Ruthenium Catalysts Promoted by Lanthanide Oxyhydrides with High Hydride‐Ion Mobility for Low‐Temperature Ammonia Synthesis

Lanthanum oxyhydrides were recently reported to be fast hydride ion conductors with the highest conductivity at 100–400 °C. Here, the relationship between the hydride‐ion conduction and the ammonia synthesis activity of ruthenium‐loaded lanthanum oxyhydrides (Ru/LaH3−2xOx) is investigated. The onset ammonia formation temperature by the Ru/LaH3−2xOx is lower by 100 °C when compared to the Ru‐loaded lanthanum oxides. The apparent activation energy of ammonia synthesis over Ru/LaH3−2xOx is 64 kJ mol−1, which is much smaller than that of hydride‐ion conductivity (≈100 kJ mol−1), indicating no direct relationship between the catalytic activity and the bulk hydride‐ion conductivity. However, the catalytic performance is strongly correlated with the surface H− ion mobility of Ru/LaH3−2xOx, which gives rise to the formation of low work function electrons at H− ion vacancies near the Ru‐support interface and high resistance for H2 poisoning on the Ru catalyst. Moreover, LaH3−2xOx has high nitridation resistance as compared with lanthanum hydride (LaH3) under ammonia synthesis condition. As a result, the high surface H− concentration of Ru/LaH3−2xOx is preserved during ammonia synthesis, exhibiting more robust activity than Ru/LaH3. Almost the same results are obtained for Ru/CeH3−2xOx implicating the common characteristics of rare‐earth oxyhydride support. Lanthanide oxyhydrides such as LaH3−2xOx and CeH3−2xOx significantly enhance the activity of a ruthenium catalyst for ammonia synthesis at low reaction temperatures. Ammonia synthesis over Ru/LaH3−2xOx is initiated even at 160 °C, which is lower by 100 °C than that using Ru/La2O3. The surface H− ion mobility is demonstrated to be a dominant factor in this low temperature ammonia synthesis.