Time Distribution of Adsorption Energies, Local Monolayer Capacities, and Local Isotherms on Heterogeneous Surfaces by Inverse Gas Chromatography

Abstract

A new inverse gas chromatographic methodology is introduced to measure local (homogeneous) adsorption energies, , local monolayer capacities, c max / , and adsorption isotherms, θ i (p,T, ) for probe gases on heterogeneous solid surfaces in the presence of nitrogen as carrier gas. The method does not depend on analytical solutions of the classical integral equation comprising the adsorption energy distribution function f( ) as unknown, nor on numerical solutions and estimations from this equation, using powerful computers.It simply uses a time function of the chromatographic peaks obtained by short flow-reversals of the carrier gas, combined with the local isotherm model of Jovanovic. All three adsorption parameters , c max / , and θ i (p,T, ) mentioned above can be calculated as a function of the experimental time and refer to instantaneous equilibration of the probe gases with the heterogeneous surface, with different kinds of active sites i being involved at different times. The kinetic physicochemical parameters for the adsorption phenomenon are also included for the gas/solid systems studied, being C

Interrelations between Adsorption Energies and Local Isotherms, Local Monolayer Capacities, and Energy Distribution Functions, as Determined for Heterogeneous Surfaces by Inverse Gas Chromatography

“…From all the above and other searches of the literature, one may easily come to the conclusion that nobody so far has tried a solution for f (ε) other than through the integral equation [1]. This is not true, however, since in four recent papers of ours (8)(9)(10)(11) we have done exactly that, employing an inverse gas chromatographic tool known as reversed-flow gas chromatography (RF-GC). A recent review on that is worth mentioning (12).…”

“…A recent review on that is worth mentioning (12). All determinations by us concerning adsorption onto heterogeneous surfaces (8)(9)(10)(11)(12) are based on experimental chromatographic data, under one single assumption, namely, that the local isotherm θ( p, T, ε) obeys the Jovanovic isotherm model (13). It is well known that this goes over to the Langmuir isotherm in middle pressures and to a linear form at low pressures.…”

“…The values of the preexponential factors A i above and the corresponding coefficients of time B i are easily and fairly accurately determined from the chromatogram by a simple PC program of nonlinear least-squares regression (12). Based on the experimental equation [4] and the Jovanovic model (13), a new inverse gas chromatography methodology was developed to measure directly local (homogeneous) adsorption energies, ε, local monolayer capacities, c * max , local adsorption isotherms, θ ( p, T, ε), and adsorption energy distribution functions, f (ε), on heterogeneous surfaces (8)(9)(10), all as functions of experimental time, circumventing altogether the integral equation [1].…”

“…(10) or in the review (12). They have been applied to several gaseous/solid systems and the results are collected either in tables or in figures (8)(9)(10)(11)(12), giving the time t, the gaseous concentration of the adsorbate c y , the adsorption energy ε, the local monolayer capacities c * max , the local adsorption isotherm values θ, and the probability density function f (ε), i.e., the adsorption energy distribution. The appearance of the graphical representation of all the above quantities with the time in the abscissa, as plotted with the PC program MATHEMATICA 3 with 3000 plot points, shows some interesting maxima and minima.…”

“…[1] and that calculated by RF-GC as described in Refs. (8)(9)(10)(11)(12), the former being based mostly on static adsorption measurements, while the latter on dynamic ones. Of course Eq.…”