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Title: Catalytic denitration of drinking water
Author: Sá, Jacinto de Paiva
Awarding Body: University of Aberdeen
Current Institution: University of Aberdeen
Date of Award: 2007
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Human demand for clean water has increase drastically in the past centuries, mainly due its demographic growth. The sources of clean drinking water have been continuously reduced due to depletion or contamination with one example being the extensive use of fertilizers in agriculture, which can lead to leaching of nitrates into groundwater and hence into surface water. The gravity of the situation was expressed by European Environmental Agency in 1998, revealing that, 87% of agricultural areas in the European Union (EU) have nitrate concentrations in groundwater above the guide level (25 ppm). Catalytic hydrogenation is one potential solution if an appropriate active, selective catalyst could be identified. One of the major drawbacks of the hydrogenation approach is the absence of robust studies which clearly describe what happens during reaction, essential for full understanding of the process. It was decided to use FTIR under operando conditions to try to disclose what happens in the catalyst surface during reaction. Pd/TiO2 catalysts were selected one. Upon adsorption at Lewis acid sites (oxygen vacancies), the nitrates are reduced by the electron enriched titania species, most likely Ti4O7 as identified by electron microscopy, which lead to the formation of nitrites, generally detected in solution during the hydrogenation tests and expected assuming a stepwise mechanism exists. The rather weak adsorption of nitrate onto the catalysts surface allied to their stability might be the reasons for their low reactivity, i.e., limiting step of the hydrogenation process. The nitrite reduction occurs essentially on Pd sites however their adsorption site is Lewis acid sites. NO is adsorbed and reduced exclusively on the noble metal. The order of reactivity of the surface species decreases with the decrease in the oxidation state of the nitrogen, i.e., NO3- NO2- NO. High surface concentration of nitrite leads to the formation of N2O, while ammonia is formed via consecutive hydrogenation of Nads. originating from the dissociation of NO. Ammonia formation takes place over Pd and is dependent on the hydrogen availability and presence of water. Ever since Becquerel discovered the photoelectric effect back 1839, researchers and engineers have been infatuated with the idea of converting light into electric power or chemical fuels. Photocatalysis is also particularly suitable for the abatement of contaminants since it offers potentially high conversions at low cost. Two aspects have dominated the research of photocatalysts, namely improvement of catalytic performances under visible light and the minimization of charge recombination. The latter can be improved by a decrease in the particle size and/or by the addition of small metal clusters of elements such as Cu that operate as electrons sinks thus allowing the system to be employed in processes such as N03- reduction. The addition of metal to TiO2 P25, led to a significant enhancement in the photocatalytic activity of the catalyst. The overall process was found to be dependent on the temperature of reaction media, and the nature and concentration of the hole scavenger, and on the metal loaded. In the case of Hombikat, the metal-free support appeared to operate better possibly as a consequence of its greater surface area as this would hinder, to a certain extent, the charge recombination process. EPR and FTIR experiments under UV irradiation carried out using the metal clusters (Au, Ag and Cu) supported on TiO2, revealed that the presence of the metal leads to the loss of signal related to the stabilized electrons on the pure TiO2 when a hole scavenger such as hydrogen is present. The Fermi level equilibration process, in a semiconductor - metal nanocomposite system, is a clear indication of the presence of electron transfer process between support and metal. Addition of the metal did not, however, modify the band gap energy of the studied semiconductor. In the generality of the cases studied, the photocatalytic approach was found to have much higher activities when compared to the hydrogenation process.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available