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

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 g_cat⁻¹ h⁻¹) and turnover frequency (0.255 s⁻¹) 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 g_cat⁻¹ h⁻¹, 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.