Adsorption and desorption of nitrogen at 77 K on micro- and meso- porous materials: Study of transport kinetics

• N2 sorption experiments at 77 K were carried out on four samples.
• Eight kinetic relations were applied to sorption kinetic data.
• Changes in rate of transport during gradual pore-filling/emptying were explained.
• Cavitation phenomenon associated with a sharp drop in rate parameter was recognized.

The paper presents and discusses the kinetic results of nitrogen adsorption and desorption experiments performed at 77 K on one aluminium-based and three carbon-based materials differing in their microporous and mesoporous nature. The paper focuses on an examination of the transport phenomena during the adsorption/desorption experiments in different-sized pores (micropores and mesopores) identified by both Horvath–Kawazoe and modified Kelvin equations. Of the eight kinetic relations applied to sorption kinetics data, Crank's intraparticle diffusion model is found to be the best-describing.It was observed that rates of transport phenomena significantly change with gradual pore-filling/emptying. Transport proceeds at a minimal rate for a fine micropore (diameter ≤ 1 nm) filling region and increases steeply upon filling of wider micropores (diameter 1–2 nm). The maximum transport rate is found to be during the filling of pores between 2 and 4 nm in size. Once irreversible capillary condensation in wider mesopores appears, transport of the gaseous phase is highly limited by a condensate. The desorption process takes place faster only for pure microporous activated carbon. In the case of mixed microporous/mesoporous materials, the transport kinetics is faster only during evaporation and desorption from larger mesopores. For the desorption a sharp drop in rate upon outward transport is identified and ascribed to a cavitation phenomenon. A low-pressure hysteresis observed (for subbituminous coal) between adsorption/desorption isotherms is not related to changes in transport kinetics.

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