Strategic ligand-induced electronic structure modulation for enhanced nitrogen reduction reaction selectivity in transition metal phthalocyanines

The electrochemical nitrogen reduction reaction (NRR) is a promising alternative to the conventional Haber-Bosch process for ammonia synthesis, which typically requires high temperatures and pressures. With the increasing interest in an ambient-condition NRR using diverse materials, transition metal (TM)-based single-atom catalysts (SACs) have emerged as key components for enhancing the activity and selectivity of the NRR. This is primarily because SACs can provide localized d orbitals that efficiently transfer electrons from TMs to N₂, thereby weakening the N₂ triple bond and suppressing the competing hydrogen evolution reaction (HER). However, conventional theoretical approaches frequently fail to accurately predict the activity and selectivity mainly because they neglect the impact of ligands that may form in solvent environments. Our study addresses this gap by systematically exploring the effects of different ligands (OH⁻, H₂O, and Cl⁻) on the activity and selectivity of the NRR using two-dimensional phthalocyanines (2D-Pcs) as substrates. Specifically, we evaluate the performance and electronic features of 27 TM-based Pc candidates. Our findings indicate that ligand adsorption significantly shifts the d-band center downward, thus weakening the hydrogen binding while maintaining N₂ interactions. This leads to the identification of three promising candidates—OsPc-OH, NbPc-H₂O, and RePc-Cl—which exhibit exceptional NRR activity and selectivity by effectively suppressing the HER. Thus, this study not only addresses the gap in existing SAC research in terms of considering ligands but also proposes SAC candidates with superior thermodynamic and electronic properties for an efficient NRR.