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

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 Fe₃O₄(001). Using X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM), we investigated the activity of such model systems towards H₂ 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 Fe₃O₄(001). In contrast, the presence of Rh_met in H₂ results in the formation of hydroxyls and subsequent etching of the Fe₃O₄(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/Fe₃O₄ 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 H₂ atmosphere.