Generalized Form of Multilayer Adsorption Isotherm and Its Application for Determining Surface Areas of Adsorbents

Abstract We propose a new approach to determining surface areas of macro-porous adsorbents by considering multilayer adsorption isotherm in generalized form. Model assumptions behind this form are significantly less restrictive than those used in conventional approaches. The BET method “continues to be the most widely used procedure for evaluating the surface area of porous and finely-divided materials, in spite of the weakness of its theoretical foundations” (quote fromThommes, M.; et al.Pure Appl. Chem.2015, 87, 1051–1069.). BET and corrected BET isotherms are based on a number of well known restrictive assumptions about intermolecular interactions. Our model is a generalization of BET where such assumptions are not necessary, but determining surface area is still possible. We demonstrated our method for nitrogen on various types of macro-porous adsorbents, including silica, graphitized and non-graphitized carbon black, and a large group of macro-porous adsorbents characterized by a “common curve” (Standard Isotherm) representing the experimental data for salts and metal oxides, such as aluminum, iron, zinc, and titanium oxides. In addition, we compared our approach with the experimental data on adsorption of argon on aerosil. Surface areas determined by our method are 10–40% higher than those of BET, which is essential correction. This result can be explained by the fact that BET range of relative pressures is below 0.35 where the first layer is not saturated, and beyond this range the BET model fails to fit experimental isotherms. Our approach does not have this limitation and allows using full range of relative pressures, including those above 0.5 where the first layer approaches saturation. Another aspect of our approach is that it allows determining contribution of the first layer (“surface layer”) in the total multilayer adsorption isotherm. This information is unachievable in existing approaches, but it can be useful for better understanding of adsorption mechanisms.