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Ordered Mesoporous Carbon Monoliths: CVD Nanocasting and Hydrogen Storage Properties Yongde Xia and Robert Mokaya* School of Chemistry, UniVersity of Nottingham, UniVersity Park, Nottingham NG7 2RD, United Kingdom ReceiVed: March 9, 2007; In Final Form: May 2, 2007
We report on the use of a chemical vapor deposition to nanocast ordered mesoporous carbon monoliths using mesoporous silica monoliths as template. The size and shape of the silica monolith template are well retained in the carbon resulting in a mechanically robust mesoporous carbon monolith. A relatively high level of mesostructural ordering is observed in the carbon monolith, which has surface area and pore volume of 1090 m2/g and 0.75 cm3/g, respectively. The carbon monolith exhibits limited microporosity (micropore surface area of 156 m2/g). The surface area and pore volume of the carbon monolith was higher than that of the silica template (553 m2/g and 0.5 cm3/g). The carbon in the monolith is mainly amorphous with a small proportion of graphitic carbon. As far as we are aware, this is the first successful attempt to generate ordered mesoporous carbon monoliths via chemical vapor nanocasting. The chemical vapor nanocasting route offers the advantage of improved mesostructural ordering in the carbon monolith. The mesoporous carbon monolith exhibits considerable hydrogen uptake; we obtained a hydrogen uptake capacity, at 20 bar and -196 °C, of up to 3.4 wt %. The hydrogen uptake capacity is comparable to that of CMK-3 mesoporous carbon powder samples.
1. Introduction Well-ordered mesoporous carbon materials that are prepared via nanocasting routes in which mesostructured silicas or aluminosilicas are used as hard templates are interesting due to their potential use in catalysis, separation or as components of electrochemical devices.1-3 The nanocasting procedures are now well established and generally involve three steps: infiltration of the carbon precursor into the pores of the inorganic “hard” template, carbonization at high temperature under nonoxidizing conditions, and finally etching out of the inorganic framework to generate porous carbons.1-5 The inverse replication process from the inorganic template to the carbon allows for a retention of particle morphology.2,6 Mesoporous carbons with a variety of particle morphology, which exist in powder form, have been prepared.1-8 However, powdered mesoporous carbon samples have limitations with respect to their use in applications such as in catalysis, high-performance liquid chromatography, or gas storage. Monolithic forms of mesoporous carbon may offer several advantages including mechanical stability, ease of handling and ease of recovery. The preparation of monolithic forms of well-ordered mesoporous carbon, however, is difficult. Indeed, to date there have been only a few reports dealing with the preparation of monolithic mesoporous carbon that are templated by monolithic mesoporous silica.9-11 The monolithic mesoporous carbons reported to date are in most cases poorly ordered due to the difficulty of transferring mesostructural order from the silica templates to the carbon.10,11 This is compounded by the generally poor mesostructural ordering in the monolithic silica templates. A number of indirect approaches, including gel-casting of powder samples,12 mechanical binding, and salt templating of powdered carbon precursor/silica composites13 or aggregation of powder particles,14 have been used in an attempt to improve the structural ordering of mesoporous carbon monoliths. A key * Corresponding author. E-mail:
[email protected].
consideration in any attempt to prepare well ordered monolithic mesoporous carbons is the extent to which the silica template is infiltrated by the carbon precursor. The silica monolith must be sufficiently infiltrated by the carbon precursor in order to generate a well ordered mesoporous carbon monolith. Low levels of infiltration will lead to carbon monoliths with poor mesostructural ordering and low mechanical stability. Infiltration of the carbon precursor into silica templates may be achieved via liquid impregnation or chemical vapor deposition. For the preparation of mesoporous carbon monoliths, infiltration of the carbon precursor into the silica monolith templates has so far been achieved mainly via liquid impregnation. It is known that under certain conditions, chemical vapor deposition (CVD) is more effective at introducing carbon into the internal space of mesoporous silica templates.6 As far as we are aware, there are no reports on the use of CVD to nanocast mesoporous carbon monoliths. Recently there has been interest in the use porous carbons prepared via hard (mesoporous silica or zeolite) templating routes as hydrogen storage media. Pang and co-workers prepared high surface area mesoporous carbon, and observed a hydrogen uptake capacity of 1.78 wt % at ambient pressure and -196 °C.15 Terres and co-workers on the other hand observed hydrogen uptake of 2.7 wt % at -196 °C and 60 bar for mesoporous carbon templated from mesoporous silica MCM48.16 Zeolite templated carbons have on the other hand been shown to absorb up to 2.6 wt % at ambient pressure and close to 7 wt % at 20 bar and -196 °C.17 While monolithic forms of carbon are known to offer certain advantages, it is not known how the presence of monolithic rather than powder samples may affect hydrogen uptake capacity. In this work, we have employed CVD nanocasting to prepare monolithic mesoporous carbon from mesoporous silica monoliths. Our aim was to maximize the extent of carbon infiltration into the silica template with the hope that this would improve the mesostructural ordering
10.1021/jp071936y CCC: $37.00 © 2007 American Chemical Society Published on Web 06/12/2007
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in the carbon monoliths. We report on the properties of the resulting carbon materials and also present hydrogen sorption data. 2. Experimental Section 2.1. Material Synthesis. Synthesis of the monolithic mesoporous silica template was via solvent evaporation from a silica gel prepared from tetraethoxysilane and P123 as previously reported by Zhao and co-workers.18 The calcined mesoporous (SBA-15) silica monolith was used as template for the preparation of mesoporous carbon monolith via CVD. Typically, the calcined mesoporous silica monolith was placed in an alumina boat and inserted into a flow-through tube furnace in a fume cupboard. Under a flow of nitrogen saturated with acetonitrile vapor at room temperature, the temperature of the furnace was increased, at a ramp rate of 20 °C/minute, to a final pyrolysis/ carbonization temperature of 950 °C, and maintained for 3 h under the acetonitrile-saturated nitrogen atmosphere. (Warning: Hot organic vapors must be handled carefully!) The furnace was then cooled to room temperature and the resulting silica/ carbon composite thoroughly washed with hydroflouric (HF) acid to etch out the silica. (Warning: HF acid is highly corrosive and must be handled carefully!) The resulting carbon monolith was air-dried dried at room temperature. 2.2. Materials Characterization. Powder XRD analysis was performed using a Philips 1830 powder diffractometer with Cu KR radiation (40 kV, 40 mA). Nitrogen sorption isotherms and textural properties were determined using data from nitrogen sorption at -196 °C in a conventional volumetric technique by an ASAP 2020 micrometrics sorptometer. Before analysis the samples were oven dried at 150 °C and evacuated for 12 h at 200 °C under vacuum. The surface area was calculated using the BET method based on adsorption data in the partial pressure (P/P0) range 0.05 to 0.2 and total pore volume was determined from the amount of the nitrogen adsorbed at P/P0 ) ca. 0.99. Micropore surface area and micropore volume were obtained via t-plot analysis. Scanning electron microscopy (SEM) images were recorded using a JEOL JSM-820 scanning electron microscope. TEM images were recorded on a JEOL 2000-FX electron microscope operating at 200kV. Thermogravimetric analysis (TGA) was performed using a Perkin-Elmer Pyris 6 TG analyzer at a heating rate of 2 °C/min under static air conditions. 2.3. Hydrogen Uptake Measurements. Gravimetric determination of hydrogen uptake capacity was performed using an Intelligent Gravimetric Analyzer (IGA-003, Hiden), which incorporates a microbalance capable of measuring weights with a resolution of (0.2 µg. Hydrogen uptake was determined at -196 °C over the pressure range 0-20 bar. The samples were outgassed (10-10 bar) under heating at 200 °C overnight before measurement. High-purity hydrogen (99.9999%) additionally purified by a molecular sieve filter was used for the uptake measurements. 3. Results and Discussion 3.1. Physicochemical Characterization of Carbon Monolith. The monolithic mesoporous silica SBA-15 hard template was prepared using established procedures.18 Figure 1 shows a photograph of the as-synthesized mesoporous silica monolith. The monolith has a slight yellow coloration due to the presence of residual surfactant molecules. The size and shape of the monolith was retained after calcination to remove the surfactant molecules. The calcined monolith was then used to prepare carbon via chemical vapor deposition using acetonitrile as
Figure 1. Photographic images of mesoporous silica monolith (left) and mesoporous carbon monolith (right) materials. The mesoporous silica monolith is 2.0 cm high with a diameter of 1.0 cm and the carbon monolith is 1.8 cm high and 0.8 cm in diameter.
carbon precursor. Following the CVD step, the resulting carbon/ silica composite monolith was immersed in HF acid to remove the silica framework, and then air-dried at room temperature. A photograph of the resulting carbon material (after the CVD process and silica etching) is shown in Figure 1. The photograph confirms that monolithic carbon was obtained. The appearance of the carbon monolith was largely similar to that of the silica template, except for a slight reduction in size involving a contraction from 2.0 to 1.8 cm (10%) in length and from 1.0 to 0.8 cm (20%) in diameter. The monolithic carbon had robust mechanical stability and could be easily handled for subsequent characterization without any damage. Figure 2 shows SEM images of particles obtained from deliberately crushing the silica and carbon monoliths. The particles generated by crushing the monolith are very large, which is consistent with the monolithic morphology from which they are derived.19 To confirm that the carbon monolith was silica free, we performed thermogravimetric analysis (TGA). The TGA curve and corresponding DTG profile are shown in Figure 3. The mass loss event below 100 °C (centered at 45 °C) is due to residual physisorbed water. The carbon monolith is thermally stable up to ca. 400 °C and is then burnt off via two mass loss events. The main mass loss event, which is centered at ca. 535 °C, is due to combustion of amorphous (non-graphitic) carbon. A further mass loss event centered at ca. 630 °C may be ascribed to the combustion of graphitic carbon.20 The assumption here is that graphitic carbon is thermally more stable compared to amorphous carbon. The TGA data (according to the magnitude of the mass loss events) indicates that there is only a small proportion of graphitic carbon compared to amorphous carbon. The most significant information from the TGA data is that the residual mass (above 700 °C) was less than 1 wt % (typically 0.5-0.8 wt %). This confirms that the carbon monolith is virtually silica free. The silica etching process therefore efficiently removes the inorganic template phase from the CVDderived carbon/silica composite. Figure 4 shows the nitrogen sorption isotherms of the calcined silica monolith and the carbon monolith. The monolithic silica exhibits an isotherm that is typical for a well ordered mesoporous material. The carbon monolith exhibits a nitrogen sorption isotherm typical of well ordered CMK-3 type mesoporous carbons.4b The sorption isotherm of the carbon is type IV with a well developed capillary condensation step into mesopores, indicating good mesostructural ordering. Both the silica and carbon monoliths exhibit relatively uniform mesopores as shown by the pore size distribution curves (inset Figure 4). The average pore size of both materials is ca. 38 Å. The mesoporous silica monolith had a surface area of 553 m2/g and pore volume of 0.5 cm3/g, and did not exhibit any microporosity. The mesoporous carbon monolith, on the other hand, had a much higher
Ordered Mesoporous Carbon Monoliths
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Figure 2. Representative SEM images of mesoporous silica monolith (a, b) and mesoporous carbon monolith (c, d) materials.
Figure 3. TGA curve (A) and corresponding DTG profile (B) of mesoporous carbon monolith.
Figure 4. Nitrogen sorption isotherms of mesoporous silica monolith and mesoporous carbon monolith. The inset shows the corresponding pore size distribution curves.
surface area of 1090 m2/g and pore volume of 0.75 cm3/g. The carbon monolith exhibited some limited microporosity (micropore surface area ) 156 m2/g). The overall trend in the textural properties of the silica and carbon monoliths, and the
development of modest microporosity in the carbon, is consistent with what has previously been observed for sucrose-derived monolithic carbons prepared via liquid impregnation.11 The high surface area of the mesoporous carbon monolith is an indication that the mesoporous silica monolith used as template was efficiently infiltrated during the CVD step. When the carbon precursor deposition step is performed via liquid impregnation, several cycles are required to achieve sufficient infiltration of the silica monolith.11 The CVD route appears to offer a much simpler non-repetitive carbon infiltration method, which also has the advantage of combining the deposition and carbonization steps into one. One of the aims of this study was to prepare mesostructurally well-ordered monolithic carbon. Mesostructural ordering can be probed by powder XRD analysis. To date it has been difficult to prepare monolithic carbon materials that exhibit XRD peaks arising from mesostructural ordering.10,11 The XRD patterns of the silica and carbon monoliths are shown in Figure 5. The low angle XRD pattern of the silica monolith exhibits two peaks, which we ascribe to the basal (100) diffraction, and (110) diffraction from a hexagonal (P6mm) array of pores.21 The silica monolith is therefore relatively well ordered. The XRD pattern of the carbon monolith exhibits one peak, which we tentatively ascribe to the (110) diffraction. Although the basal peak is not observed (perhaps due to limitations of our instrument), the presence of at least one peak suggests a significant level of mesostructural ordering. This suggests that the CVD route, in addition to generating monolithic carbon with high textural properties, may also be used to improve the overall mesostruc-
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Figure 5. Powder X-ray diffraction patterns of mesoporous silica monolith and mesoporous carbon monolith materials.
Figure 7. Hydrogen sorption isotherms at -196 °C of (A) mesoporous carbon monolith and (B) powdered mesoporous carbon CMK-3. (Carbon density of 1.5 g/cm3 was used, and hydrogen density of 0.04 g/cm3 was used for buoyancy correction of adsorbed H2.)
Figure 6. Representative TEM image of monolithic mesoporous carbon material templated from monolithic mesoporous silica.
tural ordering. The presence of relatively well ordered pore channels in the monolithic mesoporous carbon is indicated by the TEM image shown in Figure 6. Although the pore ordering is generally wormhole-like, it is possible to observe some regions of well aligned pore channels. Mesoporous materials with wormhole type channel ordering are known to exhibit at least one low angle XRD peak.22 The XRD patterns and TEM image therefore indicate that the pore structure of the SBA-15 silica monolith templates is to some extent replicated in the monolithic carbon. The wide angle XRD pattern for the monolithic carbon (inset Figure 5) exhibits a high-intensity diffraction peak at 2θ of ca. 26° and a further peak at ca. 43°. We ascribe these peaks to (002) and (101) diffractions of graphitic carbon. The presence of graphitic carbon is consistent with the TGA data in Figure 3. The formation of graphitic mesoporous carbon via the CVD route at suitably high carbonization temperatures has previously been observed for powder samples.23 The formation of monolithic forms of mesoporos carbon does not therefore appear to hinder the graphitisation process. The ability to readily tailor
the extent of ‘graphitisation’ in monolithic mesoporous carbons is an added advantage of the present CVD route. 3.2. Hydrogen Uptake Properties of Mesoporous Carbon Monolith. Hydrogen sorption isotherm of the mesoporous carbon monolith, measured by gravimetric analysis with an IGA at -196 °C over the pressure range 0-20 bar, is shown in Figure 7A. The hydrogen uptake is calculated on the basis of a density of 1.5 g cm-3 for the carbon samples, and hydrogen density of 0.04 g cm-3 is used for buoyancy correction of adsorbed hydrogen.24 The sorption isotherms in Figure 7A show no hyteresis; the desorption closely matches adsorption. This indicates that the take up of hydrogen by the carbon monolith is totally reversible. The hydrogen uptake capacity of the mesoporous carbon monolith (at -196 °C and 20 bar) is ca. 3.4 wt %. It is however clear that saturation is not achieved at 20 bar; the trajectory of the isotherm suggests that higher hydrogen uptake is possible at higher pressure. The hydrogen uptake capacity observed for the mesoporous carbon monolith (i.e., 3.4 wt % at 20 bar) is higher or comparable to that previously reported for mesoporous silica templated carbons of similar or higher surface area.15,16 Furthermore, the hydrogen uptake capacity of the mesoporous carbon monolith is generally comparable to published data for other types of porous carbon with similar or even higher surface area.17,25-27 The hydrogen uptake capacity of the carbon monolith is comparable to that reported by Pang and co-workers (1.78 wt % at ambient pressure and -196 °C) for high surface area (2314 m2/g) mesoporous carbon,15 and that observed by Terres and co-workers (2.7 wt % at -196 °C and 60 bar) for mesoporous carbon templated from mesoporous silica MCM48.16 This suggests that the formation of the mesoporous carbon in monolithic form does not adversely affect the hydrogen uptake capacity. A pertinent comparison is between powder and monolithic forms of mesoporous carbon. Figure 7B shows the hydrogen sorption isotherm for powder form of mesoporous carbon CMK-3, which had a surface area of 1404 m2/g and
Ordered Mesoporous Carbon Monoliths pore volume of 1.4 cm3/g. Although the powder sample has a higher uptake capacity at lower pressure, the maximum hydrogen sorption capacity (at 20 bar) is the same (ca. 3.4 wt %) for both samples despite the lower textural properties of the monolithic carbon. 4. Conclusions In summary, we have shown that structurally well-ordered mesoporous carbon monoliths may be nanocast using mesoporous silica SBA-15 monolith as template via a simple chemical vapor deposition route. As far as we are aware, this is the first successful attempt to prepare mesoporous monolithic carbon via CVD. Indeed, we have prepared mesoporous monolithic carbon with observable XRD peaks and considerable mesostructural ordering according to electron microscopy. The size and shape of the monolithic mesoporous silica template is generally retained through the nanocasting process to generate mechanically robust mesoporous carbon monolith. The monolithic mesoporous carbon has surface area and pore volume of 1090 m2/g and 0.75 cm3/g, respectively, and exhibits limited microporosity (micropore surface area of 156 m2/g). The surface area and pore volume of the carbon monolith is higher than that of the silica template (553 m2/g and 0.5 cm3/g). Thermogravimetric analysis indicated that the carbon in the monolith is mainly amorphous with a small proportion of graphitic carbon. The mesoporous carbon monolith was found to exhibit significant hydrogen uptake capacity. We obtained a hydrogen uptake capacity, at 20 bar and -196 °C, of up to 3.4 wt %, which is comparable or higher than what has previously been reported for powder forms of mesoporous silica-templated carbon. Acknowledgment. This work was funded by the University of Nottingham. References and Notes (1) Ryoo, R.; Joo, S. H.; Kruk, M.; Jaroniec, M. AdV. Mater. 2001, 13, 677. (2) (a) Lee, J.; Han, S.; Hyeon, T. J. Mater. Chem. 2004, 14, 478. (b) Lee, J.; Kim, J.; Hyeon, T. AdV. Mater. 2006, 18, 2073. (3) Yang, H. F.; Zhao, D. Y. J. Mater. Chem. 2005, 15, 1217. (4) (a) Ryoo, R.; Joo, S. H.; Jun, S. J. Phys. Chem. B 1999, 103, 7743. (b) Jun, S.; Joo, S. H.; Ryoo, R.; Kruk, M.; Jaroniec, M.; Liu, Z.; Ohsuna, T.; Terasaki, O. J. Am. Chem. Soc. 2000, 122, 10712. (c) Lee, J.; Yoon, S.; Hyeon, T.; Oh, S. M.; Kim, K. B. Chem. Commun. 1999, 2177. (5) (a) Tian, B.; Che, S.; Liu, Z.; Liu, X.; Fan, W.; Tatsumi, T.; Terasaki, O.; Zhao, D. Chem. Commun. 2003, 2726. (b) Fuertes, A. B. J.
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