Thermodynamics of the high temperature adsorption of some permanent gases by porous carbons

Abstract

This paper presents precise experimental data for the adsorption (n^a) of nitrogen and the noble gases by two unactivated and two activated (10.8% and 22.0% burn-off) Saran type porous carbons over the temperature range 0–125°C and at pressures (p) up to 1 atm. The data are analysed in terms of virial type expansions of (i)n^a/p as a function of p and (ii) ln n^a/p as a function of n^a. The number of terms required in each equation to give an adequate representation of the data was determined for each system at each temperature by statistical analysis. Expansion (ii) was found to be superior in the sense that where there was a difference between the number of terms required, expansion (ii) required fewer terms than (i). These differences become more marked for the adsorption of the heavier gases at the lower temperatures. Henry's law constants obtained from (i) and (ii) were, however, in good agreement.

The Henry's law constants and their temperature dependence were used to calculate differential enthalpies and standard differential entropies of adsorption (taking n^a= 10^–3 mol g^–1 as standard state) in the limit of ideal behaviour. Activation of the carbons decreased the Henry's law constants per unit surface area, and also the magnitude of the enthalpy of adsorption. The standard differential entropy of adsorption increases with activation, reflecting the larger “free volume” available to the adsorbed molecules which may be measured approximately either by the increase in micropore volume or by the increase in the monolayer equivalent surface area.