Are multiple oxygen species selective in ethylene epoxidation on silver?

Area of Science:

• Surface science
• Heterogeneous catalysis
• Chemical kinetics

Background:

• Ethylene epoxidation over silver catalysts is crucial for producing ethylene oxide.
• The exact nature of the active oxygen species in selective epoxidation is debated.
• Distinguishing between nucleophilic (total oxidation) and electrophilic (selective epoxidation) oxygen species is key.

Purpose of the Study:

• To investigate the atomic structure and reactivity of oxygen species on a silver surface during ethylene epoxidation.
• To determine if adsorbed atomic oxygen (Oads) is the electrophilic species responsible for selective ethylene epoxidation.
• To elucidate the role of different oxygen species in the partial oxidation of ethylene.

Main Methods:

• Utilized *in situ* and Ultra-High Vacuum (UHV) X-ray Photoelectron Spectroscopy (XPS) to analyze oxygen-silver interactions.
• Performed Density Functional Theory (DFT) calculations to support experimental findings on oxygen adsorption and stability.
• Investigated oxygen species under varying conditions, including low temperatures and different coverages.

Main Results:

• Experimental evidence indicates that unreconstructed adsorbed atomic oxygen (Oads) has a binding energy (BE) ≤ 528 eV.
• This Oads species does not match the BE (530-531 eV) typically assigned to the electrophilic oxygen active in epoxidation.
• Oads is observed at very low coverages *in situ* and is unstable under UHV conditions due to titration by background gases.
• DFT calculations confirm the stability of Oads only at low coverages.

Conclusions:

• Adsorbed atomic oxygen (Oads) is unlikely to be the primary electrophilic species in ethylene epoxidation on silver.
• The electrophilic oxygen species likely has a different atomic structure and higher binding energy than Oads.
• At least two distinct oxygen species may be involved in the partial oxidation of ethylene over silver catalysts.