With a high hydrogen content (17.6 wt.%), high energy density (4 kWh kg-1), facile storage and transportation, NH3 has been regarded as a potential energy carrier. The key issue for NH3 synthesis is the development of non-noble metal based, highly active and stable catalysts that can be operated under mild condition. This would require a catalyst to have strong activation to N2 but relatively weak binding to -NHx (x = 0-3) intermediates. Such a situation can hardly be achieved as long as the dissociative activation of N2 and H2 and the subsequent hydrogenation of NHx are conducted solely on the surface of transition metals. Hydrides of alkali and alkaline earth metals (AMH) are strong reducing agents capable of abstracting N from transition metal (TM) nitrides to form amides/imides. Those amides/imides undergo self-hydrogenation producing NH3 and regenerating hydrides. The unique chemistry of transition metals, AMH, N2 and H2 creates an energy-favorable pathway allowing NH3 synthesis under mild conditions, i.e., detectable NH3 formation rate was obtained at 150 °C over the TM-AMH composite catalysts. More interestingly, both early (V, Cr, & Mn) and late transition metals (Fe, Co & Ni) show comparable activities upon synergizing with AMH. Kinetic analyses indicate that the dissociative activation of N2 may not be the rate-determining step. The characterization of intermediate phases and surface clusters of catalysts provides interesting information that helps the elucidation of the step-wise reaction pathway and the interpretation of catalytic mechanism.

The role of hydrides in ammonia synthesis

255th National Meeting (2018)

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