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

This study investigated the pivotal role of oxalic acid (OA) in promoting the formation of multi-phase MnO 2 -OA complexes (α-MnO 2 -OA, β-MnO 2 -OA, and δ-MnO 2 -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-MnO 2 /OA/O 3 , β-MnO 2 /OA/O 3 , and δ-MnO 2 /OA/O 3 systems, respectively—approximately twice as high as that of O 3 alone. Pyrophosphate experiments verified that the formation of MnO 2 -OA complexes relied on the presence of Mn 3+ 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 MnO 2 -OA complexes mediated catalytic ozonation system. The results indicated that O 2 •− as dominant species responsible for ATZ removal, followed by • OH, while O 3 and 1 O 2 played comparatively minor roles. Notably, multi-phases MnO 2 -OA complexes revealed distinct active sites during the catalytic ozonation processes, as identified through a series of characterization techniques. For the α-MnO 2 -OA complex, more oxygen vacancies were generated on the surface, adsorbing O 3 and pollutants for further reaction. For the β-MnO 2 -OA complex, more manganese vacancies and enhanced lattice oxygen (O L ) mobility were key factors contributing to ROS production. As for the δ-MnO 2 -OA complex, the surface –OH groups acted as the main active site, reacting with O 3 to generate more ROS, thus improving ATZ degradation. Overall, this study provides innovative insights into the application of MnO 2 in catalytic ozonation.