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

A new inverse gas chromatographic methodology is introduced to measure local (homogeneous) adsorption energies, ε, local monolayer capacities, , and adsorption isotherms, θ i (p,T,ε) 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(ε) 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 ε, , and θ i (p,T,ε) 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 C2H4/ZnO, C2H4/PbO, C3H6/PbO, and C3H6/CaCO3. A comparison of the adsorption energy values found for these systems with heats of adsorption reported elsewhere is impermissible, since the latter refer to overall values and not to local values as in the present method. However, some literature values seem rather comparable to the present ones.