Facet-dependent diffusion of atomic oxygen on Ag surfaces

It has been proposed that the introduction of surface and subsurface atomic oxygen into Ag catalysts is crucial for selectivity enhancement of ethylene epoxidation towards ethylene oxide. However, the diffusion of atomic oxygen on various Ag crystal surfaces, which is an essential process to form surface and subsurface atomic oxygen, has not been systematically investigated. In this work, we have employed density functional theory (DFT) to study the horizontal diffusion as well as downward diffusion of oxygen atom on different crystal facets of Ag, including (111), (110), (100) and (211) surfaces. Among all studied crystal facets, the horizontal diffusion of surface atomic oxygen is most favorable on Ag (1 1 1) surface, while Ag (1 1 0) surface is the most beneficial to horizontal diffusion of subsurface atomic oxygen. It is also found that the atomic oxygen prefers to downward diffuse on Ag (1 1 0) surface rather than downward diffuse on other surfaces. The deep insight revealed here may guide to engineer Ag surface to promote the formation of surface and subsurface atomic oxygen, and accordingly pave the avenue for the rational design of Ag catalyst for ethylene epoxidation.