Cathodic reduction of bisulfite and sulfur dioxide in aqueous solutions on copper electrodes: an electrochemical ESR study

The electrochemical reduction of aqueous solutions of sulfite under acidic conditions on copper electrodes is reported, and a mechanism is proposed. Cyclic voltammetry at a copper disk suggests the operation of two reduction processes, the dominant process depending on solution pH. At very low pH (0-2), sulfur dioxide is reduced in a two-electron, two-proton reaction, but at higher pH (2-5), bisulfite is the electroactive species, being reduced by a single electron to ultimately yield the SO2*- radical anion. Simultaneous electrochemical electron spin resonance (ESR) measurements using a tubular flow cell support this proposal, and suggest that the radical anion is in equilibrium with dithionite, which is found to decay at low pH. Digisim modeling of the system is shown to be consistent with this mechanism over the experimental pH range.     

Electrochemistry of methotrexate and its metabolites: Part II. Reduction of 7-hydroxymethotrexate

The electrochemical reduction of 7-hydroxymethotrexate has been studied in the pH range of 2 to 11. At acidic and neutral pHs, 7-hydroxymethotrexate exhibits two reduction peaks. On the basis of the results obtained from experiments involving polarography, cyclic voltammetry, and high performance liquid chromatographic analysis of constant potential electrolysis of 7-hydroxymethotrexate and its analogs, it seems that the first electrochemical reduction process cleaves the side chain at the C⁹---N¹⁰ bond. The electrochemical process at more negative potentials involves the reduction of the N⁵---C⁶ double bond in the pyrazine ring. At alkaline pH only a single electrochemical peak is observed using polarography or cyclic voltammetry, because the cleavage reaction and the reduction of the ring occur at nearly the same potential, making the peaks indistinguishable from one another. Adsorption and filming of 2,4-diamino-6-methyl-7(8H)-pteridinone on the mercury electrode surface was documented using cyclic voltammetry.     

Aqueous Electrochemistry of Anthraquinone and Its Correlation with the Dissolved States of a Cationic Surfactant

Cyclic voltammetry was employed to study the electrochemical behavior of anthraquinone (AQ) in aqueous solution at a glassy carbon electrode using the sodium salt of anthraquinone-2-sulphonic acid (AQS). The cyclic voltammograms show a reduction wave and a corresponding oxidation wave. The electrochemical reaction of AQ in aqueous solution involves a two electron transfer process followed by a coupled chemical reaction and exhibits strong pH dependence at low pH (<4). A satellite peak is also observed at the cathodic side of the reduction wave for the reduction of the sulfonate group of AQS. The electrochemical investigation was also carried out in the presence of a cationic surfactant, cetyltrimethylammonium bromide (CTAB). Similar redox behavior can be observed for the electrochemical reaction of AQS in the presence of CTAB solutions similar to the aqueous media. The electrochemical responses have been found to depend on the dissolved states of the surfactant. The current-potential behavior of AQS depends on the concentration of CTAB and micellization has a profound effect on the electrochemical behavior of AQ.     

Electrochemical reduction of NO by hemin adsorbed at pyrolitic graphite

The mechanism of the electrochemical reduction of nitric oxide (NO) by hemin adsorbed at pyrolitic graphite was investigated. The selectivity of NO reduction was probed by combining the rotating ring disk electrode (RRDE) technique with a newly developed technique called on-line electrochemical mass spectroscopy (OLEMS). These techniques show that NO reduction by adsorbed heme groups results in production of hydroxylamine (NH(2)OH) with almost 100% selectivity at low potentials. Small amounts of nitrous oxide (N(2)O) were only observed at higher potentials. The rate-determining step in NO reduction most likely consists of an electrochemical equilibrium involving a proton transfer, as can be derived from the Tafel slope value of 62 mV/dec and the pH dependence of -42 mV/pH. The almost 100% selectivity toward NH(2)OH distinguishes this system both from NO reduction on bare metal electrodes, which often yields NH(3), and from biological NO reduction in cytochrome P450nor, which yields N(2)O exclusively.     

Electrochemical reduction of 5,5′-dichlorohydurilic acid. mechanism and analytical applications

The electrochemical reduction of 5,5′-dichlorohydurilic acid has been studied at the dropping mercury electrode (DME) and the pyrolytic graphite electrode (PGE). At the DME the single polarographic reduction wave observed at pH 6–11 involves a direct 4e—2H⁺ reduction of the carbon-halogen bond to give hydurilic acid and chloride. The state of hydration or ionization of the 5,5′-dichlorohydurilic acid has no effect on the electrochemical reaction. At the PGE, 5,5′-dichlorohydurilic acid shows two voltammetric peaks. Peak I_c, observed between pH 5 and 7, arises from an overall 4e—2H⁺ reduction of 5,5′-dichlorohydurilic acid via a mechanism that involves initial electron attack at a carbonyl group alpha to a carbon-halogen bond with simultaneous elimination of chloride ion. The peak II_c process involves an initial 2e—1H⁺ reduction of a partially hydrated form of 5,5′-dichlorohydurilic acid with only one unhydrated halocarbonyl moiety available for reaction. Attack is again via the carbonyl group with simultaneous elimination of chloride and formation of 5-chlorohydurilic acid. A chemical dehydration step then occurs with a rate constant of ca. 0.24 s^?1 at pH 8.2, with formation of a further reducible halocarbonyl group. This is again reduced in an overall 2e—2H⁺ reaction to give hydurilic acid and chloride ion. The peak II_c process hence proceeds via an ECE mechanism. The different mechanisms observed for reduction of 5,5′-dichlorohydurilic acid at mercury and pyrolytic graphite electrodes are unusual. Analytical methods have been developed for the polarographic determination of 5,5′-dichlorohydurilic acid via its reduction wave at the DME, and for the voltammetric determination of hydurilic acid via its first oxidation peak at the PGE.     

脉冲微波辅助化学还原合成Pt/C 催化剂及其电催化氧还原性能

采用脉冲微波辅助化学还原法制备了质子交换膜燃料电池(PEMFC)用Pt/C 催化剂. 通过透射电镜(TEM)和X射线衍射(XRD)等分析技术对催化剂的微观结构和形貌进行了表征, 并利用循环伏安(CV)、线性扫描(LSV)和恒电位测量等方法评价了催化剂催化氧还原性能. 在此基础上制备了膜电极(MEA)并组装成单电池, 考察了制备的Pt/C 催化剂作为阴极催化剂材料的电催化性能. 结果表明, 脉冲微波辅助化学还原法是一种制备PEMFC催化剂的有效方法, 溶液pH值和微波功率对Pt 颗粒直径和分散有重要影响. TEM和XRD结果显示, 当溶液pH值为10 且微波功率为2 kW时, Pt 纳米粒子较均匀地分散在碳载体上, 粒径分布在1.3-2.4 nm之间, 平均粒径为1.8 nm. CV、LSV和恒电位测试结果表明, 该催化剂电化学比表面积(ESA)为55.6 m²·g^-1, 具有良好的催化氧还原反应活性和稳定性. 单电池测试结果表明, 在溶液pH值为10条件下, 微波功率为2 kW时制备的催化剂作阴极催化剂时, 单电池最高功率密度为2.26 W·cm^-2·mg^-1, 高于微波功率为1 kW时的最高功率密度(2.15 W·cm^-2·mg^-1)和Johnson Matthey催化剂的最高功率密度(1.89 W·cm^-2·mg^-1).     

咖啡酸苯乙酯在玻碳电极上的电化学行为研究

采用循环伏安法和差分脉冲伏安法研究了咖啡酸苯乙酯(CAPE)在玻碳电极表面上的电化学行为。在0.05 mol.dm-3磷酸盐缓冲溶液(PBS,pH=7.6)中,CAPE在0.229 V和0.214 V处呈现一对明显的氧化还原峰。考察了溶液pH值和扫描速率等因素对CAPE电化学行为的影响,结果表明,电化学过程中CAPE的电子转移数为2,参与反应的质子数也为2,且为吸附控制过程。在0.05 mol.dm-3磷酸盐缓冲溶液(PBS,pH=7.6)中,CAPE的氧化峰电流与其浓度在0.34～3.40μmol.dm-3范围内成线性关系,其响应灵敏度为0.88μA/μmol.dm-3,检出限为60 nmol.dm-3(S/N=3)。本研究建立了一种简单、快速、可灵敏测定CAPE的方法。同时对CAPE在玻碳电极上的电化学行为进行了较为详细的研究。     

一氧化氮在肌红蛋白修饰电极上的电催化还原

本文研究了醋酸盐缓冲体系中NO2^-歧化产物NO在肌红蛋白修饰玻碳电极上的电催化还原反应, 其还原峰电位为-0.68V(vs. SCE)。文中考察了溶液酸度、亚硝酸根离子浓度等对修饰电极的电化学行为的影响, 并对一氧化氮在肌红蛋白修饰电极上的电催化还原机理进行了探讨。     

Mechanism for hydrogen evolution reaction on pipeline steel in near-neutral pH solution

Cyclic voltammetry, hydrogen permeation tests and electrochemical impedance spectroscopy measurements were combined to study the mechanism for hydrogen evolution reaction on X-70 pipe steel in near-neutral pH solution. It is found that hydrogen evolution reaction is dominated by the reduction of water molecules, followed by either an electrochemical hydrogen recombination reaction or a hydrogen absorption reaction. The near-neutral pH environment is capable of generating catalytic surface effect on hydrogen evolution on the pipe steel. The increasing dissolution of the cathodically pre-polarized steel could be due to the enhanced activation of the steel, rather than the increasing amount of hydrogen atoms in the steel. These results provide mechanistic information to understand the near-neutral pH stress corrosion cracking of pipelines.     

Preparative Electrochemical Reduction of Fluorinated Benzonitriles

Electrochemical reduction of fluorinated benzonitriles in aprotic media is accompanied by concurrent processes of dimerization and fragmentation of the corresponding anion-radicals. With 2-fluoro- and 3,4,5-trifluorobenzonitriles the defluorinated products can be obtained at direct electrochemical reduction; for the other compounds studied have been found mediators providing a possibility to obtain in high yield defluorinated products at preparative electrochemical reduction. A fundamental possibility was demonstrated of providing products by nucleophilic fluorine substitution by S_BN mechanism at electrochemical reduction of fluorinated benzonitriles in aprotic solvents in the presence of mediators.     

The influence of Ce3+ ions on the corrosion rate of stainless steel in acidic solutions of different pH-values

The corrosion resistance of AISI 420 stainless steel in 0.1 mol L^?1 H₂SO₄ + 0.1 mol L^?1 Na₂SO₄ solutions at different pH-values and the inhibiting effect of Ce³⁺ ions was studied using electrochemical polarization methods. The results reveal decreasing of the corrosion rate with an increasing the pH of the solution, which demonstrates the progressive protective character of the inhibitor used. At pH lower than 3.33, the corrosion inhibition was most probably a result of the competitive adsorption of Ce³⁺ with H⁺ ions on the cathodic sites of the electrode surface, and it was found to be dependent on the relative concentration of H⁺/Ce³⁺. The peroxide generated from the oxygen reduction reaction at pH 3.33 was found to be capable oxidize trivalent cerium (Ce) to the tetravalent state. As obtained hydroxide precipitates act as diffusion barrier hindering the corrosion processes, whereafter a spontaneous passivity occurs on the steel surface at this pH.     

Electropolymerization of iron tetra(<Emphasis Type="Italic">o</Emphasis>-aminophenyl)porphyrin from aqueous solution and the electrocatalytic behavior of modified electrode

Stable electroactive iron tetra(o-aminophenyl)porphyrin (FeTAPP) films are prepared by electropolymerization from aqueous solution by cycling the electrode potential between −0.4 and 1.0 V vs Ag/AgCl at 0.1 V s⁻¹. The cyclic voltammetric response indicates that polymerization takes place after the oxidation of amino groups, and the films could be produced on glassy carbon (GC) and gold electrodes. The film growth of poly(FeTAPP) was monitored by using cyclic voltammetry and electrochemical quartz crystal microbalance. The cyclic voltammetric features of Fe(III)/Fe(II) redox couple in the film resembles that of surface confined redox species. The electrochemical response of the modified electrode was found to be dependent on the pH of the contacting solution with a negative shift of 57 mV/pH. The electrocatalytic behavior of poly(FeTAPP) film-modified electrode was investigated towards reduction of hydrogen peroxide, molecular oxygen, and chloroacetic acids (mono-, di-, and tri-). The reduction of hydrogen peroxide, molecular oxygen, and dichloroacetic acid occurred at less negative potential on poly(FeTAPP) film compared to bare GC electrode. Particularly, the overpotential of hydrogen peroxide was reduced substantially. The O₂ reduction proceeds through direct four-electron reduction mechanism.     

Ni(II)cyclam Catalyzed Reduction of CO2 – Towards a Voltammetric Sensor for the Gas Phase

The detection of CO₂ in the gas phase is possible in presence of oxygen with an amalgamated Au‐poly(tetrafluoroethylene) gas diffusion electrode and an internal electrolyte solution containing Ni(II)cyclam. For concentrations between 0.1 to 1% the electrochemical cell has a sensitivity of 3.58 mA %^?1 and the detection limit is 500 ppm. In preliminary experiments at rotating disk electrodes the optimum pH‐range was found to be between 3.5 to 6 and a selectivity ratio of the catalyst for CO₂/H⁺ of 5 : 1 could be determined. The relationship between reduction current and the square root of the angular speed is linear, indicating that the electrochemical process is limited by diffusion of CO₂. Tl and Pb are presented as alternative electrode materials at which the Ni(II)cyclam catalyzed reduction of CO₂ can be observed. Problems arise from fouling effects at the sensing electrode and a non‐linearity of the calibration plot at higher concentrations.     

基于活性金电极上硫代水杨酸自组装单分子层的电化学表面增强拉曼光谱

采用原位电化学表面增强拉曼光谱(EC-SERS)研究了硫代水杨酸(TSA)吸附在活性Au电极表面的自组装单分子层(SAMs).TSA在活性Au表面的化学吸附及不同酸碱度下的TSA浸饰单层膜的SERS光谱,表明随pH值的增加,峰强呈现2个不同的下降阶段.通过EC-SERS考察不同电富集时间和电位的影响,显示在酸性介质和0.7 V及70 s富集时间下,可以获得最大EC-SERS信号,并随着电位负移,信号逐渐减弱,直至基本消失,表明TSA分子在Au表面排布状态会随外加条件的改变而发生变化.通过计算TSA在不同pH值下的分布分数以及探针分子在不同电位下的增强因子(EF),结合SERS和EC-SERS的变化走势对比,得出TSA在活性Au表面自组装形成单分子层/膜的机理,指出由于TSA不同的电化学吸附取向,以及高负电位下的还原/脱附作用,使得Au表面拉曼活性降低,造成EF显著减小,不可逆地失去了SERS的活性.     

2－乙基蒽醌电化学还原过程的研究

以Ｐｔ、Ｐｄ及几种Ｐｄ－离子注入材料为电极使用ＣＶ和ＥＳＲ法研究了２ＥＡＱ在ＤＭＳＯ中的电化学还原作用。阐明了两个电子迁移步骤所具有的准可逆性质及其后续均相化学反应。提出了电化学还原作用的历程,计算了有关步骤的动力学参数。对体系中水含量的影响进行了测量,讨论了质子给予体的作用。通过实验结果的对比,提出了在２ＥＡＱ还原中以Ｐｄ－离子注入材料代替金属Ｐｄ催化剂的可能性。     

Electrocatalytic Reduction of Dioxygen at Carbon Paste Electrode Modified with a Novel Cobalt(III) Schiff's Base Complex

A carbon paste modified electrode with a new cobalt(III) Schiff's base complex (CPME) and its application to electrocatalytic activity for dioxygen reduction is developed. The electrochemical behavior and stability of the CPME as well as the two‐electron reduction of O₂ at the electrode were investigated using cyclic voltammetry, chronoamperometry and rotating disk electrode methods. At the CPME, the reduction of dioxygen to hydrogen peroxide occurs at potentials where it is not observed at a bare carbon paste electrode. The CPME exhibited potent and persistent electrocatalysis for O₂ reduction in acetate buffer solutions of pH 4.0 with an overpotential of about 800 mV lower than unmodified CPE and drastic increase in the peak current. The heterogeneous rate constant for the reduction of O₂ at the surface of CPME was determined by hydrodynamic voltammetry using the Koutecky–Levich plot. A possible catalytic mechanism is proposed and discussed.     

[SiNi(H2O)W11O39]6-多层组装膜修饰电极及其电化学行为

采用电化学生长法制备包含杂多酸[SiNi(H2O)W11O39]6-(SiNiW11)和聚合物阳离子PDDA的多层膜修饰电极, 利用循环伏安法研究其电化学行为、 pH的影响及其对BrO3-和NO2-体系还原的电催化性能;并对多层膜电化学过程机理进行了初步探讨. 结果表明: 多层膜的增长均匀, 峰电流随层数的增加而增加;多层膜的峰电流随扫速的增加而增加;还原峰的峰电位随pH的增加而负移.     

硫代硫酸盐在铂电极上的电化学氧化行为

用循环伏安法测定了硫代硫酸盐在铂电极上的电化学氧化行为, 结果表明, 其电化学氧化行为与体系的pH和扫描速度密切相关. 当pH为5~6时, 硫代硫酸盐的循环伏安曲线出现三个氧化峰, 峰电位分别在0.05 V、0.58 V和1.02 V附近, 随pH值升高和扫描速度的降低, 0.05 V附近的氧化峰逐渐变得明显, 同时各氧化峰的峰电位与扫描速度的对数, 峰电流与扫描速度的平方根均成很好的线性关系；当pH为8~9时, 硫代硫酸盐的循环伏安曲线出现三个明显的氧化峰, 峰电位分别在0.05 V、0.91 V和1.22 V附近. 随扫描速度降低, 循环伏安曲线出现交叉, 体系呈现明显的电化学振荡行为；但当pH=10时, 1.22 V附近的氧化峰消失. 硫代硫酸盐的电化学氧化行为非常复杂, 电化学氧化机制随体系pH的变化而变化.     

超氧自由基电化学发生体系的研究

超氧自由基是人体内氧代谢的重要产物 [1 ] .迄今为止 ,超氧自由基的研究方法有电子自旋共振法、光度法和电化学法等 [2～ 5] .其中 ,通过电化学还原氧产生超氧自由基是一种简便快捷的方法 [6,7] .本文使用长链表面活性剂 DSAB作为疏水分子 ,比较了强碱性介质中氧在汞电极和 4种非汞电极上的电化学还原行为 ,建立了超氧自由基在非汞电极 (铂电极 )上电化学发生新体系 .通过石英晶体微天平和计时电量法进一步证实了超氧自由基的产生 ,并用循环伏安法比较了抗坏血酸和半胱氨酸两种抗氧化剂对超氧自由基的清除能力 .1　实验部分1.1　试剂与仪…     

457—The mechanism of low frequency a.c. electrochemical disinfection

The mechanism of low frequency a.c. electrochemical disinfection was studied. A random oriented graphite fiber-epoxy matrix composite material was employed to fabricate the electrodes. This material was found to have very good electrochemical stability when cycled anodically and cathodically in solutions containing NaCl or NaBr.The lethal species was identified as HClO and HBrO in solutions containing Cl^? and Br^?, respectively, although the free halogen molecule may contribute to the observed disinfection activity under certain experimental conditions.The efficiency of disinfection increases with increasing concentration of Cl^? (or Br^?) and with decreasing pH. Deaerating the solution decreases the efficiency of disinfection significantly. NaBr was found to be effective at much lower concentrations than NaCl at all pH values but the effect is enhanced at pH > 7 due to ionization of HClO.The unique advantage of the method described in this paper is that a high transient concentration (in time and space) of the lethal species is generated, enough to destroy the most resistant microorganisms, while the average concentration of active halogen in the effluent liquid remains well below the objectionable level.