Atomically embedded asymmetrical dual-metal dimers on N-doped graphene for ultra-efficient nitrogen reduction reaction

The atomically dispersed metal species on nitrogen-doped graphene nanosheets have exhibited impressive catalytic properties towards oxygen evolution, oxygen reduction and carbon dioxide reactions (OER/ORR/CRR), but poor performances for nitrogen reduction reactions (NRR). The main reason is ascribed to the sluggish kinetics of the first and last proton-coupled electron transfer (ET) on the unitary single-atom site. In this work, an asymmetrical dual-metal dimer catalytic center, which is inspired by the active sites of natural enzymes, such as nitrogenases, that efficiently catalyze the reduction of N2 to NH3 through FeMo cofactors, is theoretically reported. Remarkably, the density functional theory (DFT) calculations showed that the random combination of two non-precious metal atoms such as Fe, Co, Mo, W and Ru to form catalytically active bimetallic sites lead to a remarkable reduction of about two times of the first or last hydrogenation free energy barrier step. The resulting Mo-Ru, Mo-Co, Mo-W, Mo-Fe and Fe-Ru dimers exhibited ultra-low onset potentials of only 0.17, 0.27, 0.28, 0.36 and 0.39 V, respectively. Meanwhile, the HER side reaction can be well suppressed during the NRR. The superior catalytic performance in the bimetallic clusters is mainly attributed to both the electron donation from the asymmetrical metal atoms to the terminal side-on of N2 molecules, which significantly polarizes and weakens the N≡N bond, and to the synergistic effect of the dual-metal dimers that moderates the binding strength of the key intermediates. This work constitutes the first DFT study of the N2 electroreduction processes on dual-metal dimer catalytic sites and, consequently, paves the way towards the rational design of highly efficient hetero-bimetallic NRR electrocatalysts.