Irradiation titanium dioxide surface

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. 

Kemp et al. (92) operated in the close proximity mode to study the concentration of chloride ions generated near the surface of UV-irradiated titanium dioxide in the presence of aqueous 2,4-dichlorophenol. The Cl concentration was measured potentiometrically with a 50 pan diameter Ag/ AgCl microdisk. Experiments were performed at several tip-substrate distances, and the concentration of chloride was recorded as a function of time after turning on and turning off the UV irradiation. 

Kemp, T. J., Unwin, P. R., Vincze, L., Photogenerated chloride-ion concentration near-the-surface of UV-irradiated titanium-dioxide in the presence of aqueous 1,4-dichlorophenol probed using an ultramicroelectrode. J Chem Soc Earaday Trans 1995, 91(21), 3893-3896. 

Irradiation of powdered titanium dioxide suspended in solutions containing aromatic compounds and water under oxygen has recently been shown to induce hydroxylation of aromatic nuclei giving phenolic compounds and oxidation of side chains of the aromatic compounds (50-55). These reactions have been assumed to proceed through hydroxyl and other radical intermediates, but the mechanism for their generation, whether reactive free radicals result from oxidation of water, from reduction of oxygen, or from oxidation of the substrates on the surfaces of the excited titanium dioxide, has not been clear. 

Materials. The titanium dioxide powders were rutile in structure (obtained from the Titanium Division, National Lead Co., Amboy, N. J.), with nominal specific surface areas of 10 and of 100 sq. meters per gram. Chemical analysis by the supplier showed negligible impurities except for 0.8% sodium oxide in the Ti02-100 and traces of iron in both the TiO2-10 and the TiO2-100. The presence of iron was confirmed by the nature of the decay of the neutron irradiation—induced radioactivities. 

Electron donors bound to the surface of the semiconductor particles interfere with e b/h, recombination allowing e b (conduction-band electrons) to react with molecular oxygen via Eq. 120. Isotopic labeling studies have shown that H202 formed in irradiated suspensions of ZnO contained oxygen atoms derived exclusively from 02. Titanium dioxide is much less effective for H202 production (Kormann et al., 1988b). 

The most common photocatalytic processes, in terms of both mechanistic analysis and practical use, involve insoluble semiconductor metal oxides or sulfides, which upon irradiation undergo dual interfacial electron transfer between the excited semiconductor surface and adsorbed donor (D) and/or acceptor (A) molecules (Scheme 6.291). Titanium dioxide (Ti02) is a particularly popular photocatalyst due to its good redox properties (see also Special Topic 6.29), high stability, low toxicity and low price. 

In 1977 Schrauzer and Guth reported that they had synthesized NH3 from H20 and N2 [1], The process they described involved the use of titanium dioxide-based photocatalysis and UV-visible radiation. The authors reported that Ti02 powders containing chemisorbed water or surface hydroxyls produced H2 and 02 when irradiated in the near UV. The amounts of H2 and 02 so produced were found to be strongly dependent upon the catalyst pretreatment. Experiments demonstrated that N2 at one atmosphere completely inhibited the formation of H2, but had no effect on the yield of 02. The authors were aware of earlier work [57] showing that acetylene is photoreduced under conditions similar to those employed in the water photolysis reactions and they found that acetylene, like nitrogen, completely inhibited H2 production. On the basis of these results, the authors reasoned that photoreduction of N2 could be occurring. They... 

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