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

Pt/H-ZSM-5 contains independent sites for the reduction of NOx, (I) acidic sites associated with H-ZSM-5 which selectively reduce NOx to N-2 via store d hydrocarbons at low temperatures and (2) conventional Pt sites which redu ce NOx to N-2 and N2O at higher temperatures. Propylene is trapped on acid sites, undergoes polymerization but retains activity for NOx reduction prov ided both NO and O-2 are present. Neither O-2 or NO will individually react with the adsorbed polymer but together NOx reduction occurs readily suppor ting the mechanism of enhanced conversion of NOx via NO2. This phenomenon h as also been demonstrated with diesel fuel as the added reductant. This und erstanding suggests a novel strategy for diesel engines in which fuel is pu lsed into the exhaust at the conditions where the acidic sites are active ( i.e. 185 degrees C) giving enhanced NOx conversion. After aging in the auto mobile exhaust, deactivation by de-alumination of the acidic sites occurs w ith a loss of hydrocarbon trapping and low temperature NOx reduction. Thus, improved hydrothermally stable materials are still needed to satisfy the m obile source applications. (C) 1999 Elsevier Science B.V. All rights reserv ed.