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

## TL;DR

This study shows that nickel atoms in a single-atom alloy with silver can activate oxygen at very low temperatures, improving ethylene oxide production efficiency.

## Contribution

The paper provides direct experimental evidence of oxygen activation by Ni atoms in a NiAg(100) single-atom alloy at cryogenic temperatures.

## Key findings

- NiAg(100) surfaces form a distinct NiO2 species under O2 exposure at 78 K, indicating O2 dissociation.
- High-resolution STM and DFT simulations reveal the formation of an O–Ni–O species with oxygen atoms in 4-fold hollow sites.
- Ni atoms enhance O2 activation, potentially accelerating a rate-limiting step in ethylene epoxidation.

## Abstract

Silver-catalyzed ethylene epoxidation remains the only
industrially
viable route for ethylene oxide (EO) production. However, this process
requires chlorine and other promoters to achieve a high EO selectivity
while still generating substantial CO2 emissions. A recent
theory-guided approach identified Ni, in single-atom alloy (SAA) form,
as a new promoter of this reaction. Specifically, the addition of
Ni to Ag nanoparticles supported on α-Al2O3 at a highly diluted ratio (1 Ni per 200 Ag atoms) increased catalyst
selectivity to EO by ∼25%, the same increase afforded by the
ubiquitous industrial promoter chlorine. To better understand the
effect of Ni, we investigated the interaction of O2 with
NiAg(100) SAA surfaces by using scanning tunneling microscopy (STM)
and density functional theory (DFT). While only molecular O2 was present when pure Ag(100) was exposed to O2 at 78
K, a distinct NiO2 species, indicative of O2 dissociation at Ni atom sites, was identified on the NiAg(100) SAA
under the same conditions. High-resolution STM imaging backed by DFT
simulations elucidated the formation of an O–Ni–O species
with the oxygen atoms in 4-fold hollow sites. These findings provide
direct experimental evidence that Ni atoms are very effective at O2 activation, even at cryogenic temperatures. This suggests
that, in addition to the known role of Ni in stabilizing the unselective
nucleophilic oxygen on Ag, it could also accelerate O2 dissociation,
which can be rate limiting.