The pore size distribution (PSD) and the pore-network connectivity of a porous material determine its properties in applications such as gas storage, adsorptive separations, and catalysis. Methods for the characterization of the pore structure of porous carbons are widely used, but the relationship between the structural parameters measured and the real structure of the material is not yet clear. We have evaluated two widely used and powerful characterization methods based on adsorption measurements by applying the methods to a model carbon which captures the essential characteristics of real carbons but (unlike a real material) has a structure that is completely known. We used three species (CH4, CF4, and SF6) as adsorptives and analyzed the results using an intersecting capillaries model (ICM) which was modeled using a combination of Monte Carlo simulation and percolation theory to obtain the PSD and the pore-network connectivity. There was broad agreement between the PSDs measured using the ICM and the geometric PSD of the model carbon, as well as some systematic differences which are interpreted in terms of the pore structure of the carbon. The measured PSD and connectivity are shown to be able to predict adsorption in the model carbon, supporting the use of the ICM to characterize real porous carbons.
Adsorption of a model nitrogen vapor on a range of complex nanoporous carbon structures is simulated
by grand canonical Monte Carlo simulation for a single subcritical temperature above the bulk freezing
point. Adsorption and desorption isotherms, heats of adsorption, and three-dimensional singlet distribution
functions (SDFs) were generated. Inspection of the SDFs reveals significant levels of solidlike adsorbate
at saturation even in the most complex of the microporous solids considered. This strongly suggests that
solidlike adsorbate will also occur for simple subcritical vapors adsorbed on real noncrystalline solids such
as microporous carbons at temperatures above the bulk freezing point, supporting indirect experimental
observations. The presence of significant levels of solidlike adsorbate has implications for characterization
of microporous solids where adsorbate density is used (e.g., determination of pore volume from loading).
Detailed consideration of the SDF at different loadings for a model microporous solid indicates solidlike
adsorbate forms at distributed points throughout the pore space at pressures dependent on the nature of
the local porosity. The nature of the local porosity also dictates the freezing mechanism. A local freezing/melting/refreezing process is also observed. Introduction of mesoporosity into the model causes hysteresis
between the adsorption and desorption isotherms. Adsorption in the hysteresis loop occurs by a series of
local condensation events. It appears as if the presence of adjacent microporosity and/or adsorbate within
it affects the pressure at which these events occur. Reversal of the condensation during desorption occurs
throughout the mesoporosity at a single pressure; this pressure is unaffected by the presence of adjacent
microporosity or the adsorbate within it. It is also shown that the empirical concept of “pore size” is not
consistent for describing adsorption in the complex solids considered here. A new concept is, therefore,
proposed that seeks to account for the factors that affect local adsorption energy: local geometry, microtexture,
surface atom density, and surface chemistry.
The design and use of microporous solids depends on having access to characteristics such as the pore volume and surface area. Comparison methods such as the alpha(s) method are one of the most widely used means of determining these parameters. An assessment of this group of methods was undertaken by comparing estimates obtained from them using adsorption isotherms generated by grand canonical Monte Carlo simulation on a selection of model nanoporous solids with exactly known surface areas and pore volumes. Conclusions are drawn from this absolute assessment in regard to the validity of the alpha(s) method for determining the micropore volume, the mesopore surface area, and the separation of pore groups based on the concept of primary and cooperative filling, the subtracting pore effect (SPE) method, and the required character of the reference surface.
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