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Development of new highly active nano gold catalysts for selective oxidation reactions

Morad, Moataz 2014. Development of new highly active nano gold catalysts for selective oxidation reactions. PhD Thesis, Cardiff University.
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Abstract

Gold catalysts have been found to be efficient for many oxidation reactions. The performance of these catalysts depends strongly on the particle size of the Au nanoparticles. However, other factors also have a strong influence on the catalytic activity such as the preparation method and choice of support. The effect of support and the preparation methods has been investigated with regard to the selectivity and the activity of Au catalysts. The disproportionation of benzyl alcohol has been identified as a source of toluene formation in the solvent free oxidation of benzyl alcohol using supported gold palladium catalysts. The disproportionation reaction of benzyl alcohol oxidation has been performed in a conventional glass stirred reactor. Oxidation and disproportionation reactions respond slightly differently to the changes in reaction parameters, such as oxygen concentration and pressure. When MgO supported gold-palladium catalysts were used for this reaction, the toluene selectivity reduced substantially at the cost of conversion. The synthesis of bimetallic nanoalloys is of great practical importance as they exhibit size-dependent functional properties, which can be exploited in fields such as catalysis. Conventional chemical impregnation routes for generating supported bimetallic nanoparticles are facile, but often generate materials having broad particle size distributions, which typically exhibit core–shell morphologies and significant compositional variations from particle-to-particle. More complex sol-immobilisation synthesis techniques offer much better control over particle size distribution, but retain stabilising ligands on the surface, which can be deleterious for catalysis. Here, a convenient excess anion modification and post reduction step prior to the impregnation method has been used, which permits the reproducible preparation of supported bimetallic AuPd nanoparticles with a tight particle size distribution comparable to that found for sol-immobilisation materials, but without the complication of ligands adsorbed on the particle surface. These advantageous features of the modified impregnation materials resulting in higher activity and stability compared to the catalysts prepared using both conventional impregnation and sol-immobilisation methods. Detailed STEM combined with EDX analyses of individual particles have revealed that an increase in anion concentration increases the gold content of individual particles in the resultant catalyst, thus providing a method for controlling/tuning the composition of the nanoalloy particles. The improved activity and stability characteristics of these new catalysts are demonstrated using: (i) the solvent-free aerobic oxidation of benzyl alcohol and (ii) the direct synthesis of hydrogen peroxide as case studies. The study has examined using this modified impregnation catalyst in different Au:Pd ratios for the solvent-free aerobic oxidation of benzyl alcohol. These modified impregnation catalysts have been found to be exceptionally active, mainly with a Aurich composition, when compared to reduced conventional impregnation catalysts. Moreover, these modified impregnation catalysts have been found to be exceptionally active and stable when evaluated for crotyl alcohol in mild conditions when compared to sol-immobilisation methods. This new preparation protocol could be beneficial for both the academic research community and the industrial community, with a very convenient and reproducible methodology for preparing supported Au-Pd nanoalloy catalysts with high activity and stability, without using any ligands or stabilisers, while not compromising their catalytic activity.

Item Type: Thesis (PhD)
Status: Unpublished
Schools: Chemistry
Subjects: Q Science > QD Chemistry
Date of First Compliant Deposit: 30 March 2016
Last Modified: 19 Mar 2016 23:40
URI: https://orca.cardiff.ac.uk/id/eprint/59979

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