J. Phys. Chem. C 2007, 111, 2459-2464
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Novel Nanostructures of Porous Carbon Synthesized with Zeolite LTA-Template and Methanol Song Lei, Jun-ichi Miyamoto, Tomonori Ohba, Hirofumi Kanoh, and Katsumi Kaneko* Graduate School of Science and Technology, Chiba UniVersity, 1-33, Yayoi, Inage, Chiba 263-8522, Japan ReceiVed: October 6, 2006; In Final Form: December 6, 2006
Novel nanostructured carbon material was synthesized by applying zeolite LTA as a template and using methanol as a carbon source. X-ray diffraction (XRD) revealed that the higher the decomposition temperature, the more graphitic and ordered the structure. The optimum pyrolytic temperature for addition of microporosity was below 1273 K by analysis of N2 adsorption isotherm. The resultant carbons have the long-range periodic structure of a nanoscale curvature according to XRD and Raman spectroscopic examinations. The morphological similarity between zeolite LTA and synthesized carbon was evidenced by scanning electron microscopic observation. Grand canonical Monte Carlo simulation of N2 adsorption isotherm indicates that synthesized nanoporous carbon has a hollow hemispherical structure of which diameter is less than 0.7 nm.
Introduction Porous carbons are multipurpose materials that have been widely used in many fields such as air and water purification, catalyst supports, and electrodes for supercapacitors.1-3 Many novel approaches to control the pore structure have been proposed by using template-assisted routes. The previous researches have prepared porous carbon with ordered structure using a variety of porous template and these resulting carbons reflect the original template structure. Ryoo et al. prepared mesoporous carbons of regular structures with the template method.4,5 Kyotani et al. succeeded to produce high surface area carbon by use of zeolite Y as the template; their studies suggested that those carbons have unique nanostructure and physical properties.6,7 Therefore, we need to elucidate the nanostructures and physical properties of carbons prepared with the zeolite template and the growth mechanism of the nanostructured carbon in highly confined nanopore spaces of zeolites. At the same time, it is necessary to develop a convenient method for preparation of the nanocarbons with the zeolite-template. To understand the unique structure of nanocarbons prepared in the zeolite pores, a detailed study on the preparation of nanocarbons with the different kinds of the zeolite template should be carried out. In particular, the growth mechanism of nanostructured carbons in very small pore spaces of zeolites must be studied. LTA has pores whose aperture is less than 0.7 nm and thereby it can offer the lower limit nanospaces for growth of nanostructured carbons. Methanol is a very small molecule and is one of the popular chemicals. If methanol is available for the template synthesis of unique nanocarbons, nanocarbons with the template method should become applicants for an industrial application. As one of the hopeful applications of nanoporous carbons is storage of clean energy gases such as methane and hydrogen, we are intensely interested in the relationship between the nanostructures and the adsorptivity for supercritical hydrogen gas. * Corresponding author. E-mail:
[email protected]. Tel: +81-43-290-2779. Fax: +81-43-290-2788.
In this work, nanocarbons are prepared through the chemical vapor deposition of methanol using zeolite LTA as a template; the unique nanostructures and high adsorption ability are shown. Experimental Zeolite LTA from Union Showa K.K Corporation in Japan was used as the template. Zeolite LTA has a simple cubic structure with a composition Si/Al ratio of unity, that is, two different-sized R- and β-cages (diameters are about 1.1 and 0.7 nm, respectively) are arranged in a CsCl type structure. The pore diameter is defined by an eight-member oxygen ring and it is about 0.7 nm. Methanol was used as a carbon source. Pyrolytic carbon was deposited into zeolite channels by methanol chemical vapor deposition (CVD). The CVD method was performed at 773-973 K for 6 h under nitrogen flow at 100 ml min-1. The zeolite/carbon composites were carbonized at 1073∼1273 K for 4 h under nitrogen flow. The template was removed by dissolving in HF solution. Finally, the resultant was washed by deionized water, and then dried at 337 K for 1 day. The resulting sample was named as Z-PT-CT. Here, Z means zeolite template, PT and CT indicates pyrolytic temperature and carbonization temperature in Kelvin, respectively. The pore structures were evaluated by analysis of nitrogen and hydrogen adsorption isotherms measured at 77 K using a Quantachrome Autosorb-1 instrument. Raman spectra were measured with Raman spectrometer (JASCO NRS-3100) equipped with YAG laser (power 1.5 mW, wavelength 532 nm). The crystalline structures of the resultant carbons were examined with X-ray diffractometer (XRD) (Miniflex, Rigaku, Japan). The zeolite template and synthesized carbon materials were observed by means of the scanning electron microscope (SEM) (JEOL JSM-6330). GCMC Simulation The N2 adsorption isotherm at 77 K was simulated with grand canonical Monte Carlo (GCMC) method for the model of a hemispherical single wall structure using the following potential function.
10.1021/jp066564s CCC: $37.00 © 2007 American Chemical Society Published on Web 01/25/2007
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Figure 1. SEM images of zeolite template and carbon synthesized. (A) zeolite template; (B), (C) synthesized carbon materials.
An N2 molecule was expressed by a two-centered LennardJones molecule with the quadrupole moment by four Coulombic interaction centers. The intermolecular interaction is calculated by the sum of the dispersion interaction and the electrostatic interaction between partial charges on the atomic sites of an N2 molecule.
φff(r) ) 4ff
[( ) ( ) ] σff
12
-
r
σff
6
+
r
1 qiqj
∑i ∑ 4π j(*i)
0
rij
(1)
Figure 2. XRD patterns of Z-973-CT. (a) zeolite LTA; (b) CT ) 1073; (c) CT ) 1123; and (d) CT ) 1273.
Here, ff and σff are the potential well depth of the N2 molecule (ff/kB ) 35.6 K) and the effective diameter (σff ) 0.3318 nm), respectively. Both nitrogen atoms are situated at 0.05047 nm from the N2 molecular center. Two positive and two negative charge centers of |qi| ) 0.373 e are distant from the N2 molecular center by 0.084 and 0.1044 nm, respectively. Nitrogen-carbon interaction is given also by the Lennard-Jones potential.
φsf (r) ) 4sf
[( ) ( ) ] σsf r
12
-
σsf r
6
(2)
Here, ss/kB ) 30.14 K and σss ) 0.3416 nm were used for a carbon atom. The cross parameters of nitrogen and carbon atoms were given by the Lorentz-Berthelot rule. The random movement, creation, and removement of a molecule give a new configuration whose total potential energy was calculated. As the configuration is accepted in the condition of Metropolis’s sampling scheme, the system reaches an equilibrium state.8,9 Results and Discussion
Figure 3. XRD patterns of Z-PT-1173. (a) PT ) 973; (b) PT ) 873; and (c) PT ) 773.
Carbon of Periodicity of Zeolite-Template. Figure 1 shows the SEM images of zeolite template and synthesized carbon materials. The zeolite has the cubic structure as shown in the Figure 1a; the plate-shaped crystals were often observed. Figure 1b,c show that synthesized carbon materials have cubic and plate forms that are similar to the template structure, suggesting that the resultant carbon copies the template structure. Figure 2 shows the XRD patterns of zeolite LTA and porous carbons synthesized of Z-973-CT. The zeolite LTA as the template shows many sharp peaks due to the inherent crystal structure. Some of sharp peaks were still observable in the patterns of all the resultant carbons, suggesting that the zeolite framework of the template is preserved in the structure of the synthesized carbons. However, a serious restriction of the carbon growth in the zeolite pores leads to the different peak intensity from the template zeolite. The presence of XRD peaks implies formation of three-dimensional regular structure of synthesized carbon6,7,10 irrespective of the growth in highly narrow pore spaces. The sharp peak at 7° of nanocarbon indicates the periodicity of 1.2 nm, whereas peaks at 27, 44, and 54° can be ascribed to (002), (101), and (004) diffraction peaks, respec-
tively, which also appear in the zeolite LTA of template, which is an indication of long-range structural ordering, and suggest that synthesized carbon partly reflect template structure.11-14 When the heat-treatment temperature is elevated from 1073 to 1273 K, the XRD peaks become stronger and sharper. Consequently the carbon grown in the pores of zeolite should be more crystallized, leading to more stable and regular structure. Figure 3 shows the XRD patterns of Z-PT-1173 resultant carbons. XRD peaks are sharper and more intense, especially for the peaks at 7° and 54°, when the temperature is elevated during the carbon-deposition procedure. Thus, the higher the decomposition temperature, the more graphitic and ordered the structure. Raman spectra of Z-973-CT samples are shown in Figure 4A. G band comes from E2g vibration mode on an ordered graphitic structure, while the D band is associated with defective structures and disorders.15-18 The peaks of the D and G bands of all samples are observed around 1340 and 1590 cm-1, respectively. The intensity ratios of the G and D band, I(G)/ I(D) increased from 0.95 to 1.0 with carbonization temperature
Novel Nanostructures of Porous Carbon
Figure 4. Raman spectra of Z-973-CT. (a) CT ) 1073; (b) CT ) 1123; and (c) CT ) 1273. (A) High-frequency region. (B) Lowfrequency region.
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Figure 6. Nitrogen adsorption isotherms of Z-973-CT at 77 K. O, CT ) 1073; 3, CT ) 1123; and ], CT ) 1273.
Figure 7. Nitrogen adsorption isotherms of Z-PT-1173 at 77 K. O, PT ) 773; 4, PT ) 873; and ], PT ) 973. Figure 5. Raman spectra of Z-PT-1173. (a) PT ) 773; (b) PT ) 873; and (c) PT ) 973.
for Z-973-CT samples. It is noteworthy that the Raman peak is observed at 310 cm-1 for Z-973-CT samples, as shown in Figure 4B. This band is similar to the radial breathing model (RBM) band that is inherent to single-wall carbon nanotubes, doublewall carbon nanotubes, or single-wall carbon onions.19-22 Thus, the band at 310 cm-1 suggests the presence of a single wall carbon structure of nanoscale curvature, which was suggested by Kyotani et al.22 and Yoshimura et al.23 However, no peak around 300 cm-1 was observed for carbon samples prepared at 773 and 873 K of the CVD temperature. The I(G)/I(D) ratio changed from 0.85 to 0.96 with pyrolytic temperature for Z-PT-1173 samples, which are shown in Figure 5. Hence, the fundamental graphitic structure of the nanocarbon is not so sensitive to the carbonization and pyrolytic temperature examined in this works Nanopore Structures and Adsorption Supercritical Hydrogen. Figure 6 shows the effect of the carbonization temperature on the N2 adsorption isotherm at 77 K. The N2 adsorption isotherm at the lowest carbonization temperature (1073 K) is almost of type I, indicating the presence of micropores; it has also a slight uptake at about P/P0 ) 0.9, showing the contribution by adsorption on the external surfaces. The elevation of the carbonization temperature does not markedly increase the adsorption amount below P/P0 ) 0.4,
TABLE 1: Pore Structure Parameters of Prepared Carbon Materials
Z-973-1073 Z-973-1123 Z-973-1273 Z-773-1173 Z-873-1173 Z-973-1173
total surface area/m2 g-1
external surface area/m2 g-1
internal surface area/m2 g-1
micropore volume /ml g-1
290 360 410 120 230 370
50 60 70 20 30 60
200 240 260 90 210 250
0.09 0.10 0.12 0.03 0.05 0.09
indicating only a slight development of micropores by the elevation carbonization temperature. However, remarkable changes are observed above P/P0 ) 0.6 with the elevation of the carbonization temperature. In particular, carbonization at 1273 K leads to an explicit adsorption hysteresis. Accordingly, elevation of the carbonization temperature above 1123 K should give rise to the carbon deposition on the external surfaces, accompanying with mesopores. Hence, the optimum pyrolytic condition was examined below 1273 K of the carbonization temperature. Figure 7 shows the effect of the pyrolytic temperature on the N2 adsorption isotherm of carbon samples at 77 K. The adsorption isotherm of Z-773-1173 is close to type I, but it has a slight uptake about P/P0 ) 0.9. The higher the pyrolytic temperature, the greater the adsorption amount. The elevation of the pyrolytic temperature increases adsorption amount remarkably, leading to adsorption hysteresis.
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Figure 9. The extended DR plots of hydrogen adsorption isotherms of Z-973-CT. 4, CT ) 1073; O, CT ) 1123; and ], CT ) 1273.
porosity. The saturated adsorption amounts of hydrogen, WL, were evaluated by the Langmuir plot; the WL values per unit pore volume are in the range of 15 to 20 mg ml-1, as summarized in Table 2. Although WL values are smaller than adsorbed amounts of H2 on single wall carbon nanohorns at 77 K and 4 MPa (55 mg ml-1),26 it is useful to analyze these hydrogen adsorption isotherms with the extended DubininRadushkevich (DR) equation for adsorption of supercritical gases. The extend DR equation is given by eq 3.27 Figure 8. Hydrogen adsorption isotherms of Z-973-CT at 77 K. 4, CT ) 1073; O, CT ) 1123; and ], CT ) 1273.
TABLE 2: Adsorption Parameters Determined from Langmuir Plot and the Extended DR Plot for Hydrogen Adsorption Z-973-1073 Z-973-1123 Z-973-1273 Z-773-1173 Z-873-1173 Z-973-1173
WL(H2)/mg ml-1
P0q (kPa)
βE0 (kJ mol-1)
18 20 21 15 16 20
1060 1060 1070 1040 1040 1060
31 31 30 32 32 31
The internal surface area and micropore volume were determined by the subtracting pore effect (SPE) method for the Rs-plot to evaluate the internal and external surface areas without the overestimation.8 Also, the total surface area was evaluated. The pore structural parameters are summarized in Table 1. When the pyrolytic temperature is 973 K, the contribution by the internal surface to the total surface area is more than a half. Also, the micropore volume is in the order of 0.1ml g-1. These porous carbons exhibit the RBM band, as shown in Figure 4B and thereby these preparation conditions provide microporous carbon of single-wall carbon of nanoscale curvature. As the carbon samples are prepared in the very small zeolite pores, there is the possibility of the presence of ultramicropores. Then, we measured the H2 adsorption isotherms at 77 K, because an H2 molecule is a smaller probe molecule than the N2 molecule. Also, we have a great demand for a better adsorbent for supercritical H2. The hydrogen uptake of the prepared carbon materials is correlated to their micropore volume and pore size.24,25 Figure 8 shows hydrogen adsorption isotherms of Z-973-CT samples at 77 K. The H2 adsorption amounts increase with carbonization temperature, but in the low-pressure region the uptake does not significantly increase. Therefore, elevation of carbonization temperature increases the micro-
[ln(WL/W)]1/2 ) (RT/βE0)(ln P0q - ln P)
(3)
Here P0q is the quasisaturated vapor pressure in micropores. W is the micropore volume and βE0 is the characteristic adsorption energy. Extended DR plot for hydrogen adsorption isotherms of Z-973-CT samples at 77 K are shown in Figure 9. The βE0 and P0q values determined from the extended DR plots are shown in Table 2. The hydrogen adsorption isotherms of Z-PT-1173 are shown in Figure 10. When the pyrolytic temperature increases, the hydrogen adsorption amount increases especially in the lowpressure region, indicating an increase in the micropore volume. This result gives a similar tendency observed in the N2 adsorption. The linear extended DR plots of hydrogen adsorption isotherms of Z-PT-1173 samples were obtained, giving the WL, βE0, and P0q values of Z-PT-1173, which are also summarized in Table 2. The P0q and βE0 are almost constant regardless of different carbonization temperature and pyrolytic temperature. However, the P0q values are 1040-1070 kPa, indicating that the pore field of the carbon is sufficiently strong to induce a quasicondensation of supercritical hydrogen. The sum of βE0 and the enthalpy of vaporization provides the isosteric heat27-29 of adsorption at the fractional filling of e-1, qst. The obtained qst values are in the range of 31∼33 kJ mol-1, being quite larger than the isosteric heat of hydrogen adsorption on single wall carbons (12 kJ mol-1).29 Thus, nanoporous carbon developed in the present study has intensely strong adsorption sites for supercritical hydrogen. Single-Wall Hemispherical Carbon Model. The observed RBM band indicates the presence of single wall carbon nanotube like-structure of 0.7 nm in the diameter. However, the nanotube of 0.7 nm in diameter cannot be produced in the small pore space of LTA. Literatures suggest that even hemispherical single-wall carbon can give a similar Raman band to the RBM band.21,30 Also Kyotani et al. have proposed that their templated carbon produced in the pore of zeolite has a chain structure of
Novel Nanostructures of Porous Carbon
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Figure 12. Snapshots of N2 molecules adsorbed on single-wall carbon cap model.
far from the experimental one, whereas the simulated isotherm on the internal surface can describe well the experimental one. Accordingly, this simulation result supports intensely the presence of the hemispherical cap structure. Figure 12 shows the snapshots of adsorption on the internal and external surfaces of the hollow hemispherical cap. Only one N2 molecule can be accommodated in the inside of the cap. On the other hand, many N2 molecules are adsorbed on the external walls of the cap, which is far from the real carbon prepared in the restricted space of LTA zeolite. As the external surface of the positive curvature cannot provide enough adsorption sites space in the real sample, the present simulation model overestimates the adsorption amount on the external surface. Thus, the GCMC simulation can provide an indirect evidence for formation of the single wall cap structure in the zeolite pores. Acknowledgment. The Grant-in-Aid for Scientific Research (S) (No.15101003) by JSPS is acknowledged. S.L. has been in part supported by 21-COE program: Frontiers of SuperFunctionality Organic Devices. References and Notes Figure 10. Hydrogen adsorption isotherms of Z-PT-1173 at 77 K. 4, PT ) 773; O, PT ) 873; and ], PT ) 973.
Figure 11. Comparison of experimental N2 adsorption isotherm of Z-973-1273 at 77 K with GCMC simulated adsorption isotherms on total, internal, and external surfaces for the hemispherical cap model. Experimental isotherm: ], CT ) 1273. Simulated isotherm: b, total surface; 2, internal surface; and 9, external surface.
hemispherical single-wall carbon caps.22 Then, we simulated N2 adsorption isotherm of the hemispherical single-wall carbon cap of 0.7 nm in the diameter. Figure 11 compares GCMC simulated isotherms with the experimental isotherm of Z-973CT at 77 K. Here, simulated isotherms are shown for adsorption processes on the total, internal, and external surfaces of the cap. The simulated adsorption isotherm on the external surface is
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