Study of ultramicroporous carbons by high-pressure sorption. Part 2.—Nitrogen diffusion kinetics

Abstract

Sorption–desorption kinetics for nitrogen in the as-received TCM 128 ultramicroporous carbon fibre are reported at 35 °C over a wide range of pressures. The nitrogen sorption kinetics at low pressures follow the Fickian model. As the pressure increases, the deviations from the model become more pronounced, and at sufficiently high pressure a sigmoid kinetic response shape is observed which is indicative of a non-Fickian diffusion process. An additional timescale must be active to account for the non-Fickian transport behaviour. This second timescale process may correspond to adsorbate surface rearrangements leading to locally time-dependent clearing of constrictions at a rate with a characteristic kinetic constant. The nitrogen desorption kinetics are found to be less affected by the non-Fickian transport, thus leading to higher apparent diffusion coefficients for the same average pressures. At high pressures and very long sorption times, slow protracted uptake becomes apparent into restricted regions. The kinetics for this process can be described by a barrier model and are sufficiently slow to allow treatment independent of the processes occurring in the more open pore system.