Synergy effect of graphite silica and TiO2 on photocatalytic hydrogen production

Article abstract of DOI:10.1246/cl.2005.1106

The photocatalytic production of hydrogen gas from water-methanol mixtures was performed in contact with a suspended mixture of titanium dioxide (TiO ₂) and graphite silica (GS), one of natural minerals. A drastic synergy effect for hydrogen production was found for GS in collaboration with TiO₂. It is suggested that the clay fraction in GS is one of the key components for the synergy effect. Copyright

Full text of DOI:10.1246/cl.2005.1106

1106
Chemistry Letters Vol.34, No.8 (2005)
Synergy Effect of Graphite Silica and TiO2 on Photocatalytic Hydrogen Production
Miyuki Ikeda and Yoshihumi Kusumotoꢀ
Department of Chemistry and Bioscience, Faculty of Science, Kagoshima University,
1-21-35 Korimoto, Kagoshima 890-0065
(Received May 16, 2005; CL-050645)
ꢀ
matograph (detector; TCD, column packing; MS5 A or Porapak
The photocatalytic production of hydrogen gas from water–
methanol mixtures was performed in contact with a suspended
mixture of titanium dioxide (TiO2) and graphite silica (GS),
one of natural minerals. A drastic synergy effect for hydrogen
production was found for GS in collaboration with TiO2. It is
suggested that the clay fraction in GS is one of the key compo-
nents for the synergy effect.
N, carrier gas; Ar). The photoirradiation was provided by a su-
per-high-pressure mercury lamp (Ushio 500-W USH-500SC)
with an optical band pass filter (300–400 nm, Toshiba ATG
UV-D33S).
Figure 1 shows the amount of hydrogen gas from suspended
mixtures of TiO2 with GS, Pt, SiO2, calcined GS or clay in GS.
The results on TiO2 alone and platinized TiO2 (0.3 wt % Pt/
TiO2)8 alone are also included in Figure 1. Here it should be not-
ed that GS itself produces negligible H2 gas. The amount of hy-
drogen gas was increased by a factor of ca. 100 by adding GS to
TiO2 suspension. This shows that the photocatalytic hydrogen
production was enhanced by the synergy effect of TiO2 and
GS. The hydrogen evolution in the presence of platinized TiO2
under our experimental conditions was studied for comparison.
The evolution rate of H2 gas with the powdered GS–TiO2 mix-
ture was about one-eighth of that with Pt/TiO2. Since GS could
not be deposited on TiO2 in this study, the mixture of powdered
Pt and TiO2 was also examined. The synergy effect by GS was
comparable to that by powdered Pt. Therefore, the GS powder
is considered to bear comparison with cocatalytic Pt in the
photocatalytic activity.
SiO2 is the main chemical composition of GS. An addition
of powdered SiO2 (Wako) to TiO2 suspension did not enhance
the photocatalytic hydrogen production, but it was rather detri-
mental as seen in Figure 1.
The powder of GS with almost negligible carbon was pre-
pared by calcination in air. The pale-pink colored powder was
obtained by calcination at 923 K. The calcined GS had ability
to enhance the hydrogen gas evolution, almost equal to the un-
treated powder of GS. This result suggests that the carbon in
GS does not play an important role in the synergy effect between
TiO2 and GS.
The practical hydrogen production using a photocatalyst is
one of dreams for mankind. For the practical production of hy-
drogen gas,1–5 it is necessary to find a low-cost cocatalyst instead
of expensive one such as platinum. We are strongly concerned
with GS as a low-cost cocatalyst. GS is a natural mineral. It is
mined in the hills in Kaminokuni Town, Hokkaido, Japan.6
The cost of GS is much lower than platinum. Table 1 shows
the typical specific composition of GS which was estimated by
using the norm method,7 on the basis of X-ray diffraction6 and
X-ray fluorescence analysis.
Here we report the photocatalytic hydrogen production
drastically enhanced by GS in collaboration with TiO2 in
water–methanol mixtures and propose a plausible mechanism
for the synergy effect of GS and TiO2. To our best knowledge,
this is the first report showing the drastic synergy effect by using
a naturally produced mineral, GS.
The amount of photocatalytic hydrogen production from
water–methanol mixtures was measured by following proce-
dures. 1) A mixture of powdered TiO2 (15 mg, Nippon Aerosil
P25) and an additive such as GS (15 mg) was added to
40 vol % aqueous methanol (20 cm3) in a batch photoreactor of
a cylindrical flask (154 cm3) whose top was sealed with a sili-
cone rubber septum. 2) To remove oxygen gas, the suspended
solution was bubbled with Ar gas (about 1 mL/min) for 1 h after
sonication for 1 min. 3) Photoirradiation was carried out under
an Ar atomosphere of about 1 atm with stirring. 4) The evolved
gas was sampled through the silicone rubber septum by using a
locking-type syringe at a constant time interval, usually 15 min.
5) The sampled gas was quantitatively analyzed by a gas chro-
Table 1. Composition of GS
Name
Chemical formula
Content/wt %
Quartz
Sericite
Carbona
Dolomite
Kaolinite
Hematite
Rutile
SiO2
K2Al6Si6O22
C
(Ca,Mg)CO3
Al2SiO2
Fe2O3
67.9
18.9
5.80
3.50
1.77
0.87
0.77
0.48
Figure 1. Comparison of hydrogen production. Each powdered
additive (15 mg) was added to powdered TiO2 (15 mg). For TiO2
alone and Pt/TiO2 alone systems, the total amount of 30 mg was
used. Note the logarithmic abscissa.
TiO2
FeS2
Pyrite
aContains graphite.
Copyright ꢀ 2005 The Chemical Society of Japan

Chemistry Letters Vol.34, No.8 (2005)
1107
Figure 4. a: Ion exchanges between metal ions and hydrogen
ions by the clay in GS. b: Aggregation of both particles of GS
and TiO2. c: Increment of hydrogen production by the photoca-
talytic reduction of hydrogen ions held in the clay. eꢁ: Electron.
hþ: Positive hole.
Figure 2. Time variation of pH of water containing the GS
powder.
The clay fraction in GS such as kaolinite was separated by
the following method.9 At first, the powder of GS was calcined
at 923 K for 5 h in air. Then, the calcined powder was dispersed
in water by ultrasonication for 30 min. After the suspension was
allowed to stand in a dark place for 6 h, its suspension was sep-
arated into two layers of the supernatant and the sediment. The
supernatant was centrifuged at 3500 rpm for 15 min. After the
precipitate was dried on standing, the pale-pink colored clay
was obtained. The clay in GS showed the synergy effect compa-
rable to the powders of calcined and uncalcined GS as shown in
Figure 1. This fact indicates that the clay in GS is mainly respon-
sible for the synergy effect of GS and TiO2 for the hydrogen
production.
Clay is one of the functional materials in nature. It is gener-
ally known that clay has an ion-exchange property. To investi-
gate the ion-exchange property of the clay in GS, the variations
of pH with time and the amount of GS were measured. Figure 2
shows the time variation of pH of 40-cm3 water containing
30 mg of GS. The pH increased with time and reached equilibri-
um pH of about 8 after ca. 10 min. This pH variation strongly
suggests that the presence of GS results in the exchange of
cations between the metal ions in the clay and hydrogen ions
in water.
mmol cmꢁ3 at about 100 mg of GS. This result suggests that
the metal ions at all active ion-exchange sites are replaced by
the hydrogen ions in the solution used for hydrogen evolution.
Taking these results into consideration, the reaction model
composed of the various stages is illustrated as shown in
Figure 4. In Figure 4a, the clay in GS exchanges cations between
the metal ions in the clay and the hydrogen ions in water. In
Figure 4b, the hydrophilic particles of GS will be agglutinated
with the particles of TiO2 bearing the superhydrophilic surface
under photoirradiation.10 In Figure 4c, it is considered that the
hydrogen ions held in the clay are the main source of hydrogen
gas.
Though 15 mg of GS contains the clay components of about
3 mg, the cocatalytic property of GS itself was comparable to
that of 15 mg of the clay. This suggests that the synergy effect
of GS and TiO2 is not fully induced by the clay alone. Further
studies are needed to clarify all aspects of the mechanism of
the synergy effect and the surface structure of GS.
The authors are grateful to Emeritus Professor Katsutoshi
Tomita and Dr. Masamichi Hoashi for helpful discussions. We
thank Nishi-Nihon Environmental Engineering Inc. for their
kind supply of the GS powder.
Figure 3 shows the GS-amount dependence of the equilibri-
um pH and the amount of the exchanged hydrogen ions (Mp)
which was estimated from the pH values. The Mp increased with
the amount of GS and became nearly constant at about 1.5
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Figure 3. The dependence of the equilibrium pH and Mp on the
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Published on the web (Advance View) July 2, 2005; DOI 10.1246/cl.2005.1106