Only a few studies have examined oxidation of CO on pure CeO₂. Here we describe reaction on the (110) surface of CeO₂ with DFT and B3LYP calculations of small clusters, Ce₅O₁₂ and Ce₆O₁₂. We identified a weak adsorption site (0.21 eV) on top of a cerium atom and a strong site (2.31 eV) binding two oxygens to form a carbonate species. These energies are in accord with previous reports including GGA + U. Increasing the cluster size from Ce₅O₁₂ and Ce₆O₁₂ to Ce₉O₁₂ had no significant effect on the outcome. In addition a Mars-van Krevelen type reaction cycle is investigated involving the reaction of CO with a surface oxygen. A secondary pathway was discovered forming a very stable carbonate species, with the reversible reaction having a high activation barrier. This study is the first to find evidence of an activation barrier for the formation of a carbonate species as an alternative pathway to the desorption of CO₂ on CeO₂/(110), which would otherwise block the surface activity. We find the approach of modelling catalyst activity using a reduced cluster of atoms is accurate, flexible and advantageous.