New insight on N₂O formation over MnO_x/TiO₂ catalysts for selective catalytic reduction of NO_x with NH₃

High N₂O selectivity of MnO_x-TiO₂ catalyst hinders its development in ammonia selective catalytic reduction (NH₃-SCR) of NO_x. Herein, a series of MnO_x-TiO₂ catalyst with different Mn/Ti ratio were prepared by wet-impregnation method to study the N₂O formation during ammonia selective catalytic reduction (NH₃-SCR) of NO_x from the perspective of catalyst. It is found that MnO_x and TiO₂ species are the mainly redox and acid sites for the NH₃-SCR denitration reaction over MnO_x-TiO₂ catalyst, respectively. As the Mn/Ti ratio increases, MnO_x species layer expanded on TiO₂ support, leading to increase of redox sites but decrease of surface acid sites. These changes contribute to the occurrence of the no-selective oxidation of NH₃ on MnO_x species and the over-activation of NH₃ at Mn-Ti interface, resulting in the formation of N₂O via the route of 2NH₃ + 2O₂ → N₂O + 3H₂O and 4NO + 4NH₃ + 3O₂ → 4N₂O + 6H₂O, respectively. Through this work, it can be speculated that both improving MnO_x dispersion and increasing surface acidity of catalyst are feasible strategy for suppressing the N₂O formation on MnO_x-TiO₂.