scavenging experiments with benzoic acid and tetranitromethane,
an alternative electron acceptor. The adsorbed chlorine atom is
assumed to oxidize 4-CP under deprotonation to the
corresponding oxyl radical which is eventually converted to CO 2
and HCl, by analogy with the photomineralization catalyzed by
unmodified titania upon UV-excitation (36,37). It is noted that
the alternative mechanism, that is, electron injection from the
excited chloroplatinate resulting in a platinum(V) intermediate
cannot be completely ruled out.
As summarized above, the primary photoprocess generates two
surface redox centers. The oxidative one can be described as a kind
of Cl/Clredox pair weakly coordinated to a metal center. It is rea-
sonable to assume that this oxidant is weaker than the free Cl/Cl
pair (ECl/Cl¼2.6 V, value for aqueous solution( 38 ), but stronger
than the couple ½Cl 2 /Cl(E¼1.3 V ( 39 ) and the oxidation poten-
tial of 4-CP (1.18 V). Further, the potential has to be lower than
2.4 V, the estimated redox potential of the OH/OH couple,
because no hydroxyl radical formation could be observed in the
presenceofbenzoicacidandtetranitromethaneasOHandelectron
acceptor, respectively ( 18 ). Thus, the potential of the oxidative cen-
ter should be in the range of 1.3–2.4 V.
Unfortunately, the redox potential of the Pt^4 þ/3þcouple is not
known in the literature. Although some stable PtIIIcompounds
have been isolated and characterized( 40 ), the oxidation state
III is reached usually only in unstable intermediates of photo-
aquation reactions ( 41 – 43 ) and on titania surfaces as detected
by time resolved diffuse reflectance spectroscopy ( 44 ). The redox
potential of the surface Pt^4 þ/3þcouple should be equal or more
negative than 0.28 V, that is, the quasi-Fermi potential of
4.0%H 2 [PtCl 6 ]/TH at pH 7. Assuming a value of0.40 V and
recalling that the potential difference between the reactive
charges cannot be higher than the absorbed light energy divided
by the elemental charge, which is about 2.00 V upon excitation at
the absorption onset (620–650 nm), the potential ofEPt-(Cl/Cl)is
obtained as 1.6 V (Scheme 4). These chlorine based surface
centers are absent in unmodified titania and very likely enable
the degradation of otherwise reluctant cyanuric acid (vide supra).
III. Titania–Halogenorhodium(III) Complexes (X¼Cl, Br)To find out whether or not sensitization of titania as described
above is feasible also with halides of other d^6 metals, surface
modification was also attempted with RhX 3 (X¼Cl, Br) ( 45 ).
The novel hybrid photocatalysts x%RhCl 3 /TH containing 0.5,
384 HORST KISCH