Voltammetric response of iron hydroxide layers precipitated on platinum electrodes in alkaline solution

Voltammograms of thin and thick iron hydroxide layers chemically precipitated on platinum were obtained. The influence of the precipitation conditions of iron hydroxide was studied. Different types of combined potential/time perturbation programmes were employed to correlate the anodic and cathodic voltammetric peaks and to detect possible ageing-like effects. The electrochemical response of the iron hydroxide layer depends considerably on the layer thickness and precipitation conditions. The voltammetric behaviour of the precipitated iron hydroxide layer is interpreted in terms of the complex structure involving an inner poorly hydrated layer and an outer porous layer recently proposed from in situ ellipsometry data.     

Electrooxidation/electroreduction processes at composite iron hydroxide layers in carbonate-bicarbonate buffers

The electrooxidation/electroreduction processes at precipitated iron hydroxide layers on platinum electrodes have been studied in carbonate-bicarbonate buffers at 25°C by using electrochemical methods. The initial characteristics and properties of the hydrous iron hydroxide were changed by varying the precipitation conditions of the chemically formed active materials. Different potentialtime perturbation programs were employed to analyse the contribution of redox couples within the composite iron hydroxide layers on platinum electrodes and the corresponding electrochemical responses were compared with results obtained for massive iron electrodes in the same solutions. The complex electroreduction and electrooxidation processes are discussed on the basis of a reaction model which takes into account the incorporation of FeCO₃ in the hydrous iron hydroxide layer and its oxidation to FeOOH species, which in turn can participate in electroreduction reactions yielding Fe₃O₄, Fe(OH)₂, and soluble Fe(II) species.     

Kinetics of the electroreduction of anodically formed cadmium oxide layers in alkaline solutions

The kinetics of the potentiostatic electroreduction of cadmium oxide layers on polycrystalline cadmium electrodes have been investigated in 0.01–1.0m NaOH solutions. The study was undertaken to determine the influence of passive film formation conditions on the electroreduction process of the anodically produced hydrous cadmium hydroxide-oxide layers. By properly adjusting the electroreduction conditions the cathodic potentiostatic current transients can be satisfactorily described by a nucleation and growth mechanism involving the participation of soluble Cd(II) and passivating anodic species. Experimental data were analysed by the application of non linear least squares fit routines. From the parametric identification procedure coherent potential dependencies of the corresponding fitting parameters as well as reasonable values of the physicochemical constants included in the reaction model, have been obtained. In this way it is possible to correlate kinetic data of the electroreduction process of the anodic oxide layers to make a discrimination between the different mechanistic contributions to this complex reaction involving several steps.     

Nickel hydroxide film electroformation in alkaline chloride solutions

The electroformation of thick hydrous nickel hydroxide films on polycrystalline nickel electrodes in alkaline chloride solutions has been investigated. The films were obtained through the application of repetitive square-wave potential signals (RSWPS). A significant increase in the charge storage capacity of this kind of electrode with respect to those obtained by the same method in chloride-free solutions was found. It has been concluded that the halide ions do not participate directly in the metallic dissolution process which takes place at the metal-hydroxide interface, but they affect some superficial phenomena at the hydroxide-solution interface during the RSWPS treatment.     

Electrocatalytic oxidation of hydrazine on platinum electrodes in alkaline solutions

The mechanism and structure sensitivity of the electrocatalytic oxidation of hydrazine on platinum in alkaline solutions were investigated using cyclic voltammetry, steady-state current measurements, and on-line electrochemical mass spectrometry. The voltammetry of hydrazine oxidation on platinum in alkaline media is characterized by a single diffusion-controlled wave. The on-line electrochemical mass spectrometry measurements of hydrazine oxidation on Pt(1 1 1), Pt(1 0 0), and Pt(1 1 0) surfaces indicated the formation of molecular nitrogen. No oxygen-containing nitrogen compounds were detected under the given experimental conditions. A comparative analysis of voltammetric data for hydrazine oxidation in alkaline solutions on the three surfaces at low overpotentials points to a structure sensitivity of the reaction. The electrocatalytic activity of basal planes increases in the order Pt(1 1 0) > Pt(1 0 0) > Pt(1 1 1), as deduced from the onset of the oxidation wave. The structure of the electrocatalyst surface affects the mechanism of the reaction, although without affecting the selectivity. At low overpotentials the hydrazine oxidation on the Pt(1 1 0) and Pt(1 1 1) surfaces is limited by the rate of electrochemical steps, whereas on Pt(1 0 0) a chemical step that probably involves N₂H₂ adsorbed intermediate is the rate-determining step. Platinum is more active in hydrazine oxidation in alkaline solution than in acidic solutions. In contrast to hydrazine oxidation on platinum in acidic media, the stabilization of chemisorbed hydrazine does not occur to a significant extent in alkaline media.     

The voltammetric detection of intermediate electrochemical processes related to iron in alkaline aqueous solutions

The application of the triangularly modulated triangular potential sweep technique (TMTPS) to the Fe/alkaline solution interface yields information about the various reactions involving Fe(II) and Fe(III) film-forming species. Qualitative conclusions are derived about the chemical reactions of both Fe(OH)₂ and FeOOH with regard to the corresponding ageing processes of these species. The ageing processes appear to be coupled to the overall electrochemical reaction through a composite reaction pattern which, in its simplest form, approaches the square reaction pathway postulated earlier to explain the electrochemical behaviour of the Ni/alkaline solution interface.     

XPS investigation of surface oxidation layers on a platinum electrode in alkaline solution

A thick oxidation layer on a platinum electrode has been grown in 1 N NaOH at 3 V vs Ag/AgCl reference electrode. After transferring the Pt electrode into an ultrahigh vacuum chamber the surface layer was analysed by X-ray photoelectron spectroscopy. Pt4f_5/2 amd O1s electron binding energies of 74.3, 77.6 and 530.9 eV respectively, as well as the broad peak shape of the O1s signal and the oxygen-to-platinum intensity ratio of 3.08 point towards a platinum—oxyhydroxide PtO(OH)₂. This formula is in good agreement with cyclic voltammetry curves, measured for the same electrode, that revealed two cathodic reduction peaks for oxygen surface coverages equivalent to more than two hydrogen monolayers. These two peaks were assigned to PtOH and PtO. XPS analysis at elevated temperatures showed that the thick (5 nm) oxidation layer decomposes at 400 K to a mixture of several oxides and hydroxides of Pt⁴⁺ and Pt²⁺ and Pt metal with a ratio of O-to-Pt of 1. This mixture further gradually decomposes to only a monolayer of oxygen at 770–870 K. Sodium cations were found to be present in trace amounts in the adlayer and to strongly shift the O1s binding energy to lower values.     

The anodic evolution of oxygen on platinum oxide electrode in alkaline solutions

The anodic evolution of oxygen was investigated on the platinum oxide electrode, prepared by a thermal decomposition method, in alkaline solutions; the overvoltage data were reproducible on this electrode. On the basis of reliable mechanistic observations, the most probable path under Langmuir conditions was proposed, which is the same as that suggested by Krasil'shchikov for a nickel electrode in alkaline solution. The rate-determining step changes from the 2nd to the 1st step, depending on the overpotentials. This was also supported by the corresponding change in the values of activation energy. The similarity of this platinum oxide to oxide films produced by prolonged anodization of platinum in the anodic polarization characteristics was suggested.     

Electrochemical reduction of thick oxide film on platinum electrode in alkaline solutions

The electrochemical reduction of the thick oxide film formed on Pt electrode by severe preanodization has been studied in LiOH, NaOH and KOH solutions of different concentrations (0.001 ～ 1.0 M) using a galvanostatic technique.An outermost monolayer oxide and an inner multilayer oxide of the thick oxide film exhibit different potential behaviors in the cathodic reduction. In dilute solution, both the oxides are completely reduced in a potential range of 0.6-0.4 V (vs rhe in the test solution) in a single step. As the concentration is increased, however, the reduction potential of the inner oxide only shifts rapidly into a H-electrosorption potential region and the amount of the oxide reduced at this potential decreases. The remaining oxide is slowly reduced at H₂-evolution potential. The retardation of the reduction of the inner oxide is related to cations adsorbed on Pt electrode. This retardation effect increases in the order, K⁺ < Na⁺ < Li⁺.     

The potentiodynamic behaviour of iron in alkaline solutions

The potentiodynamic behaviour of iron in alkaline solutions under carefully controlled perturbation conditions reveals that the overall electrochemical process is more involved than was thought earlier. The electrochemical characteristics of the systems are explained through a series of successive conjugated redox couples principally involving Fe(OH), Fe(OH)₂ and FeOOH as limiting stoichiometric species. The yield of soluble species such as either FeO^2?₂ or HFeO^?₂ increases with the pH. Ageing effects of reactants and products are also distinguished through the potentiodynamic E/I records.     

Electrochemical behaviour of iron in alkaline sulphide solutions

Reactions of the SH^? anion on iron electrodes in deoxygenate alkaline solutions were studied using constant potential, constant current and acyclic voltammetric techniques.In the absence of sulphide, a dual oxide layer is formed. When sulphide is present, oxide growth is inhibited. At sufficiently anodic potentials the oxide layer is attacked, leading to a breakdown of passivity and the formation of sodium ferric sulphide, identified by its characteristic visible spectrum. Oxidation of the sulphide ion leads to repassivation of the electrode by a deposit of elemental sulphur. The effectiveness of this repassivation is dependent on the sulphide concentration since excess sulphide dissolves the passivating sulphur deposit to form polysulphides. Polysulphides were identified by colour and by their uv — visible spectrum.The voltammetric data were analysed according to the theory for film formation under Ohmic control. Agreement with this theory suggests that sulphide attacks the oxide in a similar manner to Cl^? (ie pitting attack) and that the repassivation is due to the local plugging of these pits with electrochemically deposited sulphur.     

Electrochemical behaviour of iron in alkaline sulphate solutions

The electrochemical behaviour of pure iron in alkaline sulphate solutions was studied using cyclic voltammetry atT = 298 K. It has been found that the results depend on the polarization pre-treatment of the electrode and the activation state of its surface. At starting potentials in the range of hydrogen evolution and at maximum activation conditions, the voltammograms show three anodic current maxima and two cathodic ones. A correlation between these different maxima is given. The electrode processes, which may take place at these maxima, are discussed in terms of possible iron dissolution mechanisms.     

The anodic dissolution process on active iron in alkaline solutions

Steady state anodic polarization curves were taken for Armco iron and in some experiments for high purity Puratronic iron in KOH solutions in the concentration range 5 x 10^?2-5M. After the 30 min cathodic pretreatment, well reproducible anodic Tafel plots are obtained. The experimentally obtained diagnostic criteria b_a = 67–70 mV dec^?1, n_OH? = 1.1 and nHFeO₂^?= ?0.45 are interpreted by the anodic reaction mechanism in which FeOH_adsand Fe(OH)_2,ads appear as the intermediates adsorbed under Temkin conditions, the primary stable product of the electrode reaction being HFeO₂^?, and the final product Fe (OH)₂, formed by precipitation.     

Anodic oxidation of copper in alkaline solutions 2—the open-circuit potential behavior of electrochemically formed cupric hydroxide films

Copper electrodes covered with Cu(OH)₂ films grown in 1 mol dm⁻³ LiOH solution have been examined using X-ray diffractometry, scanning electron microscopy and electrochemical techniques. A sharp change in potential on open-circuit, due to a change in phase from Cu(OH)₂ to Cu₂O, was observed. This phase change is accompanied by a substantial amount of film dissolution. The time required for transformation, and the final amount of Cu₂O formed, are functions of both hydroxide ion concentration and electrode rotation speed. A mechanism involving Cu(OH)₂ dissolution, disproportionation (Cu²⁺+Cu→2Cu⁺) and Cu₂O precipitation, is proposed to explain these observations.     

Electrochemical study of indoor atmospheric corrosion layers formed on ancient iron artefacts

Several indoor atmospheric corrosion layers (0-800 years old) were selected from different localisations in France. Each sample was scrapped from its iron substrate. The resulting powder was mixed with graphite in appropriate proportions and the mixture was pressed onto a stainless steel grid to constitute a composite electrode. The electrochemical responses of the different samples were recorded under galvanostatic regulation, in a near-neutral pH-buffered NaCl solution at 25 °C. The E-t reduction curves allowed the determination of two characteristic parameters, E_τ/2, the potential value obtained at half the transition time, and Q_τ, the coulombic charge obtained at the end of the reduction. The diminution of E_τ/2 and Q_τ with the age of the corrosion layer showed that the “reduction reactivity” decreases with time, suggesting a progressive stabilisation of the corrosion layer.In a second part of the work, we synthesised several common ferric or ferrous/ferric products (goethite, lepidocrocite, magnetite, maghemite, ferrihydrite) and compared their reduction responses (product alone or mixture of 2 or 3 products) to those of corrosion samples.     

Electrocatalytic oxidation of organic molecules in alkaline solutions—I. Oxidation of 1,3 dioxolane at platinum

The electroadsorption and electrooxidation of 1,3 dioxolane at platinum in 1 M KOH solutions were investigated under a wide range of experimental conditions. The potential of maximum adsorption (E^max_ad) is ca. 0.4 V and the degree of surface coverage depends linearly with the logarithm of organic concentration as predicted by a Temkin type adsorption isotherm. The kinetics of the electroadsorption process follows a Roginskii-Zel'dovich equation. From (eps)_ad, (eps)_ox and analysis of oxidation products a mechanism is postulated which consists of an adsorption with simultaneous dehydrogenation step followed by cycle rupture and electroadsorption and dehydrogenation of further residues.     

About the mechanism of formation of iron hydroxide fouling layers on reverse osmosis membranes

The rate of fouling by deposition of iron hydroxide on tubular reverse osmosis membranes was studied under closely controlled experimental conditions resembling those of brackish water desalination. Feed brines of approximately 5,000 ppm NaCl were used to which small amounts of iron hydroxide were added. The effect of brine flow rate, product water flux, pH and oil contamination on the rate of fouling layer buildup was investigated. Results obtained suggested that the rate of membrane fouling due to iron hydroxide buildup is governed by two processes: nucleation and growth.