Nitrogen adsorption at 77 K is the current standard means for pore size determination of adsorbent
materials. However, nitrogen adsorption reaches limitations when dealing with materials such as molecular
sieving carbon with a high degree of ultramicroporosity. In this investigation, methane and carbon dioxide
adsorption is explored as a possible alternative to the standard nitrogen probe. Methane and carbon
dioxide adsorption equilibria and kinetics are measured in a commercially derived carbon molecular sieve
over a range of temperatures. The pore size distribution is determined from the adsorption equilibrium,
and the kinetics of adsorption is shown to be Fickian for carbon dioxide and non-Fickian for methane. The
non-Fickian response is attributed to transport resistance at the pore mouth experienced by the methane
molecules but not by the carbon dioxide molecules. Additionally, the change in the rate of adsorption with
loading is characterized by the Darken relation in the case of carbon dioxide diffusion but is greater than
that predicted by the Darken relation for methane transport. Furthermore, the proposition of inkbottle-shaped micropores in molecular sieving carbon is supported by the determination of the activation energy
for the transport of methane and subsequent sizing of the pore-mouth barrier by molecular potential
calculations.
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