The effects of particle composition on heterogeneous catalysis were studied using dendrimer-encapsulated nanoparticles (DENs) as precursors to supported Pt-Cu catalysts. Bimetallic Pt-Cu DENs with varying Pt/Cu ratios were prepared in an anaerobic aqueous solution and deposited onto a high-purity commercial alumina support. The dendrimer template was then thermally removed to yield supported nanoparticle catalysts, which were studied with toluene hydrogenation and CO oxidation catalysis as well as infrared spectroscopy of adsorbed CO. Incorporating Cu into Pt nanoparticles had opposite effects on the two test reactions. Cu acted as a mild promoter for CO oxidation catalysis, and the promoting effect was independent of the amount of Cu present. Conversely, Cu acted as a strong poison for toluene hydrogenation catalysis, and the normalized rate tracked inversely with Cu content. Infrared spectroscopy of the supported nanoparticles indicated that electronic effects (electron donation from Cu to Pt) were minimal for these materials. Consequently, the catalysis results are interpreted in terms of potential structural differences as a function of Cu incorporation and reaction conditions. IntroductionHighly dispersed supported metal nanoparticle catalysts are an important class of industrial materials, as one-third of material U. S. gross national product involves a catalytic process somewhere in the production chain. 1 State of the art heterogeneous catalysts often contain dopant metals, which are incorporated to promote a desired reaction, prevent undesirable side reactions, or enhance catalyst longevity. 2,3 Traditional preparative routes involve impregnating metal salts onto high-surfacearea supports, followed by various thermal activation steps. 3 Most important industrial supports can also be employed as separation media; consequently, chromatographic separation of salt precursors as they pass through the support pore structure is unavoidable. Further, nanoparticle preparation via traditional routes depends on the surface (and even gas phase) mobility of the species present during the thermal treatments. 4,5 These processes vary widely for different metals and are poorly understood, at best. 4,5 Consequently, bimetallic materials prepared by such methods are often complicated, have significant compositional inhomogenaity from one particle to the next, and are difficult to characterize.Recent advances in computational methods are beginning to shed light onto the fundamental properties of bimetallic catalyst systems, and predictive models are beginning to emerge. [6][7][8][9] The potential to controllably tune catalyst properties through the nanoscale design of bimetallic nanoparticles is becoming apparent, and designed bimetallic catalysts with novel behavior for important reactions already appear in the recent literature. [10][11][12] However, the utilization of traditional preparative techniques and the complicated materials that arise from these methods remain a significant hurdle to studying new catalysts, eva...

Using PAMAM dendrimers as nanoparticle templates, a synthetic route to prepare 3 nm magnetic NiAu nanoparticles was developed. Aqueous solutions of hydroxyl-terminated generation 5 PAMAM dendrimers in 25 mM NaClO 4 were shown to bind aqueous Ni II . Coreduction of Ni II and Au III salts yielded bimetallic dendrimer stabilized nanoparticles, which were extracted into toluene with decanethiol. Characterization of the resulting monolayer protected clusters (MPCs) with transmission electron microscopy and UV-visible, atomic absorption, and X-ray photoelectron spectroscopies suggested that the MPCs had substantial surface enrichment in Au. Superconducting quantum interference device (SQUID) measurements at 5 K show the bimetallic MPCs to have low coercivity and saturation magnetization relative to bulk Ni. Solution nuclear magnetic resonance (NMR) studies using the Evans method showed the bimetallic nanoparticles retain magnetic properties at ambient temperatures.

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