Ammonia is crucial for the manufacture of nitrogen fertilizers and is also widely regarded as promising energy or hydrogen carrier. 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.
Main group elements are notable for their abundance in the Earth's crust and environmentally benign nature. Because of new dinitrogen reaction chemistry, the employment of main group elements for this vital reaction has recently aroused great interest.
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.
Transition-metal-free dinitrogen fixation mediated by barium hydride (Image by GUAN Yeqin)
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 facilitates 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 has prospects in future technologies for nitrogen fixation using transition-metal-free materials," said Prof. CHEN.
This work was supported by the National Natural Science Foundation of China and the Youth Innovation Promotion Association of CAS. (Text GUAN Yeqin)