Boosted Activity of Cobalt Catalysts for Ammonia Synthesis with BaAl<sub>2</sub>O<sub>4–<i>x</i></sub>H<sub><i>y</i></sub> Electrides
Yihao Jiang, Ryu Takashima, Takuya Nakao, Masayoshi Miyazaki, Yangfan Lu, Masato Sasase, Y. Niwa, Hitoshi Abe, Masaaki Kitano, Hideo Hosono
Abstract
Electrides are promising support materials to promote transition metal catalysts for ammonia synthesis due to their strong electron-donating ability. Cobalt (Co) is an alternative non-noble metal catalyst to ruthenium in ammonia synthesis; however, it is difficult to achieve acceptable activity at low temperatures due to the weak Co–N interaction. Here, we report a novel oxyhydride electride, BaAl 2 O 4– x H y, that can significantly promote ammonia synthesis over Co (500 mmol g Co –1 h –1 at 340 °C and 0.90 MPa) with a very low activation energy (49.6 kJ mol –1; 260–360 °C), which outperforms the state-of-the-art Co-based catalysts, being comparable to the latest Ru catalyst at 300 °C. BaAl 2 O 4– x H y with a stuffed tridymite structure has interstitial cage sites where anionic electrons are accommodated. The surface of BaAl 2 O 4– x H y with very low work functions (1.7–2.6 eV) can donate electrons strongly to Co, which largely facilitates N 2 reduction into ammonia with the aid of the lattice H – ions. The stuffed tridymite structure of BaAl 2 O 4– x H y with a three-dimensional AlO 4 -based tetrahedral framework has great chemical stability and protects the accommodated electrons and H – ions from oxidation, leading to robustness toward the ambient atmosphere and good reusability, which is a significant advantage over the reported hydride-based catalysts.