Heat of Adsorption of Butane on Multiwalled Carbon Nanotubes

Abstract

We report the isosteric heats of adsorption, qst, for butane on multiwalled carbon nanotubes (MWNTs) over a range of surface loadings at temperatures below the normal boiling point. Butane is a nonspherical, nonpolar molecule that may exhibit different orientations in multilayer adsorption. The morphology of our MWNTs is such that only surface adsorption or capillary condensation can occur: no interstitial sites between nanotubes are available to butane. For these nanotubes, exterior surface adsorption is the principal mechanism measured by gravimetric analysis as the internal pores are only a small volume fraction of the total solid volume. The isosteric heat of adsorption varied with the surface coverage, θ, defined as the ratio of adsorbed butane to the MWNT butane monolayer capacity. The initial heat of adsorption was in the region ∼22−26 kJ/mol, which is approximately 1/3 lower than the corresponding value for various graphite−butane systems. At θ = 1, qst displays a minima (13.5−15.9 kJ/mol). The isosteric heat of adsorption approaches the butane heat of condensation for θ > 2.5, suggesting that the surface film is similar to a bulk phase. The isosteric heat of adsorption of butane on MWNTs can be related to the morphology of the MWNTs. The higher heats of adsorption at low coverages are likely related to the presence of surface defects. The minima in qst near the monolayer coverage relates to a high self-association of butane. At high loadings, the butane surface film is similar to a bulk phase.