N.R. de Tacconi

The electrooxidation of CO: a test reaction in electrocatalysis☆

Abstract This review paper aims to show how the electrochemical behaviour of CO plays a key role in the understanding of the reaction mechanism of many electrocatalytic oxidations of small organic molecules. For that purpose, the adsorption of CO on noble metal electrodes, eventually modified by foreign metal adatoms, is reviewed, taking into account both experimental (electrochemical and spectroscopic techniques) and theoretical (Extended Huckel Model) approaches. Data from the gas phase—solid metal interface are also considered.

Potentiodynamic current/potential relations for film formation under OHMIC resistance control

Abstract Theoretical potentiodynamic current/potential curves for a film formation process under an ohmic resistance control are given. Results are compared with experimental data involving the formation of insoluble parathiocyanogen on platinum and the precipitation of a solid CuCl layer during copper dissolution in aqueous HCl solution.

In-situ FTIR study of the electrocatalytic oxidation of ethanol at iridium and rhodium electrodes

Abstract In-situ Fourier transform IR (FTIR) reflectance spectroscopy has been used to study the electroadsorption and oxidation of ethanol at polycrystalline Ir and Rh electrodes in HClO 4 solutions. The formation of surface and solution species has been followed optically during slow voltammetric scans. Ethanol electroadsorption leads to the formation of linearly bonded and bridge-bonded CO on Rh surfaces but only to linearly bonded CO on Ir. In the oxidation potential region, Ir electrodes show different catalytic properties from Rh electrodes, with Ir being a more selective catalyst than Rh. Oxidized states at early potentials play an activating role in ethanol electro-oxidation. Acetaldehyde is probably an intermediate product, but the major product on Ir electrodes is acetic acid whereas CO 2 is the main product at Rh electrodes.

Cathodic electrodeposition of mixed oxide thin films

Abstract Cathodic electrodeposition is a rather unexplored route for the synthesis of mixed oxides. Two different mechanisms are generally reported which lead to oxide thin film formation: (i) the direct reduction of the oxidation state of the metallic element, (ii) an interfacial pH increase and local supersaturation followed by oxide precipitation. We emphasize the major requirements for mixed thin film formation via electrosynthesis and present results obtained with two different systems of special interest. The first, ZnO/Eu(OH) x , is based on the second mechanism. The second, TiO 2 /WO 3 , is a mixed mechanism process. We describe the conditions under which europium oxide can be deposited cathodically. This oxide is obtained by co-deposition with zinc oxide. After a heat treatment at 400 °C, X-ray diffraction shows that the films are mainly made of monoclinic Eu 2 O 3 . The successful WO 3 –TiO 2 mixed film electrodeposition in a wide compositional range is also reported.

Preparation and characterization of nanocrystalline composite (nanocomposite) films of titanium dioxide and nickel by occlusion electrodeposition

A new family of nanocrystalline composite films was developed for photoelectrochemical application by occlusion deposition of TiO2 particles from a nickel plating bath. The photoactivity of the resultant Ni¦TiO2 films was examined under chopped-light illumination with a xenon arc lamp in 0.1 M KNO3. A TiO2 dose in the nickel bath of 0.8 M, a deposition potential of −0.8 V, a bath pH of 5.0, a bath temperature of 20°C and a film deposition charge of 36 mC (corresponding to a film ca. 0.7 μm thick) were found to yield the best photoresponse for the resultant Ni¦TiO2 films. The photoresponse of these nanocomposite films was compared with that of TiO2 layers on Ti supports, obtained either from thermal oxidation of titanium or by oxidative decomposition of aq. TiCl4 at 550°C. Formate and acetate ions were employed as current-doubling agents in the 0.1 M KNO3 electrolyte for this comparative study. The current-doubling effect was seen to be the highest for the Ni¦TiO2 films; possible mechanistic origins for this trend are discussed. Other characterizations of the Ni¦TiO2 films, including analyses by laser Raman spectroscopy,and X-ray photoelectron spectroscopy, are also discussed.

The electrooxidation of chemisorbed co on polycrystalline platinum: A mechanistic interpretation of the anodic current peak multiplicity

Abstract The electrooxidation of CO on polycrystalline Pt in 1 M HClO4 at 23°C under potentiodynamic conditions exhibits a double current peak. Whether a single or a double current peak is formed depends to a great extent on the amount of CO surface coverage. The present report attempts to explain the behaviour of the electrochemical reaction in terms of a single model involving the participation of two CO adsorbed states on polycrystalline Pt.

Surface state effects on the electroreflectance spectroscopy of Au single crystal surfaces

Abstract The electroreflectance spectra of Au (100), (110) and (111) surfaces are interpreted with success in terms of optical transitions involving surface states. It is revealed that there are subtle differences in structure between the metal surface in contact with an aqueous solution and the same surface in vacuum.

Electromodulated infrared spectroscopy of methanol electrooxidation on electrodispersed platinum electrodes: Enhancement of reactive intermediates

Abstract EMIRS spectra of the surface species resulting from methanol electrosorption on electrodispersed Pt were obtained by working under a wide range of experimental conditions, i.e. electrode roughness factor, methanol concentration and mean modulation potential. The intensity of the absorption band related to linearly adsorbed CO-species, COL, at ca. 2030–2080 cm−1 decreases on increasing the electrode roughness factor, a fact which is interpreted as a decrease in the CO adsorbate poisoning effect on electrodispersed Pt surfaces. As a consequence, under well-defined conditions, several bands are observed which might be assigned to adsorbed intermediates and reactive species. EMIRS also reveals a competition between COB and COL at high surface coverages. Spectral data for methanol electrooxidation on electrodispersed electrodes correlate well with the expected structure of this type of electrode.

Potentiodynamic behaviour of the rhodium/H2SO4(aq) interface in the potential range of the hydrogen and oxygen electrosorption

The electrochemical response of Rh/H2SO4(aq) interfaces at 25°C, subjected to different potential—time perturbation programs in the potential range of H and O electrosorption and H and O electrodesorption is reported. The H electrosorption and H electrodesorption involve the participation of at least two distinguishable adsorption states. The degree of reversibility of the corresponding electrochemical processes is appreciably influenced by the history of the electrode including the type of perturbation applied to the electrode in the O electrosorption potential range. The O electrosorption and O electrodesorption involve the formation of different surface species as it is deduced either from the charge balance or from the kinetic response of the electrochemical interface. The early stages of the anodic reaction are interpreted through a reversible charge transfer comprising the formation of OH adsorbed species which later undergoes electrochemical and chemical reactions yielding different O-containing species. The existence of the various O-containing surface species as well as the influence of the most stable ones in the H electrosorption potential range is evidenced through their potentiodynamic electrodesorption. The kinetic results of the O electrosorption are discussed in terms of aging mechanisms including anion adsorption.

Modulated reflectance spectroscopy and voltammetry of the sulphide/gold system

Abstract The electroadsorption of sulphide species on gold and the growth of sulphur multilayers have been investigated by optical and electrochemical techniques in sodium tetraborate buffer as the supporting electrolyte (pH 7.0 and 9.2) at 25°C. Differential reflectance spectra indicate that the adsorption of sulphide begins inside the hydrogen evolution reaction (HER) region. The HER is catalysed by the presence of sulphide species on the electrode. During the growth of sulphur multilayers, soluble polysulphide species can be optically detected either as intermediates under diffusion-controlled kinetics or as products of a chemical reaction between sulphide ions and the deposited sulphur layer. The steep fall in the integral reflectance at 0.4 V (vs. SHE) is interpreted through the incorporation of light-absorbing polysulphides into the structure of the deposited sulphur layer.