Ag(111) Remains Significantly Reduced In Situ under Simulated Ethylene Epoxidation Conditions

Area of Science:

• Chemical Engineering
• Surface Science
• Catalysis

Background:

• Direct ethylene epoxidation is a high-value industrial process.
• The reaction mechanism, particularly the state of the silver (Ag) catalyst, is debated.

Purpose of the Study:

• To determine if the Ag catalyst is metallic or oxidized under industrial reaction conditions.
• To identify the active oxidant species in ethylene epoxidation.

Main Methods:

• Ambient pressure X-ray photoelectron spectroscopy (AP-XPS).
• Simulating industrial chemical potentials and reaction conditions (5:2 ethylene-to-oxygen ratio at 433 K) on Ag(111).

Main Results:

• Under oxidizing conditions, Ag(111) forms nucleophilic oxygen (~80%) and carbonate impurities (~20%).
• Under reaction conditions, nucleophilic oxygen is consumed, leaving surface carbonate and bare Ag.
• The Ag(111) surface maintains approximately 50% exposed metallic sites.

Conclusions:

• The silver catalyst is not fully oxidized under relevant industrial conditions.
• Bare Ag sites are available, supporting mechanisms involving the oxametallacycle intermediate for selective epoxidation.