Oxygen Vacancy Engineering Unlocks Efficient Ammonia Synthesis Over Rare-Earth-Modified Ru/CeO2

Abstract Rare-earth engineering of CeO2 offers a route to enhance Ru-based catalysts for ammonia synthesis by tailoring oxygen vacancies and metal–support interactions. Herein, we systematically synthesized and benchmarked Ru catalysts supported on RE (=Gd, La, and Sm)-modified CeO2 under identical conditions to establish promoter-structure–activity relationships. Rare-earth incorporation increased the Ce3+/Ce4+ ratio, oxygen-vacancy concentration, and weak acidic sites, facilitating electron transfer to Ru, promoting N2 activation, and potentially facilitating interfacial hydrogen spillover. H2-TPD suggested improved hydrogen mobility and utilization at the Ru/CeO2 interface. Ru/GdCeO exhibited the highest NH3 synthesis rate of (19.80 ± 0.98) mmol g–1h–1 at 400 °C and 1.5 MPa, with an N2:H2 ratio of 1:2 and WHSV = 13,500 mL g–1 h–1, yielding 3.53 vol % NH3 in the reactor effluent. Such superior performance may be associated with optimized Ru-support interactions and abundant oxygen vacancies. These findings establish Gd as the most effective promoter for Ru/CeO2 under mild synthesis conditions.