<br><h3> Chapter One </h3> <b>Hybrids from Polymer Colloids and Metallic Nanoparticles: A Novel Type of "Green" Catalyst</b> <p> YAN LU and MATTHIAS BALLAUFF Soft Matter and Functional Materials, Helmholtz-Zentrum Berlin fur Materialien und Energie GmbH, Berlin, Germany <p> <p> Metallic nanoparticles have interesting perspectives in the application of catalysis as they exhibit unusual chemical and physical properties differing from the bulk material. However, for all practical applications, metallic nanoparticles must be stabilized in solution in order to prevent aggregation. Here we have reviewed our recent studies on metallic nanoparticles encapsulated in spherical polyelectrolyte brushes (SPBs) and thermosensitive core–shell microgels, respectively. SPB particles consist of a poly(styrene) (PS) core onto which long chains of polyelectrolyte brushes are densely grafted by a grafting-from technique. In the case of thermosensitive microgels, the core consists of PS whereas the network consists of poly(<i>N</i>-isopropylacrylamide) (PNIPA) crosslinked by <i>N,N</i>'-methylenebisacrylamide (BIS). Both polymeric particles present as excellent carrier systems for applications in catalysis. More importantly, the composite systems of metallic nanoparticles and polymeric carrier particles allow us to do "green chemistry," that is, low temperature, easy removal of the catalyst and low leaching of heavy metal into the product. The chemical reactions can be conducted in a very efficient way. In addition, in the case of using microgels as the carrier system, the reactivity of composite particles can be adjusted by the volume transition within the thermosensitive networks. Hence, this chapter gives clear indications on how carrier systems for metallic nanoparticles should be designed to adjust their catalytic activity. <p> <p> <b>1.1. INTRODUCTION</b> <p> Metal nanoparticles have attracted a lot of attention, because they may exhibit unusual chemical and physical properties differing strongly from the bulk material, which are due to three major factors: high surface-to-volume ratio, quantum size effect, and electrodynamic interactions. Metallic nanoparticles have interesting perspectives in the applications as catalysts, sensors, and electronics. However, the high specific surface area of the metal nanoparticles often leads to the common tendency of agglomeration, and usually requires their immobilization in mesoscopic carriers to prevent precipitation. Using suitable polymeric systems, such as microgels, dendrimers, and block copolymer micelles as carriers or "nanoreactors," metal nanoparticles can be immobilized and handled in an easier fashion. Metallic nanoparticles immobilized in such systems can then be used for catalysis in various media. Moreover, the concept of green chemistry has become a top priority item for catalysis industry, that is, low temperature, easy removal of the catalyst and low leaching of heavy metal into the product. This requires a carrier system that should allow separation (e.g., via filtration), have long-term stability, be easy to handle, and prevent the metallic nanoparticles from coagulating. Moreover, no stabilizing agent that may alter or block the surface of the nanoparticles should be used. The carrier systems should also be sufficiently stable during recycling of the catalyst. <p> In this chapter, we review recent work on two special types of polymeric carrier systems, namely, the spherical polyelectrolyte brushes (SPBs) and thermosensitive coreshell microgels, which have been used successfully for the immobilization of metal nanoparticles. Figure 1.1a gives a schematic representation of the SPB particles: Long linear polyelectrolyte chains are grafted densely to a colloidal core particle. The term brush implies that the grafting of the chains is sufficiently dense; that is, the linear dimensions of the polyelectrolyte chains are much larger than the average distance between two neighboring chains on the surface. These positively charged polyelectrolyte chains form a dense layer on the surface of the core particles and can bind metal ions. Reduction leads to metallic nanoparticles. Figure 1.1b depicts coreshell microgels that consist of a solid core of polystyrene and a shell of crosslinked poly(<i>N</i>-isopropylacrylamide) (PNIPA). The metal ions are localized within the network because of complexation between the metal ions and the nitrogen atoms of PNIPA. Reduction of these ions leads to nearly monodisperse metallic nanoparticles that are only formed within the polymer layer. <p> The focus of this chapter is the use of both the spherical polyelectrolyte brushes and microgel particles as carrier systems for novel metal nanoparticles, which can be used for catalysis in aqueous media, that is, under very mild conditions. Thus, the composite systems of metallic nanoparticles and polymeric carrier particles allows us to do "green chemistry" and conduct chemical reactions in a very efficient way. This approach can open new possibilities for catalytic application of metal composite particles in different reactions and represents a typical example of "mesotechnology": Nanoscopic objects with catalytic properties are judiciously combined with polymeric carriers to serve for a given, well-defined purpose. <p> <p> <b>1.2. SPHERICAL POLYELECTROLYTE BRUSH BASED METALLIC NANOPARTICLES</b> <p> Often, nanoparticles are stabilized by alkyl chains attached through thiol bonds to the surface of the metal. However, the strong interaction of the thiol group with the surface of the nanoparticles may profoundly alter the catalytic properties of the metal. The same problem may occur when immobilizing nanoparticles on solid substrates. <p> Recently, we reported that the SPBs are excellent carriers for various metal nanoparticles. In particular, we demonstrated that the composites of metal nanoparticles and SPBs are very stable. This fact can be understood when considering the synthesis of the composites in detail: Figure 1.2 shows the synthesis of Au nanoparticles (NPs) on a cationic SPB. The AuCl<sub>4</sub><sup>-</sup> ions are introduced as counterions of the brush layer and all metal ions not firmly bound in this layer are flushed away by ultrafiltration. Hence, only the reduction of these immobilized AuCl<sub>4</sub><sup>-</sup> ions will lead towell-defined Au NPs. All stages of nanoparticle formation within the brush layer can be followed easily by dynamic light scattering (DLS), which determines the hydrodynamic radius (<i>R</i><sub>H</sub>) of the particles. Since the radius <i>R</i> of the core particles is exactly known, the thickness <i>L</i> of the surface layer can be obtained by <i>L = R</i><sub>H</sub> - <i>R</i> throughout all stages of the synthesis of the particles. We found that even low concentrations of AuCl<sub>4</sub><sup>-</sup> ions lead to a considerable shrinking of the polyelectrolyte layer on the surface of the core particles from 71 to 59 nm. This shrinking of the surface layer is due to partial crosslinking of the polyelectrolyte chains by the AuCl<sub>4</sub> ions. <p> The AuCl<sub>4</sub><sup>-</sup> ions are partially complexed by the polyelectrolyte chains. In this way the AuCl<sub>4</sub><sup>-</sup> ions create a densely crosslinked mesh of polyelectrolyte chains. Thus, the local concentration of AuCl<sub>4</sub><sup>-</sup> ions is therefore enlarged considerably. In the next step the reducing agent NaBH<sub>4</sub> is added, which results in a collapse of the surface layer to a thickness of 21 nm. The micrographs of the resulting composite particles obtained by cryo-TEM are shown in Figure 1.3. It demonstrates that small Au NPs have been formed in this step. <p> A possible reason for the decrease of the hydrodynamic radius may be sought in a degradation of the polyelectrolyte layers by cleavage or other side reactions. However, this explanation can easily be refuted by dissolution of the Au NPs upon addition of NaCN in the presence of O<sub>2</sub>. This process leads to <i>L</i> = 69 nm, which is identical to the starting value of 71 nm within the limits of error. Hence, the polyelectrolyte chains of the brush layer are condensed by the Au NPs as shown schematically in Figure 1.2. After the dissolution of the Au NPs the chains stretch again and assume their previous conformation. Hence, the Au NPs formed by reduction within the brush layer lead to an additional crosslinking of the polyelectrolyte chains that extend far beyond the crosslinking effect of the AuCl<sub>4</sub><sup>-</sup> ions. This attractive interaction could be related to the negative charge of the Au NPs. Thus, the Au NPs crosslink the polyelectrolyte chains by ionic interaction. <p> The advantages of generating metallic nanoparticles using this method are obvious: Because of the confinement of the counterions, nanoparticles are only generated within the polyelectrolyte layer. Stabilization of the nanoparticles against aggregation is effected by the colloidal carrier particles. Because the metal nano-particles carry no group stabilizing their surface, they exhibit a high catalytic activity. Thus, these systems can be used for catalysis of various chemical reactions that proceed in aqueous solution or in two-phase systems. In the following we review the main applications established so far. <p> <p> <b>1.2.1. SPB Based Au, Pt, and Pd Nanoparticles for Catalysis in Two-Phase Systems</b> <p> In 1912, Paul Sabatier received the Nobel Prize for chemistry for his investigations into the use of finely divided metals in hydrogenation reactions. Since then, heterogeneous catalysis in organic chemistry has been developed extensively. During our research, we examined the hydrogenation of butyraldehyde to 1-butanol catalyzed by platinum nanoparticles supported on SPB particles. All reactions were carried out in aqueous solution at 40 °C and 70 bar hydrogen pressure. Product extraction was accomplished using a second (organic) liquid phase. The reactant and product concentrations (butyraldehyde and 1-butanol) were monitored by gas chromatography (GC). Excellent recyclability was observed concerning the catalytic performance and the product extraction, as shown in Figure 1.4. The catalyst system was used for 10 reactions, and the products extracted with ether after each reaction. It was found that the efficiency of the catalyst remains unaltered in nine consecutive reactions. The catalyst was stable against aggregation during the reactions and the workup procedure. Moreover, our previous work showed that platinum and palladium nanoparticles prepared by the same method can be used as catalysts for the degradation of 4-nitrophenol using NaBH<sub>4<'sub>. It is interesting to note that palladium was more effective than platinum, but their catalytic activities were in the same magnitude when normalized to the surface of metal particles. However, the hydrogenation activities of palladium nanoparticles were orders of magnitude smaller than those observed for the platinum system. <p> <p> <b>1.2.2. Pd-Nanoparticles Immobilized on SPBs for the Heck and Suzuki Coupling</b> <p> The palladium-catalyzed Heck and Suzuki reactions (cobalt nanospheres are also able to catalyze these reactions) between aryl halides and alkenes or boronic acids are well-established tools for C=C bond formation in organic synthesis, respectively. Such reactions are traditionally catalyzed using many different kinds of phosphine-based palladium catalysts and phosphine-free palladium catalysts such as Pd(PPh<sub>3</sub>)<sub>4</sub>, Pd(Oac)<sub>2</sub>, [(n<sup>3</sup>- C<sub>3</sub>H<sub>5</sub>)PdCl]<sub>2</sub>, and Pd<sub>2</sub>(dba)<sub>3</sub>C<sub>6</sub>H<sub>6</sub>. Recently, there have been many reports on the use of palladium nanoparticles as catalysts. A convenient route for cross-coupling reactions involves reusable palladium nanoparticles that promote these reactions in organic solvents or in water. However, the handling of the nanoparticles may impose problems during workup unless the particles are immobilized on suitable carriers. Recently, we reported that Pd nanoparticles immobilized in cationic spherical polyelectrolyte brushes (SPBs) present a composite system that can be used as an efficient catalyst for the Heck and Suzuki coupling reactions, as shown in Figure 1.5. Figure 1.6 shows the typical test reactions for the Suzuki- and Heck-type cross-coupling reactions using palladium nanoparticles as catalyst. We demonstrate that both reactions can be carried out under mild conditions and low temperatures (Suzuki reaction: 50 °C; Heck reaction: 70 °C). Pd loadings of 0.09 mol% (Suzuki) and 0.029 mol% (Heck) were used. For the Suzuki reaction the boronic acid gave rise to homocoupling products in 14% yield under the above-mentioned mild reaction conditions. Heterocoupling was observed for bromides (conversions of 80–90%) and iodides (around 70%). However, chlorides resulted in low yields (ca. 6%). Additionaly, substituents in the ortho and meta positions resulted in lower yields (due to steric hindrance) than para-substituted arenes. Selected results are listed in Table 1.1. <p> The Heck-type reaction (see Figure 1.6 and Table 1.2) using palladium nanoparticles as a catalyst was investigated using eight different aryl halides. With a catalyst loading of 0.029 mol% Pd a variety of aryl iodides showed almost complete conversion, whereas aryl bromides were unreactive under the conditions employed. Under such mild conditions in water it is not possible to expand the scope of the Heck reaction to bromides or chlorides. <p> The reproducibility was found to be very good for palladium nanoparticles as catalyst of both Suzuki- and Heck-type reactions. In four runs the products were removed by ether and new starting materials were added to the water phase. We found that Pd@SPB could be used repeatedly without loss of activity. After these four cycles the nanoparticles were filtered off and investigated by cryo-TEM in order to detect possible changes in the number and morphology of the nanoparticles. We found that the nanoparticles are still embedded in the SPB support. Hence, the present Pd@SPB catalyst system could easily be recycled and reused. The excellent reproducibility can be explained by the robustness of the catalyst system during catalysis and workup. <p> <p> <b>1.2.3. SPB-Based Nanoalloys of Noble Metals</b> <p> Recently, we developed a method for the immobilization of Au–Pt nanoparticles into spherical polyelectrolyte brushes. Figure 1.7 a shows the method of synthesis employed here: First, AuCl<sub>4</sub><sup>-</sup> ions are immobilized as counterions within the surface layer of cationic polyelectrolyte chains. Because we know the total number of charges on the surface of the core particles, the number of AuCl<sub>4</sub><sup>-</sup> ions can be adjusted precisely in order to replace only a certain fraction of the Cl<sup>-</sup> counterions. After ultrafiltration, PtCl<sub>6</sub><sup>2-</sup> ions are introduced in the same manner. Any excess of metal ions is flushed away by ultrafiltration. Finally, reduction by NaBH4 leads to Au–Pt alloy nanoparticles of a given composition. Figure 1.7b displays the HR-TEM of Au<sub>73</sub>Pt<sub>27</sub>. The crystal lattice is observed throughout the entire particle. Moreover, electron diffraction (inset of Figure 1.7b) demonstrates the crystalline state of the nanoparticles. Wide-angle X-ray scattering (WAXS) studies demonstrated that these alloys present solid solutions; that is, the particles consist of a random mixture of both types of metal atoms. Therefore, the lattice constants measured for the alloys vary continuously between the values found for the pure metals (Vegard-type). <p> The catalytic oxidation of alcohols to aldehydes has been applied to investigate the catalytic activity of these Au–Pt alloy particles. All reactions were carried out at room temperature using aerobic conditions. Notably, no phase transfer catalyst is needed for this reaction and the reaction conditions are very mild. GC revealed that no byproduct is obtained under these conditions. We find full conversion within the limits of error. Hence, water is the only product formed by this reaction (besides the aldehyde or ketone). We find that systems containing pure Au nanoparticles are much less stable than the alloy particles. Substrates containing phenyl groups lead to considerable leaching of gold and even coagulation. This may result from the strong interaction of Au with the phenyl groups, as determined by Miyamura et al. However, alloy nanoparticles turned out to be fully stable under the same conditions. This finding is corroborated by an analysis of the composite particles before and after catalysis by cryogenic TEM. No change or leaching of the nanoparticles is observed. Moreover, repeated use of the composite particles as catalysts did not lead to a noticeable decrease of catalytic activity. Hence, spherical polyelectrolyte brushes present a system that allows us to generate and to utilize alloy nanoparticles that exhibit properties widely differing from the properties of the respective bulk alloys. <p> <i>(Continues...)</i> <p> <p> <!-- copyright notice --> <br></pre> <blockquote><hr noshade size='1'><font size='-2'> Excerpted from <b>Hybrid Nanomaterials</b> by <b>Bhanu P. S. Chauhan</b> Copyright © 2011 by John Wiley & Sons, Inc.. Excerpted by permission of John Wiley & Sons. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.<br>Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.