CO adsorption on the interface (i) site is instable on the fully oxidized Ru/ZrO2 system, in the absence of an O-vacancy. Instead, COad will diffuse to the Ru-5a site in the middle layer during relaxation. Since CO changed the type of adsorption site during relaxation, we did not represent its adsorption energy on the interface (i) site in Figure . In fact, after relaxation the CO adsorption site and energy correspond to those on the middle layer site for this system. In the presence of an O-vacancy, CO adsorption becomes stable on the interface (i) site and shows a rather high adsorption energy, comparable to adsorption on the middle layer site, with a subtle decay with increasing distance to the O-vacancy. In this case, the adsorbed CO is also rather close to the Ru-5a atom, though not as close as for adsorption on the middle layer site (Table ). In the presence of an O-vacancy, CO adsorption on this site is further strengthened by an interaction between the O atom of the CO and the 2Zr surface cation, which is indicated by their rather close distance of about 2.3 Å (Table ). This CO -Zr interaction decreases with increasing distance of the O-vacancy from the adsorption site (Table ).