Electro-epoxidation of ethylene and propylene using atomic active oxygen derived from water electrolysis on IrN4 site in graphene at a lower applied potential and over a wide potential range

Abstract

Ethylene oxide (EO) and propylene oxide (PO) are important fine chemicals widely used in medicine and automotive industry. The direct epoxidation of ethylene and propylene to generate EO and PO through electrocatalytic methods can greatly avoid deficiencies in traditional industrial processed, such as harsh temperature and pressure, complete or excessive oxidation, and even environmental pollution. The *O (atomic active oxygen) intermediate derived from water electrolysis can serve as the direct reactive oxygen species for the epoxidation of ethylene and propylene. However, it remains a challenge to generate *O intermediates at a lower applied potential and over a wide potential range, followed by co-adsorption of ethylene/propylene and ensuring oxygen transfer until EO/PO release. To address these challenges, we explored the feasibility of 26 TMN₄ moieties in graphenes as electrocatalytic active sites for EO/PO generation using first-principles calculations via thermodynamic evaluation, Pourbaix diagrams, and kinetic evaluation through TM-predominant and C–TM synergistic mechanisms, respectively. Finally, the electro-epoxidation of ethylene and propylene demonstrated the best performance on experimentally prepared IrN₄@graphenes via the TM-predominant mechanism, which possess the applied potential of stabilize the *O intermediate over a wide range from only 1.16 V vs. SHE (below an OER equilibrium potential of 1.23 V) to 2.08 V. Subsequently, electron structure analysis indicated that electron transfer from the adsorbed ethylene/propylene to the anti-bonding orbital of Ir–O can greatly weaken the Ir–O bonding strength and facilitate the O migration to ethylene/propylene. In particular, the kinetic energy barrier for propylene epoxidation on IrN₄@graphenes was lower than that for ethylene because the Ir–O bonding is weakened more significantly after propylene adsorption than after ethylene adsorption on the *O intermediate. Hence, this study verifies the feasibility of electro-epoxidation of ethylene and propylene via single atomic electrocatalysis, and the trait of IrN₄@graphene at a lower applied potential and a wide range deserves further experimental exploration.