Low Temperature Oxygen Activation on the NiAg(100) Single-Atom Alloy Surface

Area of Science:

• Heterogeneous Catalysis
• Surface Science
• Materials Science

Background:

• Silver-catalyzed ethylene epoxidation is the sole industrial method for ethylene oxide (EO) production.
• Current processes rely on chlorine promoters, leading to significant CO2 emissions and selectivity challenges.
• A theory-guided approach identified single-atom alloy (SAA) nickel (Ni) as a potential promoter.

Purpose of the Study:

• To investigate the interaction of oxygen (O2) with NiAg(100) SAA surfaces.
• To elucidate the mechanism by which Ni enhances ethylene epoxidation selectivity.
• To provide experimental evidence for Ni's role in O2 activation.

Main Methods:

• Utilized scanning tunneling microscopy (STM) to visualize surface species at cryogenic temperatures (78 K).
• Employed density functional theory (DFT) simulations to interpret experimental observations and model surface interactions.
• Compared O2 interaction with pure Ag(100) and NiAg(100) SAA surfaces.

Main Results:

• Pure Ag(100) surfaces only showed molecular O2 upon exposure at 78 K.
• NiAg(100) SAA surfaces exhibited a distinct NiO2 species, indicating O2 dissociation at Ni sites.
• High-resolution STM and DFT confirmed the formation of O-Ni-O species in 4-fold hollow sites, demonstrating Ni's efficacy in O2 activation even at low temperatures.

Conclusions:

• Single-atom Ni is highly effective at activating O2, even at cryogenic temperatures.
• Ni likely accelerates O2 dissociation, a potentially rate-limiting step in ethylene epoxidation.
• This mechanism offers a new pathway for improving EO selectivity, potentially reducing reliance on chlorine promoters and mitigating CO2 emissions.