Density functional theory was used to explore the energy landscape of Pd atoms adsorbed on the terrace of MgO(100) and at O vacancy sites. Saddle point finding methods revealed that small Pd clusters diffused on the terrace in interesting ways. The monomer and dimer diffused via single atom hops between O sites with barriers of 0.34eV and 0.43eV respectively. The trimer and tetramer, however, formed 3D clusters by overcoming a 2D–3D transition barrier of less than 60meV. The trimer diffused along the surface either by a walking or flipping motion, with comparable barriers of about 0.5eV. The tetramer rolled along the terrace with a lower barrier of 0.42eV. Soft rotational modes at the saddle point led to an anomalously high pre-factor of 1.3 x 1014/s for tetramer diffusion. This pre-factor was two order of magnitude higher than for monomer diffusion, making the tetramer the fastest diffusing species on the terrace at all temperatures for which diffusion was active (above 200K). Neutral O vacancy sites were found to bind Pd monomers with a 2.63eV stronger binding energy than the terrace. A second Pd atom, however, binds to this trapped monomer with a smaller energy of 0.56eV, so that dimers at defects dissociate on a time scale of milliseconds at room temperature. Larger clusters bind more strongly at defects. Trimers and tetramers dissociate from monomer-bound-defects at elevated temperatures of ca. 600K. These species were also mobile on the terrace, suggesting they were important for the ripening observed at above 600K during Pd vapor deposition onto MgO(100).

Pd Diffusion on MgO(100) - the Role of Defects and Small Cluster Mobility. L.Xu, G.Henkelman, C.T.Campbell, H.Jónsson: Surface Science, 2006, 600[6], 1351-62