(197e) Potential Oscillation Controls Rates and Selectivities of Alkene Epoxidation and Oxygen Evolution Reactions on Au Anodes

The reaction mechanism of alkene epoxidations with H₂O on electrodes reveals opportunities to increase selectivity through promotion of reactive surface intermediates. We previously demonstrated on Au anodes that the 1-hexene (C₆H₁₂) epoxidation and competitive O₂ evolution reactions share common steps to form O* through electrochemical H₂O activation and then diverge. Subsequently, epoxides form when O* reacts non-Faradaically with C₆H₁₂, whereas O₂ evolves when O* reacts Faradaically with H₂O to form OOH* and then deprotonates (Figure 1a). One strategy to increase epoxidation electron selectivity involves populating the electrode surface with O* at oxidizing potentials followed by decreasing to the open circuit potential (OCP) to prevent the additional proton-electron transfer steps that generate O₂. Here, we correlate transient OCP and operando Raman spectroscopy measurements after electrode polarization to examine the creation of surface oxygen species by H₂O oxidation and their subsequent consumption by reaction with C₆H₁₂. We then develop potential oscillation programs to optimize epoxidation Faradaic efficiency.

Raman features at 580 cm^-1 show that O* species populate the Au surface after polarization to 0.96 V_Fc/Fc+. Subsequent time dependent OCP measurements display two regimes with distinct kinetic behavior that reflect electrochemical double layer dispersion (0.96-0.85 V_Fc/Fc+) and O* monolayer reduction (0.85-0.35 V_Fc/Fc+). Coupled operando Raman spectroscopy supports these interpretations through attenuation of solvated anion and Au-O features (Figure 1b). Rates at which the OCP decays increase with the value of [C₆H₁₂] and reflect surface O* consumption. We create potential oscillation programs and find that epoxidation rates and Faradaic efficiencies depend on the potential bounds and duty cycle of the applied square potential waveform. Oscillation programs with periods that equal the time scale of formation and depletion of surface O* maximize epoxidation Faradaic efficiency (Figure 1c). This work demonstrates that process development allows for modulation of selectivity and rates of electrochemical reactions.