Photoreduction of water

Although Ru(bipy)2+ alone will not split water into hydrogen and oxygen, it has been accomplished with Ru(bipy)2+ using various catalysts or radical carriers. Perhaps the most studied system for the photoreduction of water involves using methyl viologen as the quencher, EDTA as an electron donor (decomposed in the reaction) and colloidal platinum as a redox catalyst (Figure 1.19). 

Table 12.3 Compounds that have been used as components in successful systems for the photoreduction of water...

Zinc tetrasulfophthalocyanine is an attractive choice for the chromophore in multi-component systems for the photoreduction of water, but recent studies have shown that only low yields of photoproducts are obtained, and that porphyrin-based chromophores are more efficient.1181 1182... 

Cobalt(III) cage complexes can also perform as electron transfer agents in the photoreduction of water.180181 Because of the kinetic inertness of the encapsulated cobalt(II) ion, the cobalt(II)/co-balt(III) redox couple can be repeatedly cycled without decomposition. Thus these complexes are potentially, useful electron transfer agents, e.g, in the photochemical reduction of water, in energy transfer and as relays in photosensitized electron transfer reactions.180,181 The problem of the short excited-state lifetimes of these complexes can be circumvented by the formation of Co(sep)3+ ion pairs, so that the complexes can be used as photosensitizers for cyclic redox processes.182 183... 

The photooxidation and photoreduction of water in separate reaction systems are demonstrated in Sections 7.3.1 and 7.3.2, respectively. Since the redox... 

Photoreduction of Water Using Pt-loaded Ti02 Particles... 

In view of the fact that the Fong s experiments involved repeated platinization of the surfaces of the platinum and Chi a layers (74,75), the observed photoreactivity may arise from a photocatalytic effect of the composite of Chi a hydrates and platinum rather than Chi a hydrates themselves. In this respect, it has been reported that Pt dispersions catalize dye-sensitized photoreduction of water (77,78). 

Figure 8. An idealized model for CdS sensitized photoreduction of water by EDTA in polymerized 1 vesicles. The location of the particle is represented as similar to that in unpolymerized vesicles, but should not be taken for granted in this case, however.

Production of H2 fuel from water by solar energy may involve H2 complexes of transition metals, at least as intermediates, and H2 complexes have been implicated in solar energy conversion schemes based on photoreduction of water. Industrially important water gas shift and related H2-producing reactions undoubtedly proceed via transient H2 complexes [116,123],... 

Schwarz HA, Creutz C, Sutin N. (1985) Homogeneous catalysis of the photoreduction of water by visible light. 4. Cobalt(I) polypyridine complexes. Redox and substitutional kinetics and thermodynamics in the aqueous 2,2 -bipyridine and 4,4 -dimethyl-2,2 -bipyridine series studied by the pulse-radiolysis technique. Inorg Chem 24 433H39. 

Electrosorption has also been applied in electrotechnology to either catalyse or Inhibit electrochemical surface reactions. Photoreduction of water is an example of the former, corrosion inhibition of the latter. 

Topics which have formed the subjects of reviews this year include photosubstitution reactions of transition-metal complexes, redox photochemistry of mononuclear and polynuclear" complexes in solution, excited-state electron transfer processes, transition-metal complexes as mediators in photochemical and chemiluminescence reactions, lanthanide ion luminescence in coordination chemistry, inorganic photosensitive materials," and photocatalytic systems using light-sensitive co-ordination compounds. Reviews have also appeared on the photoreduction of water.Finally, various aspects of inorganic photochemistry have been reviewed in a single issue of the Journal of Chemical Education. 

Of considerable importance are the reports on the photochemical fixation of CO2 and the photoreduction of N2 that have appeared this year. In many respects, these reduction processes offer more potential for the storage of solar energy than does the photoreduction of water to H2. Several developments in this area have been noted recently. Thus, it has been reported that certain zinc porphyrins can fix CO2 upon irradiation with visible light. It has been claimed that reduced will reduce CO2 to formic acid with an overall quantum... 

For the influence of the specific surface area of the semiconductor powder on the rate of product formation, two opposite effects are of major importance [81]. One is concerned with the rate of electron-hole recombination which increases linearly with surface area, and accordingly the reaction rate should decrease. The other is a linear increase in the reaction rate of the reactive electron-hole pair with the adsorbed substrates, which should increase product formation. It is therefore expected that, depending on the nature of semiconductor and substrates, the reaction rate, or increasing surface area. This is nicely reflected by the CdS/Pt-catalyzed photoreduction of water by a mixture of sodium sulfide and sulfite. The highest p values are observed with small surface areas and are constant up to 2 m g". From there a linear decrease to almost zero at a specific surface area of 6 m g" takes place. Upon further increase to 100 m g" this low quantum yield stays constant [82]. 

Figure 6. Effect of platinization on the ZnS-catalyzed photoreduction of water by sodium formate.

The n-type character of these zinc sulfides is indicated by the observation of an anodic photocurrent upon illumination of a powder-coated electrode. The action spectrum of the photocurrent parallels the wavelength dependence of the apparent quantum yield of the photoreduction of water by sodium formate, except that hydrogen evolution starts already in the sub-bandgap region at 370 nm, a wavelength where no photocurrent was observable [98]. 

Marignier J.-L., Belloni J., Delcourt M.-O., Chevalier J.-P, New microaggregates of non noble metals and alloys prepared by radiation-induced reduction. Nature, 1985,317,344-345. Belloni J., Marignier J.-L., Delcourt M.-O., Minana M., Non noble metal catalytic microaggregates, a method for their preparation and their application in the catalysis ofthe photoreduction of water., US Pat. 1986, 4,629,709. Belloni J., Marignier J.-L., Delcourt M.-O., Minana M., New mono or multimetal microaggregates, a method for their preparation and their application in the catalysis of the photoreduction of water, US Patent 1987,4745094. 

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... 

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