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.
Easton et al. (Thu,) studied this question.