Potassium hydride-intercalated graphite as an efficient heterogeneous catalyst for ammonia synthesis

Abstract

Due to the high energy needed to break the N ≡ N bond (945 kJ mol−1), a key step in ammonia production is the activation of dinitrogen, which in industry requires the use of transition metal catalysts such as iron (Fe) or ruthenium (Ru), in combination with high temperatures and pressures. Here we demonstrate a transition-metal-free catalyst—potassium hydride-intercalated graphite (KH0.19C24)—that can activate dinitrogen at very moderate temperatures and pressures. The catalyst catalyses NH3 synthesis at atmospheric pressure and achieves NH3 productivity (µmolNH3 gcat−1 h−1) comparable to the classical noble metal catalyst Ru/MgO at temperatures of 250–400 °C and 1 MPa. Both experimental and computational calculation results demonstrate that nanoconfinement of potassium hydride between the graphene layers is crucial for the activation and conversion of dinitrogen. Hydride in the catalyst participates in the hydrogenation step to form NH3. This work shows the promise of light metal hydride materials in the catalysis of ammonia synthesis. Ammonia is industrially synthesized through an established process based on iron or ruthenium transition metal catalysts, although the quest for alternative and more sustainable processes is still ongoing. Here, the authors show that potassium hydride confined between graphene layers can reduce dinitrogen and catalyse ammonia synthesis under mild conditions.