Oxidations, photocatalyzed

The mechanism(s) by which these photocatalyzed oxidations are initiated remain uncertain. Early proposals have included involvement of either the photo-produced holes (h+) arising directly from semiconductor photo-excitation, or the (presumed) derivative hydroxyl radical (OH) which was argued to arise from the hole oxidation of adsorbed hydroxyls (h+ + OH-—> OH ). Recent subambient studies [4] with physisorbed chloromethane and oxygen suggest the dioxygen anion (02 ) as a key active species, and the photocatalytic high efficiency chain destruction of TCE is argued to be initiated by chlorine radicals (Cl) [5]. The chlorine-enhanced photocatalytic destruction of air contaminants has been proposed [1, 2, 6] to depend upon reactions initiated by chlorine radicals. 

Which is the rate-limiting step in photocatalyzed oxidations on oxide-based photocatalysts Question 6... 

H. Bruning, Photocatalyzed oxidation of alcohols and organochlorides in the presence of native Ti02 and metallized Ti02 suspensions. Part (I) Photocatalytic activity and pH influence, Water Res. 33 (1999) 661-668. 

The photocatalyzed oxidation of gas-phase contaminants in air has been demonstrated for a wide variety of organic compounds, including common aromatics like benzene, toluene, and xylenes. For gas-phase aromatic concentrations in the sub-lOO-ppm range, typical of common air contaminants in enclosed spaces (office buildings, factories, aircraft, and automobiles), photocatalytic treatment leads typically to complete oxidation to CO2 and H2O. This generality of total destruction of aromatic contaminants at ambient temperatures is attractive as a potential air purification and remediation technology. 

N—H deprotonation to form a nitrogen radical is obviously more important than alpha-deprotonation of the pendant alkyl group. Such reactions are reminiscent of the photocatalyzed oxidation of ammonia reported by Pichat and coworkers... 

Eq. (25) 5 . Various products have been reported in the photocatalyzed oxidation of dimethyl sulfide depending on the initial concentration of thioether. On TiO2, CdS, or ZnSe, a Stevens rearrangement occurs, Eq. (26) The key intermediate appears to be a dimethylsulfide dimer cation radical, and the reaction is only efficient in protic solvents. 

The course of hydrocarbon photocatalyzed oxidations seems to depend significantly on the relative positions of the valence band edge of the active photocatalyst and the oxidation potential of the substrate. For example, in contrast to the clean oxidation of toluene described above, lower activity was observed in neat benzene, a substrate whose oxidation potential lies at or slightly below the valence band edge This observation implies the importance of radical cation formation (via photoinduced electron transfer across the irradiated interface) as a preliminary step to hydrocarbon radical formation. If beitzene-saturated aqueous semiconductor suspensions are... 

Figure 24. Scheme of multilamp immersion-type photochemical installation for the photocatalyzed oxidative degradation of industrial waste water [12]. A Bypass circuit. B Reactor circuit. 1 Gas-liquid mixture and injection. 2 Reservoir. 3 Pump (ceramics). 4 Water pump. 5 Heating circuit. 6 Cooling circuit, hv Medium pressure mercury lamps (Pyrex). T Thermometers. 

Example of Application Ti02 Photocatalyzed Oxidative Degradation. As... 

TABLE 2 Variables and Experimental Region" for the Investigation of the Pilot Level Ti02 Photocatalyzed Oxidative Degradation of Organic Pollutants Contained in an Industrial Waste Water [12]... 

TABLE 3 Experimental Matrix for Investigating the Influence of Three Factors on the Ti02 Photocatalyzed Oxidative Degradation of Waste Water Pollutants in a Pilot Reactor Complete Factorial Design 23, Control Experiments 1 (Natural and Coded Variables are Indicated), and Values of the Experimental Response Y... 

TABLE 4 Influence of DOC0, Ti02 Concentration, and Temperature on the Photocatalyzed Oxidative Degradation of Waste Water Pollutants in a Pilot Reactor Coefficients (Main Effects and Interactions) Calculated from the Experimental Results of a 23 Factorial Design (Table 3)... 

Titanium dioxide photocatalyzed oxidation of neat tetralin was previously reported to give its hydroperoxide (14). Reinvestigation showed that tetralol and tetralone are also formed in acetonitrile possibly through electron transfer from tetralin to the positive holes (27). 

Lichtin, N. and Avudaithai, M., TiOz Photocatalyzed oxidative degradation of CH3CN, CH3OH, CjHCI, and CH2C12 supplied as vapors and in aqueous solution under similar conditions, Environ. Sci. Technol., 30, 2014, 1996. 

Tang, W.Z. and Huang, C., Photocatalyzed oxidation pathways of 2,4-dichlorophenol by CdS in basic and acidic aqueous solutions, Water Res., 29, 745, 1995. 

Xu, T., Cai, Y. and O Shea, K.E. (2007) Adsorption and photocatalyzed oxidation of methylated arsenic species in Ti02 suspensions. Environmental Science and Technology, 41(15), 5471-77. 

Nohara K, Hidaka H, Pelizzetti E, Serpone N. Processes of formation of NH4 and NO3 ions during the photocatalyzed oxidation of N-containing compounds at the titania/water interface. J Photochem Photobiol A Chem 1997 102 265-272. 

Serpone, N. Maruthamuthu, P. Pichat, P. Pelizzetti, E. Hidaka, H. Exploiting the interparticle electron transfer process in the photocatalyzed oxidation of phenol, 4-chlorophenol and pentachlorophenol Chemical evidence for electron and hole transfer between coupled semiconductors, J. Photochem. Photobiol., A Chem. 1995, 85, 247. 

Fornal E, Giannotti C. Photocatalyzed oxidation of cyclohexane with heterogenized decatungstate../ Photochem. Photobiol A Chem 2007 188 279-86. 

Muszkat L, Feigelson L, Bir L, Muszkat KA. Titanium dioxide photocatalyzed oxidation of proteins in biocontaminated waters. J Photochem Photobiol B Biol 2001 60 32-6. 

Krauss and Wilcke examined the TiC -photocatalyzed oxidation of 12 PCB congeners [and 20 polycyclic aromatic hydrocarbons (PAHs)] on various soil samples (four mineral topsoil horizons, six organic horizons, and four particle-site fractions in three different soils) [107]. When the Ti02/soil mixture was irradiated in the absence of H2O, no photooxidation of the chlorobiphenyls occurred. When slurried with water, however, chlorobiphenyl concentrations decreased by 40-50% after 48 hours of irradiation, while the PAH concentrations were unchanged. By way of contrast, PAHs and PCBs doped onto quartz sand diminished by 95-100% after 8 hours of photolysis. The pollutants are clearly more accessible to hydroxyl radicals on sand than on soil. It is also clear that the photooxidation occurred in the soil and not in solution. Thus, OH is generated on one surface (Ti02), diffuses in the water to the other surface (soil), where the oxidation occurs. 

Sauer ML, Ollis DF (1996) Photocatalyzed Oxidation of Ethanol and Acetaldehyde in Humidified Air, J. Catal. 158 570-582. 

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