Ammonia synthesis is a cornerstone of modern agriculture and the chemical industry. It is regarded as a promising carbon-free energy carrier for a future hydrogen economy. However, the performance of conventional transition metal-based catalysts is constrained by the linear scaling relationship (BEP relation), which give rise to a volcano-type activity curve. Early transition metals such as Mn bind N too strongly, making the hydrogenation of activated *N species difficult. As a result, efficient ammonia synthesis catalysts based on early transition metals have rarely been reported.
In a recent study published in Angewandte Chemie International Edition, a research team led by Prof. CHEN Ping from the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences (CAS), in collaboration with Prof. YUAN Shaojun from Sichuan University, developed an efficient ammonia synthesis catalyst consisting of atomically dispersed early transition metal Mn supported on the ternary hydride (LiBaH3). The catalyst enables efficient ammonia synthesis through a hydride-assisted N2 dissociation mechanism that circumvent traditional scaling relationships.
The team synthesized a LiBaH3-Mn1 catalyst, in which the activation of N2 on Mn1 sites, preventing the direct dissociation of N2 to form overly strong Mn-N bonds. Meanwhile, hydride ions (H−) from LiBaH3 further activate the adsorbed *N2 through a reductive protonation pathway, forming *N2H intermediates. Subsequent N–N bond cleavage produces surface nitride (Mn–N) and imide (*NH) species on the LiBaH3–Mn1 interface.
The catalyst exhibited an ammonia synthesis rate two orders of magnitude higher than that of manganese nitride and performs 2.5 times better than the benchmark Cs-Ru/MgO catalyst at 400 °C, representing a state-of-the-art performance among group 4–7 transition metal–based catalysts.
"Our study provides a new strategy for the rational design of highly active early transition metal-based catalysts for ammonia synthesis," said Prof. CHEN.