STM investigation and Monte-Carlo modelling of spillover in a supported metal catalyst

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 O₂ 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.