Molecular Simulation Evidence for Solidlike Adsorbate in Complex Carbonaceous Micropore Structures

Abstract

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.

New Virtual Porous Carbons Based on Carbon EDIP Potential and Monte Carlo Simulations

“…Different VPC models were proposed [1][2][3]. The most popular VPC models were proposed by Biggs et al [1,[4][5][6][7], Do et al [8][9][10][11][12][13][14], and Harris et al [15][16][17][18][19][20][21]. VPCs are also generated using a combination of experimental diffraction data and Monte Carlo simulations (reverse Monte Carlo (RMC) or hybrid reverse Monte Carlo (HRMC) techniques) -see for example [22][23][24][25][26][27][28][29].…”

Surface Area and Microporosity of Carbon Aerogels from Gas Adsorption and Small- and Wide-Angle X-ray Scattering Measurements

“…29 This feature has also two very important consequences. The first one is that the density of molecular assemblies adsorbed in micropores is different from that of bulk liquid, as already discussed by several authors 28 but generally ignored. Thus, the micropore volume deduced from adsorption measurements by assuming the validity of the Gurvitch rule can be considered only as a first approximation.…”

“…For sample J35, for which gas adsorption measurements ( Figure 2 and Table 1) indicate a wide distribution of pore sizes, the shape of the curves could be related also to fluctuations of density as a function of pore size. 27,28 Finally, carbon aerogel J contains probably, before activation, some closed porosity that disappears after activation in sample J14. For sample J, S SAXS is significantly larger than S BET or S total (S SAXS /S BET ) 1.6, Table 2).…”

Corrected thermodynamic description of adsorption via formalism of the theory of volume filling of micropores

“…In the case of heterogeneous surfaces, the adsorption takes place in pores having limited dimensions. The results of computer simulation and of adsorption in pores, carried out so frequently, show that the state of the adsorbate in pores may be distinctly different from that in a bulk liquid [37,38].…”