Exploring phase effects and ROS contributions in catalytic ozonation ofpolymorphic MnO2-Oxalic acid complexes

Abstract

This study investigated the pivotal role of oxalic acid (OA) in promoting the formation of multi-phase MnO2-OA complexes (α-MnO2-OA, β-MnO2-OA, and δ-MnO2-OA), significantly enhancing the efficiency of catalytic ozonation efficiency for pollutants degradation. At a catalyst-to-OA molar ratio of 1:0.8, the ATZ (5 mg L−1) degradation efficiencies within 3 min were 95.09 %, 93.43 %, and 96.71 % for the α-MnO2/OA/O3, β-MnO2/OA/O3, and δ-MnO2/OA/O3 systems, respectively—approximately twice as high as that of O3 alone. Pyrophosphate experiments verified that the formation of MnO2-OA complexes relied on the presence of Mn3+ on the surface of catalysts, which exhibited higher stability and catalytic ozonation ability in acidic conditions. Further probe experiments quantified the contributions of various reactive oxygen species (ROS) in the MnO2-OA complexes mediated catalytic ozonation system. The results indicated that O2•− as dominant species responsible for ATZ removal, followed by •OH, while O3 and 1O2 played comparatively minor roles. Notably, multi-phases MnO2-OA complexes revealed distinct active sites during the catalytic ozonation processes, as identified through a series of characterization techniques. For the α-MnO2-OA complex, more oxygen vacancies were generated on the surface, adsorbing O3 and pollutants for further reaction. For the β-MnO2-OA complex, more manganese vacancies and enhanced lattice oxygen (OL) mobility were key factors contributing to ROS production. As for the δ-MnO2-OA complex, the surface –OH groups acted as the main active site, reacting with O3 to generate more ROS, thus improving ATZ degradation. Overall, this study provides innovative insights into the application of MnO2 in catalytic ozonation.