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

Electrochemical oxidation of CH 4 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 CH 4 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 CH 4 to CH 3 OH, have not been developed yet. We report here the activation of CH 4 is governed by a previously unrecognized consequence of electrostatic (or Madelung) potential of metal atom in TMOs. The measured binding energies of CH 4 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 CH 4 binding energy and lower Madelung potential. Out of 12 TMOs studied here, only TiO 2 , IrO 2 , PbO 2 , and PtO 2 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 *CH 3 OH intermediates at higher potentials. The key MOR intermediate *CH 3 OH is identified on TiO 2 under operando conditions at higher potential using transient open-circuit potential measurement. To minimize the overoxidation of *CH 3 OH, a bimetallic Cu 2 O 3 on TiO 2 catalysts is developed, in which Cu reduces the barrier for the reaction of *CH 3 and *OH and facilitates the desorption of *CH 3 OH. The highest faradaic efficiency of 6% is obtained using Cu-Ti bimetallic TMO.