The evolution of model Rh/Fe3O4(001) catalysts in hydrogen environments

• The atomically dispersed Rh adatoms and in-surface octahedrally-coordinated Rh species on Fe 3 O 4 (001) do not dissociate H 2 at 300 K. • Metallic Rh clusters dissociate H 2 , with H spilling over on Fe3O4(001) and forming hydroxyls. • Elevated catalyst temperatures (≥ 500 K) lead to water formation, surface etching via the Mars van Krevelen mechanism, and the release of in-surface octahedrally -coordinated Rh onto the surface. Single metal atoms dispersed on oxides are a new emerging class of catalysts owing to their unique electronic and chemical properties. In this study, we have prepared a series of model single-atom catalysts possessing well-characterized Rh sites that include Rh adatoms (Rh ad ), mixed surface layers with octahedrally-coordinated Rh (Rh oct ), as well as metallic Rh clusters and nanoparticles (Rh met ) on Fe 3 O 4 (001). Using X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM), we investigated the activity of such model systems towards H 2 and their stability in reducing environments. Our results show that the atomically dispersed Rh ad and Rh oct species do not activate H 2, which would result in the formation of surface hydroxyls on Fe 3 O 4 (001). In contrast, the presence of Rh met in H 2 results in the formation of hydroxyls and subsequent etching of the Fe 3 O 4 (001) at higher temperatures (≥ 500 K) due to water formation via the Mars−van Krevelen mechanism. Additionally, such surface etching leads to the release of the Rh oct from the surface lattice and their sintering to Rh met . To bridge the material gap between the surface science models and high surface area catalysts, we perform parallel studies on powder Rh/Fe 3 O 4 catalysts. The XPS characterization shows remarkable similarities between these systems. Further, our surface science studies provide an atomistic picture of the behavior of high surface area catalysts in the H 2 atmosphere.