Films by anodic oxidation of mercaptoanilines AT Cu electrodes

The anodic oxidation of mercaptoanilines at different metal anodes proves that coherent coatings may be formed in situ on copper only, using the meta isomer. These coatings probably from through the oxidation of an intermediate Cu-3-mercaptoaniline complex, which leaves high copper content in the polymer deposit: however, such materials impart outstanding corrosion protection to the substrate. It is suggested that, after easy dimerization of the monometer to disulfide, C—S coupling reactions play a basic role in the constitution of the polymeric network.     

Electroorganic reactions. Part 57.† DDQ mediated anodic oxidation of 2-methyl- and 2-benzylnaphthalenes

Electron-rich 2-methyl and 2-benzylnaphthalenes are efficiently converted into the corresponding carboxaldehyde or ketone by mediated electrolysis in the presence of about 25 mol % of DDQ. The reaction conditions are simple (constant current, undivided cell of 500 ml volume, graphite electrodes, aqueous acetic acid at 80 °C) and the construction of a reaction profile (products vs charge) shows the intermediate formation of side-chain acetoxy and hydroxy derivatives and the parallel formation of a byproduct involving side-chain substitution by DDQ.     

On the anodic oxidation of cyclohexene on platinized platinum electrodes

Cyclic voltammetry experiments performed in acid solutions of cyclohexene on platinized platinum anodes show that platinum oxide must play a role in the anodic oxidation of the hydrocarbon through a mechanistic scheme of the type proposed by Gilman[10]. This fact would explain results obtained with other unsaturated hydrocarbons     

Carbon electrodes: Part 1. Hydrogen evolution in acidic solution

The hydrogen evolution reaction has been studied using electrodes of vitreous carbon and graphite. The relative activity of the surfaces in promoting the hydrogen evolution reaction was found to be pyrolytic edge vitreous carbon > pyrolytic face. Comparative measurements of the double-layer capacitances indicate that relative activity cannot be entirely explained in terms of relative surface areas. The activity of the surfaces was found to be very sensitive to electrode pretreatment. Kinetic parameters are presented for the hydrogen evolution reaction on these carbons.     

Mechanism of anodic oxidation of chlorate to perchlorate on platinum electrodes

The kinetics and mechanism of the anodic oxidation of chlorate to perchlorate on platinum electrodes have been investigated. The current efficiency for perchlorate formation and the electrode potential have been determined as a function of current density for various solution compositions, flow rates and pHs at 50°C. The results have been compared with theoretical relations between the ratio of the current efficiencies for perchlorate and oxygen formation, the electrode potential, the concentration of chlorate at the electrode surface and the current density for various possible mechanisms. It is concluded that the formation of an adsorbed hydroxyl radical is the first step in the overall electrode reaction. The mechanism proposed for the C10₄ perchlorate and oxygen formation is:

Adsorption and anodic oxidation of methanol on iridium and rhodium electrodes

Adsorption and electro-oxidation of methanol on smooth iridium and rhodium electrodes have been studied. The regularities obtained were compared with the results of previous measurements on smooth platinum. The adsorption of methanol on iridium has been established as characterized by regularities peculiar to a surface with an exponentially distributed inhomogenity of adsorption sites (Freundlich isotherm, linear change of the activation energy of adsorption with logarithm of surface coverage). The adsorption regularities for rhodium are more complex. The character of the isotherms on iridium and rhodium, as well as on platinum, does not depend on the nature of adsorbed neutral particles (methanol, hydrogen etc) and is apparently determined by the electrode surface properties. As follows from kinetic regularities (influence of potential, concentration and pH of solution, surface coverage) the rate-determining step of steady-state methanol electro-oxidation on iridium and rhodium is the oxidation of carbonaceous chemisorbed particles by adsorbed OH radicals.     

The anodic oxidation of neodymium

The anodic oxidation of neodymium was investigated by the galvanostatic technique in Na₂-HPO₄ - ethylene glycol solution. According to the combination of measurments of ac capacity and of specular reflectivity, we obtained a refractive index for the oxide films as a function of wavelength and the value of apparent dielectric constant of the films for which sesquioxide Nd₂O₃ was assumed to have been produced. It was found that the film thickness could be determined from the wavelength of both maximum and minimum specular reflectivities due to interference, and that a linear relationship existed between the film thickness and the formation voltage. The slope of this linear line was 10.0Å V^?1 or a mean field of 1.00 × 10⁷ V cm^?1 at 1 mA cm^?2. The apparent current efficiency estimated from the thickness of the film vs passed charge plots scarcely depended on latter and its value was about 0.52, assuming that the film composition was Nd₂O₃.     

The anodic oxidation of zirconium—I. growth stresses in anodic ZrO2 films

Fluoride ion used either in the surface preparation or in the subsequent anodising of zirconium has a profound effect upon the stresses in the zirconium oxide anodic layer. By a suitable choice of surface preparation, anodising electrolyte, and growth rate, anodic oxide films can be grown on the surface of zirconium having tensile, compressive, or effectively zero stress.     

The anodic behaviour of porous zinc electrodes

The passivation of horizontal and vertical porous zinc (unamalgamated) electrodes in aqueous KOH solutions has been studied. The results are interpreted in terms of the formation of a soluble anodic product which decomposes within the electrode forming an insoluble deposit. The ohmic resistance of the insoluble anodic product provides the major component of the electrode polarization and determines the useful duration of discharge of the electrode.     

The anodic oxidation of zirconium—II. Growth and morphology of anodic ZrO2 films

Measurements of the rate of growth of anodic barrier films, the stresses in these films, and the relative film thicknesses after anodising in electrolytes which may contain fluoride ions have been interpreted as showing that the incorporation of F^? ions into the oxide leads to a greater flux of zirconium ions which in turn may lead to tensile stresses in the oxide. These stresses can cause oxide fracture and the development of corrosion products.The incorporation of F^? ions into the oxide depends upon the F^? ion concentration in the electrolyte and the surface pH of the electrode. This latter is in turn dependent upon the bulk pH of the electrolyte and the applied current density. At high current densities the surface pH shifts to smaller values which leads to enhanced pick-up of F^? ions; at low current densities higher surface pH's can prevent the pick-up of fluoride ions.     

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.     

Consideration of electrodes and electrolytes for electrochemical gasification of coal by anodic oxidation

It has been found that the performance of graphite anodes in coal electrolysis is closely comparable to that reported previously for anodes of Pt gauze; the sole exception discerned thus far is that the gases formed anodically at graphite are somewhat enriched in carbon monoxide and correspondingly depleted in carbon dioxide as compared to Pt anodes. No significant consumption or degradation of the graphite was observed during short experiments in which about half an equivalent of charge passed — the weight gain measured was about 0.05% of that computed from the assumption of formation of an equivalent quantity of surface oxides. Of several different electrolytes considered and tested, aqueous solutions of H₂SO₄ appear to cause the most rapid reaction rates.     

Electrosynthesis at oxide coated electrodes Part 2. The oxidation of alcohols and amines at spinel anodes in aqueous base

The oxidation of alcohols and primary amines at a NiCo₂O₄ anode in aqueous base has been investigated. It is shown that a range of compounds undergo oxidation at the same potential, that for the conversion of the oxide surface to a higher oxidation state, and that the limiting current densities approach those expected for mass transfer control. Using a flow cell, it is confirmed that the conversation of primary alcohols to carboxylic acids, secondary alcohols to ketones and primary amines to nitriles can all be achieved with high selectivity and reasonable current efficiency at high current densities. The oxide coating appears to be quite stable during repeated electrolyses.     

The anodic oxidation of superimposed metallic layers: theory

The anodic oxidation of one metal superimposed upon another is governed by several factors, the most important being the resistivity difference between the two oxides. When the oxide of the superimposed metal is the less resistive, the metal order is conserved, the oxygen order is conserved, and the final voltage under constant current conditions is just the sum of the component potentials across the two oxide layers. When the superimposed oxide is the more resistive, however, fingers of the substrate oxide force their way through it after the manner of the Rayleigh-Taylor effect in superimposed liquid layers; the metal order is then partially inverted, the oxygen order likewise, and the final voltage is less than the sum of the components. These phenomena, however, will be modified by the effects of transport number, relative metal migration rate, oxide structure and Pilling-Bedworth ratio; the nature and possible interactions of these factors are discussed. Experimental evidence on the anodization of superimposed layers is then reviewed, and all observations can be accounted for.     

Electrochemical oxidation of several chlorophenols on diamond electrodes Part I. Reaction mechanism

The electrochemical oxidation of 4-chlorophenol, 2,4-dichlorophenol and 2,4,6-trichlorophenol aqueous wastes using boron-doped diamond electrodes was studied. This treatment led to complete mineralization of the wastes regardless of the operating conditions. A simple mechanistic model is consistent with the voltammetric and electrolysis results. According to this model, the electrochemical treatment of chlorophenol aqueous wastes involves the anodic and cathodic release of chlorine followed by the formation of non-chlorinated aromatic intermediates. Subsequent cleavage of the aromatic ring gives rise to non-chlorinated carboxylic acids. Chlorine atoms arising from the hydrodehalogenation of the chlorophenols are converted into more oxidized molecules at the anode. These molecules react with unsaturated C₄ carboxylic acid to finally yield trichloroacetic acid through a haloform reaction. The non-chlorinated organic acids are ultimately oxidized to carbon dioxide and the trichloroacetic acid into carbon dioxide and volatile organo-chlorinated molecules. Both direct and mediated electrochemical oxidation processes are involved in the electrochemical treatment of chlorophenols.     

The mechanism of anodic oxidation of alloys

The mechanism of formation of barrier-type anodic films on alloys containing valve and non-valve metals is considered in terms of data yielded by electrochemical methods, a.c. impedance measurements, conventional and scanning electron microscopy and chemical and electron probe microanalysis. The alloying elements enter the film virtually in their alloy proportions. With alloys giving oxides essentially insoluble in the electrolytes, such as Nb-Zr, the metal constituent concentration profiles across the film are determined largely by the cationic and anionic transport numbers and the cationic mobilities. Where one of the alloying elements tends to give soluble compounds, such as vanadium in V-Nb alloys, this situation is complicated by preferential dissolution of that element. The film properties depend on the solution used but the main result, that relatively small additions of valve metals to metals such as vanadium permit thick barrier-type films to be formed, is unaffected. All the as-formed films are initially pore-free, but those produced on alloys rich in non-valve metal are prone to subsequent leaching, with detectable pore formation. Analysis proves that the films on V-Nb alloys are enriched in valve metal, particularly towards the outer surface. This explains why these films can be formed and also why the impedance characteristics and resistance to leaching improve as the film thickens, with the material rich in valve metal developing into an outer, more protective and possibly thicker sheath. The presence of a mass-transfer boundary layer, rich in nonvalve metal dissolution products, also promotes film formation. Films formed in certain nonaqueous electrolytes are richer in the non-valve metal and so are more readily leached in water subsequently, but apparently are also chemically different. Special experiments, involving leaching and reanodizing in various solutions, and the formation of duplex films, provide important data, appearing to indicate substantial anionic movement during film growth.     

Wet-air oxidation of sewage sludge. Part II: The oxidation of real sludges

Experiments on the oxidation of real sludges of different origins and compositions were carried out which lead to a mathematical model which described the results obtained. On basis of this model large scale reactors can be calculated. The conversion of the Chicago installation is predicted to be 81% under conditions where the really obtained conversion is reported to be 80%.     

The anodic oxidation of allyl alcohol in acetonitrile

The kinetics of the anodic oxidation of allyl alcohol in acetonitrile has been studied. At a platinum electrode at low potentials, < 2·6 V, the reaction is charge transfer controlled and at higher potentials the process becomes diffusion controlled. The reaction is not a simple charge-transfer process, but may proceed via an adsorption step. Controlled potential electrolysis yielded acrolein and polymers as products. The oxidation of allyl alcohol on platinum is probably electrocatalysed by “surface oxides”.     

Kinetics and mechanism of anodic oxidation of chlorate ion to perchlorate ion on lead dioxide electrodes

The kinetics and mechanism of anodic oxidation of chlorate ion to perchlorate ion on titanium-substrate lead dioxide electrodes have been investigated experimentally and theoretically. It has been demonstrated that the ionic strength of the solution has a marked effect on the rate of perchlorate formation, whereas the pH of the solution does not influence the reaction rate. Experimental data have also been obtained on the dependence of the reaction rate on the concentration of chlorate ion in the solution at constant ionic strength. With these data, diagnostic kinetic criteria have been deduced and compared with corresponding quantities predicted for various possible mechanisms including double layer effects on electrode kinetics. It has thus been shown that the most probable mechanisms for anodic chlorate oxidation on lead dioxide anodes involve the discharge of a water molecule in a one-electron transfer step to give an adsorbed hydroxyl radical as the rate-determining step for the overall reaction.Nomenclature anodic energy transfer coefficient - ₂ potential of outer Helmholtz plane with respect to solution - _M potential of metal with respect to solution - dielectric constant of solution - ₂ permittivity of free space - faradaic efficiency for anodic chlorate oxidation - A adsorbed intermediate in Reaction 2 - B bulk species in Reaction 2 - c _A concentration of A at outer Helmholtz plane - c _B concentration of B at outer Helmholtz plane - c _B ⁰ concentration of B in bulk - c _ClO3 ^– /0 concentration of ClO ₃ ^– in bulk - c _ClO4 ^– /0 concentration of ClO ₄ ^– in bulk - E electrode potential corrected for ohmic drop - E _a electrode potential as measured against reference electrode - E _s ⁰ standard electrode potential of Reaction 2 - E _z potential of zero charge of the anode in test solution - F Faraday constant - f F/(RT) - I _t current at anode - I _OER current used for oxygen evolution reaction at anode - I current used for chlorate oxidation (=I _t–I _OER) at anode - i _t I _t/anode area - i _OER I _OER/anode area - i I/anode area - J total concentration of (uni-univalent) electrolytes in solution - K ₂ integral capacitance of compact part of double layer - K _s standard rate constant for Reaction 2, corrected for double layer effects - n _s number of electrons involved in Reaction 2 - p ln(–i)/lnc _ClO3 ^– /0 - q _M charge density on metal surface - Q ₁ quantity of electricity passed in given time interval - Q _OER quantity of electricity required for oxygen evolution reaction in given time interval - R ohmic resistance between anode and Luggin tip - R gas constant - r ln(–i)/lnJ - s ln(–i)/ pH - T absolute temperature - t ln(–i)/E - u (₂ RT/2)^1/2 - V volume of gases evolved in given time interval - V _H volume of hydrogen evolved in given time interval - Z _B charge on species B     

The anodic oxidation of nickel in alkaline solution

The anodic oxidation of nickel in alkaline solution was studied by cyclovoltammetric and optical techniques. The range of the scanning potential effects the resulting voltammograms. A constant I-E diagram with anodic peaks at 130 and 270 mV (at scan rate 10 mV · s^?1) is obtained after multiple scanning from ?800 to + 1200mV. The layer of Ni(OH)₂ that is formed in the anodic cycle, is only partially reduced by cathodic polarisation. Growth of the Ni(OH)₂ film on Ni occurs by repeated oxidation and reduction. This occurs via oxidation of Ni to α Ni(OH)₂ and conversion of α Ni(OH)₂ to β Ni(OH)₂.