Avrami-Erofeev Model Research Articles

Mechanistic insights into the pyrolysis of pine resin sludge based on experiments and ReaxFF-MD simulation: High-value bio-oil production, nitrogen conversion, kinetics and environmental risk assessment

Forestry and agro-industrial waste conversion into high-value chemicals and fuels is considered one of the hottest ongoing research and industrial topics toward sustainable development. As one of the forestry and agro-industrial waste, pine resin sludge (PRS) with amino acids, pine resin residues and their derivatives such as abietic acid and 4-Epidehydroabietal, have ecological toxicity and require environmentally friendly treatment. Herein, a thermal process of PRS that generates high-value biofuels while reducing environmental pollution was reported. The results of the experiment and simulation reveal that the N of bio-oil is the major evolution direction of N, and the nitrogenous gas product is mainly NH3, a useful synthesis gas. ReaxFF-MD simulations elucidated the migration and conversion mechanism of nitrogen in amino acids. The main products of amino acid pyrolysis are 3-oxo-2-Propenenitrile, 2,4-Cyclopentadien-1-ylidenemethanone and 4R)-4-amino-5,5-dihydroxypentanoic acid. PRS's thermal weightlessness was divided into three phases, and the average activation under non-isothermal conditions was calculated at 78.17, 85.68, 206.42 kJ/mol (thermal decomposition) and final 117.78, 149.31, 195.54 kJ/mol (thermal oxidation). Thermal decomposition and oxidation process followed Avrami-Erofeev model (n=2), three-dimensional diffusion model (n=3) and mampel power (n=1). PY-GC/MS analysis shows that pyrolysis at different temperatures recovered mainly 4-Epidehydroabietal (37.15–89.67 %) for potential use as the biosynthesis of dehydrofolate. The bio-oil from PRS pyrolysis shows an excellent yield (31.43 %) and a high calorific value (37.21 MJ/kg), which is comparable to biodiesel. This work focused on offering an appealing and instructive guide for the logical treatment of sludge from pine resin production.

Study on the Influence of Porous Gel on Coal Pyrolysis Performance Based on Experiments and Molecular Simulation

ABSTRACT Studying the kinetics of pyrolysis in pure coals and mixed coals (coals treated with porous gel) holds immense significance in effectively preventing and controlling the spontaneous combustion occurrences in coal. Firstly, porous gel materials were prepared based on physical mixing and chemical crosslinking reaction. Then, based on the thermogravimetric analysis method, the pyrolysis mechanism of porous gel in coal was studied using Flynn-Wall-Ozawa(FWO), Kissinger-Akahira-Sunose(KAS) and Coats Redfern models. Determine the mechanism of coal pyrolysis reaction using the Reaxx FF MD reaction mechanics model. Finally, the inhibition mechanism of porous gel on coal is analyzed through leading temperature rise experiment. The main reaction mechanisms of coal pyrolysis are the diffusion model in the low-conversion-rate range (D4), the R2 and R3 in the high-conversion-rate range, and the Avrami erofeev model (A2, A3). The molecular simulation results indicate that increasing the heating rate can accelerate the rate of coke cracking.

Hydrogen production from dry reforming of methane, using CO2 previously chemisorbed in the Li6Zn1-xNixO4 solid solution

Li6ZnO4 was chemically modified by nickel addition, in order to develop different compositions of the solid solution Li6Zn1-xNixO4. These materials were evaluated bifunctionally; analyzing their CO2 capture performances, as well as on their catalytic properties for H2 production via dry reforming of methane (DRM). The crystal structures of Li6Zn1-xNixO4 solid solution samples were determined through X-ray diffraction, which confirmed the integration of nickel ions up to a concentration around 20 mol%, meanwhile beyond this value, a secondary phase was detected. These results were supported by XPS and TEM analyses. Then, dynamic and isothermal thermogravimetric analyses of CO2 capture revealed that Li6Zn1-xNixO4 solid solution samples exhibited good CO2 chemisorption efficiencies, similarly to the pristine Li6ZnO4 chemisorption trends observed. Moreover, a kinetic analysis of CO2 isothermal chemisorptions, using the Avrami-Erofeev model, evidenced an increment of the constant rates as a function of the Ni content. Since Ni2+ ions incorporation did not reduce the CO2 capture efficiency and kinetics, the catalytic properties of these materials were evaluated in the DRM process. Results demonstrated that nickel ions favored hydrogen (H2) production over the pristine Li6ZnO4 phase, despite a second H2 production reaction was determined, methane decomposition. Thereby, Li6Zn1-xNixO4 ceramics can be employed as bifunctional materials.

Copper-based chemical looping air separation process: Thermodynamics, kinetic modeling, and simulation of the fluidized beds

Chemical looping air separation is one of the potential processes for oxygen separation from air constituents using oxygen carriers with chemical looping oxygen uncoupling properties that are circulated between reduction and oxidation reactors. Copper-based oxygen carriers have been recognized as a suitable class of materials for the process due to their promising properties and low cost. In this study, copper-based material has been investigated for chemical looping air separation to identify the process parameters that affect the performance and oxygen generation. Firstly, thermodynamic analysis was performed to predict the suitable conditions for oxygen generation and oxygen carrier oxidation, and to model oxygen equilibrium partial pressure. Then, CuO (40 wt%)/ZrO2 was prepared by impregnation synthesis technique to examine the oxygen carrier kinetics using a thermogravimetric analyzer. Results showed that 1.5-dimensional Avrami-Erofeev model best fitted the reduction data with an activation energy of 230 ± 25.2 kJ/mol, whereas the oxidation data were fitted by 1.5 reaction order model with an activation energy of 211 ± 8.0 kJ/mol. The process was simulated using Aspen Plus software package, and the reactors were represented using Fluidbed units to account for both the kinetics and hydrodynamics, to identify the suitable fluidization regimes for specific operating conditions. Simulation results showed that the highest oxygen generation occurred while operating both reactors at bubbling fluidization regimes. Moreover, the temperature effects on the process performance and output were examined, and findings indicated that operating the reducer at 1000 °C and the oxidizer at 800 °C would give out the highest solids conversion and oxygen generation flowrate (23.26 ton/day) with specific energy consumption of 132.26 kWh/tonne of oxygen. However, operating the process at high temperatures would increase the energy penalty and the operating costs, and a more comprehensive technoeconomic study will be required to decide whether lower temperatures operation would be feasible.

A study on self-shielding effect of CaCO3 in cable pyrolysis using gas product analysis and PSO optimization

This paper investigates the self-shielding mechanism of CaCO3 during the pyrolysis of PVC-CaCO3 based cable. Previous studies have focused on HCl adsorption by CaCO3, neglecting the self-shielding effect. In this paper, thermogravimetric-infrared analyzer, pyrolysis gas chromatography-mass spectrometer and particle swarm optimization (PSO) algorithm are used to investigate the self-shielding mechanism. The results show a three-stage pyrolysis process. In the first stage, CaCO3 weakens the autocatalytic decomposition of PVC by absorbing HCl in the solid phase. The 3rd Avrami Erofeev model was obtained by PSO, representing the production of CaCl2 crystals. The second stage demonstrates self-shielding due to a reduction in solid-phase porosity. CaCO3 encapsulation inhibits the pyrolysis of PVC resulting in macromolecular intermediates. The 3rd order-based model was obtained by PSO, with a lower pyrolysis reaction rate compared to pure PVC. In the third stage, the stability of Cl in the solid phase is weakened at high temperatures, which converts the Cl to the gas phase in the form of chlorinated hydrocarbons, leading to the failure of self-shielding effect of CaCO3. The 2nd order-based model was obtained by PSO, which is considered as a result of the solid phase reactions instead of pyrolysis.

The Catalytic Effect of Vanadium on Sorption Properties of MgH2-Based Nanocomposites Obtained Using Low Milling Time.

The effects of catalysis using vanadium as an additive (2 and 5 wt.%) in a high-energy ball mill on composite desorption properties were examined. The influence of microstructure on the dehydration temperature and hydrogen desorption kinetics was monitored. Morphological and microstructural studies of the synthesized sample were performed by X-ray diffraction (XRD), laser particle size distribution (PSD), and scanning electron microscopy (SEM) methods, while differential scanning calorimetry (DSC) determined thermal properties. To further access amorph species in the milling blend, the absorption spectra were obtained by FTIR-ATR analysis (Fourier transform infrared spectroscopy attenuated total reflection). The results show lower apparent activation energy (Eapp) and H2 desorption temperature are obtained for milling bland with 5 wt.% added vanadium. The best explanation of hydrogen desorption reaction shows the Avrami-Erofeev model for parameter n = 4. Since the obtained value of apparent activation energy is close to the Mg-H bond-breaking energy, one can conclude that breaking this bond would be the rate-limiting step of the process.

SiC formation on the carbon nanotube decorated with silicon nanoparticles

Ex situ methods (TEM, XRD, and Raman spectroscopy) have been used to study the processes occurring at the multi-walled carbon nanotube/silicon interfaces (MWCNT/Si) during heat treatment of MWCNT-Si composites containing highly dispersed Si particles deposited on the surface of MWCNTs by CVD method. It has been established that during heat treatment, starting from 900 °C, the formation of SiC particles occurs. A further increase in temperature leads to the formation of polycrystalline SiC particles and a significant shortening of MWCNTs due to the reaction between Si particles and the surface of MWCNTs. It is shown that one can control the size of the formed SiC crystallites by varying the time and temperature of heat treatment. The kinetic dependences of the SiC formation process were studied within the Avrami-Erofeev model. The activation energy for the formation of SiC is estimated at 470 kJ/mol. The influence of heat treatment on the electrical conductivity and porosity of MWCNT-Si composites in the pressure range of 25–175 MPa has been studied.

Hydrogen storage properties of MgH2–Tm: Ni-catalysis vs. mechanical milling

The influence of the addition of nickel on hydrogen desorption from the MgH2–Ni composite was investigated. The composite powder was ball-milled for 15, 30 and 45 min and characterized by XRD, SEM-EDS, PSD, DSC and TPD methods. It was observed that the uniform distribution of nickel decreases hydrogen desorption temperature by more than 100 °C. A kinetic model for the hydrogen desorption process was also determined. The hydrogen desorption reaction in catalyzed samples is described by the Avrami-Erofeev model with the value of parameter n = 4. The apparent activation energy of the hydrogen desorption reaction was decreased with the increase of milling time and the addition of nickel. It has been shown for the first time that two main processes (grinding and the catalytic effect) could be separately analyzed. It is concluded that for investigated short milling times, the catalytic effect of Ni is predominant.

An insight into the thermal processability of highly bioactive borosilicate glasses through kinetic approach

AbstractThe paucity of crystallization resistant bioactive glasses with desired biological functions stands as a bottleneck toward the fabrication of various biomedical constructs such as amorphous coatings, scaffolds, and fibers for advanced tissue engineering applications. In this context, a series of borosilicate‐based bioactive glasses with a range of compositions: (53.88 − x)SiO2–21.7Na2O–21.7CaO–1.7P2O5–xB2O3 (mol%) where x = 0, 13.47, 22.45, 31.43, and 40.41 were prepared to address such limitation. The glasses were primarily investigated for their potential to be processed into amorphous scaffolds through evaluation of crystallization kinetics, sintering behavior, and viscosity–temperature dependence. The inclusion of B2O3 gradually reduces the activation energy of crystallization (Ea), according to the prediction from different kinetic models, whereas Friedman's model‐free method unraveled the variation in Ea as crystallization progresses. The crystallization event is further elucidated by obtaining the Avrami parameter (n) and dimensionality (m) through Matusita–Sakka equation. The optimization of the sintering schedule for amorphous scaffold preparation was accomplished by exploiting isothermal prediction from Avrami–Erofeev model. Moreover, viscosity–temperature relationship for the studied glasses was established to identify the processing window for drawing and sintering. This study proposes a comprehensive approach adopting theoretical models to elucidate suitable high‐temperature process parameters of bioactive glasses avoiding devitrification.

Thermal behavior and kinetic mechanism of microalgae and model compounds

As the third-generation biofuel, microalgae are becoming one of the most promising biomass resources. This study investigated the thermal behavior and kinetic mechanism of microalgae and its model compounds to provide reference for thermal conversion of microalgae to produce energy products. Results showed that, there was a slight mass loss during microalgae pyrolysis among 50–180 °C, which was principally due to the dehydroxylation or deamination of proteins. Microalgae mainly pyrolyzed between 180 and 500 °C, within which, protein and carbohydrate chiefly pyrolyzed below 350 °C, while lipid violently pyrolyzed above 350 °C. Besides, NH3 released before carboxyl and CO2 during soybean protein (PT) pyrolysis, and C = O was generated before CO2 when sucrose (SC) pyrolyzed, while chain C-H generated at higher temperature. In addition, the average activation energy (Ea) for the decomposition of component models was in the order of OL > PT > SC. The Ea of OL reached 123.26 kJ/mol, but protein-rich Spirulina platensis (SP) had higher Ea than that of lipid-rich Nannochloropsis sp. (NS). Besides, the kinetic mechanism of microalgae and its components PT and SC was Reaction-order model, while it was Avrami–Erofeev model for OL pyrolysis.

Cyclic CO2 capture behavior of slag-derived Li4SiO4: A kinetic analysis of CO2 desorption

Two slag-derived Li4SiO4 were synthesized using metallurgical slags as silica sources and used as CO2 sorbents. The CO2 desorption process was studied using a fixed temperature during the sorption step, with different CO2 partial pressures and several desorption temperatures. The kinetic parameters of the CO2 desorption were studied using the Avrami-Erofeev and zero-order models. With the latter, it was possible to calculate the activation energy of the desorption during the first minutes of the process, while with the Avrami-Erofeev model, only the energy associated with the second stage of the model was obtained. The calculated energies with the Avrami model were at least three orders of magnitude greater than the sorption stages reported values. Moreover, both models showed that the Li4SiO4 derived from the electric arc furnace (EAF) slag is less dependent on temperature than the derived from the blast furnace (BF). The kinetic parameters of desorption were also calculated for several regeneration cycles, where it was found that the k values diminish when lower partial pressures of CO2 (PCO2) were used. Finally, the structural evolution after the cyclic study showed the formation of secondary phases in the slag-derived silicates.

Kinetic Study on Reduction of FeO in a Molten HIsarna Slag by Various Solid Carbon Sources

To investigate the reduction behaviour of different reductants such as charcoal (CC), thermal coal (TC), and carbon black (CB) with HIsarna slag, a series of isothermal reduction experiments were performed in a vertical tube resistance furnace (VTF), coupled with a Quadrupole mass spectrometer (QMS) at 1450 °C, 1475 °C and 1500 °C. The results confirm that the highest overall reduction rate was achieved by CC, followed by TC and CB. The reduction mechanism between FeO containing molten slag and the selected carbonaceous materials is determined by studying the morphology of the water quenched samples at the intervals of 1.5, 3 and 5 minutes, using optical and scanning electron microscopes. The results reveal that the overall reaction is controlled by two main mechanisms: (1) nucleation and growth of CO bubbles, proceeded by the gaseous intermediates CO and CO2; and (2) diffusion of FeO in the molten slag. The initial reduction period in which chemical reaction control is dominant, can be described by the Avrami–Erofeev model, whereas the final period is described by the three-dimensional diffusion model.

Products distribution and sulfur fixation during the pyrolysis of CaO conditioned textile dyeing sludge: Effects of pyrolysis temperature and heating rate

Textile dyeing sludge (TDS) is a typical industrial solid waste whose amount surged with the textile industry's development. Pyrolysis treatment is a promising technique for TDS to realize harmless disposal and resource reuse. However, the high content of organic compounds would cause sulfurous pollutants emission, reducing the economic feasibility during pyrolysis. This study aimed to fill the knowledge gaps about the thermal behavior, products distribution, kinetics, and sulfur transformation during TDS pyrolysis in 350–575 ℃ with the heating rate of 60, 600, and 6000 ℃/min, then investigate the sulfur fixation effect of CaO under representative conditions (350 ℃, 650 ℃ with 60 ℃/min, 6000 ℃/min). The primary decomposition stage of TDS is observed in 127–557 ℃, following the Avrami-Erofeev (n = 3) model, while the activation energy presents a convergent tendency with the increased heating rate. The pyrolysis temperature and heating rates impact the cracking of organic compounds, while a weakening effect is found for the sulfur distribution. CaO addition could efficiently realize sulfur fixation in char by absorbing sulfurous gas products, but SO2 escape appeared with the increased CaO fraction. Pyrolysis condition at 650 ℃-60 ℃/min with 10 wt% CaO addition is recommended to achieve high sulfur retention, and the sulfur transformation mechanism in char during the TDS pyrolysis with and without CaO is proposed. Our findings provide novel and fundamental insights into the efficient disposal and pollution control during TDS pyrolysis.

Kinetic Studies on the Steam Gasification of Chars Derived from Coals by Different Isoconversional Methods

Previous studies have successfully assessed the extent to which a kinetic model accurately represents a specific reaction mechanism by comparing the kinetic parameters derived from the kinetic model to those obtained using isoconversional methods. However, this approach remains underdeveloped for the important steam gasification reaction of char. This study addresses this issue by conducting a series of steam-assisted char gasification tests, using thermogravimetric analysis at five different heating rates. The results indicate that the carbon conversion ratio of the char gasification reaction increases with the increasing heating rate. The activation energies of the reaction process are determined with different carbon conversion ratios using three isoconversional methods, including the Flynn–Wall–Ozawa, Kissinger–Akahira–Sunose, and Starink methods. The gasification mechanism is also analyzed using model-fitting methods with a wide variety of carbon conversion models, and the accuracies of the models are evaluated, firstly, by comparing the obtained goodness of fit values of the models with the experimental results, and, secondly, by comparing the obtained activation energies with those derived using the isoconversional methods. The goodness of fit results and the results of the comparisons between the activation energies obtained using the various models with the isoconversional values demonstrate that the three-dimensional Avrami–Erofeev model best represents the steam gasification char reaction, where the difference between the two activation energy values is only 0.70 KJ.mol−1. The reliability of the proposed approach for evaluating the applicability of a given kinetic model to the steam gasification reaction of char is tested by comparing the results obtained for char samples derived from three different bituminous coal sources.

Hydrochar prepared from municipal sewage sludge as renewable fuels: Evaluation of its devolatilization performance, reaction mechanism, and thermodynamic property

Hydrothermal treatment (HT) of sewage sludge (SS) is a promising method to improve its dewaterability. Effect of HT on devolatilization performance, kinetic parameters, and thermodynamic property during the pyrolysis of SS was investigated by thermogravimetric analysis (TGA), isoconversional method, and master plots method. Derivative thermogravimetry (DTG) curves showed two distinct devolatilization peaks #1 at 283.21 °C and #2 at 327. 43 °C along with a shoulder between 400 and 600 °C. HT temperature influenced peak #2 more significantly as its peak devolatilization rate decreased consecutively from 4.25 % to 1.70 %∙min−1. Meanwhile it moved towards higher temperature and overlapped with the shoulder ultimately. The value of comprehensive pyrolysis index (CPI) decreased greatly from 3.37 × 10−5 to 1.24 × 10−6 %3∙°C−3∙min−2 with HT temperature. Activation energy was determined by Flynn-Wall-Ozawa, Kissinger-Akahira-Sunose, Starink, and Friedman methods. It generally increased against conversion with diverse variation patterns. Its average value reduced obviously after HT. Despite HT, all samples followed identical Avrami-Erofeev model with decreased order of reaction from 7.60 to 6.56. The variation trends of enthalpy, Gibbs free energy, and entropy verified that HT improved the thermal stability of SS, which coincided with CPI. Pyrolysis process of SS evolved from ordered state to disordered state, indicating increased reactivity due to the probable catalytic effect of inorganics in the ash. The work provides guidance for the design, optimization, and upscale of pyrolysis reactors and operation parameters when SS is utilized as renewable fuels.

X-ray Photoelectron Spectroscopy and Diffuse Reflectance Infrared Fourier Transform Spectroscopy Insight into the Pathways of Manganese Oxalate Thermal Decomposition to MnO and MnCO3.

X-ray photoelectron spectroscopy (XPS) and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) were employed to study the isothermal decomposition of MnC2O4 under ultrahigh vacuum and N2 environmental conditions, respectively. High-resolution core-level XP spectra, X-ray-induced Auger spectra, and infrared spectra were obtained as a function of annealing time. In XPS studies, the time-dependent thermal decomposition characteristics were elucidated by analyzing surface composition, chemical shifts, satellites in the Mn 2p3/2 and Mn LMV bands, and Auger parameters for Mn and O. Functional groups developing during the ongoing reaction were identified by DRIFTS from characteristic vibrations. For the first time, the isothermal decomposition of manganese oxalate was shown to proceed via two pathways involving nucleation and accumulation of MnO and MnCO3. The kinetics of the decomposition in vacuum could be described by the Prout-Tompkins or/and by the Avrami-Erofeev models. The results obtained by XPS, DRIFTS, and ex situ XRD allowed concluding that the final product of oxalate decomposition was composed of MnO and MnCO3 or/and unidentate/polydentate carbonate structures populating the surface of the sample. A substantial formation of graphitic carbon was also observed and associated with interface chemical reactivities between the MnO particles and the supporting gold foil.

Comparative Study on the Isothermal Reduction Kinetics of Iron Oxide Pellet Fines with Carbon-Bearing Materials

The isothermal reduction of iron oxide pellet fines–carbon composites was investigated at temperatures of 900–1100 °C. The reduction reactions were monitored using the thermogravimetric (TG) technique. Alternatively, a Quadruple Mass Spectrometer (QMS) analyzed the CO and CO2 gases evolved from the reduction reactions. The effect of temperature, carbon source, and reaction time on the rate of reduction was extensively studied. The phase composition and the morphological structure of the reduced composites were identified by X-ray diffraction (XRD) and a scanning electron microscope (SEM). The results showed that the reduction rate was affected by the temperature and source of carbon. For all composite compacts, the reduction rate, as well as the conversion degree (α) increased with increasing temperature. Under the same temperature, the conversion degree and the reduction rate of composites were greater according to using the following carbon sources order: Activated charcoal > charcoal > coal. The reduction of the different composites was shown to occur stepwise from hematite to metallic iron. The reduction, either by activated charcoal or charcoal, is characterized by two behaviors. During the initial stage, the chemical reaction model (1 − α)−2 controls the reduction process whereas the final stage is controlled by gas diffusion [1 − (1 − α)1/2]2. In the case of reduction with coal, the reduction mechanism is regulated by the Avrami–Erofeev model [−ln (1−α)2] at the initial stage. The rate-controlling mechanism is the 3-D diffusion model (Z-L-T), namely [(1−α)−1/3−1]2 at the latter stage. The results indicated that using biomass carbon sources is favorable to replace fossil-origin carbon-bearing materials for the reduction of iron oxide pellet fines.

Kinetic analysis of poly-aluminum sulfate hydrate for low-temperature thermochemical heat storage

A summary of the research undertaken on poly-aluminum sulfate is performed revealing several disagreements on important thermal properties of the material. Nevertheless, the energy density reported highlights that the material is promising for thermochemical heat storage (THS). A thorough thermal analysis (TA) of (Al2(SO4)3·xH2O) is conducted using TA devices and the ICTAC kinetics committee recommendations, to identify its thermal properties, its most stable form (Al2(SO4)3·18H2O, and the conditions of its use for low-temperature THS (80 °C and 125 °C under atmospheric pressure. The material decomposes in four endothermic stages as shown in the thermal curves and illustrated by possible reaction formulas, three of which are dehydrations followed by a final decomposition. The non-isothermal kinetics of the dehydration for PAS has been determined by the methods of Coats-Redfern (CR) and Achar-Brindley-Sharp (ABS) with 19 different reaction models. It is found that most reaction models exhibit a linear trend. The Janders reaction model is appropriate for the first dehydration with an activation energy of ca. 33.248 kJ/mol by CR and 30.759 kJ/mol by ABS, respectively. Both the power law and the Avrami-Erofeev model can be used for the second stage with an activation energy of ca. 235 kJ/mol. The overall kinetics modeling for aluminum sulfate hydrate is successful for PAS implying the substitution of aluminum sulfate with PAS in applications.

In Situ Studies of Single-Nanoparticle-Level Nickel Thermal Oxidation: From Early Oxide Nucleation to Diffusion-Balanced Oxide Thickening.

High-temperature oxidation mechanisms of metallic nanoparticles have been extensively investigated; however, it is challenging to determine whether the kinetic modeling is applicable at the nanoscale and how the differences in nanoparticle size influence the oxidation mechanisms. In this work, we study thermal oxidation of pristine Ni nanoparticles ranging from 4 to 50 nm in 1 bar 1%O2/N2 at 600 °C using in situ gas-cell environmental transmission electron microscopy. Real-space in situ oxidation videos revealed an unexpected nanoparticle surface refacetting before oxidation and a strong Ni nanoparticle size dependence, leading to distinct structural development during the oxidation and different final NiO morphology. By quantifying the NiO thickness/volume change in real space, individual nanoparticle-level oxidation kinetics was established and directly correlated with nanoparticle microstructural evolution with specified fast and slow oxidation directions. Thus, for the size-dependent Ni nanoparticle oxidation, we propose a unified oxidation theory with a two-stage oxidation process: stage 1: dominated by the early NiO nucleation (Avrami-Erofeev model) and stage 2: the Wagner diffusion-balanced NiO shell thickening (Wanger model). In particular, to what extent the oxidation would proceed into stage 2 dictates the final NiO morphology, which depends on the Ni starting radius with respect to the critical thickness under given oxidation conditions. The overall oxidation duration is controlled by both the diffusivity of Ni2+ in NiO and the Ni in Ni self-diffusion. We also compare the single-particle kinetic curve with the collective one and discuss the effects of nanoparticle size differences on kinetic model analysis.

Mechanochemical assisted hydrogenation of Mg-CNTs-Ni:kinetics modeling and reaction mechanism

MgH2, as one of the most potential hydrogen storage materials, has received considerable attention. However, its development has been plagued by its slow hydriding kinetics. Mechanochemical milling is an effective and convenient method for producing MgH2@CNTs-Ni through a solid–gas reaction between Mg-CNTs-Ni and H2. However, the hydriding kinetics and the corresponding mechanism during the milling have not been mentioned. Thus, hydriding kinetics of Mg-CNTs-Ni was researched for the purpose of gaining insights into the reaction mechanism and obtaining the best milling parameters. Also, phase transition during the milling was analyzed using XRD and TEM. It is apparent that the Avrami-Erofeev (AE) model can well describe the mechanochemical process, which contains solid solution, phase transition and growth-controlled regime. Besides, kinetics of hydrogenation is limited by nucleation and growth of MgH2. By fitting with the Arrhenius equation, average apparent activation energy for the hydrogenation of Mg-CNTs-Ni is estimated to be 47.8 kJ mol−1.