Photoluminescence properties of Mo-MCM-41 mesoporous molecular sieves and their photocatalytic reactivity for the decomposition of NOx

Article abstract of DOI:10.1246/cl.2000.408

Investigations of the mesoporous Mo-MCM-41 molecular sieves have shown that they include the tetrahedrally coordinated Mo-oxide species, which exhibits a photoluminescence spectrum attributed to the radiative decay from the charge transfer excited state. Also, the photocatalytic reaction of NO into N₂ was found to proceed much more efficiently in the presence of propane than in the absence of propane. The dynamic quenching of the photoluminescence spectrum by the addition of NO or propane indicates that the excited state of the highly dispersed Mo-oxide species having a tetrahedral coordination plays a significant role in this reaction.

Full text of DOI:10.1246/cl.2000.408

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Chemistry Letters 2000
Photoluminescence Properties of Mo-MCM-41 Mesoporous Molecular Sieves
and Their Photocatalytic Reactivity for the Decomposition of NOx
Shinya Higashimoto, Rie Tsumura, Shu Guo Zhang, Masaya Matsuoka, Hiromi Yamashita, Catherine Louis,†
†
Michel Che, and Masakazu Anpo*
Department of Applied Chemistry, Osaka Prefecture University, 1-1 Gakuen-cho, Sakai, Osaka 599-8531
†
Laboratoire de Réactivité de Surface, Université P. et M. Curie, UNR 7609-CNRS, 4 place Jussieu,
Tour 54, 75252 Paris Cedex 05, France
(Received December 27, 1999; CL-991094)
Investigations of the mesoporous Mo-MCM-41 molecular
for 2 h. The photoluminescence and lifetimes were measured at
77 K with a Shimadzu RF-501 spectrofluorophotometer and an
apparatus for lifetime measurements, respectively. The photo-
catalytic reactions of NO in the presence and absence of
propane were carried out at 298 K with a high pressure mercury
lamp through a UV cut filter (λ > 270 nm). 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 analyzed by online gaschromatography.
The results of XRD analysis indicated that Mo-MCM-41
mesoporous molecular sieves have the same structure of MCM-
sieves have shown that they include the tetrahedrally coordinat-
ed Mo-oxide species, which exhibits a photoluminescence spec-
trum attributed to the radiative decay from the charge transfer
excited state. Also, the photocatalytic reaction of NO into N2
was found to proceed much more efficiently in the presence of
propane than in the absence of propane. The dynamic quench-
ing of the photoluminescence spectrum by the addition of NO
or propane indicates that the excited state of the highly dis-
persed Mo-oxide species having a tetrahedral coordination
plays a significant role in this reaction.
4
1 molecular sieves but having mesopores larger than 20 Å and
6
Zeolites involving transition metal ions within their frame-
works are known to exhibit unique and fascinating properties
for applications not only in catalysis but also for various photo-
no other phases of the Mo-oxide species are formed.
As shown in Figure 1, Mo-MCM-41 (1.0 Mo wt%)
exhibits a photoluminescence spectrum at around 400-600 nm
upon excitation at around 295 nm (defined as X), which coin-
cides with the photoluminescence spectrum of the tetrahedrally
1
chemical processes. The transition metal ions were incorporat-
ed within the zeolite framework in a highly dispersed state and
exhibited high photocatalytic reactivity for such reactions as
hydrogenation, oxidation and the metathesis reaction of hydro-
carbon. The capacity of the Mo-oxide species in the framework
is limited due to the great strain which occurs by the insertion
of large transition metal ions in the rigid zeolite framework.
However, the Mo-oxide species can be easily incorporated into
the framework of MCM-41 due to its greater flexibility in struc-
ture.2 It was found to enable the design of tetrahedrally coordi-
nated Mo-oxide species with high loading amounts of at least
up to 1 wt% of Mo with this MCM-41, while only 0.1 wt% of
Mo for the anchoring method using a facile reaction of MoCl5
4
, 7
coordinated Mo-oxide species highly dispersed on SiO2. The
excitation and emission spectra are attributed to the following
charge transfer processes on the Mo-O moieties of the tetrahe-
2
-
dral molybdate ions (MoO ), involving an electron transfer
4
2
-
6+
from the O to Mo ions and a reverse radiative decay from
4
, 7
the charge transfer excited triplet state (eq. 1).
The width
and the wavelength at the maximum intensity of the emission
band do not change upon varying the excitation wavelength,
indicating that there is only one luminescent species (photolu-
minescence lifetime: 2.25 ms). These results indicate that the
Mo-oxide species showing absorption in the region of 295 nm,
form a tetrahedral coordination in a highly dispersed state.
with the surface OH groups on SiO was possible, showing one
2
3
of the advantages of using this MCM-41 system. Recently, we
have shown that photoluminescence measurements can provide
unique information on zeolites containing V, Ti or Cr ions in
their frameworks and thus implications for high photocatalytic
reactivity.4 The present study investigates the photolumines-
cence properties and the photocatalytic reaction of NO in the
presence and absence of propane on Mo-MCM-41.
, 5
The Mo-MCM-41 mesoporous molecular sieves (0.5, 1.0,
2
.0, 4.0 Mo wt%) were synthesized using tetraethyl orthosili-
.
cate (TEOS) and (NH ) Mo O 4H O as starting materials and
4
6
7
24
2
cetyltrimethylammonium bromide (CTMABr) as a template in
2
accordance with previous literature. After the products were
recovered by filtration, washed with distilled water several
times and dried at 373 K for 12 h, the calcination of the samples
were performed under a dry flow of air at 773 K for 8 h.
Prior to spectroscopic measurements and photocatalytic
reactions, the catalyst was degassed at 773 K for 2 h and cal-
cined in O (>20 Torr) at 773 K for 2 h, then degassed at 473 K
2
Copyright © 2000 The Chemical Society of Japan

Chemistry Letters 2000
409
Figure 3 shows the relationship between the yields of N2,
or acetone formation and relative intensity of the absorption
spectra observed in the region of 295 nm for Mo-MCM-41 (0.5,
1.0, 2.0 and 4.0 Mo wt%). As shown in Figure 3, the intensities
of the absorption spectra in the region of 295 nm have a good
On the other hand, as shown in Figure 2, there are at least
two luminescent species (the absorption spectrum can be de-
convoluted into two components having the wavelength regions
of X and Y in Figure 2: 295 and 310 nm, respectively) on the
Mo-MCM-41 (4.0 Mo wt%). The increase of Mo content (from
relationship with the yields of N or acetone formation, sug-
2
gesting that the charge transfer excited triplet state of the tetra-
hedrally coordinated Mo-oxide species in a highly dispersed
state, having absorption in the region of 295 nm, plays a signifi-
cant role in the photocatalytic reaction of NO in the presence of
1.0 to 4.0 Mo wt%) leads to the formation of the other emitted
propane, leading to the formation of N and acetone.
2
site (in the region of Y) probably induced by Mo-Mo interac-
tions via oxygen ion (Mo-O-Mo), which causes a more efficient
radiationless energy transfer leading to a decrease in the photo-
luminescence lifetime (0.91 ms).
After UV-irradiation of Mo-MCM-41 in the presence of
propane, its subsequent evacuation at 295 K did not lead to the
recovery of original photoluminescence intensity but exhibited
5+
the ESR signals of the Mo ions, showing that the charge trans-
fer excited triplet state of the Mo-oxide species abstracts the H
atom from propane, leading to the formation of the hydrocar-
The addition of propane or NO onto Mo-MCM-41 (1.0 Mo
wt%) leads to the efficient quenching of the photoluminescence
as well as a shortening of the photoluminescence lifetime,
whereas on Mo-MCM-41 (4.0 Mo wt%), NO or propane inter-
acts with the Mo-oxide species showing absorption in the
region of X more efficiently than Y. It can, therefore, be
expected that Mo-oxide species which has the absorption in the
region of X shows a high photocatalytic reactivity for the
decomposition of NO in the presence of propane.
8
bon-radical. Furthermore, only small amounts of NO or
propane were photoadsorbed under UV-irradiation, however, a
great enhancement of the photoadsorption occurred in the pres-
ence of a mixture of NO and propane. Taking these results into
consideration, we are able to propose the importance of the
intermediate species formed from NO and the hydrocarbon-radi-
cals, which were subsequently followed by further reaction with
The photocatalytic reactions of NO in the presence and
absence of propane were performed on Mo-MCM-41. No prod-
ucts could be detected under dark conditions. UV-irradiation of
NO to produce N as well as oxygen-containing compounds.
2
References
1
M. Anpo, S. G. Zhang, S. Higashimoto, M. Matsuoka, H.
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Mo-MCM-41 in the presence of NO led to the evolution of N as
2
well as N O and NO . Furthermore, the photocatalytic reaction
2
2
of NO proceeded in the presence of propane much more effi-
ciently than in the direct decomposition of NO on Mo-MCM-41,
leading to the formation of propylene and oxygen-containing
compounds such as CH COCH and CO etc. It was found that
2
W. Zhang, J. Wang, P. T. Tanev, and T. J. Pinnavaia, J. Chem.
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the total amount of oxygen involving the oxygen-containing
products corresponds well to the amount of NO consumed in the
photocatalytic reaction. Furthermore, the turnover frequency
6
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8
(
defined as to the value of the number of photo-formed N mole-
2
cules divided by the total number of Mo species involved in the
used catalyst) exceeded unity after prolonged UV irradiation in
the presence of a mixture of NO and propane. These results
clearly indicate that the reaction proceeds photocatalytically.