Abstract :
The adsorption process in activated carbon micropores can be viewed as a two-dimensional condensation on micropore walls at a critical condensation pressure, pc, which evolves into volume filling. pc on a surface of micropore walls is calculated in respect to a critical condensation pressure on the reference nonporous surface, pcref. Although theoretical treatment predicts that pcref should be a parameter of adsorption isotherms, experimental isotherm equations are expressed in terms of the bulk saturation pressure, ps. Experimental values of ps/pcref for a graphitized carbon black, which is often considered as a model of activated carbon surfaces, range from 37 to 4000 for typical adsorbates and are by no means close to unity. It is proposed that this apparent discrepancy between theory and experiments has its origin in the edge effects: carbon blacks are modeled by semi-infinite slabs, whereas the magnitudes of thickness and diameters of micropore walls are of the order of a nanometre. Factors associated with edge effects reduce the adsorption energies on surfaces of finite particles resulting in shifts of condensation pressures to higher values. For particles with diameters about 1.4 and 1.8 nm, adsorption energies decrease to ≈0.72ε1*∞ and ≈0.81ε1*∞, respectively, where ε1*∞ is the energy of adsorption on an infinite plane, and pc approaches ps. This phenomenon substantiates the application of ps in adsorption equations and the success of the Dubinin equations probably owes much to this fact.
