A Combined Theoretical and Experimental Study of Oxygen Vacancies in Co$_3$O$_4$ for Liquid-Phase Oxidation Catalysis

TL;DR

This study combines theoretical calculations and experimental analysis to understand oxygen vacancies in Co₃O₄ and their role in liquid-phase ethylene glycol oxidation, revealing that the catalyst becomes more oxidized during the reaction.

Contribution

It provides a comprehensive analysis of oxygen vacancies in Co₃O₄ using combined DFT, spectroscopy, and experimental data, highlighting their stability and oxidation behavior during catalysis.

Findings

Oxygen vacancies reduce Co³⁺ to Co²⁺ and narrow the band gap.

Spectroscopic data show catalysts become more oxidized after reaction.

Higher O₂ pressure enhances catalytic conversion and stability.

Abstract

In the present work, we investigate oxygen vacancies (V$_\mathrm{O}$) in Co$_3$O$_4$, both in the bulk phase and under liquid-phase ethylene glycol oxidation, by combining theoretical and experimental techniques. Density functional theory 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 simulations at $300$ K. Computed O and Co K-edge X-ray absorption spectra 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 O K-edge spectra of fresh and post-reaction catalysts shows that fresh samples resemble the vacancy-containing reference, whereas post-reaction 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.