The Regular/Persistent Free Radicals and Associated Reaction Mechanism for the Degradation of 1,2,4-Trichlorobenzene over Different MnO2 Polymorphs.

The role of regular/persistent free radicals on the catalytic activity of K+-tuned MnO2 tunnel structures is poorly understood to date. Herein, three MnO2 polymorphs (α-, β-, and δ-MnO2) were synthesized and examined toward the degradation of 1,2,4-trichlorobenzene (1,2,4-TrCBz) at 300 °C. δ-MnO2, with a two-dimensional-layered tunnel structure tuned by K+, exhibited the highest activity among the three MnO2 polymorphs. The electron spin resonance spectroscopy results confirmed that δ-MnO2 featured the most abundant reactive oxygen species (ROS: O2-•, •OH, and 1O2), followed by α-MnO2 (O2-• and 1O2), and β-MnO2 (O2-•), being supported by the calculated energy barrier. It was, intriguingly, noted that persistent organic free radicals, newly recognized as emerging surface-stabilized compound, were remarkably detected in α- and β-MnO2/1,2,4-TrCBz systems but not in more reactive δ-MnO2/1,2,4-TrCBz system. These might contribute to discrepant oxidative degradation process. During the oxidative process, intermediates, including benzoic acid and glycerol, formed via attack by ROS. Upon further attack, these intermediates fragmented into smaller molecules such as formic, acetic, propionic, and butyric acids. The present findings give deeper insights into the role of free radicals on the catalytic degradation of chlorinated aromatics.