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

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. (C) 2016 Elsevier Inc. All rights reserved.