Here we show, by in situ scanning tunnelling microscopy, that Pd nanoparticles enhance the rate of re-oxidation of a sub-stoichiometric TiO2(1 1 0) surface. We believe that O-2 dissociatively adsorbs at 673 K on the Pd, and "spills over" onto the support where further reaction takes place. Tin+ interstitial ions in the bulk crystal lattice react with the spillover oxygen at the surface, preferentially growing TiO2 around and over the particles. We employ a kinetic Monte-Carlo scheme to simulate this surface reaction and find a good correlation between the simulation and STM images. The simulation indicates that the surface structure may be reproduced in a model in which mobile species spillover from the metal particle, performs a random walk until it reaches a step edge where it may react (with low probability) to form TiO2 The mobile species may also down-step to a lower terrace but up-stepping is forbidden. The temporal evolution of surface structures from both experiment and simulation are compared.