Modeling water splitting

Figure 1. Model of the hydrogen device with the left chamber operating under aerobic conditions (due to water-splitting of PS2) and the right chamber under anaerobic.

The principal solar water-splitting models predict dual-band gap photoelectrolysis efficiencies of 16% [40], and 10-18% [41]. 

Based on the EPR, ENDOR, ESEEM and some other spectroscopic results models for Yz function in PS II have been developed (see references 385, 386 for recent reviews). There seems to be consensus that this amino acid is involved in proton-coupled electron-transfer. As proton acceptor His (Dl-190) was identified which is also found to be close to Yz in the recent X-ray structure.19 The postulated direct involvement of Yz in the water splitting process as a hydrogen atom abstractor or in separate or coupled proton/electron-transfer is still controversially discussed.385 386... 

Figure 17.2 illustrates our model for splitting water by solar energy. I" is important that all the redox reactions involved in thf system be reversible. The quinone compound in the organic solvent combines the two photocatalytic reactions, and its function can be compared to the electron relaying molecules in thylakoid membranes of chloroplasts. Electron transfer reactions via quinone compouncs in artificia systems have been studied as a model of photosynthesis22-23 and in an electrochemical system for acid concentration.24 ... 

Another model compound, the tris(2,2 -bipyridine)ruthenium(II) complex, has prompted considerable interest because its water-splitting photoreactivity has been demonstrated in various types of photochemical systems (77,99,100,101). Memming and Schroppel (102) have attempted to deposit a monolayer of a surfactant Ru(II) complex on a Sn02 OTE. In aqueous solution, an anodic photocurrent attributable to water oxidation by the excited triplet Ru complex was observed. A maximum quantum efficiency of 15% was obtained in alkaline solution. 

In addition to the properties of micelles described above, vesicles, which are bilayer structures and can be considered to be model membranes, separate two distinct aqueous phases an entrapped or inner water pool and the bulk aqueous phase. In principle, therefore, electron transfer may be possible across the bilayer and the sites of hydrogen and oxygen production in a water splitting system can be separated spatially. 

Figure 10. An hypothetical model for a cyclic water splitting system, based on a semiconductor particle immobilized in a polymerized membrane and having access to both aqueous solutions on each side of the membrane. Specific and selective coating by catalysts leads to simultaneous and separate hydrogen and oxygen generation on each side of the polymerized membrane.

R.M. Navarro, F. del Valle, J.A. Villoria de la Mano, M.C. Alvarez-Galva n, and J.L.G. Fierro, Photocatalytic Water Splitting Under Visible Light Concept and Catalysts Development Ajay K. Ray, Photocatalytic Reactor Configurations for Water Purification Experimentation and Modeling... 

The other approach towards artificial photosynthesis does not model the natural photosystems directly, but instead links natural and synthetic components to achieve a system capable of photochemical water splitting [83]. 

To model the water-splitting reaction and any electrochemical reaction, it is necessary to include the potential or bias. As long as only reaction energies are considered, it is possible to avoid explicit modeling of the electrochemical interface. With this approach, barriers for charge transfer reactions cannot be treated. In this section, the reference for the potential will be introduced. 

The stoichiometries of the two reactions above raise an interesting mechanistic question how is the one-electron photochemistry in PS II coupled to the four-electron water-splitting chemistry The now-classical single turnover flash experiments of Joliot and co-workers [2], which showed period four oscillation in the O2 yield with flash number, provided a clearcut answer the PS II units function independently in accumulating the four oxidizing equivalents required to split water. This observation was quickly confirmed [3,4] and Kok provided the S-state model which is now widely used to summarize the situation ... 

A substantial body of work had associated manganese with water splitting [5,10,109] and with the development of Kok s S-state model it was generally assumed that the S-state transitions correspond to valence changes in a functional manganese... 

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