On the resolution of constant isosteric heat of propylene adsorptionon graphite in the sub-monolayer coverage region

An early experimental study by Bezus, Dreving and Kiselev [1] on the adsorption of propylene on Spheron-6 carbon black (graphitized at ∼3000C) reported a plot of constant isosteric heat versus loading in the sub-monolayer region. This contrasts with their report of a linear increase in isosteric heat for propane, a similar molecule to propylene. In this paper, we report extensive Grand Canonical Monte Carlo (GCMC) simulations and a high-resolution experimental study of propylene adsorption on Carbopack F, a highly graphitized thermal carbon black, over the same temperature range studied by Bezus et al. From this combined simulation and experimental study we conclude that propylene also shows a linear increase in the isosteric heat versus loading in the sub-monolayer region, indicating that the linear increase in the fluid-fluid interaction in this region more than compensates for the decrease in the solid-fluid interaction that results from the change in orientation of the adsorbate molecules. Our study contradicts the propylene results of Bezus et al., and careful inspection of their isotherm in the sub-monolayer region shows that it does not follow Henry’s law. This calls into question their argument that π-π interactions between propylene molecules are an explanation for the constant heat

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

Computer simulations of N 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 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

We have carried out an extensive grand canonical Monte Carlo simulation to investigate the adsorption of neon and xenon on graphite. The adsorbate collision diameters of neon and xenon are smaller and greater respectively, than the commensurate graphite lattice spacing λ=3×3R30 of 0.426 nm. Simulated isotherms and isosteric heats were obtained using a graphite model that has been shown to describe successfully the adsorbate transitions for krypton, methane and nitrogen by Prasetyo et al. (2017), which have collision diameters close to λ. Neon does not exhibit commensurate (C) packing because the gain in the intermolecular potential interactions in the incommensurate (IC) packing when molecules move away from carbon hexagon centres, does not compensate for the increase in the solid-fluid potential energy. Xenon, on the other hand, exhibits IC packing because its molecular size is greater than λ. Nevertheless, at a sufficiently high chemical potential, the first layer of xenon changes from the IC to C packing (in contrast to what is observed for krypton, nitrogen and methane). This transition occurs because the decrease in the xenon intermolecular interactions is sufficiently compensated by the increase in the solid-fluid interaction, and the increase in the fluid-fluid interactions between molecules in the first layer and those in the second layer. This finding is supported by the X-ray diffraction study by Mowforth et al. (1986) and Morishige et al. (1990)

A GCMC simulation and experimental study of krypton adsorption/desorption hysteresis on a graphite surface

Adsorption isotherms and isosteric heats of krypton on a highly graphitized carbon black, Carbopack F, have been studied with a combination of Monte Carlo simulation and high-resolution experiments at 77 K and 87 K. Our investigation sheds light on the microscopic origin of the experimentally observed, horizontal hysteresis loop in the first layer, and the vertical hysteresis-loop in the second layer, and is found to be in agreement with our recent Monte Carlo simulation study (Diao et al., 2015). From detailed analysis of the adsorption isotherm, the latter is attributed to the compression of an imperfect solid-like state in the first layer, to form a hexagonally packed, solid-like state, immediately following the first order condensation of the second layer. To ensure that capillary condensation in the confined spaces between microcrystallites of Carbopack F does not interfere with these hysteresis loops, we carried out simulations of krypton adsorption in the confined space of a wedge-shaped pore that mimics the interstices between particles. These simulations show that, up to the third layer, any such interference is negligible

On the resolution of constant isosteric heat of propylene adsorption on graphite in the sub-monolayer coverage region

An early experimental study by Bezus, Dreving and Kiselev [1] on the adsorption of propylene on Spheron-6 carbon black (graphitized at ∼3000C) reported a plot of constant isosteric heat versus loading in the sub-monolayer region. This contrasts with their report of a linear increase in isosteric heat for propane, a similar molecule to propylene. In this paper, we report extensive Grand Canonical Monte Carlo (GCMC) simulations and a high-resolution experimental study of propylene adsorption on Carbopack F, a highly graphitized thermal carbon black, over the same temperature range studied by Bezus et al. From this combined simulation and experimental study we conclude that propylene also shows a linear increase in the isosteric heat versus loading in the sub-monolayer region, indicating that the linear increase in the fluid-fluid interaction in this region more than compensates for the decrease in the solid-fluid interaction that results from the change in orientation of the adsorbate molecules. Our study contradicts the propylene results of Bezus et al., and careful inspection of their isotherm in the sub-monolayer region shows that it does not follow Henry's law. This calls into question their argument that π-π interactions between propylene molecules are an explanation for the constant heat

A coherent definition of Henry constant and isosteric heat at zero loading for adsorption in solids – An absolute accessible volume

We present a new analysis of the Henry's law constant and isosteric heat at zero coverage for gas adsorption on solid surfaces, which removes the ambiguities inherent in earlier definitions (Do et al., 2008). A new definition of accessible volume ensures physical self-consistency between these two properties. We show that an earlier definition of accessible volume, as the volume within which the adsorbate-adsorbent potential energy is non-positive, is too restrictive because it neglects the penetration of molecules with large kinetic energies into the highly repulsive (positive) regions of the potential, which occurs more frequently at higher temperatures. The new definition has been tested for a number of adsorbents, and for a wide range of adsorbates commonly used in the characterization of porous solids. In particular, we have highlighted the differences between the old and new definitions of the Henry constant and the isosteric heat at zero loading. Our analysis also reveals that, contrary to a common assumption made in the adsorption literature, the van't Hoff plot is non-linear, and linearity is only satisfied over a narrow range of temperature

Characterisation of the absolute accessible volume of porous materials

We presented a new characterization method for surface area, pore volume and pore size distribution of a solid based on the recently proposed concept of absolute accessible volume (Prasetyo et al. 2018). This method provides a coherent framework to characterize nanoporous atomic models. It gives an advantage over earlier methods in the sense that the accessible properties, volume, surface area and pore size distribution, follow a single set of rules that are based on the energy of interaction between a molecular probe and the solid. The derived parameters result in the Henry constant and the differential heat of adsorption approaching zero at extremely high temperatures as physically demanded, and we illustrated this new method with a range of porous solids

The Henry constant and isosteric heat at zero loading for adsorption on energetically heterogeneous solids absolute versus excess

We present a self-consistent analysis of adsorption of gases on energetically heterogeneous solids and derive expressions for the Henry constant and the isosteric heat at zero loading, q , for both excess and absolute amounts of molecules; the latter being amenable to thermodynamic analysis. Results obtained by Monte Carlo integration are shown to depend on the system size for calculations using the total number of molecules in the adsorbed and gas phases of the system. In the limit when loading approaches zero, the thermodynamic variables obtained from the fluctuation theory in a grand canonical simulation are in agreement with the Henry constant and the isosteric heat at zero loading calculated from Monte Carlo integration, establishing the consistency of the analysis of isotherm and isosteric heat for the full range of loading. The equations for the isotherm (and the Henry constant), and the isosteric heat for a complex substrate, can be written in terms of the local isotherms and the local isosteric heats of the components comprising the substrate for all loadings. This results in a significant saving in computing time for the calculation of the thermodynamic variables of a complex substrate

Interplay between wetting and filling of argon adsorption in slit pores with different surface energies transition from filling in micropores to capillary condensation in mesopores

The concept of "micropore filling" was first introduced by Dubinin (Dubinin, A. M. M. Q. Rev., Chem. Soc. 1955, 9, 101; Bering, B. P.; Dubinin, A. M. M.; Serpinsky, V. V. J. Colloid Interface Sci. 1966, 21, 378) to describe the adsorption in pores having widths of less than 1.5 nm at temperatures less than the bulk critical point temperature. In mesopores, another phenomenon, capillary condensation to a liquidlike adsorbate, or "condensate", occurs (Do, D. D. Adsorption Analysis: Equilibria and Kinetics; World Scientific: London, 1983). The distinction between micropore filling and capillary condensation is not unambiguous. It is implicitly assumed in the literature that there is a strong attraction between the adsorbent and a given adsorbate, resulting in the formation of an adsorbed film which thickens as pressure increases. As the pressure approaches the bulk coexistence pressure, the adsorbed film on a planar surface becomes infinitely thick, a feature known as (complete) wetting. In this work, we provide a clear distinction between filling and condensation and also show how wetting behavior (nonwetting/partial wetting/continuous wetting) affects filling and condensation in slit pores and determine conditions under which filling occurs even when the surfaces are nonwetting or partial wetting. We have carried out extensive simulations of argon adsorption in slit pores to investigate the effects of substrate strength, pore size, and temperature on the transition from nonfilling to filling and condensation. From the analysis of the simulation results, we define filling as the phenomenon whereby a pore is filled before the formation of an adsorbate layer on the surface and capillary condensation as one in which adsorbate layers are formed on the pore walls prior to condensation. A parametric map or phase diagram of wetting/filling is constructed to show the interplay between the adsorbent strength, temperature, and pore size, which delineates the various regions of filling and capillary condensation