The deposition and ripening of Pd atoms on the MgO(100) surface were modeled using kinetic Monte Carlo simulations. The density of Pd islands was obtained by simulating the deposition of 0.1ML in 3 min. Two sets of kinetic parameters were tested and compared with experiment over a 200 to 800K range. One model was based upon parameters obtained by fitting rate equations to experimental data and assuming the Pd monomer was the only diffusing species. The other was based upon transition rates obtained from density functional theory calculations which showed that small Pd clusters were also mobile. In both models, oxygen vacancy defects on the MgO surface provide strong traps for Pd monomers and serve as nucleation sites for islands. Kinetic Monte Carlo simulations showed that both models reproduced the experimentally observed island density versus temperature, despite large differences in the energetics and different diffusion mechanisms. The low temperature Pd island formation at defects was attributed to fast monomer diffusion to defects in the rate-equation-based model, whereas in the density functional theory-based model, small clusters form already on terraces and diffuse to defects. In the density functional theory-based model, the strong different dimer and trimer binding energies at charged oxygen vacancy defects prevented island ripening below the experimentally observed onset temperature of 600K.

Kinetic Monte Carlo Simulations of Pd Deposition and Island Growth on MgO(100). L.Xu, C.T.Campbell, H.Jónsson, G.Henkelman: Surface Science, 2007, 601[14], 3133-42