TY - JOUR
T1 - Barium oxide encapsulating cobalt nanoparticles supported on magnesium oxide
T2 - Active non-noble metal catalysts for ammonia synthesis under mild reaction conditions
AU - Sato, Katsutoshi
AU - Nagaoka, Katsutoshi
AU - Miyahara, Shin Ichiro
AU - Tsujimaru, Kotoko
AU - Wada, Yuichiro
AU - Toriyama, Takaaki
AU - Yamamoto, Tomokazu
AU - Matsumura, Syo
AU - Inazu, Koji
AU - Mohri, Hirono
AU - Iwasa, Takeshi
AU - Taketsugu, Tetsuya
N1 - Funding Information:
This research was supported by a grant from the CREST, JST program (no. JPMJCR1341). Part of this research was financially supported by the TOYOTA Mobility Foundation and the Japan Society for the Promotion of Science (JSPS) KAKENHI grant no. 20H02522. T.I. is grateful for the financial support provided by JSPS KAKENHI grant nos. 20K05412, 20H04652, and 20K05592 and by the PRESTO, JST program (no. JPMJPR20T1). Cs-STEM observations were performed as part of a program conducted by the Advanced Characterization Nanotechnology Platform Japan, sponsored by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan (no. JPMXP09-A-18-KU-0283). X-ray absorption measurements were performed at the BL01B1 facilities of SPring-8 at the Japan Synchrotron Radiation Research Institute (JASRI) (no. 2018B1345). This work was also partly supported by the Elements Strategy Initiative (ESICB) of MEXT (grant no. JP-MXP0112101003).
Publisher Copyright:
© 2021 The Authors. Published by American Chemical Society
PY - 2021/11/5
Y1 - 2021/11/5
N2 - To realize a carbon-free society, catalysts are needed for the synthesis of ammonia under mild reaction conditions (<400 °C, <10 MPa) that use hydrogen produced from renewable energy. Ru-based catalysts are currently the most promising candidates; however, Ru is expensive and of low abundance. Here, we discovered that the encapsulation of Co nanoparticles with BaO enhanced the ammonia synthesis activity of Co and that a simple Ba-doped Co/MgO catalyst prereduced 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 (0.246 s−1) of the catalyst at 350 °C and 1.0 MPa were about 80 and 250 times higher, respectively, than those of the nondoped 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. In addition, at 1.0 MPa, our catalyst produced ammonia even at temperatures as low as 150 °C. Scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy investigations revealed that after reduction at 700 °C, the Co nanoparticles had become encapsulated by a nanofraction 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 and Co nanoparticles. 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 Ba2+ of BaO via adjacent Co atoms; this weakening accelerated the cleavage of the triple bond, which is the rate-determining step for ammonia synthesis.
AB - To realize a carbon-free society, catalysts are needed for the synthesis of ammonia under mild reaction conditions (<400 °C, <10 MPa) that use hydrogen produced from renewable energy. Ru-based catalysts are currently the most promising candidates; however, Ru is expensive and of low abundance. Here, we discovered that the encapsulation of Co nanoparticles with BaO enhanced the ammonia synthesis activity of Co and that a simple Ba-doped Co/MgO catalyst prereduced 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 (0.246 s−1) of the catalyst at 350 °C and 1.0 MPa were about 80 and 250 times higher, respectively, than those of the nondoped 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. In addition, at 1.0 MPa, our catalyst produced ammonia even at temperatures as low as 150 °C. Scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy investigations revealed that after reduction at 700 °C, the Co nanoparticles had become encapsulated by a nanofraction 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 and Co nanoparticles. 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 Ba2+ of BaO via adjacent Co atoms; this weakening accelerated the cleavage of the triple bond, which is the rate-determining step for ammonia synthesis.
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U2 - 10.1021/acscatal.1c02887
DO - 10.1021/acscatal.1c02887
M3 - Article
AN - SCOPUS:85118140754
VL - 11
SP - 13050
EP - 13061
JO - ACS Catalysis
JF - ACS Catalysis
SN - 2155-5435
IS - 21
ER -