Nitrogen molecular cross section area

An early normalizing procedure, proposed by Kiselev (1957) to compare adsorption isotherms of hydrocarbons, water vapour, etc. on a series of different adsorbents, was simply to plot the surface excess concentration F (=n/A), obtained from a knowledge of the BET-nitrogen surface area, A (BET), versus p/p°. It is also possible to plot, instead of f, the reduced adsorption , n/nm, which still relies on the BET method to determine the monolayer capacity nm but does not require knowledge of the molecular cross-sectional area a. 

Nitrogen adsorption/condensation measurements were performed using an Autosorb-1 analyzer to calculate sample surface area and pore size distribution. BET analysis at 77 K was applied for extracting the monolayer capacity from the adsorption isotherm and a N molecular cross-sectional area of 0.162 nm2 was used to relate tne monolayer capacity to surface area. PSD s were calculated from the desorption branches of the isotherms using a modified form of the BJH method [18]. Mercury intrusion measurements were performed using an Autoscan-33 continuous scanning mercury porosimeter (12-33000 psia) and a contact angle of 140°. 

The calculation of the adsorbent surface area using BET theory involves the estimation of the molecular cross-sectional area of nitrogen molecule [15]. In general it is assumed to be equal to 16.2 per nitrogen molecule on the surface, but this value is the subject of intense criticism during the past 30 years [72]. 

The surface areas in Table I were obtained from the adsorption isotherms by using the BET method (3). The relative pressure range used for BET analysis was selected to give the best linear correlation with the BET function, and the surface areas were calculated by assuming a molecular cross section for the nitrogen molecule of 0.162 nm2. 

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