Surface Chemistry of Ru/CeO₂ Catalyst as Revealed by CO and ¹⁵N₂ IR Probe Molecules

A comprehensive molecular-level characterization of the surface chemistry of Ru/CeO₂ was performed using FTIR spectroscopy of two complementary probe molecules: CO, the most commonly used IR probe for cationic and metal sites, and ¹⁵N₂, an inert molecule that selectively detects only the strongest adsorption centers. Blank experiments with bare ceria octahedra were also performed. The effects of oxidative and reductive pretreatments on CO and ¹⁵N₂ adsorption were systematically examined for both Ru/CeO₂ and CeO₂. The results show that the oxidized catalyst contains an oxidized Ru-oxide/hydroxide phase covering the metal particles and part of the support surface, thus completely blocking metallic ruthenium sites and markedly reducing the number of available CO adsorption sites on ceria and completely suppressing ¹⁵N₂ adsorption. This blocking effect is much more pronounced on the highly reactive {110} and {100} facets than on the more stable {111} surface. After reduction, metallic Ru becomes exposed, and some previously inaccessible ceria sites are restored. Structural and electronic characterization using XRD, STEM, H₂-TPR, and NAP-XPS provides key complementary information on the morphology, composition, reducibility, and chemical state of the catalysts, enabling consistent interpretation of the IR spectroscopic results.