Synthesis, Characterization, and Catalytic Properties of Colloidal Platinum Nanoparticles Protected by Poly(N-isopropylacrylamide)

We have studied the formation of platinum nanoparticles in an ethanol−water mixture with an ethanol volume fraction of 0.6 by ethanol reduction of PtCl 6 2 - in the presence of poly( N -isopropylacrylamide) (PNIPAAm−Pt) by UV−visible spectrophotometry. As predicted by calculation, a maximum at 215 nm in the absorption spectrum of the PNIPAAm−Pt sol is observed. The Pt nanoparticles can be easily transferred into distilled water, and their average particle sizes as measured by transmission electron microscopy are 10−30 Å. The colloidal Pt nanoparticles that were protected by PNIPAAm with a mean molecular weight of 6000 exhibited inverse temperature dependent solubility and a cloud-point temperature of 34.2 °C in an aqueous solution. Because of the phase separation of PNIPAAm−Pt from the continuous phase containing the substrate above 38 °C, the temperature dependence of the reaction rate did not follow normal Arrhenius behavior in an aqueous hydrogenation of allyl alcohol. The mean molecular weight of PNIPAAm and the monomeric unit/Pt molar ratio significantly affected the reduction rate of PtCl 6 2 -, as well as the average diameter and the catalytic activity of the particles.