Samarium-Driven Vacancy Engineering in CeO₂/Ru(0001) Inverse Catalysts

Ceria-based inverse catalysts derive their functionality from Ce³⁺-oxygen vacancy chemistry, yet strategies to deliberately control vacancy formation and distribution at metal-oxide interfaces remain limited. Here, we demonstrate that aliovalent samarium doping provides a direct and tunable route to vacancy engineering in the prototypical CeO₂/Ru-(0001) inverse catalyst. Using real-time, nanometer-resolved low-energy electron microscopy in combination with low-energy electron microdiffraction, X-ray absorption spectroscopy in photoemission electron microscopy, and density functional theory, we directly correlate lattice structure, oxidation state, and vacancy formation under reducing conditions. Sm incorporation generates extrinsic oxygen vacancies, which are accompanied by Ce³⁺ formation, which induces a pronounced lattice expansion that in turn fosters the formation of additional intrinsic oxygen vacancies, substantially enhancing reducibility and stabilizing reduced ceria phases relative to undoped CeO₂. The doped system exhibits accelerated reduction kinetics, ordered intrinsic vacancy formation, and modified phase-transition behavior. By establishing how aliovalent dopants govern the vacancy landscape and redox response in inverse ceria catalysts, this work provides atomic-level mechanistic insight and concrete design principles for engineering reducible oxides at metal-oxide interfaces with tailored redox properties.