Oxygen Vacancy-Triggered Performance Enhancement of Toluene Oxidation over Cu Catalysts: A Combined Kinetics and Mechanistic Investigation

Abstract

Oxygen vacancy regulation in metal catalysts has emerged as a crucial strategy for efficiently degrading volatile organic compounds (VOCs), while the acquisition of kinetics and mechanistic insights into the oxygen vacancy roles remains highly desirable yet challenging. Herein, we conduct a combined kinetics and mechanistic investigation into the oxygen vacancy effects of Cu catalysts on toluene oxidation. Cu catalysts with varying oxygen vacancy contents are prepared using three representative metal oxide supports, including reducible ZrO2 with strong oxygen storage capacity, reducible ZnO, and unreducible Al2O3. The toluene oxidation as well as CO oxidation are tested for these catalysts, revealing the superior catalytic activity of Cu/ZrO2 catalyst. Multiple charactrizations demonstrate that the strong reducibility and abundance of oxygen vacancies within the Cu/ZrO2 catalyst are the key factors for these reactions. A redox-based kinetics strategy is further applied to decouple the reduction and oxidation steps, thus affording a rate diagram to elucidate the promotional effects of oxygen vacancies on toluene decomposition. Additionally, in-situ DRIFTS study unveils the reaction pathway, showing the transformation from toluene to benzyl alcohol, benzaldehyde, benzoate, phenol, anhydride, carbonate, and ultimately to CO2. As a result, the oxygen vacancy-tuned rate-relevant step is disclosed, shifting from C-H bond cleavage for the Cu/Al2O3 catalyst with limited oxygen vacancies to the C=O and C=C bond cleavage for the Cu/ZnO and Cu/ZrO2 catalysts with more oxygen vacancies. The kinetics and mechanistic insights gained here would help guide the design of highly efficient oxidative catalysts by tuning the oxygen vacancies of support.