Titanium photo-oxidation reactions

Almquist CB, Biswas P (2001) A Mechanistic Approach to Modeling the Effect of Dissolved Oxygen in Photo-oxidation Reactions on Titanium Dioxide in Aqueous Systems, Chem. Eng. Sci. 56 3421-3430. 

Almquist, C.B., and Biswas, P.A. A mechanistic approach to modeling the effect of dissolved oxygen in photo-oxidation reactions on titanium dioxide in aqueous systems . Chem. Eng. Sci. 56, 3421 (2001). 

All these results concur to exclude the involvement of hydrogen peroxide as a coproduct of the photo-oxidation of water and intermediate of other photo-oxidation reactions at Ti02. The high rates of its photodecomposition at open circuit and photooxidation under anodic bias render unlikely the build-up of any significant concentration of H2O2 at the surface of irradiated titanium dioxide or in the nearby solution. 

In the presence of O2 and H2O, the photo-formed e and h react with these molecules on the titanium oxide surfaces to produce 02/ and OH radicals, respectively. These 02/ and OH radicals have a very high oxidation potential, inducing the complete oxidation reaction of various organic compounds such as toxic halocarbons, as shown in... 

Titanium dioxide (Ti02) has been attracting much attention for its important role in water photo-oxidation and photocatalyst, as well as a base material for dye-sensitized solar cells. A number of studies have been conducted on the mechanisms of interfacial photo-anodic reactions but the reported mechanisms still remain sketchy, and the detailed molecular mechanism has not yet been clarified. The main reason for confusion may arise from the possibility that the reaction mechanism depends on detailed chemical structures of the electrode surface. This implies that studies with well-defined surfaces are of key importance. 

With unimplanted or chemically doped titanium oxide photocatalysts, the photo catalytic reaction does not proceed under visible light irradiation (A > 450 nm). However, we have found that visible light irradiation of metal ion-implanted titanium oxide photocatalysts can initiate various significant photocatalytic... 

Reaction (39) may be seen either as a direct photo-oxidation of the surface superoxo-titanium species or as their decomposition followed by a rapid reoxidation of the resulting Ti site into Ti. ... 

The above proposed mechanism of photo-oxidation of water at titanium dioxide, including the sequence of reactions (32), (33), (38) and (39), presents some analogy with the mechanism of anodic oxygen evolution at a palladium electrode, in acid solutions, involving electrochemical oxidation of the metal into unstable palladium dioxide, Pd02, and its subsequent decomposition ... 

When referring to Ti02-based photocatalytic systems it is important to note that, in most cases, the semiconducting oxide is associated there with a noble metal or/and a noble metal oxide catalyst. While the role played by these catalysts in (partial) cathodic reactions seems relatively well understood it remains less clear with regard to the photoanodic reactions. In particular, the exact function of the extensively used ruthenium dioxide catalyst has been questioned The role of Ru02 as a hole-transfer catalyst has, for example, been established through laser-photolysis kinetic studies in the case of photo-oxidation of halide (Br and CP) ions in colloidal titanium dioxide dispersions. In fact, the yields of Brf and ClJ radical anions, photogenerated in the course of these reactions. 

M. Bideau, B. Claudel, L. Faure, and M. Rachimoellah, Photo-oxidation of formic acid by oxygen in the presence of titanium dioxide and dissolved copper ions oxygen transfer and reaction kinetics, Chem. Eng. Comm. 93, 167-179 (1990). 

Photo-oxidative self-cleaning and antifogging effects of transparent titanium dioxide films has attracted considerable attention for the past decade [334, 335]. In order to understand the photo-induced hydrophilic conversion on titanium dioxide coatings in details, it is inevitably necessary to understand the relationship between the photo reaction and the surface crystal structure this can be done, for example, by an evaluation of the photo-induced hydrophilic conversion on the different crystal faces of rutile single crystals and also... 

Although we are moving in a positive direction in the development of such clean and safe chemical systems using photofunctional materials such as titanium oxide photocatalysts, we have yet to gain a complete understeinding of the reaction mechanisms for the design of highly efficient and selective photo-induced reaction systems. 

These reactions can be considered as electron-transfer processes, in which various atoms or molecules ionize or ions become discharged. Electron transfer processes play a very important role in the photoactivity of pigments such as titanium oxide (cf. section 5.1) and zinc oxide (cf. section 5.2), and in photo-Fenton reactions (cf. section 5.6). 

Photo-initiated AOPs are subdivided into VUV and UV oxidation that are operated in a homogeneous phase, and in photocatalysis (Fig. 5-15). The latter can be conducted in a homogeneous aqueous phase (photo-enhanced Fenton reaction) or in a heterogeneous aqueous or gaseous phase (titanium dioxide and certain other metal oxide catalysts). These techniques apply UV-A lamps or solar UV/VIS radiation and they are in pre-pilot or pilot status. According to Mukhetjee and Ray (1999) the development of a viable and practical reactor system for water treatment with heterogeneous photocatalysis on industrial scales has not yet been successfully achieved. This is mainly related to difficulties with the efficient distribution of electromagnetic radiation (UV/VIS) to the phase of the nominal catalyst. 

The surfaces of mtile Ti02 have been the subject of intense research because of their photo-catalytic properties for the dissociation of water. The hydroxylation rate on the surface and the kinetics of the reaction were shown to depend strongly on the surface stoichiometry and detailed atomic structure. In addition, like the two above surfaces of sapphire and magnesium oxide, rutile titanium dioxide surfaces stand as model metal oxide surfaces. Their atomic structure is thus of fundamental interest. 

Alcohol decomposition does provide additional insight into the interaction of adsorbates with metal oxide surfaces. The reactions of alkoxides on titanium dioxide have been used to probe thermal and photo-reactivity of powder and single crystal samples. 

The observation of products at intermediate oxidation level in the photocatalytic reactions of hydrocarbons is consistent with a surface-bound radical intermediate [157]. Photocatalytic isotope exchange between cyclopentane and deuterium on bifunctional platinum/titanium dioxide catalysts indicates the importance of weakly adsorbed pentane at oxide sites. The platinum serves to attract free electrons, decreasing the efficiency of electron-hole recombination, and to regenerate the surface oxide after exchange. Much better control of the exchange is afforded with photo-electrochemical than thermal catalysis. 

Amiodarone commonly causes phototoxicity reactions (186,187). The risk of phototoxicity increases with the duration of the exposure. Window glass and sun screens do not give protection, although zinc or titanium oxide formulations and narrow band UVB photo therapy can help (188-190). For most patients this adverse effect will be no more than a nuisance, and the benefit of therapy may be worthwhile. However, in a few cases treatment may have to be withdrawn. Histological examination of skin biopsies shows intracytoplasmic inclusions of phospholipids (191). There has been a single report of a severe case of photosensitivity in conjunction with a syndrome resembling porphyria cutanea tarda, resulting in bullous lesions (192). 

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