The precise characterization and atomic-scale understandings of the catalyst materials' physical and chemical properties are essential for the design of selective and active catalytic systems. Here, we have studied the structure, electronic properties, and reactivity of Rh on the Fe3O4(001) single-crystal surface using X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). We have demonstrated that we can control the surface morphology and the nature of the interaction between Rh and Fe3O4(001) by varying the Rh loading and annealing temperatures. We have identified a series of model catalysts possessing unique Rh sites. These catalysts include Rh adatoms, mixed surface layers with octahedrally-coordinated Rh, small Rh clusters, and large Rh nanoparticles on Fe3O4 (001) surfaces. We have investigated the activity of such model systems towards H2 and their stability in reducing environments. Our results show that Rh clusters and nanoparticles are active in the dissociation of H2 at or above 400 K. The hydrogen atoms spill over to Fe3O4 and react with surface oxygen to form hydroxyls (OH). At higher temperatures (≥ 500 K), etching of the Fe3O4(001) surface was observed due to water formation via the Mars−van Krevelen mechanism. In addition, hydrogen-induced sintering of the Rh single atoms was observed at or above 400 K. To bridge the pressure and the material gap between the surface science models and high surface area catalysts, we perform parallel studies on nanoparticulate Rh/Fe3O4. The XPS characterization shows remarkable similarities between these systems. We demonstrate that the Rh nuclearity follows the same trends as a function of Rh loading and annealing temperature. Further, our surface science studies provide an atomistic picture of the behavior of high surface area catalysts in the H2 atmosphere.

Preparation, characterization, and reactivity of Rh/Fe3O4(001) model catalysts

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