Journal Tronconi

Pagina 1

Journal of Catalysis 245 (2007) 1–10 www.elsevier.com/locate/jcat Redox features in the catalytic mechanism of the “standard” and “fast” NH3-SCR of NOx over a V-based catalyst investigated by dynamic methods Enrico Tronconi a,∗, Isabella Nova a, Cristian Ciardelli a, Daniel Chatterjee b, Michel Weibelb a Dipartimento di Chimica, Materiali e Ingegneria Chimica “G. Natta”, Politecnico di Milano, Piazza Leonardo da Vinci 32, I-20133 Milano, Italy b DaimlerChrysler AG Abteilung RBP/C, HPC: 096-E220, D-70546 Stuttgart, Germany Received 29 June 2006; revised 20 August 2006; accepted 12 September 2006 Available online 16 October 2006 Abstract 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 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◦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 + NO2) compared with the “standard” SCR (NO + NH3 + O2) 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…