Regulation of oxidant activation in a peroxymonocarbonate-based system by an oxygen vacancy-rich MnO2 catalyst

Abstract

The issue concerning selective activation within the in-situ formed peroxymonocarbonate (HCO₄⁻)-based advanced oxidation process has been understudied. We selected oxygen vacancy-rich manganese dioxides (ε-MnO₂-OV) and perfect crystalline ε-MnO₂ as catalysts to initiate the activation of the coexisting oxidants (H₂O₂ and HCO₄⁻) in an in-situ formed HCO₄⁻-based system. Activation experiments, electron paramagnetic resonance (EPR) and correlation analysis between oxidant conversion and ^•OH production suggest that ε-MnO₂-OV was inert for H₂O₂ decomposition, but active for HCO₄⁻ decomposition to produce reactive oxygen species, including ^•OH and CO₃^•−. Compared with ε-MnO₂, approximately 4 times more ^•OH could be produced and many persistent organics could be removed more than 80 % in the ε-MnO₂-OV-catalyzed HCO₄⁻-based system. Moreover, a continuous-flow device was assembled and performed to remove tetracycline for 60 h with more than 80 % degradation efficiency. The catalyst characterization and density functional theory (DFT) calculations suggest that low-valent Mn species were the catalytic active sites and electron-deficient oxygen vacancy enhanced the adsorption of electron-rich HCO₄⁻, accelerated electronic conduction and charge rearrangement between ε-MnO₂-OV and HCO₄⁻, thereby promoting the O-O bond cleavage of HCO₄⁻ for producing CO₃^•− and ^•OH. Free radical quenching experiments and catalytic mechanism indicated that CO₃^•− was responsible for the degradation and the produced ^•OH likely reduced the oxidized Mn species to finish the catalytic cycle. This work offers insights into the structure–activity relations of MnO₂ catalysts in the selective activation of HCO₄⁻ for promoting ^•OH formation.