D.A.J. Rand

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.

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.

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.

A study of ruthenium electrodes by cyclic voltammetry and X-ray emission spectroscopy

Abstract Hydrogen and oxygen adsorption properties of ruthenium surfaces are characterized and shown to be analogous to those of other noble metals. Ruthenium dissolves on potential cycles, the rate of dissolution on cycles to 1.54 V being much greater than for Pd, Rh, Ir, Pt and Au. Treatment in hot chromic acid or potential cycling to 1.3 V or above results in significant changes in the voltammogram. X-ray emission spectroscopy has demonstrated that these changes are associated with the formation and growth of an oxygen-containing layer on the ruthenium surface. Electron micrographic analysis of the surface layer included in replicas revealed that the layer is essentially amorphous. Correlations between the thickness of the layer from replica shadowing experiments and X-ray emission measurements indicated that the composition of the layer at 0.03 V was predominantly RuO with a considerable degree of non-stoichiometry. The charge on the voltammogram when oxide is present is interpreted in terms of oxidation of hydrated RuO to hydrated RuO 2 by a mechanism involving the addition and removal of protons, with corresponding changes in the valence state of the metal atom. Comparisons are made with the properties of thermally-produced RuO 2 .

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.

Binary electrocatalysts for organic oxidations

Abstract The electro-oxidation of formaldehyde and methanol has been studied on a number of binary platinum electrocatalysts. These comprised mixed electro-deposits of Pt with Sb, As, Bi, Hg, Re, Te or Sn and a range of homogeneous Pt-Rh alloys of different, known, surface composition. These systems were found to exhibit an enhanced activity over that of platinum alone, and this behaviour was correlated with the ease of adsorption of oxygen on the added metal. The activities for organic oxidation were compared with predictions of a model involving reaction between adsorbed oxygen and organic species on the metal surface. The proposed mechanism accounts for the behaviour of both homogeneous and heterogeneous alloy systems.

Oxygen reduction on sulphide minerals: Part III. Comparison of activities of various copper, iron, lead and nickel mineral electrodes

Abstract The cathodic reduction of oxygen was studied at rotated electrodes of the sulphide minerals arsenopyrite (FeAsS), bornite (Cu5FeS4), chalcocite (Cu2S), chalcopyrite (CuFeS2), covellite (CuS), galena (PhS), pentlandite ((Fe,Ni)9S8), pyrite (FeS2) and pyrrhotite (Fe1−xS) in both acid and alkaline solution. The working range of each mineral (i.e. the potential range over which background currents from surface processes are negligible) was determined from analyses of linear sweep voltammograms in deoxygenated solutions. In oxygenated solutions, minerals with wide working ranges give, at low rotation speeds, voltammograms that show limiting currents corresponding to the diffusion currents calculated from the Levich equation for a 4-electron reduction of oxygen to water. Departure from Levich conditions is observed at higher rotation speeds. This is simply a consequence of the inaccessibility of the limiting current region, and the extent of the departure depends on both the sulphide and the electrolyte used. Potential vs. log i plots in the kinetically-controlled region for each mineral give characteristic Tafel slopes, which show no systematic dependence on pH. In both acid and alkaline solutions there are significant differences between the oxygen reduction activities of the sulphides. The more active minerals pyrite, pentlandite, chalcocite and covellite are only slightly less active for oxygen reduction than gold. The ranking of activity of iron containing sulphides for oxygen reduction is consistent with their requirement for oxygen in flotation with xanthate collectors.

Electrosorption characteristics of thin layers of noble metals electrodeposited on different noble metal substrates

Summary Thin electrodeposits of palladium on gold and rhodium on platinum display composite hydrogen and oxygen electrosorption properties. This behaviour is interpreted in terms of alloy formation brought about by interdiffusion of surface atoms. In contrast, platinum on gold, rhodium on gold and palladium on rhodium electrodes exhibit the electrosorption characteristics of the individual pure metals. This implies a lack of interdiffusion in these systems, consistent with the known immiscibility of the component metals. The relation between electrosorption properties and amount of metal deposited provides supporting evidence for the conclusion that the active surface in chemisorption is confined to a few atomic layers.