Remarkable assist of sulfate ion on solvent-free preparation of highly oxygen vacancy-rich MnO2 for enhancing peroxymonosulfate mediated advanced oxidation processes

Abstract

Oxygen vacancy (VO) strongly influences the catalytic activity of metal oxides. However, Vo engineering usually requires solvent- and/or post-thermal processes. Here, we developed sulfate ion (SO42-)-assisted mechanochemical preparation of VO-rich MnO2 from ball milling of only KMnO4 and Mn2+ inorganic precursors. The catalytic activity of as-prepared MnO2 depended on the Mn2+ precursors in an increasing order of Mn(NO3) < MnCO3 < MnCl2 < MnSO4, and the addition of Na2SO4 or K2SO4 during the preparation process significantly increased the catalytic activity of MnO2, indicating the SO42−-assist. MnO2 prepared from MnSO4 efficiently activated peroxymonosulfate (PMS) to degrade organic pollutants such as ofloxacin, norfloxacin, and sulfamethoxazole with 2.3–44 times larger rate constants than previously reported MnO2. Extensive characterizations suggest that SO42− increases the formation of one-electron-trapped VO (VO•) and surface Mn(III)–OH on/in MnO2 and the catalytic activity of MnO2 positively correlates with the amount of VO•, because VO• improves PMS adsorption and Mn4+/Mn3+ redox cycle and oxygen mobility in MnO2 for the generation of singlet oxygen.