Adsorption and Condensation of SO2 in Double-Walled Carbon Nanotube Arrays Studied by Monte Carlo Simulations and Simple Analytical Models

摘要

Carbon nanotubes (CNTs) have been identified as extremely promising candidates for gas capture and storage. Therefore, an understanding of the adsorption mechanisms is crucial to the improvement of CNT applications. In this work, grand-canonical Monte Carlo simulations and analytical models are used to study, at the temperature of T = 303 K, the adsorption and condensation of SO2 in hexagonal arrays of double-walled CNTs of different inner nanotube radii R-in and intertube distances d. For both the inner and the outer adsorption, type I and type IV adsorption isotherms (IUPAC classification) are observed; they can be described adequately by analytical models. At a given pressure, the maximum adsorption among different CNT geometries depends strongly on the applied pressure. For the inner adsorption, three stages of adsorption are identified with increasing pressures. At low pressures, only one monolayer is formed, where the adsorption energy dominates the adsorption. At intermediate and high pressures, multilayers are formed until finally condensation is achieved; now it is the surface area or the available volume per CNT mass unit that dominates the adsorption. The nonlinear dependence of the outer adsorption on R-in and d can be explained by similar arguments as adopted for the inner adsorption. The effective number density of SO2 molecules and isosteric heat of adsorption are also analyzed to deepen our understanding of the adsorption behavior.