M.E. Martins

The possible existence of subsurface H-atom adsorbates and H2 electrochemical evolution reaction intermediates on platinum in acid solutions

The possible existence of H-subsurface states on Pt electrodes in acid solutions was studied using Pt electrodes of different topographies which were characterized by ex-situ scanning tunnelling microscopy (STM). The H-adatom electrodesorption current peak at ca. 0.22 V versus RHE resulting from combined voltammetric and potentiostatic potential routines applied in the H-adatom potential range was assigned to H-subsurface states. Seemingly, the complementary electroreduction occurs at potentials slightly more positive than the hydrogen evolution reaction threshold potential and the reactant, in this case, appears to be related to the hydrogen evolution reaction. The charge density involved in these current contributions is only a small fraction of the H-adatom monolayer charge density. The formation of H-subsurface states appears to be favoured for those Pt surfaces exhibiting a larger contribution of weak H-atom electrosorption. A reasonable correlation between electrochemical and chemisorption data on H on Pt surfaces with different topographies and crystallographic orientations can be established. This fact provides further support for the existence of adsorbed H atom precursor states related to the formation of subsurface H adatoms and to the hydrogen evolution reaction on Pt in acid solutions.

The influence of the solution composition on the o-electroadsorption and o-electrodesorption on gold in acid electrolytes

Abstract The O-electroadsorption E/I profile obtained on polycrystalline gold after controlled-potential adsorption conditions is remarkably dependent on the electrolyte composition. The electrochemical results are interpreted in terms of the specific adsorption of the anions. According to the anion influence on the E/I profile, two groups of anions are distinguished. Group I contains those anions contributing to the electrolyte solution structure through hydrogen bonds, while group II comprises those anions where such bonds are in principle less relevant. A general electrochemical adsorption isotherm and an adsorption kinetic equation for the anions on polycrystalline gold are discussed.

A comparative study of the early stages of mercury, cadmium, lead, silver and copper electrodeposition on columnar and smooth platinum electrodes

Abstract The early stages of metal (M) electrodeposition ( M = Cu , Ag , Pb , Cd and Hg ) on columnar and smooth Pt substrates (S) were investigated in acid solutions applying potentiodynamic techniques. At moderate and high potential scanning rates M electrodeposition on columnar substrates yields M domains which are dominated either by MS interactions (MS domains) or MM interactions (MM domains) as deduced from anodic stripping data. The roughness of S, the rate of surface diffusion of M atoms and the exchange current density of the M/M z+ electrode reaction have a remarkable influence on the growth mode of the M overlayer. Rearrangements involving MS and MM domains leading to equilibrium at the M overlayer were deduced from voltammetric data. The rate of these rearrangements depends mainly on the surface diffusion of M atoms. Correlations between the charge related to MS domains and ΔH s , the enthalpy of sublimation, and T m , the melting temperature of M, were established. These parameters reflect the trend of M atoms to participate in surface diffusion processes.

The possible formation of peroxide-type species during the O-electrosorption on gold in acid electrolytes

Abstract The conditions for the appearance of an anodic current preceding the O-electrosorption on polycrystalline gold in acid electrolytes are investigated through the application of triangular potential sweep technique. The anodic prepeak is related to the electrooxidation of products which behave similarly to those obtained in the O 2 -electroreduction on gold. The origin of the minor amounts of O 2 formed during the potential cycling is explained through the formation and heterogeneous chemical decomposition of a peroxide-type structure on the metal surface when most of the surface is covered by O-atoms. Results are discussed in relation to the reaction pathway of O 2 -electroreduction on gold.

Electrochemical kinetics of molten potassium thiocyanate on platinum. Mechanisms of anodic film formation and its cathodic dissolution

Abstract The kinetics and mechanism of the anodic film formation and its cathodic dissolution on platinum, through electrolysis of molten potassium thiocyanate, were studied. Experiments were made at 190°C by applying both repetitive and single pulse linear sweep voltammetry. The kinetics of the anodic film formation is a film-growth-rate controlled process, whose mechanism comprises various steps: discharge, nucleus formation and growth. The discharge step is a fast process and the second step is postulated as rate-determining at low potential sweep rates; the corresponding Tafel slope is RT/2F . At higher potentials the larger reaction rates achieved make reaction sites undistinguishable. This fact reflects a change of reaction mechanism involving a different Tafel slope for the anodic film formation, which is twice the former slope. The corresponding E/I curves are theoretically evaluated with parameters derived from the postulated mechanisms. The dissolution process is not definitely established although the likely pathway is envisaged. The catholic reaction is characterized by a Tafel slope of RT/F . The dissolution comprises a fast electron-transfer step followed by thiocyanate-ion formation; the latter appears as the slowest step.

Early stages of copper electrodeposition on columnar structured platinum electrodes

Abstract The electrodeposition of Cu from diluted CuSO 4 in 0.5 M H 2 SO 4 on columnar structured Pt electrodes was studied through voltammetry. For a constant set of experimental conditions, as the Pt electrode roughness is increased, bulk Cu electrodeposition tends to disappear, the amount of electro-adsorbed H atoms increases, and the coverage of Pt by 2-D Cu domains decreases. This means that under non-equilibrium conditions for diluted Cu 2+ -ion-containing solutions and highly rough Pt substrates, 3-D Cu nuclei, and 2-D Cu and free Pt domains are simultaneously present at the initial stages of the process. In these cases, bulk Cu can be detected when the degree of Pt surface coverage by Cu atoms exceeds 0.5.

Chemical evidence of hydrogen sorption processes on potential cycled gold electrodes

Abstract Transient activation of polycrystalline gold electrodes for the hydrogen electrode reaction, produced either through potentiostatic or potentiodynamic treatment, is related to the adsorption of H atoms and its penetration into the bulk. Evidence for H atoms results from the application of silver crystal decoration technique. Hydrogen-loaded activated gold electrodes in contact with a soluble silver salt solution yield silver deposits which can be studied through conventional voltammetric stripping. The increase in activation for the HER correlates with the increase in the voltammetric contribution of 0 electroadsorption peak assigned to the (110) crystallographic face of gold. The development of a faceted surface structure with preferred orientation in the direction of the less dense atomic arrangement apparently favours the penetration of hydrogen into the bulk metal.

Kinetics of anodic reactions of molten potassium thiocyanate on platinum: Formation and growth of anodic film☆

Abstract The film-growth process occurring on platinum anodes when molten potassium thiocyanate is electrolysed at 190°C has been investigated. The kinetics of the process has been studied by the application of different relaxation techniques. The film growth controls the rate of the process, and its mechanism comprises a surface diffusion process occurring after SCN−-ion oxidation. The film-thickening stage approaches a parabolic growth law. The faradaic rectification of the film is discussed in terms of its physicochemical properties and the actual site where anodic and cathodic reactions take place.

Kinetics and mechanism of anodic oxidation of nitrite ion in nitrate melts on platinum electrodes

Abstract The electrochemical oxidation of nitrite ion in KNO 3 -NaNO 3 eutectic melt on a platinum rotating disk electrode has been studied in the temperature range 247 to 312°C. The current/voltage curves comprise at least two regions. At high overvoltages the process is under a net convective diffusion control whereas at low ones the kinetics of the electrode process involves both convective diffusion and activation-polarization terms. From the current/voltage curves the kinetic parameters of each contribution are evaluated as well as the diffusion coefficient of nitrite ion in the melt. The electrode process is discussed in terms of the over-all reaction NO 2 − → NO 2 + e and of the reaction mechanisms previously postulated, where the initial electron-transfer step becomes rate-determining as the nitrite ion concentration diminishes. The results are compared to those obtained with the pure nitrite melt.

New effects in the electrochemistry of carbon dioxide on platinum by the application of potential perturbations

The electroformation and electrooxidation of carbon dioxide adsorbates were studied on polycrystalline and preferentially oriented platinum surfaces by the application of medium frequency potential perturbations in saturated carbon dioxide aqueous sulphuric acid solutions. Preferentially oriented platinum surfaces were obtained by the application of fast symmetric square wave potential programmes to polycrystalline electrodes based on well-known methodology. Two new electrochemical characteristics for the electrochemistry of carbon dioxide were observed after the application of the medium frequency routines, that is, an extended adsorption potential domain for the formation of carbon dioxide adsorbates, and novel voltammetric features for the two anodic stripping peaks. These effects upon carbon dioxide electrochemistry are dependent on the morphology of the platinum electrode surface and adsorption potentials.