Insights on the mechanism of enhanced selective catalytic reduction of NO with NH₃ over Zr-doped MnCr₂O₄: A combination of in situ DRIFTS and DFT

The NH₃ selective catalytic reduction (SCR) performance of MnCr₂O₄ was shown to be greatly enhanced by doping zirconium to form Zr_0.05Mn_0.95Cr₂O₄ in our previous study. In this work, the active sites and reaction mechanisms of the catalysts were further investigated by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and density functional theory (DFT) calculations in this work. The total quantities of Lewis acid and Brønsted acid sites of Zr_0.05Mn_0.95Cr₂O₄ became 3.5 times higher than that of MnCr₂O₄, and various nitrate species were detected on the surface, which are beneficial for the promotion of SCR activity below 250 °C. The catalytic reactions over MnCr₂O₄ mainly occur between NH₃ adsorbed on Cr cations and monodentate nitrates/adsorbed NO₂ on Mn cations below 250 °C, and NH₃ on Cr cations and gaseous NO above 250 °C, while the nitrites and N₂O₂²⁻ species are not reactive. The reactions between NH₃ adsorbed on Mn and Zr cations and nitrates/adsorbed NO₂ are the dominant pathways over Zr_0.05Mn_0.95Cr₂O₄ at all temperatures, and NH₄⁺ species enrichment (4.3 times higher) contributed to the enhanced catalytic activity below 250 °C. DFT calculation show that when species including NH₃, NH₂, NO, and NO₂ adsorb on Zr_0.05Mn_0.95Cr₂O₄, they yield lower adsorption energies than on MnCr₂O₄, and the energy barrier for NH₂ and NO₂ formation is also remarkably decreased. These two energy advantages facilitate conducive intermediate formation and the catalytic reactions.