Synthesis and characterization of Pt clusters in aqueous solutions

Extended X-ray absorption fine structure (EXAFS) and UV-visible (UV-vis) spectroscopies were used to monitor the various steps involved in the synthesis of unprotected and poly(vinyl alcohol) (PVA)-protected aqueous colloidal Pt suspensions. The results indicate that on hydrolysis of the H2PtCl6 precursor, the Cl- ligands were partially replaced by aquo ligands in the first coordination shell of Pt to form [PtCl2(H2O)(4)](2+). Treatment of these species with NaBH4 under controlled pH conditions led to the formation of nearly uniform Pt-4 and Pt-6 clusters in the absence and presence of PVA, respectively. These highly dispersed colloidal Pt suspensions were stable for several months. The addition of 2-propanol (IPA) to both types of Pt suspensions led to some sintering of the Pt clusters, although both suspensions retained their colloidal nature. Less sintering was evident in the PVA-protected Pt suspension. Both the unprotected and the PVA-protected colloidal Pt suspensions were catalytically active for the liquid-phase selective oxidation of 2-propanol to acetone, with the unprotected suspension exhibiting the highest activity. (c) 2008 Elsevier Inc. All rights reserved.