甲醇在Pt表面电化学氧化过程中的化学振荡

基于甲醇电化学氧化的双途径机理,建立了能够表征甲醇电化学氧化过程电位振荡的非线性动力学模型.所建甲醇氧化系统动力学演化模型涉及三个主要的变量:电极电位(e),毒性中间体CO的表面覆盖度(x),含氧物种H2Oa的表面覆盖度(y).通过反应速率常数ki=exp(ai(e-ei))实现了化学反应与电极电位的耦合.研究发现,在不同的电流密度范围内甲醇电化学氧化呈现不同的动力学特征.甲醇电化学氧化时出现的电位振荡现象可以归因为:一是氧化过程中生成了毒性中间体CO,这是产生电化学振荡的诱因;二是强烈依赖于电极电位的非电化学反应,即,含氧物种H2Oa在Pt表面的生成与消失,则是维系振荡的直接原因.而甲醇电化学氧化体系复杂的动力学行为根源在于电极电位e对CO和含氧物种H2Oa所参与反应的耦合反馈作用.对所建模型的数值分析成功地解释了为什么甲醇电化学氧化时出现的电位振荡现象只发生在一定的电流密度范围.     

酶的光谱电化学研究（Ⅰ）：辣根过氧化物酶的电化学性质

利用薄层光谱电化学技术研究了辣根过氧化物酶(HRP)及其化合物的氧化还原过程。指出HRP可在固体电极上进行直接电子传递,该电极反应不是酶中二硫键的还原,而是血红素辅基中心金属离子的氧化态转变。测定了HRP(Fe~(3+)/Fe~(2+))电对的标准氧化还原电位和电化学动力学参数,讨论了HRP氧化性中间物的电化学性质。     

电化学振荡现象

引言电化学振荡是在远离平衡的电化学体系中产生的一种时间有序现象,按照振荡的电化学参量的不同,可分为两类:在控电位条件下的电流振荡和在控电流条件下的电位振荡。根据非平衡态热力学和耗散结构理论,产生时空     

碱性介质中葡萄糖在铂电极上的阳极氧化

为了进一步探明葡萄糖在铂电极上的氧化机理,用循环伏安法(CV)在-0.9～0.4 V(相对于饱和甘汞参比电极)内研究了葡萄糖在铂电极上催化氧化行为,首次详细报道了葡萄糖在电化学氧化过程中的电位振荡现象,并用电流扫描法表征了葡萄糖的电位振荡情况.电流扫描结果表明,在较慢的电流扫描速度下,电极过程出现了明显的电位振荡.说明电极上产生了毒化中间物,电位振荡是由于毒化中间物在电极上的吸附和在高电位下氧化除去引起的.     

双酚A在硼掺杂金刚石薄膜电极上的电化学行为研究

硼掺杂金刚石(BDD)薄膜电极是用于废水处理的理想电极材料。利用循环伏安法和电化学阻抗谱研究了环境激素双酚A(BPA)在BDD电极上的电化学行为,分析在电极/溶液界面上的电化学过程及相关的电极动力学参数。研究发现BPA在电极上的直接电化学氧化过程为不可逆过程,氧化峰电势在1.4 V左右。电极/溶液界面的双电层结构可以用一个电容与一个电阻并联的等效电路来进行拟合,当极化电位从0.5 V增加至2.0 V时,电荷转移电阻Rct由7.043×104Ω·cm2降至1.366×103Ω·cm2,下降了80.60%,表明提高电极电位可明显降低电催化反应的电阻,有利于电催化氧化反应的进行,可提高电催化反应速率。     

铜电极阳极溶解过程恒电位电流振荡的动力学模型

研究了铜电极在酸性氯化钠溶液中的恒电位电流振荡行为,分析了电极过程中的非线性步骤及电化学耦合因素,提出了一个可能的电极过程动力学模型,并借助线性稳定性分析及分支分析得到了参数坐标空间中的动力学行为区域图。在此基础上,将极化曲线视为稳定非平衡定态区态函（电流）与外控参数（电位）的关系,同时将恒电位电流振荡模拟为稳定极限环振荡,分别计算出了极化曲线与时间－电流振荡曲线,其结果与实验数据相符,表明该类电     

葡萄糖、半乳糖和乙醇恒电流氧化过程电位振荡的EQCM研究

采用电化学石英晶体微天平(EQCM)研究了0.5 mol•L^-1 NaOH水溶液中铂电极上葡萄糖、半乳糖和乙醇恒电流氧化过程中伴随的电位振荡行为. 两个糖体系的电位振荡过程伴随EQCM频率的同步振荡响应, 而乙醇体系中相应的频率响应却非常小；三个体系振荡过程的同步动态电阻响应均很小, 表明振荡过程频率响应主要为质量效应. 虽然葡萄糖和半乳糖结构相似, 电位和频率振荡的幅度相当, 但频率波数和周期明显不同, 表明电位振荡行为对两者呈现良好的分子识别能力. 本文也讨论了相关振荡机理和NaOH浓度效应及碱性介质中铂电极电化学过程, 提出了所形成的铂氧化物主要是PtO₂-3H₂O_ad以及两糖体系振荡过程中糖酸根阴离子伴随着高/低电位在铂电极上吸/脱附的新观点.     

纳米铂微粒修饰电极上甲醛电催化氧化的电位振荡和电流振荡

通过阴极还原-阳极氧化法制备了钛基纳米铂微粒修饰电极, 扫描电镜观察发现, 分布于钛基体表面的氧化钛膜三维网状孔道中的纳米铂微粒具有高度分散状态. 采用多种电化学手段在该电极上不仅观察到甲醛在恒电流条件下产生的电位振荡, 而且在循环伏安和恒电位两种条件下均观察到强烈的电流振荡, 这进一步证明高度分散的纳米铂微粒使电极的催化活性大大提高, 促进了甲醛及其毒化中间产物的电催化氧化过程, 从而有利于电极上电化学振荡的产生. 研究结果还表明, 甲醛底物浓度、硫酸介质浓度、恒电位或恒电流大小等多种因素对振荡强度、范围或类型会产生规律性的影响.     

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.     

现场压电红外反射光谱电化学研究邻联甲苯胺的电化学行为

利用现场压电红外发射光谱电化学技术研究邻联甲苯胺(OTD)在金电极上的电化学行为.压电电化学结果表明:电位范围对OTD氧化的影响很大.当电位扫至0.6 V时,氧化过程中中间体的生成和溶解的同时,部分未溶中间体在电极表面沉积.当电位大于0.65 V时,还有OTD的电化学氧化聚合.压电红外反射光谱电化学结果表明中间体是OTD第一氧化产物之间相互作用形成的.进一步考察了肝素大阴离子对OTD的氧化的影响.     

Investigation of the CO oxidation rate oscillations using electrochemical promotion of catalysis over sputtered-Pt films interfaced with YSZ

The oscillatory behaviour of CO oxidation was studied at 250 °C and atmospheric pressure using an electrochemical catalyst composed of a thin (60 nm) sputtered-Pt film interfaced with an yttria-stabilized zirconia membrane. Oscillations of CO oxidation rate showed a perfect correlation with those of the electrochemical potential values. Electrochemical promotion of catalysis was used to initiate and stop the oscillatory behaviour. Small current application induced a permanent effect on the oscillatory behaviours. An extremely small negative current (? 17 μA) led to a 4-fold increase of the catalytic activity and created oscillations that were stable even after current interruption. This permanent effect in the oscillatory behaviour of CO oxidation rate is observed for the first time using EPOC. This has been interpreted by the higher tendency of the nanometric-Pt particles to form PtO_x in thin sputtered films.     

Current oscillations during the electrochemical oxidation of sulfide in the presence of an external resistor

The electrochemical oxidation of sulfide on a polycrystalline platinum electrode was studied under potentiostatic condition when an external resistor is in series with the working electrode. Only two os- cillatory regions can be obtained in the absence of the external resistance, but four oscillatory regions, including two new current oscillations, were found in this system by controlling the external resistance. It is demonstrated that three oscillatory regimes, which arise on the positive branch of current-potential curve, can be classified as HN-NDR (Hidden N-shaped Negative Differential Resistance) oscillators. For the first oscillatory region, various transient complex phenomena, which result from the change of the electrode/electrolyte interface by accumulation of adsorbed element sulfur on the electrode, have been observed. The dynamic behavior of NDR (Negative Differential Resistance) oscillations, appearing along with negative branch of polarization curve, can transform from oscillations into bistability with a sufficient large external resistance in series. Two oscillatory regions in high-potential region classified as HN-NDR type oscillations are separated by a saddle-loop bifurcation. They displayed a sequence of bursting oscillations and irregular oscillations, respectively. The electrochemical oxidation of sulfide provides a model system for studying complex dynamics and possible application in sulfur removal.     

Novel Ni(II) Mixed Ligand Complex Modified Electrode: Catalytic Effect on Anodic Oxidation of Phenol

Cyclic voltammetry, using graphite paste electrode (GPE) and chemically modified GPE with Ni(II) mixed ligand complex (MGPE), is described for sensing and detoxification of phenol. A novel mixed ligand complex of nickel with 1,4,8,11‐tetraaza cyclotetradecane (cyclam) and thiocyanate in the molar ratio of 1 : 2 : 5 is synthesized and characterized by spectroscopic and electrochemical studies. The Ni(II)‐cyclam‐thiocyanate complex behaves as a fast electron‐transfer mediator, as nickel ions exist in higher oxidation state of Ni(III) on applying a potential of 1.25 V (vs. Ag/AgCl), which catalyses the oxidation of the target species. The mixed ligand complex, when incorporated in the graphite paste electrode, is sensitive to detect phenol as low as 10 μg/L in solution. The catalytic effect of the Ni(II)‐cyclam‐thiocyanate complex enhanced many folds the oxidation of phenol compared to GPE. The technique has potential for sensing/monitoring and detoxification of phenol released in the ecosystem from polluting industries.     

Effects of hydrogen on current oscillations during electro-oxidation of X70 carbon steel in phosphoric acid

It is the first time that the oscillatory electrodissolution of metals is used to study hydrogen-promoted corrosion, and the primary results prove that it is an effective method for investigating the effect of hydrogen on both the formation and dissolution of a passive film. Effects of hydrogen on the electrochemical oscillations of X70 carbon steel are investigated in 5.0 M H₃PO₄ solution. During the oscillatory electrodissolution of X70 steel electrode, the chemical environment near the surface of the electrode is changed artificially by the oxidation of hydrogen diffused from X70 electrode to surface. With increasing hydrogen pre-charging current density, both the induction time and the ratio of active time to passive time of the current oscillations increase, and the Flade potential also shifts positively. Oxidation of hydrogen decreases the pH value at the interface between the electrode and solution, which retards the formation of a passive film and subsequently promotes its dissolution. This investigation provides further understanding of the effect of hydrogen on the formation and dissolution processes of passive films. It is the first time to observe the periodical changes in the potential at the electrode/electrolyte interface during current oscillations by using scanning reference electrode technology. The scanning reference electrode technology is also used to verify the effect of hydrogen on the current oscillations.     

Dynamic instabilities and mechanism of the electrochemical oxidation of thiosulfate

The electrochemical oxidation of thiosulfate is revealed to have two distinct oscillatory regimes in both linear potential and galvanic voltammograms, where various nonlinear behaviors such as period-2, mixed-mode and quasi-periodic oscillations, and chaos are observed under potentiostatic or galvanostatic conditions. Electrochemical impedance spectroscopy and iR compensation characterization indicate that, depending on the operating conditions, the system could be either a strictly potentiostatic oscillator or an S-shaped negative differential resistance oscillator. Chronoamperometry measurements reveal that the first oscillatory process involves a single-electron transfer, whereas within the second oscillatory regime the average number of electrons transferred is around 3.8. Measurements with capillary electrophoresis and chemical methods illustrate that the oxidation products include S2O6(2-), S4O6(2-), S5O6(2-), S3O6(2-), and SO4(2-).     

Oxydation électrochimique de l'ion thiocyanique. Application aux dosages et ä l'étude des réactions

The author has examined the rate of reaction of the system thiocyan ate/sulphate and cyanide, by means of the current/anode potential curves obtained during the electrolytic oxidation of thiocyanate in an acid medium.Corresponding with the minimum current detectable under our experimental conditions, we find a potential which agrees well with that determined by a classical potentiométrie method, and checked chemically. Consequently, it can be used to predict the chemical reactions taking place.From these curves, the relation between the anodic potential, the intensity of the electrolytic current, and the activity of the SCN^- ions has been deduced. These methods have been applied in a new method of investigating reactions in solution, viz. the potentiometry at constant current other than zero.     

Dynamic complexity in the electrochemical oxidation of thiourea

We explored the temperature-dependent dynamics of the electrochemical oxidation of thiourea under potential-control mode and found complex oscillations with one large peak and one small peak per period. Adjusting the temperature caused the relative amplitudes and positions of the two peaks to vary. Experiments showed that there were two distinct oscillatory regimes as a function of the external current, and at some temperatures, three-frequency quasiperiodic oscillations occurred for current densities between the two oscillatory windows. We examined the species present and the component reactions by employing cyclic voltammetry and electrolysis-HPLC-MS in the two oscillatory regimes. Adsorption and desorption of multiple species, including water and chloride, contribute to the rich dynamical phenomena that were observed.     

Oscillatory kinetics in the electrochemical oxidation of formate ions during the deposition of rhodium electrode: Part I. Experimental observation

The electrochemical oxidation of sodium formate in neutral medium was studied by means of cyclic voltammetry at different given temperatures (in the range 20–90°C), using several kinds of rhodium electrodes: smooth Rh, predeposits of Rh, or in situ electrodeposits of Rh. In this last case, at temperatures above 60°C, the voltammograms show large oscillations of current which occur in the oxide region and can be correlated with the roughness of the surface and repetitive changes of the oxide coverage.The conditions under which a well-developed oscillatory system exists were systematically investigated.     

Luminescent chemical waves in the Cu(II)-catalyzed oscillatory oxidation of SCN- ions with hydrogen peroxide

The oscillatory oxidation of thiocyanate ions with hydrogen peroxide, catalyzed by Cu2+ ions in alkaline media, was so far observed as occurring simultaneously in the entire space of the batch or flow reactor. We performed this reaction for the first time in the thin-layer reactor and observed the spatiotemporal course of the above process, in the presence of luminol as the chemiluminescent indicator. A series of luminescent patterns periodically starting from the random reaction center and spreading throughout the entire solution layer was reported. For a batch-stirred system, the bursts of luminescence were found to correlate with the steep decreases of the oscillating Pt electrode potential. These novel results open possibilities for further experimental and theoretical investigations of those spatiotemporal patterns, including studies of the mechanism of this chemically complex process.     

Regioselective bond cleavage in the dissociative electron transfer to benzyl thiocyanates

The electrochemical reduction of benzyl thiocyanate and p-nitrobenzyl thiocyanate was investigated in acetonitrile at an inert electrode. These two compounds reveal a change in the reductive cleavage mechanism, and more interestingly, they show a clear-cut example of a regioselective bond dissociation. Both phenomena may be understood on the basis of the dissociative ET theory and its extension to the formation/dissociation reactions of radical ions. While the effect of the standard oxidation potential of the leaving group seems to be predominant in understanding the change in the ET mechanism by changing the driving force, the regioselective cleavage is dictated by changes in the intrinsic barrier related to the nature of the substituent on the aryl moiety.