Preparation of platinum colloids on polystyrene nanospheres and their catalytic properties in hydrogenation

Well-dispersed platinum colloids have been prepared on polystyrene nanospheres with surface-grafted poly(N-isopropylacrylamide) (PNIPAAm) via the reduction of PtCl62- by ethanol. A range of monodisperse nanospheres was obtained by emulsifier-free dispersion copolymerization of styrene with a PNIPAAm macromonomer. The surface compositions of the polystyrene nanospheres before and after immobilization of Pt colloids have been investigated by X-ray photoelectron spectroscopy. Peak fitting of the C 1s core-line spectra provides evidence for the presence of PNIPAAm chains at the surface of polystyrene nanospheres. These immobilized Pt colloids were found to be active and stable heterogeneous catalysts for the hydrogenation of allyl alcohol in water. Both the particle size and activity of platinum colloids showed a marked dependence on the level of surface PNIPAAm. The average diameter of an immobilized Pt colloid is 15.0 Angstrom and largely unchanged after seven cycles in hydrogenation, as confirmed by TEM. The immobilized Pt colloid shows considerable increased activity as compared with the PNIPAAm-stabilized colloidal Pt sol. An unusual temperature dependence of catalytic activity was observed due to the heating-induced insolubility of PNIPAAm chains.