Photocatalytic Oxidation of Water

To employ the catalysts for 02 evolution in the vesicle systems, it was essential to check whether their selectivity towards evolution of 02 remains high enough after immobilization on the lipid membrane. Shilov, Shafirovich and co-workers prepared [268-271] a membrane-bound catalyst for water oxidation by oxidation of Mn(II) salts in the presence of lipid vesicles. The Mn(IV) hydroxide catalyst 

The membrane-bound catalysts for water oxidation can also be obtained with other transition metal hydroxides. Gerasimov et al. [272] have shown that illumination of a Ru(bpy) + — persulfate system in the presence of Co(II) and lipid vesicles results in the formation of a colloid catalyst for water oxidation, viz. Co(III) hydroxide, immobilized on the lipid membranes. The same catalyst can be obtained without illumination by Co(II) oxidation with a Ru(bpy)3+ complex in the vesicle suspension. The selectivity of water oxidation with the catalysts thus obtained depends on the nature of the membrane-forming lipid. Switching from the synthetic DPL to the natural eggL the process selectivity decreases by about two orders of magnitude due to consumption of the oxidant for oxidation of organic impurities contained in lipids of natural origin [113]. 

Unfortunately, the oxidation of water in the systems discussed was not conjugated with PET across the membranes. 

The first communication about the Chl-sensitized water oxidation to 02 on the outer vesicle interface conjugated with Fe(CN)g reduction at the inner interface appeared in 1977 [42] (see System 2 in Table 1). However an attempt to reproduce this result was a failure [273]. Thus the possibility of water photooxidation in vesicular systems which contain either only Chi molecules or Chi molecules in combination with other photosynthetic pigments is still under discussion. However, embedding of the intact fragments of thylakoids in the vesicle membranes was found to provide the evolution of 02 from water upon illumination [273], But again this process was not conjugated with PET across the membranes. 

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This is a stringent requirement indeed as further elaborated in the next Section. Instead of actually photosplitting water, sacrificial agents may be added to the solution such that the HER and OER steps may be separately optimized and studied (Figures lb and lc). It must be borne in mind that now the overall photoreaction becomes thermodynamically down hill and is more appropriately termed photocata lytic (see below). Examples of sacrificial agents include sulfite for the photo driven HER case (Figure lb) or Ag+ as the electron acceptor for the photocatalytic oxidation of water (Figure lc). 

The photocatalytic oxidation of water to O2 on thin nanostructured AgCl layers has been reported.568 In subsequent work by the same group, AgCl photoanodes have been combined with either p GaMV69,570 or an amorphous silicon solar cell571 in the cathodic part of an electrochemical cell to split water. Modification of AgCl with gold colloids was found to enhance H2 and O2 production.571 572... 

The semiconductor sensitised photocatalytic oxidation of water by a sacrificial electron acceptor can be expressed by Eq. (11-6) ... 

Hara M, Lean JT, Mallouk TE (2001) Photocatalytic oxidation of water by silica-supported tris(4,4 -dialkyl-2,2 -bipyridyl)ruthenium polymeric sensitizers and colloidal iridium oxide. Chem Mater 13 4668-4675... 

On account of the reported photocatalytic activity of titania nanosheets in the oxidation of water [290], Yamada et al. [291] attempted to fabricate photoelectrodes responsive to visible light by coating Cd8 electrodes with titania nanosheets. It was considered that since the thickness of a nanosheet is as small as two atomic layers of... 

In the present article the photocatalytic reaction of water oxidation is examined from the standpoint of the electronic theory. We shall analyze here one of the possible mechanisms of the reaction [see reference (7[Pg.197]

Numerous data on the influence of adsorbed molecules on the photocatalytic activity of semiconductor catalysts in relation to the oxidation of water are evidence that acceptor molecules retard (65-68, 71-73, 77) and donor molecules speed up the reaction (65-68). 

Let us now turn to a comparison of theory with experiment. Comparing (95), (84), and (68), we find that the dependence of the photocatalytic effect K on the position of the Fermi level at the surface s and in the bulk cv of an unexcited sample for the oxidation of water is the same as for the oxidation of CO or for the hydrogen-deuterium exchange reaction. For this reason, such factors as the introduction of impurities into a specimen, the adsorption of gases on the surface of the specimen, and the preliminary treatment of the specimen will exert the same influence on the photocatalytic effect in all the three reactions indicated above. The dependence of K on the intensity I of the exciting light must also be the same in all the three cases. 

Domen, K., Kudo, A., Onishi, T., Kosugi, N., and Kuroda, H., Photocatalytic decomposition of water into hydrogen and oxygen over nickel (II) oxide-strontium titanate (SrTiOj) powder. 1. Structure of the catalysts, ]. Phys. Chem., 90, 292,1986. 

The photocatalytic oxidation of organic and inorganic compounds and the photo-catalytic production of H202 occurs also at the surface of iron(III)(hydr)oxides. It has been proposed (e.g., Hoffmann, 1990 Faust and Hoffmann, 1986) that the oxidation of S(IV) by 02 in atmospheric water is catalyzed by iron(III)(hydr)oxide particles. It is assumed that the reductant (HSO3) is specifically adsorbed at the surface of an iron(III)(hydr)oxide, forming either a monodentate or a bidentate surface complex ... 

H. Gerischer, A. Heller, Photocatalytic oxidation of organic molecules at Ti02 particles by sunlight in aerated water,... 

The titanium-mediated photocatalytic oxidation of a pyridine solution was conducted by Low et al. (1991). They proposed that the reaction of OH radicals with pyridine was initiated by the addition of a OH radical forming the 3-hydro-3-hydroxypyridine radical followed by rapid addition of oxygen forming 2,3-dihydro-2-peroxy-3-hydroxypyridine radical. This was followed by the opening of the ring to give At-(formylimino)-2-butenal which decomposes to a dialdehyde and formamide. The dialdehyde is oxidized by OH radicals yielding carbon dioxide and water. Formamide is unstable in water and decomposes to ammonia and formic acid. Final products also included ammonium, carbonate, and nitrate ions. 

Topalov, A. Molnar-Gabor, D. Csnadi, J. Photocatalytic oxidation of the fungicide Metalaxyl dissolved in water over Ti02. Wat. Res. 1999, 33, 1372. 

Byrne JA, Eggins BR, Byers W, Brown NMD (1999) Photoelectrochemical cell for the combined photocatalytic oxidation of organic pollutants and the recovery of metals from waste waters. Appl Catal B Environ 20 L85-89... 

Humidity has a significant influence on the photocatalytic oxidation of aromatic contaminants in the gas phase. This is of particular interest, because commercial photocatalytic systems will be required to operate under a broad range of relative-humidity levels. The specific influence of relative humidity on the photocatalytic reaction has generally proved rather difficult to quantify because water has a dual role It may compete with contaminants for surface adsorption sites (a negative influence) and it plays a role in the regeneration of surface hydroxyl groups during photocatalysis (a positive influence). 

The presence of gas-phase water is generally beneficial to the photocatalytic oxidation of aromatic contaminants. In continuous photoreactors, humidity appears to prolong catalyst activity and delay or prevent catalyst deactivation. The effects of humidity on reaction rates, however, appear to vary, depending on the aromatic contaminant concentration and the humidity level. For example. Petal and Ollis [18] examined the continuous photocatalytic oxidation of m-xylene in a powder-layer photoreactor at several different relative humidity levels. The m-xylene photo-oxidation reaction rate was observed to increase for gas-phase water concentrations up to 1000 mg/m (—7% relative humidity). Increasing the humidity level further (up to 5500 mg/m ) produced a gradual decrease in the observed reaction rate, possibly due to increased adsorption-site competition between xylene and water. The reported xylene removal rate for a water concentration of 5500 mg/m was approximately half that seen at 1000 mg/m. ... 

Mendez-Roman and Cardona-Martinez [55] examined titanium dioxide catalysts with FTIR spectroscopy during the photocatalytic oxidation of toluene. Reaction intermediates, believed to be benzaldehyde and benzoic acid, were reported to accumulate on catalyst samples. This accumulation of intermediates was found to be reduced in the presence of gas-phase water. Mendez-Roman and Cardona-Martinez concluded that toluene appeared to be converted to benzaldehyde, which was then oxidized further to form benzoic acid. They suggested that the accumulation of benzoic acid led to the observed apparent catalyst deactivation. Other researchers, however, have argued that benzoic acid is unlikely to be the compound responsible for apparent deactivation in the photocatalytic oxidation of aromatics. For example, Larson and Falconer [43] concluded, based on higher CO2 evolution rates for benzoic acid relative to toluene during photooxidation, that benzoic acid was not sufficiently recalcitrant to be responsible for the deactivation seen with aromatic contaminants. 

Fig. 22. L MAS NMR spectra recorded during the photocatalytic oxidation of TCE (48 pmol) with gaseous oxygen (96pinol) and water (0.1 mmol) as co-reactants on a TiO, PVG catalyst. The UV irradiation time is indicated in minutes (right). Reproduced with permission from (52). Copyright 1998 American Chemical Society.

The secondary ring carbon atoms are preferentially oxidized with respect to that of the methyl group. A simple statistical calculation demonstrates that they are 6.5 times more Teactive. This behaviour is opposite to the gas phase photocatalytic oxidation of toluene (ref. 4), which produces only traces of benzaldehyde, whereas the aromatic ring withstands oxidation, at least in pure gas or liquid organic phase and in the absence of water. The above selectivities seem to be correlated to steric factors governing the mode of adsorption of methylcyclohexane on the surface of titania. 

Another approach to wards photocatalysis is to use dy as a sensitizer instead of a semiconductor as in photosynthesis. It is not the aim of this book to cover all the aspects of the sensitized photochemical conversion system, but typical sensitized systems for photocatalytic reactions of water are described in Chapter 18 The concept of a photochemical conversion system using a sensitizer and water oxidation/reduction catalysts is mentioned in Chapter 19, accompanied by a discussion on the sensitization of semiconductors. 

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