Highly stable electrochemical oxidation of phenolic compounds at carbon ionic liquid electrode

A carbon ionic liquid electrode (CILE) was used for the investigation of the electrochemical oxidation of phenolic compounds in acidic media using cyclic voltammetry, chronoamperometry and square wave voltammetry techniques. The results indicate that, contrary to many other electrodes, the oxidation of phenolic compounds on CILE is highly stable and does not result in electrode fouling. Cyclic voltammetry showed that phenolic compounds such as phenol, 2,4-dichlorophenol and catechol were oxidized at CILE and remained electroactive after multiple cycles and at high concentrations of phenol. The cyclic voltammetric response of the CILE is very stable with more than 99% of the initial activity remaining after 20 s of stirring of a 0.5 mM solution of phenol.     

A hierarchical porous MnO2-based electrode for electrochemical capacitor

A hierarchical porous MnO₂-based electrode was prepared and its electrochemical performance for electrochemical capacitors was investigated. In this work, porous MnO₂ film with pore size of 2?C3?nm in diameter was deposited on a three-dimensional porous current collector by cathodic electrodeposition associated with subsequent controlled heat treatment at 200°C for 2?h. Transmission electron microscopy and X-ray photoelectron spectroscopy showed that the heat treatment has a great effect on the formation of the porous structure of MnO₂ layer, and the disordered porous structure was caused by dehydration during the heat treatment. Cyclic voltammetry and galvanostatic charge?Cdischarge tests showed that both energy and power densities are enhanced due to the unique hierarchical porous structure. The electrode delivers a high specific capacitance of 385?F?g^?1 at a high current density of 5?A?g^?1 within a potential window of ?0.05????0.85?V, and also exhibits excellent rate capability and electrochemical stability.     

三维有序大孔SnO2基固溶体用于有效燃烧碳烟颗粒

柴油车尾气排放的碳烟颗粒对人类的生存环境和身体健康带来了严重危害.催化燃烧是消除碳烟颗粒污染的有效途径.碳烟颗粒催化燃烧是固-固-气相反应,因此催化剂本身具有活泼的氧中心且其能与碳烟颗粒有效接触是提高反应效率的关键因素.为改善碳烟颗粒与催化剂的接触,设计制备三维有序大孔(3DOM)催化剂,使碳烟颗粒可以进入催化剂孔道内部,增加其与催化剂的有效接触,是提高反应活性的有效途径.此外,在催化剂晶格中掺杂其它金属离子形成固溶体结构,可提高其氧化还原性能,也可有效提高其碳烟燃烧活性.SnO2富含活泼的表面缺位氧和可还原的晶格氧,且其熔点高达1630 oC,具有良好的热稳定性,被广泛用于制备气体传感、电化学和催化等材料.在过去的6年中,本课题组在SnO2催化化学领域做了大量系统的工作,将SnO2基催化材料用于多种环保和能源反应.发现通过其它阳离子Fe3+,Cr3+,Ta5+,Ce4+和Nb5+等的掺杂,替换晶格中部分Sn4+形成金红石型SnO2固溶体结构,可显著提高催化剂氧物种的流动性、活性和本身的热稳定性.本文采用胶体晶体模板法制备出了Ce4+,Mn3+和Cu2+离子掺杂的SnO2三维有序大孔固溶体催化剂用于松散接触条件下的碳烟催化燃烧.采用SEM,TEM,XRD,STEM-mapping,O2-TPD和XPS等手段对催化剂进行表征,研究其碳烟催化燃烧性能.SEM和TEM结果表明已成功合成三维有序大孔结构样品.XRD,Raman和STEM-mapping结果表明,Ce4+,Mn3+和Cu2+离子均进入四方金红石型SnO2晶格形成固溶体结构.另外,Raman,H2-TPR,XPS和O2-TPD等结果发现上述离子掺杂三维大孔SnO2后,催化剂表面形成了更活泼、丰富的氧物种,有利于碳烟颗粒燃烧.其中3DOM-Cu1Sn9催化剂具有最丰富的活泼氧中心,因此表现出最高的活性.     

The kinetics of the oxidation and reduction of H2O2 at a Pt electrode: A differential electrochemical mass spectrometric study

The kinetics of the oxidation and reduction of hydrogen peroxide (HPOOR, HPORR) at a Pt electrode in 0.1 M HClO₄ + 2 mM H₂O₂ are investigated by Differential Electrochemical Mass Spectrometry (DEMS) in a flow cell. The O₂ mass signal was recorded during cyclic voltammetry, and its potential dependence follows the shape of the cyclic voltammogram. Partial currents for HPOOR and HPORR are estimated based on the O₂ mass signal and the total Faradaic current. The onset overpotential for HPORR at the Pt electrode is above 0.6 V. It is limited by the thermodynamics of OH_ad desorption, as is also the case with ORR. The onset overpotential for HPOOR is below 0.1 V, due to the faster consumption of H₂O₂ through HPORR at these potentials and the small bulk H₂O₂ concentration used.     

SnO2基固溶体用于甲烷深度氧化：X射线衍射外推法测定 SnO2晶格容量

CH4和 CO是两种主要的温室效应气体和空气污染物,催化氧化是最有效的消除 CH4和 CO的方法.研发不含贵金属的金属氧化物催化剂或者减少催化剂中贵金属用量为该领域研究热点. SnO2是一种重要的宽禁带 n型半导体材料,广泛应用于气敏器件、锂离子电池以及光电设备. SnO2表面富含活泼的缺位氧且具有良好的热稳定性,因此其在催化方面的性能近年来逐渐受到人们关注.在过去的5年中,本团队深入研究了 SnO2材料在空气污染治理和绿色能源生产等领域的应用及其催化性质.发现通过其它阳离子如 Fe3+, Cr3+, Ta5+, Ce4+和Nb5+等的掺杂,替换晶格中部分 Sn4+,形成金红石型 SnO2固溶体结构,显著提高了催化剂氧物种的流动性、活性和催化剂本身的热稳定性.固溶体材料是一类重要的催化剂,受到广泛关注.一个典型的例子是铈锆固溶体,其作为储氧材料已广泛应用于汽车尾气净化器.形成固溶体结构后,氧化铈的储氧能力和热稳定性得到显著提高.为有效形成固溶体,两个阳离子需要具有相似的离子半径和电负性.以往,人们基于结构中金属阳离子和氧阴离子的离子半径提出了容忍因子的判别方法,以此来判断固溶体是否能有效形成及所生成固溶体的稳定性.我们在前期工作中,以 Sn-Nb固溶体为例,提出了简单的X射线衍射(XRD)外推法来计算固溶体晶格容量,即形成稳定固溶体时客体阳离子取代主体晶格阳离子的最大值.作为延续工作,本文采用共沉淀法制备了一系列 Sn/M (M = Mn, Zr, Ti, Pb)摩尔比为9/1的 SnO2基催化剂,并用于 CH4和 CO催化氧化.结果表明, Mn3+, Zr4+, Ti4+和 Pb4+均可以掺杂进四方金红石型 SnO2晶格中,形成稳定的固溶体结构.其中 Sn-Mn-O固溶体表现出最高活性.为了深入研究 Mn2O3在 SnO2中的晶格容量及最优催化剂配比,采用共沉淀法制备了一系列不同 Sn/Mn摩尔比的样品,采用 XRD, N2-BET, H2-TPR, SEM和XPS等手段对其物理化学性能进行了表征,并考察了对 CH4的催化氧化性能.通过 XRD外推法测定了 Mn3+离子在 SnO2中的晶格容量为0.135 g Mn2O3/g SnO2,相当于 Sn/Mn摩尔比为79/21.这表明形成稳定的固溶体后, SnO2晶格中最多只有21% Sn4+可以被 Mn3+替代;当 Mn3+含量超过晶格容量时,过量的 Mn3+在催化剂表面形成 Mn2O3,对催化剂活性不利.类似于 Sn-Nb-O固溶体,在 Sn-Mn-O催化剂体系中亦观察到明显的晶格容量效应.纯相的 Sn-Mn-O固溶体比含过量 Mn2O3晶相的 Sn-Mn-O催化剂具有更高活性.     

Structural and electrochemical properties of a porous nanostructured SnO2 film electrode for lithium-ion batteries

A B₂O₃-doped SnO₂ thin film was prepared by a novel experimental procedure combining the electrodeposition and the hydrothermal treatment, and its structure and electrochemical properties were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) analysis, energy dispersive X-ray (EDX) spectroscopy and galvanostatic charge–discharge tests. It was found that the as-prepared modified SnO₂ film shows a porous network structure with large specific surface area and high crystallinity. The results of electrochemical tests showed that the modified SnO₂ electrode presents the largest reversible capacity of 676 mAh g^?1 at the fourth cycle, close to the theoretical capacity of SnO₂ (790 mAh g^?1); and it still delivers a reversible Li storage capacity of 524 mAh g^?1 after 50 cycles. The reasons that the modified SnO₂ film electrode shows excellent electrochemical properties were also discussed.     

Light energy conversion with pheophytina monolayer at the SnO2 optically transparent electrode

The photoelectrochemical, absorption and fluorescence properties of pheophytin a mono- and multilayers, deposited on optically transparent tin oxide electrodes and quartz slides were investigated. Spectra of photocurrents coincided with the absorption spectra of photosynthetic pigment in monolayers at the SnO₂/solution interfaces. The anodic and cathodic photocurrents were measured at various electrode potentials. Effects of pH, electrode potentials, and concentration of redox reagents on the conversion of solar energy in monolayers on optically transparent electrodes are discussed. The absorption and fluorescence spectral characteristics, and fluorescence lifetime measurements of pheophytina in monolayers and thin films are also discussed in view of the aggregation properties of the photosynthetic pigment. The thermodynamics of adsorption of large amphiphilic compounds at the interface between two immiscible liquids is considered. The adsorption behavior of pheophytin a dissolved in different solvents is investigated. The thermodynamic parameters of pheophytin a adsorption at octane/water and benzene/water interfaces were determined.Presented at the Symposium, 76^th CSC Congress, Sherbrooke, Quebec, May 30–June 3, 1993, honoring Professor Donald Patterson on the occasion of his 65^th birthday.     

Thermodynamic study of alcohol on SnO2 (100)-based gas nano-sensor

Alcohol dehydrogenation and condensation reactions are involved in chain growth pathways of SnO₂. These pathways lead to the formation of acetaldehyde and other products with high selectivity. It is recognised that together with the atmospheric oxygen, the presence of humidity greatly influences gas detection. Accordingly, it is important to understand the role of alcohol vapours in the sensing mechanism. Interaction between alcohol molecules and SnO₂ is investigated using MNDO method by semi-empirical calculations. We study the structural, total energy, thermodynamic properties of absorption of CH₃OH and C₂H₅OH on SnO₂ at 298?K. When exposed to ethanol, the SnO₂-based sensors showed oxidation products consisting of acetaldehyde, ethyl acetate and CH₄?+?CO. All the geometry optimisation structures were carried out using the Gaussian program package. Density functional theory optimised intermediates and transient states. The results show a sensitivity enhancement in resistance and capacitance when ethanol is near the surface, so converted into different products.     

Electrochemical oxidation of 2-thiouracil at pyrolytic graphite electrode

2-Thiouracil has been studied in phosphate buffers of pH 1.95-11.08 using linear and cyclic sweep voltammetry, coulometry, controlled potential electrolysis and spectral studies. One well-defined oxidation peak I(a) in the pH range 1.95-11.08 was noticed. The number of electrons involved in peak I(a) was found to be four in a thin layer cell whereas under exhaustive electrolysis condition oxidation was found to involve six electrons. A reduction peak II(c) (2e, 2H(+)) is noticed in the reverse sweep. Spectral studies during oxidation were carried out at different pH. Kinetic studies indicated that the decay of the UV-absorbing intermediate is a first order reaction. The products of the electrooxidation have been characterized and a tentative EC mechanism has been suggested for the oxidation of 2-thiouracil.     

A comparative study on direct and indirect electrochemical oxidation of polyaromatic compounds

This study has been performed to investigate the treatment of an industrial wastewater containing naphthalene- and anthraquinone-sulfonic acids, by direct and indirect electrolyses. The direct electrochemical oxidation has been carried out using boron-doped diamond and lead dioxide anodes, while the indirect electrolyses has been mediated by active chlorine electrogenerated on a platinum anode, or by hydrogen peroxide electrogenerated on a graphite felt cathode. For each type of electrolyses the effects of operating parameters, such as anode material, current density, chloride concentration, ferrous ions concentrations have been also investigated. Measurements of Chemical Oxygen Demand (COD) and colour fading have been used to compare the results of different electrolyses. Experimental data showed that the complete COD and colour removals have been obtained only by direct oxidation or by active chlorine mediated electrolyses. On the contrary using electrogenerated hydrogen peroxide the solution has been presented a residual COD and colour. In particular, it found that faster oxidation rate has been obtained by direct oxidation using a boron-doped diamond anodes at low current density.     

459—The effect of netropsin on the electrochemical oxidation of DNA at a graphite electrode

The interaction of netropsin with calf thymus, Bacillus cereus and Micrococcus luteus DNAs and with the RNA of phage f2 was studied by means of differential pulse voltammetry at a paraffin-wax-impregnated spectroscopic graphite electrode. It was found that the oxidation voltammetric peaks of double-helical calf thymus and Bacillus cereus DNAs were lowered when netropsin was added to the DNA solution. The peak corresponding to electro-oxidation of adenine residues was lowered more than that corresponding to electro-oxidation of guanine residues. Under the same experimental conditions the oxidation peaks of double-helical Micrococcus luteus DNA, double-helical RNA and thermally denatured samples of all DNAs used were almost uninfluenced by the addition of netropsin. The results observed were explained by (i) decreased flexibility of the segments to which netropsin was bound, and (ii) the specific binding of netropsin to the segments of double-helical DNA rich in adenine thymine pairs.     

Scanning electrochemical microscopic study of hydrogen oxidation and evolution at electrochemically deposited pt nanoparticulate electrode incorporated in polyaniline.

Scanning Electrochemical Microscopy (SECM) feedback mode and substrate generation-tip collection (SG-TC) mode coupled with chronoamperometric approach were used to investigate H2 oxidation and hydrogen evolution reaction (HER) at a polyaniline (PAni) coated highly oriented pyrolytic graphite (HOPG) electrode. Using the former mode, the heterogeneous electron transfer (ET) kinetics for H2 oxidation was studied, while the latter mode allowed mapping of the distribution of local [H2] at the nanoparticulate/aqueous interface, followed by monitoring the transients at the tip. These preliminary studies demonstrate that SECM is useful in evaluating the activity of nanophase electrocatalysts. Particularly, if one employs nanometer-sized tips or hydrodynamic microjet electrodes where the mass transfer rate is significantly high, it should be possible to investigate the ET kinetics more accurately.     

Voltammetric oxidation of pharmaceutical organic compounds at a new modified electrode: The aluminum graphite spray-covered electrode

A new graphite spray-modified aluminum electrode has been used to determine clozapine, clothiapine, and loxapine in acetate buffer at pH 4.7 and 20% methanol, down to 5 × 10⁻⁶ M concentrations using differential pulse voltammetry. Reproducibility exceeds that of other solid electrodes, being in the range of 1 to 2%. The electrochemical oxidation products of the latter two compounds are strongly adsorbed, causing diminished electrode response, but response is readily restored by polishing of the sprayed electrode.     

Rotating disk electrode study of electrocatalytic oxidation of ascorbate at Prussian blue modified electrode

Electrooxidation of ascorbate has been studied with the use of a rotating disk electrode. The results obtained show an efficient electrocatalytic oxidation of ascorbate at the Prussian blue (PB) modified electrode to proceed in solutions of pH 5.5 and 7.3. Depending on solution pH, the onset potential for ascorbate electrooxidation at PB modified electrode appears shifted by 0.1–0.2 V to lower values, as compared to an unmodified glassy carbon electrode. Within the electrode potential window of 0.3 to 0.5 V vs. Ag/AgCl, and electrode rotation velocity of 50–2000 rpm, the catalytic current obeys Koutecky-Levich equation at a submillimolar ascorbate concentration. Kinetic current densities, obtained from the data treatment, are higher for a pH 5.5 solution, and also at higher electrode potential.      

Quantitative structure-retention relationships of phenolic compounds without Hammett's equations

Retention times of phenolic compounds in a given pH eluent in reversed-phase liquid chromatography were predicted from dissociation constants derived from atomic partial charges and log P-values calculated by a computational chemical method. The precision of the calculation of atomic partial charges by AMI and PM3 methods of MOPAC was evaluated. The atomic partial charges obtained by AMI were the more acceptable. The atomic partial charges obtained from the hydrogen of the hydroxyl group included an ortho-effect, therefore an ortho-effect was added to the predicted values. The precision of predicted retention factors obtained using predicted pKa values was similar to that using reference pKa values.     

亚硫酸盐在乙酰二茂铁修饰碳糊电极上的电催化氧化及其电化学动力学性质研究

研究了亚硫酸盐在乙酰二茂铁(AFc)修饰碳糊电极(AFc/CPE)上的电催化行为。研究结果表明,其在裸碳糊电极(CPE)上的行为比亚硫酸盐在AFc/CPE上的氧化峰电流增加约3倍,氧化峰电位负移360 mV,表明AFc/CPE对亚硫酸盐的电化学氧化具有良好的催化作用。用循环伏安法、计时电流法测定了亚硫酸盐在AFc/CPE上的电极过程动力学参数,测得电荷传递系数α为0.70,电催化氧化反应速率常数k为(4.91±0.05)×104(mol/L)-1.s-1。催化氧化峰电流与亚硫酸盐在5.0×10-4~1.1×10-2mol/L浓度范围内呈良好的线性关系,线性回归方程为Ipa(μA)=1.345 7.956c(10-3mol/L),R=0.9988,检出限为2.0×10-5mol/L。可用于亚硫酸盐的电化学定量测定方法。     

The electrochemical oxidation of pentachlorophenol and its sensitive determination at chitosan modified carbon paste electrode

We report on a novel electrochemical method to detect trace pentachlorophenol (PCP) by using a chitosan (CS) modified carbon paste electrode (CS/CPE). Compared with that at a bare carbon paste electrode (CPE), the current response was greatly improved at the CS/CPE due to the enhancement effect of CS. Under optimal working conditions, the oxidation peak current of PCP was proportional to its concentration in the range of 1.0 × 10^?7 to 5.0 × 10^?6 and 5.0 × 10^?6 to 1.0 × 10^?4 mol/L, with an extremely low detection limit of 4.0 × 10^?8 mol/L. Our method was successfully used to detect the PCP concentration in vegetable samples.