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 PtCl62- 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 Angstrom. 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 degrees C in an aqueous solution. Because of the phase separation of PNIPAAm-Pt from the continuous phase containing the substrate above 38 degrees 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 PtCl62-, as well as the average diameter and the catalytic activity of the particles.