Suppressing Proton Affinity of Surface Oxygen for Enhanced N2 Selectivity in Mn-Based NH3–SCR Catalysts

Abstract

Inferior N2 selectivity remains a critical challenge for Mn-based catalysts in NH3–SCR denitration. Here, using atomically dispersed Mn on anatase (Mn1/TiO2) and MnO2 as model catalysts, we elucidate the origin of surface oxygen activity and the role of surface oxygen in the formation of N2O. Both catalysts achieved >99% NO conversion in the temperature range of 175–300 °C (WHSV = 60,000 mL·g–1·h–1). Compared with the poor selectivity of α-MnO2 (1.2% at 300 °C), Mn1/TiO2 maintained >88.0% N2 selectivity in 50–300 °C. Density functional theory calculations revealed that the high affinity of surface oxygen for protons results in overoxidation of NH3-derived intermediates, thus promoting N2O formation. Orbital interaction analysis manifested that surface oxygen activity in octahedrally coordinated catalysts depends on the spin polarization in transition-metal 3d bands. Atomic dispersion of Mn sites on oxides with nonspin-polarized dxy orbitals was demonstrated as a universal strategy to boost N2 selectivity. This work deepens the fundamental understanding of surface oxygen activity in catalysts, paving a way for designing Mn-based catalysts with excellent N2 selectivity.