Nitrogen reduction reaction on single cluster catalysts of defective PC 6 -trimeric or tetrameric transition metal.

The traditional Haber-Bosch method of ammonia (NH 3 ) 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 NN triple bond and the competition with the hydrogen evolution reaction (HER) limit its wide application. In order to find an effective way of reducing N 2 into NH 3 , in this work, PC 6 monolayers with good electro-optical properties and eight transition metals (V, Cr, Mn, Fe, Co, Ni, Cu, Zn) are chosen to construct PC 6 -TM 3 and PC 6 -TM 4 single cluster catalysts (SCCs), which are proved to have low overpotential, multiple active-sites and superior activity. The thermodynamic stability, N 2 adsorption, reaction paths, selectivity for the NRR and catalytic mechanism are systematically investigated. (PC 6 -Co 3 , PC 6 -Fe 4 )/(PC 6 -V 3 , PC 6 -Cr 3 )/(PC 6 -V 4 , PC 6 -Mn 4 ) prefer to adsorb N 2 rather than H in the end-on/side-on I/side-on III mode. PC 6 -Fe 4 and PC 6 -Cr 3 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 PC 6 -Cr 3 , where the back-donation behaviors of Cr atoms play an important role during the formation of NH 3 . This research may provide theoretical guidance for finding potential NRR catalysts with excellent performance and high selectivity.