Synergistic engineering of oxygen vacancies, cation vacancies, and surface acidity in MOFs-derived Co-Mn spinel oxides via acid etching: A pathway to enhanced toluene oxidation performance.

MOFs-derived spinel oxides hold great potential for the catalytic oxidation of VOCs, but their low intrinsic activity and unmanageable surface defects limit practical applications. Herein, this study proposes a method for oxalic acid etching MIL-101(CoMn) to synergistically regulate oxygen vacancies, cation vacancies, and surface acidity for enhanced toluene oxidation. Oxalic acid selectively dissolves Co, leading to the generation of cobalt vacancies. While cobalt vacancies can facilitate the migration or detachment of oxygen from the lattice, thus contributing to the formation of oxygen vacancies. The coexistence of oxygen vacancies and cobalt vacancies, with exposed surface atoms and unsaturated metal centers, enhances surface acidity. Importantly, mild oxalic acid converts strong acid sites into medium-strength ones, improving toluene adsorption and reducing carbon deposition. Meanwhile, acid etching reconfigures surface morphology to transform spherical into lamellar structures with higher specific surface area and edge atomic density. DFT calculations confirm that oxygen vacancies and cobalt vacancies optimize electronic structures of Co and Mn, boosting electron exchange and redox properties. In-situ DRIFTS reveals that this synergistic modulation improves the generation and consumption of intermediates. Hence, MIL-101(CoMn)/O shows higher activity and stability with 90 % toluene conversion at 223 °C, lower than CoMn (293 °C) and MIL-101(CoMn) (267 °C). This work offers insights for designing efficient MOFs-derived spinel oxides in environmental remediation.