Layered photocatalysis

Domen, K. and Hara, M., Water photolysis by layered compounds, in Photocatalysis, Science and Technology, Kaneko, M. and Okura, I. (Eds), Kodansha/Springer, Berlin, 2002, Chap. 16. 

The deposition of nanostructured Ti02-based material directly on glass is expected to achieve enhanced photocatalysis and mechanical strength. Sputtering and a combination of sputtering and sol-gel techniques seem to be the most adequate for such a purpose [91,97,98], In the latter case, porous alumina Trims on glass formed by anodizing sputter-deposited A1 layers were used as templates in the successive sol-gel process. 

Among these oxides, K4Nb60i7 has been studied in some detail although the structure is rather unique. Studies on this oxide will be introduced first, followed by those on other layered oxides. Tables 16.1 and 16.2 list the various systems studied for photocatalysis. 

Table 16.3 Perovskite-Related Layered Oxides Studied for Photocatalysis... 

A different type of synergy was foimd in a binary Cu(II) /Cr(VI) system imder N2 and UV light, acceleration of both Cr(VI) and Cu(II) reduction was observed (Goeringer et al., 2001). While no explanations were provided concerning this behavior, it can be proposed that the enhancement may be due to the formation of a layer or spots of metallic copper that promote photocatalysis. 

To large extent, the scientific literature is at least partially silent with respect to the effect of wavelength on photocatalysis. At most, there is some consideration to the fact that the absorption coefficient for 365 nm photons is significantly smaller than that for 254 nm photons, hence a thicker layer of photocatalyst is required with the former. Another effect of the shorter wavelength is the potential formation of active species that might play a role in the photooxidation reaction. 

A significant problem in studies on photocatalysis is the definition of positive hole. Positive hole is defined as a defect of an electron (i.e., a positive hole must be included in a substance, while an electron is a real substance). Therefore, not only h produced by photoinduced band-to-band transition in solid materials but also a hydroxyl radical, which is a one-electron deficient hydroxyl anion, can be a positive hole. If this definition is accepted, there should be no difference in the photocatalsrtic oxidation mechanisms between direct hole transfer and surface-adsorbed hydroxyl radical reaction, since it is well known that the surface of a metal oxide is covered with chemically or physically adsorbed water and a positive hole passing through this water layer into a solution may be a hydroxyl radical or its protonated or deprotonated species (Fig. 4). Actually, hydroxyl... 

Layered compounds provide unique character for electron-transfer processes owing to their low dimensionality. Especially layered materials with ion-exchange and/or intercalation capabilities show behavior that is not seen in so-called bulk-type materials. Layered materials, which have been often used in studies of photoelectron transfer as well as photocatalysis, may be classified into two groups compounds in which the host layers work as an active component for the photoexcitation and electron-transfer reactions, and materials in which the layers are inert for electron-transfer processes. Examples of the former are layered titanates and niobates and of the latter are clays. In the latter case, photoactive materials are intercalated in the interlayer spaces. Recently, the exfoliation of various layered compounds has become possible and artificial assemblies consisting of these exfoliated sheets have been formed. Electron transfer in such assemblies is also an attractive subject in this field. 

Table 10. Perovskite-related layered oxides studied for photocatalysis.

Calvert 124), sintered cuprous oxide present as a thin layer on bulk copper also shows photocatalysis of CO oxidation when irradiated with ultraviolet light at 25°. These authors observed that the addition of sulfur or antimony in solid solution, which increased the conductivity of the oxide, decreased the photoeffect, but the reason for this particular choice of dopents is obscure. 

A contribution to UV-induced photocatalysis is also expected from the cations of transitions metals such as iron, copper, and manganese dissolved in water droplets or the water layer that may cover a solid aerosol. Probable photoreactions in this case would be water splitting, or redox processes involving atmospheric contaminants that are easily absorbed by the water layer [30]. The primary step in these reactions is expected to be redox transformations of the electronically excited metal compounds [8]. 

A muitielectrode photoelectrochemlcal ceil for unassisted photocatalysis and photosynthesis, and incorporating a bipolar T102/Pt multielectrode cell with five panels has been described. It has been used for unassisted photolysis of H2O and produces an H t 0 molar ratio of 2.4 1. In a new induced junction solar cell a strong inversion layer is induced by an external bias voltage across an insulating layer. Its... 

For the last two decades, attention has been focused on redressing the ozone depletion in the earth s protective layer. It is believed that chlorine radicals dissociated from chlorofluorocarbons (CFCs), upon irradiation of sun s UV in the stratosphere, promotes the ozone depletion. Hence, in addition to development of CFC alternatives there is an urgent need for the safe disposal of CFCs. Several processes such as pyrolysis, incineration, photocatalysis, oxidative destruction over metal oxide or zeolite catalysts and destruction at very high temperatures ( by plasma technique ) are reported in the literature for the disposal of CFCs[ 1-5]. But all these processes yield harmful products like CO, HF/F2 etc. Catalytic conversion of chlorinated organics in presence of hydrogen seems to be a better technique as it yields either hydrofluorocarbons(HFCs) or hydrochlorofluorocarbons(HCFCs) whose ozone depletion potential is either zero or very low and yet most of these products act as CFC alternatives. 

L. Cui, Y. Wang, M. Niu, G. Chen, Y. Cheng, Synthesis and visible light photocatalysis of Fe-doped Ti02 mesoporous layers deposited on hollow glass microbeads . Journal of Solid State Chemistry, 182, 2785-2790, (2009). 

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