Abstract
“Isosteric heats” of adsorption of gaseous methane, ethane, propane, butane, ethene, propene, 1-butene, and ethyne on a γ-alumina gel pre-treated at 1073 K have been determined from adsorption results, obtained with a volumetric apparatus in the temperature range 213 to 773 K and pressure range 0 to 70 kPa. Special attention was paed to measurements in the low-pressure range (0 to 300 Pa). All the stages of the “isosteric heat” calculation (including conversion of isotherms into isosteres and reference to a common temperature) were carried out analytically with a computer program. “Isosteric heats” of adsorption depend on adsorbed amount and also on temperature. Surface heterogeneity plays a prominent role therein. Surface excess differential molar entropies were calculated from the “isosteric heats” of adsorption and compared with a theoretical model (mobile adsorbed layer treated as an ideal bi-dimensional gas). Surface heterogeneity again has an important influence, causing experimental plots to diverge from the theoretical model, especially for olefins. Interfacial entropies for large absorbed amounts tend approximately to values of the molar entropies of liquid hydrocarbons.
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