Characterization of Single Wall Carbon Nanotubes and Activated Carbon with Water Adsorption in Finite-Length Pore Models

Abstract

Grand Canonical Monte Carlo simulation is used to study water adsorption in activated carbon (AC) and carbon nanotubes (CNT) which is modelled as a bundle of seven tubes, and the simulation results are compared with the experimental data. To investigate the role of functional groups on water adsorption, hydroxyl groups are grafted at various locations on the surface of AC and CNT. For carbon nanotubes, the spacing between tubes can affect the isotherm. For small spacing and functional groups are located on the exterior surface of the central tube, the isotherm is not affected by the spacing and the concentration of the functional group. This is simply due to the geometrical constraint that prevents water to adsorb in the interstices between the tubes. However, the onset of adsorption occurs at lower pressures when the functional group is positioned in the larger interstices. When the spacing is larger, water clusters are readily formed around the functional group, resulting in an enhancement in adsorption, and pore-filling occurs in the interstices. For AC which is modelled as slit pores, the position and amount of functional group affect adsorption. The effects of temperature were also studied, and we observed an unusual behaviour in that the adsorptive capacity at 10 °C is lower than that at 15 °C which is confirmed with our computer simulation.