Barium chromium nitride-hydride for ammonia synthesis

Abstract

Early 3d metals such as chromium can easily dissociate N 2 , but the subsequent hydrogenation to ammonia is difficult because they bind nitrogen too strongly. Hence, investigation of Cr-based catalysts for ammonia synthesis is very rare. Here we show that when Cr compounds with Ba, N, and H forming a nitride-hydride, effective ammonia synthesis catalysis can be achieved under mild conditions. Under 573 K and 10 bar, this catalyst has an ammonia synthesis rate (6.8 mmol NH3 g cat −1 h −1 ) that is about four times that of the Cs-Ru/MgO catalyst. With low apparent activation energy (50.1 kJ mol −1 ) and positive reaction orders of H 2 and N 2 , it can produce observable ammonia at 373 K and 1 bar. The active phase has a Ba 5 CrN 4 H-like structure containing reactive hydrogen ( H - ) and nitrogen, which are involved in the ammonia formation. This work discloses a strategy to “activate” the inactive early transition metals for effective ammonia catalysis. • A quaternary barium chromium nitride-hydride is synthesized • The hydride acts as electron and hydrogen carriers for N 2 activation • Reactive lattice hydrogen ( H - ) and nitrogen are involved in the formation of NH 3 • The nitride-hydride structure enables Cr to be active for NH 3 synthesis Ammonia is a key ingredient for manufacturing fertilizers and nitrogen-containing chemicals and has also been considered recently as a promising energy carrier. Current NH 3 production is dominated by the well-developed Haber-Bosch process operated under harsh conditions. The development of more active catalysts under mild conditions is of important scientific significance and practical value. Early 3d metals such as Cr were regarded as catalytically inactive for ammonia synthesis so that the main research efforts have been focused on Ru-, Fe-, and Co-based catalysts over the years. Here we report a strategy to “activate” Cr for effective ammonia catalysis by the construction of a nitride-hydride structure with Ba, N, and H. This unique structure renders Cr highly active for ammonia synthesis especially under lower temperatures. It is envisaged that this strategy offers new opportunities for the development of novel catalysts for mild-condition and sustainable ammonia synthesis. Early 3d metals such as Cr have very low activity for ammonia synthesis catalysis. The formation of a barium chromium nitride-hydride structure leads to great change, resulting in an activity enhancement of more than three orders of magnitude compared with the Cr nitride. The barium chromium nitride-hydride structure comprising reactive hydrogen ( H - ) and nitrogen species creates a chemical environment favorable for N 2 activation and hydrogenation under mild conditions.