Elucidating the active sites and reaction mechanism for selective catalytic reduction of NOX on the MnFe2O4 catalyst at the molecular level

Abstract

The spinel-type MnFe₂O₄ catalyst exhibits excellent performance for selective catalytic reduction (SCR) of NO_X by NH₃, yet the catalytic mechanism remains to be established. Herein, density functional theory (DFT) calculations were performed to unveil the active sites and molecular-level NH₃-SCR reaction mechanism of the MnFe₂O₄ catalyst. The results indicate that the key reactants and products are chemically adsorbed on the MnFe₂O₄(100) surface. The surface 2-fold coordinated Mn atom is identified as the key active center for NH₃ and NO adsorption, which plays an important role in initiating the SCR reaction. The NH₂* species, generated from the first dehydrogenation of NH₃, serves as a vital intermediate. Its further dehydrogenation is hindered by a very high energy barrier (323.03 kJ mol⁻¹). Mechanistic analysis shows that the main reaction channel for NH₃-SCR over the MnFe₂O₄ catalyst includes two steps: (1) NH₂ production from NH₃ dehydrogenation and (2) NH₂–NO reaction. The NH₂–NO reaction is the rate-limiting step due to its relatively higher energy barrier (121.56 kJ mol⁻¹). The MnFe₂O₄ catalyst has a superior N₂ selectivity because the energy barrier of N₂O production is much higher than that of N₂ production. This work not only advances the understanding of denitration at the molecular scale but also offers a theoretical basis for the rational design of efficient catalysts for NH₃-SCR.