Temperature and pH dependence of the catalytic activity of colloidal platinum nanoparticles stabilized by poly [(vinylamine)-co-(N-vinylisobutyramide)]

Colloidal platinum nanoparticles in the size range of 5-35 Angstrom have be en successfully prepared in water at room temperature by NaBH4 reduction of ionic platinum in the presence of poly[(vinylamine)-co-(N-vinylisobutyrami de)] (PVAm-co-PNVIBA). To our knowledge, the temperature- and pH-responsive copolymer was used far the first time as the stabilizer of colloidal metal particles. Three PVAm-co-PNVIBA copolymers with PVAm contents of 4.1, 8.3, and 19.8 mol-% were examined. The particle size and morphology of the plat inum colloids varied with the copolymer composition, as confirmed by TEM me asurements. The polymer-stabilized Pt nanoparticles precipitated on heating above their critical flocculation temperatures (CFTs), which were strongly dependent on the solution pH and the copolymer composition. The CFTs were 0.2-1.6 degreesC lower than the tower critical solution temperatures (LCSTs ) of the copolymers free in water and the differences increased with increa sing PVAm content. The catalytic activity of the Pt nanoparticles was inves tigated in the aqueous hydrogenation of allyl alcohol. ft was found that th e activity was regulated through temperature- and pH-induced phase separati on. The PVAm content also strongly effected the catalytic activity and the morphology of phase separated catalysts. With a PVAm content of 4.1 mol-%, the colloidal platinum sol reversibly changed its catalytic activity with c hanges in temperature.