(574c) Computational and Experimental Insights into Reactive Forms of Oxygen Species on Dynamic Ag Surfaces Under Ethylene Epoxidation Conditions

Supported Ag particles are commercial catalysts used for partial oxidation of ethylene to ethylene oxide (EO). Most agree that dissociative adsorption of dioxygen upon Ag forms reactive atomic oxygen, however, the surface structure and coverages of reactive species under reaction conditions remain debated. Previous characterization of oxidized Ag surfaces commonly involved high vacuum or near ambient pressures. Therefore, the role of distinct forms of oxygen (surface, subsurface or lattice) in the EO reaction mechanism, and their molecular nature are still unclear. To better understand the state of the active Ag surface, we used Density Functional Theory calculations to compute temperature and pressure dependent phase diagrams to compare relative stabilities of reconstructed and unreconstructed Ag_xO_y surfaces proposed in the literature. We computed frequencies for the most stable oxygen-adlayer structures at conditions relevant for epoxidation (2–90 kPa O₂, 423–573 K). Comparisons between computed spectra and in situ surface enhanced Raman spectroscopy (SERS) provide strong evidence for distinct forms of surface, subsurface and bulk atomic O. Peaks between 800-1000 cm^-1 observed by SERS disagree with proposed models for monatomic oxygen species or reconstructed oxide layers. We then explored dioxygen species, which are consistent with this frequency range, but we found their formation to be endergonic on previously proposed surfaces. Guided by our SERS results, we examined dioxygen species upon substoichiometric oxide-like layers and found them to be stable. These species form with a variety of minima and frequencies spanning 750-940 cm^-1. Ab initio molecular dynamics simulations on these structures show that lattice O can diffuse from subsurface to the surface depending on the oxygen chemical potential, which agrees with surface dynamics inferred from transient oxidation and reduction of Ag particles observed by SERS. Taken together our results implicate presence of monatomic, diatomic, and subsurface oxygen under representative temperatures and O₂ pressures.