Reactivity of NO/NO2–NH3 SCR system for diesel exhaust aftertreatment: Identification of the reaction network as a function of temperature and NO2 feed content

We present a systematic study of the NH3-SCR reactivity over a commercial V2O5–WO3/TiO2 catalyst in a wide range of temperatures and NO/NO2 feed ratios, which cover (and exceed) those of interest for industrial applications to the aftertreatment of exhaust gases from diesel vehicles. The experiments confirm that the best deNOx efficiency is achieved with a 1/1 NO/NO2 feed ratio. The main reactions prevailing at the different operating conditions have been identified, and an overall reaction scheme is herein proposed. Particular attention has been paid to the role of ammonium nitrate, which forms rapidly at low temperatures and with excess NO2, determining a lower N2 selectivity of the deNOx process. Data are presented which show that the chemistry of the NO/NO2–NH3 reacting system can be fully interpreted according to a mechanism which involves: (i) dimerization/disproportion of NO2 and reaction with NH3 and water to give ammonium nitrite and ammonium nitrate; (ii) reduction of ammonium nitrate by NO to ammonium nitrite; (iii) decomposition of ammonium nitrite to nitrogen. Such a scheme explains the peculiar deNOx reactivity at low temperature in the presence of NO2, the optimal stoichiometry (NO/NO2=1/1), and the observed selectivities to all the major N-containing products (N2, NH4NO3, HNO3, N2O). It also provides the basis for the development of a mechanistic kinetic model of the NO/NO2–NH3 SCR reacting system.