Dual-catalyst aftertreatment of lean-burn natural gas engine exhaust

A dual-catalyst system for the reduction of NO with CH4 under lean conditions was investigated. The system is comprised of a mixed bed containing a Co/ZrO2 catalyst, active for the oxidation of NO to NO2, and a Pd/sulfated zirconia (SZ) catalyst that is active for the reduction of NO2 with CH4. Such a system is capable of taking advantage of higher reduction rates for NO2, as compared to NO, that have been previously observed. When simulated exhaust streams from lean-burn natural gas engines are used as feed, the dual-catalyst system is simultaneously active for the reduction of NOx and the oxidation of unburned hydrocarbons and CO.