Small metal nanoparticles that are also highly crystalline have the potential for showing enhanced catalytic activity. We describe the preparation of single nanocrystals of platinum that are 2 to 3 nanometers in diameter. These particles were generated and immobilized on spherical polyelectrolyte brushes consisting of a polystyrene core (diameter of approximately 100 nanometers) onto which long chains of a cationic polyelectrolyte were affixed. In a first step, a nanoalloy of gold and platinum (a solid solution) was generated within the layer of cationic polyelectrolyte chains. In a second step, the gold was slowly and selectively dissolved by cyanide ions in the presence of oxygen. Cryogenic transmission electron microscopy, wide-angle x-ray scattering, and high-resolution transmission electron microscopy showed that the resulting platinum nanoparticles are faceted single crystals that remain embedded in the polyelectrolyte-chain layer. The composite systems of the core particles and the platinum single nanocrystals exhibit an excellent colloidal stability, as well as high catalytic activity in hydrogenation reactions in the aqueous phase.

Homogeneous alloy nanoparticles as excellent catalysts. Spherical polyelectrolyte brushes can be used to generate Au‐Pt alloy nanoparticles (see figure) that exhibit properties widely differing from the properties of the respective bulk alloys. The alloy nanoparticles are shown to be homogeneous solid solutions. Moreover, they are effective catalysts for the selective oxidation of alcohols to aldehydes and ketones.

We reported on the in situ synthesis of silver nanoparticles onto polystyrene (PS) core−poly(acrylic acid) (PAA) polyelectrolyte brush particles. The synthesis of these composite particles proceeds through a photoemulsion polymerization in aqueous dispersion using the functional monomer silver acrylate. In this way the layer of the chains of poly(acrylic acid) on the surface and the Ag nanoparticles are formed at the same time. Dynamic light-scattering (DLS) measurement demonstrates the formation of polyelectrolyte brushes onto the PS core surface. TEM and cryo-TEM images show that well-dispersed Ag nanoparticles (d = 3 ± 1.2 nm) are formed in situ on the surface of PS−PAA spherical polyelectrolyte brush particles. Moreover, wide-angle X-ray scattering indicates that Ag nanoparticles formed in the brushes are crystalline. The catalytic activity is investigated by monitoring the reduction of 4-nitrophenol by NaBH4 in presence of these silver nanocomposite particles. The rate constant k app of this reaction was found to be slightly smaller than the one of Pd or Pt nanoparticles immobilized on carrier particles of the same type. All data demonstrate that the present way of synthesis leads to extremely small Ag particles immobilized on a stable carrier.

Thermosensitive core-shell microgels can be used as ''nanoreactors'' for the immobilization of metal nanoparticles. The microgels consist of a polystyrene core and a network made of poly(Nisopropylacrylamide) (PNIPA) cross-linked by N,N 0 -methylenebisacrylamide. The cross-linked PNIPA shell undergoes a volume transition at around 30 C in which most of the water is expelled. The microgel particles exhibit a weak positive charge due to the cationic initiator. Metal nanoparticles (such as Au, Rh and Pt) with high catalytic activity can be homogeneously embedded into such a network. The oxidation of alcohols to the corresponding aldehydes or ketones has been chosen as a test reaction to probe the catalytic activity of such metal-microgel nanocomposite particles