Study of ultramicroporous carbons by high-pressure sorption. Part 4.—Isotherms and kinetic transport in activated carbons

Abstract

Low-pressure hystereses, shown to exist for both nitrogen and carbon dioxide on as-received TCM carbon, disappear upon slight activation. These hystereses are related, in the case of nitrogen, to protracted penetration of restricted regions and, in the case of carbon dioxide, to dilation of tiny constrictions. A highly oxygen-activated carbon (19% weight loss) showed minute hysteresis in CO₂ sorption experiments, while the more activated carbon (32% weight loss) formed by nitric acid activation showed more extreme hysteresis. These hystereses are related to swelling of the progressively weakened carbon matrix. Nitrogen, which is a poor swelling agent, does not show any hysteresis with these highly activated carbons. A series of TCM carbons which were progressively activated by oxygen showed a maximum in their sorption affinity constant with respect to the degree of activation. This maximum is related to either the greater accessibility of new small-pore regions or to the gradual opening of tiny constrictions that regulate transport into these regions.

The lack of internal pore resistance to transport in a slightly activated carbon provides conditions conducive to observation of transient barriers formed in the entrances to the outermost pores immediately after applying a pressure step.