Exhaust system with a modified lean NO

1. An exhaust system for treating an exhaust gas from an internal combustion engine, comprising a modified lean NO x trap (LNT), a urea injection system, and an ammonia-selective catalytic reduction (NH 3 -SCR) catalyst, wherein the modified LNT stores NO x at temperatures below about 200° C. and releases the stored NO x at temperatures above about 200° C. and the modified LNT comprises:

(a) a first layer comprising a NO x adsorbent component and one or more platinum group metals; and

(b) a second layer comprising a diesel oxidation catalyst zone comprising a platinum group metal, a zeolite, and optionally an alkaline earth metal; and an NO oxidation zone comprising platinum group metal and a carrier.

2. The exhaust system of claim 1 wherein the urea injection system injects urea at temperatures above about 180° C.

3. The exhaust system of claim 1 wherein the NO x adsorbent component comprises an alkaline earth metal, an alkali metal, a rare earth metal, and mixtures thereof.

4. The exhaust system of claim 1 wherein the NO x adsorbent component comprises barium.

5. The exhaust system of claim 3 wherein the an alkaline earth metal, an alkali metal, a rare earth metal, and mixtures thereof is supported on an inorganic oxide material.

6. The exhaust system of claim 5 wherein the inorganic oxide material is a ceria-containing material or a magnesia-alumina.

7. The exhaust system of claim 1 wherein the one or more platinum group metals of the first layer is selected from the group consisting of palladium, platinum, rhodium, and mixtures thereof.

8. The exhaust system of claim 6 wherein the magnesia-alumina is a magnesium-aluminate spinel.

9. The exhaust system of claim 1 wherein the zeolite of the diesel oxidation catalyst zone is selected from the group of Framework Type consisting of an AEI, a CHA, a LEV, a BEA, a FAU, and MFI.

10. The exhaust system of claim 1 wherein the diesel oxidation catalyst zone further comprises an inorganic oxide support comprising an alumina or a silica-doped alumina.

11. The exhaust system of claim 1 wherein the carrier of the NO oxidation zone comprises alumina and cerin.

12. The exhaust system of claim 1 wherein the NO oxidation zone further comprises manganese.

13. The exhaust system of claim 1 wherein the NH 3 -SCR catalyst is selected from the group consisting of a vanadia-titania catalyst, a vanadia-tungsta-titania catalyst, and a metal/zeolite.

14. The exhaust system of claim 13 wherein the metal/zeolite comprises a metal selected from the group consisting of iron or copper and a zeolite selected from the group of Framework Type consisting of an AEI, a CHA, a LEV, a BEA (e.g., beta zeolite), or a FER (e.g, ferrierite).

15. The exhaust system of claim 1 wherein the NH 3 -SCR catalyst is an ammonia-selective catalytic reduction filter (NH 3 -SCRF).

16. The exhaust system of claim 15 wherein the NH 3 -SCRF comprises a filter having a NH 3 -SCR catalyst selected from the group consisting of a vanadia-titania catalyst, a vanadia-tungsta-titania catalyst, and a metal/zeolite.

17. A modified lean NO x trap (LNT) comprising:

(a) a first layer comprising a NO x adsorbent component and one or more platinum group metals; and

(b) a second layer comprising a diesel oxidation catalyst zone comprising a platinum group metal, a zeolite, and optionally an alkaline earth metal; and an NO oxidation zone comprising platinum group metal and a carrier.

18. A method for treating exhaust gas from an internal combustion engine of a vehicle, comprising:

(a) passing the exhaust gas over the modified lean NO x trap (LNT) of claim 17 to remove oxides of nitrogen (NO x ) from the exhaust gas at temperatures below about 200° C. and release the NO x at temperatures above about 200° C.;

(b) injecting urea into the exhaust gas downstream of the modified LNT at temperatures above about 180° C.; and

(c) passing an exhaust gas containing released NO x from the modified LNT and urea over a NH 3 -SCR catalyst to convert the NO x to nitrogen.

19. The method of claim 18 further comprising periodically subjecting the modified LNT to a temperature above about 500° C. in a rich air:fuel ratio environment to remove sulfur that has accumulated on the modified LNT.