Removal of nitrogen oxides from the exhaust of a lean-tune gasoline engine

There is an increasing interest in running passenger car engines lean of the stoichiometric air to fuel ratio (A/F) to improve fuel economy. One method being considered by vehicle manufacturers is to operate lean A/F (excess air) during cruising and return to stoichiometric operation when more power is required. The challenge for the catalyst manufacturer is to supply a product for this vehicle which will reduce NOx emissions from both lean and stoichiometric operating points to below probable EC tier III legislative limits. A technique has been developed where NOx is stored on an adsorbing catalyst while the engine runs lean. Upon switching to a short rich biased stoichiometric excursion the adsorbed NOx is released and reduced by precious metal components on the catalytic system. Selected adsorber catalysts were tested on both simulated exhaust gas and a standard bench engine with secondary air injection. The results suggest an adsorbing process whereby NO is oxidized to NO2 on Pt under lean conditions and the latter is subsequently adsorbed on an adsorbent material as a surface nitrate species. Upon return to a stoichiometric A/F ratio this species decomposes and NOx is released and subsequently reduced. The adsorber was also tested on a bench prototype lean tune engine. The results indicate an averaged NOx conversion of more than 90% over a test cycle in which the A/F ratio is cycled between lean operation and a rich biased stoichiometry.