Françoise Rouquerol

Is the BET equation applicable to microporous adsorbents

This chapter focuses on how the BET equation is applicable to microporous adsorbents. The BET method can be considered, essentially, as a mathematical means to analyze the adsorption isotherm to derive a “monolayer capacity” and then a surface area. The BET method should not be applied to adsorbents containing micropores in every case. Beyond the “linearity criterion” of the BET plot, two other criteria are found necessary, especially in the presence of micropores, to draw the specific advantage of the BET equation. Calorimetric data for adsorption on microporous adsorbents confirm the fact that the BET monolayer content mostly corresponds to the adsorbate in energetical interaction with the surface. For adsorbents containing micropores, the concept of “BET monolayer content” is misleading and could well be replaced by that of “BET strong retention capacity.” This concept includes the adsorbate present in the micropores together with the content of the statistical monolayer on the non-microporous portion of the surface.

Calorimetric determination of surface areas: Possibilities of a modified Harkins and Jura procedure

Abstract The immersion calorimetric method of Harkins and Jura for determining specific surface areas of powders was reexamined in the light of results obtained with 11 powders of different chemical nature and having specific surface areas ranging from 0.6 to 129 m2 g−1. Microcalorimetric determinations were performed, using water (and, to a smaller extent, butanol, pentanol, and n-decane) as the adsorbate and as the immersion liquid. Less than two water layers appears to be enough to “screen” completely the solid surface. This leads to a modified procedure which avoids the previous shortcomings arising from interparticulate condensation and also from the thickness of the preadsorbed layer. The experimental reproducibility is of ±0.1 m2 g−1 ± 1% (of the measured value).

Two-dimensional freezing of nitrogen or argon on differently graphitized carbons

Abstract On a set of differently graphitized carbon blacks (Sterling heated between 1500 and 2700°C), chosen to reproduce, in some respects, the set used in 1954 by Beebe and Young (J. Phys. Chem. 58, 93) in their famous experiments, adsorption microcalorimetry at 77 K gives experimental evidence of a two-dimensional fluid → solid phase change at the completion of the argon or nitrogen monolayer. That phenomenon (which is confirmed by a slight substep of the isotherms) develops earlier (i.e., on a less graphitized surface) in the case of argon than in the case of nitrogen: This is explained by the epitaxial crystallization of nitrogen which makes it very sensitive to the size of the graphitic sheets.

Possibilities of two non-isothermal procedures (temperature- or rate-controlled) for kinetical studies

The applicability of both conventional Thermal Analysis (TA) and Controlled Rate Thermal Analysis (CRTA) for kinetic analysis is discussed. It is shown that TA method can give a reliable kinetic information and meaningful kinetic parameters especially for solid state transformation. On the other hand the CRTA method is more suitable for decomposition process where one or more gasses are evolved.A consistent and reliable method of kinetic analysis is proposed for both techniques. This method is illustrated to analyze the crystallization process of chalcogenide glass and the decomposition of dolomite.ZusammenfassungEs wird die Anwendbarkeit von herkömmlicher Thermoanalyse (TA) und geschwindigkeitsgesteuerter Thermoanalyse (CRTA) bei kinetischen Untersuchungen diskutiert. Die TA Technik kann eine zuverlässige kinetische Information und sinnvolle kinetische Parameter besonders bei Feststoffumsetzungen liefern. Die CRTA Technik ist andererseits mehr für Zersetzungsprozesse geeignet, bei denen ein oder mehrere Gase freigesetzt werden.Für beide Techniken wird eine einheitliche und geeignete Methode zu kinetischen Analyse vorgeschlagen. Als Beispiel wird diese Methode zur Analyse des Kristallisationsprozesses von Chalkogenidgläsern sowie der Zersetzung von Dolomit angewendet.

Controlled rate evolved gas analysis: recent experimental set-up and typical results

Abstract The equipment described is designed for the study of complex thermal decompositions. The heating of the sample is carried out under conditions of controlled transformation rate thermal analysis (CRTA) to enhance the information obtained from the computerized quadrupole gas analyser (especially, but not only, by improving the separation between steps). It operates between −20 and +1000°C. The thermolysis of hexahydrated nickel nitrate is briefly examined as an example.

THERMAL DECOMPOSITION REACTIONS IN SOLIDS: COMPARISON OF THE CONSTANT DECOMPOSITION RATE THERMAL ANALYSIS WITH THE CONVENTIONAL TG METHOD

Abstract The thermal decomposition of CaCO3 was studied under high vacuum by means of both TG and the more recently developed constant decomposition rate thermal analysis (CRTA) which allows the monitoring of both reaction rate and the residual pressure over the sample. The reliability of the kinetic results seems to be much higher with the latter technique which actually allows the reduction of the reaction rate and therefore the heat and mass transfer effects over a broad range of sample size. For instance, it was necessary, by conventional TG started under a vacuum of 2 10−6 torr with a heating rate of 0.5 K min−1, to lower the amount of sample to 2 mg in order to obtain the same activation energy as that calculated from CRTA with various samples weighing up to 50 mg. The TG experimental conditions quoted above (and which are upper limits of mass and heating rate) are beyond the limit of sensitivity of most available conventional TG equipment.

Study of the thermal decomposition reaction mechanism of alkaline-earth carbonates under high vacuum by both thermogravimetric analysis and constant decomposition rate thermal analysis techniques

Abstract Both thermogravimetric analysis (TG) and constant rate thermal analysis (CRTA) under high vacuum were applied to the study of the mechanism of thermal decomposition of alkaline-earth carbonates. Proper experimental conditions were used in order to avoid heat and mass transfer phenomena. The kinetic analysis of TG and CRTA curves was carried out taking into account the kinetic laws of the mechanisms of solid state reactions most commonly used in the literature. Good agreement between the activation energy and the Arrhenius pre-exponential factor calculated from both techniques is only obtained when considering that the decomposition of alkaline-earth carbonates follows first-order kinetics. A compensation effect was found for the above reactions between the logarithm of the Arrhenius pre-exponential factor and the activation energy. This behaviour was interpreted by a correlation between the stability of the salts and the mobility of the CO2-3 ion present in the activated complex.

Calorimetric evidence for a bidimensional phase change in the monolayer of nitrogen or argon adsorbed on graphite at 77 K

Nitrogen adsorption on a Sterling MT 3100 graphitized carbon black is followed by isothermal microcalorimetry at 77 K. A special feature of the curve of isosteric heats is a distinct peak recorded for a coverage close to 1. Simultaneously, a sub-step is detected on the adsorption isotherm. Both effects are assigned to a bidimensional phase transition of the monolayer, changing from an hypercritical fluid state to a localized state. A similar phenomenon is observed in the case of argon adsorption: here again, a peak on the calorimetric curve and an inflexion on the isotherm are interpreted by a bidimensional fluid → solid transition; nevertheless, in the latter state, argon is not localized at graphite lattice sites.

Thermodynamics of adsorption from solution: Experimental and formal assessment of the enthalpies of displacement

Abstract This paper analyzes the operational ways to derive well-defined enthalpies of displacement from the various types of calorimetric experiments available today.These involve either immersion of a dry adsorbent into a liquid or displacement of the preadsorbed solvent by a solute. The latter experiment may in turn be carried out either in a batch or in a flow-through arrangement. In each case, both dilution and adsorption contributions are separated and the problem of the choice of the reference state is examined. Equations are obtained either for dilute solutions or for mixtures. In the case of dilute solutions, the possible simplications are considered. The enthalpy of displacement derived from the shift of the adsorption isotherms with temperature, by applying the “isosteric” method, is compared with that derived from calorimetric experiments.

Adsorption of anionic surfactants on alumina: Complementarity of the information provided by batch and liquid flow microcalorimetry

Abstract The succession of phenomena taking place during adsorption of surfactants from aqueous solutions may be expected to be better understood through a direct assessment of the energies involved, by means of displacement microcalorimetry. Nevertheless, the complexity of the phenomena and the number of parameters which deserve being controlled, at one stage or another (especially temperature, pH, ionic strength and, of course, concentration in as wide a range as possible, either during adsorption of the surfactant or during its desorption) led us to develop two complementary approaches: “batch” adsorption microcalorimetry, with a standard Tian-Calvet microcalorimeter, and “liquid flow” microcalorimetry with a specially built instrument. The results obtained from each route are discussed in the case of anionic surfactant/alumina systems, with special attention to the adsorption mechanism, desorption and influence of pH.