Studies of the mechanism of ammonia oxidation into nitrous oxide over MnBiO/α-Al2O3 catalyst

A complex of kinetic and physicochemical methods: temperature-programmed surface reaction (TPSR), pulsing NH3 or NH3/16O2 (18O2) reaction mixture, and infrared and photoelectron spectroscopies, was used for characterization of the highly selective supported manganese–bismuth oxide catalysts and for the study of the mechanism of the ammonia oxidation. Ammonia oxidation was demonstrated to proceed via alternating reduction and reoxidation of the catalyst surface with participation of the lattice oxygen. NH3 interacts with weakly bonded oxygen species through hydrogen atom abstraction to form adsorbed [N] species, which are localized on Mn2+ and Mnδ+ (2<δ<3). Manganese ions with different oxidation degrees (Mn3+ (Mn4+) and Mnδ+) serve as active sites of the catalyst surface. The correlation between the selectivity toward N2O and the portion of manganese in the Mn3+ (Mn4+) state was established. Bismuth oxide plays an important role by increasing the quantity, mobility, and thermal stability of the subsurface oxygen. The reaction kinetic scheme is suggested based on the experimental results. Numerical simulation of TPSR data confirms the reliability of the proposed reaction mechanism.