Comparative analysis of simple and advanced sorption methods for assessment of microporosity in activated carbons

The methods for assessment of microporosity in carbonaceous materials on the basis of low temperature nitrogen adsorption measurements are critically analyzed. The applicability of comparative methods, such as the t-plot and alpha(s)-plot, is thoroughly examined. The micropore volume calculated is shown to be rather independent on the choice of the reference nonporous adsorbent, but the selection of the reference significantly influences the calculated external surface area. Moreover, a proper choice of reference adsorbent is a necessary condition for the assessment of the total surface area from high resolution alpha(s)-plots and the analysis of filling and condensation swings. The procedures to calculate micropore size distributions are examined and compared. The accuracy of procedures based on the Dubinin-Radushkievich equation is questioned, since the DR equation does not adequately represent the volume filling of uniform micropores. The Horwath-Kawazoe method is shown to reasonably reproduce the shape of the micropore size distribution only when micropores below ca 0.9 nm in width prevail for a given sample. In other cases, a significant distortion of the micropore size distribution can be expected, since the method attributes the monolayer formation to the presence of nonexisting 0.6 nm micropores. The numerical methods employing local adsorption isotherms obtained from computer simulation or nonlocal density functional theory are recommended for calculations of micropore size distributions. It is shown that an incorporation of surface heterogeneity effects would be desirable to further improve these methods. (C) 1998 Elsevier Science Ltd. All rights reserved.