Photodecomposition of water

Catalytic schemes for the photodecomposition of water, for example, consist essentially of two steps i) light induced generation of strong oxidants (S+) and reductants (A ) upon absorption of visible light by the sensitizer S  

Since the electronically excited states of metal complexes are good oxidants and reductants it is possible to envisage several redox schemes to achieve one or more of the above processes. 

In such partial systems it is inevitable that one of the components be oxidized (electron donor in the case of water reduction) or reduced (electron acceptor in cases of water oxidation) irreversibly. In principle, reversible electron donors and acceptors can be used but cross reactions (as observed 

A number of electron acceptors have been identified for reaction (40) that can effectively mediate H2 evolution from water in the presence of Pt-group based redox catalysts. Typical examples are bipyridinium salts such as methyl viologen (MV ), polypyridine complexes of Rh(III), Co(II) and cage complexes of Co(II). For this reason, the oxidative quenching of various transition metal polypyridine complexes, M(LL)3] by these electron acceptors have been extensively studied  

Various aspects of excited state quenching involving these relay molecules have been discussed earlier. Table 14 presents a summary of optimal quantum yields of molecular hydrogen obtained in photocatalytic systems based on polypyridine complexes. 

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There has been intense study of the complexes of bi- and polydentate ammines since the mid-1970s, driven by interest in the catalytic photodecomposition of water using the excited states of Ru(bipy)g+ (n = 2,3) and related systems (Figure 1.18) [5, 7, 8, 71]. 

Interfacial electron transfer in colloidal metal and semiconductor dispersions and photodecomposition of water. K. Kalyanasundaram, M. Gratzel and E. Pelizzelti. Coord. Chem. Rev., 1986, 69, 57 (338). 

Figure 1. Photodecomposition of water on Ru02/BaTi40g(, H2 O-Os), Ru02/Na2Ti60i3(H,H2 .O2) and Ru02/Ba4Tii303o(A,H2 A,02).

Fujishima and Honda [16, 158] reported the photodecomposition of water using semiconductor photoelectrolysis cells (Figure 4.10). When the surface of the Ti02 electrode was irradiated with UV light, oxygen evolution was observed at the Ti02 electrode surface and hydrogen at the Pt black electrode. The overall water photodecomposition reaction ... 

Sato, S. and White, J.M. (1980) Photodecomposition of water over Pt/ Ti02 catalysts. Chemical Physics Letters, 72 (1), 83-86. 

Domen, K., Kudo, A., Tanaka, A., and Onishi, T., Overall photodecomposition of water on a layered niobate catalyst, Catal. Today, 8, 77,1990. 

Another study with Ti02 as catalyst involved the photodecomposition of water in the presence of PBN or 4PyOBN ([18] Jaeger and Bard, 1979). The results were complicated and difficult to interpret, but it was concluded that HO was formed in the photooxidation process and trapped as such. However, in view of the short half-life of HO-PBN established later (ti/2 10 s in acetonitrile and 90 s in water, see Section 15), the reported control experiments are not entirely convincing. 

The use of an electrochemical cell to effect the photodecomposition of water using visible light has been proposed by Creutz and Sutin189). This cell consists of... 

Jaeger CD, Bard AJ. Spin trapping and electron spin resonance detection of radical intermediates in the photodecomposition of water at Ti02 particulate systems. J Phys Chem 1979 83 3146-3152. 

All photocurrents from a semiconductor, when measured near the limiting current region, have the type of appearance shown in Fig. 10.5. The events that lead to the production, e.g., of hydrogen from the photodecomposition of water on illumination of, say, p-type InP, are as follows ... 

Overall Photodecomposition of Water on a Layered Niobate Catalyst Review mostly dealing with developments in the authors laboratory on particulate systems. 69... 

Sodium carbonate addition to a Pt/Ti02 suspension in water effective in promoting stoichiometric photodecomposition of water. 321, 322... 

Evidence of Photodissociation of Water Vapor on Reduced SrTiCh (III) Surfaces in a High Vacuum Environment. First report of photodecomposition of water adsorbed from the gas phase in high vacuum conditions on metal-free, reduced single crystals. 396... 

Table 12. Miscellaneous multinary oxides for the photodecomposition of water.

The possible utilization of the energy stored In the stabilized photoproducts In further chemical reactions Is discussed. Special attention Is given to the photodecomposition of water as a reaction route. 

Figure 10. Processes involved in the photodecomposition of water in a relay-free system. An amphiphilic Ru(bipy)s derivative is used as a sensitizer.

Early examples in the 1980s were aimed at the design of composite systems for photoelectrolytic generation of H2. Thus, Nafion and Si02 were used as supports for coprecipitated ZnS and CdS for photoassisted HER from aqueous sulfide media [407]. Subsequent work has addressed the mechanistic role of the support in the photoassisted HER [408]. Vectorial electron transfer was demonstrated in bipolar Ti02-Pt or CdSe-CoS photoelectrode panels arranged in series arrays for the photodecomposition of water to H2 and O2 [409, 410]. 

Layered Oxides oj Transition Metals 495 Table 3. Photodecomposition of water over K4Nb60 7 loaded with various metal oxides [14. ... 

Nickel-loaded KTiNbOs was studied by Takahashi et al. [76] for the photodecomposition of water. It was found that the activity of compounds prepared by a polymerized complex (PC) method was 10 times higher than the activity of the oxide prepared by conventional methods. The increased activity for the PC preparation was attributed partly to the larger surface area (ca. 23 vs. ca. 3 m- g ) and partly to probable improvement in surface structure. 

The ionic pair of [Co(AMMEsar)] + cation with an anthracene carboxylate anion (A-Co(III)) was used as both a photosensitizer and an ETA in the photodecomposition of water to produce hydrogen [387]. The photoreduction of encapsulated cobalt(III) ion to cobalt(II) ion occurs on excitation of anthracene chromophore (v< 25 000 cm-i). The A-Co(III) complex shows almost no fluorescence (0<2x 10 ), whereas the A-Co(II) complex produces specific violet fluorescence (Fig. 66). The cobalt(II) complex is formed in the presence of EDTA on light irradiation of the A-Co(III) solution (v> 25 641 cm i). The visible band at 21 276 cm- disappeared, and violet fluorescence was observed. The quantum yield of cobalt(II) complex formation was... 

Possible Schemes for Photodecomposition of Water via Photoredox Reactions. 41... 

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