Competition of selective catalytic reduction and non selective catalytic reduction over MnOx/TiO2 for NO removal: the relationship between gaseous NO concentration and N2O selectivity

Abstract

In this work, a novel phenomenon was discovered that N₂O selectivity of NO reduction over MnO_x/TiO₂ was related to the concentration of gaseous NO and that lower concentration of gaseous NO would cause higher N₂O selectivity. In situ DRIFTS and transient reaction studies demonstrated that both the Eley–Rideal mechanism (the reaction of over-activated NH₃ with gaseous NO) and the Langmuir–Hinshelwood mechanism (the reaction of adsorbed NO₃⁻ with adsorbed NH₃ on the adjacent sites) could contribute to the formation of N₂O. Kinetic study demonstrated that N₂O selectivity would be independent of gaseous NO concentration if NO reduction over MnO_x/TiO₂ mainly followed the Langmuir–Hinshelwood mechanism. If NO reduction over MnO_x/TiO₂ mainly followed the Eley–Rideal mechanism, there was competition between the selective catalytic reduction (SCR) reaction and non selective catalytic reduction (NSCR) reaction. As gaseous NO concentration increased, more –NH₂ was used to reduce gaseous NO to form N₂ and the further oxidization of –NH₂ to –NH was restrained, resulting in an obvious decrease of N₂O selectivity. The Eley–Rideal mechanism played an important role in NO reduction over MnO_x/TiO₂, especially at higher temperatures. Therefore, N₂O selectivity of the low temperature SCR reaction over MnO_x/TiO₂ decreased especially at higher temperatures after the increase of gaseous NO concentration.