Dimitrios Gavril

Time-resolved determination of surface diffusion coefficients for physically adsorbed or chemisorbed species on heterogeneous surfaces, by inverse gas chromatography.

A new simple method is developed for measuring surface diffusion coefficients Ds of gases adsorbed on heterogeneous surfaces, using the reversed-flow version of inverse gas chromatography. The Ds values are found in a time-resolved way, together with the corresponding adsorption energy values, the local adsorbed concentrations, and the local adsorption isotherm values. A relative dynamic adsorption rate constant, an adsorption/desorption rate constant, and a surface reaction rate constant are also found in the same experiment, together with the total diffusion coefficient of the gas in the solid bed. The method has been applied for carbon monoxide, oxygen gas, and carbon dioxide as adsorbates on 75% Pt+25% Rh catalyst supported on SiO2, at 593.8 K.

Gas chromatographic kinetic study of carbon monoxide oxidation over platinum-rhodium alloy catalysts.

The kinetics for the oxidation of carbon monoxide in the presence of excess oxygen over Pt-Rh alloy catalysts were studied by using the reversed-flow gas chromatography technique. Suitable mathematical analysis equations were derived by means of which the rate constants for the oxidation reaction of carbon monoxide, as well as for the adsorption and desorption of the reactant CO on the catalysts pure Pt, 75 atom% Pt+25 atom% Rh, 50 atom% Pt+50 atom% Rh, 25 atom% Pt+75 atom% Rh and pure Rh supported on SiO2 were determined. All the catalysts show a maximum rate constant for the production of CO2 at a characteristic temperature close to that found in the literature. The rate constants for the adsorption of CO increase generally with increasing temperature, while those for the desorption decrease with increasing temperature. From the variation of the rate constants with temperature activation energies for the oxidation reaction and adsorption of CO were determined, which are sensitive to the composition of the catalytic surface. The appearance of CO2 and carbon, when introducing pure CO into the column with the catalysts, verified a partial dissociative adsorption (e.g., disproportionation) of CO on the catalysts used. The latter indicates a mechanism for the CO oxidation through a partial dissociative adsorption of CO followed by the reaction of adsorbed CO molecules with adsorbed O atoms.

Flux of gases across the air–water interface studied by reversed-flow gas chromatography

In the present work the reversed-flow gas chromatographic technique was applied for the study of flux of gases across the air-water interface. The model system was vinyl chloride-water, which is of great significance in food and environmental chemistry. Using suitable mathematical analysis, equations were derived by means of which the following physicochemical quantities were calculated: diffusion coefficient of vinyl chloride (VC) into water, partition coefficient of VC between the water (at the interface and the bulk) and the carrier gas nitrogen, overall mass transfer coefficients of VC in the gas (nitrogen) and the liquid (water), gas and liquid film transfer coefficients of VC, gas and liquid phase resistances for the transfer of VC into the water, and finally the thickness of the stagnant film in the liquid phase, according to the two-film theory of Whitman. From the variation of the above parameters with temperature, as well as the volume and the free surface area of the water, useful conclusions concerning the mechanism for the transfer of VC into water were extracted. These are discussed in comparison with the same parameters calculated from empirical equations or determined experimentally by other techniques.

Study of the sorption of carbon monoxide, oxygen and carbon dioxide on platinum-rhodium alloy catalysts by a new gas chromatographic methodology

Abstract The mechanism of the CO oxidation reaction over Pt–Rh alloy catalysts was extracted from the adsorption of CO, O 2 and CO 2 on these catalysts. Reversed-flow gas chromatography was applied to the study of these important sorption processes. Using suitable mathematical analysis, equations were derived by means of which rate constants for the adsorption and desorption of CO, O 2 and CO 2 on and from the Pt–Rh bimetallic catalysts were determined. Temperature dependence of the rate constants for the sorption processes were also determined and the Arrhenius parameters for the adsorption and desorption were calculated. The dependence of activation energies for adsorption, desorption and disproportionation reaction on the catalyst Pt content indicates that for the CO oxidation reaction the most active is the Pt-rich catalyst, while for the CO disproportionation reaction the most active is the Rh-rich catalyst.

Size Analysis of Barley Starch Granules by Sedimentation/Steric Field Flow Fractionation

In the present work the relatively new technique of sedimentation/steric field-flow fractionation (Sd/StFFF) was used for the size analysis of the large, medium and small starch granules of barley grain. The experimental parameters varied were the time from the preparation of the starch sample dispersion - in order to study the disintegration or the aggregation of the starch granules - and the concentration of the sample -in order to study the sample overloading effect on the measured size of the starch granules. A bimodal or trimodal size distribution curve, depending on the experimental conditions used, was found by Sd/StFFF, and the number and weight average diameters of the starch granules were calculated. Finally, the experimental results are compared with those obtained by scanning electron microscopy and light scattering, and useful conclusions concerning the destruction or the aggregation of the starch granules, as well as the sample overloading effect on the granules size, measured by Sd/StFFF, are extracted.

Study of the mechanism of the interaction of vinyl chloride with water by reversed-flow gas chromatography.

The investigation of the mechanism for the interaction of vinyl chloride (VC) with liquid foods is of great significance in food science. In the present work the model system VC-water was studied by using the relatively new technique of reversed-flow gas chromatography. Using suitable mathematical analysis the following physicochemical quantities were determined: (a) diffusion coefficients of VC into water, (b) overall mass transfer coefficients of VC in the water, and in the carrier gas nitrogen, (c) partition coefficients of VC between water and nitrogen, and (d) constants of Henrys law for the adsorption of VC by water. From the variation of the above parameters with temperature, and the stirring rate of the water, useful conclusions concerning the mechanism for the VC-water interaction were extracted. The experimental results for the transfer of VC into the bulk water suggest (i) a mechanism consisted of a fast equilibrium step between the VC in the gas phase and in the interface, followed by a slow adsorption of VC into the bulk of water, which is the rate-determining step, when the water is quiescent, and (ii) a mechanism consisted of a slow diffusion of VC from the gas phase to the interface, which is the rate-determining step, followed by a fast equilibrium step between the VC in the interface and in the water bulk, when the water is stirred.

New Gas Chromatographic Instrumentation for Studying Mass Transfer Phenomena

Abstract The new method of reversed-flow gas chromatography for studying heterogeneous catalysis, diffusion, adsorption, evaporation, and other related phenomena, is now applied to study the mass transfer phenomena across the gas-gas, gas-liquid, and gas-solid interphases. As the model gas solute, the vinyl chloride monomer (VCM), which is of paramount importance in food and environmental sciences, was selected, while the second phase consisted of gaseous nitrogen, liquid water, and solid polyvinyl chloride (PVC). Using appropriate mathematical analysis, the following parameters were determined: (i) Diffusion coefficients of VCM into nitrogen, (ii) Diffusion coefficients of VCM into water, (iii) Partition coefficients of VCM between water and nitrogen, (iv) Constants of Henrys law for the adsorption of VCM by water, (v) Partition coefficients of VCM between PVC and nitrogen, and (vi) Enthalpies and entropies for the adsorption of VCM on the solid PVC. Comparison of the experimental results with those det...

Conversions of carbon monoxide oxidation over Pt−Rh alloy catalysts calculated by a new gas chromatographic technique

SummaryCatalytic fractional conversions of carbon monoxide to carbon dioxide over Pt−Rh alloy catalysts in the presence of excess oxygen, under steady-state or non steady-state conditions, as well as corresponding rate constants for the CO oxidation reaction were measured by using the reversed-flow gas chromatographic technique. From the variation of the conversions with temperature, maximum values of conversions were found, which depend on the catalysts nature (Pt content), while from the variation of the rate constants with temperature, activation energies for the CO oxidation reaction were determined, which also depend on the catalyst Pt content. The results suggest a synergism between Pt and Rh in the Pt−Rh bimetallic catalysts in accordance with previous works, showing that reversed-flow gas chromatography can be used with simplicity and accuracy for the kinetic study of the CO oxidation reaction, which is of technological importance for the control of air pollution.

NEW METHODOLOGY FOR THE MEASUREMENT OF DIFFUSION COEFFICIENTS OF PURE GASES INTO GAS MIXTURES

The new technique of reversed-flow gas chromatography has been used for the measurement of diffusion coefficients of pure gases into gas mixtures. As pure gases, carbon monoxide (CO) and carbon dioxide (CO2), were used, while the gas mixtures consisted of hydrogen (H2) and helium (He) with various percentage compositions. The experimental diffusion coefficients of CO and CO2 into various percentage mixtures of H2 and He (25% v/v H2 + 75% v/v He, 50% v/v H2 + 50% v/v He, 75% v/v H2 + 25% v/v He) were compared with those calculated theoretically, using an empirical equation in which the percentage v/v composition of the gas mixtures and the theoretical values of the diffusion coefficients of the CO and CO2 into the pure gases of the mixture were taken care of as they were found from the Fuller–Schettler–Giddings equation. All the comparisons show small deviations, indicating that both the experimental technique of reversed-flow gas chromatography, and the proposed empirical equation, can be successfully used for the determination of diffusion coefficients of pure gases into gas mixtures.

AN INVERSE GAS CHROMATOGRAPHIC TOOL FOR THE MEASUREMENT OF LOCAL ISOTHERMS ON HETEROGENEOUS SURFACES

ABSTRACT A new application of an inverse gas chromatographic technique, known as Reversed Flow Gas Chromatography (RFGC), which permits the experimental measurement of local isotherms, θ( p, T, ϵ) is presented. The method does not use analytical or numerical solutions of the classical integral equation comprising the energy distribution function, f(ϵ), as unknown, but it is based on a time function of gas chromatographic (GC) peaks obtained by short flow reversals of the carrier gas. It can be used for the measurement of local adsorption isotherms θ, as well as for the estimation of the related parameters such as local energies of adsorption ϵ, local adsorbed concentrations c s *, and local maximum monolayer capacities c max *. As a model system, we selected oxygens adsorption onto three-way silica supported 25% Pt+75% Rh alloy catalyst.