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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.

The reactivity of iron-bearing clays to catalyze the decomposition of hydrogen peroxide (H2O2) under light irradiation was investigated. Free iron oxides and structural iron in clay octahedral lattice are contained simultaneously in montmorillonite K10 (MK10), a representative natural clay mineral. By pretreatment of clay with the particular method, the reactivities of the two kinds of iron species were differentiated. It was found that free iron oxides on clay surface efficiently catalyzed the decomposition of H202 under UV light irradiation but structural iron in the octahedral lattice showed poor reactivity. This was found to result from the difference in production of Fe(II) species under UV irradiation between iron oxides and structural iron. When photoreactive substances such as N,N-dimethylaniline (DMA), rhodamine B (RhB), or malachite green (MG) were introduced, structural iron was found to promote greatly the decomposition of H202. The reduction of clay iron(II) to iron(II) is essential for the decomposition of H2O2, which is achieved by light-induced ligand to metal charge transfer (for iron oxides) or organic matters donating electrons upon irradiation (for structural iron). The light-induced redox cycling of iron did not lead to the release of iron and decomposition of H202 primarily localized on the clay surface. This work implies that iron-bearing clays could be a sink of H2O2 in the environmental system.

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.

HO-adduct radicals have been investigated and confirmed as the common initial intermediates in TiO2 photocatalysis and Fenton degradations of water-insoluble aromatics. However, the evolution of HO-adduct radicals to phenols has not been completely clarified. When 4-d-toluene and p-xylene were degraded by TiO2 photocatalysis and Fenton reactions, respectively, a portion of the 4-deuterium or 4-CH3 group (18-100 %) at the attacked ipso position shifted to the adjacent position of the ring in the formed phenols (NIH shift; NIH is short for the National Institutes of Health, to honor the place where this phenomenon was first discovered). The results, combined with the observation of a key dienyl cationic intermediate by in situ attenuated total reflectance FTIR spectroscopy, indicate that, for the evolution of HO-adduct radicals, a mixed mechanism of both the carbocation intermediate pathway and O2-capturing pathway occurs in both aqueous TiO2 photocatalysis and aqueous Fenton reactions. [ABSTRACT FROM AUTHOR]

No two ways about it: In the photocatalytic deoxygenation of epoxides, the TiO2 particle concertedly transfers two stored electrons to generate a carbanion intermediate, which dissociates to the alkene product. This pathway ensures the higher alkene and stereoselectivity of the photocatalytic deoxygenation than those involving a single‐electron transfer. [ABSTRACT FROM AUTHOR]

The title reaction provides sulfoxides together with small amounts of the corresponding sulfones. [ABSTRACT FROM AUTHOR]

Review: 52 refs. [ABSTRACT FROM AUTHOR]

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]

TiO2 acts as a photocatalyst in the synergistic aerobic oxidation of aryl sulfides and benzylamines. [ABSTRACT FROM AUTHOR]

Review: 62 refs. [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]

Abstract: The photoreactions of aromatic compounds were investigated in aqueous dispersions of -FeOOH(goethite)/H2O2 at neutral pHs. It was found that aromatic compounds could undergo rapid decomposition and mineralization (even to 100% yield) in the presence of both -FeOOH and H2O2 under UV irradiation, and the degradation rates of the organics were related to their sorption ability of the surface of -FeOOH and were in the following order: salicylic acid ≈m-hydroxylbenzoic acid>p-hydroxylbenzoic acid≈benzoic acid>p-biphthalic acid>phenol>benzenesulfonic acid. The interactions of salicylic acid and benzenesulfonic acid with goethite were examined by Fourier transform infrared spectroscopy (FTIR), and electron spin resonance (ESR) was applied to detect the intermediate radicals formed in the photoreaction. The effect of pH and foreign ligands on the degradation of salicylic acid, decomposition of H2O2, and formation of dissolved iron was also examined. [Copyright &y& Elsevier]

Review: [28 refs. [ABSTRACT FROM AUTHOR]

It is shown, that the concerted multi-electron transfer is the dominant pathway in the photocatalytic deoxygenation of epoxides on a pristine TiO2 surface to give the corresponding alkenes. [ABSTRACT FROM AUTHOR]

The article presents the steps of the chemical equations involved in selective aerobic oxidation of amines.