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 (VRHE). At lower potentials below 2.0 VRHE, 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 FeIVO 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.