CHARACTERIZATION OF MICROPORE STRUCTURE OF POROUS MATERIALS USING DFT MODELS APPLIED TO AR, N2 AND H2 ADSORPTION DATA.

Jacek Jagiello

Micromeritics, 1 Micromeritics Drive, Norcross GA, USA

Gas adsorption methods based on the density functional theory (DFT) have been well developed and used for determining pore size distributions (PSDs) of porous carbons for more than ten years. The underlying challenge in modeling PSD of a zeolite is the fact that the adsorption potential field inside a zeolite pore depends on the pore width and shape, and the type of cation, or in more general terms, on the chemical structure of the zeolite. These properties are reflected by adsorption isotherms of different gases measured on zeolite materials. Using more than one adsorbate for pore structure analysis of these materials helps to differentiate between structural and chemical effects in their adsorption properties.
Nitrogen, argon and hydrogen adsorption isotherms measured at cryogenic temperatures (77 and 87 K) were used to characterize the micropore structure of selected activated carbons and zeolite materials. A set of model isotherms (kernel) was generated using the density functional theory (DFT) for each adsorbate at each temperature used in the analysis. We apply slit pore model to carbon materials and cylindrical model for zeolites The material pore size distribution (PSD) was calculated by simultaneous fitting of the DFT isotherms to their experimental counterparts. The resulting PSD represents robust characteristics of the material pore structure that is consistent with all the data used in the analysis. Using hydrogen allows extending the range of pore size analysis to smaller pore sizes compared to the standard nitrogen adsorption analysis. In addition, due to the faster diffusion into the micropores hydrogen adsorption measurements can be performed in a shorter time than those with nitrogen or argon.