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

Abstract

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^–, O₂^2–) 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 CO_x. 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.