The traditional Haber–Bosch method of ammonia (NH₃) synthesis has low production efficiency and can lead to greenhouse gas emission due to high temperature and pressure dependent reactions. Hence, the nitrogen reduction reaction (NRR) in a mild environment has been developed. However, the inert NN triple bond and the competition with the hydrogen evolution reaction (HER) limit its wide application. In order to find an effective way of reducing N₂ into NH₃, in this work, PC₆ monolayers with good electro-optical properties and eight transition metals (V, Cr, Mn, Fe, Co, Ni, Cu, Zn) are chosen to construct PC₆-TM₃ and PC₆-TM₄ single cluster catalysts (SCCs), which are proved to have low overpotential, multiple active-sites and superior activity. The thermodynamic stability, N₂ adsorption, reaction paths, selectivity for the NRR and catalytic mechanism are systematically investigated. (PC₆-Co₃, PC₆-Fe₄)/(PC₆-V₃, PC₆-Cr₃)/(PC₆-V₄, PC₆-Mn₄) prefer to adsorb N₂ rather than H in the end-on/side-on I/side-on III mode. PC₆-Fe₄ and PC₆-Cr₃ are finally screened out which have excellent catalytic activity with an overpotential of −0.46 V and −0.26 V in the consecutive path of side-on III and I modes, respectively. Moreover, both of them have 100% faradaic efficiency and present high selectivity for the NRR. The catalytic mechanism is elucidated by discussing the electronic properties of PC₆-Cr₃, where the back-donation behaviors of Cr atoms play an important role during the formation of NH₃. This research may provide theoretical guidance for finding potential NRR catalysts with excellent performance and high selectivity.