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 alpha-Al2O3 (0001) surface (Ag-1/alpha-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/alpha-Al2O3 and find that the O-2 associative-Eley-Rideal (O(2)ass-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 O(2)ass-ER mechanism brings about obvious selectivity enhancement and higher mass activity for EO production on Ag1/alpha-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/alpha-Al2O3 and attributed it to the unstable OMC intermediate on Ag1/alpha-Al2O3, derived from the weak orbital coupling of bonding with the catalytic center. Besides, the nonequilibrium charge distribution and large spin polarization of the O-2 adsorption state facilitate gaseous ethylene to directly attack O atoms of O-2 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.