We present a study on the catalytic reduction of 4-nitrophenol by sodium borohydride in the presence of metal nanoparticles. The nanoparticles are embedded in spherical polyelectrolyte brushes, which consist of a polystyrene core onto which a dense layer of cationic polyelectrolyte brushes are grafted. The average size of the nanoparticles is approximately 2 nm. The kinetic data obtained by monitoring the reduction of 4-nitrophenol by UV/vis-spectroscopy could be explained in terms of the Langmuir−Hinshelwood model: The borohydride ions transfer a surface-hydrogen species in a reversible manner to the surface. Concomitantly 4-nitrophenol is adsorbed and the rate-determining step consists of the reduction of nitrophenol by the surface-hydrogen species. The apparent reaction rate can therefore be related to the total surface S of the nanoparticles, to the kinetic constant k related to the rate-determining step, and to the adsorption constants K Nip and K BH4 of nitrophenol and of borohydride, respectively. In all cases, an induction time t 0 was observed of the order of minutes. The reciprocal induction time can be treated as a reaction rate that is directly related to the kinetics of the surface reaction because there is a linear relation between 1/(kt 0) and the concentration of nitrophenol in the solution. All data obtained for t 0 so far and a comparison with data from literature indicate that the induction time is related to a slow surface reconstruction of the nanoparticles, the rate of which is directly related to the surface reaction.
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 PdCl4 2- 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 PdCl4 2- 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.
Nanoreactors: Metal nanoparticles must be stabilized with carrier systems against aggregation if they are to have applications in catalysis. One such system, a thermosensitive network of poly(N‐isopropylacrylamide) that surrounds a polystyrene core, enables control over the catalytic activity of the nanoparticles through a phase transition and leads to applications as a controllable nanoreactor.
We present a study on the catalytic activity of platinum nanoparticles immobilized on spherical polyelectrolyte brushes that act as carriers. The spherical polyelectrolyte brushes consist of a solid core of poly(styrene) onto which long chains of poly(2-methylpropenoyloxyethyl) trimethylammonium chloride are grafted. These positively charged chains form a dense layer of polyelectrolytes on the surface of the core particles ("spherical polyelectrolyte brush") that tightly binds divalent PtCl6-(2) ions. The reduction of these ions within the brush layer leads to nearly monodisperse nanoparticles of metallic platinum. The average size of the particles is approximately 2 nm. The composite particles exhibit excellent colloidal stability. The catalytic activity is investigated by photometrically monitoring the reduction of p-nitrophenol by an excess of NaBH4 in the presence of the nanoparticles. The kinetic data could be explained by the assumption of a pseudo-first-order reaction with regard to p-nitrophenol. In all cases, a delay time t0 has been observed, after which the reactions start. This time is shorter when the catalyst has already been used. All data demonstrate that spherical polyelectrolyte brushes present an ideal carrier system for metallic nanoparticles.
We present a new system that allows us to modulate the catalytic activity of metal nanoparticles (Ag) by a thermodynamic transition that takes place within the carrier system. Thermosensitive core-shell particles have been used as the carrier system in which the core consists of poly(styrene) (PS), whereas the shell consists of a poly(N-isopropylacrylamide) (PNIPA) network cross-linked by N,N'-methylenebisacrylamide (BIS). Immersed in water, the shell of these particles is swollen. Heating the suspension above 32 degrees C leads to a volume transition within the shell that is followed by a marked shrinking of the network of the shell. The maximum degree of swelling can be adjusted by the degree of cross-linking. Silver nanoparticles with diameters ranging from 6.5 to 8.5 nm have been embedded into thermosensitive PNIPA networks with different cross-linking densities. The Ag nanoparticles do not influence the swelling and the shrinking of the network in the shell. The surface plasmon absorption band of the nanoparticles is shifted to higher wavelengths with temperature. This is traced back to the varying distance of the nanoparticles caused by the swelling and the shrinking of the shell. The catalytic activity is investigated by monitoring photometrically the reduction of 4-nitrophenol by an excess of NaBH4 in the presence of the silver nanocomposite particles. The rate constant kapp was found to be strictly proportional to the total surface of the nanoparticles in the system. Moreover, kapp is first decreasing with increasing temperature when approaching the volume transition. This is due to the strong shrinking of the network. Only at temperatures above the volume transition is the normal Arrhenius-type dependence of kapp found again. In this way, catalytic activity of the metal nanoparticles enclosed in a "nanoreactor" can be modulated by volume transition over a wide range.
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