Molecular Adsorption of H₂ on Small Neutral Silver-Copper Bimetallic Nanoparticles: A Search for Novel Hydrogen...

In the search for novel hydrogen storage materials, neutral silver-copper bimetallic nanoparticles up to the size of eight atoms (Cu Ag : m + n ≤ 8) have been computationally studied. Density functional theory with the B3LYP exchange-correlation functional and the combined basis sets of LanL2DZ and aug-cc-pVQZ were used in all of the calculations. H₂ adsorption studies on the most stable cluster geometries of all of the neat and heterogeneous entities found that 12 potential candidates, CuAg₄, Cu₆, Cu₅Ag, Cu₄Ag₂, Cu₃Ag₃, Cu₂Ag₄, CuAg₆, Cu₅Ag₃, Cu₄Ag₄, Cu₃Ag₅, Cu₂Ag₆, and CuAg₇, fall within the recommended physisorption range of -18 to -6 kJ mol^-1. A correlation in the behavior of binding energy, vibrational frequency, average bond distance, highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gap, and chemical hardness with H₂ adsorption was observed. This analysis further revealed that the H₂ adsorption to the cluster was either a parallel or a perpendicular alignment. The analysis of the electron configuration of each atom in the cluster and the H₂ molecule and the charge transfer analysis of these 12 clusters also showed that the physisorption in the perpendicular mechanism is due to an induced dipole interaction, while that in the parallel mechanism is due to a weak ionic interaction. The clusters identified with perpendicular adsorption, CuAg₄H₂, Cu₆H₂, Cu₃Ag₃H₂, and Cu₂Ag₄H₂, polarized the H₂ molecule but had no charge transfer with the H₂ molecule and those identified with parallel adsorption, Cu₅AgH₂, Cu₄Ag₂H₂, CuAg₆H₂, Cu₅Ag₃H₂, Cu₄Ag₄H₂, Cu₃Ag₅H₂, Cu₂Ag₆H₂, and CuAg₇H₂, pulled the electrons from the H₂ molecule and had charge transfer with the H₂ molecule. The shapes of the frontier molecular orbital diagrams of the HOMO and LUMO also followed this observation.