of ZrO2/SiO2(0.01), ZrO2/SiO2(0.1), and ZrO2/SiO2(0.5)
showed a shoulder band at 300 nm (Fig. 1B b–d). Other
samples, silica (Fig. 1Ba) and ZrO2/SiO2(1.0) (Fig. 1Be), did
not exhibit such a band. These results suggest that the band in
the excitation spectrum corresponds to the fine structural
emission band centred around 520 nm. These mean that the
photoactive sites on ZrO2/SiO2 samples containing moderate
amounts of zirconium oxide are excited by the light of around
300 nm and emit the intense phosphorescence centred around
520 nm.
promoted by the surface photoexcited sites, it is very likely that
the highly dispersed zirconium oxide species exhibiting the fine
structure in photoluminescence spectra are the highly active
sites for this photo-induced reaction.
In conclusion, highly dispersed zirconium oxide species
having Zr–O–Si linkage on silica exhibit fine structure in
photoluminescence spectra, and the vibration energy of the Zr–
O–Si linkage was confirmed as 955 cm21. In addition, the
highly dispersed zirconium species were found to promote
photo-induced non-oxidative methane coupling.
This work was partly supported by a Grant-in-Aid for
Exploratory Research from the Ministry of Education, Culture,
Sports, Science and Technology (MEXT), Japan. H.Y. ac-
knowledges the Nitto Foundation and Japan Chemical Innova-
tion Institute (JCII) for the financial supports.
In the case of TS-1 (Ti-silicalite),4 only the samples of the
low Ti content (typically 0.5 mol% of Ti) show the fine
structural photoluminescence spectra. Also in the case of silica–
alumina,16 only isolated tetrahedral Al species in silica matrix
(less than 20 mol% of Al) show clearly the fine structure on the
photoluminescence spectra. In the present study, the fine
structural phosphorescence spectra were observed on the
samples of low zirconium content such as 0.01 and 0.1 mol%
(Fig. 1Ab and 1Ac). Thus, it is suggested that the luminescence
species are the highly dispersed zirconium oxide species. An
increase of Zr loading of more than 0.1 mol% reduced the
emission intensity and diminished the fine structure, probably
due to the formation of zirconium oxide aggregates at the
expense of highly dispersed zirconium oxide species.
Notes and references
† x = NZr/(NZr+NSi) 3 100 (mol%), NM is the number of M atoms in the
sample.
‡ The average is calculated as the mean of the middle five values, ignoring
the first and the last values. When all seven values are used for the
calculation, the average is 953 cm21
.
The vibration energies of the excited sites were calculated
from the difference between each maximum of the fine structure
in the emission spectra (Fig. 1Ac) to be 1044, 1006, 919, 930,
927, 991 and 852 cm21, respectively. Since the error of each
maximum value was estimated ca. ±50 cm21, the average
value‡ of 955 cm21 was employed as the vibration energy of the
luminescence moiety on the ZrO2/SiO2(0.1). This value is close
to that for Ti–O–Si in TS-1 (965 cm21)4 and Al–O–Si in silica–
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We examined the activity of the representative samples of
ZrO2/SiO2 in the photoinduced non-oxidative methane cou-
pling. On the silica sample, only a small amount of C2H6 (0.021
C%) was obtained. The ZrO2/SiO2(0.1) sample, which ex-
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