Chinese Academy of Sciences scientists make new progress in the study of the ammonia synthesis reaction mechanism

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Recently, a team of scientists from the Research Group on Composite Hydride Materials Chemistry at the Dalian Institute of Chemical Physics, Chinese Academy of Sciences, in collaboration with a team from the Research Group on Cluster Spectroscopy and Dynamics at the **Key Laboratory of Molecular Reaction Dynamics, made new progress in the study of the mechanism of the ammonia synthesis reaction. The relevant findings were published in Angewandte Chemie International Edition (Angew. Chem. Int. Ed., DOI:10.1002/ange.201703864) and were selected as a “Hot Article”. Achieving the efficient synthesis of ammonia under mild conditions has always been an important research topic in the field of catalysis. The research team reported for the first time this composite catalyst system of LiH and 3d transition metals, which exhibits excellent low-temperature activity, and proposed a \"nitrogen transfer\" catalytic mechanism: LiH acts as a second catalytic center to transfer nitrogen species from the surface of the transition metal to form Li2NH/LiNH2, which is then hydrogenated to release ammonia. This dual-center catalytic mechanism overcomes the restrictive relationship between the activation energy barrier and the adsorption energy of reaction species on a single transition metal, thereby enabling the synthesis of ammonia at low temperatures and pressures (Nature Chemistry, 2017, 9, 64). The microscopic mechanisms of nitrogen activation and transfer transformation on this catalyst still require further investigation. In this work, the research team at Dalian Institute of Chemical Physics focused on the LiH-Fe composite catalyst and discovered a strong interaction between Fe and LiH at the interface. By utilizing a self-developed cluster mass spectrometry and spectroscopy combined experimental apparatus, in close conjunction with density functional theory calculations, trihydride species of Li-Fe-H (such as Li4FeH6, Li5FeH6, etc.) present on the surface (interface) of this catalyst were successfully detected. What is even more interesting is that these hydride species can react with N2 to be directly converted into substances containing Fe-(NH2)-Li and LiNH2, thereby achieving the dissociation of N2, its transfer to Li, and hydrogenation ； At the same time, the hydrogen carrying a negative charge and bound to Fe in the ternary hydride is converted into hydrogen carrying a positive charge and bound to N, completing a two-electron transfer. These findings based on cluster reactions suggest that Li4FeH6 formed at the Fe-LiH interface is likely to be the catalytically active center, and the activation of N2 may shift from the homolysis process at the C7 site of conventional Fe-based catalysts to a \"hydrogen-assisted dissociation\" mechanism. This study deepens the understanding of the reaction mechanism for ammonia synthesis on LiH-3d transition metal catalysts, providing insights for the design and development of new, high-efficiency ammonia synthesis catalysts. http://www.mhg114.com/data/attachment/forum/201706/16/2241409fg9covp3a3n9ilv.png Uploaded at 22:41 yesterday. Download the attachment (130.74 KB). The above work was funded by the **Outstanding Young Scholars Fund, key projects of the National Natural Science Foundation of China, the iChEM Collaborative Innovation Center for Energy Materials Chemistry under the Ministry of Education, and the DICPDMTO project focused on basic research in methanol conversion and new technologies for replacing oil with coal at the Dalian Institute of Chemical Physics. Source: Dalian Institute of Chemical Physics, Chinese Academy of Sciences