Time distribution of adsorption entropy of gases on heterogeneous surfaces by reversed-flow gas chromatography

Abstract

The reversed-flow gas chromatography (RF-GC) technique has been applied to measure the adsorption entropy over time, when gaseous pentane is adsorbed on the surface of two solids (γ-alumina and a silica supported rhodium catalyst) at 393.15 and 413.15 K, respectively. Utilizing experimental chromatographic data, this novel methodology also permits the simultaneous measurement of the local adsorption energy, ɛ, local equilibrium adsorbed concentration, c s * , and local adsorption isotherm, θ( p, T, ɛ) in a time resolved way. In contrast with other inverse gas chromatographic methods, which determine the standard entropy at zero surface coverage, the present method operates over a wide range of surface coverage taking into account not only the adsorbate–adsorbent interaction, but also the adsorbate–adsorbate interaction. One of the most interesting observations of the present work is the fact that the interaction of n-pentane is spontaneous on the Rh/SiO 2 catalyst for a very short time interval compared to that on γ-Al 2O 3. This can explain the different kinetic behavior of each particular gas–solid system, and it can be attributed to the fact that large amounts of n-C 5H 12 are present on the active sites of the Rh/SiO 2 catalyst compared to those on γ-Al 2O 3, as the local equilibrium adsorbed concentration values, c s * , indicate.