Method for preventing a selective catalytic reduction (SCR) catalyst from being contaminated with platinum

Abstract

The present invention relates to a method for preventing an SCR catalyst from being contaminated with platinum group metal in an emission control system comprising, upstream of the SCR catalyst, a catalyst that contains platinum group metal, characterized in that a material zone containing a mixture of aluminum oxide and cerium oxide is located upstream of the SCR catalyst.

Claims

1. A method for avoiding platinum group metal contamination of an SCR catalyst that is positioned for exhaust gas contact in an exhaust gas treatment system; comprising passing the exhaust gas into contact with a catalyst containing platinum group metal upstream of the SCR catalyst, the method further comprising passing the exhaust gas into contact with a material zone that contains a mixture of aluminum oxide and cerium oxide, which material zone is located upstream of the SCR catalyst receiving the exhaust gas.

2. The method according to claim 1, wherein the catalyst containing platinum group metal is a platinum group metal-containing oxidation catalyst or a particulate filter provided with an oxidation-catalytically active coating that contains platinum group metal.

3. The method according to claim 1, wherein the platinum group metal is platinum.

4. The method according to claim 1, wherein the mixture of aluminum oxide and cerium oxide comprises aluminum oxide in amounts of 10 to 90% by weight relative to the weight of the mixture of aluminum oxide and cerium oxide.

5. The method according to claim 1, wherein the mixture of aluminum oxide and cerium oxide is a physical mixture.

6. The method according to claim 1, wherein the material zone that contains a mixture of aluminum oxide and cerium oxide is borne on a catalyst substrate that is located between the platinum group metal-containing catalyst and the SCR catalyst.

7. The method according to claim 1, wherein the material zone that contains a mixture of aluminum oxide and cerium oxide is located as an additional material zone on the catalyst containing platinum group metal.

8. The method according to claim 7, wherein the catalyst containing platinum group metal comprises an oxidation-catalytically active coating distributed homogeneously over the entire length of the substrate, and the material zone that contains a mixture of aluminum oxide and cerium oxide is present as an additional layer on the oxidation-catalytically active coating and covers at least a part of its entire length the oxidation-catalytically active coating, starting from the end of the platinum group metal-containing catalyst arranged downstream.

9. The method according to claim 1, wherein the catalyst containing platinum group metal comprises an oxidation-catalytically active coating, and the oxidation-catalytically active coating and the material zone that contains a mixture of aluminum oxide and cerium oxide are present in separate zones on the platinum group metal-containing catalyst.

10. The method according to claim 9, wherein the material zone that contains a mixture of aluminum oxide and cerium oxide takes up 25 to 67% of the entire length of the catalyst containing platinum group metal.

11. The method according to claim 1, wherein the catalyst containing platinum group metal has a platinum-rich coating of the platinum group metal.

12. The method of according to claim 11, wherein the platinum-rich coating contains exclusively platinum as the platinum group metal.

13. The method of according to claim 11, wherein the platinum-rich coating contains exclusively platinum and palladium as the platinum group metal with a weight ratio of platinum to palladium being 12:1 to 4:1.

14. The method according to claim 1, wherein the mixture of aluminum oxide and cerium oxide comprises aluminum oxide in amounts of 30 to 70% by weight relative to the weight of the mixture of aluminum oxide and cerium oxide.

15. The method according to claim 1, wherein the mixture of aluminum oxide and cerium oxide comprises aluminum oxide in amounts of 40 to 60% by weight relative to the weight of the mixture of aluminum oxide and cerium oxide.

16. The method according to claim 1, wherein the mixture of aluminum oxide and cerium oxide comprises aluminum oxide in amounts of 30 to 90% by weight relative to the weight of the mixture of aluminum oxide and cerium oxide.

17. The method according to claim 1, wherein the mixture of aluminum oxide and cerium oxide comprises aluminum oxide in amounts of 40 to 90% by weight relative to the weight of the mixture of aluminum oxide and cerium oxide.

Description

EXAMPLES

(1) A conventional oxidation catalyst that is a loading of 0.7 g/L (20 g/ft.sup.3) aluminum oxide on a conventional flow-through substrate was combined with different aluminum oxide/cerium oxide mixtures as platinum catchers as indicated in table 1. To this end, a catalyst was respectively coated with the corresponding amount of platinum catcher and placed downstream of the oxidation catalyst. The lengths of the oxidation catalyst and platinum catcher were selected so that 67% of the entire length was taken up by the oxidation catalyst, and 33% of the entire length was allotted to the platinum catcher.

(2) In the fresh state, they were subjected to a model gas of the following composition in a conventional model gas system with an isothermal reactor at a constant temperature of 650° C. over 18 hours in order to determine the platinum migration of the oxidation catalysts:

(3) TABLE-US-00001 CO 71 ppm NO 820 ppm O.sub.2 6.3 volume percent CO.sub.2 8.9 volume percent H.sub.2O 11 volume percent in the measurements lasting 18 hours, and 5 volume percent in the measurements lasting 60 hours N.sub.2 Balance

(4) The gas leaving downstream of the respective oxidation catalyst was guided through a flow-through substrate having a length of 5 cm (2″) and coated with an SCR catalyst of the iron-ß-zeolite type. The amount of platinum bound in the SCR catalyst was then determined for each catalyst. To this end, a drilling core of the catalyst was melted in a crucible by means of an NiS fire assay, and the platinum was then determined by means of ICP-OES.

(5) As a comparison, pure aluminum oxide and pure cerium oxide were measured. The following results were obtained:

(6) TABLE-US-00002 Composition Amount of Example [percent by weight] platinum [ppm] Example 1 90 aluminum oxide 1 + <0.2 10 cerium oxide 1 Example 2 70 aluminum oxide 1 + <0.2 30 cerium oxide 1 Example 3 70 aluminum oxide 1 + <0.2 30 cerium oxide 1 Comparative example 1 Cerium oxide 1 0.2 Comparative example 2 Cerium oxide 2 0.3 Comparative example 3 Aluminum oxide 2 1.8 Comparative example 4 Aluminum oxide 3 2.4

(7) Aluminum oxide 1 is a commercially available pure aluminum oxide

(8) Aluminum oxide 2 is a commercially available mesoporous aluminum oxide doped with 4% by weight lanthanum from manufacturer 1

(9) Aluminum oxide 3 is a commercially available mesoporous aluminum oxide doped with 4% by weight lanthanum from manufacturer 2

(10) Cerium oxide 1 is a commercially available pure cerium oxide from manufacturer 3

(11) Cerium oxide 2 is a commercially available pure cerium oxide from manufacturer 4

(12) The results show that cerium oxide, at least up to 90% by weight, can be replaced by the cheaper aluminum oxide without worsening the effect.