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 (CH) with reactive oxygen species formed by electrochemical oxidation of water (HO) on gold (Au) in an aqueous acetonitrile (CHCN) electrolyte. Cyclic voltammetry measurements demonstrate that oxygen (O) evolution competes with CH epoxidation, and the Au surface must oxidize before either reaction occurs. Raman spectroscopy reveals reactive oxygen species and spectators (CHCN) 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 O evolution rates increase linearly with the concentration of CH and depend inversely on the concentration of HO, which agree with analytical expressions that describe rates for reaction between CH and chemisorbed oxygen atoms (O*) and exclude proposals for other forms of reactive oxygen (e.g., O*, OOH*, OH*). These findings show that the epoxidation and O evolution reactions share a set of common steps that form O* through electrochemical HO activation but then diverge. Subsequently, epoxides form when O* reacts with CH through a non-Faradaic process, whereas O evolves when O* reacts with HO 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 CH epoxidation that involves reactive O*.