CATALYST FOR REDUCTION OF NITROGEN OXIDES

Abstract

The present invention relates to a catalyst comprising a support body A having a length L.sub.A designed as a flow substrate, a support body B of length L.sub.B designed as a wall-flow filter, and material zones A1, A2, B1, and B2, wherein the support body A comprises material zones A1 and A2 and the support body B comprises material zones B1 and B2, wherein material zone A1 contains a cerium oxide, an alkaline earth metal compound and/or an alkali metal compound, and also platinum and/or palladium, and material zone A2 contains cerium oxide, and also platinum and/or palladium, and is free of alkali metal and alkaline earth metal compounds, material zone B1 contains palladium supported on cerium oxide, and material zone B2 contains platinum supported on a support material.

Claims

1. Catalyst comprising a support body A having a length L.sub.A designed as a flow substrate, a support body B of length L.sub.B designed as a wall-flow filter, and material zones A1, A2, B1, and B2, wherein the support body A comprises material zones A1 and A2, and the support body B comprises material zones B1 and B2, wherein material zone A1 contains cerium oxide, an alkaline earth metal compound and/or an alkali metal compound, as well as platinum and/or palladium, and material zone A2 contains cerium oxide as well as platinum and/or palladium, and is free of alkaline earth metal and alkali metal compounds, material zone B1 contains palladium supported on cerium oxide, and material zone B2 contains platinum supported on a support material.

2. Catalyst according to claim 1, wherein the ratio of platinum to palladium in material zones A1 and A2 is the same or different and is 4:1 to 18:1.

3. Catalyst according to claim 1, wherein material zones A1 and A2 contain rhodium, independently of one another.

4. Catalyst according to claim 1, wherein the alkaline earth metal compound in material zone A1 comprises oxides, carbonates or hydroxides of magnesium, strontium, and/or barium.

5. Catalyst according to claim 1, wherein the alkali metal compound in material zone A1 comprises oxides, carbonates or hydroxides of lithium, potassium, and/or sodium,

6. Catalyst according to claim 1, wherein the alkaline earth metal or alkali metal compound is present in quantities of 10 to 50 g/L, calculated as alkaline earth metal or alkali metal oxide and in relation to the volume of support body A.

7. Catalyst according to claim 1, wherein the ratio of cerium oxide in material zone A2 to cerium oxide in material zone A1, calculated in each case in g/L and in relation to the volume of support body A, is 1:2 to 3:1.

8. Catalyst according to claim 1, wherein material zone A1 comprises cerium oxide in amounts of 110 to 180 g/L, in relation to the volume of support body A, wherein the ratio of cerium oxide in material zone A1 to cerium oxide in material zone A2, calculated respectively in g/L, in relation to the volume of support body A, is 1:1 to 5:1, the sum of cerium oxide in material zone A1 and material zone A2, calculated in g/L and in relation to the volume of support body A, is 132 to 240 g/L, the ratio of Pt:Pd, respectively calculated in g/L, in relation to the volume of support body A, in material zone A1 and material zone A2, is equal and amounts to 2:1 to 20:1, the sum of platinum and palladium, respectively calculated in g/L and in relation to the volume of support body A, in material zone A1 and material zone A2 is equal, and the ratio of the concentrations of platinum and palladium in material zone A1 to platinum and palladium in material zone A2, respectively in relation to the total mass of the respective material zone, calculated respectively in g/L, in relation to the volume of support body 1 is 1:1 to 1:5.

9. Catalyst according to claim 1, wherein material zone A2 is present in an amount of 50 to 200 g/L, in relation to the volume of support body A, and the minimum mass fraction in % of cerium oxide in material zone A2 is calculated from the formula
0.1amount of material zone B1 in g/L+30.

10. Catalyst according to claim 1, wherein material zone A1 lies directly on support body A over its entire length L.sub.A, and material zone A2 lies over the entire length L.sub.A on material zone A1.

11. Catalyst according to claim 1, wherein material zone A1, starting from one end of support body A, extends to 10 to 80% of its length L.sub.A, and material zone A2, starting from the other end of the support body A, extends to 10 to 80% of its length L.sub.A.

12. Catalyst according to claim 11, wherein
L.sub.A=L.sub.A1+L.sub.A2 or
L.sub.A<L.sub.A1+L.sub.A2 or
L.sub.A>L.sub.A1+L.sub.A2 applies, where L.sub.A is the length of support body A, L.sub.A1 is the length of material zone A1, and LA2 is the length of material zone A2.

13. Catalyst according to claim 1, wherein both material zones B1 and B2 are present only on one part of the length L.sub.B of support body B.

14. Catalyst according to claim 13, wherein
L.sub.B=L.sub.B1+L.sub.B2 or
L.sub.B>L.sub.B1+L.sub.B2 applies, where L.sub.B is the length of support body B, L.sub.B1 is the length of material zone B1, and L.sub.B2 is the length of material zone B2.

15. Catalyst according to claim 13, wherein material zones B1 and B2 are located within the porous walls of support body B.

16. Catalyst according to claim 1, material zone B1 extends along the entire length L.sub.B of support body B and is located within its porous wails.

17. Catalyst according to claim 16, wherein material zone B2 is located on the porous walls of support body B in the channels, which are sealed gas-tight on the first end B.sub.E1 of support body B.

18. Catalyst according to claim 17, wherein support body A is arranged upstream, and support body B is arranged downstream.

19. Method for converting NO.sub.x in exhaust gases of motor vehicles that are operated with lean-burn engines, wherein the exhaust gas is guided over a catalyst according to claim 1.

20. Method according to claim 19, wherein the exhaust gas is first guided through support body A and thereafter through support body B.

Description

EXAMPLE 1

[0069] a) To produce a catalyst according to the invention, a commercially available honeycomb flow ceramic support is coated with a first material zone A1 which contains Pt, Pd, and Rh supported on a lanthanum-stabilized alumina, cerium oxide in an amount of 125 g/L, as well as 20 g/L barium oxide and 15 g/L magnesium oxide. In this case, the loading of Pt and Pd amounts to 1.766 g/L (50 g/cft) and 0.177 g/L (5 g/cft), and the total loading of the washcoat layer is 300 g/L in relation to the volume of the ceramic support.

[0070] b) An additional material zone A2, which also contains Pt and Pd, as well as Rh supported on a lanthanum-stabilized alumina, is applied to the first material zone A1 The loading of Pt, Pd, and Rh in this washcoat layer is 1.766 g/L (50 g/cft), 0.177 g/L (5 g/cft), and 0,177 g/L (5 g/cft). Material zone A2 additionally contains 55 g/L cerium oxide in the case of a washcoat loading of layer B of 101 g/L.

[0071] c) In the next step, a commercially available wall-flow filter made of cordierite is coated such that material zones B1 and B2 are both located within the porous wall between the channels. However, both material zones are coated only over 50% of the length of the wall-flow filter, viz., material zone B1, starting from one end of the wall-flow filter, and material zone B2, starting from the other end.

[0072] Material zone B1 consists of 1.11 g/L (3 g/ft.sup.3) palladium on 80 g/L cerium oxide and 20 g/L aluminum oxide, while material zone B2 consists of 1.11 g/L (3 g/ft.sup.3) platinum on 70 g/L aluminum oxide.

[0073] d) The coated flow-through ceramic support according to (a) and (b) and the wall-flow filter according to (c) are combined such that, during operation, the flow-through ceramic support is arranged upstream, and the wall-flow filter is arranged downstream.

[0074] It is to be noted that the exhaust gas enters the flow-through ceramic support in such a way that it first comes into contact with material zone A2.

[0075] It is further to be noted that the exhaust gas enters the wall-flow filter in such a way that it first comes into contact with material zone B1.