Reactive hydrogen in KH or KOH for catalytic NH3 synthesis

Public summary

* K has long been employed as a key promoter for ammonia synthesis; however, its exact role remains elusive.

* KH and KOH promote the hydrogenation of Fe nitride intermediates to form NH3 via distinct reaction pathways.

* Hydrogen in KH (hydridic) and KOH (protonic) is involved in NH3 formation via formation of KNHx intermediates.

* This study provides new insights into the role of alkali metals, guiding the design of efficient catalysts.

Abstract

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 (Nad) on Fe to form KNHx 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 Nad 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.