Development of noble metal alloy nanoparticles and mesoporous metal oxides for applications in catalysis and charge storage
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The creation of next-generation functional materials will require fine control of nanoscale surface characteristics. Two common approaches to this problem are the formation of nanoparticles and the synthesis of mesoporous materials. By incorporating nanoscale structural features, not only can a greater proportion of the material be devoted to active surface area, but properties can also emerge that are absent in the corresponding bulk material. For example, metals can be miscible on the nanoscale despite being immiscible in the bulk, and restrictive mesopores can lead to increased catalytic selectivity. Mesoporous LaMnO₃-SiO₂ composites were synthesized by several different nanocasting routes using SBA-15 silica as the hard template. The final composites were stable in refluxing NaOH solution, indicating encapsulation of the remaining SiO₂ in each case, although the exact structure of the composites depended on the solvent mixture in which they were prepared. All three composites displayed respectable pseudocapacitative capabilities, with normalized specific capacitances over 200 F g⁻¹. The next project revolved around the synthesis of RhPd alloy nanoparticles and the examination of their hydrogenation activity. RhPd alloy nanoparticles were synthesized using both microwave and conventional heating and a range of reaction times. Application of these particles to the hydrogenation of cyclohexene revealed that particles synthesized at very short reaction times showed comparable reactivity to particles that had been heated for hours longer. In addition, the empirical finding that RhPd alloys have a hydrogenation activity between those of the two pure metals was further supported by DFT calculations. Third, RhPdAu alloy nanoparticles were synthesized for use as hydrogenation catalysts. Tuning the exact composition of this alloy system is expected to influence the catalysts’ activity, and the inclusion of gold may promote selective hydrogenation of the carbonyl bond in unsaturated aldehydes. A series of alloy compositions has been successfully synthesized, but their catalytic properties remain untested. Finally, Rh nanoparticles supported on Co₃O₄ are also being studied for this selective hydrogenation of unsaturated aldehydes. Preliminary results suggest the mesoporosity of the support may play a crucial role in controlling the orientation of the substrate molecule, and therefore the selectivity toward the desired product.