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

High N2O selectivity of MnOx-TiO2 catalyst hinders its development in ammonia selective catalytic reduction (NH3-SCR) of NOx. Herein, a series of MnOx-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 NOx from the perspective of catalyst. It is found that MnOx and TiO2 species are the mainly redox and acid sites for the NH3-SCR denitration reaction over MnOx-TiO2 catalyst, respectively. As the Mn/Ti ratio increases, MnOx 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 MnOx 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 MnOx dispersion and increasing surface acidity of catalyst are feasible strategy for suppressing the N2O formation on MnOx-TiO2.