Other Photocatalytic Processes

Metal oxides are an important elass of heterogeneous catalysts. They find direct application in a variety of reactions, from acid-base to redox reactions, in photocatalytic processes, and as catalysts for environmental protection. In addition, they are widely used as supports for other active components (metal particles or other metal oxides), although often they act not only as a support, but actively participate in the reaction mechanism." ... 

The key requirement for a SET step in the photocatalytic process seems to be the surface complexation of the substrate, according to an exponential dependence of the probability of electronic tunneling from the distance between the two redox centers [66]. However, as was pointed out in the preceding section on the key role of back reactions, the presence of a SET mechanism could be a disadvantage from an applicative point of view. If the formed SET intermediate (e.g., a radical cation) strongly adsorbs and/or does not transform irreversibly [e.g., by loss of CO from a carboxylic acid or fast reaction with other species (e.g., superoxide or oxygen)], it can act as a recombination center, lowering the overall photon efficiency of the photocatalytic process. 

The presence of others species in the reaction environment can enhance or decrease the rate of the photocatalytic process depending on the mechanism of reaction. As reported by Kavita et al. [2], small quantities of some ions, such as Cu2 + or Mn2 +, increase the rate of oxidation of organic compounds thanks to the inhibition of... 

The demand to develop efficient systems for pollutant abatement and wastewater treatment has brought some researchers to develop systems in which the photocatalytic process is combined with other methods with the purpose to increase its efficiency and to decrease the reaction time thanks to a synergistic effect. 

The photocatalytic processes that could have been involved in the synthesis of prebiotic molecules are not limited to iron sulphides. Photocatalytic processes occurring on the surface of other sulphide and oxide materials should be considered as well. 

Several other photocatalytic environmentally friendly processes may also be considered. Photocatalytic synthesis of chemicals may serve as a clean alternative route to traditional synthetic methods (see section 6.5). Palmisano et al. reviewed various types of organic reactions that could be achieved by heterogeneous photocatalysis... 

Holes react with adsorbed substances, in particular with adsorbed water or OH ions, generating HO radicals and/or other radicals, as in other AOTs. Normally, in environmental applications, the photocatalytic processes take place in aerobic environments, and adsorbed oxygen is the principal electron acceptor species ... 

Ortiz-Gomez, A. Ph.D., Enhanced Mineralization of Phenol and other Hydroxylated Compounds in a Photocatalytic Process assisted with Ferric Ions . Thesis Dissertation, The University of Western Ontario, Ontario (2006). 

Further studies indicate that the phenomena characterized here can be extended to the study of other bound-free transitions, furnishing a means of mapping the repulsive states which characterize small metal clusters (51). This may have significant importance for the characterization of photocatalytic processes. Further it appears that one can make use of the shallow nature of the bonding potentials characterizing many metal clusters and supersonic expansions in general to obtain "hot band" structure... 

It is now realized that copper as metal next to iron and chromium participates in photoredox cycles and its role cannot be ignored. The most important part of the cycle is photoreduction of Cu(II) to Cu(I) induced by solar light and oxidation of ligands to their environmentally benign forms. Then Cu(I) is easily re-oxidized to Cu(II), which can coordinate the next ligand molecule, and thereby the Cu photocatalytic cycles contribute to continuous environmental cleaning. Besides oxida-tion/reduction, other critical processes relevant to the copper cycles are adsorption/desorption and precipitation/dissolution... 

Two semiconductor materials—n-type CdS and TiOi—have been the focus of much attention in such photocatalytic processes as the photocleavage of water and hydrogen sulphide, and the photooxidation of alcohols, carboxylates and various other organics. The majority of photoinduced processes on these materials are examined when they are in powder form. Enquiries into photocatalytic processes centre on the... 

The most important and widespread applications of photochemistry are, of course, photoimaging techniques, such as photography and xerography (Special Topic 6.32).1313 Some other large-scale applications, such as environmental remediation of anthropogenic pollutants by photocatalytic processes (Special Topic 6.28) and potential applications such as photoelectrochemical cells (Special Topic 6.31) and artificial photosynthesis (Special Topic 6.26), are also discussed in the following text. 

Other investigations revealed the importance of this technique in the fabrication of microcircuitry (4), which is now totally dependent on the photolithographic techniques. Electroless process (photocatalytic process) was also proved to be useful for the same purpose (5). 

Other supports, such as polymeric materials is that the glass substrates are inert and non-degradable under the photocatalytic process. The immobilization methods of NS-Ti02 on the different kinds of glass substrates are presented in Table 10. 

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