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Abstract

We show that CO adsorption on Pd aggregates of varying size and order gives rise to several absorption bands in the range of CO stretching frequencies which we assign to different absorption sites. At low temperature (90 K) and saturation coverage we find the population of terminal as well as bridging sites. The CO molecules are preferentially terminally bound, but these exhibit the lower binding energies in agreement with earlier TDS studies. The CO-molecules on two-fold bridging sites are more tightly bound and the relative intensity of the corresponding absorption band increases with increasing size and order of the Pd aggregates. The observed bands may be assigned according to IR results on Pd(111) single crystals which is the orientation of the aggregate surfaces observed for the present deposits. The bands previously assigned to Pd(100), which gain intensity for the well-ordered aggregates with preferentially (111) oriented surfaces, we reassign to CO molecules bound to edges of the (111) facets.