Physisorption of Nitrogen by Porous Materials

Nitrogen adsorption at 77 K is widely used for the determination of the surface area and pore size distribution of various porous materials. The first stage in the interpretation of a nitrogen isotherm is the identification of the physisorption mechanism(s): that is monolayer-multilayer adsorption, capillary condensation or micropore filling. The Brunauer-Emmett-Teller (BET) method cannot be used to provide a reliable evaluation of the surface area if the solid contains pores of molecular dimensions (i.e. narrow micropores). Furthermore, with such materials, it is necessary to use different probe molecules to determine the pore size distribution. Capillary condensation is involved as a secondary process in the filling of mesopores (pore width in the range 2-50 nm). Recent work on MCM-41, a model mesoporous adsorbent, has confirmed that capillary condensation-evaporation can occur reversibly in open-ended pores of about 4 nm width.