Morphology Study And Mechanism Analysis Of Au(111) Terrace Evolution Duringthe Electrochemical Oxidation Of Sodium Thiosulfate

As a catalyst of heterogeneous oxidation reactions,gold is attracted much attention from the wold,and its demand is increases exponentially every year.Currently,gold is mainly obtained from ores leaching,and thiosulfate becomes a potential gold lixiviant to replace cyanidation method applying in gold leaching industry since its advantages of environmental friendliness and low cost.However,the problems of high lixiviant consumption and complex reaction kinetics/mechanismthiosulfate in gold leaching limited it to completely replace cyanidation presently.Therefore,it is a key point to study the molecular level dynamics and reaction mechanism of electrochemical dissolution and deposition of gold electrode.In this work,in-situ electrochemical atomic force microscope?EC-AFM?was used to study the surface morphology evolution of Au?111?terrace during the electrooxidation of sodium thiosulfate,and the mechanism of thiosulfate electrochemical leaching of gold was analyzed.In the neutral and alkaline media at room temperature,as Na₂S₂O₃ or NH₃ existed singly,no dissolution is happened on Au?111?electrode surface.The electro-oxidation of Na₂S₂O₃was accelerated by the increase of potential,with the chemisorbed of polythionates,sulfate and elemental sulfur on the Au?111?surface.In Cu²⁺-NH₃-S₂O₃^2-solutions with air atmosphere,on one hand,the lower concentration of Cu²⁺leading to the high overpotential of gold dissolution.On the other hand,the higher concentration of Cu²⁺could lower the overpotential of Au dissolution and accelerate the dissolution.However,copper could reacted with S₂O₃^2-and catalyzed thiosulfate oxidation,making the fast consumption of substrate thiosulfate.The initial potential of Au?111?electrode was set at-0.20 V to prevent electrochemical dissolution of gold in the reaction process.At the initial stage of Au?111?dissolution,the preferred reaction sites were terrace rather than steps or defect edges.With increasing electrode potential,pit defects appeared on the terrace,deepened and enlarged to present layer-by-layer dissolution of Au?111?electrode.As the potential at the open-circuitpotential,the dissolution of Au?111?electrode transferred from layer-by-layer type dissolution to pits typed dissolution,and finally the crystal surface was passivated by produced elemental sulfur.At potential of higher than open circuit potential,the dissolution of Au?111?presented as pits type dissolution.The increasing potential accelerate the dissolution of Au atom as pits.The deposition potential of Au at-0.50V,and Au atoms were initially deposited on the steps and pits edges,then on the terrace.In the atmosphere of Arogen,the Au?111?crystal surface presents as pits type with layer-by-layer dissolution,and no passivation occurred during the reaction process.In the absence of oxygen,the rate of dissolution of Au was decreased as well as the rate of oxidation of S₂O₃^2-.Based on the experimental results,the electrochemical reaction mechanism of Au?111?crystal surface dissolution in Cu²⁺-NH₃-S₂O₃^2- system solution was analyzed.The adsorption of elemental sulfur on Au?111? as the passivation film inhibited the dissolution of gold during the electrochemical reaction processes.The higher active energy of step edges and defect edges making the stronger bonding of these sites with elemental sulfur than that on the terrace.Therefore,[Cu?NH₃?₄]²⁺ in the lixiviant solution reacted more easily withthe bonded sulfur on the terrace than that on the steps and defect edges.Therefore,the adsorption on the terrace of elemental sulfur first begins to dissolve,leading to the expose of Au atoms firstlyon theterrace,then S₂O₃^2-combined with Au atoms,producing the stable Au?S₂O₃?₂^3- complex into solution,representing the pits type dissolution occurring.After[Cu?NH₃?₄]²⁺ is reduced to[Cu?S₂O₃?₃]^5-,it can be oxidized by oxygen in the solution,also can be oxidized to[Cu?NH₃?₄]²⁺ in the anodic region of the Au electrode,which repeate the dissolution of elemental sulfur on the electrode surface.