Photocatalytic oxidation mechanism

The sequence of elementary steps which Turchi and Ollis suggest describe the initial photocatalytic oxidation mechanism is shown in Table... 

Giuseppe Marci, Agatino Di Paola, Elisa Garda-L6pez, et al. Photocatalytic oxidation mechanism of benzonitrile in aqueous suspensions of titanium dioxide. Catalysis Today, 129(1-2) 16-21,2007. 

Catalysis (qv) refers to a process by which a substance (the catalyst) accelerates an otherwise thermodynamically favored but kiaeticahy slow reaction and the catalyst is fully regenerated at the end of each catalytic cycle (1). When photons are also impHcated in the process, photocatalysis is defined without the implication of some special or specific mechanism as the acceleration of the prate of a photoreaction by the presence of a catalyst. The catalyst may accelerate the photoreaction by interaction with a substrate either in its ground state or in its excited state and/or with the primary photoproduct, depending on the mechanism of the photoreaction (2). Therefore, the nondescriptive term photocatalysis is a general label to indicate that light and some substance, the catalyst or the initiator, are necessary entities to influence a reaction (3,4). The process must be shown to be truly catalytic by some acceptable and attainable parameter. Reaction 1, in which the titanium dioxide serves as a catalyst, may be taken as both a photocatalytic oxidation and a photocatalytic dehydrogenation (5). 

Intensification can be achieved using this approach of combination of cavitation and advanced oxidation process such as use of hydrogen peroxide, ozone and photocatalytic oxidation, only for chemical synthesis applications where free radical attack is the governing mechanism. For reactions governed by pyrolysis type mechanism, use of process intensifying parameters which result in overall increase in the cavitational intensity such as solid particles, sparging of gases etc. is recommended. 

The ambient temperature and the possible use of solar UV are the advantages of photocatalysis moreover, Ti02 is not toxic. The reaction mechanisms of Ti02 photocatalytic oxidation of azo dyes was similar to the biodegradation process of oxidation of azo dyes with OH radical. 

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... 

Regardless of the exact mechanism at work, HCl catalyst pretreatment have been demonstrated to enhance the photocatalytic oxidation of toluene at low concentrations [68,69]. The apparent deactivation of the photocatalyst is noticeably delayed over HCl-pretreated catalyst samples in a manner similar to that seen with cofed toluene and TCE (Fig. 13). However, the pseudo-steady-state level of conversion appears to be nearly identical on both untreated and HCl-pretreated catalysts. Because the batch HCl pretreatment process incorporates a limited quantity of HCl into the catalyst surface structure, this similarity in longterm activity may be the result of surface chlorine depletion. 

Infrared spectroscopy was used to gain a better understanding of the mechanism of photocatalytic oxidation of TCE (Fan and Yates, 1996). IR spectroscopy also was used to determine intermediates formed from the reaction. Chemisorption of oxygen onto the TiOz surface plays an important role in the oxidation of TCE. The reaction was also temperature dependent. TCE is more easily degraded in the gas phase than in the aqueous phase. For this reason, a process that strips TCE from the groundwater and then treats the vapor containing TCE can be used. 

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... 

Sun, Y. and Pignatello, J., Evidence for a surface dual hole-radical mechanism in the Ti02 photocatalytic oxidation of 2,4-dichlorophenoxyacetic acid, Environ. Sci. Technol, 29, 2065, 1995. 

Lee, H. and Choi, W. (2002) Photocatalytic oxidation of arsenite in Ti02 suspension kinetics and mechanisms. Environmental Science and Technology, 36(17), 3872-78. 

Scheme 54. Putative mechanism for the ruthenium(l I l)-mediated enantioselective photocatalytic oxidative dimerization of 2-naphthol (68a).

Yang J-K, Davis AP. Photocatalytic Oxidation of Cu(II)-EDTA with illuminated Ti02 mechanisms. Environ Sci Technol 2000 34 3796-801. 

Muneer et al. [129] examined the photocatalytic oxidation of three pesticide derivatives, propham, propachlor and tebuthiuron in aqueous TiC>2 suspensions. The rates of degradation of each compound were found to be strongly affected by the type of TiC>2 used, pH, catalyst and substrate concentration. For each compound several intermediate products were identified using GCMS. This study indicated that the photocatalytic oxidation process proceeded by reactions involving electron transfer, hydroxyl radical and superoxide radical anions. Scheme 2 displays the proposed mechanism for the photocatalytic decomposition of propham. 

However, the detailed mechanism of photocatalytic oxidations is still a controversial issue. 

Muggli DS, McCue JT, Falconer JL (1998) Mechanism of the Photocatalytic Oxidation of Ethanol on Ti02, J. Catal. 173 470-483. 

Nimlos MR, Jacoby WA, Blake DM, Milne TA (1993) Direct Mass Spectrometric Studies of the Destruction of Hazardous Wastes. 2. Gas-Phase Photocatalytic Oxidation of Trichloroethylene over Ti02 Products and Mechanisms, Environ. Sci. Technol. 27 rn-TAO. 

The initial steps OF photocatalytic oxidation of aromatic compounds in aqueous suspensions of TiOa or other semiconductors have been described in terms of several mechanisms (1-13). The most important and widely accepted of these mechanisms is the generation from water decomposition of hydroxyl radicals that attack the aromatic ring. 

Reaction Mechanisms. Our analysis of intermediates and reactions reported by other researchers leads to proposed reaction pathways describing the photocatalytic oxidation of 4-chlorophenol in TiOz aqueous suspensions. The photocatalytic oxidation reaction is brought about by OH radicals, which are formed mainly from water decomposition on the Ti02 surface upon UV light irradiation (9-13). The OH radicals can either directly react with the adsorbed organic species on the TiOa surface or diffuse to the solution and then react with the dissolved organic species in the solution phase. Both reactions lead to formation of hydroxylated products such as 4-chlorocatechol, hydroquinone, 4-chlororesorcinol, and hydroxyhydroquinone as the initial products (Figure 6). Eventually, the reaction will mineralize these interme-... 

Comparison with Direct Photolysis Process. The Ti02-mediated photocatalytic oxidation reaction involves a complex free-radical reaction mechanism in which OH radicals are responsible for the oxidation of 4-chlorophenol. The initial reaction step produces 4-chlorocatechol as the main product. In contrast, the direct photolysis of 4-chlorophenol produces a different set of reaction products. Figure 8 shows that the direct photolysis of... 

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