T. Biegler

Limiting oxygen coverage on platinized platinum; Relevance to determination of real platinum area by hydrogen adsorption

Summary A limiting oxygen coverage is found on platinized platinum electrodes and identified as a monolayer of chemisorbed oxygen atoms. Evidence is presented to support a realistic separation of hydrogen adsorption and evolution currents on linear sweep voltammograms in order to determine the hydrogen monolayer charge. Comparison with the hydrogen monolayer charge shows the stoichiometry of the oxygen monolayer to be 2 oxygen atoms/surface platinum atom for both smooth and platinized platinum electrodes. The problems involved in interpreting hydrogen adsorption measurements in terms of real electrode areas are discussed.

The Electrochemistry of Surface Oxidation of Chalcopyrite

Linear sweep voltammetry with a freshly renewed surface of a chalcopyrite (CuFeS/sub 2/) mineral electrode shows a small anodic prewave in acid electrolytes. The main anodic reaction, with complete dissolution of copper and iron, occurs at more positive potentials. Experiments under nitrogen show that the prewave is not a consequence of atmospheric oxidation during surface preparation. The prewave represents a surface oxidation of chalcopyrite to form a thin passivating film. The charge passed in forming this film is around 2 mC cm/sup -2/ for a polishe surface. The prewave reaction is identified with the initial phase of the oxidative dissolution or leaching of chalcopyrite. Contrary to the reported behavior at 80/sup 0/C, the surface film is stable at 25/sup 0/C for long periods at open circuit Various evidence, including the relative charges passed in forming and then reducing the surface film, and the presence of dissolved copper, leads to the following suggested stoichiometry for the prewave reaction CuFeS/sub 2/ ..-->.. 0.75CuS + 0.25Cu/sup 2 +/ + Fe/sup 2 +/ + 1.25S + 2.5e. The product layer containing CuS and S is about 3 nm thick.

Anodic electrochemistry of chalcopyrite

The anodic behaviour of chalcopyrite in 1 M H2SO4 and 1 M HCl was studied by linear sweep voltammetry and potentiostatic electrolysis. Voltammograms with a fresh surface showed a small prewave, attributed to a surface oxidation process, followed by a region of active dissolution characterized by a steeply rising anodic current. At still higher potentials the behaviour differed in the two electrolytes, but, contrary to a previous report, there was no evidence of a photosensitive limiting current. Anodic characteristics of chalcopyrite specimens from different sources, including synthetic material, were similar. The anode reaction was found to involve 6·7±0·3 F per mole of chalcopyrite, from which it was calculated that 86% of the sulphide sulphur was oxidized to the elemental form (some of which was plastic sulphur) and 14% to sulphate. The surfaces of oxidized electrodes were examined and photographed after varying periods of potentiostatic electrolysis. Chalcopyrite dissolution occurred at localized sites, the number of which depended strongly on potential. Current-voltage and potentiostatic current-time curves were interpreted in terms of the kinetics of nucleation, growth and overlap of these discrete corrosion centres.

Oxygen reduction on sulphide minerals: Part I. Kinetics and mechanism at rotated pyrite electrodes

Summary The electro-reduction of oxygen was studied at rotated electrodes of the sulphide mineral pyrite (FeS2). Kinetic parameters were obtained from currentpotential measurements at the foot of the oxygen reduction wave. In oxygensaturated 1 M acid solutions, the Tafel slopes and exchange currents were of the order of −130 mV and 10−11 A cm−2 respectively. The results at low pH indicated that the first electron transfer step to form O2− is rate-determining, while in alkaline solution the rates of subsequent steps become important. At low rotation speeds, linear sweep voltammograms reached a limiting current corresponding to 4-electron reduction of oxygen to water. The limiting current plateau was not reached at higher speeds because of the loss of a soluble intermediate, identified as hydrogen peroxide. The dependence of current on rotation speed, potential and surface roughness was analysed in terms of a mechanism involving kinetic control of peroxide reduction and diffusion control of its escape into the solution bulk.

Oxygen reduction on sulphide minerals: Part II. Relation between activity and semiconducting properties of pyrite electrodes

Abstract The electrochemical reduction of oxygen in acid solution was studied at rotated electrodes of eight different pyrite specimens which were electrically characterized and included n-type, n-type metallic and p-type examples. Shapes of voltammograms, Tafel slopes and electrocatalytic activities for oxygen reduction all showed a degree of variation from sample to sample, but no correlation of behaviour with semiconducting type could be established.

The electrolytic reduction of chalcopyrite in acid solution

Chalcopyrite electrodes of both natural and synthetic material were cathodically reduced in several acid electrolytes. At current densities below 10mA cm−2 chalcocite was formed and at higher currents the solid product contained elemental copper. The other reduction products were H2S and Fe++. The current efficiency, measured in terms of the Fe++ produced, generally fell with the amount of charge passed, the extent of the decrease depending on current density and the composition and temperature of the electrolyte. The decrease was attributed to evolution of hydrogen on the increasing surface area of the porous solid product as a consequence of inhibition of the chalcopyrite reduction process by ferrous salt precipitated in the pores. At sufficiently low current densities and short periods of electrolysis, chalcopyrite reduction was found to be activation controlled over a three order of magnitude range of currents, with a Tafel slope consistent with a one electron rate-determining step.

The coulombic efficiency of zinc electrowinning in high-purity synthetic electrolytes

Measurements of coulombic efficiency (QE) for zinc electrodeposition were carried out under mass transfer-controlled conditions using a rotating disc electrode in synthetic acidic zinc sulphate electrolytes. At 25°C in 0.8 M ZnSO4+1.07 M H2SO4 prepared from reagent grade chemicals, the QE at an aluminium cathode was 95.7–97.6%. In order to study the influence of electrolyte purity on QE several preparation and purification techniques were employed. While different sources of chemicals produced different QEs, the main source of impurities seemed to be the zinc-containing reagent rather than the sulphuric acid. Improvements in purity either had a negligible effect or lowered the QE, indicating that some impurities are beneficial to electrolyte performance. In the purest solutions prepared, an effect due to residual impurities still seemed to be present. The maximum QE obtainable through variation of the three parameters, i.e. temperature, current density and electrode rotation rate, was determined for two electrolytes of different purities; the values of QE obtained were 98.4 and 98.8%, with temperature as the dominant factor. ‘Warks Rule’ (the dependence of QE on zinc/acid ratio) was obeyed approximately in the purest electrolyte prepared, over a limited range of composition.

Reduction kinetics of a chalcopyrite electrode surface

Abstract The first cathodic linear sweep voltammogram with a freshly-polished chalcopyrite (CuFeS 2 ) electrode shows a small peak (prewave) preceding the main voltammetric wave for reduction to chalcocite (Cu 2 S). The charge under the peak is of the order of 1 mC cm −2 and is independent of sweep rate. Current-time curves at potentials in the prewave region have maxima, indicative of surface phase formation involving expansion and overlap of growing nuclei. The results are interpreted in terms of nucleation and two-dimensional phase growth of a thin product layer prior to the bulk reduction of chalcopyrite to chalcocite. The possible identity of the product phase is discussed.

Continuous electrolytic reduction of a chalcopyrite slurry

A laboratory-scale cell for the continuous electrolytic reduction of chalcopyrite contained in copper concentrate is described. Dry concentrate is fed into one end of the cell and forms a particulate bed electrode, with fluidized properties, resting on the copper cathode constituting the cell base. At the opposite end, the reduced product containing copper sulphide of composition Cu1.8S to Cu2S flows over a weir. The feed rate determines the coulombic efficiency and extent of reduction of the sulphide. A copper extraction process incorporating slurry electrolysis is proposed and scale-up problems discussed

Effects of neglecting pre-exponential terms in electrochemical kinetic equations

Summary The consequences of excluding coverage-dependent pre-exponential terms from rate equations involving chemisorbed species are examined. Expressions are derived for: (1) the slope of the isotherm for a reversible adsorption step with charge transfer, (2) the Tafel slope and reaction order when such a step is followed by a rate-determining combination step, and (3) the slopes of kinetic plots for irreversible adsorption steps with charge transfer. Comparisons are made between the expressions obtained including and neglecting the pre-exponential terms. It is shown that significant differences between these expressions can exist in the range of coverages where it is commonly assumed that the pre-exponential terms can be ignored.