Manganese containing oxides catalytic ozonation in aqueous solution: Catalytic mechanism on acid sites

Abstract

• Protonation of hydroxyl groups on acid sites is a necessary condition for electron transfer reaction. • Solution pH is a significant variable in the linear sweep voltammetry test. • Electron transfer ability of acid sites in MgMn x O y is positively related to Mg content. Transition metal oxides and transition metal containing oxides have been extensively researched in heterogeneous catalytic ozonation for water treatment. However, catalytic mechanism for typical transition metal oxide – manganese oxide (MnO 2 ) in catalytic ozonation is still ambiguous. Herein, MnO 2 and magnesium manganese oxides (MgMn x O y ) with different molar ratios of Mg to Mn were analysed by linear sweep voltammetry (LSV) test to explore a necessary condition for electron transfer reaction between ozone and acid sites of the catalysts in aqueous solution. And their catalytic performances were investigated in ozonation of acetic acid aqueous solution at a neutral pH. It is found that electron transfer reaction occurs in manganese containing oxides catalytic ozonation when ozone is adsorbed on protonated surface hydroxyl groups of acid sites (Mn 2+ /Mn 3+ in the lattice). But ozone that is absorbed on protonated hydroxyl groups of Mg 2+ in the lattice cannot react with acid sites to generate reactive oxygen species in MgMn x O y catalytic ozonation. Protonation of hydroxyl groups on acid sites can be achieved by enhancement in point of zero charge (pH PZC ) of the catalysts or decrease in initial pH of aqueous solution. Protonation ability of hydroxyl groups on acid sites and electron transfer ability of acid sites are positively related to Mg content in MgMn x O y . MgMn x O y with 2:1 molar ratio of Mg to Mn (Mg 2 MnO y ) exhibits the highest catalytic activity and good stability in ozonation of acetic acid. On basis of catalytic mechanism on acid sites of manganese containing oxides, modification and application of manganese containing oxides would be further developed in heterogeneous catalytic ozonation for water treatment.