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Colloids of RuO;2H20 and MnO, have been produced by chemical and radiolytic methods. The colloids slowly aggregate upon standing in water over several weeks but radiolysis gives much smaller particles. The colloids are active in catalysing water oxidation under chemical and photochemical conditions although there is some corrosion. The chemically formed RuO, -2H,O colloids are quite efficient catalysts for water reduction to H, but the radiolytically formed colloids do not promote H, formation. Colloidal MnO, is a poor catalyst for both H, and 0, generation and it tends to aggregate under operating conditions. The RuO, -2H,O colloids obtained by gamma radiolysis are selective for oxidation processes. They are stable and catalyse 0, formation with high efficiencies. Such materials appear to be the best catalysts available for use in model systems for the photodissociation of water.Considerable research activity has been expended on the design of photosystems capable of the cyclic cleavage of water into H, and 0,. Any such system that could operate under visible light excitation would have genuine applications for solar energy conversion and storage. However, no satisfactory system has been devised to date.' One of the more serious barriers preventing construction of an overall photosystem is the absence of a highly effective and selective 0, evolving catalyst.2 Because of its low overpotential, most ~y s t e m s ~-~ have employed RuO, as the anodic catalyst, but this material is far from selective and it corrodes' badly under operating conditions. Corrosion can be inhibited by careful choice of the support,' by thermal treatment' and (possibly) by thermodynamic considerations.* Even so, these stabilised materials are not selective, and in most cases they exist as large-grain powders that are unsuitable for use in photochemical systems. In this paper we describe the properties of some colloidal RuO, catalysts which, whilst not being particularly reactive, represent a marked improvement over existing materials.

We have confirmed an earlier report by Kalanasundaram and Gratzel that a positively charged, water-soluble zinc porphyrin photosensitises the reduction of water to H2 with high efficiency. Using MV2+ as electron relay and EDTA as sacrificial electron donor. the quantum yield for production of $H2 is ca. 0.6. The reaction mechanism involves reduction of MV2+ by triplet porphyrin and the porphyrin v-radical cation so produced is reduced by EDTA. The concentrations of reactants have been optimised for production of H2 and to limit destruction of the porphyrin. Under optimised conditions the turnover with respect to the porphyrin can reach 6000.In addition, some consideration has been given to ways of improving c $ ~~ and of increasing the fraction of sunlight that can be harvested. In this respect, Cd porphyrins may possess suitable properties.Finally, we note that the porphyrin w-radical cation may possess the thermodynamic capacity to oxidise water to 02, but this is very much borderline.

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A series of compounds has been synthesised having two identical viologen groups separated by organic bridges of varying degree of flexibility. Chemical or radiolytic reduction results in formation of the mono viologen .rr-radical cations, which undergo disproportionation to form the doubly reduced viologen. In this latter species both viologen units are reduced and there is a distinct stabilisation effect if the molecule can form an intramolecular cofacial dimer. The disproportionation constants have been determined by cyclic voltammetry and depend markedly upon the type of bridging group employed. Using pulse radiolysis it was shown that the mono radical could be stabilised by intramolecular association with the unreduced viologen. This is achieved by fairly slow conformational rearrangement in competition with bimolecular electron transfer to the preferred conformation. All the mono radicals disproportionate and the bimolecular rate constants have been measured. Although the viologens can store several electrons on one molecule, they do not reduce water to H2 in the absence of a catalyst.

Tetrasulphophthalocyanine and the Zn" and Cu" complexes have intense absorption in the near i.r. region but the compounds aggregate in aqueous solution. Aggregation is hindered by addition of organic solvents, such as pyridine, and here H2PcS4-and ZnPcS4-show strong fluorescence. Photophysical data have been collected for both compounds and for the Cu" complex and the observed values related to the ideal properties of a chromophore for a three-component [chromophore/donor/methyl viologen (MV2 ')] H,-producing system. The ZnT1 complex appears as the most attractive chromophore but irradiation in the presence of EDTA, MV2+ and colloidal Pt gives no H2 production due to efficient reverse electron-transfer. The yield of H2 is increased in the absence of MV2+ but the sulphonated phthalocyanines compare unfavourably with metalloporphyrins.In recent years there has been considerable interest in the development of photochemical processes capable of evolution of hydrogen from water upon irradiation with visible light. Successful systems have been pioneered by Shilov,' Lehn2 and Gratze13 and their coworkers and subsequently developed by other researcher^.^The essential features of successful systems have been the use of an irreversible donor to minimise reverse electron-transfer and the incorporation of a catalyst to promote electron-transfer between the reduced acceptor and a proton. Such systems can be termed "three-component processes" and below is a summary of the reactants used in the best documented ones.