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 °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 g_cat⁻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 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 °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.