From single Ni atom to tetranuclear Ni clusters on graphdiyne: Unraveling size-dependent catalysis toward formic acid dehydrogenation

Designing stable, efficient, and low-cost catalysts based on earth-abundant elements is vital for advancing HCOOH as a hydrogen storage medium. In this work, four graphdiyne-supported nickel-based single-atom/cluster catalysts, Ni x @GDY (x = 1–4), were designed and their stability and dehydrogenation performance were systematically studied. AIMD simulations confirm all four catalysts possess good thermodynamic stability. DFT calculations reveal that the activation barrier of the rate-limiting elementary step decreases with increasing active sites: Ni 4 @GDY has the smallest barrier, Ni 2 @GDY and Ni 3 @GDY have similar slightly higher barriers, and Ni 1 @GDY the largest, suggesting an activity order of Ni 4 @GDY > Ni 2 @GDY ≈ Ni 3 @GDY > Ni 1 @GDY. However, kinetic analysis reveals that Ni 3 @GDY actually delivers outstanding overall catalytic activity and excellent hydrogen selectivity, while Ni 2 @GDY and Ni 4 @GDY exhibit lower TOF values but greater temperature sensitivity, indicating their promise as novel catalysts. Electronic structure analyses reveal zero band gaps after Ni loading, and increasing Ni content leads to greater Ni-3d/C-2p orbital overlap, enhancing charge transfer. The d-band center shifts negatively, with optimal activity at an intermediate Fermi-level-to-d-band distance. Importantly, in Ni 4 @GDY, vertex Ni atoms transfer less charge and retain stronger unsaturation, causing excessive formic acid adsorption that inhibits activity, which explains its lower TOF than Ni 3 @GDY and Ni 2 @GDY and corroborates the microscopic kinetic findings.