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Barium Oxide Encapsulating Cobalt Nanoparticles Supported on Magnesium Oxide: Active Non-noble Metal Catalyst for Ammonia Synthesis under Mild Reaction Condition

preprint
submitted on 24.02.2021, 10:31 and posted on 25.02.2021, 05:26 by Katsutoshi Sato, Shin-ichiro Miyahara, Kotoko Tsujimaru, Yuichiro Wada, Takaaki Toriyama, Tomokazu Yamamoto, Syo Matsumura, Koji Inazu, Hirono Mohri, Takeshi Iwasa, Tetsuya Taketsugu, Katsutoshi Nagaoka

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 gcat1 h1) and turnover frequency (0.255 s1) 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 gcat1 h1, 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.

History

Email Address of Submitting Author

nagaoka.katsutoshi@material.nagoya-u.ac.jp

Institution

Nagoya University

Country

Japan

ORCID For Submitting Author

0000-0003-1774-1537

Declaration of Conflict of Interest

The authors declare no competing financial interest.

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