H.T. Mishima

Electrochemical oxidation of ammonia in alkaline solutions : its application to an amperometric sensor

The electrochemical oxidation of ammonia was studied at room temperature on a black platinum electrode, iridium black, a mixture of both and PtIr electrodeposits in alkaline solutions of potassium hydroxide. The current density for the ammonia discharge was proportional to the ammonia concentration in the range from 50 to 200 ppm of the functioning of the sensor. It was found that the current density on the platinum-iridium mixture is higher than on platinum black and iridium black. On the other hand, PtIr electrodeposits on Au and Pt were prepared and characterized by X-ray analysis. The diffractograms evidenced the presence of a continual platinum-iridium solid solution where platinum is replaced by iridium atoms in the fcc structure. The PtIr electrodeposits showed a good response to the ammonia gas.

Characterization of platinum-rutheniun electrodeposits using XRD, AES and XPS analysis

Abstract Metallic deposits of platinum-ruthenium were produced on a gold substrate by electrodeposition from salts in platinum and ruthenium solutions. These deposits were characterized by X-ray diffraction, Auger electron spectroscopy and X-ray photoelectron spectroscopy. The XRD studies showed a crystalline alloy whose structure was cubic centered on the face; that is, an alloy of one phase as a substitutional solid solution where the platinum atoms were replaced by ruthenium atoms. Some deposits grew with the crystals oriented according to a preferential direction and developed a crystallographic texture. The XPS analysis revealed that, in principle, there are no compounds with oxidation states different from metal ruthenium and metal platinum. Good correlations were obtained between the values of the ratios in atomic composition of Pt–Ru for the different samples calculated for XPS and Auger spectroscopy. The catalytic activity of the electrodeposits was measured in methanol solutions. From the XRD studies, the network parameters and the crystallographic texture were compared before and after using them in electrochemical methanol oxidation. The distribution of the observed relative integrated intensities of the diffraction patterns of the deposits showed a variation which is more evident when the deposit has a preferential grain orientation. The reflective power of planes (111) decreases after methanol oxidation.

An electrochemical impedance spectroscopy study of electrodeposited manganese oxide films in borate buffers

The reduction-oxidation processes and conducting properties of manganese dioxide films prepared by electrochemical deposition on Pt substrate were studied in borate/boric acid solutions using electrochemical impedance spectroscopy (EIS). The characteristic parameters of the film can be determined as a function of the manganese oxide deposition routine as well as film thickness by application of transfer function analysis using non-linear fit routines. A model developed for thin layer intercalation electrodes, which is based on the insertion of protons and electrons, was used and the diffusion coefficients of protons were calculated for different manganese oxide thicknesses. The reversible behaviour of the manganese oxides during discharge-charge processes as a function of deposition routines was analyzed. Data obtained under a wide variety of experimental conditions show that the film generated on Pt substrate behaves as a n-type semiconductor.

Characterization of passive films on zinc electrodes by impedance measurements and XPS

Abstract The behaviour of the passivating layers formed by anodic polarization on zinc electrodes in borate and arsenate and in borate arsenate mixture solutions of pH 8.9 was studied by means of impedance measurements and XPS analysis. An equivalent circuit for the system is employed for the data fitting. Since an analysis based on the Mott—Schottky relationship leads to an excessively high carrier density with respect to the corresponding resistivity value, the potential drop should occur through the whole passive film. In accordance with the XPS results, a model can be proposed consisting of a layer of hydrated zinc oxide which should be responsible for the passivation. The passivating film on zinc electrodes has a more protective effect when it is formed in borate 0.1 M and arsenate 0.1 M solutions than in arsenate 0.5M solutions.

Surface studies of Pt-Ru electrodeposits on gold

Abstract Auger electron spectroscopy, energy dispersive X-ray techniques and X-ray diffraction were used for the characterization of platinum-ruthenium electrodeposits prepared on gold. SEM-EDAX analysis showed that the gold surface is totally covered by a homogeneous electrodeposit after 30 min of potentiostatic electrodeposition at 0.05 V vs. rhe . The AES analysis of the low kinetic energy peaks revealed that the electrodeposits with more Pt, presented more oxygen on the surface and those deposits with higher intensity ratio of Ru showed more oxygen in bulk. The composition of the deposits calculated from MNN transitions showed that the surface is enriched in Pt. On the basis of the XRD spectra, the electrodeposits, with solPt Ru ratio higher than 1.5, form alloys. The catalytic activity was observed in 3.5 NH 2 SO 4 + 2 M methanol solution at 70 °C. The electrodeposit with 80% Pt and 19% Ru and 1% O (at 5 nm) exhibited the highest apparent current density at 450 mV vs. rhe , for the oxidation of methanol.

Extracción de naringina de Citrus paradisi L. estudio comparativo y optimización de técnicas extractivas

The comparative study of the different extractive techniques of naringin was made, using grapefruit peel (Citrus paradisi). The best conditions for previous preparation of the material are the fragmentation of fresh peel and drying through a drying chamber and subsequent fragmentation. For fresh peel it turns to be more convenient the extraction with ethyl alcohol by Soxhlet system (obtaining 26 g/kg of fresh weight), while for dried peel the best results are obtained in the extraction with the mixture ethyl alcohol - water in the proportions 70:30 by maceration (20 g/kg of fresh weight) and reflux (22 g/kg of fresh weight). Evidences of the importance of water in the extraction process were observed. The values of naringin concentration in peel of different varieties of Citrus paradisi of the region of Argentinean North West were determined.

The electrochemical response of manganese hydroxide-oxide films in slightly alkaline solutions. II. Voltammetric and ellipsometric studies

Abstract The potentiodynamic behaviour of colloidal manganese hydroxide–oxide films obtained by chemical precipitation on a conducting substrate in alkaline solutions is described. The voltammetric response of cycled electrodes shows a main redox couple which is associated with the Mn(III)/Mn(IV) reaction occurring in solid state. The observed dependence of peak currents and peak potentials on sweep rate, charge and solution pH is similar to that reported for manganese and iridium oxides obtained by cycling the base metal. The optical response of cycled hydrous manganese oxide films is studied by in-situ ellipsometry. Data can be explained using an anisotropic single layer model. The optical anisotropy is related to the morphology of the hydrous oxide layer. The electroreduction process produces a decrease of the refractive indices and a pronounced diminution of the extinction coefficients yielding a practically transparent film.

The electrochemical response of manganese hydroxide—oxide films in slightly alkaline solutions—I. The redox couple

The electrochemical response of the Pt/Mn(OH)2 interface subjected to potentiodynamic sweep and XPS analyses shows the Mn(III)/Mn(IV) for the reaction—oxidation process. The XPS study shows that the oxygen spectrum changes during the cathodic discharge process and can be deconvoluted into two components: MnO and MnOH. The surface composition of the film has been described according to the 0.1 s spectrum analysis. The MnOH/MnO ratio increases during the reduction process and the oxidation number calculated at different potentials changes from 4 to 3.4.

An Ellipsometric Study of Manganese Oxide Films In Situ Characterization of the Deposition and Electroreduction of MnO 2

Inst. de Ciencias Quimicas Fac. de Agronom. y Agroindustrias Univ. Nac. de Santiago del Estero, 4200 Santiago del Estero

Effect of arsenate anion on the dissolution and passivation of zinc electrode in slightly alkaline solutions

Abstract Zinc dissolution and passivation in either arsenate or borate and a mixture of arsenate-borate solutions at pH 8.9 was studied by polarization curves and cyclic voltammetry. The arsenate anion produces an increase on zinc dissolution. Potential sweep rate, rotation speed, temperature, dissolved oxygen and anion nature determine the current-potential response in the corrosion region of the metal. Thick films are obtained by repeated potentiostatic steps in a still electrode and by a potentiostatic step in a rotating disc electrode. Repassivation are observed in the current-time response. EDAX analyses indicate the presence of arsenic and sodium in the film.