Comparative analysis of the dual origins of the N2O byproduct on MnOx, FeOx, and MnFeOx sphere catalysts for a low-temperature SCR of NO with NH3

Abstract

Herein, the origin and evolution mechanism of N₂O formation over the MnFeO_x spherical catalyst were investigated in detail, accompanied by a comparison analysis of pristine MnO_x and FeO_x catalysts. The results showed that the MnFeO_x catalyst possessed higher catalytic activity but poorer N₂ selectivity than the sole oxide catalysts, and the N₂O concentration could even reach 300 ppm over 175 °C. Further activation tests under separate NH₃/NO oxidation conditions revealed the N₂O accumulation of the MnFeO_x catalyst that was primarily derived from the NSCR pathway below 150 °C and then gradually turned to NH₃ over-oxidation with temperature. The coupling of Mn and Fe cations induced electron migration and released more Mn⁴⁺ and Os, contributing to the upgraded redox capacity, which served as the inherent motivation of high activity but poor N₂ selectivity over the MnFeO_x catalyst. For the NH₃ over-oxidation pathway, both gaseous O and catalyst oxygen acted as the oxidizers, and the double –NH₂/NH species with the assistance of O was deemed as the restricted step for N₂O generation. For the NSCR pathway, the byproduct N₂O was generated through both E–R and L–H mechanisms, and the former contributed to more N₂O. In addition, the kinetic analysis further verified the obtained conclusions. Finally, a possible mechanism model of N₂O origins over the MnFeO_x catalyst was proposed.