The Nature and Stability of Transition Metal-Anchored Nitrogen-Doped Graphene Single-Atom Catalysts

Transition metal-anchored nitrogen-doped graphene single-atom catalysts (SACs) represent an emerging class of catalysts that combine the advantages of both homogeneous and heterogeneous catalysis. To prevent demetallation and ensure catalyst stability, sufficiently strong bonds between the transition metal and the support are essential. We have quantum chemically analyzed the trend in bonding interaction between period 4 transition metals (TM = Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) and the four-nitrogen-doped graphene support. We find that the metal-support interactions strengthen from Ti to Ni but weaken from Ni to Zn. Activation strain and Kohn-Sham molecular orbital (KS-MO) analyses reveal that this trend stems from changes in the interaction between the metal's 3d_xy atomic orbital and the nitrogen lone pair orbitals of the support. As we move along period 4, the bonding mechanism changes from an increasingly more stabilizing HOMO-LUMO interaction (TM = Ti-Ni), due to the higher effective nuclear charge of the metal, to a less favorable HOMO-SOMO (TM = Cu) and unfavorable HOMO-HOMO (TM = Zn) interaction, as a result of the filling of the metal's 3d_xy atomic orbital. This results in the observed strengthening, followed by weakening, of the metal-support interaction. These insights could guide the rational design of future single-atom catalysts.