Calcium-Mediated Reconfiguration and Turnover of Surface Lattice Oxygen in γ-MnO2 for Promoted NO Oxidation Catalysis

Abstract NO-to-NO2 oxidation is critical for improving NOx abatement efficiency, yet it relies on costly platinum-group metals (PGMs) as catalysts. Transition-metal oxides, such as MnO2, are promising PGM alternatives, but their performance is limited by the insufficient reactivity and mobility of surface lattice oxygen. Here, we show that trace Ca doping can reconfigure the chemistry and availability of surface lattice oxygen in γ-MnO2, and an optimized Ca loading of 0.6 wt % increased the NO conversion from 36% to 65% at 300 °C. Such promotion was sustained in repeated runs and after hydrothermal aging at 800 °C. Combined Raman spectroscopy, XPS, O2-TPD and H2-TPR investigations show that Ca induced a measurable Mn–O lattice perturbation and a more electron-deficient surface environment on the γ-MnO2, thereby increasing the availability of surface lattice oxygen and favoring Mn redox. A semi-quantitative empirical descriptor integrating these two factors, namely surface lattice oxygen availability and Mn redox, was developed and found to correlate linearly with NO conversion over 150–300 °C, emphasizing their synergistic contribution to NO oxidation catalysis. Temperature-programmed desorption and in situ infrared spectroscopy (both with NO or NO + O2 as the probe) further revealed that Ca doping promoted NO activation while suppressing the accumulation of undesired nitrate/nitrite intermediates. This work paves a new avenue of oxygen-chemistry-centered optimization of MnOx-based catalysts for high-efficiency NO oxidation.