Multi-step Reaction Mechanism Research Articles

Reaction Kinetics of Germanium Nanowire Growth on Inductively Heated Copper Surfaces

This article describes the chemical kinetics of germanium nanowire growth on inductively heated copper surfaces using diphenylgermane as a precursor. Inductive heating of metal surfaces presents a simple, rapid, and contact-free method to activate the direct growth of nanowires on metal surfaces. We show the main effects of synthesis temperature, duration, precursor concentration on the morphology, and loading of the nanowire film. We describe the complex interplay of precursor degradation, nucleation, and growth in context of a multistep reaction mechanism. We studied the temporal evolution of nanowire loading and morphology to develop a kinetic model, which predicts critical thresholds that define the onset of sequential axial and radial nanowire growth modes. These results may be used to commercially scale a nanowire growth process.

Multi-Physics Simulation of SOFC Button Cell with Multi-Step Charge Transfer Model in Composite LSM/YSZ Cathode

In the present study, a numerical simulation of a solid oxide fuel cell (SOFC) button cell with a multi-step oxygen reduction reaction (ORR) mechanism in a composite cathode is developed for better understanding of processes within the SOFC. The ORR mechanism involves both surface charge transfer through the triple phase boundary (3PB) and bulk charge transfer via a two phase boundary (2PB) pathway. The developed multi-physics simulation considers the charge conservation and species transportation through the porous media. Finally, the simulations of a full SOFC button cell were calibrated with measured cell performance (polarization curves and impedance behavior) at three different air/fuel utilization conditions. The surface absorption and dissociation in the proposed mechanism was identified as the rate-determining step. The cell performance is determined via combination of this rate-determining step, charge transfer steps in parallel pathways, and diffusion phenomena. The dependence of cell resistance on oxygen partial pressure was also quantified.

Investigation of thermal degradation of pine needles using multi-step reaction mechanisms

The objective of this study is to assess the relevance of several multi-step reaction mechanisms to describe the mass loss and the mass loss rate of pine needles in TGA at different heating rates in inert and oxidative atmospheres. The kinetic parameters of the different reactions were optimized using the Shuffled complex evolution (SCE) technique. Model results show that both mass loss and mass loss rate should be considered in order to evaluate properly the mechanism. The drying process is described accurately by a single reaction with a well-established set of kinetic parameters. The conversion of dry pine into char requires a five-step reaction mechanism that is combined of three reactions to describe the pyrolysis under inert atmosphere and another two reactions to describe the oxidative process. Less detailed mechanisms were found to be unable to reproduce the mass loss rate. In particular, the one-step reaction mechanism, widely used to model the pyrolysis process in wildland fire simulations, should be used with care. Finally, the char oxidation process can be described with a single step-reaction mechanism. The final complex mechanism is comprised of one reaction for drying, five reactions for the conversion of dry pine into char, and one reaction for the char oxidation, is promising. Further studies are required for its validation in large-scale experiments.

The Degree of Rate Control: A Powerful Tool for Catalysis Research

The “degree of rate control” (DRC) is a mathematical approach for analyzing multistep reaction mechanisms that has proven very useful in catalysis research. It identifies the “rate-controlling transition states and intermediates” (i.e., those whose DRCs are large in magnitude). Even in mechanisms with over 30 intermediates and transition states, these are generally just a few distinct chemical species whose energies, if they could be independently changed, would achieve a faster net reaction rate to the product of interest. For example, when there is a single “rate-determining step”, the DRC for its transition state (TS) is 1, which means (by definition) that if this TS’s energy could be decreased by kBT (where kB is Boltzmann’s constant and T is temperature), the net rate would increase by a factor of e. Because the (relative) energies of these key adsorbed intermediates and transition states can be adjusted by modifying the catalyst or solvent, or even a reactant’s molecular structure, the DRC values pr...

Surface Reaction Kinetics of the Oxidation and Reforming of Propane over Rh/Al2O3 Catalysts

AbstractA multi‐step surface reaction mechanism for partial oxidation and steam reforming of propane over Rh/Al2O3 catalysts is presented. The mechanism is also applicable to model reactions of the subsystems H2/CO/H2O/CO2/O2/CH4. A stagnation–flow reactor with a catalytically coated disk is used to determine the surface reaction rate and spatial concentration profiles on top of the catalytic plate using a micro‐probe sampling technique. The reactor configuration facilitates one‐dimensional modeling of coupled diffusive and convective transport within the gas‐phase boundary layer coupled with detailed heterogeneous chemistry models of the zero‐dimensional surface. The reaction system is studied at varying inlet concentrations and temperatures. The established reaction kinetics are furthermore tested by simulation of autothermal reforming of propane in an annular reactor previously described by Pagani.

Developing a multistep surface reaction mechanism to model the impact of H2 and CO on the performance and defect chemistry of [formula omitted] mixed-conductors

This study examines the impact of hydrogen (H2) and carbon monoxide (CO) on the oxygen permeation and defect chemistry of La0.9Ca0.1FeO3−δ (LCF) membranes. By conducting two separate experiments with hydrogen-argon (Ar) and carbon monoxide-argon mixtures, we show that the oxygen flux (JO2) increases by more than one order of magnitude when either fuel is introduced at the fuel side as compared to non-reactive experiments. Our measurements reveal that H2 oxidation is faster compared to CO oxidation, hence higher fluxes can be achieved with the former. Using X-Ray Diffraction (XRD), we verify that the aforementioned performance rise is achieved without sacrificing the stability of the material. We show that JO2 increases due to surface reactions of fuel with lattice oxygen ions and hence, two 4-step surface reaction mechanisms are proposed for either fuel. The mechanisms are thermodynamically consistent and coupled with detailed diffusion of charged species within the material by utilizing a Poisson-Nernst-Planck (PNP) model in the dilute limit. The model demonstrates that surface reactions impose a limit on the rise of the oxygen flux, while charged species diffusion within the material is fast. For LCF, reduction of Fe+3 to Fe+2 during fuel oxidation is faster compared to Fe+4 reduction to Fe+3 and this behavior significantly impacts the kinetic rates.

Investigation of carbon steel anodic dissolution in ammonium chloride solutions using electrochemical impedance spectroscopy

The anodic dissolution of carbon steel in ammonium chloride (NH4Cl) solutions (5, 10, and 20 wt%) is investigated via various electrochemical techniques and other complementary techniques. The polarization measurements reveals that the carbon steel is susceptible to general corrosion. The impedance data taken at various overpotentials shows multiple loops, corresponding to capacitance, inductance, and negative capacitance, and the number of time constants observed is also not the same for various NH4Cl concentrations. From reaction mechanism analysis, a multi-step reaction mechanism with three adsorbed intermediates and three dissolution paths (one chemical path and two electrochemical paths) is proposed to describe the observed patterns in impedance measurements. The surface coverage of intermediate species and the contribution of chemical reaction and electrochemical reaction to the overall corrosion rate are also estimated from the proposed model. The results obtained from field emission scanning electron microscopy and Raman spectroscopy measurements are also reported.

Theory and Modeling of Fuel Cells: Extravaganza to Panacea?

<i>Theory and Modeling of Fuel Cells: Extravaganza to Panacea?</i>

Multistep Reaction Mechanisms in Nonaqueous Lithium–Oxygen Batteries with Redox Mediator: A Model-Based Study

Lithium–oxygen cells with nonaqueous electrolyte show high overpotentials during charge, indicating asymmetric charge/discharge reaction mechanisms. We present a kinetic modeling and simulation study of the lithium–oxygen cell cycling behavior. The model includes a multistep reaction mechanism of the cell reaction (2Li + O2 ⇄ Li2O2) forming lithium peroxide by precipitation, coupled to a 1D porous-electrode transport model. We apply the model to study the asymmetric discharge/charge characteristics and analyze the influence of a redox mediator dissolved homogeneously in the liquid electrolyte. Model predictions are compared to experimental galvanostatic cycling data of cells without and with 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO) as redox mediator. The predicted discharge behavior shows good agreement with the experimental results. A spatiotemporal analysis of species concentrations reveals inhomogeneous distributions of dissolved oxygen and reaction products within the cathode during discharge. The e...

Non-isothermal pyrolysis of de-oiled microalgal biomass: Kinetics and evolved gas analysis

Non-isothermal (β=5, 10, 20, 35°C/min) pyrolysis of de-oiled microalgal biomass (DMB) of Chlorella variabilis was investigated by TGA-MS (30–900°C, Argon atmosphere) to understand thermal decomposition and evolved gas analysis (EGA). The results showed that three-stage thermal decomposition and three volatilization zone (100–400°C, 400–550°C and 600–750°C) of organic matters during pyrolysis. The highest rate of weight-loss is 8.91%/min at 302°C for 35°C/min heating-rate. Kinetics of pyrolysis were investigated by iso-conversional (KAS, FWO) and model-fitting (Coats-Redfern) method. For Zone-1and3, similar activation energy (Ea) is found in between KAS (α=0.4), FWO (α=0.4) and Avrami–Erofe’ev (n=4) model. Using the best-fitted kinetic model Avrami–Erofe’ev (n=4), Ea values (R2=>0.96) are 171.12 (Zone-1), 404.65 (Zone-2) and 691.42kJ/mol (Zone-3). EGA indicate the abundance of most gases observed consequently between 200–300°C and 400–500°C. The pyrolysis of DMB involved multi-step reaction mechanisms for solid-state reactions having different Ea values.

Multi-step reaction mechanism for F atom interactions with organosilicate glass and SiOx films

An ab initio approach with the density functional theory (DFT) method was used to study F atom interactions with organosilicate glass (OSG)-based low-k dielectric films. Because of the complexity and significant modifications of the OSG surface structure during the interaction with radicals and etching, a variety of reactions between the surface groups and thermal F atoms can happen. For OSG film etching and damage, we propose a multi-step mechanism based on DFT static and dynamic simulations, which is consistent with the previously reported experimental observations. The important part of the proposed mechanism is the formation of pentavalent Si atoms on the OSG surface due to a quasi-chemisorption of the incident F atoms. The revealed mechanism of F atom incorporation into the OSG matrix explains the experimentally observed phenomena of fast fluorination without significant modification of the chemical structure. We demonstrate that the pentavalent Si states induce the weakening of adjacent Si–O bonds and their breaking under F atom flux. The calculated results allow us to propose a set of elementary chemical reactions of successive removal of CH3 and CH2 groups and fluorinated SiOx matrix etching.

Simplified models for the thermodynamic properties along a combustor and their effect on thermoacoustic instability prediction

Accurately predicting the thermoacoustic modes of a combustor depends upon knowledge of the thermodynamic properties within the combustor; flame temperature, heat release rate, speed of sound and ratio of specific heats all have a strong effect. Calculating the global equilibrium properties resulting from fuel combustion is not straightforward due to the presence of complex multi-species and multi-step reaction mechanisms. A method which decouples the calculations of species dissociations is proposed in this work: this improves the precision of calculation when using few species and reduces the computational cost and complexity to a degree that embedding within low order thermoacoustic network codes is feasible. When used to calculate the combustion product mole fractions, temperature, heat release rate, speed of sound and ratio of specific heats for hydrocarbon-air flames, the method is found to be accurate and highly efficient across different operating conditions and fuel types. The method is then combined with improved low-order wave-based network modelling, the latter employing wave-based acoustic models which account for the variation of thermodynamic properties along the combustion chamber. For a laboratory-scale combustor with a large downstream temperature variation, it is shown that accurate prediction of thermoacoustic modal frequencies and growth rates does depend on accounting for the variation in thermodynamic properties.

Dynamic Modeling of the Reaction Mechanisms in a Li/O2 Battery Cell

Extensive research efforts on metal-air batteries are being carried out due to their high theoretical energy density. Specifically, the lithium-oxygen (Li/O2) based batteries have drawn attention of the researchers as a promising alternative to Li-ion batteries for long range electric vehicles. However, this system suffers from high overpotentials during charge, indicating asymmetrical charge/discharge reaction mechanisms. High charge overpotentials lower the energy efficiency and can increase potential-driven side reactions (e.g., electrolyte decomposition). Furthermore, the discharge product forms as a new solid phase in the cathode by a nucleation and growth process, which is in general different from lithium-ion battery technologies. Thus, a detailed understanding of the reaction mechanism is crucial for enhancing battery performance and durability. Our approach towards understanding the charge/discharge reaction mechanism is by development and exploitation of a continuum model of a lithium-oxygen cell [1]. The one-dimensional computational domain consists of a gas reservoir filled with oxygen, a porous carbon cathode flooded with an organic electrolyte, a porous separator, and a lithium metal anode. We present a multi-step reaction mechanism of the electrochemistry in the cathode, consisting of O2 dissolution, O2 reduction, LiO2 precipitation, LiO2 disproportionation during discharge (Figure 1, left), as well as Li2O2 electrostripping during charge. In particular, the mechanism is asymmetric with respect to discharge and charge. The model is parameterized by experimental Swagelok cells [2] and galvanostatic cycling simulations are compared to experimental data. Furthermore, possible reasons for the high charge overpotential are analyzed, options to reduce the charge overpotential (e.g. redox mediator) are discussed, and a charge reaction mechanism including a surface-mediated Li2O2 dissolution is proposed (Figure 1, right). [1] D. Grübl, B. J. Bergner, J. Janek and W. G. Bessler, “Dynamic Modeling of the Reaction Mechanism in a Li/O2Cell: Influence of a Redox Mediator,” ECS Transactions 69(19), 11-21 (2015). [2] B. J. Bergner, A. Schürmann, K. Peppler, A. Garsuch, and J. Janek, “TEMPO: A Mobile Catalyst for Rechargeable Li-O2 Batteries,” J. Am. Chem. Soc. 136, 15054–15064 (2014). Figure 1

Mechanistic Insights into Water-Catalyzed Formation of Levoglucosenone from Anhydrosugar Intermediates by Means of High-Level Theoretical Procedures

Levoglucosenone (LGO) is an important anhydrosugar product of fast pyrolysis of cellulose and biomass. We use the high-level G4(MP2) thermochemical protocol to study the reaction mechanism for the formation of LGO from the 1,4:3,6-dianhydro-α-d-glucopyranose (DGP) pyrolysis intermediate. We find that the DGP-to-LGO conversion proceeds via a multistep reaction mechanism, which involves ring-opening, ring-closing, enol-to-keto tautomerization, hydration, and dehydration reactions. The rate-determining step for the uncatalyzed process is the enol-to-keto tautomerization (ΔG‡298 = 68.6 kcal mol–1). We find that a water molecule can catalyze five of the seven steps in the reaction pathway. In the water-catalyzed process, the barrier for the enol-to-keto tautomerization is reduced by as much as 15.1 kcal mol–1, and the hydration step becomes the rate-determining step with an activation energy of ΔG‡298 = 58.1 kcal mol–1.

59Co($\vec p$,α) reaction at 100 MeV incident energy – statistical multistep direct reaction into the continuum of outgoing energies

The pre-equilibrium proton induced emission of light complex nuclei with energies in the continuum has been studied comprehensively for many years. Double–differential cross sections and especially analyzing power distributions are typical of an intranuclear nucleon-nucleon multistep statistical reaction mechanism. The final stage of the reaction may be a result of a direct pickup or knockout of the ejectile. The discussion on this subject continues to be a hot topic for both theoretical and experimental investigations. In this paper the results from the latest studies of the inclusive (, α) reactions on 59 Co to the continuum will be reported. The formalism based on the statistical multistep direct emission formulation of Feshbach, Kerman and Koonin is found to give a reasonably good reproduction of cross section and analyzing power distributions at various emission energies. Special attention will be paid to the details of the calculations.

The chemistry of nucleation

Nucleation phenomena are of critical importance in numerous areas of science and everyday life. For decades the prevailing models to explain nucleation have been based on thermodynamic arguments without consideration of the chemical nature of the specific system. Even though newer models have included system-dependent variables, the quantitative atomistic differences are largely ignored. As a consequence, nucleation processes are treated on a “particle” or “monomer” level without discussion of the true atomic scale “chemistry of nucleation”. In the past couple of years, in situ studies of solvothermal reactions have considerably changed the experimental insight into nucleation phenomena, and especially the measurement of X-ray total scattering data and the subsequent pair distribution function analysis have proven to be vital tools. Here we discuss in situ results obtained for ten different material systems, and show that nucleation of nanoparticles in solvothermal reactions expose a fascinating chemical richness spanning from mono-metal to complex polymer precursor species, which, through a specific system-dependent multistep reaction mechanism, develop into pristine nanocrystals. It is argued that it is time to introduce a paradigm shift in the general nucleation theory and move away from the “one model fits all” to a chemistry-based approach rooted in atomic scale insight.

Oxidation of Antitubercular Drug Isoniazid by a Lipopathic Oxidant, Cetyltrimethylammonium Dichromate: A Mechanistic Study

The oxidation of an antitubercular drug isoniazid by a lipopathic oxidant cetyltrimethylammonium dichromate (CTADC) in a nonpolar medium generates isonicotinic acid both in the presence and the absence of acetic acid. The conventional UV–vis spectrophotometric method is used to study the reaction kinetics. The occurrence of the Michaelis–Menten–type kinetics with respect to isoniazid confirms the binding of oxidant and substrate to form a complex before the rate-determining step. The existence of the inverse solvent kinetic isotope effect, k(H2O)/ k(D2O) = 0.7, in an acid-catalyzed reaction proposes a multistep reaction mechanism. A decrease in the rate constant with an increase in [CTADC] reveals the formation of reverse micellar–type aggregates of CTADC in nonpolar solvents. In the presence of different ionic and nonionic surfactants, CTADC forms mixed aggregates and controls the reaction due to the charge on the interface and also due to partition of oxidant and substrate in two different domains. High negative entropy of activation (ΔS‡ = –145 and –159 J K−1 mol−1 in the absence and presence of acetic acid) proposes a more ordered and highly solvated transition state than the reactants. Furthermore, the solvent polarity-reactivity relationship reveals (i) the presence of less polar and less ionic transition state compared to the reactants during the oxidation, (ii) differential contribution from nonpolar and dipolar aprotic solvents toward the reaction process, and (iii) the existence of polarity/hydrophobic switch at log P = 0.73. A suitable mechanism has been proposed on the basis of experimental results. These results may provide insight into the mechanism of isoniazid oxidation in hydrophobic environment and may assist in understanding the drug resistance in different location.

Mechanism-Based Trapping of the Quinonoid Intermediate by Using the K276R Mutant of PLP-Dependent 3-Aminobenzoate Synthase PctV in the Biosynthesis of Pactamycin.

Mutational analysis of the pyridoxal 5'-phosphate (PLP)-dependent enzyme PctV was carried out to elucidate the multi-step reaction mechanism for the formation of 3-aminobenzoate (3-ABA) from 3-dehydroshikimate (3-DSA). Introduction of mutation K276R led to the accumulation of a quinonoid intermediate with an absorption maximum at 580 nm after the reaction of pyridoxamine 5'-phosphate (PMP) with 3-DSA. The chemical structure of this intermediate was supported by X-ray crystallographic analysis of the complex formed between the K276R mutant and the quinonoid intermediate. These results clearly show that a quinonoid intermediate is involved in the formation of 3-ABA. They also indicate that Lys276 (in the active site of PctV) plays multiple roles, including acid/base catalysis during the dehydration reaction of the quinonoid intermediate.

Real-Time and In Situ Monitoring of Mechanochemical Reactions: A New Playground for All Chemists.

We provide a brief overview of the first techniques for direct, real-time observation of mechanochemical reactions by milling. Whereas mechanisms and kinetics of solid-state reactions induced by temperature or pressure have been extensively investigated, transformations of materials under continuous impact in a milling assembly remain largely unexplored and based on ex situ studies. The recent introduction and development of techniques for in situ monitoring of milling reactions by synchrotron X-ray powder diffraction and Raman spectroscopy has enabled the first direct insight into milling mechanochemistry, opening a new area for studies of chemical reactivity. So far, these techniques have revealed rapid, multistep reaction mechanisms and metastable intermediates that are impossible or difficult to observe or isolate in solution and have highlighted shortcomings of ex situ mechanistic studies. These pioneering advances also highlight the low level of mechanistic understanding and future challenges in developing a clear mechanistic picture of physicochemical transformations by milling.

Decomposition kinetic study, spectroscopic and pyrolytic analyses of Isoberlinia doka and Pinus ponderosa

Two woody species of different origins were subjected to Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis, and analytical pyrolysis gas chromatography/mass spectrometry (Py-GC/MS). Flynn-Wall-Ozawa, a model-free technique, was adopted for the decomposition kinetic study of Isoberlinia doka (ID) and Pinus ponderosa (PP). FTIR spectroscopy was employed to determine the level of cellulose crystallinity in the samples under investigation using the total crystallinity and lateral order indices methods. The apparent activation energy appeared as a function of conversion with significant influence from biomass diversity. The apparent activation energy recorded values of 202 to 365 kJ mol−1 for ID and 205 to 583 kJ mol−1 for PP. Thus, biomass decomposition kinetics is better modeled as a multi-step reaction mechanism. The analytical Py-GC/MS showed the presence of acids, sugars, and phenolic compounds in significant proportions for the two biomass samples. There were marked distinctions in both the quantity and the individual compounds detected in the biomass samples that were investigated.