Elucidating d-d orbital hybridization in metal-doped MnO2 for N2O formation inhibition mechanism in NH3-SCR

Abstract

Manganese-based oxides, demonstrating exceptional catalytic performance in NH3-Selective Catalytic Reduction of NO below 200 °C, are considered a promising solution in the field of low-temperature catalysis. However, their excessive dehydrogenation activity can cause NH3 to be converted into the undesired byproduct N2O, thereby limiting their application. Here, we employed density functional theory (DFT) calculations to investigate the inhibiting mechanism of N2O formation through single 3d transition metal doping (Sc, Ti, V, Cr, Fe, Co, Ni, and Cu) on MnO2. At the Mn active site, the d orbitals split into five non-degenerate localized electronic states, and the pz orbital of NH3 preferentially couples with the dz2 orbital to achieve the most stable adsorption. Based on this selective orbital coupling behavior, we identified that the energy level of the dz2 orbital center is a dominant factor in N2O formation, and we also elucidated how the energy barrier of N2O formation can be tailored by modulating the energy alignment between the pz and selected dz2 orbitals, which exhibit a volcano plot relationship. Our work not only reveals the adsorption characteristics of selective orbital coupling in doped MnO2 but also uncovers the role of d-d orbital hybridization in inhibiting N2O formation.