Multistage Electrochemical Reactions with the Transfer of Intermediates between the Near-Electrode Layer and the Bulk Solution: The Accumulation of the Intermediates and the Current Redistribution between the Stages during Electrolysis

Dependences of reactant concentrations and current densities of individual stages on the electrolysis time for a multistage electrode process complicated by the transfer of intermediates between the near-electrode layer and the bulk electrolyte are obtained by numerical computer-aided modeling. Effect of various factors on the dynamics of variations in the concentration of the initial reactant and intermediate in the bulk electrolyte is analyzed. The possibility of a substantial increase in the current efficiency of the second stage of discharge during electrolysis is shown.     

Multistep Electrochemical Reactions Involving Transport of Intermediates between the Near-electrode Layer and the Bulk Solution: The Kinetics of Two-Step Processes in Conditions of Non-steady-state Diffusion

Results of numerical simulation of the kinetics of a two-stage electrochemical reaction with partial transport of intermediates between the near-electrode layer and the bulk solution in conditions of linear nonsteady-state diffusion of reactants are analyzed with use made of a theory of generalized variables. Dimensionless complexes (similitude criteria) that characterize the electrochemical reaction under consideration are obtained. Combinations of kinetic parameters of the first and second steps of the reaction, at which the reaction proceeds in a quasi-steady-state regime, are established. The conditions, at which rates of individual steps of electrochemical conversion are equal, are determined.     

铬(Ⅵ)与谷胱甘肽作用及其中间态配合物形成的电化学表征

本文采用电化学方法，对谷胱甘肽(GSH)与重铬酸钾的相互作用及其中间态配合物的形成过程进行研究。结果表明：在pH=5.6的HAc-NaAc缓冲溶液中，GSH浓度为Cr(Ⅵ)浓度5倍以上时，Cr(Ⅵ)与GSH作用完全并形成一新的中间态配合物，该中间态配合物于+0.21 V和+0.36 V(vs SCE)处产生一对新的氧化还原峰，UV-Vis的吸收光谱进一步证明了中间态配合物的形成。该配合物不稳定，在一定时间内缓慢分解，其电化学与UV-Vis光谱动力学信息同步。进一步探讨了GSH与Cr(Ⅵ)作用的电极反应机理。当Zn(Ⅱ)存在于该体系时，Zn(Ⅱ)对中间态配合物的生成和分解过程起着双向催化作用。     

Kinetic Study of the Thermopolymerization of Furfuryl Methacrylate in Bulk by Mathematical Modeling. Part A: Simulation of Experimental Data and Sensitivity Analysis of Kinetic Parameters

Mathematical modeling of the thermopolymerization of FM and CMFMA was carried out using a cross‐linked kinetic model proposed for the photo‐initiated polymerization of acryl‐furanic compounds. In this model, the photochemical initiation step was substituted by a thermal one and it was assumed that the constant of radical termination was time‐dependent, which allowed the gel effect (Trommsdorff) at high monomer conversion to be simulated. Optimization of all kinetic constants was achieved and the results of simulation suitably fitted the experimental data of the monomer conversion. The contribution of each step in the mechanism and its dependence on the experimental conditions were estimated by a sensitivity analysis technique.

     

Multistep electrochemical reactions involving transport of intermediates between the near-electrode layer and the bulk solution: A kinetics analysis based on theory of generalized variables (theory of similarity)

A kinetic analysis of multistep electrochemical reactions involving partial diffusion transport of intermediates between the near-electrode layer and the bulk electrolyte is performed within theory of generalized variables (theory of similarity). Dimensionless complexes (similarity criteria) that characterize such reactions are obtained, and their values corresponding to the transport of intermediates are determined.Translated from Elektrokhimiya, Vol. 41, No. 1, 2005, pp. 121–125.Original Russian Text Copyright © 2005 by Protsenko, Danilov.     

Electron Correlation Effects for Adiabatic Electrochemical Reactions of Electron Transfer in a Model for an Electrode with an Infinitely Wide Conduction Band: Diagrams of Kinetic Modes

For adiabatic electrochemical reactions, within the framework of a model for an electrode with an infinitely wide conduction band, exact results are obtained for critical regions that correspond to different possible types of electron transfer processes (transfer of a single electron with and without an intermediate state, simultaneous transfer of two electrons) and regions that correspond to electroadsorption of the reactant in certain charge states. These regions form a diagram of kinetic modes (DKM) in the space of model parameters. Analytical expressions for outermost curves of DKM are obtained for some extreme cases. For the general case of a model for an electrode with an infinitely wide conduction band, a DKM is constructed and investigated with exact allowance for the effects of electron–electron correlations.     

Computer-aided Modeling of Porous Electrodes with an Immobilized Enzyme: Substrates with a Partially Regular Structure

In porous electrodes with an immobilized enzyme the substrate must have a high specific surface area, which must be accessible to landing onto it a maximum number of enzyme molecules. These demands are not easy to meet. Conceivable are limiting versions as follows: a stochastic substrate, where substrate particles (SP) are distributed in the volume randomly, and a regular substrate, where SP are distributed strictly regularly. Both versions of the organization of SP have advantages and disadvantages; therefore, in the paper studied is a third, intermediate, version, specifically, substrates with a partially regular structure. Shown is that there exists an optimum of values of fractal dimensionality for a regular base of a substrate, where, by somewhat sacrificing the amount of active enzymes, one can attain a considerable ease of the process of landing enzymes on the surface of a porous substrate. Calculations also show that of practical interest may be a porous substrate with a purely regular structure.     

甲基肼/四氧化二氮反应化学动力学模型构建及分析

甲基肼(MMH)和四氧化二氮(NTO)是常用的液体火箭发动机推进剂,但目前对其反应机理的研究还十分有限.本文首先构建了一个包含23种组分和20个基元反应的MMH/NTO反应动力学模型;对MMH/NTO自燃着火过程进行的验证计算表明,该机理能够合理地描述MMH/NTO的自燃温升过程,准确预测反应物系统的着火延迟时间及平衡温度,并能合理地反映MMH/NTO反应物系统着火延迟时间对反应初始压力以及氧燃比的依赖关系;通过灵敏度分析方法指出了影响MMH/NTO着火过程的关键反应.模拟分析了在不同压力和氧燃比条件下MMH/NTO系统的自燃温升过程,结果表明,随着压力的升高,系统着火延迟时间变短,平衡温度升高;在一定范围内增大氧燃比,着火延迟时间变长,平衡温度先升高后减小.     

Modeling and Experimentation of RAFT Solution Copolymerization of Styrene and Butyl Acrylate,Effect of Chain Transfer Reactions on Polymer Molecular Weight Distribution

Chain transfer reactions widely exist in the free radical polymerization and controlled radical polymerization, which can significantly influence polymer molecular weight and molecular weight distribution. In this work, the chain transfer reactions in modeling the reversible addition–fragmentation transfer (RAFT) solution copolymerization are included and the effects of chain transfer rate constant, monomer concentration, and comonomer ratio on the polymerization kinetics and polymer molecular weight development are investigated. The model is verified with the experimental RAFT solution copolymerization of styrene and butyl acrylate, with good agreements achieved. This work has demonstrated that the chain transfer reactions to monomer and solvent can have significant impacts on the number‐average molecular weight (M_n) and dispersity (Ð).     

模拟混凝土孔溶液中钢筋电化学噪音测量及小波分析

电化学噪音(Electrochemical Noise,ECN)测量可用于在无电信号扰动条件下检测腐蚀体系的暂态行为,获得有关腐蚀类型和腐蚀速率的信息.小波分析不需要对ECN作稳态假设,并同时具有时间分辨和频率分辨的特点,在ECN信号处理中表现出一定的优势.本工作考察了氯离子对钢筋在模拟混凝土孔溶液中电化学噪音的影响,并采用离散小波变换(DWT)及能量分布图(EDP)分析ECN信号的时间～频率特征.结果表明,在含NaCl0.0001mol/L的饱和Ca(OH)2溶液中,时间常数为16～32s的暂态占优势;在Cl-浓度更高的溶液中,去钝化趋势为主导事件,表明钢筋在SPS溶液中活化/钝化的临界Cl-浓度介于10-4～10-3mol/L.     

Numerical Simulation of Electrochemical Systems by Coupling Analytical Calculation of the Diffuse Double Layer and Finite Element Description of Solution Bulk

Numerical modelling of electrochemical systems covers length scales from the nanometers up to the macroscopic scale. With finite element methods, the mesh must be extremely fine to describe the diffuse double layer, thus increasing the needed computational resources. We propose a method to describe the diffuse double layer by analytical equations, expressed as boundary conditions for the partial differential equations describing the solution bulk. We apply the method to a one-dimensional system, i. e. to a cell with plane parallel electrodes, in the presence of a redox couple and a supporting electrolyte. We provide evidence of the precision of the method.     

应用扫描电化学显微镜研究电子在室温离子液体与1,2-二氯乙烷混合溶液/水相界面上的转移反应

应用扫描电化学显微镜研究了室温离子液体(Omim·Tf2N)与1,2-二氯乙烷(DCE)混合溶液/水界面上的电子转移反应. 在保持共同离子(Tf2N-)的浓度比恒定及异相电子转移反应由界面电势差所决定的条件下, 研究了离子液体和DCE混合溶液中二茂铁(Fc)与水相中亚铁氰化钾[K4Fe(CN)6]之间异相电子转移反应. 探讨了混合溶液中离子液体的体积分数(xRTIL)的变化对混合溶液/水界面上电子转移反应的影响. 结果表明, 随着xRTIL的减小(从1减小到0.1), Fc在混合溶液中的扩散系数单调递增(从2.730×10-7 cm2·s-1增加到9.131×10-6 cm2·s-1); 而异相电子转移反应速率常数(k)则先逐渐减小(从8.0 mol-1·cm·s-1减小到0.32 mol-1·cm·s-1), 之后又略有增大(从0.32 mol-1·cm·s-1增大到0.48 mol-1·cm·s-1). 对这种现象可能的原因进行了较详细探讨.     

Electrochemistry of the Silicon Oxide Dielectric Layer: Principles,Electrochemical Reactions,and Perspectives

Electrochemistry of the silicon oxide dielectric layer, a notable insulator often used as a gate oxide, is counterintuitive, but addresses fundamental questions to yield novel scientific discoveries. In this minireview, the fundamental electron transfer mechanism of silicon oxide in the electrolyte solution is elucidated. The possible electrochemical reactions to date are discussed in detail, providing numerous potential areas of application which are elaborated and justified. This minireview not only provides background but also guides future research.     

Effects of Electronic Correlations for Adiabatic Electrochemical Reactions of Electron Transfer in Electrode Models with Nearly Empty and Nearly Filled Conduction Bands: Diagram of Kinetic Modes

A diagram of kinetic modes (DKM), whose parts correspond to different electron transfer types, for any electrode's Fermi energy _F and any finite value of its conduction band's width B is studied. Important differences between DKM for any _F and B and DKM in a model of a surface molecule for an electrode-reactant system are pointed out. DKM for electrode models with nearly empty and nearly filled conduction bands proposed earlier are discussed. Expressions for outermost curves of DKM are derived for some special cases within these models.     

Decoupling Study of Melt Polycondensation of Polycarbonate: Experimental and Modeling of Reaction Kinetics and Mass Transfer in Thin Films Polycondensation Process

During the melt polycondensation process of polycarbonate, reaction and mass transfer are deeply coupled owing to relatively high melt viscosity. In this work, the polycondensation reaction kinetics and mass transfer behavior of volatile phenol are decoupling studied in detail by using thin‐film experiments with 250–280 °C, 10–1000 Pa and 0.085–0.68 mm film thickness. A realistic apparent rate model coupled the reaction kinetics with thermodynamic equilibrium and diffusion behavior is developed to describe the polycondensation process, while the diffusion characteristic of small molecule (phenol) is further obtained based on penetration theory. The obtained polycondensation equilibrium constant ranges from 0.3 to 0.55, while the activation energy and pre‐exponential factor of temperature‐dependent diffusion coefficients of phenol are 87.9 kJ mol⁻¹ and 5.08 × 10² m² s⁻¹, respectively. It is also observed that the overall apparent rate of polycarbonate (PC) polycondensation process increases with higher temperature, lower pressure, and thinner film thickness. Coupling the reaction kinetics with mass transfer, the predictions of the realistic apparent rate model are in quite satisfactory agreement with experimental data.     

Kinetic Modeling as a Tool to Understand and Improve the Nitroxide Mediated Polymerization of Styrene

Kinetic modeling is used to obtain insight in the complex interplay between reaction rates and obtained polymer properties in the SG1 and the TEMPO mediated bulk polymerization of styrene at 396 K. The increase of the viscosity during NMP is accounted for. At higher targeted chain lengths, chain transfer to dimer and transfer from nitroxide to dimer are shown to cause the experimentally observed reduced control over the average polymer properties and to result in a clear fronting of the polymer chain length distribution. The potential of kinetic modeling to design tailor‐made synthesis strategies is illustrated. Simulations indicate that careful control of the polymerization conditions allows to obtain an important improvement of the polymer properties. The approach is also applicable for NMP mediated by other alkoxyamines/nitroxides and allows to expand the application range of NMP for styrene polymerization in particular to synthesize complex polymer architectures by assembly of functionalized polymers.

     

Rate‐Based Modeling of CO2 Absorption into Piperazine‐Activated Aqueous N‐Methyldiethanolamine Solution: Kinetic and Mass Transfer Analysis

A comprehensive two‐dimensional mathematical model based on surface renewal theory has been developed to analyze the CO₂ absorption into piperazine (PZ)‐activated aqueous N‐methyldiethanolamine (MDEA) solvent by taking into account the structured packed bed column hydraulics, mass transfer resistances, and chemical reactions. The modeling results have been validated with the experimental data reported in the literature, and they have been found to be in good agreement with the experimental results. The effects of amine concentration, liquid temperature, initial CO₂ partial pressure, liquid flow rate, and CO₂ loading on the mass transfer performance have been evaluated in terms of overall mass transfer coefficient (K _Ga_v). The overall mass transfer coefficient and absorption flux of CO₂ into aqueous MDEA+PZ blended solution have been calculated over the CO₂ partial pressure range of 4–16 kPa, temperature range of 298–333 K, and solvent concentration of 1–3 M. To evaluate the performance of different solvents on separation process, some common industrial chemical absorbents including monoethanolamine (MEA), diethanolamine (DEA), triethylamine (TEA), MDEA and PZ were compared with a MDEA+PZ blended solution. The results indicate that CO₂ absorption reaction with PZ is faster than that with MDEA, but also adding small amounts of PZ as a promoter to MDEA solvents improves significantly the absorption rate. The results show that CO₂ absorption reaction with the MDEA+PZ blended solution is faster than that with TEA and MDEA, also comparable with DEA, but slower than those with MEA and PZ. The modeling results illustrate that the K _Ga_v enhances with increasing the solvent concentration, liquid temperature, and liquid flow rate, but reduces with increasing the CO₂ loading and initial CO₂ partial pressure. In addition, the reaction kinetics in terms of enhancement factor was found to decrease as the CO₂ loading enhances and increase as the operating temperature rises.     

The Kinetic Modeling of Strong Diffusion‐Limited Reaction with Cyclization for Three Isomers – Diallyl Ortho‐, Iso‐ and Tere‐Phthalate in a Bulk

Free radical polymerization kinetic in a bulk for three diallyl phthalate isomers – diallyl orthophthalate, diallyl isophthalate and diallyl terephthalate was investigated in a temperature range from 50 to 70 °C initiated with dicyclohexan peroxydicarbonate as initiator at three different initiator concentrations. Conversion points were measured using Fourier Transform Raman measurements. A new kinetic model for polymerization of three diallyl phthalate isomers was developed. It demands the inclusion of only two new kinetic parameters k_DegC and k_pc which were obtained as a ratio k_DegC/k_pc from an additional set of experiments conducted. Computed conversions from the proposed kinetic model show good agreement with the conversion and molecular weight measured data for all three investigated diallyl phthalate isomers.