Exhaust system with a modified lean NO
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) NOx treatment catalysts, and
(b) a urea injection system;
wherein the NOx treatment catalysts consist essentially of a single layer lean NO x trap (LNT), comprising platinum, palladium, barium, and a ceria-containing material and a platinum:palladium molar ratio of at least 3:1; and an ammonia-selective catalytic reduction (NH 3 -SCR) catalyst;
wherein the urea injection system is upstream of the NH 3 -SCR catalyst and downstream of the LNT; and
wherein the LNT stores NO x at temperatures below about 200° C. and releases the stored NO x at temperatures above about 200° C.
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 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 exhaust system 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.