CATALYST FOR REDUCTION OF NITROGEN OXIDES

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 an upper washcoat layer B located above washcoat layer A comprises cerium oxide, platinum and palladium, does not contain any alkali and alkaline-earth compounds, and has macropores. Also disclosed is a method for converting NOx in exhaust gases from motor vehicles 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, as well as platinum and palladium; 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 or alkaline earth compounds, characterized in that the upper washcoat layer B has macropores of an average pore size of less than 15 m, wherein the macropores form a pore volume in the upper washcoat layer B of 5 to 25 vol %.

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 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 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.

7. Nitrogen oxide storage catalyst according to claim 1, characterized in that washcoat layer A contains manganese oxide.

8. Nitrogen oxide storage catalyst according to claim 7, 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 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.

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

11. Nitrogen oxide storage catalyst according to claim 10, 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.

12. Nitrogen oxide storage catalyst according to claim 1, characterized in that the macropores of the upper washcoat layer B have an average pore size of 2 to 12 m.

13. Nitrogen oxide storage catalyst according to claim 1, characterized in that the macropores form a pore volume in the upper washcoat layer B of 5 to 10 vol %.

14. Nitrogen oxide storage catalyst according to claim 1, characterized in that the macropores form a pore volume in the upper washcoat layer B of 10 to 15 vol %.

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 claim 1.

Description

[0063] The invention is explained in more detail in the examples and figures below.

[0064] FIG. 1: NOx storage amount in g/L at 50% and at 75% of the catalysts K1, K2, and VK1.

EXAMPLE 1

[0065] a) In order to produce a catalyst according to the invention, a commercially available, honeycombed, ceramic substrate is coated with a first coating suspension 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. After coating, the obtained washcoat layer A was dried.

[0066] b) Another washcoat layer B was applied to the first washcoat layer A. For this purpose, the coating took place with a coating suspension that also contained Pt and Pd carried on aluminum oxide, as well as Rh carried on a lanthanum-stabilized aluminum oxide. The loading of Pt, Pd, and Rh in washcoat layer B thus amounted 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 coating suspension moreover contained 55 g/L cerium oxide in a washcoat loading of layer B of approximately 81 g/L in the calcined catalyst.

[0067] In addition to the aforementioned components, the coating suspension also contained 5 g/L of a pore builder composed of a cross-linked polymethylmethacrylate resin of an average particle size of 5 to 7 m. The coating was dried, and calcination took place thereafter. After calcination, the pore volume in washcoat layer B was 6.5 vol %.

[0068] The catalyst thus obtained is referred to below as K1.

EXAMPLE 2

[0069] Example 1 was repeated, with the difference that the coating suspension for washcoat layer B contained the pore builder in a quantity of 7.5 g/L pore builder. After calcination, the pore volume in washcoat layer B was 9.7 vol %.

[0070] The catalyst thus obtained is referred to below as K2.

COMPARATIVE EXAMPLE 1

[0071] Example 1 was repeated, with the difference that the coating suspension for washcoat layer B did not contain any pore builder. The catalyst thus obtained is referred to below as VK1.

[0072] Comparative Tests

[0073] a) The catalysts K1, K2, and VK1 were aged hydrothermally for 16 hours at 800 C.

[0074] b) Their nitrogen oxide storage capacity was, subsequently, respectively determined as follows:

[0075] First, the sample was conditioned at 450 C. To this end, a lean gas composition according to table 1 and a rich gas composition were alternatingly guided over the catalyst for 80 s and 10 s respectively for a duration of 15 min.

TABLE-US-00001 TABLE 1 Lean Rich Adsorption GHSV [1/h] 50,000 50,000 50,000 NO [ppm] 0 0 500 O.sub.2 [vol %] 8 0 8 CO [ppm] 0 40,000 0 CO.sub.2 [vol %] 10 10 10 H.sub.2O [vol %] 10 10 10

[0076] The sample was subsequently cooled in a nitrogen atmosphere to measuring temperature (175 C. or 300 C.) or kept at 450 C. At a constant measuring temperature, the NOx adsorption in the gas composition Adsorption according to table 1 is then measured. The NOx storage capacity is calculated from the difference in the dosed NOx amount in relation to the catalyst volume from the amount of NOx slip measured behind the catalyst sample in relation to the catalyst volume at that point in time when the NOx conversion over the sample is 75% or only 50%, and is illustrated in FIG. 1 as NOx storage amount.

[0077] As a result, the NOx storage amount in g/L at 50% and at 75% conversion was specified, wherein the storage amounts of VK1 were respectively set to 100%, and the storage amounts of K1 and K2 were related thereto.

[0078] The results can be taken from FIG. 1.

EXAMPLE 3

[0079] Example 1 was repeated, with the difference that the coating suspension for washcoat layer B contained 5 g/L of a pore builder composed of a cross-linked polymethylmethacrylate resin of an average particle size of 8 to 12 m.

EXAMPLE 4

[0080] Example 1 was repeated, with the difference that the coating suspension for washcoat layer B contained 7.5 g/L of a pore builder composed of a cross-linked polymethylmethacrylate resin of an average particle size of 4 to 5 m.

[0081] Other examples are listed in Table 2

TABLE-US-00002 CeO.sub.2 CeO.sub.2 MnO MnO Pore Washcoat Washcoat Washcoat Washcoat builder/ A B A B quantity in Example [g/L] [g/L] [g/L] [g/L] (g/L) 5 110 25 5 1 a/7.5 6 125 40 c/5 7 140 60 2.5 b/5 8 155 100 2.5 2.5 c/5 9 155 22 7.5 0.5 b/7.5 10 110 129 a/5 In Table 2: a means pore builder composed of a cross-linked polymethylmethacrylate resin of an average particle size of 8 to 12 m. b means pore builder composed of a cross-linked polymethylmethacrylate resin of an average particle size of 5 to 7 m. c means pore builder composed of a polyacrylonitrile resin of an average particle size of 8 m.