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 Ni-3 and Fe-3 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 Fe-3@Mn4N approximate to Ni-3@ 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 N-2 activation (E-TS) as descriptors, we concluded that the beneficial effects owing to interfacial N-v sites are the most pronounced when BEN is either too strong or too weak while E-TS is high; otherwise, excessive N-v sites may hinder catalyst performance.

Automated summary

Using dual-site models for NH3 synthesis with Mn4N-supported Ni-3 and Fe-3 clusters showed that Fe-3@Mn4N and Ni-3@Mn4N had the highest turnover frequencies. The beneficial effects of interfacial N-v sites were most pronounced when N binding energy and N-2 activation transition-state energy were either too strong or too weak while being high, otherwise excessive N-v sites may hinder catalyst performance.