Adsorption of light alkanes and alkenes onto single-walled carbon nanotube bundles: Langmuirian analysis and molecular simulations

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Abstract

Grand canonical Monte Carlo (GCMC) simulations are employed to study the adsorption equilibrium properties of methane, ethane, ethylene, propane, and propylene onto homogeneous bundles of single-walled carbon nanotubes, at room temperature, from 10-4 bar up to 90% vapor pressure. Individual adsorption isotherms for the internal volume of a bundle and for its external adsorption sites are separately calculated for individual nanotube diameters in the range 11.0 Å ≤ D ≤ 18.1 Å. External adsorption is further decomposed into the contributions from its two main adsorption sites - external grooves and exposed surfaces of the peripheral tubes - based on a geometrical model for the average groove volume that takes into account the molecular nature of the adsorbate. Both intrabundle confinement and adsorption onto the grooves lead to type I isotherms, which are modeled with Langmuirian-type equations. Adsorption on the exposed surfaces of the peripheral tubes in a bundle gives rise to a type II isotherm, which is described by the BET model with a finite number of adsorbed layers. The linear combination of the Langmuir isotherm model for adsorption onto groove sites and the BET isotherm model produces a composite isotherm that is in good agreement with the GCMC isotherm for overall adsorption onto the external sites of a bundle. The influence of adsorbate molecular length and existence of an unsaturated chemical bond in its molecular skeleton are studied by monitoring the dependence of the Henry constant and zero-coverage isosteric heat of adsorption with the dispersive energy for the solid-fluid pair potential of each adsorbate. Our results show that the adsorptive properties are especially influenced by the presence of a double bond in the case of small molecules, such as the ethane/ethylene pair.

Original languageEnglish
Pages (from-to)43-52
Number of pages10
JournalColloids and Surfaces A: Physicochemical and Engineering Aspects
Volume357
Issue number1-3
DOIs
Publication statusPublished - 20 Mar 2010

Keywords

  • Adsorption
  • Alkanes
  • Alkenes
  • Henry constant
  • Molecular simulation
  • Single-walled carbon nanotubes

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