Steric and electronic effects of sulfur on CO adsorbed on Pd(100)

Abstract

The mechanism of sulfur poisoning was studied on the Pd(100) surface using TPD, HREELS, and LEED for sulfur coverages ranging from zero to saturation (0.5 monolayer). A rapid initial decrease in the CO saturation coverage at 80 K was observed up to a sulfur coverage of about 0.1 ML. Coincident with this coverage decrease the formation of high-density CO compression structures is blocked as revealed by LEED. Thereafter the CO coverage declines linearly in a nearly 1:1 ratio with the addition of adsorbed sulfur. For low θ CO the CO stretching frequency increased from 1895 to 1925 cm −1 as the sulfur coverage was increased from zero to saturation. The CO singleton frequency at θ s=0.50 is slightly less than the reported C 12O singleton stretching frequency in the presence of 0.5 monolayer of C 13O, indicating that adsorbed sulfur has less effect on the CO bond than adsorbed CO itself at this coverage. The observed value of the CO stretching frequency at CO saturation coverage decreases with increasing sulfur coverage. This net decrease in ν(CO) with θ s for saturation coverage by CO coincides with the reduction in CO coverage in the presence of adsorbed sulfur. Sulfur does not appear to change the bridge-site preference of adsorbed CO, and the decrease in the saturation coverage for 0< θ s<0.10 is ascribed to disorder in the CO adlayer induced by the preadsorbed sulfur. When compared to the sulfur-free surface at the same CO coverage there is an increase in the CO stretching frequency. This increase in the CO stretching frequency at saturation coverage by CO due to adsorbed sulfur rises rapidly at low sulfur coverage and remains constant above θ s=0.15. This effect appears to be due to COCO coupling transferring the local changes in frequency caused by sulfur to the entire adsorbed layer.