Adsorption properties of single-walled carbon nanotubes for air quality engineering

This study explored the adsorption properties of single-walled carbon nanotubes (SWNTs) for their future applicability as gas-phase adsorbents. Nanotubes were observed to undergo "aging" in which the surface area and pore volumes of the tested samples changed over a period of several months. It is hypothesized that "aging" of nanotubes is due to slow changes in their surface chemistry; and it is believed that "aging" will be an important factor in deciding their use as adsorbents for air quality applications. The adsorption mechanisms were studied by combining molecular simulation with experiments; and a methodology was developed to determine the fraction of open nanotubes in samples and the adsorptive contribution of common impurities. These results are of interest to a wide community of researchers, e.g., scientists can now use our procedure to determine if the hydrogen storage capacity of their sample is the limiting capacity or if it can be improved by opening and purifying the nanotube samples.; The adsorption capacities of nanotubes were also determined for pure organic vapors, water vapor and mixtures of organic and water vapor. Adsorption of organic vapors depended mainly on the sample morphology with a maximum uptake equivalent to only ≈50% of the total pore volume of the sample. The adsorption capacities appeared unaffected by the physical properties of the organic vapors. The adsorption of water vapor was equivalent to only 10--15% of the total pore volume, which indicated that nanotubes are extremely hydrophobic. It was also observed that when mixtures of organic and water vapor were allowed to adsorb onto nanotubes, water adsorbed in the internal volume of the nanotubes while organic vapors adsorbed onto the outer surface of the nanotubes. This information is useful to understand how nanotubes can be used for conditions such as when organic vapors exist at very low concentrations (10's of ppbv) and or when adsorption of organic vapors is desired from gas streams saturated with water vapor.