Dendrimer-encapsulated bimetallic nanoparticles: Synthesis, characterization, and applications to homogeneous and heterogeneous catalysis

We review the preparation, characterization, and properties of dendrimer-templated bimetallic nanoparticles. Polyamidoamine (PAMAM) dendrimers can be used to template and stabilize a wide variety of mono- and bimetallic nanoparticles. Depending on the specific requirements of the metal system, a variety of synthetic methodologies are available for preparing nanoparticles with diameters on the order of 1-3 nm with narrow particle size distributions. The resulting dendrimer-encapsulated nanoparticles, or DENs, have been physically characterized with electron microscopy techniques, as well as UV-visible and X-ray photoelectron spectroscopies. For certain metal systems, the chemical properties of bimetallic DENs include selective extraction from the dendrimer interior into organic solvents. Catalytic properties include homogeneous hydrogenation catalysis; heterogeneous hydrogenation and oxidation catalysis have also been examined. Homogeneous hydrogenation studies indicate that synergism in catalytic activity often occurs when two metals are intimately mixed in nanoparticles. DENs can also be deposited onto a variety of solid substrates and the organic dendrimer template thermally removed. The resulting activated nanoparticles are also active catalysts, and have been further characterized with infrared spectroscopy of adsorbed CO. Relationships between these heterogenized systems and the solution DENs are also discussed.