Portugaliae
Electrochimica Acta

To evaluate the response of pyrite and arsenopyrite to oxidizing conditions, an electrochemical approach was employed, to set boundaries for the influence of the arsenopyrite content. The effects of galvanic interactions and local pH on the oxidation reaction of pyrite were also studied. With this purpose, artificial two-mineral electrodes were constructed ranging in concentration from 20-80 % arsenopyrite. The resulting cyclic voltammograms were analyzed and relative quantities of oxidation products were evaluated.

The aim of this paper is to evaluate the use of molibdate solutions for pre-treatments of zirconiun surfaces. Zirconium electrodes were held immersed in molibdate solutions for predetermined time intervals. With the subsequent determination of the pitting nucleation potential and the analysis of the surface morphology, it was possible to evaluate the effect of the molibdate concentration and of the solution pH on the pitting corrosion processes. Among the most important results, two are worthy of attention: (i) for all pH values, the immersion of zirconium electrodes in molibdate solutions hinders the pitting corrosion process of the metal, shifting the pitting nucleation potential to more positive values, (ii) the more pronounced shifts of the pitting nucleation potential were observed in alkaline solutions with high molibdate concentration and in acid solutions with low molibdate concentration; this effect was attributed to the adsorption of different species formed at different pH values.

The influence of potential-scan number on the Schiff base self-assembled monolayer was probed by the Cyclic Voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS) techniques for the first time. The results showed that the charge transferring resistance (Rct) could be reduced with the potential-scan, suggesting that the structure of the self-assembled monolayer was altered. Also, the relationship between the content of C=N group, represented approximately by the reduction electric charge (Qr), and the monolayer’s packing degree which was represented by the value of Rct, was described. Providing a new way to change the interface characterization is the main contribution of this paper.

Hybrid films of poly(o-aminophenol) and nickel sulfonated phtalocyanine are produced by electrochemical polymerization of o-aminophenol in the presence of the metal complex. The irreversible incorporation of the metal complex into the hybrid films is tested using reflectance infrared spectroscopy. Electrodes modified with poly(o-aminophenol) alone and the hybrid film show a higher electrocatalytic activity, for NO oxidation, than the base substrate electrode (glassy carbon). The hybrid films show higher currents and lower oxidation overpotential. The oxidation peak current is linear with concentration up to 200 sm M of NO.

Copolymers of aniline and 2-aminobenzoic acid are prepared by chemical copolymerization. The copolymers are soluble in basic media and thin films are prepared, onto glassy carbon electrodes, by evaporation of the solutions. The modified electrodes show a voltammetric response, with two peaks, similar to polyaniline with two peaks. The dependence of both peak potentials on pH is similar to that of PANI, suggesting that proton expulsion is the mechanism of charge compensation on oxidation. The mechanism is confirmed by Probe Beam Deflection, which shows proton expulsion on oxidation. However, while polyaniline films exchange mainly anions at pH>0, poly(aniline-co-(2-aminobenzoic acid)) (p2ABA) film exchange protons up to pH 4 .

The complete theory corresponding to a CE mechanism when applying cyclic chronopotentiometry to a spherical electrode of any size is developed. The influence of several variables on the transition time ratios, such as the electrode radius, rate constants of the homogeneous chemical reaction and current density, is discussed. A simple and practical criterion based on the variation of current density applied to the electrode is proposed for the detection of a CE mechanism.

Polyelectrolyte thin films constructed using the method proposed by Decher and co-workers are studied using several electrochemical techniques, such as cyclic voltammetry and linear sweep voltammetry with rotative disc electrode. The main goal of this work is to evaluate the influence of the number of layers and the charge of the external layer in thin film permeability. These films seem to have electrochemical properties strongly influenced by deposition conditions. It’s shown that the increased number of layers lowers film permeability.

The inhibition behavior of chromate ions towards corrosion and dezincification of α-brass (Cu/Zn: 64/36) in acetate buffer solutions (pH 2.4, 7.0 and 8.5) containing 3% sodium chloride was studied. Weight loss and solution analysis were used for a long period (four weeks) and electrochemical polarization for a short period (one hour). Chromate accelerated corrosion and dezincification at pH 2.4 by acting as depolarizer but chromate suppressed efficiently the corrosion and dezincification at pH 8.5. At pH 7.0 the corrosion and dezincification suppressed in the presence of chromate but the inhibition action started to deteriorate at chromate concentration less than 0.005 mol/L. The inhibition effect is attributed to adsorption of chromate ions on the corroding surface and improvement of the passivity. Chromate led to an increase in the concentration of copper species in solution due to the formation of the more soluble CuCrO4 than Cu2O, which contributed to the apparent suppression of dezincification.

The electrochemical behavior of Cu-(0.5-13 wt.%)Al alloys in K2SO4 0.5 mol dm-3 (m = 1.5 M, pH 2-5) was studied using open-circuit potential (Eoc) measurements and polarization curves, in order to know the conditions of copper oxides formation, influence of Al concentration and solution pH on the electrode processes. The open-circuit potential measurements with time indicated that the Eoc decreases with the Al content, due to the lower Al potential compared to Cu. The electrode potentials decrease ca. 20 mV per unity of solution pH as increase pH of the solution. The polarization curves showed changes with the increase of Al concentration. For alloys with up to 3 wt.%Al the obtained Tafel anodic coefficient was 40 mV/decade for low potentials region (E < -320 mV) and 130 mV/decade for the other region (-320 < E/mV < -250), indicating that up to this concentration there is no considerable change in the electrochemical behavior of the alloys, when compared to pure Cu. For alloys with an Al content from 8 to 13 wt.% the Tafel anodic coefficient was 60 mV/decade. No influence of the solution pH on the Tafel anodic coefficient was observed.

The electrochemical behaviour of coated Cr3C2-NiCr steel in aerated 0.5 M H2SO4 solution was studied by means of electrochemical ac and dc measurements. The structural characterisation of the coated steel, before and after electrochemical tests, was also performed in order to identify the mechanism of the electrolyte penetration through the coating up to the steel substrate, causing its corrosion. This characterisation may also help to explain electrochemical results. Three types of Cr3C2-NiCr coatings performed by a High Velocity Oxy-Fuel Spraying system (HVOF) were analysed. The facility for the electrolyte penetration through the coating and the corresponding electrochemical behaviour of the samples were strongly influenced by the spray parameters used and heat treatment applied. It was observed that heat-treated coatings (Q1 and Q3) showed better corrosion resistance than the as-sprayed coating (A). For coatings Q1 and Q3, the electrolyte did not reach the steel substrate during the measurement, leading to a better protection of the steel substrate against corrosion.

Cyclic voltammetry and surface analysis techniques (EDX microanalysis, mapping of elements and SEM images) have been used to study the solid state reactions of Ir in blank solution or Hg(I)-containing solution. The studies in KNO3/HNO3 solution permit to suggest the formation of a thin oxide layer film deposited in the substrate. The RCV process exhibits the decrease of capacitive current associated to the substrate surface restructuring. The electro deposition of Hg occurs initially in UPD condition from Hg(I)-containing solution, C[Hg(I)] = 3.51x10-7 mol/L. The electro deposition of bulk mercury occurs from Hg(I)-containing solution, C[Hg(I)] = 3.51x10-5 mol/L. It was observed the deposition of Hg in the substrate in open circuit for all employed conditions.

A new amperometric biosensor for lactose determinaion in raw milk was developed through the simultaneous immobilization of β-galactosidase and galactose oxidase on a derivatised polyethersulphone membrane. β-galactosidase catalyses the hydrolysis of lactose into galactose and glucose and galactose oxidase catalyses the oxidation of galactose into galactonic acid and H2O2. The membranes with the two immobilized enzymes were then used in an amperometric sensor, by oxidation of the H2O2 formed, at a Pt electrode of an Universal Sensors electrode base system. The sensitivity and the reproducibility of the biosensor thus formed were found to be 6.81 and 0.72 nA.M-1, respectively. Biosensors were found to be stable for 20 days.

The zinc electrochemical behaviour from sulphate and sulphate-gluconate baths was studied both on a vitreous carbon and a copper electrode. The voltammetric response depends on the complexing agent concentration, the electrode material, the pH and the cathodic potential limit. The reduction of zinc ions proceeds with hydrogen evolution, and at less negative potentials on the copper electrode than on the vitreous carbon electrode, specially in absence of gluconate. Gluconate decreases the efficiency of the process. On the other hand, gluconate favours zinc electrodeposition on vitreous carbon but retards it on copper.

The electrochemical intramolecular cyclisation of allyl 2-bromophenyl ethers in N,N'-dimethylformamide at constant current in a diaphragmless cell has been developed using Ni(II) and Co(II) complexes as electron-transfer mediators. Cyclic compounds are obtained in good yields under appropriate experimental conditions.

Iron disulphide molten salt electrochemical cells are among the most promising technological options for batteries. The electrochemical behaviour of pyrite allies excellent cathodic characteristics to optimal performance and low operational costs. The cathodic iron disulphide mechanism involves many processes, encompassing from polysulphides formation to the reduction of iron to the metallic state. The use of X-ray diffraction together with scanning electronic microscopy analysis on cells of the Li/KCl-LiCl/FeS2 system made possible to identify intrinsic and extrinsic parameters to the electrochemical process involved in the establishment of the polysulphides stoichiometry. It is necessary to note that the augmentation of the cell's internal resistance and loss of electrical capacity are directly related with the formation of these same substances. Researches in the electrochemistry of these phenomena aim to elucidate the cathodic interphase processes and the effect of every reaction in the global mechanism.

On the basis of an ohmic model and a Tafel equation describing relations between current density and overpotentials in the film and at the metal/film interface, respectively, it is shown that a quantitative analysis of galvanostatic transients for the growth of passivating ultra-thin films on the so-called non-noble metals can be obtained. As an example, the growth of ZnO on Zn in a boric/borate buffer solution is considered. In this case, the values of the transfer coefficient and the exchange current density of the reaction at the metal/film interface were found to be 1.2 and 0.11 mA cm-2, respectively. It was shown that a single, first film occurred at low current densities and two films at high ones. The ionic resistivity inside the single, first film, during the transients, has an initial constant value region followed by a final increase indicating the aging process. For this variation the evolution of the point defect concentrations is taken into account. For the variation of the ionic resistivity with the galvanostatic current density two types of behaviors were found, depending on the current density. An interpretation of these results is advanced in terms of the concentrations, mobilities and recombination rate of point defects inside the film.

CoFe films were electrodeposited on platinum and copper from an acidic sulfate bath. The deposits surface morphology was analyzed using SEM and composition was determined by XPS. For deposition potentials lower than -0.80 V (SCE), the anodic stripping voltammogram showed a complex anodic current peak evidencing the complexity of the deposition process. Analysis of the deposition current transient curves showed that the nature of the substrate influenced the kinetic of the process: it was observed an instantaneous nucleation on Pt and a progressive nucleation on Cu. Pure cobalt and cobalt-iron films formed on Pt and on Cu presented binding energies for Co2p3/2 signal corresponding to cobalt oxide, while the Fe2p3/2 signal is related to metallic iron. The corresponding metallic cobalt was observed for the Co2p3/2 signal with binding energy of about 778.0 eV.

The degradation of a reactive blue 4 dye in acidic solution has been compared using electrochemical reduction and oxidation at reticulated vitreous carbon electrode (RVC) and Ti/SnO2/SbOx (3%mol)/RuO2 (30%mol) electrodes and photo-Fenton method under UV irradiation. The reduction of RB4 dye at -0.6 V on RVC electrode results in 50% of color removal and up to 64% of TOC removal. The direct oxidation of RB4 dye at +1 V on RVC promotes only the oxidation of amine group and there is no color removal. On SnO2 electrode, 58% TOC was removed and the decolorization is around 100 % after 1 hour of electrolysis at 2.4 V indicating good efficiency. Best performance was obtained by photo-Fenton method that indicates 80% elimination of TOC and 100% of color removal but the method requires oxidant addition and time of rest to remove the generated residues.