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 (BaH2) can synergize with Co, leading to a much better low-temperature activity, i.e., the BaH2-Co/carbon nanotube (CNT) catalyst exhibits ammonia synthesis activity right above 150 degrees C; at 300 degrees 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-BaH2 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. BaH2, 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 NH3 and BaH2 can be facilely carried out at 150 degrees C. The relayed catalysis by Co and BaH2 sites creates an synthesis under milder conditions.