Infrared vibrational studies of CO adsorption on ultrathin Cu films on a Rh(100) surface

Adsorption of CO on ultrathin Cu films supported on a Rh(100) surface has been studied using infrared reflection absorption spectroscopy (IRAS). At low Cu coverage (<0.3 ML), CO on Cu shows a higher vibrational frequency than the gas phase CO stretching frequency (2143 cm−1). This blue shift is interpreted as arising from CO adsorption on Cu atoms and/or two-dimensional atom clusters with a small positive charge, i.e., Cuδ+n, formed via charge transfer at the Cu–Rh(100) interface. As the Cu coverage increases, the CO vibrational frequency for Cu–CO red shifts. On an annealed Cu overlayer (≳0.3 ML), the IR spectra show a sharp peak at 2095 cm−1 which is interpreted to correspond to CO adsorbed on well-ordered pseudomorphic Cu domains. CO adsorption on Rh(100) was found to be blocked by 0.6 ML Cu. For a high coverage of Cu (∼7 ML) annealed to 850 K, a peak at 2076 cm−1 is observed and identified as corresponding to CO adsorption on three-dimensional Cu clusters.