Nicholas A. Katsanos

Diffusion, adsorption and catalytic studies by gas chromatography

Three topics important in many areas of physical chemistry, namely measurements of diffusion coefficients, adsorption kinetics and adsorption isotherms, as well as rates and conversions in heterogeneous catalysis, are reviewed from the gas chromatography viewpoint. After a general introduction, various theoretical sections include synopses of important mathematical models and developments, followed by experimental sections describing basic instrumentation and set-ups, and sections exemplifying the main results with brief discussions. The article is concluded with a methodology embracing simultaneous determination of all three properties (diffusion, adsorption and catalytic rates) in a single experiment by GC.

Measurement of diffusion coefficients by reversed-flow gas chromatography instrumentation

Abstract A method is reported for measuring mutual diffusion coefficients in gases. It is a gas chromatographic method, but not a broadening technique. Two short empty columns are placed perpendicular to one another: a diffusion column through which no carrier gas flows, and a chromatographic colum. The carrier gas which flows through the latter carries over to the detector the diffusion flux established in the diffusion column. Analysis of the concentration-time curve recorded gives the value of the diffusion coefficient of an injected solute into the carrier gas. More reliable results are obtained by the so-called “chromatographic sampling”, i.e. reversing the direction of flow of the carrier gas at definite known times. The analytical mathematical expression, describing the elution curves when the gas flow is reversed, is derived and used to determine diffusion coefficients for fifteen gas pairs. The results are of high precision, and comparison with the theoretical values shows that they have also high accuracy.

Measurement of activity coefficients by reversed-flow gas chromatography

Abstract The determination of activity coefficients for various two-component liquid—vapour systems was accomplished by reversed-flow gas chromatography. By means of derived mathematical equations, the gaseous equilibrium concentration of each component under study, in the pure state and in the binary mixture, was determined. From these equilibrium concentrations, activity coefficients for the alcohol component in the binary mixtures methanol—water, ethanol—water and 1-propanol—water were calculated. The coefficients found depend on the nature of the alcohol component, its molar fraction and temperature. From the activity coefficients measured and their variation with temperature, excess partial molar thermodynamic functions of mixing were calculated. The activity coefficients found are compared with those at infinite dilution, calculated by an empirical equation.

Simple determination of experimental isotherms using diffusion denuder tubes

If the internal wall of the diffusion column used in the reversed-flow gas chromatography (RFGC) technique is covered with an adsorbent material or a supported liquid, then isotherm determination becomes much simpler. Diffusion and resistance to mass transfer are not neglected, the sorption effects is non-existent and the pressure gradients are negligible along the bed. The method leads directly to independent experimental isotherms over a wide range of concentrations, without specifying a priori an isotherm equation. The isotherm can also be determined in the presence of a surface reaction of the adsorbate. The necessary theoretical analysis has been developed, and the equations derived were applied to benzene and cyclohexane adsorbed on graphitized carbon black, to ethene, ethyne, propene and sulfur dioxide on marble powder, and to propene on silica gel.

Deposition parameters of air pollutants on solid surfaces, measured in the presence of surface and gaseous reactions, with a simultaneous determination of the experimental isotherms

Abstract A method is described for measuring deposition velocities and reaction probabilities of air pollutants with solid surfaces, in the presence of a chemical reaction between two pollutants in the gas phase above the solid. The theoretical analysis is based on experimental adsorption isotherms measured simultaneously through the local adsorption parameters, together with the rate constants for desorption, first-order surface reaction and gaseous chemical reactions. The experimental procedure is that of the reversed-flow gas chromatography (RF-GC) technique, the analysis of the diffusion bands being effected by a personal computer programme. The methodology has been applied to the action of gaseous hydrocarbons on two metal oxides, and of dimethyl sulfide on Penteli marble particles and on pieces obtained from three archaeological statues. All physicochemical parameters mentioned above were measured both in the absence and in the presence of a second gaseous pollutant (NO2). The synergistic effects in the gas phase on the various deposition parameters was fairly obvious.

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.

Denuder tubes used with gas chromatographic instrumentation to measure rate coefficients and equilbrium constans

To provide a scientific basis for conservation of objects of cultural heritage from air pollution, the use of denuder tubes in conjunction with gas chromatographic instrumentation is described. This leads to the determination of deposition velocities, reaction probabilities, adsorption isotherms and other physico-chemical parameters for the action of air pollutants on solid surfaces. Cylindrical, annular, parallel plate and wet effluent denuders are theoretically described, in addition to simple cylindrical without air flowing through them. The latter are combined with the reversed-flow gas chromatographic technique.

Measurement of diffusion coefficients in multicomponent gas mixtures by the reversed-flow technique

SummaryThe reversed-flow method for measurement of gas diffusion coefficients in binary mixtures is now extended to simultaneous determination of effective diffusion coefficients for each substance in a multicomponent gas mixture. The method is applied to six ternary mixtures, each consisting of two gaseous hydrocarbons and H2, He or N2. The results are in agreement with a limiting case of the Stefan-Maxwell equations.

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.

Temperature variation of gas diffusion coefficients measured by the reversed-flow sampling technique

Abstract The temperature dependence of mutual diffusion coefficients in the binary gas mixtures C 2 H 6 + He, C 2 H 4 + He, C 3 H 6 + He, C 2 H 6 + N 2 , C 2 H 4 + N 2 and C 3 H 6 + N 2 has been studied experimentally, using the reversed-flow gas chromatography sampling technique reported earlier. An improved sampling procedure was developed theoretically and applied, giving reversal-peaks twice the height of those obtained previously, thus increasing the sensitivity and precision of the method. The 43 diffusion coefficients determined for the above six binary gas mixtures at various temperatures show an average difference of 4.4 % from those calculated using the Fuller-Schettler-Giddings equation. The mean exponents in T n , giving the temperature dependences of gas diffusion coefficients, were in accord with results given in the literature.