A qualitative approach to adsorption mechanism identification on microporous carbonaceous surfaces

Abstract

The paper presents results of research on identification of localized and other adsorption mechanisms, on geometrically heterogeneous graphite-like carbonaceous surfaces. It attempts to get an insight into properties of individual adsorptive molecule movement near attractive adsorption sites, arising from adsorbent surface geometrical heterogeneities. In particular, a shape and volume of space occupied by the continuously moving molecule mass center are investigated. To this aim, kinematic equilibrium of the particle moving near a hypothetical microporous carbonaceous adsorbent wall is considered, and then compared with thermodynamic equilibrium. The proposed approach enables to examine effects of certain surface geometry on the shape and volume of space occupied by adsorbed particles, and so to outline temperature conditions for the localized adsorption mechanism predomination. Thus, it provides a cognitive basis to answer the question, what particular mechanism (localized or other-e.g. mobile) should be assumed for a class of adsorption systems in order to select the most appropriate mathematical adsorption model. Hence, it makes it possible for more reliable examination of real porous structures, based on adsorption measurements. Lennard-Jones potential intermolecular diameter for carbon atom interaction with adsorbate particle, at k-th position nm ð Þ s i i-th particle ''temperature'', average in phase KeywordsSpecific surface area of a movement space of i-th adsorbate particle mass center nm 2 ð Þ d fiDiameter of an equivalent sphere of volumeDiameter of an equivalent sphere of volumeMaxwell-Boltzmann distribution function at a given temperature T of isothermal adsorption; dimensionless Averaged in space v, boundary equipotential surface of an individual i-th particle moving

Characterization of hard coal properties applying multiple sorption model with parameters determined from qualitative approach to adsorption mechanism identification

“…A separation of the sorbate molecule from its liquidlike reference state and its transition to a vacuum, at the b Obtained in the evaporation temperature, in accordance to the procedure described in (Reid et al 1987) c Calculated for the sorption conditions within MSM model d Calculated applying adsorption mechanism identification procedure (Ziółkowska et al 2016a) where δ p is sorbate solubility parameter, V s is sorbate molar volume in sorption system, R is a gas constant and T is temperature. At the second stage, the nature of each subprocess may be expressed parameterizing an effective radius of a created hole R ha (Fig.…”

“…Hence, the space of free movement V fi is bounded by an equipotential surface U bdry (V fi ), at which kinetic energy of two degrees of freedom k B τ i is equilibrated by a decrease in an average solid-fluid potential U avge (V fi ). In such a view, kinematic equilibrium of the particle free movement may be expressed by the following formulae (Ziółkowska et al 2016a) (5)…”

“…for adsorption mechanism identification, see (Ziółkowska et al 2016a). When considering isothermal adsorption, an excess energy evolved due to the adsorbate particle interaction with adsorbent surface may be seen as totally withdrawn to keep the constant temperature T. Simultaneously, having regard to accepted assumptions, the volume V mpi and the quantities U madh (V mpi ), U m (V mpi ) may be seen as an additive (in the range τ ind < τ i ≤ τ max the quantities has to be proportionally shared in terms of the number of j particles, to keep the consistency of the description).…”

“…Hence, it makes it possible for averaging of the quantities in Eq. (6) with Maxwell-Boltzmann distribution function f(τ i ,T) at a given temperature T, more detailed see (Ziółkowska et al 2016a). Assuming that τ i temperature distribution is the Maxwell-Boltzmann distribution, for each i-th particle being in the kinematic equilibrium (i.e.…”

Effect of adsorbate properties on adsorption mechanisms: computational study

“…The background for the model design was to especially consider the surface geometrical heterogeneity and energy distribution effects on the adsorption system properties . Recently, the author's team works were also focused on the examination of structural diversity effects, on the molecule behavior near the adsorbents surface, with special regard in what extent the local surface deformation reduces the particle movement making it closer to the localized mechanism (Ziółkowska and Duda 2015). Having in mind that system properties are closely related to the surface geometrical heterogeneity and energy distribution (Rudzinski and Everett 1992) for adsorbents of irregular, randomly distributed porous structure localized adsorption may be assumed.…”