Relative Activation Energies Research Articles

Leaching Kinetics of Praseodymium in Sulfuric Acid of Rare Earth Elements (REE) Slag Concentrated by Pyrometallurgy from Magnetite Ore

A leaching kinetics was conducted for the purpose of recovery of praseodymium in sulfuric acid (<TEX>$H_2SO_4$</TEX>) from REE slag concentrated by the smelting reduction process in an arc furnace as a reactant. The concentration of <TEX>$H_2SO_4$</TEX> was fixed at an excess ratio under the condition of slurry density of 1.500 g slag/L, 0.3 mol <TEX>$H_2SO_4$</TEX>, and the effect of temperatures was investigated under the condition of 30 to <TEX>$80^{\circ}C$</TEX>. As a result, praseodymium oxide (<TEX>$Pr_6O_{11}$</TEX>) existing in the slag was completely converted into praseodymium sulfate (<TEX>$Pr_2(SO_4)_3{\cdot}8H_2O$</TEX>) after the leaching of 5 h. On the basis of the shrinking core model with a shape of sphere, the first leaching reaction was determined by chemical reaction mechanism. Generally, the solubility of pure REEs decreases with the increase of leaching temperatures in sulfuric acid, but REE slag was oppositely increased with increasing temperatures. It occurs because the ash layer included in the slag is affected as a resistance against the leaching. By using the Arrhenius expression, the apparent activation energy of the first chemical reaction was determined to be <TEX>$9.195kJmol^{-1}$</TEX>. In the second stage, the leaching rate is determined by the ash layer diffusion mechanism. The apparent activation energy of the second ash layer diffusion was determined to be <TEX>$19.106kJmol^{-1}$</TEX>. These relative low activation energy values were obtained by the existence of unreacted ash layer in the REE slag.

Catalytic complete oxidation of acetylene and propene over clay versus cordierite honeycomb monoliths without and with chemical vapor deposited cobalt oxide

The catalytic oxidation of acetylene (C2H2) and propene (C3H6), selected as model unsaturated hydrocarbons, was investigated over uncoated- and Co3O4-coated natural clay honeycomb monoliths and their performance was compared with commercial cordierite. Co3O4 was deposited using chemical vapor deposition (CVD) and chosen based on its recently demonstrated high activity. The catalytic behavior of the clay and cordierite-based monolithic samples has been investigated for the determination of the temperature-dependent conversion yield, specific reaction rate and related activation energy. The obtained results revealed a promising intrinsic catalytic performance of the uncoated clay monoliths. Furthermore, a synergetic interaction between the clay and the Co3O4 film has enabled an improved performance relative to the coated cordierite. The present paper reveals a promising potential application of the natural clay as catalyst and/or catalyst-support through the association of the advantages offered by the chemical composition and honeycomb monolithic structure of clay.

Modeling the constitutive relationship of powder metallurgy Al–W alloy at elevated temperature

The deformation behavior of Al–W alloy was researched with isothermal compression tests at various deformation temperatures and strain rates to evaluate the deformation activation energy and to develop the constitutive relationship equation, which is in pursuit of revealing the dependence of the flow stress on the strain, strain rate and deformation temperature. The compression tests were conducted in the temperature range between 420 and 570°C and at strain rates between 0.001 and 5.0s−1. With the help of determination of related material constants (such as A, β and α) and activation energy Q (451.15kJmol−1), the Arrhenius-type constitutive relationship equation of Al–W alloy is developed. It was found that the correlation coefficient R and the AARE is 0.997% and 4.08%, respectively. The results show that the Arrhenius-type model, which considers the combined influence of strain rate and deformation temperature, is able to provide the accurate prediction of high temperature flow stress for the researched alloy.

Use of the Bell–Evans–Polanyi Principle to predict regioselectivity of nucleophilic aromatic photosubstitution reactions

Regioselectivity of nucleophilic aromatic photosubstitution has been shown experimentally to depend upon activation energies of the competing transition states. Computational means of determining relative activation energies were sought, therefore, in order to predict regioselectivity. Optimization of the three triplet transition states of 2-chloro-4-nitroanisole with hydroxide ion gave energies of insufficient accuracy to predict regioselectivity. Computed enthalpy changes from the first triplet transition state to the triplet σ-complexes correlated precisely with the experimental activation energies. This exemplifies the Bell–Evans–Polanyi Principle, and it provides an accurate means of assessing regioselectivity.

A Physicochemical Evaluation of Modified HZSM-5 Catalyst Utilized for Production of Dimethyl Ether From Methanol

A number of HZSM-5 catalysts modified with 5 wt% Mg, Na, Zr, and Al as well as others modified with 5–60 wt% Zn prepared by wet impregnation. These materials were characterized by the NH3-TPD, XRF, and XRD analyses and tested in a slurry reactor to determine their activities in dehydration of methanol solution in kerosene. Reactions were carried out at 230°C and 19 bar for 4 h of residence time in the reactor. Results showed that in the first series, the catalyst modified with Zr and in the second series, the one modified with 10 wt% Zn led to the highest methanol conversion. It was deduced that elimination of strong acid sites and partial replacement of active cations in the HZSM-5 zeolite lattice enhanced the performance and improved resistance against formation of hydrocarbons and other by-products such as olefins all of which acted as coke precursors. In order to determine the activation energy, the rate equation of Bercic and Levec was utilized and assumptions consistent with reaction conditions considered. Hence, rate constants determined empirically through fitting experimental data to the aforementioned rate equation. Ultimately, the related activation energies were calculated. In comparison with common dehydration catalysts such as alumina, the activation energies of the modified catalyst decreased. Nonetheless, the superior performance of the modified HZSM-5 attributed mainly to the elimination of strong surface acid sites by partial substitution of cations resulted in prevention of coke and undesired hydrocarbon formation.

A Simple and Effective Model for Modeling of Convective Drying of Sewage Sludge: The Reaction Engineering Approach (REA)

Sludge drying is an essential step to reduce the cost of waste treatment but design of sewage sludge dryer is still relatively complicated since the drying kinetics is influenced signficantly by shrinkage, cracking and skin formation which occur during drying. In this study, the REA (reaction engineering approach) is implemented for the first time to model the convective drying of sewage sludge, a highly shrinkable material. The equilibrium activation energy (ΔEv,b) is evaluated according to the corresponding drying air humidity and temperature. This is combined with the relative activation energy (ΔEv/ΔEv,b), generated from one accurate drying run to produce the activation energy, which describes the internal behavior of the samples during drying. A good agreement between experimental and predicted data is observed. The REA is shown to perform comparable or even better than the classical diffusion-based model. It can be said that the REA can model the convective drying of sewage sludge well. The REA can be implemented for further applications including design and evaluation of sludge dryer.

Cobalt-Mediated Decarboxylative Homocoupling of Alkynyl Carboxylic Acids

Cobalt-mediated decarboxylative Glaser-like C–C bond coupling of carboxylates has been studied in the gas phase using collision-induced dissociation (CID) multistage mass spectrometry (MSn) experiments. Both the identity of the carboxylate RCO2– (R = Me, HC≡C, MeC≡C, and PhC≡C) and the nuclearity of the complex ([CoCl(O2CR)2]– versus [Co2Cl3(O2CR)2]–) play a role in the types of reactions observed and their relative activation energies. In the first stage of CID, the mononuclear complex [CoCl(O2CMe)2]– undergoes decarboxylation, while the dinuclear [Co2Cl3(O2CMe)2]– undergoes cluster fission to yield [CoCl3]–; all acetylenic carboxylate complexes [CoCl(O2CR)2]– and [Co2Cl3(O2CR)2]– undergo decarboxylation. Isolation of the decarboxylated products followed by a second stage of CID results in a second decarboxylation event for all systems except for [CoCl(Me)(O2CMe)]–, which undergoes bond homolysis. In the final stage of CID, all acetylenic complexes undergo Glaser coupling, forming reduced Co anions. Overall dinuclear cobalt clusters are superior to mononuclear complexes at promoting decarboxylation and reductive coupling. The order of reactivity among the acetylide ligands is PhC≡C &gt; MeC≡C &gt; HC≡C.

Examining the Suitability of the Reaction Engineering Approach (REA) to Modeling Local Evaporation/Condensation Rates of Materials with Various Thicknesses

The reaction engineering approach (REA) is examined here to investigate its suitability as the local evaporation rate to be used in multiphase drying. For this purpose, REA is first implemented to model the convective drying of materials with various thicknesses. The relative activation energy, as the fingerprint of REA, generated from one size of a material is used to model the convective drying of the same material with different thicknesses. Because the results indicate that REA parameters can model the drying of materials with various thicknesses, REA can be scaled down to describe the local evaporation rate (at the microscale as affected by local composition and temperature). The relative activation energy is used to describe the global drying rate in modeling the local evaporation rate. REA is combined with a system of equations of conservation of heat and mass transfer in order to yield the spatial reaction engineering approach (S-REA) as a nonequilibrium multiphase drying model. By using S-REA, the spatial profiles of moisture content, concentration of water vapor, temperature, and local evaporation rate can be generated, which can assist in comprehending the transport phenomena.

Investigation of the Etching of Silicon under Subcritical Water Conditions

Herein, we present a study investigating the reaction conditions to wet etch monocrystalline silicon in the subcritical region of water, specifically from 200 to 300 °C. In the subcritical region there is a departure of thermodynamic, transport, and chemical properties from normal conditions. This has the potential to affect the reaction paths for the dissolution of silicon, by modifying the local concentration of reactants and products, by altering the relative activation energy of different reactions, or by changing the polarity of the water. Furthermore, our methodology uses concentrations of etching agent markedly lower than previous works (our reactions were conducted at [KOH] = 0.03–0.7%/wt). The results of the study suggest that dilute etchant solutions at high temperatures can produce results comparable to those reported at atmospheric conditions using higher concentrations of etchants. Based on the results obtained, the main key factors in the etch rates were crystallographic orientation of the wafer, composition of the etching solution, and temperature.

Modeling of water vapor sorption process by employing the reaction engineering approach (REA)

For dehumidification process, sorption process is commonly applied. Mathematical modeling can be implemented in order to assist in minimization of energy consumption and better process design. In this study, the reaction engineering approach (REA), which has been proven to model various challenging drying process successfully, is developed and implemented for the first time to model completely reverse process of drying. The equilibrium activation energy (ΔEv,b) is evaluated according to the environmental temperature and humidity and combined with the relative activation energy to describe the wetting process. Interestingly, the relative activation energy generated from one accurate drying/desorption run can be used to describe accurately the water vapor sorption/wetting process. The REA implemented as the mass balance is combined with the heat balance incorporating the heat of sorption. A good agreement between the predicted and experimental data is shown. The REA is able to model accurately the wetting process. The applicability of the REA could be because of the flexibility of the relative activation energy to capture the environmental conditions and reflect this in the change of internal behavior, similar to the relative activation energy used for intermittent drying process. This indicates that the relative activation energy can be used as fingerprint of both drying and water vapor sorption process.

Polymer/carbon nanotube composite film formation: A fluorescence study

In this study, the effect of multi‐walled Carbon nanotube (MWNT) on film formation behavior of Polystrene (PS) latex film was investigated by using steady state fluorescence technique. Films were prepared by mixing of pyrene (P)‐labeled PS latex with different amounts of MWNTs varying in the range between 0 and 20 wt%. After drying, MWNT containing films were separately annealed above glass transition temperature (Tg) of PS ranging from 100 to 270°C for 10 min. In order to monitor film formation behavior of PS/MWNT composites, Scattered light (Is) and fluorescence intensities (IP) from P were measured after each annealing step to monitor the stages of film formation. At 0–20 wt% range of MWNT content films, minimum film formation (To), void closure (Tv), and healing, (Th) temperatures were determined. Void closure and interdiffusion stages were modeled and related activation energies were determined. It was observed that while void closure activation energies increased, backbone activation energies decreased as the percent of MWNT is increased in the composite films. POLYM. COMPOS., 35:817–826, 2014. © 2013 Society of Plastics Engineers

Anomalous capacitance response induced by the superconducting gap in an Au/BiFeO3/La1.84Sr0.16CuO4/LaSrAlO4 heterostructure

Film capacitor characteristics of Au/BiFeO3/La1.84Sr0.16CuO4 on LaSrAlO4 (001) substrate were studied in the temperature range 10–300 K under magnetic fields up to 7 T. Apparent capacitance anomalies were observed at the superconducting transition temperature Tc of La1.84Sr0.16CuO4. Furthermore, the magnetic field dependences of the dielectric relaxation related activation energy can be well fitted from 10 K to Tc by the superconducting gap versus magnetic field. These results suggest an alternative technique for detecting the superconductivity related features in superconducting film, and also may be useful for future tunable multifunctional devices.

Comprehensive study of hydrogen sensing phenomena of an electroless plating (EP)-based Pd/AlGaN/GaN heterostructure field-effect transistor (HFET)

A Pd/AlGaN/GaN heterostructure field-effect transistor (HFET) type hydrogen sensor, with sensitization, activation, and electroless plating (EP) deposition approaches, is fabricated and investigated. Based on sensitization and activation processes, the dense and uniform Pd layer exhibits small grain size (38.14±7.24nm) and less surface roughness in gate region. Experimentally, for a 1μm gate-length HFET device, advantages of lower temperature-dependent variation of the maximum drain saturation current (−0.52mA/mmK), maximum extrinsic transconductance (−0.13mS/mmK), and threshold voltage (−1.05mV/K) are obtained over the temperature range from 300 to 600K. For three-terminal-controlled hydrogen gas sensing phenomena, a high current variation of 34.9mA/mm is found in a 1% H2/air gas at 375K. The exothermic hydrogen adsorption reaction with the enthalpy and entropy of −45.3kJmol−1 and −118.2Jmol−1K−1, respectively, are obtained. Furthermore, the response analysis reveals that the initial response rate is significantly increased with the hydrogen concentration and temperature. The related activation energy for hydrogen adsorption is about 11.65kJmol−1.

Synthesis, characterization, and dielectric properties of low loss LaBO3 ceramics

Abstract The preparation, sintering behaviour, and dielectric properties of low loss LaBO 3 ceramics have been investigated. Single-phase LaBO 3 powder was synthesized by the conventional solid-state ceramics route and dense ceramics (relative density >96%) with uniform microstructure (grain size ∼30 μm) were obtained by sintering at 1300 °C in air. The electrical conductivity of LaBO 3 follows the Arrhenius law and the related activation energies for electrical conduction of bulk and grain boundary are 0.62 eV and 0.90 eV, respectively. The LaBO 3 ceramics sintered at 1300 °C exhibit excellent microwave dielectric properties with a relative permittivity, ɛ r ∼ 11.8, a quality factor, Q × f 0 value ∼76,869 GHz (at ∼15 GHz), and a negative temperature coefficient of resonant frequency τ f ∼ −52 ppm/°C.

Zr diffusion coefficient and activation energy calculations based on EDXRF measurement and evaluation of mechanical characteristics of YBa2Cu3O7−x bulk superconducting ceramics diffused with Zr nanoparticles

Abstract This study reports two main mechanisms on the YBa 2 Cu 3 O 7 − x bulk superconductor for the researchers: (I) determination of the diffusion coefficient and activation energy of Zr nanoparticles in the YBa 2 Cu 3 O 7 − x matrix over the annealing temperature range of 500–945 °C by means of the energy dispersive X-ray fluorescence (EDXRF) technique, and (II) change of the mechanical performances belonging to the YBa 2 Cu 3 O 7 − x bulk superconducting materials by Zr inclusions with the aid of Vickers microhardness (H v ) measurements performed at various indentation loads (0.245 N ≤ F ≤ 2.940 N) for the first time. The results obtained from the first part show that the zirconium diffusion coefficient increases dramatically with the enhancement of the diffusion annealing temperature, presenting that at higher temperatures more Zr nanoparticles begin to penetrate into the Y123 matrix. Thus, two different diffusion coefficients (in grains and over grain boundaries) are determined for the Zr individuals in the Y123 materials. The thickness dependences of the Zr diffusion coefficients are defined by the equations D 1 = 1.47 × 10 − 4 exp (− 1.05 eV/k B T) and D 2 = 2.23 × 10 − 3 exp (− 1.03 eV/k B T) in grains and over grain boundaries, respectively. The related activation energies (− 1.05 eV and − 1.03 eV) may be attributed to the relatively slow migration in the former region and rapid migration over the latter region, respectively. In other words, the physical and mechanical properties of the Y123 superconducting samples improve with the Zr additives owing to the elimination of the defects. The second part also indicates that the Zr nanoparticles lead to change in the microhardness characteristic of the Y123 bulk superconductors. Whereas the microhardness parameters such as Vickers microhardness ( H v ), elastic modulus and yield strength of the undiffused sample tend to increase systematically, those of the diffused samples decrease with the enhancement of the applied load, meaning that the former sample shows the reverse indentation size effect ( RISE ) behavior while the latter superconductors exhibit Indentation Size Effect ( ISE ) feature. Additionally, in the second part, the load independent (true) microhardness values are analyzed by Meyer's law, elastic/plastic deformation ( EPD ), proportional sample resistance ( PSR ), indentation-induced cracking ( IIC ) and Hays–Kendall ( HK ) approach. It is found that the three models (Meyer's law, EPD and PSR ) fail to explain the evaluation of the microhardness with the applied load. However, the IIC model is found to be superior to other models for the pure sample exhibiting the RISE behavior, whereas the HK approach is observed to be the most successful model for the description of the mechanical properties of the Zr diffused Y123 bulk superconductors obeying ISE feature. To sum up, it is not wrong to generalize that the IIC model is the best approach for the Y123 superconductor presenting the RISE feature, while the HK approach defines perfectly the mechanical properties of the Y123 materials obtaining the ISE behavior.

Why Are Some Reactions Slower at Higher Temperatures?

It is well understood by most chemistry students at advanced undergraduate levels that chemical reactions generally follow the Arrhenius law of temperature dependence with positive activation energies, proceeding faster at elevated temperatures. It is much less widely known that the rates of some Arrhenius-compliant reactions are retarded by increasing the temperature and are therefore associated with negative activation energies. The few readily accessible explanations of this phenomenon are rather cursory and limited to enthalpic considerations relating to the relative activation energies and thermicities of the component elementary steps of the process. However, these explanations are incomplete and consequently counterintuitive. Here we provide the required augmentation via entropic considerations.

Charge transport in diatomaceous earth studied by broadband dielectric spectroscopy

In the present work, broadband dielectric spectroscopy measurements in the frequency range of 10mHz to 1MHz and at elevated temperatures (room temperature up to 950°C) were carried out in industrial diatomaceous earth samples, in order to study charge transport phenomena which play a key role to the transport of both organic and inorganic contaminants in this kind of minerals. Cole–Cole plots of complex impedance measurements may serve to distinguish the contributions of grain interior, grain boundaries and electrode polarization to the overall electrical behavior of diatomite. Complex electrical conductivity and electric modulus representations have been combined under the fulfillment of the BNN relationship over a wide temperature range, in order to investigate the temperature dependence of diffusion coefficients and concentration of charge carriers in thermally modified diatomite. Both diffusivity and mobility follow the same Arrhenius-type behavior at two distinct temperature regions as DC-conductivity does. The related activation energies were estimated and ascribed to proton conduction (Ea≈1.00eV) at low temperatures (T<650°C) and hopping conduction of small polarons (Ea≈1.67eV) at higher temperatures, due to the presence of transition metals in diatomite sample. The thermal treatment of diatomite at 950°C enhances the available charge carriers and increases the diffusivity which might improve its adsorption capacity.

Dielectric Properties of PbNb2O6 up to 700°C from Impedance Spectroscopy

Piezoelectric materials have wide band gap and no inversion symmetry. Only the orthorhombic phase of lead metaniobate (PbNb2O6) can be ferroelectric and piezoelectric below Curie temperature, but not the rhombohedral phase. High temperature piezoelectric applications in current decades have revived international interest in orthorhombic PbNb2O6, synthesis of which in pure form is difficult and not well documented. Second problem is that its impedance spectroscopy (IS) data analysis is still incomplete. Present work attempts to fill up these two gaps. Presently found synthesis parameters yield purely orthorhombic PbNb2O6, as checked by X-ray Rietveld analysis and TEM. Present 20 Hz to 5.5 MHz IS from room temperature to 700°C shows its ferroelectric Curie temperature to be one of the highest reported, &gt;574°C for 0.5 kHz and &gt;580°C for 5.5 MHz. Dielectric characteristics and electrical properties (like capacitance, resistance and relaxation time of the equivalent CR circuit, AC and DC conductivities, and related activation energies), as derived here from a complete analysis of the IS data, are more extensive than what has yet been reported in the literature. All the properties show sharp changes across the Curie temperature. The temperature dependence of activation energies corresponding to AC and DC conductivities has been reexamined.

H2 oxidation on doped yttrium chromites/yttrium stabilized zirconia anode of solid oxide fuel cell

Co and Ni doped yttrium chromites as potential anodes for solid oxide fuel cell (SOFC) are studied with respect to the electrode performance and anode reaction mechanisms. Both electrical conductivity and electrode performance of yttrium chromites have been enhanced after Co and Ni doping. Electrochemical impedance spectroscopy (EIS) results indicate that charge transfer process at high frequency and surface adsorption/diffusion processes at low frequency domain can be the dominant anode reaction steps. Ni doping accelerates the surface processes by reducing the related activation energy from 1.2 to 0.5eV. It also substantially improves the charge transfer process probably by increasing the amount of adsorbed H on electrode surface. The resistance of high frequency is found to be dependent on H2 content. The observed reaction order is 1/4 for Co doped and 1/3–1/2 for Ni doped yttrium chromites. A model of H2 oxidation reaction is proposed, revealing this dependence stems from the reaction between adsorbed H and the lattice oxygen.

Tuning of the Dienophilic Reactivity of Imidazo[1,2-a]pyridine: A Comparison of DFT and Dispersion-Corrected DFT Calculations

Abstract A comparative study of the DFT calculations with and without dispersion correction for the determination of the relative activation energy barriers and reaction energies of the normal electron demand Diels–Alder (NED DA) reactions of imidazo[1,2-a]pyridine and its derivatives with 1,3-butadiene at the B3LYP/6-31+G** level reveals that on inclusion of the dispersion correction, the relative activation energy barriers are lowered by ca. 10–13 kcal mol−1 whereas relative reaction energies are increased by ca. 9–13 kcal mol−1 as compared to the DFT calculations without dispersion correction. Similar results are obtained for the reverse electron demand (RED) DA reactions of imidazo[1,2-a]pyridine and its derivatives with tetrachloro-o-benzoquinone. The relative free energies of solvation of different species determined from SCRF calculations allow to detect solvent-induced geometry relaxation of the reactant molecules and the transition states which rationalises lowering of the relative activation energies in methylene chloride. The reactions are found to occur without the involvement of the reactants complexes.