Carbon–metal versus metal–metal synergistic mechanism of ethylene electro-oxidation via electrolysis of water on TM2N6 sites in graphene

Abstract

Acetaldehyde (AA) and ethylene oxide (EO) are important fine chemicals, and are also substrates with wide applications for high-value chemical products. Direct electrocatalytic oxidation of ethylene to AA and EO can avoid the untoward effects from harmful byproducts and high energy emissions. The most central intermediate state is the co-adsorption and coupling of ethylene and active oxygen intermediates (*O) at the active site(s), which is restricted by two factors: the stability of the *O intermediate generated during the electrolysis of water on the active site at a certain applied potential and pH range; and the lower kinetic energy barriers of the oxidation process based on the thermo-migration barrier from the *O intermediate to produce AA/EO. The benefit of two adjacent active atoms is more promising, since diverse adsorption and flexible catalytic sites may be provided for elementary reaction steps. Motivated by this strategy, we explored the feasibility of various homonuclear TM₂N₆@graphenes with dual-atomic-site catalysts (DASCs) for ethylene electro-oxidation through first-principles calculations via thermodynamic evaluation, analysis of the surface Pourbaix diagram, and kinetic evaluation. Two reaction mechanisms through C–TM versus TM–TM synergism were determined. Between them, a TM–TM mechanism on 4 TM₂N₆@graphenes and a C–TM mechanism on 5 TM₂N₆@graphenes are built. All 5 TM₂N₆@graphenes through the C–TM mechanism exhibit lower kinetic energy barriers for AA and EO generation than the 4 TM₂N₆@graphenes through the TM–TM mechanism. In particular, Pd₂N₆@graphene exhibits the most excellent catalytic activity, with energy barriers for generating AA and EO of only 0.02 and 0.65 eV at an applied potential of 1.77 V vs. RHE for the generation of an active oxygen intermediate. Electronic structure analysis indicates that the intrinsic C–TM mechanism is more advantageous than the TM–TM mechanism for ethylene electro-oxidation, and this study also provides valuable clues for further experimental exploration.