Surface complexation photoredox reaction

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 ... 

The solid state and the surface chemistry of some of the solid Fe-phases impart to these oxides and sulfides the ability to catalyze redox reactions. Surface complexes and the solid phases themselves acting as semiconductors can participate in photoredox reactions, where light energy is used to drive a thermodynamically unfavorable reaction (heterogeneous photosynthesis) or to catalyze a thermodynamically favorable reaction (heterogeneous photocatalysis). 

The various elementary steps involved in the surface photoredox reaction, leading to dissolution of hematite in the presence of oxalate, are outlined in Figure 12.10. The two-dimensional stmcture of the surface of an iron(III) hydroxide given in this figure is highly schematic. The charges indicated correspond to relative charges. An important step is the formation of a hypothetical bidentate, mononuclear surface complex. With pressure jump relaxation technique, it has... 

We have chosen hematite oxalate as a model system, since the photochemical properties of colloidal hematite (Stramel and Thomas, 1986) and the photochemistry of iron(III) oxalato complexes in solution (Parker and Hatchard, 1959) have been studied extensively. The experiments presented in this section were carried out as batch experiments with monodispersed suspensions of hematite (diameter of the particles 50 and 100 nm), synthesized according to Penners and Koopal (1986) and checked by electron microscopy and X-ray diffraction. An experimental technique developed for the study of photoredox reactions with colloidal systems (Sulzberger, 1983) has been used. A pH of 3 was chosen to maximize the adsorption of oxalate at the hematite surface. This case study is described in detail by Siffert (1989) and Siffert et al. (manuscript in preparation). 

Apart from the photoredox reaction occurring at the surface of hematite and leading to dissolved iron(II), Fe" is also produced through photolysis of dissolved iron(III) trioxalato complexes. Dissolved iron(III) is formed via thermal pathways, where Fe11 acts as a catalyst. 

As discussed in the previous section, a ligand-to-metal charge-transfer transition of the surface complex (mechanism 1) and/or a Fe -O"11 charge-transfer of hematite (mechanism 2) are the oscillators involved in the surface photoredox reaction, leading to reductive dissolution of hematite in the presence of oxalate. The elementary steps and the derivations of the rate expressions of photochemical surface iron(II) formation of mechanism 1 and 2 are outlined in reactions 16-19, Eqs. 20-26, reactions 27-31, and Eqs. 32-37, respectively. 

Since the rate of the photochemical dissolution via surface photoredox reaction depends on the surface concentration of the adsorbed oxalate and not the solution concentration, it is constant under the experimental conditions of this case study, whereas the rate of the homogeneous photoreduction of iron(III) depends on the concentration of dissolved iron(lll) trioxalato complexes. The rate of the photochemical iron(II) production is the sum of the rates of the heterogeneous and the homogeneous photoredox reaction. Since, under the... 

Consideration of the nature of the LMCT transitions, redox energetics, and photoredox behaviour of transition-metal ammine complexes has allowed Endicott and co-workers18 to propose new models for the potential energy surfaces describing their photoredox reactions. These models have been used to discuss the differences in photoreactivity of [Co(NH3)5Br]2+ and [Co(NH3)5-N03]2+.21 These differences are ascribed to (i) more Co-radical bonding in the... 

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