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

Abstract

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.