Reactions of Surface Peroxides Contribute to Rates and Selectivities for C₂H₄ Epoxidation on Silver

Area of Science:

• Catalysis
• Surface Chemistry
• Chemical Engineering

Background:

• Ethylene oxide (EO) is a key industrial chemical produced via ethylene partial oxidation.
• The precise mechanism of silver-catalyzed ethylene epoxidation remains debated.
• Understanding active sites and intermediates is crucial for catalyst optimization.

Purpose of the Study:

• To elucidate the role of active silver surface species in ethylene epoxidation.
• To identify reactive oxygen species and dominant reaction pathways.
• To correlate surface species with ethylene oxide selectivity.

Main Methods:

• Operando Raman spectroscopy to probe surface species during reaction.
• Transient kinetic measurements to study reaction dynamics.
• Ab initio density functional theory calculations for mechanistic insights.

Main Results:

• Higher coverages of peroxide species on in situ formed Ag oxide surfaces correlate with increased EO selectivity.
• Ag oxide surfaces stabilize diatomic oxygen species (peroxide and superoxide), favoring selective EO formation.
• Reaction pathways involve activation of bound O₂ by ethylene, forming peroxo- and oxometallacycle intermediates.

Conclusions:

• Peroxide intermediates, stabilized by oxidized silver surfaces, are crucial for selective ethylene epoxidation.
• Surface oxidation of silver is necessary for peroxide formation and high EO selectivity.
• Industrial promoters likely enhance EO selectivity by promoting silver oxidation and peroxide formation.