Methane adsorption in single-walled carbon nanotubes arrays by molecular simulation and density functional theory

Abstract

A combination of grand canonical ensemble Monte Carlo (GCMC) and density functional theory (DFT) methods has been used to study the adsorption of methane molecules in single-walled carbon nanotubes (SWNTs) square arrays for the tubes of diameters of 2.04 and 4.077 nm at 74.95, 148 K and ambient temperature, 300 K. On one hand, the adsorption of methane molecules inside the tubes of SWNTs arrays is calculated by the DFT method. The effect of temperature and tube diameter on phase transition and adsorption is discussed. Layering transitions and capillary condensations are observed at 74.05 K in two different pore diameters. By using the grand potential, the positions of phase transitions are exactly located. On the other hand, the adsorption of methane molecules in interstices formed by outer surfaces of the neighboring carbon tubes in SWNTs square arrays has been studied by the GCMC method. It is observed that the adsorption in the interstices for large size tubes is not insignificant. Simulation results suggest that in the interstice formed by outer surfaces of square arranged carbon tubes with diameter being 4.077 nm, the solid-like structure can be recognized at 120 K or below this temperature. Moreover, the total amount of adsorptive storage of methane in SWNT square array can reach 22 mmol/g for the tube of 4.077 nm at 6 MPa and 300 K.