Co Nanoparticle Catalysts Encapsulated by BaO-La₂O₃ Nanofractions for Efficient Ammonia Synthesis Under Mild Reaction...

Ruthenium catalysts may allow for realization of renewable energy-based ammonia synthesis processes using mild reaction conditions (<400 °C, <10 MPa). However, ruthenium is relatively rare and therefore expensive. Here, we report a Co nanoparticle catalyst loaded on a basic Ba/La₂O₃ support and prereduced at 700 °C (Co/Ba/La₂O₃_700red) that showed higher ammonia synthesis activity at 350 °C and 1.0-3.0 MPa than two benchmark Ru catalysts, Cs⁺/Ru/MgO and Ru/CeO₂. The synthesis rate of the catalyst at 350 °C and 1.0 MPa (19.3 mmol h^-1 g^-1) was 8.0 times that of Co/Ba/La₂O₃_500red and 6.9 times that of Co/La₂O₃_700red. The catalyst showed ammonia synthesis activity at temperatures down to 200 °C. Reduction at the high temperature induced the formation of BaO-La₂O₃ nanofractions around the Co nanoparticles by decomposition of BaCO₃, which increased turnover frequency, inhibited the sintering of Co nanoparticles, and suppressed ammonia poisoning. These strategies may also be applicable to other non-noble metal catalysts, such as nickel.