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

摘要

In this paper, we demonstrate the facile and general synthesis of alkaline-earth metal manganites, denoted as A(Mg, Ca, Ba)MnxOy, for efficient degradation of high-concentration phenolic compounds via catalytic ozonation. The representative CaMnxOy oxides show a hierarchical spherical structure constructed by crystalline nanorods and numerous macropores. They possess mixed Mn4+/Mn3+ chemical valences and abundant surface hydroxyl (OH) groups. The ozone (O-3) decomposition rate on the CaMnxOy 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 CaMnxOy catalysts show 100% degradation and 50-70% mineralization within 1.0 h. The Ca2+ ions are essential to create redox Mn4+/Mn3+ couples and to significantly reduce manganese leaching. High surface ratios of Mn4+/Mn3+. and OH/lattice oxygen (O-lat) are beneficial for enhancing the catalytic performance. Superoxide anion free radicals (O-center dot(2)-) and singlet oxygen (O-1(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-center dot(2)- and O-1(2) play a role in oxidation. The redox Mn4+/Mn3+ 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. (C) 2019 Elsevier Inc. All rights reserved.