Superoxide radical-mediated photocatalytic oxidation of phenolic compounds over Ag+/TiO2: Influence of electron donating and withdrawing substituents

A comparative study was constructed to correlate the electronic property of the substituents with the degradation rates of phenolic compounds and their oxidation pathways under UV with Ag + /TiO 2 suspensions. It was verified that a weak electron withdrawing substituent benefited photocatalytic oxidation the most, while an adverse impact appeared when a substituent was present with stronger electron donating or withdrawing ability. The addition of p -benzoquinone dramatically blocked the degradation, confirming superoxide radicals ( O 2 − ) as the dominant photooxidant, rather than hydroxyl radicals, singlet oxygen or positive holes, which was also independent of the substituent. Hammett relationship was established based on pseudo-first-order reaction kinetics, and it revealed two disparate reaction patterns between O 2 − and phenolic compounds, which was further verified by the quantum chemical computation on the frontier molecular orbitals and Mulliken charge distributions of O 2 − and phenolic compounds. It was found that electron donating group (EDG) substituted phenols were more likely nucleophilically attacked by O 2 − , while O 2 − preferred to electrophilically assault electron withdrawing group (EWG) substituted phenols. Exceptionally, electrophilic and nucleophilic attack by O 2 − could simultaneously occur in p -chlorophenol degradation, consequently leading to its highest rate constant. Possible reactive positions on the phenolic compounds were also detailedly uncovered.