CO chemisorption on Cr(110): Evidence for a precursor to dissociation

Abstract

High resolution electron energy loss spectroscopy(EELS),electron stimulated desorption ion angular distributions (ESDIAD), low energy electron diffraction(LEED), and Auger electron spectroscopy(AES) have been combined to study CO chemisorption on the Cr(110) surface. At 120 K, two molecular binding modes are sequentially populated. The first, α1CO, is a novel binding mode for a clean metal surface in which the molecules are bound with the C–O axis nearly parallel to the Cr(110) surface. These α1CO molecules exhibit a νCO of 1150–1330 cm− 1, no detectable ion emission in ESDIAD, and form a c(4×2) α1CO overlayer at saturation (θCO≈0.25). A model is proposed in which the α1CO are π bonded to the Cr(110) surface at twofold symmetric surface hollow sites. Annealing experiments demonstrate that the α1 binding mode is the precursor to dissociation on Cr(110). At higher CO coverages, 0.25<θCO<0.35, the α1CO overlayer is disordered by the addition of the second binding mode, α2CO. Three new vibrations, at 495, 1865, and 1975 cm− 1, are observed by EELS while O+ion emission along the [110] direction is seen in ESDIAD. These α2CO results are indicative of terminally bound molecules adsorbed at atop and bridge sites and aligned approximately perpendicular to the surface. Annealing studies of CO and O/CO adlayers provide evidence for a surface reaction in addition to direct dissociation above 170 K. In contrast, adsorption at 300 K leads to direct dissociativechemisorption with no detectable intermediate molecular adsorbate. The implications of these results to catalytic reactions of CO are considered and comparisons to CO adsorption on clean and ‘‘promoted’’ transition metals are made.