Molecular Simulation of Adsorption of Gases on Nanotubes

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ABSTRACT:The effect of pore morphology on the hydrogen-storage capacity of carbon materials at room temperature (298 K) has been studied using Grand Canonical Monte Carlo (GCMC) molecular simulation. Prototypical pore geometries such as slit pores and nanotubes were considered along with carbon foams and a random disordered carbon structure. Both physisorption and chemisorption models were considered in order to take into account the most favourable adsorption scenarios. Fluid-fluid and fluid-solid interactions were assumed to follow a Lennard-Jones-type potential.It was seen that physisorption alone cannot account for the adsorption of more than a few weight percentages, regardless of the pore morphology. Under physisorption conditions, the simulation results show that carbons with a slitshaped pore geometry are more efficient than other geometries at storing hydrogen, particularly at pore distances which allow the formation of fluid layers commensurate with the pore geometry. Slit pores appear to store a maximum of 1.77 wt% hydrogen by physisorption as opposed to carbon foams (1.48 wt%), carbons with a random structure (1.04 wt%) and carbon nanotubes (0.31 wt%). Simulation with hypothetical chemisorption models where the solid-fluid interaction is artificially enhanced showed that the storage capacity is higher in pore geometries with a larger accessible volume. Strong chemisorption gave loadings of up to 17 g/ (1.5 wt%), 91 g/ (13.8 wt%), 57.33 g/ (9.6 wt%) and 42.94 g/ (8.29 wt%) in nanotubes, foams, random structures and slit pores, respectively. Desorption is a problem in this scenario, as only ca. 1-3 wt% hydrogen can be delivered from these structures. Ultimately, the comparison of the different fluid-solid potentials revealed that if solid-fluid interactions could be enhanced beyond the classical dispersion, the more open structures could be ideal candidates for hydrogen-storage materials.

Section: Simulation Detailsmentioning

confidence: 99%

Molecular Simulation of Hydrogen Physisorption and Chemisorption in Nanoporous Carbon Structures

Kumar

Salih

Lü

et al. 2011

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“…higher electron density and an increased dispersion interaction. However, geometric effects appear to cancel these phenomena, which therefore seem to have little effect on the resulting equilibrium potentials and, as pointed out by Simonyan et al (2001), and also discussed by Müller (2010), by assuming the carbon atoms to be a collection of Lennard-Jones spheres with parameters similar to those used to describe carbon atoms in planar graphene, the fluid-fluid and solid-fluid interaction parameters can be estimated using the classical Lorentz-Berthelot combining rules as explained in the "Carbon Surface Reactivity" section.…”

Section: Carbon Nanotubesmentioning

confidence: 72%

“…To improve the dispersion of CNTs in aqueous solutions and particularly to improve the reactivity of MWCNTs with potential matrix compounds in composite structures, it is necessary to add polar functional groups. There has been much discussion in the literature on the functionalization of SWCNTs; this is only feasible using very controlled reactions involving, for example, either addition or substitution of heteroatoms at specific graphene carbons, otherwise the tube structure is severely compromised (Mawhinney 2000;Rosca et al 2005;Marshall et al 2006).…”

Section: Carbon Nanotubesmentioning

confidence: 99%

“…Raman spectroscopy is well established and has been used extensively in the characterization of graphene-based carbon materials. In 1974, Walker reported results from studies of a wide range of carbons (natural graphite, pyrolytic graphite, glassy carbon, carbon black and coal) (Nakamizo et al 1974) and subsequently the technique has been applied widely in the field-see for example Werner et al (2009). The work of Dresselhaus and co-workers has provided detailed firstprinciples understanding of the application of the technique to carbons and, most specifically, to nanotubes (see Dresselhaus et al 2005 and references therein).…”

Section: Carbon Nanotubesmentioning

confidence: 99%

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Structure, Chemistry and Energy of Carbon Surfaces

Bradley

Pendleton

2013

In this paper, the graphene layer is considered as the basic structural unit in a range of carbon materials. The effects of 2D and 3D order in defining the microstructures and macrostructures of carbons are reviewed along with the related concept of graphene layer orientation within the structure and the resulting ratio of basal plane to edge site as well as its effects on surface chemical reactivity and energy. Both non-specific and specific surface forces and the resulting potentials are considered. Theoretical physical models based on the Lennard-Jones potential applied to the basic structural units, either as an ideal, i.e. perfect, single graphene layer or as multiple layers of varied inter-layer spacing and size, are presented. Mention is then made of non-ideal surfaces (stepped or containing vacancies) and their resulting enhanced potential. The effects of incorporating chemical heterogeneity into the surfaces of these structures are also considered: carbon oxidation mechanisms are discussed in terms of bond energy and disruption of the (conjugated) graphene structure and the resulting polar functional groups are shown to increase interactivity with molecules that possess permanent dipoles or multipoles. The resulting increases in the polar component of surface-free energy can be used to control properties of adhesion, adsorption and wettability.

“…In the GCMC simulations the temperature and the activity (Mu¨ller, 2010) (which is directly related to the chemical potential) are specified. Simulations are also performed for bulk fluids that would hypothetically be in equilibrium with the adsorbed fluid (i.e.…”

Section: Molecular Simulation Of the Adsorbed Phasementioning

confidence: 99%

Predicting the adsorption of n-perfluorohexane in BAM-P109 standard activated carbon by molecular simulation using SAFT-γ Mie coarse-grained force fields

Herdes

Forte

Jackson

et al. 2016

This work is framed within the Eighth Industrial Fluid Properties Simulation Challenge, with the aim of assessing the capability of molecular simulation methods and force fields to accurately predict adsorption in porous media for systems of relevant practical interest. The current challenge focuses on predicting adsorption isotherms of n-perfluorohexane in the certified reference material BAM-P109 standard activated carbon. A temperature of T ¼ 273 K and pressures of p=p 0 ¼ 0:1, 0.3, and 0.6 relative to the bulk saturation pressure p 0 (as predicted by the model) are the conditions selected in this challenge. In our methodology we use coarse-grained intermolecular models and a top-down technique where an accurate equation of state is used to link the experimental macroscopic properties of a fluid to the force-field parameters. The stateof-the-art version of the statistical associating fluid theory (SAFT) for potentials of variable range as reformulated in the Mie group contribution incarnation (SAFT-Mie) is employed here. The parameters of the SAFT-Mie force field are estimated directly from the vapour pressure and saturated liquid density data of the pure fluids using the equation of state, and further validated by molecular dynamic simulations. The coarse-grained intermolecular potential models are then used to obtain the adsorption isotherm kernels for argon, carbon dioxide, and n-perfluorohexane in graphite slit pores of various widths using Grand Canonical Monte Carlo simulations. A unique and fluid-independent pore size distribution curve with total micropore volume of 0.5802 cm 3 /g is proposed for the BAM-P109. The pore size distribution is obtained by applying a non-linear regression procedure over the adsorption integral equation to minimise the quadratic error between the available experimental adsorption isotherms for argon and carbon dioxide and