8
38 Bull. Chem. Soc. Jpn. Vol. 81, No. 7 (2008)
IR-Induced Reaction on MoO3
couple attached to the pan. Then, the sample cell was put on a
three-axis stage with x, y, and z motions controlled using a person-
al computer. The positions were determined with 100-mm preci-
sion.
(
a)
The MoO3 was first calcined at 673 K for 2 h in the presence of
oxygen and evacuated for 1 h at 673 K. After cooling the sample to
room temperature in vacuo for more than 8 h, 6 Torr ethanol was
introduced into the chamber and infrared irradiation was started.
The pulse interval was 2 s to avoid heating the sample. No overall
temperature increase was observed using the thermocouple.
The sample was shifted horizontally after each five pulses to re-
duce effects of radiation damage. The gas phase was analyzed us-
ing gas chromatography (GC5A; Shimadzu Corp.) for C2H5OH,
CH3CHO, and C2H4 using a Porapak Q column and for CO,
CO2, and C2H4 using an active carbon column. The retention time
and peak intensity were calibrated using corresponding standard
gases.
(
b)
The thermal reaction was carried out in the same cell in a batch
reaction at a reaction temperature range of 403–443 K. The gas
phase was analyzed using gas chromatography after 30 min
reaction.
Characterization of the Irradiated Sample.
photoelectron spectroscopy (XPS) were measured using JPS
200 (JEOL) in a micro-mode (field of view = 30 mm) with an
The X-ray
9
Al Kꢁ target at 20 kV–10 mA. The samples were taken from the
IR reaction chamber and were loaded into the XPS chamber. Dur-
ing that procedure, the samples were exposed to air and measure-
ments were carried out without further treatment. The binding
energies were calibrated against bulk O1s photoelectron to be
Figure 4. Gas chromatography analyses with different
excitation wavelengths, using Porapak Q (a) and active
carbon (b). 1, C2H4; 2, H2O; 3, CH3CHO; 4, C2H5OH;
and 5, CO2.
5
30 eV.
Results
Reactions. Figure 4 shows gas chromatograms of C2H5OH
reaction products on the MoO3 sample after irradiation of 1200,
Table 1 lists the reaction products after IR-FEL irradiation
and thermal reactions. The products were CH3CHO and C2H4:
C2H4 was the main product in the IR-induced reactions,
whereas the CH3CHO was formed in the thermal reaction.
ꢁ1
9
cusing condition using a focusing lens. When the sample was
67, and 814 cm . The samples were placed almost at the fo-
ꢁ
1
irradiated with 967 cm , which is a resonant wavenumber to
asymmetric stretching of Mo(=O)2 bonds, the gas chromatog-
raphy yielded peaks at 55, 68, 126, and 178 s which correspond-
ed respectively to C2H4, H2O, CH3CHO, and unreacted
C2H5OH. The gas chromatograms using an active carbon col-
umn in Figure 4b revealed C2H4 and CO2 formation, the latter
formation being 1/10 of the C2H4 formation. When the IR
ꢁ
1
The amount of photons at the 967 cm irradiation of Figure 4
1
7
was evaluated as 2 ꢂ 10 photons per pulse from the power
ꢁ1
(3.2 mJ pulse ). We introduced 1800 macropulses into the
2
0
sample. Consequently, about 4 ꢂ 10
photons (0:7 ꢂ
ꢁ
3
10 mol) were introduced into the sample. The total convert-
ꢁ
7
ed C2H5OH was 3:6 ꢂ 10 mol. The apparent quantum yield
ꢁ
1
ꢁ4
ꢁ1
.
wavenumber was changed to 1200 cm , which had no reso-
nant vibration mode of MoO3, we observed two peaks in the
Porapak column corresponding to H2O and C2H5OH; no peak
was observed in the active carbon column. The reaction scarce-
was ca. 5 ꢂ 10 at 967 cm
To determine the product dependence on the intensity
roughly, we changed the IR intensity by modifying the focus
position. Figure 6 shows photographs of MoO3 after irradia-
tion and gas chromatography outputs using a Porapak column.
We irradiated two infrared wavenumbers that were at 1200 and
ꢁ1
ly occurred at this wavelength. At 814 cm , which is another
resonance frequency of the symmetric stretching of M(=O)2
bonds, we were able to observe production of C2H4. At that
wavelength, the power was not so large, about 60% of that at
ꢁ1
967 cm under different focusing conditions.
We decreased the power density of IR by removing the lens
and increasing the focus size. Figures 6a and 6b respectively
display photographs of MoO3 after gently focused irradiation
ꢁ ꢁ1
1
9
67 cm and 50% of that at 1200 cm . Although the produc-
tion was small, we observed a distinctive amount of C2H4 and
little formation of CO2.
ꢁ
1
of FEL at 1200 and 967 cm , using only a focusing mirror.
The beam size was about 1 mm in diameter. We observed no
product formation.
The thermal reaction was carried out at 403–443 K using the
same sample cell in a batch reaction. The gas was sampled af-
ter 30 min of reaction. Figure 5 shows the gas chromatography
output of the reaction at 413 K. The main product was
ꢁ
1
Figure 6c shows MoO3 after irradiation with 1200 cm
FEL using a focusing lens, where beam size was about
0.1 mmꢀ on the sample. We found a plume after every laser
pulse shot, indicating the occurrence of optical breakdown.
We also found holes in the sample after irradiation, as por-
5
CH3CHO, as reported in the literature. The activation ener-
gies are 24 kJ mol as evaluated from Figure 5b. Even at
43 K no peak other than CH3CHO was detected.
ꢁ1
4