Selective Enhancement of Ethylene Epoxidation via Directing Reaction Pathways over Ag Single-Atom Catalyst

The current industrial synthesis of ethylene oxide (EO) relies on the ethylene epoxidation process catalyzed by Ag-based catalysts, the EO selectivity of which is limited by the competitive formation of EO and byproduct acetaldehyde (AA) upon the isomerization of oxametallacycle (OMC) intermediates. Here, we propose a strategy that an anchored Ag single atom on the α-Al2O3 (0001) surface (Ag1/α-Al2O3) acts as a heterogeneous catalyst for ethylene epoxidation by density functional theory (DFT) calculations, energy span model (ESM), and microkinetic analysis. We study the whole reaction network of ethylene epoxidation on Ag1/α-Al2O3 and find that the O2 associative-Eley–Rideal (O2ass-ER) mechanism dominates over any other reaction pathways, in which the two O atoms of adsorbed O2 successionally interact with free-standing ethylene molecules to form EO. The O2ass-ER mechanism brings about obvious selectivity enhancement and higher mass activity for EO production on Ag1/α-Al2O3, compared with the Ag(111) surface, due to the mechanism conversion and avoiding the emergent of the OMC intermediate. We investigated the conversion of dominant reaction pathways between Ag(111) and Ag1/α-Al2O3 and attributed it to the unstable OMC intermediate on Ag1/α-Al2O3, derived from the weak orbital coupling of bonding with the catalytic center. Besides, the nonequilibrium charge distribution and large spin polarization of the O2 adsorption state facilitate gaseous ethylene to directly attack O atoms of O2 one by one instead of interacting with them simultaneously, inhibiting the formation of OMC intermediates. Our model of single-atom catalysts can be utilized as an effective strategy to tune the reaction pathway avoiding precursors of byproducts. Implementing the concepts of single-atom catalysis into ethylene epoxidation would provide new perspectives for the design of advanced catalysts.