Using N≡N dipole as a theoretical indicator for estimating electrocatalytic performance of active sites in nitrogen reduction reaction: single transition metal atoms embedded in the two dimensional phthalocyanine

The electrocatalytic reduction of nitrogen (N2) has recently emerged as an attractive technology for producing ammonia (NH3) at mild conditions. Nevertheless, achieving a high selectivity of N2 reduction with respect to the hydrogen evolution at relatively low overpotential, and thus increasing the energy efficiency of ammonia production, has remained a key challenge. Herein, with systematic density functional theory (DFT) calculations, we report that the the dipole of N≡N triple bond in the adsorbed N2 molecule as a more efficient and accurate theoretical indicator comparing with the previous ones for predicting the catalytic performance of active sites in nitrogen reduction reaction (NRR). By screening the single transition metal atoms anchoring on the two-dimensional phthalocyanine (2D Pc) organic frameworks, we show that the 2D Mo-Pc is a promising single-atom-catalyst in NRR with an extremely low onset potential of -0.25 V. The origin of such high catalytic activity can be interpreted by the large dipole moment introduced to the N≡N bond via strong Mo-N interactions. As a result, the injection of electrons to the anti-bonding orbitals of nitrogen molecules are promoted. In addition, the competing hydrogen evolution reaction (HER) can be effectively inhibited due to the unfavourable bonding interactions between the H and the single Mo atom.