Computational Study of Model Pd−Zn Nanoclusters and Their Adsorption Complexes with CO Molecules

Abstract

Using an all-electron scalar relativistic density functional method, we studied bimetallic cuboctahedral nanoscale clusters Pd140-nZnn (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.