Electroless Reduction Of Gold(iii) On Pyrite From Chloride Electrolytes

The electroless reduction of aqueous precious metal salts on arsenopyrite, galena, zinc blende, stibnite, wolframite and molybdenite was reported as early as 1870. Since this initial work, very little has been reported about the electroless reduction mechanism of gold(III) on pyrite, despite the geochemical importance of this reaction. Recently, surface analytical techniques have been used to study the chemical and morphological characteristics of laboratory-controlled metal deposition. Since the reduction of gold(III) on pyrite is rapid, surface analytical techniques have not been adequate to elucidate the entire mechanism.;On the other hand, electrochemical techniques in conjunction with surface analytical measurements have proven valuable in determining the reduction mechanism. In oxygen-free chloride solutions at room temperature, reproducible steady state rest potentials are set up on electrodes either of pyrite alone, or of pyrite upon which gold is depositing from solutions containing gold(III) chloride. Tafel measurements have provided the number of electrons in the rate-determining step, as well as the rate dependence on hydrogen ion, chloride ion and gold(III) concentrations for gold reduction on pyrite and metallic gold. The rate law has been confirmed using X-ray photoelectron spectroscopy (XPS). A reduction mechanism consisting of two slow one-electron reductions of gold(III) chloride ion from the neutral gold(III) chloride complex, followed by a fast one-electron reduction, with nucleation and growth of metallic gold, was determined from Tafel plots and linear sweep voltammetry. The rate of the gold(III) reduction reaction was not transport limited. X-ray photoelectron spectroscopy showed that a Au(I) intermediate may be partially stabilized on the surface