Electro-epoxidation of ethylene and propylene via atomic active oxygen from water electrolysis on IrN4 site in graphene with lower applied potential and wide range

Abstract

Ethylene oxide (EO) and propylene oxide (PO) are important fine chemicals, and widely applied in medicine and automotive industry. The electrocatalytic direct epoxidation of ethylene and propylene to generate EO and PO 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 to epoxidate ethylene and propylene. However, it remains a challenge to generate *O intermediates at lower applied potential and even in wide potential range, and further to co-adsorb ethylene/propylene and then ensure oxygen transfer until EO/PO release. According to these essential requirements, we explored the feasibility of 26 TMN4 moiety in graphene as electrocatalytic active sites for EO/PO generation through first-principle calculations via thermodynamic evaluation, Pourbaix diagrams, and kinetic evaluation through TM predominant and C-TM synergistic mechanism, respectively. Finally, the electro-epoxidation of ethylene and propylene exhibits the best performance on experimentally prepared IrN4@graphene though TM predominant mechanism, which possess computational applied potential to stabilize *O intermediate in a wide range from only 1.16 V vs. SHE (below OER equilibrium potential of 1.23 V) until 2.08 V. Subsequently, electron structure analysis indicates that electron transfers from the adsorbed ethylene/propylene to the anti-bonding orbital of Ir-O, which can greatly weaken the Ir-O bonding strength and facilitate the O migration to the ethylene/propylene. In particular, the kinetic energy barrier for propylene epoxidation on IrN4@graphene is below that for ethylene, because the Ir-O bonding is weakened more significantly after propylene adsorption than ethylene adsorption on the *O intermediate. Hence, this study verified the feasibility of electro-epoxidation of ethylene and propylene on single atomic electrocatalysis, and the trait on IrN4@graphene with lower applied potential and wide range deserve further experimental exploration.