Titanium dioxide photooxidation

Draper R. B. and Eox M. A. (1990), Titanium dioxide photooxidation of thiocyanate (SCN)i studied by diffuse reflectance flash photolysis , J. Phys. Chem. 94, 4628-4634. 

Photoactivities of Commercial Pigments. The photoactivities of titanium dioxide pigments, as indicated by the quantum yields for photooxidation of isopropyl alcohol to acetone, vary by a factor of 10 (Table 1). The observation of generally lower photoactivity of the rutile relative to the anatase pigment is consistent with previous observations ( 5). 

Pigment-Sensitized Dye Photooxidation. Dye degradation sensitized by titanium dioxide is a recognized phenomenon (16). 

Photolytic. Low et al. (1991) reported that the photooxidation of aqueous primary amine solutions by UV light in the presence of titanium dioxide resulted in the formation of ammonium and nitrate ions. 

When an aqueous solution containing pentachlorophenol (45 pM) and a suspension of titanium dioxide (2 g/L) was irradiated with UV light, carbon dioxide and HCl formed in quantitative amounts. The half-life for this reaction at 45-50 °C is 8 min (Barbeni et al, 1985). When an aqueous solution containing pentachlorophenol was photooxidized by UV light at 90-95 °C, 25, 50, and 75% degraded to carbon dioxide after 31.7, 66.0, and 180.7 h, respectively (Knoevenagel and Himmelreich, 1976). The photolysis half-lives of pentachlorophenol under sunlight irradiation in distilled water and river water were 27 and 53 h, respectively (Mansour et al., 1989). 

Photolytic. Low et al. (1991) reported that the photooxidation of a saturated solution of atrazine by UV light in the presence of titanium dioxide resulted in the formation of ammonium, carbonate, chloride, and nitrate ions. Evgenidou and Fytianos (2002) also studied the photodegradation of atrazine using UV light (X >290 nm) in distilled water (pH 7.1, conductivity 456 mS/cm, dissolved oxygen 8.0 mg/L, total organic carbon 0.8 mg/L), lake water (pH 8.7,... 

Mendez-Roman and Cardona-Martinez [55] examined titanium dioxide catalysts with FTIR spectroscopy during the photocatalytic oxidation of toluene. Reaction intermediates, believed to be benzaldehyde and benzoic acid, were reported to accumulate on catalyst samples. This accumulation of intermediates was found to be reduced in the presence of gas-phase water. Mendez-Roman and Cardona-Martinez concluded that toluene appeared to be converted to benzaldehyde, which was then oxidized further to form benzoic acid. They suggested that the accumulation of benzoic acid led to the observed apparent catalyst deactivation. Other researchers, however, have argued that benzoic acid is unlikely to be the compound responsible for apparent deactivation in the photocatalytic oxidation of aromatics. For example, Larson and Falconer [43] concluded, based on higher CO2 evolution rates for benzoic acid relative to toluene during photooxidation, that benzoic acid was not sufficiently recalcitrant to be responsible for the deactivation seen with aromatic contaminants. 

In the photooxidation of phenol induced by excited titanium dioxide, the hydroxy radical was directly implicated as the reactive species, the observed organic products having incorporated oxygen, Eq. (16) On further photolysis, aldehydes, acids, and CO2 could be obtained. 

Moza, P.N., Sukul, P, Hustert, K., Kettrup, A. (1994) Photooxidation of metalaxyl in aqueous solution in the presence of hydrogen peroxide and titanium dioxide. Chemosphere 28, 341-347. 

Paz Y, Luo Z, Rabenberg L, Heller A. Photooxidative self-cleaning transparent titanium dioxide films on glass. J Mater Res 1995 10 2842-2848. 

Serpone et al. have examined colloidal titanium dioxide sols (prepared by hydrolysis of TiCl4) with mean particle diameters of 2.1, 13.3, and 26.7 nm by picosecond transient absorption and emission spectroscopy [5]. Absorption decay for the 2.1 nm sols was found to be a simple first-order process, and electron/hole recombination was 100% complete by 10 ns. For the 13.3 and 26.7 nm sols absorption decay follows distinct second-order biphasic kinetics the decay times of the fast components decrease with increase in particle size. 10 ns after the excitation pulse, about 90% or more of the photogenerated electron/hole pairs have recombined such that the quantum yield of photooxidations must be 10% or less. The faster components are due to the recombination of shallow-trapped charge carriers, whereas the slower components (x > 20 ns) reflect recombination of deep-trapped electrons and holes. 

The strongest evidence for direct hole oxidation as the principal step in the photooxidation step comes from a recent study performed by Draper and Fox that failed to detect any of the expected intermediate hydroxyl radical adducts following diffuse reflectance flash photolysis of several titanium dioxide/substrate combinations [33]. In each case where the product of hydroxyl radical-mediated oxidation was known to be different from that of direct electron transfer oxidation, the authors observed only the products of the direct electron-transfer oxidation. 

In the photooxidation of phenol on an irradiated suspension of titanium dioxide, the hydroxyl radical was also directly implicated as the reactive species [122], with oxygen incorporated in the observed organic products (Eq. 14) [118]. 

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. 

Sayre RM, Dowdy JC. Titanium dioxide and zinc oxide induce photooxidation of unsaturated lipids. Cosmet Toilet 2000 115 ... 

Hsu ER, Gebert MS, Becker NT, Gaermer AL. Effects of plasticizers and titanium dioxide on the properties of poly(vinyl alcohol) coatings. Pharm Dev Technol 2001 6(2) 277-284. Kakinoki K, Yamane K, Teraoka R, et al. Effect of relative humidity on the photocatalytic activity of titanium dioxide and photostability of famotidine. J Pharm Sci 2004 93(3) 582-589. Sayre RM, Dowdy JC. Titanium dioxide and zinc oxide induce photooxidation of unsaturated lipids. Cosmet Toilet 2000 115 75-80, 82. 

A number of reviews of topical interest have appeared with degradable plastics making a come back. These include the role of plastics in the environment " and agricultural film , ethylene carbon monoxide copolymers and their analysis. Several other topic reviews of interest relate to the role of titanium dioxide pigments 22 laser ablation and photolysis, degradation and stabilisation of polyolefins , conformation defects in poly(vinyl chloride)and poly(ethylene terephthalate) photooxidation . 

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