The H and H2 interaction with Pd3Cu, Pd4, and Cu4 fcc (111) clusters:: A DFT comparative study

A comparative study of adsorption of H atoms and H, molecules on Pd3Cu, Cu-4, and Pd-4 clusters has been performed through density functional calculations, using the hybrid B3LYP exchange-correlation functional as implemented in the Gaussian98 program. For Pd atoms the relativistic small-core effective core potential LANL and LANL2DZ basis set was used and for hydrogen a 6-31G** basis set was used. The main emphasis is set in the reaction behavior of the different clusters with hydrogen atoms and molecules. We find that full geometry optimization does not appreciably change the metal cluster geometry either for certain reaction modes or the H and H-2 capture parameters, but increases the number of reactive sites of the metal clusters. Also, we found that there is charge transfer competition between H and Cu atoms, which drastically diminishes H, adsorption energy, related to the Pd cluster observed value. Edges and threefold sites are the principal hydrogen adsorption sites. Hydrogen has a great mobility over the metal clusters for different minima, especially when Cu is present; many initial pathways end in the same adsorption site. The observed hydrogen adsorption and binding energies are well reproduced by the calculations. Also, the adsorption mechanisms were determined. (c) 2005 Wiley Periodicals, Inc.