Role of MnO2 as an activator of molecular oxygen for singlet oxygen production

• 1 O 2 -dominated air oxidation was realizied under catalysis of one-dimensional MnO 2 . • The activation pathway of O 2 is regulated by the temerpature. • Mn valence state and exposed facet feature determine catalytic activity of MnO 2 . • O 2 is activated at oxygen vacancy through electron transfer with multiple Mn species. • MnO 2 shows activity for pollutants removal under ambient condition as O 2 activator. Activating molecular oxygen (O 2 ) into singlet oxygen ( 1 O 2 ) 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 O 2 activation pathways. Here, we address this challenge by exploiting one-dimensional tunnel structured MnO 2 to initiate 1 O 2 -dominated wet air oxidation reaction under ambient condition. Experimental results and theoretical calculations suggest an O 2 activation pathway at the oxygen vacancy of MnO 2 through the Mn redox cycle. Temperature demonstrates an important role in regulating the products from O 2 activation. The wet air oxidation system applies 1 O 2 oxidation pathway at more than 80 % selectivity below 100 °C, but •OH is involved to mediate the oxidation reaction as the temperature rises above 100 °C. The various MnO 2 polymorphs exhibit discrepant catalytic activities dependent upon Mn valence state and facet exposure. α-MnO 2 possesses higher 1 O 2 -triggering capacity than β- and γ-MnO 2 , due to more abundant oxygen vacancies on its (310) facet that favors the adsorption of O 2 . In contrast, •OH is more easily triggered by β-MnO 2 because of more facile adsorption and dissociation of H 2 O on its defective (100) surface. The wet air oxidation system with α-MnO 2 obtains COD removal efficiency approximating 100 % on a landfill leachate under ambient condition. These findings provide insight into the principles regulating the activation of O 2 towards 1 O 2 production, and demonstrate a great potential of MnO 2 as an O 2 activator for on-site remediation of contaminated surface water bodies.