Design and Activity of Supported Transition Metal Catalysts for Methane to Methanol Oxidation in Electrochemical Fuel Cell

Methane is a relatively inexpensive and abundant resource and its partial transformation to chemicals and chemical fuels presents attractive yet challenging pathways for its utilization. Conventional synthesis for methanol involve a multistep process involving steam reforming of methane with subsequent catalytic reactions, which require high energy input and run at high cost [1]. Alternatively, methane oxidation over catalyst surfaces in an electrochemical cell is a promising single-step approach to achieve a direct conversion of methane to methanol at lower temperatures and low cost [2]. Previous studies have demonstrated that methane can be electrochemically converted into methanol with high selectivity but with low overall conversion efficiency. Increasing the rate of methane conversion can be achieved by systematic improvement of the electrochemical cell design along with the concurrent development of new efficient catalysts materials. Methane oxidation is energetically challenging process and dual role of the catalysts involve first, activation of the relatively inert C-H bond enabling the oxidative hydroxylation, and methanol formation. Second, the effective catalysts should simultaneously inhibit the methanol oxidation, which proceeds with a much lower energy barrier and can result in the formation of formaldehyde, formic acid, carbon monoxide, and carbon dioxide. To date, a variety of the catalysts, among different supported metals (Pd, Ru, Au, Ag) and metal oxides (V 2 O 5 , Fe 2 O 3 , CoO, Mn 2 O 3 , MoO 3 , CrO), have been tested in electrochemical cell and shown promise for the direct oxidation of methane [2]. However, further systematic studies are essential for understanding the mechanism for methane oxidation, enabling catalysts rational design. On the other hand, bioinspired supported binuclear metal catalysts show potential for high selectivity and conversion [3], but have not yet been explored for electrochemical methane conversion. In this study, first-row transition metal oxides as well as single and binuclear catalysts, supported on well-defined crystalline 2D materials including carbides, oxides, and nitrides will be presented. The catalysts are synthesized with wet-chemical synthesis routes and subsequently fabricated in membrane electrode assembly for testing their activity, methanol selectivity, and conversion efficiency in an electrochemical fuel-cell-type reactor. In-depth structural and chemical analyses of catalysts using a combination of various transmission electron microscopy techniques, complimented with spectroscopy analyses is used to establish structure-property relationship. These insights will provide valuable basis for a scientific-guided approach toward the optimization of the known, and the identification of the new metal oxide and single-site supported catalysts for this challenging process. Ravi, M., et al, Angew. Chemie Int. Ed., 2017 , 56 , 26464. Tomita, A., et al, Angew. Chemie Int. Ed., 2008 , 47 , 1462. Starokon, E. V., Phys Chem. C. , 2011 , 115 , 2155.