EXHAUST SYSTEM WITH A MODIFIED LEAN NOx TRAP

Abstract

An exhaust system for treating an exhaust gas from an internal combustion engine is disclosed. The system comprises a modified lean NO x trap (LNT), a urea injection system, and an ammonia-selective catalytic reduction (NH 3 -SCR) catalyst. The modified LNT comprises platinum, palladium, barium, and a ceria-containing material, and has a platinum:palladium molar ratio of at least 3:1. The modified LNT stores NO x at temperatures below about 200° C. and releases the stored NO x at temperatures above about 200° C. The urea injection system injects urea at temperatures above about 180° C.

1 . An exhaust system for treating an exhaust gas from an internal combustion engine, comprising:
 (a) a lean NO x  trap (LNT), wherein the LNT comprises platinum, palladium, barium, and a ceria-containing material and the LNT has a platinum:palladium molar ratio of at least 3:1;   (b) a urea injection system; and   (c) an ammonia-selective catalytic reduction (NH 3 -SCR) catalyst,   wherein there is no intervening catalysts between the LNT and the NH 3 -SCR catalyst.   
     
     
         2 . (canceled) 
     
     
         3 . The exhaust system of  claim 1  wherein the LNT has a platinum:palladium molar ratio of at least 4:1. 
     
     
         4 . The exhaust system of  claim 1  wherein the LNT has a barium loading of greater than 150 g/ft 3 . 
     
     
         5 . The method of  claim 1  wherein the LNT has a barium loading of greater than 400 g/ft 3 . 
     
     
         6 . 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. 
     
     
         7 . The exhaust system of  claim 6  wherein the metal/zeolite comprises a metal selected from the group consisting of iron or copper and a zeolite selected from the group consisting of a beta zeolite, a faujasite, an L-zeolite, a ZSM zeolite, an SSZ-zeolite, a ferrierite, a mordenite, a chabazite, an offretite, an erionite, a clinoptilolite, a silicalite, an aluminum phosphate zeolite, and a mesoporous zeolite. 
     
     
         8 . The exhaust system of  claim 1  wherein the NH 3 -SCR catalyst is an ammonia-selective catalytic reduction filter (NH 3 -SCRF). 
     
     
         9 . The exhaust system of  claim 8  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. 
     
     
         10 . A method for treating exhaust gas from an internal combustion engine of a vehicle, comprising:
 (a) passing the exhaust gas over a 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., wherein the modified LNT comprises platinum, palladium, barium, and a ceria-containing material and the LNT has a platinum:palladium molar ratio of at least 3:1;   (b) injecting urea into the exhaust gas downstream of the LNT at temperatures above about 180° C.; and   (c) passing an exhaust gas containing released NO x  from the LNT and urea over a NH 3 -SCR catalyst to convert the NO x  to nitrogen;   wherein there is no intervening catalysts between the LNT and the NH 3 -SCR catalyst.   
     
     
         11 . The method of  claim 10  further comprising periodically subjecting the 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. 
     
     
         12 . The method of  claim 10  wherein the LNT has a platinum:palladium molar ratio of at least 4:1. 
     
     
         13 . The method of  claim 10  wherein the LNT has a barium loading of greater than 150 g/ft 3 . 
     
     
         14 . The method of  claim 10  wherein the LNT has a barium loading of greater than 400 g/ft 3 . 
     
     
         15 . An exhaust system for treating an exhaust gas from an internal combustion engine, comprising:
 a lean NOx trap (LNT), wherein the LNT comprises platinum, palladium, barium, and a ceria-containing material and the modified LNT has a platinum:palladium molar ratio of greater than 4:1; and   an ammonia-selective catalytic reduction (NH 3 -SCR) catalyst.   
     
     
         16 . The exhaust system of  claim 15  wherein there is no intervening catalysts between the LNT and the NH 3 -SCR catalyst. 
     
     
         17 . The exhaust system of  claim 15 , further comprising a urea injection system.