Activation of N₂ on Manganese Nitride-Supported Ni₃ and Fe₃ Clusters and Relevance to Ammonia Formation

Dual-site models were constructed to represent manganese nitride (Mn₄N)-supported Ni₃ and Fe₃ clusters for NH₃ synthesis. Density functional theory calculations produced an energy barrier of approximately 0.55 eV for N-N bond activation at the interfacial nitrogen vacancy sites (N_v); also, the hydrogenation and removal of interfacial N is promoted by earth-abundant Ni and Fe metals. Steady-state microkinetic modeling revealed that the turnover frequencies of NH₃ production follow an order of Fe₃@Mn₄N ≈ Ni₃@Mn₄N > Mn₄N > Fe ≫ Ni. Moreover, we present clear evidence that, before NH₃ formation, NH migrates from N_v onto the metallic sites. Using N binding energy (BE_N) and the transition-state energy of N₂ activation (E_TS) as descriptors, we concluded that the beneficial effects owing to interfacial N_v sites are the most pronounced when BE_N is either too strong or too weak while E_TS is high; otherwise, excessive N_v sites may hinder catalyst performance.