Determination of Heats of Adsorption by Transient-Response Techniques

Abstract

THE classical equations of chromatography¹ have been applied to the determination of the heat of adsorption of a gas on a solid^2,3 by recognizing that the measured residence time, within a packed column, of a pulse of adsorbable gas relative to that of the carrier (or non-adsorbable component) may be related to an ‘equilibrium constant’ K _a by the equation: where t _a and t _c represent the retention times, respectively, of the active and carrier species. By assuming that K _a ≫ 1 and utilizing the usual temperature functionality for K _a, Greene and Pust² show that the logarithm of the retention time of the adsorbing component is linearly related to the reciprocal of the absolute temperature. Thus the observed data on retention time versus temperature permit the evaluation of the adsorption heat, ΔH. Good agreement between values of ΔH determined by this technique and those obtained by others calorimetrically and/or isosterically was found by Greene and Pust for the adsorption of some low boiling gases on charcoal and for light hydrocarbons on alumina and silica gel at temperatures below 100° C. However, these authors correctly noted that no simple correspondence should exist between heats of adsorption derived from continuous-flow transient-response data (adsorption chromatography in their work) and those obtained by calorimetric methods, since the latter are functions of surface coverage.