Modulating N2 Activation and Reduction Activity by Tuning the Metal–Metal Distance in M2–Phen2 (M=Ti, Fe) Catalysts

In our study, density functional theory (DFT) is used to investigate how the metal–metal (M–M) spacing in bimetallic M2–Phen2 (M=Ti, Fe; Phen = 1,10-phenanthroline) regulates N2 activation and Nitrogen Reduction Reaction (NRR) activity. The results indicate that M–M distance significantly influences catalytic efficiency. N2 adsorbs on M2–Phen2 in a side-on:side-on configuration. Increasing the M–M spacing initially enhances N2 activation, but excessive separation weakens this effect. In comparison, Ti2–Phen2 activates N2 more effectively than Fe2–Phen2. The NRR pathway calculations identify *NH2 → *NH3 as the rate-determining step (RDS), where the electronic energy difference between the two states exhibits a parabolic relationship with M–M distance. By scanning Ti–Ti (dTi–Ti) spacing from 2.6 Å to 3.4 Å and Fe–Fe spacing (dFe–Fe) from 2.1 Å to 2.7 Å, the lowest RDS electronic energy differences were observed at 2.9 Å (1.01 eV) for Ti2–Phen2 and 2.2 Å (1.13 eV) for Fe2–Phen2. Conversely, the highest electronic energy differences within the scanned ranges were found at a Ti–Ti spacing of 3.4 Å (1.33 eV) and a Fe–Fe spacing of 2.7 Å (1.34 eV). This study reveals that interatomic distance is a key factor influencing catalytic activity, providing theoretical guidance for optimizing bimetallic catalysts through precise structural design.