SIMS/XPS study of CO chemisorption on polycrystalline Pd, Ag and a PdAg alloy

The SIMS spectra of Ag, Pd and a 79% PdAg alloy exposed to CO have been investigated. Exposure of a Ag foil to 10^-6 Torr CO resulted in no measurable metal CO clusters. On Pd foil, however, CO adsorption reached saturation after 3 L exposure as indicated by the rise in Pd⁺, Pd₂⁺, PdCO⁺ and Pd₂CO⁺ ions. Further increases in these ions were noticed as the background pressure of CO was increased from ambient up to 10^-6 Torr. On 79% PdAg, the CO adsorption was found to be very slow, reaching saturation after 3000 s at 10^-6 Torr CO. Of special interest was that both the Pd⁺ and Ag⁺ ions were observed to increase during the chemisorption even though the CO is believed to interact only with Pd atoms. The SIMS spectra also produced other combinations of cluster ions including PdAg⁺, Pd₂⁺, Ag₂⁺, Ag₃⁺, PdCO⁺, AgCO⁺, PdAgCO⁺ and an undetermined mixture of M₃CO⁺ clusters. The presence of AgCO⁺ in large quantities provides strong evidence that the cluster ions do not leave the surface as a single entity, but form from atoms that were not necessarily neighbors.