Effects of Heterogeneous Surface Geometry on Adsorption

We demonstrate through simulations that surface geometry heterogeneity is sufficient to induce an adsorption/desorption hysteresis loop. The simulations show that the hysteresis originates from the quasiequilibrium nature of the adsorption branch. Different shapes of hystereses were obtained (parallel, widening, and unclosed at low relative pressure), according to the geometry of the surface (low microporosity, high microporosity, and mesoporosity). A unique population of permanently trapped gas phase molecules was identified and the dynamic nature of this population, termed “latent adsorbed” molecules, was demonstrated. The behavior of the BET equation in the presence of geometrical surface heterogeneity and lateral interactions was examined. It was found that the error in calculating the BET surface area, for the same surface-adsorbate and adsorbate-adsorbate interactions, is greater for irregular surfaces than for smooth surfaces. This observation is attributed to the effect of surface heterogeneity on higher adsorbed layers, which in turn increases the influence of adsorbate-adsorbate interactions. For weak lateral interactions the BET surface area estimation was within 10% of the theoretical prediction. The effects of surface heterogeneity on the thermodynamic functions of the adsorption/desorption process were calculated. The important role of the entropy of adsorption in the generation of the hysteresis loop was identified.