Polyoxotungstate stabilized palladium, gold, and silver nanoclusters
A study of cluster stability, catalysis, and effects of the stabilizing anions
Polyoxometalates (POMs) have demonstrated the unusual ability to both stabilize nanoclusters and facilitate catalytic activity by allowing substrate access to sufficient metallic surface area. This paper presents a wet chemical synthetic method to prepare Pd, Ag, and Au metal nanoclusters stabilized by tetrabutylammonium salts of Phosphotungstates with the well-known Keggin [alpha-PW12O40](3-) (PW12), Wells-Dawson [P2W18O62](6-) (P2W18), and their lacunary derivatives [alpha-PW11O39](7-) (PW11) and [P2W15O56](12-) (P2W15) structures. The preparation of this series of nanoclusters has facilitated a comparison of the effect these anions have in nanocluster stabilization and catalysis. The nanocluster systems were found to be re-dispersible in acetonitrile and acetone. The effect of aging on stability was followed over 3 months and it was found that many systems were very stable for more than 70 days without precipitation, depending on type of polytungstate stabilizer and the presence of competing ions. The PW12, P2W18, PW11, and P2W15 stabilized Pd nanoclusters have been investigated for catalytic activity in the hydrogenation of 1-hexene and were found to be very active with the P2W18 stabilized Pd nanoclusters demonstrating a best of 139,000 Total Turnovers, thus ranking this "soluble heterogeneous catalyst" among one of the longest lived reported in the literature. The nanocluster systems were characterized using instrumental techniques such as UV-Visible Spectroscopy, FTIR Spectroscopy, Powder X-ray Diffraction, Energy Dispersive Spectroscopy, X-ray Photoelectron Spectroscopy, and Transmission Electron Microscopy.