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Seminar on Adsorption : Pore Size Distribution

Micropore Analysis

There are t-plot, HK, SF, DR-plot, NLDFT and GCMC method for the evaluation of micropore. t-plot and DR-plot are used to determine the pore volume and separation of internal and external surface area of the particle. HK, SF, NLDFT and GCMC method are used to determine the pore size distribution.

Since the micropore analysis theories must describe the short-range interaction of adsorbate and pore wall, it is not as easy as describing the flat surface adsorption or mesopore adsorption. The typical assumption of these theories is that the pore shape is a slit or cylinder. As the parameters, the surface atoms of pore wall and adsorbate molecules must be selected (e.g. oxygen/carbon, N2/Ar). If the sample has uniform and homogeneous pores, the calculated pore size will be accurate. However, most of real materials have nonuniform and heterogeneous pores which are not fit to the assumption of the theories. This disagreement is true not only for the pore size distribution obtained from the gas adsorption but also for other porosimetry and particle size measurement. The gas adsorption so far is the best method for the evaluation of micropores compared to other methods because the probe gas molecule size is below nm to detect micropores.

Our recommend method of micropore analysis is as follows: For zeolitic materials, measure them with the Ar adsorption isotherm at 87K and analyze by the cylindrical pore model theory (SF, NLDFT, and GCMC). N2 molecules, which have strong quadrupole moment, strongly attract to the cation sites and OH group on the surface. For activated carbon materials, they are often measured with N2 adsorption isotherm at 77K and analyzed by the slit pore model theory (HK, NLDFT, and GCMC).

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