CO chemisorption on Ni-n, Pd-n, and Pt-n clusters

Vibrationally resolved photoelectron spectra of mass-selected negatively charged Ni-n(-) (n=1-3), Pd-n(-) (n=2, 3) and Pt-n(-) (n=1-4) clusters are compared with the corresponding spectra of these clusters ligated with m CO molecules (m=1-8). The spectra of the Pt-n(CO)(m)(-) species reveal part of the valence orbitals, which actually form the chemisorption bond. The data are in good agreement with the Blyholder model for CO chemisorption (sigma-donation-pi-backdonation scheme) and indicate that saturation corresponds to the formation of a closed electronic shell of the neutral. The strength of the pi-backdonation is found to be larger for small particles compared with the corresponding single crystal surfaces, which may be related to the catalytic properties of small particles. The spectra of the unsaturated Ni(?)n(CO)(m)(-) show fundamental differences compared with the ones of Pt-n(CO)(m)(-) clusters owing to the high degree of localization of the Ni 3d orbitals. The spectra of the Pd-n(CO)(m)(-) species show a rather irregular behaviour.