Alkene Epoxidation and Oxygen Evolution Reactions Compete for Reactive Surface Oxygen Atoms on Gold Anodes

Rates and selectivities for the partial oxidation of organic molecules on reactive electrodes depend on the identity and prevalence of reactive and spectator species. Here, we investigate the mechanism for the epoxidation of 1-hexene (C₆H₁₂) with reactive oxygen species formed by electrochemical oxidation of water (H₂O) on gold (Au) in an aqueous acetonitrile (CH₃CN) electrolyte. Cyclic voltammetry measurements demonstrate that oxygen (O₂) evolution competes with C₆H₁₂ epoxidation, and the Au surface must oxidize before either reaction occurs. In situ Raman spectroscopy reveals reactive oxygen species and spectators (CH₃CN) on the active anode as well as species within the electrochemical double layer. The Faradaic efficiencies toward epoxidation and the ratios of epoxide formation to O₂ evolution rates increase linearly with the concentration of C₆H₁₂ and depend inversely on the concentration of H₂O, which agree with analytical expressions that describe rates for reaction between C₆H₁₂ and chemisorbed oxygen atoms (O*) and exclude proposals for other forms of reactive oxygen (e.g., O₂*, OOH*, OH*). These findings show that the epoxidation and O₂ evolution reactions share a set of common steps that form O* through electrochemical H₂O activation but then diverge. Subsequently, epoxides form when O* reacts with C₆H₁₂ through a non-Faradaic process, whereas O₂ evolves when O* reacts with H₂O through a Faradaic process to form OOH*, which then deprotonates. These differences lead to distinct changes in rates in response to electrode potential, and hence, disparate Tafel slopes. Collectively, these results provide a self-consistent mechanism for C₆H₁₂ epoxidation that involves reactive O*.