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

Reversed-Flow Gas Chromatography as a Tool for Studying the Interaction between Aroma Compounds and Starch

The versatile technique of reversed-flow gas chromatography was introduced to calculate physicochemical quantities for the interaction between aroma compounds and starch. Adsorption, adsorption/desorption, and surface reaction rate constants as well as surface diffusion coefficients for the vapors of aroma compounds over the different starch surfaces were calculated in the temperature range of 303.15-333.15 K. Enthalpies of adsorption between -45.5 and -109.0 kJ mol^-1 and enthalpies of physicochemical interaction between 6.8 and 47.4 kJ mol^-1 were also calculated for all the systems studied. From the obtained results, it is concluded that the interaction forces between aroma compounds and starch correspond to weak energy bonds such as hydrogen bonds and dipole-dipole interactions. For all the systems studied, except for the system heptanal/potato, physical sorption of aroma compounds on starch granules was indicated according to the calculated activation energies.

Inverse gas chromatography applications: a review

Inverse gas chromatography (IGC) is a versatile, powerful, sensitive and relatively fast technique for characterizing the physicochemical properties of materials. Due to its applicability in determining surface properties of solids in any form such as films, fibres and powders of both crystalline and amorphous structures, IGC became a popular technique for surface characterization, used extensively soon after its development. One of the most appealing features of IGC that led to its popularity among analytical scientists in early years was its similarity in principle to analytical gas chromatography (GC). The main aspect which distinguishes IGC experiments from conventional GC is the role of mobile and stationary phases. Contrary to conventional GC, the material under investigation is placed in the chromatographic column and a known probe vapour is used to provide information on the surface. In this review, information concerning the history, instrumentation and applications is discussed. Examples of the many experiments developed for IGC method are selected and described. Materials that have been analysed include polymers, pharmaceuticals, minerals, surfactants, and nanomaterials. The properties that can be determined using the IGC technique include enthalpy and entropy of sorption, surface energy (dispersive and specific components), work of co/adhesion, miscibility and solubility parameters, surface heterogeneity, glass transition temperature, and specific surface area.

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.

New gas chromatographic instrumentation for studying the action of sulfur dioxide on marbles

Reversed-flow gas chromatography, which is a sub-technique of inverse gas chromatography, is an experimental arrangement simulating a simple model for the action of air pollutants on buildings and monuments, in laboratory scale. By using a commercial gas chromatograph and an appropriate mathematical analysis, kinetic parameters such as rate constants for adsorption k1, adsorption/desorption kR and surface reaction k2, as well as surface diffusion coefficients Dgamma, deposition velocities Vd and reaction probabilities gamma of SO2 on marble surfaces at different temperatures (303.15-353.15 K) in the presence or in the absence of protective materials (an acrylic copolymer, Paraloid B-72 or a siloxane, CTS Silo 111) were calculated. From the above mentioned physicochemical quantities the ability of the examined materials to minimize the dry deposition of SO2 on marble is carrying out and a possible mechanism for the interaction between SO2 and Paraloid B-72 was suggested. Both materials (CTS SILO 111 and Paraloid B-72) are good enough for protecting marble against SO2 at low temperatures (303.15-323.15), while at high temperatures (333.15-353.15), siloxane seems to protect marble better than acrylic copolymer.

Diffusion and Adsorption Measurements in Porous Solids by Inverse Gas Chromatography

The new inverse gas chromatography methodology of reversed-flow gas chromatography has been applied to measure diffusion and adsorption in porous solids. The theoretical analysis leads to equations describing the experimental data with very few approximations. From these, the total overall rate constant of transference (k = k(R) + k(2)), its probable error, the diffusion coefficient (D(1)) into porous solids (alpha-alumina and gamma-alumina), and its probable error are calculated by means of a simple PC program. The methodology was applied to pentane, hexane, and heptane diffusing into porous alpha- and gamma-alumina at various temperatures. A comparison of the results is made with those obtained by the Knudsen formula and with those of other researchers.

Gas chromatographic investigation of the competition between mass transfer and kinetics on a solid catalyst

The reversed-flow gas chromatography (RF-GC) method is used to investigate the competition between mass transfer and kinetics in heterogeneous catalysis. The well-studied dissociative adsorption of carbon monoxide over a silica supported rhodium catalyst at various temperatures is used as model system. The Thiele-type modulus phis, and the effectiveness factor eta are calculated for both adsorbate (CO) and product (CO2), from the experimental chromatographic peaks. The values experimentally found are similar to those predicted theoretically and give interesting information for the mechanism of the interaction of carbon monoxide with the catalyst studied.

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 diffusion coefficients by gas chromatography

Gas chromatography (GC), apart from the qualitative and quantitative analysis of gaseous mixtures, offers many possibilities for physicochemical measurements, among which the most important is the determination of diffusion coefficients of gases in gases and liquids and on solids. The gas chromatographic techniques used for the measurement of diffusion coefficients, namely the methods based on the broadening of the chromatographic elution peaks, and those based on the perturbation of the carrier gas flow-rate, are reviewed from the GC viewpoint, considering their running though the history, the experimental arrangement and procedure, the appropriate mathematical analysis and the main results with brief discussions. The experimental data on diffusion coefficients, determined by the various gas chromatographic techniques, are compared with those quoted in the literature or estimated by the known empirical equations predicting diffusion coefficients. This comparison permits the calculation of the precision and accuracy of the techniques applied to the measurement of diffusion coefficients.

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.

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.

Heterogeneous catalysis on solids of gases diffusing through a liquid layer, studied by inverse gas chromatography

Physicochemical parameters for heterogeneous catalytic reactions when the catalytic bed was under a liquid phase have been determined, using a non-linear adsorption isotherm by the reversed-flow version of inverse gas chromatography (RF-GC). The mathematical analysis developed in heterogeneous catalysis, mass transfer across gas-liquid boundaries, and diffusion coefficients of gases in liquids was associated with a non-linear adsorption isotherm to find the relevant equations pertaining to the problem. These equations were then used to calculate the adsorption/desorption rate constant, the rate constant for the first-order catalytic reaction and the equilibrium constant for the non-linear adsorption isotherm. The diffusion coefficients of the reactant in the liquid and gaseous phases and the partition coefficients for the distribution of the reactant between the gaseous and liquid phase were also determined.

Physicochemical measurements by the reversed-flow version of inverse gas chromatography

Since the first publication on the method, reversed-flow gas chromatography has been used to "separate" physicochemical quantities by measuring the value of one in the presence of another. The experimental arrangement consists of a small modification of a commercial gas chromatograph, so that it includes a four- or six-port gas sampling valve, and a simple cell placed inside the chromatographic oven. This cell suppresses the effects of the carrier gas flow on the physicochemical phenomena taking place in the stationary phase. These phenomena pertain to chemical kinetics, diffusion in gases, liquids and surfaces, mass transfer across gas-liquid and gas-solid boundaries, local adsorption on heterogeneous solid surfaces, etc.

Experimental adsorption isotherms based on inverse gas chromatography

A new chromatographic perturbation method is used for studying the adsorption-desorption equilibrium in various gas-solid heterogeneous systems. It is the reversed-flow method giving accurate and precise values of many physicochemical constants including the basic and necessary adsorption isotherm values. For four inorganic oxides, namely, Cr2O3, Fe2O3, TiO2 and PbO, and two aromatic hydrocarbons (benzene, toluene) these adsorption isotherms have been determined through a non-linear model.

Physicochemical Characterization of Inorganic Pigments in the Presence of Gaseous Pollutants. The Role of Ozone

The Reversed–Flow Gas Chromatography technique was used to study the interaction of volatile hydrocarbons on three inorganic pigments, namely, CdS, ZnS and Cr

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.

Corrosive effects from the deposition of gaseous pollutants on surfaces of cultural and artistic value inside museums

The objectives of the project were to assess the critical relationships between environmental factors and damage of the artifacts and other cultural property exposed inside museums, by studying: (a) the outdoor/indoor pollutant concentration and their transfer inside the museum; (b) the distribution and circulation of pollutants inside the museum influenced by various factors; (c) chemical interactions between pollutants in the gas phase leading to removal and/or formation of secondary pollutants; (d) the final deposition of the indoor pollutants on surfaces of artistic interest and the damage on them, governed by strictly defined physicochemical parameters. All the above information, together with the main factors influencing each stage, were obtained by applying the methodology developed and described in detail here. Measurements of rate constants of reactions in the gas phase, of physicochemical deposition parameters on artefacts, and the synergistic effects of pollutants on the deposition parameters, were conducted. Seven PC programmes for analysing the experimental data were written and used. The pollutants, the solid materials and the museums chosen in this programme are only examples needed to develop the necessary methodology. The numerical results obtained serve the purpose of exemplifying the procedures and not enriching the world's bibliography with useless empirical information. Two commercially available protectives for marble were investigated from the point of view of their reactivity towards SO2 by using a diffusional technique. From measurements of SO2 concentration carried out on three types of marble, the deposition velocities have been calculated. Indoor monitoring of the church of San Luigi dei Francesi and of the Museo della Civiltá Romana in Rome has shown that indoor production of nitrous acid most likely results from heterogeneous reactions indoors, on the walls and the exposed surfaces.