Simulation of methane adsorption and diffusion in a carbon nanotube channel

• We modeled the pore-connectivity feature of nanoporous carbons as a constricted CNT.
• The methane loadings are in the same order of magnitude as in real NPCs.
• The diffusions are 1–2 orders of magnitude lower than that of slit and CNT.
• The diffusions are comparable with those of the C-slit and VPC models.
• The diffusivity decreases with the constriction length.

The effect of pore-connectivity feature of nanoporous carbons (NPCs) on the adsorptive and diffusive behavior of methane was studied by a simple constricted carbon nanotube (C-CNT) model using grand canonical Monte Carlo and molecular dynamics simulations. In spite of slight overprediction, C-CNT model can properly predict the adsorption of methane in some real NPCs. The calculated heat of adsorption was well agreed with the reported values for the hypothetical Schwarzite (C168) model and activated carbon. The obtained results show that the most preferred site for the adsorption of methane is the constricted region of the pore. The effect of constriction length on the methane uptake was studied at various pressures. It was shown that at low pressures the methane uptake increased with the length of constriction, while at high pressures the uptake is larger for short constriction. The diffusion coefficients of methane inside the C-CNT were calculated by very long MD simulations (220 ns). The calculated methane diffusion coefficients in the C-CNT were one to two orders of magnitude lower than that in the regular slit pores as well as the straight carbon nanotubes. In addition, the diffusion coefficients in the C-CNT were in the same order of magnitude as in the constricted slit and virtual porous carbon (VPC) models of carbon pores. MD simulations revealed that the diffusivity decreases with the constriction length and approaches a constant value for large constriction length.

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