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During the last several decades TiO2 photocatalytic oxidation using the molecular oxygen in air has emerged as a promising method for the degradation of recalcitrant organic pollutants and selective transformations of valuable organic chemicals. Despite extensive studies, the mechanisms of these photocatalytic reactions are still poorly understood due to their complexity. In this review, we will highlight how the oxygen-18 isotope labeling technique can be a powerful tool to elucidate complicated photocatalytic mechanisms taking place on the TiO2 surface. To this end, the application of the oxygen-18 isotopic-labeling method to three representative photocatalytic reactions is discussed: (1) the photocatalytic hydroxylation of aromatics; (2) oxidative cleavage of aryl rings on the TiO2 surface; and (3) photocatalytic decarboxylation of saturated carboxylic acids. The results show that the oxygen atoms of molecular oxygen can incorporate into the corresponding products in aqueous solution in all three of these reactions, but the detailed incorporation pathways are completely different in each case. For the hydroxylation process, the O atom in O2 is shown to be incorporated through activation of O2 by conduction band electrons. In the cleavage of aryl rings, O atoms are inserted into the aryl ring through the site-dependent coordination of reactants on the TiO2 surface. A new pathway for the decarboxylation of saturated carboxylic acids with pyruvic acid as an intermediate is identified, and the O2 is incorporated into the products through the further oxidation of pyruvic acid by active species from the activation of O2 by conduction band electrons.

Photooxidation of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (DMDBT) sensitized by N-methylquinolinium tetrafluoborate (NMQ(+)BF4-) has been investigated in O2-saturated acetonitrile solutions. Nearly 100% oxidation of DBT and DMDBT was observed, and the oxidized products are predominantly composed of sulfoxides and sulfones, which are formed via photoinduced electron transfer (ET). Such ET processes were studied with fluorescence quenching of NMQ+, time-resolved transient absorption measurement, and ESR experiments. The fluorescence of NMQ+ is efficiently quenched by DBT and DMDBT via diffusion-controlled processes, with bimolecular quenching constants of 1.6 x 10(10) M(-1) s(-1) for DBT and 2.3 x 10(10) M(-1) s(-1) for DMDBT. The electron-transfer nature of the quenching is evidenced by the transient absorption measurement of the neutral radical NMQ*, which is formed by electron transfer from the substrates (DBT or DMDBT) to the excited singlet state of NMQ+. The ESR spectra of the superoxide radical anion (O2*-) trapped by 5,5-dimethyl-1-pyrroline N-oxide (DMPO) in the photooxygenation of DBT and DMDBT as well as their sulfoxides manifest that O2 traps an electron from NMQ* to form O2*-. The fact that the formation of sulfoxides and sulfones is greatly suppressed in the presence of benzoquinone (BQ), an efficient electron trap for NMQ* and O2*-, further indicates an ET process in the photooxidation of DBT and DMDBT. As inferred from the control experiments, the role of singlet oxygen (1O2) in the photooxidation is negligible. The intermediates responsible for the formation of sulfoxides and sulfones have been examined in detail.

As the main source of atmospheric sulfate aerosol, the aqueous-phase oxidation of SO 2 has been of great concern. Traditionally, the aqueous-phase oxidation of SO 2 is considered a consumption process of atmospheric oxidation capacity; however, we report here that •OH radicals and other reactive oxygen species (ROS) can be generated in the photochemical reactions of bulk HSO 3 − solution. This •OH production pathway results in the fast sulfate formation in the photooxidation of dissolved SO 2. The photochemical process is determined to be pH-dependent in which sulfate formation is promoted by decreasing pH. Hence, a light - driven acidic positive feedback mechanism is proposed that the oxidation of dissolved SO 2 and the formation of •OH radicals have a synergistic promoting effect. These results indicate that the photochemistry of dissolved SO 2 could be a missing source of aqueous •OH radicals as well as sulfate at regional to global scales. [Display omitted] • Aqueous-phase photooxidation of SO 2 is an important sulfate formation pathway. • Acidity promotes rather than inhibits the aqueous-phase photooxidation of SO 2. • SO 2 is not only a consumer but also an initiator of reactive oxygen species. [ABSTRACT FROM AUTHOR]

Highlights: [•] The actual role of F− in electron transfer reactions was investigated. [•] ESR results show that NDI − and NDI +–F− complex coexist in deaerated solvents. [•] The complexation of F− with the cation radical induced electron transfer reactions. [Copyright &y& Elsevier]