Cold start catalyst and its use in exhaust systems
A cold start catalyst is disclosed. The cold start catalyst is effective to adsorb NO x and hydrocarbons (HC) at or below a low temperature and to covert and release the adsorbed NO x and HC at temperatures above the low temperature. The cold start catalyst comprises a molecular sieve catalyst and a supported platinum group metal catalyst. The molecular sieve catalyst consists essentially of a noble metal and a molecular sieve. The supported platinum group metal catalyst comprises one or more platinum group metals and one or more inorganic oxide carriers. The invention also includes an exhaust system comprising the cold start catalyst, and a method for treating exhaust gas from an internal combustion engine utilizing the cold start catalyst.
1. A cold start catalyst article comprising: (1) a molecular sieve catalyst coating consisting of palladium on a molecular sieve having a maximum ring size of eight tetrahedral atoms for adsorbing NO x from a diesel engine exhaust gas (2) a supported platinum group metal catalyst coating comprising one or more platinum group metals and one or more inorganic oxide carriers, and (3) a flow-through honeycomb monolith or a wall-flow filter substrate, wherein the molecular sieve catalyst is disposed on or in the substrate as a bottom coating and the platinum group metal catalyst is disposed on the molecular sieve catalyst coating as a top coating.
2. The cold start catalyst article of claim 1 wherein the platinum group metal is platinum.
3. The cold start catalyst article of claim 1 wherein the one or more platinum group metals is selected from the group consisting of platinum, palladium, rhodium, iridium, and mixtures thereof.
4. The cold start catalyst article of claim 1 wherein the one or more inorganic oxide carriers is selected from the group consisting of alumina, silica, titania, zirconia, ceria, niobia, tantalum oxides, molybdenum oxides, tungsten oxides.
5. The cold start catalyst article of claim 1 wherein the substrate is a filter.
6. The cold start catalyst article of claim 5 wherein the substrate is a flow-through honeycomb monolith.
7. The article of claim 1 , wherein the molecular sieve has an AEI framework.
8. The article of claim 1 , wherein the molecular sieve has a CHA framework.
9. An exhaust system for diesel engines comprising the cold start catalyst article of claim 1 .
10. The exhaust system of claim 9 further comprising a catalyst component selected from the group consisting a selective catalytic reduction (SCR) catalyst, a particulate filter, a SCR filter, a NO x adsorber catalyst, an oxidation catalyst, and combinations thereof.
11. A method for treating exhaust gas from a diesel engine, said method comprising adsorbing NO x onto the cold start catalyst article of claim 1 below a low temperature, converting and thermally desorbing NO x from the cold start catalyst article at temperatures above the low temperature, and catalytically removing the desorbed NO x on a catalyst component downstream of the cold start catalyst.
12. The method of claim 11 wherein the low temperature is 200° C.
13. The method of claim 11 wherein the catalyst component downstream of the cold start catalyst article is selected from the group consisting of a SCR catalyst, a particulate filter, a SCR filter, a NO x adsorber catalyst, an oxidation catalyst, and combinations thereof.