Conventionally used argon and nitrogen adsorption isotherms are of limited value for the characterization of very narrow carbon micropores due to the slow rate of adsorbate diffusion at cryogenic temperatures. In order to access the micropores of molecular dimensions, it is necessary to use an adsorbate such as CO2, which may adsorb at elevated temperatures. Recent advances in sorption technology have enabled the accurate measurement of CO2 isotherms at 273K over a wide range of pressures (up to 35 bar), allowing the resolution of both micropores (0.4 – 2 nm) and mesopores (2 - 50 nm) in a single adsorption experiment. Several such adsorption experiments were undertaken on characteristic non-porous carbons as well as hierarchical CMK-3 structures.
In this work, a novel method is developed for calculating the pore size distribution from high-pressure CO2 isotherms. For modeling adsorption, the method employs the Quenched Solid Density Functional Theory (QSDFT, ) with the roughness parameter that is kept constant in the mesopores range and is gradually reduced with the pore size in the range of micropores, converting QSDFT into Non-Local DFT (NLDFT) in the narrowest micropores of molecular dimensions (<0.5 nm). A cylindrical pore model is used for mesopores and a slit-shaped model for micropores. The pore size distributions obtained using the constructed hybrid kernel of QSDFT and NLDFT isotherms are shown to be in agreement with the conventional characterization methods based of N2 and Ar isotherms. We demonstrate the capabilities of high pressure CO2 characterization as an advanced characterization tool for micro-mesoporous carbons of hierarchical structure drawing on the example CMK-3 materials .
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