Uncovering Electrochemical Methane Oxidation Mechanism through the In Situ Detection of Reaction Intermediates

The electrochemical partial oxidation of methane (CH₄) to value-added chemicals under ambient conditions provides a solution for harnessing abundant natural gas resources. Here, we investigate α-Fe₂O₃ as a model catalyst to gain a mechanistic understanding of the electrochemical CH₄ oxidation reaction (eCH₄OR). During chronoamperometric experiments, we obtain liquid products (formic acid, acetic acid, and acetone) with ∼6.5% total Faradaic efficiency at 2.3 V versus the reversible hydrogen electrode (V_RHE). At lower potentials below 2.0 V_RHE, non-Faradaic CH₄ adsorption occurred, confirmed by in situ ATR-SEIRAS (attenuated total reflectance-surface-enhanced infrared absorption spectroscopy) and impedance spectroscopies. In addition to verifying the presence of the Fe^IVO species, in situ spectroelectrochemical measurements revealed that CH₄ oxidation initiates via H-abstraction to form •OCH₃ species. The reaction undergoes further oxidation steps, leading to formate. Coupling between •OCH₃ and formate generates •OCOCH₃ species. Further, C-C coupling between - COCH₃ and - CH₃ resulted in acetone formation. Real-time proton-transfer-reaction time-of-flight mass spectrometry (PTR-TOF-MS) confirms the proposed pathways. Based on these observations, we propose a mechanistic pathway for selective CH₄ electrooxidation.