Three-step Reaction Mechanism Research Articles

Unveiling the Nature of Ultrastable Potassium Storage in Bi0.48Sb1.52Se3 Composite.

The conversion and alloying-type anodes for potassium-ion batteries (PIBs) have drawn attention. However, it is still a challenge to relieve the huge volume expansion/electrode pulverization. Herein, we synthesized a composite material comprising Bi0.48Sb1.52Se3 nanoparticles uniformly dispersed in carbon nanofibers (Bi0.48Sb1.52Se3@C). Benefiting from the synergistic effects of the high electronic conductivity of Bi0.48Sb1.52Se3 and the mechanical confinement of the carbon fiber that buffers the large chemomechanical stress, the Bi0.48Sb1.52Se3@C//K half cells deliver a high reversible capacity (491.4 mAh g-1, 100 cycles at 100 mA g-1) and an extraordinary cyclability (80% capacity retention, 1000 cycles at 1000 mA g-1). Furthermore, the Bi0.48Sb1.52Se3@C-based PIB full cells achieve a high energy density of 230 Wh kg-1. In situ transmission electron microscopy (TEM) reveals an intercalation, conversion, and alloying three-step reaction mechanism and a reversible amorphous transient phase. More impressively, the nanofiber electrode can almost return to its original diameter after the potassiation and depotassiation reaction, indicating a highly reversible volume change process, which is distinct from the other conversion type electrodes. This work reveals the stable potassium storage mechanisms of Bi0.48Sb1.52Se3@C composite material, which provides an effective strategy to enable conversion/alloying-type anodes for high performance PIBs for energy storage applications.

Investigation of Economical Design Strategies for the Solid Oxide Fuel Cell Afterburner Based on Computational Fluid Dynamics Simulation

This study simulates the computational fluid dynamics (CFD) to explore the design optimization strategies of the solid oxide fuel cell (SOFC) afterburner. The CFD modeling with combustion is carried out with the eddy dissipation concept (EDC), applying a three-step rate-dependent reaction mechanism to predict the determining step of the combustion reactions by changes of the control and design parameters. The implemented CFD model is validated by comparison of the precedent experimental data and CFD studies. The simulation results examine the temperature distribution and reactivity of the fuel inside the SOFC afterburner with design parameters at start-up operation, such as the equivalence ratio and swirl number. In particular, the results of the parametric study show the possibility of design optimization to minimize the wall temperature and constraints to reduce the emission of toxicants (i.e., CO and NOx). The alternative burner design derived from the parametric study, when starting the SOFC system, has been proven to keep on the stable thermal stress and fuel reactivity, despite lowering the maximum wall temperature to 80.9%, compared to the conventional burner design.

Elucidation of Discharge Mechanism in CFx As a High Energy Density Cathode Material for Lithium Primary Battery

Fluorinated graphite (CF x ) is a class of cathode materials with the high theoretical capacity (865 mAh g-1 in case of x=1) for primary (non-rechargeable) batteries. When using Li metal as the anode material, the system possesses a very low self-discharge rate (< 0.5% per year at 25 °C) compared to other current alternative chemistries. This system, with the proposed general governing reaction of CF x +Li→LiF+C, is one of the best candidates for a wide range of applications, such as implantable medical devices and equipment for extreme environment.Although the lithium fluorinated graphite system has been under investigation for a few decades, there is still a lack of fundamental understanding of the reaction mechanism in this system. Here, a multiscale investigation on the CFx discharge mechanism was performed using a novel cathode structure to minimize the carbon and fluorine additives for precise cathode characterizations. Titration gas chromatography (TGC), X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), cross-sectional focused ion beam (FIB), high-resolution transmission electron microscopy (HRTEM), and scanning transmission electron microscopy with electron energy loss spectroscopy (STEM-EELS) were utilized to investigate this system.The results demonstrated that: (i) There is no lithium deposition or intercalation through the entire discharge. (ii) The CFx structure transforms to a hard-carbon like structure with less sp2 content by increasing depth of discharge. (iii) The crystalline LiF particles uniformly covered the layers of CFx structure with a size range of < 10 nm. A three-step discharge reaction mechanism is proposed in agreement with our electrochemical performances. This work deepens the understanding of CFx as a high energy density cathode material and highlights the need for future investigations on primary battery materials to advance performance. Figure 1

Kinetics mechanism of inert and oxidative torrefaction of biomass

Torrefaction of biomass is a promising pre-treatment process capable of substantially improving the properties of raw biomass for its use as a solid biofuel, among many other potential applications. Oxygen-lean torrefaction can effectively reduce the cost and complexity of inert torrefaction. In this work, torrefaction tests of crushed olive stones were conducted in a thermogravimetric analyzer in various oxygen concentrations, under both non-isothermal and isothermal conditions. Non-isothermal torrefaction measurements were used to gain fundamental knowledge on the inert and oxidative torrefaction processes by applying a model-fitting kinetics method. Analysis of these measurements showed that an accurate description of inert torrefaction based on a two-step reaction mechanism is possible, whereas a three-step reaction mechanism is necessary for oxidative torrefaction. The new three-step mechanism was found to be accurate for describing the global mass loss during isothermal torrefaction, obtaining average root mean squared errors between the model predictions and the TGA measurements below 2.0 % for inert and oxidative torrefaction of olive stones. Furthermore, predictions using the extended mechanism were in good agreement with the more fundamental torrefaction reactions derived from the kinetics analysis of the non-isothermal torrefaction measurements.

Coupling mesoscale transport to catalytic surface reactions in a hybrid model.

In heterogeneous catalysis, reactivity and selectivity are not only influenced by chemical processes occurring on catalytic surfaces but also by physical transport phenomena in the bulk fluid and fluid near the reactive surfaces. Because these processes take place at a large range of time and length scales, it is a challenge to model catalytic reactors, especially when dealing with complex surface reactions that cannot be reduced to simple mean-field boundary conditions. As a particle-based mesoscale method, Stochastic Rotation Dynamics (SRD) is well suited for studying problems that include both microscale effects on surfaces and transport phenomena in fluids. In this work, we demonstrate how to simulate heterogeneous catalytic reactors by coupling an SRD fluid with a catalytic surface on which complex surface reactions are explicitly modeled. We provide a theoretical background for modeling different stages of heterogeneous surface reactions. After validating the simulation method for surface reactions with mean-field assumptions, we apply the method to non-mean-field reactions in which surface species interact with each other through a Monte Carlo scheme, leading to island formation on the catalytic surface. We show the potential of the method by simulating a more complex three-step reaction mechanism with reactant dissociation.

The Effect of Swirl Intensity on the Flow Behavior and Combustion Characteristics of a Lean Propane-Air Flame

The effect of swirl number (Sn) on the flow behavior and combustion characteristics of a lean premixed propane Flame Ф = 0.5 in a swirl burner configuration was numerically verified in this study. Two-dimensional numerical simulations were performed using ANSYS-Fluent software. For turbulence closure, a standard K-ε turbulence model was applied. The turbulence-chemistry interaction scheme was modeled using the Finite Rate-Eddy Dissipation hybrid model (FR/EDM) with a reduced three-step reaction mechanism. The P1 radiation model was used for the flame radiation inside the combustion chamber. Four different swirl numbers were selected (0, 0.72, 1.05, and 1.4) corresponding to different angles (0°, 39°, 50°, and 57.8°). The results show that the predicted model agrees very well with the experimental data, especially with respect to the axial and radial velocity and temperature profiles. An outer recirculation zone (ORZ) is present in the combustor corner at Sn = 0 and an inner recirculation zone (IRZ) appears at the combustor centerline inlet at a critical Sn = 0.72. When the Sn reaches an excessive value, the IRZ moves toward the premixing tube, leading to a flame flashback. The flame structure and its length are strongly affected by changes in the Sn as well as the formation of NOx and CO at the combustor exit.

Absorption performance and reaction mechanism study on a novel anhydrous phase change absorbent for CO2 capture

• A novel phase change absorbent was proposed for CO 2 capture with low energy consumption. • Ethanol promoted phase change behavior and increased CO 2 loading. • Reaction of CO 2 capture with [N 1111 ][Gly]/ethanol was a three-step reaction. • Intramolecular hydrogen bonds affected products’ solubility, causing phase change. Carbon capture and utilization technologies are urged more than ever due to the excessive emissions of CO 2 that could lead to serious environmental problems. Phase change absorbents are quite competitive in reducing the energy consumption of carbon capture. In this work, tetramethylammonium glycinate ([N 1111 ][Gly]), an efficient absorbent, was dissolved in ethanol and 1-propanol, respectively, to absorb CO 2 . After CO 2 absorption, the saturated solution separated into two phases. CO 2 was enriched in the lower phase, so that only the CO 2 -rich phase needed to be regenerated and the energy consumption of regeneration could be thus reduced. Under the optimal conditions, the CO 2 loading of fresh absorbents could reach 0.85 mol CO 2 /mol IL. On the basis of the 13 C NMR analysis and quantum chemical calculation, a three-step reaction mechanism of CO 2 capture with [N 1111 ][Gly]/ethanol was proposed. CO 2 reacted with [N 1111 ][Gly] to form carbamate, which was similar to the two-step Zwitterionic mechanism of MEA with CO 2 . Then the formed carbamate could react with ethanol to form ethyl carbonate and [N 1111 ][Gly], which resulted in higher CO 2 loading. Moreover, hydrogen bond characteristics were analysed to clarify the phase change mechanism. The number and bonding energy of intramolecular hydrogen bonds were increased after capturing CO 2 , leading to the formation of precipitation.

The effect of CO on CO2-char gasification

The effect of CO on the gasification of a Polish coal-derived char was investigated in a fluidised bed from 1123 to 1248 K. Rate expressions developed from Ergun's mechanism or a modified three-step reaction mechanism, coupled with Cylindrical Pore Interpolation Model (CPIM) to account for the intra-particle mass transfer, were developed to predict gasification and the effect of CO. Compared to the Ergun's rate expression, the three-step expression has an extra term for pco, making the inhibition effect of CO more pronounced. The agreement between experimental and numerical results was satisfactory for both models simulating gasification by CO2/N2. Then, when CO (1% or 3%) was intentionally introduced in the feed gas (CO2/N2), the gasification rates significantly decreased. It was found that the model based on Ergun's mechanism over-estimated the gasification rate, while the results from the model with the three-step mechanism agreed with the experimental data.

Model-free and model-based kinetic analysis of Poplar fluff (Populus alba) pyrolysis process under dynamic conditions

As representative among biomass feedstocks, Poplar fluff was chosen to analyze the pyrolysis properties of this new biofuel. Thermogravimetry under inert (argon) atmosphere at the heating rates of 5, 10, 15, and 20 °C min−1 was employed in the investigation of thermal processing of studied biomass. The determination of kinetic parameters and estimation of reaction mechanism function were done through application of the state-of-the-art NETZSCH Kinetics NEO software. With this aim, the various model-free models and model-based calculations were implemented. The obtained results showed that the most appropriate kinetic model to describe the thermal decomposition of Poplar fluff sample was n-dimensional acceleratory Avrami–Erofeev model (An) for three-step consecutive reaction mechanism. Governed reaction pathway in pyrolysis process represents fragmentation reactions which arise from levoglucosan decomposing that permits permanent gases and refractory tars, where diffusion-controlled process is involved in the sample devolatilization. The modulated and multiple-step predictions of pyrolysis process were also carried out.

Effect of different precursors on CVD growth of molybdenum disulfide

Control over thickness, size, and area of chemical vapor deposition (CVD) grown molybdenum disulfide (MoS2) flakes is crucial for device application. Herein, we report a quantitative comparison of CVD synthesis of MoS2 on SiO2/Si substrate using three different precursors viz., molybdenum trioxide (MoO3), ammonium heptamolybdate (AHM), and tellurium (Te). A three-step chemical reaction mechanism of evolution of MoS2 from MoO3 micro-crystals is proposed for MoO3 precursor. Furthermore, a strategy based on growth temperature and ratio of amount of precursors is developed to systematically control thickness and area of MoS2 flakes. Our findings show that for large-sized crystalline monolayer MoS2 flakes, MoO3 is a better choice than AHM and Te-assisted synthesis. Moreover, Te as growth promoter, can lower down growth temperature by ∼250 °C. This study can be further used to fabricate MoS2 based high-performance electronic devices such as photodetectors, thin film transistors, and sensors.

Unveiling Biosynthesis of the Phytohormone Abscisic Acid in Fungi: Unprecedented Mechanism of Core Scaffold Formation Catalyzed by an Unusual Sesquiterpene Synthase.

Abscisic acid (ABA) is a well-known phytohormone that regulates abiotic stresses. ABA produced by fungi is also proposed to be a virulence factor of fungal pathogens. Although its biosynthetic pathway in fungi was proposed by a series of feeding experiments, the enzyme catalyzing the reaction from farnesyl diphosphate to α-ionylideneethane remains to be identified. In this work, we identified the novel type of sesquiterpene synthase BcABA3 and its unprecedented three-step reaction mechanism involving two neutral intermediates, β-farnesene and allofarnesene. Database searches showed that BcABA3 has no homology with typical sesquiterpene synthases and that the homologous enzyme genes are found in more than 100 bacteria, suggesting that these enzymes form a new family of sesquiterpene synthases.

Substrate Specificity and Chemical Mechanism for the Reaction Catalyzed by Glutamine Kinase.

Campylobacter jejuni, a leading cause of gastroenteritis worldwide, has a unique O-methyl phosphoramidate (MeOPN) moiety attached to its capsular polysaccharide. Investigations into the biological role of MeOPN have revealed that it contributes to the pathogenicity of C. jejuni, and this modification is important for the colonization of C. jejuni. Previously, the reactions catalyzed by four enzymes (Cj1418-Cj1415) from C. jejuni that are required for the biosynthesis of the phosphoramidate modification have been elucidated. Cj1418 (l-glutamine kinase) catalyzes the formation of the initial phosphoramidate bond with the ATP-dependent phosphorylation of the amide nitrogen of l-glutamine. Here we show that Cj1418 catalyzes the phosphorylation of l-glutamine through a three-step reaction mechanism via the formation of covalent pyrophosphorylated ( Enz-X-Pβ-Pγ) and phosphorylated ( Enz-X-Pβ) intermediates. In the absence of l-glutamine, the enzyme was shown to catalyze a positional isotope exchange (PIX) reaction within β-[18O4]-ATP in support of the formation of the Enz-X-Pβ-Pγintermediate. In the absence of ATP, the enzyme was shown to catalyze a molecular isotope exchange (MIX) reaction between l-glutamine phosphate and [15N-amide]-l-glutamine in direct support of the Enz-X-Pβintermediate. The active site nucleophile has been identified as His-737 based on the lack of activity of the H737N mutant and amino acid sequence comparisons. The enzyme was shown to also catalyze the phosphorylation of d-glutamine, γ-l-glutamyl hydroxamate, γ-l-glutamyl hydrazide, and β-l-aspartyl hydroxamate, in addition to l-glutamine.

Synthesis, characterization and in situ monitoring of the mechanochemical reaction process of two manganese(II)-phosphonates with N-containing ligands

Two divalent manganese aminophosphonates, manganese mono(nitrilotrimethylphosphonate) (MnNP3) and manganese bis(N-(carboxymethyl)iminodi(methylphosphonate)) (Mn(NP2AH)2), have been prepared by mechanochemical synthesis and characterized by powder X-ray diffraction (PXRD). The structure of the novel compound Mn(NP2AH)2 was determined from PXRD data. MnNP3 as well as Mn(NP2AH)2 exhibits a chain-like structure. In both cases, the manganese atom is coordinated by six oxygen atoms in a distorted octahedron. The local coordination around Mn was further characterized by extended X-ray absorption fine structure. The synthesis process was followed in situ by synchrotron X-ray diffraction revealing a three-step reaction mechanism. The as-prepared manganese(II) phosphonates were calcined on air. All samples were successfully tested for their suitability as catalyst material in the oxygen evolution reaction.

Electrochemical Properties and Redox Mechanism of Na2Ni0.4Co0.6[Fe(CN)6] Nanocrystallites as High-Capacity Cathode for Aqueous Sodium-Ion Batteries

Nickel-based ferrocyanides have attracted wide interest as cathode for aqueous sodium-ion batteries in fields of sustainable energies such as wind and solar, owing to excellent cycling stability and high-rate capability. However, they suffer from low specific capacities (∼60 mAh g–1). Herein, Na2Ni0.4Co0.6[Fe(CN)6] nanocrystallites are reported for the first time as high-capacity cathode for aqueous sodium-ion batteries. Its electrochemical properties and redox mechanism have been understood by combining the X-ray diffraction technique, Fourier-transform infrared spectroscopy, cyclic voltammetry, electrochemical impedance microscopy, and charge/discharge measurements. It is revealed that the material undergoes a reversible three-step single-phase reaction mechanism during Na extraction through sequential electrochemical oxidation of nitrogen-coordinated Co2+ ions and carbon-coordinated Fe2+ ions and achieves superior electrochemical performance with a high reversible capacity of 85 mAh g–1 at 0.5 C, an av...

Electrochemistry of Selenium with Sodium and Lithium: Kinetics and Reaction Mechanism.

There are economic and environmental advantages by replacing Li with Na in energy storage. However, sluggishness in the charge/discharge reaction and low capacity are among the major obstacles to development of high-power sodium-ion batteries. Among the electrode materials recently developed for sodium-ion batteries, selenium shows considerable promise because of its high capacity and good cycling ability. Herein, we have investigated the mechanism and kinetics of both sodiation and lithiation reactions with selenium nanotubes, using in situ transmission electron microscopy. Sodiation of a selenium nanotube exhibits a three-step reaction mechanism: (1) the selenium single crystal transforms into an amorphous phase Na0.5Se; (2) the Na0.5Se amorphous phase crystallizes to form a polycrystalline Na2Se2 phase; and (3) Na2Se2 transforms into the Na2Se phase. Under similar conditions, the lithiation of Se exhibits a one-step reaction mechanism, with phase transformation from single-crystalline Se to a Li2Se. Intriguingly, sodiation kinetics is generally about 4-5 times faster than that of lithiation, and the kinetics during the different stages of sodiation is different. Na-based intermediate phases are found to have improved electronic and ionic conductivity compared to those of Li compounds by first-principles density functional theory calculations.

Numerical investigation of turbulent swirling flames with validation in a gas turbine model combustor

Numerical investigation of turbulent swirling flames is performed in a model gas turbine combustor. The calculations are performed using the CFD code OpenFOAM. Large Eddy Simulation (LES) approach based on the Smagorinsky model is used as the main turbulence modelling strategy, whereas Unsteady Reynolds Averaged Numerical Simulations (URANS) are also applied, employing the Shear Stress Transport model as the turbulence model. Turbulence-chemistry interactions are modelled by the Eddy Dissipation Concept (EDC) and the Laminar Flamelet Model (LFM). In EDC, a three-step global reaction mechanism is used. In LFM, limitations of the standard non-premixed approach, based on the mixture fraction and the scalar dissipation rate, for lifted flames like the present one, is overcome by adding the progress variable as an additional dimension to the flamelet libraries. URANS is applied only with combination with LFM. LES is applied in combination with EDC and LFM. Special attention is paid to obtaining an adequate grid resolution. Predictions are compared with measurements. It is observed that LES provides a better accuracy compared to URANS, whereas the latter may still be seen useful, since its computational time is shorter. For LES, it is observed that EDC provides a similar, or even slightly better overall-accuracy compared to LFM. On the other hand, it is observed that LFM requires substantially shorter computational times compared to EDC. This makes LFM attractive especially for LES of real combustors requiring much larger grids and/or for cases where a detailed reaction mechanism is of interest.

Silicon Diphosphide: A Si-Based Three-Dimensional Crystalline Framework as a High-Performance Li-Ion Battery Anode.

The development of an electrode material for rechargeable Li-ion batteries (LIBs) and the understanding of its reaction mechanism play key roles in enhancing the electrochemical characteristics of LIBs for use in various portable electronics and electric vehicles. Here, we report a three-dimensional (3D) crystalline-framework-structured silicon diphosphide (SiP2) and its interesting electrochemical behaviors for superior LIBs. During Li insertion in the SiP2, a three-step electrochemical reaction mechanism, sequentially comprised of a topotactic transition (0.55-2 V), an amorphization (0.25-2 V), and a conversion (0-2 V), was thoroughly analyzed. On the basis of the three-step electrochemical reaction mechanism, excellent electrochemical properties, such as high initial capacities, high initial Coulombic efficiencies, stable cycle behaviors, and fast-rate capabilities, were attained from the preparation of a nanostructured SiP2/C composite. This 3D crystalline-framework-structured SiP2 compound will be a promising alternative anode material in the realization and mass production of excellent, rechargeable LIBs.

Numerical computations of premixed propane flame in a swirl-stabilized burner: Effects of hydrogen enrichment, swirl number and equivalence ratio on flame characteristics

This paper investigates the effect of hydrogen addition and swirl intensity and equivalence ratio on characteristics of premixed C3H8–air flame, in a model burner. The numerical simulation is carried out using Reynolds Average Navier–Stokes (RANS) technique with Realizable k–ε as a turbulence closure model. The turbulence–chemistry interaction scheme is modeled using FR/EDM with three-step global reaction mechanism. The study is performed with two lean regimes of Φ = 0.3 and 0.5 and four volumetric fractions of hydrogen XH2 = 10%, 20%, 40% and 80% for two swirl numbers Sn = 0.6 and 1.05. First, validations of the computational models with the experimental data are performed, and a good agreement is found. Second, the effect of H2 addition to extended lean flammability limits is addressed. Third, the differential diffusion effects on the accuracy of the predictions are studied and therefore should always be accounted for. Fourth, the effect of H2 addition on the flow development and the flame characteristics is investigated. Results indicate that the H2 addition affects strongly the flame structure. Two types of enriched flame are found, Balloon-type and M-type flames. The effects of H2 addition on the reaction rates of C3H8 and H2, the flow species and CO emissions are also analyzed.

Density functional theory study of the reactivity and electronic structure of the transesterification of triacetin in biodiesel production via a sulfated zirconia heterogeneous catalysis

This DFT study examined the interaction of a sulfated zirconia (SZ) slab model system (heterogeneous catalyst) and triacetin (a precursor in biodiesel production) using explicit methanol solvent molecules. Full geometry optimizations of the systems were performed at the B3LYP level of theory. Gibbs free energies provide insight into the spontaneity of the reactions along a three-step reaction mechanism for the transesterification of triacetin. Charge decomposition analysis revealed electronic charge transfer between the metallic oxide and the organic moieties involved in the reaction mechanism. Fukui indices indicate the likely locations on the SZ surface where catalysis may occur. The quadratic synchronous transit scheme was used to locate transition structures for each step of the transesterification process. The results are in agreement with the strongly acidic catalytic character of zirconium observed experimentally in the production of biodiesel. © 2016 Wiley Periodicals, Inc.

Computational Study of the Mechanism of the Oxidation of Hydrazine / Hydrazinium Ion by Iodine in the Gas Phase

The reaction mechanisms of the oxidation of hydrazine / hydrazinium ion by iodine have been studied using 6311+G** basis set of the density functional theory (DFT) method at the B3LYP level of computation. The study shows that the oxidation reactions can proceed via four independent routes or pathways that can be separately monitored. Two of the proposed pathways involved a two-step reaction mechanism each, in which two transition states were produced while each of the other two routes involved three-step reaction mechanism in which three activated complexes were produced. The results obtained were based on the analyses of the computational energetics of the optimized reactants, intermediates, transition states and products of the reaction of iodine with hydrazine / hydrazinium ion. The study showed that all the four proposed routes were possible by comparing the enthalpies of reactions of the four proposed pathways as well as the activation barriers of the respective rate determining steps which were found to be reasonably acceptable. Rate laws, which were consistent with the written mechanisms, were also derived for each of the proposed mechanisms.