Redox couples photo potential

The valence band top of d° or d10 stable oxide semiconductor with good photo-catalytic activity consists of an O 2p orbital. Commonly, the electrochemical potential of this VB is 2.5-3.5 V vs. NHE, which is considerably more positive than the redox potential of the O2/H2O redox couple. Strategies to narrow the band gap of these oxides and better match the redox potentials are ... 

Extended comparative measurements of the current efficiencies for the photo-oxidation of a large variety of compounds, in competition with the oxygen evolution, have essentially confirmed the same general tendency for the decrease of the oxidation rate with increasing the standard potential of the redox couple -The highest current efficiency for the competitive photo-oxidation at Ti02 in add solutions has been reported for hydroquinone - which (i.e., the quinone/hydroquinone couple) has the standard redox potential slightly more positive than that of the I2/I couple. 

It appears that the more thermodynamically favoured redox reactions also become kinetically favoured, so that these reactions predominate. This effect has been used to stabilize semiconductor electrodes by establishing a redox couple in the electrolyte with a redox potential more negative than the oxidative decomposition potential (or more positive than the reductive decomposition potential), such that this electrolyte redox reaction occurs preferentially compared to the decomposition reaction, and scavenges the photo-generated minority carriers. However, this stabilization technique can only be used for electrochemical photovoltaic cells, and it is discussed later in further detail. 

A combination of cat. Ybt and A1 is effective for the photo-induced catalytic hydrogenative debromination of alkyl bromide (Scheme 28) [69]. The ytterbium catalyst forms a reversible redox cycle in the presence of Al. In both vanadium- and ytterbium-catalyzed reactions, the multi-component redox systems are achieved by an appropriate combination of a catalyst and a co-reductant as described in the pinacol coupling, which is mostly dependent on their redox potentials. 

From the discussion of the Marcus theory above and equations (20) and (21), we see that the experimental data needed to judge the feasibility of ET steps involving spin traps and spin adducts are the redox potentials and A values of the ST +/ST and ST/ST - couples, as well as those for hydroxylamine derivatives related to the operation of reactions (4) or (5). The electroactivity of the spin adducts themselves is also of interest since it must somehow be related to their lifetimes in a redox-active environment. Moreover, the excited-state redox potentials (of ST /ST and ST,+/ST ) are also necessary for the understanding of photo-ET processes of spin traps. 

A semiconductor suitable for an efficient photo-oxidation reactions of environmental relevance should fulfil several requirements. Its bandgap should allow the use of solar radiation, ie the catalyst has to absorb in the visible or near-UV light region. The redox potentials of 0H7H20 and 02/02 couples should lie within this bandgap ( 0H./H20 = 2.8V [62], E°02/0.- = -0.16V [63]) in order to facilitate... 

---