Clusters in Action

In this presentation, we highlight several examples of clusters in action. We first present results on the activation of methane on free tantalum and tantalumoxide clusters and show that intrinsic cluster reactivities behave in a non-predictable way with reaction paths strongly dependent on cluster size and composition. Ab initio calculations confirm the existence of a size-dependent rate-determining step measured by kinetic and isotope exchange experiments. In a second part of the talk, strategies for optimizing cluster stability against ripening will be presented, 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. As a third example of clusters in action, results on the hydrogenation of ethylene on Pt_n clusters are presented. Surprisingly we observed an onset of reactivity going from Pt₉ to Pt₁₀ and Pt₁₃ being the most active cluster size. The origin of the size dependent reactivity is the local charging of the reactive atoms in the cluster. We present strategies of how to tune the reactivity through the design of the local environment of the clusters. Finally, we discuss the reaction mechanisms of the hydrogen evolution during methanol photoreforming on small Pt clusters on rutile TiO₂ single crystals. It is shown that the metal clusters on the semiconductor in first place act as an active site for the formation of molecular hydrogen. The found reaction mechanism suggests that some of the general concepts in photocatalysis must be revised and extended.