COATING SUSPENSION

Abstract

The invention relates to a coating suspension containing at least one platinum group metal on a support material, as well as manganese(II) carbonate, and to a method for coating a catalyst support substrate.

Claims

1. Coating suspension containing at least one platinum group metal on a support material and manganese(II) carbonate.

2. Coating suspension according to claim 1, characterized in that it contains platinum, palladium, and/or rhodium as platinum group metals.

3. Coating suspension according to claim 1, characterized in that it contains platinum and palladium or platinum alone as platinum group metals.

4. Coating suspension according to claim 1, characterized in that it contains platinum and palladium in a ratio of 15:1 to 1:5 as platinum group metals.

5. Coating suspension according to claim 1, characterized in that it contains aluminum oxide, rare-earth doped aluminum oxide, silicon dioxide, titanium dioxide, cerium oxide, aluminum-silicon mixed oxides, cerium-zirconium mixed oxides, or rare-earth doped cerium-zirconium mixed oxides as support material.

6. Coating suspension according to claim 1, characterized in that it contains aluminum oxide stabilized with 1 to 6 wt % La.sub.2O.sub.3 as support material.

7. Coating suspension according to claim 1, characterized in that it contains manganese(II) carbonate in quantities of 3 to 20 kg/m.sup.3 (from 3 to 20 g/L) with respect to the volume of the catalyst support substrate to be coated.

8. Coating suspension according to claim 1, characterized in that it contains one or more zeolite compounds.

9. Coating suspension according to claim 8, characterized in that the zeolite compound is selected from the group consisting of zeolite β (zeolite beta), zeolite Y, ZSM-5, and mixtures of two or more of these.

10. Method for coating a catalyst support substrate with a coating which includes a hydrogen sulfide blocking function and an oxidation function, characterized in that the catalyst support substrate is brought into contact with a coating suspension according to claim 1 and then dried and calcined.

11. Method according to claim 10, characterized in that monolithic through-flow honeycomb structures made of ceramic or metal are used as catalyst support substrate.

12. Method according to claim 10, characterized in that ceramic through-flow honeycomb structures or ceramic wall-flow filter substrates made of cordierite, aluminum titanate, or silicon carbide are used as catalyst support substrate.

Description

[0038] The invention is explained in more detail by means of figures and examples below:

[0039] FIG. 1 shows the results of a test in which the effectiveness of filter K3 as a hydrogen sulfide blocking catalytic converter, said filter being coated with a coating suspension according to the invention, was investigated.

[0040] FIG. 2 shows the results of a test in which the effectiveness of filter VK1 as a hydrogen sulfide blocking catalytic converter, said filter being coated with a conventional coating suspension, was investigated.

EXAMPLE 1

[0041] (a) In order to prepare a coating suspension according to the invention, a suspension in water of 15 kg/m.sup.3 (15 g/L) (with respect to the volume of the wall-flow filter substrate to be coated (see step (b)) of a γ-aluminum oxide stabilized with 4% La.sub.2O.sub.3 was created by constant stirring and coated by the injection method with 0.176 kg/m.sup.3 (5 g/ft.sup.3) (with respect to the volume of the wall-flow filter substrate to be coated (see step (b)) of platinum and palladium in a 2:1 ratio. In the next step, 9.5 kg/m.sup.3 (9.5 g/L) (with respect to the volume of the wall-flow filter substrate to be coated (see step (b)) of powdered manganese(II) carbonate was added and the mixture ground in a ball mill until a D.sub.99 value of <5 μm was reached. (b) A commercially available wall-flow filter substrate made of silicon carbide was coated with the coating suspension obtained in step (a). Here, the filter substrate was oriented such that the flow ducts were perpendicular. The suspension was then pumped into the substrate via the lower end face. After a short interval, the excess suspension was suctioned downwards. With this method, the coating was substantially introduced into the pores of the substrate walls. The coating step was not repeated.

[0042] The filter was then dried and calcined at 550° C. The fully-coated and calcined wall-flow filter substrate no longer contains any manganese(II) carbonate, but only manganese oxide.

EXAMPLE 2

[0043] In order to prepare a coating suspension according to the invention, a suspension in water of 8 kg/m.sup.3 (8 g/L) (with respect to the volume of the wall-flow filter substrate to be coated (see step (b)) of a γ-aluminum oxide stabilized with 4% La.sub.2O.sub.3, which was coated with 0.7 kg/m.sup.3 (20 g/ft.sup.3) (with respect to the volume of the wall-flow filter substrate to be coated (see step (b)) of platinum and palladium in a 2:1 ratio, and 4 kg/m.sup.3 (4 g/L) (with respect to the volume of the wall-flow filter substrate to be coated (see step (b)) of the identical γ-aluminum oxide stabilized with 4% La.sub.2O.sub.3, but not coated with precious metal, was created by constant stirring. In the next step, 10 kg/m.sup.3 (10 g/L) (with respect to the volume of the wall flow filter substrate to be coated (see step (b)) of powdered manganese(II) carbonate was added and the mixture ground in a ball mill until a D value of <5 μm was reached.

[0044] With the coating suspension thus obtained, a commercially available wall-flow filter substrate of silicon carbide was coated as indicated in Example 1, step (b).

EXAMPLE 3

[0045] In order to prepare a coating suspension according to the invention, a suspension in water of 5 kg/m.sup.3 (5 g/L) (with respect to the volume of the wall-flow filter substrate to be coated (see step (b)) of a γ-aluminum oxide stabilized with 4% La.sub.2O.sub.3, which was coated with 0.53 kg/m.sup.3 (15 g/ft.sup.3) (with respect to the volume of the wall-flow filter substrate to be coated (see step (b)) of platinum and palladium in a 6:1 ratio, and 6 kg/m.sup.3 (6 g/L) (with respect to the volume of the wall-flow filter substrate to be coated (see step (b)) of the identical γ-aluminum oxide stabilized with 4% La.sub.2O.sub.3, but not coated with precious metal, was created by constant stirring. In the next step, 16 kg/m.sup.3 (16 g/L) (with respect to the volume of the wall-flow filter substrate to be coated (see step (b)) of powdered manganese(II) carbonate was added and the mixture ground in a ball mill until a D.sub.99 value of <7 μm was reached. With the coating suspension thus obtained, a commercially available wall-flow filter substrate of silicon carbide was coated as indicated in Example 1, step (b). The target loading was 25.03 kg/m.sup.3 (25.06 g/L). A pH value of >8 was maintained, not only during the preparation of the coating suspension, but also during the coating process. The filter thus obtained is referred to below as K3.

COMPARATIVE EXAMPLE 1

[0046] Example 3 was repeated with the difference that, instead of 16 kg/m.sup.3 (16 g/L) of powdered manganese(II) carbonate, 12 kg/m.sup.3 (12 g/L) of powdered manganese(IV) oxide was used.

[0047] The filter thus obtained is referred to below as VK1.

[0048] After the grinding process, the washcoat tubes of the ball mill were contaminated with a brown layer and had to be disposed of. In the case of Example 3, on the other hand, the washcoat tubes were still in very good condition and could continue to be used.

[0049] The catalytic activity of both filter VK1, prepared in the comparative example, and filter K3 from Example 3 was investigated,

[0050] Before the investigation, the filters underwent artificial aging under hydrothermal conditions. To do so, they were exposed in an oven to a temperature of 800° C. for 16 hours in an atmosphere containing 10 vol % steam and 10 vol % oxygen in nitrogen. The ability to oxidize H.sub.2S catalytically to SO.sub.2 under the conditions that may occur during the desulfurization of an upstream NOx storage catalytic converter was tested in a model gas test. To do so, in each case a system consisting of a commercially available NOx storage catalytic converter and a particulate filter directly downstream of it was sulfurized. The volumes of the NOx storage catalytic converter and particulate filter were identical in both systems. The selected quantity of sulfur was approx. 2.4 kg/m.sup.3 (2.4 of catalytic converter volume and was introduced by adding 300 vol.ppm SO.sub.2 to the exhaust gas stream. In each case, a desulfurization with a catalytic converter inlet temperature of 600° C. was then simulated. Here, the air-fuel equivalence ratio λ was reduced to about 0.93 in 40 intervals of 8 seconds each by increasing the CO and HC concentrations (CO: 1.85 vol %; HC: 5700 vol.ppm) or by reducing the oxygen concentration in the exhaust gas stream (interval between these rich phases: 30 s).

[0051] The results are shown in FIG. 1 for filter K3 coated with a coating suspension according to the invention and in FIG. 2 for the comparison filter VK1. It is evident that the H.sub.2S concentration measured after filtering is considerably lower in the case of K1.