Highly dispersed magnesium oxide species on silica as photoactive sites for photoinduced direct methane coupling and photoluminescence

Article abstract of DOI:10.1039/b410089a

Photoinduced direct methane coupling proceeded around room temperature over highly dispersed magnesium oxide species on silica, which exhibited fine structure in photoluminescence emission spectra. It was found that increasing the emission intensity tends

Full text of DOI:10.1039/b410089a

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R E S E A R C H P A P E R
Highly dispersed magnesium oxide species on silica as photoactive
sites for photoinduced direct methane coupling and
photoluminescence
a
a
b
Leny Yuliati, Tadashi Hattori and Hisao Yoshida*
a
Department of Applied Chemistry, Graduate School of Engineering, Nagoya University,
Furo-cho, Chikusa-ku, Nagoya, 464-8603, Japan
Division of Environmental Research, EcoTopia Science Institute, Nagoya University, Furo-cho,
Chikusa-ku, Nagoya, 464-8603, Japan. E-mail: h-yoshida@esi.nagoya-u.ac.jp;
Fax: þ81-52-789-5849
b
Received 2nd July 2004, Accepted 25th October 2004
First published as an Advance Article on the web 15th November 2004
Photoinduced direct methane coupling proceeded around room temperature over highly dispersed magnesium
oxide species on silica, which exhibited fine structure in photoluminescence emission spectra. It was found that
increasing the emission intensity tends to give an increase in the photoactivity for this reaction. The emission sites
ꢀ1
in the silica-supported magnesia have vibrational energy around 950 cm and lifetime of excited state around
8 ms, which were similar properties to the previously reported other silica-based photoactive systems for this
3
reaction, such as silica–alumina and silica-supported zirconia. These photoluminescence spectra could be similarly
quenched by methane molecules. Thus, it is commonly suggested that in the systems of highly dispersed metal
oxide species (MO ) on silica, the surface Si–O–M bonds are deeply related to the dominant photoactive sites
x
for both the fine structural photoluminescence spectra and photoinduced direct methane coupling.
the unique emission sites are suggested to be the active sites for
photoinduced non-oxidative methane coupling.
Silica–magnesia is well known as acid and base catalyst for
Introduction
Direct production of higher hydrocarbon from methane is one
of the most attractive routes for the efficient use of natural gas
and has been investigated over the past two decades. Especially
the oxidative coupling of methane (OCM) has been extensively
studied. However, it tends to produce CO
reached the criteria for industrial application.
researches in the field have been gradually weakened. On the
other hand, the movement of fossil fuels age to future hydro-
gen energy age makes the non-oxidative direct methane con-
version become one of the alternative approaches that still
attracts the attention of many researchers. However, the
conversion of methane to higher valuable chemicals such as
ethane and hydrogen under non-oxidative condition is thermo-
dynamically unfavorable and very difficult at low temperature.
In our research, this became feasible by using photoenergy
even at room temperature. It was reported that photoinduced
non-oxidative methane coupling proceeded at room tempera-
13
dehydration of alcohol, aldolic condensation of ketone, etc.
14
Due to the discovery of photoluminescence spectra exhibited
15,16
by silica-supported magnesia,
some studies in photocata-
x
, and no catalyst has
lytic reactions have been examined using silica-supported
magnesia as a photocatalyst. It has been reported that silica-
supported magnesia showed activity in photooxidation of
1
,2
Thus, the
15
CO and photoepoxidation of propene by using molecular
17–19
oxygen.
In the present study, we found that silica-sup-
ported magnesia exhibited activity in the photoinduced direct
methane coupling. Then, by photoluminescence spectroscopy
we investigated the photoactive sites in silica-supported mag-
nesia and discussed the photocatalytic active sites. In addition,
we considered a general aspect for photoactive sites in the
systems of highly dispersed metal oxide on silica.
1
,3
ture without any formation of CO over silica-based materials
x
4
–6
7,8
silica-supported zirconia, silica–
Experimental
such as silica–alumina,
,10
9
11
and H-zeolites. The starting of this reac-
alumina–titania,
tion was:
Materials
Amorphous silica was synthesized from Si(OEt)
method, followed by calcination in a flow of dry air (1 cm s
at 773 K for 5 h. MgO/SiO
MgO were prepared by impregnation of the silica (554 m g
with an aqueous solution of Mg(NO O (Kishida). The
water was slowly removed by stirring the mixture on a hot
plate temperature of which was 393 K. Then, the sample was
dried at 393 K overnight in an oven, followed by calcination in
4
by a sol–gel
3
ꢀ1
, DG ¼ 68.6 kJ molꢀ
1
,
)
2CH
4
- C
2
H
6
þ H
2
1
7
2
samples with different amount of
2
ꢀ1
)
where the photoenergy is converted to chemical potential of
product.
3
)
2
ꢁ 6H
2
Photoluminescence spectroscopy is a good method for ob-
taining significant information about the structures of photo-
active sites in the catalyst. Especially because of its high
sensitivity, it is applicable for samples with low concentration,
3
ꢀ1
a flow of dry air (1 cm s ) at 773 K for 5 h. The Mg content (x
mol%) was defined as x ¼ NMg(mol)/(NMg(mol) þ N (mol)) ꢂ
Si
1
2
e.g. o1%. In the case of silica–alumina and silica-supported
zirconia, it was found that the catalysts exhibiting the char-
acteristic fine structure in photoluminescence spectra showed
much higher activity in photoinduced direct methane coupling
2
100 and referred to as to MgO/SiO (x). For comparison, silica–
alumina (SiO –Al O , Al ¼ 9.1 mol%) and silica-supported
2
2
3
zirconia (ZrO
according to the methods in previous studies.
2
/SiO
2
, Zr ¼ 0.1 mol%) samples were prepared
4
,5,7,8
4,7
than the ones exhibiting no or broad emission band.
Thus,
T h i s j o u r n a l i s & T h e O w n e r S o c i e t i e s 2 0 0 5
P h y s . C h e m . C h e m . P h y s . , 2 0 0 5 , 7 , 1 9 5 – 2 0 1
195

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Photoreactions
supported magnesia samples, the main product was C
490%) which was obtained as a gaseous product. Trace
amounts of C and C were observed as adsorbed pro-
ducts that were detected through a thermal desorption proce-
dure at 573 K. After a 3-h irradiation, the production of H
could not be detected by gas chromatography in most cases
Table 1, entry 1–4) due to less sensitivity, but it was confirmed
after 24 h of irradiation (Table 1, entry 5). The stoichiometric
ratio of C and H formation is confirmed here (C /H
.96) within experimental error.
In order to compare the activity of the silica-supported
magnesia system to other silica-based systems, we prepared
SiO -Al and ZrO /SiO samples and applied them in the
2 6
H
(
The activities of catalysts were tested in the photoinduced
direct methane coupling in a similar way to the previous
2
H
4
2 6
H
4
–11
studies.
(
The sample (0.2 g) was spread on the flat bottom
2
14 cm ) of a closed quartz reaction vessel (30 cm ), and treated
3
2
with 100 Torr (1 Torr ¼ 133 Pa) oxygen for 1 h at 1073 K,
followed by evacuation for 1 h at 1073 K as pre-treatment.
After cooling to room temperature, 200 mmol of methane was
introduced and the photocatalytic reaction was carried out for
(
2
H
6
2
H
2 6
2
¼
0
3
1
h upon irradiation from a 300 W Xe lamp (used at 17.5 A,
5 V, intensity of light measured at 220–300 nm was ca. 10 mW
ꢀ2
cm ). The products in the gaseous phase were separately
collected and analyzed by gas chromatography. The products
that were strongly adsorbed on the catalyst were collected by
heating at 573 K for 15 min and analyzed by gas chromato-
graphy. Two kinds of detectors were used to analyze the
products. Thermal conductivity detector with Ar carrier was
applied for detection of hydrogen and the detection limit in this
case was estimated as 0.04 mmol. Flame ionization detector was
applied for detection of hydrocarbons and the measurements
were reproducible within 96%. The experimental error on the
reaction tests was estimated within 4%.
2
2
O
3
2
2
photoreaction under the same condition. The amount of
alumina and zirconia loading on silica were chosen to exhibit
the fine structural emission spectra and to show high activity in
4,7
the photoreaction. The difference in optimal loading amount
of metal cations on silica will be described in the Discussion
section. It was demonstrated that the silica-supported magne-
sia system showed a similar level of activity to silica–alumina
and silica-supported zirconia systems.
2
Structure of MgO/SiO samples
Characterizations
It was confirmed that there was no crystalline structure of
MgO observed from an X-ray diffraction pattern, even in the
sample with the highest Mg content (5 mol%). This indicates
that all samples may contain very small particles or less
crystallinity of MgO.
BET specific surface area of samples was calculated from the
amount of nitrogen adsorption at 77 K. Before the measure-
ment, the sample was heated at 673 K for 30 min in a flow of
He. The amount of N adsorbed was determined by a thermal
2
conductivity detector.
Phosphorescence spectra and decay curves were recorded at
The specific surface area of MgO/SiO samples are listed in
2
Table 1. They were not drastically different from that of the
pure SiO . The surface area per unit weight of SiO was almost
7
7 K by Hitachi F-4500 fluorescence spectrophotometer, using
2
2
a UV-cut filter (ltransmittance 4 330 nm) to remove the scattered
light from Xe lamp and an attachment for phosphorescence
measurement. Before recording phosphorescence spectra and
decay curves, 0.15 g of sample was treated with 100 Torr
oxygen for 1 h at 1073 K in a specially designed in situ cell,
followed by evacuation for 1 h at 1073 K. Then, the sample was
transferred to the optical part without exposing to air.
the same as the pure SiO . These indicate that the magnesium
2
oxide species on the samples were well dispersed on the silica
surface without significantly changing the surface area of silica
support. Similar results have been observed in the ZnO/SiO
2
2
0
system.
The amount of magnesium oxide species on the silica surface
ꢀ
2
corresponds to 1 atom nm for MgO/SiO (2). Monolayer
2
coverage of the highly dispersed magnesium oxide on the silica
surface corresponds to ca. 36 mol% (32 wt%) of magnesium
content. This means that the present samples have enough high
surface area to provide highly dispersed magnesium oxide
species on the silica surface. Besides, from XANES spectra it
was reported that silica-supported magnesia prepared by im-
pregnation of silica with aqueous solution of magnesium
nitrate (up to 30 wt% MgO) produced highly dispersed
Results
Photoactivity in direct methane coupling
The photoactivities of silica and silica-supported magnesia
samples in photoinduced direct methane coupling are listed
in Table 1. There were no CO molecules formed under this
x
non-oxidative photoreaction. All silica-supported magnesia
samples showed much higher activity than silica itself
1
8,21
magnesium oxide species on the silica surface. Thus, it is
considered that all of the samples in the present study would
(
Table 1, entry 1–4 and 6). On either the silica or the silica-
a
Table 1 Photoactivity of the samples in photoinduced non-oxidative direct methane coupling
Yield of gaseous
phase products/
Yield of thermally
desorbed products /10
c
ꢀ2
Yield of
d
ꢀ2
b
b
ꢀ2
1
0
C%
C%
H
2
/10 mmol
BET specific
surface area/m g
Total
yield/10 C% Experimental Theoretical
2
ꢀ1
ꢀ2
b
e
Entry Sample
C
2
H
6
C
H
3 8
C
2
H
4
C
2
H
6
3 6
C H
1
2
3
4
5
6
7
8
a
MgO/SiO
2
2
2
2
2
(0.3) 509
1.77
2.62
2.72
1.32
8.16
0.42
1.42
2.14
0.07
0.08
0.07
0.07
0.73
0.03
0.04
0.11
0
tr.
0
0
0
0
0
0
1.83
2.73
2.81
1.39
8.93
0.45
1.66
2.25
n.d.
n.d.
n.d.
n.d.
7.8
1.8
2.9
2.8
1.4
9.2
0.5
1.8
2.4
MgO/SiO
MgO/SiO
MgO/SiO
MgO/SiO
(0.5) 495
(2.0) 471
(5.0) 445
0.08
tr.
0.03
0.03
tr.
tr.
f
(0.5) 495
554
0.02
0
0.02
0
SiO
SiO
2
2
n.d.
n.d.
n.d.
-Al
2
O
3
(9.1) 517
(0.1) 500
tr.
0.02
0.09
tr.
0.13
0
ZrO
2
/SiO
2
Reaction temperature was ca. 310 K, sample was 0.2 g, initial methane was 200 mmol, irradiation time was 3 h. tr. ¼ trace, n.d. ¼ not
b
c
detected. Based on the initial amount of methane, analyzed by FID (reproducibility 96%). These products were desorbed by heating at 573 K for
d
e
1
5 min after photoreaction. Analyzed by TCD (limit detection 0.04 mmol). Based on the yield of gaseous phase and thermal desorption
f
products. Irradiation time was 24 h.
1
96
P h y s . C h e m . C h e m . P h y s . , 2 0 0 5 , 7 , 1 9 5 – 2 0 1
T h i s j o u r n a l i s & T h e O w n e r S o c i e t i e s 2 0 0 5

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have the structure of highly dispersed magnesia oxide species
on the silica surface.
Table 2 The maxima and their intervals of the fine structure in the
a
emission spectra of MgO/SiO
2
Maximum
The interval
to the next
maximum/cm
Photoluminescence spectra
4
ꢀ1
ꢀ1
No.
nm
ꢂ 10 cm
Photoluminescence excitation and emission spectra of MgO/
SiO samples evacuated at 1073 K are shown in Fig. 1. The
intensities of both excitation and emission spectra increased
with an increase of Mg loading amount, then decreased at
higher loading of Mg (more than 2 mol%). The excitation
1
2
3
4
5
6
7
8
a
435.8
454.8
475.4
499.6
523.6
548.4
579.4
613.8
2.2946
2.1988
2.1035
2.0016
1.9098
1.8235
1.7259
1.6292
959
953
1019
917
864
975
967
2
spectra of MgO/SiO samples showed two maxima around 240
2
and 300 nm (Fig. 1A). The band intensity at 300 nm was lower
than that at 240 nm. These two kinds of excitation maxima
suggested the presence of different types of magnesium species
on the silica surface or different excitation transitions. As the
shape of excitation spectra did not change by changing the
monitoring emission wavelength, it can be considered that
there would be only one kind of photoactive sites that has at
least two dominant absorption transitions from ground state to
2
MgO/SiO (2), recorded at 77 K, excitation wavelength was 300 nm.
value is in agreement with the vibrational energy described in
15
the previous work.
2
2
excited state.
As shown in Fig. 1B, MgO/SiO
weak and broad emission band centered around 440 nm wich is
2
(0.1) sample exhibited a
Interaction of methane molecules with the emission sites
Quenching experiments were carried out to get information
about interactions between the emission sites of silica-sup-
ported magnesia and methane molecules. Little amount of
7
,8,16,18
probably due to surface silanols of silica.
Other MgO/
SiO samples exhibited emission spectra with fine structure
2
centered around 525 nm, which were similar to each other in
spectral shape. This suggests the presence of the similar emis-
2
methane was introduced to the MgO/SiO (0.5) sample step
by step. The admission of the methane molecules decreased the
intensity of the emission spectrum, as shown in Fig. 2. This
quenching effect clearly indicates that there were interactions
between the emission sites of silica-supported magnesia and the
added methane molecules.
As described elsewhere,
phorescence quenching are possible, dynamic quenching (colli-
sional or weak interaction), and static quenching via formation
of surface complexes. In silica-supported magnesia, the emis-
sion spectra were quenched by methane molecules, but about
sion sites in 0.3–5.0 mol% MgO/SiO
the emission spectra are clearly different from the emission
2
samples. The shapes of
2
3,24
spectra of either silica or bulk MgO,
reported one which is assigned to the highly dispersed magne-
but are similar to the
1
5,18
sium oxide species on the silica surface.
The shape of the
12,23–25
two mechanisms of phos-
fine structural emission spectra did not vary with the excitation
wavelength, suggesting that there was probably only one kind
of phosphorescence emission sites in the samples.
There are eight maxima on the emission spectra and the
energy position of each maximum is listed in Table 2. The
intervals between the positions of the maxima are constant,
corresponding to the vibrational energy of the photoexcited
sites. The vibrational energy of the photoexcited sites can be
8
0% of the initial intensity was recovered by evacuation at
room temperature for 15 min. It shows that there were weak
interactions between the emission sites and methane, though
some small parts were not reversible at room temperature, as
shown in Fig. 2. Thus, it suggests that the quenching of
emission spectra in silica-supported magnesia by methane
molecules follows the dynamic quenching mechanism domi-
nantly.
ꢀ
1
estimated as the average of the intervals at 950 ꢃ 48 cm . This
Analysis of decay curve
Fig. 3 shows the decay curves of the emission light from MgO/
SiO2 samples that exhibited the fine structure of emission
spectra (0.3–5 mol%). The decay curves were measured at
excitation wavelength 300 nm and emission wavelength 525 nm.
2
The decay curve of MgO/SiO sample can not be described by
Fig. 1 Photoluminescence excitation (A) and emission (B) spectra
recorded at 77 K of (a) MgO/SiO (0.1), (b) MgO/SiO (0.3), (c) MgO/
SiO (0.5), (d) MgO/SiO (2), and (e) MgO/SiO (5). Monitoring emis-
sion wavelength for A was 525 nm, excitation wavelength for B was
00 nm.
Fig. 2 Effect of methane admission upon phosphorescence spectra of
MgO/SiO (0.5), recorded at 77 K. Excitation wavelength was 300 nm.
The amount of methane was (a) 0, (b) 21, (c) 84, (d) 170 and (e) 320 Pa.
The spectrum f (bold) was recorded after vacuum at room temperature
for 15 min.
2
2
2
2
2
2
3
T h i s j o u r n a l i s & T h e O w n e r S o c i e t i e s 2 0 0 5
P h y s . C h e m . C h e m . P h y s . , 2 0 0 5 , 7 , 1 9 5 – 2 0 1
197

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2 2 2 2
Fig. 3 Experimental (circle) and simulation (line) decay curve of (a) MgO/SiO (0.3), (b) MgO/SiO (0.5), (c) MgO/SiO (2) and (d) MgO/SiO (5).
The spectra were recorded at 77 K. Excitation and emission wavelengths were 300 nm and 525 nm, respectively.
one exponential function, but it can be described by two
exponential functions as the following equation:
As shown in Table 3, the SLE and LLE fraction ratios were
obtained as almost constant in the present samples by the
present fitting procedures. Since all the samples presented in
Table 3 showed clear fine structural spectra, it was predicted
that the ratios would be similar to each other. The sample of
I ¼ I
0
[A
1
exp(ꢀt/t
1
) þ A
2
1
exp(ꢀt/t )]
where I and I are the initial intensity of the emission light and
0
the intensity at t ms, t shows the real time in measurement, A is
i
2
MgO/SiO (0.1) exhibiting a broad spectrum would show a
different kind of emission sites with different lifetimes from
those on these samples. Even though the decay curve of the
MgO/SiO (0.1) was difficult to be analyzed due to large noise
the emission fraction (A þ A ¼ 1) and t is the lifetime of
1
2
i
excitation state. The simulation curves calculated by the equa-
tion above are also shown in Fig. 3.
2
and low intensity (not shown), it was confirmed that the
emission sites in this sample have the character of longer
lifetime emission. On the other hand, the optical cell also
exhibited a very low intensity of broad emission spectra with
a long lifetime (not shown), which may partly be contributed to
the LLE fraction. Although the intensity was very low, the
contribution of this optical cell will be higher when the emis-
sion intensity is lower. Thus, the measured decay curve of the
sample exhibiting low intensity would have a lower accuracy of
determining the fraction originated from the sample. At least,
however, it is clear that the LLE was a minor component,
i.e. the SLE was the dominant emission sites in these samples.
The best fitting parameters are presented in Table 3. At least
two kinds of emission fractions that have different lifetime can
be deduced from the decay curve. The shorter lifetime emission
(
SLE) was dominant for all samples (0.3–0.5 mol%) that
exhibited the fine structural spectra, and the lifetime did not
show remarkable changes although the Mg content is increas-
ing. It shows that the same kind of emission sites was formed in
all samples, and the fractions in the emission sites were almost
the same in all of these samples. The lifetime of the SLE
fraction (t
The longer lifetime emission (LLE) was found as the minor
component in all MgO/SiO samples exhibiting fine structural
spectra. The lifetime (t ) was around 500–700 ms. In the silica–
1
) was estimated to be 38 ms.
2
2
alumina system, it has been suggested that the LLE fractions
are due to surface OH groups and did not play an important
Discussion
The emission sites on MgO/SiO
2
2
6
role in photoinduced direct methane coupling. In other
words, the SLE fraction was clarified as the fraction exhibiting
the fine structure in the emission spectra and the emission sites
would be the expected active sites responsible for the photo-
The relationship between the total intensity of emission spectra
(excitation wavelength was 300 nm, emission wavelength was
525 nm) and Mg content is shown in Fig. 4. Fig. 4 also displays
the relationship between the emission intensity of shorter life-
time (SLE) and Mg content. The SLE intensity and total
intensity of emission spectra did not show much difference
since the SLE is the dominant emission fraction, as shown in
Table 3. The variation of Mg content resulted in the variation
of emission intensity. It is clear that the Mg content affects the
number of emission sites formed in silica-supported magnesia.
Since the emission intensity increased with increasing Mg
content up to 2 mol%, the amount of SLE species would
increase up to around 2 mol%. When assuming that the
emission efficiency is constant or not drastically changed, the
curve in Fig. 4 means that the number of emission sites in
silica-supported magnesia would initially increase with the
increasing of Mg content, but too much Mg loaded on silica
would cause the reduction of the emission sites in number. This
phenomenon indicates that only highly dispersed magnesium
7
,26
reaction.
Table 3 The best fitting parameter for decay curve of the phosphor-
a
escence in MgO/SiO
2
SLE
LLE
b
b
Sample
A
1
t
1
/ms
A
2
2
t /ms
2
2
2
2
MgO/SiO
MgO/SiO
MgO/SiO
MgO/SiO
a
2
2
2
2
(0.3)
(0.5)
(2)
0.92
0.97
0.95
0.89
38(5)
38(5)
38(5)
38(5)
0.08
0.03
0.05
0.11
5(1) ꢂ 10
7(1) ꢂ 10
5(1) ꢂ 10
7(1) ꢂ 10
(5)
Recorded at 77 K, excitation wavelength was 300 nm, emission
b
wavelength was 525 nm, measurement time was 750 ms. The value
in parentheses is the error allowed for the fitting.
1
98
P h y s . C h e m . C h e m . P h y s . , 2 0 0 5 , 7 , 1 9 5 – 2 0 1
T h i s j o u r n a l i s & T h e O w n e r S o c i e t i e s 2 0 0 5

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2
Fig. 4 Plot of the emission intensity of the MgO/SiO samples (open
circle), SLE intensity (cross) and the total yield in the photoinduced
direct methane coupling (closed circle, broken line) versus Mg content.
Fig. 5 Relation between SLE intensity of the MgO/SiO
2
samples
/SiO (0.1)
triangle) and the total yield on them in the photoinduced direct
(
closed circle), the SiO
2
-Al
2
3
O (9.1) (opened circle), the ZrO
2
2
(
methane coupling.
species on the silica surface would exhibit the fine structure in
1
8
emission spectra.
As reported previously, SLE is suggested as the emission
sites that play an important role in photoinduced direct
of this saturation has also been reported in silica-supported
7
zirconia.
7
,26
methane coupling.
is a Si–O–Al linkage, and in silica-supported zirconia the
In silica–alumina, the proposed SLE site
2
6
7
SLE site proposed is a Si–O–Zr linkage. These linkages have
General aspect on a silica-based photocatalyst
already been confirmed by Raman and infrared spectro-
2
Fig. 5 also shows the photoactivity of silica–alumina and silica-
supported zirconia in this reaction. It was elucidated that the
silica-supported magnesia showed an activity at almost the
same level as other reported systems; or rather, when they were
compared at the same SLE intensity, the silica-supported
magnesia was found to be slightly superior to others.
7–31
scopy.
In a separate experiment, it has been confirmed
that the fine structure of emission spectra in silica-supported
magnesia was not obtained when the vacuum pretreatment was
1
5
done at low temperature, such as 773 K. We also confirmed
that the fine structure of emission spectra on the present
samples could not be obtained by vacuum pre-treatment at
Even though these three systems showed a similar level of
activity, the optimum loading amounts of each metal oxide on
the silica support were different. The difference of the cation or
the sample preparation method will affect the optimum loading
amount of the cation on the silica support. The ratio of the
amount of the formed active species to the loading amount on
silica was high in the order of silica-supported zirconia 4
silica-supported magnesia 4 silica–alumina. When the activity
was normalized by the SLE intensity, these three different
kinds of cations showed almost the same activity even though
the apparent optimum concentrations on silica were different
from each other. In addition, the lifetimes of the excitation
state and vibrational energy were also similar to each other
8
the fine structure of emission spectra in silica-supported mag-
73 K. The high temperature (1073 K) is necessary to obtain
5
nesia, as also needed in silica–alumina and silica-supported
7
,8
zirconia. The evacuation at high temperature is important
for desorption of hydroxyl groups on the surface, which would
produce coordinatively unsaturated sites and would corres-
pond to photoexcitation sites. In silica-supported magnesia, it
has been reported that the emission site is a coordinatively
unsaturated Mg (Mg–O bond) on the surface of the tetrahedral
silica network which was only produced by evacuation at high
1
5,16,18
temperature.
These facts mentioned above suggest that
the Si–O–Mg linkages as the emission sites would be respon-
sible for exhibiting the fine structure in the phosphorescence
emission spectra.
(
Table 4). These facts suggest that there might be similar active
sites for the photoinduced methane coupling which do not
strongly depend on the kind of cations.
These similar properties may bring a similar performance in
the interaction with the methane molecules, which can be
proved by studying the quenching effect by methane molecules.
When the illumination intensity and concentration of the
molecules are maintained to be constant, in the presence of
quencher molecules the relative photoluminescence intensity
will be expressed as a function of the concentration of the
quencher molecules, according to the following Stern–Volmer
The active sites in photoinduced direct methane coupling
Fig. 4 also shows the photoactivity of silica-supported magne-
sia samples (total yield) as a function of Mg content. As shown
in Fig. 4, both the emission intensity and the total yield
similarly varied with Mg content. This good relationship
suggests a possibility that the emission sites in silica-supported
magnesia are the dominant active sites in the photoinduced
direct methane coupling. This suggestion is also supported by
the result of the quenching effect (see Fig. 2) that showed the
interactions between methane and the emission sites of silica-
supported magnesia.
1
2
expression:
I0
¼ 1 þ tk½Qꢄ
I
In Fig. 5, we can see the relationship between the SLE
emission intensity and the photocatalytic activity of the sam-
ples in the photoinduced direct methane coupling. The increase
of the emission intensity tended to give an increase in the
photoactivity of the sample, suggesting that the emission sites
would be the active sites in the reaction. However, the increase
of the emission intensity did not show a proportional increase
with the photoactivity of the sample. It would be reasonable to
assume that in the sample with higher Mg content, the excita-
tion state would be delocalized over Mg oligomer which might
cause a decrease in the photocatalytic activity. We confirmed
similar phenomena in the silica–alumina system from the
Table 4 Characteristics of emission sites in silica-supported magnesia,
silica–alumina, and silica-supported zirconia
a
Vibrational energy /cm
ꢀ1
b
Lifetime (SLE) /ms
Sample
c
MgO/SiO
2
950(48)
985(54)
955(50)
38(5)
33(3)
28(5)
d
SiO
ZrO
2
-Al
/SiO
2
2
O
3
e
2
a
Calculated from the average interval of the maximum in fine structure
b
of emission spectra. The dominant fraction calculated from the decay
c
curve, see text. The value in parentheses is the error allowed for the
d
fitting. Data from ref. 26. Data from ref. 7.
e
4
reported data of intensity and photoactivity. The tendency
T h i s j o u r n a l i s & T h e O w n e r S o c i e t i e s 2 0 0 5
P h y s . C h e m . C h e m . P h y s . , 2 0 0 5 , 7 , 1 9 5 – 2 0 1
199

View Article Online
Scheme 1 NBOHC as photoactive sites on the silica surface.
It has been reported that silica itself also shows photocata-
4
–10
lytic activity in photoinduced direct methane coupling
and
photoepox-
and photooxidations. Also in the present
3
2,33
some other reactions, such as photometathesis,
Fig. 6 Stern–Volmer plots of relative emission intensity versus partial
pressure of methane molecules on MgO/SiO (0.5) (circle), SiO -Al
sample (triangle) and ZrO /SiO sample (square). The broken line is
the interaction between SiO -Al sample and N molecules. The data
for SiO -Al and ZrO /SiO samples were taken from refs. 5 and 7,
respectively.
20,34,35
36
idation
2
2
2 3
O
study, the activity in photoinduced direct methane coupling
was confirmed as listed in Table 1 (entry 6). The pure silica
materials evacuated at higher temperature have a kind of
surface quantum defect that has a function as photoactive sites
2
2
2
2
O
3
2
2
2
O
3
2
2
3
7
for photoreactions such as photometathesis. It has been
ꢅ
clarified that the radical sites RSi–O (NBOHC, non-bridging
oxygen hole center) can be photoexcited upon light at 4.8 eV
0
where I and I are the intensities of emission spectra in the
absence and presence of quencher molecules, t shows the
lifetime of the emission sites, k is the quenching rate constant,
and [Q] is the concentration (partial pressure) of quencher
molecules. In the present study, we plotted the relative emis-
(
orbital of Si–O to 2p nonbonding orbital of nonbridging
258 nm) and the charge transfer occurs from the bonding
3
2,38
oxygen,
of NBOHC on the silica has been revealed to be 891 and
as shown in Scheme 1. The stretching vibration
ꢀ
1 32
0
sion intensity (I /I) of the silica-supported magnesia as a
9
10 cm
On the other hand, it has been reported that in highly
dispersed TiO species on silica, the photoexcitation on Ti–O
.
function of partial pressure of methane molecules (Fig. 6).
As shown in Fig. 6, in the presence of methane molecules the
relative emission intensity is a linear function of methane
pressure, with an intercept equal to 1. Here, the slope corre-
sponds to kt. This clearly indicates that the quenching of
emission intensity occurred by interactions between the added
methane molecules and the emission sites of the silica-sup-
ported magnesia through non-radiative deactivation pathways.
In other words, dynamic quenching mainly happened, in which
the quenching depended on the amount of the quencher
molecules. When we also plotted the results on the silica–
4
is explained as a charge transfer from ligand oxygen to
titanium (LMCT, ligand-to-metal charge transfer) as shown
1
in Scheme 2. The vibrational energy of the excited sites for
2
ꢀ1
Ti–O–Si in TS-1 was around 965 cm , which was estimated by
3
9
the photoluminescence study. However, a recent study by
UV resonance Raman spectroscopy revealed that the band at
ꢀ
1
9
65 cm
should be assigned to silicalite moiety that was
affected by the introduced Ti cation, not to Ti–O–Si bond
4
0
vibration. A previous study on photoluminescence of silica–
alumina also revealed that the photoactive AlO species on
silica were not acid sites at Al cations, implying that the Si–O
bond affected by incorporated Al cation would be the active
sites.
5
7
alumina and the silica-supported zirconia in Fig. 6, it was
clarified that these three systems have similar slope in the Stern–
Volmer plot. Since the lifetimes of the emission sites in all
systems are similar to each other (Table 4), the similar slope
of Stern–Volmer plot indicates that the silica-supported magne-
sia exhibits similar methane quenching rate to the silica–alumina
and the silica-supported zirconia. This result suggests that the
emission sites in these systems would be concerned with activa-
tion of methane molecules in a similar way. As a comparison,
4
2
6
Since evacuation at high temperature is necessary to gen-
2
0,32–35
erate the active sites in the pure silica,
the silica–alumi-
5
7,8
na, the silica-supported zirconia, and the silica-supported
magnesia, we propose that the active sites in highly dispersed
metal oxide in a silica system would be formed in a similar way
to those in the pure silica materials. The presence of the metal
cation in silica would result in an ionic character bond to some
extent which has different electron density from the covalent
Si–O bond and causes the easiness in vibration. From the
discussion above, the difference of the element (M ¼ Al, Zr,
Mg) in silica did not show any remarkable changes in either
emission properties or photoactivity in the reaction, although
these three elements have many differences, e.g. group classi-
fication, valence, and ionic radius. It brings us to a considera-
tion that the photoexcitation of the Si–O–M linkage would be
strongly influenced by the character of the Si–O bond moiety,
which might be the charge transfer in the Si–O bond. In our
opinion, this might be expressed as shown in Scheme 3. This
also gives us one possible explanation for the difference
between the photoexcitation of metal oxide (via LMCT) and
the photoexcitation of the highly dispersed metal oxide in silica
introducing the same amount of an inert gas (N ) to the silica–
2
5
alumina system instead of methane reduced much less the
emission intensity, as shown in Fig. 6 as a broken line, which
confirmed a certain interaction between the emission site and the
methane molecules.
Since the photoreaction could not occur without irradiation,
it is most likely that the emission sites give the photoexcitation
energy to the methane molecules through adsorption, or the
complex of the active sites and adsorbed methane might be
7
activated by the photoenergy. Similar quenching efficiency of
the silica-supported magnesia to the silica–alumina and the
silica-supported zirconia gives us information that the perfor-
mance of the photoactive sites on the silica-supported
magnesia is similar to those on the silica–alumina and the
silica-supported zirconia systems. It indicates that they have a
similar key-step in the activation of methane; probably
methane would be activated by the same mechanism in photo-
(via CT on Si–O bond). At present, no direct and clear results
have been elucidated to describe the details of the charge
7
induced direct methane coupling, which would bring similar
photoactivity in the reaction as shown in Fig. 5. Thus, it can be
expected that other cations than Mg, Al, and Zr will show
activity toward photoinduced direct methane coupling with
similar mechanism if they could be highly dispersed on a silica
support.
Scheme 2 LMCT on highly dispersed Ti–O bond on silica.
2
00
P h y s . C h e m . C h e m . P h y s . , 2 0 0 5 , 7 , 1 9 5 – 2 0 1
T h i s j o u r n a l i s & T h e O w n e r S o c i e t i e s 2 0 0 5

View Article Online
7
8
9
0
1
H. Yoshida, M. G. Chaskar, Y. Kato and T. Hattori, J. Photo-
chem. Photobiol. A, Chem., 2003, 160, 47.
H. Yoshida, M. G. Chaskar, Y. Kato and T. Hattori, Chem.
Commun., 2002, 2014.
Y. Kato, N. Matsushita, H. Yoshida and T. Hattori, Catal.
Commun., 2002, 3, 99.
H. Yoshida, N. Matsuhita, Y. Kato and T. Hattori, J. Phys.
Chem. B, 2003, 107, 8355.
1
1
Scheme 3 Plausible model of photoactive sites on a silica-based
photocatalyst.
Y. Kato, H. Yoshida, A. Satsuma and T. Hattori, Microporous
Mesoporous Mater., 2002, 51, 223.
M. Anpo and M. Che, Adv. Catal., 1999, 44, 119.
1
1
2
3
transfer. Further study on this idea is required to get more
understanding of the electron charge transfer mechanism for
the photocatalytic field.
T. Lo
Lett., 1999, 38, 283.
T. Lopez, R. Gomez, M. E. Llanos, E. Garcı
J. Navarrete and E. Lopez-Salinas, Mater. Lett., 1999, 39, 51.
pez, R. Gomez, M. E. Llanos and E. Lopez-Salinas, Mater.
´ ´ ´
14
15
16
17
´
´
´
a-Figueroa,
´
T. Tanaka, H. Yoshida, K. Nakatsuka, T. Funabiki and
S. Yoshida, J. Chem. Soc., Faraday Trans., 1992, 88, 2297.
H. Yoshida, T. Tanaka, T. Funabiki and S. Yoshida, J. Chem.
Soc., Faraday Trans., 1994, 90, 2107.
Conclusion
Silica-supported magnesia promoted photoinduced direct
methane coupling. The highly dispersed magnesium oxide
species on silica would be the active sites responsible for both
exhibiting the fine structural photoemission spectra and the
photoactivity in the reaction.
The similar photoactivity and properties of the emission sites
in the silica-supported magnesia, the silica–alumina, and the
silica-supported zirconia suggest the general aspect on a silica-
based photocatalyst, as follows:
H. Yoshida, C. Murata and T. Hattori, J. Catal., 2000, 194, 364.
18 H. Yoshida, T. Tanaka, M. Yamamoto, T. Yoshida, T. Funabiki
and S. Yoshida, J. Catal., 1997, 171, 351.
19 H. Yoshida, T. Tanaka, M. Yamamoto, T. Funabiki and S.
Yoshida, Chem. Commun., 1996, 2125.
2
0
H. Yoshida, T. Shimizu, C. Murata and T. Hattori, J. Catal.,
003, 220, 226.
H. Yoshida, T. Yoshida, T. Tanaka, T. Funabiki, S. Yoshida,
2
2
1
T. Abe, K. Kimura and T. Hattori, J. Phys. IV, 1997, 7, C2–911.
22 B. Shelimov, V. Dellarocca, G. Martra, S. Coluccia and M. Che,
Catal. Lett., 2003, 87, 73.
(
i) The highly dispersed metal oxide species on silica having
Si–O–M linkages (M ¼ metal cation) that are generated by
desorption of hydroxyl groups on the silica surface at high
temperature would become the photoemission sites and the
active sites for the photoinduced direct methane coupling.
2
3
M. Anpo, Y. Yamada, Y. Kubokawa, S. Coluccia, A. Zecchina
and M. Che, J. Chem. Soc., Faraday Trans., 1988, 84, 751.
M. Anpo and Y. Yamada, Mater. Chem. Phys., 1988, 18, 465.
M. Anpo, M. Kondo, S. Coluccia, C. Louis and M. Che, J. Am.
Chem. Soc., 1989, 111, 8791.
2
2
4
5
(ii) These emission sites would be concerned with the activa-
tion of methane molecules in a similar way. Probably the
reactions would proceed with similar mechanisms.
26 Y. Kato, H. Yoshida and T. Hattori, Phys. Chem. Chem. Phys.,
000, 2, 4231.
2
H. Yoshida, T. Tanaka, A. Satsuma, T. Hattori, T. Funabiki and
S. Yoshida, Chem. Commun., 1996, 1153.
2
7
(iii) The photoexcitation of the species would be deeply
related to the Si–O bond moiety.
2
8
G. Mestl and H. Kno
lysis, ed. G. Ertl, H. Kno
heim, 1997, vol. 2, p. 539.
¨
zinger, Handbook of Heterogeneous Cata-
¨
zinger and J. Weitkamp, VCH, Wein-
2
3
9
0
S. W. Lee and R. A. Condrate Sr, J. Mater. Sci., 1988, 23, 2951.
Z. Dang, B. G. Anderson, Y. Amenomiya and B. A. Morrow,
J. Phys. Chem., 1995, 99, 14437.
Acknowledgements
This work was partially supported by a Grant-in-Aid for
Scientific Research on Priority Areas (417) from the Ministry
of Education, Culture, Sports, Science and Technology
(MEXT) of the Japanese Government. We acknowledge the
invaluable contribution by the reviewers.
3
1
2
B. Rakshe, V. Ramaswamy and A. V. Ramaswamy, J. Catal.,
1
996, 163, 501.
3
Y. Inaki, H. Yoshida, T. Yoshida and T. Hattori, J. Phys. Chem.
B, 2002, 106, 9098.
33 T. Tanaka, S. Matsuo, T. Maeda, H. Yoshida, T. Funabiki and
S. Yoshida, Appl. Surf. Sci., 1997, 121–122, 296.
3
4
5
H. Yoshida, C. Murata and T. Hattori, Chem. Commun., 1999,
551.
C. Murata, H. Yoshida and T. Hattori, Chem. Commun., 2001,
1
References
3
1
T. V. Choudhary, E. Aksoylu and D. W. Goodman, Catal. Rev.,
003, 45, 151.
Y. Xu and L. Lin, Appl. Catal. A, 1999, 188, 53.
Y. Xu, X. Bao and L. Lin, J. Catal., 2003, 216, 386.
H. Yoshida, N. Matsushita, Y. Kato and T. Hattori, Phys. Chem.
Chem. Phys., 2002, 4, 2459.
2412.
36 A. Ogata, A. Kazusaka and M. Enyo, J. Phys. Chem., 1986, 90,
5201.
37 H. Yoshida, Curr. Opin. Solid Mater. Sci., 2003, 7, 435.
38 L. Skuja, Solid State Commun., 1992, 84, 613.
´
39 Allison S. Soult, Duke D. Poore, Elizabeth I. Mayo and A. E.
2
2
3
4
5
6
H. Yoshida, Y. Kato and T. Hattori, Stud. Surf. Sci. Catal., 2000,
130, 659.
Y. Kato, H. Yoshida and T. Hattori, Chem. Commun., 1998, 2389.
Stiegman, J. Phys. Chem. B, 2001, 105, 2687.
40 C. Li, G. Xiong, J. Liu, P. Ying, Q. Xin and Z. Feng, J. Phys.
Chem. B, 2001, 105, 2993.
T h i s j o u r n a l i s & T h e O w n e r S o c i e t i e s 2 0 0 5
P h y s . C h e m . C h e m . P h y s . , 2 0 0 5 , 7 , 1 9 5 – 2 0 1
201