Once formed, the primary redox products are converted in
subsequent thermal reactions steps to the final compounds Ared
and Dox. When oxygen is the electron acceptor and a pollutant
like phenol is the electron donor, carbon dioxide and water are
the final redox products (Scheme 2). The primary reductive redox
product is superoxide which can be converted to the strongly
oxidizing OH radical via protonation, disproportionation of HO 2
and reductive photocleavage of the produced H 2 O 2. Instead of
water oxidation, the oxidative primary step may consist of the
oxidation of the pollutant producing a phenoxy radical and a pro-
ton. Such complete photooxidation reactions are often termed as
mineralization and in general titania is employed as the
photocatalyst ( 4 – 7 ).
Thermodynamics requires that the redox potentials of
acceptors and donors (D and A) are located within the potential
range given by the reactive electron–hole pair. Since the latter
in general is not known, the positions of the band edges may be
taken as approximate values. It is noted that this is reliable only
in the absence of any crystal defects prone of charge trapping, a
rather rare case in experimental photocatalysis. Thus, the reduc-
tion and oxidation potentials of these surface centers in general
may be a few hundred of millivolts smaller than estimated by
this approach. A more detailed discussion is given by Gerischer
for an n-type semiconductor (Scheme 3) ( 8 ). Absorption of light
SCHEME2. Simplifiedmechanisticschemeofthe titaniaphotocatalyzed
mineralization of phenol.
VISIBLE LIGHT PHOTOCATALYSIS 375