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 Pt4 and Pt6 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.