10.1002/cssc.201802799
ChemSusChem
FULL PAPER
Conclusions
LIII-edge XAFS (X-ray absorption fine structure) of the Pt-loaded
samples were recorded at BL-9C of the Photon Factory at the
High Energy Accelerator Research Organization (KEK, Tsukuba
Japan) with a Si(111) double-crystal monochromator in a Quick
XAFS mode. An ion chamber of the I0 detector was filled with a
flow of N2(85%)/Ar(15%). X-ray fluorescence from the sample
was recorded by using a Lytle detector filled with an Ar(100%)
flow with a Ga filter (μt=6). The spectra were analyzed with a REX
2000 software (Rigaku).
The photocatalytic reaction tests for water vapor splitting
(2H2O ➝ 2H2 + O2) were carried out with a fixed-bed flow reactor.
The photocatalyst was pressed at 24 MPa and granulated to the
size of 300–600 mm. The quartz cell (60×20×1 mm3) was filled
with a mixture of the photocatalyst (0.8 g) and required amount of
quartz granules to fill the cell. The water vapor (1.5%) with argon
carrier was introduced into the reactor at the flow rate of 40 mL
min−1 and the reaction was carried out upon photoirradiation with
the 300 W xenon lamp without using any optical filters, where the
light intensity measured in the range of 245±10 nm was 10 mW
cm−2. The temperature around the reaction cell rose up to ca. 308
K upon photoirradiation. The outlet gas was analyzed by on-line
In the present study, it was found that the Pt cocatalyst can
function on the CaTiO3 photocatalyst to produce hydrogen and
oxygen in the photocatalytic water vapor splitting when it was
loaded by the conventional impregnation method around 0.01
wt% or by the photodeposition method with very low Pt loading
amount. The Pt species loaded in the oxidized state are
catalytically inactive for the reverse reaction of water splitting,
which enables to provide the hydrogen and oxygen in the
photocatalytic water vapor splitting.
Experimental Section
A powder CaTiO3 sample was prepared by a solid-state reaction
method reported in the previous study [21]. Starting materials,
CaCO3 (Kojundo, 99.99%) and rutile TiO2 (Kojundo, 99.9%), were
dried at 383 K for 2 h, weighted in equimolar ratio, and mixed
physically by a wet ball-milling method with acetone for 24 h. The
obtained powder was dried at 343 K overnight, calcined on an
alumina boat in air at 1273 K for 10 h, and grained.
gas chromatography with
a thermal conductivity detector
(Shimadzu, GC-8A, Shincarbon ST, 12 m).
Two methods were used for loading Pt cocatalyst on the
The catalytic reaction tests for the reverse reaction of water
splitting (2H2 + O2 ➝ 2H2O) without photoirradiation were carried
out in a closed reactor of 62 mL, which was connected to a
vacuum system. A part (0.3 g) of the photocatalyst sample used
for the photocatalytic reaction test of water vapor splitting was
recovered and introduced in the reactor followed by evacuation at
318 K for 0.5 h. Since the temperature of the photocatalyst
surface during the photocatalytic reaction test mentioned above
was presumed to be higher than that of the outside of the reaction
cell, the reaction temperature was maintained at 318 K for this
catalytic reverse reaction test. And then hydrogen and oxygen (3
and 1.5 kPa, respectively) were introduced at 318 K, which was
the start of the reaction time. The decrease of the total pressure
of the reactant gases with time was monitored by an absolute
pressure transducer (MKS Baratron).
CaTiO3 sample, i.e., an impregnation method and
a
photodeposition method. The precursor employed was
H2PtCl6∙H2O (Wako 99.9%) in both methods. The loading amount
was in the range of 0.005–0.2 wt%, which values are based on
the weight of the starting materials used. In the impregnation
method, 2.0 g of the CaTiO3 powder was impregnated with 50 mL
of an aqueous H2PtCl6 solution of a desired concentration, stirred
in a rotary evaporator at 120 rpm for 30 min, and continuously
stirred at 331 K under evacuation at 120 mbar for 2.5 h, followed
by drying at 343 K in an electric oven overnight. Thus obtained
powder was calcined on an alumina boat in air at 773 K for 6 h. In
the photodeposition method, 360 mL of an aqueous H2PtCl6
solution containing 2.0 g of the CaTiO3 powder was magnetically
stirred in a 500 mL beaker covered with a quartz watch glass
under photoirradiation by using a 300 W xenon lamp (PE300BUV)
from the upper side for 0.5 h, followed by the addition of 40 mL
methanol and stirring for 1 h in dark. Then, the suspension was
photoirradiated for 2 h with stirring, and filtered by a membrane
filter. The powder was washed by hot water, and dried in the oven
overnight at 343 K.
Field emission scanning electron microscopy (SEM) image
was recorded by a S-5200 (HITACHI) with the accelerating
voltage of 1.0 kV and the emission current of 10 μA. Powder X-
ray diffraction (XRD) pattern was recorded at room temperature
on a MiniFlex-II/AP (Rigaku) using Cu Kα radiation (30 kV, 15 mA).
Diffuse reflectance UV-visible spectrum was recorded at room
temperature on a V-640 (JASCO) equipped with an integrating
sphere covered with BaSO4, where BaSO4 was used as the
reference. The Brunauer–Emmett–Teller (BET) specific surface
area was calculated from the amount of nitrogen adsorption at 77
K, which was measured by a Monosorb (Quantachrome).
Acknowledgements
The SEM measurements were carried out by the support of Nano-
fabrication platform at Venture business laboratory (VBL),
Nagoya University, under Nanotechnology platform, the Ministry
of Education, Culture, Sports, Science and Technology (MEXT),
Japan. The Pt LIII-edge XAFS experiments were performed under
the approval of the Photon Factory Program Advisory Committee
(Proposal No. 2011G575, 2012G586, and 2014G548).
This work was partially supported by a Grant-in-Aid for
Scientific Research (C), (No.21560799), a Grant-in-Aid for
Scientific Research (B) (No. 25289285), and a Grant-in-Aid for
Scientific Research on Innovative Areas “Artificial photosynthesis
(AnApple)” (No. 25107515) from the Japan Society for the
Promotion of Science (JSPS).
Transmission electron microscope (TEM) images were
recorded by a JEOL electron microscope (JEM-2100M, 200 kV)
equipped with a CCD camera (Gatan, erlangshen ES500W). Pt
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