Adsorption of CO on nanosize Pd particles was studied theoretically by density functional method and spectroscopically by means of infrared reflection absorption spectroscopy (IRAS) and sum frequency generation (SFG). A density functional approach was applied to three-dimensional crystallites of about 140 atoms. The model clusters were chosen as octahedral fragments of the face centered cubic (fcc) bulk, exhibiting (111) and (001) facets. Bare and adsorbate-decorated cluster models were calculated with O h symmetry constraints. Various types of adsorption sites were inspected:  3-fold hollow, bridge, and on-top positions at (111) facets; 4-fold hollow and on-top sites at (001) facets; bridge positions at cluster edges; on-top positions at cluster corners; and on single Pd atoms deposited at regular (111) facets. Adsorption properties of the relatively small regular cluster facets (111) and (001) are calculated similar to those of corresponding ideal (infinite) Pd surfaces. However, the strongest CO bonding was calculated for the bridge positions at cluster edges. The energy of adsorption on-top of low-coordinated Pd centers (kinks) is also larger than that for on-top sites of (111) and (001) facets. To correlate the theoretical results with spectroscopic data, vibrational spectra of CO adsorbed on supported Pd nanocrystallites of different size and structure (well-faceted and defect-rich) were measured using IRAS and SFG. For CO adsorption under ultrahigh vacuum conditions, a characteristic absorption in the frequency region 1950−1970 cm-1 was observed, which in agreement with the theoretical data was assigned to vibrations of bridge-bonded CO at particle edges and defects. SFG studies carried out at CO pressures up to 200 mbar showed that the edge-related species was still present under catalytic reaction conditions. By decomposition of methanol leading to the formation of carbon species, these sites can be selectively modified. As a result, CO occupies on-top positions at particle edges and defects. On the basis of the computational data, the experimentally observed differences in CO adsorption on alumina-supported Pd nanoparticles of different size and surface quality are interpreted. Differences between adsorption properties of Pd nanoparticles with a large fraction of (111) facets and adsorption properties of an ideal Pd(111) surface are also discussed.

Large octahedral and cuboctahedral palladium clusters, ranging from Pd55 to Pd146, have been investigated by means of all-electron relativistic density functional calculations. Adsorption of CO molecules on the (111) facets of these clusters was also studied. In particular, we focused on the interaction of CO (a single molecule per facet) with threefold hollow sites to inspect the variation of the calculated adsorption parameters with cluster size. We considered how observables calculated for that adsorption position on cluster facets relate to adsorption properties of the corresponding site at the single crystal surface Pd(111). We demonstrated for the first time that, with three-dimensional cluster models proposed here, one can reach cluster size convergence even for such a sensitive observable as the adsorption energy on a metal surface. We also addressed size effects on interatomic distances and the cohesive energy of bare Pd nanoclusters whose structure was fully optimized under the imposed Oh symmetry constraint. These quantities were found to correlate linearly with the average coordination number and the inverse of the cluster radius, respectively, allowing a rather accurate extrapolation to the corresponding values of Pd bulk. Finally, we considered the size convergence of adsorption properties of the optimized Pd clusters, as probed by CO adsorption. We also outlined implications of using these symmetric clusters for investigating adsorption and reactions on oxide-supported nanoparticles of model Pd catalysts.

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

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