Heterogeneous semiconductor photocatalysis

In most studies, heterogeneous photocatalysis refers to semiconductor photocatalysis or semiconductor-sensitized photoreactions, especially if there is no evidence of a marked loss in semiconductor photoactivity with extended use. It is meant here that the initial photoexcitation takes place in the semiconductor catalyst substrate and the photoexcited catalyst then interacts with the ground state adsorbate molecule [209]. 

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] ... 

In this chapter, an attempt has been made to address fundamental mechanistic and kinetic aspects of TiO2 photocatalysis of organophosphorus compounds. Comparisons between homogeneous (radiolysis) and heterogeneous (photocatalysis) hydroxyl-generating processes have helped to elucidate the reaction pathways and led to number of important mechanistic conclusions. From the various kinetic parameters, the overall rates and efficiencies for the degradation of organophosphorus compounds can be predicted and may find direct application in evaluation and implementation of semiconductor photocatalysis. 

Given the importance of the oxidation states of the actinides in reprocessing and the capability of heterogeneous semiconductor photocatalysts to manipulate the valence states of a wide range of metal ions, attention has been given to the potential applications of photocatalysis in nuclear reprocessing scenarios [59-65]. The attractions of this are twofold, and lead to a further minimisation of the environmental impact associated with fuel use and processing ... 

Hetmninger JC (1999) Heterogeneous chemistiy in the troposphere A modem surface chemistry approach to the study of fundamental processes. Int l Rev Phys Chem 18 387-417 Herman JR, Bhartia PK, Torres O, Hsu C, Seftor C, Celarier E (1997) Global distribution of uv-absorbing aerosols from Nimbus 7/TOMS data. J Geophys Res 102 16911-16922 Hoffmaim MR, Martin ST, Choi W, Bahnemann DW (1995) Enviromnental applications of semiconductor photocatalysis. Chem Rev 95 69-96... 

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). 

Bard AJ (1979) Photoelectrochemistry and heterogeneous photocatalysis at semiconductors. J Photochem 10 59-75... 

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,... 

The solid state and the surface chemistry of some of the solid Fe-phases impart to these oxides and sulfides the ability to catalyze redox reactions. Surface complexes and the solid phases themselves acting as semiconductors can participate in photoredox reactions, where light energy is used to drive a thermodynamically unfavorable reaction (heterogeneous photosynthesis) or to catalyze a thermodynamically favorable reaction (heterogeneous photocatalysis). 

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

Optical Properties. It should not be forgotten that the absorption of photons by the semiconductor is the initial step of heterogeneous photocatalysis. Therefore, the utilization of incident photons must be carefully considered. 

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. 

This review is a discussion of the kinetic modelling of the photoelectrochemistry of colloidal semiconductor systems. This area is currently attracting significant attention from the scientific community due to the applications of colloidal semiconductors within two rapidly advancing research fronts heterogeneous photocatalysis and nanocrystalline particle technology. 

This AOT will be discussed later in this book therefore, only a brief introduction is included here. Heterogeneous photocatalysis is a process based on the direct or indirect absorption of visible or UV radiant energy by a solid, normally a wide-band semiconductor. In the interfacial region between the excited solid and the solution, destruction or removal of contaminants takes place, with no chemical change in the catalyst. 

---