Photocatalytic destruction

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. 

Studies on the gas-phase photocatalytic destruction of VOCs have shown the efficiency of such catalysts in the destruction of chlorinated compounds [203], even using relatively small amounts of TiOz on the wall of the monolith [204],... 

In a similar way, a well-adhered surface modification of BC fibers can be achieved with Ti(>2 nanoparticles (with a diameter of about 10 nm) by the hydrolysis of titanium tetraisopropanolate adsorbed onto the fibers. It was observed that the titania-coated surface appears to be dense and have low porosity and to consist of near-spherical grains. By washing with sodium carbonate solution, the TiC>2 films were not removed during neutralization. It seems that the particles have formed strong interactions with BC. The coated membranes showed substantial bactericidal properties under UV radiation and white light (containing a small fraction of UV) conditions, too. This effect is caused by the photocatalytic destruction of the bacterial cells. 

Amalric L, Guillard C, Pichat P. Use of catalase and superoxide dismutase to assess the roles of hydrogen peroxide and superoxide in the Ti02 or ZnO photocatalytic destruction of 1,2-dimethoxybenzene in water. Res Chem Interned 1994 20 579-594. 

Zhao et al. (2002) studied the photodestruction of sulforhodamine-B in Pt/Ti02 suspension under visible light illumination. The electrons excited from dye molecules were accepted by Pt islands. The accumulated electrons then react with 02 to form 02- and subsequently OH, which ultimately caused the self-destruction of the dye molecules. Photocatalytic destruction of oxalic acid (Iliev et al. 2006) and H2 generation (Sreethawong et al. 2006) from aqueous methanol solution were also promoted by platinizing Ti02. Under anoxic conditions, products from... 

Theurich et al. [59] in a study of the degradation of 4-chlorophenol also found the kinetics of the photocatalytic destruction process could also be described by the L-H model. Although the rate of 4-CP was uninfluenced by pH it was found the level of mineralisation was particularly sensitive to changes in this parameter. The mechanism of destruction also appeared to be affected by the nature of titania used. With Degussa P25 seven intermediates were detected while with Hombicat UV100 only three by-products were generated (Scheme 1). 

Scheme 1 Mechanism for the photocatalytic destruction of 4-chlorophenol (reproduced from [59] with permission from the American Chemical Society)...

Wang and Hong investigated the effect of additives such as hydrogen peroxide, periodate and persulphate on the photocatalytic destruction of 2-chlorobiphenyl in water [105]. No rate enhancement for the destruction process was observed when any of these compounds were used as additives. In this study greater destruction rates of the chlorobiphenyl were in fact... 

The photocatalytic destruction of haloaliphatic compounds has been studied in detail by Hoffmann and co-workers [107,108]. A detailed study of the mechanism of the photocatalytic destruction of chloroform using Ti(>2 was investigated by Kormann et al. [107]. The process was believed to be initiated by hydrogen abstraction by the hydroxyl radicals generated on the surface of the TiC>2 (Eqs. 27-32). The chloroform radical subsequently reacts with oxygen forming trichloromethanol oxidation to trichloroformaldehyde and ultimately complete mineralisation. 

Catechol and hydroquinone by-products were also detected in the photocatalytic destruction of nitrophenols [117]. Ninety-nine percent destruction of 4-nitrophenol was achieved within 3 h, although for complete mineralisation up to 9 h irradiation was necessary. The presence of oxygen not only affected the level of mineralisation of the nitrophenol but also the extent of denitration, believed to be the major initial step of the photocatalytic process. Hydrogen peroxide was reported to significantly improve the rate of photocatalytic process increasing the destruction rate by up to 3 fold. Addition of Cu ions, however, had a detrimental effect [118]. 

The solar photocatalytic destruction of Aldrin has been reported by Bandala et al. using both concentrated and nonconcentrated solar processes [126]. As with other processes the addition of peroxide enhanced... 

The photocatalytic destruction of the fungicide 2-phenylphenol generated a range of by-products including hydroquinones, benzoquinones and dihy-droxybiphenyls [132]. This indicated that the major routes of degradation included hydroxylation and scission of the phenyl-phenol ring. The rate of... 

The hydroxyl radical attack on aromatic structures appeared also to be an important step in the photocatalytic destruction of a series of other dye molecules including Crocein orange G, methyl red, Congo red and Alizarin... 

The role of oxygen in the photocatalytic destruction process is also unclear. For some studies oxygen was found to enhance, or in some cases be a prerequisite for, photocatalytic-induced cell death, although the optimum concentration required is equivocal [173,190]. However, Herrera Melian et al. [179] showed that it was not necessary to bubble oxygen into a waste water sample undergoing photocatalytic treatment since comparison of an air-bubbled sample and a stirred sample resulted in similar rates of destruction. 

Table 1 Compounds which have undergone gas-phase photocatalytic destruction...

Many have found that during the photocatalytic destruction of toluene the catalyst was susceptible to deactivation. This may be due to the accumulation of benzoic acid on the catalyst surface. Studies have also shown that the addition of TCE or ozone can either have a positive or negative effect on toluene degradation [235]. Larson and Falconer [235] observed that toluene reacts quickly on TiC to form strongly bound intermediates which react more slowly to form CO2 and water. 

Howell BC (1996) Photolytic and Photocatalytic Destruction of Formaldehyde in Aqueous Media, J. Electrochem. Soc. 143, No. 5 1562-1570. 

Computational fluid dynamics approach was utilized in the study of photocatalytic destruction of gas-phase vinyl chloride in an annular flow reactor (Mohseni and Taghipour, 2004). The kinetic data for the model was obtained from a differential glass plate reactor. The modeling results indicated significant gradient of vinyl chloride in the radial direction and nonuniform flow distributions, which resulted in reduced efficiency over the entire range of inlet concentrations. 

Herrera-Melian, J. A. Dona-Rodriguez, J. M. Rendon, E. Tello Soler Vila, A. Brunet Quetglas, M. Azcarate, A. A. Pascual Pari-ente, L. Solar Photocatalytic Destruction of p-Nitrophenol A Pedagogical Use of Lab Wastes, J. Chem. Educ. 2001, 78, 775-777. 

Soltzberg, L. J. Brown, V. Note on Photocatalytic Destruction of Organic Wastes Methyl Red as a Substrate, J. Chem. Educ. 2005, 82, 526. 

Pichat PG, MaiUard C, Amalric L, ITOliveira JC. Titanium dioxide photocatalytic destruction of water aromatic pollutants intermediates properties-degradabiUty correlation effects of inorganic ions and titanium dioxide surface area comparisons with hydrogen peroxide processes. Trace Metals Environ 1993 3 207-223. 

Photocatalytic destruction of chlorinated solvents in water with solar energy was investigated [436] using a nearcommercial scale, single-axis tracking parabolic trough system with a glass pipe reactor mounted at its focus. In... 

Recently, it was found that carbon and other species such as the fullerenes are also active in UV photocatalysis. In addition, these can experience photocatalytic destruction [28, 29]. This suggests a possible contribution to atmospheric photocatalysis by soot particles. 

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