Facile synthesis of yolk shell Mn₂O₃@Mn₅O₈ as an effective catalyst for peroxymonosulfate activation

Yolk shell Mn₂O₃@Mn₅O₈ was prepared through a facile synthetic procedure and was demonstrated to be a highly efficient and stable catalyst in peroxymonosulfate (PMS) activation for the catalytic degradation of organic contaminants. Mn₂O₃@Mn₅O₈ exhibits much improved activity compared with other classic manganese catalysts such as ϵ-MnO₂, Mn₂O₃ and Mn₃O₄, and this performance was due to its yolk shell structure, mesoporous shell, well-defined interior voids, particular particle size and mixed valence states. The long-term stability and efficiency of Mn₂O₃@Mn₅O₈ was observed in activating PMS to generate sulfate radicals for the removal of various organic pollutants such as phenol, 4-chlorophenol (4-CP), 2,4-dichlorophenol (2,4-DP) and 2,4,6-trichlorophenol (2,4,6-TCP) in aqueous medium. The effects of the initial solution pH, influence of anions, catalyst stability and the temperature effect on 4-CP degradation were also investigated. Furthermore, electron paramagnetic resonance (EPR) spectroscopy and radical quenching tests were employed to investigate sulfate, hydroxyl, superoxide radicals and even ¹O₂ for organic degradation processes. Finally, a possible activation pathway of Mn₂O₃@Mn₅O₈/PMS was proposed that involved the inner-sphere interactions between the HSO₅^- and the catalyst surface, electron transfer from Mn species to PMS, and the generation of sulfate radicals. These findings provide new insights into PMS activation by using nano-particle catalysts of non-toxic metal oxides.