Effects of Adsorbates (CO, COH, and HCO) on the Arrangement of Pd Atoms in PdCu(111)

We investigated the arrangement of Pd atoms in PdCu(111) when CO, COH, and HCO are introduced as adsorbates, by performing density functional theory (DFT) based calculations. We modeled several Pd alloyed Cu(111) surfaces, i.e., PdCu(111), by substituting small numbers of Cu atoms with Pd atoms in the topmost and subsurface layers of Cu(111). The arrangement of Pd atoms in the presence of adsorbates is evaluated by comparing the energy profiles of adsorbate–PdCu configurations with aggregated and nonaggregated surface and subsurface Pd atoms. In clean PdCu(111) surfaces, the Pd atoms prefer the nonaggregated arrangement. In the presence of the adsorbed molecules, however, we found that the Pd atoms will favor the aggregated configuration. CO and HCO adsorption structures are determined by the coordination of Pd atoms in the topmost layer. Their adsorption energies do not depend on the number of Pd atoms in the topmost layer alone but are also influenced by subsurface Pd atoms. On the other hand, COH is al...