Poisoning of platinized platinum electrodes by hydrogen in dimethylsulphoxide

A study of hydrogen sorption on platinized platinum in dimethylsulphoxide solutions has shown that there is a heterogeneous reaction between hydrogen and dimethylsulphoxide to form a tightly bound film of reduction products, probably dimethylsulphide.     

Radiotracer study of the adsorption of pimelic acid at a platinized platinum electrode

The adsorption of pimelic acid has been studied in 1 mol dm⁻³ HClO₄ supporting electrolyte. The potential dependence of the adsorption, the mobility of the adsorbed species and the effect of other saturated aliphatic acids (acetic, malonic, succinic and adipic acids) and Cl⁻ ions on the adsorption were investigated. A reversible adsorption was found in the potential range from 400 to 800 mV. At lower potentials (100-400 mV) a slow chemisorption takes place resulting in strongly chemisorbed species which can be eliminated by reduction and oxidation. The behaviour of loosely adsorbed species involved in the reversible process is similar to that observed for other saturated acids. A sequence for the relative adsorbability of different dicarboxylic acids was established on the basis of the experimental results. Factors influencing the adsorbability were discussed.     

The influence of carbon monoxide on the voltammetric behaviour of nickel in acid solutions

The voltammetric behaviour of nickel in acid solutions is considerably modified by the presence of CO. As in plain acid the voltammogram also exhibits two main anodic peaks, but in this case, the adsorption of CO on the nickel surface shifts the metal electrodissolution to more positive potentials, and increases the contribution of the second electrooxidation current peak at the expense of the first one. The latter is the main peak in CO-free acids. The presence of CO adsorbates also decreases the charge of the Ni(OH)₂/NiOOH redox couple in alkaline solutions. The potential shift of the nickel electrooxidation reaction in acids follows the trend which is expected from the energy difference between CO and H₂O adsorption on nickel in the gas phase.     

Cyclic voltammetric behaviour and some surface characteristics of the lead electrode in aqueous HCL solutions

The positive linear sweep voltammograms of lead were investigated in 0.01–2.00 M aqueous HCl solutions. The results revealed the occurrence of one anodic current peak as the result of formation of a thin PbCl₂ surface layer. Intensive study of the electrochemical behaviour of the lead electrode in 1.00 M HCl solution was carried out by cyclic voltammetry. The results indicated that the formation of PbCl₂ is a reversible process controlled by diffusion of Cl^? ions to the electrode surface. Controlling the voltammetric conditions enabled the building up of a relatively thick and porous PbCl₂ layer that could resist rupture or breakdown. In addition, the electrode surface products were examined by scanning electron microscopy and X-ray diffractometry. The morphology of the as-formed PbCl₂ layers showed a primitive dependence on the voltammetric conditions. X-ray diffraction analysis proved that only PbCl₂ was formed on the electrode surface and no other products could be detected.     

Electrochemical behaviour of the chloride/chlorine system at platinum electrodes in acetonitrile solutions

The electrochemical behaviour of chlorine and chloride solutions in acetonitrile at platinum electrodes has been studied. The rotating disk electrode technique was applied in a temperature range of −6 to 30 ± 0·1°C. The over-all reaction Cl₂ + 2 e⁻ ? 2 Cl⁻ undergoes a two-electron exchange per mol of chlorine and only one anodic and one cathodic wave are found.     

The electrochemical reduction of ruthenium(IV) in perchloric acid solutions on platinum electrodes

Two reversible waves are observed for the reduction of Ru(IV) on platinum electrodes. Each wave is the sum of two one-electron steps according to:

Potentials (vs nhe) and species are given for 1.0 M HClO₄ solutions; more hydrolysed products are formed in less acid solutions. The tetrameric Ru(III) ion slowly decomposes to a more stable Ru(III) species, probably a dimeric ion.     

Comparative voltammetric study of methanol oxidation and adsorption on noble metal electrodes in perchloric acid solutions

Periodic current/potential curves were measured potentiostatically on smooth electrodes of platinum, iridium, rhodium, and gold at 30 mV/s in perchloric acid solutions with different additions of methanol (10⁻³ M to 1 M). Information on methanol adsorption was obtained from impedance measurements at 1000 c/s by volcammetry with superimposed a.c. voltage. The current/potential curves look similar on Pt and Pd on the one hand, and on Ir and Rh on the other hand. There is no noticeable methanol oxidation on gold. The peak current of the first wave during the anodic sweep decreases considerably in the order Pt> Pd> Rh> Ir. The capacitive component of the impedance shows methanol adsorption in the hydrogen and double layer region. The shape of the current/potential curves and the methanol adsorption are strongly influenced on platinum metals by oxygen layers which are formed or reduced electrochemically during the sweeps in different potential ranges.     

Electrochemical and photoelectrochemical behaviour of passivated Ti and Nb electrodes in nitric acid solutions

Ti and Nb electrodes were passivated galvanostatically in nitric acid solutions. The electrochemical and photoelectrochemical behaviour of these electrodes was studied and compared with the behaviour of similar electrodes passivated in sulphuric acid solutions under the same conditions. The flatband potential and donor concentration of the passive films were obtained from the corresponding Mott-Schottky plots. The photoelectrochemical investigations were carried out using a pulsed laser which rendered measurements with photon energies smaller than the band gap. The results showed that the behaviour of passivated titanium in nitric acid was differing from its behaviour in sulphuric acid. The behaviour of the passivated niobium was not much affected by the exchange of the two acids.     

The behaviour of high polar organic solvents on platinum electrodes—I. The study of adsorption and electrode reactions of dimethylsulphoxide

The adsorption of dimethylsulphoxide and reactions of adsorbed molecules at platinized platinum electrode were studied by potentiodynamic and radiometric techniques. The adsorption of dimethylsulphoxide is described by Temkin type isotherm. The adsorbed dimethylsulphoxide is reduced in two steps to dimethylsulphide in the hydrogen range of potentials and oxidized to dimethylsulphone above 0.6 V. The latter product is desorbed from the surface of the electrode.     

Kinetics and mechanism of the oxygen electroreduction reaction on faceted platinum electrodes in trifluoromethanesulfonic acid solutions

The kinetics of the oxygen electroreduction reaction (OERR) were investigated on (1 1 1)-type and (1 0 0)-type faceted, and polycrystalline platinum electrodes in aqueous (0.05–1.0)m trifluoromethanesulfonic acid (TFMSA) using the rotating disc and ring-disc electrode techniques at 25°C. Reaction orders with respect to oxygen close to either 1/2 or 1 were found, depending on the TFMSA concentration and platinum surface morphology. At all TFMSA concentrations the formation of H₂O₂ was enhanced at (1 0 0)-type platinum surfaces. The difference in the electrocatalytic activity of platinum surfaces can be explained through data derived from the OERR formalism proposed by Damjanovicet al. The rate of the direct O₂ to H₂O electroreduction reaction increased steadily with the cathodic overvoltage irrespective of the platinum surface morphology, whereas a maximum H₂O₂ formation rate was found at about 0.5 V, depending on the TFMSA concentration. The H₂O₂ decomposition rate on (1 0 0)-type platinum electrode yielding H₂O approached zero within a certain potential range.     

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.     

Triangular potential sweep voltammetric study of porous iron electrodes in alkali solutions

The cyclic voltammograms of pure iron and sintered iron electrodes in 6.0m KOH solutions revealed a plateau and two anodic peaks in the forward direction and two cathodic peaks in the backward direction when polarized from –1.3 to –0.3 V vs Hg/HgO. In the forward scan the formation of Fe(OH)₂ and FeOOH occurs and these are subsequently reduced to Fe(OH)₂ and iron in the backward scan. The peak potential separation of the Fe/Fe(II) and Fe(II)/Fe(III) couples at zero sweep rate and the ratio of cathodic to anodic charges at zero sweep rates for the above two redox couples have been used to evaluate the reversibility of porous iron electrodes. Additions of LiOH, Na₂S, FeS, sulphur, Sb₂O₃ and As₂O₃ on the reversibility of these redox couples have been discussed. A suitable electrode fabrication condition has been suggested.     

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.     

Potentiodynamic behaviour of graphite and platinum electrodes in sodium nitrite-potassium nitrite melts

The potentiodynamic behaviour of graphite and platinum electrodes was studied in alkaline nitrite melts. No appreciable corrosion of graphite was observed, due to the apparently low concentration of highly oxidising species. Rate processes such as NO₂^? ion oxidation. NO₂ reduction and OH^? ion oxidation were studied and interpreted with formerly discussed reaction pathways.     

The electrochemical behaviour of polycrystalline nickel electrodes in different carbonate-bicarbonate ion-containing solutions

The dissolution and passivation of polycrystalline nickel in carbonate-bicarbonate ion-containing solutions covering wide ranges of pH and electrolyte concentration were investigated by employing voltammetric, galvanostatic and potentiostatic transient techniques. Results obtained with a rotating disc electrode allow the competing reactions related to the active-passive transition to be distinguished through the influence of the potential sweep rate and the rotation speed on the electrochemical behaviour of the system at fixed concentrations of either carbonate or bicarbonate ion. The first oxidation level of nickel corresponds mainly to Ni(OH)₂ formation, the chemical dissolution of the surface layer and the precipitation of NiCO₃ and Ni(OH)₂. The partial removal of the prepassive layer is predominantly assisted by both the bicarbonate ion concentration and the electrode rotation. In the presence of chloride ions the formation of soluble Ni(II) species and NiCo₃ in the potential range of the first oxidation level appears to be enhanced. This effect can be interpreted by taking into account competitive adsorption processes at the base metal between Cl^– and OH^– ions.     

Potentiodynamic behaviour of graphite and platinum electrodes in molten sodium nitrate-potassium nitrate eutectic

The electrochemical behaviour of graphite and platinum electrodes has been studied in molten alkaline nitrates using cyclic voltammetry. The voltammograms obtained with graphite exhibit two current peaks not observed on platinum under similar conditions. An electrochemical reaction involving O₂^? ion and NO has been shown to proceed via an oxide on the graphite electrode surface which can be electrochemically reduced. It was assumed the formation of the NO₃ intermediate is mainly responsible of the corrosion of the graphite in this melt. The thermal and electrochemical properties of graphite in the molten alkaline nitrate have been correlated.     

Nitric oxide reduction at noble metal electrodes: a voltammetric study in acid solution

Features of the electrochemical reduction of nitric oxide on platinum, palladium, rhodium and ruthenium in aqueous perchloric acid solutions (0.33–1.0 M) are compared. The results from voltammetric studies (ie linear potential sweep and rotating disc electrode) using the bulk metal electrodes are described and compared with residual current voltage plots in acid electrolyte alone. In general, three nitric oxide reduction peaks are observed on the metals. The most anodic peak, at ca E = 0.15 V vs sce is attributed to the one-electron reduction of nitric oxide to an adsorbed NOH intermediate on a bare metal surface (ie one free of oxides or adsorbed hydrogen). The other two peaks occur in potential regions where adsorbed hydrogen is present on the metal surface (ca E = 0.0 and −0.20 V, respectively). The co-adsorbed hydrogen complicates the analysis and precludes an unambiguous interpretation of these two peaks. However, they apparently reflect nitric oxide reduction to nitrogen, hydroxylamine and/or ammonia. In a cathodic scan on the rhodium electrode, a current plateau is seen instead of the first (most anodic) peak, a probable consequence of oxide film formation with subsequent chemical complications. On the ruthenium electrode the first two (most anodic) peaks are not observed probably due to a relatively stable oxide layer. Reaction selectivities at metal black gas diffusion cathodes operating in an electrogenerative (ie galvanic) mode with perchloric acid electrolyte are compared with the voltammetric results at the corresponding bulk electrodes. Dinitrogen formation is observed on the platinum and rhodium black electrodes as suggested from voltammetric results. A series-parallel reaction sequence is proposed to explain the results. Limitations of using simple voltammetric techniques for predicting behavior of large scale preparative electrochemical reactors are discussed.     

Voltammetric behaviour of platinum in aqueous solutions containing sodium hypophosphite

The anodic and cathodic behaviour of polycrystalline Pt in sodium hypophosphite solutions has been investigated by cyclic voltammetry. In acid solutions the voltammograms show that, owing to the presence of the reducing agent, electrooxidation of species related to H₃PO₃ occurs in the potential range 0.6 to 1.4 V with respect to NHE. In alkaline solutins the presence of NaH₂PO₂ partially inhibits the HER and displaces the formation of the oxygen monolayer on platinum to higher potentials. The voltammetric peaks are attributed to the electrooxidation of the products resulting from the chemical decomposition of NaH₂PO₂ in contact with the Pt surface.     

Electrochemical behaviour of iron electrode in acidic acetonitrile solutions

Iron dissolution as well as hydrogen evolution was studied in HCl and HBr acetonitrile solutions in a concentration range between 2·8 × 10^?4 M. Halide concentration was changed by adding lithium halide to some of the previous solution. Measurements were performed at 0·0°C.Current efficiency for a two-electron transfer was about 100 per cent for both anodic and cathodic reactions.Corrosion and kinetic parameters were obtained from the polarization curves performed under different experimental conditions. Tafel slopes of 2·3 × 2 RT/F were found for both anodic and cathodic reactions.A reaction mechanism for the anodic reaction is postulated on the basis of anion adsorption. An inhibitory effect, probably due to solvent adsorption at the electrode, occurs as the acid concentration decreases, affecting both the anodic and cathodic polarization curves.     

Partial oxidation of butadiene on platinized Pt and Au anodes in acid aqueous solutions

At 30°C butadiene is only partially oxidized on Pt and Au anodes in H₂SO₄Na₂SO₄ solutions. On Pt, ethanol is the principal resulting substance at this temperature. On Au, tert-butyl alcohol and propanone are obtained at 30°C and a hydroxylated polymer, together with butanone, are produced at 70°C. Polarization curves run at this temperature and at 30°C allow some mechanistic conclusions: on Pt electrodes the role of surface must be recognized in analyzing this kind of reactions; on Au, it is suggested that hydration of butadiene must take place homogeneously prior to adsorption of the organic molecule.