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Chemistry Letters Vol.36, No.3 (2007)
Photocatalytic Steam Reforming of Methane over Platinum-loaded Semiconductors
for Hydrogen Production
Hisao Yoshida,ꢀ1 Satoru Kato,2 Kazuhisa Hirao,2 Jun-ichi Nishimoto,2 and Tadashi Hattori2
1Division of Environmental Research, EcoTopia Science Institute, Nagoya University,
Furo-cho, Chikusa-ku, Nagoya 464-8603
2Department of Applied Chemistry, Graduate School of Engineering, Nagoya University,
Furo-cho, Chikusa-ku, Nagoya 464-8603
(Received November 22, 2006; CL-061383; E-mail: h-yoshida@esi.nagoya-u.ac.jp)
Hydrogen can be photocatalytically produced from methane
50% methanol by photodeposition method. Two samples of lan-
8
and water by using platinum-loaded semiconductor photocata-
lysts. Platinum-loaded lanthanum-doped NaTaO3 (Pt/NaTaO3:
La) showed higher photocatalytic activity for this reaction than
platinum-loaded TiO2 did. The apparent quantum yield of this
system employing Pt/NaTaO3:La was estimated to be higher
than that of photocatalytic water splitting system employing
NiO/NaTaO3:La.
thanum-doped NaTaO3 (NaTaO3:La) were prepared through
solid-phase reaction at 1373 K from La2O3, Na2CO3, and Ta2O5
where doping amount of La was 2%; for one sample these start
materials were mixed by a conventional mortar, and for another
sample they were mechanically mixed well by wet ball-milling
using acetone, then both were followed by calcination at
1373 K, referred to as NaTaO3:La(C) and NaTaO3:La(BM),
respectively. The former had higher BET specific surface area
(3.6 m2 gꢂ1, calculated from N2 adsorption) and smaller average
crystallites size (50 nm, calculated from X-ray diffraction) than
those of the latter (1.9 m2 gꢂ1 and 62 nm, respectively). Pt was
loaded on NaTaO3:La by impregnation method using an aqueous
nitric acid solution of Pt(NO2)2(NH3)2 followed by calcination
in dry air and H2 reduction at 773 K. NiO was loaded by impreg-
nation method using an aqueous Ni(NO3)2 solution followed by
calcination at 543 K.8 The semiconductor photocatalysts loading
metal or metal oxide are referred to as, e.g., Pt(x)/TiO2 and
NiO(x)/NaTaO3:La, where x is the loading amount (wt %) of
metal or metal oxide.
Reaction tests were carried out in a fixed-bed flow-type re-
actor. The catalysts were granulated to the size of 400–600 mm.
A thin quartz cell (60 ꢃ 20 ꢃ 1 mm3) was filled with the photo-
catalyst sample. As for the NaTaO3:La samples, the sample was
diluted by quartz granules to fill the cell. Prior to the photoreac-
tion test, to clean the catalyst surface, the catalyst was photoirra-
diated by a 300-W Xe lamp in a flow of H2O vapor (3%) with Ar
carrier. H2O vapor was introduced by bubbling the carrier gas
through a saturator containing distilled water. The reaction
gas, a mixture of H2O vapor and CH4 with Ar carrier, was
introduced into the reactor at a flow rate of 50 mLꢁminꢂ1, and
the reaction was carried out without heating at atmospheric
pressure upon photoirradiation. The temperature of the reactor
was raised up to around 348 K by the photoirradiation. The inci-
dent light intensity was measured by a photodiode (TOPCON:
UD-25, UVR-2) in the range of 230–280-nm wavelength. The
outlet gas was analyzed by on-line gas chromatography with a
thermal conductivity detector.
Hydrogen is an environmentally benign fuel, and it is well
accepted that the hydrogen should be produced from renewable
resources and natural energy for the sustainable society in the
near future. Methane is not only used as a fuel and a chemical
resource, but it also attracts much attention as a hydrogen re-
source due to the highest H/C value among hydrocarbons, and
a system for steam reforming of methane including water–gas
shift reaction has been established for hydrogen production.1
Although methane is converted to CO2 in the steam reforming
of methane, CO2 could become biomass through photosynthesis
and the biomass could be again converted to methane by the
aid of biotechnology or biomass technology. Thus, methane
is essentially recognized as one of the renewable resources.
However, it is known that the steam reforming system is endo-
thermic reaction and requires high temperature, i.e., consuming
huge energy.
Photocatalytic system can function even at room tempera-
ture by using photoenergy, implying utilization of solar energy,
where photoenergy compensates an increase of Gibbs free ener-
gy of the reaction system. Thus, up-hill type reaction (ꢀG > 0)
can proceed even at low temperature. It has been well known
that the photocatalytic reactions to produce hydrogen could
proceed between water and many kinds of reductants such as
alcohols,2 CO,3 ethene,4 and some kinds of carbon-related
materials such as active carbon,4,5 coal, tar sand,6 and also some
kinds of biomass such as sugar, starch, and cellulose.7 Compared
with them, methane, as the most stable hydrocarbon, gives a
largely positive Gibbs free energy for this kind of reaction:
Photocatalytic activities of TiO2 and Pt(0.1)/TiO2 samples
were examined in the flow of CH4 and H2O vapor. TiO2 gave
only a trace amount of H2 in the present condition. However,
H2 and CO2 were obtained as main products on Pt(0.1)/TiO2
(Table 1, Entry 1) with a trace amount of C2H6 (0.014
mmolꢁminꢂ1) and CO (0.003 mmolꢁminꢂ1) as by-products, while
no O2 was detected. The molar ratio of H2 to CO2 in the
outlet gas was about four, suggesting that the main reaction
would be PSRM as shown in eq 1.
2H2O(g) þ CH4 ! 4H2 þ CO2; ꢀG298K ¼ 113 kJꢁmolꢂ1: ð1Þ
To our knowledge, methane has never been applied to
the photocatalytic reaction with water. Here, we studied a new
photocatalytic system producing hydrogen from methane and
water, i.e., photocatalytic steam reforming of methane (PSRM),
in a mild condition upon photoirradiation.
TiO2 (JRC-TIO-8, anatase, 338 m2 gꢂ1) was supplied from
the Catalyst Society of Japan, and Pt/TiO2 (335 m2 gꢂ1) was
prepared from the TiO2 and an aqueous H2PtCl6 solution with
During the reaction the color of the catalyst changed to pale
Copyright ꢀ 2007 The Chemical Society of Japan