Temperature-responsive catalytic activity of poly(N-isopropylacrylamide)-protected Au/Pt bimetallic nanoparticles in aqueous solutions

Abstract

Colloidal gold/platinum bimetallic nanoparticles with various Au/Pt ratios have been prepared by the reduction of HAuCl4 and H2PtCl6 from ethanol/water (6/4 v/v) solution in the presence of poly(N-isopropylacrylamide) (PNIPAAm). The colloids obtained have been characterized by means of transmission electron microscopy, X-ray diffraction and UV-vis spectroscopy. The physical measurements demonstrated that the Au/Pt bimetallic colloids (19.6 Å in mean diameter) have alloy structures. The alloy colloids are more active catalysts than the Pt monometallic colloids for the hydrogenation of allyl alcohol in water at room temperature. Further, the rate of hydrogenation depends on the molar ratio of the Au/Pt in the bimetallic nanopartides. Both Pt monometallic and Au/Pt bimetallic colloids protected by PNIPAAm with a mean molecular weight of 6000 exhibited the inverse temperature-dependent solubility and cloud-point temperatures at ca. 34.2°C in aqueous solutions. Because of the phase separation of the catalysts from the reaction mixtures above 39°C, the catalytic activity was lower at 40°C than at 10°C for the aqueous hydrogenation of allyl alcohol. On cooling the reaction mixture to below 30°C, the catalyst redissolved and showed the same activity as before. © 1999 John Wiley & Sons, Ltd.