Zhao Yue, Cui Jia-Tong, Wang Ming, Valdivielso David Yubero, Fielicke André, Hu Lian-Rui, Cheng Xin, Liu Qing-Yu, Li Zi-Yu, He Sheng-Gui, Ma Jia-Bi
Key Laboratory of Cluster Science of Ministry of Education, Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, School of Chemistry and Chemical Engineering , Beijing Institute of Technology , 100081 Beijing , China.
Institute for Optics and Atomic Physics , Technische Universität Berlin , 10623 and Fritz-Haber-Institut der Max-Planck-Gesellschaft Faradayweg 4-6 , 14195 Berlin , Germany.
J Am Chem Soc. 2019 Aug 14;141(32):12592-12600. doi: 10.1021/jacs.9b03168. Epub 2019 Jul 31.
Dinitrogen activation and reduction is one of the most challenging and important subjects in chemistry. Herein, we report the N binding and reduction at the well-defined TaNH and TaN gas-phase clusters by using mass spectrometry (MS), anion photoelectron spectroscopy (PES), and quantum-chemical calculations. The PES and calculation results show clear evidence that N can be adsorbed and completely activated by TaNH and TaN clusters, yielding to the products TaNH and TaN, but the reactivity of TaNH is five times higher than that of the dehydrogenated TaN clusters. The detailed mechanistic investigations further indicate that a dissociative mechanism dominates the N activation reactions mediated by TaNH and TaN; two and three Ta atoms are active sites and also electron donors for the N reduction, respectively. Although the hydrogen atom in TaNH is not directly involved in the reaction, its very presence modifies the charge distribution and the geometry of TaNH, which is crucial to increase the reactivity. The mechanisms revealed in this gas-phase study stress the fundamental rules for N activation and the important role of transition metals as active sites as well as the new significant role of metal hydride bonds in the process of N reduction, which provides molecular-level insights into the rational design of tantalum nitride-based catalysts for N fixation and activation or NH synthesis.
二氮的活化与还原是化学领域中最具挑战性和重要性的课题之一。在此，我们通过质谱（MS）、阴离子光电子能谱（PES）和量子化学计算，报道了在明确的TaNH和TaN气相团簇上的氮结合与还原情况。PES和计算结果清楚地表明，氮能够被TaNH和TaN团簇吸附并完全活化，生成产物TaNH和TaN，但TaNH的反应活性比脱氢后的TaN团簇高五倍。详细的机理研究进一步表明，解离机理主导了由TaNH和TaN介导的氮活化反应；两个和三个Ta原子分别是氮还原的活性位点和电子供体。尽管TaNH中的氢原子不直接参与反应，但其存在改变了TaNH的电荷分布和几何结构，这对提高反应活性至关重要。该气相研究中揭示的机理强调了氮活化的基本规则、过渡金属作为活性位点的重要作用以及金属氢键在氮还原过程中的新的重要作用，这为基于氮化钽的固氮、活化或合成氨催化剂的合理设计提供了分子层面的见解。
