Assessing the Surface Area of Porous Solids: Limitations, Probe Molecules, and Methods

Abstract

In this modeling study, the uses of nitrogen (77.3 K), probe molecule of choice for decades, and argon, opted as alternative in the 2015 IUPAC report on adsorptive characterization, as probe molecules for geometric surface area determination are compared. Graphene sheets possessing slit-shaped pores with varying size (width) are chosen as model porous solids, and different methods for the determination of specific surface areas are investigated. The BET method, which is the most commonly applied analysis, is compared to the Langmuir and relatively recently proposed ESW (excess sorption work) method. We show that either using argon or nitrogen as adsorptive, the physical meaningfulness of adsorption-derived surface areas highly depends on the pore size. When less than two full layers of adsorbate molecules can be formed within slitlike pores of a graphitic material (D < 5.8 Å for Ar/N), adsorption-derived surface areas are about half that of the geometric surface area. Between two and four layers (6.8 < D < 12.8 Å), adsorption surface areas can be significantly larger (up to 75%) than the geometric surface area because monolayer-multilayer formation and pore filling cannot be distinguished. For four or more layers of adsorbate molecules (D > 12.8 Å), adsorption-derived surface areas are comparable to their geometrically accessible counterparts. Note that for the Langmuir method this only holds if pore-filling effects are excluded during determination. This occurs in activated carbon materials as well. In the literature, this indistinguishability issue has been largely overlooked, and erroneous claims of materials with extremely large surface areas have been made. Both the BET and Langmuir areas, for D > 12.8 Å, correspond to geometric surface areas, whereas the ESW method yields significantly lower values. For the 6.8 Å < D < 12.8 Å range, all methods erroneously overestimate the specific surface area. For the energetically homogeneous graphene sheets, differences between argon and nitrogen for the assessment of surface areas are minor.

π‐Interpenetrated 3D Covalent Organic Frameworks from Distorted Polycyclic Aromatic Hydrocarbons

“…The positive deviation of the BET surface area from the theoretical value in the case of Marta-COF-3 can be ascribed to the known overestimation of surface areas by the BET method in materials with micropores between 0.6 and 1.3 nm. [16] The pore size distributions calculated with Density Functional Theory (DFT) (Figures 2 C and G) give a dominant pore centered at 0.63 nm in both COFs, consistent with the theoretical pore sizes of 0.60 and 0.64 nm, respectively for Marta-COF-3 and 4.…”

iRASPA: GPU-accelerated visualization software for materials scientists

“…For small pore systems, e.g. smaller than about 12-13 Å, BET values are questionable because monolayer-multilayer formation and pore filling cannot be distinguished [58]. iRASPA computes an extension of the geometric surface area, i.e.…”

Evaluation of the BET Theory for the Characterization of Meso and Microporous MOFs

“…Typically, N 2 gas is used for this purpose . However, other gases, for instance Ar or CO 2 or even organic vapors (dynamic vapor sorption) may also be used . While there have been attempts to use H 2 , they remained in the form of discussion .…”