Redox features in the catalytic mechanism of the “standard” and “fast” NH3-SCR of NOx over a V-based catalyst investigated by dynamic methods |
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| Affiliation: | 1. Dipartimento di Chimica, Materiali e Ingegneria Chimica “G. Natta”, Politecnico di Milano, Piazza Leonardo da Vinci 32, I-20133 Milano, Italy;2. DaimlerChrysler AG Abteilung RBP/C, HPC: 096-E220, D-70546 Stuttgart, Germany;1. School of Environmental and biological Engineering, Nanjing University of Science and Technology, Nanjing, 210094 PR China;2. State Key Joint Laboratory of Environment Simulation and Pollution Control (SKLESPC), School of Environment, Tsinghua University, Beijing, 100084 PR China;3. School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai, 200240 PR China;4. Department of Municipal Engineering, Suzhou University of Science and Technology, Suzhou, 215011, PR China;1. Center for Materials Architecturing, Korea Institute of Science and Technology, Cheongryang, Seoul 130-650, Republic of Korea;2. Department of Materials Science, University of Science and Technology, Daejon 305-350, Republic of Korea;1. School of Energy Source and Mechanical Engineering, Shanghai University of Electric Power, Shanghai 200090, PR China;2. Shanghai Engineering Research Center of Power Generation Environment Protection, Shanghai 200090, PR China;3. Shanghai Institute of Pollution Control and Ecological Security, Shanghai 200092, PR China;1. Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland;2. École Polytechnique Fédérale de Lausanne (EPFL), Institute of Chemical Science and Engineering, CH-1015 Lausanne, Switzerland;3. Department of Chemistry R&D, Treibacher Industrie AG, A-9330 Althofen, Austria |
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| Abstract: | The redox mechanism governing the selective catalytic reduction (SCR) of NO/NO2 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 O2: 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 deNOx 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 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. |
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