Revealing the Nature of Active Oxygen Species and Reaction Mechanism of Ethylene Epoxidation by Supported Ag/α-Al₂O₃ Catalysts

The oxygen species on Ag catalysts and reaction mechanisms for ethylene epoxidation and ethylene combustion continue to be debated in the literature despite decades of investigation. Fundamental details of ethylene oxidation by supported Ag/α-Al₂O₃ catalysts were revealed with the application of high-angle annular dark-field-scanning transmission electron microscopy-energy-dispersive X-ray spectroscopy (HAADF-STEM-EDS), in situ techniques (Raman, UV–vis, X-ray diffraction (XRD), HS-LEIS), chemical probes (C₂H₄-TPSR and C₂H₄ + O₂-TPSR), and steady-state ethylene oxidation and SSITKA (¹⁶O₂ → ¹⁸O₂ switch) studies. The Ag nanoparticles are found to carry a considerable amount of oxygen after the reaction. Density functional theory (DFT) calculations indicate the oxidative reconstructed p(4 × 4)–O–Ag(111) surface is stable relative to metallic Ag(111) under the relevant reaction environment. Multiple configurations of reactive oxygen species are present, and their relevant concentrations depend on treatment conditions. Selective ethylene oxidation to EO proceeds with surface Ag₄–O₂* species (dioxygen species occupying an oxygen site on a p(4 × 4)–O–Ag(111) surface) only present after strong oxidation of Ag. These experimental findings are strongly supported by the associated DFT calculations. Ethylene epoxidation proceeds via a Langmuir–Hinshelwood mechanism, and ethylene combustion proceeds via combined Langmuir–Hinshelwood (predominant) and Mars–van Krevelen (minor) mechanisms.