Palladium atomic energy levels

To explain the different behavior for nanoparticles, it is important to realize that for palladium not only absorption in the bulk of the material is important, but also the sorption of hydrogen near the surface should be considered. It is known that palladium forms surface hydrides, with about 1 H per Pd surface atom. However, sorption in subsurface sites is also important, as evidenced by studies on (flat (110) surfaces [45]. Figure 10.9 shows a schematical representation of the energy levels for the hydrogen atom at the surface, in subsurface sites, and absorbed in the bulk. 

Room temperature deposition of silver on Pd(lOO) produces a rather sharp Ag/Pd interface [62]. The interaction with a palladium surface induces a shift of Ag 3d core levels to lower binding energies (up to 0.7 eV) while the Pd 3d level BE, is virtually unchanged. In the same time silver deposition alters the palladium valence band already at small silver coverage. Annealing of the Ag/Pd system at 520 K induces inter-diffusion of Ag and Pd atoms at all silver coverage. In the case when silver multilayer was deposited on the palladium surface, the layered silver transforms into a clustered structure slightly enriched with Pd atoms. A hybridization of the localized Pd 4d level and the silver sp-band produces virtual bound state at 2eV below the Fermi level. 

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