Photocatalysis, semiconductors

Many of the commercial applications of semiconductor photocatalysis involve the oxidative breakdown of organic pollutants in aqueous solution or of volatile organic compounds in air by oxygen, a process called photomineralisation. 

Adapted from A. Mills and S. Le Hunte, An Overview of Semiconductor Photocatalysis , Journal of Photochemistry and Photobiology A, 108 (1997) Elsevier 

Mineral acids are only formed if heteroatoms such as nitrogen, sulphur and chlorine are present in the pollutant. For example, feni-trothion, an insecticide, undergoes photocatalytic degradation  

Titanium dioxide has also been involved in the photocatalysis of toxic inorganic substances to yield harmless or less-toxic species. Sterilisation of drinking water by chlorine yields potentially carcinogenic compounds so that ozone has been used as an alternative sterilising agent. Bromate 

Heavy metals may be removed from waste water by photosensitisation using Ti02 where the metal is deposited on the surface of the photocatalyst  

The cathodic decomposition of Bi2S3 by electrons in the conduction band occurs at the standard potential of the 61283 electrode by the reaction 

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]. 

One advantage with semiconductor particulate systems is that light-induced oxidation/reduction is very often irreversible, unlike homogeneous solutions of 

The primary target of studies on photocatalytic semiconductor suspensions has been water cleavage by visible light. Suspension-based photocatalytic processes are also useful for the removal of inorganic (metal ions) and organic pollutants, the reduction of CO2, the photodestruction of bacteria and viruses, and various organic reactions an example is the use of Pt-loaded CdS for the photocatalytic racemization of L-lysine [210]. 

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Mills A, Le Hunte S (1997) An overview of semiconductor photocatalysis. J Photochem Photobiol A 108 1-35... 

Goebl JA, Black RW, Puthussery J, Gibhn J, Kosel TH, Kuno M (2008) Solution-based II-VI core/shell nanowire heterostructures. J Am Chem Soc 130 14822-14833 Hoffmann MR, Martin ST, Choi W, Bahnemann DW (1995) Environmental applications of semiconductor photocatalysis. Chem Rev 95 69-96... 

One of the major limitations in semiconductor photocatalysis is the relatively low value of the overall quantum efficiency mainly due to the high rate of recombination of photoinduced electron-hole pairs at or near the surface. Some success in enhancing the efficiency of photocatalysts... 

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] ... 

Mills, A Lee, S-K. Semiconductor photocatalysis. In Parsons SA, editor. Advanced oxidation processes for water and wastewater treatment. London IWA Publishing 2004 137-166. 

The basic principle of semiconductor photocatalysis involves photogenerated electrons and holes migrating to the surface. (From Wang et ah, 2006)... 

Hoffmann MR, Martin ST, Choi W, Bahneman DW (1995) Environmental applications of semiconductor photocatalysis. Chem Rev 95 69-96... 

Stroyuk AL, Kryukov AI, Kuchmii SY, Pokhodenko VD (2005) Quantum size effect in semiconductor photocatalysis. Theoretical and Experimental Chemistry 41 207-228... 

Ti02. - Semiconductor photocatalysis based on titanium dioxide (Ti02)... 

A variety of models have been derived to describe the kinetics of semiconductor photocatalysis, but the most commonly used model is the Langmuir-Hinshel-wood (LH) model [77-79]. The LH model relates the rate of surface-catalyzed reactions to the surface covered by the substrate. The simplest representation of the LH model [Eq. (7)] assumes no competition with reaction by-products and is normally applied to the initial stages of photocatalysis under air- or oxygen-saturated conditions. Assuming that the surface coverage is related to initial concentration of the substrate and to the adsorption equilibrium constant, K, tire initial... 

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. 

Titanium dioxide is widely used in the production of plastics, enamels, artificial fibers, electronic materials, and rubber (Hadjiivanov and Klissur-ski, 1996). Its ability to photocatalyze the oxidation of organic materials has been known for years in the paint industry. For this reason, TiOz is used as a white paint pigment (Stafford et al., 1996). TiOz is also known as an excellent catalyst for semiconductor photocatalysis due to its nonselectivity for environmental engineering applications it is nontoxic, insoluble,... 

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