MICROPOROSITY DEVELOPMENT BY CO2 ACTIVATION OF AN ANTHRACITE STUDIES BY PHYSICAL ADSORPTION OF GASES, MERCURY POROSIMETRY, AND SCANNING ELECTRON-MICROSCOPY

A series of CO2-activated carbons (18%-97% burn-off) prepared from a low-ash-content anthracite has been characterized by maceral analysis, physical adsorption of gases (Ar/77 K, N2/77 K, n-butane/273 K, CO2/273 K), n-nonane preadsorption, mercury porosimetry and scanning electron microscopy. Apparent surface areas greater than 1600 m2/g and pore volumes near 1 cm3/g were obtained. There is a continuous increase in adsorption capacity with the burn-off due to the enlargement of the micropores of the carbons. The meso- and macroporosity were poorly developed, and almost exclusively microporosity was created in the carbons. The development of this microporosity was produced in three consecutive stages; for burn-off values smaller than 35% only small micropores were created, and the carbons exhibited molecular sieve properties towards n-butane; at burn-off percentages between 35% and 60% medium micropores were developed, and for greater activation degrees large micropores were produced. The small micropores can be evaluated from the CO2/273 K adsorption isotherms by application of the Dubini-Radushkevich (DR) eqn. Medium micropores can be obtained from application of the DR eq to the adsorption isotherms of Ar/77 K, N2/77 K or n-butane/273 K. Large micropores are evaluated either by n-nonane preadsorption or from the alpha(s) plots of Ar/77 K, N2/77 K or n-butane/273 K adsorption On the other hand, the total pore volumes of carbons evaluated from the He and Hg densities were in good agreement with the gas volumes adsorbed at P/P(o) = 0.95 (Ar/77 K, N2/77 K and n-butane/273 K). The results obtained for these carbons are very similar to those previously reported for different series of CO2-activated carbons prepared from lignocellulosic materials.