Barium hydride activates Ni for ammonia synthesis catalysis

Abstract

Nickel (Ni) metal has long been considered to be far less active for catalytic ammonia synthesis as compared to iron, cobalt, and ruthenium. Herein, we show that Ni metal synergized with barium hydride (BaH₂) can catalyse ammonia synthesis with an activity comparable to that of an active Cs–Ru/MgO catalyst typically below 300 °C. Kinetic analyses show that the addition of BaH₂ makes the apparent activation energy for the Ni catalyst decrease dramatically from 150 kJ mol⁻¹ to 87 kJ mol⁻¹. This result together with N₂-TPR experiments suggests a strong synergistic effect between Ni and BaH₂ for promoting N₂ activation and hydrogenation to NH₃. It is suggested that an intermediate [N–H] species is generated upon N₂ fixation and then is hydrogenated to NH₃ with the regeneration of hydride species, forming a catalytic cycle.