An improved model for N2 adsorption on graphitic adsorbents and graphitized thermal carbon black—The importance of the anisotropy of graphene

Abstract

Computer simulations of N 2 adsorption on graphite frequently use the 10-4-3 equation with Steele's molecular parameters to describe the dispersive-repulsive interaction between a molecule and graphite. This model assumes that graphite is a uniformly homogeneous continuum solid, and its derivation implies the following assumptions: (1) the solid is built from stacked, equally spaced graphene layers, (2) there is an infinite number of layers, and (3) the carbon atom molecular parameters are invariant for all layers (collision diameter of 0.34 nm and reduced well depth of interaction energy of 28 K). Despite the fact that this model can give an acceptable description of experimental data for this system, there are experimental observations that simulation results fail to account for. First, the isotherm does not exhibit a step in the sub-monolayer coverage region at 77 K, which is attributed to a transition from the supercritical state of the adsorbate to the commensurate state, and therefore fails to reproduce the cusp and heat spike in the experimental isosteric heat curve versus loading at close to monolayer coverage. Second, the simulation results overpredict the experimental data in the multilayer region. These discrepancies suggest that (1) the absence of lateral corrugation in the 10-4-3 potential misses the commensurate to incommensurate transition and (2) the long-range solid-fluid potential, experienced by the second and higher layers onwards, is too strong. Here we examine a revised graphite potential model that incorporates three features absent from the 10-4-3 model: (1) an energetic corrugation of the potential arising from the discrete atom structure of the adsorbent, (2) the unequal spacing of the graphene layers due to the anisotropic force field acting on graphene layers at the surface, and (3) the different polarizabilities of carbon atoms in graphite, parallel and normal to the graphene surface. These features are corroborated by a number of experimental measurements and quantum-mechanical calculations: (1) the Low-Energy Electron Diffraction (LEED) and Surface-Extended X-ray Absorption Fine Structure (SEXAFS) experiments show that the first adsorbate layer is smaller than predicted by the 10-4-3 model with the traditional molecular parameters suggested by Steele, and (2) the potential well depth for atoms in graphene is stronger than for C-atoms in graphite. The simulation results using this revised graphite model give an improved description of the fine features of adsorption of N 2 on graphite: the sub-step in the first layer of the isotherm, the spike in the isosteric heat curve versus loading, and the coverage at higher loadings.

The role of adsorbate size on adsorption of Ne and Xe on graphite

“…Recently we proposed a model for graphite [1,27], which goes beyond the homogeneous model [28], and achieves better agreement between simulation results and experimental data, particularly the transition from the commensurate (C) to incommensurate (IC) packing ( Figure 1). The C-packing has a   0 3 3 30 R  lattice spacing of 0.426 nm and an adsorbate density of 10.56 μmol/m 2 , and the IC structure has 2D-hexagonal packing of adsorbate molecules and the adsorbate density depends on the size of the adsorbate.…”

“…However, this model is unable to show the experimentally observed commensurate structure of the first layer for simple adsorbates [34][35][36]. For example, it fails to describe the sub-step in the experimental isotherm of nitrogen at its boiling point (77K) and the corresponding cusp and spike in the plot of the isosteric heat versus loading [1,26].…”

A re-assessment of the isosteric heat for CCl4 adsorption on graphite

“…The second is the Corrugation and Anisotropy (CA) model that accounts for the hexagonal arrangement of carbon atoms and the anisotropy of polarizability of graphite [32]. Details of this model are given in references [33,34]. 8…”

Fundamental study of adsorption of non-polar and polar fluids on hydrophobic ordered solids

“…The C-packing has a lattice spacing of 0.426 nm and an adsorbate density of 10.56 μmol/m 2 , and the IC structure has 2D-hexagonal packing of adsorbate molecules and the adsorbate density depends on the size of the adsorbate. This new model has been proven to describe well the ordering transition for N2 (previous chapter 157 ), Kr 158 , CH4 158 , whose intermolecular separations at the LJ potential minimum…”

“…The corrugation-anisotropy (CA) model of graphite that is proposed in Chapters 4 and 5 has been tested for simple gases (Ne 110 , Ar 109 , Kr 158 , Xe 110 , N2 157 , CH4 158 ). It is therefore recommended to investigate the validity of this model for more complex fluids such as methanol, ammonia, water and tert-butanol.…”