James P. Hoare

Sorption of oxygen from solution by noble metals: I. Bright platinum

Platinum electrodes in the form of beads or wires were permitted to sorb oxygen from O2-saturation H2SO4 solutions under steady-state conditions of constant current, constant potential, and open circuit. True areas were determined from constant current anodic pulses, and the amount of charge associated with the sorbed oxygen was determined from the measured transition times of potential-time traces obtained with constant current, cathodic stripping pulses. Two kinds of adsorption sites are identified with respect to the diffuculty with which the sorbed oxygen is removed by a cathodic stripping pulse. The strong adsorption sites are located in the skin of the metal (first 1 or 2 atom layers of the metal) and the sorbed oxygen on these sites corresponds to the so-called dermasorbed oxygen. Weak adsorption sites are located on the metal surface, and the sorbed oxygen on these sites is called adsorbed oxygen. Oxygen which is dissolved in the interior of the metal beyond the dermasorb region is called absorbed oxygen. The amount of oxygen sorbed is a function of the potential. Below 800 mV, sorbed oxygen could not be detected. The first oxygen to be sorbed appears above 800 mV on the surface adsorption sites but dermasorbed oxygen is not detected until potentials greater than 1000 mV are reached. Under open-circuit conditions, the maximum amount of surface adsorbed oxygen present at steady state (potential = 1060 mV) in O2-saturated acid solution is 30% of a monolayer. A complete layer of adsorbed oxygen (Pt—O) may be obtained with anodization at potentials of about 1600 mV. At higher potentials (>2000 mV), the Pt—O sites may be converted to PtO2 sites until a maximum of a monolayer of PtO2 is formed. Under open-circuit conditions, the PtO2 sites in the presence of Pt decompose to Pt—O sites. The equivalent of many complete layers of oxygen may be dissolved in the interior of the metal with anodic polarization, and this absorbed oxygen may replace any dermasorbed oxygen removed.

On the interaction of oxygen with platinum

Abstract The cyclic voltammograms were obtained on both bright polycrystalline Pt and PtO alloy bead electrodes in both 2 N H2SO4 and 1 N HF solutions saturated with purified N2. The samples were scanned between 0 and 1500 mV vs nhe at rates from 100 to 17 mV s−1. In rigorously purified systems, the chief effect of polarizing Pt above 1 V is the charging of Pt with dissolved oxygen. Such a Pt O alloy has different electrochemical properties than Pt. On Pt there are two adsorption and desorption sites for hydrogen and possibly two adsorption sites for sulfate ions. A third hydrogen desorption site exists on PtO alloy.

Some effects of alternating current polarization on the surface of noble metal electrodes

Abstract Small electrodes of platinum, rhodium, palladium, iridium and gold were polarized with alternating current and alternating current biased with direct current. Only those metals which dissolve hydrogen —platinum and palladium—produced metal-black surfaces with alternating current polarization. The dissolved hydrogen plays an essential role in the break-up of the surface. Using a single-pulse technique, double layer capacitance data were obtained; they support these conclusions. The superior behaviour of an anodized platinum indicator electrode may be traced to the presence of “oxygen bridges”.

Potentiostatic polarization studies on platinum, gold and rhodium oxygen electrodes☆

Abstract Potentiostatic polarization measurements have been made on platinum, gold and rhodium oxygen electrodes in acid solutions. The thin films of adsorbed oxygen atoms on these electrodes are good electronic conductors. The film of Au 2 O 3 , on Au is a poor electronic conductor, and the Au/Au 2 O 3 electrode is a true metal/metal-oxide electrode system with E ° of 1.36 V (nhe). Evidence presented favours the contention that oxygen is adsorbed on gold, possibly in only small amounts, in the potential range of 900–1300 mV (nhe). With anodic polarization of Pt, some PtO sites are converted to PtO 2 sites. It appears that anodic polarization of a Rh/O 2 , electrode produces an alloy of Rh and O atoms by dissolution of the oxygen in the first few surface layers of the Rh metal.

Sorption of oxygen from solution by noble metals: II. Nitric acid-passivated bright platinum

The nature of the adsorbed films produced on platinum electrodes treated with concentrated nitric acid was studied by means of constant current stripping pulses as a function of the time of passivation and the contact with atmospheric oxygen. It is concluded that the dissolved oxygen of the Pt-O alloy does not come from the HNO3. Instead, the HNO3 treatment makes the Pt more accommodating to the sorption of oxygen (in the manner of an oxygen sponge), and the Pt then sorbs oxygen from the surroundings producing the Pt-O alloy. The presence of the oxygen in the Pt lattice modifies the electronic structure of the Pt, generating an electrode material with different catalytic and electronic properties from untreated Pt. In the presence of oxygen, the HNO3 treatment produces a Pt-O alloy electrode with a complete monolayer of Pt-O adsorbed on the surface. Such an electrode exhibits the reversible oxygen potential in O2-saturated, 2 N H2SO4 solution. The rest potential is a linear function of the degree of surface coverage. No more than a monolayer is adsorbed by this procedure, but the equivalent of two layers may be formed by anodizing the Pt-O alloy electrode. Apparently the Pt-O sites are converted to PtO2 sites which are unstable at potentials below 1500 mV in the presence of Pt metal. The Pt-O alloy structure is rather stable and is converted to that of untreated Pt by heating the metal in the vicinity of its melting point.

Concerning the structure of platinum—oxygen alloys

Strips of Pt foil were cleaned and cut in half. One half was converted to a PtO alloy by treatment in concentrated HNO3; the other half was not treated. The treated and untreated Pt samples were analyzed with X-ray diffraction, Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) and scanning electron microscope (SEM). Reflections from the lattice planes were shifted for PtO alloy showing an expansion of the lattice. Certain reflections for Pt were missing for PtO alloy, indicating preferred orientation of the sample. The SEM micrographs showed that the PtO alloy was covered with hexagonal-shaped etch pits. From the XPS data, dissolved oxygen was detected to a depth of 360A in the Pt0 alloy.

Oxygen overvoltage on bright gold—I

Oxygen overvoltage measurements have been made on Au/Au-O and Au/Au2O3 electrodes in oxygen-saturated 2 N sulphuric acid solution. The rate-determining step in the reduction of oxygen on an Au/Au-O electrode is O2(ads) + e ⇌ O2−(ads), with an i0 of 1·3 × 10−11 A/cm2. An Au/Au-O electrode is a poly-electrode system, and the open-circuit corrosion current is 3·6 × 10−8 A/cm2. Oxygen evolution could not be studied on an Au/Au-O electode because the anodization process converts the electrode to an Au/Au2O3 electrode. The Au/Au2O3 electrode is a single electrode system with an E0 of + 1·360 V, and the electrode process is the formation and reduction of Au2O3, Au2O3 + 6H+ + 6e ⇌ 2Au + 3H2O. At high anodic current densities two parallel and independent processes occur—the evolution of the oxygen and the formation of Au2O3. Gold is a poor catalyst for oxygen reactions in acid solutions because the adsorbed oxide films are poor electronic conductors, and the electrode surface is a poor peroxide-decomposing catalyst.

Oxygen overvoltage on bright iridium

Summary Steady-state oxygen overvoltage measurements were made both galvanostatically and potentiostatically on bright Ir bead anodes and cathodes in O 2 -saturated sulfuric acid solutions. The mechanism for the reduction of O 2 on Ir cathodes is most likely the same as that suggested for Pt cathodes. Although an oxide, IrO 2 , is formed on Ir anodes, the mechanism for the evolution of oxygen does not take place by an oxide mechanism as for Pd anodes, but by the discharge of H 2 O as for Pt anodes. From the experimental evidence that a limiting current region appears in the constant potential anodic polarization curve and that the double-layer capacity is increased as oxygen is adsorbed on the Ir surface, it is concluded that the adsorbed oxygen layers on Ir are good electronic conductors. Evidence is presented which supports the contention that the presence of adsorbed oxygen on Ir cathodes hinders the reduction of O 2 to H 2 O 2 but promotes the reduction of H 2 O 2 to H 2 O. A stable intermediate, H 2 O 2 , is formed during the oxygen reduction process.

Current efficiency during the electrochemical machining of iron and nickel

Current efficiency determinations from weight-loss measurements were made on pure iron and pure nickel anodes in 4M NaClO3 solution in a flow cell at flow rates between 500 and 3000 cm/s in a current range from 5 to 50 A/cm2. The current efficiency for metal removal was virtually independent of current density and flow rate on iron anodes. On nickel anodes the current efficiency increased strongly with current density. In the high current density region, the current efficiency decreases with flow rate up to 2000 cm/s and then increases with higher flow rates. This behavior was accounted for by differences in the nature and properties of the anodic films formed on iron and nickel anodes.

Effect of dermasorbed oxygen on the catalytic activity of platinum indicator electrodes

Abstract Both the reduction of ferric ion and the oxidation of ferrous ion in 2N H 2 SO 4 solutions were carried out on reduced and pre-anodized Pt and Au electrodes. From an analysis of the steady-state polarization data it is concluded that the presence of dermasorbed oxygen in the skin of the Pt metal facilitates the transfer of electrons from the metal to reactants in the double layer. Since Au does not dissolve oxygen, these effects were not observed on reduced and pre-anodized Au electrodes. In addition, oxygen adsorbed on the surface of either Pt or Au electrodes inhibits the transfer of electrons due to the layer of negative dipoles of the MO bonds. Similar effects were obtained in the investigation of the reduction of ceric or vanadate ions on reduced and pre-anodized Pt electrodes.