Reactive hydrogen in KH or KOH for catalytic NH₃ synthesis

Potassium (K) has long been employed as a key catalyst promoter for ammonia synthesis. Its chemical state and promoting mechanism, however, remain elusive. Here, we study the influence of different anionic counterparts of K (i.e., H^- and OH^-) on its promoting capability and function in ammonia synthesis catalysis. When potassium is in the form of hydride (KH), it can enhance the ammonia formation rate of the Fe/carbon nanotube (CNT) catalyst by two orders of magnitude; while it is in the form of hydroxide (KOH), the rate increment is only ∼5 times. We show that both KH and KOH can promote N removal from Fe nitride under a hydrogen atmosphere, where an intermediate KNH₂ species may form and further be hydrogenated to NH₃. Isotopic measurements show that H atoms in KH and KOH are involved in the formation of NH₃. For the KH-Fe catalyst, the hydridic H of KH is reductive and may undergo a redox reaction with adsorbed N (N_ad) on Fe to form KNH_x species and further be hydrogenated to NH₃ and create a clean Fe surface for further nitrogen activation. For the KOH-Fe catalyst, the protonic H of KOH may protonate N_ad on Fe to form NH₃ through the intermediate species of K-Fe-O and KNH₂. This work reveals the crucial role of hydrogen in ammonia synthesis catalysis and, moreover, could inspire revisiting of the role of alkali promoters in industrial ammonia synthesis catalysts.