Synthesis, characterization and catalytic application of dendrimer-stabilized metal nanoparticles

Several research groups, including our own, use poly(amidoamine) (or PAMAM) dendrimers to synthesize noble metal nanoparticles in aqueous solutions. The concept is straightforward: we are pursuing "atom-up" design and fabrication of dendrimer-templated nanoparticles in solution, and we are seeking ways to self-assemble and immobilize them on supports to create new, nanostructured, heterogeneous catalytic materials. Our efforts build on previous work that demonstrates the utility of poly(amido amine) dendrimers (PAMAM) for templated synthesis of sub-two nm metal nanoparticles, as well as the activity of the resulting nanoparticles as homogeneous catalysts. Building on this foundation, we are exploring new possibilities for making heterogeneous catalysts with unprecedented control of the structure, composition, and location of the active sites on the support.;The first part of this work aims for a fundamental understanding of how PAMAM dendrimers function in controlling platinum nanoparticle size. We have carried out careful studies of Pt-PAMAM complexation and the nanoparticles produced by treatment with reducing agents. With respect to complexation, we have used UV-visible spectrophotometry, flame atomic absorption spectroscopy, and ion chromatography to elucidate the details of Pt complexation with PAMAM. The results show substantial amounts of non-complexed Pt present in the complex solution due to the slow, incomplete Pt ligand exchange reaction with PAMAM.;The second part of the dissertation focuses on direct deposition of mono- or bimetallic PAMAM complexes onto solid supports to prepare highly dispersed heterogeneous catalysts. Pt-PAMAM complexes can be used as precursors to prepare sol-gel Pt catalysts because PAMAM protects Pt against reduction during sol-gel synthesis. The resulting catalysts have small (1-3 nm) particle size with uniform distribution as well as high thermal stability. Bimetallic Pt-Ru-PAMAM complexes with various Pt:Ru ratios have been employed as novel precursors to produce Pt-Ru/C electrocatalysts. The size, composition and structure of supported particles were probed by TEM, XPS and XRD. The electrocatalytic activities of these Pt-Ru/C catalysts were evaluated by using the methanol oxidation reaction. The results shows the Pt-RU/C catalyst with Pt:Ru = 1 (molar ratio) treated at 673K shows the best electrochemical performance.