Luigi Stradella

The chemistry of silica-supported Cr ions: A characterization of the reduced and oxidized forms of chromia/silica catalyst by calorimetry and ultraviolet-visible spectroscopy

CrO3SiO2 samples with low Cr loading (0.5% Cr by weight) have been prepared by impregnation. Formation of the oxidized surface phase, reduction, thermal deactivation and reoxidation have been studied. During the initial thermal treatment of the samples, two processes occur. These are anchorage of CrO3 onto the surface, yielding surface chromates and dichromates, stable toward temperature and second, degradation of nonanchored CrO3 to lower valence states, easily reoxidized at 823 K. Reduction with CO at 623 K yields a 98% population of CrII. Four species of such ions (with different electronic transitions) are present at the surface: two are reactive (CrA and CrB) and two less reactive (Crc1 and Crc2). Thermal deactivation converts CrA into Crc and does not affect CrB. A fraction of Crc (at least some 30%) is unavoidably present at the surface. In the most active samples CrA is about 45% and CrB about 25%. Reoxidation of CrA and CrB occurs with comparable reaction heats (480 and 425 kJ, respectively) via a nonactivated process. Crc oxidation is strongly activated and occurs under O2 pressure.

A study of the thermal decomposition of urea, of related compounds and thiourea using DSC and TG-EGA

The thermal decomposition of urea, of related compounds and of thiourea was investigated by means of TGA and DSC measurements together with evolved gas analysis (EGA), achieved by connecting an FTIR spectrometer to the outlet of the thermobalance. Biuret, ammelide and cyanuric acid were identified as intermediate compounds from their thermograms and IR spectra. An inexplicable exothermic phenomenon was detected during the decomposition of thiourea.

Enthalpies of solvation in cyclohexane and in water for homologous aliphatic ketones and esters

The standard enthalpies of solution at infinite dilution were determined for homologous aliphatic ketones and esters in water and in cyclohexane, using a rotating Calvet calorimeter, and solution concentrations about 5×10−4 mole fraction. Vaporization enthalpies, obtained for each compound with an effusion calorimetric cell, were added to calculate the solvation enthalpies. Their dependence on the number of carbon atoms in the chain is discussed in terms of the Friedman and Krishnan treatment. The effect of polarization of the functional groups is evaluated, and separation from the influence of chain length and the hydrophobic interactions of the methylenes is attempted. For the aqueous solutions, the rearrangement in the structure of the solvent around solute molecules is also considered in relation to deviations from linearity. Comparisons are made with solvation enthalpies obtained for ketones and esters with branched or cyclic substitutes.

Energies of different surface rehydration processes on “eta”, “theta” and “alpha” aluminas

The adsorption of water vapour on completely dehydroxylated samples of η-, θ- and α-alumina has been investigated at 150°C by a microcalorimetric technique.The same rehydration mechanisms have been found on all three aluminas, essentially dissociative and coordinative chemisorption. The hydroxylation process appears to be inhibited on passing from η- to θ- and α-alumina, the coordination of molecular water in different forms being favoured. However decreases in adsorption capacity and energies are evidenced, which may be correlated both with different alumina structures and with the modification of surface reactivity induced by the increasing preparation temperatures.

A DSC, TG, IR study of the thermal decomposition of some alkyl- and aryl-ureas

The melting, vaporization and thermal decomposition of monoethyl-, 1,3-diethyl-, monophenyl-, 1,3-diphenyl-urea were investigated by means of TGA and DSC measurements together with evolved products analysis by I.R. spectrometry. I.R. spectra allowed to identify isocyanate, amine and trimer among the compounds coming from the thermal treatment. DSC data seem to indicate hydrogen bonds are determinant for the enthalpy of vaporization of these derivatives of urea.

TG AND DTA STUDY OF OXYGEN DEPLETION AND REOXIDATION OF BISMUTH MOLYBDATES (2:1; 2:3)

Oxygen depletion and reoxidation of bismuth molybdates (2∶1; 2∶3) have been studied by means of isothermal thermogravimetry and DTA measurements. From the isothermal curves, Arrhenius energies were obtained between 411 and 683 K. The activation energies for oxygen depletion from Bi2O3·MoO3 were lower than those for Bi2O3·3MoO3. Two kinetically different types of oxygen release were identified for both molybdates. Arrhenius plots were also obtained from reoxidation experiments: Bi2O3·MoO3 was more easily reoxidable than Bi2O3·3MoO3. The substantial closeness of the respective activation energies suggests that depletion and reoxidation follow the same mechanistic steps.Some DTA measurements confirm the existence of at least two types of reoxidation sites for both oxysalts.ZusammenfassungDeoxygenierung und Reoxydation von Wismutmolybdaten—2∶1; 2∶3—wurden mittels isothermer TG und DTA untersucht. Aus den isothermen Kurven wurden im Temperaturbereich von 411 bis 683 K die Arrhenius-Energien ermittelt. Die Deoxygenierung von Bi2O3·MoO3 erfordert eine geringere Aktivierungsenergie als die von Bi2O3·3MoO3. Zwei sich kinetisch unterscheidende Arten der Sauerstoffabspaltung wurden bei beiden Molybdaten festgestellt. Auch die Daten der Reoxydation wurden in Arrhenius-Diagrammen dargestellt: Bi2O3·MoO3 ist leichter als Bi2O3·3MoO3 zu reoxydieren. Die nahe beeinander liegenden Werte der betreffenden Aktivierungsenergien lassen auf gleiche Teilschritte im Mechanismus der Deoxygenierung und Reoxydation schließen. Einige DTA-Messungen bestätigen das Vorliegen von zumindest zwei Typen von Reoxydationszentren bei beiden Oxysalzen.РезюмеМетодом изотермической термогравиметрии и ДТА иеучено уменшенние киспорода и повторное окисление молибдатов висмута состава 2∶1 и 2∶3. Исходя из изотермических кривых получены аррениусовские знергии в интервале 411 и 683 К. Энергии активации уменьшения кислорода для Bi2O3·MoO3 были ниже, чем для Bi2O3·3MoO3. Для обоих молибдатов были идентифицированы два кинетически различных типов высвобождения кислорода. Из опытов по повторному окислению были получены аррениусовские графики. Система Bi2O3·MoO3 была более легко окисляемой по сравнению с Bi2O3·3MoO3. Близость значений соответствующих энергий активации предполагает, что уменьшение кислорода и повторное окисление следуют тем зе самым механистическим стадиам. Некоторые измерения ДТА подтвердили существование, по крайней мере, двух типов участков повторного окисления для обоих оксисолей.

Energetics of the adsorption-desorption cycles of oxygen on doped titanium dioxide

Abstract The adsorption-desorption cycles of dioxygen on doped TiO 2 have been investigated at 305 K by means of a microcalorimetric technique. O 2 chemisorption occurs at low equilibrium pressure, while at higher pressure the process goes through physically adsorbed molecules. Interaction energies ranging from 5 to 40 kJ/mol were detected for the chemisorbed species. The oxygen adsorption in the second and in the successive cycles is reversible.

Calorimetric study of adduct formation at 300 K between H2Os3(CO)10 and Lewis bases

The enthalpies of the reactions H2Os3(CO)10(I)+L=H2Os3(CO)10L(II),L=PR3, P(OR)3, in chloroform were determined by means of microcalorimetry. Satisfactory agreement was found between the basicity parameters and the measured reaction enthalpies. As concerns the kinetics, qualitative correlation of a thermokinetics parameter with the steric factor of the ligand was observed.

Energetics of the adsorption of Cu(II) on activated sludge

Abstract Dried activated sludge (DS) and thermally modified samples (MDS) [1] were used as adsorbents of Cu(II) from water at zero and 0.1 ionic strength. Three mechanisms are essentially implied at the solid-liquid interface: 1. ion exchange of free metals and hydroxo-complexes with surface protons 2. adsorption and hydrolysis at the surface 3. hydrolysis followed by an adsorption reaction The use of activated sludge as adsorbent is also justified by its bacterial nature [2]. Experimental Two kinds of adsorbent were prepared: the samples obtained from a pilot plant were thickened and dried at 100 °C in air, powdered and sieved in the 42–200 mesh range (DS), and heated at 110 °C in air for 24 h (MDS). The BET surface evaluation resulted in 2±0.1 m2/g for DS and 0.5±0.1 m2/g for MDS; this may be interpreted assuming that grain agglomeration takes place. The calometric experiments were carried out using a SETARAM rotating C.R.M.T. calorimeter [3], with a cell we constructed because of the impossibility of any handling from outside after introduction of the sample. The method has already been described in detail [4]. Results and Discussion In Fig. 1 we have reported the enthalpies of displacement, -H12 (J/g), as a function of the specific quantities of Cu(II) adsorbed, n2s (mg/g). Heat changes measured are actually heats of displacement, the water being gradually replaced by the solute [3]. The trend of the enthalpies of displacement in Fig. 1 considering influence of the thermal treatment shows that the MDS exhibit at every surface coverage a higher enthalpy of displacement; this fact is interpreted by supposing that the thermal treatment favours the exhibition at the surface of chemical and polar groups capable of strong interaction with the Cu(II) ions. This fact could be interpreted as due to an increase in the electrical potential of the Stern Gouy diffuse layer [5] with a consequent more difficult diffusion of Cu(II) ions toward the polar surface: analogous results have been obtained on an oxide surface [6]. From the analysis of calometric peaks, relative to DS and MDS samples, we can deduce that the thermokinetic trend is somewhat different, the MDS showing an apparently smaller reaction rate. Starting from the hypothesis that the rate determining step is the diffusion of the solute, we have tried to apply a semiempirical treatment assuming a time dependence of the diffusion equation [8]. In Fig.2 we have reported the function: F(t) = log P + 3 2 log t where P ≅KW is the instantaneous deviation of the calorimeter vs. the reciprocal of time. The linearity of the curves in Fig. 2 is satisfactory enough both for DS and MDS samples. The MDS samples show a greater slope, that means a smaller diffusion coefficient; that would correspond to a more dense Gouy layer; in other words, the MDS sample that exhibits a greater number of polar and ionic groups would have a higher repulsive potential field in the diffuse layer. We may conclude that kinetic [1] and thermokinetic studies are in agreement and show that the initial uptake rate is smaller on MDS than DS; but the limiting adsorption quantities are higher for the former. This situation could be interpreted by assuming that the determining step is the diffusion of the ions through the double layer which should be characterized by a higher electrical potential in the case of MDS.

Calorimetry study of the exchange equilibrium between ligands or metal-ligands and an anion-exchange resin

Abstract The exchange equilibrium between the arylsulphonate ion, free or bound to a metal, and the chloride ion of an anion-exchange resin has been studied by means of a spectrophotometric and calorimetric method. The correction terms due to the dilution of the anions and to the swelling effect of the resin have been evaluated. On the basis of these calorimetric data, a thermodynamic parameter may be obtained that is related to the selectivity coefficient of the anion-exchange resin.