I. Horváth

Kinetics and compensation effect in kaolinite dehydroxylation

Activation parameters (E, A, ΔH≠, and ΔS≠) and the effect of various factors involved in kaolinite dehydroxylation reaction have been studied using isothermal TG measurements. Thermal decomposition of kaolinite is found to be diffusion controlled within 0 < α < 0.65–0.70. The rate of the water release in dehydroxylation has obeyed the expression for diffusion in a spherical particle and/or the rate equation of the second order reaction, depending upon the kaolinite nature. The values of activation energy (140–300 kJ/mol) increased with increasing of partial water pressure in the reaction environment and increasing of structural disorder (natural, or induced by grinding). n nActivation parameters have shown a compensation behavior, following the equations; {E} = 51.1 + 12.961log {A} and {ΔH≠}= 222.0 + 0.680{ΔS≠} Isokinetic temperature β = (679 ± 2)K derived from these relations is in good agreement with the temperature range of the isothermal measurements performed.

Study of copper-chromium oxide catalyst: I. Thermal decomposition of copper(III) chromate, CuCrO4

Abstract The kinetics, mechanism, and activation energy of the isothermal decomposition of CuCrO4 was studied using an isothermal TG method and an X-ray high-temperature diffraction technique in either air or a flowing atmosphere of N2. The enthalpy change ΔH of the decomposition reaction 2 CuCrO 4 → CuO + CuO + CuCr 2 O 4 + 3 2 O 2 was determined by DSC analysis. The mechanism of the thermal decomposition of CuCrO4 is well represented by the standard Avrami-Erofeev kinetic equation [− ln (1 − α)] 1 2 = kt . According to this mechanism, the reaction rate is controlled by the formation and growth of nuclei on the surface of the reactant. The activation energy EA of the process in air is EA = (248 ± 8) kJ mole−1, in flowing atmosphere of nitrogen EA = (229 ± 8) kJ mole−1. ΔH in air is 110 kJ mole−1, in flowing nitrogen 67 kJ mole−1. The lower values of ΔH and EA in the flowing atmosphere of nitrogen are due to the fast elimination of O2 from the reaction interface. However, the decay of the crystalline portion of CuCrO4 during its thermal decomposition, studied by the X-ray diffraction, is controlled by a different reaction mechanism (first-order kinetics). The reaction mechanism is discussed in the relation to the crystal structure of the reactants.

Study of copper-chromium oxide catalyst: II. Crystal data and thermal decomposition of basic copper(II) ammonium chromate

Abstract Basic copper(II) ammonium chromate (BCAC) of the composition CuCrO4(Cu(OH)2)x-(NH4OH)y, (NH3)z has been prepared. Depending on the pH of the precipitation reaction, quotients x, y and z vary in the intervals 0 C 2 m , Z is 4 and Dm = (2.98±0.04) Mg m−3). The thermal decomposition of BCAC proceeds in four steps. The activation parameters, Ea, A, ΔH≠ and ΔS≠, of the first step of the thermal decomposition were derived.

Thermal decomposition reactions of nickel(II) complexes under quasi-equilibrium conditions: Part 3. The study of the relations between the thermal, spectral and structural properties of the Werner clathrates [Ni(4-Mepy)4(NCS)2] · 2G (where G represents naphthalene derivatives)

Abstract The relationship between the quasi-equilibrium decomposition temperature TD, the band shift Δṽ(3A2g → 3T1g) in the electronic spectra and the degree of tetragonal distortion ( R e R a ) of the host complex in the [Ni(4-Mepy)4(NCS)2] · 2G clathrates (where G is 1-methyl-, 2-methyl-, 1-bromo-, and 2-bromonaphthalene) was studied. It was found that with increasing Td the positive band shift also increases, whereas the degree of tetragonal distortion of the coordination polyhedra decreases. The reasons for these differences are discussed.

Thermal decomposition reactions of nickel(II) complexes under quasi-equilibrium conditions: Part I. Study of the relationship between thermal parameters and degree of distortion of the coordination polyhedron of the complexes NiL4(NCS)2 (L = piperidine, pyridine or 3-ethylpyridine)

Abstract The relationship between the thermal parameters (quasi-equilibrium decomposition temperature, activation energy and frequency factor) and the degree of the tetragonal distortion of the NiL 4 (NCS) 2 complexes (L = piperidine, pyridine or 3-ethylpyridine) was studied. It was found that with increasing degree of tetragonal distortion of the coordination polyhedron of these complexes, the activation energy E a also increases while the decomposition temperature (determined by quasi-isothermal measurements) decreases. The reasons for these differences are discussed.

Thermal decomposition of CuCrO4

Abstract The kinetics, mechanism and activation energy of isothermal decomposition of CuCrO4. was studied using isothermal TG measurements and X-ray high temperature technique in air and flowing atmosphere. The rate of oxygen release in the decomposition measured by the isothermal TG method obeys Avrami-Erofeev equation with n = 2 and with activation energy values /248±8/ and /229±8/ kJ mol in air and flowing atmosphere of nitrogen, respectively. Decomposition of the crystalline portion of CuCrO4 studied by X-ray diffraction follows the first order kinetics with-actlvation energy E = /262±4/ kJ mol .

Thermal decomposition reactions of nickel(II) complexes under quasi-equilibrium conditions

The stoichiometry of thermal decomposition and thermal (thermodynamic) stability was studied for the Werner clathrates [Ni(4-Mepy)4(NCS)2]·G, whereG = benzene(I), toluene(II) andp-xylene(III). The loss of the volatile components occurs in five steps in compounds I and II and in four steps in the complex III.According to the quasi-equilibrium data the thermodynamic stability of these compounds can be ordered in the following sequence: I<II<III. The increasing host-guest interaction (larger positive band shift in the visible spectra) was accompanied by increasing in the quasi-equilibrium temperature (TD) for the complexes under study.ZusammenfassungAn den Werner-Klathratverbindungen [Ni(4-Mepy)4(NCS)2]G mitG=Benzol (I), Toluol (II) und p-Xylol (III) wurde die Stöchiometrie der thermischen Zersetzung und die thermische (thermodynamische) Stabilität untersucht. Der Verlust der flüchtigen Komponenten erfolgt in fünf Schritten bei den Verbindungen I und II und in vier Schritten bei Komplex III. In Übereinstimmung mit den Quasigleichgewichtsangaben kann die Reihenfolge der thermodynamische Stabilität dieser Verbindungen wie folgt angegeben werden: I II III. Die steigende Wechselwirkung zwischen Wirts- und Gastkomponente (größeres positives Band wird im sichtbaren Bereich verschoben) wird von einem Anwachsen der QuasigleichgewichtstemperaturTD der untersuchten Komplexe begleitet.

Thermogravimetric study of the transition copper valency in the superconductive system Y-Ba-Cu-O

Abstract The transition copper valency in the Y-Ba-Cu-O superconductive system was studied with respect to its dependence on the sample processing, doping, the heating and cooling conditions, and on the atmosphere. Significant differences existed in the oxidation reaction between the reduced phases Y(Ce)Ba2Cu3O5.9 and YBa2Cu3O5.9 in the temperature interval 430–550°C. The presence of Ce3+ accelerated the oxidation reaction.

The acidity of dealuminated clinoptilolite investigated by DSC, DTA and TG methods

Abstract Dealuminated NH 4 + -clinoptilolite samples, prepared using (NH 4 ) 2 SiF 6 and calcined at various temperatures, were used for the investigation of acidity properties. The strength of the acid sites was measured by a combination of TG and DSC measurements. It was found that the dealumination process starts preferentially at the weak acid sites. The intensive removal of aluminium from the strong acid sites started at the highest extent of dealumination.

Acidity of montmorillonite pillared by alumina, zirconia and chromia

Acidic properties of Al-, Zr- and Cr- pillared montmorillonites prepared from Jelšový Potok bentonite (Central Slovakia) have been investigated using temperature-programmed desorption of ammonia. The number of acid sites varied with different interlayer cations from 0.06 to 0.23 mmol g−1. The character of acid sites was similar in the case of Al- and Zr- forms. It is obvious that the chromium analogue contains fewer acid sites, nevertheless, they seem to be stronger compared to Al- and Zr- pillared montmorillonites.