CROSS-SECTIONAL AREAS OF ADSORBED N2, AR, KR, AND O2 ON CARBONS AND FUMED SILICAS AT LIQUID-NITROGEN TEMPERATURE

Adsorption of N2, Ar, Kr, and O2 at 76.6 K was investigated on Aerosil samples and three different categories of carbons: graphitized, ungraphitized, and activated graphitized substrates. With the BET equation, the appropriate ranges of relative pressure are defined and the corresponding experimental cross-sectional areas, sigma (nm2/adsorbed species), are reported, based on sigma(N2) = 0.162. Values of sigma on the carbons were fitted to physical models illustrating possible arrangements of adsorbed species on the surface. With graphitized carbons, sigma(O2) = 0.147 and sigma(Ar) = 0.138. With ungraphitized or activated carbons, values of sigma are 0.157 for Ar, 0.214 for Kr (at P/P-degrees = 0.02-0.25), and 0.148 for O2. With Aerosils, however, the value of sigma(Ar) = 0.186, obtained by the calibration, is high and could not be fitted to a model. An alternative approach has been considered. When comparing the present data with those reported in the literature, the recommended new values on Aerosils (and other silicas) are sigma(N2) = 0.143, sigma(O2) = 0.147 and sigma(Ar) = 0.165 nm2/mol. The present results indicate that oxygen could be considered as the standard gas for adsorption on carbons and silicas, provided that it does not change the carbon structure or its surface characteristics when adsorbed at 77 K.