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Author:    date: 2018-10-23

Recently, a research team led by Prof. Ping Chen and Dr. Jianping Guo from the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences developed high efficiency manganese-based catalysts for ammonia synthesis. This research entitled “Alkali and alkaline earth hydrides-driven N2activation and transformation over Mn nitride catalyst” was published in J. Am. Chem. Soc.(DOI: 10.1021/jacs.8b08334).

Ammonia synthesis over transition metals is a very important topic in the research field of heterogeneous catalysis. Iron and ruthenium have moderate nitrogen adsorption energies, and therefore exhibit excellent ammonia synthesis performances and have been used extensively in industry. Early transition metals such as V, Cr, and Mn, however, own strong nitrogen adsorption capability and usually form stable nitride phases under the reaction conditions of ammonia synthesis, which hinders the sequent hydrogenation steps and thus showing poor catalytic activities.

Based on previous research on ammonia synthesis (NatureChem.ACS Catal.Angew. Chem. Int. Ed.), this research team thoroughly investigated the influence of alkali or alkaline earth metal hydrides (AH) on the catalytic performance of Mn-based catalysts. Experimental results indicate that the catalytic activities of Mn-AH composites are 1-3 orders of magnitude higher than that of Mn catalyst itself. Among them, the performances of Mn-LiH and Mn-BaH2are comparable to that of Ru-based catalyst. Moreover, the order of promotion effect of AH for Mn is distinctly different from that of conventional alkali or alkaline earth (hydr)oxide electronic promoters. Thermodynamic analyses and characterization results reveal that the phase transformation between alkali or alkaline earth metal hydrides and their imides, as well as their interactions with Mn nitride under the reaction conditions of ammonia synthesis should account for the promotion effect. More detailed investigation on the activity-phase structure relationship ofMn-LiH catalystindicates the active phases and kinetic behaviors of these composite catalysts strongly depend on the reaction conditions especially temperature and space velocity. This is another unique feature of these composite catalysts that is different from the conventional ammonia synthesis catalysts. This work propose a promising strategy for “activating” the early transition metals in ammonia synthesis catalysis.

Alkali or alkaline earth metal hydrides activate manganese nitride in ammonia synthesis catalysis (Image byFeiCHANG)

This workwas conducted in collaboration with Prof.Xingguo Li from Peking University. This work was supported by the National Natural Science Foundation of China, Sino-Japanese Research Cooperative Program of Ministry of Science and Technology, Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), DICP DMTO, andYouth Innovation Promotion Association of Chinese Academy of Sciences.(Text by Fei CHANG and Jianping GUO).

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