J. González Velasco

Influence of changes in the total surface area and in the crystalline surface composition of Pt electrodes on their electrocatalytic properties with respect to the electro-oxidation of hydrazine

Abstract The electro-oxidation of hydrazine on electrodispersed and electrofacetted Pt electrodes in acid solutions takes place without noticeable changes in the mechanism with respect to that followed on smooth polycrystalline Pt electrodes. Nonetheless, a negative shift in the oxidation peak potential is observed with increasing roughness factor, R , of the electrodes. By working with preferentially oriented electrodes it was possible to demonstrate that the shift towards more negative potential values of the hydrazine oxidation peak is due to a slightly higher proportion on the electrodispersed electrodes of (100) microfacets.

On the composition and stability of some D(+) saccharic acid complexes

Abstract We have investigated the composition and stability of the systems formed by: Mn(II), Fe(III), Co(II), Ni(II), Cu(II) and Cd(II) ions with D(+) saccharic acid, using a method of electrophoresis on paper at 25°C, with an ionic strength of 0·05 and by potentiometric determination of the proton association functions with an ionic strength of 1 at 25°C. The results obtained by using both methods were consistent with each other. The Rajan Martell treatment was used for the study of the proton association curves of the Cu(II)-D(+) saccharate system and the formation of the (Cu(LH3)−)n was proved. The formation equilibrium constants of the metal chelates were determined and the IR spectrum of the complex formed with Cu(II) was obtained. The groups involved in the bond with the central metal ion were the carboxyl groups and the OH in the α position. The possible structures of the complex species formed are given.

In situ FTIRS study of ‘reduced’ CO2 on columnar-structured platinum electrodes in different acid media

In situ Fourier transform infrared spectra (FTIRS) of species related to ‘reduced’ CO2 (r-CO2) adsorbates formed on columnar-structured Pt electrodes in aqueous 0.5 M HClO4, 0.5 M H2SO4 and 0.5 M H3PO4 at 25°C are presented. From spectral, electrochemical and electrode topography data, the appearance of different types of complex adsorbates involving CO2, Hads, multi, bridge-, and linearly bonded CO has been concluded. The influence of anions on the CO adsorbate adsorption band is tentatively interpreted through changes in the water-electrolyte structure caused by different types of anions.

On the surface diffusion of Pt adatoms in acidic solutions

Abstract Surface diffusion coefficients, D s , of platinum adatoms were determined by means of in situ electrochemical methods on columnar-structured Pt electrodes immersed in 1M HClO 4 , 1M H 3 PO 4 and 0.5M H 2 SO 4 electrolytic solutions. Measurements were made at different potential values covering the range from hydrogen coverage formation to that in which platinum oxides begin to be formed. The D s values observed confirm an adsorbate induced relaxation of the Pt surface atoms and could explain why restructuring of Pt electrodes has been observed under some experimental conditions. A study of the variation of D s with temperature leads to values of ΔH ≢ f and ΔS ≢ at three potentials, which can be explained according to interactions between adsorbates and Pt surface sites. According to the data obtained, a surface diffusion mechanism in three steps is formulated, in which different steps become rate determining at different potential ranges.

Open-Circuit Study of Stain Etching Processes Leading to the Formation of Porous Silicon Layers

Layers of monocrystalline silicon were used as a working electrode in a three-electrode electrochemical cell with platinum as reference electrode in order to carry out stain etching processes leading to the formation of porous silicon layers. The potential at which the corrosion process takes place was measured as a function of the concentration of the oxidizing agent, and the reaction order with respect to the nitrate concentration was found to be close to one. From the experimental results a mechanism is formed for the stain etching process and a model proposed for growth of the porous layer.

MEASUREMENT OF SURFACE DIFFUSION COEFFICIENTS IN AIR OF COLUMNAR STRUCTURED PLATINUM NEAR ROOM TEMPERATURE

Abstract The surface self-diffusion coefficients, D S , of columnar structured Pt were measured in air following the decrease with time of the roughness factor obtained by means of electrochemical techniques. From D S values in a narrow temperature range (20–60°C), the activation enthalpy for the surface self-diffusion process and the pre-exponential factor D 0 have been calculated as 47.5 kJ mol −1 and 3.3×10 −11 cm 2 s −1 , respectively. The low values of D S are the consequence of low D 0 , since the activation enthalpy is lower than that necessary to induce surface diffusion at temperatures close to the melting point of Pt.

Electro‐Oxidation of Glyoxylic Acid on Pt Polycrystalline Electrodes in Acidic Solutions

The electrocatalytic oxidation of glyoxylic acid in acid medium on platinum electrodes has been investigated. The oxidation mechanism proposed excludes any dissociative adsorption of the organic molecules. This fact is explained as the consequence of the stabilization of the C-H bond in the aldehyde group induced by the deactivating and polarizing effects exerted by the carboxylic group. Ideas for the explanation of oxidation currents produced by aldehydes, alchols, and other organic compounds in the potential range corresponding to the double-layer charging are given in the context of the discussion of the glyoxylic acid electro-oxidation mechanism

Electrocatalytic oxidation of glyoxalate in alkaline medium on platinum adatom electrodes

The influence of the foreign metal adatoms deposited at underpotentials on the oxidation of glyoxalate on platinum was studied in alkaline medium. Pronounced catalytic effects caused by underpotential submonolayers of Pb, Tl, Cd, Cu and Cr were observed. The enhancement of the oxidation processes by underpotential submonolayers has been interpreted according to the bifunctional theory of the electrocatalysis.

Residual Currents Associated with Surface Reorganization Processes for Solid Metal Electrodes in Contact with Electrolytes

Residual currents were measured at an open circuit by connecting a columnar structured gold or platinum electrode of high roughness factor with another flat electrode of the same material, both immersed in the same electrolytic solution. Such currents are interpreted as the consequence of capacitance changes associated to the potential difference between both electrodes provenient from surface-free energy differences. An equation has been derived which allows an estimation of the surface diffusion coefficients for metal atoms in contact with electrolytes from current vs. time plots.

Study of complexes between D(+) saccharic acid and A1(III), Ga(III), Cr(III) and Mo(VI) ions, by electrophoretic methods

Abstract A study was carried out on the composition and stability of the complexes formed by the ions: A1(III), Ga(III), Cr(III) and molybdate with D(+) saccharic acid, using the method of electrophoresis on paper at 25·C and with an ionic strength of 0.05. A study was also made of the variation of the electrophoretic mobilities as a function of the concentration of D(+) saccharic acid at different pH values, in order to determine (Jokls method [4, 5]) the composition and stability constants of the possible complexes formed. In the cases of A1(III), Cr(III) and Ga(III) it was concluded that there exist olation equilibria and complexes of the type (LH2Me(III))−. Once again, it has been noted that the chemical behaviour of the A1(III) and Ga(III) ions is very similar.