Low-energy electron diffraction, Auger electron spectroscopy, and thermal desorption studies of chemisorbed CO and O 2 on the (111) and stepped [6(111) × (100)] iridium surfaces

Abstract

The adsorption of CO, O 2, and H 2O was studied on both the (111) and [6(111) × (100)] crystal faces of iridium. The techniques used were LEED, AES, and thermal desorption. Marked differences were found in surface structures and heats of adsorption on these crystal faces. Oxygen is adsorbed in a single bonding state on the (111) face. On the stepped iridium surface an additional bonding state with a higher heat of adsorption was detected which can be attributed to oxygen adsorbed at steps. On both (111) and stepped iridium crystal faces the adsorption of oxygen at room temperature produced a (2 × 1) surface structure. Two surface structures were found for CO adsorbed on Ir(111); a (√3 × √3)R30° at an exposure of 1.5–2.5 L and a (2√3 × 2√3)R30° at higher coverage. No indication for ordering of adsorbed CO was found on the Ir(S)-[6(111) × (100)] surface. No significant differences in thermal desorption spectra of CO were found on these two faces. H 2O is not adsorbed at 300 K on either iridium crystal face. The reaction of CO with O 2 was studied on Ir(111) and the results are discussed. The influence of steps on the adsorption behaviour of CO and O 2 on iridium and the correlation with the results found previously on the same platinum crystal faces are discussed.