Barium chromium nitride-hydride for ammonia synthesis

Summary Early 3d metals such as chromium can easily dissociate N2, 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 mmolNH3 gcat−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 H2 and N2, it can produce observable ammonia at 373 K and 1 bar. The active phase has a Ba5CrN4H-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.