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Materials for the photocatalytic treatment of recalcitrant organic waste

Bouleghlimat, Emir 2017. Materials for the photocatalytic treatment of recalcitrant organic waste. PhD Thesis, Cardiff University.
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The photocatalytic degradation of cinnamic acid, by TiO2, has been studied extensively in aerobic and anaerobic conditions and in the presence of common salts often found in industrial waste waters. Analysis of the intermediates formed found that molecular oxygen is central to forming the important radicals for the main benzaldehyde mechanism by which cinnamic acid initially degrades, as well as a key component required for the mineralisation to carbon dioxide. In the absence of molecular oxygen an alternate, but slower, pathway becomes the prevalent. The hydroxyl driven mechanism is capable of decarboxylation of the initial carboxyl group but further oxidation does not occur at a reasonable rate. By investigating the effect of salts in solution we found that sulfates and chlorides both interfere with degradation mechanisms and decrease the mineralisation efficiency of titania photocatalysis. Sulfates blocked important surface sites needed for substrate binding that inhibited the benzaldehyde pathway and slowed down the degradation pathway. Whilst chloride formed radical chlorine species (Cl∙) in the presence of TiO2 that resulted in the increase of cinnamic acid removal and the emergence of new reaction pathways. Cl∙ competed with the superoxide radical anion (O2∙-) to attack across the double bond of cinnamic acid, decarboxylate and form several new acetophenone-derived intermediates. A widening of the intermediate map, through the formation of new intermediates, is coupled with a significant slowing of total mineralisation which presents real issues for photocatalytic waste water treatment where chloride ions are present. Additionally, the chlorine radical induced pathways produce intermediates of a greater toxicity; bringing the implication that partial oxidative degradation could result in waste water with an increased toxicity. Anodic nanotubes were explored as an option for alternative materials to be used within photocatalytic reactors. Nanotubes anodised for 8 hours were found to be the most photoactive in the liquid phase, and in the surface degradation of contaminants, due to the wider pores that were structurally sound enough to not slope and reduce light penetration. The surface topography was identified as the key factor for promoting photocatalysis. It was also found that the materials had a cross-phase applicability, in that the most active liquid phase nanotubes were also the most efficient for surface degradation. The incorporation of tungsten into the anodisation process did not improve the photocatalytic activity. Photodeposition of palladium and gold resulted in a decrease in the degradative efficiency of the nanotube arrays. Pd/TiO2 and Au/TiO2 powders were found to reduce the degradation rate of cinnamic acid in oxygenated conditions, although both metals improved the oxidation of surface deposits of carbon. In deoxygenated conditions, Pd/TiO2 catalysts exhibited superior degradation of cinnamic acid in comparison to plain TiO2 and gold doped catalysts. Enhancements in the mineralisation rates, to CO2, were also found. The improvements were attributed to the presence of palladium improving charge separation and introducing new reaction sites capable of decarboxylating the alcohol and aldehyde functionalities, respectively. While the gold nanoparticles were poorly dispersed, they were found to increase the selectivity for phenylacetaldehyde, in deoxygenated conditions, by a factor of 5.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Chemistry
Subjects: Q Science > QD Chemistry
Date of First Compliant Deposit: 19 December 2017
Last Modified: 18 May 2021 14:21

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