A new and reliable method to obtain micropore volume in nanoporous solids by gas adsorption based on Dubinin works and the thickness of the adsorbed layer

Abstract

It is known that the use of the Dubinin–Radushkevich method in micro-mesoporous samples does not give adequate values of micropore volumes, unlike when the samples contain only microporous. Based on that, in this work, we propose an easy method to calculate a reliable micropore volume (V_μP) of micro-mesoporous (nanopores) samples, separating the microporous region from the experimental isotherm. For this, the original isotherm is modified, estimating the thickness of the adsorbed layer (t) as a function of relative pressure and changing the external surface area (S_ext) to obtain a Type I adsorption isotherm in the microporous region; then, the DR method can be applied to the modified isotherm. This proposal, named the DR_t method, allows the calculation of a reliable V_μP of any nanoporous material using different adsorbates. Using this method, we analyzed adsorbents of distinct nature (i.e., carbons and silicas) with different adsorbates as N₂ and O₂ at 77 K, Ar at 87 K, and CO₂ at 273 K. We used this method to calculate V_μP in different samples and compare them with those obtained with the traditional DR method, highlighting that unlike the latter the DR_t method showed similar and consistent results with the different adsorbates. Therefore, the values of micropore volume calculated using the DR_t method demonstrate consistency across various adsorbates, not only for N₂ but especially for CO₂, which is suggested to analyze narrow micropore volumes.