Origin of Transition Metal Binding Preferences in Pyridinic and Pyrrolic Nitrogen-Doped Graphene Single-Atom Catalysts

Abstract

Transition metal-anchored nitrogen-doped graphene single-atom catalysts (SACs) have emerged as promising catalysts that combine the advantages of both homogeneous and heterogeneous catalysis. Two common vacancy motifs in nitrogen-doped graphene-based SACs are the pyridinic and pyrrolic vacancy sites. Despite their importance for catalyst design, quantitative insights into the intrinsic differences in their metal–support bonding interactions and their impact on the preferred metal binding site are still lacking. Here, we have quantum chemically investigated the trends in bonding interactions between Period 4 transition metals (TM = Ti, V, Cr, Mn, Fe Co, Ni, Cu, and Zn) and these vacancy motifs using relativistic, dispersion-corrected periodic density functional theory in combination with the activation strain model and Kohn-Sham molecular orbital (KS-MO) analyses. Regardless of the vacancy motif, the metal–support interaction follows the same trend across Period 4, namely a strengthening from Ti to Ni followed by a weakening from Ni to Zn. This trend originates from the interplay between the increasing effective nuclear charge and the gradual filling of the metal's 3d atomic orbitals. Additionally, the vacancy motif plays a key role in determining the preferred transition metal binding site. Ti and V preferentially bind to the larger pyrrolic vacancy site because the planar coordination minimizes Pauli repulsion between the TM and the support. In contrast, Cr through Zn favor the pyridinic motif, where shorter TMN bonds and a larger CNC angle enhance both electrostatic and orbital interactions. Together, these insights could aid in the rational design of future SACs.