Ammonia synthesis under mild conditions is a goal that has been long sought after. Previous investigations have shown that adsorption and transition-state energies of intermediates in this process on transition metals (TMs) scale with each other. This prevents the independent optimization of these energies that would result in the ideal catalyst: one that activates reactants well, but binds intermediates relatively weakly. Here we demonstrate that these scaling relations can be broken by intervening in the TM-mediated catalysis with a second catalytic site, LiH. The negatively charged hydrogen atoms of LiH act as strong reducing agents, which remove activated nitrogen atoms from the TM or its nitride (TMN), and as an immediate source of hydrogen, which binds nitrogen atoms to form LiNH2. LiNH2 further splits H2 heterolytically to give off NH3 and regenerate LiH. This synergy between TM (or TMN) and LiH creates a favourable pathway that allows both early and late 3d TM–LiH composites to exhibit unprecedented lower-temperature catalytic activities.
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This work is dedicated to K. R. Tsai for his enlightenment and encouragement. The authors thank Z. Xiong for his earlier efforts in this study, and T. Zhang and X. Bao for beneficial discussions. The authors also thank the Dalian Institute of Chemical Physics (DICP DMTO201504), the Project of National Science Funds for Distinguished Young Scholars (51225206) and the Collaborative Innovation Center of Chemistry for Energy Materials (2011-iChEM) for financial support. We also thank the Shanghai Synchrotron Radiation Facility (BL14W1) for providing beam time.
The authors declare no competing financial interests.
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Wang, P., Chang, F., Gao, W. et al. Breaking scaling relations to achieve low-temperature ammonia synthesis through LiH-mediated nitrogen transfer and hydrogenation. Nature Chem 9, 64–70 (2017) doi:10.1038/nchem.2595
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