Transition-metal–free barium hydride mediates dinitrogen fixation and ammonia synthesis

Ammonia is crucial for the manufacture of nitrogen fertilizers. Due to the high energy consumption of industrial ammonia production, the development of alternative materials and approaches for efficient N₂ reduction to ammonia under mild conditions is a long-sought goal.

Recently, a research group led by Prof. Chen Ping and Prof. Guo Jianping from the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences, in collaboration with Prof. Tejs Vegge from Technical University of Denmark, synthesized ammonia via a chemical looping process mediated by a transition-metal-free barium hydride (BaH₂) and revealed its mechanism.

This study was published in Angewandte Chemie International Edition on August 2.

Alkali or alkaline earth metal hydrides can fix N₂, forming corresponding metal imides and H₂. The metal imides then undergo hydrogenation to NH₃ and metal hydrides. However, the reaction mechanisms of N₂ activation, H₂ release and NH₃ formation over alkali hydrides are still unclear.

The researchers indicated that the creation of hydrogen vacancies played an important role in N₂ fixation process mediated by BaH₂.

The creation of hydrogen vacancies led to the formation of multiple coordinatively unsaturated Ba sites, which were responsible for the adsorption and activation of N₂. The hydridic hydrogen acted as an electron donor and facilitated the activation of N₂ with concurrent H₂ release.

They found that the process functionally resembled the molecular hydrido complexes and FeMo cofactor in nitrogenase. Both the hydridic hydrogen and gaseous H₂ were involved in the NH₃ formation process.

"This is a helpful model for understanding the activation and hydrogenation of N₂ to NH₃ mediated by alkali and alkaline earth metal hydrides, which is promising in future technologies for nitrogen fixation using transition-metal-free materials," said Prof. Chen.