Complex Reaction Mechanisms Research Articles

Evaluating the effectiveness ofin situcharacterization techniques in overcoming mechanistic limitations in lithium–sulfur batteries

Li–S batteries hold great promise for electric vehicles but complex reaction mechanisms during operation have, to date, prevented commercialization.In situtechniques provide insights that may overcome these limitations.

Scaling of catalytic cracking fluidized bed downer reactor based on CFD simulations-Part II: effect of reactor scale.

The practical realization of the scaling up of gas–solid multiphase flow reactors with chemical reactions is hindered by chaotic flow behaviors and complex heat and mass transfers in the reactor. In addition, a law to scale up complex reaction mechanisms in multiphase flow systems has been rarely proposed in the existing literature. Thus, this study aims to investigate the scaling up of the catalytic cracking fluidized bed downer reactor based on the similitude method of chemical reaction performance. Three downer reactor scales with a height of 5, 15, and 30 m, were investigated. To anticipate the behavior of reactive flow, a Eulerian–Eulerian CFD model, two-fluid model, was constructed, which was combined with the kinetic theory of granular flow. A four-lump kinetic model was chosen to represent the mechanism of the catalytic cracking reaction of heavy oil from the pyrolysis of waste plastic. The CFD model accurately predicted the species composition distribution. The scaling law based on the geometric similarity, kinematic similarity, and chemical reaction similarity, was proposed. The catalytic cracking performance similarity of the downer reactor was obtained. With variances in the range of 10% and mean relative absolute error less than 5%, the axial and lateral distributions of chemical performance (heavy oil conversion, gasoline mass fraction, and gasoline selectivity) were found to be extremely similar.

INFLUENCE OF ADDITIONAL SLOW STAGES ON KINETIC REGULARITIES OF CHEMICAL REACTIONS

It is known that mechanisms with a different set of elementary stages are alternatively used to describethe kinetic patterns of chemical reactions involving the same reagents. In this regard, the article introduces two concepts–the "initial" reaction mechanism, which is obtained as a result N.I. Kol’tsovChemChemTech. 2022. V. 65. N 833of experimental studies, and alternative reaction mechanisms thatcan demonstrate dynamic be-havior close to the original mechanism. Alternative mechanisms include all stages of the original mechanism and additional slow stages satisfying certain stoichiometric constraints. The influence of additional dependent and independent slow stages on the patterns of chemical reactions in closed and open reactors of ideal mixing is investigated. It is established that the introduction of slow linearly dependent stages into the reaction mechanism does not significantly affect its kinetic pat-terns. The addition of the reaction mechanism by slow linearly independent stages leads to a change in its dynamics, which does not allow them to be used as part of alternative mechanisms. The article describes an approach that allows us to detectdependent stages, check the possibility of their im-plementation by comparing stationary and non-stationary characteristics (coordinates of stationary states, linear and nonlinear relaxation times, etc.), analyzing the properties of the matrix of stoi-chiometric coefficients (the number of rows and columns) and its rank (the maximum number of independent rows or columns). In this case, the main role is played by stoichiometric conservation laws (stationary and non-stationary), taking into account the peculiarities of their implementation in closed, quasi-open and fully open reactors of ideal mixing. Examples are given demonstrating the influence of additional dependent and independent slow stages on the dynamics of reactions. The results obtained can be useful in practice when solving inverse problems of chemical kinetics to clarify the mechanisms of specific complex reactions.

Pseudoelementary Steps: A Key Concept and Tool for Studying the Kinetics and Mechanisms of Complex Chemical Systems.

The concept of a pseudoelementary step (PEStep) is reviewed, a key concept for approaching the analysis of kinetics data and associated, underlying mechanisms of complex chemical systems. Following a brief Introduction, a definition of a PEStep is given: a PEStep is an initial building block for more complex reactions, that is a starting point for the initial analysis of the observed kinetics and then constructing initial, deliberately minimalistic mechanistic models for complex reactions. PESteps are, therefore and typically, composites of underlying elementary step reactions and can be very useful if not required for the inverse problem of discovering mechanisms from experimental observables for complex reactions. It is the use of PESteps in the inverse problem of mechanism determination that is a primary focus of this review. After a section detailing the results of a literature search of "pseudoelementary step" and related terms such as "pseudoelementary process", pedagogically illustrative examples are given of the use of the PEStep concept in approaching and elucidating the mechanisms of complex reactions. This review shows how the underlying elementary steps of a catalytic cycle were successfully uncovered via a PEStep approach, addresses the classic case of the use of PESteps in determining the mechanisms of oscillating reactions, and examines a well-studied case of an Ir(0)n nanoparticle formation reaction. This latter example is illustrative in that the Ir(0)n nanoparticle formation reaction consisting of thousands of underlying elementary steps that, however, can be treated initially kinetically as just two PESteps, a reduction in complexity of 3 orders of magnitude. Known weaknesses and caveats of the PEStep approach are also summarized and discussed. A short summary of Horituti's "Stoichiometric Number" concept is provided, a concept that would appear to merit further investigation and use in the study of complex reactions. Finally, a section is provided that lists a few, selected areas where the PEStep concept and methodology are expected to prove especially important in the future, and a Conclusions section is provided that lists 11 bullet points. The latter serves as a summary of this first review of the PEStep concept and its importance in dealing with the kinetics and in elucidating the mechanisms of more complex, multistep reactions.

Stable sodium-sulfur electrochemistry enabled by phosphorus-based complexation

A series of sodium phosphorothioate complexes are shown to have electrochemical properties attractive for sodium-sulfur battery applications across a wide operating temperature range. As cathode materials, they resolve a long-standing issue of cyclic liquid-solid phase transition that causes sluggish reaction kinetics and poor cycling stability in conventional, room-temperature sodium-sulfur batteries. The cathode chemistry yields 80% cyclic retention after 400 cycles at room temperature and a superior low-temperature performance down to -60 °C. Coupled experimental characterization and density functional theory calculations revealed the complex structures and electrochemical reaction mechanisms. The desirable electrochemical properties are attributed to the ability of the complexes to prevent the formation of solid precipitates over a fairly wide range of voltage.

Application of the genetic algorithm method in the problem of chemical kinetics of the oxidative regeneration process

Annotation. One of the main problems in chemical kinetics is the establishment of the mechanisms of complex chemical reactions. The inverse problem of chemical kinetics is understood as the determination of the dependence of the concentration of the participating components on the basis of experimental data obtained from a laboratory installation for the oxidative regeneration of coked catalysts. One of the main methods used in inverse problems the genetic algorithm. The algorithms considered in the article make it possible to determine the values of the rate constants of the considered chemical stages.

Synthetically Produced Isocubanite as an Anode Material for Sodium-Ion Batteries: Understanding the Reaction Mechanism During Sodium Uptake and Release.

Bulk isocubanite (CuFe2S3) was synthesized via a multistep high-temperature synthesis and was investigated as an anode material for sodium-ion batteries. CuFe2S3 exhibits an excellent electrochemical performance with a capacity retention of 422 mA h g-1 for more than 1000 cycles at a current rate of 0.5 A g-1 (0.85 C). The complex reaction mechanism of the first cycle was investigated via PXRD and X-ray absorption spectroscopy. At the early stages of Na uptake, CuFe2S3 is converted to form crystalline CuFeS2 and nanocrystalline NaFe1.5S2 simultaneously. By increasing the Na content, Cu+ is reduced to nanocrystalline Cu, followed by the reduction of Fe2+ to amorphous Fe0 while reflections of nanocrystalline Na2S appear. During charging up to -5 Na/f.u., the intermediate NaFe1.5S2 appears again, which transforms in the last step of charging to a new unknown phase. This unknown phase together with NaFe1.5S2 plays a key role in the mechanism for the following cycles, evidenced by the PXRD investigation of the second cycle. Even after 400 cycles, the occurrence of nanocrystalline phases made it possible to gain insights into the alteration of the mechanism, which shows that CuxS phases play an important role in the region of constant specific capacity.

Recent Advances in Dynamic Modeling and Process Control of PVA Degradation by Biological and Advanced Oxidation Processes: A Review on Trends and Advances

Polyvinyl alcohol (PVA) is an emerging pollutant commonly found in industrial wastewater, owing to its extensive usage as an additive in the manufacturing industry. PVA’s popularity has made wastewater treatment technologies for PVA degradation a popular research topic in industrial wastewater treatment. Although many PVA degradation technologies are studied in bench-scale processes, recent advancements in process optimization and control of wastewater treatment technologies such as advanced oxidation processes (AOPs) show the feasibility of these processes by monitoring and controlling processes to meet desired regulatory standards. These wastewater treatment technologies exhibit complex reaction mechanisms leading to nonlinear and nonstationary behavior related to variability in operational conditions. Thus, black-box dynamic modeling is a promising tool for designing control schemes since dynamic modeling is more complicated in terms of first principles and reaction mechanisms. This study seeks to provide a survey of process control methods via a comprehensive review focusing on PVA degradation methods, including biological and advanced oxidation processes, along with their reaction mechanisms, control-oriented dynamic modeling (i.e., state-space, transfer function, and artificial neural network modeling), and control strategies (i.e., proportional-integral-derivative control and predictive control) associated with wastewater treatment technologies utilized for PVA degradation.

The crystal structures of the enzyme hydroxymethylbilane synthase, also known as porphobilinogen deaminase.

The enzyme hydroxymethylbilane synthase (HMBS; EC 4.3.1.8), also known as porphobilinogen deaminase, catalyses the stepwise addition of four molecules of porphobilinogen to form the linear tetrapyrrole 1-hydroxymethylbilane. Thirty years of crystal structures are surveyed in this topical review. These crystal structures aim at the elucidation of the structural basis of the complex reaction mechanism involving the formation of tetrapyrrole from individual porphobilinogen units. The consistency between the various structures is assessed. This includes an evaluation of the precision of each molecular model and what was not modelled. A survey is also made of the crystallization conditions used in the context of the operational pH of the enzyme. The combination of 3D structural techniques, seeking accuracy, has also been a feature of this research effort. Thus, SAXS, NMR and computational molecular dynamics have also been applied. The general framework is also a considerable chemistry research effort to understand the function of the enzyme and its medical pathologies in acute intermittent porphyria (AIP). Mutational studies and their impact on the catalytic reaction provide insight into the basis of AIP and are also invaluable for guiding the understanding of the crystal structure results. Future directions for research on HMBS are described, including the need to determine the protonation states of key amino-acid residues identified as being catalytically important. The question remains - what is the molecular engine for this complex reaction? Thermal fluctuations are the only suggestion thus far.

How to Optimize the Synthesis of Ni1-y-ZMnyCoz(OH)2, y+z≤0.20, in a Continuous Stirred Tank Reactor (CSTR)

LixNi1-y-zMnyCoz(OH)2 for y+z≤0.20 or Ni-rich NMCs are state-of-the-art cathode materials. As they have high energy density, they are used in present electric vehicles like Nissan Leaf S Plus, BMW i3, Chevrolet Bolt and many more. Pioneering research has been going on in the battery community to push the Ni-content further or to stabilize the capacity retention of current NMC materials used in market. These efforts include, studying the capacity fading on long cycling, coating, or doping the pristine NMCs, etc. All these research works need a good baseline material to work with which is often complex to synthesize due to the involvement of the complex reaction mechanism, which is not very well understood. Hence most of the researcher limits themselves to commercially available NMC materials which may differ from batch to batch, and it also reduces the flexibility to modify the material.In this presentation, we will discuss our detailed study of the reaction mechanism to optimize the Ni-rich NMCs from their respective sulfate salts on a 2L bench-top continuous-stirred tank reactor. The synergistic effect between the pH control, strong base, complexing agent, temperature, and other physical factors which are involved in the formation of the desired dense spherical particles and how these factors can be tuned to achieve the desired morphology will be shown. Finally, different compositions of Ni-rich NMCs synthesized via same method will be discussed along with their electrochemical performances using a commercial NMC811 as the baseline.Acknowledgement.This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy, through the Advanced Battery Materials Research Program (Battery500 Consortium).

Thermal decomposition behaviour and chemical kinetics of tungsten based electrically controlled solid propellants

In recent times, there is a growing interest in the development of electrically controlled solid propellants (ECSPs) because they offer various advantages over conventional solid propellants. However, the fundamental understanding on the complex reaction mechanism of decomposition/combustion of these ECSPs is still limited. The present study attempts to expand the knowledge on thermal decomposition mechanisms of ECSPs based on lithium perchlorate (LP) oxidizer and poly vinyl alcohol (PVA) binder, and to further investigate the influence of tungsten (W) as a metal additive, using thermal analysis. The chemical kinetic parameters for different metallized samples are obtained and compared with the non-metallized baseline propellant. Decomposition of non-metallized sample occurs over three stages involving primary reactions between molten oxidizer and pyrolysis products of PVA, and secondary gas-phase reactions overlapping with pyrolysis of unreacted LP. However, metallized ECSPs exhibit only single step decomposition that incorporates metal oxidation as well. It is obtained that the initial decomposition temperature of non-metallized ECSP is 349 °C with a heat release from the overall decomposition reaction of 2043 J/g. Addition of W increases this overall heat release up to 43% for ECSP-M1 containing 5% W, whereas the decomposition temperature is reduced by ∼60 °C, when compared to the non-metallized case. This indicates that the thermal stability of ECSPs is lowered due to the inclusion of W particles. Further increase in W content diminishes the overall heat release by 10% and 13% with correspondingly more unreacted samples, and rises the effective activation energy by 36% and 60% for ECSP-M2 and ECSP-M3, respectively, relative to ECSP-M1. Based on the present findings, we propose the global reaction mechanism for both non-metallized and metallized ECSPs.

Trace Key Mechanistic Features of the Arsenite Sequestration Reaction with Nanoscale Zerovalent Iron.

Nanoscale zerovalent iron (nZVI) is considered as a highly efficient material for sequestrating arsenite, but the origin of its high efficacy as well as the chemical transformations of arsenite during reaction is not well understood. Here, we report an in situ X-ray absorption spectroscopy (XAS) study to investigate the complex mechanism of nZVI reaction with arsenite under anaerobic conditions at the time scale from seconds to days. The time-resolved XAS analysis revealed a gradual oxidation of AsIII to AsV in the course of minutes to hours in both the solid and liquid phase for the high (above 0.5 g/L) nZVI dose system. When the reaction time increased up to 60 days, AsV became the dominant species. The quick-scanning extended X-ray absorption fine structure (QEAXFS) was introduced to discover the transient intermediate at the highly reactive stage, and a small red-shift in As K-edge absorption edge was observed. The QEAXFS combined with density functional theory (DFT) calculation suggested that the red-shift is likely due to the electron donation in a Fe-O-As complex and possible active sites of As sequestrations include Fe(OH)4 and 4-Fe cluster. This is the first time that the transient reaction intermediate was identified in the As-nZVI sequestration system at the fast-reacting early stage. This study also demonstrated usefulness of in situ monitoring techniques in environmental water research.

Gamma Radiation Chemistry of Polydimethylsiloxane Foam in Radiation-Thermal Environments: Experiments and Simulations.

The γ radiolysis behavior of polydimethylsiloxane (PDMS) in the radiation-thermal environments (dose rate, 0.2 Gy/s) is studied to pinpoint the basic knowledge of the temperature (20-70 °C) effects. The non-monotonous temperature effects on the formation of gas products, paramagnetic species in silica, and cross-linking density are proposed to correlate with the complex chemical reaction mechanisms. Besides, molecular dynamics simulation and theoretical calculation are first performed simultaneously based on the radical chemistry and intricate material composition, making it easier to comprehend and further harness the radiolysis mechanisms and structure deterioration of PDMS. The γ radiation-induced primary gas products and dominant cross-linking phenomena are reproduced by the molecular dynamics simulations with a reactive force field, and the reaction mechanisms and physicochemical interactions among PDMS chains, gas products, reactive radicals, and silica fillers are thoroughly studied at the atomic scale. The thermochemistry of the barrierless radical coupling reactions and reactions with explicit high-barrier transition states is calculated at the M06-2X theoretical level with the 6-311g(d, p) basis set. The barrierless reactions are all exothermal with the heat release of 321-618 kJ/mol, while the potential barriers for reactions with explicit transition states vary between 37 and 229 kJ/mol. The results show that γ radiation-induced radicals are crucial for the ensuing gas formation and cross-linking reactions, especially for the radical coupling reactions. The radical chemistry involved in the radiolytic PDMS is the key to understand and simulate its radiolysis behavior, according to the experimental and simulated results.

Novel lithium-chalcogenide batteries combining S, Se and C characteristics supported by chitosan-derived carbon intertwined with CNTs

Lithium-sulfur (Li-S) batteries exhibit a high specific capacity (1675 mAh g−1) but poor S conductivity and severe shuttle effects. Selenium (Se) can greatly mitigate these disadvantages, but its theoretical capacity is relatively low (675 mAh g−1). A series of Sx-Sey compounds supported by chitosan-derived carbon with intertwined carbon nanotube (CC + CNT) carriers exhibit the high capacity of S, the stability of Se, and the excellent conductivity of C, complementing their individual defects. S2Se1/CC + CNT shows the best electrochemical performance (833.2 mAh g−1 after 500 cycles). Characterization results reveal the structure–activity relationships and complex reaction mechanisms of the prepared electrode materials. The overall reactions are a combination of basic Li-S and Li-Se reactions. The discharge steps are clearly identifiable, while the reaction sequences are slightly adjusted and fully integrated when charging. This work provides a foundation for future research and for the practical application of chalcogenides in lithium-metal batteries.

Initial Decomposition Mechanism of 3-Nitro-1,2,4-triazol-5-one (NTO) under Shock Loading: ReaxFF Parameterization and Molecular Dynamic Study

We report a reactive molecular dynamic (ReaxFF-MD) study using the newly parameterized ReaxFF-lg reactive force field to explore the initial decomposition mechanism of 3-Nitro-1,2,4-triazol-5-one (NTO) under shock loading (shock velocity >6 km/s). The new ReaxFF-lg parameters were trained from massive quantum mechanics data and experimental values, especially including the bond dissociation curves, valence angle bending curves, dihedral angle torsion curves, and unimolecular decomposition paths of 3-Nitro-1,2,4-triazol-5-one (NTO), 1,3,5-Trinitro-1,3,5-triazine (RDX), and 1,1-Diamino-2,2-dinitroethylene (FOX-7). The simulation results were obtained by analyzing the ReaxFF dynamic trajectories, which predicted the most frequent chain reactions that occurred before NTO decomposition was the unimolecular NTO merged into clusters ((C2H2O3N4)n). Then, the NTO dissociated from (C2H2O3N4)n and started to decompose. In addition, the paths of NO2 elimination and skeleton heterocycle cleavage were considered as the dominant initial decomposition mechanisms of NTO. A small amount of NTO dissociation was triggered by the intermolecular hydrogen transfer, instead of the intramolecular one. For α-NTO, the calculated equation of state was in excellent agreement with the experimental data. Moreover, the discontinuity slope of the shock-particle velocity equation was presented at a shock velocity of 4 km/s. However, the slope of the shock-particle velocity equation for β-NTO showed no discontinuity in the shock wave velocity range of 3–11 km/s. These studies showed that MD by using a suitable ReaxFF-lg parameter set, could provided detailed atomistic information to explain the shock-induced complex reaction mechanisms of energetic materials. With the ReaxFF-MD coupling MSST method and a cheap computational cost, one could also obtain the deformation behaviors and equation of states for energetic materials under conditions of extreme pressure.

Surface‐Enhanced Raman Spectroscopic Evidence of Key Intermediate Species and Role of NiFe Dual‐Catalytic Center in Water Oxidation

AbstractNiFe‐based electrocatalysts have attracted great interests due to the low price and high activity in oxygen evolution reaction (OER). However, the complex reaction mechanism of NiFe‐catalyzed OER has not been fully explored yet. Detection of intermediate species can bridge the gap between OER performances and catalyst component/structure properties. Here, we performed label‐free surface‐enhanced Raman spectroscopic (SERS) monitoring of interfacial OER process on Ni3FeOx nanoparticles (NPs) in alkaline medium. By using bifunctional Au@Ni3FeOx core‐satellite superstructures as Raman signal enhancer, we found direct spectroscopic evidence of intermediate O‐O− species. According to the SERS results, Fe atoms are the catalytic sites for the initial OH− to O‐O− oxidation. The O‐O− species adsorbed across neighboring Fe and Ni sites experiences further oxidation caused by electron transfer to NiIII and eventually forms O2 product.

Surface-Enhanced Raman Spectroscopic Evidence of Key Intermediate Species and Role of NiFe Dual-Catalytic Center in Water Oxidation.

NiFe-based electrocatalysts have attracted great interests due to the low price and high activity in oxygen evolution reaction (OER). However, the complex reaction mechanism of NiFe-catalyzed OER has not been fully explored yet. Detection of intermediate species can bridge the gap between OER performances and catalyst component/structure properties. Here, we performed label-free surface-enhanced Raman spectroscopic (SERS) monitoring of interfacial OER process on Ni3 FeOx nanoparticles (NPs) in alkaline medium. By using bifunctional Au@Ni3 FeOx core-satellite superstructures as Raman signal enhancer, we found direct spectroscopic evidence of intermediate O-O- species. According to the SERS results, Fe atoms are the catalytic sites for the initial OH- to O-O- oxidation. The O-O- species adsorbed across neighboring Fe and Ni sites experiences further oxidation caused by electron transfer to NiIII and eventually forms O2 product.

Data-Driven Analysis of Nonlinear Heterogeneous Reactions through Sparse Modeling and Bayesian Statistical Approaches.

Heterogeneous reactions are chemical reactions that occur at the interfaces of multiple phases, and often show a nonlinear dynamical behavior due to the effect of the time-variant surface area with complex reaction mechanisms. It is important to specify the kinetics of heterogeneous reactions in order to elucidate the microscopic elementary processes and predict the macroscopic future evolution of the system. In this study, we propose a data-driven method based on a sparse modeling algorithm and sequential Monte Carlo algorithm for simultaneously extracting substantial reaction terms and surface models from a number of candidates by using partial observation data. We introduce a sparse modeling approach with non-uniform sparsity levels in order to accurately estimate rate constants, and the sequential Monte Carlo algorithm is employed to estimate time courses of multi-dimensional hidden variables. The results estimated using the proposed method show that the rate constants of dissolution and precipitation reactions that are typical examples of surface heterogeneous reactions, necessary surface models, and reaction terms underlying observable data were successfully estimated from only observable temporal changes in the concentration of the dissolved intermediate products.

Spatially-resolved reaction profiles in Fischer-Tropsch synthesis – influence of operating conditions and promotion for iron-based catalysts

The complex reaction mechanism of iron-based Fischer-Tropsch synthesis comprises of manifold primary and secondary reaction pathways. Within this work, an experimental setup for spatially-resolved measurements on promoted iron-based Fischer-Tropsch catalysts is presented, allowing for a general study of the influence of operating conditions and promotion as well as of the influence of secondary reactions on product selectivity. It was found that, depending on catalyst composition, considerable hydrogenation of 1-alcohols and olefins occurs, while no considerable influence of reinitiation of chain growth on product distribution could be found.

Kinetics of pyrolysis of cotton stalk using model-fitting and model-free methods

With the depletion of fossil fuels, the conversion of biomass to biofuel is increasingly seen as a possible solution. Pyrolysis is one of these biomass-to-biofuel conversion technologies. In the present study, cotton stalk pyrolysis was investigated to determine the intrinsic kinetic parameters to understand the pyrolysis process. Model-fitting and model-free methods are applied to assess the kinetics. Pyrolysis of the cotton stalk was performed at different heating rates (10, 20, 30, and 40 K min−1) using a thermogravimetric analyzer. The obtained data from TGA was used to estimate the key kinetic parameters using various models/methods including Single reaction model, Series reaction model, Kissinger-Akahira-Sunose (KAS) method, and Flynn–Wall–Ozawa (FWO) method. The results of model-fitting kinetics showed that the pyrolysis mechanism could not be explained by the simple reaction models and obtained unrealistic kinetic parameters. Further, model-free methods were investigated to address the complex reaction mechanism. Besides, the predicted activation energy using KAS method values ranged from 223 to 230 kJ mol−1. In the FWO method, activation energy ranges from 213 to 240 kJ mol−1. In both methods, the calculated average activation energy value is in the proximity of 226 kJ mol−1 for fractional conversion value of 0.05 > α > 0.7.