Chemistry Letters Vol.33, No.12 (2004)
1611
100
90
80
70
60
50
40
30
20
10
0
Propanal
A
B
Hexanal
Propanoic acid
Hexanoic acid
10.0
8.0
6.0
4.0
2.0
0.0
SiO -TiO
SiO -TiO -C
10
SiO -TiO
SiO -TiO -C
10
2
2
2
2
2
2
2
2
0
20
40
60
80
100
Acetonitrile / vol%
Figure 2. Photocatalytic activities for oxidation of aldehydes
on TiO2 poweders. A: Oxidation of propanal, B: Oxidation of
hexanal. SiO2–TiO2–C10: surface modified TiO2, SiO2–TiO2:
porous silica covered pure TiO2.
Figure 1. Dependence of amount of floating particles on the
volume fraction of acetonitrile in water. Closed circles, SiO2–
TiO2–C10; Closed squares, SiO2–TiO2.
silyl groups, are currently being investigated.
toirradiated under aerated conditions. Photoirradiation was per-
formed using a super-high-pressure mercury lamp (Wakom
BMS-350S, 350 W) from the top of a cylindrical reaction vessel
(transparent at >300 nm, 2.5 cm in diameter) at room tempera-
ture. The intensity of the incident light was 7.0 mW cmꢀ2. The
reaction mixture was agitated vigorously with a magnetic stirrer
during photoirradiation. Decrease of aldehydes and calboxylic
acids generated in the aqueous solution were analyzed using a
capillary gas chromatograph equipped with an RTx-5 capillary
column.
Figure 2 shows the photocatalytic activities of TiO2 powders
for oxidation of aldehydes by irradiation for 1 h at room temper-
ature. Using any kind of aldehyde, the main products are calbox-
ylic acids (propanoic acid or hexanoic acid). When hexanal was
used as a substrate, photooxidation proceeded on SiO2–TiO2
photocatalysts with a fairly high yield. Marked acceleration
was observed when SiO2–TiO2–C10 was used as a photocatalyst.
The photocatalytic activity level of SiO2–TiO2–C10 was about
three-times higher than that of SiO2–TiO2. The hydrophobic in-
teraction between hexanal and alkylsilyl groups on the surfaces
of TiO2 particles is thought to be the most important factor for
improving their reactivity. On the other hand, enhancement of
the activity of SiO2–TiO2–C10 was not observed in the case of
propanal. The hydrophobic interaction between propanal and al-
kylsilyl groups is very weak because propanal is less hydropho-
bic than is hexanal. These results were supported by the differ-
ence in adsorbtivities of aldehydes on SiO2–TiO2–C10 and
SiO2–TiO2. The adsorbtivities of hexanal on SiO2–TiO2–C10
and SiO2–TiO2 were about 9.1 and 3.2 mmol dmꢀ3/100 mg, re-
spectively. On the other hand, no difference was found in the ad-
sorbtivities of propanal between on SiO2–TiO2–C10 and SiO2–
TiO2, the values being 2.6 and 2.4 mmol dmꢀ3/100 mg, respec-
tively.
This work was supported by a Grant-in-Aid for Scientific
Research on Priority Areas (417) from the Ministry of Educa-
tion, Culture, Sports, Science and Technology (MEXT) of the
Japanese Government.
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Published on the web (Advance View) November 24, 2004; DOI 10.1246/cl.2004.1610