Density functional theory study of palladium cluster adsorption on a graphene support

The geometric, thermodynamic and electronic properties of Pd-graphene nanocomposites are comprehensively studied through quantum mechanical methods. Geometries of these clusters are optimized with the well-calibrated Minnesota functional M06-2X. The adsorption energies calculated at the M06-2X/LANL2DZ level show better agreement with those calculated from MP2/ANO-RCC-VDZP. Two different representative models for graphene, coronene and hexabenzocoronene, are used. The adsorption energies analysis reveals that the interaction energies increase with the size of the adsorbed cluster. However, for Pd-n/hexabenzocoronene, the interaction energies show a sudden drop at Pd-8/hexabenzocoronene. The difference in behavior between the interaction energies of Pd-n/hexabenzocoronene and Pd-n/coronene is attributed to the edge effect present in coronene. The electronic properties, including highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), Fermi level, molecular electrostatic potential (MEP), dipole moment, vertical ionization potential (VIP), vertical electron affinity (VEA), chemical hardness (eta), softness (S) and chemical potential (mu) are studied. The VIP and VEA reveal that Pd-n/coronene clusters are stable in nature with the least reactivity. The HOMO-LUMO energy gaps are reduced with the increase in cluster size. The electronic properties show irregular trends, where the most favorable electronic properties are obtained for Pd-7/coronene and Pd-10/coronene.