Reduction mechanism of Au(III) species adsorbed on -MnO₂

Au(III)-chloride complex and its hydrolysis products, [AuCl4-n(OH)(n)](-), are typical Au species in the hydrosphere, and are known to be adsorbed and then reduced to Au(0) on some metal oxides. However, the process from adsorption to reduction has not been made clear. In this paper, adsorption of [AuCl4-n(OH)(n)](-) was performed using two different metal oxides, Al2O3 and delta-MnO2. The experiments were performed for 72 h at pH 4.0, with Au (III) initial concentration of 0.10 mmol/dm(3) and 1.0 g/dm(3) of delta-MnO2 or Al2O3. The adsorption behavior of [AuCl4-n(OH)(n)](-) differed greatly between the solids. On Al2O3, Au(III) complex ions formed outer sphere complexes and maintained their coordination structure even after adsorption. On the other hand, Au(III) complex ions formed inner sphere complexes on delta-MnO2 and the adsorption ratio reached 100% within 30 min. And then, the adsorbed Au(III) complex ions spontaneously reduced to Au(0). Detailed analysis of both the solid and liquid phases proved that the Au(III) complex ion, [AuCl1.67(OH)(2.33)](-), first adsorbed onto delta-MnO2 by exchanging its OH- with surface OH groups. After that, Cl-, which was originally coordinated to Au(III) complex ions, was gradually changed to OH- in water to form pseudo [Au(OH)(4)](-) on the surface, and Au(III) was reduced. Our experimental results suggested that ligand exchange caused by the formation of inner sphere complexes is the key to inducing redox reactions after adsorption.