In this work, we developed Monte Carlo simulations to understand the transient experiments on CO 2 formation at the oxygen-rich ruthenium surface observed in recent experiments [A. Bottcher et al., J. Phys. Chem. B 193, 6267 (1999)]. We modeled this very complex system by a semi-infinite cubic lattice, the catalyst having been represented by the (001) free surface, and the typical profile shapes obtained by employing the TRIM (Transport and Range of Ions in Matter). When the surface is exposed to a constant flux of CO molecules we observe the formation of CO 2 due to the Langmuir-Hinshelwood process. The number of active sites over the surface is taken proportional to the number of monolayers of oxygen deposited due to the sputtering mechanism. Increasing the temperature, the active sites on the free surface become more mobile, and the subsurface oxygen atoms migrate to surface through diffusion. We show that desorption of CO molecules from the surface is very important at higher temperatures. Even taking high values for the diffusion of active sites and subsurface oxygen, a small desorption rate of CO leads the surface to become quickly poisoned by these molecules, therefore blocking the diffusion of oxygen atoms towards the topmost layer.