Cooperative Nitrogen Activation and Ammonia Synthesis on Densely Monodispersed Mo-N-C Sites.

The production of ammonia from nitrogen reduction reaction (NRR) under mild conditions is one of the most challenging issues in modern chemistry. The linear scaling relationship between the adsorption energies of -NH and -NH on a single active site is a well-established bottleneck. By investigating a series of densely monodispersed Mo-N-C sites embedded in graphene using first-principles calculations, we found that previously underappreciated neighboring effects between adjacent active sites may help break the limit: they not only improve the energetics of potential determining steps of NRR but also promote an alternative associative mechanism based on a cooperative bridge-on adsorption of N by two Mo-N-C sites of ∼6 Å apart. Further, a barrier of 0.71 eV for N-N bond dissociation is achieved by proper ratio of coordinated C/N atoms of Mo. Our work suggests the cooperation of two adjacent active sites may offer an alternative strategy of nitrogen fixation.