ADSORPTION OF NITROGEN AND KRYPTON AT 77-K ON SILICAS AND GRAPHITIZED CARBONS

Volumetric adsorption of nitrogen and krypton on silicas and graphitized carbons at 77 K has been investigated and the specific surface areas are calculated by using the Brunauer, Emmett, and Teller (BET) equation at the normal operating conditions, i.e., between relative pressures ( P P°) of 0.05 and 0.21. For the graphitized carbons, the monolayer volumes (Vm) obtained with N2 and Kr, and hence the corresponding apparent cross-sectional areas (σ(X)) of the adsorbed species (X), are not the same. However, based on the results obtained from other techniques, e.g., neutron diffraction, X-ray scattering, and LEED during the past 15 years, the cross-sectional areas of an adsorbed Kr atom or N2 molecule are the same. It is equivalent to the area of three graphitic basal plane hexagons, i.e., to 0.157 nm2. This implies that the monolayer volume obtained by N2 or Kr should be the same. When full isotherms, with 60-80 data points covering P P° = 0.001-0.95, were analyzed, it was found that the BET linearities are only valid below P P° of 0.1, the N2 and Kr plots coincide, and the true value of Vm obtained with both gases is the same. Vm is also in excellent agreement with the value obtained directly from the N2 and Kr isotherm at the inflection point (point B). In the two latter cases, the product of Vm and 0.157 is considered to yield the true surface area of the graphitized carbon under investigation. The same value of true surface area is obtained with the apparent value of Vm obtained from the BET equation operating at P P° = 0.05-0.21 using σ(N2) = 0.162 nm2/molecule and σ(Kr) = 0.214 nm2/atom. In the case of silicas, the corresponding cross-sectional areas at P P° = 0.05-0.21 are 0.162 nm2/N2 molecule and 0.270 nm2/Kr atom. © 1990.