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41602-AEF
Development of Nanoparticle Filaments, Sponge & Treelike Aggregates for Hydrogen Storage Applications
Our work dealt with the vapor phase synthesis of different classes of nanostructured materials. The vapor phase synthesis of nanoparticles involves the generation of the vapor of the material of interest, followed by the condensation of clusters and nanoparticles from the vapor phase. Specifically, we have reported on the synthesis of Ge, Mg, Pd, and Pt nanowires can be controlled by adjusting the experimental parameters of the VLS process. Magnesium nanowires may find important applications for hydrogen storage. However, neither nanoparticles nor nanobelts displayed hydrogen storage capabilities exceeding those of their bulk counterpart. Thus we have turned our attention to CO oxidation. Based on this, we have concluded the following: (1) Formation of alloys: intermetallic PdX and PtX (where X = Co, Fe and Ni) can be synthesized in the vapor phase via the LVCC method from their bulk powder counterparts, the intermetallic nanoparticles prepared in the vapor phase from the elemental powder mixtures display similar characteristics to nanoparticles formed from ablation of intermetallic bulk targets, (2) the incorporation of Au and Pd nanoparticles within the metal oxide catalysts improves both the activity and stability of these catalysts in the CO oxidation process, (3) the activity of the unsupported bimetallic nanoparticles can be tuned to the desired performance depending on the composition of the catalyst prepared by the LVCC method.
We then developed a method for the synthesis of supported nanoparticle catalysts via microwave irradiation. This method offers extremely short reaction times and produces high purity and high yield of efficient nanoparticle catalysts. Synthesis and characterization of Au and Pd nanoparticle catalysts supported on CeO2, CuO and ZnO nanoparticles for CO oxidation. The results indicate that supported Au/CeO2 catalysts exhibit excellent activity for low temperature CO oxidation. The Pd/CeO2 catalyst shows a uniform dispersion of Pd nanoparticles with a narrow size distribution within the ceria support. A remarkable enhancement of the catalytic activity is observed and directly correlated with the change in the morphology of the supported catalyst and the efficient dispersion of the active metal on the support achieved by using capping agents during the microwave synthesis. The significance of the current method lies mainly in its simplicity, flexibility and the control of the different factors that determine the activity of the nanoparticle catalysts.
Using our previous skills to synthesize nanostructures we synthesized and characterized supported (Au, Pd) and unsupported MgO nanocubes and ZnO nanobelts prepared via thermal evaporation. We have also utilized laser ablation in conjunction with thermal evaporation to generate MgO nanobelts. The results indicate that the Pd supported nanocube and nanobelt catalysts display excellent activity for CO oxidation. We have also extended this study to show the catlaysis can further be improved by the addition of metal loaded ceria to the surface. Thus, the high activity and stability of the nanocubes and nanobelts prepared via thermal evaporation imply that a variety of efficient catalysts can be synthesized using this approach.
Finally, we have demonstrated the synthesis of transition metal-doped TiO2 nanoparticles also using microwave irradiation. The XRD data of the Co-and Fe-doped TiO2 show the formation of the anatase phase without any indication of the presence of metallic dopants. While it is generally accepted that XRD does not detect small quantities (<2%), we believe that the absence of a blocking temperature and the small domain size calculated for the Co and Fe doped samples, even at 10% loading, provides strong evidence that the metals are well dispersed within the titania lattice. We have shown that the as-prepared samples are paramagnetic, hydrogenating the samples transform the samples to ferromagnetic, and that this phenomenon is reversible. Theoretically, it has been suggested that oxygen vacancies are essential to provide the exchange coupling between the doped ions leading to RTFM; thus, the hydrogenation experiments are likely to extract oxygen from the titania lattice producing oxygen vacancies which may account for the reversibility of the RTFM.
Due to the success of the CO oxidation studies, I am plan on submitting a Type G Starter Grant entitled “Catalysis on Supported Nanostructures”
