Effect of O2 and CO Exposure on the Photoelectron Spectroscopy of Size-Selected Pdn Clusters Supported on TiO2 (110)

Ultraviolet photoelectron spectroscopy of size-selected Pd-n clusters supported on TiO2 shows a decrease in density of states in the TiO2 gap after the absorption of CO, while O-2 does not result in a decrease in density. Oxygen is more electron withdrawing than CO, so the Pd clusters should become more positively charged when exposed to O-2 than CO. More positively charged clusters are expected to have larger electron binding energies; thus, the observed shifts in the UPS spectra are at odds with conventional wisdom. We have performed a combined experimental and theoretical investigation of the UPS spectra of Pd-n clusters with adsorbed O-2 and CO. Bonding of both CO molecules and O atoms to the Pd clusters results in a decrease in the density of states in the TiO2 gap; however, O atom binding on top of the clusters also results in significant final state stabilization, restoring the density of states in the TiO2 gap. We also find that 4d-5s hybridization plays a critical role in the initial state energies in X-ray photoelectron spectroscopy and evaluate two methods for determining the final state shift via periodic calculations.