Photon Factory Activity Report 2005 #23Part B (2006)
Surface and Interface

10B/2003G248

Photocatalytic Active Sites of Silica-supported Gallium Oxide
for Non-oxidative Direct Methane Coupling

Leny YULIATI1, Tadashi HATTORI1, Hisao YOSHIDA*2
Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan.
2
EcoTopia Science Institute, Nagoya University, Nagoya 464-8603, Japan.

Introduction
Gallium oxide, in which gallium can be found as
tetrahedral and/or octahedral coordination states, has beenreported to be important for many catalytic reactions,such as reduction of NO and alkane dehydrogenation.1,2 It
was found that the local structure of gallium oxide
affected the catalytic activity for NO reduction.1
On the other hand, XANES spectroscopy has been
proposed to be a good method to analyze the coordination
states of gallium oxides quantitatively.3 In the present
study, a series of silica-supported gallium oxide samples
were tested for photocatalytic non-oxidative direct
methane coupling and Ga K-edge XANES spectroscopy
was carried out in order to reveal the local structure ofphotoactive sites in the samples.
Experimental
As support we used amorphous silica that was preparedby sol-gel method, followed by calcining at 773 K for 5 h,with specific surface area was 554 m2g-1. Silica-supported
gallium oxide samples, Ga/SiO2(x), were prepared byimpregnation of the silica support with an aqueous
solution of gallium nitrate, drying at 383 K in an oven
overnight and calcination at 773 K for 5 h, where x
showed mol% of Ga to total cation of Ga and Si. β-Ga2O3
was commercially obtained and employed as a referencecompound.
The Ga K-edge XANES measurement was performedat room temperature in transmission mode (Ga > 1 mol%)and fluorescence mode (Ga < 1 mol%) at the BL-10B
station of KEK-PF, Tsukuba, Japan, with a Si(311)channel-cut monochromator. Before recording XANESspectra, the sample was pretreated with 13.3 kPa ofoxygen for 1 h at 1073 K, followed by evacuation for 1 h
at 1073 K. Then the sample was sealed with apolyethylene film in a dry atmosphere. The photocatalytic
test was carried out as in the previous study.4 Before
photoreaction, the samples were treated as thepretreatment before recording the XANES spectra.
Results and discussion
For analysis of XANES spectra, β-Ga2O3 was used as areference, where the ratio of tetrahedral (Td) to octahedral(Oh) species was reported to be near to one.3 Ga(Td)species exhibits an absorption peak at 10375.2 eV, whileGa(Oh) species exhibits an absorption peak at 10379.1 eV,
as shown in Fig.1. The samples with low loading of Ga
on silica support showed the dominant peak of Ga(Td),50

Users' Reportsuggesting the presence of Ga(Td) as the main species,
while samples with high loading of Ga showed thepresence of both Ga(Td) and Ga(Oh) species. The ratio oftetrahedral to octahedral species obtained bydeconvolution using the curve-fitting method3 is also
shown for each sample in Fig. 1.
Photoreaction results showed that the silica-supported
gallium oxides samples exhibited higher activity thanTd Oh

Ga (Td)/Ga(Oh)Normalized intensity

110370

10380

Ga2O3

1.0

Ga/SiO2(10)1.5Ga/SiO2(5)1.6

Ga/SiO2(2)2.0

Ga/SiO2(1)2.1

Ga/SiO2(0.5)

2.5Ga/SiO2(0.1)

3.3

10390

X-ray energy / eVFig. 1 Normalized Ga K-edge XANES spectra of Ga/SiO2
samples and Ga2O3 as a reference compound.
TheGa(Td)/Ga(Oh) for each sample is also shown.

silica support. The main product was ethane. It wasconfirmed that the hydrocarbons and hydrogen were
produced almost stoichiometrically over samples withlow loading of Ga. However, in the case of high loading
samples, the amount of detected hydrogen was lower than
the amount calculated from detected hydrocarbons. It
seemed that the hydrogen would be consumed during thephotoreaction. Thus, it is proposed that the highly
dispersed Ga(Td) species is the photocatalytic active sites
for non-oxidative direct methane coupling.
References[1] K. Shimizu et al., J. Phys. Chem. B, 103 1542 (1999).
[2] Y. Ono, Catal. Rev. Sci. Eng., 34, 179 (1992).
[3] K. Nishi et al., J. Phys. Chem. B, 102, 10190 (1998).
[4] L. Yuliati et al., Phys. Chem. Chem. Phys.,7, 195 (2005).

*h-yoshida@esi.nagoya-u.ac.jp