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

Abstract

The NH3 selective catalytic reduction (SCR) performance of MnCr2O4 was shown to be greatly enhanced by doping zirconium to form Zr0.05Mn0.95Cr2O4 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 Zr0.05Mn0.95Cr2O4 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 Zr0.05Mn0.95Cr2O4 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 Zr0.05Mn0.95Cr2O4, 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.