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 (Nv); 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 Nv onto the metallic sites. Using N binding energy (BEN) and the transition-state energy of N2 activation (ETS) as descriptors, we concluded that the beneficial effects owing to interfacial Nv sites are the most pronounced when BEN is either too strong or too weak while ETS is high; otherwise, excessive Nv sites may hinder catalyst performance.