Exhaust system including NOx reduction catalyst and EGR circuit

1. An exhaust system for a vehicular lean burn internal combustion engine that emits oxides of nitrogen (NO x ) and particulate matter (PM), the system comprising a NO x reduction catalyst for reducing NO x in the presence of a reductant, means for introducing reductant into a flowing exhaust gas and a source of reductant selected from the group consisting of a hydrocarbon reductant, a nitrogenous reductant, hydrogen and mixtures of any two or more thereof, a filter for removing PM from exhaust gas flowing in the exhaust system and a low pressure exhaust gas recirculation (EGR) circuit for connecting the exhaust system downstream of the filter to an air intake of the engine, wherein the EGR circuit comprises a NO x , adsorber catalyst (NAC) comprising a nitric oxide (NO) adsorbent, and wherein the NO adsorbent consists of palladium and a cerium oxide or a mixed oxide or composite oxide containing cerium and at least one other transition metal.

2. An exhaust system according to claim 1 , wherein the at least one other transition metal in the mixed oxide or composite oxide is zirconium.

3. An exhaust system according to claim 1 , wherein the NO adsorbent has a palladium loading ranging from 0.1 to 200 g/ft −3 .

4. An exhaust system according to claim 1 , comprising an oxidation catalyst for oxidising NO to nitrogen dioxide located upstream of (i) the filter, (ii) the NO x reduction catalyst, or (iii) the filter and the NO x reduction catalyst.

5. An exhaust system according to claim 1 , wherein the NO x reduction catalyst selectively catalyzes the reduction of NO x using a nitrogenous reductant.

6. An exhaust system according to claim 1 , wherein the nitrogenous reductant is ammonia, hydrazine or an ammonia precursor selected from the group consisting of urea ((NH 2 ) 2 CO), ammonium carbonate, ammonium carbamate, ammonium hydrogen carbonate and ammonium formate.

7. An exhaust system according to claim 1 , wherein the NO adsorbent is combined with a thermally regenerable NO x adsorbent for net adsorbing NO x at above 200° C. and net desorbing NO x at above 250° C. which thermally regenerable NO x absorbent comprising platinum and a metal oxide.

8. An exhaust system according to claim 7 , wherein the thermally regenerable NO x absorbent comprises platinum dispersed on (i) alumina, (ii) zirconia, or (iii) alumina and zirconia.

9. An exhaust system according to claim 7 , wherein the NO adsorbent is present in an underlayer and the thermally regenerable NO x absorbent is present in a layer overlying the underlayer.

10. An exhaust system according to claim 1 , wherein the NO x reduction catalyst is located downstream of the filter.

11. An exhaust system according to claim 10 , wherein the means for introducing reductant into a flowing exhaust gas is located between the filter and the NO x reduction catalyst.

12. An exhaust system according to claim 1 , wherein the NO x reduction catalyst is located on the filter.

13. An exhaust system according to claim 12 , wherein the means for introducing reductant into a flowing exhaust gas is located upstream of the filter.

14. An exhaust system according to claim 1 , comprising means for controlling the introduction of reductant into the exhaust gas in order to reduce NO x therein.

15. An exhaust system according to claim 14 , wherein the control means comprises an electronic control unit.

16. An exhaust system according to claim 14 , wherein the control means comprises a NO x sensor located downstream of the NO reduction catalyst.

17. A lean-burn internal combustion engine comprising an exhaust system according to claim 1 .

18. The lean-burn internal combustion engine according to claim 17 , wherein said engine is a compression ignition engine.

19. An exhaust system for a vehicular lean burn internal combustion engine that emits oxides of nitrogen (NO x ) and particulate matter (PM), the system comprising a NO x reduction catalyst for reducing NO x in the presence of a reductant, means for introducing reductant into a flowing exhaust gas and a source of reductant selected from the group consisting of a hydrocarbon reductant, a nitrogenous reductant, hydrogen and mixtures of any two or more thereof, a filter for removing PM from exhaust gas flowing in the exhaust system and a low pressure exhaust gas recirculation (EGR) circuit for connecting the exhaust system downstream of the filter to an air intake of the engine, wherein the EGR circuit comprises a NO x adsorber catalyst (NAC) comprising a nitric oxide (NO) adsorbent, and wherein the NO adsorbent includes palladium dispersed on ceria or a mixed oxide or composite oxide containing cerium and at least one other transition metal.

20. An exhaust system according to claim 19 , wherein the at least one other transition metal in the mixed oxide or composite oxide is zirconium.

21. An exhaust system according to claim 19 , wherein the NO adsorbent has a palladium loading ranging from 0.1 to 200 g/ft −3 .

22. An exhaust system according to claim 19 , comprising an oxidation catalyst for oxidising NO to nitrogen dioxide located upstream of (i) the filter, (ii) the NO x reduction catalyst, or (iii) the filter and the NO x reduction catalyst.

23. An exhaust system according to claim 19 , wherein the NO x reduction catalyst selectively catalyzes the reduction of NO x using a nitrogenous reductant.

24. An exhaust system according to claim 19 , wherein the nitrogenous reductant is ammonia, hydrazine or an ammonia precursor selected from the group consisting of urea ((NH 2 ) 2 CO), ammonium carbonate, ammonium carbamate, ammonium hydrogen carbonate and ammonium formate.

25. An exhaust system according to claim 19 , wherein the NO adsorbent is combined with a thermally regenerable NO x adsorbent for net adsorbing NO x at above 200° C. and net desorbing NO x at above 250° C., which thermally regenerable NO x absorbent comprising platinum and a metal oxide.

26. An exhaust system according to claim 25 , wherein the thermally regenerable NO x absorbent comprises platinum dispersed on (i) alumina, (ii) zirconia, or (iii) alumina and zirconia.

27. An exhaust system according to claim 25 , wherein the NO adsorbent is present in an underlayer and the thermally regenerable NO x absorbent is present in a layer overlying the underlay.

28. An exhaust system according to claim 19 , wherein the NO x reduction catalyst is located downstream of the filter.

29. An exhaust system according to claim 28 , wherein the means for introducing reductant into a flowing exhaust gas is located between the filter and the NO x reduction catalyst.

30. An exhaust system according to claim 19 , wherein the NO x reduction catalyst is located on the filter.

31. An exhaust system according to claim 30 , wherein the means for introducing reductant into a flowing exhaust gas is located upstream of the filter.

32. An exhaust system according to claim 19 , comprising means for controlling the introduction of reductant into the exhaust gas in order to reduce NO x therein.

33. An exhaust system according to claim 32 , wherein the control means comprises an electronic control unit.

34. An exhaust system according to claim 32 , wherein the control means comprises a NO x sensor located downstream of the NO reduction catalyst.

35. A lean-burn internal combustion engine comprising an exhaust system according to claim 19 .

36. The lean-burn internal combustion engine according to claim 35 , wherein said engine is a compression ignition engine.