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

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: AuCl4 - > Ag+ > PtCl6 2- > Pd(NH3)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