Supported Dendrimer Templated Nanoparticles

Infrared Spectroscopy of Supported Bimetallic Dendrimer Templated Nanoparticles... 

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. 

Infrared spectroscopy of adsorbed CO is a useful characterization tool for dendrimer-templated supported nanoparticles, because it directly probes particle surface features. In these experiments, which are performed in a standard infrared spectrometer using an in-situ transmission or DRIFTS cell, a sample of supported DENs is first treated to remove the organic dendrimer. Samples are often reduced under H2 at elevated temperature, flushed with He, and cooled to room temperature. Dosing with CO followed by flushing to remove the gas-phase CO allows for the spectrum of surface-bound CO to be collected and evaluated. Because adsorbed CO stretching frequencies are sensitive to surface geometric and electronic effects, it is potentially possible to evaluate the relative effects of each on nanoparticle properties. 

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. 

H. Lang, S. Maldonado, K.J. Stevenson, B.D. Chandler, Synthesis and Characterization of Dendrimer-Templated Supported Bimetallic Pt-Au Nanoparticles, Journal of the American Chemical Society 126, 12949, 2004. 

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