Barium Oxide Encapsulating Cobalt Nanoparticles Supported on Magnesium Oxide: Active Non-noble Metal Catalyst for Ammonia Synthesis under Mild Reaction Condition
2021
To
realize a sustainable, carbon-free society, catalysts for the synthesis of
ammonia using renewable energy under mild reaction conditions (<400 °C,
<10 MPa) are needed. Ru-based
catalysts are currently the most promising candidates; however, Ru is expensive and of low abundance. Here,
we discovered that encapsulation of Co
nanoparticles with BaO enhanced the ammonia synthesis activity of the
Co, and that a simple Ba-doped Co/MgO catalyst pre-reduced at an
unusually high temperature of 700 °C (Co@BaO/MgO-700red)
showed outstanding ammonia synthesis activity. The ammonia synthesis rate (24.6 mmol gcat−1 h−1)
and turnover frequency (0.255 s−1)
of the catalyst at 350 °C and 1.0 MPa were 22 and 64 times higher,
respectively, than those of the non-doped parent catalyst. At the same temperature but
higher pressure (3.0 MPa), the ammonia
synthesis rate was increased to 48.4 mmol gcat−1
h−1, which is higher than that of active Ru-based catalysts. Scanning
transmission electron microscopy and energy dispersive X-ray spectrometry investigations revealed that after
reduction at 700 °C the Co
nanoparticles had become encapsulated by a nano-fraction of BaO. The mechanism
underlying the formation of this unique structure was considered to comprise
reduction of oxidic Co to metallic Co, decomposition of BaCO3 to
BaO, and migration of BaO to the Co nanoparticle surface. Spectroscopic and density-functional theory investigations revealed that
adsorption of N2 on the Co atoms at the catalyst surface weakened
the N2 triple bond to the strength of a double bond due to electron
donation from the Ba atom of BaO via adjacent Co atoms; this weakening
accelerated cleavage of the triple bond, which is the rate-determining step for
ammonia synthesis.
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