Metal oxide photoredox reactions

In heterogeneous photoredox reactions not only the solid phase, i.e. the semiconducting mineral, may act as the chromophore (as discussed in Chapter 10.2) but also a surface species (i) a surface complex formed from a surface metal ion of a metal (hydr)oxide and a ligand that is specifically adsorbed at the surface of the solid phase, and (ii) a chromophore that is specifically adsorbed at the surface of a solid phase. In the following these three cases will briefly be discussed. 

In heterogeneous photoredox systems also a surface complex may act as the chromophore. This means that in this case not a bimolecular but a unimolecular photoredox reaction takes place, since electron transfer occurs within the lightabsorbing species, i.e. through a ligand-to-metal charge-transfer transition within the surface complex. This has been suggested for instance for the photochemical reductive dissolution of iron(III)(hydr)oxides (Waite and Morel, 1984 Siffert and Sulzberger, 1991). For continuous irradiation the quantum yield is then ... 

Sulzberger, B. (1990), "Photoredox Reactions at Hydrous Metal Oxide Surfaces a Surface Coordination Chemistry Approach", in W. Stumm, Ed., Aquatic Chemical Kinetics, Wiley-lnterscience, New York, pp. 401-429. 

A large number of other metal complexes have received long and detailed attention, but activity in recent years has revealed few new principles appropriate for discussion here and some systems have been treated in detail elsewhere.2 Included among these are oxalato complex photochemistry where oxidation of the oxalato ligands is coupled to the central metal reduction Ag(I) photochemistry related to imaging systems uranyl ion photochemical reactions coupled to organic oxidations and aquo ion photoredox reactions. Two specific topics have recently emerged as... 

See, for example, the reviews by B. Sulzberger, Photoredox reactions at hydrous metal oxide surfaces A surface coordination chemistry approach, Chap. 14 in W. Stumm, op. cit.,3 and T. D. Waite, op. cit.27... 

The electrons that are provided by photosystem I are finally used to reduce CO2 to carbohydrates, while in photosystem II, water is oxidized to oxygen. Intense research over many decades has partially revealed the extremely complicated mechanism of natural photosynthesis. It follows that it is obviously rather difficult to imitate this in an artificial photosynthesis that is intended to convert and store solar energy in simple but energy-rich chemicals. Different approaches have been developed to solve this problem (i). It has been suggested to facilitate artificial photosynthesis by the assistance of redoxactive metal complexes in homogeneous systems. Generally, photoredox reactions of metal... 

There are essentially two possibilities to accomplish two-electron or multielectron transfer at metal complexes without formation of one-electron transfer intermediates (e.g., radicals). Appropriate metal centers should have available stable oxidation states that differ by at least two emits. T5rpical examples that represent such photoredox reactions are reductive eliminations such as (X = halide, pseudohalide) (5) ... 

The partitioning of the POM excited state between productive processes (photoredox reactions involving substrate) and nonproductive processes (radiationless decay including internal conversion, bimolecular quenching, or emission) depends on the structure and elemental composition of the POM, and to a lesser extent other parameters. The quantum yields for the photoredox processes of POMs in solution can be quite high (well over 50%). The presence of -electron containing metal centers lowers the quantum yields for substrate oxidation by quenching the excited states. 

PHOTOREDOX REACTIONS AT HYDROUS METAL OXIDE SURFACES A SURFACE COORDINATION CHEMISTRY APPROACH... 

Photoredox reactions at hydrous metal oxide surfaces... 

Pichat and Fox (1988)]. As will be discussed later in this chapter, in many cases the surface structure, especially the coordinative interactions taking place at the surface, determines the ellieiency of a photoredox reaction at the solid -liquid interface of a hydrous metal oxide. 

In photoredox reactions occurring at the surface of hydrous metal oxides, a surface compound can have different functions it can act merely as electron donor or acceptor without being involved as the chromophore, or it can act as electron donor or acceptor and as the light absorbing species, relevant for the heterogeneous photoredox reaction. 

The examples given above illustrate that the coordinative interactions taking place at the surface of a hydrous metal oxide play an important role in these heterogeneous photoredox reactions. This is especially true for the light-induced dissolution of iron(III) hydroxides, as will be discussed in the next section. 

---