Photocatalytic oxidation of water on TiO2 (rutile)
powder proceeded with a fairly high efficiency (about 9%)
when iron(III) ions were used as the electron acceptor. The
reaction continued until all iron(III) ions added
to the solution were reduced into iron(II) ions. This
behavior was in marked contrast to other reversible
photocatalytic reactions, whose reaction rates decelerate as the result
of the back reactions of the products on
the photocatalysts. The efficient oxidation of water in the
presence of iron(III) ions was attributed to
preferential
adsorption of iron(III) ions on TiO2 over
iron(II) ions, which enabled efficient oxidation of water,
although
this reaction was thermodynamically less favorable than oxidation of
iron(II) ions. Furthermore, from the
measurements of photocurrents at crystalline TiO2
electrodes, iron(III) ions were concluded to have a
catalytic
function for the oxidation of water on photoirradiated
TiO2.
When organic compounds such as 2-propanol are in aqueous solutions and are photooxidized on the surface
of semiconductor electrodes, the current quantum efficiency can reach close to 200% (current-doubling effect).
However, we found that the current-doubling effect in the photooxidation of 2-propanol on TiO2 electrodes
disappeared in solutions containing Fe(III) ions. From the analyses of the products, we found that acetone
was produced in quantitative correlation to the photocurrent, and that practically no Fe(II) ions were generated
in the solution. We also found that Fe(II) ions were not oxidized at a TiO2 photoanode in a solution containing
Fe(III) ions, while they were oxidized in quantitative correlation to the photocurrent in the solution without
Fe(III) ions. These results refuted the possibility that Fe(III) ions accepted electrons from the oxidation
intermediates of 2-propanol. The plausible reason for the disappearance of the current-doubling effect is that
2-propanol is oxidized by two holes on the Fe(III)-adsorbed TiO2 surface.
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