Iron oxide photoredox reactions

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 ... 

Especially important for cleaning of aquatic systems are iron(III) complexes with HSs. These thermally very stable compounds undergo photoredox reactions, creating ferrous ion and free HS radicals. In a neutral or alkaline medium, the Fe11 species is reoxidized to Fe111 by molecular oxygen, whereas in acidic medium a more efficient oxidant (H202) is needed (Fenton reaction) [11,19]. 

Sulzberger B (1990) Photoredox reactions at hydrous metal oxide surfaces A surface coordination chemistry approach. In Aquatic Chemical Kinetics. Stumm W (ed) p 401-430. New York Wiley Sulzberger B, Siffert C, Stumm W (1988) Surface coordination chemistry and redox processes— Photoinduced iron (III) oxide dissolution. Chimia 42 257-261 Swap R, Garstang M, Greco S, Talbot R, Kallberg P (1992) Saharan dust in the Amazon basin. Tellus B 44 133-149... 

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. 

Iron-thiazine photogalvanic cells use the photoredox reactions of Fe(II) with thiazine dyes, represented for thionine by Reactions 1, 2, 3, 4, and 5, to convert photon energy into chemical potential. The spontaneous ground state reactions represented by Reactions 6, 7, 8, and 9 also occur in homogeneous solution during illumination. Photogalvanic action results when homogeneous Reactions 7, 8, and 9 are replaced by anodic oxidation of and TH2 coupled with cathodic reduction of... 

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