Abstract
In the search for novel hydrogen storage materials, neutral silver-copper bimetallic nanoparticles up to the size of eight atoms (Cu (m) Ag (n) : 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. H(2) adsorption studies on the most stable cluster geometries of all of the neat and heterogeneous entities found that 12 potential candidates, CuAg(4), Cu(6), Cu(5)Ag, Cu(4)Ag(2), Cu(3)Ag(3), Cu(2)Ag(4), CuAg(6), Cu(5)Ag(3), Cu(4)Ag(4), Cu(3)Ag(5), Cu(2)Ag(6), and CuAg(7), 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 H(2) adsorption was observed. This analysis further revealed that the H(2) 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 H(2) 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, CuAg(4)H(2), Cu(6)H(2), Cu(3)Ag(3)H(2), and Cu(2)Ag(4)H(2), polarized the H(2) molecule but had no charge transfer with the H(2) molecule and those identified with parallel adsorption, Cu(5)AgH(2), Cu(4)Ag(2)H(2), CuAg(6)H(2), Cu(5)Ag(3)H(2), Cu(4)Ag(4)H(2), Cu(3)Ag(5)H(2), Cu(2)Ag(6)H(2), and CuAg(7)H(2), pulled the electrons from the H(2) molecule and had charge transfer with the H(2) molecule. The shapes of the frontier molecular orbital diagrams of the HOMO and LUMO also followed this observation.