Barium Hydride-Mediated Nitrogen Transfer and Hydrogenation for Ammonia Synthesis: A Case Study of Cobalt

Industrial ammonia synthesis catalyzed by Fe- and Ru-based catalysts is an energy-consuming process. The development of low-temperature active catalyst has been pursued for a century. Herein, we report that barium hydride (BaH 2 ) can synergize with Co, leading to a much better low-temperature activity, i.e., the BaH 2 -Co/carbon nanotube (CNT) catalyst exhibits ammonia synthesis activity right above 150 °C; at 300 °C, it is 2 orders of magnitude higher than that of the BaO-Co/CNTs and more than 2.5-times higher than Cs-promoted Ru/MgO. Kinetic analyses reveal that the dissociative adsorption of N 2 on the Co-BaH 2 catalyst may not be the rate-determining step, as evidenced by the much smaller reaction order of N 2 (0.43) and the lower apparent activation energy (58 kJ mol –1 ) compared with those of the unpromoted and BaO-promoted Co-based catalysts. BaH 2, with a negative hydride ion, may act as a strong reducing agent, removing activated N from the Co surface and forming a BaNH species. The hydrogenation of the BaNH species to NH 3 and BaH 2 can be facilely carried out at 150 °C. The relayed catalysis by Co and BaH 2 sites creates an energy-favored pathway that allows ammonia synthesis under milder conditions.