PNNL

Abstract

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 (Rhad), mixed surface layers with octahedrally-coordinated Rh (Rhoct), as well as metallic Rh clusters and nanoparticles (Rhmet) 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 Rhad and Rhoct species do not activate H2, which would result in the formation of surface hydroxyls on Fe3O4(001). In contrast, the presence of Rhmet 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 Rhoct from the surface lattice and their sintering to Rhmet. 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. This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division, Catalysis Science Program, FWP 47319. PNNL is a multiprogram national laboratory operated for DOE by Battelle under Contract DE-AC05-76RL01830