Industrial
ammonia synthesis catalyzed by Fe- and Ru-based catalysts
is an energy-consuming process. The development of low-temperature
active catalyst has been pursued for a century. Herein, we report
that barium hydride (BaH2) can synergize with Co, leading
to a much better low-temperature activity, i.e., the BaH2-Co/carbon nanotube (CNT) catalyst exhibits ammonia synthesis activity
right above 150 °C; at 300 °C, it is 2 orders of magnitude
higher than that of the BaO-Co/CNTs and more than 2.5-times higher
than Cs-promoted Ru/MgO. Kinetic analyses reveal that the dissociative
adsorption of N2 on the Co-BaH2 catalyst may
not be the rate-determining step, as evidenced by the much smaller
reaction order of N2 (0.43) and the lower apparent activation
energy (58 kJ mol–1) compared with those of the
unpromoted and BaO-promoted Co-based catalysts. BaH2, with
a negative hydride ion, may act as a strong reducing agent, removing
activated N from the Co surface and forming a BaNH species. The hydrogenation
of the BaNH species to NH3 and BaH2 can be facilely
carried out at 150 °C. The relayed catalysis by Co and BaH2 sites creates an energy-favored pathway that allows ammonia
synthesis under milder conditions.
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Chemical looping has been gained increasing attention as an emerging approach for sustainable ammonia synthesis. Nitrogen carriers with high efficiencies are the key to the practical feasibility and competitiveness of...
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