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 is in the form of a first washcoat;

wherein the second SCR catalyst composition is in the form of a second washcoat;

wherein the first washcoat and the second washcoat are coated on one or more substrates in a laterally zoned configuration such that the first SCR catalyst composition is located laterally upstream of the second SCR catalyst composition according to a flow direction of an engine exhaust gas stream from an engine towards a tailpipe;

wherein the first SCR catalyst composition comprises a mixed oxide comprising vanadia/titania; and

wherein the second SCR catalyst composition comprises a metal-exchanged zeolite, excluding Cu-exchanged or Fe-exchanged ZSM-5 or Beta zeolites;

wherein the SCR catalyst system provides an improved NOx conversion efficiency as compared to a SCR catalyst system comprising the first SCR catalyst composition and substantially free of the second SCR catalyst composition; and

wherein the SCR catalyst system provides a lower N 2 O formation as compared to a SCR catalyst system comprising the second SCR catalyst composition and substantially free of the first SCR catalyst composition.

2. The SCR catalyst system of claim 1 , wherein the first SCR catalyst composition and the second SCR catalyst composition are disposed on a common substrate.

3. The SCR catalyst system of claim 1 , wherein the first SCR catalyst composition and the second SCR catalyst composition are disposed on different substrates.

4. The SCR catalyst system of claim 1 , wherein the vanadia/titania is stabilized with tungsten.

5. The SCR catalyst system of claim 1 , wherein the zeolite is a metal-exchanged 8-ring small pore zeolite.

6. The SCR catalyst system of claim 5 , wherein the zeolite has a structure type selected from the group consisting of AEI, AFT, AFX, CHA, EAB, ERI, KFI, LEV, SAS, SAT, DDR, and SAV.

7. The SCR catalyst system of claim 5 , wherein the zeolite is an aluminosilicate zeolite and has the CHA structure type.

8. The SCR catalyst of claim 7 , wherein the zeolite is selected from SSZ-13 and SSZ-62.

9. The SCR catalyst system of claim 5 , wherein the metal is selected from the group consisting of Cu, Fe, Co, Ce and Ni.

10. The SCR catalyst system of claim 9 , wherein the metal is Cu.

11. The SCR catalyst system of claim 7 , wherein the zeolite is exchanged with Cu in the range of 2% to 8% by weight.

12. 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 is in the form of a first washcoat;

wherein the second SCR catalyst composition is in the form of a second washcoat;

wherein the first washcoat and the second washcoat are coated on one or more substrates in a laterally zoned configuration such that the first SCR catalyst composition is located laterally upstream of the second SCR catalyst composition according to a flow direction of an engine exhaust gas stream from an engine towards a tailpipe;

wherein the metal-exchanged 8-ring small pore molecular sieve is a metal-exchanged zeolite, excluding Cu-exchanged or Fe-exchanged ZSM-5 or Beta zeolites;

wherein the SCR catalyst system provides an improved NOx conversion efficiency as compared to a SCR catalyst system comprising the first SCR catalyst composition and substantially free of the second SCR catalyst composition; and

wherein the SCR catalyst system provides a lower N 2 O formation as compared to a SCR catalyst system comprising the second SCR catalyst composition and substantially free of the first SCR catalyst composition.

13. The SCR catalyst system of claim 12 , wherein the second SCR catalyst composition comprises Cu and an aluminosilicate zeolite having the CHA structure type.

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

15. The system of claim 14 , wherein the engine is a heavy duty diesel engine.

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

17. The method of claim 16 , wherein the lean burn engine is a heavy duty diesel engine.

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

19. The SCR catalyst system of claim 12 , 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 8-ring small pore molecular sieve has a higher ammonia storage capacity than the vanadia/titania.

20. The SCR catalyst system of claim 1 , wherein the first SCR catalyst composition and the second SCR catalyst composition are disposed on a single flow-through substrate, or wherein the first SCR catalyst composition and the second SCR catalyst composition are disposed on separate flow-through substrates.