Influence of surface properties of transition metal oxides for permanganate activation: Key role of Lewis acid sites

Permanganate (PM) activation to remove contaminants in aqueous solution has gained increasing interest, and effective approaches to enhance its oxidation ability are becoming one of the hot spots. Transition metal oxides (TMOs) stand out as the most potential catalysts, but have received little attention in this area until now. Their mechanism towards PM activation also remains controversial. In this study, a series of TMOs with diverse surface properties were synthesized, and their effectiveness was compared. It was observed that α-MnO₂, NiO and Co₃O₄ exhibited significantly higher rates of degrading sulfadiazine, levofloxacin and phenol through PM activation than CeO₂, α-Fe₂O₃ and CuO. A performance evolution depended on the surface Lewis acid (LA) strength was established, while a less consistent correlation was observed between the degradation rate and other surface properties. Based on electrochemical analysis and density functional theory calculations, the spontaneous adsorption of MnO₄^- anions on LA sites in TMOs with an increase in oxidation potential of PM was further confirmed. This research would deepen the understanding of PM activation mechanism, and offer new guidance in the design of more efficient catalysts.