Catalyst systems for the reduction of the oxides of nitrogen

Claim 1

A process for treating the exhaust gases from an internal combustion engine to obtain the reduction of the oxides of nitrogen therein while suppressing the formation of ammonia by-products during said reduction which comprises contacting said exhaust gases prior to the introduction of air or neutralizing fuel gases thereto with a catalytic device comprising a monolithic refractory support structure of the honeycomb type supporting on the surface thereof a mixed noble metal catalyst consisting essentially, in weight percent, of 10-70% iridium and the remainder noble metals selected from the group consisting of platinum, palladium, and mixtures thereof, said mixed noble metal catalyst making up about 0.1-0.5% by weight of said support structure, said engine being operated at a substoichiometric air-to-fuel ratio ranging from 90% of stoichiometric up to but not including a stoichiometric ratio to maintain an overall reducing gaseous environment, said exhaust gases being passed through said support structure at a space velocity in the range of about 15,000-200,000 hr. -1 , and said exhaust gases being at a temperature at least sufficient to initiate the catalytic reduction of the oxides of nitrogen upon contact with said mixed noble metal catalyst.