Emission treatment system with NSR and SCR catalysts

1. An emissions treatment system for an exhaust stream, comprising: a NOx storage reduction (NSR) catalyst comprising a NOx sorbent at a concentration of at least 0.1 g/in 3 and a platinum group metal component dispersed on a refractory metal oxide support; and, an SCR catalyst disposed downstream of the NSR catalyst, wherein the NSR catalyst further comprises an oxygen storage component selected from one or more oxides of cerium and praseodymium in an amount less than 0.5 g/in 3 .

2. The emissions treatment system of claim 1 , wherein the platinum group metal components of the NSR catalyst are selected from the group consisting of platinum, palladium, rhodium components and mixtures thereof

3. The emissions treatment system of claim 1 , wherein the oxygen storage component is ceria.

4. The emissions treatment system of claim 1 , wherein the NOx sorbent is at least one metal oxide selected from the oxides of lithium, sodium, potassium, cesium, magnesium, calcium, strontium, barium, lanthanum and neodymium.

5. The emissions treatment system of claim 1 , wherein there is less than 0.35 g/in 3 of the oxygen storage component present in the NSR catalyst.

6. The emissions treatment system of claim 1 , wherein amount of the oxygen storage component present in the NSR catalyst is in the range of from about 0.05 g/in 3 to about 0.5 g/in 3 .

7. The emissions treatment system of claim 1 , wherein the refractory metal oxide of the NSR catalyst is selected from the group consisting of alumina, titania, zirconia, zeolites, ceria dispersed on alumina, titania dispersed on alumina and silica on alumina.

8. The emissions treatment system of claim 1 , wherein the SCR catalyst comprises at least one component is selected from the group consisting of zeolite component, V 2 O 5 , WO 3 and TiO 2 .

9. The emissions treatment system of claim 1 , wherein the SCR catalyst comprises a zeolite component.

10. The emissions treatment system of claim 9 , wherein the SCR catalyst comprises a zeolite ion exchanged with ions selected from the group consisting of iron and copper.

11. The emissions treatment system of claim 10 , wherein the SCR catalyst comprises a zeolite ion exchanged with iron.

12. The emissions treatment system of claim 1 , wherein the NSR catalyst comprises at least two catalyst layers deposited on a substrate.

13. The emission treatment system of claim 12 , wherein the two layers comprise a top layer and a bottom layer, and the NOx sorbent in the bottom layer is present in an amount greater than about 0.3 g/in 3 and that the NOx sorbent in the top layer is present in the amount of 0.0 to less than about 0.3 g/in 3 .

14. The emissions treatment system of claim 1 , wherein the NSR catalyst is a single catalyst layer deposited on a substrate.

15. The emissions treatment system of claim 1 , further comprising a diesel engine or a lean burn gasoline engine.

16. The emissions treatment system of claim 1 , wherein the system further comprises a controller to periodically lower the air/fuel ratio in the exhaust stream upstream of the NSR catalyst.

17. The emissions treatment system of claim 16 , wherein the controller comprises an injector that periodically meters a reducing agent selected from at least one of a hydrocarbon fuel, carbon monoxide and hydrogen into the exhaust stream upstream of the NSR catalyst to form a rich gaseous stream.

18. The emissions treatment system of claim 1 , wherein the SCR catalyst is disposed on a ceramic or metallic honeycomb flow through substrate.

19. The emissions treatment system of claim 1 , wherein the NSR catalyst is disposed on a honeycomb flow through substrate.

20. The emissions treatment system of claim 1 , further comprising an upstream diesel oxidation catalyst upstream of the NSR catalyst and/or a downstream oxidation catalyst downstream of the SCR catalyst.

21. The emissions treatment system of claim 20 , further comprising the upstream diesel oxidation catalyst upstream of the NSR catalyst.

22. The emissions treatment system of claim 20 , further comprising the downstream diesel oxidation catalyst downstream of the SCR catalyst.

23. The emissions treatment system of claim 20 , further comprising the upstream diesel oxidation catalyst upstream of the NSR catalyst and the downstream oxidation catalyst downstream of the SCR catalyst.

24. The emissions treatment system of claim 1 , wherein there is a soot filter disposed upstream of the NSR or between the NSR catalyst and the SCR catalyst.

25. The emission treatment system of claim 1 , wherein the NSR is present in an amount sufficient to sorb NOx in a gaseous stream and to reduce the sorbed NOx to NH 3 upon contact with the gaseous stream.

26. The emission treatment system of claim 1 , wherein the NOx sorbent is present in an amount from about 0.1 g/in 3 to 1.0 g/in 3 .

27. The emission treatment system of claim 1 , wherein the NOx sorbent is present in an amount of at least about 0.3 g/in 3 .