Atomic-Level Regulation of Mn Monovacancies of 2D-Mn2O3 for High-Efficient Catalytic Diesel Oxidation

Abstract

Recent intensive research has reported that oxygen vacancies on transition metal oxides (TMOs) are crucial for improving catalytic performance in diesel oxidation, particularly for catalytic NO oxidation. However, the impact of metal defects on the intrinsic properties of TMOs remains ambiguous. Herein, we report an original MOF-templated strategy to fabricate Mn-defected 2D-Mn₂O₃ nanomaterials, which demonstrate a prominent performance for NO oxidation (93.3% at 275 °C under a GHSV of 240,000 h^–1), rivaling Pt/Al₂O₃ (54.7% at 350 °C) and recently reported good-performing NO oxidation catalysts. The high-angle annular dark-field scanning transmission electron microscopy image manifests the formation of Mn monovacancies with different concentrations, confirmed by positron annihilation lifetime spectroscopy (PLAS). Furthermore, X-ray absorption near-edge spectroscopy, O₂ temperature-programmed desorption, and Raman and X-ray photoelectron spectroscopy confirm that Mn monovacancies can soften the binding strength of neighboring oxygen atoms and induce the generation of more unsaturated oxygen sites, which efficiently lower the formation barrier of oxygen vacancies and boost the reactivity of surface lattice oxygen. More importantly, in situ DRIFTS analysis combined with theoretical calculations reveals that the introduction of Mn monovacancies into 2D-Mn₂O₃ shifts the O₂ adsorption configuration from Yeager-type mode to Pauling-type mode, which can promote the generation of labile monodentate NO₃^– and lower the energy barrier of the rate-determining NO₂ desorption step (0.80–1.04 eV). By quantitatively correlating the reaction rates normalized by the specific surface area with the Mn monovacancies estimated by PLAS, we uncover that the increased concentration of Mn monovacancies is accountable for improving the intrinsic activity of NO oxidation. Moreover, these attributes also impart the as-obtained Mn-defected Mn₂O₃ with enhanced oxidative capabilities toward a series of other atmospheric pollutants, including CO, C₃H₈, and NO-assisted soot. This discovery highlights the pivotal role of metal defects in modulating the electronic state of lattice oxygen and provides an innovative strategy for developing prospective redox catalysts.