Journal
ACS SUSTAINABLE CHEMISTRY & ENGINEERING
Volume 8, Issue 7, Pages 2726-2734Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acssuschemeng.9b06299
Keywords
Sustainable energy; Energy carrier; Rare-earth oxide; Ruthenium; Alkali earth metal
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Funding
- CREST, JST program [JPMJCR1341]
- Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan [JPMXP09-A-18-KU-0283]
- Program for Elements Strategy Initiative for Catalysts & Batteries (ESICB) by MEXT [JPMXP0112101003]
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To mitigate global problems related to energy and global warming, it is helpful to develop an ammonia synthesis process using catalysts that are highly active under mild conditions. Here we show that the ammonia synthesis activity per weight of catalyst of Ru/Ba/LaCeOx, prereduced at 700 degrees C, is the highest among reported oxide-supported Ru catalysts, 52.3 mmol h(-1) g(cat)(-1) at 350 degrees C, 1.0 MPa. The turnover frequency of Ru/Ba/LaCeOx at 350 degrees C was more than 8 times that of Cs+/Ru/MgO, which is a well-known active catalyst used as a benchmark; furthermore, hydrogen poisoning, a typical drawback for oxide-supported Ru catalysts, was effectively suppressed. Scanning transmission electron microscopy observations with energy dispersive X-ray spectrometry and electron energy loss spectroscopy analysis revealed that a low-crystalline, oxygen-deficient nanofraction including Ba2+, Ce3+, and La3+ had accumulated on the Ru particles. This unique structure was obtained by exploiting the surface dynamics of alkaline earth compounds and thermostable rare earth oxides that contain redox-active atoms during the reduction at an unusually high temperature. The nanofraction showed strong electron-donating ability because of the strong basicity of the included cations, removal of carbonate, and formation of oxygen defect sites that eliminated electron-withdrawing O2- anions from the interface between the nanofraction and Ru atom. Electrons were therefore effectively donated to antibonding pi-orbitals of the N-2 molecules via Ru in contact with the nanofraction, and N N triple bond cleavage, which is the rate-determining step for ammonia synthesis, was promoted.
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