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 (V O ) in Co 3 O 4, 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 Co 3 O 4 show that introducing an oxygen vacancy reduces two adjacent Co 3+ ions to Co 2+ and narrows the band gap. The newly formed Co 2+ 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 Co 3 O 4 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, Co 3 O 4 becomes more oxidized rather than reduced, by refilling preexisting oxygen vacancies. This is further supported by the observation that higher O 2 pressures increase the conversion and that the catalyst remains stable and active over several cycles.