Contribution of Nitrogen Vacancies to Ammonia Synthesis over Metal Nitride Catalysts

Ammonia is one of the most important feedstocks for the production of fertilizer and as a potential energy carrier. Nitride compounds such as LaN have recently attracted considerable attention due to their nitrogen vacancy sites that can activate N₂ for ammonia synthesis. Here, we propose a general rule for the design of nitride-based catalysts for ammonia synthesis, in which the nitrogen vacancy formation energy (E_NV) dominates the catalytic performance. The relatively low E_NV (ca. 1.3 eV) of CeN means it can serve as an efficient and stable catalyst upon Ni loading. The catalytic activity of Ni/CeN reached 6.5 mmol·g^-1·h^-1 with an effluent NH₃ concentration (E_NH3) of 0.45 vol %, reaching the thermodynamic equilibrium (E_NH3 = 0.45 vol %) at 400 °C and 0.1 MPa, thereby circumventing the bottleneck for N₂ activation on Ni metal with an extremely weak nitrogen binding energy. The activity far exceeds those for other Co- and Ni-based catalysts, and is even comparable to those for Ru-based catalysts. It was determined that CeN itself can produce ammonia without Ni-loading at almost the same activation energy. Kinetic analysis and isotope experiments combined with density functional theory (DFT) calculations indicate that the nitrogen vacancies in CeN can activate both N₂ and H₂ during the reaction, which accounts for the much higher catalytic performance than other reported nonloaded catalysts for ammonia synthesis.