In situ characterization of growth, structure, and surface chemistry in mixed rare-earth oxide model catalysts

The need to develop efficient catalysts for CO₂ conversion is pressing, given the challenges posed by climate change. Inverse oxide/metal model catalysts based on ceria have attracted significant attention in recent years due to their unique redox properties and ability to activate the CO₂ molecule. The key to this activation lies in the Ce³⁺ active sites, but our understanding of their specific role in catalytic activity, as well as how to control their formation and stabilization, remains incomplete. This thesis focuses on rare-earth oxide model system to systematically study the following aspects: (i) the influence of the metal substrate (Ru(0001), Cu(111) and Au(111)) on the growth and redox properties of cerium and samarium oxides; (ii) the effect of samarium doping on promoting Ce³⁺ sites and oxygen vacancy formation; (iii) and the role of the oxygen chemical potential in the growth and structural properties of cerium oxide. In combination with in situ X-ray photoelectron spectroscopy (XPS) measurements conducted at our home facilities, advanced surface science techniques at synchrotron-radiation facilities, including low-energy electron microscopy (LEEM), X-ray absorption spectroscopy (XAS), and micro-diffraction (µ-LEED), are used for in situ and in real-time characterization with nanoscale resolution. Key experiments span the individual growth of samarium oxide and cerium oxide, and the combination of this gained expertise in the growth and characterization of epitaxial (100)- and (111)-oriented ceria islands doped with Sm. Among the most important findings, this work shows that the interaction between Sm₂O₃ and the metal support (Ru(0001) and Cu(111)) facilitates the stabilization of exotic phases at mild conditions. Moreover, the formation of Ce³⁺ exhibits a remarkable structural sensitivity in the Sm-doped ceria nanoislands grown on Cu(111). The ceria (100)-oriented islands undergo a significant reduction upon Sm deposition, in contrast to the (111)-oriented islands that remain almost completely oxidized. Finally, changes in the oxygen chemical potential strongly influence the crystallinity level and thermal stability of ceria in Au(111). These insights contribute to our understanding of how substrate selection, growth conditions, and controlled doping can promote the formation and stabilization of Ce³⁺ active sites. This surface science work provides an atomic-level understanding that can be transferred to the optimization of realistic powder catalysts.