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Theoretical insights into the gaseous and heterogeneous reactions of halogenated phenols with ˙OH radicals: mechanism, kinetics and role of (TiO2)n clusters in degradation processes.

Halogenated phenols are highly toxic chemicals with serious health risks, and the removal of these persistent environmental pollutants remains a challenge. Based on quantum chemistry calculations, the homogeneous/heterogeneous degradation mechanism and kinetics of C6X5OH (X = F, Cl, and Br) initiated by ˙OH radicals in the gas phase and TiO2 cluster surfaces are investigated in this work. Four ˙OH-addition and one proton-coupled electron-transfer (PCET) reaction channels for each halogenated phenol were found and the ˙OH-addition channels were more favorable than the PCET pathway without TiO2 clusters. At 296 K, the calculated total rate constant for ˙OH with C6F5OH in the atmosphere well agreed with the limited experimental data of (6.88 ± 1.37) × 10-12 cm3 molecule-1 s-1. The lifetimes of C6F5OH, C6Cl5OH, and C6Br5OH were about 12.04-12.86 h at 296 K, which favored their medium-range transport in the atmosphere. In the presence of (TiO2)n clusters (n = 4, 6, 8, 12, and 16), the PCET mechanism for hydrogen transfer reaction of C6F5OH with ˙OH radicals was changed from the previous four-electron/three-center into four-electron/two-center, which results in the PCET pathway becoming more favorable than the ˙OH-addition channels. Meanwhile, the heterogeneous degradation rate constants of C6F5OH were accelerated by more than 10 orders of magnitude within 200-430 K compared with those of the naked reaction. The effects of (TiO2)n cluster (n = 4, 6, 8, 12, and 16) size on the degradation rates were analyzed at 200-430 K, and the reaction on the (TiO2)8 cluster had a faster rate. The subsequent reactions including the bond cleavage of the benzene ring and O2 addition or abstraction were studied. This work provides new insights into halogenated aromatic atmospheric chemistry and nanoscale TiO2 photocatalysis in air or wastewater management.