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Preparation of ordered mesoporous NbTa mixed oxide with crystallized wall
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
251
Preparation of ordered mesoporous NbTa mixed oxide with crystallized wall T. Katou ^ B. Lee \ D. Lu ^ J. N. Kondo \ M. H a r a ' and K. Domen''^
^Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama, 226-8503, Japan
''Core Research for Evolutional Science and Technology, Japan Science and Technology
An ordered 2D-hexagonal mesoporous Nb and Ta (Nb:Ta=l:l) mixed oxide was successfully crystallized.
A new strategy for preserving mesoporous structure after the
crystallization of the walls was performed by re-filling a templating material to the mesopores before crystallization for suppressing the destruction of mesoporous structure. Transmission electron microscopy and electron diffraction analyses revealed the formation of crystallized mesoporous NbTa mixed oxide with single crystal phases in periodical mesoporous structure (denoted NbTa-TIT-2) at ca. 100-nm range.
1. INTRODUCTION
Compared with silica-based mesoporous materials, less work has been directed to non-silica mesoporous materials although they have high potential in wide range of applications.' oxides
For example, in terms of photocatalysis, mesoporous pure and mixed Ta
showed
considerable
photocatalytic
activity
decomposition, in spite of the amorphous wall structure."^'^
for
the
stoichiometric
water
The crystallization of the walls
of mesoporous materials would be one of the important subjects for advancing various applications, although at present, the wall structure of almost all the mesoporous materials is amorphous and the preservation of the original mesoporous structure after crystallization is difficult.
We have attempted to solve this problem."^'^
One of the strategies is to re-fill the
This research was supported by Core Research for Evolutional Science and Technology (CREST) program of the Japan Science and Technology Corporation.
252
mesopores and cover the whole particles by thermally stable materials.
If this re-templating
material could be removed after crystallization of mesoporous substance, mesoporous structure would be preserved.
In this report, among various re-templating sources (sucrose,
ftirfuryl alcohol, glucose, silica, BaCh, etc), the results obtained by using amorphous carbon using furftiryl alcohol are shown. 2. EXPERIMENTAL The 2D-hexagonally ordered mesoporous NbTa oxide (amorphous precursor) was synthesized by the method introduced in previous report.^
The crystallized ordered
mesoporous NbTa oxide, NbTa-TIT-2, was prepared as follows.
The ftirfuryl alcohol vapor
(0.023 mol/min) with nitrogen gas (30 ml/min) was passed on the amorphous precursor fixed in reactor at 473 K.
After the furftiryl alcohol vapor treatment, polymerized fiirftiryl alcohol
is accumulated in pores of amorphous precursor.
Then, the carbonization of polymerized
furfuryl alcohol was performed at 823 K for 3 h in vacuo.
Then, the crystallization of the
carbon-filled and coated mesoporous NbTa oxide was conducted by the calcination at 923 K for 2 h in helium to avoid the removal of re-templated carbon during the high-temperature treatment.
Finally, the carbon included in the crystallized sample was eliminated by the
calcination at 773 K in air. X-ray diffraction (XRD) patterns were obtained using a Rigaku RINT 2100 diffractometer with CuKa radiation.
N2 sorption isotherms were recorded using a Coulter SA3100 system.
The sorption data were analyzed by Barrett-Joyner-Hallenda (BJH) method.
Transmission
electron microcopy (TBM) images and electron diffraction (ED) patterns were obtained by JEOLJEM2010F(200kV). 3. RESULTS AND DISCUSSION The physicochemical properties of mesoporous NbTa mixed oxide were considerably affected by the amount of metal sources, as well as the amount of added water.^
The results
of XRD, N2 sorption isotherm and a TEM image together with the ED pattern of mesoporous NbTa oxide prepared under the optimized condition are shown in Fig.l.
As shown in Fig.l,
the low-angle (1-6 degree) XRD pattern and the N2 adsorption isotherm are consistent with hexagonally ordered mesoporous structure.
The TEM image and ED pattern ftirther support
the proposed 2D-hexagonal mesoporous structure of NbTa oxide.
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Fig. 1. Low-angle XRD pattern (left), N2 sorption isotherm and representative TEM image and diffraction pattern (right) for NbTa oxide sample prepared under the optimized conditions of water and metal sources.
It is noted that the wall thickness estimated by assuming a hexagonal structure was ca. 2.6 nm. Crystallization of the sample after calcination of 923 K was confirmed by wide-angle XRD pattern (not shown).
Fig.2(a) displays a typical TEM image of NbTa-TIT-2, crystallized
NbTa oxide with periodical mesoporous structure.
The mixed spot ED pattern (inset of
Fig.2(a)) obtained from a whole particle reveals the presence of multi crystal phases.
In
order to estimate the size of a single crystal domain, ED patterns were collected from places of different sizes.
The ED pattern (Fig.2(b)) taken from a 100-nm range (shown image)
shows a spot ED pattern, which indicates a single crystal phase in the range.
From high
resolution TEM (HRTEM) image of walls displayed in Fig.2(c), lattice fringes in a limited place, where ordered pores are directly observed, run in the same direction.
From these
results, it is considered that the ED pattern of mixed spots (inset of Fig.2(a)) taken from a whole particle is resulted from the fact that a crystallized particle with the ordered mesoporous structure consists of phases by ca. 100-nm ranged single crystal domains.
This
means that the ordered crystallized mesoporous NbTa oxide, NbTa-TIT-2, is different from single crystal particles of worm-hole mesoporous NbTa oxide, NbTa-TIT-1, in the size of a single crystal domain.
In the case of worm-hole structure, single crystal domain spreads to
several hundreds nanometer size, whereas ordered mesoporous structure
254
:5^" ii--#
m
-•K-
SO.Onm
(^\
Fig. 2. Typical TEM images and ED patterns of NbTa-TIT-2.
(a) A particle with periodical
mesoporous structure (inset : ED pattern collected from whole particle), (b) periodical mesoporous structure in 100-nm ranges single crystal domain (inset : ED pattern collected from the image) and (c) HRTEM image of walls.
suppresses the size to ca. 100 nm.
It is also mentioned that in the case of the
2D-hexagonally ordered mesoporous NbTa oxide calcined under the same condition without re-filling template, the crystallized sample as NbTa-TIT-1 was obtained.
Therefore, the
presence of re-filling template appeared to be effective for preserving mesoporous structure during the crystallization. In conclusion, it was found that NbTa-TIT-2 prepared by the use of furfuryl alcohol as re-templating source possessed ca. 100-nm ranged single crystal domains in a particle with the original 2D-hexagonally ordered mesoporous structure.
We expect that this strategy
would be improved and become one of the general methods applicable to various materials. REFERENCES 1. U. Ciesla and F. Schiith, Microporous and Mesoporous Materials, 1999, 27, 131. 2. Y. Takahara, J. N. Kondo, T Takata, D. Lu and K. Domen, Chem. Mater., 13, 1200. 3. M. Uchida, J. N. Kondo, D. Lu and K. Domen, Chem. Lett., 498 (2002). 4. B. Lee, D. Lu, J. N. Kondo and K. Domen, Chem. Commun., 2001, 2118. 5. B. Lee, T. Yamashita, D. Lu, J. N. Kondo and K. Domen., Chem. Mater, 2002, 14, 867. 6. T. Katou, D. Lu, J. N. Kondo and K. Domen, J. Mater. Chem., 2002, 12 1480.
251
Preparation of ordered mesoporous NbTa mixed oxide with crystallized wall T. Katou ^ B. Lee \ D. Lu ^ J. N. Kondo \ M. H a r a ' and K. Domen''^
^Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama, 226-8503, Japan
''Core Research for Evolutional Science and Technology, Japan Science and Technology
An ordered 2D-hexagonal mesoporous Nb and Ta (Nb:Ta=l:l) mixed oxide was successfully crystallized.
A new strategy for preserving mesoporous structure after the
crystallization of the walls was performed by re-filling a templating material to the mesopores before crystallization for suppressing the destruction of mesoporous structure. Transmission electron microscopy and electron diffraction analyses revealed the formation of crystallized mesoporous NbTa mixed oxide with single crystal phases in periodical mesoporous structure (denoted NbTa-TIT-2) at ca. 100-nm range.
1. INTRODUCTION
Compared with silica-based mesoporous materials, less work has been directed to non-silica mesoporous materials although they have high potential in wide range of applications.' oxides
For example, in terms of photocatalysis, mesoporous pure and mixed Ta
showed
considerable
photocatalytic
activity
decomposition, in spite of the amorphous wall structure."^'^
for
the
stoichiometric
water
The crystallization of the walls
of mesoporous materials would be one of the important subjects for advancing various applications, although at present, the wall structure of almost all the mesoporous materials is amorphous and the preservation of the original mesoporous structure after crystallization is difficult.
We have attempted to solve this problem."^'^
One of the strategies is to re-fill the
This research was supported by Core Research for Evolutional Science and Technology (CREST) program of the Japan Science and Technology Corporation.
252
mesopores and cover the whole particles by thermally stable materials.
If this re-templating
material could be removed after crystallization of mesoporous substance, mesoporous structure would be preserved.
In this report, among various re-templating sources (sucrose,
ftirfuryl alcohol, glucose, silica, BaCh, etc), the results obtained by using amorphous carbon using furftiryl alcohol are shown. 2. EXPERIMENTAL The 2D-hexagonally ordered mesoporous NbTa oxide (amorphous precursor) was synthesized by the method introduced in previous report.^
The crystallized ordered
mesoporous NbTa oxide, NbTa-TIT-2, was prepared as follows.
The ftirfuryl alcohol vapor
(0.023 mol/min) with nitrogen gas (30 ml/min) was passed on the amorphous precursor fixed in reactor at 473 K.
After the furftiryl alcohol vapor treatment, polymerized fiirftiryl alcohol
is accumulated in pores of amorphous precursor.
Then, the carbonization of polymerized
furfuryl alcohol was performed at 823 K for 3 h in vacuo.
Then, the crystallization of the
carbon-filled and coated mesoporous NbTa oxide was conducted by the calcination at 923 K for 2 h in helium to avoid the removal of re-templated carbon during the high-temperature treatment.
Finally, the carbon included in the crystallized sample was eliminated by the
calcination at 773 K in air. X-ray diffraction (XRD) patterns were obtained using a Rigaku RINT 2100 diffractometer with CuKa radiation.
N2 sorption isotherms were recorded using a Coulter SA3100 system.
The sorption data were analyzed by Barrett-Joyner-Hallenda (BJH) method.
Transmission
electron microcopy (TBM) images and electron diffraction (ED) patterns were obtained by JEOLJEM2010F(200kV). 3. RESULTS AND DISCUSSION The physicochemical properties of mesoporous NbTa mixed oxide were considerably affected by the amount of metal sources, as well as the amount of added water.^
The results
of XRD, N2 sorption isotherm and a TEM image together with the ED pattern of mesoporous NbTa oxide prepared under the optimized condition are shown in Fig.l.
As shown in Fig.l,
the low-angle (1-6 degree) XRD pattern and the N2 adsorption isotherm are consistent with hexagonally ordered mesoporous structure.
The TEM image and ED pattern ftirther support
the proposed 2D-hexagonal mesoporous structure of NbTa oxide.
253 200 150
ra
100
"c
13
e
'o
^
50
>
CD
10.0 nm
:^;-?-
0 •
0
^ €
02
04
oe P/Pu
>2. 3r '(/) c
_
0)
^ _J
2
•
1
^- — 1
r
1
'Mi^
1
3 4 5 26 (degree)
Fig. 1. Low-angle XRD pattern (left), N2 sorption isotherm and representative TEM image and diffraction pattern (right) for NbTa oxide sample prepared under the optimized conditions of water and metal sources.
It is noted that the wall thickness estimated by assuming a hexagonal structure was ca. 2.6 nm. Crystallization of the sample after calcination of 923 K was confirmed by wide-angle XRD pattern (not shown).
Fig.2(a) displays a typical TEM image of NbTa-TIT-2, crystallized
NbTa oxide with periodical mesoporous structure.
The mixed spot ED pattern (inset of
Fig.2(a)) obtained from a whole particle reveals the presence of multi crystal phases.
In
order to estimate the size of a single crystal domain, ED patterns were collected from places of different sizes.
The ED pattern (Fig.2(b)) taken from a 100-nm range (shown image)
shows a spot ED pattern, which indicates a single crystal phase in the range.
From high
resolution TEM (HRTEM) image of walls displayed in Fig.2(c), lattice fringes in a limited place, where ordered pores are directly observed, run in the same direction.
From these
results, it is considered that the ED pattern of mixed spots (inset of Fig.2(a)) taken from a whole particle is resulted from the fact that a crystallized particle with the ordered mesoporous structure consists of phases by ca. 100-nm ranged single crystal domains.
This
means that the ordered crystallized mesoporous NbTa oxide, NbTa-TIT-2, is different from single crystal particles of worm-hole mesoporous NbTa oxide, NbTa-TIT-1, in the size of a single crystal domain.
In the case of worm-hole structure, single crystal domain spreads to
several hundreds nanometer size, whereas ordered mesoporous structure
254
:5^" ii--#
m
-•K-
SO.Onm
(^\
Fig. 2. Typical TEM images and ED patterns of NbTa-TIT-2.
(a) A particle with periodical
mesoporous structure (inset : ED pattern collected from whole particle), (b) periodical mesoporous structure in 100-nm ranges single crystal domain (inset : ED pattern collected from the image) and (c) HRTEM image of walls.
suppresses the size to ca. 100 nm.
It is also mentioned that in the case of the
2D-hexagonally ordered mesoporous NbTa oxide calcined under the same condition without re-filling template, the crystallized sample as NbTa-TIT-1 was obtained.
Therefore, the
presence of re-filling template appeared to be effective for preserving mesoporous structure during the crystallization. In conclusion, it was found that NbTa-TIT-2 prepared by the use of furfuryl alcohol as re-templating source possessed ca. 100-nm ranged single crystal domains in a particle with the original 2D-hexagonally ordered mesoporous structure.
We expect that this strategy
would be improved and become one of the general methods applicable to various materials. REFERENCES 1. U. Ciesla and F. Schiith, Microporous and Mesoporous Materials, 1999, 27, 131. 2. Y. Takahara, J. N. Kondo, T Takata, D. Lu and K. Domen, Chem. Mater., 13, 1200. 3. M. Uchida, J. N. Kondo, D. Lu and K. Domen, Chem. Lett., 498 (2002). 4. B. Lee, D. Lu, J. N. Kondo and K. Domen, Chem. Commun., 2001, 2118. 5. B. Lee, T. Yamashita, D. Lu, J. N. Kondo and K. Domen., Chem. Mater, 2002, 14, 867. 6. T. Katou, D. Lu, J. N. Kondo and K. Domen, J. Mater. Chem., 2002, 12 1480.