Crystallization of highly ordered mesoporous niobium and tantalum mixed oxide

Crystallization of highly ordered mesoporous niobium and tantalum mixed oxide

Studies in Surface Science and Catalysis, volume 154 E. van Steen, L.H. Callanan and M. Claeys (Editors) © 2004 Elsevier B.V. All rights reserved. 95...

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Studies in Surface Science and Catalysis, volume 154 E. van Steen, L.H. Callanan and M. Claeys (Editors) © 2004 Elsevier B.V. All rights reserved.

951

CRYSTALLIZATION OF HIGHLY ORDERED MESOPOROUS NIOBIUM AND TANTALUM MIXED OXIDE Kondo, J . N . \ Katou, T.\ Lu, D.^ Hara, M J and Domen, K.^'^ ^Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku Yokohama 226-8503, Japan. ^CREST (Core Research for Evolutional Science and Technology) of JST (Japan Science and Technology). ABSTRACT A highly ordered (2D-hexagonal) mesoporous Nb-Ta mixed oxide was synthesized as a amorphous precursor for crystal. It showed 184 m^g'^ BET surface area and 5.6 nm pore size, and TEM images and electron diffraction patterns verified the 2D-hexagonal mesoporous structure. Crystallization was performed by calcination at 650°C for 1 h under various conditions. When Nb-Ta oxide was calcined in air, single crystal particles of wormhole mesoporous material with 66 m^g"^ surface area and 13 nm pore size were obtained due to a drastic rearrangement of mesostructure accompanied by crystallization. Next, carbon deposition inside the pores and over the particles was attempted followed by crystallization in He. The deposited carbon was removed by calcination in air after crystallization. BET surface area decreased to 93 m^g\ but preservation of porosity was improved compared with the sample crystallized in air. 20-30% of the original ordered mesopores were maintained and coexisted with the original structure. The rest resulted in the rearrangement to 13 nm pores. In addition, crystallization in UHV chamber for TEM observation preserved the original ordered mesoporous structure, while the amorphous wall was converted into poly crystalline domains. INTRODUCTION Among various mesoporous materials involving surface-modified mesoporous silica, mesoporous organnosilica hybrid materials, non-siliceous mesoporous materials, and so on, only a few of them consist of ordered or crystallized inorganic phases. Mesoporous zeolite single crystals were synthesized with carbon nanoparticles [1] or with carbon nanotubes [2] as templates of mesopores. In a class of mesoporous hybrid organosilica, alternate phenylene (-C6H4-) and silica layers are arranged in crystal-like wall structure in 2D-hexagonal mesoporous organosilica hybrid material [3]. The ordered structures in the wall of these mesoporous materials are constructed during the initial formation of mesostructure, and do not accompany phase transition of the amorphous wall structure. On the other hand, the crystallization of amorphous inorganic phases of mesoporous metal oxides is known to resuh in destruction of porous structure to form bulk materials with low surface area or to generate nanocrystals, in which interparticle voids appear as expanded mesopores. Only the reported exception is the case of mesoporous Nb and Ta mixed oxide (Nb-Ta oxide). When wormhole-like mesoporous Nb-Ta oxide with amorphous wall was crystallized by calcination in air, mesoporous single crystal particle were obtained; wormhole-like mesopores were observed in a single crystal particle, which showed clear spot electron diffraction pattern and lattice fringes in an uniform direction althrough a particle [4]. However, the structural change before and after crystallization could not be discussed in detail because both amorphous and crystallized mesoporous materials had non-ordered mesoporus structure. Therefore, we optimised the preparation condition and obtained a highly ordered 2D-hexagonal mesoporous Nb-Ta oxide [5]. Crystallization of the highly ordered mesoporous Nb-Ta oxide under various conditions is investigated in the present study. EXPERIMENTAL The detailed preparation method of 2-D hexagonal mesoporous Nb-Ta oxide is described elsewhere [5]. Briefly, 1 g block co-polymer template (Pluronic P123) was dissolved in 10 g ethanol, and equal amounts of NbCls and TaCls (total 5.5 mmol) were added with vigorous stirring. Then, 18 mmol water was added before

952 aging. The aging was performed at 40 °C for 7 days in an oven. The block co-polymer template was removed by calcination at 400°C for 5 h to provide 2D-hexagonal mesoporous Nb-Ta oxide. The carbonfiUing and coating was performed by using condensation and coking of furfuryl alcohol. Furfuryl alcohol vapor (0.023 molmin"^) in nitrogen gas (30 mLmin"^) was passed through the amorphous precursor fixed in a reactor at 200°C for 2 h. The brown color of the resulting sample is due to the accumulation of polymerized furfuryl alcohol. The polymerized furfuryl alcohol in the brown sample was then changed to black carbon by carbonization at 500 °C for 3 h in evacuation. The sample was subsequently crystallized by heating in a He atmosphere. The carbon template in the crystallized sample was then removed by calcination at 500°C K for 15 h in air. Small angle powder XRD patterns of the products were obtained on a PHILIPS X'Pert-MPD PW3050 diffractometer using Cu-Ka radiation (40 kV, 40 mA) at a 0.02 step size. Nitrogen adsorption-desorption isotherms at 77 K were measured using a Micrometrics COULTER OMNISORP lOOCX system. Samples were normally prepared for nitrogen adsorption measurement by outgassing at 100°C under vacuum until a final pressure of 1 x 10'^ Torr was reached. BET surface areas were estimated over a relative pressure (P/Po) ranging from 0.05 to 0.30. Pore size distribution was obtained from the analysis of the adsorption branch of the isotherms using the BJH (Barrett-Joyner-Halenda) method. Images of TEM were obtained on a JEOL 201 OF electron microscope operated at 200 keV. The samples for TEM observation were prepared by dropping mesoporous Mg-Ta oxide powder dispersed in 2-propanol on a copper grid covered with carbon film.

RESULTS AND DISCUSSION A small angle X-ray diffraction (XRD) pattern of the highly ordered 2D-hexagonal mesoporous Nb-Ta oxide are shown in Figure lA. A sharp peak corresponding to d(lOO) = lA nm together with small peaks at higher angle region is indicative of 2D-hexagonal mesoporous structure. By the analysis of nitrogen adsorption-desorption isotherm (Figure IB), which is typical to mesoporous materials (type IV pattern), BET surface area, pore volume and pore size were estimated as 184 m ^ g \ 0.34 mLg"^ and 5.6 nm, respectively [5]. The wall thickness is thus estimated as 2.9 nm from d(lOO) value and the pore size. The 2-D hexagonal mesoporous structure was further confirmed by transmission electron microscope (TEM) observation and electron diffraction patterns as shown in Figure 2. The highly ordered structure is supported by the presence of diffraction spots in wide range. The homogeneous mixing of Nb and Ta in oxide phase was confirmed by energy dispersive X-ray analysis (EDS) in about 5 nm range using TEM apparatus. The crystallization was accomplished by the same method as that performed on wormhole-like mesoporous Nb-Ta oxide, i.e., calcination in air at 650°C for 1 h [4]. Three different environments were selected for crystallization by calcination at 650°C for 1 h; 1) calcination in air, 2) calcination in He after the mesopores were filled and particles were covered by carbon followed by removal of carbon by calcination in air, 3) and calcinations in ultrahigh vacuum (UHV) chamber for in-situ TEM observations. The complete crystallization in any cases was confirmed by XRD patterns similarly to the case of cry stallization of wormhole-like mesoporous Nb-Ta oxide [4]. tm •

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Figure 2. TEM images and electron diffraction patterns of 2-D hexagonal mesoporous Nb-Ta oxide. First, the material crystallized by simple calcination in air is shown. The nitrogen adsorption-desorption isotherm of the crystallized Nb-Ta oxide is compared with that before crystallization in Figure 3. Although a type IV isotherm pattern was still observed, the original mesoporous structure apparently disappeared. BET surface area and pore volume decreased from 184 to 66 m^g'^ and from 0.34 to 0.28 mLg\ respectively, with increasing in pore size from 5.6 to 13.0 nm (Figure 3 inset).

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Figure 3. Change of mesoporous Nb-Ta oxide in nitrogen adsorption-desorption isotherm and pore size distribution before (A) and after (B) crystallization in air. Therefore, rigorous rearrangement of mesoporous structure occurred upon crystallization, which caused extreme mass transfer, and the original ordered 2D-hexagonal mesoporous structure could not be sustained. Because the crystallized Nb-Ta oxide still indicated type IV isotherm pattern with expanded mesopores, the local relation between the single crystal domains and mesopores was studied by TEM observations and electron diffractions. A typical TEM image of Nb-Ta oxide crystallized by calcination in air is shown in Figure 4 (left), where wormhole-like mesopores were found. These wormhole-like mesopores are regarded as being responsible for the capillary condensation of nitrogen molecules at P/PQ = 0.8 (13.0 nm pores). There are two possible interpretation of the expanded mesopores; 1) interparticle voids in secondary particles, and 2) mesopores in single crystal domains. These are distinguished by electron diffraction patterns. If a crystallized porous particle consists of small single crystals (case 1), an electron diffraction pattern measured from whole particle would result in a ring pattern typical to a secondary poly-crystal particle. However, the electron diffraction pattern taken from whole particle shown in Figure 4 demonstrated a clear spot pattern (Figure 4A), which is characteristic to a single crystal structure. From this result the presence of only one single crystal domain in the particle is evidenced. Since this particle could be interpreted as almost amorphous porous material with a small single crystal domain, we attempted to observe electron diffraction patterns from several different parts of the same particle with the purpose to study the crystal Unity.

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Figure 4. TEM images and electron diffraction patterns of mesoporous Nb-Ta oxide aftercrystallization by calcination in air at 650 °C. The electron diffraction A was measured from whole particle and those B-E were collected from corresponding places indicated in the image. The electron diffraction patterns shown in (B)-(E) in Figure 4 were collected from the corresponding parts indicated by circles, and sharp spot patterns were obtained. Accordingly, only one single crystal domain exists in each part. It should be noted that all the electron diffraction patterns are identical to that obtained from whole particle. Therefore, this mesoporous particle is not a secondary poly-crystalline particle, but is regarded as a single crystal. In addition, high resolution TEM images reveal the presence of mesopores in a single crystal domain. One of such images is shown in Figure 4 (right), where a mesopore is observed at the center surrounded by lattice fringes in the uniform direction. This directly evidences that a mesopore is not resulted from an inter-particle void, but is present in a continuous crystal structure. Consequently, it is found that the amorphous Nb-Ta oxide with a highly ordered 2D-hexagonal mesoporous structure is converted into single crystals with wormhole mesoporous structure when crystallized in air, similarly to the crystallized Nb-Ta oxide obtained by calcination of amorphous precursor with wormhole-like mesoporous structure. Secondly, stabilization of the ordered mesoporous structure was attempted by filling the mesopores of ordered mesoporous Nb-Ta oxide with carbon. The presence of carbon inside mesopores and outside particles after the treatment was directly observed by TEM [6]. Then, carbon-containing 2D-haxagonal mesoporous Nb-Ta oxide was calcined at 650 °C in He for crystallization, followed by calcination in air at 500 °C for 15 h in order to remove the carbon. Nitrogen adsorption-desorption isotherm of the crystallized mesoporous Nb-Ta oxide is compared with that of the original one in Figure 5. The nitrogen uptake at P/Po = 0.5-0.7 for the crystallized sample is ca. 1/4 of that of the amorphous precursor. The nitrogen uptake at P/Po = 0.8 is derived from large pores, which increased markedly in size after crystallization of the amorphous precursor. In addition, the mesopore volume decreased from 0.34 to 0.23 ml g\ most probably due to the collapse of mesopores. Therefore, it is considered that ca. 3/4 of the original mesopores (ca. 6.0 nm) in the amorphous precursor was converted to larger pores or collapsed after crystallization. This is consistent with the XRD pattern of the crystallized sample (not shown): the presence and decrease in intensity of the d(lOO) peak was observed for the sample after crystallization [6]. In some TEM images, both ordered mesoporous structure and lattice fringes due to the crystallized material were simultaneously observed. Typical high resolution TEM images are shown in Figure 6, where the presence of ordered mesopores with the original pore size and structure are observed together with lattice fringe patterns in uniform directions over the mesopores. Therefore, mesopores are found to exist within single crystal domains. By detailed analysis of electron diffraction, the size of these single crystal domains with ordered mesopores was estimated to be about 100 nm [6]. The insufficient carbon content inside the pores of the sample is regarded as one of the reasons for low yield of crystallized and highly ordered mesoporous parts, with the rest resulting in the same material as that crystallized without carbon treatment. Although the yield of single crystal Nb-Ta oxide maintaining a hexagonal ordered mesoporous structure after crystallization is still low at present, it is considered that this carbon templating method is effective for controlling the mesoporous structure of crystallized mesoporous materials.

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Figure 5. Change of mesoporous Nb-Ta oxide in nitrogen adsorption-desorption isotherm before (A) and after (B) crystallization with carbon templating.

Figure 6. High resolution TEM images of crystallized with carbon templating method. Finally, crystallization was carried out in UHV chamber for in-situ TEM observation. The 2D-hexagonal mesoporous Nb-Ta oxide with amorphous inorganic phase was set on a heating sample holder in an UHV chamber of TEM apparatus. A thin particle, in which ordered mesopores were clearly observed in parallel with the channel direction, was selected. The sample was heated gradually (5°C min') and kept for 10 min for stabilization of the particle before TEM observation at certain temperatures. TEM image in Figure 7A was the amorphous precursor of 2D-hexagonal mesoporous Nb-Ta oxide measured at room temperature and the presence of the highly ordered structure all over the particle was confirmed. The sample was once heated to 450°C, and TEM images were measured from 500 to 800°C at 50°C interval. By increasing temperature the particle was gradually tilted, and finally at 800°C (Figure 8B), the flat particle seems to be narrowed due to inclination. Consequently, the clearly observed ordered mesopores at the left edge of the sample measured at room temperature became ambiguous at 800°C, while mesopores were obvious at the lower right comer of the particle at 800°C.

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Figure 7. TEM images of 2D-hexagonal mesoporous Nb-Ta oxide measured at room temperature (A) and 800°C (B).

Figure 8. Electron diffraction of crystallized 2D-hexagonal mesoporous Nb-Ta oxide at 800°C. For examining the mesoporous and crystalline structures in detail, electron diffraction patterns of the sample at each temperature were collected. Two electron diffraction patterns as shown in Figure 8 were observed from the whole particle heated at 800°C. The electron diffraction in Figure 8A is analogous to ring pattern typical to poly-crystal structure, and therefore, crystallization of the Nb-Ta oxide under the UHV condition is convinced. The gradual crystallization process was observed by measuring the electron diffractions at TEM-observed temperatures during the heating procedure. The contract of the ring electron diffraction pattern became stronger as increasing in temperature, and seemed to be saturated at 800°C, indicating the complete crystallization at this temperature. Although the 2D-hexagonal mesoporous structure was indistinct in TEM image in Figure 7B, electron diffraction pattern in Figure 8B supports the presence of the original ordered structure in the crystallized mesoporous Nb-Ta oxide. From these results, it is found that the 2D-hexagonal ordered mesoporous Nb-Ta oxide was crystallized into poly-crystal preserving the original ordered structure by heating in UHV condition. Since XRD measurement was not available under the present experimental condition, the size of a single crystal domain and their arrangement were studied by high resolution TEM images. Figure 9 is one of the representatives of high resolution image. In the sample heated to 750°C ordered mesopores are apparent but seem to be smaller than the original pore size with thicker walls. This is due to the inclined sample from the complete parallel position with the electron beam. Consequently, lattice fringes appear inside mesopores.

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Figure 9. High resolution TEM image of crystallized 2D-hexagonal mesoporious Nb-Ta oxide measured at 750°C. In agreement with the electron diffraction pattern in Figure 8A, presence of small crystal domains are visible in Figure 9. The precise size of each crystal domain is not determined by Figure 9, but is expected to be less than 10 nm. It is worth noting that these small crystal domains connect closely without demonstrating apparent domain boundaries, and thus, inter-domain spaces are absent. Therefore, mesopores are considered to be surrounded by considerably dense poly-crystalline inorganic phase in this material. Repetition of similar experiments certificate the high reproducibility of the present results, i.e., the original 2D-hexagonal mesoporous structure is maintained while the amorphous inorganic phase is transformed into dense poly-crystal structure. When the same amorphous precursor was calcined in air, a drastic mass transfer occurs to form single crystal particles with mesopores inside. Thus, carbon filling inside the pores plays an important role for preservation of the original mesoporous structure. On the other hand, crystallization in UHV condition results in the complete maintenance of the original ordered mesoporous structure, and the inorganic phase is converted into dense poly-crystalline domains. The difference is, at present, considered to be attributable to the presence or the absence of oxygen in the atmosphere. The crystallization temperature is high enough for the exchange of lattice and atmospheric oxygen on the oxide surfaces [7]. Therefore, rearrangement of mesopores due to the mass transfer is probably assisted by oxygen exchange reaction, while this is suppressed in the absence of atmospheric oxygen, i.e., UHV condition. In conclusion, crystallization of a highly ordered 2D-hexagonal mesoporous transition metal mixed oxide, Nb-Ta oxide, was studied under various conditions. Simple calcination in air generated single crystal particle of Nb-Ta oxide with expanded mesopores inside each particle. Carbon filling inside the mesopores and coating over particles was found to be effective to preserve the original mesoporous structure, yet, the yield of such crystallized and ordered mesopores was low. The crystallization in UHV condition formed poly-crystal inorganic phase with sustaining the original well ordered mesoporous structure. ACKNOWLEDGEMENT This work was supported by CREST (Core Research for Evolutional Science and Technology) of JST (Japan Science and Technology).

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