Dendrimer Templated Nanoparticle Catalysts

Evaluating dendrimer templated nanoparticles in the absence of the dendrimer provides opportunities for insights into these new materials. In order to pursue these investigations, it is first necessary to immobilize DENs onto an appropriate substrate and to gently remove the dendrimer shell see Scheme 5. Opportunities for controlling nanoparticle size and composition make DENs potentially important precursors for heterogeneous catalysts and electrocatalysts, and DEN deposition and thermolysis are similarly critically important steps in pursuing these applications [45]. 

Immobilizing DENs within a sol-gel matrix is another potential method for preparing new supported catalysts. PAMAM and PPI dendrimers can be added to sol-gel preparations of silicas " and zinc arsenates to template mesopores. In one early report, the dendrimer bound Cu + ions were added to sol-gel silica and calcined to yield supported copper oxide nanoparticles. Sol-gel chemistry can also be used to prepare titania supported Pd, Au, and Pd-Au nanoparticle catalysts. Aqueous solutions of Pd and Au DENs were added to titanium isopropoxide to coprecipitate the DENs with Ti02. Activation at 500°C resulted in particles approximately 4 nm in diameter. In this preparation, the PAMAM dendrimers served two roles, templating both nanoparticles and the pores of the titania support. 

Section II is about the new structure and understanding of nanocatalysts. Chapters 4 and 5 provide insight for understanding the structure and reactivity of gold catalyst. Chapters 6 and 7 disclose new methods for making nanoparticle catalysts in a control way by using the sol-gel technique and dendrimer template, respectively. Chapter 8 reviews the synthesis, structure, and applications of tungsten oxide nanorods. 

Fig. 3. Schematic illustration of the synthesis of metal nanoparticles within dendrimer templates. The composites are prepared by mixing of the dendrimer and metal ion, and subsequent chemical reduction. These materials can be immobilized on electrode surfaces where they serve as electrocatalysts or dissolved in essentially any solvent (after appropriate end-group functionalization) as homogeneous catalysts for hydrogenation and other reactions...

We are developing a new method for preparing heterogeneous catalysts utilizing polyamidoamine (PAMAM) dendrimers to template metal nanoparticles. (1) In this study, generation 4 PAMAM dendrimers were used to template Pt or Au Dendrimer Encapsulated Nanoparticles (DENs) in solution. For Au nanoparticles prepared by this route, particle sizes and distributions are particularly small and narrow, with average sizes of 1.3 + 0.3 nm.(2) For Pt DENs, particle sizes were around 2 nm.(3) The DENs were deposited onto silica and Degussa P-25 titania, and conditions for dendrimer removal were examined. 

A complementary study evaluated composition effects on dendrimer-templated PtCu nanoparticles [23]. Although Cu-CO bands were not observed1, a similar red shift in the Pt-CO stretching frequency to the PtAu system was observed, indicating the presence of well-mixed bimetallic nanoparticles throughout the composition range. Infrared spectroscopy of CO adsorbed on both the PtAu and PtCu catalysts showed that the shifts in the CO stretching frequency upon Cu or Au incorporation were small relative... 

Heterogeneous catalysis also directly probes the surface properties of supported nanoparticles, and has been employed for dendrimer templated PtAu [37], PdAu [79], and PtCu [36] nanoparticles. Figure 4.17 shows CO oxidation for Pti Aui / Si02 catalysts compared with monometallic catalysts. Similar to the homogeneous catalysis studies, all three metal systems show synergism in catalytic activity CO oxidation catalysis, with the bimetalHc catalysts being more active than any of the corresponding monometallic catalysts. 

As prepared and purified G4-OH(Pt5o) DENs solutions (ca. 20 /xM) can also be used to prepare high surface area sol-gel Pt/silica catalysts. This process can be used to carefully control metal loadings without condensing the solution provided the loadings remain low (less than 0.2 % Pt). The low concentrations do not allow the dendrimer to serve as a porosity template. Eurther, low loadings combined with delayed introduction of the DENs solution to a preformed sol likely lead to the occlusion of metal nanoparticles within the oxide matrix. ... 

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