Dendrimer-Encapsulated Bimetallic Nanoparticles

In 1998 we reported a strategy for preparing metal nanoparticles within the interior of poly(amidoamine) (PAMAM) dendrimers (J. Am. Chem. Soc. 1998, 120, 4877). A typical synthetic procedure involves first sequestering metal ions, such as Cu, Pd, or Pt, within the dendrimer, followed by chemical reduction of the metal ions to give near-monodisperse metal dendrimer-encapsulated nanoparticles (DENs). Pd and Pt DENs were used as catalysts for hydrogenation, carbon-carbon coupling, and oxygen reduction reactions (Acc. Chem. Res. 2001, 34, 181). DENs are interesting catalysts, because the dendrimer provides a steric template for nanoparticle growth without passivating the surface, and the dendrimer periphery can be modified to tune catalyst solubility.

Recently, research group members Dr. Robert Scott and Orla Wilson have examined the synthesis, characterization, and catalytic activity of bimetallic DENs. These materials are prepared by co-complexation of different ratios of two different metal salts to the interior tertiary amines of PAMAM dendrimers followed by chemical reduction, or alternatively by deposition of a second metal onto metallic DEN seeds. This work is important, because it provides a relatively simple route to nearly size-monodisperse bimetallic metal catalysts in the interesting size range of < 3 nm. In addition, dendrimer templates offer the possibility for preparation of bimetallic and multimetallic nanoparticles having well-controlled architectures (e.g., bimetallic core/shell materials) and tunable catalytic properties.

Scheme 1 shows the general synthetic route for preparing Pd-Pt DENs via the co-complexation method (J. Am. Chem. Soc. 2003, 125, 3708). Pd and Pt salts are loaded into hydroxyl-terminated PAMAM dendrimers, followed by chemical reduction with BH₄^-. Figure 1a shows a high-resolution transmission electron micrograph (HRTEM) image of G4-OH(Pd₃₀Pt₁₀) DENs.  The data indicate that the particles have an average size of 1.9 ± 0.4 nm. Single-particle energy dispersive spectroscopy (EDS) measurements indicate that the individual nanoparticles have elemental compositions that approximate the original molar ratios of metal salts used to prepare them.  For example, EDS analysis of the entire area in Figure1b indicates a molar elemental composition of 74% Pd and 26% Au, while analysis of two single particles (shown in the inset) gave compositions of 73% Pd, 27% Au and 63% Pd, 37% Au, respectively. These Pd-Pt DENs exhibited enhanced catalytic activity towards the hydrogenation of allyl alcohol compared to physical mixtures of monometallic Pd and Pt DENs, as shown in Figure 2. The enhanced turnover frequencies at high Pd compositions is characteristic of a synergistic electronic ligand effect, in which the more electronegative Pt atoms withdraw electron density from the Pd atoms thereby increasing their hydrogenation activity.

We have also observed enhanced catalytic activities for Pd-Au DENs prepared by co-complexation of Pd and Au salts within PAMAM dendrimers. In addition, core-shell nanoparticles have been prepared by catalytic reduction of a Pd²⁺ complex onto Au DEN seeds (and vice versa) using mild reducing agents. Further work being pursued involves the formation of heterogeneous catalysts either by impregnation of DENs onto solid catalyst supports, and by embedding DENs in sol-gel networks followed by calcination. We are also exploring the feasibility of extracting bimetallic DENs from within dendrimer using surfactants (see Fall 2003 Crooks Group Research Highlight in the archives section of this webpage).