Photooxidation of water

Again with platinized Ti02, ultraviolet irradiation can lead to oxidation of aqueous CN [323] and to the water-gas shift reaction, CO + H2O = H2 + CO2 [324]. Some mechanistic aspects of the photooxidation of water (to O2) at the Ti02-aqueous interface are discussed by Bocarsly et al. [325]. 

The nature of the intermediates impHcated in the photooxidation of water with Ti02 has been identified in several reports using spin traps by the electron spin resonance (esr) technique under ambient conditions (53). No evidence for OH species, even at 4.2 K, was found (43), but the esr signal... 

No clear picture of the primary radical intermediate(s) in the HO2 photooxidation of water has appeared. The nature of the observed radical species depends on the origin and pretreatment of the HO2 sample, on the conditions and extent of its reduction, on the extent of surface hydroxylation, and on the presence of adventitious electron acceptors such as molecular oxygen (41). The hole is trapped on the terminal OH group (54). 

Stephens and co-workers (69) have found that the preheating of CdS specimens in an atmosphere of nitrogen (the purpose of preheating is to enrich a CdS specimen in an acceptor impurity) reduced the catalyst activity in relation to the photooxidation of water. 

Glaze and Kang (1989a,b) presented a model describing the photooxidation of water-soluble hazardous organic waste and also observed the rate increase with a decrease in pH of reaction medium. 

In addition to acid catalyzed reactions, this paper deals with charge and/or energy transfer reactions between adsorbed species and the lattice, or between co-adsorbed species. Reduction of lattice iron(III, as well as luminescence quenching of Ru(bpy), photoaquation of Cr(bpy)3 and photooxidation of water, are examples chosen for illustrating this aspect of the reactivity of clays. 

Figure 7. Models suggested for explaining the observation of the photooxidation of water by a sensitizer S, (Ru(bpy)32+), which in the excited state S transfer an electron to a sacrificial acceptor A, [CoCNH Cl], in the presence of a colloidal catalyst (RuCy or a molecular catalyst (cwRu(bpy)2(H20)22+). See text for the significance of a, b, and c. (Reprinted with permission from ref. 32. Copyright 1983.)...

Suspensions of hematite have also been used and studied for other aims than photooxidation of water, e.g. catalytic oxidation of sulphur dioxide in aqueous solutions [52]. Aqueous dispersion of semiconductor particles could be an easy and attractive way to photooxidise water, but they have the drawback that dihydrogen and dioxygen are produced simultaneously in the same suspension. Apart from the separation problem the two produced gases may create a pathway for back reactions that reduces the yield of the overall photo-oxidation process. The latter obstacle can partly be avoided by addition of Na2C03, which was successfully shown by Arakawa et al [115]. 

Higher-valent manganese porphyrins have been of interest since they have been shown to be important intermediates in the activation of hydrocarbons under mild conditions (Section 41.4.8). Their use as sensitizers for the photooxidation of water to oxygen has also provided a motivation,479 although the involvement of a manganese porphyrin in the natural system has not been fully documented. 

Irradiation at 300 nm in CCl, CHCl, or CH2CI2 promotes am irreversible reaction and formation of what is probably [Os(IV)(TTP)Cl2l. Optimum conditions have been reported for determination of osmium by measurement of the luminescence of its 1 3-complex with 1,10-phenanthroline.A catalyst, prepared by reducing the product of grafting OsO on to the C-C bond of sepiolite, has been found to mediate the photooxidation of water but to do so less efficiently than RuOjj. This is the first exaunple of a dispersed water oxidation catalyst grafted on to a solid support. 

As always, the photooxidation of water to 2 has received much less attention than the much easier photoreduction of water to H2 However, some notable achievements have appeared and genuine progress has been made. The mechanisms for oxidation of water by metalloporphyrins and cobalt(III) ammines have been studied, with particular attention being given to kinetic parameters. Several new 02 evolving catalysts have been... 

S Saphon and AR Crofts (1977) Protolytic reactions in photosystem II A new model for the release of protons accompanying the photooxidation of water. Z Naturforsch 32C 617-626... 

MG Goldfeld, LA Blumenfeld, LG Dmitrovsky and VD Mikoyan (1980) Plastoquinone function in photosystem 2. Mol Biol 14 804-813 AG Volkov (1986) Molecular mechanism of the photooxidation of water during photosynthesis Cluster catalysis of synchronous multielectron reactions. Mol Biol 20 728-736... 

G.S. Nahor, S. Mosseri, P. Neta, and A. Harriman, Polyelectrolyte-Stabilized Metal Oxide Hydrosols as Catalysts for the Photooxidation of Water by Zinc Porphyrins, J. Phys. Chem., 92 (1988) 4499. 

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