Effect of surface diffusion on rapid gas transport in carbon nanotubes

Abstract

Abstract Many studies on carbon nanotubes (CNTs) have revealed that gas transport behaves differently at the nanoscale compared to the macroscale. While traditional gas transport models do not account for the effect of adsorption, as the adsorbed gases at the macroscopic level are relatively minor and can be ignored, the significant specific surface area of nanochannels leads to a significant increase in the proportion of adsorbed gases. Therefore, the impact of adsorption on gas transport must be taken into consideration. In this study, the diffusion and adsorption behaviors of different gas molecules inside CNTs of different diameters were simulated based on equilibrium molecular dynamic and Grand canonical Monte Carlo methods. The transport diffusion coefficients of gases were calculated using flux correlation functions, and the surface diffusion coefficients of gases were derived by combining with rarefied gas kinetics. The results show that the transport diffusion coefficients of gases in carbon nanotubes are several times or even one order of magnitude higher than those calculated by Knudsen's equation, and the surface diffusion of adsorbed molecules in carbon nanotubes is responsible for the rapid transport, and for the first time, the surface diffusion coefficient of different adsorbed gases in CNTs at different pressures and diameters is derived.