Decoding gas-solid interaction effects on adsorption isotherm shape: I. Non-polar adsorptives

Abstract

A suite of non-polar adsorptives of different kinetic diameters and shape were used to determine adsorption and pore filling mechanism of a well-characterised poly(furfuryl alcohol)-based activated carbon. Triplicate measured Type I adsorption isotherms for each adsorptive were averaged to provide standard deviation in relative pressures and associated amounts adsorbed. Plateau amounts adsorbed for N2, Ar, CH4, and C6H6, provided Gurvitsch volumes averaged to 0.368 ± 0.015 cm 3 (liq)/g. The calculated Gurvitsch volumes were compared with those derived via the Dubinin-Radushkevich (DR) equation. Additional adsorptives were CO2, iso-butane and SF6. The results of these 7 adsorptives were used to qualitatively analyse and decode a micropore filling adsorption mechanism. The DR equation was also used for further analysis of the pore filling mechanism. Based on the adsorbate-adsorbate and adsorbate-adsorbent interactions, adsorbates were classified into three groups: (a) Non-polar with nonspecific interactions (no dipole, no quadrupole, not readily polarizable: Ar, N2, CH4 and iso-butane), adsorbing as a continuous uptake over the observed relative pressure range; (b) Non-polar adsorptives with potential for specific interactions (no dipole, quadrupole moment: CO2 and C6H6), adsorbing as a condensation process over a relatively narrow relative pressure range in a medium pressure range; (c) Halogenated adsorptives (no dipole, no quadrupole, polarizable: SF6), adsorbing with an S-shaped uptake extending over a relatively broad relative pressure range.

Fluctuation adsorption theory: quantifying adsorbate–adsorbate interaction and interfacial phase transition from an isotherm

“…3 Adsorbate structure, or adsorbate-adsorbate interaction, has been considered not only to play an important role in the surface properties and reactivity of nanoparticles 2,3 but it is also one of the key factors determining the type (functional shape) of an isotherm. [4][5][6][7][8] However, precisely how the balance between adsorbatesurface and adsorbate-adsorbate interactions gives rise to each type of adsorption is still an unresolved question. [4][5][6][7][8] Furthermore, the surface structure expected from the type of isotherm may be different from reality.…”

“…[4][5][6][7][8] However, precisely how the balance between adsorbatesurface and adsorbate-adsorbate interactions gives rise to each type of adsorption is still an unresolved question. [4][5][6][7][8] Furthermore, the surface structure expected from the type of isotherm may be different from reality. 9 Hence, a better link should be provided between an isotherm and adsorbate structure, which is the goal of this paper.…”

Decoding gas-solid interaction effects on adsorption isotherm shape: II. Polar adsorptives

“…Previous adsorption analyses of this carbon focused on non-polar adsorptives whose predominant mode of interaction would be non-specific, but with fluid-fluid interactions sufficient to promote condensation within the micropores [2]. It would be clear from the details in Fig.…”

“…Typical adsorptives would be N 2 , Ar, CO 2 , SF 6 , and C 6 H 6 . Adsorption of these by the carbon used in the current work has been discussed in detail [2]. The latter class exhibit specific interactions with either polar or polarizable surface groups.…”

“…Differences in the amounts adsorbed in the current work as p/p o → 1.0 suggests that these micropore volumes would approximate adsorptive dimension-controlled, specific micropore volumes, molecular sieve exclusion effects defined as criterion 1 above.Differences in evaluated Gurvitsch volumes are not necessarily attributable to adsorptive dimensions alone; molecular packing effects themselves manifest as an effect due either to dependence on the critical ratio of adsorptive size: pore size[20] or to long-range structure development in the adsorbed phase within pores of suitable dimensions. These influences are expressed by both non-polar[2] and polar molecules in the adsorbed phase, defined as criterion 2 exclusion effects. The concept of a critical ratio between adsorptive dimensions and pore dimensions affecting adsorbed phase packing…”