Electroreduction and oxidation of terephthalaldehyde and the role of hydration

Electroreduction of terephthalaldehyde (benzene-1,4-dicarboxaldehyde) (I) in protic media is complicated by acid-base and hydration-dehydration equilibria. At pH 0-5.5 the diprotonated form of I is reduced in a reversible two-electron step to a diradical-quinonemethide. This species is at pH 1-3 converted into an aldol (4-HOCH₂C₆H₄CHO), which is further reduced at a more negative potential. At pH 5.5-8.5 the monoprotonated form is reduced. The protonated formyl group is reduced first and then, at a more negative potential, the second formyl group in the aldol, formed in the first step, is further reduced. As terephthalaldehyde is in aqueous solutions present in about 15% as a monohydrate, the limiting currents in this pH range are controlled by the hydration-dehydration equilibrium, the establishment of which is acid and base-catalyzed. The reduction of the unprotonated form occurs at pH > 8.5. At pH > 10 addition of OH⁻ ions takes place. The formation of a geminaldiol anion results in a decrease of reduction waves of the aldehyde, but an increase in the oxidation anodic wave of the diol. Dependence of anodic waves on activity of OH⁻ ions indicates a consecutive addition of OH⁻ ions to the two formyl groups. The understanding of the sequence of chemical (protonation, hydration) and electrochemical reaction steps enabled application of changes of polarographic i-E curves with time as an analytical method for following (in real time) concentration changes of the parent compound, its monohydrazones and monoximes as well as dihydrazones and dioximes.     

Electrochemical and spectroscopic characterization of a tungsten electrode as a sensitive amperometric sensor of small inorganic ions

Cyclic voltammetry was used to investigate the electrochemical behaviour of the tungsten oxide films toward the electroreduction of BrO₃⁻, ClO₂⁻ and NO₂⁻ ions in acidic medium. The effects of the temperature, scan rate, pH, chemical composition of the electrolytic solutions, were investigated and the reduction mechanism was critically discussed.The reduction currents, evaluated in cyclic voltammetry and measured at −0.250 V versus SCE, increased linearly on increasing analyte concentration up to 20, 55 and 45 mM for nitrite, chlorite and bromate ions, respectively. The detection limits, evaluated in cyclic voltammetry, were 0.1, 0.4 and 0.7 mM for BrO₃⁻, ClO₂⁻ and NO₂⁻, respectively.The tungsten oxide film was successfully characterized as an amperometric sensor for the analytical determination of BrO₃⁻, ClO₂⁻ and NO₂⁻ ions in flowing stream. Operating under constant applied potential of −0.3 V versus Ag/AgCl the good reproducibility of the peak height and background current level during consecutive injections, indicates the absence of fouling effects and the potential applicability of the amperometric sensor for the routine analytical determination of the investigated inorganic ions. Considering the low values of the background currents (ca. 1.1 ± 0.1 μA) obtained in acidic and not deoxygenated carrier electrolyte, the tungsten sensing electrode seems to compete favourably with other common sensors for the amperometric determination of electroactive molecules under cathodic conditions.The X-ray photoelectron spectroscopy technique (XPS) was used in order to evaluate the chemical composition of the tungsten film upon electrochemical treatment in 0.1 M H₂SO₄ solution. Independently of the electrochemical treatment in acid solution, the tungsten surface electrode is generally composed by 50-60% of W⁰, 35-40% of W⁶⁺ and traces of W²⁺ oxide species.     

Preparation and activity of mono- or bi-metallic nanoparticles for electrocatalytic reactions

Improvement of Pt activity for electrocatalytic reactions is possible by modifying Pt nanoparticles with other metals able to activate water. Selected examples are discussed with the electrooxidations of methanol and ethanol or the electroreduction of dioxygen. Nanoparticles electrodes of Pt-Ru (for methanol oxidation), of Pt-Sn (for ethanol oxidation) or of Pt-Cr (for oxygen reduction) supported on carbon powder can be prepared from colloidal precursors. This kind of preparation allows varying the composition and/or the structure of the electrode. The formulation of improved electrodes can be obtained after complete study of the reaction mechanism by “in situ” spectroscopic experiment.     

Electrosynthesis of lactic acid on copper and lead cathodes in aqueous media

The present study concerns the electrochemical reduction of pyruvic acid in different aqueous media on lead and copper electrodes. Some parameters such as electrode material, nature of the electrolyte, reduction potential and concentration of the organic substrate were systematically studied. The cathodic behavior of pyruvic acid was studied by cyclic voltammetry and suitable conditions were found in order to optimize the conversion of pyruvic acid during prolonged electrolyses. The results obtained showed that the pH of the solution (pH 10), the electrode material (Pb) and potential of electrode (−2.0 V (MSE)) allowed to enhance the selectivity of lactic acid which reached 90% with a conversion yield of 92%.     

Macroporous defective tungsten oxide nanostructure electrodes deliver ultrahigh capacitance and remarkable long cyclic durability in LiCl electrolyte

Tungsten oxide (WO₃) has been considered as an fascinating candidate for supercapacitors (SCs) material because of its desirable physico-chemical properties and electrochemical behaviors. Nevertheless, it is still a significant challenge to enhance its electrochemical properties and stability. Herein, we report an electroreduction strategy to fabricate the macroporous defective tungsten oxide nanostructure (ER-WO₃) as a negative electrode material with outstanding electrochemical behavior and remarkable cycling durability in 5?M LiCl aqueous electrolyte, which attributes to the introduction of oxygen deficiencies. The ER-WO₃ electrode exhibits a large areal capacitance of 244.7?mF?cm^?2 and an ultrahigh gravimetric specific capacitance of 266.6?F?g^?1 at scan rate of 50?mV?s^?1. More importantly, the ER-WO₃ product also delivers an ultralong cyclic stability with 97.4% capacitance retention after 5000 cycles. Such these optimized properties of the ER-WO₃ nanostructure electrode will promote its applications in the field of science and technology.     

Scale-up of the perforated bipole trickle-bed electrochemical reactor for the generation of alkaline peroxide

This paper reports work on the scale-up of a perforated bipole trickle-bed electrochemical reactor for the electro-synthesis of alkaline peroxide. The reactor uses a relatively simple cell configuration in which a single electrolyte flows with oxygen gas in a flow-by graphite felt cathode, sandwiched between a microporous polyolefin diaphragm and a nickel mesh/perforated Grafoil anode/bipole. Both one and two-cell reactors are scaled-up from cathode dimensions 120 mm high by 25 mm wide and 3.2 mm thick (reactor-A) to 630 mm high by 40 mm wide and 3.2 mm thick (reactor-B). The scale-up is achieved by the use of constrictions that prevent segregation of the 2-phase flow in the larger cell, combined with switching from a polypropylene to a polyethylene diaphragm with improved transport properties and raising the electrolyte feed concentration from 1 to 2 M NaOH.For the one-cell reactor-B with a polypropylene diaphragm, operating on a feed of 1 M NaOH and oxygen at 900 kPa(abs)/20 °C, the peroxide current efficiency at a superficial current density of 5 kA m⁻² increases from 27% (un-constricted cathode) to 57% with a constricted cathode. The corresponding current efficiencies at 3–5 kAm⁻² for reactor-A and the constricted reactor-B are respectively 69–64% and 66–57%. Under similar conditions at 3–5 kA m⁻² the one-cell constricted reactor-B with a polyethylene diaphragm gives current efficiencies of 88–64%, and changing to an electrolyte of 2 M NaOH raises this range to 90–80%. At 3–5 kA m⁻² the equivalent two-cell (bipolar) constricted reactor-B shows current efficiencies of 82–74% and at 5 kA m⁻² obtains 0.6 M peroxide in 2 M NaOH with specific energy 6.5 kWh per kg H₂O₂.     

Electrochemical transient techniques for determination of uranium and rare-earth metal separation coefficients in molten salts

The main step in the pyrometallurgical recycling process of spent nuclear fuel is a molten salt electrorefining. The knowledge of separation coefficients of actinides (U, Np, Pu and Am) and rare-earth metals (Y, La, Ce, Nd and Gd) is very important for this step. Usually the separation coefficients are evaluated from the formal standard potentials of metals in melts containing their own ions, i.e. values obtained by potentiometric method. Electrochemical experiments were carried out at 723-823 K in order to estimate separation coefficients in LiCl-KCl eutectic melt containing uranium and lanthanum trichlorides. The electrochemical behaviour of UCl₃ in LiCl-KCl melt was studied by different electrochemical methods. The diffusion coefficients of U(III) were determined by linear sweep voltammetry, chronopotentiometry and chronoamperometry. The standard rate constants of charge transfer for electroreduction of uranium, U(III) + 3e⁻ → U, were calculated by the impedance spectroscopy method. The values of constants testify that electroreduction of U(III) to U is mainly controlled by the rate of charge transfer. La(III) discharge on uranium electrode was also investigated. It was shown that for the calculation of uranium and lanthanum separation coefficients it is necessary to determine the voltammetric peak potentials of U(III) and La(III), their concentration in the melt and the kinetic parameters relating to U(III) discharge such as transfer and diffusion coefficients, and standard rate constants of charge transfer.     

Electroreduction of lichexanthone

This paper reports the first study on the electrochemical reduction of lichexanthone (1H) (1-hydroxy-3,6-dimethoxy-8-methylxanthen-9-one) on glassy carbon (GC) electrodes in DMSO, using cyclic voltammetry, rotating disc and ring electrodes, and long-term controlled-potential electrolysis. Parameters involving data from cyclic voltammetry and rotating disc electrodes, such as current functions, E_pc1 vs. log ν, E_pc2 vs. log ν, E_pc/2,1 − E_pc1, −I_pc1ox/I_pc1red, I_pc2/I_pc1, E_1/2 vs. log ω, and collection efficiency (rotating disc and ring electrode data), were used to elucidate the reduction mechanism of 1H that involves two one-electron transfers (two reduction peaks in the voltammograms), the first of which, with reversible characteristics, involves electroreduction of 1H, producing a radical anion 1H⁻, whereas the second, with irreversible characteristics, involves electroreduction of 1H⁻, producing a dianion 1H²⁻. Both transfers appear to involve an E_rC_slow-type mechanism with a chemical step consisting of breakage of a bond followed by protonation of residual water, or parent compound, or solvent, etc., to yield 2-hydroxy-4-methoxy-6-methylphenyl 2-hydroxy-4-methoxyphenyl ketone (1H₃), directly, in the case of 1H²⁻ involved. Compound 1H₃ was elucidated by 1D- and 2D-NMR methods. D₀ = 2.66 × 10⁻⁶ cm² s⁻¹ was found for the electrochemical reduction of 1H.     

离子液体在硝基化合物电还原中的应用

概述了离子液体应用于硝基化合物电还原的进展,总结介绍了硝基化合物在不同性质离子液体中的电还原特性,并重点归纳出不同性质离子液体中硝基化合物电还原的优缺点及其前景.     

UO_2(NO_3)_2-HNO_3-N_2H_5NO_3(H_2O)/30%TBP(煤油)体系中组分浓度对U(Ⅵ)电解还原速率影响的研究

本文在振动搅拌槽中,研究了UO_2(NO_3)_2-HNO_3-N_2H_5NO_3(H_2O)/30％TBP(煤油)体系的水相电解液组分浓度对U(Ⅵ)电解还原速率的影响。根据实验所得数据,经回归分析得到反应动力学微分方程 式中速度常数k一般说是温度的函数。25℃时,k=0.00187。在实验浓度范围内,U(Ⅵ)还原速率随U(Ⅵ)浓度升高而增大,表观反应级数为0.75级;而[N_2H_5~+]及[HNO_3]影响不大,反应级数近于0。