Ternary ruthenium complex hydrides for ammonia synthesis via the associative mechanism

Abstract

Ammonia is the feedstock for nitrogen fertilizers and a potential carbon-free energy carrier; however, its production is highly energy intensive. Conventional heterogeneous catalysts based on metallic iron or ruthenium mediate dinitrogen dissociation and hydrogenation through a relatively energy-expensive pathway. Here we report the ternary ruthenium complex hydrides Li4RuH6 and Ba2RuH6 as an alternative class of catalysts, composed of electron- and hydrogen-rich [RuH6] anionic centres, for non-dissociative dinitrogen reduction, where hydridic hydrogen transports electrons and protons between the centres, and the Li/Ba cations stabilize NxHy (x = 0–2, y = 0–3) intermediates. The dynamic and synergistic involvement of all the components of the ternary complex hydrides facilitates an associative reaction mechanism with a narrow energy span and perfectly balanced kinetic barriers for the multistep process, leading to ammonia production from N2 + H2 with superior kinetics under mild conditions. The mechanism of ammonia synthesis on traditional iron or ruthenium catalysts features a high energetic span. Here, the authors introduce ternary ruthenium complex hydrides of lithium and barium that can activate dinitrogen via a lower-energy path, resulting in the highly efficient production of ammonia under milder conditions.