Variation Of Apparent Activation Energy Research Articles

Analytical fast pyrolysis of nitrogen-rich mosquito species via pyrolysis-FTIR and pyrolysis-GC/MS

This study presents a thorough thermal analysis of the dead remains of mosquitoes, in order to unravel the composition of vapors evolved from the bug zappers decimating them. Analytical pyrolysis-gas chromatograph/mass spectrometry (Py-GC/MS) and pyrolysis-Fourier transform infrared (Py-FTIR) spectrometric techniques were used to understand the pyrolysate composition and their time evolution from mosquitoes. Thermogravimetric mass loss data obtained at different heating rates (5–30 °C min−1) were used to determine the variation of apparent activation energy with conversion using isoconversional methods of Friedman, Flynn-Wall-Ozawa and Kissinger–Akahira–Sunose. The apparent activation energies varied in the range of 90–200 kJ mol−1. Increase in pyrolysis temperature from 400 to 700 °C resulted in the formation of alcohols, indoles, nitriles, monoaromatics and alkenes in higher selectivities, while the selectivity to long chain carboxylic acids decreased with increasing temperature. The formation of various organics was corroborated by the evolution of functional groups at short timescales. The significant nitrogen content in the char (∼9.7 wt.%) was evidenced in the form of NH and NO groups in the FTIR spectrum. The deconstruction of proteins and fatty acids present in mosquito was similar to that of microalgae in terms of the pyrolysis products.

Comparison of bituminous coal apparent activation energy in different heating rates and oxygen concentrations based on thermo gravimetric analysis

Simultaneous thermal analysis (STA) experiments are carried on to study the relationship between the coal apparent activation energy and the heating rate as well as the oxygen concentration, respectively. Bituminous coal samples are analyzed using the simultaneous thermal analyzer. Experimental condition involves two aspects: different heating rates (2, 5, 10, 20 and 30 °C min−1) and different oxygen concentrations (5, 10, 15, 21 and 30%). A mixture of oxygen and nitrogen flows through the coal sample. Characteristic points at different temperatures are obtained; Coats–Redfern integral method is utilized to calculate the apparent activation energy in different experimental conditions. The results indicate that the variation of apparent activation energy in different heating rates follows the exponential function mathematically, which falls in order as the heating rate increases. The apparent activation energy of coal increases in order as the oxygen concentration increases, and the variation of apparent activation energy in different oxygen concentrations follows the logarithmic function mathematically.

Comparison of thermal degradation behavior of epoxy/ammonium perchlorate composite propellants

Non-isothermal degradation process of different mass ratios of epoxy/ammonium perchlorate (EP/AP) composites was studied by thermogravimetric (TG) analysis and differential scanning calorimetry. TG data were treated with AKTS-Thermokinetics software, Flynn–Wall–Ozawa and Friedman’s isoconversional methods for measuring the variation of apparent activation energy (E a) with degree of conversion (α). It was found that thermal degradation process of EP/AP composites is complex, and theirs thermal degradation temperatures are lower than that for epoxy and ammonium perchlorate (AP). The obtained results showed that the degradation activation energy of the EP/AP composites is equal or higher than that for epoxy at the beginning of the degradation. The most interesting finding was that epoxy resin can shift high-temperature decomposition peak of AP to a lower temperature. Self-accelerating decomposition temperature (T SADT) and explosion critical temperature (T b) of EP/AP composites were determined to evaluate the thermal stability of the samples. Finally, lifetime prediction of the samples was done by AKTS software.

Using rheometry to study the curing kinetics of glycidyl azide polymer spherical propellant by non-isothermal method

The curing kinetics of GAP spherical propellant was studied by rheological non-isothermal measurement. The kinetic parameters were determined by rheometry based on conversion degree α described by storage modulus. The non-isothermal method was first used in rheometry as conversion α determined by a novel method, considering that storage modulus variation was a complex process which increased with polymerization and decreased as the temperature increased. The variation of apparent activation energy was obtained by the isoconversional method of Kissinger–Akahira–Sunose. It was found that Ea is little lower in the range of 0.05 < α < 0.50 than it is in the range of 0.50 < α < 0.70. During the complete curing process, apparent activation energy can be considered a constant. A new kinetic equation had been established and showed great agreement with experimental data. Altogether, the rheological non-isothermal method can be used to investigate the mechanism and kinetics of thermosetting reactions, and the obtained knowledge of the curing kinetic will form a contribution to the study of GAP spherical propellant.

Erratum to: Elucidation of the Nature of Structural Heterogeneity During Alkali Leaching of Non-activated and Mechanically Activated Boehmite (γ-AlOOH)

Crystal joints and faces in non-activated boehmite and, state of agglomeration of particles, degree of amorphization, microcrystallite dimension and, strain in mechanically activated boehmite are indicators of structural heterogeneity which influences reactivity of the solid phase. The focus of this paper is on understanding the manifestation of the heterogeneity during alkali leaching of a boehmite (specific surface area—263 m2/g), without and with mechanical activation using planetary milling up to 240 minutes. A two-prong strategy is used for this purpose which involved analysis of the kinetics of leaching by a model-free approach using ‘isoconversional method’ and, in parallel, characterization of the reacting solid after different durations of leaching. Unlike model-fitting methods, the kinetic analysis revealed sample-dependent variation of apparent activation energy with fraction leached. Changes observed in the morphology of samples (by SEM), particle size distribution (by laser diffraction), and crystalline nature (by powder X-ray diffraction) are used to explain activation energy changes and propose mechanisms of leaching. The effect of mechanical activation on rate constant is assessed and it has been found that up to ~23-fold increase in rate is possible depending on the activation time, leaching temperature, and fraction leached. Further, based on binary correlations between activation energy at different fractions leached and initial characteristics of the samples, it is found that the leaching is predominantly influenced by structural changes during milling, namely, degree of amorphization, microcrystallite dimension, and strain, vis-a-vis specific surface area. Significantly, the paper highlights limitation of model-fitting methods used by most researchers to analyze the kinetics of leaching, especially for mechanically activated minerals.

Reduced Grain Boundary Resistance by Surface Modification

Interfacial resistance is one of the severe problems in composite electrodes of all solid state batteries (ASSBs), especially oxide-type ASSBs. Conflicts between poor sinterability and possible unfavorable reaction with active materials limit applicable materials and processes. In this report, a novel approach is proposed to decrease grain boundary resistance among nonsintered solid electrolyte particles. The concept is successfully demonstrated, and the nonsintered grain boundary resistance of a highly conducting solid electrolyte (Li1.3Al0.3Ti1.7(PO4)3) was suppressed by being coated with poorly conducting solid electrolyte (Li2SiO3). Increased total conductivity and variation of apparent activation energy are well explained from the viewpoint of defect chemistry.

The influence of CO2 and H2O on selective catalytic reduction of NO by NH3 over Cu/SAPO-34 catalyst

Abstract The effect of CO2 and H2O addition on selective catalytic reduction of NO by NH3 (NH3-SCR) over Cu/SAPO-34 catalysts was confirmed and we tried to study their influence in this research. Two Cu/SAPO-34 samples with the equivalent Cu loading were utilized, and their acidities were adjusted by the Si amounts in SAPO-34 supports. NH3-SCR, NH3-TPD, NH3 oxidation and kinetic analysis were conducted to evaluate the role of CO2 and H2O. NH3-SCR results revealed that CO2 in the inlets presented less impact on NO conversion, while the presence of H2O enhanced NO conversion during the whole temperature range. NH3-TPD results showed H2O improved the acidity of Bronsted acid sites and the reducibility of Cu2+ species, both of which benefited the NH3-SCR activity. In addition, the presence of H2O at high temperature inhibited NH3 conversion and side-products during NH3 oxidation process. Kinetic analysis of NH3-SCR and NH3 oxidation at high temperature were further conducted to estimate the variation of apparent activation energy (Ea) for the presence of water vapor, and finally the relation between the variation of acidity, Cu2+ species reducibility and the SCR mechanism over Cu/SAPO-34 sample at different temperature ranges was proposed, respectively.

A Critical Analysis of the Compensation Effect and Its Application to Heat Exchanger Fouling Studies

Occurrences of the compensation effect in published studies on the initial rate of heat exchanger fouling are discussed. When fouling-rate experiments are performed on a series of related fluids, it is shown that random errors in the measurements can create an apparent compensation effect. This explains some reported occurrences in the literature. Other reported instances of the compensation effect show a systematic variation in apparent activation energy with fluid flow rate. It is shown that these may be the result of analyzing the temperature dependence of the overall rate of reaction. The conditions under which the compensation effect is expected when flow rate is varied are explored using synthetic data generated by a model for chemical reaction fouling.

Corrosion Inhibition of Mild Steel by Safflower (Carthamus tinctorius) Extract: Polarization, EIS, AFM, SEM, EDS, and Artificial Neural Network Modeling

Inhibitory action of safflower (Carthamus tinctorius) extract (SE) was investigated in hydrochloric acid solution through electrochemical (polarization, EIS), and surface analysis (optical microscopy/atomic force microscopy (AFM)/scanning electron microscopy (SEM)/energy dispersive spectroscopy (EDS)) techniques. In addition, Inhibition efficiency was predicted by neural network (NN) modeling in elevated temperatures and different acid concentrations. The thermodynamic adsorption parameters propose that this inhibitor retard both cathodic and anodic processes through physical adsorption and blocking the active corrosion sites. Surface analysis techniques confirm the inhibitor adsorption on the metal surface, which is in accordance with the variation of apparent activation energy of corrosion. Finally, inhibition efficiency is discussed in terms of protective film formation.

Benzimidazole and its derivatives as corrosion inhibitors for mild steel in 1M HCl solution

Abstract In this paper, the inhibition ability of benzimidazole and its derivatives against the corrosion of mild steel in 1M HCl solution was studied. The change of impedance parameters observed by variation of inhibitors concentration within the range of 50–250 ppm was an indication of their adsorption. The thermodynamic adsorption parameters proposed that these inhibitors retard both cathodic and anodic processes through physical adsorption and blocking the active corrosion sites. The adsorption of these compounds obeyed the Langmuir’s adsorption isotherm. The inhibition efficiency was increased with inhibitor concentration in the order of 2-mercaptobenzimidazole > 2-methylbenzimidazole > benzimidazole, which is in accordance with the variation of apparent activation energy of corrosion.

Isoconversion analysis of the glass transition

Abstract The activation energy of a kinetic process can be determined as a function of conversion using isoconversion methods. In the past, such isoconversion methods were used to examine the variation in apparent activation energy (Ex) through the glass transition using differential scanning calorimtery (DSC) heating data. In this work we apply a similar isoconversion analysis to data obtained by cooling polystyrene through the glass transition using both capillary dilatometry and differential scanning calorimetry (DSC), and we compare the results to an analysis of DSC heating data for the same material. Although the values of Ex obtained from the isoconversion analyses of cooling data are found to be in good agreement with values of average activation energy (Eave), the values of Ex from analysis of heating data are considerably lower. Hence, we demonstrate that to obtain estimates of the activation energy, the isoconversion analysis should be performed on cooling rather than on heating. We also show that the conversion dependence of Ex can be directly related to the temperature dependence of the equilibrium relaxation time.

Application of the Arrhenius equation to solid state kinetics: can this be justified?

Application of the Arrhenius equation to the kinetics of solid state reactions has been criticised on several grounds. One of the main theoretical objections has been that the energy distribution amongst the immobilised constituents of a crystalline reactant is not represented by the Maxwell–Boltzmann equation. The detailed band structure of a solid may, however, include interface energy levels (analogous to impurity levels in semiconductors) that participate in the crucial bond redistribution step. Occupancy of such levels would be determined by energy distribution functions based on Fermi–Dirac statistics for electrons, and Bose–Einstein statistics for phonons. For the highest energies, necessary for reaction, both distributions approximate to an exponential energy term, thereby countering the above objection to the application of the Arrhenius equation to reactions of solids. The existence of such interface levels, with a limited range of energies, would also allow for the variation of apparent activation energy with extent of reaction, and also with temperature, reported for many complex solid state reactions.

Low-temperature carbon monoxide oxidation on Pt(111). Dependence of apparent activation energy on reactant gas composition

The low-temperature catalytic oxidation of CO by O2 over a pure Pt(111) surface has been studied over a range of CO:O2 partial pressure ratios at a total pressure of 10 Torr. A marked variation in apparent activation energy with gas composition is found, with a pronounced minimum in the vicinity of the stoichiometric ratio (2:1). The results may be understood in terms of changes in the strength of MCO and MO bonding.

Conductivity variations in hydrated lanthanum-exchanged zeolites X, Y and A

Sodium-form zeolite X, Y and A parent materials have been lanthanum-exchanged to varying degrees. Temperature dependences (−40°C ⪕ T ⪕ 80°C) of ionic conductivities have been determined using impedance techniques for materials stored at a fixed relative humidity (RH ≈ 30% at ambient temperature) and approximate to Arrhenius behaviour in all cases at T ⪕ 30°C. Conductivities decrease on lanthanum exchange. The dominant mobile charge carrier in these systems is hydrated Na+. A minimum in the variation of apparent activation energy (E) with sodium content is apparent for both X and Y zeolites.

The diffusivity of hydrogen through solid solutions of palladium with the transition metals

Abstract Measurements of the diffusivity of hydrogen made by electrolytic techniques at low temperatures (270–330 K) in Pd - U “binary matrixes” have been analyzed in terms of a constant-pressure statistical model. The U -elements (substitutional solutes) were the transition metals Ti, V, Cr, Mn, Fe, Co, and Ni. The statistical model, based upon reaction rate theory, is in good accord with the variation of diffusivity with U -concentration, θ u , and in reasonable accord with variation of apparent activation energy of the H-atoms with θ u .

The multiple-trapping model and hole transport in SiO2

An understanding of the charge-transport process in SiO2 is vital to an understanding of radiation effects in MOS devices. To account for hole transport in SiO2 below room temperature, a stochastic hopping transport model has been employed by others with considerable success. An alternate model is presented here which explains many experimental data in an even more satisfactory fashion. This model is based on multiple trapping at a continuum of trapping levels, the concentrations of which decrease exponentially with energy from the valence band edge. By adjusting the parameters of the model to fit the data at a single temperature, behavior at other temperatures for the same specimen is accurately predicted, including the variation of apparent activation energy with the amount of transport that has occurred. This feature and others which are discussed are not shared by the hopping model previously employed.

Effect of Cold Work on the Oxidation of Nickel at High Temperature

The oxidation of cold‐worked and annealed pure Ni in 1 atm O2 was investigated from 700° to 1270°C in continuous weighing experiments for periods from 1 min to 20 hr and the oxidized specimens examined by diffraction, electron‐optical techniques, and metallographic cross sections. Cold‐worked Ni was found to oxidize faster than annealed Ni and form finer‐grained oxide. Plots of the apparent parabolic rate constant, Kp, for cold‐worked Ni show an initial high value that decreases rapidly as the oxide coarsens with time. For annealed Ni, Kp is lower, grain size larger, and both change little with time. Accordingly, a range of Kp values is obtained at each temperature and, on an Arrhenius plot of Ni oxidation, there is a corresponding variation in apparent activation energy, EA. These results can be plausibly interpreted on the basis of oxide grain boundaries acting as easy diffusion paths for Ni through the layer: for fine‐grained oxide, Kp is higher and EA lower. The oxide is thinnest (and the derived EA highest) on Ni grains which form a single crystal overgrowth. A break in the Arrhenius plot around 1000°C is the result of transport below 1000° being largely by leakage paths while at higher temperatures volume transport is more important. The estimated activation energy for growth of the oxide layer is for transport by lattice diffusion and appreciably less for transport by grain boundary diffusion.

Catalysis on Wide-Band-Gap Semiconductors

This paper presents two novel models of catalysis on wide-band-gap semiconductors: injection of electrons (holes) into the conduction (valence) band by donor (acceptor) intermediates and extraction of electrons (holes) by acceptor intermediates in concurrence, while the catalytic surface reaction via ionic intermediates is in progress. At the stationary state of catalysis, the two concurrent rates of charge transfer are assumed exactly equal in magnitude so that conservation of charges are maintained. The models of catalysis provide an interpretation of the variation in catalytic activity with the surface potential of an extrinsic semiconductor induced by an external electrostatic field, and of the variation in apparent activation energy with the bulk potential. Moreover, this theory sheds light on the complexity and the significance of catalytic experiments carried out under dynamic conditions.

Mechanism of the Oxidation of Zirconium in Oxygen and Liquid Sodium

A rigorous analysis using Fick's second law diffusion equation (for film growth and diffusion into the metal) is given which explains the observed variation of apparent activation energy between about 28 and 50 kcal/mole for the oxidation process. This analysis also explains the observed differences between oxidation in oxygen and oxidation by oxygen in liquid sodium. An empirical equation is given that will accurately describe the experimental data of weight gain vs. time (for Zr, Ti, and Cu) as this relationship goes from approximately parabolic, to quasi‐cubic, to parabolic again. The derivation of this equation stems from the hypothesis that the diffusion coefficient (for oxygen into the metal) decreases as the oxygen consumption increases.