Understanding Carbon Dioxide Adsorption in Carbon Nanotube Arrays: Molecular Simulation and Adsorption Measurements

Grand-canonical Monte Carlo simulations and adsorption experiments are conducted to understand the adsorption of CO2 onto bundles of 3D aligned double-walled carbon-nanotubes of diameter 5 nm at 303 K. The simulation of partial adsorption isotherms, i.e., only inner tube volume, only interstices between tubes, and unrestricted, allows a breakdown of the experimental adsorption isotherms into contributions of different regions. The results are compatible with microscopic observations of the majority of the inner tube volumes being accessible for CO2. Further, the unrestricted adsorption isotherm is quantitatively equivalent to the sum of inner and outer adsorption for the pressure range considered in this work, p < 40 bar, indicating no significant interference between inner and outer regions. The intertube distance, which is varied from 0 to 15 nm, dramatically affects the isosteric heat of adsorption and adsorption capacity. Excess adsorption is found to display a nonlinear behavior with d, for unrestricted and outer cases. For low pressures (p ≤ 14 bar), maximum adsorption occurs at d = 0.5 nm. However, for higher pressures, 14 < p < 40 bar, the adsorption peaks at d = 1 nm. The Freundlich isotherm is found to fit the experimental and simulation data. The adsorption sequence changes with the intertube distance for the unrestricted case. At d ≤ 0.5 nm, adsorption proceeds with increasing loading in the following order: grooves and inner surface adsorption → fill interstitial region → fill inner region. However, at higher distances, d > 0.5 nm, the sequence changes the following: inner surface adsorption + partial outer surface adsorption → complete outer surface adsorption → fill interstitial, groove, inner adsorption. The change in mechanism of adsorption is clearly reflected in the behavior of the heat of adsorption, where we observed a crossover behavior at around d = 0.5 nm.