Hierarchically Structured Anti-Perovskite Oxynitride-Hydride Enabling Ammonia Synthesis via a Cooperative Mechanism Mediated by (N2)− Ions

Abstract

Ba3AlO4-xNyHz with an antiperovskite structure is employed as a support for transition-metal catalysts for ammonia synthesis. This metastable material is synthesized by employing multiple anions with different valences and ionic radii. The optimized non-noble metal catalyst, Co(5.8 wt %)/Ba3AlO4-xNyHz/Al2O3, exhibits an exceptional ammonia synthesis rate of 15.9 mmol g–1 h–1 at 0.9 MPa and 300 °C, outperforming benchmark Cs–Ru/MgO, industrial Fe catalyst, and recently reported non-noble metal catalysts. Structural analyses show that Ba3AlO4-xNyHz and Co species self-organize on γ-Al2O3 nanorods to form a hierarchically structured catalyst. Combined experimental and density functional theory analyses reveal that the high activity originates from incorporated (N2)− ions at Co–Ba–Al interfacial sites, where the combination of Ba and Al plays distinct roles in N2 activation and NHx formation, respectively. These findings not only provide a strategy for stabilizing metastable crystal structures through the incorporation of diverse anionic species but also demonstrate a unique cooperative mechanism for efficient ammonia synthesis mediated by highly activated molecular (N2)− ions.