Infrared vibrational studies of CO adsorption on Cu/Pt(111) and CuPt(111) surfaces

Adsorption of CO on ultrathin Cu films supported on Pt(111) has been studied using infrared reflection absorption spectroscopy (IRAS). Our results indicate that the infrared intensities of adsorbed CO are not representative of the relative composition of the Cu<1.0/Pt(111) surfaces. The Cu-bonded CO molecules screen CO molecules bonded to Pt, making them invisible in the infrared spectrum. The ''screening'' effect depends on the morphology and polarizability of the Cu overlayer. Changes in the morphology of the Cu adlayer produce large variations in the position and line shape of the Cu--CO signal in the infrared spectrum. CO molecules bonded to small Cu clusters show a higher (∼40 cm-1) C--O stretch frequency than CO molecules adsorbed on large Cu islands. The present results were compared with those reported in the literature for the CO/Cu/Ru(0001) and CO/Cu/Rh(100) systems. For CO adsorbed on supported monolayers of Cu, a correlation was found between the strength of the Cu--CO bond, the amount of π backdonation, and the C--O stretch frequency. This correlation cannot be explained using simple models of molecular orbital theory. On the bimetallic surfaces, the C--O stretch frequency is more sensitive to the charge on the metal center to which CO is bonded, than to the electron population of the CO(2π) orbitals