This can be rationalized by their shorter distances to the O-vacancy of 2.27 Å (1Zr) and 2.49 Å (2Zr), respectively, while for the O-vacancies on the (iii) and (iv) sites these distances are much larger (Table ). Apparently, the very high CO adsorption energies at the interface (i) and (ii) sites in the presence of an O-vacancy on the (ii) site are mainly caused and dominated by an additional direct interaction between the O atom of the adsorbed CO and the 1Zr and 2Zr surface ions, respectively, overriding the effects caused by the charge on the Ru adsorption site.