Change In Activation Energy Research Articles

Structure modulation for synergistically optimizing photoluminescence and conductive property in EuNbO4–CaMoO4 ceramics

AbstractAs new multifunctional semiconductor materials, solid‐solution ceramics demonstrate great potential in the field of electronic and optoelectronic devices. CaEuNbMoO8 ceramic is the solid solution of EuNbO4 and CaMoO4, but how to synergistically regulate its transmission of electricity and light still remains challenging. Herein, the structure modulations by adjusting the ratio of EuNbO4 and CaMoO4 and the changes in electrical transport and photoluminescence (PL) properties have been investigated. Meanwhile, the asymmetric ratio (5D0–7F2/5D0–7F1) further determines the symmetry variation of the local microstructure with the solid‐solution ratio of EuNbO4. The correspondence between the structure and phase transition of EuNbO4 ceramics at high temperature is investigated, and it is found that the point symmetry changes from C2h to C4h, and the activation energy changes significantly with increasing temperature. This work provides a solution for the synergistic optimization of PL and conductivity of solid‐solution ceramics by modulating the symmetry of the crystal structure.

Thermogravimetric analysis of coking during dry reforming of methane

Nickel-based catalysts used for dry reforming of methane (DRM) suffer from coking and sintering, which hinders the broad application of the process in the industry. Thermogravimetric analysis was employed to investigate coking on a commercial nickel catalyst with an anti-coking additive (CaO). It was found that the catalyst sintered at temperatures between 850 and 900 °C, which resulted in permanent catalyst deactivation. For the tested Ni/CaO–Al2O3 catalyst, the coking and carbon gasification rates are equal at the temperatures of 796–860 °C, depending on the heating rate (5–20 K/min). Significant differences in the temperatures related to the maxima on TG curves for various heating rates follow from DRM kinetics. This work reveals that the coking rate is lower at higher temperatures. After 50 min, the weight gains amount to about 20% and 40% at 800 °C and 600 °C, respectively. Lower sample weight gains were observed at higher temperatures for a methane decomposition reaction over the Ni/CaO catalyst, unlike for the second tested catalyst – activated carbon. For the nickel catalyst, the reaction order for methane decomposition is 0.6 in the temperature range 640–800 °C, while the sign of the activation energy changes at 700 °C. The elaborated kinetic equation predicts the initial CH4 decomposition rate with 15% accuracy.

The onset of mass transport limitations triggers the stimulus responsiveness of polymer coated catalysts

A series of Pd/Al2O3 catalysts coated with N-isopropylacrylamide polymer (p-NIPAM) brushes with increasing particle size of the support were prepared to study the interplay between mass transport limitations and the ability of the thermo-responsive catalysts to modify reactivity during reaction. Nitrite hydrogenation in water was chosen as probe reaction, which is a key step in the reduction of nitrites and nitrates from drinking water. The results show that the apparent activation energy decreases on increasing temperature above the LCST, i.e. the temperature where the polymer undergoes a phase change, for catalyst with particle sizes between 38 and 100 μm, alluding to a significant increase in mass transfer limitations. In sharp contrast, both p-NIPAM modified catalysts with smaller particle size and uncoated catalysts do not show this change in activation energy with temperature. Detailed multi-physics mass transport and reaction modelling indicated that the transport of the limiting reagent, hydrogen in this case, is severely reduced when the p-NIPAM collapses at temperatures above its LCST. It is concluded that effective reaction control using stimulus responsive polymers requires the system to be close to the mass transport limitation regime, to maximize the effect of the conformation change of the polymer on the catalyst performance and achieve sharp and reversible transitions from active to inactive.

Kinetics of poly(vinyl chloride) thermal and thermo‐mechanical degradation in the presence of epoxidized plasticizer. Induction effect

AbstractKinetics of thermal and thermo‐mechanical degradation of poly(vinyl chloride) (PVC) plasticized with di‐(2‐ethylhexyl)phthalate and epoxidized linseed oil were monitored using differential scanning calorimetry, thermogravimetry‐mass spectrometry and viscosity and temperature change during high shear mastication. Linear dependence of induction period prior to PVC chain‐degradation on epoxidized plasticizer concentration was observed using each method (not reported previously). Zero‐order degradation kinetics controlled by the rate of hydrochloric acid elimination from saturated PVC molecules and consequent fast 2nd order scavenging of HCl by oxirane groups were consistent with experimental data. Apparent activation energies of 128.6, 146.3 kJ mol−1 for thermal degradation and 164.4 kJ mol−1 for thermo‐mechanical degradation were deduced from the temperature dependence of the degradation induction time detected using thermogravimetry‐mass spectrometry, differential scanning calorimetry and mastication, respectively. Change of activation energy with degradation mechanism is examined.

Study on Characteristics of Coal Spontaneous Combustion in Kerjian Mining Area, Xinjiang, China

The spontaneous combustion of coal is a disaster associated with coal mining. In this study, the authors investigated the characteristics of spontaneous combustion of coal at different temperatures (room temperature, 50–500 °C with 50 °C interval) using Fourier transform infrared spectroscopy (FTIR), high-resolution transmission electron microscopy (HRTEM), etc. The results showed the aromatic structure was mainly naphthalene. The aliphatic hydrocarbons were long chain. Oxygen, nitrogen, and sulphur existed as C-O, pyridine, pyrrole nitrogen, aliphatic sulphur, and sulfone. The molecular structural formula is C142H112N2O22. The stable 3D structural was obtained through optimization. Thermogravimetric analysis results showed the critical and dry-cracking temperatures of coal samples showed downward trends overall, whereas the acceleration and thermal-decomposition temperatures varied greatly with increase in oxidation temperature. The activation energy change pattern of 4 stages is not obvious. The FTIR results showed the contents of self-associated OH changed greatly. The aliphatic hydrocarbons changed greatly at 30–150 °C and 300–500 °C. The C-O showed increasing trends, whereas the C=O decreased consistently. The HRTEM results showed the aromatic fringes in coal samples were dominated by 1 × 1 and 2 × 2, the contents of which accounted for more than 80% of the total fringes.

Production and characterization of microsphere reinforced polyester composite: Modeling of thermal decomposition with ANN and optimization studies by RSM

In this research, hollow inorganic microsphere (Q-cel) reinforced unsaturated polyester composite is produced, and its thermophysical properties have been characterized. The thermal decomposition kinetics of the composite obtained at different heating rates and various compositions are modeled using artificial neural networks. Also, the production optimization of the polyester composite has been evaluated using response surface methodology. The study has been repeated for certain heating rates (5, 10, and 20 K/min). Activation energies of polyester composites have been calculated using thermogravimetric data and kinetic methods. Changes in activation energy during the thermal decomposition (4 wt.% Q-cel, 94 wt.% UP, 580–660 K, and 20 K/min) of the composite are compared using FWO (129.4 kJ/mol), KAS (127.6 kJ/mol), and CR (126.5 kJ/mol). According to the results, it is seen that the activation energy goes up as the temperature and Q-cel ratio by the mass increase. When the amount of the filler in the polyester composite increases, the thermal conductivity coefficient also rises. As well as, it is determined that as Q-cel ratio in the mixture raises, the density of the composite decreases and Shore D hardness goes up. HIGHLIGHTS A new and original composite has been produced with Qcel-reinforced unsaturated polyester. The density of the composite is low, its thermal stability is high, and its easy processability will provide a great convenience. Modeling the Qcel ratio of the composite with RSM and the activation energy with ANN have been provided fast, efficient and optimum results. With the composite obtained, both lightweight and thermally stable polymer materials will be produced.

A comparative study on pyrolysis kinetics and thermodynamic parameters of little millet and sunflower stems biomass using thermogravimetric analysis

Several biochemical and thermochemical routes including pyrolysis, liquefaction, combustion and gasification are used to convert biomass to several bioproducts and green fuels. The current investigation included two important biomass namely, little millet stem (LMS) and sunflower stem (SS), whose potentiality as useful feedstocks is largely unexplored. The presence of considerable level of cellulose accumulation (approx. 30 %), volatiles (approx. 67 %) and high heating value (approx. 14 MJ/kg) in both the biomass, inferred their potentiality to be used as feedstocks in the pyrolysis process. The estimate of activation energy for LMS was reported as 191.14 kJ/mol (FWO), 191.46 kJ/mol (KAS) whereas for SS, the activation energy was estimated as 166.52 kJ/mol (FWO) and 162.68 kJ/mol (KAS). The difference between change in enthalpy and activation energy was small (5 to 6 kJ/mol) for both the biomasses, indicating the feasibility of combustion process. From Z(α) analyses, the experimental curve was seen passing through different theoretical curves, indicating complex nature of pyrolysis process for both the biomass.

SYNTHESIS OF GO-PEDOT:PSS NANOCOMPOSITES AND STUDY OF THEIR ELECTRICAL PROPERTIES FOR NO2 GAS SENSOR APPLICATION

In this study, GO and GO-PEDOT:PSS nanocomposite films were prepared by using the modified Hummer method and spin-coating, respectively. GO-PEDOT:PSS films with different weight ratios of GO (0.015, 0.03, 0.045 and 0.06) were prepared to study the effect of the GO additive on nitrogen dioxide (NO2) sensing performance. XRD and AFM were used to determine the crystal structure and the topography of the GO-PEDOT:PSS films. The effects of GO concentration and temperature on electrical conductivity and the change in activation energy of PEDOT:PSS films were also investigated. The findings show that as the temperature rises, the electrical resistance reduces, and as the concentration increases, the activation energy decreases.

High temperature dielectric and impedance spectroscopy study of LaCo0.7Nb0.3O3

We report the high temperature dielectric and ac impedance spectroscopy investigation of Nb substituted LaCo0.7Nb0.3O3 polycrystalline sample. The maximum dielectric constant value was observed ≈1400 at around 400 K where the peak value shows a decreasing trend at higher temperatures and frequency. Similar variation was reflected in the dielectric loss behavior with temperature, which shows the thermal activation of the charge carriers in the material. The analysis of high temperature impedance spectroscopy data shows the grain and grain boundary contributions by fitting the Nyquist plots to the equivalent circuit. From the analysis of the impedance and modulus spectra, it was possible to discern between the effects of overlapping grains, grain boundaries, and electrode interfaces. The relaxation time decreases with an increase in the temperature and the activation energy changes from 0.44 eV to 0.56 eV at around 400 K, which is due to involvement of thermal activation in the conduction of charge carriers.

Investigating the Thermo-Oxidative Degradation and Operational Limits for Hybrid AlOX–PET Fabrics Created Via Vapor Phase Infiltration (VPI)

In vapor phase infiltration (VPI), vapor phase metalorganic precursors are diffused into polymers and further reacted with water vapor to form hybrid organic-inorganic materials. Hybrid materials created through VPI have shown utility in industrially relevant applications including membranes for chemical separation, photovoltaics, and catalysis. VPI can be conducted at different temperatures, with each processing temperature resulting in different chemical states that alter the properties of the hybrid material. In this project, polyethylene terephthalate (PET) fabrics are infiltrated with a trimethylaluminum (TMA) precursor and co-reacted with water vapor at a range of infiltration temperatures (60-140 ˚C) to create hybrid AlOX - PET fabrics. The purpose of this project is to develop a set of thermal operational limits for these hybrid organic-inorganic materials and translate techniques for thermal characterization from the textile community to the VPI community. Thermo-oxidative degradation for the hybrid fabrics was quantified through changes in degradation temperature and activation energies. Thermogravimetric analysis (TGA) was employed to quantify inorganic loadings (~20 weight percent inorganic), degradation temperatures, and the activation energy for degradation using the Flynn/Wall/Ozawa (FWO) method for decomposition kinetics. Most hybrid fabrics demonstrated lower degradation temperatures than neat PET, except for the fabric infiltrated at 140˚C which had increased degradation temperatures. Activation energies for the first thermal-oxidative degradation event for the 80˚C, 100˚C, and 140˚C fabrics decreased by a range of 62˚C to 84˚C compared to neat PET, while the 120˚C fabric’s activation energy slightly increased by 10˚C. Additionally, a new degradation event occurred for fabrics infiltrated at low temperatures which was explored through FTIR characterization and attributed to structural rearrangements of the inorganic component. Overall, this work demonstrates how VPI process conditions alter both the organic-to-inorganic chemical binding as well as the inorganic cluster structure in these hybrids and how these structural features affect thermophysical operational limits of the material.

Quantitative Assessment of Structure Reactivity Correlation on Rapid Kinetics of Halogenated Aromatic Compounds: A Hydrodynamic Voltammetry Study

Halogenations are mostly rapid and cannot be studied by the conventional method, hence, hydrodynamic voltammetry which is based on voltammetric principles can be adopted to study these fast reactions, provided either a reactant or a product in the reaction is electro reducible. These reactions are mostly electrophillic substitution reactions and second order when molecular halogens are used as a reagent. Rapid kinetic studies on some important halogenated aromatic compounds at rotating platinum electrode (RPE) using hydrodynamic voltammetry are summarized and reviewed for a period of the last few years. The advancement made to the technique as well as its application for mechanistic purposes, electrode kinetic measurements, pharmaceutical, and analytical applications have been addressed. As an investigative tool, kinetics has been used to obtain thermodynamic data that allows a quantitative comparison of the reactivity of the two substrates such as activation energy, velocity constants, frequency factors, and entropy change. This aids in configuring the mechanistic route, from among those theoretically possible by checking the compatibility with experimentally observed facts.

Influence of Strong Ionic Interaction on the Kinetics of Graphite Intercalation Compound Formation.

Graphite intercalation compounds (GICs) of anions have attracted much attention as a positive electrode for use in dual-ion batteries. In this study, GICs of bis(trifluoromethanesulfonyl)amide (TFSA) anions were electrochemically synthesized in ionic liquids with lithium or magnesium salts to investigate the kinetics of GICs formation. By varying the concentrations of LiTFSA or Mg(TFSA)2 , the activation energy of interfacial anion transfer resistance was discovered to be correlated with the viscosity of the ionic liquid electrolytes. Such a change in activation energy is unique to ionic liquids and not seen in common molecular solvent electrolytes. In addition, ionic liquids with nearly identical viscosities exhibited similar resistances and activation energies. The relationship between the viscosity of ionic liquids and the kinetic parameters of interfacial anion transfer found in this study provides important insights for the construction of dual-ion batteries that aim for both high capacity and high rate capability.

Effect of hydroxyl (·OH) radicals on the progression of NaBH4 hydrolysis reaction on fcc-Co surfaces: A DFT study

The effect of ·OH radical in the medium for the sodium borohydride (NaBH4) hydrolysis reaction in presence of face-centered cubic (fcc) Cobalt (Co) catalyst was investigated based on density functional theory (DFT). The main purpose is to determine the effective cobalt surface for the disassociation of hydrogen from NaBH4, and the catalyst performance for the intermediate steps formed by the decomposition of water on the cobalt surface. Low index fcc-Co catalyst surfaces were used for the catalyzed NaBH4 hydrolysis reaction. As a result of the calculations, Co(111) is the most effective surface among the low index fcc-Co surfaces for the hydrolysis reaction. Therefore, the possible steps of the NaBH4 hydrolysis reaction are modeled on the Co(111) surface for ambient conditions containing different concentrations of H* atoms, ·OH* radicals, and H2O* molecules. According to our results, the concentration of ·OH* radicals adsorbed on the surface is directly proportional to the change in activation energy. In addition, calculations for the possible second and third steps of the hydrolysis reaction showed that the steps involving NaBH3OH and NaBH3O base molecular structures, respectively, are more feasible than others. This will be a guide for experimental studies in terms of time and cost.

Effect of solvent swelling with different enhancement methods on the microstructure and pyrolysis performance of Hefeng subbituminous coal

Hefeng subbituminous coal (HSBC) was swelled with different enhancement method, including mechanical activation (THSBC2), ultrasonic radiation (THSBC3) and microwave radiation (THSBC4), with isometric N-methylpyrrolidone (NMP) and 1-ethyl-3-methylimidazole acetate (IL) mixture as the solvent (the un-enhancement coal was marked as THSBC1). Pore structure and pyrolysis performance of each sample were discussed. Results showed that the five samples presented abundant micropores, mesopores and some macropores from N2 adsorption-desorption profiles. In lower P/P0 range, the pore type was mainly semi-open pore closed at one end that did not affect the hysteresis loop of the samples; in higher P/P0 range, the pore types were mainly open pore (including cylindrical pore with two open ends and parallel plate pore with four open sides), ink-bottle pore and semi-open pore closed at one end. Hysteresis loops of THSBC3 and THSBC4 were similar to THSBC1, and the pore types were similar to some extent. The hysteresis loop in higher P/P0 range of THSBC2 was not obvious, indicating that there were more semi-open pores closed at one end (with no ink-bottle pore). Pore structure and pyrolysis kinetics of each sample were investigated, and fractal dimension and kinetic parameter were calculated by Frenkel-Halsey-Hill model and Coats-Redfern model, respectively. Results demonstrated that the fractal dimensions D1 (related to surface roughness) and D2 (related to complexity of inner pore structure) were closely related to the pore structure. Compared to THSBC1, the value of D1 is larger for THSBC2, presenting more obvious surface roughness. While the complexity of the pore structure of the five samples is little different, showing very close D2. In addition, the change of pyrolysis activation energy was more obvious for the sample of THSBC2 (29.027 kJ∙mol−1), 4.75% less than THSBC1 (30.482 kJ∙mol−1), which might be attributed to the violent collision process caused by mechanical activation. HSBC presents higher solid yield during pyrolysis, up to 64%, and the solid yield of others samples is around 60%. Compared with HSBC, the gas yields of other four samples have little change, while the liquid yields are increased, indicating that the swelling treatment can promote the formation of tar.

Synthesis, Thermal Analysis, and Thermodynamic Properties Study of New Quinoline Derivative and Their V(IV), Co(II), and Cu(II) Complexes

A new ligand (E)-2-((2-chloro-6-methylquinoline-3-yl)methylene)-N'-((E)-(2-chloro-6-methylquinoline-3-yl)methylene)hydrazine-1-carbothiohydrazide (QH) was prepared by reacting hydrazine hydrate with carbon disulfide to yield thiocarbohydrazide. The thiocarbohydrazide in the second step was treated with a quinoline derivative 2-chloro-6-methylquinoline-3-carbaldehyde to yield the ligand. The ligand was identified by spectroscopic techniques FTIR, 1H-NMR, and 13C-NMR. Next, vanadium (V), cobalt (Co), and copper (Cu) complexes were prepared in the [M:L] ratio of 1:1 (QV, QCo, QCu). The complexes were characterized using FTIR, ESI, magnetic susceptibility, and molar conductivity. The thermal analysis (TGA) of V(IV), Co(II), and Cu(II) complexes were studied. The activation thermodynamic parameters, such as the energy of activation, enthalpy, entropy, and free energy change of the complexes, were evaluated, and the stabilities of the thermal decomposition of the complexes were discussed.

Tuning the workfunction of ZnO through surface doping with Mn from first-principles simulations

Surface doping of ZnO allows for the tailoring of the surface chemistry of the material while preserving the superb electronic structure of the bulk. Apart from obvious changes in adsorption energies and activation energies for catalysis, surface doping can alter the workfunction of the material and allow it to be tuned for specific photocatalytic and optoelectronic applications. We present first-principles electronic structure calculations for surface doping of Mn on the ZnO (0001) surface. Various dopant concentrations have been considered at the out-most (surface) layer of Zn atoms, while the interior of the material is kept at the ideal wurtzite structure. For each system, the surface energy and surface workfunction have been calculated. Both workfunction and surface energy drop with increasing Mn concentration for O-terminated surfaces, while more complex behaviour is observed in metal-terminated ones. We discuss trends in surface stability and surface electronic structure of this material and how they affect its properties.

Carbon nanotubes decorated with Pt as a viable electrocatalyst system using electrochemical atomic layer deposition

Surface limited redox replacement (SLRR) is an electrochemical deposition method designed to deposit metal thin films, typically onto other metals, that are mere monolayers in thickness, where such low dimensions allows catalyst-support interactions to affect catalyst efficiency. Here we explore the growth of Pt, using iterative Cu-mediated SLRR cycles, directly onto carbon nanotubes (CNTs) which are potentially good candidates as electrocatalyst supports due to their electrical conductivity and chemical resistance. Scanning electron microscope images showed that Pt grew as nanoparticles, and hydrogen underpotential deposition from cyclic voltammetry showed that the active surface area approached an asymptotic value around eight iterations. Catalytic activity, measured using the oxygen reduction reaction, reached a maximum at ten iterations. Both are shown to be influenced by the growth mode, but electrochemical modeling indicates that the trend in activity is also due to a change in activation energy, possibly due to changes in Pt electronic structure due to interactions with the CNTs. Durability cycling showed a greater than 85% retention of surface area for the first 10,000 cycles of accelerated stress testing but decayed steadily to about 50% after 30,000 cycles. This performance may be related to the phenomenon that limits surface area growth.

Effect of interface carbonization on dielectric properties of potassium nitrate nanocomposite based on porous glasses

Dielectric properties and structure of pure and carbon-modified nanocomposites on the base of porous glasses with an average pore diameter of 6 nm (PG6) with embedded KNO3 have been studied at the temperature diapason of 300–430 K and at frequencies of 0.1–3 × 106 Hz on cooling. X-ray diffraction studies of these samples have shown, that in modified and unmodified composites there is a mixture of the low-temperature paraelectric phase ([Formula: see text]-phase) and the ferroelectric [Formula: see text]-phase. In modified composites, a decrease in permittivity and conductivity is observed. Dielectric response has been analyzed in the framework of modern theoretical models. Two relaxation processes have been identified and their origin has been determined. It has been found that the main contribution to the dielectric response of nanocomposite material PG6+KNO3 is provided by charge polarization on interfaces, which can be governed by modifying the inner pore surfaces. DC-conductivity of both composites has been estimated and the activation energies have been determined. Activation energy change observed in a vicinity of 360 K is attributed to the phase transformation and the appearance of KNO3 [Formula: see text]-phase.

Phytochemical Constituent and Anti-corrosion Properties of the Root Extract of Phyllanthus mellerianus (Nvo-nkwu) Plant on Mild Steel in 1.5M HCl Medium

The research reports on the phytochemical constituents and anti-corrosion properties of the root extract of Phyllanthus mellerianus on mild steel in 1.5M HCl. The powdered root was extracted with ethanol and concentrated with a rotary evaporator. The phytochemical constituents’ were achieved through GC-MS, FTIR and wet analysis. The anticorrosion properties were investigated via weight loss, hydrogen evolution techniques, and SEM analysis. The phytochemical investigation reveals the presence of tannins, flavonoids, phenols, and terpenoids at reasonable percentages. At different temperatures of 303K, 313K, and 323K, the inhibition efficiency, enthalpy, entropy, activation energy, Gibbs free energy, and adsorption isotherms were extrapolated with some models. The inhibition efficiency increased with an increase in the concentration of the extract, an indication of the inhibitive property of the root extract. This was supported through the extrapolation of Gibbs free energy, activation energy, enthalpy, and entropy change. The thermodynamic variable shows that the mechanism of inhibition was physisorption, whereas the kinetics study confirmed a first-order kinetic on the corrosion of mild steel in hydrochloric acid. The values of the linear regression R2 confirm the inhibitive impact of the root extract on mild steel in an acid medium. The presence of the heteroatoms N, O, and S in the root extracts of Phyllanthus mellerianus has been attributed to their inhibitory effectiveness.

A quantum chemistry study of the deoxidation of lignite during hydrothermal dewatering treatment

ABSTRACT To investigate the deoxidation mechanism of lignite under hydrothermal dewatering (HTD) condition, the removal mechanism of carboxyl and methoxy groups was calculated at the B3LYP-D3/6-311 G(d,p) level by using density functional theory. The feasibility of deoxidation was analyzed from the perspective of thermodynamics and kinetics, and the influences of hydrogen bond on the deoxidation process were analyzed. The results showed that the activation energy of decarboxylation was lower than that of demethoxide, indicating that the carboxyl group had high reactivity. Through the hydrogen bonding between the carboxyl group and water, the activation energy of indirect decarboxylation (lignite-C6H5-COOH→lignite-C6H6-COO→lignite-C6H6+ CO2) decreased by 10 kJ/mol, whereas the activation energy of direct decarboxylation increased by 8 kJ/mol. The activation energy of demethylation was reduced by 25 kJ/mol, and the change in activation energy of subsequent decarbonylation was similar to that of decarboxylation. The two-step elementary reaction benefitted the deoxidation reaction.