Finally, we calculated the influence of O-vacancies in deeper layers underneath the Ru nanorod on the CO adsorption strength, using the same positions of the O-vacancies and adsorption sites as before (see section 3.1.3). The resulting CO adsorption energies are summarized in Figure . When an O-vacancy is present at the (iv) site (indicated by the orange bar), the CO adsorbed at the edge site of the top layer (bridge site between 7a and 8a) will relax to the hollow site formed by 5a, 7a, and 8a Ru atoms, and CO adsorbed on the middle layer site (Ru-5a) will relax to a bridge site between 1a and 5a (Table ). For adsorption on the interface (ii) site, the presence of an O-vacancy on the (ii) site causes the adsorbed CO to relax from the initial bridge site (Ru-2a, Ru-6a) to a bridge site formed by the Ru-2a and Ru-3a atoms. Furthermore, in this case CO adsorption also causes a spontaneous migration of the O-vacancy from the (ii) site to the (1i) site, where it is stabilized by interaction with the Ru nanorod. Therefore, the related energies are indicated as hatched bars in Figure . Similar to the effect of O-vacancies aside the Ru nanorod, O-vacancies in deeper layers have almost no influence on the CO adsorption strength, independent of the adsorption site. The only exception are O-vacancies in the second layer, at the (ii) site, which result in a significant increase of the CO adsorption strength on the two interface sites. Following our above explanation of the enhanced CO bond strength for adsorption on interface site (ii) in the presence of a surface O-vacancy at the 16i site aside the Ru nanorod, the adsorption energy contains also contributions from a direct interaction between the adsorbed CO molecule and a partly reduced surface 1Zr ion neighboring to the O-vacancy on the (i) site. Note that the enhanced CO adsorption strength is observed only for CO adsorption on the two interface sites, not for adsorption on the middle layer site (Table ). This is tentatively explained by the larger O(CO) -2Zr distance (3.96 Å) on the middle layer site, while for CO adsorption at the interface (i) and (ii) sites, the distances to the closest Zr ion are only 2.34 Å (O(CO) -2Zr) or 2.14 Å (O(CO) -1Zr), respectively (Table ). Furthermore, for adsorption on the interface sites, the CO adsorption energy also includes the gain in energy obtained upon the spontaneous migration of O-vacancy from the (ii) site to the (2i) site, where it is stabilized by interaction with the Ru nanorod.