Preparation of Size- and Composition-Controlled PtnSnx/SiO2 (n=4, 7, 24) Bimetallic Model Catalysts with Atomic Layer Deposition

摘要

Model catalysts prepared by mass-selected cluster deposition are useful in fundamental studies, but for bimetallic clusters, a precise selection of both size and composition can be difficult except for very small clusters. We discuss an alternative method for preparing bimetallic PtSn model catalysts, in which size-selected Pt-4, Pt-7, and Pt-24 clusters, deposited on SiO2 and then hydrogenated, are used to seed selective addition of Sn by a self-limiting reaction with SnCl4. Catalysts are characterized using X-ray photoelectron spectroscopy, ion scattering spectroscopy, and CO temperature-programmed desorption, and structures are calculated by plane-wave density functional theory. It is found that Sn deposition saturates at a relatively low SnCl4 exposure, indicating that the reaction is self-limiting, and that nonselective Sn deposition on the SiO2 support is similar to 40 times less efficient than Sn deposition on hydrogenated Pt cluster sites. The resulting PtnSnx stoichiometry is near 1:1 for Pt-4 and Pt-7, clusters, whereas for Pt-24 clusters, x is similar to 7.5. Nearly, all of the Cl in the adsorbed SnCl4 desorb as HCl during the room-temperature Sn deposition process, and the balance can be desorbed by mild heating.