Selective photocatalysis

Selective photocatalysis can be achieved through the use of proper molecular sieves, such as the titanosilicate ETS-10, an alternative approach. The photocatalytic activity of ETS-10 is due to the presence of photoexcitable Ti-O-Ti chains and the three-dimensional interconnected pore system of large 12-membered ring channels in its structure, which endow the material with excellent diffusion properties [83], This regular channel system has a direct effect in determining the shape selectivity of the degradation process [84],... 

Conclusion The earlier discussion clearly shows the significant advantages of selective photocatalysis. It is evident that this route requires milder conditions, far less capital investment while yielding high selectivity under proper conditions. Thus, it can bring about radical changes in certain chemical syntheses. It must be emphasized that considerable efforts will be needed to develop the concepts discussed earlier to the level of mature technologies. 

Fukuzumi, S., Tanii, K., and Tanaka, T., Protonated pteridine and flavin analogues acting as efficient and substrate-selective photocatalysis in the oxidation of benzyhc alcohols derivatives by oxygen,... 

Photocatalysis is a fundamental feature of life processes on our planet [1] (it provides photosynthesis in plants and bacteria) and of the chemistry of its atmosphere [2]. Work is under way to develop photocatalytic technologies for abatement of environmental problems [3,4]. Photocatalysis is anticipated to become in the coming years important also for selective organic synthesis [4]. In a more distant future thermal catalytic processes induced by heating with solcir radiation, together with photocatalytic processes may become important for environmentally friendly technologies of solar energy utilization [5-9]. 

Of the numerous works described in the literature, particular examples are selected, which hopefully will let a reader to feel the flavour of R D, as well as potential applications in various areas of photocatalysis. 

In classical kinetic theory the activity of a catalyst is explained by the reduction in the energy barrier of the intermediate, formed on the surface of the catalyst. The rate constant of the formation of that complex is written as k = k0 cxp(-AG/RT). Photocatalysts can also be used in order to selectively promote one of many possible parallel reactions. One example of photocatalysis is the photochemical synthesis in which a semiconductor surface mediates the photoinduced electron transfer. The surface of the semiconductor is restored to the initial state, provided it resists decomposition. Nanoparticles have been successfully used as photocatalysts, and the selectivity of these reactions can be further influenced by the applied electrical potential. Absorption chemistry and the current flow play an important role as well. The kinetics of photocatalysis are dominated by the Langmuir-Hinshelwood adsorption curve [4], where the surface coverage PHY = KC/( 1 + PC) (K is the adsorption coefficient and C the initial reactant concentration). Diffusion and mass transfer to and from the photocatalyst are important and are influenced by the substrate surface preparation. 

Photocatalysis has become a very important tool both in organic syntheses and environmental chemistry. It represents not only a very selective instrument to achieve synthesis or decomposition of complex structures but also a green chemistry approach since the electron is a very clean reagent. 

Doll TE, Friimnel FH (2005) Removal of selected persistent organic pollutants by heterogeneous photocatalysis in water. Catal Today 101 195-202... 

Serpone N, Pelizzetti E (1989) Photocatalysis Fundamentals and Applications, Wiley, New York Schiavello M, Dordrecht H (1985) Photoelectrochemistiy, Photocatalysis, and Photoreactors Fundamentals and Developments Kluwer Academic, Boston, MA Linsebigler AL, Lu G, Yates JT (1995) Photocatalysis on Ti02 surfaces principles, mechanisms, and selected results. Chem Rev 95 735-758... 

There are a number of examples of applications of environmental photocatalysis that are already at or near the stage of implementation or commercialization. Some selected applications of photocatalytic technology are listed in Table 2.1. 

Sonophotocatalysis is photocatalysis with ultrasonic irradiation or the simultaneous irradiation of ultrasound and light with photocatalyst. Tnis method includes irradiation with alternating ultrasound and light. Ultrasound effects on heterogeneous photocatalytic reaction systems have been demonstrated by Mason,1 Sawada et al.,2) Kado et al.,3) and Suzuki et al.4) In these papers, not only acceleration of photocatalytic reactions but increase in product selectivity by ultrasonic irradiation has also been reported. It was postulated that ultrasound effects, such as surface cleaning, particle size reduction and increased mass transfer, were the result of the mechanical effects of ultrasound.1,5) Lindley reviewed these and other effects.5)... 

Sonication is a tool for improvement of chemical processes such as photocatalytic reaction. The improvements of reaction rates, yields and selectivity, the generation of reactive intermediate species and so on were reviewed.36) Some examples have been also shown in this chapter. The development of a new reaction pass by the combined effect of photocatalysis and sonolysis is expected in the near future. The contribution to Green Chemistry is one of typical examples. 

Various polyoxometalates can be reduced electrochemically and reversibly by several electrons at modest potentials (Section VILA), and these properties are exploited in photocatalysis and eiectrocatalysis. In both cases, redox properties of heteropolyanions (Fig. 49) and the organic reactants (Table XXXV) are the principal properties that control the catalytic performance. The selection of the electrode is also important in eiectrocatalysis. Photocatalysis by hereopoly-anions has been reported extensively, but there are only a few reports of eiectrocatalysis by these compounds. 

Metal oxides, such as TiOz, can sometimes act as high-temperature thermal oxidation catalysts, but oxidative selectivity can be observed at room-temperature photocatalytic oxidations. For example, the oxidation of cyclohexane by 02 and TiOz is thermodynamically possible, but its rate at room temperature is impossibly slow without irradiation. At higher temperatures, little oxidative selectivity is obtained. With the use of TiOz photocatalysis, high oxidative selectivity is obtained. 

By careful selection of the semiconductors it is possible to use heterogeneous photocatalysis as an alternative approach to more conventional synthetic pathways. 

The wide potentiality of semiconductor-mediated photocatalysis was also applied for the transformation of functional groups such as selective cyclization of amino adds in aqueous suspensions [59]. 

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