Reversed-flow Inverse Gas Chromatography Research Articles

Determination of Sulfur Dioxide Diffusion Coefficients in Hydrogen, Helium, and Nitrogen by Reversed-Flow Inverse Gas Chromatography

Diffusion coefficients of sulfur dioxide in hydrogen, helium, and nitrogen were determined by reversed-flow gas chromatography. Experiments were performed at temperatures ranging from 323.3 to 573.3 K and reduced at 101.3 kPa. The precision of the method is 96.6%, while its accuracy is 95.5%. Comparison with values predicted by Fuller–Schettler–Giddings and Huang equations indicates small deviations: the Chen–Otmer equation is successful only for the SO2–N2 system. The temperature exponents of SO2 diffusion are as follows: 1.77, 1.71, and 1.75 in H2, He, and N2, respectively. Comparisons with available experimental values for the SO2–H2 and SO2–N2 systems showed small average deviations: 5.7 and 1.3%, respectively.

Determination of Dichlorodifluoromethane’s Diffusion Coefficients in Hydrogen, Helium, Nitrogen, and Air by Reversed-Flow Inverse Gas Chromatography

Diffusion coefficients of dichlorodifluoromethane in hydrogen, helium, nitrogen, and air were determined by means of reversed-flow inverse gas chromatography, at atmospheric pressure and temperatur...

New inverse gas chromatographic methodology for studying mass transfer caused by the evaporation of volatile liquids

Physico-chemical quantities related to evaporation have been simply and directly calculated by a new methodology of reversed-flow inverse gas chromatography. The only demand is a preliminary determination of quantification coefficient(s), converting the signal to solute(s) molar or volume units. Time-dependent, kc(t) and overall mass transfer, kc, coefficients were determined for the evaporation of methanol at temperatures from 305 to 336 K, using two diffusion column lengths (10.0 and 40.5 cm) to evaluate the effect of the mass transfer route. Mass transfer coefficients indicate the endothermic nature of evaporation, while the longer diffusion route resulted in lower kc values. The precision in the determination of evaporation rates was high (98.3%). Time-resolved diffusion coefficients, Dg(t), were also estimated. The smaller diffusion route resulted in lower values due to the higher enrichment of the mobile phase in the solute vapors. In the case of the smaller diffusion route, the change of Dg(t) values increased from 0.1% to 0.8%, while in the longer it was less than 0.1%. The respective temperature exponent increased with the diffusion route length from 1.68 to 1.74. Removing the effect caused by the different lengths of diffusion routes, the activation energy corresponding to pure evaporation was equal to 35.2 kJ mol−1, rather close to the literature values for methanol’s vaporization enthalpy, indicating the reliability of the new methodology. Finally, the effect of mass transfer route was estimated to be 229 MJ mol−1 km−1, revealing the huge amounts of heat transfer accompanying evaporation at high altitudes.

Optimization of a microcontroller for the simultaneous logging of temperature and reversed-flow inverse gas chromatography measurements

ABSTRACTThe utilization of a microcontroller board, optimized for the simultaneous data logging of chromatographic information, the oven temperature, and remote transfer to a personal computer, is investigated, with the goal to extend the capabilities of a gas chromatograph (Shimadzu GC-14A) which does not include a built-in data logging module. An initial comparison of the chromatographic peaks obtained from solute injections, collected both by a dedicated analog-to-digital expansion card and the microcontroller board, demonstrated the latter’s ability to perform accurate analog signal digitization. Afterward, the evaporation of three organic liquids (methanol, acetone, and n-pentane) was studied by reversed-flow inverse gas chromatography. From the temperature variation of the experimentally determined rates, the respective thermodynamic functions of enthalpy and entropy changes of acetone and methanol evaporation were determined. The small deviations from the theoretical values indicate the reliability of the proposed methodology. More importantly, it was made apparent that by utilizing automatic data logging instead of the carrier gas flow reversal procedure, the monitoring of rapid physicochemical phenomena was performed with better accuracy, while the duration of the experiments was decreased with the reduced consumption of gas chromatographic consumables.

New Approach to the Acidity Characterization of Pristine Zeolite Crystals by Ethylene Using Reversed-Flow Inverse Gas Chromatography (RF-IGC)

The adsorption and diffusion are key parameters for the catalytic conversions of ethylene to hydrocarbons. Using microcrystals of beta zeolites and the reversed-flow inverse gas chromatography technique, a new approach is developed for the catalytic characterization of pristine crystals. On the basis of the monitoring of the dynamic concentration in a time-resolved way, this method allows one to follow the surface coverage and adsorption energies among other physicochemical criteria. Pristine crystals were synthesized to rule out the effect of surface defects. The impact of porosity and acidity was studied by ethylene adsorption over porous, nonporous, and acid materials. The acidity of the mother zeolite (acid and porous) was decreased by retro-exchanges and partial cokage. The modification of the ethylene sorption due to the Bronsted acidity was then highlighted. The local surface coverage was not affected by the presence of porosity, while a second adsorption phase of the ethylene was induced by the mo...

An inverse gas chromatographic methodology for studying gas-liquid mass transfer

A novel methodology of reversed flow inverse gas chromatography (RF-IGC) is presented. It permits the simultaneous determination of mass transfer coefficients across the gas liquid interface as well as the respective solubility parameters and thermodynamic functions of dissolution of gases into liquids. The standard deviation of the experimentally determined parameters is estimated for first time, which combined with the successful comparison of the values of the present parameters with other literature ones ascertain the reliability of the methodology. Another novelty of the present work is that the chromatographic sampling of the physicochemical phenomena is done without performing the usual flow reversals procedure. Vinyl chloride monomer’s (VCM) interaction with various composition liquid foods: orange juice, milk and olive oil was used as model system. The present transfer rates are controlled by the gas film at lower temperatures, but at higher temperatures the resistances in both films tend to become equal. The found liquid diffusivity values express the total mass transfer from the gas phase into the liquid’s bulk and they decrease with rising temperature, as the solubilities of gases in liquids do. Solubility, expressed by Henry’s law constant and the mean values of interfacial thickness are of the same order of magnitude to literature ones. From the thermodynamic point of view, VCM dissolution in all liquids is accompanied by significant heat release and it is a slightly non-spontaneous process, near equilibrium, while the entropy change values are negative.

Interactions of H2on the Isobutane Adsorption over Bifunctional Catalyst PtHBEA Revealed by Reversed-Flow Inverse Gas Chromatography

The interactions of isobutane (i-C4) over a bifunctional catalyst PtHBEA are investigated at 343 K in both neutral (helium) and reactive (hydrogen) atmospheres. Reverse flow inverse gas chromatography, used in conjunction with numerical computation, allowed the determination of physicochemical quantities such as local monolayer capacities, probability density functions, and diffusion coefficients in a time-resolved way. The energetic heterogeneity is modified by the dissociative adsorption of hydrogen over the platinum particles. Effective diffusion coefficient of isobutane is mainly affected by the concentration of protonic sites, while the surface diffusion involves more complex phenomena related to the platinum/zeolite interactions and the hydrogen spillover.

CO oxidation on nanosized Au/Al2O3 by surface hydroxyl groups and in the absence of O2, studied by inverse gas chromatography

In catalysis by gold, the term “decomposition” is used in order to describe CO oxidation by surface hydroxyl groups in the absence of O2, at which carbon dioxide is formed without the deposition of carbon. This process is not only important due to its contribution in the activity of CO oxidation but also because it can offer significant information concerning the mechanism of CO oxidation on gold catalysts. In the present work reversed-flow inverse gas chromatography (RF-IGC) is utilized in order to compare the kinetics of CO oxidation by surface hydroxyl groups, the adsorptive capacity and the nature of the active sites related to CO sorption over bare γ-Al2O3 and Au/γ-Al2O3 catalyst, in a wide temperature range (50–300°C).The rates related to CO surface binding exhibit exactly the same behavior, as the activity of CO oxidation by surface hydroxyl groups does over bare γ-alumina and Au/Al2O3 respectively, indicating that at higher temperatures a much stronger binding of CO molecules over γ-Al2O3 can occur. While, the strong bonding of CO over Au/Al2O3 catalyst, is attributed to CO on active sites of cationic Aux+ near the alumina support, in the case of bare γ-Al2O3 is related to CO insertion in surface Al-hydroxyls, which are inactive at lower temperatures.Rise of temperature affects the topography in a different way resulting in homogeneity in the case of γ-Al2O3 and heterogeneity in that of Au/Al2O3. Gold phase enhances the sorption of bigger amounts of CO over Au/Al2O3 catalyst in comparison to bare γ-Al2O3 support, in agreement with the fact that CO is activated preferably at small particles of metallic Au. Although, a certain amount of CO is stored on the studied solids as inactive spectators of CO oxidation e.g. as carbonate like species during CO adsorption particularly over γ-Al2O3, Au containing catalyst exhibits higher adsorptive capacity towards CO. Slight decrease of the amount of surface CO with rising temperature is observed due to the enhancement of CO oxidation by surface hydroxyl groups over both bare alumina and Au/Al2O3.CO oxidation by surface hydroxyl groups indicates a “periphery” mechanism in which CO molecule activated on metallic gold (Au0…CO) is attacked by an hydroxyl group either on a support cation or on a peripheral AuIII ion, forming an intermediate carboxylate group attached to the latter.

Impact of the BEA zeolite morphology on isobutane adsorption followed by Reversed-Flow Inverse Gas Chromatography

The mass transfer phenomena of isobutane (i-C4) were investigated at 343K on three protonic BEA zeolites. Defined by their crystallites sizes and degrees of aggregation, these samples were characterized by Reversed-Flow Inverse Gas Chromatography (RF-GC). This simple technique, used in conjunction with numerical computation, allowed the determination of physicochemical quantities like local monolayer capacities, probability density functions and diffusion coefficients in a time-resolved way. This study enabled to conclude that the effective diffusion coefficient was affected by the size of the zeolite agglomerate whereas the surface diffusion depended on the zeolite crystallite size.

Kinetic Study of Oxygen Adsorption over Nanosized Au/γ-Al2O3 Supported Catalysts under Selective CO Oxidation Conditions

O2 adsorption is a key process for further understanding the mechanism of selective CO oxidation (SCO) on gold catalysts. Rate constants related to the elementary steps of O2 adsorption, desorption and surface bonding, as well as the respective activation energies, over a nanosized Au/γ-Al2O3 catalyst, were determined by Reversed-Flow Inverse Gas Chromatography (RF-IGC). The present study, carried-out in a wide temperature range (50–300 °C), both in excess as well as in the absence of H2, resulted in mechanistic insights and kinetic as well as energetic comparisons, on the sorption processes of SCO reactants. In the absence of H2, the rate of O2 binding, over Au/γ-Al2O3, drastically changes with rising temperature, indicating possible O2 dissociation at elevated temperatures. H2 facilitates stronger O2 bonding at higher temperatures, while low temperature binding remains practically unaffected. The lower energy barriers observed, under H2 rich conditions, can be correlated to O2 dissociation after hydrogenation. Although, H2 enhances both selective CO reactant’s desorption, O2 desorption is more favored than that of CO, in agreement with the well-known mild bonding of SCO reactant’s at lower temperatures. The experimentally observed drastic change in the strength of CO and O2 binding is consistent both with well-known high activity of SCO at ambient temperatures, as well as with the loss of selectivity at higher temperatures.

Elucidation of the Local Character of Chemical Reactivity through the Time-Resolved Chromatographic Analysis of Local Molecular Properties of Gaseous Molecules Adsorbed on Solid Surfaces

The basic objective of this research work is to contribute to the understanding of “how adsorption processes could be used in the determination of Lennard-Jones parameters for gaseous molecules adsorbed on a solid surface, by means of the time-resolved analysis of molecular properties, such as polarizabilities, ionization energies, electron affinities, electronegativities and hardness, which mirror the local character of chemical reactivity”. To this end, the well-known methodology of reversed-flow inverse gas chromatography, which provides us with real experimental values for significant physicochemical quantities illustrating the adsorption–desorption phenomenon step by step, is appropriately associated with quantitative structure–property relationship model and density functional theory. Finally, local molecular properties and Lennard-Jones parameters are determined for nine gas–solid systems at 323.2 K, namely: C2H6(g)/TiO2(s), C2H4(g)/TiO2(s), C2H2(g)/TiO2(s), C2H6(g)/Fe2O3(s), C2H4(g)/Fe2O3(s), C2H2(g)/Fe2O3(s), C2H6(g)/ZnO(s), C2H4(g)/ZnO(s), and C2H2(g)/ZnO(s).

Surface studies by reversed-flow inverse gas chromatography: A review

In the present work, the application of reversed-flow inverse gas chromatography (RF-IGC) to adsorption studies related to heterogeneous catalytic processes on noble metal supported catalysts is reviewed. RF-IGC methodologies are technically simple and they are combined with suitable mathematical treatments which give the possibility for studying the kinetics of catalyzed surface reactions, as well as the topography and the nature of the active sites of solid catalysts. Adsorption parameters, such as rate constants, adsorption energies, adsorption entropies, local isotherms, surface diffusion coefficients, lateral interaction energies and energy distribution functions, are simultaneously determined by RF-IGC. The potential of the novel methodologies has been evaluated by utilising as model systems CO adsorption/oxidation over well-studied Pt–Rh/SiO 2 and nanosized-Au/γ-Al 2O 3 catalysts. The main findings are presented and future goals are also discussed.

Study of the Adsorption of Ozone on the Surface of Ferric Oxide by Reversed-Flow Inverse Gas Chromatography

In this work, in order to describe the impact of ozone on ferric oxide – a common constituent of building materials – in purely scientific terms, a novel methodology that of Reversed Flow–Inverse Gas Chromatography (RF–IGC or RF–GC) is proposed. Five important physicochemical quantities concerning adsorption of ozone on the solid surface of ferric oxide are determined, in five temperatures, in a time-resolved way. By means of a simple PC-program, the values of local adsorption energy, local adsorption isotherm, local monolayer capacity, probability density function for adsorption energy, and energy from lateral interactions are calculated. Thus, the role of surface heterogeneity, lateral interactions and different distributions of active sites (surface topography) on the adsorption of ozone on the surface of ferric oxide is analyzed in the way the RF–IGC method has it developed.

Contribution of Adsorption Phenomena to the Degradation Study of Marble Statues in the Archaeological Museum of Philippi, Greece

A description is provided of the contribution of time-resolved analysis in understanding adsorption phenomena and its contribution to an examination of the overall degradation mechanism of authentic marble pieces brought about by air pollution. As a consequence, a scientific answer has been provided to the question how are materials of cultural heritage affected by air pollution? The use of reversed-flow inverse gas chromatography (RF-IGC) has allowed the determination of important local physicochemical quantities relating to the influence of three light hydrocarbons, either in the presence or absence of sulphur dioxide. A detailed analysis of the results as well as a correlation study is given.

Air pollution effect of SO 2 and/or aliphatic hydrocarbons on marble statues in Archaeological Museums

This study allowed the identification of the main physicochemical characteristics of deterioration of the materials used in the construction of Greek ancient statues in order to plan a correct methodology of restoration. The method of Reversed-Flow Inverse Gas Chromatography is appropriate to investigate the influence of air pollutants on authentic pieces from the Greek Archaeological Museum of Kavala, near Salonica. Six local physicochemical quantities which refer to the influence of one or two pollutants (synergistic effect) were determined for each system. These quantities answer the question “when, why and how materials of cultural heritage are attacked”.

Hydrogenation of 1-butene on nanosized Pd/ZnO catalysts

The reaction concerning the hydrogenation of 1-butene has occupied the researchers conducting research based on the method of reversed-flow inverse gas chromatography (RF-IGC), for extended time periods. This work aims to define and record, with the utmost accuracy, the phenomena and their possible parallel reactions. It was a challenge for the RF-IGC, which was met. The venture consisted of many parts. Answers had to be provided to the following questions: (a) Can RF-IGC deal with issues of catalysis? (b) Can RF-IGC be applied to thin films? (c) Can RF-IGC identify peaks? (d) Can RF-IGC define the gnostic regions of adsorption, desorption, surface diffusion, surface reaction, the existence of more than one reaction? (e) Can it kinetically follow the above? The answer is certainly yes. The effort made is presented in this work and aims to answer all the above questions.

Gas chromatographic investigation of the effects of hydrogen and temperature on the nature of the active sites related to CO adsorption on nanosized Au/γ-Al 2O 3

Nanometer-sized Au particles on oxide supports exhibit extraordinary activity for the selective oxidation of CO (SCO) under conditions compatible with the operation of polymer electrolyte membrane fuel cells (PEM-FCs). In the present work, the novel methodology of reversed flow inverse gas chromatography (RF-IGC) is further extended to the study of the nature of the active sites related to CO adsorption over Au/γ-Al 2O 3 catalyst both in the presence as well as in the absence of hydrogen in a wide temperature range. The main findings are as follows: (i) higher amounts of CO can be bound on the catalyst active sites, at conditions compatible with the operation of PEM-FCs. (ii) At rising temperatures, catalyst adsorptive capacity decreases while the degree of surface heterogeneity increases since new groups of active sites appear, both in the presence and in the absence of hydrogen. (iii) The experimentally observed high-activity of Au/γ-Al 2O 3 for SCO at ambient temperatures can be explained as the consequence of CO weaker bonding over metallic Au actives sites in comparison with stronger CO bonding taking place at active sites located on γ-Al 2O 3 support, which is related to deactivation.

Investigation of the surface heterogeneity of solids from reversed-flow inverse gas chromatography

In the present work, the novel methodology of the inverse gas chromatographic technique of reversed-flow gas chromatography (RF-GC) was applied to the well-studied catalytic oxidation of carbon monoxide over silica supported Pt, Rh and Pt–Rh alloy catalysts. Adsorption energies, local isotherms, local monolayer capacities, surface diffusion coefficients, lateral interaction energies and energy distribution functions are simultaneously determined in a single experiment. The variation of the determined physicochemical parameters against the nature of the studied catalysts (Pt content) is consistent with the observed catalytic activity. The energy distribution functions, estimated by means of RF-GC, give useful information about the “ topography” and the nature of the active sites on the catalyst surface, similar to those of experimental techniques, such as Thermal Desorption Spectroscopy studies of the adsorption of CO on group VIII noble metal surfaces. The experimentally found results explain the superior activity of Pt 0.25 + Rh 0.75 alloy, in comparison to that of the pure Pt and Rh catalysts.

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.