The Selective Catalytic Reduction of Nitric Oxide in the Presence of Excess Oxygen

The selectivities of iridium (Ir), platinum (Pt) and rhodium (Rh) catalysts for reducing NO in the presence of excess O₂ in two synthetic fuel-lean exhaust gases have been determined, before and after a severe heat treatment. When tested at 500°C, the order of selectivity was Ir>>Rh>Pt for fresh catalysts, and Pt>Rh for treated catalysts. When tested at 400°C, the treated Pt and Rh catalysts were identical in selectivity to the fresh Ir catalyst. A catalyst called Leanox* was developed which showed higher NO selectivity than the fresh Ir catalyst. The advantages of incorporating this catalyst upstream of a conventional three-component-control catalyst to improve NO conversion on the fuel-lean side of stoichiometry were demonstrated in laboratory tests.