Electrochemical reduction of 1,4-dihalobutanes at carbon cathodes in dimethylformamide

Cyclic voltammetry and controlled-potential electrolysis have been employed to investigate the electrochemical behavior of 1,4-dibromo-, 1,4-diiodo-, 1-bromo-4-chloro- and 1-chloro-4-iodobutane at glassy carbon cathodes in dimethylformamide containing tetramethylammonium perchlorate. Depending on the identity of the 1,4-dihalobutane electrolyzed and the choice of potential, reduction of these compounds leads to a myriad of products including cyclobutane, n-butane, n-octane, 1-butene, cis-and trans-2-butene, 1,3-butadiene, ethylene, 1-chlorobutane, 1-bromobutane, 1-iodobutane, 1-iodooctane, 1,4-dichlorobutane, 1,8-dichlorooctane, and 1,8-diiodooctane. Experiments involving the use of proton donors (phenol and 1,1,1,3,3,3-hexafluoro-2-propanol), a radical trap (norbornylene), and several deuterium ion or atom donors have been utilized to elucidate the mechanisms by which the various electrolysis products are formed.     

Electrochemical reduction of halogenated pyrimidines at mercury cathodes in acetonitrile

Cyclic voltammetry and controlled-potential electrolysis have been employed to investigate the reduction of some mono-, di-, tri-, and tetrahalopyrimidines at mercury cathodes in acetonitrile containing tetramethylammonium tetrafluoroborate. Two irreversible cyclic voltammetric waves are observed for reduction of 2-bromo-, 5-bromo-, and 2-chloropyrimidine; the first wave is due to cleavage of the carbon---halogen bond, and the second wave is attributable to reduction of pyrimidine. Cyclic voltammograms for 2,4-dichloro- and 4,6-dichloropyrimidine exhibit three cathodic waves, whereas that for 2,4,6-trichloropyrimidine shows four cathodic waves, arising from sequential cleavage of carbon---chlorine bonds as well as the reduction of pyrimidine. For the reduction of 2,4,5,6-tetrachloropyrimidine, a cyclic voltammogram exhibits four major irreversible cathodic waves corresponding to the cleavage of carbon---chlorine bonds, but the wave for reduction of pyrimidine is poorly defined. Bulk electrolyses of halopyrimidines at potentials for different stages of reduction lead to products that are consistent with expectations based upon cyclic voltammetry. In addition, our findings agree well with theoretical calculations of the relative stabilities of the various reduction intermediates. Mechanistic aspects of the reduction of halopyrimidines are discussed and, using homogeneous redox catalysis, we have determined the lifetimes of the electrogenerated radical-anions of 2-bromo- and 2-chloropyrimidine.     

Electrochemical behavior of glassy carbon electrodes modified by electrochemical oxidation

Glassy carbon electrodes were modified electrochemically by pretreatment in sulfate, phosphate or carbonate solutions by means of cycling the potential well into the positive limit of the solvent. Electrodes treated in this manner were then used to incorporate and concentrate a variety of redox species that were either cations or aromatic containing compounds, including Ru(bpy)²⁺₃, Ru(NH₃)³⁺₆, Cu(NH₃)²⁺₄, ferrocene, methylviologen, 1,4-benzoquinone, anthraquinone-2-sulfonate, riboflavin, flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD). Surface-equivalent concentrations ranged from 5 × 10^?9 to 1 × 10^?7 mol cm^?2 for electrodes pretreated for 10 min in sulfuric acid. An E_1/2 vs. pH study of 1,4-benzoquinone, riboflavin, FMN and FAD in modified electrodes shows that the pK_a values shift toward higher pH (nearly 2 pH units). Results concerning the incorporation of redox compounds detected only by mediation with other electroactive complexes and the study of the modified electrodes in electrocatalysis are also discussed.     

羟胺在碳纳米管修饰玻碳电极上的电催化还原及电化学动力学性质研究

研究了羟胺在碳纳米管修饰玻碳电极(CNT/GC)上的电化学行为。研究结果表明,碳纳米管对羟胺的电化学行为有良好的电催化作用,在-0.62 V有一还原峰,是羟胺获得2个电子还原为铵所形成,同时测定了该电化学过程的动力学参数:电子转移数n为2,电子转移系数α为0.287,电极反应速率常数k为1.35×10-3cm/s。     

盐酸阿霉素在玻碳电极上的电化学行为研究及分析应用

采用线性扫描伏安法和循环伏安法研究了盐酸阿霉素在玻碳电极上的电化学行为及电极反应机理,优化了测定盐酸阿霉素的各实验参数。结果表明,在0.01 mol/L的HCl溶液中,盐酸阿霉素在-0.40 V处出现(vs.SCE)一灵敏的还原峰,峰电流与其溶液浓度在5.0×10-8~1.0×10-6mol/L(r=0.999)和1.0×10-6~1.0×10-5mol/L(r=0.998)范围内呈良好的线性关系,检出限为1.0×10-8mol/L。并用循环伏安法研究了盐酸阿霉素的峰电流性质,发现电极反应属于准可逆过程,出现一对灵敏的氧化还原峰,体系属准可逆吸附波。利用盐酸阿霉素在玻碳电极的电化学行为建立的分析方法可用于盐酸阿霉素的质量监控及药代动力学研究。     

Catalytic reduction of 1- and 2-bromooctanes by a dinickel(I) Schiff base complex containing two salen units electrogenerated at carbon cathodes in dimethylformamide

A dinickel Schiff base complex (Ni₂(II)L) containing two salen units has been synthesized and characterized by NMR and mass spectrometry. The complex was used for the catalytic reduction of 1- and 2-bromooctanes and its electrochemical behavior, which is similar to that of nickel salen, was studied by cyclic voltammetry and controlled-potential electrolysis. Due to the proximity of the two nickel centers, the yield of octyl dimers is slightly higher for the electrochemical reduction of 2-bromooctane catalyzed by the dinickel complex than that by nickel salen. However, the yield of hexadecane does not increase for the catalytic reduction of 1-bromooctane, which can give more reactive primary radicals. In addition to the formation of various products, octylation of the dinickel complex takes place to a further extent for the reduction of 2-bromooctane than 1-bromooctane. Compared with nickel salen-catalyzed reduction, the electrochemical data also indicate that more octyl groups may incorporate into the dinickel complex. Mechanistically, the catalytic reduction of the bromooctanes involves the intermediacy of primary or secondary octyl radicals.     

Oxygen reduction at the surface of glassy carbon electrodes modified with anthraquinone derivatives and dyes

The preparation, electrochemical and catalytic behaviour of glassy carbon electrodes modified by anthra-9,10-quinone, its amino derivatives and dyes were investigated. The stability of the modified electrodes was studied by cyclic voltammetry in acidic and neutral media. The electrocatalytic ability of the modified electrodes for the reduction of dioxygen to hydrogen peroxide was examined by cyclic voltammetry, chronoamperometry and chronocoulometry techniques. The influence of pH on the electrochemical and catalytic behaviour was studied and pH 5.0–8.0 was chosen as the optimum working pH by comparing the shift in oxygen reduction potential. The anthraquinone-adsorbed glassy carbon electrodes possess excellent electrocatalytic abilities for dioxygen reduction with overpotential ranging from 280 to 560 mV lower than that at a plain glassy carbon electrode. Hydrodynamic voltammetric studies were performed to determine the heterogeneous rate constants for the reduction of O₂ at the surface of the modified electrodes, mass specific activity of the anthraquinones used and the apparent diffusion coefficient of O₂ in buffered aqueous O₂-saturated solutions. Studies showed the involvement of two electrons in dioxygen reduction.     

The electrochemical polymerization of redox dye-nile blue for the amperometric determination of hemoglobin

A semi conducting polymer-modified glassy carbon electrode was fabricated by cyclically sweeping in the potential range of −0.4–1.2 V(vs. SCE) or by electrolyzation at constant potential of 1.4 V in 0.5 M sulfuric acid solution containing 1.0 mM Nile Blue. This redox active polymer could successfully be used for the amperometric determination of hemoglobin. The effect of pH was studied for the polymerization of Nile Blue. It was found that the polymerization did not occur in neutral or basic solutions. The formal potential of this film is −410 mV (pH 7.0) which is more negative than that obtained at the bare electrode in the Nile Blue bulk solution. The electrochemical characteristics of this polymer are discussed. The modified electrode could electrocatalyze the reduction of hemoglobin, and a good linear amperometric response could be obtained over the range of 0.05–7.0 mg/mL (correlation coefficient: 0.985). This modified electrode exhibited good stability and reproducibility for long-term use.     

四种药物中席夫碱基团在玻碳电极上的电化学行为

选择呋喃妥因、盐酸二甲双胍、西咪替丁和醋甲唑胺4种含有席夫碱基团的常见药物,运用电化学循环伏安法对其中的-C=N-基团在玻碳电极上的电化学氧化还原行为进行了研究。呋喃妥因、盐酸二甲双胍和西咪替丁中的席夫碱基团(-CH=N-)在玻碳电极上能够被还原,而且是一个电化学的不可逆过程,其还原电位分别为-0.864V,-1.36...     

Simple and rapid voltammetric determination of morphine at electrochemically pretreated glassy carbon electrodes

A simple and rapid method for morphine detection has been described based on electrochemical pretreatment of glassy carbon electrode (GCE) which was treated by anodic oxidation at 1.75 V, following potential cycling in the potential range from 0 V to 1.0 V vs. Ag|AgCl reference electrode. The sensitivity for morphine detection was improved greatly and the detection limit was 0.2 μM. The reproducibility of the voltammetric measurements was usually less than 3% RSD for six replicate measurements. Moreover, this method could readily discriminate morphine from codeine. And an electrochemical detection of morphine in spiked urine sample was succeeded with satisfactory results.     

Transition metal-nitrogen sites for electrochemical carbon dioxide reduction reaction

Electrochemical CO₂ reduction reaction (CO₂RR) powered by renewable electricity has emerged as the most promising technique for CO₂ conversion, making it possible to realize a carbon-neutral cycle. Highly efficient, robust, and cost-effective catalysts are highly demanded for the near-future practical applications of CO₂RR. Previous studies on atomically dispersed metal-nitrogen (M-N_x) sites constituted of earth abundant elements with maximum atom-utilization efficiency have demonstrated their performance towards CO₂RR. This review summarizes recent advances on a variety of M-N_x sites-containing transition metal-centered macrocyclic complexes, metal organic frameworks, and M-N_x-doped carbon materials for efficient CO₂RR, including both experimental and theoretical studies. The roles of metal centers, coordinated ligands, and conductive supports on the intrinsic activity and selectivity, together with the importance of reaction conditions for improved performance are discussed. The mechanisms of CO₂RR over these M-N_x-containing materials are presented to provide useful guidance for the rational design of efficient catalysts towards CO₂RR.     

Nanostructured heterogeneous catalysts for electrochemical reduction of CO2

Electrochemical reduction of CO₂ provides a sustainable solution to address the intermittent renewable electricity storage while recycling CO₂ to produce fuels and chemicals. Highly efficient catalytic materials and reaction systems are required to drive this process economically. This Review highlights the new trends in advancing the electrochemical reduction of CO₂ by developing and designing nanostructured heterogeneous catalysts. The activity, selectivity and reaction mechanism are significantly affected by the nano effects in nanostructured heterogeneous catalysts. In the future, energy efficiency and current density in electrochemical reduction of CO₂ need to be further improved to meet the requirements for practical applications.     

Oxygen reduction on rotating porous cylinder of modified reticulated vitreous carbon

A rotating cylinder porous electrode (RCPE) of reticulated vitreous carbon (RVC) matrix was used for oxygen reduction reaction (ORR) in H₂SO₄ solutions. Cyclic voltammetry and hydrodynamic voltammetric techniques were used for electrochemical characterization of the ORR. Cyclic voltammograms in stationary solutions showed better performance of the anodically oxidized RVC (for periods of 1 and 5 min) for the ORR than the untreated RVC in which the first scan (ORR) after the surface treatment was of no utility, and the second scan was presented here. The hydrodynamic voltammograms obtained at the treated RCPE gave well-defined limiting current plateau with positively shifted onset potential as compared with the untreated (plain) RVC electrode. The analysis of the limiting current data on RCPE and the determination of a limiting current enhancement factor α enabled us to quantify the enhancement extent exerted by the anodic oxidation treatment. An enhancement factor of up to ∼3 was obtained at the RCPE electrode anodically oxidized for 5 min. It was found that the α slightly decreased with the rotation speed depending on the extent of anodic oxidation of RVC. This was attributed to the different mode of mass transfer (diffusion) to the interior of the micropores with different microstructure resulting from different extent of anodic oxidation. X-ray photoelectron spectroscopic and scanning electron microscopic measurements helped us to characterize the anodically oxidized RVC surface.     

Electroreductive intramolecular cyclization of a bromo propargyloxy ester catalyzed by nickel(i) tetramethylcyclam electrogenerated at carbon cathodes in dimethylformamide

Cyclic voltammetry and controlled-potential electrolysis have been employed to investigate and characterize the reductive intramolecular cyclization of ethyl 2-bromo-3-(3',4'-dimethoxyphenyl)-3-(propargyloxy)propanoate (1) promoted by (1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane)nickel(I), [Ni(tmc)](+), electrogenerated at glassy carbon cathodes in dimethylformamide containing tetraalkylammonium salts. Cyclic voltammograms for reduction of [Ni(tmc)](2+) in the presence of 1 reveal that [Ni(tmc)](+) catalytically reduces 1 at potentials more positive than those required for direct reduction of 1. During controlled-potential electrolyses of solutions containing [Ni(tmc)](2+) and 1, catalytic reduction of the latter proceeds via one-electron cleavage of the carbon-bromine bond to form a radical intermediate that undergoes cyclization to afford 2-(3',4'-dimethoxyphenyl)-3-(ethoxycarbonyl)-4-methylenetetrahydrofuran (2). In the presence of a base (either electrogenerated or deliberately added as potassium tert-butoxide), 2 rearranges to give 2-(3',4'-dimethoxyphenyl)-3-(ethoxycarbonyl)-4-methyl-2,5-dihydrofuran (3). A mechanistic scheme is proposed to explain the results obtained by means of cyclic voltammetry and controlled-potential electrolysis.     

苦参碱在多壁碳纳米管修饰玻碳电极上的电催化氧化及其电分析方法

研究了苦参碱(Matrine, MT) 在多壁碳纳米管修饰玻碳电极(MWCNT/GCE)上的电化学行为. 与GCE相比, MT在MWCNT/GCE上峰电位负移120 mV, 峰电流增大约2.5倍, 表明MWCNT/GCE对MT的电化学氧化具有良好的催化作用. 同时测定并计算了MT在MWCNT/GCE上的电极过程动力学参数: 电子转移系数α、电极反应速率常数ks、扩散系数D. 运用差分脉冲伏安法对苦参碱样品含量进行测定, 相对标准偏差为0.12%～2.9%, 加标回收率为98.4%～99.0%. 该方法可用于MT的电化学定量测定.     

Enhanced electrochemical performance at screen-printed carbon electrodes by a new pretreating procedure

A new method for the pretreatment of screen-printed carbon electrodes (SPCEs) by two successive steps was proposed. In step one, fresh SPCEs were soaked into NaOH with high concentration (e.g. 3 M) for tens to hundreds of minutes, and the resulted electrodes were called as SPCE-I. In step two, SPCE-I were pre-anodized in low concentration of NaOH, which were designated as SPCE-II. The pretreated electrodes showed remarkable enhancement in heterogeneous electron transfer rate constant (k⁰) increased from 1.6 × 10⁻⁴ cm s⁻¹ at the fresh SPCE to 1.1 × 10⁻² cm s⁻¹ at SPCE-I for Fe(CN)₆^3−/4− couple. The peak to peak separation (ΔE_p) in cyclic voltammetry was reduced from ca. 480 to 84 mV, indicating that the electrochemical reversibility was greatly promoted, possibly due to the removing of polymers/oil binder from the electrode surfaces. The electroactive area (A_ea) of the electrode was increased by a factor of 17 after pretreatment in step one. Further analysis by the electrochemical impedance method showed that the electron transfer resistance (R_ct) decreased from ca. 2100 to 1.4 Ω. These pretreated electrodes, especially SPCE-II, exhibited excellent electrocatalytic behavior for the redox of dopamine (DA). Interference from ascorbic acid (AA) in the detection of DA at SPCE-II could be effectively eliminated due to the anodic peak separation (190 mV) between DA and AA, which resulted from the functionalization of the electrode surface in the pretreatment of step two. Under optimum conditions, current responses to DA were linearly changed in two concentration intervals, one was from 3.0 × 10⁻⁷ to 9.8 × 10⁻⁶ M, and the other was from 9.8 × 10⁻⁶ to 3.3 × 10⁻⁴ M. The detection limit for DA was down to 1.0 × 10⁻⁷ M.     

黄芩甙在玻碳电极上的电化学行为及其应用

用线性扫描伏安法(LSV)、循环伏安法(CV)和常规脉冲伏安法(NPV)等多种电化学方法研究了黄芩甙在玻碳电极上的电化学行为,并建立了尿样和血清样品中黄芩甙的微分脉冲伏安(DPV)测定法。实验结果表明:黄芩甙在玻碳电极上的吸附符合Frumkin吸附等温式,吸附自由能为-35.01kJ mol。采用整体电解法求得电极反应电子数为2,并结合Nernst方程式推导了反应机理。黄芩甙在玻碳电极上预富集后,在4.20×10-10～1.05×10-5mol L范围内分段呈线性关系,对5.60×10-7mol L黄芩甙溶液连续6次测定的RSD=2.0%,检出限为2.8×10-10mol L。     

Overwhelming electrochemical oxygen reduction reaction of zinc-nitrogen-carbon from biomass resource chitosan via a facile carbon bath method

Developing high efficiency and low cost electrocatalysts is critical for the enhancement of oxygen reduction reaction (ORR), which is the fundamental for the development and commercialization of renewable energy conversion technology. Herein, zinc-nitrogen-carbon (Zn-N-C) was prepared by using biomass resource chitosan via a facile carbon bath method. The obtained Zn-N-C delivered a high specific surface area (794.7 cm²/g) together with pore volume (0.49 cm³/g). During the electrochemical evaluation of oxygen reduction reaction (ORR), Zn-N-C displayed high activity for ORR with an onset potential E₀ = 0.96 V_RHE and a half wave potential E_1/2 = 0.86 V_RHE, which were more positive than those of the commercial 20 wt% Pt/C benchmark catalyst (E₀ = 0.96 V_RHE and E_1/2 = 0.81 V_RHE). In addition, the Zn-N-C catalyst also had a better stability and methanol tolerance than those of the Pt/C catalyst.     

Is glassy carbon a really inert electrode material for the reduction of carbon–halogen bonds?

Dissociative electron transfer (DET) to various organic halides has been investigated at glassy carbon electrodes (GCEs) in acetonitrile. It is shown that the surface composition of GCEs significantly affects the reduction peak potential of RX, an increase of the oxygen-to-carbon ratio apparently causing a positive shift of E_p. This catalytic effect depends on both GCE surface composition and DET mechanism, appreciable catalysis being observed only when the initial electron transfer plays a crucial role in the overall kinetics of the process. The highest observed peak potential difference is of the order of 0.1 V found for some alkyl halides undergoing a concerted DET.     

苯甲酰肼在多壁碳纳米管修饰玻碳电极上的电催化氧化及其电化学动力学性质

研究了苯甲酰肼(BH)在MWCNT/GCE上的电化学行为。实验结果表明,BH在GCE上的直接电化学氧化十分迟缓,无氧化峰出现,但在MWCNT/GCE上BH在0.20 V处出现了一个不可逆氧化峰,且峰电流大幅度增大,表明MWCNT/GCE对BH电化学氧化具有良好的催化作用。同时用计时库仑法(Chro-nocoulometry,CC)和计时电流法(Chronoamperometry,CA)测定了电极过程动力学参数:扩散系数D=8.73×10-5cm2.s-1,电子转移系数α=0.85,电极反应速率常数kf=1.45×10-3s-1。稳态电流-时间实验结果表明,电流响应信号随其浓度成比例增长,响应时间小于6 s,最低响应浓度为1×10-6mol/L。该方法可用于BH电化学定量测定。