A Combined Theoretical and Experimental Study of Oxygen Vacancies in Co 3 O 4 for Liquid-Phase Oxidation Catalysis

In the present work, we investigate oxygen vacancies (VO) in Co3O4, both in the bulk phase and under liquid-phase ethylene glycol (EG) oxidation by combining theoretical and experimental techniques. Density functional theory (DFT) calculations for bulk Co3O4 show that introducing an oxygen vacancy reduces two adjacent Co3+ ions to Co2+ and narrows the band gap. The newly formed Co2+ ions adopt high-spin configurations in distorted octahedral sites and remain stable in this state in ab initio molecular dynamics (AIMD) simulations at 300 K. Computed O and Co K-edge X-ray absorption spectra (XAS) for ideal and vacancy-containing Co3O4 show excellent agreement with the experimental data and serve as references to analyze the liquid-phase ethylene glycol oxidation. The comparison with experimental K-edge spectra of fresh and postreaction catalysts shows that fresh samples resemble the vacancy-containing reference, whereas postreaction spectra shift toward the ideal reference. These results suggest that under liquid-phase ethylene glycol oxidation conditions, Co3O4 becomes more oxidized rather than reduced, by refilling preexisting oxygen vacancies. This is further supported by the observation that higher O2 pressures increase the conversion and that the catalyst remains stable and active over several cycles.