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Abstract

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 mmol_NH3g_cat⁻¹h⁻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.