Redox features in the catalytic mechanism of the “standard” and “fast” NH3-SCR of NOx over a V-based catalyst investigated by dynamic methods

The redox mechanism governing the selective catalytic reduction (SCR) of NO/NO 2 by ammonia at low temperature was investigated by transient reactive experiments over a commercial V 2 O 5 /WO 3 /TiO 2 catalyst for diesel exhaust aftertreatment. NO + NH 3 temperature-programmed reaction runs over reduced catalyst samples pretreated with various oxidizing species showed that both NO 2 and HNO 3 were able to reoxidize the V catalyst at much lower temperature than gaseous O 2 : furthermore, they significantly enhanced the NO + NH 3 reactivity below 250 °C via the buildup of adsorbed nitrates, which act as a surface pool of oxidizing agents but are decomposed above that temperature. Both such features, which were not observed in comparative experiments over a V-free WO 3 /TiO 2 catalyst, point out a key catalytic role of the vanadium redox properties and can explain the greater deNO x efficiency of the “fast” SCR (NO + NH 3 + NO 2 ) compared with the “standard” SCR (NO + NH 3 + O 2 ) reaction. A unifying redox approach is proposed to interpret the overall NO/NO 2 –NH 3 SCR chemistry over V-based catalysts, in which vanadium sites are reduced by the reaction between NO and NH 3 and are reoxidized either by oxygen (standard SCR) or by nitrates (fast SCR), with the latter formed via NO 2 disproportion over other nonreducible oxide catalyst components.