Barium Oxide Encapsulating Cobalt Nanoparticles Supported on Magnesium Oxide: Active Non-Noble Metal Catalysts for Ammonia Synthesis under Mild Reaction Conditions

To realize a carbon-free society, catalysts are needed for the synthesis of ammonia under mild reaction conditions (<400 degrees 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 degrees C (Co@BaO/MgO-700red) showed outstanding ammonia synthesis activity. The ammonia synthesis rate (24.6 mmol g(cat)(-1)h(-1)) and turnover frequency (0.246 s(-1)) of the catalyst at 350 degrees 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 g(cat)(-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 degrees C. Scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy investigations revealed that after reduction at 700 degrees 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 N-2 on the Co atoms at the catalyst surface weakened the N-2 triple bond to the strength of a double bond due to electron donation from Ba (2+) of BaO via adjacent Co atoms; this weakening accelerated the cleavage of the triple bond, which is the rate-determining step for ammonia synthesis.

Automated summary

Encapsulating cobalt nanoparticles with barium oxide enhanced the ammonia synthesis activity, leading to exceptional performance of a simple barium-doped cobalt/MgO catalyst pre-reduced at high temperature. This catalyst showed significantly improved synthesis rates compared to conventional ruthenium-based catalysts, even operating at lower temperatures. The unique structure formed by the encapsulation process facilitated electron donation and accelerated the cleavage of the triple bond, enhancing the rate-limiting step for ammonia synthesis.