Mechanochemical preparation of perovskite-type oxyhydrides BaLnO₂H (Ln = Tb–Lu) and their catalytic activity for ammonia synthesis

Oxyhydrides have recently attracted attention as promising candidates for hydrogenation catalysts and electrochemical device applications. The functions of these oxyhydrides are expected to be influenced by the coordination environment around the H^- sites. Here, we report novel lanthanide-based oxyhydrides BaLnO₂H (Ln = Tb-Lu) synthesized by a mechanochemical method and their catalytic performance for ammonia synthesis. X-ray diffraction (XRD) measurements indicated that these compounds have a cubic perovskite structure, and the formability of the perovskite phase could be predicted according to the Goldschmidt tolerance factor and octahedral factor. The compositions of the oxyhydrides were determined by thermal desorption spectroscopy (TDS) measurements to be BaLnO₂H_0.69-0.75, which suggests that H^- vacancies were introduced under the mechanochemical processes. These compounds effectively promoted the activity of Ru catalysts for ammonia synthesis at a low reaction temperature. The turnover frequencies (TOFs) of the Ru/BaLnO₂H catalysts increase with a decrease in the work functions (WFs) of the hydride-deficient oxyhydride surfaces, and the electron-donating ability of the oxyhydrides can be tuned according to Ln-H interactions.