Electrochemical CH_4 Oxidation on Spinel NiCo_2O_4-Based Composite Catalysts

摘要

CH_4 is the main component of natural gas, which represents 21.4% of the total primary sources of energy in the world; it is an essential fuel for both daily human life and chemical industry. The increasing emissions of methane is considered as a crucial issue because its effect as a greenhouse gas is more than 30 times potent than that of CO_2. One attractive solution to cut-off the impact of methane emission is performed by oxidizing methane into liquid high-value fuels, such as methanol. For decades, the conversion of methane into liquid fuels has been a significant challenge for heterogeneous catalysis. Nevertheless, to yield higher efficiencies, complicated multistage processes should be utilized, which require higher pressures and temperatures and consequently extended infrastructure. On the contrary, the electrochemical oxidation of methane could be a far more promising approach. The challenge of producing liquid intermediate products at elevated rates and selectivity lies in activating CH_4 while suppressing the competing oxygen evolution reaction (OER) and its complete oxidation into carbon dioxide, which is the most thermodynamically favorable product. It is worth mentioning that the direct electrochemical oxidation of CH_4 at mild conditions is extremely sluggish. Alternatively, the CH_4 oxidation could be achieved via exploiting the partial OER. The reaction would take place purely through a thermal catalytic step, whereby coupling the adsorbed oxygen atom (*O) on the active sites governed by the first two elementary reactions in OER with CH_4 in the vicinity to it. Here, nickel-cobalt oxide (NiCo_2O_4)-based nanowire electrodes for partial oxidation of CH_4 are reported. The NiCo_2O_4-based materials could achieve elevated electrochemical performance by its resilience in terms of controlling the pore properties and morphology. Nanowire catalysts with porous structures are attained by the facile decomposition of the adsorbed cetrimonium bromide during the calcination process. The spinel NiCo_2O_4 is further improved by incorporating metal oxides, i.e., RuO_2 and V_2O_5, oxygen delivery sources. Moreover, the synergistic effects of the NiCo_2O_4 and metal oxides would enhance the conductivity and its bifunctional catalytic activity towards favoring CH_4 and O* coupling. The electrocatalysts are investigated at room temperature in a liquid-phase electrolyzer, where the CH_4 gas and anolyte flows are separated by a carbon-based gas diffusion layer that sandwiches the reaction interface between separate conductive and hydrophobic supports.