Similar trends are observed for CO adsorption on the interface (i) site (Ru-1a, Ru-5a). As discussed before, CO adsorption on this site is not stable on the fully oxidized Ru/ZrO2, therefore there is no bar for this case in Figure . Furthermore, similar to adsorption on the middle layer site, CO tends to spontaneously diffuse to the hollow site formed by the Ru-1a, Ru-2a, and Ru-5a atoms for the O-vacancy at the 16i site (Table ). This results in a significantly higher CO adsorption energy, identical to that obtained for adsorption on the middle layer site for this system (hatched bar in Figure ). Again, the stronger bond with CO is partly due to the interaction of CO with the partly reduced 2Zr ion adjacent to the O-vacancy, and the direct electron transfer from the partly reduced Zr ion to the CO molecule (see Figure and Tables and). In this case, the distance between the 2Zr ion and the O atom of the CO molecule, decreases from 2.52 Å on the fully oxidized system to 2.17 Å in the presence of a vacancy at the 16i site (Table ). In the presence of O-vacancies on the more distant 18i or 13i sites, changes in the physical properties of the adsorption site are little, in agreement with the trend in the CO adsorption energy.