Adsorption methods for the characterization of porous materials

No other type of adsorption method can offer the same scope for the characterization of porous solids as gas adsorption. Adsorption from solution measurements are easy to carry out, but are often difficult to interpret. Although immersion calorimetry is experimentally demanding, the technique can yield useful information provided that the corresponding adsorption isotherm data are also available. Nitrogen (at 77 K) is the most widely used adsorptive for the characterization of porous materials. Although the Brunauer–Emmett–Teller (BET) theory is based on an over-simplified model of multilayer adsorption, the BET method continues to be used as a standard procedure for the determination of surface area. Generally, the derived values of BET-area can be regarded as effective areas unless the material is ultramicroporous (i.e. containing pores of molecular dimensions). It is advisable to check the validity of the BET-area by using an empirical method of isotherm analysis. In favourable cases, this approach can be used to evaluate the internal and external areas. The computation of mesopore size distribution should be undertaken only if the nitrogen isotherm is of Type IV. Because of network–percolation effects, analysis of the desorption branch of the hysteresis loop may give a misleading picture of the pore size distribution; also, a considerable range of delayed condensation is to be expected if the pores are slit-shaped. Recent work on MCM-41, a model mesoporous adsorbent has improved our understanding of the mechanisms of mesopore filling. Adsorptive molecules of different size are required to provide a realistic evaluation of the micropore size distribution.