Effect of alkali-metal ion addition to silica-supported molybdenum oxide on photocatalysis: Photooxidation of propane and propene, and photo-assisted metathesis of propene

Article abstract of DOI:10.1039/a707471i

The effect of adding alkali-metal ions (Li⁺, Na⁺, K⁺, and Rb⁺) to silica-supported molybdenum oxide (MoO₃/SiO₂) has been investigated by photooxidation of propane and propene with gaseous oxygen, and photo-assisted metathesis of propene at room temperature. Over alkali-metal ion modified MoO₃/SiO₂, propanone has been mainly produced from propane, and acrolein from propene, whereas over MoO₃/SiO₂ a variety of products have been formed, such as ethanal, propene, propanone and propanal from propane, and ethanal and acrolein from propene. The photooxidation selectivities change upon the addition of alkali-metal ions because the electrophilicity of the oxygen atom in the molybdenum species in the excited state (i.e., it is active for photooxidation) is weakened by the interaction with the alkali-metal ions. In the photo-assisted metathesis reaction of propene, the conversion is lower over alkali-metal ion modified MoO₃/SiO₂ than over MoO₃/SiO₂. The activity for the metathesis reaction has been related to the reducibility of the surface molybdenum species by CO under UV light irradiation. In addition, the Raman spectra suggest that the surface molybdenum species in alkali-metal ion modified MoO₃/SiO₂ is like Mo₂O₇^2- besides a small amount of MoO₄^2-, whereas the molybdenum species in MoO₃/SiO₂ exists as a mononuclear species.

Full text of DOI:10.1039/a707471i

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Effect of alkali-metal ion addition to silica-supported molybdenum
oxide on photocatalysis
Photooxidation of propane and propene, and photo-assisted metathesis of propene
Sakae Takenaka, Tsunehiro Tanaka, Takuzo Funabiki and Satohiro Yoshida*
Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Kyoto
606-01, Japan
The e†ect of adding alkali-metal ions (Li`, Na`, K`, and Rb`) to silica-supported molybdenum oxide (MoO /SiO ) has been
3
2
investigated by photooxidation of propane and propene with gaseous oxygen, and photo-assisted metathesis of propene at room
temperature. Over alkali-metal ion modiÐed MoO /SiO , propanone has been mainly produced from propane, and acrolein from
3
2
propene, whereas over MoO /SiO a variety of products have been formed, such as ethanal, propene, propanone and propanal
3
2
from propane, and ethanal and acrolein from propene. The photooxidation selectivities change upon the addition of alkali-metal
ions because the electrophilicity of the oxygen atom in the molybdenum species in the excited state (i.e., it is active for
photooxidation) is weakened by the interaction with the alkali-metal ions. In the photo-assisted metathesis reaction of propene,
the conversion is lower over alkali-metal ion modiÐed MoO /SiO than over MoO /SiO . The activity for the metathesis
3
2
3
2
reaction has been related to the reducibility of the surface molybdenum species by CO under UV light irradiation. In addition,
the Raman spectra suggest that the surface molybdenum species in alkali-metal ion modiÐed MoO /SiO is like Mo O 2~
3
2
2 7
besides a small amount of MoO 2~, whereas the molybdenum species in MoO /SiO exists as a mononuclear species.
4
3
2
Alkali-metal ions are frequently used as promoters in many
catalytic systems to improve the selectivity and the activity, as
surface acidity can be considered as one of the main factors
controlling the catalytic properties. One distinct function of
alkali-metal ions is their electron-donating ability, which leads
to an enhancement of basicity of metal oxides, but the exact
nature and role of the alkali-metal ions are still not well
understood. Alkali-metal ions act as both promoters and
poisons, for example, potassium ions are often added to iron-
based catalysts for FischerÈTropsch reactions in order to
promote activity,1 whereas the addition of potassium ions to
vanadyl pyrophosphate poisons the selective oxidation of n-
butane to maleic anhydride.2
the e†ect of alkali-metal ion addition on photocatalysis by
silica-supported vanadium oxide (V O /SiO ).9h14 Alkali-
2
5
2
metal ion modiÐed V O /SiO selectively catalyses the photo-
2
5
2
oxidation of propane to propanone by gaseous oxygen at
room temperature, while a variety of products, such as
ethanal, propanal, propanone, are produced from propane
over V O /SiO .9,10 The added alkali-metal ion is found to
be located adjacent to the surface vanadium species, VO as
determined by spectroscopic methods, such as XAFS and
2
5
2
4
Raman spectroscopy.13 We have concluded that the change in
photocatalytic performance on addition of alkali-metal ions
results from a change in the electronic states of the vanadium
species, i.e., the electrophilicity of the oxygen atom in the
vanadium species in the triplet state, which is active for the
photooxidation, becomes weak upon the interaction with
alkali-metal ions.14 Silica-supported molybdenum oxide
(MoO /SiO ) also exhibits photocatalytic activity under UV
Supported molybdenum oxide catalysts are excellent for the
selective oxidation of hydrocarbons,3 the oxidative dehydro-
genation of alcohols,4 and the metathesis reaction of alkenes.5
Supported molybdenum oxide catalysts are also modiÐed by
alkali-metal ions in order to improve catalytic performance.
For example, the modiÐcation of bismuth- and molybdenum-
containing systems by alkali-metal ion improves selectivity to
partial oxidation products in the ammoxidation of alkanes.6
3
2
light irradiation, and catalyses the oxidation of propane15 and
the metathesis reaction of propene.16 An investigation into
photocatalysis by alkali-ion modiÐed metal oxides would be
helpful in order to understand the e†ect of alkali-metal ion
addition, as the photocatalytic performance of the metal oxide
is sensitive to the addition of alkali-metal ions, as mentioned
above.
For oxidation of methane by N O over silica-supported
2
molybdenum oxide catalyst, modiÐcation by sodium ion
poisons the formation of formaldehyde at low loadings of
molybdenum oxides, whereas the formation of formaldehyde
is promoted by the addition of sodium ions at fairly high
loadings of molybdenum oxides.7 Ban8 ares et al. reported the
In this work, we carried out the oxidation of hydrocarbons
(propane and propene) by gaseous oxygen and the metathesis
of propene over alkali-metal ion modiÐed MoO /SiO under
3
2
e†ect of the addition of alkali-metal ions to silica-supported
molybdenum oxide for oxidation of methanol and methane.8
Addition of alkali-metal ions decreases the number of isolated
surface molybdenum oxide species and brings about the for-
mation of new alkali-metalÈmolybdate compounds. The
oxygen associated with the alkali-metalÈmolybdate com-
pounds is generally not available for oxidation reactions and
the addition of alkali-metal ions decreases catalytic activity
for the oxidation of methane and methanol. As for supported
molybdenum oxide, the addition of alkali-metal ions often
leads to a decrease in activity for oxidation.
UV light irradiation, and we investigated the e†ect of alkali-
metal ion addition on photocatalysis.
Experimental
Materials
The reactants (propane, propene and oxygen), tetra-
ethylorthosilicate (TEOS, Nacalai Tesque, EP grade), ammon-
ium heptamolybdate [(NH ) Mo O É 4H O, Nacalai
4 6
7
24
2
Tesque, GR grade], LiOHÉ H O (Wako Pure Chemical
2
Photocatalysis by supported metal oxides is also inÑuenced
by the addition of alkali-metal ions. We have already reported
Industries, GR grade), NaOH (Nacalai Tesque, GR grade),
KOH (Nacalai Tesque, GR grade), and RbOHÉ H O
2
J. Chem. Soc., Faraday T rans., 1998, 94(5), 695È700
695

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(Mitsuwa's Pure Chemicals, GR grade) were commercially
MS at room temperature for 10 min. The photooxidation of
supplied. The gaseous reactants were reÐned by vacuum distil-
propane took place efficiently over all the samples and no sig-
niÐcant di†erence in the activity could be found among the
Ðve samples. The activities of the alkali-metal ion modiÐed
samples were almost the same, regardless of the type of alkali-
metal ion added. In the oxidation over MS, propene, ethanal,
propanone, propanal etc., were formed, i.e., the reaction was
not selective. However, the selectivity to propanone was
higher and the selectivities to propene and ethanal were lower
over alkali-metal ion modiÐed MS, compared with those over
MS, i.e., the alkali-metal ion modiÐed MS samples are e†ec-
tive at incorporating an oxygen atom into a propane molecule
without Ðssion of a CwC bond. The trend is remarkable with
the large atomic numbers of the added alkali-metal ions. The
change in selectivities on addition of the alkali-metal ions to
MS indicates that the alkali-metal ions interact with the active
molybdenum species for the photooxidation.
lation at low temperatures. 18O was supplied from Isotec Inc.
2
(purity 98.1%) and was used without further puriÐcation.
Preparation of samples
A silica support (BET surface area: 556 m2 g~1) was prepared
by hydrolysis of distilled TEOS and calcination at 773 K for
5 h in a dry air stream as described elsewhere.17 Silica-
supported molybdenum oxide (MS) was prepared by impreg-
nating the silica with an aqueous solution of ammonium
heptamolybdate [(NH ) Mo O É 4H O] at 353 K for 2 h
4 6
7
24
2
and by drying up impregnated sample. The sample was dried
at 363 K for 12 h and calcined in a dry air stream at 773 K for
5 h. The amount of molybdenum oxide loaded was 4.0 wt.%
as MoO . Alkali-metal ion modiÐed silica-supported molyb-
3
denum oxide samples (Li-MS, Na-MS, K-MS, and Rb-MS)
We carried out photooxidation of propane over Na`-
modiÐed MS samples with di†erent amounts of sodium ions
in order to investigate further the interaction of the alkali-
metal ions with the surface molybdenum species. The results
are shown in Fig. 1. The Na : Mo molar ratio was changed
from 0 to 1.7. Although the activities for the photooxidation
of propane decreased with an increase in the amounts of
sodium ions added, the change in the conversion was not so
appreciable as that for the yields of partial oxidation products,
in particular, propanone. The yield of each product changed
linearly with the molar ratio up to Na : Mo\ 1.0, i.e., the
yields of propene and ethanal decreased, and those of propa-
nal and propanone increased. These results suggest that the
addition of sodium ions up to Na : Mo\ 1.0 brings about the
formation of the molybdenum species that interacts with the
sodium ions, which mainly converts propane to propanone. In
contrast, with Na : Mo[ 1.0, the yield of each product did
not change very much. A sluggish decrease in the conversion
were prepared by impregnating MS with aqueous solutions of
LiOHÉ H O, NaOH, KOH, and RbOHÉ H O, respectively,
2
2
followed by calcination in a dry air stream at 773 K for 5 h, in
the same manner as mentioned above. Unless otherwise
noted, alkali-metal ions equimolar with the molybdenum
atoms in MS were added.
Pretreatment of the samples
MS and alkali-metal ion modiÐed MS samples were pretreat-
ed by evacuation at 673 K for 1 h, treatment of gaseous
oxygen (60 Torr) at 673 K for 2 h and evacuation at 673 K for
10 min prior to reactions and spectral measurements.
Reactions
The reactions were performed in a conventional closed gas-
circulating system (dead space: 280 cm3) described else-
where.17 The powder sample (0.5 g) was spread on the Ñat
bottom of a quartz reactor and irradiated by the light from a
250 W ultrahigh pressure Hg lamp at room temperature. The
experiments were repeated more than twice to examine the
reproducibility of the data. In the dark, the reactions did not
take place. After each run, the catalyst bed was heated to 573
K for the collection of products that tightly adsorbed on the
catalyst at room temperature. The desorbed products were
frozen out in a trap cooled by liquid nitrogen, and were
analysed by GLC and GLCÈmass spectrometry.
Raman spectra
The laser Raman spectra were taken with the 514.5 nm line of
an argon laser (JASCO NRS-2000). The incident laser power
at the sample was 30 mW. Spectra were recorded with a
resolution of 4 cm~1.
Results
Photooxidation of propane
Fig. 1 Change in the conversion and yield of partial oxidation pro-
ducts on photooxidation of propane over Na`-modiÐed MS samples
with di†erent amounts of sodium ions
Table 1 shows the results of the photooxidation of propane
with gaseous oxygen over MS and alkali-metal ion modiÐed
Table 1 Photooxidation of propane with gaseous oxygen over MS and alkali-metal ion modiÐed MS at room temperaturea
selectivityc (%)
sample
conv.b (%)
C H
CH CHO
C H CHO
(CH ) CO
C H CHO
CO
3
6
3
2
5
3 2
2
3
x
MS
33.0
32.5
28.0
25.8
35.8
26
15
5
3
2
22
18
14
7
10
11
19
8
16
23
54
72
66
1
21
24
1
3
18
Li-MS
Na-MS
K-MS
Rb-MS
2
trd
1
5
5
1
a MS and alkali-metal ion modiÐed MS: sample, 0.5 g; propane, 70 lmol; oxygen, 70 lmol; and irradiation time, 10 min. b Based on introduced
propane. c Based on converted propane. The other products were trace amounts of ethene, propylene oxide, propan-2-ol, and propan-1-ol.
d tr\ traces.
696
J. Chem. Soc., Faraday T rans., 1998, V ol. 94

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and yield of propanone was observed on increasing the
amounts of sodium ions added. It is likely that the addition of
sodium ions to more than the molar ratio Na : Mo\ 1 does
not bring about the formation of the active species, which
mainly produces propanone from propane, but leads to the
formation of an alkali-metalÈmolybdate species, which is inac-
tive for photooxidation. Therefore, the molybdenum species
that forms propanone from propane would be a complex
comprised of a molybdenum ion and a sodium ion.
Photooxidation of propene
Table 2 shows the results of the photooxidation of propene
with gaseous oxygen over MS and alkali-metal ion modiÐed
MS. Photoirradiation was carried out at room temperature
for 20 min. Conversion of propene over Li-MS and Na-MS
was almost the same as that over MS, whereas the conversion
over K-MS and Rb-MS was twice that over MS. In the pho-
tooxidation of propene over MS, ethanal, acrolein, propanal,
ethene, etc., were formed. The addition of alkali-metal ions to
MS resulted in an increase in selectivity to acrolein. Over
Fig. 2 Change in the conversion and yield of partial oxidation pro-
ducts on photooxidation of propene over Rb`-modiÐed MS samples
with di†erent amounts of rubidium ions
Li-MS and Na-MS, the formation of ethanal and CO was
depressed and that of acrolein was preferred, compared with
those over MS. Over K-MS and Rb-MS, selectivities to
ethene, ethanal, propanal, and propanone decreased, and
that convert mainly propene to acrolein are formed, and the
addition of rubidium ions to more than Rb : Mo\ 1.0 causes
the formation of a species inactive for the photooxidation.
x
those to acrolein and CO increased in comparison with those
over MS. Because the conversion levels of propene over
Photo-assisted metathesis reaction of propene
x
K-MS and Rb-MS were higher than those over the other
We carried out a photo-assisted metathesis reaction of
propene over MS and alkali-metal ion modiÐed MS at room
temperature in order to investigate further the e†ect of the
addition of alkali-metal ions to MS. The results are found in
Table 3. Although the metathesis reaction took place over all
the samples under UV light irradiation, the activity became
less on the addition of alkali-metal ions to MS and the
decrease in the activity became more noticeable with the
larger atomic numbers of the added alkali-metal ions. It has
been reported that prereduction of MoO /SiO by hydrogen,
samples, the selectivity to CO would be higher over K-MS
x
and Rb-MS. These results indicate that the molybdenum
species interacts with some of the alkali-metal ions to convert
propene to oxygen-containing products, especially acrolein,
without Ðssion of a CwC bond.
We also carried out photooxidation of propene over Rb`-
modiÐed MS samples with di†erent amounts of rubidium
ions. The results were shown in Fig. 2. The conversion of
propene and the yield of acrolein became higher with an
increase in the amounts of rubidium ions added up to a molar
ratio Rb : Mo\ 1.0. In the range Rb : Mo\ 1.0È2.0, a
decrease in conversion and in the yield of acrolein was
observed. These results are similar to those for the photooxi-
dation of propane over Na`-modiÐed MS with di†erent
amounts of sodium ions, suggesting that on addition of rubid-
ium ions up to a molar ratio Rb : Mo\ 1.0, the active species
3
2
carbon monoxide, or propene, which are known to produce
molybdenum ions in a low oxidation state, results in an
increase in activity of the metathesis reaction,18,19 whereas
unreduced MoO /SiO is a poor catalyst for the reac-
3
2
tions.20,21 This suggests that the active species for the metath-
esis reaction over supported molybdenum oxide catalysts is
the surface molybdenum oxide in a low oxidation state.
Table 2 Photooxidation of propene with gaseous oxygen over MS and alkali-metal ion modiÐed MS at room temperaturea
selectivityc (%)
sample
conv.b (%)
C H
CH CHO
C H CHO
(CH ) CO
C H CHO
CO
2
4
3
2
5
3 2
2
3
x
MS
11.6
12.1
13.2
22.5
21.1
12
21
14
6
32
27
21
11
9
12
10
10
5
6
6
4
2
1
24
30
48
41
39
13
trd
trd
35
40
Li-MS
Na-MS
K-MS
Rb-MS
4
5
a MS and alkali-ion-modiÐed MS: sample 0.5 g; propene, 70 lmol; O , 70 lmol; and irradiation time, 20 min. b Based on introduced propene.
2
c Based on converted propene. The other product was trace amounts of propylene oxide. d tr\ traces.
Table 3 Photo-assisted metathesis of propene over MS and alkali-metal ion modiÐed MS at room temperaturea
product distributionc/mol%
sample
conv.b (%)
C H
1-C H
trans-2-C H
cis-2-C H
2
4
4
8
4
8
4
8
MS
42.7
40.7
33.3
14.5
5.5
63
61
58
55
57
8
7
51
3
19
19
25
23
24
8
8
11
12
14
Li-MS
Na-MS
K-MS
Rb-MS
3
a MS and alkali-metal ion modiÐed MS: sample, 0.5 g; propene, 70 lmol, and irradiation time, 20 min. b Based on introduced propene. c The
other products were trace amounts of hexenes, ethanal, propylene oxide, propanone and propanal.
J. Chem. Soc., Faraday T rans., 1998, V ol. 94
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Martin et al.22 and O'Young23 have also reported that the
reducibility of the molybdenum species on the supports is
decreased by the addition of alkali-metal ions. Hence, we
expected that the activity for photo-assisted metathesis would
decrease on addition of alkali-metal ions because the surface
molybdenum oxide that interacts with the alkali-metal ions is
difficult to reduce under UV irradiation. In order to conÐrm
whether or not our speculation was reasonable, we carried out
photoreduction of the molybdenum species in MS and alkali-
metal ion modiÐed MS by gaseous CO at room temperature,
and investigated the amount of carbon dioxide formed. The
photoreduction of MS and alkali-metal ion modiÐed MS by
CO gas was carried out in the closed gas-circulating system
with the trap cooled by liquid nitrogen in a circulating path.
The CO formed by photoreduction of the molybdenum
2
species was trapped immediately, hence we could monitor the
change in pressure of the gaseous CO. Fig. 3 shows the results.
Over all the samples a decrease in the pressure of CO was
observed under UV light irradiation, suggesting that all the
samples were reduced. The rate of decrease in the pressure of
CO over MS was the fastest of all the samples, and the rate
became slower the larger the atomic number of the alkali-
metal ion added, i.e., Li[ Na[ K[ Rb. In addition, the
largest total amount of CO formed was in the photoreduc-
tion over MS, and the amount decreased for the alkali-metal
2
Fig. 4 Raman spectra of MS and Na-MS
ion modiÐed MS in the order Li-MS[ Na-MS[ K-
MS[ Rb-MS. These results suggest that the molybdenum
species in alkali-metal ion modiÐed MS are more difficult to
reduce than those in MS; the interaction of the alkali-metal
ion with the surface molybdenum species would cause this
e†ect. The reducibility of the molybdenum species by CO
under UV light irradiation is compatible with the activity for
the photo-assisted metathesis of propene. Therefore, the
reducibility of the surface molybdenum species under UV light
irradiation would be one of the factors determining the activ-
ity for photo-assisted metathesis of propene.
ions. The small band at 888 cm~1 could be assignable to
Na MoO ,25 suggesting that a small amount of Na MoO is
2
4
2
4
present in Na-MS.
We carried out photooxidation of propane by 18O over
2
Na-MS at room temperature, with subsequent treatment of
the Na-MS by 16O at 673 K, and we then recorded the
2
Raman spectrum of the Na-MS sample. The spectrum is
shown in Fig. 5. In the Raman spectrum [Fig. 5(b)] of Na-MS
used in photooxidation of propane with 18O , four of the
2
bands (not the band at 888 cm~1) were shifted to lower wave-
numbers by ca. 5 cm~1 and the bandwidths became broader
Raman spectra
than those [Fig. 5(a)] for Na-MS before use in the reaction. In
the oxidation of propane by 18O , propanone was formed
The Raman spectra gave helpful information about the active
species on a silica support. Fig. 4 shows the Raman spectra of
MS and Na-MS. A band was found at 986 cm~1 in the
Raman spectrum of MS. The band at 986 cm~1 was assigned
to terminal MoxO stretching vibration of an isolated molyb-
denum species on the silica support.24 Five sharp bands at
936, 918, 888, 870, and 830 cm~1 were observed in the Raman
spectrum of Na-MS, indicating that the alkali-metalÈ
molybdate species were formed by the addition of sodium
2
mainly and the content of 16O atoms contained in the propa-
none formed was higher than that of 18O atoms in the initial
period of the reaction. The formation of 16O18O or 16O was
2
not conÐrmed during photooxidation, suggesting that the
lattice oxygen in Na-MS was not exchanged for gaseous
oxygen under these conditions. We also recorded the Raman
spectra of Na-MS treated by 18O at 673 K, and Na-MS irra-
2
diated by the light from an ultrahigh pressure Hg lamp in the
Fig. 5 Raman spectra of Na-MS samples. (a) Na-MS after pretreat-
ment (described in Experimental section). (b) Na-MS used in photoox-
idation of propane by 18O and successively treated by 16O at 673
Fig. 3 Change in the pressure of CO on reduction of MS and alkali-
metal ion modiÐed MS samples by CO under UV irradiation at room
2
2
temperature. P , pressure of introduced CO; P, pressure of CO at
K. (c) Na-MS treated by 18O at 673 K. (d) Na-MS irradiated by UV
0
2
each photoirradiation time.
light in the presence of 18O .
2
698
J. Chem. Soc., Faraday T rans., 1998, V ol. 94

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presence of 18O2 at room temperature. The spectra of the
by CO under UV irradiation at room temperature. We
Na-MS samples were the same as the spectrum [Fig. 5(a)] of
suppose that one of the factors determining the reducibility is
the energy level of the LUMO (lowest unoccupied molecular
orbital) of the molybdenum species, because electrons would
be incorporated into the LUMO in the reduction process. We
have found that the energy levels of the LUMO and the
HOMO (highest occupied molecular orbital) of the vanadium
cluster are destabilized by the interaction of an alkali-metal
ion when calculated by density functional theory,14 this is
because of the donation of the electron from the alkali-metal
ion to the vanadium cluster. For molybdenum species modi-
Ðed by alkali-metal ions, the same phenomenon occurs.
the Na-MS before use in the reaction by 18O . The results
2
suggest that the lattice oxygen of the molybdenum species in
Na-MS is not exchanged with gaseous oxygen by the treat-
ment at 673 K or by irradiation with UV light at room tem-
perature. Therefore, the band shift observed after
photooxidation of propane with 18O was caused by the inter-
2
action of the reactants (C H and 18O ) with the molybdenum
3
8
2
species active for the reaction, i.e., the lattice oxygen in the
molybdenum species in Na-MS was incorporated into the
propane and was then restored by gaseous oxygen during
photooxidation of propane. From the results mentioned
above, we conclude that the molybdenum species that gives
the Raman bands at 936, 918, 870, and 830 cm~1 is the species
active for photooxidation of propane. Na MoO on silica
The structures of the molybdenum species in MS and
alkali-metal ion modiÐed MS were investigated by Raman
spectroscopy. In the Raman spectrum of MS, a band was
observed at 986 cm~1 but bands assigned to MoO (995 and
2
4
3
would be inactive for the photooxidation, because the band at
820 cm~1)27 and MowOwMo (deformation mode observed
888 cm~1, due to Na MoO , was not shifted after the pho-
at 180È240 cm~1)27 were not detected. The results indicate
that the molybdenum species on silica is isolated, i.e., mono-
meric in MS. There are many reports that the surface molyb-
2
4
tooxidation of propane by 18O .
2
denum species is tetracoordinated (OxMoxO) O at a low
Discussion
2
loading of MoO in MoO /SiO in a dehydrated state.27h30
3
3
2
In the photooxidation of propane and propene, we found that
the addition of alkali-metal ions to MS brought about a
change in the photocatalytic performance, in particular, in
selectivity. Over MS, a variety of products were formed, i.e.,
the reaction over MS was not selective. However, the forma-
tion of propanone from propane, and that of acrolein from
propene became signiÐcant on the addition of alkali-metal
ions to MS, i.e., alkali-metal ion modiÐed MS samples are
e†ective at putting an oxygen atom into the hydrocarbons
without the Ðssion of CwC bonds. The same phenomena were
also observed in photooxidation of light alkanes over alkali-
metal ion modiÐed V O /SiO samples.9h12 It has been
However, it has been proposed that the structure of a dis-
persed surface molybdenum species in MoO /SiO is octa-
3
2
hedral in a dehydrated state.31h33 Hu et al. concluded from
the results of Raman spectra and Mo L -edge XANES that
the surface molybdenum species on silica are isolated and
3
highly distorted, and possess a symmetry somewhat between
octahedral and tetrahedral coordination.34 We can not deter-
mine the structure of the surface molybdenum species in MS
by only the Raman spectrum.
On the other hand, bands di†erent from those of MS were
observed in the Raman spectrum of Na-MS. This indicates
that with the addition of sodium ions alkali-metalÈmolybdate
species are formed on the silica. In these bands of the Raman
spectrum of Na-MS, a small band due to Na MoO was iden-
2
5
2
reported that the species active for photooxidation of propane
over MoO /SiO is an isolated molybdenum species in the
3
2
2
4
triplet state formed by photoirradiation.26 The excitation
from the ground state to the triplet state was assigned to a
charge-transfer transition from the oxygen atom to the molyb-
denum atom of MoxO in the surface molybdenum species. It
is expected that the oxygen atom of the MoxO in the triplet
state has a strong electrophilicity, such as is the nature of O~,
and the oxygen is active in the photooxidation of hydrocar-
bons because an electron from the oxygen atom in MoxO is
transferred to the molybdenum atom by the excitation from
the ground state to the triplet state. As the addition of alkali-
metal ions to MS brought about a change in the catalytic
performance for photooxidation, the alkali-metal ions would
be located adjacent or close to the molybdenum species in the
alkali-metal ion modiÐed MS samples. In general, alkali-metal
ions have a tendency to donate electrons. The electrophilicity
of the oxygen of the MoxO in the excited molybdenum
species is possibly weakened by the presence of alkali-metal
ions, and consequently the catalytic performance for photoox-
idation should be changed. The di†erences in photocatalytic
performance among the alkali-metal ion modiÐed MS samples
result from the distinction in their ability to donate electrons
owing to the types of alkali-metal ions.
tiÐed, suggesting that the addition of sodium ions to MS
brought about the formation of Na MoO . However, we
2
4
found that Na MoO was inactive for photooxidation of
2
4
propane and that the Raman band due to Na MoO was not
shifted by photooxidation of propane with 18O , while the
other Raman bands were shifted to lower wavenumber during
2
4
2
the reaction. Hence, the formation of Na MoO is concluded
2
4
to be independent of the change in the photocatalytic per-
formance a†orded by the addition of sodium ions to MS. The
Raman bands of Na-MS at 936, 918, 870, and 830 cm~1 were
similar to the bands due to Na Mo O (940, 920, 880, and
2
2 7
820 cm~1).35 The Raman bands were shifted by the photooxi-
dation of propane with 18O , suggesting that the molyb-
denum species that gave rise to them were the active species
2
for the photooxidation and the formation of the molybdenum
species caused the change in the photocatalytic performance.
The structure of the molybdenum species can not only be
determined by the Raman spectra. We expect from the results
of the reactions in the present study that the di†erent molyb-
denum species formed are dependent on the type of alkali-
metal ions added and/or the amount added. In the future, we
will describe the structures of the molybdenum species in the
MS and alkali-metal ion modiÐed MS samples by combining
these results with those of other spectroscopic methods.
In the photo-assisted metathesis of propene, it was found
that the addition of alkali-metal ions depressed the activity.
The decrease in activity was correlated to the reducibility
of the molybdenum species by CO under photoirradiation.
The present work is partially supported by Grants-in-Aid
from the Ministry of Education, Science, Sports and Culture
of Japan (Nos. 06239237 and 08405052). S. T. acknowledges
support by the Fellowship of JSPS for Japanese Junior
Scientists.
Ban
8 ares et al. reported that the molybdate compounds formed
upon addition of alkali-metal ions to MoO /SiO were less
3
2
reducible than the dispersed surface molybdenum species in
MoO /SiO .8 Martin et al. also stated that the reducibility of
3
2
the molybdenum species in MoO /TiO decreased on addi-
3
2
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