Kinetics Of Homogeneous Reactions Research Articles

Controlling Current Distribution in Slurry Electrolytes for Redox Flow Batteries

Slurry electrodes have the potential to decouple the power and storage capacities of hybrid redox flow batteries1 (RFB). In more commonly studied true RFB chemistries, such as all-vanadium, the power and storage capacities can be considered separately2; the power capacity is dependent on the flow cell design while the storage capacity is dependent on the amount of positive and negative electrolyte that can be stored. These decoupled capacities allow true RFB’s to be highly scalable. However, in hybrid RFB chemistries, such as all-iron, solid metal is deposited within the flow cell during charge, limiting the storage capacity of the battery based on the geometry of the flow cell3. This limitation can be addressed by incorporating a slurry electrode in the negative electrolyte. By depositing the reduced metal on this mobile dispersion of electrically conductive carbon particles, hybrid redox RFB’s may achieve high scalability while utilizing less expensive materials, such as iron or zinc1,4. To successfully augment the scalability of hybrid RFB’s, the current distribution of the metal deposition reaction must occur predominantly on the mobile slurry electrode, rather than on the stationary current collector. This current distribution is dependent on numerous factors, including the electrical conductivity of the slurry electrode, the ionic conductivity of the electrolyte, the kinetics of the faradaic reaction, and mass transfer to both the stationary current collector and to the slurry particles. In this work, we explore three approaches to controlling the current distribution in slurry electrodes. In the first approach, we explore the utility of slurry electrodes consisting of multiple types of carbon particles. Our earlier work has shown that slurries of varying carbon type can have significantly different electrochemical performance5. Slurries consisting of carbon black particles with high specific surface area can minimize the kinetic resistance of the faradaic reactions but struggle to distribute current away from the current collector due to their low electrical conductivity. Meanwhile, slurries consisting of more electrically conductive, but lower surface area graphitic particles can facilitate faradaic current away from the current collector but may not minimize kinetic resistance to the same degree. By using a slurry electrode consisting of mostly graphitic particles with a small portion of high surface area carbon black, the faradaic current may be kinetically facile in addition to well distributed through the slurry. In the second approach, we explore options to hinder the ionic conductivity of the electrolyte. The faradaic current distribution through the slurry electrode depends heavily on the competition between the ionically and electronically conductive routes. Typically, the ionic conductivity is heavily favored due to the desire for high per-volume energy density and the low pH required to keep the metallic reactants soluble. By hindering the ionic conductivity of the electrolyte, either by utilizing viscosity increasing additives or by lessening the ionic concentration, the faradaic current may be better spread through the slurry. In the third approach, we explore options for controlling the current distribution via varying reaction kinetics on various electrode surfaces. While considerable work6,7 has been done comparing the reaction kinetics of homogeneous redox reactions on various electrode materials (glassy carbon, basal/edge plane graphite, platinum...) relatively little work has been done regarding metal deposition reactions. By changing the current collector material to one with poor deposition kinetics relative to the slurry electrode material, the current distribution may be enhanced on the slurry.(1) Petek, T. J. Enhancing the Capacity of All-Iron Flow Batteries: Understanding Crossover and Slurry Electrodes. Ph.D. Thesis 2015, No. May.(2) Weber, A. Z.; Mench, M. M.; Meyers, J. P.; Ross, P. N.; Jeffrey, T.; Liu, Q. Redox Flow Batteries, a Review. J. Appl. Electrochem. 2011, 41 (10), 1–72. https://doi.org/https://doi.org/10.1007/s10800-011-0348-2.(3) Dinesh, A.; Olivera, S.; Venkatesh, K.; Santosh, M. S.; Priya, M. G.; Inamuddin; Asiri, A. M.; Muralidhara, H. B. Iron-Based Flow Batteries to Store Renewable Energies. Environ. Chem. Lett. 2018, 16 (3), 683–694. https://doi.org/10.1007/s10311-018-0709-8.(4) Petek, T. J.; Hoyt, N. C.; Savinell, R. F.; Wainright, J. S. Slurry Electrodes for Iron Plating in an All-Iron Flow Battery. J. Power Sources 2015, 294, 620–626. https://doi.org/10.1016/j.jpowsour.2015.06.050.(5) Tam, V.; Wainright, J. Electrochemical Behavior of Low Loading Slurry Electrodes for Redox Flow Batteries. J. Electrochem. Soc. 2023, 170 (1), 010538. https://doi.org/10.1149/1945-7111/acb10a.(6) Mccreery, R. L.; Mcdermott, M. T. Comment on Electrochemical Kinetics at Ordered Graphite Electrodes. Anal. Chem 2012, 84. https://doi.org/10.1021/ac2031578.(7) Chen, P.; McCreery, R. L. Control of Electron Transfer Kinetics at Glassy Carbon Electrodes by Specific Surface Modification. Anal. Chem. 1996, 68 (22), 3958–3965. https://doi.org/10.1021/ac960492r. Figure 1

Hyperbranched RCA-mediated construction of a dendritic DNA nanostructure for ratiometric fluorescence biosensing

As a robust isothermal strand amplification technique, rolling circle amplification (RCA) is popularly adopted for constructing various homogeneous fluorescence biosensing strategies. However, the limited functional unit decoration on linear RCA products and the slow homogeneous reaction kinetics usually restrict their performance. Herein, by designing a target biorecognition-triggered hyperbranched RCA reaction to construct an Mg2+-dependent DNAzyme (MNAzyme)-decorated dendritic DNA nanostructure, a novel ratiometric fluorescence biosensing method was proposed. The catalyzed cleavage of MNAzymes toward a short stand labeled with the FAM fluorophore and its quencher realized a “signal-on” fluorescence output. Meanwhile, a “signal-off” fluorescence output of 2-aminopurine (2-AP) labeled at a DNA hairpin was realized through the DNA hybridization with the nanostructure. Besides efficient acceleration of homogeneous reaction kinetics, these fluorescence responses were dramatically amplified by the high decoration of MNAzymes on the dendritic nanostructure and its strong signal suppression toward 2-AP. Together with the exonuclease III-amplified target biorecognition reaction, a very low detection limit of 0.53 fg mL–1 and a six-order magnitude linear range were endowed for kanamycin antibiotic assay. This excellent analytical performance and its high selectivity, and good operability determine promising prospect of the method for practical applications.

Kinetics of Homogeneous Reaction of Potassium Methoxide Based on K2CO3 Catalyst in Transesterification of RBDPO to Biodiesel

Biodiesel production is generally catalyzed by potassium methylate or sodium methylate catalysts based on KOH and NaOH and these catalysts are still imported. The search for a cheap and effective catalyst continues to be carried out by researchers. One of the catalyst support materials currently in use involves impregnating K2CO3 with various substances, resulting in a heterogeneous catalyst. In this study, it was tried to use K2CO3 dissolved in methanol to produce a homogeneous potassium methylate catalyst. Potassium methylate-based homogeneous catalyst K2CO3-methanol is proven to have a very high function in the transesterification reaction of Refined Bleached Deodorized Palm Oil (RBDPO) into biodiesel, this is evidenced by the use of a catalyst percentage of 2% w and 30% w methanol to the weight of RBDPO resulting in an acid content in biodiesel of only 0.12% and a total glycerol of 0.124% in reaction time 3 hours, with the purity of the methyl ester in biodiesel reaching 98.80%. Meanwhile, for the calculation of homogeneous reaction kinetics, a reaction rate equation is produced where the order of the RBDPO transesterification reaction is order 2 (two) and the reaction rate constant is 0.0044.

Quartic autocatalysis on horizontal surfaces with an asymmetric concentration: water-based ternary-hybrid nanofluid carrying titania, copper, and alumina nanoparticles

Understanding the mechanisms and kinetics of homogeneous (i.e. water-based ternary-hybrid nanofluid) and heterogeneous (i.e. catalyst) reactions is capable of creating a more effective distribution of species and quality of fluids at the outlet. Thus, the present study focuses on analyzing the quartic type homogeneous-heterogeneous reactions in ternary-hybrid nanofluid of copper, alumina, and titania nanoparticles with water at the surface of a stationary/moving flat plate. The leading non-linear partial differential equations were transformed into a set of ordinary differential equations using local similarity transformations. The resultant non-linear ordinary differential equations were numerically solved using MATLAB’s built-in BVP4C tool to explore the impact of pertinent factors. From the design side of view, the Tiwari-Das model for nano-liquid was employed to study the flow-thermal behaviour of the nanofluid. The thermal conductivity formulations were taken from experimental works to incorporate the spherical, platelet and cylindrical-shaped nanoparticles. The proposed concept is comparatively analyzed for Blasius and Sakiadis flows. The homogeneous bulk fluid and heterogeneous catalyst concentrations are observed for small and large nanoparticle volume fractions. The investigation shows that the strength of homogeneous and heterogeneous reactions has the opposite effect on the concentration fields. Strong correlations exist between the heterogeneous catalyst’s diffusivity and the homogeneous bulk fluid’s viscosity. Mass diffusion became more prevalent at the surface as the Schmidt number surged. Conversely, the bulk fluid concentration was highest in areas far from the surface.

Steam reforming of methane over nickel catalyst using a one-dimensional model

Steam reforming of hydrocarbons is a well established chemical process which provides synthesis gas (H2 and CO). These synthesis products can hence be converted to numerous valuable basic chemicals. For the industrial application of steam reforming, a detailed understanding of the process is a prerequisite. Models that capture the detailed homogeneous and heterogeneous reaction kinetics and the comprehensive transport processes as well as their interaction have the potential to optimize the catalytic process without expensive experimental campaigns.
 In this paper, a detailed investigation has been done using a multi-step reaction mechanism for modeling steam reforming of methane over nickel-based catalyst using a one-dimensional (1D) model, LOGEcat [1]. The model is applicable to the simulation of all standard after-treatment catalytic processes of combustion exhaust gas along with other chemical processes involving heterogeneous catalysis, such as, the Sabatier process [27]. It is a 1D tool, thus is computationally cost effective and is based on a series of perfectly stirred reactors (PSR).
 The model is used to perform the simulations for various reactor conditions in terms of temperature, pressure, flow rates and steam-to-carbon (S/C) ratio. Several chemical reaction terms, such as, selectivity, yield, conversion, and mole fraction have been shown with respect to the varied parameters and the results are compared with 2D simulations and experimental reference data. We report a very good agreement of the various profiles produced with 1D model as compared to the reference data.
 Note that the main aim of this study is to check how far the 1D model can capture the basic chemistry for modeling steam reforming of methane over nickel-based catalysts. It is interesting to note that the cost effective reduced order model is capable to capture the physics and chemistry involved with a multi-step reaction mechanism showing the predictive capability of the model. This study forms the basis for further analysis towards the thermochemistry of the species to develop a kinetically consistent reaction mechanism.

Insights into kinetics extraction of the homogeneous electrocatalytic reaction between TMPD and ascorbic acid by cyclic voltammetry

• Reaction rate constant between TMPD •+ and AA is determined by a finite element model. • The waveform transition behavior hinders the kinetics extraction of redox mediation. • A rate constant of 2.61 (mol m −3 ) −1 s −1 is obtained from the proposed methodology. As a continued research interest for energy conversion and chemical analysis purpose, illustrating the reaction kinetics of homogeneous electrocatalytic reaction is crucial in the understanding of redox mediation process. In this paper, the reaction kinetics of the homogeneous electrocatalytic reaction between N,N,N’,N’-tetramethyl- para -phenylene-diamine (TMPD) and ascorbic acid (AA) is extracted via a 1D homogeneous electrocatalytic reaction model based on finite element methods. The experimental voltammograms reveal the waveform transition in terms of kinetic parameter and excess factor and an oxidative pre peak is observed at the scan rate of 1 mV s −1 . Based on the results above, the same 1D homogeneous electrocatalytic reaction model is employed for the extraction of reaction rate constant between TMPD •+ and AA. A transition region of the voltammetric waveform is illustrated under different scan rates and the methodology for the kinetics extraction under specific scan rate is defined. The obtained k e value is (2.61 ± 0.11 (mol m −3 ) −1 s −1 ) and this value is further verified by experimental voltammograms under various scan rates and substrate concentrations. In conclusion, the proposed kinetics extraction methodology reflects the electrocatalytic reaction between TMPD and AA. Also, the employment of waveform transition region facilitates the understanding of the redox mediation process.

Correlating Reaction Dynamics and Size Change during the Photomechanical Transformation of 9-Methylanthracene Single Crystals.

Photomechanical molecular crystals that expand under illumination could potentially be used as photon-powered actuators. In this study, we find that the use of high-quality single crystals of 9-methylanthracene (9MA) leads to more homogeneous reaction kinetics than that previously seen for polycrystalline samples, presumably due to a lower concentration of defects. Furthermore, simultaneous observation of absorbance and shape changes in single crystals revealed that the dimensional change mirrors the reaction progress, resulting in a smooth expansion of 7 % along the c-axis that is linearly correlated with reaction progress. The same expansion dynamics are highly reproducible across different single crystal samples. Organic single crystals exhibit well-defined linear expansions during 100 % photoconversion, suggesting that this class of solid-state phase change material could be used for actuation.

Correlating Reaction Dynamics and Size Change during the Photomechanical Transformation of 9‐Methylanthracene Single Crystals

AbstractPhotomechanical molecular crystals that expand under illumination could potentially be used as photon‐powered actuators. In this study, we find that the use of high‐quality single crystals of 9‐methylanthracene (9MA) leads to more homogeneous reaction kinetics than that previously seen for polycrystalline samples, presumably due to a lower concentration of defects. Furthermore, simultaneous observation of absorbance and shape changes in single crystals revealed that the dimensional change mirrors the reaction progress, resulting in a smooth expansion of 7 % along the c‐axis that is linearly correlated with reaction progress. The same expansion dynamics are highly reproducible across different single crystal samples. Organic single crystals exhibit well‐defined linear expansions during 100 % photoconversion, suggesting that this class of solid‐state phase change material could be used for actuation.

Spectroelectrochemistry Using Optically Transparent Electrodes – Ted Kuwana and the Early Years

AbstractTheodore “Ted” Kuwana and his research group played a central role in the invention and early development of spectroelectrochemistry at optically transparent electrodes (OTEs). The history of the “idea” and the first ten years of research are reviewed. Topics include OTEs and spectroelectrochemical cells for transmission and internal reflection experiments, rapid scan spectrometer development, and applications to kinetics of homogeneous chemical reactions associated with electrode processes and redox active biological compounds such as cytochromes. Relevant work of other contemporary research groups is also presented.

Reaction kinetics for the heterogeneously resin-catalyzed and homogeneously self-catalyzed esterification of thioglycolic acid with 2-ethyl-1-hexanol

• Esterification of thioglycolic acid (TGA) with 2-ethyl-1-hexanol (EHL) was studied. • Heterogeneous and homogeneous reaction kinetics were determined. • The adsorption of water on resin catalyst improved the conversion of TGA. • Heterogeneous kinetics were well described by the PH, ER and LHHW models. • Homogeneous kinetics were described by an improved homogeneous model. Producing 2-ethyl-1-hexyl thioglycolate (ETE) via esterification reaction with thioglycolic acid (TGA) aqueous solution as raw material by reactive-separation coupling technology is a promising process intensification method. To choose suitable reactive-separation coupling strategy, the kinetic studies of the esterification of TGA with 2-ethyl-1-hexanol (EHL) were carried out in a batch system. The commercial ion exchange resin was employed as an eco-friendly catalyst. The effects of temperature, catalyst concentration and molar ratio were determined. It was interesting to observe that the equilibrium conversion of TGA increased with the increase of catalyst mass fraction due to the adsorption of product water onto resin surface. The activity-based pseudo-homogeneous (PH), Eley-Rideal (ER) and Langmuir-Hinshelwood-Hougen-Watson (LHHW) models were used to fit the kinetics data of the resin-catalyzed reaction. The models of ER and LHHW performed better than the PH model. The kinetics of the TGA-self-catalyzed reaction was also determined. An activity-based homogeneous kinetics model could well describe this self-catalyzed reaction. These results would be meaningful to the selection and design of an appropriate reaction-separation strategy for the production of ETE, to realize the process intensification.

Comparative kinetics of homogeneous reaction of CO2 and unloaded/loaded amine using stopped-flow technique: A case study of MDEA solution

The stopped-flow technique was applied for the investigation of kinetics of CO2 absorption into loaded MDEA solution over the range of MDEA concentration of 0.4 to 1.0 kmol/m3, CO2 loading of 0.14 to 0.17 mol CO2/mol amine and the temperature of 293 to 308 K. The kinetics of unloaded MDEA was also investigated for the comparison and a huge difference of results which should be equal theoretically was indicated between loaded and unloaded systems. Then, the effect of CO2 loading on the kinetics was studied. As the loading increases, there is a turning point around loading of 0.10, below which the overall reaction constant decreases strongly. It was found that contribution of hydroxyl is a crucial to overall reaction rate constant and cannot be ignored and the influence of reverse reaction of MDEA and CO2 can be ignored. The kinetics of loaded system obtained in this work was modified and interpreted well by the base-catalyzed mechanism with AARD of 6.29% and compares favorably with literature using gas-liquid apparatus. The kinetics of unloaded system in this work is approximately twice as that of literature and loaded system. This work provided literature review and interpretation of MDEA-CO2-H2O and is hopeful to shed light on kinetic study of CO2 absorption in tertiary amine using stopped-flow apparatus.

Probe effects on concentration profiles in the diffusion layer: Computational modeling and near-surface pH measurements using microelectrodes

A finite-element method was applied to the problem of measuring near-surface concentration values with a probe placed in front of the electrochemical interface. Those probes, like pH-sensing microelectrodes, interfere with mass transport and unreliable results may be obtained due to the local distortions. A pure diffusion problem was solved for a one-component two-dimensional axisymmetric model, under Dirichlet and Neumann-type boundary conditions at the substrate surface. The generalized results may be used to estimate the effects of the sensor tip-substrate distance on local current density and surface concentrations, and on measurement accuracy in absence of homogeneous reactions. To account for the presence of homogeneous reactions, simulations were performed for specific system compositions, using the near-surface pH measurement in solutions containing a buffering agent as an example. An IrOx microelectrode was used for experime ntal measurements of near-surface pH carried out in solutions displaying different buffering capacities. The results were compared to simulations performed for a one-dimensional model with simplified electrode kinetics, but fully describing the kinetics of homogeneous reactions. It is shown that valuable information on the electrochemical processes may be extracted by using this experimental approach, even when the measured pH does not strictly correspond to the pH effectively experienced by the substrate surface.

Incorporation of Si during vapor phase epitaxy of III-V compounds: Evidence of an enthalpy-entropy compensation effect

The incorporation of dopants in III-V compound semiconductor epilayers during chemical vapor deposition involves complex homogeneous and surface reaction kinetics and is expected to be an activated process. In particular, silicon is an element of choice for n-type doping of various III-V compound epilayers for which there is a wealth of data involving different growth and doping precursors. Kinetic arguments such as the role of multi-excitation entropy or quasi-equilibration at the growth interface during incorporation predict an enthalpy-entropy compensation effect (EECE), which is observed from compiled data for more than 14 orders of magnitude of the prefactor describing the activated incorporation rate. For this particular observation of the EECE, an explanation involving data pre-selection imposed by restrictive growth conditions may also be invoked.

NOX formation in oxy-fuel combustion of lignite in a bubbling fluidized bed – Modelling and experimental verification

This paper reports results of experimental and numerical studies of NOx formation in a 30 kWth lab-scale bubbling fluidized bed running in oxy-fuel mode. The numerical model is based on the GRI-mech 3.0 mechanism to compute the kinetics of homogeneous reactions of volatiles and char combustion products and takes into account the flue gas recirculation. The impact of the oxygen excess and of the fluidized bed temperature was examined. Both the numerical simulations and the experiments shown a significant correlation of NOX formation with the excess of oxygen, where a higher oxygen concentration enhances the fuel-bound nitrogen oxidation to NOX. It was also found that there is no correlation of the NOx formation and resulting emissions with the fluidized bed temperature in temperature range typical for bubbling fluidized bed combustors (840–960 °C), but the numerical simulations showed an increased NOX concentration when the temperature raised more (up to 1360 °C). The agreement of experimental and numerical results shows that the numerical model can provide useful insight into the mechanism of NOX formation.

Homogeneous and Heterogeneous Catalyzed Esterification of Acrylic Acid with Ethanol: Reaction Kinetics and Modeling

ABSTRACTKinetics of esterification of acrylic acid with ethanol in the presence of homogeneous (H2SO4, HCl, p‐TSA, HI) catalysts as well as heterogeneous catalysts (Dowex 50WX, Amberlyst 15) was studied. The effects and performance of these catalysts on the conversion of acrylic acid were evaluated. In the kinetics of homogeneous catalyzed reaction, both concentration and activity‐based model were employed. Activity coefficients were predicted by the Universal Functional group Contribution (UNIFAC) method to consider nonideal behavior of the liquid phase. The heterogeneous catalyzed reaction mechanisms were developed using Eley–Rideal theory. The model results were compared with the experimental results and were in good agreement. The temperature dependency of the constants, reaction enthalpy, and entropy, and activation energy were determined. The conversion of acrylic acid was obtained as 63.2%, 61.02%, 53.3%, 21.4%, 34.96%, and 14.84% for H2SO4, p‐TSA, HCl, HI, Dowex 50WX, and Amberlyst 15, respectively, under process temperature of 70°C, reactant molar ratio of 1:1, and catalyst concentration of 2% (v/v) for homogeneous and 2.17 g for heterogeneous catalyst. These outcomes provide an approach to understand the significant effect of each catalyst on the esterification kinetics of acrylic acid and ethanol.

Kinetics of homogeneous and heterogeneous reactions in the reductive aminolysis of glucose with dimethylamine

The reductive aminolysis of glucose with dimethylamine (DMA) as aminating agent has been investigated experimentally as well as via kinetic model construction. A fed-batch reactor configuration was used at following conditions: temperatures ranging between 383 K–398 K, total pressures from 6.0 MPa to 7.5 MPa, an overall catalyst to glucose ratio of 9–31 gcat mol−1, an overall DMA to glucose ratio of 12–24 mol mol−1 and an overall hydrogen to glucose ratio of 5–10 mol mol−1. Such a reactor configuration, combined with a controlled feeding rate of the sugar, allowed to significantly avoid degradation reactions. The main desired products, i.e., dimethylaminoethanol (DMAE) and tetramethylethylenediamine (TMEDA), were obtained after amination followed by retro-aldol cleavage with maximum selectivity, amounting up to 15% and 60% respectively. Retro-aldol cleavage after amination proceeds at lower temperatures, evidenced by an activation energy of 60 kJ mol−1, than without amination, where activation energies amounting to 140 kJ mol−1 have been reported. A higher catalyst to glucose ratio leads to more parallel side products such as N,N-dimethylglucamine and 4‐dimethylamino-1,2,3-butanetriol. The effects of temperature and catalyst to glucose ratio were much more pronounced than that of the total pressure or the ratio of the aminating agent to glucose. The developed kinetic model is based on the most prominent homogeneous bulk phase and heterogeneously catalyzed reactions and accounts quantitatively for degradation reactions. A statistically and physically significant model which could satisfactorily reproduce the experimental observations, was thus obtained.

Homogeneous and heterogeneous reaction mechanisms and kinetics of mercury oxidation in coal-fired flue gas with bromine addition

A detailed kinetic model consisting of homogeneous mechanism with 352 elementary reactions and a new heterogeneous mechanism on fly ash surface was developed to predict mercury oxidation in coal-fired flue gas with bromine addition. The heterogeneous mechanism involves the catalytic oxidation activity of unburned carbon (UBC) and Fe2O3 within fly ash, and includes 38 irreversible elementary reactions governing the adsorption, oxidation and desorption of mercury on fly ash. The competitive adsorption among flue gas compositions is also involved in the heterogeneous mechanism. Kinetic parameters of some homogeneous reactions in the Hg/Br and Hg/Cl sub-mechanisms have been updated based on the recent experimental data obtained by the improved aqueous chemistry methods. This kinetic model was validated by comparison to lab-, pilot- and full-scale experimental data. The comparison shows that the model predictions are in good agreement with the experimental data. The rate of production analysis illustrates that the heterogeneous pathway is dominant for mercury oxidation in coal-fired flue gas with bromine addition. The sensitivity analysis and reaction path analysis indicate that the dominant pathway of heterogeneous mercury oxidation by bromine on fly ash surface is Hg0→StHgBr(s)→HgBr2, whereby a brominated carbon site partially oxidizes Hg0 into StHgBr(s) which is subsequently oxidized into HgBr2. NO and SO2 weakly inhibit mercury oxidation. H2O exhibits an inhibitory effect on heterogeneous Hg oxidation because of the elimination of brominated carbon sites.

Cyclic multipulse voltammetric techniques. Part II: EC mechanism

A theoretical analysis of electrode processes controlled by kinetics of follow-up homogeneous chemical reaction under conditions of (simple) cyclic multi pulse voltammetry (CMPV), cyclic differential multi pulse voltammetry (CDMPV) and cyclic square-wave voltammetry (CSWV) is presented. The study was performed under linear diffusion conditions and for equal diffusion coefficients of all participants of the electrode process. Chemical reaction changes the shape and position of CMPV response, compared to a simple reversible electrode reaction. As the chemical process becomes faster, forward/reduction wave decreases whereas a pronounced maximum appears on it. On the other hand, the influence of following chemical reaction on the shape of CDMPV and CSWV reduction signal is not significant, while their voltammograms shifted towards more positive potentials as well. Experimentally, the rate constant of follow-up chemical reaction (kf) can be estimated from the linear dependence of limiting to maximum reduction currents ratio (il,c/imax,c) on log(tp) in CMPV, as well as of anodic to cathodic net peak currents ratio (Δip,a/Δip,c) on log(tp) in CDMPV and on log(f) in CSWV. Combination of two or more cyclic multipulse voltammetric techniques gives more information about kinetics i.e. more precise determination/estimation of the rate constant of follow-up chemical reaction, compared with application of only one technique.

Theory of single potential step absorbance transient at an optically transparent rough and finite fractal electrode: EC′ mechanism

A theoretical model for single potential step absorbance transient has been developed for the pseudo first-order catalytic coupled mechanism (EC′) at a rough optically transparent electrode (OTE). A second-order perturbation theory is developed for an arbitrary surface roughness profile. Results are further extended for the ensemble averaged absorbance response on the random surface roughness model in term of the power-spectrum of roughness. A general result derived showing relation between the absorbance of pseudo first-order catalytic reaction and the absorbance of reversible charge transfer. The dependence of absorbance transient on morphological characteristics and homogeneous rate constant is highlighted. Our results show an enhancement in the absorbance-transients for the band limited self-affine fractal roughness of OTE and coupled kinetics of homogeneous reaction with a heterogeneous charge transfer step.

Computational Fluid Dynamics Modeling of Biomass Gasification in Circulating Fluidized-Bed Reactor Using the Eulerian–Eulerian Approach

A three-dimensional CFD (computational fluid dynamics) steady-state model was established to simulate biomass gasification in a circulating fluidized-bed (CFB) reactor. The standard k–ε turbulence model was coupled with the kinetic theory of granular flow to simulate the hydrodynamics in the gasifier. The kinetics of homogeneous and heterogeneous reactions were studied and integrated with the equations of continuity, motion, and energy to describe the distributions of velocity, temperature, and concentration. The simulation results were compared to experimental data. The impacts of turbulence models, radiation model, water–gas shift (WGS) reaction, and equivalence ratio (ER) were investigated to present a reliable understanding of biomass gasification in a CFB reactor.