Fig. 6.
An illustrative pathway for dihydrogen dissociation and hydrogen atom migration on a palladium overlayer on Ag(111) at high hydrogen atom coverage. The models corresponding to the different states depicted are below the energy diagram (silver atoms, silver; palladium atoms, green; initially adsorbed hydrogen atoms on the surface, blue; additional hydrogens from dihydrogen, red). The surface was initially covered by 0.75 ML of hydrogen atoms (blue; IS); after further dihydrogen adsorption, additional hydrogen atoms (red) occupy a fcc site and an hcp site (IM1). The hydrogen atom on a hcp site moves on the surface via a bridge site (TS1) and eventually migrates to the Ag(111) surface in the FS. The silver lattice is included in all calculations; however, it is adsorbate-free except in the FS. Since the hydrogen atom coverage is high, the silver lattice periodicity was used for the palladium overlayer (see Density Functional Analysis of the Bonding of Atomic Hydrogen and Its Migration on the Surface). The structure considered has a coverage of 0.75 ML on a 2 × 2 unit cell, but isolated fcc sites occur starting at 2/3 ML. All energies are given per two hydrogen atoms adsorbed, referred to as H2 in the gas phase.