Equal Activation Energies Research Articles

Structural properties, dielectric relaxation, and impedance spectroscopy of NASICON type Na3+xZr2−xPrxSi2PO12${\rm Na}_{3+x} {\rm Zr}_{2-x} {\rm Pr}_{x} {\rm Si}_2 {\rm PO}_{12}$ ceramics

AbstractWe investigate the dielectric and impedance spectroscopic investigation of Pr‐doped NASICON type () samples as a function of temperature and frequency. The Rietveld refinement of X‐ray diffraction patterns confirms the monoclinic phase having C2/c space groups for all the samples. The scanning electron microscopy shows the granular‐like structure and energy dispersive X‐ray analysis confirms the desired compositions. The temperature (90–400 K) and frequency (20 Hz–2 MHz) dependence of electric permittivity are explained using Maxwell–Wagner–Sillars (MWS) polarization and space charge polarization mechanisms. The dielectric relaxation shows nearly equal activation energy for all the samples with a non‐Debye type of relaxation in the measured temperature range. The complex impedance analysis shows the presence of broad grain and grain boundary relaxation peaks. The stretched exponent analysis of electric modulus using the Kohlrausch–Williams–Watts (KWW) function further confirms the non‐Debye type of relaxation. Moreover, scaling analysis of the electric modulus shows a similar type of relaxation for all the samples. The ac conductivity data are fitted using modified power law, where the temperature dependence of exponent () confirms the correlated barrier hopping (CBH) type conduction for all the samples. Our results indicate that the Pr‐doped NASICON samples are potential candidates for charge storage devices due to their large electric permittivity.

Radical Polymerization Kinetics of Hexyl Methacrylate in Dimethylsulfoxide Solution

In this study, we conducted a series of polymerization studies of hexyl methacrylate in dimethyl sulfoxide with (0.1 - 0.4) mol dm-3 of monomer and (1 10-3 – 4 10-3) mol dm-3 of benzoyl peroxide as initiators at 70 °C. Using the well-known conversion vs. time technique, the effects of initiator and monomer concentration on the rate of polymerization (Rp) were studied. An initiator of order 0.35 was obtained in accordance with theory and a divergence from normal kinetics was detected with an order of 1.53 with respect to monomer concentration. The activation energy was determined to be (72.90) kJ mol-1, which does not correspond to the value of most thermally initiated monomers. The observed value of activation energy suggests that propagation and termination reactions have equal activation energy and the difference between them is nearly zero. The average degree of polymerization (DPn) decreased as benzoyl peroxide concentration increase whereas an increase in solvent polarity has slightly increased rate of polymerization value.

Improving the Coke Resistance of Ni-Ceria Catalysts for Partial Oxidation of Methane to Syngas: Experimental and Computational Study.

The synthesis of syngas (H2 : CO=2) via catalytic partial oxidation of methane (CPOM) is studied over noble metal doped Ni-CeO2 bimetallic catalysts for CPOM reaction. The catalysts were synthesized via a controlled deposition approach and were characterized using XRD, BET-surface area analysis, H2 -TPR, TEM, Raman and TGA analysis. The catalysts were experimentally and computationally studied for their activity, selectivity, and long-term stability. Although the pure 5Ni/CeO2 catalyst showed high initial activity (∼90%) of CH4 conversion, it rapidly deactivates around 20% of its initial activity within 140 hours of TOS. Doping of Ni/CeO2 catalyst with noble metal was found to be coke resistant with the best-performing Ni-Pt/CeO2 catalyst showed ∼95% methane conversion with >90% selectivity at a temperature of 800 °C, having exceptional stability for about 300 hours of time-on-stream (TOS). DFT studies were performed to calculate the activation barrier for the C-H activation of methane over the Ni, Ni3 Pt, Ni3 Pd, and Ni3 Ru (111) surfaces showed nearly equal activation energy over all the studied surfaces. DFT studies showed high coke formation tendency of the pure Ni (111) having a very small C-C coupling activation barrier (14.2 kJ/mol). In contrast, the Ni3 Pt, Ni3 Pd, and Ni3 Ru (111) surfaces show appreciably higher C-C coupling activation barrier (∼70 kJ/mol) and hence are more resistant against coke formation as observed in the experiments. The combined experimental and DFT study showed Ni-Pt/CeO2 as a promising CPOM catalyst for producing syngas with high conversion, selectivity and long-term stability suited for future industrial applications.

Unraveling the reaction mechanisms for furfural electroreduction on copper.

Electrochemical routes for the valorization of biomass-derived feedstock molecules offer sustainable pathways to produce chemicals and fuels. However, the underlying reaction mechanisms for their electrochemical conversion remain elusive. In particular, the exact role of proton-electron coupled transfer and electrocatalytic hydrogenation in the reaction mechanisms for biomass electroreduction are disputed. In this work, we study the reaction mechanism underlying the electroreduction of furfural, an important biomass-derived platform chemical, combining grand-canonical (constant-potential) density functional theory-based microkinetic simulations and pH dependent experiments on Cu under acidic conditions. Our simulations indicate the second PCET step in the reaction pathway to be the rate- and selectivity-determining step for the production of the two main products of furfural electroreduction on Cu, i.e., furfuryl alcohol and 2-methyl furan, at moderate overpotentials. We further identify the source of Cu's ability to produce both products with comparable activity in their nearly equal activation energies. Furthermore, our microkinetic simulations suggest that surface hydrogenation steps play a minor role in determining the overall activity of furfural electroreduction compared to PCET steps due to the low steady-state hydrogen coverage predicted under reaction conditions, the high activation barriers for surface hydrogenation and the observed pH dependence of the reaction. As a theoretical guideline, low pH (<1.5) and moderate potential (ca. -0.5 V vs. SHE) conditions are suggested for selective 2-MF production.

Insight into carbon deposition associated with NiCo/MgO catalyzed CH4/CO2 reforming by using density functional theory

A density-functional theory method has been conducted to investigate carbon deposition associated with NiCo/MgO catalyzed CH4/CO2 reforming. In the process of carbon deposition formation, CH4 dissociation to produce C, further C accumulates to form carbon deposition. In addition, C and its precursor can be eliminated by oxygen species from CO2 dissociation. Based on above analysis about carbon deposition, three possible ways are considered to contribute to resist carbon deposition (1) reducing CH4 dissociation to decrease C produced; if not, (2) accelerating CHx(x=0∼3) oxidation to eliminate C, (3) blocking C accumulation, further suppressing carbon deposition, that is, the rate of CHx oxidation is more than that of C accumulation. The calculation results show that the dissociation of CH4 has almost equal activation energy on perfect and defective NiCo/MgO, and they are lower than those on Ni/MgO and NiCo(111), indicating that it is impossible to reduce C produce through reducing CH4 dissociation when addition a second metal Co to Ni/MgO or using different support MgO; CH oxidation is favorable to produce CHO compared to CH dissociation into C and H, further it is favorable for CHO dissociation into CO and H on perfect and defective NiCo/MgO catalyst. In addition, C oxidation is easier on defective NiCo/MgO than that on perfect NiCo/MgO, meanwhile it is easier on NiCo/MgO than that on NiCo(111), indicating that using defective MgO support accelerates C oxidation, further elimination carbon deposition. Carbon accumulation is easier on perfect NiCo/MgO than that on defective NiCo/MgO. The results indicate that the defective MgO surface support NiCo bimetal catalyst is more resistant to carbon deposition than perfect one. One can modify the Ni-based catalyst behavior via addition a second metal Co or using MgO support with oxygen-vacancy. This might be given a clue for experimental catalyst to develop Ni-based catalyst with the property of resisting carbon deposition.

Ultrasound-Assisted Alkaline Hydrolysis of Waste Poly(Ethylene Terephthalate) in Aqueous and Non-aqueous Media at Low Temperature

This work demonstrates an attempt to depolymerize the waste poly(ethylene terephthalate) (PET) at low temperature using 20 kHz frequency ultrasound. Alkaline hydrolysis of PET was carried out in an aqueous as well as in methanolic medium. Ultrasound has shown a significant enhancement in the rate of alkaline hydrolysis of PET in methanol compared to an aqueous medium. The rate of alkaline hydrolysis of PET increases with an increase in the temperature and alkali concentration. PET conversion of 69% and 46% was observed with and without ultrasound, respectively, using 10% (w/w) sodium hydroxide (NaOH)-methanol solution at 50°C temperature in 60 min. Pure terephthalic acid was recovered from the PET and characterized using Fourier-transform infrared spectroscopy and acid number. The shrinking core model was proposed for the reaction mechanism with surface reaction as the rate of the controlling step. Kinetic analysis indicates almost equal activation energy and yet shows a significant difference in the frequency factor with and without ultrasound-assisted alkaline hydrolysis. Thus, ultrasound does not alter the intrinsic mechanism of PET hydrolysis. The reason for enhancement in the rate of reaction may be the ultrasound-induced micro-mixing and thereby the frequency of interaction between PET and NaOH molecules.

Theoretical study of the molecular structure and intramolecular proton transfer in benzohydroxamic acid

Benzohydroxamic acid (BHA), an important member of the hydroxamic acid family (R-CO-NH-OH), has been investigated in detail in the gas and aqueous phase using density functional methods. Like the other members, it displays amide/imide tautomerism. Each tautomer may also exist as Z or E conformer. The Z amide conformer is found to be the most stable in both phases. The electronic properties of benzohydroxamic acid, along with its NBO analysis, suggest that the molecule is hard (stable). It is found that Z amide can be converted to Z imide via two paths, having almost equal activation energies. An explanation of their acid character, which had been eluding scientists, has emerged from this work. The relative stabilities of the anions suggest that BHA behaves as an N-acid, i.e. deprotonation of the N-H group occurs from the 1Z tautomer. It can also be considered as an O-acid formed by deprotonation from the 2Z tautomer. On consideration of the charge distribution of the anion, it is concluded that the latter description is more apt, and it is an O-acid formed by deprotonation of the iminol tautomer. Furthermore, the probable protonation sites of benzohydroxamic acid have been studied and the carbonyl oxygen is found to be the preferred site for protonation.

Computational Chemistry Approach to Interpret the Crystal Violet Adsorption on Golbasi Lignite Activated Carbon

In this paper, adsorption mechanism of Crystal Violet (CV) dye from the aqueous solution on the activated carbon prepared from Golbasi lignite was explained and interpreted by a computational chemistry approach and experimental studies. Molecular dynamic simulations and Ab initio frontier orbital analysis indicated relatively high energy and electron transfer processes during adsorption, and molecular dynamics simulations showed CV dye molecules moving around on the activated carbon surface after adsorption, facilitating penetration into cracks and pores. The experimental results supported to molecular dynamic simulation and showed that the monolayer coverage occurred on the activated carbon surface and each CV dye ion had equal sorption activation energy.

Structure-modulated ionic transport behavior in zirconia–dysprosia mixed oxides

In the present study, we report a systematic study on Zr1−xDyxO2−x/2 system exploring the effect of structure on ion-transport. All the compositions were synthesized by combustion route followed by high-temperature sintering and thoroughly characterized by X-ray diffraction, Raman spectroscopy, Scanning electron microscopy and AC impedance studies. These studies revealed a phase relation wherein Zr0.9Dy0.1O1.95 exhibited coexistence of a monoclinic and cubic phase. The Dy3+ content greater than 10 mol% led to complete stabilization of cubic zirconia solid solutions. The structural evolution has also been monitored with Raman spectroscopy wherein very weak pyrochlore-type ordering in compositions with x ≥ 0.4 could be observed even when XRD showed a fluorite-type structure. The effect of structure-induced conductivity control was revealed by AC impedance analysis which bestowed a high ionic conductivity, 0.0048 Scm−1 at 700 K, on the nominal composition Zr0.8Dy0.2O1.90. This has been attributed to a very high disorder prevailing in this system due to the substitution of Zr4+ by Dy3+. It was revealed that the ionic conductivity behavior was driven by high pre-exponential factor which led to substantially high ionic conductivity for Zr0.8Dy0.2O1.90 despite almost equal activation energies observed for all the nominal compositions studied. Such high ionic conductivity for a zirconia-based system makes it a potential candidate for electrolyte for intermediate-temperature solid oxide fuel cell, as they are inherently stabler than ceria-based electrolytes.

Comparative study of hot pressing and high-speed electropulse plasma sintering of Al2O3/ZrO2/Ti(C,N) powders

A comparative study was made of the kinetics of the sintering of Al2O3/ZrO2/Ti(C,N) powders under hot pressing and spark plasma sintering conditions. It was found that the consolidation processes under identical hot pressing and spark plasma sintering conditions at low heating rates at the intense densification stage are characterized by identical kinetics and have equal activation energies.

A (Diborole)cobalt Complex with a C–H···B Bridge, CpCo(1,3‐C3B2Me5H), and Its Thallium Derivative: Synthesis, Structure, and Bonding

AbstractThe reaction of CpCo(1,3‐C3B2Me5H) (1) with CpTl affords the thallium derivative CpCo(1,3‐C3B2Me5)Tl (2). The structures of 1 and 2 were determined by X‐ray diffraction at 100 K. An “extra” hydrogen atom in 1 occupies a C–H···B bridging position. According to DFT calculations, 1 exists as a mixture of two enantiomers with an enantiomerization barrier of only 0.5 kcal mol–1. The transition state has Cs symmetry with an endo‐CH hydrogen atom. The isomeric iso‐1with the Co–H···B bridge is less stable than 1 by 10 kcal mol–1.The formation of 1 and iso‐1 from C3B2Me5H and CpCo(C2H4)2 have almost equal activation energies. The isomerization of iso‐1 to 1 was shown to proceed as a two‐step hydrogen transfer. The bonding of the “extra” hydrogen atom in the related CHB‐bridged carborane nido‐2,3,5‐C3B3R5H2 and metallacarboranes M(C5R5)(C3B2R′5H) (R, R′ = H, Me; M = Co, Rh, Ir) as well as their hydridic isomers MH(C5R5)(C3B2R′5) was compared. According to energy decomposition analysis, the bonding of the parent anion [CpCo(1,3‐C3B2H5)]– with metal cations becomes stronger in the following order: K+ &lt; Na+ &lt; Tl+ &lt; Li+ &lt; [RuCp]+. The attractive interactions between the [CpCo(1,3‐C3B2H5)]– and Tl+ fragments are 68 % electrostatic and 32 % covalent.

Thermal expansion coefficient and relaxation parameters of glasses in the system GaX2-GeX2-Sb2X3 (X = S, Se)

The concentration behavior of the thermal expansion coefficient of glasses in the system GaX2-GeX2-Sb2X3 (X = S, Se) has been studied in two sections. Parameters characterizing the relaxation properties of the glasses have been calculated on the basis of the Tool-Narayanaswamy theory from experimental temperature dependences of the thermal expansion coefficient in the softening range. Temperature dependences of the viscosity have been calculated under the assumption of equal free activation energies for the relaxation and viscous flow.

Temperature and Excitation Wavelength Dependence of Surface-Plasmon-Mediated Emission from CdSe Nanocrystals

The authors demonstrate the temperature-dependent behavior of surface plasmon (SP) coupling emission of CdSe nanocrystals (NCs) on Au colloids at different excitation wavelengths. It is found that the photoluminescence (PL) enhancement factor of CdSe NCs on Au colloids increases with the measuring temperature from 8 to 300 K. Such a trend is independent of the excitation wavelength, which whereas greatly influences the PL enhancement factor and the critical temperature for PL quenching switching to enhancement. We attribute the temperature-dependent PL enhancement factor to the increase of the SP coupling rate kSP and the red shift of the PL energy of CdSe NCs with the rising temperature. Both the PL intensity and the energy of CdSe nanocrystals are modeled against the temperature (7−300 K). Compared with the bare nanocrystals, the nanocrystals on Au colloids have an equal activation energy, a larger temperature coefficient, and a temperature-dependent red shift of the PL energy.

Optical and Electrical Properties of Mechanochemically Synthesized Nanocrystalline Delafossite CuAlO2

Nanocrystalline p-type semiconductor copper aluminum oxide (CuAlO2) has been synthesized by mechanical alloying using freshly prepared Cu2O and alpha-AlO2O3 nanocrystals in toluene medium. A study on structural property performed with different alloying and post annealing durations, by X-ray diffraction (XRD) reveals the formation of single phase with average crystallite size approximately 45 nm. Optical absorbance onset at 364.5 nm confirms its wide band gap nature (E(g) = 3.4 eV) and the fluorescence emission behaviour (390 nm) confirms its direct band type transition. The activation energy for electrical conduction has been calculated by Arrhenius plots using impedance measurement. Both grain and grain boundary conductivity takes place with almost equal activation energies of approximately 0.45 eV. The paper discusses synthesis, structural, optical and electrical properties of delafossite CuAlO2 in detail.

Coupling exothermic and endothermic reactions in adiabatic reactors

The steady state and the dynamic behavior of coupling exothermic and endothermic reactions in directly coupled adiabatic packed bed reactors (DCAR) are analyzed using one-dimensional pseudo-homogeneous plug flow model. Two different configurations of DCAR (simultaneous DCAR—SIMDCAR and sequential DCAR—SEQDCAR) are investigated. In SIMDCAR, the catalyst bed favors both exothermic and endothermic reactions and both reactions occur simultaneously. SEQDCAR has alternating layers of catalyst beds for exothermic and endothermic reactions and hence the exothermic and endothermic reactions occur in a sequential fashion. The performance of both reactors, in terms of conversion achieved and manifested hot spot behavior, is compared with that of the co-current heat exchanger type reactor. Various possible operational regimes in SIMDCAR have been classified and the conditions for the existence of hot spots or cold spots in SIMDCAR are obtained analytically for the first order reactions with equal activation energies. The reactor behavior for the reactions with non-equal activation energies is also presented. The preliminary criteria for the selection of suitable reactor type and the general bounds on the reaction parameters to obtain the desired conversion for endothermic reaction with minimal temperature rise are proposed. The dynamic behavior of these reactors is important for control applications and we have reported some of the transient behavior.

Extreme values of relative permittivity and dielectric relaxation in Sr2SbMnO6 ceramics

A polycrystalline ceramic of Sr2SbMnO6 (SSM) was fabricated using the powders obtained by the conventional solid-state reaction route. The dielectric measurements of the ceramic were carried out as a function of frequency (100 Hz–10 MHz) and temperature (190–360 K). SSM exhibited higher relative permittivity (∼2 × 105 for 1 kHz at 360 K) than that for the recently known high permittivity ceramic CaCu3Ti4O12 (CCTO). It decreased to a relatively low value (∼2 × 103 for 1 kHz) on decreasing the temperature down to 195 K. The modified Cole–Cole equation that included the conductivity term was used to describe the dielectric spectra of the ceramic. Impedance spectroscopy was employed to determine the electrical parameters (resistance, capacitance and relaxation time) of the grain and the grain boundary. The grain and grain boundary conduction and the dielectric relaxation time followed an Arrhenius law associated with activation energies of 0.22 eV, 0.35 eV and 0.24 eV, respectively. Nearly equal activation energies for dielectric relaxation and grain conduction revealed a possibility of invoking a common mechanism to rationalize the permittivity data. The capacitance and resistance associated with the grain boundary were found to be higher than that associated with the grain to a great extent. The incidence of giant relative permittivity and the other experimental findings support the Maxwell–Wagner type of mechanism.

Influence of H2on the Gas-Phase Decomposition of Formic Acid: A Theoretical Study

Gas-phase decomposition of formic acid results in final products CO + H2O and CO2 + H2. Experimentally, the CO/CO2 ratio tends to be large, in contradiction with mechanism studies, which show almost equal activation energies for dehydration and decarboxylation. In this work, the influence of H2 on the decomposition mechanism of HCOOH was explored using ab initio calculations at the CCSD(T)/6-311++G**//MP2/6-311++G** level. It was found that, in the presence of H2, the reaction channels leading to CO + H2O are more than those leading to CO2 + H2. With competitive energy, H2 addition to HCOOH can reduce the latter into HCHO, which then dissociates into CO + H2 catalyzed by H2O. Compared to trans-HCOOH, cis-HCOOH and cis-C(OH)2, conformers required for decarboxylation, are less populated due to interactions with H2.

LARGE-ACTIVATION-ENERGY ASYMPTOTIC ANALYSIS OF MULTICOMPONENT-FUEL DIFFUSION FLAMES

ABSTRACT This analysis addresses some aspects of the internal structure and extinction of diffusion flames established by burning of multicomponent fuel (n-heptane diffusion flame enriched with hydrogen). By presuming one-global-step reactions with large activation energy, the zone where all reactions take place is described by large-activation-energy asymptotics. In this study, it is also supposed that the values of the Damköhler numbers for the reactions are very close and the values of the activation energies are also very close. Under these conditions, all reactions will be performed inside the same region. The results show that the reaction with small Damköhler number and/or large activation energy controls the extinction of the multicomponent-fuel diffusion flame. Under the condition of equal Damköhler numbers and equal activation energies, Hamins and Seshadri's (Hamins, A. and Seshadri, K. (1984b) Proc. Combust. Instit., 20, 1905–1913) results are recovered.

Molecular Dynamics Study of Anisotropic Translational and Rotational Diffusion in Liquid Benzene

Equilibrium NPT and NVT molecular dynamics simulations were performed on liquid benzene over an extended range of temperature (from 260 to 360 K) using the COMPASS force field. Densities and enthalpies of vaporization (from cohesive energy densities) were within 1% of experiment at all temperatures. tumbling and spinning rotational diffusion coefficients, D(perpendicular) and D(parallel), computed as a function of temperature, agreed qualitatively with the results of earlier reported experimental and computational investigations. Generally, it was found that D(parallel)/D(perpendicular) approximately 1.4-2.5 and the activation energy for tumbling was significantly greater than for spinning about the C6 axis [Ea(D(perpendicular)) = 8.1 kJ mol(-1) and Ea(D(parallel)) = 4.5 kJ mol(-1)]. Calculated translational diffusion coefficients were found to be in quantitative agreement with experimental values at all temperatures [deviations were less than the scatter between different reported measurements]. In addition, translational diffusion coefficients were computed in the molecule-fixed frame to yield values for Dxy (diffusion in the plane of the molecule) and Dz (diffusion perpendicular to the plane). It was found that the ratio Dxy/Dz approximately 2.0, and that the two coefficients have roughly equal activation energies. This represents the first atomistic molecular dynamics study of translational diffusion in the molecular frame.

MBE-Grown Si : Er Light-Emitting Structures: Effect of Implantation and Annealing on the Luminescence Properties

Features appearing in the photo-and electroluminescence spectra of light-emitting structures based on MBE-grown Si: Er layers are studied. The luminescence properties of Si layers implanted by Er and O ions were used as a reference. The temperature quenching of the photoluminescence intensity of Er-containing centers in MBE-grown and implanted layers can be approximated adequately by the same functional relationships with equal activation energies but with preexponential factors differing by more than two orders of magnitude. It is shown that the electroluminescence of Er3+ ions can be increased by additional coimplantation of erbium and oxygen ions into MBE-grown light-emitting diode structures and subsequent annealing. After this treatment, the Er-containing centers continue to dominate the luminescence spectrum.