Endohedral Adsorption Research Articles

Storage and permeation of hydrogen molecule, atom and ions (H+ and H−) through silicon carbide nanotube; a DFT approach

The barriers for the encapsulation and decapsulation of hydrogen ions (cationic hydrogen and hydride), atom, and molecule through silicon carbide nanotube are thoroughly studied. DFT method is selected to measure the kinetic barriers for the passage of hydrogen atom, ions and molecule through nanotube via scanning potential energy surface. The kinetic barriers for the passage (encapsulation and decapsulation) of hydrogen are very important to understand the mechanism of hydrogen storage and release. The barriers for the permeation of H, H+ and H− across SiC nanosheet are lower compared to hydrogen molecule (H2). The exohedral and endohedral adsorption of hydrogen ions (cation and anion), atom and exohedral hydrogen molecule on silicon carbide are exothermic in nature. Whereas the encapsulation of hydrogen molecule in silicon carbide is endothermic. Electronic properties are analyzed through measurement of energy gap between highest occupied and lowest unoccupied molecular orbitals gap (GH-L) and the density of state (DOS) spectra. The GH-L analysis reveals that endohedral complexes have more pronounced effect on electronic properties compared to exohedral complexes. The SiC nanotube has highly favorable properties for storage and release of hydrogen ions, and atom.

Theoretical Study of Interaction Between Methanol and Metal Encapsulated Single Walled Carbon Nanotubes

Interaction of methanol (MeOH) with the outer (exohedral) and inner (endohedral) sides of (10, 0) single walled carbon nanotube (SWCNT) and transition metal (Ni, Pd) encapsulated SWCNT were investigated using density functional theory (DFT). MeOH was inserted to the nanotube from both –CH3 and –OH sides and all structures were fully relaxed. Binding energies show the exothermic interaction of MeOH and SWCNT which increases by inserting Metals to SWCNT. It was found that Endohedral adsorption is more favored over the exohedral one in all structures. Electronic results show a few modifications in band structures and density of states of pristine SWCNT. However endohedral adsorption of MeOH to Ni@SWCNT alters its electronic and magnetic properties. Methanol adsorption reduces the band gap of Pd@SWCNT too. Spin polarized partial densities of states (PDOS) were also investigated for further comparative studies. According to our results Ni or Pd encapsulated SWCNT are expected to be a potential candidate for MeOH adsorption.

Noble gas encapsulation into carbon nanotubes: predictions from analytical model and DFT studies.

The energetics for the interaction of the noble gas atoms with the carbon nanotubes (CNTs) are investigated using an analytical model and density functional theory calculations. Encapsulation of the noble gas atoms, He, Ne, Ar, Kr, and Xe into CNTs of various chiralities is studied in detail using an analytical model, developed earlier by Hill and co-workers. The constrained motion of the noble gas atoms along the axes of the CNTs as well as the off-axis motion are discussed. Analyses of the forces, interaction energies, acceptance and suction energies for the encapsulation enable us to predict the optimal CNTs that can encapsulate each of the noble gas atoms. We find that CNTs of radii 2.98 - 4.20 Å (chiral indices, (5,4), (6,4), (9,1), (6,6), and (9,3)) can efficiently encapsulate the He, Ne, Ar, Kr, and Xe atoms, respectively. Endohedral adsorption of all the noble gas atoms is preferred over exohedral adsorption on various CNTs. The results obtained using the analytical model are subsequently compared with the calculations performed with the dispersion-including density functional theory at the M06 - 2X level using a triple-zeta basis set and good qualitative agreement is found. The analytical model is however found to be computationally cheap as the equations can be numerically programmed and the results obtained in comparatively very less time.

Molecular Dynamics Simulation of Noble Gases Adsorption on Carbon Nanotube Bundles

To study the possible application of nanotubes as gas storage materials, noble gases adsorption on carbon nanotube bundles has been studied using the molecular dynamics simulation. Adsorption isotherms of Ar, Kr, and Xe on bundles of (10,10) single walled carbon nanotubes have been investigated at various pressures and temperatures. The results indicate that adsorption occur both internal pore volume and external surface of the open-ended carbon nanotubes. It is found that endohedral and exohedral adsorption coverages depend on temperature. We have also calculated isosteric heat of adsorption, binding energy, and isometric heat capacity. The values of these quantities indicate that carbon nanotube bundles can be used as materials for storage of noble gases.

Reactivity of the interior surface of (5,5) single-walled carbon nanotubes with and without a Stone–Wales defect

The adsorption of hydrogen and fluorine atoms on the interior surface of defect-free and Stone–Wales defect armchair (5,5) single-walled carbon nanotubes were investigated using density functional theory. The reaction energy values for the hydrogenation and fluorination at all the possible unique sites were obtained at the UB3LYP/6-31G* level. The calculated reaction energy values were used to corroborate the reactivities of different sites on the interior surface of SWCNTs. The results indicate that for defect-free nanotube, endohedral adsorption is rather unfavorable both for hydrogen and fluorine. The computed exothermicities range from 4.8 to 13.4kcal/mol for hydrogenation and from 16.6 to 23.2kcal/mol for fluorination. However, the introduction of a SW defect on the (5,5) SWCNT improves the chemical reactivity of the interior surface of the defected tube. The computed exothermicities for the endohedral hydrogen and fluorine atoms are within the ranges of 7.0–33.4 and 18.5–41.8kcal/mol, respectively. The most exothermic and reactive site on the interior surface of SW defective (5,5) SWCNT is the C2 site shared by two seven-membered rings and one five-membered ring. This is different from the results reported on the reactivity of the exterior surface of SW defective nanotubes. The electronic and vibrational properties of the (5,5) SWCNTs with adsorption of hydrogen and fluorine atoms on the interior surface were also explored.

Exohedral and endohedral adsorption of alkaline earth cations in BN nanocluster

Adsorption of three alkaline earth cations inside and outside of a B(12)N(12) nano-cage in aqueous medium was investigated using density functional theory. The results obtained are discussed in terms of thermodynamic, geometric, and electronic properties. Based on the calculation of enthalpy changes at 298 K and 1 atm, the adsorption of the considered cations was found to be exothermic outside the cluster while it is endothermic inside. It was also found that the exohedral adsorption favorability of the cluster increases in the series: Ca(2+) < Mg(2+) << Be(2+) with Gibbs free energy changes in the range of -0.08 to -1.53 eV at B3LYP/6-31G (d) level of theory. Overall, interaction of the cations with the cluster influences the electronic properties of the cluster through stabilizing the HOMO and LUMO as well as reducing the energy gap between them. However, the electronic properties changed much more in the case of endohedral adsorption in comparison with the exohedral adsorption.

Theoretical study of chemisorption of hydrogen atoms on the sidewalls of armchair single-walled carbon nanotubes with Stone–Wales defect

The geometrical structures and electronic properties as well as thermal stabilities of hydrogenated armchair (n,n) (n=4–8) SWCNTs with Stone–Wales (SW) defect were investigated using density functional theory (DFT-B3LYP) in combination with the 6-31G* basis set. It is found that the chemisorptions of two hydrogen atoms inside and outside the SW defective SWCNTs are exothermic processes. As the nanotube diameter increases, the reaction energy of exohedral addition decreases, whereas that of endohedral addition increases. Exohedral nanotube adsorption is energetically more favorable than endohedral adsorption for smaller diameter (4,4)-SW, (5,5)-SW SWCNTs, but not for larger diameter (7,7)-SW and (8,8)-SW SWCNTs. The result is different from hydrogen on perfect nanotubes. Significantly, compared with the adsorption on the exterior sidewalls of perfect SWCNTs, the reaction energies of the endo-hydrogenated (7,7)-SW and (8,8)-SW SWCNTs are more exothermic, meaning that the central CC bond of SW defect inside the armchair (7,7)-SW and (8,8)-SW SWCNT sidewalls is more reactive than that in perfect sites. This is different from the result reported on the sidewall reactivity of SW defects outside the armchair SWCNTs. The calculated energy gaps indicate that the hydrogen-chemisorbed SW defective armchair tubes are always wide energy gap structures. The energy gaps of endohedral hydrogen-chemisorbed tubes are higher than those of the corresponding perfect tubes.

HELIUM ADSORPTION ON CARBON NANOCONES WITH DIFFERENT DISCLINATION ANGLE: MOLECULAR DYNAMICS SIMULATION

We have used molecular dynamics simulation to study the influence of disclination angle on helium adsorption capacity of carbon nanocones (CNCs). Adsorption capacities of 180°, 240° and 300° CNCs have been compared at various temperatures. The results indicate that helium atoms adsorb on internal and external surfaces of the CNCs. At saturation conditions, the endohedral adsorption coverage is decreased by increasing the disclination angle, while the exohedral adsorption coverage is independent of this angle. It is also found that temperature has a considerable effect on the adsorbed amount. Comparison of the adsorption on CNCs with carbon nanotubes indicates that the adsorption capacity of cones is greater than those of tubes. Our results confirm the fact that the CNCs are good candidate for gas storage.

NMR Study of Preferential Endohedral Adsorption of Methanol in Multiwalled Carbon Nanotubes

Adsorption of 13C-enriched methanol in multiwalled carbon nanotubes (MWNTs) was systematically studied under well-controlled environment by 13C MAS NMR. The results of variable-pressure experiments indicate that the endohedral adsorption of methanol in carbon nanotubes (CNTs) is preferential over the exohedral adsorption. The exohedral adsorption does not occur until the endohedral adsorption saturates. The 13C chemical shift of endohedral methanol exhibits a large upfield shift due to the strong spatial diamagnetic shielding effect induced by the delocalized electrons of nanotubes under the influence of external magnetic field. The exohedral methanol is also shielded, but to a lesser extent. DFT calculations support these experimental results. The 13C spin–lattice relaxation times, T1, of endohedral and exohedral methanol were also measured, and they have different vapor pressure dependence at room temperature. Furthermore, variable-temperature experiments suggest that methanol molecules inside CNTs may ...

Adsorption properties of chalcogen atoms on a golden buckyball Au16− from first principles

Using first-principles density functional theory, we investigate the adsorption properties of chalcogen elements (oxygen and sulfur) on an anionic golden nanocage Au16− and its effects on the structural and electronic properties of the golden cage. In particular, we find that when a sulfur atom is encapsulated inside Au16−, its bonding character with Au atoms appears ionic due to electron transfer from sulfur to the gold nanocage. In contrast, the exohedrally adsorbed S atom tends to have strong orbital hybridization with the golden nanocage. For an oxygen adsorption case, electrons from the golden cage tend to be shared with the adsorbed O atom exhibiting strong orbital hybridization, regardless of its adsorption sites. To investigate the transition behaviors between the most stable exohedral and endohedral adsorption configurations, we calculate the activation and reaction energies in the transition. The oxygen atom experiences a lower energy barrier than the sulfur atom due to its smaller atomic radius. Finally, we explore the vibrational properties of S- or O-adsorbed Au16− buckyballs by calculating their infrared spectra.

Density functional theory study of the hydrogen chemisorption of single-walled carbon nanotubes with carbon ad-dimer defect

The structural and electronic properties of hydrogenated armchair and zigzag SWCNTs with carbon ad-dimer (CD) defect were investigated by means of the B3LYP hybrid density functional method using 6-31G∗ basis set. It is found that the chemisorptions of two hydrogen atoms inside and outside the CD defective SWCNTs are exothermic processes. Exohedral nanotube adsorption is energetically more favorable than endohedral adsorption. These results are in agreement with hydrogen on pristine nanotubes. The positional preference for the chemisorption of two hydrogen atoms is the same for the CD defective armchair and zigzag nanotubes. However, the reaction energy of two hydrogen atoms on the exterior sidewalls of CD defective SWCNTs is almost independent of the tube diameter. This is different from the results reported on pristine nanotubes. The calculated energy gaps indicate that the hydrogen-chemisorbed CD defective armchair tubes are always wide energy gap structures, while the hydrogen-chemisorbed CD defective zigzag tubes have significantly lower gaps. The HOMO–LUMO gap and reaction energy for the chemisorption of more hydrogen atoms on the exterior sidewalls of CD defective armchair SWCNTs were also explored.

Interaction of the alcohol molecules methanol and ethanol with single-walled carbon nanotubes – A computational study

Interactions of a primary alcohol molecule, methanol (MeOH) or ethanol (EtOH), with the outer (exohedral) and inner (endohedral) sides of various armchair single-walled carbon nanotubes (SWCNTs), with diameters from 0.53 to 1.62 nm, are investigated by means of density functional theory including an empirical energy correction for dispersion (DFT-D). In a previous computational study, the accuracy of this DFT-D approach was demonstrated by benchmarking it to high-level wavefunction calculations on model systems for the SWCNT–MeOH interaction. In the present study, for both alcohols, endohedral adsorption sites are found to be favoured over the exohedral ones.

Behavior of ethylene and ethane within single-walled carbon nanotubes. 1-Adsorption and equilibrium properties

Endohedral adsorption properties of ethylene and ethane onto single-walled carbon nanotubes were investigated using a united atom (2CLJQ) and a fully atomistic (AA-OPLS) force fields, by Grand Canonical Monte Carlo and Molecular Dynamics techniques. Pure fluids were studied at room temperature, T=300 K, and in the pressure ranges 4×10−4<p<47.1 bar (C2H4) and 4×10−4<p<37.9 bar (C2H6). In the low pressure region, isotherms differ quantitatively depending on the intermolecular potential used, but show the same qualitative features. Both potentials predict that ethane is preferentially adsorbed at low pressures, and the opposite behavior was observed at high loadings. Isosteric heats of adsorption and estimates of low pressure Henry’s constants, confirmed that ethane adsorption is the thermodynamically favored process at low pressures. Binary mixtures of C2H4/C2H6 were studied under several (p,T) conditions and the corresponding selectivities towards ethane, S, were evaluated. Small values of S<4 were found in all cases studied. Nanotube geometry plays a minor role on the adsorption properties, which seem to be driven at lower pressures primarily by the larger affinity of the alkane towards the carbon surface and at higher pressures by molecular volume and packing effects. The fact that the selectivity towards ethane is similar to that found earlier on carbon slit pores and larger diameter nanotubes points to the fact that the peculiar 1-D geometry of the nanotubes provides no particular incentive for the adsorption of either species.

Determination of the surface area and porosity of carbon nanotube bundles from a Langmuirian analysis of sub- and supercritical adsorption data

In this paper we revisit the problem of how to obtain reliable information on the porosity and surface area of single-walled carbon nanotube (SWCNT) bundles from adsorption data. We report on adsorption measurements obtained on bundles of high-purity electric-arc nanotubes, with various gases (methane, ethane, ethylene, propane, and propylene) spanning reduced temperatures between 0.8 and 1.6, and pressures from 200 Pa up to 8 MPa, exploring low uptakes as well as multilayering conditions. Using these data we demonstrate the applicability of an analytical procedure for interpreting adsorption isotherms of light alkanes and alkenes on SWCNT bundles in quantitative terms. In particular, its use for the reliable assessment of the surface area and accessible pore volume of the bundles is demonstrated. Our results show that the same set of endohedral sites, as well as the same set of external sites, is available on our sample to all adsorbates investigated. We also show a crossover behavior in endohedral adsorption between the alkene and its saturated counterpart when moving from low pressure to near saturation conditions.

Statistical mechanical lattice models of endohedral and exohedral xenon adsorption in carbon nanotubes and comparison with Monte-Carlo simulations

Adsorption of xenon in carbon nanotubes has been investigated by Kuznetsova et al. [A. Kuznetsova, J.T. Yates Jr., J. Liu, R.E. Smalley, J. Chem. Phys. 112 (2000) 9590] and Simonyan et al. [V. Simonyan, J.K. Johnson, A Kuznetsova, J.T. Yates Jr., J. Chem. Phys. 114 (2001) 4180] where endohedral adsorption isotherms show a step-like structure. A matrix method is used for calculation of the statistical mechanics of a lattice model of xenon endohedral adsorption which reproduces the isotherm structure while exohedral adsorption is treated by mean-field theory.

Ab initio study of curvature effects on the physical properties ofCH4-doped nanotubes and nanoropes

We have performed an ab initio study of the energetics, structural, electronic, and optical propertiesof the CH4-doped ultrathin 4 Å and large diameter carbon nanotubes (CNTs). No adsorption of theCH4 molecule has been seen either on the groove or on the interstitial sites of the achiral4 Å nanoropes. In the case of weakly bonded systems such as the adsorption of theCH4 molecules on the nanotubes, a prominent role of the dispersion forces in the binding isobserved. The negative contribution of the zero point vibrational energy to thebinding energy is seen to be appreciable. The effects of the tube diameter and thedifferent chiralities of the carbon nanotubes on the adsorbate-induced physicalproperties have been investigated. In the large diameter tubes, the binding of theCH4 moleculein the endohedral adsorption is much stronger than that in the exohedral adsorption. The binding ofthe CH4 molecules depends upon the chirality of the nanotube and we find no adsorption on thechiral (4, 2) tube. We find that the local density approximation (LDA) over-binds theCH4 molecule and the generalized gradient approximation (GGA) under-binds it, andfor a reliable theoretical estimate one should take some weighted averageof the binding energies (BEs) determined in the LDA and the GGA. Thecurrently calculated BE and the adsorbate concentration are in reasonableagreement with the measured data available for the (10, 10) nanotubes. Theelectronic structure of the pristine tube is quite altered by the adsorption of theCH4 molecule on the surface of the tube because of the breaking of the symmetry of the hostlattice except the chiral (4, 2) tube, which has practically no symmetry. The adsorptionincurs splitting in the states in the whole energy range, especially in the large curvature4 Å tubes. The bandgap of the semiconducting achiral zigzag nanotube is reduced,whereas that of a chiral semiconducting tube is enhanced, by the adsorption of theCH4 molecules. Theadsorption of CH4 molecules does not alter significantly the peak structure in the optical absorption of thepristine tube, except for some changes in the energy locations and the relative intensities inthe achiral tubes. Most of the calculated peaks in the optical absorption of the pristinelarge diameter (10, 0) and (10, 10) nanotubes have been observed in the experimentalmeasurements.

Gas adsorption in single-walled carbon nanotubes studied by NMR

Adsorption isotherms of methane and ethane in single-walled carbon nanotubes (SWNTs) were measured by ${}^{1}\mathrm{H}$ nuclear magnetic resonance (NMR) at room temperature. It is shown that the interior of SWNTs becomes available for methane and ethane adsorption after cutting of SWNTs. Such endohedral adsorption dominates methane and ethane adsorption in SWNTs, at least below 1 MPa. The average exchange time between molecules adsorbed inside SWNTs and free gas molecules outside was estimated to be on the order of 80 ms. It is shown that exposure to oxygen has no effect on methane and ethane endohedral adsorption in SWNTs, suggesting that the adsorption energy of oxygen molecules inside SWNTs is small compared to that of methane. ${}^{13}\mathrm{C}$ NMR indicates that under atmospheric pressure and room temperature helium atoms could access the interstitial sites of SWNT bundles whereas ${\mathrm{H}}_{2},$ ${\mathrm{CO}}_{2},$ and ${\mathrm{N}}_{2}$ molecules could not.

Hydrogen Adsorption Storage on Single-Walled Carbon Nanotube Arrays by a Combination of Classical Potential and Density Functional Theory

The adsorptions of hydrogen both on square-packed single-walled carbon nanotube (SWCNT) arrays and on isolated nanotubes were investigated by a combination of a classical potential and density functional theory (DFT) method. Excess adsorption of hydrogen on the SWCNT with diameters of 1.225, 2.04, and 2.719 nm at 77 K and at ambient temperature, T = 300 K, has been calculated. DFT calculations indicate that the excess gravimetric storage capacity of exohedral adsorption of hydrogen on SWCNT array is as much as 30% of that of endohedral adsorption when the van der Walls (VDW) gap was fixed at 0.335 nm, while excess adsorption of hydrogen outside the nanotubes is close to or exceeds the excess endohedral adsorption for isolated nanotubes. The total excess gravimetric storage capacity (including endohedral and exohedral adsorptions) of hydrogen on open nanotube arrays of 2.719 nm is 7.1 wt % at 77 K and 4 MPa, while the total excess adsorption of hydrogen on open isolated nanotubes of 2.719 nm reaches 9.5 wt...

Exohedral and endohedral adsorption of nitrogen on the sidewall of single-walled carbon nanotubes

Different configurations of a single nitrogen atom adsorbed at different sites on the sidewall of single-walled carbon nanotubes (SWNT's) are studied through first-principles calculations. It is found that the outer surface of the tube wall is reactive to the nitrogen atom, and a nitrogen atom can be chemically adsorbed on it forming an exohedral complex with a binding energy of $\ensuremath{\sim}3.86\mathrm{eV}$ for a (5,5) SWNT. Although the inner surface of the tube wall is less reactive than the outer surface, a nitrogen atom can still be adsorbed on the inner wall of the (5,5) tube forming an endohedral complex with the binding energy on the order of $\ensuremath{\sim}1.78\mathrm{eV}.$ The local structure of the SWNT around the adsorption sites is substantially changed. From studying the binding energies of a nitrogen atom adsorbed on different armchair tubes $(n,n)$ with n ranging from 5 to 16, it is found that the binding energy for exohedral adsorption decreases significantly with increasing radius (or increasing $n)$ of the tube, while for endohedral adsorption the binding energy varies slightly. The influence of endohedral adsorption of a single nitrogen atom on the local density of states of a (5,5) SWNT is also discussed.

Adsorption of Nitrogen in Carbon Nanotube Arrays

Grand canonical Monte Carlo molecular simulations have been carried out for adsorption of nitrogen at 77 K on square arrays of open and closed single-wall carbon nanotubes with diameters of 6−30 A and nanotube separations of 4−30 A. Exohedral spaces for arrays of closed nanotubes with small tube separations are microporous. For large separations, two-stage adsorption occurs, corresponding to monolayer formation followed by a condensation step. Filling of the exohedral space is dominated by geometrical factors that allow the calculation of adsorbate molecular packing factors, which are close to that for liquid nitrogen. The amounts adsorbed in arrays of open nanotubes are higher than for closed tube arrays, as expected. The highest increase is for arrays with small tube separations and large tube diameters, where endohedral adsorption dominates; for wider tube separations the increase in amount adsorbed is less because exohedral adsorption becomes important. Arrays of single-wall carbon nanotubes have the ...