(667c) Utilizing Open Circuit Potential Decay to Track Consumption of Reactive Surface Intermediates during Electrochemical Alkene Epoxidations

Design of selective electrochemical epoxidation catalysts requires knowledge of mechanisms and methods to selectively stabilize reactive surface intermediates. We demonstrated that 1-hexene (C₆H₁₂) electroepoxidation with H₂O in aqueous-CH₃CN electrolyte proceeds through a non-Faradaic reaction between C₆H₁₂ and an electrochemically derived O*-atom on Au. Populating surfaces with O* with an applied potential followed by reactions at open circuit potential (OCP) may increase selectivities by inhibiting the competing O₂ evolution reaction (OER), which requires an additional proton-electron transfer to generate OOH*. Here, we correlate transient OCP and operando Raman spectroscopy measurements after polarization of Au surfaces to examine the creation of reactive oxygen species by H₂O oxidation and their subsequent consumption by reactions with reductants (e.g., C₆H₁₂).

Following polarization to 1.0 V_Fc/Fc+, Raman features at 580 cm^-1 show that O* species populate the Au surface. Subsequent time dependent measurements of the OCP display three regimes with distinct kinetic behavior that reflect the dispersion of the electrochemical double layer (1.0-0.85 V_Fc/Fc+), reduction of the O* monolayer (0.85-0.6 V_Fc/Fc+), and reduction of Au(OH)₃ under the O* monolayer (0.6-0.35 V_Fc/Fc+). Coupled operando Raman spectroscopy supports these interpretations through attenuation of features indicative of solvated anions and Au-O. The addition of chemical reductants (e.g., C₆H₁₂, NaBH₄) changes rates of OCP decay both for processes that reflect the dispersion of the electrochemical double-layer (1.0-0.85 V_Fc/Fc+­­­) and consumption of surface intermediates (0.85-0.35 V_Fc/Fc+). The changes in these rates differ with identity and concentration of the chemical reductants. We extend this analysis to other aqueous electrolyte systems on Au and find that the OCP decay and consumption of surface oxygen species depends on electrolyte solution composition, supporting electrolyte anion, and flow rate. Insights from this work can be applied to other relevant electrochemical reactions to understand how reaction conditions affect electrochemical double-layer formation and reactive species consumption.