Optimizing Oxygen Vacancies through p-Band Center Modulation of Oxygen in the Li₂WO₄/Mg₆MnO₈ Catalyst for Enhanced Oxidative Coupling of Methane: An Experimental and Theoretical Study

Herein, we demonstrate a one-pot sol–gel-assisted procedure to prepare a defect-rich Li2WO4/Mg6MnO8 catalyst having surface oxygen vacancies, which facilitates the adsorption of O2 molecules to generate active oxygen species (O2–, O22–) by incorporating Li and W into the Mg6MnO8 lattice. These active oxygen species serve as primary active sites, selectively dissociating CH4 into CH3• and promoting CH3• coupling into C2H6, while hindering excessive oxidation of CH3• into COx. Various analytical methods such as XPS, O2-TPD, EPR, CH4-TPSR, in situ DRIFTS, and in situ Raman spectroscopy studies demonstrated that surface reactive oxygen species are more active and selective than lattice oxygen toward the formation of C2 products. The controlled addition of Li and W plays a crucial role in stabilizing surface Li species through the formation of Li–O–W bonds by forming the Li2WO4 phase, ensuring stable catalyst performance up to 100 h. DOS analysis shows a positive shift in the p-band center, which effectively promotes the formation of oxygen vacancies. Analytical studies confirmed that surface active oxygen species are more active and selective than lattice oxygen in forming C2 hydrocarbons. The Li2WO4/Mg6MnO8 catalyst exhibited superior performance, achieving ∼82% C2 selectivity and ∼25% C2 yield at 700 °C. We found that the stable formation of active oxygen species (O2–) and a high Mn4+/Mn3+ ratio over the surface are the key factors for achieving high C2 selectivity and yield during OCM. DFT results show that the concentration of oxygen defect sites is higher on the surface of the Li2WO4/Mg6MnO8 catalyst, which synergistically binds Li2WO4 and Mg6MnO8, in comparison with pure Mg6MnO8 surfaces. Furthermore, DFT calculations also indicate that oxygen vacancies are energetically more favorable on the surface of the Li2WO4/Mg6MnO8 catalyst rather than in its subsurface. In situ XRD and in situ Raman analysis demonstrated that Li2WO4 undergoes a reversible phase change, transitioning into a molten state at higher temperatures, potentially forming Li2O2 species that may serve as active centers during the reaction.