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Seminar on Adsorption : Specific Surface Area

Method of Specific Surface Area Measurement for Microporous Materials (Type I Isotherm)

In the literature, it is said that BET theory should not be applied to Type I isotherm. However, the surface area is an important parameter for the characterization of materials, and actually, the surface areas of microporous materials are often calculated. Then what are the problems and how can we evaluate the surface area?

Problems:
1.The diameter of micropore is less than 2nm, and because of the pore size limitation, the gas molecules cannot form the multi-layer adsorption. It is contradictory to BET theory
Adsorption image into the micropore
2.Since there is a big curvature in micropores, the use of cross sectional area to calculate the surface area underestimates the value.
Evaluation of surface area in the micropore
Solutions:
1.BET-plot should be applied to the low relative pressure range (0.05 - 0.1) because in this range, multi-layer adsorption of BET theory occurs and there is linearity in the plot. In this case, C parameter must be positive.
Type I isotherm and BET-plot
2.The calculated surface area of type I isotherm should be considered as the BET specific surface area, and not the actual geometrical specific surface area. This BET specific surface area has a good agreement with the adsorption amount and reactivity. So it is not necessary to evaluate the geometrical specific surface area for the actual applications.

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