Ethylene Dehydrogenation on Pt_4,7,8 Clusters on Al₂O₃: Strong Cluster Size Dependence Linked to Preferred Catalyst Morphologies

Catalytic dehydrogenation of ethylene on size-selected Pt_n (n = 4, 7, 8) clusters deposited on the surface of Al₂O₃ was studied experimentally and theoretically. Clusters were mass-selected, deposited on the alumina support, and probed by a combination of low energy ion scattering, temperature-programmed desorption and reaction of C₂D₄ and D₂, X-ray photoelectron spectroscopy, density functional theory, and statistical mechanical theory. Pt₇ is identified as the most catalytically active cluster, while Pt₄ and Pt₈ exhibit comparable activities. The higher activity can be related to the cluster structure and particularly to the distribution of cluster morphologies accessible at the temperatures and coverage with ethylene in catalytic conditions. Specifically, while Pt₇ and Pt₈ on alumina have very similar prismatic global minimum geometries, Pt₇ at higher temperatures also has access to single-layer isomers, which become more and more predominant in the cluster catalyst ensemble upon increasing ethylene coverage. Single-layer isomers feature greater charge transfer from the support and more binding sites that activate ethylene for dehydrogenation rather than hydrogenation or desorption. Size-dependent susceptibility to coking and deactivation was also investigated. Our results show that size-dependent catalytic activity of clusters is not a simple property of single cluster geometry but the average over a statistical ensemble at relevant conditions.