separation and IFET can be described. Detailed information is
available from recent original publications (13,18– 20 ).
A. PREPARATION ANDCHARACTERIZATION OFPHOTOCATALYSTS
Upon stirring a suspension of titania powders in an aqueous
solution of H 2 [PtCl 6 ] in the dark, different maximum amounts
were adsorbed. For the anatase hydrate and anatase/rutile
samplesTH(Kronos) andP25(Degussa), there were taken up
4.0% and 1.1%, respectively, whereas only traces were adsorbed
onto the rutile material Ald. The almost four times larger
amount adsorbed byTHcorresponds with the about four times
larger surface area as compared toP25(vide infra). Subsequent
heat treatment at 200C afforded the desired photocatalyst
( 21 ). Surface modification may be performed also by simple
grinding with PtCl 4 , but the resulting powders are of lower
photocatalytic activity and less stability (18,20).
Analysis by transmission electron microscopy (TEM) and X-ray
powder diffraction (XRD) of 4.0%H 2 [PtCl 6 ]/TH revealed the pres-
ence of about 200 nm large aggregates composed of 2–4 nm sized
anatase crystallites ( 22 ). Specific surface areas of unmodified
P25(50 m^2 /g) andAld(3 m^2 /g) were not changed upon modifica-
tion whereas a significant decrease from 334 to 260 m^2 /g was
found forTH.
To find out if the chlorplatinate is physically or chemically
adsorbed onto the tiania surface, 4.0%H 2 [PtCl 6 ]/TH was stirred
in 0.01 M potassium fluoride at ambient conditions. It is known
that fluoride irreversibly chemisorbs through replacement of
titania bound OH groups( 23 – 27 ). Surprisingly, no desorption of
a chloroplatinate complex was observable under these experi-
mental conditions. However, desorption became feasible
upon decreasing the pH value. These observations strongly sug-
gest that chloroplatinate is chemically bound to titania as
summarized in Eq. 14) (L¼Cl, OH) and that desorption is an
acid catalyzed process. From
½ TiO 2 OHþPtCl 5 Ln!½ TiO 2 OPtCl 4 LnþHCl ð 12 Þthe amount of chloride released after stirring an NaOH suspen-
sion of 4.0%H 2 [PtCl 6 ]/TH in the dark, the composition
[Ti]OPtCl 4 Ln,L¼H 2 O, OH,n¼1, 2, was proposed for the tita-
nia surface complex. In corresponding photochemical desorption
experiments (l455 nm) with 4.0%H 2 [PtCl 6 ]/TH suspended in
VISIBLE LIGHT PHOTOCATALYSIS 379