For further information on the CO adsorption behavior and on the effect of O-vacancy formation thereon we also evaluated the C-O vibrational properties. ), where CO adsorption was most stable. Adsorption on the fully oxidized Ru/ZrO2 is denoted in these panels as the 0 th row and 0VO-vacancy cluster. As evident from Figure , the CO molecules adsorbed on the top layer terrace sites and, except for the fully oxidized system, at the middle layer exhibit the highest vibrational frequencies, which fits well to the generally higher frequencies of CO adsorbed on on-top sites. For adsorption on the top layer edge site the frequencies are generally lower, as expected for adsorption on bridge sites (see Tables and). Changes in the frequencies with O-vacancy position are either fully absent (top layer terrace sites) or small (top layer edge sites). This fully agrees with expectations based on the constant adsorption energies and charges on the top layer sites due to metallic screening. For adsorption on the middle layer site we find a distinct up-shift by about 160 cm -1 when introducing an O-vacancy, independent of the position of the vacancy. This change in vibrational frequency is much more pronounced than the small increase in CO adsorption energy on this site (see Figure ). It may be related to a change in the orientation of the adsorbed CO adsorbed. In the 0VO configuration, CO points downward towards one Zr surface ion in the support, while in the presence of an O-vacancy the C-O bond is parallel to the support surface. While a simple explanation of the physical origin for this change in orientation is still missing, it nevertheless demonstrates the complexity of these adsorption systems. Adsorption on the interface sites results in the lowest frequencies, as expected for CO adsorption on bridge or threefold hollow sites. For adsorption on the interface (i) site, the vibrational frequency is lower than expected for adsorption on bridge sites (see Table ), which may be related to the additional interaction with the 2Zr surface ion. Here we find a small increase of the C-O frequency when moving the O-vacancy away from the adsorption site, from the 1 st to the 2 nd and then to the 3 rd row. (There is no stable CO adsorption on this site in the absence of an O-vacancy). The small upshift of about 57 cm -1 fits well to the subtle decrease in CO adsorption energy (see Figure ). Both trends are related to the decreasing electron charge transfer (see Figure ) to the adsorption site upon O-vacancy formation with increasing distance of the vacancy from the adsorption site. site, which varies from -0.24 e via -0.48 e and -0,28 e to finally -0.44 e (Figure ). This is different for the CO adsorption energy, which shows a very small steady increase (Figure ).