Lean NOx: dual sites for NOx reduction on Pt/H-ZSM-5 Original Research Article

Pt/H-ZSM-5 contains independent sites for the reduction of NOx, (1) acidic sites associated with H-ZSM-5 which selectively reduce NOx to N2 via stored hydrocarbons at low temperatures and (2) conventional Pt sites which reduce NOx to N2 and N2O at higher temperatures. Propylene is trapped on acid sites, undergoes polymerization but retains activity for NOx reduction provided both NO and O2 are present. Neither O2 or NO will individually react with the adsorbed polymer but together NOx reduction occurs readily supporting the mechanism of enhanced conversion of NOx via NO2. This phenomenon has also been demonstrated with diesel fuel as the added reductant. This understanding suggests a novel strategy for diesel engines in which fuel is pulsed into the exhaust at the conditions where the acidic sites are active (i.e. 185°C) giving enhanced NOx conversion. After aging in the automobile exhaust, deactivation by de-alumination of the acidic sites occurs with a loss of hydrocarbon trapping and low temperature NOx reduction. Thus, improved hydrothermally stable materials are still needed to satisfy the mobile source applications.