title: Superoxide radical-mediated photocatalytic oxidation of phenolic compounds over Ag⁺/TiO₂: Influence of electron donating and withdrawing substituents.
abstract: 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(+)/TiO2 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 (O2(-)) 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 O2(-) and phenolic compounds, which was further verified by the quantum chemical computation on the frontier molecular orbitals and Mulliken charge distributions of O2(-) and phenolic compounds. It was found that electron donating group (EDG) substituted phenols were more likely nucleophilically attacked by O2(-), while O2(-) preferred to electrophilically assault electron withdrawing group (EWG) substituted phenols. Exceptionally, electrophilic and nucleophilic attack by O2(-) 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.
authors.affiliation: National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Beijing Engineering Research Center of Process Pollution Control, Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China; University of Chinese Academy of Sciences, Beijing 100049, China.
authors.forename: Jiadong
authors.initials: J
authors.lastname: Xiao
authors.affiliation: National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Beijing Engineering Research Center of Process Pollution Control, Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China. Electronic address: ybxie@ipe.ac.cn.
authors.forename: Yongbing
authors.initials: Y
authors.lastname: Xie
authors.affiliation: State Key Laboratory of Multi-phase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China.
authors.forename: Qingzhen
authors.initials: Q
authors.lastname: Han
authors.affiliation: National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Beijing Engineering Research Center of Process Pollution Control, Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China; Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China.
authors.forename: Hongbin
authors.initials: H
authors.lastname: Cao
authors.affiliation: Department of Chemical and Biomedical Engineering, University of Science and Technology Beijing, China.
authors.forename: Yujiao
authors.initials: Y
authors.lastname: Wang
authors.affiliation: National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Beijing Engineering Research Center of Process Pollution Control, Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China; University of Chinese Academy of Sciences, Beijing 100049, China.
authors.forename: Faheem
authors.initials: F
authors.lastname: Nawaz
authors.affiliation: National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Beijing Engineering Research Center of Process Pollution Control, Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China; University of Chinese Academy of Sciences, Beijing 100049, China.
authors.forename: Feng
authors.initials: F
authors.lastname: Duan
keywords: Mechanism; Phenolic compounds; Substituent; Superoxide radical; TiO(2) Photocatalysis