Photocatalytic oxidation process

In these photocatalytic oxidation processes the organic compound is either directly oxidized by the valence band holes ... 

Davis, A. P. Huang, C. P. (1990). The removal of substituted phenols by a photocatalytic oxidation process with cadmium sulfide. Water Research, 24, 534-50. 

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. 

Photocatalytic oxidation process Washing away by rainwater... 

Fig. 6 Comparison of self-cleaning effects (left) conventional photocatalytic oxidation process, (right) washing away by rainwater due to photo-induced highly hydrophilic surface...

The production of carbon dioxide under these three experimental conditions is shown in Figure 1C. UV light alone yields only 2% carbon dioxide, whereas the presence of both Ti02 and UV light produces greater than 95% carbon dioxide under similar experimental conditions. This product clearly indicates that the photocatalytic oxidation process is effective in completely mineralizing 4-chlorophenol to carbon dioxide and chloride ions. 

The Ti02-mediated photocatalytic oxidation process can readily degrade 4-chlorophenol in aqueous solutions, with a complete mineralization to carbon dioxide and chloride ions, whereas the direct photolysis of 4-chlorophenol generates only a small amount of carbon dioxide. The distribution of intermediates during the course of the reaction shows that the reaction mechanism of the photocatalytic oxidation process is clearly different from that of the direct photolysis reaction. 

The superior photocatalytic activity of P25 makes it as a standard reference in photocatalytic oxidation processes. The mechanism of superior activity of P25 is still a controversy among the photocatalytic community [166]. When scientists realize that CO2 can also reduce by the photocatalytic phenomenon, the followers used P25 as the reference material, even though it s not a champion photocatalyst for CO2 reduction. In a process like CO2 reduction, the product yield depends on many factors interde-pendently as compared to the normal photocatalytic oxidation processes (See Table 6) [167]. The electron as well as proton transfer plays a crucial role in the thermodynamic uphill processes such as water or carbon dioxide splitting into valuable fuels that means the reaction led not alone by light irradiation. 

TABLE 17.1 pKa Values of Various Oxygen Intermediates in Photocatalytic Oxidation Processes... 

Photocatalytic oxidation processes involving the use of semiconductors have gained increased attention for innovative treatment of hazardous wastes as well as photoreduction of toxic compounds and purification and disinfection of drinking water. 

R. Thiiuvenkatachari, S. Vigneswaian, l.S. Moon, A review on UV/Ti02 photocatalytic oxidation process. Korean J. Chem. Eng. 25,64-72 (2008)... 

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