TuningMetal-Phthalocyanine 2D Covalent Organic Frameworksfor the Nitrogen Reduction Reaction: Cooperativity of Transition Metalsand Organic Linkers

Two-dimensional covalent organic frameworks (2D-COFs) exhibit high activity in the electrochemical nitrogen reduction reaction (NRR), while the structure–performance relationship toward which remains an unsolved problem, therefore hindering the development of efficient catalysts. In this work, a total of 145 candidate transition metal phthalocyanine (TMPc-DX) materials comprised of 29 transition metals and X-phthalocyanine (X = N, O, F, S, and Cl) are evaluated for their capabilities in the NRR by density functional theory calculations. Our computations showed that metal atoms in TMPc-DN/DF/DCl materials could efficiently activate N₂ molecules by doping with electron-withdrawing groups. The catalysts MoPc-DN and NbPc-DN are recognized to exhibit the highest electrocatalytic performance for the NRR with the lowest limit potentials of 0.31 and 0.32 V, respectively. Moreover, NbPc-DN showed a significant advantage by suppressing the hydrogen evolution reaction, making it a top contender among TMPc-DX materials for the electrocatalytic NRR. Through detailed orbital interaction analysis, our research has demonstrated that embedding transition metals into the substrate (Pc-DX) results in a direct correlation between the number of electrons transferred to the substrate and the reduction of energy splitting of the metal’s d orbital, which directly causes a decrease in the metal’s magnetic moment. Consequently, the affinity of the metal for N₂ adsorption is directly proportional to the extent of the electron transfer. Our findings underscore the significance of the material’s magnetic moment as a preliminary indicator for gauging the effectiveness of N₂ adsorption.