Fundamental insight into the substrate-dependent ripening of monodisperse Clusters

Sintering is one of the major routes for the deactivation of surface-supported catalytic particles in heterogeneous catalysis. The two mechanisms that are responsible for such coarsening phenomena are Ostwald ripening, in which larger clusters tend to grow at the expense of smaller ones, and Smoluchowski ripening, in which entire clusters diffuse and coalesce. The sintering properties of cluster-assembled materials can be influenced by tuning the interactions between the particle and the substrate. To explore the fundamental factors that control cluster-ripening mechanisms, we deposited truly monodisperse Pd clusters onto three different model catalysts: bare Rh(111), graphene-Moiré films that were grown on Rh(111) and Ru(0001), and a hexagonal boron-nitride film that was grown on Rh(111). The evolution of particle size and density was tracked by high-resolution scanning tunneling microscopy. The principal microscopic mechanisms that govern the ripening processes on each of these three substrates have been determined from thorough analyses of the cluster heights and size distributions. The ripening mechanisms were related to the distinct cluster-adsorption and atom-detachment energies that were obtained from first-principle calculations. These results elucidate the ripening processes and underlie the formulation of a strategy for optimizing cluster stability against ripening, where both, the binding of the clusters to the surface and that of the individual atoms, must be controlled. Such tuning of the interactions may be achieved through the judicious selection of surfaces with laterally modulated wettability.