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

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 + H-2) 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 N-2. 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 Bronsted 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 N-2 selectivity, together with the prevention of the ammonia slip. Published by Elsevier B.V.