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1.4 CHEMICAL SCIENCES
Three supported gold nanocomposites, gold on titanium dioxide (Au/TiO2), gold on single walled Au/SWCNT and gold on carbon black (Au/P90) were prepared and systematically characterised using Scanning Electron Microscopy/Scanning Transmission Electron Microscopy/Transmission Electron Microscopy (SEM/STEM/TEM), Energy Dispersive X-Ray Analysis (EDX), X-Ray Diffraction Spectroscopy (XRD), Atomic Absorption Spectroscopy (AAS), (Nitrogen) N2 adsorption, Ultra Violet-Visible (UV-Vis) absorption spectroscopy and Raman spectroscopy to probe the nanoparticle and support physical and chemical characteristics as well as the metal-support interactions. For all composites, the Au nanoparticles were found to be crystalline with a well-defined and narrow particle-size distribution centered between 4-10nm.
EDX analysis provided the initial confirmation of the presence of gold in the composite samples and XRD provided evidence of the formation of metallic gold nanoparticles on the surface of the supports. AAS was used to determine the % gold by weight in each of the three composites. N2 adsorption analysis showed an increase in surface area of the supports both before and after deposition of gold, indicative of deposition of crystalline nanoparticles. UV-Vis absorption spectroscopy showed the existence of a gold plasmon resonance bond in the composite samples and by using the absorption maximum allowed estimation of the gold particle sizes to be determined. Raman spectroscopy showed the dampening in intensity of the characteristic bands in each sample due to the deposition of gold nanoparticles on the surface of the supports. In addition, evidence of changes in the characteristics of the single walled carbon nanotube support spectrum was seen, most specifically charge transfer which lends some insight into the behaviour of the Au/SWCNT composite. The activity and selectivity of the composites as heterogeneous catalysts for solventless aerobic selective oxidations of aromatic alcohols were examined in three reactions, firstly the aerobic oxidation of 1-phenylethanol as a probe reaction. For this reaction, both Au/TiO2 and Au/SWCNT catalysed the reaction with >95% conversion and 100% selectivity, whereas the Au/P90 resulted in a side reaction attributed primarily to the surface carbon acting as a reductant. Au/SWCNT gave the highest reaction rates. All composite materials were shown to be recyclable and the Au/P90 showed improved selectivity after the 2nd cycle of use. With these initial results, it was necessary to expand the activity and selectivity testing to more challenging reactions. For the second reaction, selective oxidation of 2-phenylethanol, all three composites catalysed the reaction to the corresponding aldehyde all with a % conversion of ~25% and 100% selectivity. However the aldehyde spontaneously trimerises to 2,4,6-tribenzyl-s-trioxane and requires an additional clean up step to distill the aldehyde. Again, the Au/SWCNT gave the fastest reaction rates. Finally, for the aerobic selective oxidation of benzylalcohol, all three composites catalysed the reaction to the corresponding aldehyde with >99% conversion. There are no other products formed which shows all three catalysts to be 100% selective. The Au/SWCNT showed superior reaction rates in this reaction also. Overall the novel composite material shows good initial promise for use as a heterogeneous catalyst in green reactions of importance in synthetic chemistry compared to the more typical industry standard catalysts based on these preliminary studies.
Shanahan, A. E. (2013).Preparation and characterisation of gold nanocomposites for use as heterogeneous catalysts in green chemistry. Doctoral thesis. Technological University Dublin. doi: 10.21427/D74G6J