Features of BET method application to various adsorbents

Abstract

For various adsorbents, especially nanoporous, there is an applicability problem of the Brunauer-Emmett-Teller (BET) method using nitrogen as a probe adsorbate. Therefore, the nitrogen adsorption (a(p/p0)) isotherms in several pressure ranges of the BET method at p/p0 = 0.05–0.3, 0.06–0.22, and narrower are analyzed for a large set (about 200 samples) of essentially different adsorbents such as fumed oxides (individual, binary and ternary, initial and modified), porous silicas, activated carbons and porous polymers. Graphitized carbon black ENVI–Carb composed of nonporous nanoparticles aggregated into microparticles is used as a standard adsorbent characterized by the standard area occupied by nitrogen molecule σm(N2) = 0.162 nm2. For initial nanooxides composed of nonporous nanoparticles, the standard value of σm = 0.162 nm2 results in the overestimation of the SBET values by ca. 10 % because of non-parallel-to-surface orientation of slightly polarized N2 molecules interacting with polar surface functionalities (e.g., various hydroxyls). For nanooxides modified by low- and high-molecular (linear, 2D and 3D polymers and proteins) compounds, the overestimation of SBET at σm = 0.162 nm2 could reach 30 %, as well as for some activated carbons. For adsorbents possessing nanopores (at half-width x or radius R < 1 nm) and narrow mesopores (1 nm < R < 3 nm), an overlap of monolayer and multilayer sorption (giving apparent underestimation of SBET at σm = 0.162 nm2) and non-parallel-to-surface orientation of the N2 molecules (causing σm lower than 0.162 nm2) could lead to various location of the normalized nitrogen adsorption isotherms (in the BET range) with respect to that for ENVI–Carb. It could be characterized by positive or negative values of the BET constant cBET. Two main criteria showing the inapplicability or applicability of the BET method (with nitrogen as a probe) related to the cBET values and a course of reduced adsorption a´(1- p/p0) vs. p/p0 in the BET range could not be in agreement for adsorbents, which are not pure nanoporous, but they are in agreement for pure nanoporous or meso/macroporous adsorbents