Computational study of model Pd-Zn nanoclusters and their adsorption complexes with CO molecules

Using an all-electron scalar relativistic density functional method, we studied bimetallic cuboctahedral nanoscale clusters Pd_140-nZn _n (n = 0, 8, 24, 32) as local models of the active component of novel Pd/ZnO catalysts for methanol steam reforming. As recently demonstrated (Yudanov, I. V., et al. J. Chem. Phys. 2002, 117, 9887; Yudanov, I. V., et al. J. Phys. Chem. B 2003, 107, 255), such compact model clusters provide a quantitatively accurate description of adsorption properties of single-crystal metal surfaces as well as supported metal particles. The calculated average cluster cohesive energy decreases gradually when the number of Zn atoms increases: each of them introduces a destabilization by ∼1 eV. Zn atoms preferentially occupy positions in the surface layer of the clusters. A small transfer of electron density from Zn to Pd atoms was found. To probe how adsorption properties of bimetallic species change relative to those of the reference cluster Pd₁₄₀, we studied complexes with CO molecules adsorbed on 3-fold hollow Pd₃ sites of (111) cluster facets. CO adsorption energies were calculated notably smaller when Zn atoms are located in the subsurface layer of the clusters; on the other hand, Zn atoms in the surface layer affected the CO adsorption energy only slightly. Calculated CO adsorption energies and vibrational C-O frequencies do not correlate, reflecting the different origin of these properties.