Yeonuk Choi

Quantitative SHINERS analysis of temporal changes in the passive layer at a gold electrode surface in a thiosulfate solution.

Shell-isolated gold nanoparticles (SHINs) were employed to record shell-isolated nanoparticle-enhanced Raman spectra (SHINERS) of a passive layer formed at a gold surface during gold leaching from thiosulfate solutions. The (3-aminopropyl)triethoxysilane (APTES) and a sodium silicate solution were used to coat gold nanoparticles with a protective silica layer. This protective silica layer prevented interactions between the thiosulfate electrolyte and the gold core of the SHINs when the SHINs-modified gold electrode was immersed into the thiosulfate lixiviant. The SHINERS spectra of the passive layer, formed from thiosulfate decomposition, contained bands indicative of hydrolyzed APTES. We have demonstrated how to exploit the presence of these APTES bands as an internal standard to compensate for fluctuations of the surface enhancement of the electric field of the photon. We have also developed a procedure that allows for removal of the interfering APTES bands from the SHINERS spectra. These methodological advancements have enabled us to identify the species forming the passive layer and to determine that the formation of elemental sulfur, cyclo-S8, and polymeric sulfur chains is responsible for inhibition of gold dissolution in oxygen rich thiosulfate solutions.

Active and passive behaviors of gold in cyanide solutions

Abstract Active and passive behaviors of pure gold (Au) and roasted gold ore (RGO) electrodes were investigated at 25 °C in de-aerated agitated cyanide media. Cyclic voltammetry and potentiodynamic polarization with agitation at 100 r/min in 0.04 mol/L NaCN solution showed different peak positions and current densities. Potentiodynamic tests illustrate that the peak current densities increase greatly with increasing the cyanide concentration. Increasing the pH value from 10 to 11 resultes in a great decrease of current density, while it increases noticeably by decreasing the agitation from 100 to 60 r/min. In the presence of oxygen, Au and RGO electrodes show different characteristics of peak positions and corrosion rates. The potentiostatic studies show that increasing the potential from 1 to 1.4 V at pH value of 11 results in an 80% decrease of current density while decreasing the pH value from 11 to 10 at 1 V gives a 1.7 fold increase of current density, possibly due to more effective passive layer. Following polarization, electrochemical noise measurements (ENM) during decay periods show that Au results in more passive states at high potentials, showing pitting corrosion. The ENM results show that this technique can be a promising tool for a better understanding of gold leaching. The XPS studies prove the presence of passive oxides.

Passive behavior of gold in sulfuric acid medium

Anodic behavior of pure Au as compared to platinum (Pt) in H2SO4 solutions was considered by different electrochemical techniques for an appropriate insight. Cyclic voltammetry studies showed two oxidation and one film reduction peaks for Au, while one oxygen evolution reaction for Pt. Increasing H2SO4 concentration (from 0.5 to 1 mol/L) caused 2-fold increases in peak current density of Au. Increase in agitation promoted passive zone of Au, while it was negligible for Pt by potentiodynamic studies. Potentiostatic studies (2 h) at three anodic passive potentials in 1 mol/L H2SO4 showed that the admittance of Au was found to be the lowest at 1.4 V. Electrochemical noise measurements during the decay periods (16 h) after polarization showed that the thin passive film formed during potentiostatic polarization has been dissolved.