Facile synthesis of alkaline-earth metal manganites for the efficient degradation of phenolic compounds via catalytic ozonation and evaluation of the reaction mechanism

In this paper, we demonstrate the facile and general synthesis of alkaline-earth metal manganites , denoted as A(Mg, Ca, Ba)Mn x O y , for efficient degradation of high-concentration phenolic compounds via catalytic ozonation . The representative CaMn x O y oxides show a hierarchical spherical structure constructed by crystalline nanorods and numerous macropores. They possess mixed Mn 4+ /Mn 3+ chemical valences and abundant surface hydroxyl (OH) groups. The ozone (O 3 ) decomposition rate on the CaMn x O y catalysts is greatly accelerated and follows the first-order law. These catalysts are promising for the degradation of phenolic compounds via catalytic ozonation, exhibiting rapid pseudofirst-order degradation kinetics, a high total organic carbon (TOC) removal efficiency and an excellent stability. Under optimized conditions (a low O 3 dosage of 1.5 mg/min and a catalyst dosage of 7.5 g/L), for the treatment of concentrated phenol (50–240 mg/L), the CaMn x O y catalysts show 100% degradation and 50–70% mineralization within 1.0 h. The Ca 2+ ions are essential to create redox Mn 4+ /Mn 3+ couples and to significantly reduce manganese leaching. High surface ratios of Mn 4+ /Mn 3+ and OH/lattice oxygen (O lat ) are beneficial for enhancing the catalytic performance. Superoxide anion free radicals ( O 2 – ) and singlet oxygen ( 1 O 2 ) are the predominant reactive species for the oxidation degradation. The O 2 – reaction pathway is proposed. Specifically, the surface OH sites activate O 3 , displaying highly enhanced decomposition rates. The generated O 2 – and 1 O 2 play a role in oxidation. The redox Mn 4+ /Mn 3+ and the O lat /oxygen vacancy (O lat /O vac ) couples play important roles in electron transfer . The proposed mechanism is supported by active site probing, radical scavenging, spectroscopic studies, and the results in the degradation of substituted phenols.