Sorption and surface diffusion in microporous carbon cylinders

Abstract

An investigation has been made of adsorption and diffusion of helium, neon, hydrogen, argon, nitrogen and krypton in microporous cylinders of carbon compressed to different porosities e, and having very high internal surfaces, A . On the uncompressed carbon powder sorption of argon occurred at 90 and 78° K without hysteresis; in a compressed cylinder with e = 0.64 cm 3 per cm 3 and 2e A = 18.8 Å the isotherms changed from type II to type IV in Brunauer’s classification and a pronounced hysteresis loop appeared. In a highly compressed cylinder with e = 0.37 cm 3 per cm 3 and 2e/ A = 6.5 Å hysteresis again disappeared and the isotherms changed from type IV nearly to type I. The areas A and heats of sorption were measured, and special attention was paid to sorption equilibria involving dilute films of nitrogen, argon and krypton between 273 and 323° K. Surface and volume diffusion were then investigated over the same temperature interval, 273 to 323° K, in the compressed cylinders already carefully characterized by the equilibrium studies. Surface diffusion coefficients, D s , were evaluated for all the gases including helium. The properties of D s indicate that dilute films of helium, neon and hydrogen are best regarded as one- or two-dimensional gases but that argon, nitrogen and krypton still give localized adsorbed films in which surface flow is by jumps, each unit diffusion requiring an energy of activation which was 0.53 to 0.62 of the heat of adsorption. It was further shown that adsorption isotherms in the Henry’s law region can be measured by flow methods, and are in agreement with those obtained by direct measurement. The surface diffusion coefficients increase and the activation energies for diffusion decrease for argon, nitrogen and krypton as the surface concentration of adsorbate increases. In some cases, especially for krypton, surface flow was much the most important mode of transport within the micropore system.