Selective catalytic reduction catalyst system

1. A selective catalytic reduction (SCR) catalyst system comprising a first SCR catalyst composition and a second SCR catalyst composition arranged in the system, the first SCR catalyst composition promoting higher N 2 formation and lower N 2 O formation than the second SCR catalyst composition, and the second SCR catalyst composition having a different composition than the first SCR catalyst composition, the second SCR catalyst composition promoting lower N 2 formation and higher N 2 O formation than the first SCR catalyst composition, wherein the first SCR catalyst composition and the second SCR catalyst composition are in a layered relationship, with the first SCR catalyst composition layered on top of the second SCR catalyst composition.

2. The SCR catalyst system of claim 1 , wherein the first SCR catalyst composition comprises a mixed oxide.

3. The SCR catalyst system of claim 2 , wherein the mixed oxide is selected from Fe/titania, Fe/alumina, Mg/titania, Cu/titania, Ce/Zr, vanadia/titania, and mixtures thereof.

4. The SCR catalyst system of claim 3 , wherein the mixed oxide comprises vanadia/titania.

5. The SCR catalyst system of claim 4 , wherein the vanadia/titania is stabilized with tungsten.

6. The SCR catalyst system of claim 2 , wherein the second SCR catalyst comprises a metal-exchanged 8-ring small pore molecular sieve.

7. The SCR catalyst system of claim 6 , wherein the molecular sieve has a structure type selected from the group consisting of AEI, AFT, AFX, CHA, EAB, ERI, KFI, LEV, SAS, SAT, DDR, and SAV.

8. The SCR catalyst system of claim 7 , wherein the molecular sieve is an aluminosilicate zeolite and has the CHA structure type.

9. The catalyst of claim 8 , wherein the zeolite is selected from SSZ-13 and SSZ-62.

10. The catalyst system of claim 6 , wherein the metal is selected from the group consisting of Cu, Fe, Co, Ce and Ni.

11. The catalyst system of claim 10 , wherein the metal is selected from Cu.

12. The catalyst system of claim 8 , wherein the zeolite is exchanged with Cu in the range of 2% to 8% by weight.

13. A selective catalytic reduction (SCR) catalyst system comprising a first SCR catalyst composition comprising vanadia/titania disposed on a substrate and a second SCR catalyst composition comprising a metal-exchanged 8-ring small pore molecular sieve disposed on a substrate, wherein the first SCR catalyst composition and the second SCR catalyst composition are in a layered relationship, with the first SCR catalyst composition layered on top of the second SCR catalyst composition.

14. The catalyst system of claim 13 , wherein the second catalyst composition comprises Cu and an aluminosilicate zeolite having the CHA structure type.

15. The catalyst system of claim 14 , wherein the zeolite is selected from SSZ-13 and SSZ-62.

16. The catalyst system of claim 13 , wherein vanadia/titania promotes higher N 2 formation and lower N 2 O formation than the metal-exchanged 8-ring small pore molecular sieve, and wherein the metal-exchanged 8-ring small pore molecular sieve promotes lower N 2 formation and higher N 2 O formation than the vanadia/titania, and the metal-exchanged 8-ring small pore molecular sieve has a higher ammonia storage capacity than the vanadia/titania.

17. A lean burn engine exhaust gas treatment system comprising the catalyst system of claim 1 , a lean burn engine, and an exhaust gas conduit in fluid communication with the lean burn engine, wherein the catalyst system is downstream of the engine.

18. The system of claim 17 , wherein the engine is a heavy duty diesel engine.

19. A method of removing nitrogen oxides from exhaust gas from a lean burn engine, the method comprising contacting an exhaust gas stream the catalyst system of claim 1 .

20. The method of claim 19 , wherein the lean burn engine is a heavy duty diesel engine.