Coking-Resistant Sub-Nano Dehydrogenation Catalysts: PtnSnx/SiO2 (n = 4, 7)

We present a combined experimental/theoretical study of Pt n /SiO 2 and Pt n Sn x /SiO 2 ( n = 4, 7) model catalysts for the endothermic dehydrogenation of hydrocarbons, using the ethylene intermediate as a model reactant. Mass-selected Pt n clusters were deposited onto amorphous SiO 2 /Si(100) to make the Pt n SiO 2 model catalysts. To produce Pt n Sn x clusters, size-selected Pt n was used to seed selective deposition of Sn on Pt via a self-limiting H 2 /SnCl 4 /H 2 reaction sequence. Model catalysts were analyzed using C 2 D 4 and CO temperature-programmed desorption (TPD), low-energy ion scattering (ISS), X-ray photoelectron spectroscopy (XPS), plane wave density functional theory (DFT) global optimization combined with a statistical mechanical description of the catalytic interface, and a DFT mechanistic study. Supported pure Pt n clusters are found to be highly active toward dehydrogenation of C 2 D 4, quickly deactivating due to a combination of carbon deposition and sintering, resulting in loss of accessible Pt sites. Addition of Sn to Pt n clusters results in the complete suppression of C 2 D 4 dehydrogenation and carbon deposition and also stabilizes the clusters against thermal sintering. Theory shows that both systems have thermal access to a multitude of cluster structures and adsorbate configurations that form a statistical ensemble. While Pt 4 /SiO 2 clusters bind ethylene in both di-σ- and π-bonded configurations, Pt 4 Sn 3 /SiO 2 binds C 2 H 4 only in the π mode, with di-σ bonding suppressed by a combination of electronic and geometric features of the PtSn clusters. Dehydrogenation reaction profiles on the accessible cluster isomers were calculated using the climbing image nudged elastic band (CI-NEB) method. Dehydrogenation of di-σ-bound ethylene is computed to dominate and is suppressed by Sn addition, in agreement with the experiments. DFT indicates that, after Sn alloying, the barrier for ethane-to-ethylene conversion is lower than that for unwanted ethylene dehydrogenation.