Lattice density functional theory investigation of pore shape effects. I. Adsorption in single nonperiodic pores

Abstract

A fully explicit in three dimensions lattice density functional theory is used to investigate adsorption in single nonperiodic pores. The effect of varying pore shape from the slits and cylinders that are normally simulated was our primary interest. A secondary concern was the results for pores with very large diameters. The shapes investigated were square pores with or without surface roughness, cylinders, right triangle pores, and trapezoidal pores. It was found that pores with very similar shape factors gave similar results but that the introduction of acute angled corners or very large side ratio lengths in rectangular pores gave results that were significantly different. Further, a rectangular pore going towards the limit of infinite side ratio does not approach the results of a slit pore. In all of these cases, the importance of features that are present for only a small portion of the pore is demonstrated.

On the condensation/evaporation pressures and isosteric heats for argon adsorption in pores of different cross-sections

“…Adsorption proceeds in a similar way in the hexagonal pore, but the affinity of the junction in the triangular pore is greater than in the hexagonal pore because potential overlap effects are stronger. Similar phenomena were found by Malanoski and van Swol using DFT calculations [8].…”

“…2b). Malanoski and van Swol [8] also found similar phenomena in their DFT calculation for an arbitrary pore with acute-, right-and obtuse-angled corners. Since the solid-fluid interaction decays approximately as the inverse 3rd power of the distance from the surface, and becomes negligible at distances of about two collision diameters, the process of filling the gas-like core (condensation) is governed mainly by the fluid-fluid interactions and the interface curvature.…”

“…This differs from the criteria adopted by Ustinov [9] who used only the surface area and by Malanoski and van Swol [8] who used cross-sectional area or circumference as a basis for comparison. Our choice leads to the following relationships between sizes of cylindrical, hexagonal and triangular pores:…”

“…The equivalent diameter of a cylindrical pore is 8.1 nm from Eq. (8), and the side length of the triangular pore is 14.0 nm. …”

“…Previous studies of the effects of pore shape on condensation and evaporation include a density functional investigation of adsorption in single pores with cylindrical, square and triangular cross sections by Malanoski and van Swol [8]; simulations of nitrogen adsorption in hexagonal and cylindrical pores of MCM-41 by Ustinov [9], and a simulation study of the effects of surface curvature and adsorption strength on 2D transitions of argon adsorbed in carbon nanotubes by Liu et al [10]. Triangular pores have also been studied in earlier work [11][12][13][14] as a kernel for the characterization of activated carbon.…”