X-ray Absorption Spectroscopy Probing of Gold Electrooxidation Reveals Intermediate Surficial Au(I)

While Au electrooxidation in acidic aqueous media on a phenomenological level proceeds directly from Au(0) to Au(III), it has previously been suggested that Au(I) states are intermediate species of the oxidation mechanism. Here we add to the evidence for the transient Au(I) by probing Au oxidation operando in a pH=3 HClO4 electrolyte by high energy resolution fluorescence detected X-ray absorption near-edge structure (HERFD-XANES) at potentials up to 1.8 V vs the reversible hydrogen electrode. The perchlorate ions in the electrolyte are used as sacrificial oxidizing agents. The reduced perchlorate compounds in turn produces chlorine ions which reacts with Au ions to form chlorinated Au-Cl compounds. The operando HERFD-XANES detects the chlorinated compounds yielding separate peaks based on the oxidation state of Au. Through this methodology, we observe the formation of Au(I) during the early stages of Au oxidation, and we further infer that Au(I) is accessed by the electrolyte These observations are consistent with the previously hypothesized route for Au electrooxidation involving charge transfer after a dipole induced place-exchange step.