Cooperative Adsorption of Supercritical CH4 in Single-Walled Carbon

Sep 27, 2012 - High-density CH4 storage using adsorption techniques is an important issue in the use of CH4 as a clean energy source. The CH4 adsorpti...
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Cooperative Adsorption of Supercritical CH4 in Single-Walled Carbon Nanohorns for Compensation of Nanopore Potential Tomonori Ohba,*,† Katsumi Kaneko,‡ Masako Yudasaka,§ Sumio Iijima,§ Atsushi Takase,† and Hirofumi Kanoh† †

Graduate School of Science, Chiba University, 1-33 Yayoi, Inage, Chiba 263-8522, Japan Research Center for Exotic Nanocarbons, Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japan § Nanotube Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Central 5 1-1-1 Higashi, Japan ‡

S Supporting Information *

ABSTRACT: High-density CH4 storage using adsorption techniques is an important issue in the use of CH4 as a clean energy source. The CH4 adsorption mechanism has to be understood to enable innovative improvements in CH4 adsorption storage. Here, we describe the adsorption mechanism, based on CH4 structure, and stabilities in the internal and external nanopores of single-walled carbon nanohorns, which have wide and narrow diameters, respectively. The adsorption of larger amounts of CH4 in the narrow nanopores at pressures lower than 3 MPa was the result of strong adsorption potential fields; in contrast, the wider nanopores achieve higher-density adsorption above 3 MPa, despite the relatively weak adsorption potential fields. In the wider nanopores, CH4 molecules were stabilized by trimer formation. Formation of CH4 clusters therefore compensates for the weak potential fields in the wider nanopores and enables high-density adsorption and adsorption of large amounts of CH4.

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target values.13−17 Molecular simulations have shown the amounts and structures of CH4 adsorbed in various models of nanoporous carbons, and these results can be used to improve the design of nanoporous carbons.18,19 Nanoporous carbons with pore sizes of two to three CH4 layers are promising materials for CH4 adsorption. These nanopores have high adsorption potential fields, but small nanopore volumes, and this is one of the major barriers to achieving higher levels of adsorption. We therefore have to overcome these issues by developing an understanding of the behavior of adsorbed CH4 as well as its structure. Such understanding is necessary for the fabrication of carbon nanotubes with high adsorption abilities. The objectives of this study are to examine the CH4 structures in the internal and external nanopores of single-walled carbon nanohorns (NHs) and to elucidate the relationship between CH4 adsorption and the nanopore sizes to evaluate the potential ability of carbon nanotubelites to adsorb larger amounts of CH4.

INTRODUCTION Energy sources as an alternative to conventional fuels such as gasoline and diesel are required for a future sustainable energy supply and environmental constraints. CH4 has one of the highest potentials as an alternative fuel because the levels of CO2 and SOx emissions from CH4 combustion are lower than those from conventional fuels. CH4 can therefore be considered as a new energy resource. However, CH4 is one of the most important greenhouse gases, so efficient storage of CH4 is necessary. There are currently three techniques for storing CH4: liquefaction, compression, and adsorption. CH4 liquefaction permits high-density storage, but consumes large amounts of energy. Storage of high densities of compressed CH4 requires large and heavy containers to withstand high pressure. Adsorption in nanoporous media has the advantage of storage at relatively low pressure and ambient temperature. Many studies have therefore been carried out on the storage of large amounts of CH4. The fabrication and tuning of nanoporous media such as activated carbons, carbon nanotubes, zeolites, and metal−organic frameworks are important research areas with respect to achieving storage of large amounts of CH4.1−8 Some metal−organic frameworks show high adsorption densities of CH4, surpassing the target value of 180 v/v at 3.5 MPa at room temperature, set by the U.S. Department of Energy.9,10 Nanoporous carbons have a weight ratio advantage, and therefore they can adsorb significantly high amounts of CH4 in their nanopores.11,12 These nanoporous carbons have the potential to achieve CH4 adsorption densities exceeding the © 2012 American Chemical Society

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EXPERIMENTAL AND SIMULATION SECTIONS The internal nanopores of NHs cannot be accessed by adsorbed molecules, because those are closed by graphene walls. However, the internal nanopores can be made available for molecular adsorption by partial oxidation of the NHs at 673 Received: July 18, 2012 Revised: September 12, 2012 Published: September 27, 2012 21870

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K for 1 h (an oxidized NH is referred to as ox-NH).20 The adsorption by the internal nanopores is estimated by subtraction of the adsorption for ox-NH from that for NH. Adsorption isotherms of CH4 at 303 K were measured using a high-pressure Cahn microbalance, after heat treatment of the NHs and ox-NHs below 0.01 Pa at 423 K. The N2 adsorption isotherms were also measured using a volumetric adsorption apparatus (AUTOSORB-1, Quantachrome Co.). The filling factor of the adsorbed CH4 was obtained by αS analysis of the N2 adsorption isotherms. In situ small-angle X-ray scattering (SAXS) analysis of CH4-adsorbed NHs was conducted at 303 K using Cu Kα X-rays (30 kV, 20 mA) in transmission mode using an atmosphere-controlled sample holder and an accumulation time of 3600 s. The structure analysis technique of SAXS associated with simulation can be available for any adsorbed molecule in carbon nanopores. The technique has the advantage of structure analysis of molecules in those nanopores, because of high X-ray transmission through carbon walls and direct observation of associated molecular structure mainly in the range of subnanoscale to nanoscale.21 CH4 gas under a given pressure was introduced into the sample holder prior to the SAXS measurements. Ornstein−Zernike analysis of the SAXS profile was carried out to evaluate the proportional density fluctuations of the CH4 adsorbed on the NHs.22 These density fluctuations were associated with the structure of the adsorbed CH4 using simulated density fluctuations in adsorbed CH4 models, as shown in Figure S1. CH4 molecules are primarily adsorbed in the internal and external nanopores for ox-NH and NH, respectively. Pore evaluation studies showed that the internal and external nanopores have effective cylindrical diameters of 2.9 and 0.6 nm, respectively.23,24 Simulated density fluctuations for the above models were therefore calculated on the basis of the electron density differences between the boundaries of CH4, carbon, and voids (see the Supporting Information for details). The adsorption potentials of CH4 on NHs were evaluated using the intermolecular potential profiles of CH4 in the internal and external nanopores of NHs. The intermolecular interactions of CH4 were described by a 12−6 Lennard-Jones potential (potential well depth: ε/kB = 161.3 K; effective diameter: σ = 0.3721 nm). The interaction potential between CH4 and the NHs was evaluated using the smooth-structure model, as previously reported.25 Here, the intermolecular potential parameters between CH4 and a carbon atom of an NH were calculated using the Lorentz−Berthelot mixing rules: ε/kB = 69.73 K and σ = 0.3569 nm. In the adsorption potential calculation, we assumed that NH particles are of infinite cylindrical form and that these particles form bundles (hexagonal packing array with distance of 0.7 nm between graphene walls).24 Grand canonical Monte Carlo (GCMC) simulation of CH4 adsorbed on NH particles was also performed to obtain the adsorption isotherms. We applied to Metropolis’s sampling scheme in proportion to exp(−ΔE/kT), where ΔE is defined as the total energy change calculated from configuration change by the random movement, creation, or removal of a CH4 molecule. We used the three-dimensional periodic boundary conditions in the unit cell size as 10 × 10 × 10 nm3.

Figure 1. High-pressure CH4 adsorption isotherms at 303 K. Gas density at 303 K and liquid density at 112 K are shown for comparison.

isotherms at 77 K in Figure S2. Here, the adsorbed density in the internal nanopores was obtained by subtracting the amount of CH4 adsorbed on ox-NH from that adsorbed on NH. The adsorbed CH4 densities in the internal and external nanopores were significantly higher than the bulk gas densities, although they were less than 80% of the bulk liquid density of 422.6 mg mL−1 at 111 K, as previously reported.11 The CH4 adsorption densities in the external nanopores were larger than those in the internal nanopores in the low-pressure region and/or at low adsorbed densities. In contrast, at pressures higher than 4 MPa, the densities in the internal nanopores became larger than those in the external nanopores. The interaction potential between a CH4 molecule and NHs was simply obtained from the Lennard-Jones potential. Figure 2

Figure 2. Adsorption potential profiles of CH4 in the internal and external nanopores of NH (left) and NH model (right).

shows the adsorption potential fields for single CH4 molecules in the internal and external nanopores. Intermolecular interactions of CH4 are neglected in this calculation. A CH4 molecule in an internal nanopore has a stabilization energy of 9.5 kJ mol−1 at the site of contact with a graphene wall. In the external nanopores, a CH4 molecule gains stabilization energies of 8.0 kJ mol−1 at the interstitial sites and 13.5 kJ mol−1 at the junction between two NH particles. The maximum stabilization energies of CH4 are therefore 9.5 and 13.5 kJ mol−1 in the internal and external nanopores, respectively. The average stabilized energies at adsorption sites in the internal and external nanopores are 6.0 and 7.4 kJ mol−1, respectively. Here, the adsorption sites are defined as sites having larger adsorption potential fields than the CH4 thermal energy of 2.5 kJ mol−1 at 303 K. The difference from the average stabilized energy of 1.4 kJ mol−1 imposes different adsorbed densities in the internal

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RESULTS AND DISCUSSION The adsorbed CH4 densities in the internal and external nanopores of the NHs are shown in Figure 1, evaluated from the CH4 adsorption isotherms at 303 K and the N2 adsorption 21871

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adsorbed NH corresponded with those for nonadsorbed NH in all SAXS scattering regions. A larger scattering intensity in a SAXS region indicates a more heterogeneous structure, so the adsorbed CH4 in the internal nanopores forms a heterogeneous adsorption phase, whereas that in the external nanopores forms a homogeneous adsorption phase. Ornstein−Zernike analysis gives the density fluctuations evaluated from the scattering intensity at s = 0, as shown in Figure 3b. The abscissa here represents the relative intensities of CH4-adsorbed NHs against nonadsorbed NHs. A significant increase in the relative intensities for the internal nanopores was observed with increasing filling factor, whereas the relative intensities for the external nanopores changed little. These results suggest promotion of the above-mentioned heterogeneous structure of CH4 in the internal nanopores with increasing filling factor. The detailed assembled structures of adsorbed CH4 were evaluated by comparison with simulated density fluctuations using adsorbed structural models. Typical adsorption structural models of monolayers, dispersion, and clusters on the side views of NHs for the calculation of simulated density fluctuations are shown in Figure 4 (see the Supporting Information for details). CH4

and external nanopores in the low pressure region and/or at low adsorbed densities. Thus, higher adsorbed densities in the external nanopores could be explained by stronger adsorption potential fields in the external nanopores, as a result of formation of NH bundles. However, the reason for the larger adsorbed densities in the internal nanopores in the highpressure region could not be understood based on the adsorption potential fields. Intermolecular interactions among CH4 molecules have to be taken into account to explain the anomalously large adsorbed densities in the internal nanopores. GCMC simulation provides a simulated adsorption isotherm by considering intermolecular interaction between CH4 molecules and CH4 molecule−nanopore walls. Thus, GCMC simulation could evaluate the effect of intermolecular interactions on the adsorption isotherm. The simulated adsorption isotherms in the internal and external nanopores are shown in Figure S3. The adsorbed density of CH4 in the external nanopores was larger than that in the internal nanopores at low pressure. However, the adsorbed densities in both nanopores were reversed above 10 MPa, proposing the synergy effect of CH4 adsorption on the intermolecular interaction at high pressure. The tendency agrees with the experimental one well. However, the reverse densities were observed at rather higher pressure than that in experiment. Therefore, we also have to observe the adsorption mechanism by evaluating the intermolecular structure of adsorbed CH4 experimentally. SAXS profiles of the adsorbed CH4 were correlated with the structures of CH4 adsorbed on NHs by simulation analysis.16 Figure 3a shows SAXS profiles of CH4-adsorbed NHs at 3 MPa and NHs in vacuo at 303 K. Here, a scattering factor s is defined as s = 4π sin(θ)/λ. The scattering factors for CH4-adsorbed oxNHs were larger than those for nonadsorbed ox-NHs, especially at low scattering factors, whereas those for CH4-

Figure 4. Typical structural models of CH4 molecules adsorbed in the internal (a) and external (b) nanopores of NHs.

molecules are adsorbed on a graphene wall in the monolayer models, whereas they are dispersed in the internal and external nanopores in the dispersion models. In the cluster models, three kinds of clusters were tried: dimers, trimers, and hexamers. Figure 5 shows the simulated density fluctuations obtained from evaluation of the structural models in Figure 4. The CH4 molecular structure was presumed to be clusters in the internal nanopores, based on a comparison of the simulated and experimental density fluctuations. The clusters grew from dimers to trimers above a filling factor of 0.5. In contrast, the CH4 molecular structure was presumed to be a dispersed form in the external nanopores. Monolayers could not be distinguished from dispersed structures in the external nanopores because of size restrictions, and clusters could not be formed. The CH4 molecules in the internal nanopores therefore formed clusters from dimers to trimers, even in a supercritical gas, whereas they were dispersed in the external nanopores. As the intermolecular potential well between CH4 molecules is 1.3 kJ mol−1, CH4 dimer or trimer formation in the nanopores gains an additional stability of 1.3 or 2.6 kJ mol−1, respectively. Here, the averaged adsorption potential field in the internal nanopores is 1.4 kJ mol−1 shallower than that in the external nanopores. The adsorption stability in the internal nanopores therefore exceeds that in the external nanopores as a result of trimer formation in the internal nanopores. The ratio of dimer or trimer formation over monomer was 1.7 or 2.8 at 303 K, estimated by Boltzmann distribution exp(−ΔE/kBT) of

Figure 3. (a) SAXS profiles of CH4-adsorbed NH (blue) and ox-NH (red) at 3 MPa, and NH (black) and ox-NH (green) in vacuo. (b) Relative density fluctuations of CH4. 21872

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Promotion of Ion Engineering, Nippon Sheet Glass Foundation, and Global COE program, MEXT, Japan.

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Figure 5. Simulated density fluctuations of CH4-adsorbed NH (external nanopores, a) and ox-NH (internal nanopores, b). Solid curves, simulations; dashed curves, experimental.

the additional stabilized energy ΔE = 1.3 or 2.6 kJ mol−1, respectively. Cluster formation is promoted by low temperature, inducing cooperative adsorption, whereas the effect of cooperative adsorption on an adsorbed density disappears at high temperature. This cooperative adsorption mechanism by self-stabilization leads to a higher adsorbed density in the internal nanopores of diameter 2.9 nm (wide nanopores) than in the external nanopores of diameter 0.6 nm (narrow nanopores) above a filling factor of 0.5. The cooperative adsorption might have occurred above diameter 0.8 nm, corresponding to the diameter of CH4 dimer or trimer. However, no excessively large nanopores are necessary for providing strong adsorption potential. It is therefore possible to increase the CH4 adsorbed density by fabricating nanocarbons with relatively wide nanopores and inducing cooperative adsorption of CH4. Further study using a spectroscopic method could also clarify the detailed structure of adsorbed methane in such nanoporous carbons.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

REFERENCES

S Supporting Information *

Evaluation of simulated density fluctuations, and N2 and CH4 adsorption isotherms. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by a Research Fellowship from the Kurita Water and Environment Foundation, Foundation for the 21873

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