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Towards the continuous production of supported metal nanoparticles and their application in hydrogenation reactions

Cattaneo, Stefano 2018. Towards the continuous production of supported metal nanoparticles and their application in hydrogenation reactions. PhD Thesis, Cardiff University.
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The research carried out in this thesis describes the synthesis and study of mono- and bi-metallic Au-Pd catalysts and their application in catalytic reactions, namely the nitrophenol reduction reaction and the selective hydrogenation of cinnamaldehyde. In the first part, an innovative setup was assembled for the production of supported monometallic Au and Pd and bimetallic AuPd nanoparticles in a continuous fashion. The millifluidic reactor was firstly optimised for the production of Au colloid having a smaller mean particle size and size distribution compared to the batch benchmark method. During this procedure, several operational parameters, such as reaction conditions, solution residence time and mixing, were studied. A stream of a suspended solution of the support, namely TiO2, was then integrated in the metal colloid stream in order to produce the whole supported metal nanoparticle catalyst in continuous mode. Moreover, different monometallic Au and Pd and bimetallic AuPd based catalysts were synthesised to demonstrate the applicability of the apparatus for the synthesis of various supported metal nanoparticles catalysts. Interestingly, no size-dependent alloy composition was observed during the synthesis of bimetallic AuPd catalysts in continuous, most likely due to a very fast deposition of the nanoparticles on the support surface that inhibits the Oswald ripening process responsible for metal rearrangements over time. The continuous prepared catalysts were finally tested in the model reaction of reduction of 4-nitrophenol to 4-aminophenol and showed consistently higher activity compared to that of the corresponding batch produced materials due to smaller mean nanoparticle size and uniform alloy composition. In the second section of this thesis, the relation between metal particle size and catalytic activity was studied in the 4-nitrophenol reduction to 4-aminophenol using Au catalysts with different particle size. Despite the catalytic activity being ascribed indiscriminately to surface metal atoms by previous results, the tests carried out on Au/TiO2 catalysts having mean particle size ranging from 4 nm to 30 nm showed a different behaviour. In order to isolate the active sites directly involved in the reaction, a mathematical model was applied: the Au nanoparticles were approximated to perfect hemi-icosahedrons and the number of Au atoms in vertex, edge, periphery and face positions were calculated from geometrical considerations. The rate of conversion of 4-nitrophenol was found to increase linearly with the number of peripheral and edge Au atoms in the catalyst. Finally, in the third part, bimetallic AuPd-based catalysts were employed in the selective catalytic hydrogenation of cinnamaldehyde. Several parameters were varied in order to observe changes in activity and selectivity, such as stirring speed, catalyst amount, hydrogen pressure, reaction temperature, solvent and support. After initial optimisation of the reaction conditions, such as stirring speed, catalyst amount, hydrogen pressure and reaction temperature, the effect of metal composition was studied. It was observed that a maximum in activity was reached with a Au : Pd relative molar composition of 50 : 50, while high selectivity towards the hydrogenation of the vinyl group was observed increasing the Pd concentration. The enhanced activity was attributed to a combination of ligand and ensemble effect caused by formation of bimetallic alloy, while the higher selectivity of Pd-rich catalysts was attributed to the presence of a Pd surface that favoured η4 adsorption of the substrate and therefore the hydrogenation of the C=C bond. Deactivation phenomena caused by metal leaching and particle sintering was addressed by treating the catalysts at high temperature. In addition to a higher stability, the catalysts showed an enhanced selectivity towards the hydrogenation of the vinyl bond, attributed to the formation of Pd surface segregation during the high temperature oxidation step.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Chemistry
Subjects: Q Science > QD Chemistry
Date of First Compliant Deposit: 22 May 2019
Last Modified: 03 Aug 2022 01:48

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