Ruthenium Complex Hydride Catalyst as a Platform for Ammonia Synthesis: The Effect of Alkali or Alkaline Earth Elements

Mild-condition ammonia synthesis from N2 and H2 is a long-sought-after scientific goal and a practical need, especially for the intensively pursued ammonia production using renewable H2. However, effective ammonia production on conventional transition metal catalyst surfaces is unattainable because of the limitations imposed by scaling relations. Here, we reported a series of ruthenium complex hydrides AMn[RuHm] (AM = Li, Na, K, Ca, or Ba) capable of catalyzing ammonia synthesis under mild conditions, which exhibit universal and high activities that far exceed the benchmark Cs–Ru/MgO catalyst. Importantly, their catalytic behaviors depend on the identity of AM cation and follow a trend that is different from the electronegativity-dependent one observed in conventional promoted Ru metal catalysts. Detailed investigations disclose that such an AM effect is a consequence of the differences in interaction strengths between AM cations and the reacting NxHy species, which can be quantitatively mapped out by using the NxHy adsorption energies as the descriptor. Based on the knowledge of the unique configuration and synergized scenario in the Ru complex hydride catalyst system, we propose a strategy for the development of efficient catalysts toward ammonia synthesis, that is designing active site motifs with multicomponent and electron-rich characters.