Multi-step Reaction Mechanism Research Articles

Combined Experimental and Numerical Analysis of Surface-Modified Solid Oxide Fuel Cell Cathodes

In this study, the effects of surface modification on the performance of composite solid oxide fuel cell (SOFC) cathodes are investigated through a series of experiments and relevant multiphysics simulations. Experimentally, the LSM/YSZ composite cathodes of commercial fuel cells are surface-treated with various electrocatalysts, and datasets of both polarization curves and electrochemical impedance spectra are collected. The multiphysics simulations consider both charge conservation and species transportation, as well as a multi-step oxygen reduction reaction (ORR) mechanism with parallel pathways. The numerical simulations are developed and then calibrated against data from baseline cells. Based on changes in microstructural properties and rate constants, the performance of the modified fuel cells is then predicted using reaction mechanism parameters recalibrated from experimental data. Analysis is also performed to separate and quantify the effect on cell performance from the changes in microstructural properties and reaction mechanism parameters resulting from surface modification. This study provides insights into the tuning of electrochemical processes when modifying the surface of cathodes and guides further performance improvements.

Numerical Study on Premixed Methane–Air Flame Propagation in a Confined Vessel at Low Initial Temperature

In this article, premixed methane–air flame propagation in a confined vessel at low initial temperature was simulated using a multistep chemical reaction mechanism. The confined vessel was a cylinder with aspect ratio of 3 with asymmetrical position of the ignition source near the side cover. The equivalence ratio and the initial temperature of the premixed unburned combustible gas were 1.0 and 150 K, respectively. The overall evolution of the flame and the flame dynamics were obtained, respectively. Through the entire flow field variation, vortex movement, and pressure wave propagation characteristics during the whole process of combustion, the flame propagation mechanism of methane combustion at low initial temperature was established finally. Results indicate that five stages are divided during the methane combustion in a confined vessel: spherical flame propagation, “fingertip” shaped flame propagation, flame “skirt edge” contacts the side wall, “crescent” flame propagation, and typical “tulip” flame ...

A paramedic treatment for modeling explicitly solvated chemical reaction mechanisms.

We report a static quantum chemistry modeling treatment to study how solvent molecules affect chemical reaction mechanisms without dynamics simulations. This modeling scheme uses a global optimization procedure to identify low energy intermediate states with different numbers of explicit solvent molecules and then the growing string method to locate sequential transition states along a reaction pathway. Testing this approach on the acid-catalyzed Morita-Baylis-Hillman (MBH) reaction in methanol, we found a reaction mechanism that is consistent with both recent experiments and computationally intensive dynamics simulations with explicit solvation. In doing so, we explain unphysical pitfalls that obfuscate computational modeling that uses microsolvated reaction intermediates. This new paramedic approach can promisingly capture essential physical chemistry of the complicated and multistep MBH reaction mechanism, and the energy profiles found with this model appear reasonably insensitive to the level of theory used for energy calculations. Thus, it should be a useful and computationally cost-effective approach for modeling solvent mediated reaction mechanisms when dynamics simulations are not possible.

Multiscale modeling of methane catalytic partial oxidation: From the mesopore to the full‐scale reactor operation

A multiscale methodology combining three different reactor length‐scales is presented to investigate the role of the catalyst internal pore structure and metal loading and dispersion on the catalyst layer and full‐scale reactor performances. At the catalyst level, the methodology involves pore‐scale simulations in the three‐dimensional mesopore and macropore space. The information gathered at the catalyst level is delivered to the full‐scale reactor model. The methodology is applied to a honeycomb reactor performing methane partial oxidation considering reaction kinetics described through a detailed multistep reaction mechanism. Realistic mesopore and macropore structures were reconstructed and combined to form specific bidisperse porous washcoat layers. The study shows that species effective diffusivities vary significantly but not in the same proportion for different structures. For structures featuring poor transport characteristics, the integral methane conversion and hydrogen selectivity are strongly affected while the reactor temperatures increase substantially. © 2017 American Institute of Chemical Engineers AIChE J, 64: 578–594, 2018

Modeling chemical reactions in classical molecular dynamics simulations

An algorithm capable of incorporating multi-step reaction mechanisms into atomistic molecular dynamics simulations using traditional fixed valence force fields is proposed and implemented within the framework of LAMMPS (Large-scale Atomic Molecular Massively Parallel Simulator). This extension, referred to as fix bond/react, enables bonding topology modifications during a running MD simulation using pre- and post-reaction bonding templates to carry out a pre-specified reaction. Candidate reactants are first identified by interatomic separation, followed by the application of a generalized topology matching algorithm to confirm they match the pre-reaction template. This is followed by a topology conversion to match the post-reaction template and a dynamic relaxation to minimize high energy configurations. Two case studies, the condensation polymerization of nylon 6,6 and the formation of a highly-crosslinked epoxy, are simulated to demonstrate the robustness, stability, and speed of the algorithm. Improvements which could increase its utility are discussed.

Thermo-kinetic, spectroscopic study of brewer’s spent grains and characterisation of their pyrolysis products

Brewer’s spent grains, major by-products of the beverage industries, were obtained from a Nigerian brewery and subjected to pyrolysis at 500°C. Analytical pyrolysis (Py-GC/MS) was conducted on the original BSG, extractive-free BSG and BSG CH2Cl2 extract. There was a marked difference in the concentration of levoglucosan between the extractive-free BSG (40.5%) and the original BSG (11.5%). The Py-GC/MS detected significant amount of palmitic and oleic acids in the three samples. Prior to pyrolysis, the extractives content were isolated through methanolysis, transesterified and subjected to GC/MS analysis. The analysis revealed the presence of linoleic, oleic and palmitic acids in an appreciable quantity. The BSG and its biochar were also characterised according to Fourier transform infrared and Raman spectroscopic, and thermogravimetric analyses alongside some conventional physico-chemical analysis. The total crystallinity and condensation indices were evaluated from specific FTIR spectra peaks so as to monitor structural transformations in the bio-char. Kinetic data were deduced from TGA measurements and it was apparent that thermal decomposition of BSG is better modelled as a multi-step reaction mechanism. The nitrogen content of the bio-char was relatively high making it a suitable feedstock for the production of in situ nitrogenised activated bio-carbon. There was a significant variation between the structural configuration of the original BSG and the bio-char as revealed by the change in the value of the condensation index as well as the cellulose crystallinity content. It may thus be concluded that the BSG biomass would be a viable feedstock for thermal decomposition and other valorisation processes.

Insights into pyrolysis and co-pyrolysis of biomass and polystyrene: Thermochemical behaviors, kinetics and evolved gas analysis

The purpose of this study was to investigate the effect on polystyrene (PS) during co-pyrolysis with biomass through thermal decomposition. The model-free iso-conversional methods (Kissinger, Friedman, Flynn-Wall-Ozawa, Kissinger-Akahira-Sunose, Starink and Vyazovkin) were adopted to calculate activation energy of the pyrolysis and co-pyrolysis process of two biomass samples (walnut shell: WS and peach stones: PST) with PS. It is found that biomass blending to PS decreased activation energy values and resulted in multi-step reaction mechanisms. Furthermore, changes in the evolution profiles of methyl, water, methoxy, carbon dioxide, benzene and styrene was monitored through evolved gas analysis via TGA/FT-IR and TGA/MS. Detection of temperature dependent release of volatiles indicated the differences occur as a result of compositional differences of biomass.

A genetically optimized kinetic model for ethanol electro-oxidation on Pt-based binary catalysts used in direct ethanol fuel cells

A one-dimensional model is proposed for the anode of a liquid-feed direct ethanol fuel cell. The complex kinetics of the ethanol electro-oxidation reaction is described using a multi-step reaction mechanism that considers free and adsorbed intermediate species on Pt-based binary catalysts. The adsorbed species are modeled using coverage factors to account for the blockage of the active reaction sites on the catalyst surface. The reaction rates are described by Butler-Volmer equations that are coupled to a one-dimensional mass transport model, which incorporates the effect of ethanol and acetaldehyde crossover. The proposed kinetic model circumvents the acetaldehyde bottleneck effect observed in previous studies by incorporating CH3CHOHads among the adsorbed intermediates. A multi-objetive genetic algorithm is used to determine the reaction constants using anode polarization and product selectivity data obtained from the literature. By adjusting the reaction constants using the methodology developed here, different catalyst layers could be modeled and their selectivities could be successfully reproduced.

Predicting thermal degradation mechanisms in urea–formaldehyde cellulose composites filled with tin particles

This article reports a study on structural characterization, thermal stability, and prediction of the thermal degradation mechanisms of urea–formaldehyde cellulose (UFC) composites filled with tin particles by condensed phase kinetic approaches. Structural characterization of UFC/Sn composites performed by scanning electron microscopy, X‐ray diffraction, and Fourier‐transform infrared analyses demonstrates that the composites are fairly homogenous, and the interactions between UFC and tin in the composites are physical in nature. Measurements of inherent thermal stabilities, apparent reaction profiles, and thermal degradation kinetics of UFC and UFC/Sn composites have been carried out on the thermoanalytical data of materials. The integral procedure decompositions temperature elucidates the increasing thermal stabilities trend of UFC/Sn composites with the increase in tin contents in UFC matrix. Reaction profiles of UFC and UFC/Sn composites suggest considerably complex thermal degradation pathways of the materials which comprise various competitive/consecutive reactions. Substantial variations in the activation energies of matrix and composites with the advancement of reaction by generalized integral isoconversional method verify their multistep reaction mechanisms. Advanced reaction model determination methodology with the help of a novel kinetic function F(α, T) reveals that the multistep thermal degradation of UFC goes to completion by overall following intricate nucleation/growth mechanisms. A detailed account of the mechanistic information related to thermal degradation of UFC and UFC/Sn composites is also given and discussed in this study. POLYM. COMPOS., 39:4341–4354, 2018. © 2017 Society of Plastics Engineers

Mechanisms of the Formation of Adenine, Guanine, and Their Analogues in UV-Irradiated Mixed NH3:H2O Molecular Ices Containing Purine.

We investigated the formation mechanisms of the nucleobases adenine and guanine and the nucleobase analogues hypoxanthine, xanthine, isoguanine, and 2,6-diaminopurine in a UV-irradiated mixed 10:1 H2O:NH3 ice seeded with precursor purine by using ab initio and density functional theory computations. Our quantum chemical investigations suggest that a multistep reaction mechanism involving purine cation, hydroxyl and amino radicals, together with water and ammonia, explains the experimentally obtained products in an independent study. The relative abundances of these products appear to largely follow from relative thermodynamic stabilities. The key role of the purine cation is likely to be the reason why purine is not functionalized in pure ammonia ice, where cations are promptly neutralized by free electrons from NH3 ionization. Amine group addition to purine is slightly favored over hydroxyl group attachment based on energetics, but hydroxyl is much more abundant due to higher abundance of H2O. The amino group is preferentially attached to the 6 position, giving 6-aminopurine, that is, adenine, while the hydroxyl group is preferentially attached to the 2 position, leading to 2-hydroxypurine. A second substitution by hydroxyl or amino group occurs at either the 6 or the 2 position depending on the first substitution. Given that H2O is far more abundant than NH3 in the experimentally studied ices (as well as based on interstellar abundances), xanthine and isoguanine are expected to be the most abundant bi-substituted photoproducts. Key Words: Astrophysical ice-Abiotic organic synthesis-Nucleic acids-Origin of life-RNA world. Astrobiology 17, 771-785.

Study of non-premixed turbulent flame of hydrogen/air downstream Co-Current injector

For three decades, hydrogen has been identified as a versatile potential fuel concurrent to the conventional fuel such as gasoline. In order to fully implement it and to develop the combustion based power devices that may supply much higher energy density, it is very essential to understand the mechanism of Hydrogen/Air combustion. In this work, Computational Fluid Dynamics (CFD) numerical simulations have been performed to study the combustion of non-premixed turbulent hydrogen-air mixture with different equivalence ratios and different mass flow rates and its effect on different species formation, peak temperature and NOx formation. The performance of the combustor is evaluated by using FLUENT software under adiabatic wall condition. Generalized finite rate chemistry model was used to analyze the hydrogen-air combustion system. The combustion is modeled using multi-step reaction mechanism with 14 species, until complete conversion of fuel to H2O. Through such a systematic analysis, a proper controlled operation condition for the combustor is suggested which may be used as a guideline for combustor design. Results reported in this work illustrate that the CFD simulation can be one of the most powerful, beneficial and economical tool for combustor design and for optimization and performance analysis. They are more sensitive to the model of the transport properties while the reasonable results can be achieved even with the use of global reaction mechanism and a simple turbulence model as k- ε, which are not excessively time and memory consuming. From an environmental point view, this study shows that the radical production (OH and NO) is very small although maximum temperature reached exceeded 2000 (K). The mass fraction of NO is much lower if we increase the air inlet velocity, which makes the cold reaction mixture do not promote the NO formation by dissociation.

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

In the present study, numerical simulation of a solid oxide fuel cell (SOFC) button cell with a multi-step oxygen reduction reaction (ORR) mechanism in a composite cathode was developed for better understanding of processes within the SOFC. The ORR mechanism involves both surface charge transfer through triple phase boundary (3PB) and bulk charge transfer via a two phase boundary (2PB) pathway. The developed multi-physics simulation considered the charge conservation and species transportation through porous media. The developed simulation was first applied to a dense thin film study only involving surface adsorption and the 2PB pathway. By calibrating the simulation with measurements of dense thin film cathodes, the kinetic rates of rate-determining steps in ORR model were decided. Finally, the simulations of full SOFC button cell were calibrated with measured cell performance (polarization curves and impedance behavior) at three different air/fuel utilization conditions. It was found that it is beneficial to determine the kinetic rates of rate-determining steps in a multi-step charge transfer model before calibrating the simulations for full cell system. With the determined rates, the various performance of different cells can be attributed to the microstructure versus material variations. Therefore, the present study not only helps better understand the details of processes in SOFC but also provides guidance for electrode design and manufacturing.

The Electrochemical Performance of LSM with A-site Non-Stoichiometry under Cathodic Polarization

The activation behavior of a LSM based-cathode in the SOFC is characterized by a rapid reduction in the electrode polarization resistance after cathodic current passage treatment. This activation behavior has been attributed to mechanisms that either enhance the kinetics of oxygen reduction or remove impediments from the oxygen reduction mechanism. Due to the sensitivity of the activation process to the operation history, a comprehensive study was carried out to reveal the relationship between the activation conditions, the steady state cathode performance and the long term stability. This work demonstrates the application of a two electrode configuration half-cell operated for the impedance analysis of LSM-based electrodes. The effects of current density and overpotentials on the activation behavior of LSM based electrode were investigated. The impacts of temperature and oxygen partial pressure to the activation process and final performance are also discussed. Long term degradation test of LSM electrodes were carried out on both half-cells and full cells with various activation processes. Numerical simulation of the LSM-based cathode was conducted with a multi-step oxygen reduction reaction (ORR) mechanism for a better understanding of the nature of the activation process. The results are of great relevance as the electrode performance depends on not only the fabrication but also on the initial break-in procedure.

Reaction mechanism in solid BaCe0.9Y0.1O3 – liquid BaWO4 system

BaCeO3–based protonic conductors may be considered as perspective materials for construction of various electrochemical devices as fuel cells, gas sensors, electrochemical pumps, separators or membrane chemical reactors. The desired properties, as high electrical conductivity or chemical stability of these materials may be further improved by formation of composite, by introduction of modifying phase as BaWO4 into the host Y-doped BaCeO3 material. The high temperature process of composite formation requires the recognition of chemical reactivity between the used component. In this work the mutual reactivity and reaction mechanism in the solid BaCe0.9Y0.1O3 – liquid BaWO4 system were investigated. The synthesis of BaCe0.9Y0.1O3 and BaWO4 compounds was done by solid-state reaction method. The formation of BaCe0.9Y0.1O3 – BaWO4 materials was obtained by high temperature impregnation of melted BaWO4 into solid BaCe0.9Y0.1O3 with different impregnation time applied (2, 4 and 8 h). The evolution of crystal structure, phase composition, microstructure, chemical composition and electrical properties with impregnation time, for surface and interior of the samples allowed to analyse the mutual reactivity of composite components and to propose the multi-step reaction mechanism in the solid BaCe0.9Y0.1O3 – liquid BaWO4 system.

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.