Size-dependent activation of ammonia borane on copper clusters (Cu _x , x = 2-10): a DFT study of bonding, electronic...

Ammonia borane (NH₃BH₃, AB) is a promising hydrogen-storage material; however, its practical implementation requires efficient and low-cost catalysts for hydrogen release. In this study, density functional theory calculations were performed to investigate the size-dependent activation of AB on copper clusters (Cu _x , x = 2-10) in both gas and aqueous phases. Adsorption of AB on the copper clusters induces elongation of the N-H and B-H bonds by up to 0.027 Å, accompanied by contraction of the B-N bond, indicating activation toward dehydrogenation. The calculated binding energies range from -0.810 to -1.282 eV in the gas phase and become less negative in aqueous solution due to solvent effects. Frontier orbital analysis reveals a pronounced increase in HOMO-LUMO gaps upon adsorption, reflecting electronic stabilization and strong orbital coupling. Natural bond orbital analysis identifies σ(B-H) → n*(Cu) donor-acceptor interactions as the primary charge-transfer pathway. Furthermore, non-covalent interaction and quantum theory of atoms in molecules analyses reveal that the adsorption is governed by a combination of partially covalent Cu-B bonding and weak Cu⋯H-B interactions. Among all investigated systems, Cu₃ exhibits the most favorable balance between adsorption strength and bond activation. Mechanistic investigation of AB dehydrogenation on Cu₃ demonstrates that the first and second H₂-release steps are both thermodynamically and kinetically accessible under mild conditions. In contrast, the third dehydrogenation step is associated with substantially higher energetic barriers and unfavorable reaction energetics.