(401g) Lean-NOx Trap/SCR Aftertreatment Catalysts for Automotive Emission Control

Lean burn engines offer fuel economy benefits and reduced CO2 emissions. However, conventional three-way automotive emission catalysts (TWC) are not effective for NOx reduction under lean conditions. One solution has been to use lean NOx trap (LNT) catalysts that store NOx during the lean engine cycle, and then catalytically reduce NOx when the engine operates under rich condition. During fuel combustion in the engine, steam is generated and represents one of the major components in automotive exhaust. While steam effects have been widely investigated for TWC catalysts, less is known about how steam effects NOx reduction over LNT catalysts. During the rich cycle the primary NOx reduction product over Pt?BaO/Al2O3 is NH3 that is formed via the reduction of NOx by H2 that is generated by water gas shift when CO used as reductant or by steam reforming when hydrocarbons (C3H8) are used as reductant. A method was established using a dual-bed reactor system containing a zeolite catalyst down-stream of the LNT bed to capture NH3 generated during the rich cycle. The zeolite serves then as NH3-SCR catalyst to eliminate lean NOx by using desorbed NH3 as reductant when the rich cycle is switched to the lean cycle.