INFRARED SPECTROSCOPIC INVESTIGATION OF CO ADSORBED ON PD AGGREGATES DEPOSITED ON AN ALUMINA MODEL SUPPORT

We show that CO absorption on Pd aggregates of varying size and order gives rise to several absorption bands in the range of CO stretching f requencies which we assign to different absorption sites. At low tempe rature (90 K) and saturation coverage we find the population of termin al as well as bridging sites. The CO molecules are preferentially term inally bound, but these exhibit the lower binding energies in agreemen t with earlier TDS studies. The CO-molecules on two-fold bridging site s are more tightly bound and the relative intensity of the correspondi ng 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 assig ned to Pd(100). which gain intensity for the well-ordered aggregates w ith preferentially (111) oriented surfaces, we reassign to CO molecule s bound to edges of the (111) facets. (C) 1998 Elsevier Science B.V.