Kammert James, Moon Jisue, Cheng Yongqiang, Daemen Luke, Irle Stephan, Fung Victor, Liu Jue, Page Katharine, Ma Xiaohan, Phaneuf Vincent, Tong Jianhua, Ramirez-Cuesta Anibal J, Wu Zili
Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States.
Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States.
J Am Chem Soc. 2020 Apr 22;142(16):7655-7667. doi: 10.1021/jacs.0c02345. Epub 2020 Apr 14.
Recently, there have been renewed interests in exploring new catalysts for ammonia synthesis under mild conditions. Electride-based catalysts are among the emerging ones. Ruthenium particles supported on an electride composed of a mixture of calcium and aluminum oxides (C12A7) have attracted great attention for ammonia synthesis due to their facile ability in activating N under ambient pressure. However, the exact nature of the reactive hydrogen species and the role of electride support still remain elusive for this catalytic system. In this work, we report for the first time that the surface-adsorbed hydrogen, rather than the hydride encaged in the C12A7 electride, plays a major role in ammonia synthesis over the Ru/C12A7 electride catalyst with the aid of neutron scattering techniques. Combining neutron diffraction, inelastic neutron spectroscopy, density functional theory (DFT) calculation, and temperature-programmed reactions, the results provide direct evidence for not only the presence of encaged hydrides during ammonia synthesis but also the strong thermal and chemical stability of the hydride species in the Ru/C12A7 electride. Steady state isotopic transient kinetic analysis (SSITKA) of ammonia synthesis showed that the coverage of reactive intermediates increased significantly when the Ru particles were promoted by the electride form (coverage up to 84%) of the C12A7 support rather than the oxide form (coverage up to 15%). Such a drastic change in the intermediate coverage on the Ru surface is attributed to the positive role of electride support where the H poisoning effect is absent during ammonia synthesis over Ru. The finding of this work has significant implications for understanding catalysis by electride-based materials for ammonia synthesis and hydrogenation reactions in general.
最近，人们对探索在温和条件下合成氨的新型催化剂重新产生了兴趣。基于电子化物的催化剂是其中新兴的一类。负载在由钙和铝的氧化物混合物（C12A7）组成的电子化物上的钌颗粒，因其在常压下活化氮的能力而在氨合成中备受关注。然而，对于该催化体系，活性氢物种的确切性质以及电子化物载体的作用仍然难以捉摸。在这项工作中，我们首次报告，借助中子散射技术，在Ru/C12A7电子化物催化剂上，表面吸附的氢而非包裹在C12A7电子化物中的氢化物在氨合成中起主要作用。结合中子衍射、非弹性中子光谱、密度泛函理论（DFT）计算和程序升温反应，结果不仅为氨合成过程中包裹氢化物的存在提供了直接证据，也为Ru/C12A7电子化物中氢化物物种的强热稳定性和化学稳定性提供了直接证据。氨合成的稳态同位素瞬变动力学分析（SSITKA）表明，当Ru颗粒由C12A7载体的电子化物形式（覆盖率高达84%）而非氧化物形式（覆盖率高达15%）促进时，反应中间体的覆盖率显著增加。Ru表面中间体覆盖率的这种剧烈变化归因于电子化物载体的积极作用，即在Ru上进行氨合成时不存在氢中毒效应。这项工作的发现对于理解基于电子化物的材料在氨合成和一般氢化反应中的催化作用具有重要意义。
