inorganic chemistry

HO 2 þHO 2 !H 2 O 2 þO 2 ð 17 Þ

H 2 O 2 þO 2 !OH



þOH



þO 2 ð 18 Þ

H 2 O 2 þeCB!OHþOH ð 19 Þ

Visible light excitation within the rhodium-to-titanium charge-
transfer band of RhCl 3 /TH affords as primary products an elec-
tron in the titania conduction band and a Rh(IV) center
(Eq. 13). The energetic position of the latter can be estimated
by adding the energy of the Vis absorption onset (2.25 eV) to
the quasi-Fermi level as depicted in Scheme 5. Approximate
the same position has been assigned to the Rh(III/IV) level in
Rh(III) doped BaTiO 3 ( 53 ). The resulting potential of 1.91 V is
positive enough to oxidize water or more likely 4-chlorophenol
to the radical cation (Eq. 14), which finally breaks down to CO 2 ,
HCl, and H 2 O, as well known from the UV photodegradation in
the presence of unmodified TiO 2 ( 54 – 56 ). The electron generated
in the conduction band reduces oxygen to superoxide (Eq. 15)
which through the reaction sequence according to Eqs. (16)–(19)

1.91 V

VB

CB

[RhIV]

4-CP, 1.18 V

+

• OH/OH–
  ca. 2.4 V

+

O 2 /O 2 –

–0.16 V

–0.34 V

2.87 V

[Ti]ORhIIICl 3 –

hn

SCHEME5. Mechanistic scheme of titania sensitization by rhodium
(III) complexes. Depicted values apply for 5.0%RhCl 3 /TH at pH 7.

390 HORST KISCH