Semiconductor powders photocatalysis

Nonbiological methods for removal of trichloroethylene from water are also being studied. These include the use of a hollow fiber membrane contactor (Dr. A.K. Zander, Clarkson University), photocatalysis by solar or artificially irradiated semiconductor powders (Dr. G. Cooper, Photo-catalytics, Inc.), and micellar-enhanced ultrafiltration (Dr. B.L. Roberts, Surfactant Associates, Inc.). 

Sakata, T. and Kawai, T., Photosynthesis and photocatalysis with semiconductor powders, in Energy Resources through Photochemistry and Catalysis, Gratzel, M., Ed., Academic Press, New York, 1983. 

Heterogeneous Photocatalysis with Semiconductor Powders 237 Ti02/Pt hv... 

Kisch H, Kiinneth R Photocatalysis by semiconductor powders Preparative and mechanistic aspects. In Photochemistry and Photophysics. Edited by Ra-bekj. CRC Press Inc. 1991 131—175. 

Photocatalysis by semiconductor powders, most usually titanium dioxide, has been applied to the oxidation of several aromatics. In many such photocatalytic reactions, the key intermediates are hydroxyl radicals formed by oxidation of water. With rather good donors such as aromatic compounds, hole transfer on the excited semiconductor surface is a viable alternative in this case, the reaction of the radical cation of the substrate or further intermediates arising from it with oxygen or superoxide anion may have a role. Photocatalyzed oxidation of naphthalene yields 2-formyldrmamaldehyde and 1,4-naphthoquinone as the primary products (Eq. (45.16)), similarly to what occurs upon direct irradiation of naphthalene in water ... 

The different types of quinones active in photosynthesis are being used as electron acceptors in solar cells. The compounds such as Fd and NADP could also be used as electron/proton acceptors in the photoelectrochemical cells. Several researchers have attempted the same approach with a combination of two or more solid-state junctions or semiconductor-electrolyte junctions using bulk materials and powders. Here, the semiconductors can be chosen to carry out either oxygen- or hydrogen-evolving photocatalysis based on the semiconductor electronic band structure. 

See also Ti02-based heterogeneous photocatalysis, 18 58 powder used in cosmetics, 7 841t preparation of, 25 18 production of, 19 385, 387-393 PVC and, 25 684 reactivity of, 25 17-18 semiconductor, 5 600 as soap bar additive, 22 744 sulfate production process for,... 

Hydrazone cyclization and hydroalkylation [138-140] are rare examples of reactions conducted on a preparative scale, since the products were isolated in milligram amounts and not just identified in solution. As already mentioned in Section 6.2.5, photocorrosion of the semiconductor photocatalyst often prevents its use in preparative chemistry. This is very true also for colloidal semiconductors although the pseudo-homogeneous nature of their solutions allows one to conduct classical mechanistic investigations, until now they were too labile to be used in preparative chemistry [107, 141, 142]. In contrast to the above-mentioned reactions, in recent years we have isolated novel compounds on a gram-scale employing photostable zinc and cadmium sulfide powders as photocatalysts [97, 107, 143-145]. During this work we found also a new reaction type which was classified as semiconductor photocatalysis type B [45]. In contrast to type A reactions, where at least one oxidized and one reduced product is formed, type B reactions afford only one unique product, i.e., the semiconductor catalyzes a photoaddition reaction (see below). 

---