Dependence Of Activation Energy Research Articles

Effective activation energies of chemical reactions and viscous flow of liquids depending on pressure in tribosystems with boundary and mixed friction

This work presents a kinetic analysis of the probable chemical reactions in a frictional contact zone depending on their energetics, temperature, and pressure, as well as the impact of these factors on the viscous flow of liquids, which can be used as an oily base in tribosystems with mixed boundary friction. The dependences of the effective activation energy and viscous flow of liquids on pressure have been analyzed. It has generally been shown that the effective activation energy of thermally activated chemical reactions and viscous flow depends on the activation volume and consists of standard activation energy under standard conditions and additional components connected with the influence of pressure on these processes.

Investigation of diffusion and cure kinetic in nanoclay-reinforced unsaturated polyester resin

The cure kinetics parameters of unsaturated polyester (UP) resin and unsaturated polyester resin containing 3wt% Cloisite 10A (UP/10A) have been evaluated by three different kinetic approaches using non-isothermal differential scanning calorimetry (DSC) data. In the first method, dependence of activation energy, pre-exponential factor and reaction model on the degree of conversion has been calculated using an Advanced Integral Isoconversional method. In the second method, kinetic parameters of Kamal model and Chern diffusion factor have been computed using experimental data. In the third method, the Sbirrazzuoli diffusion factor kinetic parameters have been evaluated using the activation energy dependence on the degree of conversion. The results show that the addition of Cloisite 10A to UP resin causes the diffusion occurs earlier. According to the results, by adding nanoclay, activation energy and pre-exponential factor of chemical reaction and diffusion are decreased.

Product selectivity in plasmonic photocatalysis for carbon dioxide hydrogenation

Photocatalysis has not found widespread industrial adoption, in spite of decades of active research, because the challenges associated with catalyst illumination and turnover outweigh the touted advantages of replacing heat with light. A demonstration that light can control product selectivity in complex chemical reactions could prove to be transformative. Here, we show how the recently demonstrated plasmonic behaviour of rhodium nanoparticles profoundly improves their already excellent catalytic properties by simultaneously reducing the activation energy and selectively producing a desired but kinetically unfavourable product for the important carbon dioxide hydrogenation reaction. Methane is almost exclusively produced when rhodium nanoparticles are mildly illuminated as hot electrons are injected into the anti-bonding orbital of a critical intermediate, while carbon monoxide and methane are equally produced without illumination. The reduced activation energy and super-linear dependence on light intensity cause the unheated photocatalytic methane production rate to exceed the thermocatalytic rate at 350 °C.

Kinetic analysis of non-isothermal dehydration of poly(acrylic acid)-g-gelatin hydrogel using distributed activation energy model

The applicability of distributed activation energy model (DAEM) for describing non-isothermal dehydration kinetics of poly(acrylic acid)-g-gelatin (PAG) hydrogel has been examined. Thermogravimetric curves were recorded at different heating rates from 5 to 20 K min−1. The activation energy dependence on dehydration degree was determined by Kissinger–Akahira–Sunose and Vyazovkin’s methods. The possibility of applying thermodynamically inhibited nucleation and growth reaction model on investigating process has been examined. Miura–Maki method was used to establish the shape of activation energy distribution function and the dependence of pre-exponential factor on dehydration degree. Non-isothermal dehydration kinetics of PAG hydrogel can be successfully described by the DAEM. Here we defined the procedure by which DAEM can be successfully connected with kinetic models commonly used for reactions in solid state. The dependences of specific rate (i.e., rate constant) and reaction model function on dehydration degree were established for all of the investigated heating rates. It has been concluded that the specific rate changes with dehydration degree and that this is the one of the reasons why non-isothermal dehydration kinetics of PAG hydrogel is complex.

Diffusion in the aged aluminium film on iron**Project supported by the Ministry of Science and Education of the Russian Federation (Grant No. 14.607.21.0035, unique identifier RFMEFI60714X0035).

Metallic coatings of many types can be applied to steel to provide outstanding, long-term corrosion protection. A thin Al film is studied at an Fe substrate by the molecular dynamics method at temperatures ranging from 300 K to 1173 K. Al atoms are found to penetrate the Fe matrix at a temperature of 873 K. The potential energy of the system changes step-like at a temperature of 1173 K. At such temperature mean square atomic displacement significantly changes. The behaviors of the Al and Fe diffusion coefficients are mainly determined by the temperature dependence of the diffusion activation energy.

Catalytic Dephosphorylation Using Ceria Nanocrystals

Phosphorus is a crucial element for living systems and plays significant roles in plant growth. The world’s supply of phosphorus today, however, relies on depleting feedstocks such as phosphate rocks, while the demand for phosphorus fertilizers escalates as the population continues to grow. It is thus urgent to develop sustainable sources and production methods for phosphorus. Here, we report on catalytic dephosphorylation for phosphorus recovery from organic and biological molecules. Ceria (CeO2) nanocrystals were synthesized with shape control and applied as artificial phosphatases to cleave the phosphate ester bond in para-nitrophenyl phosphate and release free phosphate anions in aqueous solutions. The dephosphorylation reaction was studied on the CeO2 nanocrystals at various temperatures to evaluate the dependences of rate constant, activation energy, and recyclability on the particle shape. The structure–property relationships established in these studies suggest that the oxygen vacancies on the sur...

Modelling the non-isothermal crystallization of polymers: Application to poly(ethylene 2,5-furandicarboxylate)

Several equations have been compared for the modelling of semi-cristalline polymer crystallization on cooling from the melt and on heating from the glass. The temperature dependence was described with the Hoffman–Lauritzen (HL) equation, where U* and Kg were first determined by fitting the activation energy (Eα) dependency with temperature computed with an advanced isoconversional method (AIC) from DSC data. Several models were used to describe the extent of crystallization dependence, such as Sestak-Berggren and Avrami models. Simulations using Ozawa and AIC methods were performed for comparisons. It was shown that the model-free approach is able to take into account additional crystallization phenomena that are not considered in the classical HL or Avrami equations. Then, a new equation has been proposed to simulate additional phenomena occurring at the end of crystallization on heating from the glass, which are not described by the Hoffman–Lauritzen theory. The experimental finding that application of an isoconversional method to crystallization data following a HL mechanism, should lead to positive decreasing values for crystallization from the glass and to negative increasing values for crystallization from the melt was confirmed by simulations.

ТЕРМИЧЕСКИЙ АНАЛИЗ ДРЕВЕСИНЫ ЛИСТВЕННИЦЫ (LARIX GMELINII (RUPR.) RUPR.)

The thermal decomposition of tree rings (separately, latewood and earlywood) of Larix gmelinii (Rupr.) Rupr.) during the period from 1988 to 1998 was studied using thermogravimetric analysis (TG / DTG) and differential scanning calorimetry (DSC). Thermal analysis was performed in oxidizing (air) condition. The stages of thermal decomposition of wood were analyzed by heating the sample from 30 to700 °C at the heating rates of 10, 20, 40 °C/min for TG / DTG and from 30 to590 °C at the heating rates of 10, 40 °C/min for DSC. The temperature ranges, mass loss, mass loss rate, temperature DTG / DSC peaks, thermal evaporation effects of moisture and the processes of thermal decomposition for each annual layer were studied. The result of thermogravimetry measurements were carried out using the Broido and Ozawa – Flynn – Wall methods. Tree rings of larch wood on the basis of the analysis of the activation energy values of the individual stages of thermal decomposition and activation energy dependency on the degree of conversion of wood material different tree rings; and based on the comparison of the mass loss for corresponding stages of thermal destruction, thermal effects, and other parameters of TG / DSC and DTG were characterized. The material presented in the study would, in our view, to study the impact of growing conditions on wood physical properties and chemical composition of wood, which are the basis for determining the direction of ways to use it, as well as indicators of the reaction of wood plants on endogenous and exogenous conditions.

Temperature dependence of activation energy of endothermic processes and related imperfections of non-isothermal kinetic evaluations

The of observed dependences between activation energy and equilibrium temperature of process is sourced by insufficiency of kinetic equation dα/dt = f(α) Z exp(−E/RT) which ensures no regards to the fact that endothermic (i.e. enantiotropic) processes (like melting) can be realized only if equilibrium temperature Teq (e.g. Tmelt for melting) is reached. The imperfection of non-isothermal kinetic evaluations are revealed recalling the habitual cases of Kissinger and Piloyan methods which are ignoring the impact of the Newton cooling laws known for centuries.

Low temperature photoluminescence properties of CsPbBr3 quantum dots embedded in glasses.

The photoluminescence properties of perovskite CsPbBr3 QDs embedded in glasses were investigated at cryogenic temperature in the range of 40-240 K. CsPbBr3 QDs with radii of 3.3 nm, 4.2 nm and 4.8 nm were precipitated in phosphate glasses using conventional thermal treatment. Photoluminescence (PL) integral intensities, bandgap energies and full with at half maximum of the PL bands of CsPbBr3 QDs showed a strong dependence on temperature. An exciton binding energy of ∼40 meV was derived from the temperature-dependent emission intensity. Optical phonon energy involved in the exciton-phonon interaction was found to be ∼56 meV, about three times as that of the single phonon energy. Exciton-phonon coupling strength and the lattice thermal expansion coefficient were strongly dependent on the size of CsPbBr3 QDs, and as a result, inflection temperature of the PL peak energies of CsPbBr3 QDs increased as the size increased.

Inactivation Kinetics and Structural Changes of High Pressure Treated Actinidin

Plant sulfydryl proteases such as actinidin are mainly used as meat tenderisers, milk clotting agents for the development of novel dairy products and proteolytic agents in biotechnological procedures. The main issue with regards their application is the method for the control of their enzymatic activity after the desired extent of proteolysis, considering that thermal treatment is not applicable. High Pressure (HP) processing could be applied for the enzyme activity regulation. Actinidin, extracted from kiwi fruit (Hayward var., A. chinensis), was purified using anion-exchange and gel filtration chromatography. After purification procedure, only one single peak was finally received for this enzyme corresponding to a protein of molecular mass of 24 kDa with an isoelectric point equal to or lower than 3.5. The effect of HP (500–900MPa and 50- 70°C) on the remaining activity of purified actinidin in phosphate buffer solution (pH 6.0) was studied. Inactivation of purified actinidin was described by a first-order kinetic model both for thermal and HP treatment at the studied processing conditions. Aiming at an optimal process design, a composite mathematical model, which describes the actinidin inactivation rate as a function of pressure and temperature conditions, taking into account the dependence of both activation energy and activation volume on the applied pressure and temperature respectively, was used. HP-induced conformational changes of the purified actinidin were also investigated using Circular Dichroism spectroscopy (CD). A direct comparison of the CD results for the treated and the untreated enzymes reveals that reversible changes were depicted in the far- and near-UV. It is suggested that the exposure to HP may affect the linkage between thiolate ion of cysteine group and positively charged nitrogen of the imidazole group of the active site of actinidin leading to the reduction of the enzyme activity.

Study of the transport of alkali metal ions in a nonaqueous polymer electrolyte based on Nafion

Single-ion conducting polymer electrolytes have received great attention due to their potential applications in electrochemical energy sources. In this work we have in detail studied features of ion transport of alkali metal ions in the polymer electrolytes based on Nafion plasticized with DMSO. The samples have been characterized by DSC, FTIR, and impedance spectroscopy. Based on the obtained results the speculations about the reasons for the presence of extremes in dependencies of both ion conductivity and activation energy on cation radius have been made. It is also suggested that solvate Li+(DMSO)4 is the minimum transport unit in Nafion-Li+ swollen in DMSO.

The Glass Transition Temperature and Temperature Dependence of Activation Energy of Viscous Flow of Ovalbumin

Abstract The paper presents the results of viscosity determinations on aqueous solutions of ovalbumin at a wide range of concentrations and at temperatures ranging from 5°C to 55°C. On the basis of these measurements and three models of viscosity for glass-forming liquids: Avramov’s model, free-volume model and power-law model, the activation energy of viscous flow for solutions and ovalbumin molecules, at different temperatures, was calculated. The obtained results show that activation energy monotonically decreases with increasing temperature both for solutions and ovalbumin molecules. The influence of the energy of translational heat motion, protein-protein and protein-solvent interactions, flexibility and hydrodynamic radius of ovalbumin on the rate of decrease in activation energy with temperature has been discussed. One of the parameters in the Avramov’s equation is the glass transition temperature Tg. It turns out that the Tg of ovalbumin solutions increases with increasing concentration. To obtain the glass transition temperature of the dry ovalbumin, a modified Gordon-Taylor equation is used. Thus determined the glass transition temperature for dry ovalbumin is equal to (231.8 ± 6.1) K.

Non-isothermal kinetic study of bituminous coal and lignite conversion in air and in argon/air mixtures

The exact characteristics of the pyrolysis and combustion kinetics for Siberian bituminous coals and lignites were studied for better understanding of the fuel conversion processes. The bituminous coal samples of Kuznetskiy deposit and lignite samples of Kansko-Achinsk deposit were investigated in argon and in argon/air gas mixtures. The pyrolysis and oxidation experiments were executed at four heating rates (5, 10, 20 and 30°C/min) under TG/DSC analyses coupled with mass-spectrometry. The activation energy for bituminous coal and lignite samples was decreased with conversion degree during oxidation. The combustion products of highest oxidation degree (CO2 and H2O) were dominated in gases released in oxidizing atmosphere. Contrariwise, activation energy was increased during pyrolysis with high content of CO, CO2, H2O, CH4, and H2 in released gases.The two isoconversional models were applied to determine the activation energy dependence on fuel samples conversion: Starink model and Ozawa iterative procedure. The mean arithmetic values of the resulted activation energy were 60kJ/mole and 400kJ/mole for oxidation and pyrolysis processes, respectively. These values are in good agreement with the results, presented previously for the other coals. The Starink model showed higher accuracy and lower activation energy values. The heating rate by non-isothermal oxidation and pyrolysis had the significant influence on the reaction rate because of evolution processes of the reactive surface and pore structure of the coal samples.

Phase behaviour of dehydrated phosphatidylcholines

Dehydrated DLPC, DMPC, DPPC and DSPC have been characterised at temperatures below the diacyl carbon chain-melting transition (Tm), using DSC. For the first time, the existence of pre-Tm transition processes, which are, usually, only observed in the colloidal/liposomal state of saturated phospholipids, has been detected for the dehydrated phosphatidylcholines. Temperature-modulated differential scanning calorimetry was used to characterise the several complexes, overlapping pre-Tm transition processes. Kinetic studies of the chain-melting (Tm) transition show the activation energy dependence on α (conversion rate), i.e. activation energy decreases as the transition progresses, pointing to the importance of initial cooperative (intra- and inter-molecular) mobility. Furthermore, the activation energy increases with increase in diacyl chain length of the phosphatidylcholines which supports the finding that greater molecular interactions of the polymer chain and its head groups in the dehydrated solid state lead to enhanced stability of dehydrated phosphatidylcholines.

Deformed transition-state theory: Deviation from Arrhenius behavior and application to bimolecular hydrogen transfer reaction rates in the tunneling regime.

A formulation is presented for the application of tools from quantum chemistry and transition-state theory to phenomenologically cover cases where reaction rates deviate from Arrhenius law at low temperatures. A parameter d is introduced to describe the deviation for the systems from reaching the thermodynamic limit and is identified as the linearizing coefficient in the dependence of the inverse activation energy with inverse temperature. Its physical meaning is given and when deviation can be ascribed to quantum mechanical tunneling its value is calculated explicitly. Here, a new derivation is given of the previously established relationship of the parameter d with features of the barrier in the potential energy surface. The proposed variant of transition state theory permits comparison with experiments and tests against alternative formulations. Prescriptions are provided and implemented to three hydrogen transfer reactions: CH4 + OH → CH3 + H2 O, CH3 Cl + OH → CH2 Cl + H2 O and H2 + CN → H + HCN, widely investigated both experimentally and theoretically. © 2016 Wiley Periodicals, Inc.

Flux-flow and vortex-glass phase in iron pnictide single crystals with K

We analysed the flux-flow region of isofield magnetoresistivity data obtained on three crystals of NixAs2 with Tc ∼ 20 K for three different geometries relative to the angle formed between the applied magnetic field and the c-axis of the crystals. The field dependent activation energy, U0, was obtained from the thermal assisted flux-flow (TAFF) and modified vortex-glass models, which were compared with the values of U0 obtained from flux-creep available in the literature. We observed that the U0 obtained from the TAFF model show deviations among the different crystals, while the correspondent glass lines obtained from the vortex-glass model are virtually coincident. It is shown that the data is well explained by the modified vortex-glass model, allowing extract of values of Tg, the glass transition temperature, and , a temperature which scales with the mean field critical temperature . The resulting glass lines obey the anisotropic Ginzburg–Landau theory and are well fitted by a theory developed in the literature by considering the effect of disorder.

Nonlinear and complex cure kinetics of ultra-thin glass fiber epoxy prepreg with highly-loaded silica bead under isothermal and dynamic-heating conditions

In the advent of the miniaturized mobile devices, the packaging technology is required utmost high performance of the thin composite laminates in such materials properties as the coefficient of thermal expansion (CTE) and stiffness. Accordingly, the composition of a thermosetting resin becomes extremely complicated often including multiple fillers, monomers and/or catalysts in thermoset-based glass fiber prepregs. The prepreg systems is so complicated that it is usually difficult to obtain a reliable kinetic description and methodology that could be used for the complex thermal cycles including both isothermal and dynamic-heating segments in a facile manner. In this investigation, we propose an isoconversional kinetic using an ultra-thin glass fiber epoxy prepreg with highly loaded silica filler (the ultra-thin glass fiber/silica bead epoxy prepreg) as a model system. The activation energy was determined as a function of the conversion of curing reactions, which was fitted to linear models. The kinetic prediction using the linear models showed an excellent agreement to isothermal experiments. The master curve of a conversion-dependent function which derived from activation energy dependency used to investigate the complex reactions of the ultra-thin glass fiber/silica bead epoxy prepreg.

Low Temperature Broad Band Dielectric Spectroscopy of Multiferroic Bi6Fe2Ti3O18 Ceramics

AbstractIn the present research the tool of broadband dielectric spectroscopy was utilized to characterize dielectric behavior of Bi6Fe2Ti3O18(BFTO) Aurivillius-type multiferroic ceramics. Dielectric response of BFTO ceramics was studied in the frequency domain (Δν=0.1Hz – 10MHz) within the temperature range ΔT=-100°C – 200°C. The Kramers-Kronig data validation test was employed to validate the impedance data measurements and it was found that the measured impedance data exhibited good quality justifying further analysis. The residuals were found to be less than 1%, whereas the “chi-square” parameter was within the range χ2~10−7−10−5. Experimental data were analyzed using the circle fit of simple impedance arc plotted in the complex Z”-Z’ plane (Nyquist plot). The total ac conductivity of the grain boundaries was thus revealed and the activation energy of ac conductivity for the grain boundaries was calculated. It was found that activation energy of ac conductivity of grain boundaries changes from EA=0.20eV to EA=0.55eV while temperature rises from T=-100°C up to T=200°C. On the base of maxima of the impedance semicircles (ωmτm=1) the relaxation phenomena were characterized in terms of the temperature dependence of relaxation times and relevant activation energy was calculated (EA=0.55eV).

Thermal decomposition study of chloride-containing complex ammonium nitrate-based fertilizers by thermogravimetry and differential scanning calorimetry

Thermogravimetry and differential scanning calorimetry were used to study the thermal properties of chloride-containing complex ammonium nitrate-based fertilizers produced with various degrees of phosphoric acid ammonization on the basis of ammonium nitrate. Isoconversion integral methods were used to calculate the dependence of the activation energy on the degree of exothermic decomposition. It was shown that the exothermic decomposition process occurs in several stages. A conclusion was made on the basis of the results obtained that the thermal decomposition of these types of fertilizers is affected by the degree of ammonization and by the component ratio.