Thermodynamical Considerations and Mechanistic Pathways

In the direct reduction (a), the initial electron transfer step, reaction (11), is usually considered as the rate determining one (Mills and Valenzuela, 2004). For predicting the feasibility of the transformation, the reduction potential of the first step related to the energy of the conduction band has to be considered. The conjugate anodic reaction of Equation (11) is oxidation of water by holes or HO, initiated by reaction (3) and ending in protons and oxygen  

The photogenerated holes or HO radicals can reoxidize also the species to the original one, causing a nonproductive short-circuiting of the overall process  

As the water oxidation reaction (13) is a very slow four-electron process, metal reductive transformation can be generally improved by addition of sacrificial organic agents (Prairie and Stange, 1993 Prairie et al., 1993a). The process can be improved even more if a strong reducing species is formed 

Indirect reduction (mechanism (b)) was suggested by Baba et al. (1986), who analyzed the photodeposition of Ft, Ag, and Au on an anodically biased Ti02 electrode in a solution containing alcohols (MeOH, EtOH, or 2-PrOEI). As no metal deposition was observed in the absence of alcohols, these authors proposed that photogenerated conduction band Ti02 electrons do not take part directly in the deposition. As a result, the reduction is then assumed to be driven by intermediates generated by attack of or 

For the cases of methanol, ethanol, or 2-propanol, R are 1-hydroxyalkyl radicals, while for formic or oxalic acids the strong reducing is 

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