Transition metals are frequently utilized as catalysts due to their ability to participate in oxidation-reduction reactions. Metals used for heterogenous catalysts heavily influence industrial practices such as synthesis of many bulk materials and other useful commodities. One such catalytic application involves an oxygen-rich silver surface employed to promote the partial oxidation of ethylene-to-ethylene oxide. Commodities produced by this reaction include ethylene glycol, which is required to produce antifreeze and some polyester fibers, and precursors used in sterilization procedures. This reaction is paramount to industrial production, but the structure of the active catalyst, specifically as how oxygen adsorbs to the silver surface, has yet to be established. It has been hypothesized that some electrophilic oxygen species is the active species of the catalyst, but this has yet to be concluded. Past studies, including those by Nobel laureate Gerhard Ertl suggest that atomic oxygen adsorbs to the surface as well as the region below the surface, the subsurface, and that this subsurface adsorbed species may be related to the electrophilic oxygen species. However, studies performed in a laboratory use different pressures than what is industrially used, as most laboratories require the use of a vacuum and industrial applications are performed around 0.75 bar. To investigate the possible active species of the system and bridge the gap between experiment and industrial results, we have studied the formation of surface and subsurface atomic oxygen, via adsorption, on Ag(111) at different oxygen temperatures and surface coverages using a combination of density functional theory (DFT), a pairwise-additive site model, and Monte Carlo simulations. Current results show that at surface temperatures greater than ~475 K and oxygen coverages in excess of 1/3 ML oxygen accumulate in the subsurface, suggesting that subsurface oxygen participates in industrial oxidative catalysis. Our future studies will explore the role of subsurface oxygen in surface reconstruction and the mechanism(s) for catalytic activity of Ag(111), which we hope to extend to the Ag(110) and Ag(100) surfaces.

Bridging the coverage gap in adsorption models of atomic oxygen on the Ag(111) surface

ACS Fall 2021

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