Catalytic activity of palladium nanoparticles encapsulated in spherical polyelectrolyte brushes and core-shell microgels

We present a quantitative comparison of the catalytic activity of palladium nanoparticles immobilized in different colloidal carrier systems, namely, in (i) spherical polyelectrolyte brushes (SPB) and (ii) core-shell microgels. The first system given by the SPB carrier particles consist of a solid core of polystyrene onto which long chains of poly((2-methylpropenoyloxyethyl) trimethylammonium chloride) (PMPTAC) are grafted. These positively charged polyelectrolyte chains form a dense layer on the surface of the core particles which binds the divalent PdCl42- ions. Reduction leads to metallic Pd particles. System 2 is given by core-shell microgels which consists of a solid core of polystyrene and a shell of cross-linked poly(N-isopropylacrylamide) (PNIPA). The metal ions were strongly localized within the network because of complexation of the PdCl42- ions and the nitrogen atoms of PNIPA. Reduction of these ions leads to nearly monodisperse nanoparticles of metallic palladium that are only formed within the polymer layer. The average diameter d of the particles is approximately 2.4 nm (system 1; SPB) and 3.8 nm (system 2; microgel). Both types of composite particles exhibit an excellent colloidal stability. The catalytic activities of the Pd nanoparticles in both carrier systems were investigated by monitoring photometrically the reduction of p-nitrophenol by an excess of NaBH4. We find that the catalytic activity of the palladium nanoparticles is strongly influenced by the carrier system: The measured rate constants of Pd nanoparticles immobilized in spherical polyelectrolyte brushes (system 1) is much higher than the one measured for Pd particles in the network of the microgels (system 2). The dependence of the rate constants obtained for both carrier systems demonstrates that these differences must solely be traced back to the different diffusional barriers in both carrier systems; there is no indication for any specific interaction of the polymer chains with the metallic nanoparticles. A comparison with data from literature demonstrates that both types of core-shell particles are excellent carrier systems that may be used to tune the catalytic activity of the metallic nanoparticles.