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

Abstract

The spinel-type MnFe2O4 catalyst possesses excellent performance for selective catalytic reduction (SCR) of NOx 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 MnFe2O4 catalyst. The results indicate that the key reactants and products are chemically adsorbed on 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). Mechanistic analysis shows that the main reaction channel for NH3-SCR over 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). 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 offering a theoretical basis for the rational design of efficient catalysts for NH3-SCR.