Preparation and Characterization of Model Homotopic Catalysts: Rh Adatoms, Nanoparticles, and Mixed Oxide Surfaces on Fe3O4(001)

The atomic-level characterization of active sites is essential to understanding the mechanisms behind catalytic reactions. In this study, using scanning tunneling microscopy and X-ray photoelectron spectroscopy, we follow the morphological changes of a model Rh catalyst supported on Fe₃O₄(001) as a function of temperature and Rh coverage. We identify the preparation conditions leading to model catalysts containing homotopic or nearly homotopic Rh species bound as adatoms, substitutional octahedral sites within the Fe₃O₄(001), and nanoparticles. Adsorbates such as CO and CO₂ are subsequently used to characterize the properties of different Rh sites. Using temperature-programmed desorption, we demonstrate that adatoms and nanoparticles exhibit high-temperature CO desorption (250–600 K). Strong binding on such sites further allows for CO oxidation to CO₂ via the Mars–van Krevelen mechanism. In contrast, CO₂ was found to interact weakly with all Rh sites. Differences in desorption temperature enable the use of CO and CO₂ as titration methods for nanoparticles and Fe₃O₄(001), respectively. A small quantity of CO₂ was found to be reduced to CO on Rh adatoms and small nanoparticles.