Portugaliae
Electrochimica Acta

Special Issue dedicated to the XVIII Congress of SIBAE (Sociedad Iberoamericana de Electroquímica)

Carbon-supported binary PtSn/C and ternary PtSnNi/C catalysts were prepared for the electro-oxidation of ethanol. The carbon-supported nanoparticles were synthesised by employing a modified polyol methodology and characterised in terms of structure, morphology and composition by using XRD, EDX and TEM techniques. Their electro-catalytic behaviour for ethanol oxidation (EO) was investigated by employing a disc-composite electrode covered by a thin layer of catalyst imbedded in a Nafion polymer electrolyte film. An enhancement of the EO through a negative shift of the onset oxidation potential and higher current density at constant potentials were observed on the PtSnNi/C electrodes as compared to those for PtSn/C electrodes. The temperature was changed in the range 40 ºC ≤ t ≤ 80 ºC, and the variation of the activation energy calculated by linear regression of the Arrhenius plots.

The electro-oxidation, over platinized titanium and ruthenium oxide anodes, of nitrogen containing molecules (urea, reactive Blue 4 dye, acetonitrile, formamide, guanidine and pyridazine) was investigated, monitoring the products distribution. The N-mineralization leads to have inorganic pollutants (NH3/NH4+ and/or NO2-/NO3-). Amidic and aminic compounds react both in homogeneous (acid hydrolysis) and in heterogeneous phase (direct electroxidation) with a rate depending on the original state of oxidation of nitrogen. Heterocyclic and multiple-bond carbon-nitrogen molecules were effectively converted with negligible mineralization of nitrogen due to the stability of their first oxidation intermediates. The obtained results (high rate of nitrate generation) evidence the need of coupling of the direct electroxidation with other process to limit the nitrate concentration to an accepted level; in accordance, dialysis (of the ammonia cation) and indirect oxidation (chlorine-mediated) were proved to be valid alternatives.

Laccase is a poliphenoloxidase enzyme that catalyzes the oxidation of catechol compounds in the corresponding quinones. The current obtained in this redox process can be used for quantitative analysis. In this work, a carbon paste biosensor modified with a crude enzymatic extract of the Pycnoporus sanguineus fungi as a lacase source is proposed for catechol determination and other phenolic compounds. The effect of carbon paste and electrolyte composition, pH from 3.0 to 8.0, scan rate from 10 to 40 mVs-1 and potential pulse amplitude from 10 to 60 mV on the differential pulse voltammetric response was investigated. The analytical curves were linear in the catechol concentration range from 2.0 x 10-5 to 7.0 x 10-4 molL-1, with detection limits of 4,5 x 10-6 molL-1. This biosensor was used for the determination of different kind of phenolic compounds, presenting better response for catechol, hydroquinone and resorcine.

Removal of acid orange 24 color was studied in an electrochemical cell composed of a dimensional stable anode (DSA), a stainless steel cathode and an electrolyte media containing sodium sulfate, sodium chloride, and two commercial surfactants named Uniperol y Leophen, corresponding to wetting and dispersant agents, both of them required to favor the color incorporation inside the fiber. DSA used in this study comprises a titanium mesh covered by a layer of iridium, and a stainless steel plate covered by a layer of iridium-tin mixture. Experimental approach allowed getting an inside of the rol played by sulfate, chloride and surfactants for both color and chemical oxygen demand (COD) removal. Obtained results have shown that the Ir-Sn electrode provides the highest color removal under chloride mediated oxidant conditions. Evaluation of the DSA performance in solutions containing surfactants has shown that obtained color removal is similar for both electrodes; otherwise the COD removal attained with the Ir DSA is about six times the one obtained with the Ir-Sn electrode.

The electrochemical and mechanical behavior were evaluated for specimens of concrete clean and contaminated with 2 and 4% of NaCl (of the weight of cement), elaborated with two types of cements: compound and waterproof. The dosage of the mixture of concrete was done on the basis of weight in two 0.45 and 0.65 water/cement ratios. The specimens under study were exposed in two media: clean water and 3% of NaCl solution. The test to evaluate the presence of corrosion of the reinforced steel consisted in the monitoring of the half cell potentials as the norm ASTM C-876-91 indicates. For the mechanical behavior the compression test was used, according to the ASTM C-39/C 39-M-99 norms. This paper presents the results obtained for the first 4 months of monitoring, identifying the influence of the attack of chlorides in the corrosion of steel and its mechanical properties. There is no a significant influence of the type of cement on the parameters of the study.

This work is part of the DURACON iberoamerican project, which deals with the characterization of the concrete durability exposed to environmental conditions at Iberoamerica, through the exposure of reinforced concrete specimens in at least two different environments, one marine and one urban, in each participating country. In this paper, results of the first eight months of the specimen exposition to environmental conditions in the Xalapa city, Mexico, are evaluated. The specimens were built with two types of concretes at 0.45 and 0.65 water/cement ratios. In all cases, the same type of cement with three thicknesses (1.5, 2.0 and 3.0 cm) of covering were used. The tests to evaluate the susceptibility to corrosion of the reinforced steel were resistance to linear polarization and monitoring of the corrosion potentials according to the ASTM C-876 norm.

The use of silane formulations for the pre-treatment of metallic substrates prior to painting or as topcoat has been increasing during the recent years. The proposed formulations are very attractive because they present environmental friendliness, enhance adhesion properties and provide a barrier layer that delays the corrosion processes. One of the most used techniques to synthesized based-SiO2 coatings is sol-gel. The preparation of sol-gel coatings with specific chemical functions offers potential advantages over traditional methods as it offers tailoring of their structure, texture and thickness and allows the fabrication of large coatings. The present work investigates the electrochemical behaviour of commercial carbon steel coated with ceria doped SiO2 by sol-gel process in basic media. To evaluate the influence of temperature on adhesion and protective properties, the coatings were sintered at three different temperatures, 623 K, 723 K and 873 K, for 3 h. The anticorrosion behaviour, of the films has been characterized in an aggressive medium by electrochemical techniques, while structural characteristics and morphology was followed by X-Ray Diffraction, Fourier Transformed Infra-Red and Scanning Electron Microscopy techniques. The results indicated that the thin films act as a protective barrier against exposure to the corrosive medium (3.5 wt-% NaCl solution) and increase the lifetime of the susbtrate.

Nickel 200 is used in the petrochemical industry in the handling of caustic solutions using a variety of components and equipments made of materials highly resistant to corrosion. However, due to their high costs and their impact on failures by corrosion, it is necessary to resort to other alternative materials that could mitigate this phenomenon. One of them is steel coated with electroless nickel (without passing flow), which in certain situations and under certain conditions offers high resistance. The aim of this study was to evaluate the corrosion resistance of the coated steel test coupons by electroless nickel (EN) in different concentrations of caustic soda and establish the optimal coating thickness for each concentration of NaOH. The chemical analysis conducted by EDX indicates that the composition of coatings in phosphorus compounds are phosphorus by 10.5% and 89.5% nickel with hardnesses average of 550 HV. The corrosion rate in steel EN was in the order between 10.0 and 13.6 mm/yr (25 °C) and 10.2 and 14.7 mm/yr (85 ° C). While for nickel 200 it was in the order between 4.7 and 5.2 mm/yr (25 °C) and 7.0 and 9.9 mm/yr (85 °C). Nickel 200 was comparatively more corrosion resistant than steel coated with electroless nickel, but its cost is higher.

In this work, the electrochemical synthesis and characterization of rhodium nanoparticles are reported. The Rh nanoparticles were electrochemically synthesized at room temperature by direct electroreduction of rhodium (III) chloride (RhCl3) salt. We have applied different values of current intensity and potential in order to control the formed size of Rh nanoparticles. We have used tetrapropyl ammonium bromide (TPAB), as the support electrolyte and at the same time, effectively stabilizing the nanoparticles formed in aqueous medium. Size and morphology of nanoparticles formed, are subsequently studied by Transmission Electronic Microscopy (TEM). Electrocatalytic activity and stability for the oxidation of methanol, were studied using cyclic voltammetry. Nanoparticles of Rh synthesized present an electrocatalytical activity toward the electrooxidation of methanol.

The adverse effects of endocrine disruptors on human health have created a need for screening systems to detect xenoestrogens. In this project, we develop a system to detect one of these xenestrogens, Bisphenol A (BPA), through the development of a biosensor based on impedance measurements. The biosensor consists of a supported lipid bilayer that incorporates a protein receptor sensitive to the presence of BPA. This setup was originally proposed by V. Granek and J. Rishpon [19]; we now present an alternative method of analysis to increase sensibility. As a preliminary evaluation of bioactivity of BPA we measured the direct interaction of this xenoestrogen with the membrane by analyzing changes in the membrane impedance as a function of BPA concentration. The results indicate that BPA inserts and disrupts the membrane, but only at very high unphysiological BPA concentrations (> 1 ppm). We then developed a high sensitivity biosensor, based on the detection of the electric signal by means of impedance spectroscopy, resulting from the interaction between BPA and an estrogen receptor reconstituted in a lipid membrane. The results show that the presence of the receptor increases by several orders of magnitude the sensitivity of the system, making it possible to detect BPA at ppb.

Cations present in the soil solution interact with soil charged surfaces by electrostatic forces. An applied electric field promotes principally, the migration of ions of the soil solution, whose interactions with charged surfaces are not so strong. If, by means of an applied electric field to the soil, the transport of a higher valence ion such as Ba2+ is promoted, it would be expected that cations of lower valence should be desorbed from the soil exchange sites. The main goal of the present work was to study the effect, not only of the applied electric field, but the Ba2+ ion migration, in the extraction of a major cation (K+), naturally founded in a volcanic soil. Experimentally, it was found that Ba2+ migration through the soil, increased in an important way the amount of K+ ions which are mobilized by effects of the applied electric field.

Amorphous and/or nanocrystalline Al-based alloys have better mechanical properties when compared with crystalline conventional Al-alloys. Different techniques can be used to obtain amorphous Al-based alloys, usually in a ribbon form (from melt-spinning processing) or powders (from gas atomization or ball milling processing). To obtain bulk samples, that is, samples with dimensions typically much larger (mm scales) than the ones obtained from the above mentioned techniques (<40 m), the ribbons or powders must be hot-consolidated. One of the most important challenges in the development of such alloys is to keep a refined microstructure after the necessary heating. The present work focuses the pitting resistance by electrochemical corrosion resistance test in solution 0.9 % NaCl and pH 7.0, for Al90Fe7Nb3 and Al90Fe7Zr3 nanocrystalline alloys, which were obtained by hot-extrusion of mechanically alloyed powders, and melt-spinning process. The results of the polarization curves indicated that the Al90Fe7Nb3 and Al90Fe7Zr3 ribbons present better corrosion properties than the extruded alloys.

The electro-synthesis of a nickel metalloporphyrin by using the 5,10,15,20-tetraquis(p-hydroxyphenyl)porphyrin (TPPOH) as binding agent is reported. Before the electrosynthesis, cyclic voltammetry technique was applied to establish the best conditions for the electrochemical reduction of the porphyrin to the di-anion radical. The electrosynthesis of the metalloporphyrin was done under the conditions found from voltammetry. The electrolysis was performed in a non-divided cell, with a nickel sacrificial anode and at controlled potential, in order to favouring the chemical formation of the metal-complex by reaction between the porphyrin dianion radical and Ni(II) ions, both of them electro-generated during the process. The effect of the use of an ultrasonic wave of 20 kHz in the metalloporphyrin electro-synthesis process was studied. The porphyrin, of free base, and the products obtained in both the absence and the presence of ultrasonic were characterized through Atomic Absorption, UV-Visible, IR and elemental analysis. The results of this characterization make available to establish that the process leads to the formation of the metalloporphyrin of Ni(II) with TPPOH, with an electrolytic efficiency greater than 80%. The application of the 20 kHz ultrasonic wave at the amplitude of the study favoured the yield of the reaction and did not lead to changes in the electrochemical mechanism.

Using chronoamperometry technique and a ferrous ammonium sulphate bath, an iron film was electroplated on a silver electrode. The characterization of the iron film was carried out using the analytical Electron Probe for Microanalysis (EPMA), X ray Diffraction (DRX) and Spectroscopy Mössbauer Transmission (TMS). The obtained results indicate that the film had an approximated thickness of 20 μm and it was compound mainly of metallic iron with crystalline structure bcc. Analysis EPMA indicated that the iron film presented islands of silver of different sizes in all the area.

The electrical properties of conducting polymers make them useful materials in a large number of technological applications. In the last decade, it has been showed an important effect on the properties of the conducting polymer when iron oxide particles are incorporated into the conductive matrix. In the present study, films of polypyrrole were synthesized in the presence of magnetite particles. The effect of the magnetite particles on the structure of the polymer matrix was determined using Raman spectroscopy and mass variations at different concentrations of Fe3O4 incorporated into the conducting matrix were also calculated by means of quartz crystal microbalance

PtRu/C electrocatalysts were prepared by hydrothermal carbonization process using starch as carbon sources and reducing agents and platinum and ruthenium salts as catalysts of carbonization process and metals source. pH of the reaction medium was adjusted using KOH or TPAOH (tetrapropylammonium hydroxide). The obtained PtRu/C electrocatalysts were characterized by SEM/EDX, TGA, XRD and cyclic voltammetry. The electro-oxidation of methanol was studied by cyclic voltammetry and chronoamperometry. The PtRu/C electrocatalyst prepared using TPAOH was more active for methanol electro-oxidation than the material prepared with KOH.

The goal of this work is the evaluation of the analytical characteristics of the determinations performed using glucose oxidase and acetylcholinesterase based electrochemical sensors, developed applying original or optimized conventional methods of enzyme immobilization. It was found that the sensitivity of glucose determination, for example, varies from 0.048 to 3.36 mA L mol-1 cm-2 and the response time of the glucose oxidase based sensors – from 5 to 30 s, according to the method of the bioreceptor immobilization. The sensitivity of the analysis is affected from the activity of the immobilized biocomponent, from the composition of the solution (concentration of the substrate, of the mediator and of the inhibitor), and from the experimental conditions (pH, temperature, agitation), as well as from the kinetic parameters of the studied process. It was found that the immobilized glucose oxidase conserves its substrate specificity in the presence of a number of glucides (galactose, maltose, fructose, and saccharose) in 100 fold higher concentrations. The selectivity of glucose analysis is ensured applying a suitable potential. Interferences free glucose amperometric determination was performed at 0.00 V/SCE, in the presence of ascorbates and urates. The electrochemical quantification of enzyme inhibitors allows reaching particularly low limits of detection (10-9 - 10-14 mol/L).

Piezoelectric ceramics with perovskite structure based on lead zirconate titanate (PZT) show excellent electrical properties near the morphotropic phase boundary (MPB) and thus are widely used for actuators, sensors as well as microelectronic devices. However, because of the high toxicity of lead oxide and its high vapour pressure during sintering, the use of lead-based ceramics has caused serious lead pollution and environmental problems; for this reason, the development of lead-free piezoelectric ceramics has been demanded to replace lead-based ceramics. Therefore, it is necessary to develop lead-free piezoelectric ceramics with good properties. Among the lead-free piezoelectric materials, the alkaline niobate-based perovskite compounds and Bi-containing materials, have attracted a large amount of attention because of their superior characteristics. The alkaline niobates (K,Na)NbO3 (KNN)-based material exhibits especially good piezoelectric properties and have been studied as substitutes for PZT piezoelectrics. In this work, alkaline niobate-based piezoelectric ceramics were sintetized, system (K0.5Na0.5)xLi(1-x)NbyTa(1-y)O3, and were characterized by X-ray diffraction (DRX) and scanning electron microscopy (SEM). In order to determine their transition temperature, electrochemical impedance spectroscopy (EIS) was used, and it was found the transition temperature was about 420 ºC, which is a desirable value in this kind of materials.

Atrazine is a highly used herbicide and it has been found in both deep and superficial waters. Its solubility in water is reduced and is relatively stable in humid environments, where it has a half-life of one hundred days. Atrazine can be degraded by oxidative photolysis or by microorganisms. It is moderately toxic in humans, animals and plants, because it can be absorbed by inhalation, ingestion or through the skin. In this work, we study the degradation of atrazine in aqueous solution using current controlled electrolysis at a platinum electrode. The effects of pH, current magnitude and direction, and temperature, were evaluated. The atrazine decomposition was monitored during electrolysis by UV-Vis spectrophotometry; quantification of atrazine was done by GC/MS, and quantification of cyanuric acid was done by HPLC. It was found that at 25 ºC in acid media, atrazine is degraded partially to cyanuric acid with formation of persistent intermediate compounds, but at 60 ºC atrazine is completely degraded to cyanuric acid. The TOC results indicate no electrochemical combustion and no mineralization was observed under the experimental conditions studied.

This paper deals with the use of reticulated vitreous carbon (RVC) and graphite felt (GF) as porous electrode for the removal of 20 ppm Cu(II) in 0.5 mol dm-3 Na2SO4 at pH 2 (which resembles a rinsing wastewater generated by a plating industry). The experimental mass transport characterization (kma = buc) showed that for 100 ppi (RVC), the value of the coefficient b, associated with magnitude of porous electrode, is 0.88, while for (GF) is 3.38. On the other hand, c value for 100 ppi (RVC) is 0.06, while for (GF) is 0.62, indicating that the flow pattern is a complex function of the shape of the electrode. The experimental potential drop for 100 ppi (RVC) and (GF) (1.2 cm thick), indicated the absence of hydrogen evolution. Current efficiencies for RVC and GF were function of convection, giving values comprised between 45    68% and 51    73%, respectively, and energy consumption values of 0.3 < Econs < 1.7 and 0.4 < Ec < 1.1 kWh m-3, respectively. Theoretical number of identical cells in the stack (N) necessary to the cupric depletion from 20 to 1 ppm, for 100 ppi (RVC) and (GF) were calculated.

The fuel cells are new alternatives for energy production with low environmental impact. Nowadays, conducting polymers are considered the most promising material to manufacture these type of cells because of their easy application and operation, being polypyrrole (PPy) one of them. The electrochemical synthesis of single-layer polypyrrole and two-layers or multiple-layers polypyrrole/platinum (PPy/Pt) films is reported in this paper. The films were obtained by electropolymerisation of pyrrole and cathodic deposition of platinum from ammonium hexachloroplatinate salt on stainless steel 304. The composition, morphology, conductivity and the electrochemical properties of the films were study by Raman Spectroscopy, SEM, Profilometer, impedance and cyclic voltammetry. The possible use of PPy and PPy/Pt films as electrodes for fuel cells is also discussed.

Paz de Rio, municipality of the North of the department of Boyacá (Colombia), has a coal waste dump for the residues of washing of the coal produced by the preparation of this commodity and locates them without any treatment, action that originates the sterilization of the soil. The big volume produced of the waste causes environmental problems and a health risk for the nearby populations because the toxic metals contained, which flow and polluted the waters of the rivers Soapagua and Chicamocha, and decreases the suitable land for the agricultural uses. In consequence, it is necessary to apply a treatment that decreases the negative effects over the water and soil resources of the region. By this reason, it is required a treatment of the waste, such as the electrokinetic remediation, for applying a voltage gradient for a time (Zhou, 2004). The electric current acts as an agent that moves the contaminants to the anode or the cathode, in agreement with their charges and direction of flux (Acar y Alshawabkeh, 1993). This way, the research found the ability of the electrochemical remediation to handle 6 Kg of the waste, with a humidity of 30 %, applying a voltage of 30-40V.

In this work the physical-chemical properties of six different soils original from Tenerife Island, Spain, are related with the measured electrical current in an electrokinetic experiment. Results show that total electrical charge has relation with properties such as: total metal content, pH, carbon organic content, and cationic exchange capacity. Since all these properties are between those which allow to evaluate the fertility of a soil, an electrokinetic experiment with samples of this soil is proposed like diagnosis tool.