Nitrogen reduction reaction on single cluster catalysts of defective PC₆-trimeric or tetrameric transition metal

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.