Density functional calculations have been performed for the Rh2 and Pd2 dimers and for Rh4 and Pd4 clusters, in order to compare the Rh-Rh and Pd-Pd bonds. Using the Rh4 and Pd4 clusters as models for (111) surfaces, chemisorption of CO at top, bridge, and 3-fold sites has also been studied. The calculated optimized geometries and normal frequencies for the adsorbed CO are reported, and the trends are compared with experimental results on Rh and Pd surfaces. Chemisorption energies as well as electronic properties of the Rh4CO and Pd4CO models are compared for the three sites. The gradient corrected binding energies of CO are clearly different for the three sites of Pd clusters, the largest corresponding to the 3-fold site adsorption. In contrast, these energies are rather similar for the three sites of the Rh clusters. Analysis of the Mulliken gross atomic populations shows that the atomic configuration of the metal atom(s) bonded to CO is a characteristic of the adsorption site. These results are rationalized by assuming that the metal atom(s) of the surface, to which CO is bonded, has some “memory” of the energetic properties of the isolated Rh or Pd atom, which governs its ability to adopt a specific configuration and hence its bonding capability.
ASJC Scopus subject areas
- Colloid and Surface Chemistry