MULTI-LAYER NITROGEN OXIDE STORAGE CATALYST WITH MANGANESE

Abstract

The 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 compound, platinum and palladium, and manganese oxide, and an upper washcoat layer B disposed on the washcoat layer A comprises cerium oxide, platinum and palladium and does not contain any alkali and alkaline-earth compounds, and to a method for 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, an alkaline earth compound and/or an alkali compound, platinum and palladium, as well as manganese oxide; and an upper washcoat layer B arranged above washcoat layer A contains cerium oxide, as well as platinum and palladium, and is free of alkali compounds and alkaline earth compounds.

2. Nitrogen oxide storage catalyst according to claim 1, characterized in that washcoat layer A contains cerium oxide in a quantity of 110 to 160 g/L.

3. Nitrogen oxide storage catalyst according to claim 1, characterized in that washcoat layer B contains cerium oxide in a quantity of 22 to 120 g/L.

4. Nitrogen oxide storage catalyst according to claim 1, characterized in that the alkaline earth compound in washcoat layer A is an oxide, carbonate, and/or hydroxide of magnesium, strontium, and/or barium.

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

6. Nitrogen oxide storage catalyst according to claim 1, characterized in that the alkali compound in washcoat layer A is an oxide, carbonate, and/or hydroxide of lithium, potassium, and/or sodium.

7. Nitrogen oxide storage catalyst according to claim 1, characterized in that the alkaline earth or alkali compound in washcoat layer A is present in quantities of 10 to 50 g/L, calculated as alkaline earth or alkali oxide and in relation to the volume of the support body.

8. Nitrogen oxide storage catalyst according to claim 1, characterized in that manganese oxide is present in washcoat layer A in quantities of 1 to 10 wt % in relation to the total of washcoat layers A and B and calculated as MnO.

9. Nitrogen oxide storage catalyst according to claim 1, characterized in that manganese oxide is present in washcoat layer B in quantities of up to 2.5 wt % in relation to the total of washcoat layers A and B and calculated as MnO.

10. Nitrogen oxide storage catalyst according to claim 1, characterized in that the ratio of platinum to palladium in washcoat layer A and in washcoat layer B is respectively 4:1 to 18:1, independently of each other.

11. Nitrogen oxide storage catalyst according to claim 1, characterized in that washcoat layer B contains rhodium.

12. Nitrogen oxide storage catalyst according to claim 11, characterized in that rhodium is present in quantities of 0.003 to 0.35 g/L in relation to the volume of the support body.

13. Nitrogen oxide storage catalyst according to claim 1, characterized in that the total washcoat loading of the support body is 300 to 600 g/L in relation to the volume of the support body.

14. Nitrogen oxide storage catalyst according to claim 1, characterized in that it contains a lower washcoat layer A cerium oxide in a quantity of 100 to 160 g/L, platinum and palladium in a mass ratio of 10:1, magnesium oxide and/or barium oxide; as well as manganese oxide in a quantity of 10 to 20 g/L, and an upper washcoat layer B arranged above lower washcoat layer A and containing no alkaline earth compound and no alkali compound, platinum and palladium in a mass ratio of 10:1, as well as cerium oxide in a quantity of 45 to 65 g/L, wherein washcoat layer A is present in quantities of 250 to 350 g/L, and washcoat layer B is present in quantities of 80 to 130 g/L, and wherein the quantity g/L respectively relates to the volume of the support body.

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

Description

[0050] The invention is explained in more detail in the examples and figures below.

[0051] FIG. 1: NOx conversion of catalysts K1 (dashed line) and VK1 (solid line) as a function of the temperature.

[0052] FIG. 2: NOx conversion of catalysts K2 (solid line) and K3 (dashed) as a function of the temperature.

[0053] FIG. 3: NOx conversion of catalysts K2 (solid line) and K4 (dashed) as a function of the temperature.

EXAMPLE 1

[0054] a) In order to produce a catalyst according to the invention, a honeycombed ceramic substrate is coated with a first washcoat layer A containing Pt and Pd carried on aluminum oxide, cerium oxide in a quantity of 125 g/L, 21 g/L barium oxide, 15 g/L magnesium oxide, and 7.5 g/L MnO in the form of manganese carbonate. In this case, the loading of Pt and Pd amounts to 1.236 g/L (35 g/ft.sup.3) and 0.124 g/L (3.5 g/ft.sup.3), and the total loading of the washcoat layer is approximately 293 g/L in relation to the volume of the ceramic substrate.

[0055] b) Another washcoat layer B, which also contains Pt and Pd carried on aluminum oxide, as well as Rh carried on a lanthanum-stabilized aluminum oxide, is applied to the first washcoat layer. The loading of Pt, Pd, and Rh in this washcoat layer amounts to 1.236 g/L (35 g/ft.sup.3), 0.124 g/L (3.5 g/ft.sup.3), and 0.177 g/L (5 g/ft.sup.3). The washcoat layer B also contains 55 g/L of cerium oxide in the case of a washcoat loading of layer B of approximately 81 g/L.

[0056] The catalyst thus obtained is referred to below as K1.

Comparative Example 1

[0057] Example 1 was repeated, with the difference that washcoat layer A did not contain any manganese oxide. The catalyst thus obtained is referred to below as VK1.

[0058] Determining the NOx conversion of K1 and VK1

a) K1 and VK1 were first aged for 16 h at 800 C. in a hydrothermal atmosphere.
b) The NOx conversion of K1 and VK1 as a function of the temperature upstream of the catalyst was determined in a model gas reactor in the so-called NOx conversion test.

[0059] 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 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 air/fuel ratio 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 of 0.92; by adding 5.5 vol % of carbon monoxide with simultaneous reduction of the residual oxygen content to 1 vol %).

[0060] 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. FIG. 1 shows the NOx conversions determined in this way of the catalyst K1 according to the invention and of the comparison catalyst VK1. The conversion of K1 in the entire temperature range, accordingly, is above the conversion of VK1.

EXAMPLE 2

[0061] a) In order to produce another catalyst according to the invention, a honeycombed ceramic substrate is coated with a first washcoat layer A containing Pt and Pd carried on aluminum oxide, cerium oxide in a quantity of 125 g/L, 21 g/L barium oxide, 7.5 magnesium oxide, and 7.5 g/L manganese oxide in the form of manganese carbonate. In this case, the loading of Pt and Pd amounts to 1.236 g/L (35 g/ft.sup.3) and 0.124 (3.5 g/ft.sup.3), and the total loading of the washcoat layer is approximately 299 g/L in relation to the volume of the ceramic substrate.

[0062] b) Another washcoat layer B, which also contains Pt and Pd, as well as Rh carried on aluminum oxide, is applied to the first washcoat layer. The loading of Pt, Pd, and Rh in this washcoat layer amounts to 1.236 g/L. (35 g/ft.sup.3), 0.124 g/L (3.5 g/ft.sup.3), and 0.177 g/L (5 g/ft.sup.3). The washcoat layer B also contains 55 g/L of cerium oxide in the case of a washcoat loading of layer B of approximately 94 g/L. The catalyst thus obtained is referred to below as K2.

EXAMPLE 3

[0063] Example 2 was repeated, with the difference that washcoat layer B additionally contained 2.5 g/L manganese oxide in the form of manganese carbonate.

[0064] The catalyst thus obtained is referred to below as K3.

[0065] The NOx conversion of K2 and K3 was measured as described above. FIG. 2 shows the result.

EXAMPLE 4

[0066] Example 2 was repeated, with the difference that washcoat layer A contained 15 g/L manganese oxide in the form of manganese carbonate.

[0067] The catalyst thus obtained is referred to below as K4.

[0068] The NOx conversion of K2 and K4 was measured as described above. FIG. 3 shows the result.

EXAMPLES 5 THROUGH 10

[0069] Example 1 was repeated, with the difference that the quantities of cerium oxide and manganese oxide specified in Table 1 below were used.

[0070] The catalysts thus obtained are called K2 through K6.

TABLE-US-00001 TABLE 1 CeO.sub.2 CeO.sub.2 MnO MnO Washcoat A Washcoat B Washcoat A Washcoat B Catalyst [g/L] [g/L] [g/L] [g/L] K5 110 25 5 1 K6 125 40 7.5 K7 140 60 2.5 K8 155 100 2.5 2.5 K9 155 22 7.5 0.5 K10 110 129 2 2.5