The CO adsorption energies remained nearly unchanged for adsorption on the top layer terrace site, top layer edge site, middle layer site and interface (i) sites, regardless of the size of the Ovacancy cluster aside the Ru nanorod. In contrast, for adsorption on the interface (ii) site, there was a significant increase in CO adsorption energy upon the formation of the second O-vacancy in the 2VO system.