Experimental adsorption isotherms based on inverse gas chromatography

Time-resolved chromatographic analysis and mechanisms in adsorption and catalysis

The main object of this review is the study of fundamentals of adsorption and heterogeneous catalysis, a benefit for the understanding of adsorptive and catalytic properties. This work aims to define and record, with the utmost accuracy, the phenomena and the possible reactions. A new methodology for the study of the adsorption is presented, which is a version of the well-known inverse gas chromatography. This reversed-flow inverse gas chromatography (RF-IGC) is technically very simple, and it is combined with a mathematical analysis that gives the possibility for the estimation of various physicochemical parameters related to adsorbent or catalyst characterization, under conditions compatible with the operation of real adsorbents and catalysts. On this base, this methodology has been successfully applied to the study of the impact of air pollutants, volatile organic and/or inorganic, on many solids such as marbles, ceramics, oxide-pigments of works of art, building materials, authentic statues of the Greek Archaeological Museums. Moreover, this methodology proved to be a powerful tool for studying the topography of active sites of heterogeneous surfaces in the nano-scale domain. Thus, some very important local quantities for the surface chemistry have been determined experimentally for many solids including thin films. These physicochemical local quantities (among which adsorption energy and entropy, surface diffusion coefficient, probability density function) have been determined from the experimental pairs of height of extra chromatographic peaks and time by a nonlinear least-squares method, through personal computer programs written in GW BASIC and lately in FORTRAN. Through the time-resolved analysis the surface characterization of the examined materials took place. In addition, the kinetic constants responsible for adsorption/desorption and surface chemical reactions have also been calculated. Thus, important answers have been provided to the following essential questions: (1) Can RF-IGC define the gnostic regions of adsorption/desorption, surface diffusion, surface reaction? Yes, irrefutably and undeniably. (2) Can RF-IGC deal with issues of catalysis, the existence of more than one reaction? Certainly yes. Indeed, it is impressive to observe the reactions "on line". (3) Can RF-IGC identify peaks of products and reactants simultaneously? Certainly yes. (4) Can RF-IGC be applied to thin films in a nano-scale domain? The answer is "definitely yes". (5) Can it kinetically follow the above? Yes, again.

Physicochemical characterization of interfaces

Reversed-flow inverse gas chromatography (RF-IGC) was used to measure, directly from experimental data, adsorption energies, local adsorption isotherms, the probability density function for the adsorption energies, and lateral interaction parameters, as distributed over experimental time. Local isotherms and the distribution energy function were correlated with adsorption energy. The results obtained are comparable to those calculated on the basis of the well-known integral equation. The RF-IGC method was used with the two most common hydrocarbons, acetylene and 1-butene, as probe gases, in the presence and absence of ozone, and with magnesium oxide and silicon oxide as solid adsorbents.

Determination of isotherms by gas–solid chromatography

This literature review of the fundamental developments in gas-solid adsorption isotherms includes articles published from 1933 until now. Analytical and numerical methods used for calculating the adsorption energy distribution function, as a quantitative measure of surface heterogeneity, are included. Special attention is paid to inverse gas chromatography (IGC) and more precisely to a new version of IGC known as reversed-flow gas chromatography (RF-IGC or RF-GC). RF-GC is presented as a quick, precise and effective method to investigate physicochemical properties of different kinds of adsorbents, through adsorption isotherms and related energetic parameter determinations. Advantages of the RF-GC method over traditional chromatographic methods are discussed.

Determination of chemical kinetic properties of heterogeneous catalysts

The importance of cooperation of heterogeneous catalysis with surface science is stressed for simultaneous adsorptive and catalytic measurements. Inverse gas chromatography and reversed-flow gas chromatography offer a suitable research ground for such collaboration. After a short introduction, adsorption physicochemical quantities of heterogeneous catalysts with typical recent results, chemical kinetic properties and surface energy of catalysts are described, stressing the important aspect of time-resolved chromatography, due to the heterogeneity of the solid surface of catalysts. Adsorption energies, local monolayer capacities, local isotherms and energy distribution functions are extensively described. Also, lateral molecular interactions, surface diffusion and adsorption rates on heterogeneous catalysts are described.

Experimental determination of adsorption energies, adsorption isotherms, probability density functions, and lateral molecular interactions on CXHY/CaO systems

Reversed-flow gas chromatography (RF-GC) is extended to the measurements of the probability density function for the adsorption energies as well as the differential energies of adsorption due to lateral interactions of molecules adsorbed on different heterogeneous solid surfaces. All these calculations are based on a non-linear adsorption isotherm model as it is well accepted that the linear one is inadequate for substances such as these used in this work. Thus, some new important physicochemical parameters have been obtained for the characterization of the heterogeneous systems studied. The adsorbent used in this study was calcium oxide. The adsorption of many significant hydrocarbons was investigated. With these systematic experiments under conditions which are similar to the atmospheric ones, an extrapolation of the results obtained to "real" atmospheres with a high degree of confidence is possible.