Unprecedented CO2 uptake in vertically aligned carbon nanotubes

Abstract

As an adsorbent, carbon nanotubes integrate the advantages of activated carbon with a robust sp2 carbon framework and a defined pore structure. Due to the presence of multiple adsorption sites, vertically aligned carbon nanotubes (VACNTs) exhibit excellent adsorption characteristics at high pressures (∼30 bar). However, it is known that a large fraction of the as-synthesized VACNTs are closed and are inaccessible for the adsorptive. In the present study, we employ a high temperature CO2 treatment process to open the VACNTs under controlled conditions without altering their alignment. Opening of the CNTs is confirmed by TEM and N2 adsorption measurements and subsequently the surface area of the opened VACNTs increases by 60%. Consequently, a so far unreported adsorption step, corresponding to adsorption in the interior of the CNTs is observed in the N2 adsorption isotherm. High pressure adsorption studies on these opened VACNTs revealed a dramatic increase of more than 140% in CO2 adsorption compared to as-prepared VACNTs, which is almost double that of zeolite 13X and is at par with certain metal-organic frameworks.