Electronic heterogeneous photocatalysis

Heterogeneous Photocatalysis. Heterogeneous photocatalysis is a technology based on the irradiation of a semiconductor (SC) photocatalyst, for example, titanium dioxide [13463-67-7] Ti02, zinc oxide [1314-13-2] ZnO, or cadmium sulfide [1306-23-6] CdS. Semiconductor materials have electrical conductivity properties between those of metals and insulators, and have narrow energy gaps (band gap) between the filled valence band and the conduction band (see Electronic materials Semiconductors). 

Photoinduced ET at liquid-liquid interfaces has been widely recognized as a model system for natural photosynthesis and heterogeneous photocatalysis [114-119]. One of the key aspects of photochemical reactions in these systems is that the efficiency of product separation can be enhanced by differences in solvation energy, diminishing the probability of a back electron-transfer process (see Fig. 11). For instance, Brugger and Gratzel reported that the efficiency of the photoreduction of the amphiphilic methyl viologen by Ru(bpy)3+ is effectively enhanced in the presence of cationic micelles formed by cetyltrimethylammonium chloride [120]. Flash photolysis studies indicated that while the kinetics of the photoinduced reaction,... 

High-surface-area inorganic materials with ordered mesoporous structures have also been oT major interest Tor numerous applications including photocatalysis [99-102], The ultra-high-surface-area of mesoporous materials is appealing in applications of heterogeneous photocatalysis where it is desirable to minimize the distance between the site of photon absorption and electron-hole redox reactions to improve efficiency [103-105],... 

Heterogeneous photocatalysis is based on the photonie exeitation of a solid, which renders it more complex. The term photocatalysis may designate several phenomena that involve photons and catalyst, while this part of the ehapter will consider only semiconductor photocatalysis. Photocatalytic activity of Ti02 is based on its semiconductor properties. Radiation by photons, whieh have higher transfer energy, of such semiconductor leads to generation of electron-hole pairs [94] ... 

Meissner, D., Sinn, C., Memming, R., Notten, P.H.L. and Kelly, J.J. 1986. On the nature of the inhibition of electron transfer at illuminated p-type semiconductor electrodes. In Homogenous and Heterogenous Photocatalysis" (Ed. E.Pelizzetti andN.Serpone). (D.Reidel, Dordrecht), pp. 317-333. 

A varying and much more complex mechanistic situation exists in heterogeneous photocatalysis (Fig. 5-13). With respect to the transient oxygen species, comparable overall oxidation reactions are usually observed, but the set of primary reactive oxygen species is slightly different. It is commonly assumed, that superoxide radical anions and hydroxyl radicals are the primary species formed after photogeneration of the electron-hole pair of a semiconductor catalyst in the presence of water and air (Serpone, 1996). In the presence of ozone, ozonide radical anions or are formed by fast electron transfer reaction of superoxide radical anions with O3 molecules. The combination Ti02-03-UV/VIS is called photocatalytic ozonation (Kopf et al., 2000). For example, it was applied for the decomposition of tri-chloroethene in the gas phase (Shen and Kub, 2002). 

Heterogeneous photocatalysis of Pb(II) systems has received scarce attention. In our previous review (Litter, 1999), we cited a few early papers (Inoue et al., 1978, 1980a, b Kobayashi et al., 1983 Lawless et al., 1990 Maillard-Dupuy et al., 1994 Rajh et al., 1996a, b Termakone, 1984 Tennakone and Wijayantha, 1998 Thumauer et al., 1997 Torres and Cervera-March, 1992), and although some new papers appeared later (Aarthi and Madras, 2008 Chen and Ray, 2001 Chenthamarakshan et al., 1999 Kabra et al., 2007, 2008 Kobayashi et al., 1983 Mishra et al., 2007 Rajeshwar et al., 2002), information was and continues to be scant. The mechanisms of transformation of lead (II) in water by UV-Ti02 are especially attractive because they depend very much on the reaction conditions, related to the nature of the photocatalyst, the effect of oxygen, and the presence of electron donors. 

There are very few reports in the literature concerning heterogeneous photocatalysis for uranium treatment in water. In our previous review, only one case of photocatalytic reaction on uranium salts was reported (Amadelli et al., 1991). Taking into account the standard reduction potentials, U(VI) can be photocatalytically reduced by Ti02 conduction band electrons to U(V) and then to U(IV) (E° = +0.16 V and +0.58 V, respectively, Bard et al., 1985). However, more reduced U(III) and U(0) forms cannot be generated because of very negative redox potentials (Bard et al., 1985). In addition, U(V) rapidly disproportionates to U(VI) and U(IV), and its chemistry is very complex (Selbin and Ortego, 1969). For example, uranyl... 

Although the lifetime of the reactive electron-hole pair is not known, the reasonable estimate of 10 -10 s leads to an electron transfer rate constant between 10 and 10 M s . In general, electron transfer reactions at the semiconductor-liquid interface are very fast [see Ref [33] and N. Serpone, E. Pellizetti (Eds.), Homogeneous and Heterogeneous Photocatalysis, Reidel, Dordrecht, 1986, p. 51]. 

As advanced in chapter I, the basic mechanism of heterogeneous photocatalysis is related to the exciting of the TiOa or other metal oxides. It is generally accepted that this photon-induced state of excitation promotes an electron from the valence band level to... 

Heterogeneous photocatalysis steps were reviewed by Cunningham and Hodnett, (1981), Jacoby etal., (1996) and Yue( 1993) among others. These steps include the mass transfer of substrate from the bulk of the fluid to the catalyst surface, the transport of the reactants within the catalyst particle and the adsorption of substrates on the active catalytic surface. Once the Ti02 is irradiated, photon energy is absorbed, followed by the generation of electron-hole pairs, the formation of radicals, the surface reaction, radical recombination and finally the desoiption and mass transfer of products from the particle surface into the bulk of the fluid. 

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