Investigation of activated carbon surface heterogeneity by argon and nitrogen low-pressure quasi-equilibrium volumetry

BET and Kelvin Analyses by Thermogravimetric Desorption

Porous solids with nanometer-sized pores and large surface areas are a highly important class of materials. Uses of such materials include filtration, batteries, catalysts, and carbon sequestration. These porous solids are characterized by their surface areas, typically >100 m²/g, and pore size distributions. These parameters are typically measured using cryogenic physisorption, frequently referred to as Brunauer-Emmett-Teller (BET) analysis when BET theory is applied to interpret experimental results. Cryogenic physisorption and related analysis elucidate how a particular solid interacts with the cryogenic adsorbate, but this can be a poor predictor of how that solid will interact with other adsorbates, limiting the applicability of the results. Additionally, the cryogenic temperatures and deep vacuum required for cryogenic physisorption can cause kinetic limitations and experimental difficulties. This method nevertheless remains the standard technique to characterize porous materials for a wide variety of applications due to limited other options. In this work, a thermogravimetric desorption technique for determining surface areas and pore size distributions of porous solids available to adsorbates having boiling points above ambient temperature at ambient pressure is presented. A thermogravimetric analyzer (TGA) is used to measure temperature-dependent adsorbate mass loss, and isotherms are derived. For systems that exhibit multilayer formation, BET theory is applied to isotherms to derive specific surface areas. For systems that do not exhibit multilayer formation, the Kelvin equation is applied to determine pore size distributions and surface areas for the porous materials. In this study, the thermogravimetric method is applied to four adsorbents and two adsorbates─water and toluene─and results are compared to cryogenic physisorption results.

Low-pressure argon adsorption assessment of micropore connectivities in activated carbons

Low-pressure argon adsorption has been used to study the energetic distribution of microporous activated carbons differing by their burn-off. The collected isotherms were analyzed using the derivative isotherm summation method. Some oscillations on the experimental curves for very low partial pressures were detected. The results are analyzed and discussed according to the literature and could be attributed to local overheating caused by spontaneous mass transfer of argon through constrictions between former pores and the new opening pore or deadend pores. We used the dynamic character of the experimental method and mainly the discrepancy of the quasi-equilibrium state to deduce key parameters related to the porosity topology.