Supports and Pt particle size effects were investigated on Pt catalysts for selective NO_x reduction with hydrocarbons in oxidizing atmosphere. Metallic Pt was found as the important active site of NO_x reduction with alkene. Pt catalysts supported on SiO₂ and zeolites have high activity for NO_x reduction with alkene since supported Pt on them remains as a metallic state even in oxidizing atmosphere. Pt catalysts supported on acidic oxides such as zeolite and g-Al₂O₃ have high activity for NO_x reduction with alkane. This result indicates that the acidic sites in oxide supports should contribute to NO_x reduction by the activation of alkane.

As for the particle size effects, highly dispersed and small particle size Pt was active for NO_x reduction with alkene while the optimum Pt particle size existed for NO_x reduction with alkane. The Pt particle size effetcs on the NO_x reduction activity depends only on the type of hydrocarbons, and not on the number of carbons in them.

�@