Electrochemical reduction of noble metal compounds in ethylene glycol

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 degrees C up to 60 degrees 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: AuCl4- >Ag+ > PtCl62- > Pd(NH3)(4)(2+). 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 degrees 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. (C) 1999 Elsevier Science Ltd. All rights reserved.