G. Lodi

Ruthenium dioxide-based film electrodes

Oxygen evolution from acid solutions has been investigated on two kinds of RuO2-based electrodes; compact and cracked (porous) films. The kinetic study has been carried out by means of potentiostatic curves. A method is suggested to estimate the surface concentration of active sites. The reaction order with respect to active sites and the activation energy have been determined. The mechanism is shown to differ on the two kinds of film in relation to theS(site)-OH bond strength and the surface concentration of intermediates. The behaviour of films on silica glass substrates is reported for the first time.

Electrochemical Incineration of Glucose as a Model Organic Substrate. II. Role of Active Chlorine Mediation

The electrochemical incineration of glucose mediated by active chlorine in alkaline media has been studied under different elec trolysis conditions. For the sake of comparison, the electrolysis has been carried out in the presence of 1 M Na 2SO4 1 0.01 M NaOH at Pt, SnO2-Pt composite electrodes and PbO2 electrodes in the absence of sodium chloride. At the first two electrode materials, only partial oxidation could be achieved and complete mineralization was observed only at PbO 2 electrode. While the other parameters remain constant, addition of NaCl to the solution causes a sharp increase of the reactivity of glucose and its oxidat ion intermediates, toward the electrochemical incineration. At a NaCl concentration as low as 1 g dm 23 , the mediation of the incineration process by active chlorine is already significant. A maximum is achieved at [NaCl] 5 5 g dm 23 (in 0.01 M NaOH). At this sodium chloride concentration, the chemical oxygen demand of glucose solutions has been found to decrease faster, the lower the solution temperature and the higher the current density. This acceleration of the mineralization is accompanied by an increase of faradaic efficiency.

Mechanistic study of C12 evolution at Ti-supported Co3O4 anodes

Co3O4 layers were prepared by thermal decomposition of Co(NO3)2 at various temperatures in the range 200–500° C on a Ti support with and without an interlayer of RuO2. Kinetic studies were carried out with and without dissolved Cl2 at various partial pressures in NaCl solutions of concentration in the range 0.5–5 mol dm−3. The effect of the solution pH was especially investigated. Kinetic measurements were carried out both close to and far from equilibrium. The following parameters were determined: transfer coefficient, Tafel slope, stoichiometric number, reaction orders with respect to Cl−, H+ and surface sites, activation energy. The most intriguing feature observed was the retarding effect of acidity on the anodic Cl2 reaction. This has been ascribed to the complex surface behaviour of oxides in solution. A detailed mechanistic scheme has been proposed and discussed. The stability of the oxide surface was monitored by measuring the voltammetric charge in alkaline solution after sets of experiments.

Influence of the Valve Metal Oxide on the Properties of Ruthenium Based Mixed Oxide Electrodes II . Coatings

Depth profiling of RuO{sub 2}/TiO{sub 2} mixed oxide coatings supported on titanium foil, was performed by means of Rutherford backscattering spectrometry and x-ray photoelectron spectroscopy. Results of the two techniques indicate the occurrence of surface segregation of the valve-metal oxide and a general enrichment with it across the first 100-150 nm. The segregation phenomenon takes place for all of the coating compositions investigated. However, it seems less pronounced for Ru contents higher than 60/ac. The nature of the ruthenium salt used in the coating preparation seems to affect the extent of the surface enrichment.

Fractional reaction orders in oxygen evolution from acidic solutions at ruthenium oxide anodes

Oxygen evolution at RuO2 anodes from acidic solutions exhibits a fractional reaction order with respect to H+. This is close to −1 for a set of “compact” film electrodes, and close to −2 for a set of “cracked” film electrodes. The difference in mechanism is related to the different defect structure of the surface. Possible reasons for the observed fractional reaction orders are discussed. It is suggested that the experimental observation may be explained in terms of the reaction proceeding with different mechanisms in parallel on different patches of the surface whose structure is governed by the acid-base properties of the oxide.

Hydrogen detection in ruthenium oxide layers by means of the 1H(15N,αγ)12C nuclear reaction

Hydrogen in RuOx layers on quartz prepared by the thermal decomposition of RuCl3 has been detected by means of a nuclear technique. RuOx absorbs water rapidly from the liquid and a constant H/Ru ratio is found throughout the thickness of the layer. RuOx also absorbs water from the ambient moisture, but no saturation is observed even after several months. The permeability of RuOx to water is explained in terms of grain boundary diffusion.

Effect of the support on the surface properties of ruthenium dioxide on silica glass

The surface properties of SiO2-supported RuO2 have been found to depend on the temperature and the procedure of sample preparation. In particular, the point of zero charge in KNO3 solutions has been observed to pass through a maximum at ca. 600 °C and then to drop at higher temperatures. Comparison with results obtained with RuO2 powders and the surface analysis of supported RuO2 by Auger electron spectroscopy have shown that substantial segregation of SiO2 to the surface of RuO2 takes place at temperatures higher than ca. 500 °C. These results suggest that caution should be used when interpreting data on the surface activity of SiO2-supported oxides.

PROTON EXCHANGE IN GROUP VIII METAL-OXIDE FILMS

Abstract In the present work a comparison has been tried between the proton-exchange properties of thermally prepared films of ruthenium oxide and iridium oxide, by means of nuclear reaction analysis and a radiometric method based on exchange of tritium-labelled species. Hydrogen concentration depth-profiles have been determined before and after anodic polarization. Results indicated that a thickness of about 50 nm below the surface is involved in the exchange process. Larger amounts of hydrogenated species were found in iridium oxide samples. Tritium-exchange experiments confirmed these data. Comparison with electrochemical results has also been carried out.