Oxydation électrochimique de l'acétylène en solution aqueuse a des électrodes d'or

The electro-oxidation of aqueous solutions of acetylene at gold electrodes is studied by voltammetry and controlled potential electrolysis. A mechanism involving the oxidation of the gold electrode is presented for the electro-oxidation of acetylene at gold electrodes.     

The influence of electrode structure on the adsorption and electro-oxidation of ethylene on platinum

The adsorption and electro-oxidation of ethylene in acid electrolyte were studied on differently prepared platinum electrodes. A clear dependence of the apparent specific catalytic activity for ethylene electro-oxidation on platinum electrode structure was observed. Platinized platinum electrodes exhibit a low apparent specific catalytic activity which decreases on increasing the roughness factor due to additional overpotential contributions within the pores. The specific adsorptive characteristics and catalytic activity of non-porous platinum electrodes prepared by electroreduction of thick platinum oxide layers are independent of roughness factor and similar to those of smooth platinum electrodes.     

Electrooxidation of methanol on polycrystalline and single crystal gold electrodes

Oxidation of methanol has been investigated on polycrystalline and single crystal gold electrodes: Au(poly), Au(1 1 1) and Au(2 1 0), in acidic, neutral and alkaline solutions. As expected, catalytic activity of gold towards methanol oxidation increases with increasing pH of the solution. It has been found that in all studied solutions methanol is oxidised in two potential regions, prior to gold surface oxide monolayer formation and in more positive potentials, on gold surface oxide after so called “turn over”. Surface structure of the electrode has little influence on the oxidation current, however potentials at which oxidation is observed depends on the crystallographic orientation. The mechanism of electro-oxidation of methanol on gold is discussed.     

Electrochemical performance for the electro-oxidation of ethylene glycol on a carbon-supported platinum catalyst at intermediate temperature

To determine the kinetic performance of the electro-oxidation of a polyalcohol operating at relatively high temperatures, direct electrochemical oxidation of ethylene glycol on a carbon supported platinum catalyst (Pt/C) was investigated at intermediate temperatures (235–255 °C) using a single cell fabricated with a proton-conducting solid electrolyte, CsH₂PO₄, which has high proton conductivity (>10⁻² S cm⁻¹) in the intermediate temperature region. A high oxidation current density was observed, comparable to that for methanol electro-oxidation and also higher than that for ethanol electro-oxidation. The main products of ethylene glycol electro-oxidation were H₂, CO₂, CO and a small amount of CH₄ formation was also observed. On the other hand, the amounts of C₂ products such as acetaldehyde, acetic acid and glycolaldehyde were quite small and were lower by about two orders of magnitude than the gaseous reaction products. This clearly shows that C–C bond dissociation proceeds almost to completion at intermediate temperatures and the dissociation ratio reached a value above 95%. The present observations and kinetic analysis suggest the effective application of direct alcohol fuel cells operating at intermediate temperatures and indicate the possibility of total oxidation of alcohol fuels.     

Electro-oxidation of glycerol on platinum dispersed in polyaniline matrices

Films of polyaniline (PAni) were electrosynthesized on gold and glassy carbon substrates. The morphology of the films was verified using scanning electron microscopy (SEM) and, as expected, the PAni film formed on glassy carbon presented fibrillar morphology, while that formed on gold presented fibrils on top of a more compact structure. Different amounts of platinum were electrodeposited into the polymer matrices at constant potential and the electrocatalytic activities of the electrodes were evaluated for glycerol electro-oxidation in acidic medium. Furthermore, the active areas of such modified electrodes were determined from the charges involved in the electro-oxidation of an adsorbed carbon monoxide monolayer. Considering the real active areas, the modified electrode with the gold substrate presents higher electrocatalytic activity for glycerol oxidation than that with the glassy carbon substrate. This difference is mainly related to their morphological characteristics and platinum particle sizes.     

Electrocatalytic activities of metal electrodes in anodic oxidation of hydrazine in alkaline solution

The electrocatalytic activities of various metals and alloys in the anodic oxidation of hydrazine in alkaline solution have been studied by means of palladium membrane method in which the contact side of the membrane was electrodeposited with a thin layer of the electrocatalytic metals. The electrode materials studied can be divided into two groups. In the first group, platinum, rhodium, cobalt, cobalt—phosphor and cobalt—boron, anodic current of hydrogen oxidation on the diffusion side decreased remarkably with an increase of the electro-oxidation of hydrazine on the contact side. The anodic oxidation of hydrazine occurs through the preliminary stepwise dehydrogenation on this group metals.On the other hand, the amount of sorbed hydrogen in the palladium, gold, nickel and nickel—phosphor electrodes increased with an increase of the electro-oxidation of hydrazine on the contact side. Thus, the anodic oxidation of hydrazine on the latter group metals may proceed through the anodic formation of the intermediate radicals which readily decompose into hydrogen and the related compounds.     

Oxydation electrocatalitique du glycerol sur electrodes d'or et de platine en milieu aqueux

The electrocatalytic oxidation of glycerol has been investigated at platinum and gold electrodes, both in acid and in alkaline medium. In acid medium only platinum electrodes are electroactive, whereas in alkaline medium both electrodes, particularly gold electrodes, give relatively high current densities (about 20 mA cm^?2 for 0.1 M glycerol at 25°C).By varying the reaction parameters (electrode potential, concentrations of glycerol and of hydroxylions), it is possible to determine the transfer coefficient and the reaction orders. The fractional values obtained may be interpreted by formulating a reaction mechanism involving adsorbed intermediates.     

The electro-oxidation of dimethylamine borane: Part 2, in situ FTIR on single-crystal gold electrodes

Dimethylamine borane (DMAB) electro-oxidation in alkaline media was studied using static and rotating gold single-crystal electrodes in the hanging-meniscus configuration. DMAB oxidation showed a strong sensitivity towards surface structure on gold electrodes. In situ Fourier transform infrared (FTIR) spectroscopy revealed that the potential-dependent cleavage of the B–N bond is one of the initial steps of the oxidation process. FTIR also showed that dimethylamine is oxidised in the potential range of gold oxide formation.     

Mathematical Modeling and Experimental Studies of the Joint Electrodeposition of Gold and Silver from Sulfuric Acid Thiourea Solutions on Flow-Through 3D Electrode Taking into Account Its Nonstationary State

Mathematical models of electrochemical processes in reactors with flow-through 3D electrodes for the electrodeposition of metals from a polycomponent electrolyte solution have been presented. The parameters of the process and electrode, such as the flow rate of the electrolyte, porosity, specific surface, and the specific conductivity of the electrode and solution, at each electrode point during electrolysis have been taken into account. The mathematical models and methods for determining the electrochemical and hydrodynamic parameters of multicomponent electrochemical systems have been described. The data have been used to calculate industrial electrochemical processes in flow-through 3D electrodes. The adequacy of these methods has been shown. The experimental and calculated data on the electrodeposition of gold and silver from sulfuric acid thiourea solutions to carbon-graphite fiber cathodes have been analyzed. The dependences of electrodeposition parameters on the electrolysis conditions, the initial properties of the electrode-solution system, and the concentration ratio of gold and silver in the solution taking into account the joint reduction of hydrogen ions and molecular oxygen have been examined. The current and resulting regularities of the processes have been revealed. The adequacy of the mathematical models for industrial joint electrodeposition of gold and silver from sulfuric acid thiourea solutions on flow-through 3D electrodes taking into account the side electrode reactions has been shown. The appearance of anode zones on a cathode polarized carbon fiber flow electrode during the deposition of metals from multicomponent systems has been considered.     

Comparative EIS study of the adsorption and electro-oxidation of thiourea and tetramethylthiourea on gold electrodes

A comparative study of the electrochemical behaviour of thiourea (TU) and tetramethylthiourea (TMTU) on gold in sulphuric acid was performed using a gold rotating disc electrode. The electrochemical impedance spectra are interpreted through an equivalent circuit involving the electrolyte resistance, a constant phase element for the capacity double layer, a charge transfer resistance and, depending on the electrode potential, a second parallel impedance element whose interpretation depends on the potential region considered. Thus, for both thioureas, the Nyquist plots for E < ?0.6 V (vs. MSE) exhibit a single capacitive time constant related to the adsorption of the molecule on the electrode surface. As E increases, the Nyquist plots exhibit a new time constant assigned to the formation of a soluble complex species. This time constant appears in the potential region also related to the electro-oxidation of the thioureas to the corresponding formamidinium disulphide. This means that these processes are coupled and, therefore, only one time constant can be observed. An inductive loop at low frequencies is associated with the pitting of the gold electrode for E ≥ ?0.20 V, in agreement with SEM micrographs. The value of the corresponding charge transfer resistance decreases markedly with the electrode potential, indicating the increase in the rate of the electrochemical processes. Electrodissolution of gold results more importantly in the presence of TMTU. At potential values associated with the formation of the anodic oxide layer on gold, a negative resistance is recorded, indicating the passivation of the electrode surface. Eventually, at E > 0.6 V, the electrode surface passivation disappears and the Nyquist plots exhibit two strongly overlapped capacitive constants assigned to the oxide film growth at the monolayer level and the second electro-oxidation process of thioureas, the latter resulting in the formation of carbon dioxide and sulphate ions according to FTIRRAS data.     

Effects of tunable acidity and basicity of Nb‐KIT‐6 catalysts on ethanol conversion: Experiments and kinetic modeling

The relative amounts of acidic and basic sites in niobium‐containing ordered mesoporous silicates (Nb‐KIT‐6) are tunable with metal loading. The acidity and basicity change from 0.11 to 0.34 mmol NH₃‐equiv./g cat. and 0.17 to 0.31 mmol CO₂‐equiv./g cat., respectively, as the Nb loading is increased from 1.5 to 10.9%. This work harnesses this unique feature to better understand acidity and basicity effects on ethanol conversion activity and product selectivity. It is shown that the yields of ethylene and acetaldehyde, the dominant products, are tuned with Nb loading. Catalyst characterization results and designed kinetic experiments provide evidence of acid‐base pairs involved in the formation of various products. A macrosite kinetic model based on these observations and published mechanistic pathways fits the data for formation of the major products (ethylene, acetaldehyde, diethyl ether, and ethane) remarkably well. These results provide guidance for the rational design of bifunctional mesoporous materials exhibiting tunable acidity and basicity. © 2017 American Institute of Chemical Engineers AIChE J, 63: 2888–2899, 2017     

High catalytic activity of chemically activated gold electrodes towards electro-oxidation of methanol

Electro-oxidation of methanol has been investigated on activated, rough gold electrodes in alkaline solutions. The electrodes were activated by formation and decomposition of gold amalgam. The oxidation of methanol starts at potentials about 400 mV less positive as compared with smooth poly and single crystal gold electrodes and the oxidation current is much higher. For freshly prepared, activated gold electrodes the oxidation current is similar to that obtained on smooth platinum, however it diminishes with time. The formation of small crystallites, which could trap OH⁻ anions, seems to be the most important factor for this unusual catalytic activity. The dependence of the oxidation process on electrode topography is discussed.     

鸟嘌呤和腺嘌呤吸附和电氧化行为的电化学石英晶体微天平研究

基于压电电化学石英晶体微天平(EQCM)技术,研究了腺嘌呤(A)和鸟嘌呤(G)在裸金电极和多壁碳纳米管(MWCNTs)修饰金电极上的吸附和电氧化行为。结果表明,在金电极上,A的吸附量和氧化电流均比G大,而在MWCNTs/Au电极上,A和G可类似地吸附,但其氧化峰电位负移且氧化峰电流增大,表明MWCNTs对A和G的氧化具有催化作用。实验发现,A和G在裸金电极上氧化的电子转移数分别为5.4(RSD=±2.3%)和1.9(RSD=±1.3%),而在MWCNTs/Au电极上的氧化电子转移数分别为5.1(RSD=±1.8%)和1.5(RSD=±1.6%)。     

Electrochemical fabrication and electrocatalytic characteristics studies of gold nanopillar array electrode (AuNPE) for development of a novel electrochemical sensor

Gold nanopillar array electrodes were prepared by electrochemical deposition of gold into the nanopores of anodic aluminum oxide membrane placed onto the gold thin film electrode surface, which was in advance modified with cysteamine self-assembled monolayer as an anchoring layer. The Au nanopillar electrode is electrochemically stable and consists of highly dense, upstanding pillars assembled on the cysteamine monolayer. The structural morphology and chemical composition of the nanoarray electrode was characterized by field emission scanning electron microscopy, X-ray photoelectron spectroscopy, energy dispersive X-ray spectroscopy, and X-ray diffraction. Electrochemical measurements indicate that the Au nanopillar electrode possesses high electrocatalytic activities in the reduction of hydrogen peroxide and molecular oxygen, especially in glucose oxidation due to its higher electroactive surface area. The electro-oxidation studies of several electroactive neurotransmitters demonstrate that the nanopillar electrode can be utilized as a promising material for the construction of novel electrochemical sensor.     

Synchronized current oscillations of formic acid electro-oxidation in a microchip-based dual-electrode flow cell

We investigate the oscillatory electro-oxidation of formic acid on platinum in a microchip-based dual-electrode cell with microfluidic flow control. The main dynamical features of current oscillations on single Pt electrode that had been observed in macro-cells are reproduced in the microfabricated electrochemical cell. In dual-electrode configuration nearly in-phase synchronized current oscillations occur when the reference/counter electrodes are placed far away from the micro-electrodes. The synchronization disappears with close reference/counter electrode placements. We show that the cause for synchronization is weak albeit important, bidirectional electrical coupling between the electrodes; therefore the unidirectional mass transfer interactions are negligible. The experimental design enables the investigation of the dynamical behavior in micro-electrode arrays with well-defined control of flow of the electrolyte in a manner where the size and spacing of the electrodes can be easily varied.     

Electrocatalytic oxidation of ethanol on Sn<Subscript>(1−<Emphasis Type="Italic">x</Emphasis>)</Subscript>Ir<Subscript>(<Emphasis Type="Italic">x</Emphasis>)</Subscript>O<Subscript>2</Subscript> electrodes in acid medium

The electrochemical oxidation of ethanol at Sn_(1−x)Ir _x O₂ electrodes (with x = 0.01, 0.05, 0.1 and 0.3) was studied in 0.1 mol L⁻¹ HClO₄ solution. Electrolysis experiments were carried out and the reaction products were analyzed by Liquid Chromatography. It was found that the amounts of the reaction products depended on the composition of the electrode. In situ infrared reflectance spectroscopy measurements were performed to identify the adsorbed intermediates and to postulate a reaction mechanism for ethanol electrooxidation on these electrode materials. As evidence, acetaldehyde and acetic acid were formed through a successive reaction process. Carbon dioxide was also identified as the end product, showing that the cleavage of the carbon–carbon bond occurred. These results indicate that the synthesized catalysts are able to lead to the total combustion of organic compounds. Analysis of the water bending band at different potentials illustrated its role at the electrode interface.     

Electrocatalytic activity of surface adsorbed ruthenium-alizarin complexone toward the oxidation of benzyl alcohol

The surface electrochemical behavior of an adsorbed alizarin complexone (abbreviated as AC) and its surface coordination with Ru(II) were studied in aqueous solution at a pH range of 0-6. The surface complex of ruthenium with AC displays strong electrocatalytic activities toward benzyl alcohol. Based on the rotating disk electrode measurement, it is believed that the electrocatalytic oxidation of benzyl alcohol is a two-electron and two-proton process with benzaldehyde as a major product. On the other hand, ruthenium-AC surface complex has also shown catalytic activities toward electro-oxidation of several small organic molecules such as methanol, formic acid, formaldehyde, ethanol, and acetaldehyde.     

A comparative study of commercial and laboratory-made Ti/Ru0.3Ti0.7O2 DSA electrodes: “In situ” and “ex situ” surface characterisation and organic oxidation activity

This paper presents the study of two dimensionally stable anodes (DSA^®) of nominal composition Ti/Ru_0.3Ti_0.7O₂, one obtained commercially and the other prepared in the laboratory. The materials were studied by ex situ (scanning electron microscopy (SEM)) and by in situ techniques (cyclic voltammetry, determination of the morphology factor (φ_morph) and electrochemical impedance spectroscopy (EIS)). The results obtained indicate that both electrodes present similar morphology and voltammetric profiles, with the main difference between the two materials studied being the internal electrode area, with the commercial electrode being demonstrated to possess the greater. The EIS study demonstrates that the differences in the equivalent circuit fitting parameters can be related to the different surface characteristics of the electrodes. However, the abnormal dependence of the solution ohmic resistance on the potential demonstrates that the model should be refined to be adequately applied to commercial electrodes. A study of the activity for organic oxidation was also performed, with the aim of verifying the effect of chain length on the rate of oxidation of different aldehydes (formaldehyde, acetaldehyde, propionaldehyde and n-butyraldehyde). It can be observed that the rate of carboxylic formation can be related to the working electrode potential during electrolysis, with higher potentials (commercial electrode) seeming to favour the more rapid formation of the carboxylic acid.     

The electrochemical facetting of metal surfaces: Preferred crystallographic orientation and roughening effects in electrocatalysis

New electrochemical procedures to develop highly rough and preferentially oriented surfaces at face-centered cubic metals are described. The structure and morphology of the different surfaces have been studied through scanning electron and scanning tunneling microscopy. The mechanisms of the processes involved in the development of roughening and preferred crystallographic orientation are discussed on the basis of electrochemical data for platinum in acid electrolytes. The electrochemical behaviour of the new electrode surfaces has been tested for different reactions, such as the electro-oxidation of adsorbed carbon monoxide on preferentially oriented platinum and the adsorption and electrooxidation of both ethylene and reduced carbon dioxide on electrodispersed platinum.     

Continuous thin gold films electroless deposited on fibrous mats of polyacrylonitrile and their electrocatalytic activity towards the oxidation of methanol

Polyacrylonitrile nanofibers containing different amounts of gold nanoparticles have been prepared by electrospinning technique. By using the gold nanoparticles as seeds, thin continuous gold films have been deposited on the surface of polyacrylonitrile fibers through self-catalyzed reduction of chloroauric acid in solution. The conductivities of the fibrous mats increase with the amount of gold deposited on the fibers increase. The smooth continuous thin gold films tend to form on the fibers surface when the organic fibers contain more gold seeds, while coarse films tended to form on the fibers containing less gold seeds. The electrocatalytic activity of the fibrous mats electrodes towards the methanol oxidation in alkaline medium has been investigated, indicating that these electrodes exhibit higher electrocatalytic activity than pure gold electrode because of their three-dimensional structures. The results also indicate that the mats with smooth gold coating exhibit higher electrocatalytic activity than that with coarse gold coating.