Manganese oxide activation of peroxydisulfate: Evaluation via redox potential difference (ΔE) index

In advanced oxidation processes (AOPs), heterogeneous catalysts mediate coupled oxidation-reduction steps. Therefore, assessing both oxidative and reductive tendencies of catalysts is essential for rational screening. Here, we propose the redox potential difference, ΔE (pollutant oxidation potential minus PDS reduction potential), as a quantitative descriptor for PDS activation. Three MnO₂ polymorphs (α, β, γ) were studied: their PDS reduction and pollutant oxidation potentials were measured, ΔE values computed, and correlated with PDS activation performance. To validate the universality and mechanism, we tested various phenolic and non-phenolic pollutants, monitored PDS degradation, and conducted ROS quenching, electrocatalytic tests, and structural characterization. Results show that smaller ΔE corresponds to stronger electron-transfer ability, faster PDS consumption, and higher degradation rates; β-MnO₂ exhibited the smallest ΔE and the highest activity toward pollutant degradation. A Butler-Volmer-based theoretical framework rationalizes the ΔE-activity link and delineates the limits imposed by internal charge transport. Overall, ΔE provides a transferable, experimentally accessible metric to guide design and screening of transition-metal oxide catalysts for AOPs.