(97a) Synthesis and Characterization of Pt Clusters in Aqueous Solutions and on a Al2O3 Surface

Conventional preparation techniques such as incipient wetness, wet impregnation, and deposition-precipitation are frequently used for preparation of supported Pt catalysts. However, these techniques involve deposition of cationic Pt precursors from an aqueous solution onto various oxide supports, and do not allow for adequate control of the size of the resulting metal particles. Alternative synthetic routes based on the deposition of organometallic cluster precursors and metal colloids have been employed for a better control of the size of the supported metal particles. The use of metal colloids typically requires the chemical reduction of transition metal salts in aqueous media in the presence of templating agents and surfactants in order to stabilize the metal nanoparticles formed. A novel approach to the synthesis of stable Pt colloids in aqueous solution based on the chemical reduction of H₂PtCl₆ with NaBH₄ was examined in this work. This technique does not require the use of any surfactants or templating agents and leads to the formation of nearly uniform Pt clusters that remain intact upon deposition onto a γ-Al₂O₃ support. UV-Vis spectroscopy, Extended X-ray Adsorption Fine Structure spectroscopy (EXAFS), and FT-IR spectroscopy were used to monitor the solution chemistry and characterize the morphology of the Pt clusters in the solution and on the γ-Al₂O₃ surface.