Exhaust gas treatment system

1. An exhaust gas treatment system comprising:

an ammonia generating catalyst comprising a NO x storage component, a refractory metal oxide support, a platinum component, and a palladium component, wherein the platinum component and the palladium components are present in a platinum to palladium ratio of greater than 6 to 1, and wherein the ammonia generating catalyst is substantially free of ceria;

wherein the NO x storage component is present in an amount of about 15 to about 25 wt. % on an oxide basis, based on total weight of the ammonia generating catalyst;

and wherein the ammonia generating catalyst is disposed on a flow-through monolith substrate carrier; and

an ammonia selective catalytic reduction (SCR) catalyst downstream of the ammonia generating catalyst.

2. The exhaust gas treatment system of claim 1 , wherein the NO x storage component is selected from an oxide or carbonate of an alkali or alkaline earth metal.

3. The exhaust gas treatment system of claim 2 , wherein the NO x storage component is selected from an oxide or carbonate of one or more of cesium, barium, magnesium, calcium, and strontium.

4. The exhaust gas treatment system of claim 3 , wherein the NO x storage component is selected from an oxide or carbonate of barium.

5. The exhaust gas treatment system of claim 1 , wherein the refractory metal oxide support is selected from alumina, silica, titania, zirconia and combinations thereof.

6. The exhaust gas treatment system of claim 1 , wherein the Pt/Pd ratio is in the range of 6/1 to 100/1.

7. The exhaust gas treatment system of claim 6 , wherein the Pt/Pd ratio is in the range of 6/1 to 20/1.

8. The exhaust gas treatment system of claim 1 , wherein the ammonia generating catalyst and the SCR catalyst are on separate substrates.

9. The exhaust gas treatment system of claim 1 , wherein the ammonia generating catalyst and the SCR catalyst are on a single substrate.

10. The exhaust gas treatment system of claim 9 , wherein the substrate comprises a wall-flow filter having inlet passages and outlet passages, and wherein the ammonia generating catalyst is on the inlet passages and the SCR catalyst is on the outlet passages.

11. The exhaust gas treatment system of claim 1 , wherein the ammonia generating catalyst comprises barium, alumina, platinum, and palladium.

12. The exhaust gas treatment system of claim 1 , wherein the SCR catalyst comprises one or more of a molecular sieve material, and a mixed oxide.

13. The exhaust gas treatment system of claim 12 , wherein the molecular sieve material has a double six-ring (d6r) unit.

14. The exhaust gas treatment system of claim 13 , wherein the molecular sieve material is selected from framework types AEI, CHA, and AFX.

15. The exhaust gas treatment system of claim 14 , wherein the molecular sieve material has the CHA framework type.

16. The exhaust gas treatment system of claim 12 , wherein the molecular sieve material has a silica to alumina ratio in the range of 2 and 100.

17. The exhaust gas treatment system of claim 12 , wherein the molecular sieve material is promoted with a metal selected from Cu, Fe, Co, Ni, La, Ce, Mn, V, Ag, and combinations thereof.

18. The exhaust gas treatment system of claim 1 , wherein the SCR catalyst is disposed on a wall-flow filter.

19. The exhaust gas treatment system of claim 1 , wherein the SCR catalyst is disposed on a flow-through substrate.

20. The exhaust gas treatment system of claim 1 , wherein a wall flow filter is disposed upstream from the SCR catalyst, the filter having a three-way conversion (TWC) catalyst and the ammonia generating catalyst thereon.

21. The exhaust gas treatment system of claim 1 , further comprising a three-way conversion (TWC) catalyst.

22. The exhaust gas treatment system of claim 21 , wherein the TWC catalyst is upstream of the ammonia generating catalyst.

23. The exhaust gas treatment system of claim 22 , wherein the TWC catalyst comprises a TWC catalyst on a filter.

24. The exhaust gas treatment system of claim 21 , wherein the TWC catalyst is downstream of the ammonia generating catalyst.

25. The exhaust gas treatment system of claim 1 , further comprising an ammonia oxidation catalyst downstream of the SCR catalyst.

26. An exhaust gas treatment system comprising:

an ammonia generating catalyst that is substantially free of ceria and that comprises a NO x storage component, a refractory metal oxide support, a platinum component, and a palladium component, wherein the platinum component and the palladium components are present in a platinum to palladium ratio of greater than 6 to 1;

and wherein the NO x storage component is present in an amount of about 15 to about 25 wt. % on an oxide basis, based on total weight of the ammonia generating catalyst; and

an ammonia selective catalytic reduction (SCR) catalyst comprising a molecular sieve material having a double six-ring (d6r) unit downstream of the ammonia generating catalyst.

27. A method of treating an engine exhaust gas stream of a gasoline engine, the method comprising flowing the engine exhaust gas stream over an ammonia generating catalyst; and directing the exhaust gas stream through a downstream SCR catalyst, wherein the ammonia generating catalyst is substantially free of ceria and comprises a NO x storage component, a refractory metal oxide support, a platinum component, and a palladium component, wherein the platinum component and the palladium component are present in a platinum to palladium ratio of greater than 6 to 1; and wherein the NO x storage component is present in an amount of about 15 to about 25 wt. % on an oxide basis, based on total weight of the ammonia generating catalyst.