Unveiling the Size Effects of Oxygen Vacancy Clusters in Toluene Combustion under Humid Conditions

Oxygen vacancy (OV) engineering, which involves increasing OV concentration, is one of the most effective strategies for enhancing the catalytic performance of environmental catalysts, particularly for volatile organic compounds (VOCs) combustion. However, this strategy often results in the aggregation of OV clusters, a phenomenon that has been largely overlooked in previous studies, despite its pronounced impact on catalytic performance. Herein, we demonstrate that the size of OV clusters governs the volcano-shaped dependence of catalytic activity for toluene oxidation under humid conditions. Experimental results and DFT calculations reveal that variations in OV cluster size significantly modulate H₂O activation. In particular, dimer OV clusters promote rapid electron transfer and exhibit the lowest energy barrier for H₂O dissociation, facilitating the formation of OH species. These OH species not only enhance the adsorption of toluene and intermediates but also act as reactive oxygen species that directly participate in the oxidation pathway, thereby improving toluene combustion under humid conditions. Consequently, MnO₂-M with dimer OV clusters exhibits the highest activity (T₉₀ = 222 °C) in the presence of 5 vol % H₂O. This work provides new insights into the size-dependent effects of OV clusters and contributes to the rational design of high-performance catalysts based on OV engineering.