“…The use of ultrasmall supported clusters, down to single atoms, have gained considerable attention in heterogeneous catalysis as a way of optimizing activity and selectivity of catalytic reactions. − In general, the atomic structure and electronic properties of ultrasmall clusters are highly sensitive to changes of 1–2 atoms and are not representative of the bulk material. − In this nonscalable size regime, increased sensitivity to cluster structure and support effects can change the reaction energetics and thus alter the mechanism, activity, and selectivity. ,,− Cluster-support interactions also strongly influence cluster mobility, which, in turn, determines preferred binding sites, e.g., steps or defects, and their ability to aggregate into larger 2D islands or 3D nanoparticles under reaction conditions. − Although significant progress has been made in the preparation of atomically precise supported clusters and single atoms, ,,, aggregation processes at elevated temperatures and/or pressures can lead to loss of active sites and a decrease in catalytic activity. , Hence, the use of ultrasmall clusters for catalysis applications requires detailed knowledge of the structure and thermal stability of the clusters on the surface since aggregation can largely negate any advantages of controlled size.…”