Photocatalytic degradation, mechanism

SCHEME 16.7 Photocatalytic degradation mechanism of PE-(Ti02/CuPc) films [69]. 

Fig. 3.17 Proposed photocatalytic degradation mechanism by using CB[6]-HPA composite as heterogeneous photocatalysts (Reprinted with permission from Ref. [128], Copyright 2011, Elsevier)...

Tang WZ, An H (1995) Photocatalytic degradation kinetics and mechanism of acid blue 40 by UV/Ti02 in aqueous solution. Chemosphere 31 4171-4183... 

Mills G, Hoffmann MR (1993) Photocatalytic degradation of pentachlorophenol on Ti02 particles identification of intermediates and mechanism of reaction. Environ Sci Technol 27 1681-1689... 

Luo and Ollis have shown that this cofeeding strategy may also be used to enhance the photocatalytic oxidation of toluene [12]. Near-complete photocatalytic oxidation of toluene at less than —200 mg/m in the gas-phase was achieved in the presence of TCE (either 226 or 753 mg/m ). Based on die homogenous gas-phase TCE degradation mechanism proposed by Sanhueza et al. [64], which... 

Photocatalytic oxidation of 2,4-dichlorophenoxyacetic acid (2,4-D) was investigated (Sun and Pignatello, 1995). In addition to the dominant hydroxyl radical mechanism, Sun and Pignatello found evidence that direct hole oxidation may be the mechanism for the photocatalytic degradation of some organic compounds. The assumed mechanism for this oxidation is H+ acting as an electron-transfer oxidant, while O behaves like a free OH and abstracts H or adds to C=C multiple bonds. Hole oxidation has been used to explain the oxidation of oxalate and trichloroacetate ions, which lack abstractable hydrogens or unsaturated C-C bonds. Whether the reaction... 

The addition of Fe ions to the solution also increased the rate of photo-catalytic degradation. The TOC reduction was 80% in the presence of Fe ions and 10% in the absence of Fe ions. pH is the key parameter in determining the photocatalytic degradation of anilines as well as other compounds previously studied. The high photocatalytic degradation near the pHpzc has also been observed for other compounds studied with Ti and Zn. The major mechanism is the attack of photogenerated OH radicals on the aniline molecule under alkaline conditions. The addition of Fe ions to the solution can enhance the formation of OH radicals and lead to higher photocatalytic rates. 

The proposed mechanism for photocatalytic degradation of the /V-(l-mcthylcthyl) group involves reaction of an hydroxyl radical with either a methinyl or the methyl carbon, which via the corresponding hydroperoxide subsequently leads to formation of the acetamido and formylamino (via aminopropionaldehyde) products, respectively... 

Figure 23.4 Proposed mechanism for the photocatalytic degradation of the IV-(l-inethylethyl) group in the presence of Ti02.

To substantiate this mechanism, haloquinolines (75) were used. The strategy was to hinder sterically the addition of superoxide. In the case of 6-chloroquinoline, the products were the same as those formed from quinoline, except that they were chlorinated, which was expected because position 6 is not involved in either mechanism. Halogen substitution on the pyridine moiety in part directed oxygen addition to the benzene moiety, which was consistent with superoxide addition onto the more accessible positions on the benzene ring of the halogenated radical cation. This result supports the fact that a cycloaddition mechanism can take place in the photocatalytic degradation of quinoline. This mechanism has been proposed in the case of other aromatics, such as 4-chlorophenol (76) and 4-chloro-catechol (77). 

Turchi CS, Ollis DF. Photocatalytic degradation of organic water contaminants mechanisms involving hydroxyl radical attack. J Catal 1990 122 178-192. 

Mao, Y. Schoeneich, C. Asmus, K. D. Radical mediated degradation mechanisms of halogenated organic compounds as studied by photocatalysis at Ti02 and by radiation chemistry, in Photocatalytic Purification and Treatment of Water and Air D. F. Ollis H. Al-Ekabi, eds., Elsevier Science Publishers B.V. Amsterdam, 1993, p. 49. 

The photocatalytic degradation of TCE was studied intensively, both in the liquid phase and in the gas phase. Unlike in the aqueous phase, where the quantum efficiency is no more than a few percents (Alberici and Jardim, 1997 Pruden and Ollis, 1983), the quantum efficiency in the gas phase can be higher than 100% (Upadhya and Ollis, 1998). This difference was attributed to the existence of two mechanisms. That the mechanism in the liquid phase was different than that in the gas phase could be deduced also by the fact that the intermediate products dichloroacetaldehyde (DCA) and dichloroacetic acid (DCAA) were identified only during liquid-phase photocatalysis (Pruden and Ollis, 1983). 

Photocatalytic degradation of l,3-dihydroxy-5-methoxybenzene in the presence of Ti02 follows zero-order kinetics. The product formed from this process is CO2 with the best results obtained at pH 9 . Other dihydroxybenzenes can also be photooxidized using dye-mediated oxidation involving 02( Ag). A charge transfer mechanism is thought to be... 

Fig. 2. Mechanism ot Orange II photocatalytic degradation mediated by Fe/Nation/ glass mats under visible light irradiation during the degradation ot Orange H.

Scheme 1 - Mechanism of the photocatalytic degradation of phenol in UV-vis photocatalyzed process.

In support of the argument for the surface-bound hydroxyl radical mechanism, there is an intermediate presence of hydroxylated structures during the photocatalytic degradation of halogenated aromatics. These hydroxylated intermediates are also found when similar aromatics react with a known source of hydroxyl radicals. Furthermore, ESR studies confirm the existence of hydroxyl and hydro-peroxy radicals in aqueous solutions of illuminated Ti02 (Hoffman et al., 1995 Linsebigler et al., 1995). 

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