The redox mechanism governing the selective catalytic reduction (SCR) of NO/NO2 by ammonia at low temperature was investigated by transient reactive experiments overacommercial V2O5/WO3/TiO2 catalyst for diesel exhaust aftertreatment. NO+NH3 temperature-programmed reaction runs over reduced catalyst samples pretreated with various oxidizing species showed that both NO2 and HNO3 were able to reoxidize the V catalyst at much lower temperature than gaseous O-2: furthermore, they significantly enhanced the NO + NH3 reactivity below 250 degrees 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 WO3/TiO2 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 + NH3 + NOD compared with the "standard" SCR (NO + NH3 + O-2) reaction. A unifying redox approach is proposed to interpret the overall NO/NO2-NH3 SCR chemistry over V-based catalysts, in which vanadium sites are reduced by the reaction between NO and NH3 and are reoxidized either by oxygen (standard SCR) or by nitrates (fast SCR), with the latter formed via NO2 disproportion over other nonreducible oxide catalyst components. (c) 2006 Elsevier Inc. All rights reserved.