Surface-Supported Cluster Catalysts: Fluxionality, Statistical Nature, and Design*

Sub-nano surface-supported catalytic clusters, generally have many low-energy isomers accessible at elevated temperatures of catalysis. The most stable isomer may not be the most catalytically active. We present methodology that, for moderate-sized systems, thoroughly samples the potential energy surface, at the theory level such as density functional theory. It gives access to the ensemble of thermally-accessible structures of surface-mounted clusters. Additionally, isomers may interconvert across barriers, i.e. exhibit fluxionality, during catalysis. We model such process as an isomerization network using structural matching and paralleled nudged elastic band method. We will show how all practically-important properties of cluster catalysts are in fact ensemble-average. Side-by-side with the experiment, we explain why Pt₇ on alumina is much more active toward alkene dehydrogenation than Pt₈ and Pt₄ on this support. We also present the design of nano-alloys of Pt clusters, which are remarkably selective against coke in this reaction, as again confirmed experimentally. These findings were possible only within the ensemble representation of the catalyst.