Mo-Doped Metal-Organic Frameworks for Efficient Nitrogen Reduction Reaction: A Density Functional Theory Study

The activation of nitrogen molecules is one of the main obstacles to electrocatalytic nitrogen reduction reaction (eNRR), which is closely related to the d-p degree of coupling between d-orbitals of metal active sites and p-orbitals of nitrogen. Herein, two types of MOFs M2(DOBDC) and M2Cl2(BBTA) (M = V, Cr, Mn, Fe, Co, and Ni) were systematically studied via density functional theory (DFT) calculations. In addition, modification strategies including Mo doping and coordination environment optimization were successfully implemented. These modified MOFs can not only effectively activate nitrogen but also maintain smooth ammonia desorption in the NRR process. Especially, Mo(II)-Fe2(DOAnBDC) and Mo(II)-Fe2F2(BBTA) were considered potential eNRR electrocatalysts, because of their high eNRR activity (UL = -0.60 V and -0.73 V, respectively) and selectivity. Furthermore, the detailed electronic structural analysis revealed that the increased eNRR activity is dependent on the unique d-orbitals arrangement of Mo itself. Moreover, the charge transfer between the -Fe-X-Mo- chains (X is a nonmetal anion) regulated the Mo-d orbitals to enhance the d-p coupling, contributing to the enhanced eNRR performance. These findings are of great significance to stimulate the enthusiasm for studying ligand effects to find high-performance eNRR catalysts.