Comparative study of methane adsorption on single-walled carbon nanotubes

Acidic layer-enhanced nanoconfinement of anions in cylindrical pore of single-walled carbon nanotube

The adsorption of the nitrate ion by the cylindrical pore of single-walled carbon nanotubes (SWCNT) was found to be aided by an acidic adsorbed layer. Adsorbed water in the vicinity of the pore wall can supply protons through ionization, forming the acidic layer, according to Raman spectra and results of solution pH fluctuations caused by ion species adsorption. Such an acidic adsorbed layer leads to surplus adsorption of anionic species where the adsorbed amount of nitrate ions is much larger than that of cations. Also, we could observe the Raman bands being assignable to the symmetrical stretching mode at an extremely high-frequency region for nano-restricted nitrate ions compared to any other bulk phases. The abnormal band shift of adsorbed nitrate ions indicates that the nitrate ions are confined in the pore under the effects of nanoconfinement by the pore and the strong interaction with the acidic layer in the pore. Our results warn that we have to construct the adsorption model of aqueous electrolytes confined in carbon pores by deliberating the acid layer formed by the adsorbed water.

Porous carbon-based adsorption systems for natural gas (methane) storage

The use of gas fuel has considerable environmental advantages over traditional types of fuel based on petroleum products or fossil coal. The adsorption technology for methane (natural gas) uptake, storage and transportation can serve as a connecting link in gas supply to customers. The review considers characteristics of adsorption systems for methane storage. The key methods used for the synthesis of microporous carbon adsorbents based on various organic and inorganic carbon-containing compounds are described. Methods for increasing the efficiency of methane storage systems based on carbon adsorbents are analyzed. Theoretical approaches to optimization of the properties of methane – carbon adsorbent systems are discussed.

The bibliography includes 174 references.

Inhibitory Effect of Adsorbed Water on the Transport of Methane in Carbon Nanotubes

We investigate the transport diffusion of methane at 300 K and pressures of up to 15 bar in dry and wetted carbon nanotubes (CNTs) having diameters ranging from 0.95 to 2.034 nm using nonequilibrium molecular dynamics (NEMD) simulation. Because of their strong hydrogen bonding, preadsorbed water molecules transport in the form of clusters and block the diffusion of methane, reducing the Onsager coefficient of methane dramatically compared to that in dry CNTs. The reduction in the methane Onsager coefficient is greater in narrower CNTs or at higher water densities. Because the diameter of the water clusters is almost invariant with water density, the Onsager coefficient of water in the (10, 10) CNT increases linearly with water density. It is further found that whereas decreasing the CNT diameter from 2.034 to 0.95 nm enhances the Onsager coefficient of pure methane by about 1 order of magnitude, the Onsager coefficient of water is almost independent of the CNT diameter at a water density of 0.05 g/cm³. We propose a theoretical model for the strong dependency of methane diffusion in wetted CNTs on the Onsager coefficient of water, the preadsorbed water density, and the CNT diameter. The model predicts the Onsager coefficients of the methane/water mixture from the Onsager coefficients of the pure components. Our study provides a basic understanding of the coupled diffusion of immiscible components in nanochannels and will facilitate progress in gas storage and carbon capture as well as nanofiltration and biomedical and biotechnological applications.

Experimental Information on the Adsorbed Phase of Water Formed in the Inner Pore of Single-Walled Carbon Nanotube Itself

Thus far, nobody has successfully obtained the accurate information on the properties of the adsorbed phases of gases or vapors formed inside a cylindrical micropore of single-walled carbon nanotube (SWCNT) itself based on the experimental procedure. In this work, we succeeded in analyzing experimentally the properties of adsorbed nitrogen and water confined in the inner pore of SWCNT itself by opening the pore composed of close-ended SWCNT without any changes in the surface state and also by applying the unique method for characterization; both the amounts, as well as properties, of surface functional groups and the bundle structure are the same even after the treatments for introducing an open-ended structure to a close-ended one. As a result, the average pore sizes, as well as characteristic adsorption behavior, on the two types of sample were available from the analysis of respective difference adsorption isotherms of nitrogen measured at 77 K between the adsorbed amounts on the open-ended SWCNT and that on the close-ended one. The evaluated pore sizes well coincide with the results estimated by Raman data. These results strongly support that we could analyze the adsorbed phases formed only in the inner pore of SWCNTs by applying the present method. Furthermore, we could analyze the adsorbed phase of water formed inside the cylindrical micropore of SWCNTs, showing the difference in the densities of adsorbed water depending on the pore sizes from the value of bulk water; the densities of the adsorbed water were evaluated to be 0.62 and 0.71 g mL(-1) for SWCNTs having average pore sizes of 1.3 and 1.7 nm, respectively, which were in harmony with those obtained by the theoretical calculations reported by other researchers. The proposed analysis method makes it possible to recognize the focused states of the adsorbed water formed inside the cylindrical micropore of SWCNT more precisely and correctly. The method proposed will shed light on the discussion related to the detailed nature of various adsorbed gases into SWCNT, to the detailed role of adsorbed species formed inside pore in various phenomena, and to the designing the useful materials based on the gained knowledge.

Adsorption of Polar and Nonpolar Molecules on Isolated Cationic C60 , C70 , and Their Aggregates

Physisorption on graphite, graphene, nanotubes, and other graphitic structures has been the subject of numerous studies, partly driven by interest in the nature of order in two-dimensional systems, their phase transitions, and the use of graphitic scaffolds for reversible storage of hydrogen at high volumetric density and low mass. In contrast, physisorption on individual fullerenes or small aggregates of fullerenes has remained largely unexplored, last but not least, because of technical challenges. A summary of recent progress in identifying specific adsorption sites on positively charged C₆₀ , C₇₀ , and their aggregates is given in this Minireview. Adsorption energies and storage capacities for helium, hydrogen, methane, oxygen, nitrogen, water, and ammonia are determined. Mass spectrometric data reveal the formation of a commensurate phase in which all hollow sites of C₆₀ or C₇₀ are occupied. This phase is identified for all nonpolar molecules, including oxygen, which does not form a commensurate phase on planar graphite. The polar molecules, on the other hand, do not wet fullerenes and they do not form this commensurate phase. A hierarchy of other distinct adsorption sites are identified for nonpolar molecules, namely, groove sites for fullerene dimers and beyond, and dimple sites for fullerene trimers and beyond. Furthermore, evidence is presented for the preferential adsorption of hydrogen and methane in registered sites on fullerene dimers. The interpretation of experimental data that merely count the number of preferred adsorption sites is aided by molecular dynamics simulations, which utilize interaction potentials derived from ab initio calculations to determine adsorption energies.

Methane adsorption on aggregates of fullerenes: site-selective storage capacities and adsorption energies

Methane adsorption on positively charged aggregates of C60 is investigated by both mass spectrometry and computer simulations. Calculated adsorption energies of 118-281 meV are in the optimal range for high-density storage of natural gas. Groove sites, dimple sites, and the first complete adsorption shells are identified experimentally and confirmed by molecular dynamics simulations, using a newly developed force field for methane-methane and fullerene-methane interaction. The effects of corrugation and curvature are discussed and compared with data for adsorption on graphite, graphene, and carbon nanotubes.

Methane Storage in Spherical Fullerenes

In this paper, we investigate methane encapsulation in five spherical fullerenes C60,C240,C540,C960, and C1500. We exploit the 6–12 Lennard-Jones potential function and the continuum approximation to model the surface binding energies between methane and spherical fullerenes of varying sizes. Our results show that for a methane molecule interacting inside a spherical fullerene, the binding energies are minimized at locations which become closer to the fullerene wall as the size of the fullerene increases. However, we find that the methane molecule would require an applied external force to overcome the repulsive energy barrier in order to be encapsulated into a C60 fullerene. The present modeling indicates that the optimal minimum energy for methane storage in any spherical fullerene occurs for a fullerene with radius ≃6.17 Å, with a corresponding potential energy of ≃0.22 eV which occurs for a fullerene bigger than a C60 but slightly smaller than a C240 as the ideal spherical fullerene for methane encapsulation. Overall, our results are in very good agreement with other theoretical studies and molecular dynamics simulations, and show that fullerenes might be good candidates for gas storage. However, the major advantage of the approach adopted here is the derivation of explicit analytical formulae from which numerical results for varying physical scenarios may be readily obtained.

Co-adsorption of N2 in the presence of CH4 within carbon nanospaces: evidence from molecular simulations

Molecular simulations were performed to study the separation of CH(4) and N(2) from mixtures of composition x(CH(4))/x(N(2)) = 5/95 and x(CH(4))/x(N(2)) = 10/90 at 50 bar and 298 K on prototype carbon materials with different pore structures. The studied carbon structures include a slit and a tubular pore, that represent the simplest form of activated carbon and carbon nanotubes, respectively, in addition to a realistic porous carbon model with disordered pore structure and a recently introduced carbon foam model, which has a three-dimensional pore structure. The results indicate that, depending on the pressure and composition, the pore structure influences both the CH(4)/N(2) selectivity and the adsorption behaviour of the fluid molecules. The selectivity was decided by the interactions between CH(4) and N(2) molecules within the pore structure, in addition to the solid-fluid interactions. The simulation results indicate that, at least for the case of activated carbons (slit and random pores), it would not be appropriate to predict the binary adsorption behaviour of methane and nitrogen by means of pure component information. Regardless of the pore structure, the simulation results indicate that carbon materials show a CH(4)/N(2) (thermodynamic) selectivity of only 2-3 up to 2 bar at 298 K, and above this pressure, at equilibrium, none of the carbon materials is adequate for the efficient separation of this mixture.

Methane Adsorption on Graphitic Nanostructures: Every Molecule Counts

Bundles of single-walled nanotubes are promising candidates for storage of hydrogen, methane, and other hydrogen-rich molecules, but experiments are hindered by nonuniformity of the tubes. We overcome the problem by investigating methane adsorption on aggregates of fullerenes containing up to six C(60); the systems feature adsorption sites similar to those of nanotube bundles. Four different types of adsorption sites are distinguished, namely, registered sites above the carbon hexagons and pentagons, groove sites between adjacent fullerenes, dimple sites between three adjacent fullerenes, and exterior sites. The nature and adsorption energies of the sites in C(60) aggregates are determined by density functional theory and molecular dynamics (MD) simulations. Excellent agreement between experiment and theory is obtained for the adsorption capacity in these sites.

Methane storage in molecular nanostructures

We survey various molecular structures which have been proposed as possible nanocontainers for methane storage. These are molecular structures that have been investigated through either experiments, molecular dynamics simulations or mathematical modelling. Computational simulation and mathematical modelling play an important role in predicting and verifying experimental outcomes, but both have their limitations. Even though recent advances have greatly improved computations, due to the large number of atoms and force field calculations involved, computational simulations can still be time consuming as compared to an instantaneous mathematical modelling approach. On the other hand, underlying an ideal mathematical model, there are many assumptions and approximations, but such modelling often reveals the key physical parameters and optimal configurations. Here, we review methane adsorption for three conventional nanostructures, namely graphite, single and multi-walled carbon nanotubes, and nanotube bundles (including interstitial and groove sites), and we survey methane adsorption in other molecular structures including metal organic frameworks. We also include an examination of minimum binding energies, equilibrium distances, gravimetric and volumetric uptakes, volume available for adsorption, as well as the effects of temperature and pressure on the adsorption of methane onto these molecular structures.

Ethane/ethylene adsorption on carbon nanotubes: temperature and size effects on separation capacity

We present the results of Monte Carlo simulations of the adsorption of single-component ethane and ethylene and of equimolar mixtures of these two gases on bundles of closed, single-walled carbon nanotubes. Two types of nanotube bundles were used in the simulations: homogeneous (i.e., those in which all the nanotubes have identical diameters) and heterogeneous (those in which nanotubes of different diameters are allowed). We found that at the same pressure and temperature more ethane than ethylene adsorbs on the bundles over the entire range of pressures and temperatures explored. The simulation results for the equimolar mixtures show that the pressure at which maximum separation is attained is a very sensitive function of the diameter of the nanotubes present in the bundles. Simulations using heterogeneous bundles yield better agreement with single-component experimental data for isotherms and isosteric heats than those obtained from simulations using homogeneous bundles. Possible applications of nanotubes in gas separation are discussed. We explored the effect of the diameter of the nanotubes on the separation ability of these sorbents, both for the internal and for the external sites. We found that substrate selectivity is a decreasing function of temperature.