200° C. is important for reproducing driving behavior in urban areas—at 450° C. for highway journeys. In order to meet the Euro 6 exhaust emissions standard, it is particularly important to demonstrate a high NOx regenerative capacity across this entire temperature range.

[0120] FIG. 1 shows the NOx conversion of the catalyst systems KS1, KS2, KS3 according to the invention and of the comparison system VKS1 determined in this way.

[0121] It follows that the NOx conversion of the comparison catalyst system VKS1 at temperatures up to approx. 350° C. is considerably poorer than the catalyst systems KS1 through KS3 according to the invention. Therefore, for example, the NOx conversion of VKS1 at 250° C. is approx. 67%, while it is approx. 75% for KS1 through KS3.