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_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 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 (${\text{H}}^{\text{-}}$) 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.

早期的 3d 金属如铬可以很容易地离解 N ₂，但随后的氢化成氨很困难，因为它们与氮结合太强。因此，对用于氨合成的 Cr 基催化剂的研究非常罕见。在这里，我们表明当 Cr 与 Ba、N 和 H 化合物形成氮化物氢化物时，可以在温和条件下实现有效的氨合成催化。在 573 K 和 10 bar 下，该催化剂的氨合成速率（6.8 mmol _NH3 g _cat ^-1 h ^-1）约为 Cs-Ru/MgO 催化剂的四倍。具有低表观活化能 (50.1 kJ mol ^-1 ) 和 H ₂和 N _{2 的}正反应顺序，它可以在 373 K 和 1 bar 下产生可观察到的氨。活性相具有类似 Ba ₅ CrN ₄ H 的结构，含有活性氢（${\text{H}}^{\text{——}}$) 和氮，它们参与氨的形成。这项工作揭示了一种“激活”非活性早期过渡金属以实现有效氨催化的策略。