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Segregation and reactivity in bi-cationic oxide catalysts

Hellier, Pip 2019. Segregation and reactivity in bi-cationic oxide catalysts. PhD Thesis, Cardiff University.
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Abstract

The in uence of segregation on shell-core selective oxidation catalysts has been investigated in this thesis in three main areas. Novel shell-core catalysts, namely VOx/Fe2O3 catalysts, are selective to formaldehyde during reaction with methanol, indicating that the core Fe2O3 is sufficiently segregated, while VOx remains at the surface. Significant structural understanding has been gained, suggesting VO4 tetrahedra constitute the active site at the surface. With the soundness of the shell-core model further confirmed by VOx/Fe2O3, other reactions of interest can be investigated with shell-core catalysts. While VOx and MoOx/Fe2O3 function well as shell-core catalysts, benefitting from greater surface area and amenability to analysis, NbOx/Fe2O3 catalysts are unable to achieve the necessary segregation during calcination, resulting in exposed Fe2O3 at the surface, worsening their selectivity. This is attributed to the notably higher Tamman temperature of NbOx which is never reached during calcination, preventing the spreading of the NbOx across the Fe2O3 required for shell-core formation. The properties of Al dopants in Fe2O3 cores have also been examined. The addition of up to 20 wt% Al in Fe2O3 can increase surface area fourfold, enhancing catalytic activity in turn; however, detrimental effects on catalyst selectivity are seen for higher Al loadings, indicating a degree of structural disruption. It is now known that Al can only occupy sites in the Fe2O3 structure at low Al loadings, above which the impetus to phase separate increases. Magnetocatalysis has also been investigated using shell-core catalysts based on zincdoped cobalt ferrite. Clear evidence of selectivity manipulation by applying an external field during reaction has been obtained. Additionally, strong indications of internal magnetocatalytic effects have been observed, which are those effects on selectivity caused by magnetisation changes in the ferrite support. Overall, these studies have further emphasised the importance of multicomponent catalysts, and the need to carefully control catalyst speciation.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Chemistry
Date of First Compliant Deposit: 6 December 2019
Last Modified: 09 Dec 2019 10:20
URI: http://orca.cf.ac.uk/id/eprint/127377

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