Apparent Pre-exponential Factor Research Articles

Evaluating the Reaction Kinetics on the H2 Reduction of a Manganese Ore at Elevated Temperatures

AbstractThis study investigates the hydrogen reduction of Nchwaning manganese ore at elevated temperatures to enhance understanding of reaction kinetics and optimize industrial applications. Experimental investigations were conducted across temperatures ranging from 600 °C to 900 °C to observe reduction behavior and identify rate determining steps. Thermogravimetric analysis (TGA) was employed to monitor manganese ore weight loss, facilitating precise measurement of reduction rates. Various kinetic models validated experimental outcomes for H2 reduction, revealing an apparent activation energy (Ea) of 65.76 kJ/mol and an apparent pre-exponential factor (k0) of 319.66 min⁻1. The rate constant (k) exhibited a significant temperature-dependent increase, following the Arrhenius equation where rates approximately doubled every 100 °C, rising from 0.037 min⁻1 at 600 °C to 0.377 min⁻1 at 900 °C. Morphological and compositional analyses using scanning electron microscopy (SEM) and X-ray diffraction (XRD) assessed structural changes post-reduction. Results demonstrated that pre-reduction temperature critically influences the physical and microstructural properties of the ore particles, particularly above 700 °C, where a notable reduction in BET (Brunauer–Emmett–Teller) surface area and pore volume indicated sintering within the ore. The rate determining step for this reduction process is most likely the chemical reaction at the gas–solid interface between hydrogen and the manganese ore. These findings highlight advancements in efficient manganese ore reduction processes, with significant implications for metallurgical practices and the hydrogen economy. Graphical Abstract

Thermal behavior of ammonium fluorosilicates complexes: Obtaining and kinetic analysis

In this work, a mixture of (NH4)3SiF7/(NH4)2SiF6 powders was obtained as a by-product of the Li extraction process from α-spodumene aluminosilicate with NH4HF2. The thermal behavior of the powders was analyzed by non-isothermal thermogravimetric experiments. The kinetic parameters that describe the processes involved were obtained using mathematical deconvolution, Friedman's method, combined kinetic analysis, and nonlinear regression optimization. The results show that the process occurs in two partially overlapping steps, the thermal decomposition of (NH4)3SiF7 into (NH4)2SiF6 and the subsequent sublimation of (NH4)2SiF6. The apparent activation energies were 72.6 and 79.8 kJ/mol for steps 1 and 2, respectively. The apparent pre-exponential factors were 1.19 × 106 and 2.50 × 105 s-1, respectively. The kinetic models indicated that Step 1 follows an A2 model, while Step 2 follows an F0 model. Finally, the resulting kinetic parameters allowed the reconstruction of the original experimental curves and obtaining predictions of curves recorded under other heating programs.

An Innovative Technology for Utilization of Oolitic Hematite via Microwave Fluidization Roasting: Phase, Structure and Reaction Kinetics Analyses

ABSTRACT Oolitic hematite, as a complex and refractory iron ore resource, can be efficiently utilized by magnetic roasting. In this study, to ascertain the mechanism of microwave fluidization roasting technology (MFR) for the utilization of oolitic hematite, the influence of MFR on mineral magnetism, ore microstructure, mineral phase and kinetics was further explored and clarified. The results show that under the optimized conditions of the roasting temperature 650°C for 5 min, CO concentration 30%, and ore fineness with −74 μm accounting for 90%, the saturation magnetization and saturation susceptibility of the roasted product reached the maximum respectively. Microstructure analysis shows that microcracks were generated between iron minerals and gangue minerals and gradually extended to the core of the oolitic structure with a prolongation in pretreatment time, which was particularly propitious to upgrade the iron minerals in the subsequent grinding and magnetic separation process. The isothermal kinetics shows that the apparent activation energy and pre-exponential factor of kinetic parameters in MFR process decreased gradually with a decrease in ore particle size, and the mechanism function for ore with the particle of −0.15 mm was in accordance with the one-dimensional diffusion model k = 3.805exp(−24075/RT).

Novel synthesis of poly(2-acryloyloxyethyl ferrocenecarboxylate) as quasi-reversible redox-active gel polymer electrolytes

In this research, the electrochemical behavior of poly(2-acryloyloxyethyl ferrocenecarboxylate) (PAEFc) synthesized via an atom transfer radical polymerization (ATRP) strategy was investigated in a series of tetrabutylammonium perchlorate (TBAP)/acetonitrile vs. Ag/AgNO3 system. In comparison, series of PAEFc electrolyte with specific molecular weight and depositing concentration were analyzed via cyclic voltammetry (CV) method. Furthermore, chronocoulometry (CC) technology has been employed on the dynamic electron/mass transfer mechanism and the determination of kinetic parameters (diffusion coefficients, reaction rate constant, etc.). Finally, it is demonstrated that anodic and cathodic process of PAEFc was controlled by diffusion and adsorption–diffusion, respectively. Meanwhile, the kinetic parameters of the typical quasi-reversible redox process were obtained, of which the apparent activation energy (Ea) and pre-exponential factor (A) were 121.38 kJ mol−1 4.28 × 1018 s−1, respectively.

The roles of ZnFe2O4 and α-Fe2O3 in the biphasic catalyst for the oxidative dehydrogenation of n-butene

Iron-based catalysts were one of the most effective catalysts in the oxidative dehydrogenation of n-butene, however, the roles of spinel and α-Fe2O3 in the biphasic catalyst have remained controversial. Herein, we found that biphasic catalyst exhibited much superior catalytic performance compared to each individual component. According to the temperature programmed desorption, XPS and kinetics studies results, it was speculated that ZnFe2O4 was primarily responsible for activating n-butene and α-Fe2O3 was used to activate O2. Moreover, the key factor for the catalytic activity of the biphasic catalyst were investigated and kinetic studies showed that reaction rates were well correlated with the apparent pre-exponential factor. Therefore, the efficient contact between ZnFe2O4 and α-Fe2O3 may lead to the improvement of the activity. In addition, the reaction mechanism over α-Fe2O3/ZnFe2O4 was proposed to be the interface reaction based on the catalytic evaluation results of substituting ZnFe2O4 by the redox-inactive ZnAl2O4.

Thermal characteristics and kinetic analysis of coal-oxygen reaction under the condition of inert gas

ABSTRACT To ascertain fire prevention and extinguishing mechanism of inert gas in coal mine goaf, TG-DTG-DSC experiments of coal-oxygen reaction at 30–850℃ under N2 and CO2 conditions were carried out, and the thermal characteristics and kinetic factors were analyzed. The process of coal-oxygen reaction was divided into five stages by the four characteristic temperatures of T1 (temperature at which water evaporates completely), T2 (temperature at which coal begins to decompose intensely), T3 (ignition temperature) and T4 (burnout temperature). With the decrease of O2 concentration, T2, T3, and T4 increased, the total mass loss and heat release decreased, and the decomposition degree increased. The injection of N2 and CO2 reduced the apparent activation energy and pre-exponential factor of T1–T2, T2–T3 and T3–T4 stages, and there was dynamic compensation effect between activation energy and pre-exponential factor. Under the same O2 concentration conditions, the effect of CO2 was more significant than that of N2. Reducing oxygen concentration by injecting N2 and CO2 could rapidly reduce the degree of coal-oxygen reaction rate. This study is of great significance for the prediction of coal spontaneous combustion and the using of N2 and CO2 coal fire prevention and extinguishing technology.

Diffusion effect and evolution of kinetic parameters during coal char-CO2 gasification

Large coal char particle (LCC, 12 mm and 20 mm cube) and small coal char particle (SCC, less than 50 μm) were gasified under different temperatures in fixed bed reactor and thermogravimetric analyzer, respectively. The effectiveness factor and kinetic parameters (apparent activation energy and apparent pre-exponential factor) were also calculated to investigate the diffusion effect and the evolution of kinetic parameters during coal char-CO2 gasification. It was found that the internal effectiveness factor was not constant during the conversion process of coal char. In the intermediate and later stage of gasification reaction, there was a remarkable increase for the effectiveness factor. The effectiveness factor of LCC and SCC in the higher temperature range were all much less than 1, this indicated that the gasification of LCC and SCC at high temperature was limited by pore diffusion. Apparent activation energy and apparent pre-exponential factor evolved gradually during the whole conversion process, the existence of diffusion effect would lead to a certain change for the evolution law. A kinetic compensation effect was observed between the apparent activation energy and apparent pre-exponential factor during the conversion process of coal char. When the diffusion effect was very significant, it would have a certain weakening effect on the compensation effect.

Mechanistic insights into the kinetic compensation effects during the gasification of biochar in H2O

This study aims to gain insight into the mechanism and kinetics during the gasification of biochar in steam, which was formed in situ in a fluidised-bed reactor using mallee wood in two particle size ranges of 0.80–1.0 mm and 2.0–3.3 mm. The overall biochar gasification rate and the formation rates of key product components were calculated by continuously monitoring the product gas stream with a quadrupole mass spectrometer. The kinetic compensation effects reveal that CO and CO2 are both formed from the heterogeneous reactions between the biochar surface and H2O. CO2 is formed either by the surface (biochar)-catalysed water-gas-shift reaction or directly from the carbon active sites involving the same intermediate for the formation of CO, as revealed by the apparent activation energies and apparent pre-exponential factors for CO and CO2 formation. The changes in the particle size of biomass substrate do not affect the extent of the kinetic compensation effects of biochar consumption and formation of CO, CO2 and H2 in the kinetics-controlled and mixed regimes. The similar extent of the kinetic compensation effects of H2 formation and biochar consumption for both particle sizes indicates that the formation of H2 also mainly involve the carbon active sites on the biochar surface instead of the gas-phase water-gas-shift reaction.

Reduction Kinetics and Microscopic Properties Transformation of Boron-Bearing Iron Concentrate–Carbon-Mixed Pellets

ABSTRACTSelective reduction followed by magnetic or melting separation is an innovative and effective method for the comprehensive utilization of boron-bearing iron concentrate. As the raw material of boron products, the reserve of szaibelyite ore is significantly depleted. Therefore, the comprehensive utilization of boron-bearing iron ore resources becomes more and more important. To study the reduction kinetics in depth, a boron-bearing iron concentrate containing 52.18% total iron and 5.50% B2O3 was reduced isothermally with coke as the reductant. The results showed that both the reduction degree and the maximum reduction rate increased with temperature and C/O molar ratio (fixed carbon in the coke to oxygen in iron oxides in the ore). 98.00% reduction degree and 0.00414 s−1 maximum reduction rate were achieved at 1523K of reduction temperature and 1.4 of C/O molar ratio. The reduction kinetics can be best described by Avrami–Erofeev model, g(α) = [−ln(1-α)]3/2. The apparent activation energy and pre-exponential factor were 264.565 kJ·mol−1 and 1.87 × 107 s−1, respectively. The phase transformation observed by XRD agreed well with the results of reduction degree analysis. The micro-structure evolution behavior of the pellets was analyzed by SEM and EDS. As the reduction time increased from 0 to 600 s, most of the iron oxides were reduced to metallic iron, leaving other major elements in the slag phase. The slag phase mainly consisted of the solid solution of Mg2SiO4 and Mg3B2O6. With further increase to 1200 s, the metallic iron particles migrated and flocculated together forming bigger particles.

Theoretical and numerical study on ignition behaviour of coal dust layers on a hot surface with corrected kinetic parameters.

Industrial fires and explosions initiating from self-ignition of combustible porous dust deposits represent a serious hazard for human beings, environment and industry. Understanding the fundamental basis of combustible dust ignition behaviours at different geometries is of importance to prevent and mitigate the accidental risks. A correlation of self-ignition temperatures (SITs) measured by hot-oven tests and minimum ignition temperatures of dust layers (MITLs) determined by hot-plate tests has been established previously. However, this analogy approach based on Frank-Kamenetzkii model is limited by ignoring the influence of oxygen diffusion. In this work, an improved method is developed by implementing a correction factor for the pre-exponential factor caused by the boundary geometry. This method is testified by comparing with experimental data, previous analogy method and numerical simulation. Results show that our proposed method performs a better predictability of MITLs and simplicity. The improved analogy method indicates that the different boundary geometries of a dust deposit significantly impact the apparent pre-exponential factor, while have an ignorable influence on the activation energy, which is also verified by numerical investigations. Furthermore, the numerical model with the corrected kinetic parameters provides a satisfactory explanation compared with experimental observations regarding to temperature and concentration evolutions of dust layers.

Mathematical modeling of temperature effect on algal growth for biodiesel application

Microalgae biomass is promising feedstock for the industrial production of biodiesel. Hence, research and development are required in various domains especially optimizations of growth conditions including temperature effect for mass scale operation (production of biomass, harvesting, extraction of lipid, etc). Since in middle east region, seasonal temperature variation and more rapid daily fluctuations are amenable to alter the growth kinetics of microalgae in outdoor culture and hence affect algae biomass production efficiency. Therefore, in this study, a mathematical model was developed to calculate how the algae sp. (Chlorella kessleri) will react at different temperatures. The model integrates Monod model and Arrhenius equation, and as such it describes the relationship of algal growth rate with culturing temperature and limiting nutrient concentration. The apparent activation energy and pre-exponential factors were calculated to be 2537 cal/mol and 0.0077 day−1, respectively. The developed models could be useful to anticipate the effective impacts of temperature on outdoor algae culture.

Kinetic compensation effects in the chemical reaction-controlled regime and mass transfer-controlled regime during the gasification of biochar in O2

This study aims to investigate the kinetic compensation effects during the char-O2 reaction in a fluidised-bed reactor for two particle sizes of 0.80–1.0 mm and 2.0–3.35 mm. The rate of char-O2 reaction was determined by analysing the gasification product gas composition in a quadrupole mass spectrometer. The char-O2 reaction exhibited different kinetic compensation effects between apparent activation energy and apparent pre-exponential factor in the kinetics-controlled, diffusion-controlled and mixed regimes for both particle sizes. The same reaction mechanism is followed during the char-O2 reaction in the kinetic regime at same or at different pyrolysis temperatures as revealed by the kinetic compensation effects. In the mixed regime, higher diffusion limitations increased the m and c values in the kinetic compensation effect lnAapp = mEapp + c for any given particle size. Due to higher rates of reaction at higher char conversions, the char-O2 reaction switched from kinetics-controlled to mixed regimes, resulting in higher slopes ‘m’ and y-intercepts ‘c’ in the kinetic compensation effects. The absence of isokinetic temperature at higher conversions indicates that char properties changed significantly at higher conversions compared with those at lower conversions.

Phenanthroimidazole-based monomers: synthesis, properties and self-polymerization

Phenanthroimidazole-based monomers with reactive vinyl groups were synthesized, and their thermal, optical, photophysical and electrochemical properties were investigated. The monomers exhibited high thermal stability with 5% weight loss temperatures (Td) ranging from 378 to 409 °C. Thermal degradation of the polymerization products apparently takes place in this temperature range. The solutions of the monomers exhibit emission peaks in the range from 388 to 398 nm. In the solid state, the emission of these molecules shows red shift which is coherent with the similar red shifts of the corresponding absorption spectra. Ionization potential values of the compounds estimated by cyclic voltammetry were found to be close and varied in the range from 5.44 to 5.63 eV. Solid-state ionization potentials estimated by photoelectron emission spectrometry varied in the range from 5.54 to 5.66 eV. Self-polymerization of the synthesized monomers was demonstrated by differential scanning calorimetry. The number average molecular weights of the polymerization products of monomers containing substituents at phenyl rings linked to C-2 and N-1 positions of imidazole ring were found to be 100,100 and 196,000, respectively. The apparent activation energy and pre-exponential factor of self-polymerization were found to be dependent on conversion degree. The values of activation energy for self-polymerization of monomers varied in the range from 78.7 to 136.0 kJ/mol (estimated by Ozawa method) and from 78.3 to 139.0 kJ/mol (estimated by Kissinger method).

Kinetics of the hydrolysis of xylan based on ether bond cleavage in subcritical water

In this study, the rate of xylan hydrolysis was investigated. The hydrolysis of tulip poplar was conducted in subcritical water from 180 to 220 °C for 1–240 min. An equation for estimating the hydrolysis of ether bonds in both xylan and xylooligosaccharide from the yield of xylan and xylooligosaccharide was derived, and the rate constant of ether bond hydrolysis was calculated using the derived equation. It was found that our kinetic study can be applied to subcritical water treatment of various lignocellulosic biomass. In addition, linearity between the apparent activation energy and the logarithmic value of the apparent pre-exponential factor was observed. An equation for the apparent reaction rate constant was derived by introducing a factor, α, that reflects the changes in the reaction rate arising from various factors.

Deactivation and Characterization of SCR Catalysts Used in Municipal Waste Incineration Applications

Catalysts used for selective catalytic reduction were deactivated for various times in a slipstream from a municipal solid waste incineration plant and then characterized. The activity for NO reduction with NH3 was measured. The Brunauer–Emmett–Teller surface areas were determined by N2 adsorption from which the pore size distributions in the mesopore region were obtained. Micropore areas and volumes were also obtained. The composition of fresh and deactivated catalysts as well as fly ash was determined by atomic absorption spectroscopy and scanning electron microscopy with energy dispersive X-ray analysis. The changes in surface area (8% decrease in BET surface area over 2311 h) and pore structure were small, while the change in activity was considerable. The apparent pre-exponential factor was 1.63 × 105 (1/min) in the most deactivated catalyst, compared to 2.65 × 106 (1/min) in the fresh catalyst, i.e. a reduction of 94%. The apparent activation energy for the fresh catalyst was 40 kJ/mol, decreasing to 27 kJ/mol with increasing deactivation. Characterization showed that catalytic poisoning is mainly due to decreased acidity of the catalyst caused due to increasing amounts of Na and K.Graphical

In-situ steam reforming of biomass tar over sawdust biochar in mild catalytic temperature

In order to fully understand the catalytic activities of sawdust biochar on in-situ tar steam reforming in mild catalysis temperatures (650–800 °C), the experiment was carried out in a two-stage fluidized bed/fixed bed reactor. The structural characteristics of biochar were analyzed by the Raman and XPS spectroscopies. The results of in-situ tar steam reforming over biochar were performed by gas chromatograph/mass spectrometer (GC/MS). Kinetic aspects of tar steam reforming in mild temperatures were calculated to evaluate the catalytic effect of sawdust biochar on in-situ tar reforming. The results indicate that sawdust biochar has a positive effect on in-situ tar steam reforming, while the effect is not proportional to biochar amount. During the in-situ tar steam reforming over biochar, the changes of oxygen-containing functional groups on biochar surface are mainly caused by the C-O bonds. The first order kinetic rate constant of sawdust biochar for the heterogeneous reforming of biomass tar at 650–800 °C is found to have an apparent activation energy (Eapp) of 35.03 kJ/mol with the apparent pre-exponential factor (kapp) of 1.8 × 104 m3 kg−1h−1. The biochar structures were considerably transformed during the tar steam reforming. According to GC/MS analysis of biomass tar, the biochar seems to promote the cracking of large ring polyaromatic tar compounds to form 1-ring aromatic ones.

Thermal behavior and compatibility study of dihydroxylammonium 3,4-dinitraminofurazan

ABSTRACTA large number of nitramino-featured energetic salts have been reported and some of them show promising properties. Among them, the dihydroxylammonium 3,4-dinitraminofurazan (HADNAF) is easy to synthesize and shows high calculated detonation performances and acceptable thermal stability. The non-isothermal kinetics parameters of HADNAF including the apparent activation energy (E) and pre-exponential factor (A) of the exothermic decomposition reaction, and activation entropy (ΔS≠), activation enthalpy (ΔH≠), activation Gibbs free energy (ΔG≠) at TP0 of the reaction and the critical temperature of thermal explosion (Tb) were obtained by Kissinger’s and Ozawa’s method, respectively. Additionally, the compatibility of HADNAF with other materials (e.g. TNT, RDX, HMX, B, Mg) was tested by DSC method.

Boosting electrocatalytic activities of plasmonic metallic nanostructures by tuning the kinetic pre-exponential factor

The electrocatalytic activity of metallic nanostructures is commonly tuned by changing their structure, composition, size and morphology to decrease the activation energy of a rate-determining elementary step. Here, we report that the electrocatalytic activity of the excited plasmonic model Pd nanostructured electrocatalyst for oxygen reduction reaction (ORR) can be enhanced significantly by increasing the kinetic pre-exponential factor. The localized surface plasmon resonance (SPR) effect promotes a two-orders of magnitude increase in the apparent pre-exponential factor in the Arrhenius equation. Combined experimental and theoretical analyses of the kinetic H/D isotope effect indicate that the observed increase in the apparent pre-exponential factor is attributable to an increased quantum–mechanical proton tunneling arising from the local electric field enhancement upon SPR excitation. Intriguingly, the ORR activity enhancement can be observed under simulated solar light illumination, implying potential for fuel cell applications. The methodology developed in this study is applicable to other proton-coupled electron-transfer reactions, thus opening a promising route to manipulating electrocatalytic reactivity by tuning the kinetic pre-exponential factor and suggesting potential for fuel cell applications.

Investigation on thermal degradation properties of oleic acid and its methyl and ethyl esters through TG-FTIR

Thermogravimetric analysis in conjunction with Fourier transform infrared spectroscopy (TG-FTIR) is used to investigate thermal degradation properties of oleic acid and its esterification derivants of methyl ester and ethyl ester. Based on TG experimental results, which are operated under nitrogen atmosphere from 298 to 773K at the sample temperature heated rates of 5, 10, and 15Kmin−1, respectively, two degradation iso-conversional approaches of Kissinger-Akahira-Sunose (KAS) and Flynn-Wall-Ozawa (FWO) are mentioned to calculate the apparent activation energy, based on which the apparent pre-exponential factor is obtained. Meanwhile, Avrami theory is used for the apparent reaction order estimation. Further, thermodynamic parameters of the apparent enthalpy change, apparent Gibbs free energy change and apparent entropy change are calculated to label the thermal degradation process directly. Also, the evolved gaseous products from TG are detected by FTIR online to understand the thermal degradation comprehensively. Information about fatty acid and its methyl ester and ethyl ester derived from TG-FTIR, especially the comparison between esterification derivants and their feedstock, has been rarely reported and is expected to contribute to the commercial application of biodiesel.

Neon diffusion kinetics and implications for cosmogenic neon paleothermometry in feldspars

Observations of cosmogenic neon concentrations in feldspars can potentially be used to constrain the surface exposure duration or surface temperature history of geologic samples. The applicability of cosmogenic neon to either application depends on the temperature-dependent diffusivity of neon isotopes. In this work, we investigate the kinetics of neon diffusion in feldspars of different compositions and geologic origins through stepwise degassing experiments on single, proton-irradiated crystals. To understand the potential causes of complex diffusion behavior that is sometimes manifest as nonlinearity in Arrhenius plots, we compare our results to argon stepwise degassing experiments previously conducted on the same feldspars. Many of the feldspars we studied exhibit linear Arrhenius behavior for neon whereas argon degassing from the same feldspars did not. This suggests that nonlinear behavior in argon experiments is an artifact of structural changes during laboratory heating. However, other feldspars that we examined exhibit nonlinear Arrhenius behavior for neon diffusion at temperatures far below any known structural changes, which suggests that some preexisting material property is responsible for the complex behavior. In general, neon diffusion kinetics vary widely across the different feldspars studied, with estimated activation energies (Ea) ranging from 83.3 to 110.7kJ/mol and apparent pre-exponential factors (D0) spanning three orders of magnitude from 2.4×10−3 to 8.9×10−1cm2s−1. As a consequence of this variability, the ability to reconstruct temperatures or exposure durations from cosmogenic neon abundances will depend on both the specific feldspar and the surface temperature conditions at the geologic site of interest.