On the basis of an experimental study in a large temperature range, it is shown that “helium densities” of adsorbents measured at room temperature could be erroneous due to a non-negligible effect of helium adsorption. It is proposed that the density obtained with helium at high temperature, for instance, at the regeneration temperature of the adsorbent, be considered as the adsorbent density. Using the corrected densities of 3A, 4A, 5A, and 13X zeolites and of activated and graphitized carbons and of silica gel, we experimentally determined the adsorption of helium on the above mentioned adsorbents at room temperature and in a large pressure range up to 500 MPa. The shape of the adsorption isotherm reveals no saturation at high pressure. These experimental data are in agreement with Monte Carlo simulations of adsorption of a Lennard-Jones gas by a rigid plane as well as by a microporous rigid solid interface. We also examined implications of the new helium density of activated carbon for our previous measurements of adsorption at high pressure. The result is the disappearance of the inexplicable negative part of the isotherms and even a renewed increase in the curves at high pressure. Moreover, a comparison with Monte Carlo simulations of argon adsorption on microporous graphite is in good agreement with the shape of the adsorption curve at high pressure. Finally, the role of the microporous structure of adsorbents and of the gas−adsorbent interaction in adsorption at high pressure is discussed.

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The excess surface adsorptions of nitrogen and hydrogen gases on graphite basal planes at a temperature of 293 K have been computed by numerical simulations. The results of the simulations are compared to experimental data on the adsorption of these two gases measured on activated carbons. The adsorption isotherms have been calculated up to 600 MPa, and the experimental isotherms have been measured in the pressure range from 0 to 500 MPa for nitrogen gas and up to 60 MPa for hydrogen gas. Both theoretical and experimental nitrogen adsorption isotherms have similar shapes. They present a maximum at moderate pressures, then decrease toward a constant value or a shallow minimum followed by a slight increase at very high pressures. In addition, we compare the values of the hydrogen adsorption computed by simulation at a temperature of 77 K and low pressures with those recently measured. The qualitative agreement between experimental and simulations results is rather good for both gases at the temperatures considered.

High-pressure adsorption of helium, neon, argon, nitrogen, krypton, and methane on zeolites 3A, 4A, 5A, and 13X was measured at room temperature up to 500 MPa. The corresponding isotherms are qualitatively comparable to those previously obtained with the same gases adsorbed on activated carbon. They are analyzed and compared to those calculated on the basis of the Ruthven thermodynamic model of adsorption on zeolites. The respective importance of the contributions of solid−gas and gas−gas interactions and the exact role of pores were established. Attractive forces involve the isotherm maximum whereas repulsive forces are predominant at high pressure and involve a slight reincrease of adsorption. As concerns the well-calibrated pores of zeolites, in addition to their molecular sieving effect, they contribute mainly to increasing the specific area.