Unraveling the site-resolved synergy between tetrahedral Co2+ and octahedral Co3+ in spinel oxides for toluene oxidation.

The development of efficient non-noble-metal catalysts and clarification of their structure-activity relationships are crucial for volatile organic compounds (VOCs) abatement. Although cobalt spinel oxides show promising activity for VOCs oxidation, the distinct functions and cooperative effects of cobalt cations in different geometric configurations remain insufficiently understood. Here, Co3O4, ZnCo2O4, and CoCr2O4 were constructed as site-differentiated model catalysts through targeted cation substitution, in which tetrahedral Co2+ and octahedral Co3+ configurations were preferentially perturbed by Zn2+ and Cr3+, respectively. Combined experimental characterization and density functional theory calculations were used to elucidate their roles in toluene oxidation. Co3O4 exhibited the highest activity, together with superior reducibility, a higher abundance of surface-adsorbed oxygen, and enhanced lattice oxygen mobility, indicating that preservation of the coupled tetrahedral/octahedral cobalt framework is beneficial for efficient redox cycling. Theoretical results showed that tetrahedral Co2+ sites are more favorable for O2 adsorption and activation, whereas octahedral Co3+ sites preferentially promote toluene adsorption and initial activation. Mechanistic analyses further indicated the cooperative involvement of surface-adsorbed oxygen and lattice oxygen, with increasing lattice oxygen participation at elevated temperatures. Toluene was sequentially converted through benzyl, benzyl alcohol, benzaldehyde, benzoate, and maleic anhydride intermediates before complete oxidation to CO2 and H2O. These findings provide a site-resolved understanding of cobalt spinel catalysts and offer guidance for designing efficient spinel oxides by preserving and regulating complementary tetrahedral and octahedral active-site functions.