Phase Boundary-controlled Reaction Research Articles

Kinetic Studies of Isothermal Decomposition of Unirradiated and Γ-Irradiated Gallium Acetylacetonate: New Route for Synthesis of Gallium Oxide Nanoparticles

Isothermal decomposition of unirradiated and γ-irradiated gallium acetylacetonate Ga(acac)3 with 103 kGy total γ-ray dose was carried out in static air. The isothermal operating temperatures were 160, 170, 180 and 190°C. The kinetics of decomposition were followed using both model-fitting and model-free approaches. The results of model fitting application on the investigated data showed that the decomposition behaviour was best described by phase-boundary controlled reaction (R2). Kinetic parameters of the decomposition process were calculated and evaluated. Analysis of the data using model free approach signifies the dependency of Ea on extent of conversion (α). Pre-γ-irradiation of gallium acetylacetonate Ga(acac)3 with 103 kGy total γ-ray dose has almost no effect on the kinetic parameters.

Kinetics of Isothermal Ethanol Adsorption onto a Carbon Molecular Sieve under Conventional and Microwave Heating

AbstractThe isothermal kinetics of ethanol adsorption from aqueous solution onto a zeolite type carbon molecular sieve (CMS‐3A) under conventional heating (CH) and microwave heating (MWH) was investigated. The adsorption kinetics is described by the model of a phase‐boundary controlled reaction for both heating modes. The activation energy (Ea) for the adsorption process under MWH is lower than under CH while the preexponential factor (lnA) is higher. Ethanol adsorption is a kinetically complex process whose complexity changes with the mode of heating. The established decrease in Ea and increase in lnA under MWH compared to CH is explained with the increase in the ground vibrational level of the – OH twisting vibrations in the ethanol molecule and with the decrease in its anharmonicity factor which is caused by the selective resonant transfer of energy from CMS‐3A to the OH oscillators.

Effect of thermal dehydration and rehydration on Na-magadiite structure

The effect caused by dehydration and rehydration of the synthetic Na-magadiite was investigated by TGA, XRD, SEM, and 29Si NMR. Thermal analysis of Na-magadiite presented two well-defined loss mass stages between 20 and 150 °C and another between 270 and 310 °C, both related to the removal of interlayer water. The swelling behavior of Na-magadiite was studied by thermal dehydration data obtained at 150 and 300 °C, and respective rehydration by water addition. X-ray patterns showed that the dehydration of Na-magadiite at 150 and 300 °C provoked the basal spacing decrease. The XRD also showed that only the material treated at 150 °C returned to the original structure with the rehydration. 29Si NMR spectra showed that after rehydration, the Q 3/Q 4 relationship presented the same value for Na-magadiite treated at 150 °C. However, this Q 3/Q 4 value decreased when the treatment was done at 300 °C. Kinetic studies of thermal decomposition showed that the dehydration of magadiite is based on a phase boundary-controlled reaction, caused by contracting areas. The exfoliation of lamellas with thermal treatment can explain this behavior, as observed in SEM images.

Metamorphic reactions in dry and aluminous granulites: a Perple_X P–T pseudosection analysis of the influence of effective reaction volume

In high-Mg, Al metapelites, monophase sapphirine corona occur around spinel–corundum aggregates in monomineralic cordierite layers, and bi-phase orthopyroxene–sillimanite aggregates replace locally warped sapphirine in polygonized cordierite aggregates. P–T phase topologies computed (Perple_X software) using compositions of cordierite-rich layers that host the reaction textures did not match the assemblages for the discontinuous reactions spinel + corundum + cordierite → sapphirine and sapphirine + cordierite → orthopyroxene + sillimanite. Instead, the reaction assemblages were reproduced using P–T pseudosection analysis for micro-domain reaction volumes estimated from compositions of product phases in the volume proportion they occur. The results are consistent with known phase relations deduced using Schreinmakers P–T grids. Apparently, the compositions of cordierite-rich layers that hosted the reaction textures were inadequate chemical proxies for determining P–T–X relations of phase-boundary controlled reactions influenced by compositions of the nearest-neighbor minerals in the proportion they react (effective composition), and not in the proportion they existed in the layer/bulk rock. In other words, P–T–X phase topologies and reconstructed P–T paths in dry and aluminous rocks may be best understood by thermodynamic modeling of reactions using effective reaction volume compositions rather than the bulk composition of the rock or the mineralogical layer that host the reaction textures.

Non-oxidative thermal degradation of poly(ethylene oxide): kinetic and thermoanalytical study

Study of the thermal decomposition of well-characterised poly(ethylene oxide) (PEO) under non-oxidative conditions has been conducted by thermogravimetry (TG) in both dynamic and isothermal mode and by thermogravimetry coupled on-line with infrared spectroscopy (TG/FTIR) or mass spectrometry (TG/MS). Kinetic analysis, based on isoconversional methods, yielded value of (apparent) activation energy at the level of 145–180 kJ/mol. Further evaluation of the kinetic model function f( α) by non-regression analysis revealed that phase boundary-controlled reaction is a rate-determining kinetic approach which can have the character either of a surface chemical reaction or of diffusion. Analysis of the evolution of low-molecular weight decomposition products by TG/FTIR and TG/MS techniques showed that main decomposition products are ethyl alcohol, methyl alcohol, alkenes, non-cyclic ethers (ethoxymethane, ethoxyetane and methoxymethane), formaldehyde, acetic aldehyde, ethylene oxide, water, CO and CO 2. Condensed phase FTIR spectra do not show formation of any stable functional groups or structures which is an additional confirmation of the kinetic data that indicate on the dominating role of phase boundary-controlled reactions during the thermal decomposition; volatile low-molecular weight decomposition products that are formed on the surface with the reaction rate proportional to the surface area/diffusion coefficient undergo eventually a rapid vaporization at the temperature higher than 300 °C.

γ-irradiation effects on kinetics and mechanism of the thermal decomposition of zinc acetate

The kinetics of the thermal decomposition of un-irradiated (pristine) and pre-γ-irradiated dehydrated zinc acetate was studied in the temperature range (553–573 K) and in air using both isothermal and dynamic thermogravimetric techniques. The data were analyzed using various solid state reaction models. The integral method using the Coats–Redfern equation was applied in the dynamic data analysis. The results showed that the kinetics of isothermal decomposition of dehydrated zinc acetate was governed by a phase-boundary controlled reaction (R 2) while in non-isothermal (dynamic) decomposition the kinetic was controlled by a nucleation process. The activation energies of the main decomposition process for un-irradiated and pre-γ-irradiated samples were calculated and the results of the isothermal and dynamic integral methods were compared and discussed. The changes in texture and crystal structure of the investigated zinc acetate by γ-irradiation were studied using electron microscopy and X-ray diffraction techniques.

Synthesis, spectral and thermal studies of monomeric, homobimetallic and polymeric complexes containing benzoyldithiocarbazate

New mixed ligand complexes of benzoyldithiocarbazate (H2BDT) have been synthesized and characterized by elemental analyses, spectral studies (i.r., u.v.–vis., mass), thermal analysis and electrical conductivity measurements. The complexes have the general formulae: [M2(BDT)(OX)2] · xH2O; [Co2(BDT)(OX)2(H2O)4]; [M′(HBDT)(OX)-(H2O)], [Ni(BDT)(py)2] n and [Ni(BDT)(L)] n where M = MnII, NiII and CuII; BDT = dithiocarbazate dianion; OX = 8-hydroxyquinolinate; x = 1 or 2; M′ = ZnII or CdII; HBDT = dithiocarbazate anion and L = 2,2′-bipyridyl or 1,10-o-phenanthroline. For the [M2(BDT)(OX)2] · xH2O, [Co2(BDT)(OX)2(H2O)4], [Ni(BDT)(py)2] n and [Ni(BDT)(L)] n complexes, benzoyldithiocarbazate acts as a dibasic-tetradentate ligand in the enol form via the enolic oxygen, the hydrazide nitrogens and the thiolate sulphur, while it acts as a monobasic-tridentate ligand in the keto form in the [M′(HBDT)(OX)(H2O)] complexes. The thermal behaviour of the complexes has been studied by t.g.–d.t.g. techniques. Kinetic parameters of the thermal decomposition process have been computed by Coats–Redfern and Horowitz–Metzger methods. It is obvious that the thermal decomposition in the complexes occurs directly at the metal–ligand bonds except for the ZnII and CdII complexes in which decomposition seems to be at a point in the benzoyldithiocarbazate moiety. From the calculated kinetic data it can be concluded that the dehydration processes in all complexes have been described as phase-boundary controlled reactions. The activation energy values reveal that the thermal stabilities of the homobimetallic complexes lie in the order: MnII < NiII < CoII, while the monomeric CdII complex has more enhanced thermal stability than the ZnII complex.

Kinetic study of the thermal dehydration of copper(II) acetate monohydrate: II. Crushed crystals

Kinetic analysis of the thermal dehydration of crushed crystals of Cu(CH 3COO) 2· H 2O was made by use of TG and DSC curves recorded under comparable experimental conditions. Isothermal TG was also employed, and the result was compared with that obtained nonisothermally. The isothermal reaction was described by a phase-boundary controlled reaction (R n ) law, 1 −(1-α) 1 n = kt, with 1 < n < 2. As for the nonisothermal reaction, there was a tendency for the appropriate kinetic law determined from TG to change from an Avrami-Erofeyev ( A m ) law, [−ln(1-α)] 1 m = kt, to one of diffusion-controlled reaction (D L laws as the reaction advanced, particularly for a larger particle-size fraction of the sample. According to DSC, the nonisothermal kinetics were described exclusively by an A m law. The difference in kinetics determined from TG and DSC was explained by the nature of the two methods.

Kinetic study of the thermal dehydration of copper (II) acetate monohydrate I. Single crystal material

Kinetic analysis of the thermogravimetric curves obtained for the thermal dehydration of single crystalline Cu(CH 3COO) 2·H 2O revealed that the isothermal dehydration is regulated, as a whole, by a phase-boundary controlled reaction law, R n = kt, with 2< n<3. During the early stage of the reaction, surface nucleation of the solid product plays an important role. The non-isothermal dehydration followed a diffusion-controlled reaction law, D 4= kt, particularly at higher heating rates and at later stages of the reaction. Kinetic compensation behaviors were established among the Arrhenius parameters obtained under different measuring conditions, in different ranges of α, and in terms of different kinetic laws. Polarizing microscopic observation of internal surface of partially-dehydrated single crystals showed that the dehydration proceeds exclusively through advancement of the reaction front. An optically different layer, in the reactant adjacent to the reaction front, was recognized. This is an evidence of a strain zone of appreciable thickness.

The kinetic study of thermal dehydration of oxalic acid dihydrate

The thermal dehydration kinetics of crystalline powders of (COOH)2 · 2 H2O was examined by thermogravimetry both at constant and linearly increasing temperatures. An Avrami-Erofeyev law is found to hold with a possibility that a phase-boundary controlled reaction proceeds simultaneously. This is supported by the particle size effect on the rate constant. The activation energyE for the dehydration is estimated as around 80 kJ mol−1. It is likely that the effects of particle size and temperature onE are compensated by those on the frequency factorA.

On the determination of the activation energy of solid-state reactions from the maximum reaction rate of isothermal runs

The method of kinetic analysis of isothermal traces recently proposed by Dharwadkar et al. is criticized. It is concluded that this method would be a proper one in the case of reactions following the mechanism of Prout and Tompkins or Avrami-Erofeev, but it would lead to misinterpretation when diffusion or phase-boundarycontrolled reactions are involved.

Reactivity and point defects of double oxides with emphasis on simple silicates

Heterogeneous solid-state reactions in quasibinary oxide systems are analyzed. As long as local thermodynamic equilibrium prevails, diffusion processes through the reaction product are rate-controlling. The diffusion coefficients are governed by point-defect concentrations. Point-defect thermodynamics allow calculation of relative point-defect concentrations as a function of the relevant thermodynamic variables, if the disorder type of the crystalline product is known. Disorder types in ternary ionic crystals are introduced. On this basis, several reactions leading to simple silicates of the form A 2 BO 4 are discussed in terms of ion mobilities and Gibbs energies of formation, and the possible reaction mechanisms are analyzed. Finally some remarks are included on the influence of the gas atmosphere on the reaction rate, on powder reactions and on phase boundary-controlled reaction rates.

Initial Oxidation of Pure Iron and the Effect of the Allotropic Transformation

AbstractA volumetric method has been used to study the kinetics of the initial oxidation of oure iron at two pressures of dry oxygen (viz. p O2 = 7·6 and 29 cm mercury) in the range 800–1000° in which the first allotropic transformation point of iron occurs. It was found that the oxidation of both body-centred cubic (b.c.c.) and face-centred cubic (f.c.c.)–iron at p O2 = 7·6 cm and of only b.c.c. at p O2 = 29 cm follows a modified parabolic law; a true parabolic law was found to operate in the oxidation of f.c.c.–iron only at p O2 = 29 cm. An anomaly, which is obtained in the Arrhenius plot at the allotropic change point of iron (∼911°), is attributed to the effect of phase-boundary reactions on the oxidation kinetics. It would appear from the concept of vacancy distribution in iron, that for a modified parabolic law, it is the metal/oxide interfacial reaction in f.c.c.–iron and the oxide/oxygeninterfacial reaction in b.c.c.–iron that governs the phase-boundary controlled reaction rate.