Confined Oxygen Promotes Radical Generation for Methane Oxidation Toward Liquid Oxygenates

Abstract

The activation of methane (CH4) gas to produce value-added liquid hydrocarbons has been a challenging issue in catalytic research. Electrochemical oxidation is one of the alternative methods to oxidize methane and can be usually performed at ambient temperature, which can solve the problems of methane steam reforming caused by high temperature. However, electrochemical direct methane oxidation still have limitations that it should competes with oxygen evolution reaction, resulting in low efficiency. Also, oxygenates generated from methane oxidation can be easily overoxidized to carbon dioxide which is not appropriate product for the purpose of electrochemical system to make liquid hydrocarbons , suggesting solution to transport problem in the industry.Therefore, we suggest new system to convert methane into liquid products via oxygen reduction reaction using cathode. Recently in some papers, oxygen is used as an oxidant in photocatalytic system to oxidize methane into liquid hydrocarbons. Herein, hydrogen peroxide (H2O2) generated from oxygen reduction reaction can produce hydroxyl radical, superoxide radical, and hydroperoxide radical with strong activity to break first C-H bond from CH4. By combining methyl radical activated from methane and radicals generated from H2O2, we could make formic acid with high selectivity. Through the reduction reaction on cathode and oxidation reaction by radicals, intermediate products, such as methanol and methyl hydroperoxide, were finally converted into formic acid. Furthermore, overoxidation, one of important limitation in direct methane oxidation, can be prevented by using cathode where reduction reaction occurs on electrode.In this work, we synthesized metal organic framework (MOF)-derived cobalt single atom using ZIF-67. The cobalt single atom has high faradaic efficiency (FE) in acidic media with low onset potential, which is over 80% at 0.5 V (vs RHE). To efficiently utilize H2O2 produced by oxygen reduction reaction, additional hydrophobic layer was introduced with gas diffusion electrode (GDE) by coating poly tetra fluoro ethylene (PTFE) on catalyst layer. Additional PTFE layer on GDE confined H2O2 and CH4 together, offering higher possibilities to be reacted. In addition, liquid oxygenates generated from the reaction have hydrophilic properties that prevents further reactions by reducing the diffusion of liquid products toward cathode due to hydrophobic layer. By applying new system into flow cell, the low solubility of methane in the electrolyte can be alleviated to achieve high production rate. At last, This new system using oxygen reduction reaction to oxidize methane can make a new pathway for many chemical reaction area in the point that the system can be applied in any industrial reactions where H2O2 included. Figure 1