The formation of a lithium–iridium complex hydride toward ammonia synthesis

Abstract

Noble metal elements are focal catalytic candidates for many chemical processes, but have received little attention in the field of nitrogen fixation except ruthenium and osmium. Iridium (Ir), as a representative, has been shown to be catalytically inactive for ammonia synthesis because of its weak nitrogen adsorption and severe competitive adsorption of H over N that strongly inhibits the activation of N₂ molecules. Here we show that, upon compositing with lithium hydride (LiH), iridium can catalyze ammonia formation at much enhanced reaction rates. The catalytic performance of the LiH–Ir composite can be further improved by dispersion on a MgO support with a high specific surface area. At 400 °C and 10 bar, the MgO-supported LiH–Ir (LiH–Ir/MgO) catalyst shows a ca. 100-fold increase in activity compared to the bulk LiH–Ir composite and the MgO-supported Ir metal catalyst (Ir/MgO). The formation of a lithium–iridium complex hydride phase was identified and characterized, and this phase may be responsible for the activation and hydrogenation of N₂ to NH₃.