Filter catalyzed with SCR catalyst, systems and methods
Provided are catalyst articles, methods and systems for treating lean burn engine exhaust. Catalyst articles include selective catalytic reduction catalyst on a particulate filter. The SCR catalyst on the particulate filter provides limited NOx conversion so that unconverted NO 2 is available to facilitate passive oxidation of soot trapped on the particulate filter by reaction with NO 2 . Systems and methods utilize such catalytic articles, and further include, e.g., a downstream selective catalytic reduction catalyst on a flow through substrate.
1. A vehicle emission system for removing NOx and particulates from exhaust gas of a lean burn engine, the system comprising:
a catalyst which oxidizes NO to form NO 2 ;
a first ammonia selective catalytic reduction (SCR) catalyst disposed on a particulate filter, the first ammonia SCR catalyst comprises a CHA zeolitic framework material promoted with Cu and/or Fe and is loaded at a rear zone of the filter at a loading of 0.2 g/in 3 in the rear zone; and
a second ammonia SCR catalyst downstream from the first ammonia SCR catalyst, wherein the second ammonia SCR catalyst is disposed on a flow through substrate designed to promote SCR of NOx, the loading of the first ammonia SCR catalyst at the rear zone of the particulate filter is lower than a loading of the second ammonia SCR catalyst at the flow through substrate, and the loading of the second ammonia SCR catalyst at the flow through substrate is 3.0 g/in 3 ;
wherein the system is configured to provide a NOx conversion exceeding 90% and wherein the first ammonia SCR catalyst is configured to limit NOx conversion on the particulate filter to a range of 5-75%, facilitate passive oxidation of soot trapped by the particulate filter by reaction of the soot with unconverted NO 2 , and allow ammonia slip through the particulate filter to facilitate the SCR of NOx on the second ammonia SCR catalyst.
2. The system of claim 1 , wherein limiting NOx conversion and facilitating passive oxidation of soot trapped by the particulate filter comprises retarding the SCR activity on the particulate filter to enable the passive oxidation of soot on the particulate filter.
3. The system of claim 1 , wherein the first ammonia SCR catalyst is configured to balance the rate of reaction of particulate combustions by NO 2 and rate of SCR reaction based upon NOx and particulate matter exiting the engine, and the second ammonia SCR catalyst is designed to provide additional NOx conversion to meet a targeted total system NOx conversion.
4. The system of claim 1 , wherein the first ammonia SCR catalyst and the second ammonia SCR catalyst are the same SCR catalyst.