Photocatalysis heterogeneous

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. 

Colloidal semiconductor particles have been found to act as heterogeneous photocatalysts in a number of environmentally important reactions. 

This has led to an increased interest in the elucidation of the mechanistic details of such reactions and the photocatalytic properties of particulate semiconductors for the purposes of either trying to improve process efficiency prior to engineering scale-up or, in the case of acid rain production, deliberate inhibition. Indeed, as has been noted by Ollis and Al-Ekabi [44] and more recently reiterated by Hoffmann et al. [45], the average publication rate over the last 10 years in the areas of water, air and wastewater treatment alone exceeds 200 papers per year. The publication of authoritative review articles on colloidal semiconductors over the last decade has been correspondingly prolific [44-68] and the interested reader is referred to them for information beyond the scope of this review. 

A semiconductor suitable for an efficient photo-oxidation reactions of environmental relevance should fulfil several requirements. Its bandgap should allow the use of solar radiation, ie the catalyst has to absorb in the visible or near-UV light region. The redox potentials of 0H7H20 and 02/02 couples should lie within this bandgap ( 0H./H20 = 2.8V [62], E°02/0.- = -0.16V [63]) in order to facilitate 

Mineralization (meaning a complete oxidation) of numerous organic species cannot be regarded only as an oxidation process. In many cases oxidation must be preceded by reduction steps, eg photocatalyzed transformation of CC14 to C02 and Cl requires first reduction of carbon(IV) to lower oxidation states, followed by its reoxidation to C02 [30], In this context redox properties of an excited semiconductor play a crucial role. Photogenerated holes should support highly oxidative potential, but at the other surface sites an efficient reducer (electron) should also be available [64,65], 

The primary redox processes leading to formation of superoxide and hydroxyl radicals (see equations 7.29 and 7.30) participate in secondary chemical transformations. 

Another form of reactive oxygen that can be formed on Ti02 irradiation is singlet oxygen. The mechanisms of 02 formation that are usually discussed are  

The most likely singlet oxygen is formed according to the first mechanism, ie in the photosensitization process [70], 

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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). 

Water Treatment. Several components must be treated simultaneously in a multicomponent mixture as available in wastewaters to prove the technology of heterogeneous photocatalysis. The formation and subsequent elimination of intermediates in the photooxidative process must be monitored, identifying all intermediates and final products. 

A debate centers on the mechanistic details of heterogeneous photocatalysis. The goal is to improve the photocatalytic activity of Ti02, and understand the role and importance of mineralisation by (/) free versus surface bound oxidising radicals, OH, and (2) by surface OH radicals versus direct hole oxidation. 

Schiavello, M. (1997) Heterogeneous Photocatalysis, John Wiley Sons, Ltd, Chichester. 

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]. 

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

See e.g. Schiavello M (ed) (1985) Fundamentals and developments of photocatalytic and photoelectrochemical processes, Reidel, Dordrecht Pelizzetti E (ed) (1986) Homogeneous and heterogeneous photocatalysis, Reidel, Dordrecht... 

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,... 

Pelizetti, E., and N. Serponne (Eds) Homogeneous and Heterogeneous Photocatalysis, D. Reidel, Dordrecht, 1986. 

Oxidation reactions are the most studied processes owing to the well-known ability of illuminated Ti02 in water to produce reactive oxygen species. In this context, heterogeneous photocatalysis could contribute to the replacement of hazardous compounds such as KMn04 and K2Cr20 [13]. 

Herrmann, J.M. (1999) Heterogeneous photocatalysis fundamentals and applications to the removal of various types of aqueous pollutants. Catalysis Today, 53 (1), 115-129. 

Heterogeneous particle morphology, in polymer colloids, 20 387 Heterogeneous photocatalysis, 19 73, 103 principles of, 29 74-75 Heterogeneous polymer blends, 20 343. 

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,... 

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],... 

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