Nano-CeO2-Derived BaCeO3– x N y H z : A High-Purity Oxynitride-Hydride Support for Enhanced Ammonia Synthesis

Hetero-anion doping has emerged as an effective approach to modulate the physicochemical and catalytic properties of perovskite-type oxides. In this study, nanosized CeO2 synthesized by a continuous flow-type supercritical hydrothermal synthesis (SHS) process was employed as a precursor for a solid-state reaction with Ba(NH2)2, enabling the low-temperature synthesis of single-phase perovskite oxynitride–hydride BaCeO3–xNyHz (BaCeO3–xNyHz–SHS) with a high surface area and abundant N3– and H– species. Ru-loaded BaCeO3–xNyHz–SHS exhibited a maximum ammonia synthesis rate of 9.0 mmol g–1 h–1 at 300 °C and 0.9 MPa and showed up to a 6-fold enhancement in activity compared with the corresponding Ru/BaCeO3–xNyHz prepared from commercial CeO2, owing to the enhanced generation of anion vacancies at the Ru–support interface. Moreover, BaCeO3–xNyHz–SHS acted as a highly efficient support for various transition metals in ammonia synthesis and for Ni in ammonia decomposition, achieving >99% conversion at 560 °C with a low apparent activation energy (75 kJ mol–1). These findings demonstrate that enhancing the solid-state reactivity of oxide precursors through nanoscale engineering provides a general strategy for the low-temperature synthesis of highly active hetero-anion perovskite catalysts for ammonia synthesis and decomposition.