CHaracterization of CO binding sites on Rh{111} and Rh{331} surfaces by XPS and LEED: Comparison to EELS results

Changes in the nature of the binding site of chemisorbed CO on the Rh{111} and Rh{331} single crystal surfaces during adsorption and desorption have been monitored by X-ray Photoelectron Spectroscopy (XPS) and Low Energy Electron Diffraction (LEED). Two bonding states of molecular CO have been identified from the O 1s photoemission line. These states are assigned as atop and bridge-bonded species and are observed to be coverage and temperature dependent. On both surfaces atop sites are populated first and at higher CO coverages bridge sites are filled. On Rh{111} the bridge sites are filled at a CO coverage of θ_CO ～ 0.50 and their presence is correlated with a change in the LEED pattern. The presence of the step atoms on the Rh{331} surface markedly influenced the sequential filling of binding sites in comparison to that observed on the Rh{111} surface. A comparison of our data to previous Electron Energy Loss Spectroscopy (EELS) work on Rh{111} is in remarkable quantitative agreement with EELS peak heights.