DOI: 10.1002/adma.200702421 Stable Bimetallic Gold–Platinum Nanoparticles Immobilized on Spherical Polyelectrolyte Brushes: Synthesis, Characterization, and Application for the Oxidation of Alcohols** By Marc Schrinner, Sebastian Proch, Yu Mei, Rhett Kempe, * Nobuyoshi Miyajima, and Matthias Ballauff* Metallic nanoparticles have recently received much atten- tion as catalysts. In particular, gold nanoparticles have been shown to be excellent catalysts for oxidation reactions. [1–5] Thus, in 1989 Haruta reported the low-temperature oxidation of CO by molecular oxygen using gold nanoclusters. [3] Catalysis under very mild conditions at room temperature is of particular interest. Recently, Miyamura et al. demonstrated that the aerobic oxidation of alcohols can be performed at room temperature by using gold nanoclusters stabilized by a polymer. [6] Nanometer-scale alloy particles composed of gold and platinum metals may be prepared that exhibit improved catalytic activities as compared to the pure Au nanoparti- cles. [5,7–10] In particular, Enache et al. were able to show that Au–Pd nanocrystals present excellent catalysts for the oxidation of primary alcohols. [11] Bimetallic Au–Pt nanopar- ticles (Au–Pt NPs) have been used recently in electrocata- lysis. [12,13] Here, the question arises whether the alloy exhibits a miscibility gap, as in the bulk phase, or whether a homo- geneous solid solution can be achieved. [14–19] Lou et al. showed that the lattice parameter of alloy nanoparticles scales linearly with the relative Au–Pt content in the composition. [14] On the other hand, De and Rao reported the formation of core/shell nanoparticles of Au and Pt at elevated temperatures. [16] Recent theoretical work suggests that the catalytic properties of Au–Pt-NPs are superior to those containing Pt or Au alone. [19] Hence, Au–Pt NPs should be highly suitable for oxidation reactions under very mild conditions as, for example, in aqueous solutions at room temperature. A central problem of the usage of catalytic nanoparticles is their stabilization against coagulation as well as their handling. Often, nanoparticles are stabilized by alkyl chains attached through thiol bonds to the surface of the metal. [5,20,21] However, the strong interaction of the thiol group with the surface of the nanoparticles may alter the catalytic properties of the metal profoundly. Also, filtering off nanoparticles after use in catalysis may be difficult. Leaching of heavy metal or even dissolution may be another problem when using metal nanoparticles as catalyst. Recently, we have found that spherical polyelectrolyte brushes [22] (SPBs) present ideal carrier systems for nanopar- ticles. [23–25] Figure 1 shows these colloidal particles in a schematic manner: Long polyelectrolyte chains are attached to colloidal core particles (diameter: ca. 100 nm) made of polystyrene. Most of the counterions balancing the charge of the polyelectrolyte chain are confined [26] within the surface layer when the particles are immersed in water. Hence, metal ions such as, for example, [AuCl 4 ]  can be immobilized within the polyelectrolyte layer. Reduction by NaBH 4 under suitable conditions then leads to the formation of metal nanoparti- cles [23–25] localized on the surface of the colloidal carrier particles. Here we use the spherical polyelectrolyte brushes for the generation of stable homogeneous nanoparticles composed of a Au–Pt alloy and analyze their catalytic activity. Figure 1 shows the method of synthesis employed here: First, [AuCl 4 ]  -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 4 ]  ions can be adjusted precisely in order to replace only a certain fraction of the Cl  -counterions. After ultrafiltration, [PtCl 6 ] 2 ions are introduced in the same manner. A possible excess of metal ions is flushed away by ultrafiltration. Finally, reduction by NaBH 4 leads to nanoparticles of the Au–Pt alloy of a given composition. The advantages of this way of generating metallic nano- particles are obvious: Because of the confinement of the COMMUNICATION [*] Prof. R. Kempe, S. Proch Inorganic Chemistry II University of Bayreuth 95440 Bayreuth (Germany) E-mail: kempe@uni-bayreuth.de Prof. M. Ballauff, M. Schrinner, Y. Mei Physical Chemistry I University of Bayreuth 95440 Bayreuth (Germany) E-mail: matthias.ballauff@uni-bayreuth.de Dr. N. Miyajima Bayerisches Geoinstitut University of Bayreuth 95440 Bayreuth (Germany) [**] We acknowledge financial support by the Deutsche Forschungsgemeinschaft, SFB 481, Bayreuth, and by BASF AG. This work was also supported by NanoCat, an International Graduate Program within the Elitenetzwerk Bayern. Supporting Information is available online from Wiley InterScience or from the authors. 1928 ß 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2008, 20, 1928–1933