The use of nitrogen adsorption for the characterisation of porous materials

Problems, which may arise when low-temperature nitrogen adsorption is used for the characterisation of porous materials, are discussed in this review. Continuous or discontinuous manometric techniques can be employed for nitr ogen adsorption measurements at 77 K. For pore structure analysis, the nitr ogen adsorption-desorption isotherms should be determined over the widest p ossible range of relative pressure, while allowing for slow equilibration a nd other operational problems, particularly at very low pressures. In spite of its artificial nature, the Brunauer-Emmett-Teller (BET) method is still used for the determination of Surface area. In principle, nitrogen isother ms of Types II and IV are amenable to BET analysis provided that pores of m olecular dimensions are absent and that the BET plot is obtained over an ap propriate range of the isotherm. An empirical method based on the applicati on of standard adsorption data is useful for checking the validity of the B ET-area. All the computational procedures for pore size analysis have limit ations of one sort or another. The various assumptions include an ideal por e shape, rigidity of the structure and an oversimplified model (capillary c ondensation or; micropore filling). The derived pore widths and pore volume s should be regarded as effective (or apparent) values with respect to the adsorption of nitrogen at 77 K. (C) 2001 Elsevier Science B.V. All rights r eserved.