Alloying with Sn Suppresses Sintering of Size-Selected Subnano Pt Clusters on SiO₂ with and without Adsorbates

Using Pt in the form of subnanometer dispersed clusters is a way to save precious metals in catalysis, but making such clusters stable against sintering is an uphill battle. Sn is a known agent used to increase the selectivity of dehydrogenation of alkanes on Pt. Through a joint experimental and theoretical approach, we show that adding Sn to size-selected Pt clusters deposited on amorphous SiO2 also dramatically enhances the thermal stability of the clusters against sintering. CO temperature-programmed desorption (TPD) and He+ ion scattering indicate that no Pt sites are lost, and X-ray photoelectron spectroscopy (XPS) shows no change in the electronic structure of Pt, upon repeated system heating and cooling. Density functional theory (DFT) results indicate that the binding energy of Pt clusters to the support increases by >1 eV upon adding Sn, and Sn forms strong polar covalent bonds with Pt within the clusters and quenches all of the unpaired spins. As a result, the energy needed to remove a Pt atom from Pt4Sn3/SiO2 and put it on the support is 0.15 eV larger than that from Pt4/SiO2, and in fact, it is significantly easier to dissociate a Sn atom. Both factors would tend to stabilize the Pt core of the clusters against sintering, as is observed experimentally. CO adsorbates further facilitate Ostwald ripening of the pure Pt clusters, and even in that case, alloying with Sn suppresses sintering.