Sensitivity photocatalysis

Direct and sensitized photocatalysis creates similar products and involves primarily oxidative steps when oxygen is present. In deoxygenated systems, both oxidative and reductive steps are involved in the degradation process. The presence of oxygen in direct and sensitized photocatalytic systems affects the extent of mineralization and the denitration of 4-NP (Dieckmann and Gray, 1996). 

Degradation pathways of 4-nitrophenol via sensitized photocatalysis. (From Dieckmann, M. and Gray, K., Water Res., 30(5) 1169,1996. With permission.)... 

Percherancier, J., Chapelon, B., and Pouyet, B., Semiconductor-sensitized photocatalysis of pesticides in water the case of carbetamide, J. Photochem. Photobiol. A, 87, 261, 1995. 

To demonstrate the ability of this system to achieve sensitized photocatalysis, the authors measured the electron transfer from the zinc porphyrin to methyl viologen. Adding p-mercaptoethanol as a sacrifidal reductant enabled regeneration of the resulting porphyrin radical cations, thus dosing the catalytic cycle and allowing for continued photoreduction. When capsids that were modified with donor dyes inside and porphyrins outside were illuminated at the peak donor exdtation (where the porphyrin has a very low extinction coeffident), an almost fourfold increase in the photocatalytic efficiency of the porphyrin was observed. 

R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki and Y. Taga, 2001, Visible-Light Photocatalysis in Nitrogen-Doped Titanium Oxides, Science, 293, 269-271. R. Asahi and T. Morikawa, 2007, Nitrogen complex species and its chemical nature in Ti02 for visible-light sensitized photocatalysis, 2007, Chem. Phys. 339, 57-63. 

In most studies, heterogeneous photocatalysis refers to semiconductor photocatalysis or semiconductor-sensitized photoreactions, especially if there is no evidence of a marked loss in semiconductor photoactivity with extended use. It is meant here that the initial photoexcitation takes place in the semiconductor catalyst substrate and the photoexcited catalyst then interacts with the ground state adsorbate molecule [209]. 

Applications of titania nanotube arrays have been focused up to now on (i) photoelectrochemical and water photolysis properties, (ii) dye-sensitized solar cells, (iii) photocatalysis, (iv) hydrogen sensing, self-cleaning sensors, and biosensors, (v) materials for photo- and/or electro-chromic effects, and (vi) materials for fabrication of Li-batteries and advanced membranes and/or electrodes for fuel cells. A large part of recent developments in these areas have been discussed in recent reviews.We focus here on the use of these materials as catalysts, even though results are still limited, apart from the use as photocatalysts for which more results are available. 

Another approach to wards photocatalysis is to use dy as a sensitizer instead of a semiconductor as in photosynthesis. It is not the aim of this book to cover all the aspects of the sensitized photochemical conversion system, but typical sensitized systems for photocatalytic reactions of water are described in Chapter 18 The concept of a photochemical conversion system using a sensitizer and water oxidation/reduction catalysts is mentioned in Chapter 19, accompanied by a discussion on the sensitization of semiconductors. 

Many dyes are decolorized and ultimately mineralized by photocatalysis. In the degradation of azo dye,18 21) the degradation rate decreases in the order monoazo > diazo > triazo.1819) Three processes including oxidation and reduction are considered to occur simultaneously in the photocatalytic degradation of dye. These are illustrated in Fig. 9.9. Process 1 is the common photocatalytic degradation process of organic compounds. Process 2 is spectral sensitization as observed in a wet-type solar cell. In process 3 one moiety of dye molecule serves as the electron acceptor, suppressing recombination between electron and positive hole. 

A comparison of sensitized and direct photocatalysis was performed for the degradation of 4-nitrophenol (Dieckmann and Gray, 1996). The application of sensitization to the degradation of nitrophenolic compounds and azo... 

A [2 + 2] photocycloaddition with two alkenes can also be induced by photochemical electron transfer [16,17]. In such cases, sensitizers are frequently used and the reactions therefore occur under photocatalysis [18]. Under photochemical electron transfer (PET) conditions, the diene 10 yielded in an intramolecular reaction the cyclobutane 11 (Scheme 5.2) [19], such that in this reaction a 12-membered cyclic polyether is built up. The reaction starts with excitation of the sensitizer 1,4-dicyanonaphthalene (DCN) only 0.1 equivalents of the sensitizer are added to the reaction mixture. Electron transfer occurs from the substrate 10 to the excited sensitizer, leading to the radical cation I. This intermediate then undergoes cycli-zation to the radical cation of the cyclobutane (II). Electron transfer from the radical anion of the sensitizer to the intermediate II leads to the final product 11, and regenerates the sensitizer. In some cases, for example the cydodimerization of N-vinylcarbazole, the effidency is particularly high because a chain mechanism is involved [20]. 

Nitroaromatics also sensitize the oxidation of methylarenes and it has been found that silica-grafted 2,4,6-trinitrobenzene is a convenient heterogeneous sensitizer, giving the aldehydes in 79-90% yields with 100% selectivity [229]. Bibenzyls, pinacols and pinacol ethers are likewise oxidized to ketones or respectively esters through carbon-carbon bond fragmentation upon dicyanonaphthalene sensitization [230]. A good method for benzylic oxidation is based on titanium dioxide photocatalysis [231-233]. 

Because of their properties they are widely used in electrochemistry, in electrophotography, as well as in photocatalysis and photochemical water cleavage. Thus Hennig and Rehorek in 1986 reported the sensitization of photochemical reactions by Prussian blue analogues of molybdenum octacyanide, and Kaneko and his group in 1984 found that water can be photolyzed with visible light in presence of Pmssian blue and tris (2, 2 -bipyridine) ruthenium (II) complex. 

Figure 6.12 Classification of photocatalysis and summary of various mechanism-specific labels. Assignments C, catalytic entity pC, catalyst precursor R, substrate P, product C, photoinitiator pC, photoinitiator precursor Sens, sensitizer D, electron donor A, electron acceptor...

The different microorganisms sensitivities toward Ti02 photocatalysis follows the order virus > bacterial cell gram positive > gram negative > bacterial spores... 

Fig. 3 Energy levels in TiC>2 photocatalysis (given in terms of reduction potentials at pH 0) (A) shows the reduction of acceptor A, (B) the direct oxidation of donor D, and (C) the oxidation of D mediated by a sensitizer S...

---