Reaction mechanisms of lean-burn hydrocarbon SCR over zeolite catalysts

Acid zeolites loaded with transition metals have been intensively investigated as catalysts for selective catalytic reduction of NOx using hydrocarbons. Detailed insight into reaction mechanisms and zeolite effects was reached through experimentation in minireactors with simulated exhaust gas and compressed zeolite powder, detailed reaction product analysis and in situ spectroscopic investigations. Irrespective of the nature of the transition metal (Fe, Ag, Ce, Cu, Co, Pd, Pt, In, Sn. Mn) and of the zeolite type (MFI, FER, MOR), a unified mechanistic picture emerges from the vast amount of literature data. Water plays a key role in this catalytic chemistry. Reaction mechanisms in the presence and absence of water are different. Evidence for the involvement of NO2 as a reaction intermediate in NO reduction is provided, and the means to generate NO oxidation activity in the catalyst discussed. It is explained how organo nitrogen compounds are involved in the pairing of nitrogen atoms. Hydrocarbon SCR can be achieved in two steps, comprising the trapping of NOx by reaction with adsorbed hydrocarbons at low temperatures followed by conversion of the organo nitrogen compounds into nitrogen at higher temperatures. Molecular sieving properties of zeolites can be exploited in order to suppress undesired oxidation of the hydrocarbons with oxygen. The dual pore concept for achieving NO oxidation and NO, reduction in small and large pores, respectively, is presented.