Chapter 15. Structure of porous adsorbents: Analysis using density functional theory and molecular simulation

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Abstract

The pore size distribution (PSD) analysis method based on nonlocal density functional theory (DFT) and on molecular simulation is reviewed and compared with classical PSD methods. Applications to carbons and oxides are given. The DFT method offers several advantages over classical methods: (a) a valid and accurate description for small pores; (b) a description of the full adsorption isotherm (not just the capillary condensation pressure), as well as other properties such as heats of adsorption; (c) it can be used for supercritical conditions; (d) it accounts for effects of pore shape; (e) it can be improved in a systematic way, since it rests on fundamental statistical mechanics. A critique of the method as currently applied is also offered. In common with most other PSD methods, the model neglects connectivity and pore blocking, and changes in pore size and geometry with pressure and temperature, and assumes that heterogeneity due to differences in pore shape and surface chemical groups can be approximated by an effective porous material, in which all heterogeneity is due to a distribution in pore sizes. Additional tests, using molecular simulation and experiment, are needed to determine whether these neglected effects exhibit signatures in experimental results that are distinct from the PSD effects. Molecular simulation studies of pore connectivity effects have been made for a simple network model; the model seems able to provide a detailed molecular explanation for the several hysteresis types found in Type IV and V isotherms.

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