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Title: Experimental and computer modelling studies of photocatalytic oxidation of chloroaromatic compounds on semiconductor surfaces
Author: Axelsson, Anna-Karin
Awarding Body: London South Bank University
Current Institution: London South Bank University
Date of Award: 2001
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Heterogeneous photocatalysis is a promising technique for treating contaminated waste waters, producing harmless end products. The degradation, which occurs by illumination of a semiconducting surface (which includes an adsorbed phase), is typically suggested to be achieved by oxidative pathways involving photogenerated surface-trapped holes, creating hydroxy radicals (OH). In this thesis the importance is established of the reductive pathway provided by trapped electrons, which are present at surface Ti3+ sites, that coexists with the oxidative pathway. Chlorinated hydrocarbons are widely used in the manufacture of plastics, pesticides and wood preservatives, and are found as contaminants in drinking and ground waters. Such compounds are highly stable chemically, and their high lipid solubility renders them a high health risk due to bioaccumulation in humans. In this thesis, degradation of 1,2-dichlorobenzene was investigated. Extensive work has been undertaken on the photocatalytic reaction pathway and characterisation of the many intermediates formed in this process. This is of importance, as an incomplete photocatalytic treatment may create more toxic products than the starting compound. With chloroaromatic compounds there is a significant risk of forming extremely toxic polychlorinated biphenols (PCB). However, no traces of this type of compound were found. Reactor runs were timed so that at the end no significant amount of aromatic compounds could be detected. By such a time it was found that the end products were hydrochloric acid and various non-chlorinated aliphatic acids, as well as water and carbon dioxide. An effective photocatalytic system requires a good performance from the catalyst. In this thesis, enhanced catalytic performance by coupled semiconductor catalysts was examined, as well as thin film sol-gel derived catalysts. It was found that a coupled Sn02ffi02 catalyst had a performance very similar to a commercially available Ti02 (Degussa P25, 75% anatase 25% rutile), which is the most commonly used catalyst material used in this type of research. Kinetic modelling of the photocatalysis of 1,2-dichlorobenzene was successfully undertaken, and kinetic parameters derived by computer modelling and fitting to experimental data. Further kinetic models were derived for the photocatalytic degradation of chlorophenols. Electron-hole recombination is often suggested to be decreased by increasing molecular oxygen levels, as it traps the excited electron on the catalyst surface. In this work it is suggested that a hydroxyl radical can be photo-ejected from the catalyst surface, leading to a translationally hot OHe1t) radical, rather than react on the surface as is commonly believed. In presence of dissolved oxygen, the major reaction is hydroxyl addition to the chlorinated aromatic compound by a direct OH radical attack. In low oxygen concentrations, an electron transfer from a Ti3+ site to the aromatic ring causes partial dechlorination of the ring, before subsequent hydroxy radical attack. This important finding suggests that dissolved oxygen has two important roles in photocatalysis: One as a H-atom acceptor required in direct hydroyl radical addition to the phenyl ring, and the other as an electron acceptor on the Te+ surface sites. A theoretical model of the kinetics is proposed, which is able to account semi-quantitatively for the overall features of the reaction state space. Significantly, monitoring of the intermediate species produced by these two routes shows that the relative yields can be inverted by changing the dissolved oxygen concentration, which significantly is in accord with the theoretical predictions.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available