Ruthenium Catalysts Promoted by Lanthanide Oxyhydrides with High Hydride‐Ion Mobility for Low‐Temperature Ammonia Synthesis
Kayato Ooya, Jiang Li, Keiga Fukui, Soshi Iimura, Takuya Nakao, Kiya Ogasawara, Masato Sasase, Hitoshi Abe, Y. Niwa, Masaaki Kitano, Hideo Hosono
Abstract
Abstract 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/LaH 3 − 2 x O x ) is investigated. The onset ammonia formation temperature by the Ru/LaH 3 − 2 x O x is lower by 100 °C when compared to the Ru‐loaded lanthanum oxides. The apparent activation energy of ammonia synthesis over Ru/LaH 3 − 2 x O x 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/LaH 3 − 2 x O x , 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, LaH 3 − 2 x O x has high nitridation resistance as compared with lanthanum hydride (LaH 3 ) under ammonia synthesis condition. As a result, the high surface H − concentration of Ru/LaH 3 − 2 x O x is preserved during ammonia synthesis, exhibiting more robust activity than Ru/LaH 3 . Almost the same results are obtained for Ru/CeH 3 − 2 x O x implicating the common characteristics of rare‐earth oxyhydride support.