R. Woods

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.

A study of the dissolution of platinum, palladium, rhodium and gold electrodes in 1 m sulphuric acid by cyclic voltammetry

Summary Platinum, palladium, rhodium and gold electrodes have been shown to dissolve in 1 M H 2 SO 4 during cyclic voltammetry. For all metals, the difference between the total anodic and cathodic charges on a cycle corresponds to the amount of metal detected in solution. Evidence in support of an anodic mechanism for noble metal corrosion has been obtained from studies of the variation of dissolution rate with both potential and temperature. It is emphasized that metal dissolution currents should not be ignored when examining electrochemical processes on noble metals at anodic potentials.

The nature of adsorbed oxygen on rhodium, palladium and gold electrodes

Summary Oxygen coverage has been determined on rhodium, palladium and gold anodes as a function of potential and time. The oxygen-containing film consists of chemisorbed oxygen atoms, but phase oxide can nucleate and grow under severe conditions of anodization. A basis for distinguishing between chemisorption and phase formation is presented. Comparison with previous work on platinum allows the stoichiometry of chemisorbed oxygen at coverage steps and limiting coverage regions to be determined. This stoichiometry can be utilized for accurate measurement of real surface area of rhodium and palladium electrodes.

The surface oxidation of pyrite

Abstract The surface oxidation in air and air-saturated aqueous solutions of the iron sulfide mineral, pyrite, has been studied by X-ray photoelectron spectroscopy. Iron sulfate was produced on fracture surfaces within the first few minutes of exposure to air under ambient conditions. Iron oxide was also included in the oxidation products after prolonged exposure which implies that a sulfur product in addition to sulfate must be formed. It is suggested that this product is an iron-deficient sulfide. Elemental sulfur was not evident at surfaces exposed to air. Iron oxide rather than sulfate was present at abraded surfaces exposed to air for a few minutes. Oxidation of pyrite in air-saturated acid solutions resulted in the formation of a surface sulfur layer the extent and nature of which depended on solution composition and exposure time. Sulfate was the only sulfur oxidation product identified in alkaline solutions not containing soluble sulfide, and iron oxide remained at the surface after such treatment. Thin layers of elemental sulfur were observed at fracture surfaces immersed in aerated, dilute sodium sulfide solutions.

An investigation of surface oxidation of pyrite and pyrrhotite by linear potential sweep voltammetry

Abstract The products of surface oxidation of pyrite and pyrrhotite have been determined from analysis of linear potential sweep voltammograms. Pyrite oxidizes to both sulphur and sulphate. The formation of sulphur is restricted to the order of a monolayer at pH 9.2 and 13, but significant yield occurs at pH 4.6. The proportion of sulphate formed increases rapidly with increase in potential. Sulphur is the major product of pyrrhotite oxidation at pH 4.6, 9.2 and 13. Sulphate is also formed in significant quantities, particularly in the alkaline solutions. Oxidation of pyrrhotite is strongly inhibited by the surface ferric oxide produced.

An electrochemical investigation of the natural flotability of chalcopyrite

Abstract The flotation of chalcopyrite particles was investigated using a modified Hallimond tube cell, in which the potential at the mineral-solution interface was controlled potentiostatically. Previous reports that this mineral displays natural flotability at oxidizing potentials and non-flotability at reducing potentials have been confirmed. Anodic oxidation of the mineral surface is responsible for the change from a hydrophilic to a hydrophobic condition. Linear potential sweep voltammetry has identified the products of the anodic reaction as CuS, Fe(OH) 3 and S. The presence of sulphur on the mineral surface is considered to be the critical factor in rendering chalcopyrite flotable.

Cyclic voltammetric studies on iridium electrodes in sulphuric acid solutions: Nature of oxygen layer and metal dissolution

Summary Iridium electrodes have been shown to dissolve in 1 M H2SO4 during potential cycling. The rate of dissolution has been compared with those of other noble metals under identical conditions, the case of dissolution increasing as the standard potential of the metal/metal ion couple decreases. Oxygen chemisorption has been studied on iridium in sulphuric acid solutions. The process exhibits increasing hysteresis as the anodic limit of the sweep is increased. A coverage of one oxygen atom per iridium surface site is reached at 1.5 V on a triangular potential sweep at 40 mV s−1. Additional current peaks appear and grow on the anodic and cathodic traces of voltammograms for iridium electrodes cycled continuously in 1 or 0.1 M H2SO4. This behaviour is attributed to the irreversible formation of an iridium phase oxide and the charge associated with these peaks is considered to arise from changes in the stoichiometry of the oxide during potential cycling. Phenomena previously reported in the literature for iridium electrodes are identified with the phase oxide rather than with chemisorption on bare metal sites.

Hydrogen adsorption on platinum, iridium and rhodium electrodes at reduced temperatures and the determination of real surface area

Summary Cyclic voltammograms of platinum and iridium electrodes in 5 M sulphuric acid at −72°C display much greater separation of the hydrogen adsorption and evolution currents than at 20°C. This enables the saturation hydrogen coverage and the real surface area of an electrode to be determined directly from the voltammogram. The adsorption of hydrogen on rhodium is not sufficiently rapid to maintain equilibrium on a voltammogram at reduced temperatures, and the saturation hydrogen coverage cannot be obtained directly. The accuracy of determination of surface area for the three metals from measurements at 20–25°C is discussed. The voltammograms for platinum electrodes at −72°C do not show the “third anodic hydrogen peak” observed at 20°C. The processes which have been suggested to account for this peak are discussed.

Determination of the surface composition of smooth noble metal alloys by cyclic voltammetry

Summary Homogeneous platinum-rhodium, palladium-rhodium and palladium-gold alloy surfaces display composite hydrogen and oxygen electrosorption properties. The potential of the oxygen desorption peak on a voltammogram varies linearly with surface composition. This relationship presents a method for analyzing the alloy surface. The dissimilarity between the electrosorption behaviour of these homogeneous alloys with heterogeneous systems can be used to detect phase separation at the surface. Changes in surface composition during continuous potential cycling are due to the preferential dissolution of a component metal. Comparison of the change in electrosorption properties with the amount of metal dissolved leads to the conclusion that the “surface” involved in chemisorption reactions consists of no more than a few atomic layers. This result is discussed in relation to theories of chemisorption and catalysis.

An x-ray photoelectron spectroscopic study of the oxidation of galena

Abstract The oxidation of cleaved surfaces of galena (natural lead sulfide) in air at ambient temperature and humidity has been studied by X-ray photoelectron spectroscopy. Analyses were carried out with the sample held below 150 K, a temperature at which elemental sulfur was shown to be retained at a galena surface, and subsequently at ambient temperature. During the initial stages of air-oxidation, the surfaces became enriched with lead due to the formation of lead hydroxide, oxide and carbonate. No oxidised sulfur species were observed in this period and it was concluded that a metal-deficient sulfide was formed. Exposure of galena to air for extended periods resulted in the formation of sulfate. After oxidising galena in air-saturated acetic acid solution, the presence of polysulfide phases was revealed. Minor elements present in the galena were found to migrate to fill some of the vacancies in the metal-deficient sulfide and polysulfide phases.