Performance of Modified La_<i>x</i>Sr_1–<i>x</i>MnO₃ Perovskite Catalysts for NH₃ Oxidation: TPD, DFT, and Kinetic Studies

Abstract

The modified perovskites (La _xSr_{1- x}MnO₃) were prepared using the selective dissolution method for the selective catalytic oxidation (SCO) of NH₃. We found that more Mn⁴⁺ cations and active surface oxygen species formed on the catalyst's surface with increasing the dissolution time (dis). The 1h-dis catalyst exhibited excellent NH₃ conversion, and it performed well in the presence of SO₂ and H₂O. The 10h-dis and 72h-dis catalysts produced considerable N₂O and NO at high temperatures, while they were not detected from the fresh catalyst. Both temperature-programmed experiments and density functional theory calculations proved that NH₃ strongly and mostly bonded to the B-site cations of the perovskite framework rather than A-site cations: this framework limited the bonding of SO₂ to the surface. The reducibility increased superfluously after more than 10 h of immersion. The adsorptions of NH₃ on Mn⁴⁺ exposed surface were stronger than that on La³⁺ or Sr⁴⁺ exposed surfaces. The selective catalytic reduction, nonselective catalytic reduction, and catalytic oxidation reactions all contributed to NH₃ conversion. The formed NO from catalytic oxidation preferred to react with -NH₂/-NH to form N₂/N₂O.