Fundamental insight into electrochemical oxidation of methane towards methanol on transition metal oxides

Electrochemical oxidation of CH₄ is known to be inefficient in aqueous electrolytes. The lower activity of methane oxidation reaction (MOR) is primarily attributed to the dominant oxygen evolution reaction (OER) and the higher barrier for CH₄ activation on transition metal oxides (TMOs). However, a satisfactory explanation for the origins of such lower activity of MOR on TMOs, along with the enabling strategies to partially oxidize CH₄ to CH₃OH, have not been developed yet. We report here the activation of CH₄ is governed by a previously unrecognized consequence of electrostatic (or Madelung) potential of metal atom in TMOs. The measured binding energies of CH₄ on 12 different TMOs scale linearly with the Madelung potentials of the metal in the TMOs. The MOR active TMOs are the ones with higher CH₄ binding energy and lower Madelung potential. Out of 12 TMOs studied here, only TiO₂, IrO₂, PbO₂, and PtO₂ are active for MOR, where the stable active site is the O on top of the metal in TMOs. The reaction pathway for MOR proceeds primarily through *CH_x intermediates at lower potentials and through *CH₃OH intermediates at higher potentials. The key MOR intermediate *CH₃OH is identified on TiO₂ under operando conditions at higher potential using transient open-circuit potential measurement. To minimize the overoxidation of *CH₃OH, a bimetallic Cu₂O₃ on TiO₂ catalysts is developed, in which Cu reduces the barrier for the reaction of *CH₃ and *OH and facilitates the desorption of *CH₃OH. The highest faradaic efficiency of 6% is obtained using Cu-Ti bimetallic TMO.