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

摘要

Lanthanum oxyhydrides were recently reported to be fast hydride ion conductors with the highest conductivity at 100-400 degrees 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 degrees 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 (approximate to 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 H-2 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.