Surface Area Determination of Porous Materials Using the Brunauer-Emmett-Teller (BET) Method: Limitations and Improvements

Surface areas of metal-organic frameworks (MOFs) have been commonly characterized using the Brunauer-Emmett-Teller (BET) method based on adsorption isotherms of nonreactive nitrogen or argon. Recently, some discrepancies between surface areas computed from the BET method and those from geometric methods were reported in the literature. In this study, we systematically evaluated the BET and geometric surface areas of over 200 geometrically diverse real MOFs as well as CNTs with varying pore sizes as model systems to achieve a comprehensive understanding of the limitations of the BET and geometric methods. We compared the BET and geometric surface areas to the true monolayer area, which is determined by directly counting the number of molecules included in the monolayer of the surface from molecular simulation snapshots. Furthermore, we found that the excess sorption work (ESW) method or a combination of ESW and BET methods can potentially help facilitate a more accurate estimation of the surface area, particularly in cases where the structures show relatively less complex isotherms having distinct steps.