Patent ID: 8661788

Claim:
A method for treating an exhaust flow from an engine that is combusting a stoichiometric mixture of air and fuel, the method comprising: supplying a plurality of cylinders of a multi-cylinder reciprocating spark-ignited engine with a mixture of air and fuel defined by an air to fuel mass ratio that averages, over time, to about stoichiometry; combusting the mixture of air and fuel in the cylinders to produce a low-oxygen content exhaust flow that comprises carbon monoxide, unburned and partially burned hydrocarbons, and NO X ; passing the exhaust flow over a close-coupled three-way-catalyst that catalytically oxidizes carbon monoxide and unburned and partially burned hydrocarbons, reduces NO X , and passively generates ammonia from NO X ; and passing the exhaust flow over an ammonia-selective catalytic reduction catalyst positioned downstream from the close-coupled three-way-catalyst, relative to a flow direction of the exhaust flow, to catalytically reduce NO X that escapes past the close-coupled three-way-catalyst in the presence of oxygen and ammonia, the ammonia-selective catalytic reduction catalyst comprising (1) fine particles of at least one of a base metal ion-substituted zeolite or a base metal ion-substituted silicoaluminophosphate and (2) an oxygen storage material selected from the group consisting of CeO 2 , Pr 6 O 11 , CeO 2 —ZrO 2 , CuO—CeO 2 , FeO X —CeO 2 , MnO X —CeO 2 , Pr 6 O 11 —CeO 2 and mixtures thereof.