Gas Adsorption on Graphite Surfaces
In this review, the adsorption of non-polar and polar molecules on highly graphitized carbon black (GCB) which has a very homogeneous flat carbon surface is introduced along with the adsorption isotherms and the isosteric heats. There are two different adsorption mechanisms; (1) molecular layering adsorption on carbon surface, and (2) molecular clustering on very strong sites such as functional groups whose affinity is much greater than that of the carbon surface. Since the difference between the mechanisms give the difference to the uptake of isotherms and the isosteric heats, each example of gas adsorption is shown and the details of the mechanisms are explained. For simple gases, e.g. argon, nitrogen, and krypton adsorption proceeds by molecular layering on the carbon surface. Particularly, in adsorption at temperatures below the bulk triple point, the stepwise adsorption isotherm, type VI according to IUPAC classification, is observed due to the layering adsorption mechanism. On the other hands, for polar gas especially water molecules adsorb by clustering on the functional groups of GCB due to the hydrogen bonding, but do not adsorb on the carbon surface. The polar molecules weaker in polarity than water, e.g. methanol, and ammonia, adsorb by both layering and clustering mechanisms on GCB (called spill-over phenomenon).
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