Electrochemical study of thiourea and substituted thiourea adsorbates on polycrystalline platinum electrodes in aqueous sulfuric acid

The electrochemical response of adsorbates produced on polycrystalline Pt from thiourea, methyl thiourea, 1,3 dimethyl thiourea and tetramethyl thiourea dissolved in aqueous 0.5 M sulfuric acid are comparatively studied using electrochemical routines. The adsorption kinetics of thioureas on Pt follows an Elovich-type equation. Their saturation coverage, measured from the decrease in the H-atom electrosorption charge after correction for molecular size for steric effects, decreases as the size of the molecule producing adsorbates increases. Adsorption data fulfill the empirical Frumkin isotherm with a repulsive adsorbate–adsorbate lateral interaction term. Adsorbate electrooxidation starts at about 0.65 V vs SHE. The deprotonation of hydrogen-containing thioureas yields soluble products, their electrochemical behaviour being to some extent similar to that of formamidine disulfide. For E > 0.65 V vs SHE, the oxidation of thioureas can be described as complex processes in which intermediates compete with oxide layer formation on platinum.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada

The surface diffusion of gold atoms on gold electrodes in acid solution and its dependence on the presence of foreign adsorbates

The surface diffusion coefficient (Ds) of Au atoms in 0.5 M H2SO4 at 298 K has been determined from the time dependence of the surface roughness factor of electrodispersed Au electrodes in 0.5 M H2SO4. The value of Ds for 0.5 M H2SO4 is 5 × 10−14 cm2 s−1, a figure which depends considerably on the presence of adsorbable species in solution. Thus, the adsorption of pyridine produces a remarkable decrease of Ds, whereas the adsorption of Cl− ion results in the opposite effect. The corresponding values of Ds are 1.2 × 10−14 cm2 s−1 for 0.5 M H2SO4 + 0.1 M pyridine, and 2.5 × 10−13 cm2 s−1 for 0.5 M H2SO4 + 5 × 10−4 M KCl. These results are interpreted through the influence of the adsorbate on the Au surface atom diffusion mechanism.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas (INIFTA)Facultad de Ciencias Exacta

Competition between the rearrangement of electrodeposited overlayers of copper on platinum and the hydrogen electrode reactions

The electrodeposition of Cu2+ ions on polycrystalline Pt from acid aqueous CuSO4, solutions was studied in the underpotential (upd) and overpotential (opd) ranges by using potentiostatic and potentiodynamic techniques. In the upd region, monotonic current transients were recorded for potentials (Ed) more positive than the reversible potential (Er) of the Cu2+/Cu electrode. For Cu electrodeposits involving a charge density close to that of the complete Cu monolayer, the hydrogen electrode reactions were strongly inhibited. Otherwise, non-monotonic current transients were recorded for Ed → Er. These transients suggest rearrangement of the monolayer structure when additional Cu atoms are deposited. In the opd region where the early stages of threodimensional(3D) nucleation and growth take place, the contribution of the hydrogen electrode reaction reappeared due to bare platinum sites which are present in the early stages of 3D growth. The present results in the limiting case can be interpreted in terms of a 2D to 3D transition at the overlayer level implying a certain distribution of clusters and the reappearance of the bare substrate surface. This process, in principle, accounts for the anomalies observed systematicly in vohammetric measurements for this system.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada

Electrochemical study of hydrogen absorption in polycrystalline palladium

The hydrogen reactions on polycrystalline Pd in 0.1 M NaOH at 25°C have been studied by using transients at constant potential, and by impedance spectroscopy and X-ray diffraction techniques. At potentials, Es, more positive than the reversible potential, Er, for the H2 evolution reaction, the current-time response and the impedance data indicate H atom diffusion into the bulk Pd. The X-ray diffraction pattern of electrodes cathodized during 20 min at these potentials are similar to those obtained for Pd. At Es s moves in the negative direction. The Nyquist plot for the rising part of the transients indicates the H atom diffusion into the bulk metal and H2 evolution on the Pd surface at high frequencies. The X-ray diffraction pattern of the electrodes cathodized at Es r, shows the presence of the βPdH phase and Pd. The experimental results indicate that different reactions take place simultaneously in this potential range: (i) Hs evolution, (ii) H diffusion into the bulk Pd, (iii) nucleation and diffusion-controlled growth of the βPdH phase. Taking into account the contribution of these reactions, a model, which is able to reproduce the experimental current transients, is presented.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada

Stochastic and deterministic behaviours of 316 stainless steel pitting corrosion in phosphate-borate buffer containing sodium chloride

The pitting corrosion of 316 stainless steel (SS) in phosphate-borate buffer (pH 8.00) containing 0.5 M NaCl at 25°C is studied through current transients run under a constant potential step, potentiodynamic runs and scanning electron microscopy. According to the potential and time windows different laws for pitting kinetics are observed. For small number of pits, the kinetics of pit growth is governed by a stochastic law whereas for a large number of pits a deterministic law which corresponds to an instantaneous nucleation and growth is obeyed. At the initial stages two different rate constants for pit nucleation are obtained. This fact correlates with two well distinguishable types of pits observed through SEM observations. Results are discussed through a model involving the penetration of chloride ions through the passive film and the existence of at least two types of sites at the metal surface which in turn become centers of pit nucleation. Pit stability is directly related to the Cr(III) content in the passive layer which can be shifted by changing the applied potential with respect to that of the Cr2O3/CrO2−4 redox couple.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada

The surface diffusion of gold atoms on gold electrodes in acid solution and its dependence on the presence of foreign adsorbates

The surface diffusion coefficient (Ds) of Au atoms in 0.5 M H2SO4 at 298 K has been determined from the time dependence of the surface roughness factor of electrodispersed Au electrodes in 0.5 M H2SO4. The value of Ds for 0.5 M H2SO4 is 5 × 10−14 cm2 s−1, a figure which depends considerably on the presence of adsorbable species in solution. Thus, the adsorption of pyridine produces a remarkable decrease of Ds, whereas the adsorption of Cl− ion results in the opposite effect. The corresponding values of Ds are 1.2 × 10−14 cm2 s−1 for 0.5 M H2SO4 + 0.1 M pyridine, and 2.5 × 10−13 cm2 s−1 for 0.5 M H2SO4 + 5 × 10−4 M KCl. These results are interpreted through the influence of the adsorbate on the Au surface atom diffusion mechanism.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas (INIFTA)Facultad de Ciencias Exacta

The behaviour of copper anodes in aqueous thiourea-containing sulphuric acid solutions. Open circuit potentials and electrochemical kinetics

The electrochemical behaviour of copper anodes in aqueous thiourea (TU)-containing sulphuric acid was followed from open circuit potential measurements, voltammetry and rotating disk and ring-disk electrode data. The open circuit potential of copper is controlled by equilibria involving different Cu(I)–TU complex species. The thiourea electro-oxidation which yields formamidine disulphide and Cu(I)–TU complex species can be described as a process under intermediate kinetics taking place on a partially blocked copper anode. The degree of copper surface coverage and the type of predominant adsorbate depend on the concentration of thiourea and applied electric potential. Depending on the potential range adsorbates can be formed directly from either a strong TU–copper interaction or a relatively weaker formamidine disulphide interaction. The passivating layer can be related to the formation of polymer-like Cu(I)–TU layers and residues resulting both from thiourea and formamidine disulphide electro-oxidation. A previously proposed complex reaction pathway can explain the general behaviour of these systems, including adsorbate formation and the influence of the electrolyte solution constituents.Instituto de Investigaciones Fisicoquímicas Teóricas y AplicadasFacultad de Ciencias Exacta

The behaviour of copper anodes in aqueous thiourea-containing sulphuric acid solutions. Open circuit potentials and electrochemical kinetics

The electrochemical behaviour of copper anodes in aqueous thiourea (TU)-containing sulphuric acid was followed from open circuit potential measurements, voltammetry and rotating disk and ring-disk electrode data. The open circuit potential of copper is controlled by equilibria involving different Cu(I)–TU complex species. The thiourea electro-oxidation which yields formamidine disulphide and Cu(I)–TU complex species can be described as a process under intermediate kinetics taking place on a partially blocked copper anode. The degree of copper surface coverage and the type of predominant adsorbate depend on the concentration of thiourea and applied electric potential. Depending on the potential range adsorbates can be formed directly from either a strong TU–copper interaction or a relatively weaker formamidine disulphide interaction. The passivating layer can be related to the formation of polymer-like Cu(I)–TU layers and residues resulting both from thiourea and formamidine disulphide electro-oxidation. A previously proposed complex reaction pathway can explain the general behaviour of these systems, including adsorbate formation and the influence of the electrolyte solution constituents.Instituto de Investigaciones Fisicoquímicas Teóricas y AplicadasFacultad de Ciencias Exacta

Steric effects on molecular adsorption at columnar surfaces : A model for the adsorption of pyridine on gold electrodes

Results for pyridine (Py) adsorption on columnar-structured (cs) gold substrates from acid solutions at different potentials and 298 K are compared to the predictions of a recently proposed Langmuirian adsorption model at columnar surfaces. The difference in the apparent adsorption Gibbs energy derived from the adsorption isotherm of Py on cs- and smooth gold substrates was accounted for by a correction factor (f) that depends on the roughness (R) of the substrate according to f∝R−n, as expected from Monte Carlo simulation of the model. This agreement indicates that the substrate geometry plays a key role in the interpretation of adsorption isotherms.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada

Changes in the composition of the passive layer and pitting corrosion of stainless steel in phosphate-borate buffer containing chloride ions

The influence of the passive layer properties on the pitting corrosion of 316SS was studied in phosphate—borate buffer containing chloride ions by using potential step and potentiodynamic techniques complemented with scanning electron microscopy. The increase of the anodization time in the passive region decreases the nucleation rate and the mean number of corrosion pits formed onthe 316SS surface. Results are explained through changes in the structure and composition of the passive layer during anodization. Two different Cr(III) species can be voltammetrically detected at short anodization times, an outer weakly bound Cr(III) species which is electroaoxidized to soluble CrO42− and an inner Cr(III) species which is electrooxidized to Cr(VI) but retained in the film at potentials lying in the transpassive region. As the anodization time in the passive region increases, the weakly bound Cr(III) species is transformed into another more stable one, probably an iron chromite, which exhibits an electooxidation potential more positive than that of Cr(III) species. The aged passive layer becomes more resistant to pit initiation, due to either a decrease in the density of active sites or a decrease in the nucleation rate constant for pit initiation.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada