R. Herrera Urbina

Synthesis of monodisperse Au, Pt, Pd, Ru and Ir nanoparticles in ethylene glycol

Au, Pt, Pd, Ru and Ir nanoparticles with a narrow size distribution have been synthesized by chemical reduction of their corresponding metal species in ethylene glycol. In all cases, the average particle size was found to be smaller than 10 nm. Particle size was mainly controlled by varying the initial total metal concentration, the reaction temperature, and the concentration of PVP. With the exception of Ir, metal particle agglomeration and sintering was prevented by the addition of PVP, a well known protective agent that also aids particle dispersion.

Electrochemical reduction of noble metal compounds in ethylene glycol

Abstract The effect of temperature on both the electrochemical oxidation of pure ethylene glycol and the reduction of AuCl4− in ethylene glycol at a rotating disk glassy carbon electrode has been investigated using linear sweep voltammetry. As the temperature is increased from 25°C up to 60°C, ethylene glycol begins to oxidize at lower potentials, whereas the reduction potential of AuCl4− is independent of temperature. Reduction current densities, however, increase as temperature increases. Room temperature reduction of several noble metal species in ethylene glycol was also investigated. Metal reduction potentials at both a platinum and a glassy carbon electrodes follow the sequence: AuC14−>Ag+>PtC162−>Pd(NH3)42+. The oxidation potential of ethylene glycol at both electrodes was found to be more positive than the reduction potential of the gold, silver, platinum and palladium precursors. These results predict that the spontaneous formation of noble metal particles by chemical reduction with ethylene glycol is thermodynamically unfavorable at 25°C. Gold and silver particles, however, are easily prepared at room temperature using the polyol process, which is a redox based process for the preparation of finely divided metals by chemical reduction of the corresponding metal precursors with ethylene glycol. Since measured potentials are the sum of a thermodynamic and a kinetic contribution (the overpotential), metal reduction in the polyol process seems to be aided by the overpotential. Therefore, measured potentials have been correlated to the chemical conditions at which noble metal particles are synthesized in the polyol process. It was found that as the potential difference between ethylene glycol oxidation and metal reduction increases, both the reaction temperature and time needed for metal synthesis increases. These electrochemical results may contribute to have a better understanding of the fundamentals of the polyol process, and for optimizing such reaction parameters as temperature, time and solution chemistry.

Flotation of oxidized minerals of copper using a new synthetic chelating reagent as collector

Abstract A new synthetic reagent containing a mixed aliphatic-aromatic structure, with a hydrocarbon chain and an aminothiophenol chelating group, has proven to be an effective collector for the flotation of chrysocolla minerals. The flotation is optimum in the narrow pH range of 5.5 to 6, falls sharply at pH 5, and is moderate in the pH range 7 to 11. Infrared spectra indicate that copper aminothiophenolate chelates are formed on the surface of the chrysocolla under the conditions of maximum flotation.

Electrochemical reduction of noble metal species in ethylene glycol at platinum and glassy carbon rotating disk electrodes

Abstract Linear sweep voltammetry has been used to delineate the electrochemical behavior of ethylene glycol, and to determine the reduction potential of several noble metal species in this solvent at room temperature. Ethylene glycol was found to be electrochemically inactive between −1.15 and 1.65 V at a glassy carbon electrode, and between −0.82 and 2.0 V at a Pt electrode. Metal reduction potentials determined using both rotating electrodes follow the sequence: AuCl 4 − >Ag + >PtCl 6 2− >Pd(NH 3 ) 4 2+ . Under all conditions tested, ethylene glycol oxidation began at potentials more positive than metal reduction ones, thus suggesting that ethylene glycol cannot reduce these noble metal species. However, finely divided Ag and Au, were synthesized at room temperature by reduction of their corresponding ions with ethylene glycol (the basis of the polyol process). This observed difference between electrochemical results and chemical synthesis can be explained by recognizing that measured potentials are the sum of a thermodynamic potential and overpotential. Comparison between metal reduction potentials and temperature for metal particle synthesis indicates that the potential becomes more negative as the temperature increases. These results may provide useful information to better understand the fundamentals of the polyol process.

In situ deposition of silver and palladium nanoparticles prepared by the polyol process, and their performance as catalytic converters of automobile exhaust gases

Abstract In situ deposition of silver particles onto alumina and palladium particles onto mixed CeZr oxides has been achieved upon chemical reduction of the corresponding metal species (AgNO 3 and PdCl 2 ) by ethylene glycol in the presence of polyvinylpyrrolidone. The support oxide powders were found to keep their crystalline structure and morphology after treatment with hot ethylene glycol while the BET surface area decreased after metal deposition. Microprobe maps obtained from energy dispersive X-ray analysis revealed a homogeneous distribution of metal nanoparticles on the surfaces of alumina and of the mixed CeZr oxides. Supported silver and palladium were tested as catalytic converters of simulated exhaust automobile gases. The catalytic activity of silver-loaded alumina powder catalyst for CO and hydrocarbon oxidation as well as NO and NO x reduction, was found to be higher than that of a reference silver catalyst. Palladium-loaded mixed CeZr oxides powder catalyst showed a similar performance to that of a reference palladium catalyst as a three-way catalyst converter.

Recent developments and advances in formulations and applications of chemical reagents used in froth flotation

Over the last two decades, significant advances have been made in the field of chemical reagents used in froth flotation, a solids separation technique whose success depends on the use of a wide variety of both inorganic and organic reagents, including polymers, for controlling the wetting behavior of solid surfaces, particle dispersion or aggregation, and bubble stability. Novel discoveries include new chemistries of reagents, especially collectors, depressants and frothers, new blends of known chemical compounds, new applications of known reagents, and the development of scientific methodologies for reagents design and testing of potential formulations. Even though the most important application of froth flotation has been the processing of raw materials, in recent years new applications of flotation continue to grow. Depletion of easy-to-process, high-grade ores and an increasing demand for clean technologies require new flotation reagents, that are environmentally friendly, more versatile, and economic. This article reviews recent developments of new chemistries for froth flotation, some of which are now available as commercial products, and their applications.

A reversible inorganic electrochromic solution system

Abstract Electrochromic devices that respond to varied levels of applied electrical potential by changing light opacity are presently prepared from either insertion compounds or reversible electrodeposition of metals from solution. We report here another approach to electrochromics based on the reversible deposition/dissolution of silver oxide onto FTO-coated glass from an aqueous solution containing silver (I)-ammonia complexes. The basic redox chemistry underlying the functioning of this electrochromic system has been identified using traditional techniques such as X-ray diffraction, scanning electron microscopy, and cyclic voltammetry, as well as a quartz nanobalance for measuring the working electrode weight at different potentials. The AgNH3+↔AgO reaction is responsible for the coloration/decoloration of the working electrode, which can be repeatedly cycled between various states of visual opacity. Optical measurements reveal that the transmittance of the working electrode in the visible region drastically drops from 80% without silver oxide deposition to 4% when it is coated with a silver oxide film.