Effect of Pd nanoparticle size on the catalytic hydrogenation of allyl alcohol.

We report a particle size dependence for the rate of hydrogenation of allyl alcohol using 1.3-1.9 nm Pd dendrimer-encapsulated nanoparticle (DEN) catalysts. For particles with diameters of <1.5 nm and containing <147 Pd atoms, the modulation in catalytic activity is due to the electronic properties of the particle. For the larger particles, 1.5-1.9 nm in diameter and containing an average of 147-250 Pd atoms, the size effect is a result of geometrical constraints. Specifically, the hydrogenation reaction is shown to occur preferentially on the face atoms of the larger nanoparticles.

Titania-supported PdAu bimetallic catalysts prepared from dendrimer-encapsulated nanoparticle precursors.

We report the synthesis, characterization, and catalytic activity of titania-supported bimetallic PdAu particles prepared using dendrimer-encapsulated nanoparticle (DEN) precursors. Single-particle energy-dispersive spectroscopy indicates a homogeneous distribution of bimetallic nanoparticles having compositions closely related to the metal-ion ratios used to prepare the DEN precursors. The catalytic activity of the supported PdAu catalysts was compared to that of supported Pd-only and Au-only catalysts; the enhanced CO oxidation activity of the PdAu catalysts is indicative of a synergetic bimetallic interaction.

Synthesis, characterization, and structure-selective extraction of 1-3-nm diameter AuAg dendrimer-encapsulated bimetallic nanoparticles.

The synthesis and characterization of 1-3-nm diameter, structurally well-defined, bimetallic AuAg dendrimer-encapsulated nanoparticles (DENs) are reported. Three different bimetallic structures were examined: AuAg alloys synthesized by cocomplexation and subsequent reduction of dendrimer-encapsulated Au3+ and Ag+ and core/shell [Au](Ag) and [AuAg alloy](Ag) structures (for structured materials, brackets indicate the core metal and parentheses indicate the shell metal) synthesized by a sequential loading method. Depending on the shell metal and its oxidation state, the AuAg nanoparticles can be extracted from the dendrimer into an organic phase using different surfactants. This provides a means for analyzing the composition of the shell. UV-vis, TEM, and single-particle X-ray energy dispersive spectroscopy (EDS) were used to characterize the bimetallic DENs before and after extraction and show that the extraction step does not alter the size or composition of the bimetallic nanoparticles.

Synthesis, characterization, and applications of dendrimer-encapsulated nanoparticles.

In this article we describe the synthesis, characterization, and applications of dendrimer-encapsulated nanoparticles (DENs). These materials are synthesized by a template approach in which metal ions are extracted into the interior of dendrimers and then subsequently chemically reduced to yield nearly size-monodisperse particles having dimensions of less than 3 nm. Monometallic, bimetallic (including core/shell), and semiconductor nanoparticles have been prepared by this route. The dendrimer component of these composites serves not only as a template for preparing the nanoparticle replica but also to stabilize the nanoparticle, makes it possible to tune solubility, and provides a means for immobilization of the nanoparticle on solid supports. These materials have a number of potential applications, but the focus here is on catalysis. Homogeneous catalytic reactions, including hydrogenations, Heck coupling, and Suzuki reactions, in water, organic solvents, biphasic fluorous/organic solvents, and liquid and supercritical CO2 are discussed. In many cases it is easy to recycle catalytic DENs. DENs can also be immobilized on supports, such as silica and titania, and used for heterogeneous catalysis. Bimetallic DENs are shown to have particularly interesting catalytic properties. In addition to a discussion of current progress in this field, a number of intriguing questions related to the properties and potential applications of these materials are examined.

Bimetallic palladium-gold dendrimer-encapsulated catalysts.

The synthesis, characterization, and catalytic properties of 1-3 nm-diameter bimetallic PdAu dendrimer-encapsulated catalysts are reported. Both alloy and core/shell PdAu nanoparticles were prepared. The catalytic hydrogenation of allyl alcohol was significantly enhanced in the presence of the alloy and core/shell PdAu nanoparticles as compared to mixtures of single-metal nanoparticles.