Plasmonic nano-photocatalysts for light-induced alcohol oxidation
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Photocatalytic oxidation of alcohols in the presence of plasmonic metals using oxygen as oxidizing agent is considered as a promising approach for efficient and green chemical transformations under mild conditions. In this direction, this Thesis demonstrates the synthesis of commercially valuable products such as benzaldehyde via the development of plasmonic-based nanomaterials as photocatalysts, along with design of continuous flow reactor, enabling high reaction rate and high selectivity of formed carbonyl compounds. Initially, the current state of photocatalytic oxidation of alcohols is summarized, the importance and fundamentals of plasmonic metal nanoparticles is presented while challenges and research gaps of the field are highlighted. Three experimental strategies were adopted in the Thesis. The first experimental part of the thesis focused on understanding the plasmonic heating effect generated by Au nanoparticles and particularly the surface plasmon resonance of Au nanoparticles by performing a well controlled experiment using a continuous flow system. The results revealed a significant temperature rise of Au-based nanofluids, with different Au loadings, compared to bare TiO2 nanofluid or pure water, which arise from the localized surface plasmon resonance effect of Au nanoparticles. The second experimental part of the thesis aimed to investigate the performance of plasmonic Au nanoparticles decorated on Cr2O3 microspheres towards the photocatalytic oxidation of benzyl alcohol. It has been shown that the amount of Au loading affected significantly the reaction efficiency, with 1.18 wt.% of Au loaded photocatalyst converting 81.4% of benzyl alcohol to benzaldehyde with a selectivity of 98.3% after 3 hours of laser irradiation. Additionally the plasmonic heating effect of Au nanoparticles contributed to a 26% oxidation rate enhancement using 1.18 wt.% Au. In the third experimental part, the effect of incorporating a second noble metal like Ag as well as the fabrication of a continuous flow reactor for comparison with a typical batch reactor were demonstrated. Bimetallic 0.34Ag-1.21Au/Cr2O3 photocatalyst converted 92.4% of benzyl alcohol to benzaldehyde with 98.8 % selectivity, which is 4.5 times that of pure Cr2O3 and 1.3 times of monometallic 1.18Au/Cr2O3. The results showed that the reaction rate under continuous flow conditions was almost an order of magnitude superior to the values achieved using batch reactor. Therefore, the findings of this Thesis highlight the need for development and optimization of continuous flow synthesis of carbonyl compounds in the presence of plasmonic metals, which can favour the conversion of alcohols in terms of reaction rate and selectivity.