inorganic chemistry

C. MECHANISM

To test if the photocatalytic activity of RhCl 3 /TH is also initiated
by a homolytic MCl bond cleavage, as proposed previously for plat-
inum(IV) chloride modified TH, the photodegradation of phenol
undervisiblelightirradiation(l455 nm)wascarriedout.Forma-
tion of chlorophenol would evidence the presence of intermediate
chlorine atoms. However, no significant amount of chlorophenol
was detectable( 51 ). This differs from Pt(IV)/TH, in which case
chlorophenol formation was observable ( 18 ). Further, cyanuric
acid, a molecule which is mineralized in the presence of platinum
(IV) modified TH ( 20 ) (vide supra), is not decomposed by 4.0%
RhCl 3 /TH. These significant differences indicate that in the case
of rhodium(III) modification visible light-induced cleavage of the
metal–halogen bond is not a major primary photoprocess. More
likely seems a mechanism as proposed for UV light-induced oxida-
tion reactions in the presence of Rh(III) doped titania colloids ( 52 ).

½ŠTiO 2 ORh^3 þþhn!½TiO 2 ŠORh^4 þþeCB ð 13 Þ

½ŠTiO 2 ORh^4 þþ 4 CP!½ŠTiO 2 ORh^3 þþ 4 CPþ ð 14 Þ

O 2 þeCB!O 2  ð 15 Þ

O 2 þHþ!HO 2  ð 16 Þ

0 10 20 30 40 50 60

0.0

0.2

0.4

0.6

0.8

1.0

d

c

b

a

4-CP, TOC/TOC

0

Time (min)

FIG. 8. Mineralization of 4-CP as function of rhodium content;
photocatalyst concentration: 0.5 gL^1 ; (a) 0.5%RhCl 3 /TH, (b) 1.0%
RhCl 3 /TH, (c) 2.0%RhCl 3 /TH, (d) 5.0%RhCl 3 /TH.

VISIBLE LIGHT PHOTOCATALYSIS 389