An overview of the production and use of ammonia in NSR + SCR coupled system for NOx reduction from lean exhaust gas

Abstract

This paper gives a critical overview of the recent advances in NOx abatement in excess of oxygen based on the combination of the NOx storage–reduction (NSR) and selective catalytic reduction (SCR) processes. Ammonia may be produced during the regeneration step of NSR catalyst, by the direct reaction (NOx+H2) or/and the isocyanate route. Recent literature highlights that the ammonia production rate is higher than the ammonia reaction rate with the remaining NOx in order to form N2. In order to optimize the use of the in situ produced ammonia, a catalyst dedicated to the NOx-SCR by NH3 can be added. Zeolites are the main studied materials for this application. Catalytic reduction of NOx by NH3 relates a complex mechanism, in which the nuclearity of the active sites is still an open question. Over zeolites, the NO to NO2 oxidation step is reported as the rate-determining step of the SCR reaction, even if the first step of the reaction is ammonia adsorption on zeolite Brønsted acid sites. Thus, the addition of a NH3-SCR material to the NSR catalyst is a possible way to increase the global NOx abatement and maximize the N2 selectivity, together with the prevention of the ammonia slip.