Regulation Strategy for the Generation of Singlet Oxygen Activated from Molecule Oxygen on the Manganese Oxides

Abstract

Activation of molecular oxygen (O2) into singlet oxygen (1O2) represents a promising way for selective oxidation of the electron-rich pollutants in wastewaters. However, such potential remains largely untapped due to poor knowledge regarding the mechanism governing the O2 activation pathways. Here, we address this challenge by exploiting one-dimensional tunnel structured MnO2 to initiate 1O2-dominated wet air oxidation reaction under ambient condition. Combining experimental results and theoretical calculations, O2 is confirmed to be activated at the oxygen vacancy of MnO2 through a redox cycle of multiple Mn valences. Temperature demonstrates an important role in regulating the O2 activation pathways. The wet air oxidation system applies 1O2 oxidation pathway at more than 80% selectivity below 100 °C, and •OH is involved to mediate the oxidation reaction as the temperature rises above 100 °C. The various MnO2 polymorphs exhibit discrepant catalytic activities dependent upon Mn valence state and facet exposure. α-MnO2 possesses higher 1O2-triggering capacity than β- and γ-MnO2, due to more abundant oxygen vacancies on its (310) facet that favors the adsorption of O2. In contrast, •OH is more easily triggered by β-MnO2 because of more facile adsorption and dissociation of H2O on its defective (100) surface. Using α-MnO2 as an O2 activator, application of the wet air oxidation system in the treatment of a landfill leachate obtains COD removal efficiency approximating 100% under ambient condition. These findings provide insight into the principles regulating the O2 activation pathways towards 1O2 production, and demonstrate a great potential of MnO2 as an O2 activator for on-site remediation of contaminated surface water bodies.

Keywords: Manganese dioxide, Wet air oxidation, Singlet oxygen, Catalytic selectivity, Ambient condition