Angel M. Meléndez

The Effect of the Cu2 + ∕ Cu + Step on Copper Electrocrystallization in Acid Noncomplexing Electrolytes

in all of the solutions employed. In cyclic voltammetry, a larger reduction peak was obtained in the nitrate electrolyte, in comparison with the other two media. This higher current has been associated with the catalytic reduction of nitrate ions on recently deposited metallic copper. This effect was confirmed in the potential-step analysis. Despite the fact that Cu + is not thermodynamically stable in these three solutions, the reduction step Cu 2+ /Cu + has been considered as an electron-transfer reaction. The anion type has a direct effect on this electron- transfer reaction and consequently, on the electrocrystallization process; sulfates provoke the most important current decrease associated with the Cu 2+ /Cu + step and also strongly influence the nuclei formation, while the nitrate ions accelerate the Cu 2+ /Cu +

On the Reactivity of Sulfosalts in Cyanide Aqueous Media: Structural, Bonding and Electronic Aspects

The reactivity of the ruby silver minerals proustite (3Ag(2)S⋅As(2)S(3)) and pyrargyrite (3Ag(2)S⋅Sb(2)S(3)) was studied with two types of electrodes: a carbon-paste electroactive electrode (CPEE) and a paraffin-impregnated graphite electrode (PIGE). Polycrystalline samples of α-Ag(2)S (acanthite), As(2)S(3) (orpiment), Sb(2)S(3) (stibnite), Ag(3)AsS(3) (proustite), Ag(3)SbS(3) (pyrargyrite), and three samples of the proustite-pyrargyrite solid solution series were synthesized from pure elements by a solid-state reaction method. Phase identification of samples was carried out by XRD and chemical homogeneity was checked by SEM-EDS. Besides, sulfosalts were characterized by diffuse reflectance spectroscopy (DRS). Flat-band and formal potentials of sulfosalts were determined by the Mott-Schottky method and differential pulse abrasive stripping voltammetry, respectively. Band structure, bonding and solid-state structure are considered to investigate the oxidation and reduction of the solids. A ligand-to-metal charge transfer (LMCT) transition from the AsS(3) (or SbS(3)) group to Ag is related to ease of reducing the pyrargyrite-proustite series. Despite the increase in the amount of As (Sb) in Ag(3)SbS(3) (Ag(3)AsS(3)), reactivity is similar due to the similarity of the solid-state structures, and the same oxidation states of S, As, Sb and Ag species in the lattice. However, the nature of the pnictogen (As or Sb) changes the position of the conduction and valence band edges and modulates the reactivity of the pyrargyrite-proustite series. Anodic dissolution occurs by hole transfer from the top of the valence band that is formed mainly by the states of the AsS(3) and SbS(3) groups. Meanwhile, silver reduction occurs by electron transfer from the Ag 5s orbitals located at the bottom of the conduction band. The difficulty in dissolving proustite and pyrargyrite in cyanide is related to the presence of pyramidal AsS(3) and SbS(3) groups in these sulfosalts.

An Approach to the Reactivity of Isomorphous Proustite (Ag3AsS3) and Pyrargyrite (Ag3SbS3) in Cyanide Solutions

Cyanidation is the main process for recovering silver from its ores. Its impact on hydrometallurgy over the last 100 years is very great. The complexity of the silver mineralogy is the main problem during cyanidation, since each ore is a problem in itself due to mineralogical and physical differences. Ruby silver ores, proustite (Ag3SbS3) and pyrargyrite (Ag3SbS3) are refractory to leaching with cyanide. Unfortunately, the chemistry of recovery of silver-bearing minerals in cyanide has been few understood. In order to understand the main factors associated to the reactivity of the ruby silver ores in the cyanide solution, synthesis and characterization of proustite and pyrargyrite has been performed.

Platinum leaching from automotive catalytic converters with aqua regia

Herein, kinetics extraction of platinum from spent auto catalysts, using nitric acid as an oxidant in hydrochloric acid solution, was investigated. The parameters such as temperature, hydrochloric and nitric acid concentrations, stirring speed, particle size and liquid/solid ratio, were analysed. The kinetic data were analysed using the shrinking core model. A variant of this model fits the kinetic data more appropriately. At a temperature of 90°C, the values of R2 in surface chemical reactions and diffusion were 0.819 and 0.937, respectively. With the alternative model, however, 0.991 was obtained. The activation energy for the dissolution was 35.75kJ/mol.

Electrochemical Determination of Minor Elements in Zinc Flotation Concentrates

Characterization of complex assemblages of sulfides, such as ores or concentrates, is critical to pre-mine planning and post-mine waste characterization. An inexpensive and rapid method based on electrochemistry detection is proposed to determine minor phases usually associated with sphalerite (o marmatite), in zinc flotation concentrates. The achieved information with this method has been compared with the chemical and mineralogical composition obtained from X-ray fluorescence (XRF), Xray diffraction (XRD) and scanning electron microscopy (SEM). In addition, a study of the influence of the particle size on electrochemical response was undertaken.

The role of boron in the carrier transport improvement of CdSe-sensitized B,N,F-TiO2 nanotube solar cells: a synergistic strategy

The synergistic effects of different engineering strategies, especially interface engineering, band structure engineering, and micro/nano engineering, can be exploited for the development of efficient photoanodes for quantum dot-sensitized solar cells (QDSSCs). Herein, we investigate the energy transfer mechanism and the charge carrier transport capacity of a set of photoanodes developed for a CdSe QDSSC. Boron, nitrogen and fluorine-tridoped TiO2 nanotube (BNF-TNT) membranes were obtained by anodization of titanium to self-organized TiO2 nanotube (TNT) layers, followed by a lift-off process. Then BNF-TNT membranes were adhered onto indium–tin oxide (ITO) conductive glass and sensitized by varying the load of CdSe quantum dots (BNF-Y-CdSe) using the SILAR method. The as-prepared electrode materials were characterized by FESEM, HR-TEM, DRS, XPS and Raman spectroscopy. The photochemical, photoelectrochemical, and semiconducting properties of the electrode materials were investigated by photopotential, photovoltammetry, photocurrent transient measurements, and Mott–Schottky analyses in 1.0 M Na2S. CdSe quantum dots (QDs) were homogeneously and intimately coated on BNF-TNT, which favored electron transport to the ITO substrate, and promoted a red-shift in the light harvesting of the composite toward the visible region (1.65 eV) from UV (2.75 eV). The highest photoresponse was obtained for BNF-TNT grown in 0.06 wt% H3BO3, and sensitized with CdSe QDs after five SILAR cycles. Boron doping in BNF-5-CdSe increased the photoconversion efficiency with respect to the CdSe-sensitized nanotubes without B-doping (NF-5-CdSe) by around 176% under one sun illumination (AM 1.5 G, 100 mW cm−2). The results showed that B-doping/sensitization synergism occurs by a Ti3+ states-to-CdSe QD electron transfer, which increases electron flow toward back contact. This allowed the enhancement of the electron lifetime, charge-collection efficiency and incident-to-electron conversion efficiency.

Simultaneous leaching of Pt, Pd and Rh from automotive catalytic converters in chloride-containing solutions

Dissolution of platinum group metals (PGM; herein Pt, Pd and Rh) in different chloride-based leaching systems from spent auto catalysts was performed. Response surface methodology and a five-level-five-factor central composite design were used to evaluate the effects of 1) temperature, 2) liquid-to-solid ratio, 3) stirring speed, 4) acid concentration and 5) particle size on extraction yield of PGM by aqua regia. Analysis of variance was used to determine the optimum conditions and most significant factors affecting the overall metal extraction. In the optimum conditions, leaching of Pt, Pd and Rh was 91.58%, 93.49% and 60.15%, respectively. The effect of different oxidizing agents on the PGM dissolution in chloride medium was studied comparatively in the following leaching systems: a) aqua regia/sulfuric acid mixture, b) hydrogen peroxide in sulfuric acid (piranha solution), c) sodium hypochlorite and d) copper(II). Dissolution of Rh is increased in both aqua regia and hydrogen peroxide/hydrochloric acid solutions by adding sulfuric acid.

Effect of substrate nature on the electrochemical deposition of calcium-deficient hydroxyapatites

Calcium phosphates were obtained by reducing nitrate ions to produce hydroxide ions on TiO2/stainless steel and TiO2/titanium electrodes. TiO2 coatings on metallic substrates were prepared by sol-gel dip-coating method. The morphology of deposits was observed by FESEM. Chemical nature of calcium phosphate deposits was identified by Raman micro-spectroscopy and FESEM/EDS microanalysis. Electrochemical behavior of nitrate and nitrite reduction on stainless steel and titanium electrodes was studied by linear sweep voltammetry. In addition, voltammetric study of the calcium phosphate electrodeposition on both electrodes was performed. From these measurements was selected the potential to form a calcium phosphate. A catalytic current associated to nitrate reduction reaction was obtained for stainless steel electrode, leading to significant deposition of calcium phosphate. Ca/P ratio for both substrates was less than 1.67. The formation of calcium deficient hydroxyapatite was confirmed by Raman spectroscopy.

Considerations on electrical impedance measurements of electrolyte solutions in a four-electrode cell

A tetrapolar probe to measure the electrical properties of electrolyte solutions was implemented with gold electrodes according to the van der Pauw method. Electrical impedance spectroscopy (EIS) measurements of different concentrations of phosphate buffer saline (PBS) solution and an oral mucosal tissue sample dispersed in PBS were performed in the galvanostatic mode using a four-electrode cell (tetrapolar probe). Taking advantage of using a potentiostat/galvanostat for carrying out the electrical measurements, a simple and rapid method using a three-electrode electrochemical cell is described for: a) cleaning of electrodes, b) verification of surface chemical state of electrode material and c) choice of current supplied to electrodes for EIS measurements. Results of this research shown a depolarization effect due to the addition of oral mucosa tissue cells into the PBS solution.

Electrosynthesis and characterization of Hg1-xCdxSe films

An electrodeposition process from an aqueous bath of CdSO4, SeO2 and Hg2Cl2 at pH 2.3 was performed to synthesize two Hg1-xCdxSe alloys with x ≥ 0.8 onto TiO2. In order to establish an intimate contact between Hg1-xCdxSe and the TiO2 substrate, electrodeposition of Hg1-xCdxSe films was investigated. Reduction processes of H2SeO3 and Cd2+ to produce CdSe and Hg1-xCdxSe were studied by cyclic voltammetry. Initially, deposition of selenium allows codeposition of Cd2+ and Hg2+. Hg1-xCdxSe film was obtained by holding the electrode at a constant potential. The characterization of the films was performed by GIXRD, FESEM, and the chemical composition of the film was obtained by energy-dispersive X-ray spectroscopy, photoluminescence and Raman spectroscopy.