Exhaust system with a modified lean NOx trap

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 comprising copper and a zeolite selected from the group of Framework Type consisting of an AEI or a CHA, 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 selected from the group consisting of palladium, platinum, and mixtures thereof; 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 2 , 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 3 , wherein the alkaline earth metal, an alkali metal, a rare-earth metal, and mixtures thereof is supported on an inorganic oxide material.

5. The exhaust system of claim 4 , wherein the inorganic oxide material is a ceria-containing material or a magnesia-alumina.

6. The exhaust system of claim 5 , wherein the magnesia-alumina is a magnesium-aluminate spinel.

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

8. The exhaust system of claim 5 , 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.

9. The exhaust system of claim 8 , wherein the diesel oxidation catalyst zone further comprises an inorganic oxide support comprising an alumina or a silica-doped alumina.

10. The exhaust system of claim 9 , wherein the carrier of the NO oxidation zone comprises alumina and cerin.

11. The exhaust system of claim 10 , wherein the NO oxidation zone further comprises manganese.

12. The exhaust system of claim 11 wherein the NH 3 -SCR catalyst is an ammonia-selective catalytic reduction filter (NH 3 -SCRF).

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

(a) passing the exhaust gas over a modified lean NO x trap (LNT) 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 comprising copper and a zeolite selected from the group of Framework Type consisting of an AEI or a CHA to convert the NO x to nitrogen.

14. The method of claim 13 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.