Crossing the Oxo-Peroxo Wall for Selective Electrochemical Epoxidation

Electrochemical oxidation in water requires a formation of reactive oxygen species to be able to oxidize organic compounds to ketones, aldehydes and epoxides. These reactions, broadly classified as alternative oxidation reactions (AOR), directly compete with prevalent oxygen evolution reaction (OER). In molecular catalysis, the Oxo-Wall dictates a transition from a stable oxo intermediate (OER active) to a meta-stable metal-oxo (OER inactive) generally occurs. In this work, we find that in heterogeneous catalysis the same Oxo-Wall applies, but a meta-stable oxo preferentially coordinates with lattice oxygen to form a more stable surface peroxo intermediate. We identify a universal free energy onset of this process at 3.39 eV under electrochemical activation in water and show that it is completely decoupled from the OER oxo species. Such decoupling gives rise to a new region of oxygen reactivity relevant for AOR where we predict a selective oxidation of the unsaturated C-C bonds to occur instead of OER. We construct a distinct AOR overpotential volcano and identify recently reported electrocatalysts, including palladium-platinum for propylene epoxidation and silver-nickel for ethylene epoxidation, along with others on TiO2 and CuO. We discuss broader implications and limitations of electrochemical AOR, highlighting their potential to enable electrochemically enhanced thermal catalysis.