Monte Carlo Simulations of SO2, H2S, and CO2 Adsorption in Charged Single-Walled Carbon Nanotube Arrays

摘要

Carbon nanotubes (CNTs), as a gas adsorbent with electrical conductivity, are one of the most promising functional materials for electric swing adsorption. By using a grand-canonical Monte Carlo method, the adsorption and the orientation ordering of SO2, H2S, and CO2 in hexagonal arrays of charged and uncharged armchair single-walled carbon nanotubes (SWCNTs) of diameter similar to 3 nm and different intertube distances were investigated at 303 K with the applied pressure up to their saturation vapor pressure. In the SWCNT model systems, each carbon atom carries an extra charge q between -0.04e and +0.04e. For the inner adsorption of SO2 molecules, it is found that the capacity of adsorption in positively charged SWCNTs is always larger than in neutral SWCNTs. In contrast, for negatively charged SWCNTs, their SO2 adsorption capacity is larger than that in neutral SWCNTs only at low pressures. For the inner adsorption of H2S molecules, a similar trend is found but with an opposite dependence on the sign of the CNT charges. In the case of CO2, however, positive CNT charging always enhances the adsorption, while negative CNT charging always suppresses it. By analyzing the isosteric heat of adsorption, molecular orientation, and CNT-adsorbate interactions, it is proven that the inner adsorption behavior of these polar or nonpolar molecules having different dipole moments in charged CNTs mainly depends on the cooperative effect of CNT-adsorbate interactions and the geometry of CNT space and the topology of the gas molecules. Furthermore, such a cooperative effect is believed to be universal and can also be used to understand the outer and unrestricted adsorptions of these molecules in charged CNTs.