The demand for decarbonizing the ammonia industry by using renewable energy has invoked increasing research interests into catalyst development for effective N₂ reduction under mild conditions. Hydride-based materials are among some of the emerging catalysts for ammonia synthesis at ambient pressure and low temperatures (< 673 K). A recent chemical looping process based on Ni/BaH₂ showed the most promise as it can realize ammonia production at a temperature as low as 373 K and under ambient pressure. However, the chemical transformation of the hydride catalyst at the molecular level remains unclear in this process. In this work, we report detailed in situ neutron spectroscopy and diffraction investigations along with first-principles simulations on the structural transformation of Ni/BaH₂ during the nitridation and hydrogenation steps in the chemical looping process for ammonia synthesis. It was shown that a ball-milling process of the starting Ni/BaH₂ could significantly decrease the size of BaH₂ and increase the density of defects, thus potentially enhancing the reactivity of the hydride. The evolution from BaH₂ to barium imide (BaNH) was evidenced in the inelastic neutron scattering (INS) and neutron diffraction results during the N₂ reaction step. During the hydrogenation study, in addition to the recovery of BaH₂, a possible intermediate species, N-deficient barium imide, was also detected. In comparing the N₂ and H₂ reaction steps, the neutron results indicate that the hydrogenation step appears more difficult than the nitridation step, confirming the facile N₂ fixation property of Ni/BaH₂ catalyst in ammonia synthesis.

通过使用可再生能源对氨工业进行脱碳的需求已引起人们对在温和条件下有效还原N _2的催化剂开发的研究兴趣日益增加。氢化物基材料是在环境压力和低温（<673 K）下合成氨的一些新兴催化剂之一。最近基于Ni / BaH _2的化学成环过程展示了最有希望的前景，因为它可以在低至373 K的温度和环境压力下实现氨的生产。然而，在该过程中，氢化物催化剂在分子水平上的化学转化仍然不清楚。在这项工作中，我们报告了详细的原位中子光谱和衍射研究，以及关于氨合成过程中Ni / BaH ₂在氮化和氢化步骤中Ni / BaH ₂的结构转变的第一性原理模拟。已经表明，起始Ni / BaH ₂的球磨工艺可以显着减小BaH ₂的尺寸并增加缺陷的密度，从而潜在地提高氢化物的反应性。BaH ₂的演变在N ₂反应步骤中，非弹性中子散射（INS）和中子衍射结果证明了亚氨基钡（BaNH）的生成。在氢化研究期间，除了回收了BaH _2之外，还检测到了可能的中间物种，即N缺乏的钡酰亚胺。在比较N ₂和H ₂反应步骤时，中子结果表明加氢步骤似乎比氮化步骤更困难，从而证实了Ni / BaH ₂催化剂在氨合成中的简便的N ₂固定性能。