Catalyst for reduction of nitrogen oxides

Abstract

The present invention relates to a nitrogen oxide storage catalyst composed of at least two catalytically active washcoat layers on a support body, wherein a lower washcoat layer A comprises cerium oxide, an alkaline earth metal compound and/or an alkali metal compound, and platinum and palladium, and an upper washcoat layer B disposed atop the washcoat layer A comprises cerium oxide, platinum and palladium, and no alkali metal or alkaline earth metal compound, and to a method of converting NO.sub.x in exhaust gases from motor vehicles which are operated with lean-burn engines.

Claims

1. Nitrogen oxide storage catalyst composed of at least two catalytically active washcoat layers on a support body, wherein a lower washcoat layer A contains cerium oxide in a quantity of 110 to 180 kg/m.sup.3 (110 to 180 g/L) in relation to the volume of the support body, an alkaline earth metal compound and/or an alkali metal compound, as well as platinum and palladium; an upper washcoat layer B disposed atop the washcoat layer A contains cerium oxide, as well as platinum and palladium, and no alkali metal or alkaline earth metal compounds; the ratio of the cerium oxide in washcoat layer A to the cerium oxide in washcoat layer B is greater than 1:1 up to 5:1, wherein the total quantity 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 support body, is 132 to 240 kg/m.sup.3 (132 to 240 g/L); the ratio Pt:Pd in washcoat layer A and washcoat layer B is equal, and the ratio Pt:Pd amounts to 2:1 to 20:1; the total quantity of platinum and palladium, calculated respectively in kg/m.sup.3 (g/L) and in relation to the volume of the support body, in washcoat layer A and washcoat layer B is equal; and the ratio of the concentrations of platinum and palladium in washcoat layer A to platinum and palladium in washcoat layer B, respectively in relation to the total mass of the respective washcoat layer, is 1:1 to 1:5, and wherein the quantity of cerium oxide in lower washcoat layer A exceeds the quantity of alkaline earth metal oxide or alkali metal oxide in lower washcoat layer A.

2. Nitrogen oxide storage catalyst according to claim 1, wherein the washcoat layer B contains cerium oxide in a quantity of 22 to 120 kg/m.sup.3 (22 to 120 g/L).

3. Nitrogen oxide storage catalyst according to claim 1, wherein the washcoat layer A contains cerium oxide in a quantity of 110 to 160 kg/m.sup.3 (110 to 160 g/L).

4. Nitrogen oxide storage catalyst according to claim 1, wherein the total washcoat loading of the support body is 300 to 600 kg/m.sup.3 (300 to 600 g/L) in relation to the volume of the support body.

5. Nitrogen oxide storage catalyst according to claim 4, wherein the loading with washcoat layer A is 150 to 500 kg/m.sup.3 (150 to 500 g/L), and the loading with washcoat layer B is 50 to 300 kg/m.sup.3 (50 to 300 g/L), respectively in relation to the volume of the support body.

6. Nitrogen oxide storage catalyst according to claim 4, wherein the loading with washcoat layer A is 250 to 300 kg/m.sup.3 (250 to 300 g/L), and the loading with washcoat layer B is 50 to 150 kg/m.sup.3 (50 to 150 g/L), respectively in relation to the volume of the support body.

7. Nitrogen oxide storage catalyst according to claim 1, wherein the ratio of platinum to palladium is 4:1 to 18:1.

8. Nitrogen oxide storage catalyst according to claim 1, wherein the ratio of platinum to palladium is 6:1 to 16:1.

9. Nitrogen oxide storage catalyst according to claim 1 wherein washcoat layer A and/or washcoat layer B contain rhodium.

10. Nitrogen oxide storage catalyst according to claim 9, wherein rhodium is provided in quantities of 0.003 to 0.35 kg/m.sup.3 (0.003 to 0.35 g/L) in relation to the volume of the support body.

11. Nitrogen oxide storage catalyst according to claim 1, wherein the alkaline earth metal compound in washcoat layer A is magnesium oxide, barium oxide, and/or strontium oxide.

12. Nitrogen oxide storage catalyst composed of least two catalytically active washcoat layers on a support body, wherein a lower washcoat layer A contains cerium oxide in a quantity of 110 to 180 kg/m.sup.3 (110 to 180 g/L) in relation to the volume of the support body, an alkaline earth metal compound and/or an alkali metal compound, as well as platinum and palladium; an upper washcoat layer B disposed atop the washcoat layer A contains cerium oxide, as well as platinum and palladium, and no alkali metal or alkaline earth metal compounds; the ratio of the cerium oxide in washcoat layer A to the cerium oxide in washcoat layer B is 1:1 to 5:1, wherein the total quantity 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 support body, is 132 to 240 kg/m.sup.3 (132 to 240 g/L); the ratio Pt:Pd in washcoat layer A and washcoat layer B Is equal, and the ratio Pt:Pd amounts to 2:1 to 20:1; the total quantity of platinum and palladium, calculated respectively in kg/m.sup.3 (g/L) and in relation to the volume of the support body, in washcoat layer A and washcoat layer B is equal; and the ratio of the concentrations of platinum and palladium in washcoat layer A to platinum and palladium in washcoat layer B, respectively in relation to the total mass of the respective washcoat layer, is 1:1 to 1:5, and wherein the nitrogen oxide storage catalyst comprises a lower washcoat layer A that contains cerium oxide in a quantity of 100 to 160 kg/m.sup.3 (100 to 160 g/L), platinum and palladium in a ratio of 10:1, as well as magnesium oxide and/or barium oxide: and an upper washcoat layer B that is disposed atop the washcoat layer A and contains no alkaline earth metal compound and no alkali metal compound, platinum and palladium in a ratio of 10:1, as well as cerium oxide in a quantity of 45 to 65 kg/m.sup.3 (45 to 65 g/L), wherein washcoat layer A is provided in quantities of 250 to 350 kg/m (250 to 350 g/L), and washcoat layer B is provided in quantities of 80 to 130 kg/m (80 to 130 g/L), wherein the specification of quantity kg/m (g/L) respectively relates to the volume of the support body, and wherein the quantity of cerium oxide in lower washcoat layer A exceeds the quantity of alkaline earth metal oxide or alkali metal oxide in lower washcoat layer A.

13. Method of converting NOx in exhaust gases of motor vehicles that are operated with lean-burn engines, wherein the exhaust gas is guided over a nitrogen oxide storage catalyst composed of at least two catalytically active washcoat layers on a support body, wherein a lower washcoat layer A contains cerium oxide in a quantity of 110 to 180 kg/m.sup.3 (110 to 180 g/L) in relation to the volume of the support body, an alkaline earth metal compound and/or an alkali metal compound, as well as platinum and palladium; an upper washcoat layer B disposed atop the washcoat layer A contains cerium oxide, as well as platinum and palladium, and no alkali metal or alkaline earth metal compounds; the ratio of the cerium oxide in washcoat layer A to the cerium oxide in washcoat layer B is equal to or greater than 2.27:1 up to 5:1, wherein the total quantity 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 support body, is 132 to 240 kg/m.sup.3 (132 to 240 g/L); the ratio Pt:Pd in washcoat layer A and washcoat layer B is equal, and the ratio Pt:Pd amounts to 2:1 to 20:1; the total quantity of platinum and palladium, calculated respectively in kg/m.sup.3 (g/L) and in relation to the volume of the support body, in washcoat layer A and washcoat layer B is equal; and the ratio of the concentrations of platinum and palladium in washcoat layer A to platinum and palladium in washcoat layer B, respectively in relation to the total mass of the respective washcoat layer is 1:1 to 1:5, and wherein the quantity of cerium oxide in lower washcoat layer A exceeds the quantity of alkaline earth metal oxide or alkali metal oxide in lower washcoat layer A.

14. Nitrogen oxide storage catalyst according to claim 1, wherein the alkaline earth metal compound in washcoat layer A is selected from the group consisting of magnesium oxide, barium oxide, strontium oxide, and mixtures thereof.

15. Nitrogen oxide storage catalyst according to claim 1, wherein the cerium oxide quantity in lower washcoat layer A is greater than the cerium oxide quantity in upper washcoat layer B by a ratio of 2.27:1 up to 5:1; and wherein the ratio of the concentrations of platinum and palladium in washcoat layer A to platinum and palladium in washcoat layer B, respectively in relation to the total mass of the respective washcoat layer, is greater than 1.83:1 and at or below 4.3:1.

16. Nitrogen oxide storage catalyst according to claim 1, wherein the nitrogen storage catalyst provides for NOx conversion removal of 54% or higher in a temperature range of 200 degrees Celsius to 500 degrees Celsius, wherein the nitrogen storage catalyst provides for maximum NOx conversion removal within a range of 350 to 375 degrees Celsius, and wherein the nitrogen storage catalyst provides for a conversion of 50% below 200 degrees Celsius.

17. Nitrogen oxide storage catalyst according to claim 1, wherein lower washcoat layer A contains cerium oxide in a quantity of 125 to 145 kg/m.sup.3 (125 to 145 g/L) and upper washcoat layer B contain cerium oxide in a quantity of 40 to 100 kg/m.sup.3 (40 to 100 g/L).

18. Nitrogen oxide storage catalyst according to claim 1, wherein the quantity of alkaline earth metal oxide or alkali metal oxide in lower washcoat layer A is less than the amount of cerium oxide in upper washcoat B.

19. Nitrogen oxide storage catalyst according to claim 18, wherein lower washcoat layer A contains 10 to 50 kg/m.sup.3 (15 to 50 g/L) alkaline earth metal oxide or alkali metal oxide.

Description

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

(2) FIG. 1: NOx conversion of catalyst K1 as a function of the temperature.

EXAMPLE 1

(3) In order to produce a catalyst according to the invention, a honeycombed ceramic substrate is coated with a first washcoat layer A, which contains Pt, Pd, and Rh supported on an alumina stabilized by lanthanum, cerium oxide in a quantity of 125 kg/m.sup.3 (125 g/L), as well as 20 kg/m.sup.3 (20 g/L) barium oxide and 15 kg/m.sup.3 (15 g/L) magnesium oxide. In this case, the loading of Pt and Pd amounts to 1.766 kg/m.sup.3 (1.766 g/L (50 g/cft)) and 0.177 kg/m.sup.3 (0.177 g/L (5 g/cft)), and the total loading of the washcoat layer is 300 kg/m.sup.3 (300 g/L) in relation to the volume of the ceramic substrate. Another washcoat layer B, which also contains Pt and Pd as well as Rh supported on an alumina stabilized by lanthanum, is applied to the first washcoat layer. The loading of Pt, Pd, and Rh in this washcoat layer amounts to 1.766 kg/m.sup.3 (1.766 g/L (50 g/cft)), 0.177 kg/m.sup.3 (0.177 g/L (5 g/cft)), and 0.177 kg/m.sup.3 (0.177 g/L (5 g/cft)). Washcoat layer B additionally contains 55 kg/m.sup.3 (55 g/L) cerium oxide with a washcoat loading of layer B of 101 kg/m.sup.3 (101 g/L).

(4) The catalyst thus obtained is referred to below as K1.

EXAMPLES 2 through 6

(5) Example 1 was repeated with the difference that the quantities of cerium oxide or noble metals specified in Table 1 below were used. The catalysts thus obtained are called K2 through K6.

(6) TABLE-US-00001 TABLE 1 Cerium oxide Cerium oxide Cerium (Pt + Pd) Washcoat A Washcoat B oxide concentration Catalyst [kg/m.sup.3 (g/L)] [kg/m.sup.3 (g/L)] ratio/B/A B/A K1 125 55 1/2.27 2.98/1 K2 150 30 1/5.00 4.30/1 K3 110 110 1/1.00 1.83/1 K4 152.8 67.2 1/2.27 2.91/1 K5 110 22 1/5.00 4.22/1 K6 180 60 1/3.00 3.36/1

(7) Determination of the NOx Conversion of K1 a) K1 was first aged for 16 h at 800? C. in a hydrothermal atmosphere. b) The NOx conversion of the catalyst K1 according to the invention as a function of the temperature upstream of the catalyst was determined in a model gas reactor in the so-called NOx conversion test.

(8) In this test, synthetic exhaust gas with a nitrogen monoxide concentration of 500 ppm, respectively 10 vol % carbon dioxide and water, a concentration of 50 ppm of a short-chained hydrocarbon mixture (consisting of 33 ppm propene and 17 ppm propane), as well as a residual oxygen content of 7 vol %, is guided 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 an air/fuel ratio ? of 1.47), during which time nitrogen oxides are stored, and exhibits an oxygen deficit for 10 s for regenerating the catalyst sample (rich gas mixture with an air/fuel ratio ? of 0.92; by admixing 5.5 vol % carbon monoxide while simultaneously reducing the residual oxygen content to 1 vol %).

(9) In the process, the temperature is reduced by 7.5? C./min from 600? C. to 150? C., and the conversion during each 90-second-long lean/fat cycle is determined.

(10) The NOx regeneration capacity at 200? C. is important, in order to model the driving behavior in urban areas, and at 450? C. for freeway driving. In order to fulfill the Euro 6 emission standard, it is, in particular, important in this respect to show a high NOx regeneration capacity across this entire temperature range.

(11) FIG. 1 shows the thus determined NOx conversion, of the catalyst 1 according to the invention. Accordingly, the conversion is 54% at 200? C. and 74% at 450? C.