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 nitrogen adsorption measurements at 77 K. For pore structure analysis, the nitrogen adsorption-desorption isotherms should be determined over the widest possible range of relative pressure, while allowing for slow equilibration and 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 isotherms of Types II and IV are amenable to BET analysis provided that pores of molecular dimensions are absent and that the BET plot is obtained over an appropriate range of the isotherm. An empirical method based on the application of standard adsorption data is useful for checking the validity of the BET-area. All the computational procedures for pore size analysis have limitations of one sort or another. The various assumptions include an ideal pore shape, rigidity of the structure and an oversimplified model (capillary condensation or; micropore filling). The derived pore widths and pore volumes 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 reserved.