Determination of Henry's law constants, enthalpies and potential energies of adsorption, and surface areas by gas-solid chromatography: inert gases, nitrogen and methane on active carbon

Abstract

Henry's law constants, isosteric enthalpies and potential energies of adsorption, and the surface area of the adsorbent, as given by each gas, have been determined by gas-solid chromatography using hydrogen as carrier gas and helium as marker gas, for Ne, Ar, Kr, Xe, CH₄ and N₂ on an active carbon. The enthalpies of adsorption and the mean temperature of the experiments are: Ne, 6.90 kJ mol^–1, 270.7 K; Ar, 16.82 kJ mol^–1, 255.7 K; Kr, 22.13 kJ mol^–1, 333.2 K; Xe, 32.51 kJ mol^–1, 343.2 K; CH₄, 22.26 kJ mol^–1, 333.2 K; and N₂, 17.64 kJ mol^–1, 255.7 K. There is good agreement between these values and the limited number of reported enthalpies of adsorption on similar carbons. The dependence of retention times and volumes on sample size, flow rate and method of analysis were examined. Potential energies of adsorption ε* and surface areas A were obtained by the approximate method of Hansen, Murphy and McGee which gives a simplified expression for the retention valume in terms of ε*/k and Ar*, where r* is the equilibrium separation distance between an adsorbate molecule and the surface. The adsorption potentials appear to be reliable, being about RT/3 lower than the corresponding enthalpies of adsorption, but the apparent areas, like those found by other workers from application of high-temperature adsorption theory, decrease markedly with increase in r*.