How to Appropriately Characterize the BET Area of a Microporous Material

The Brunauer-Emmett-Teller (BET) method is the method most widely used to evaluate the surface area of porous material.

The BET equation is given by the formula:

Where W is the weight of gas adsorbed at a relative pressure of P/P0, and Wm is the weight of the adsorbate constituting a monolayer of surface coverage. The term C, the BET C constant, is related to the energy of adsorption in the first adsorbed layer and consequently, its value is an indication of the magnitude of the adsorbent/adsorbate interactions.

The BET equation is applicable for surface area analysis of nonporous, macro-, and mesoporous materials (with sufficiently large pores) and is applied in the relative pressure range P/P0 from 0.05 – 0.3. However, this method is in a strict sense not applicable in the case of microporous adsorbents – a critical appraisal of the BET method is given in various textbooks on porosity measurement. The BET method is based on the assumption that a monolayer, single layer of adsorbate, is formed on the pore walls. In materials possessing micropores, it is difficult to differentiate between the monolayer formation process and the micro-pore filling process that usually occurs at P/P0 below 0.15. The application of the BET method is also problematic for estimating the specific surface area of mesoporous molecular sieves of pore widths less than ~4 nm because pore condensation is observed at pressures very close to the pressure range where monolayer-multi-layer formation occurs on the pore walls. This may lead to a significant overestimation of the monolayer capacity.

For these reasons, the surface area obtained using the BET equation for microporous materials should be considered a “characteristic” or “equivalent” surface area, or it should just be called BET area. In this case, it is, of course, obligatory to report the range of linearity for the BET plot. If the classical BET range (P/P0 - 0.05 to 0.3) is applied to a microporous material isotherm, a non-linear plot with an unacceptable correlation coefficient as well as a negative y-intercept and negative C constant will result - an unphysical result.

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This information has been sourced, reviewed and adapted from materials provided by Anton Paar GmbH.

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