Infrared reflectance absorbance spectroscopy of coadsorbed CO and H2O on Pt(111) surfaces

We have used polarization modulated infrared reflectance absorbance spectroscopy (IRRAS) to study the interactions between coadsorbed CO and H2O on Pt(111) surfaces at 150 K. Two types of behavior are observed, depending on CO coverage. For CO coverages greater than θ≊1/2, the vibrational band for linear CO species shifts from 2100 cm−1 for an H2O-free surface to 2095 cm−1 for H2O exposures up to 15 L. For larger H2O exposures a second, broader band appears at around 2080 cm−1. These results indicate that H2O adsorbs in clusters on top of a saturated CO adlayer, without strongly perturbing the CO molecules underneath. In contrast, for CO coverages less than θ≊1/4, adsorption of H2O at 150 K results in the disappearance of the linear CO vibrational band, with its subsequent reappearance upon desorbing the H2O layer. This indicates that for low CO coverages, coadsorbed H2O can displace CO out of its preferred linear coordination mode.