Selective Reduction of NO_x over Iridium Catalyst Supported on MFI-type Metallosilicate for Lean Burn Gasoline Engine

Abstract

Iridium catalyst supported on MFI-type metallosilicate with activation treatment in 10% H₂O–N₂ at 700 °C was prepared and its catalytic activity for the selective reduction of NO_x with hydrocarbons under oxgen rich conditions was investigated. The primary objective is the development of a catalyst system which reduce NO_x and the emission of hydrocarbons at a wide temperature range. The two stage catalysts with activated iridium catalyst and Pt–Rh/Al₂O₃ (Three-way catalyst), were prepared and showed NO_x reduction activity at a wide temperature range of 250–500 °C. Hydrocarbons were completely removed at higher temperatures than 300 °C. This system revealed high NO_x reduction activity both a lean and stoichiometry atmosphere, and showed superior transient NO_x reduction activity from lean to a stoichiometry atmosphere. The activated iridium catalyst showed a maximum NO_x conversion of 70% at 400 °C in the actual exhaust gas of a lean burn gasoline engine. It is inferred that the activated iridium catalyst had sufficient NO_x reduction activity and durability after aging treatment of a lean atmosphere at 660 °C and a rich atmosphere at 800 °C for a lean burn gasoline engine.