Role of reactive oxygen species on the activity of noble metal-doped TiO2 photocatalysts

Modified TiO2 catalysts are of interest in environmental water remediation since they can lead to efficient electron-hole separation and greatly enhance the photocatalytic properties of TiO2. Reactive oxygen species (ROS), such as the superoxide radical (O2 −), hydroxyl radical ( OH), and positive valence band holes (h+VB), have been reported as the main oxidative species involved in photocatalytic degradation processes. In this work, the role of these species using TiO2, TiO2/Pt 0.5 wt%, and TiO2/Ag 10 wt% has been examined in order to clarify the oxidation pathways. For this purpose, the contribution of the main oxidative species was analyzed in the photocatalytic degradation of dichloroacetic acid (DCA) solutions using specific scavengers (benzoquinone, tert-butyl alcohol, and formic acid). Moreover, the hydroxyl radicals were quantitatively determined in order better understand the results. Regardless of the catalyst used, it is concluded that OH radicals are the major reactive species responsible for DCA degradation and no significant degradation is due to O2 − radicals. Nevertheless, different OH generation pathways were found, depending on the nature of the catalysts. Degradation using TiO2 was conducted mainly via OH radicals generated in the positive holes, while noble metal-doped TiO2 catalysts generated OH radicals through the transformation of O2 − radicals.Financial support through projects CTM2015-69845-R and CTQ2015-66078-R (MINECO/FEDER, UE) is gratefully acknowledged. Paula Ribao thanks the University of Cantabria for her research grant. Juan Corredor would also like to thank the FPI postgraduate research grant (BES-2016-079201)

Photocatalytic degradation and mineralization of perfluorooctanoic acid (PFOA) using a composite TiO2 -rGO catalyst

The inherent resistance of perfluoroalkyl substances (PFASs) to biological degradation makes necessary to develop advanced technologies for the abatement of this group of hazardous substances. The present work investigated the photocatalytic decomposition of perfluorooctanoic acid (PFOA) using a composite catalyst based on TiO2 and reduced graphene oxide (95% TiO2/5% rGO) that was synthesized using a facile hydrothermal method. The efficient photoactivity of the TiO2-rGO (0.1 g L-1) composite was confirmed for PFOA (0.24 mmol L-1) degradation that reached 93 ± 7% after 12 h of UV-vis irradiation using a medium pressure mercury lamp, a great improvement compared to the TiO2 photocatalysis (24 ± 11% PFOA removal) and direct photolysis (58 ± 9%). These findings indicate that rGO provided the suited properties of TiO2-rGO, possibly as a result of acting as electron acceptor and avoiding the high recombination electron/hole pairs. The release of fluoride and the formation of shorter-chain perfluorocarboxilyc acids, that were progressively eliminated in a good match with the analysed reduction of total organic carbon, is consistent with a step-by-step PFOA decomposition via photogenerated hydroxyl radicals. Finally, the apparent first order rate constants of the TiO2-rGO UV-vis PFOA decompositions, and the intermediate perfluorcarboxylic acids were found to increase as the length of the carbon chain was shorter.Financial support from projects CTM2013-44081-R, CTM2015-69845-R and CTM2016-75509-R (MINECO, SPAIN-FEDER 2014–2020) is acknowledged. B. Gomez thanks the FPI scholarship (BES-2014-071045)

Diseño de materiales a base de TiO2 para aplicaciones fotocatalíticas medioambientales

RESUMEN: La contaminación y disponibilidad del agua y la crisis energética se han convertido en dos de los principales desafíos para la sociedad en un futuro cercano. Por lo tanto, existe una necesidad urgente de buscar tecnologías sostenibles para satisfacer las demandas de los seres humanos. En las últimas décadas, los procesos de oxidación avanzada (POAs) han atraído un gran interés debido a su versatilidad y sus amplias áreas de aplicación. Específicamente, la fotocatálisis heterogénea ha surgido como una alternativa factible ya que permite trabajar a temperatura ambiente y presión atmosférica, siendo el dióxido de titanio (TiO2) es uno de los fotocatalizadores más prometedores debido a que es abundante y económico. Sin embargo, presenta algunos inconvenientes que dificultan su aplicación comercial. Se hacen necesarios fotocatalizadores capaces de trabajar bajo luz visible y con bajas tasas de recombinación de los portadores de carga para la mejora de la tecnología de fotocatálisis. Con este objetivo, el óxido de grafeno y los metales nobles han sido empleados para mejorar la actividad limitada del TiO2 comercial. Por tanto, la presente tesis doctoral tiene como objetivo el diseño, síntesis y caracterización de materiales a base de TiO2 para el tratamiento de soluciones acuosas de ácido dicloroacético (DCA) y la producción fotocatalítica de hidrógeno, un prometedor vector energético, a partir de glicerol.ABSTRACT: Pollution and water scarcity and the energy crisis have become two of the main challenges for society in the near future. Therefore, there is urgent need to search for sustainable technologies to meet demands of human beings. Over the last decades, advanced oxidation processes (AOPs) have been attracting a great interest due to their versatility and potential areas of application. Specifically, heterogeneous photocatalysis has emerged as a feasible alternative since it allows working at room temperature and atmospheric pressure. Until now, titanium dioxide (TiO2) is one of the most promising photocatalyst because it is abundant and cost-effective. However, some drawbacks hinder its wider commercial application. Photocatalysts capable of working under visible light and with low recombination rates of charge carriers are necessary for the improvement of photocatalysis technology. With this objective, graphene oxide and noble metals have been selected to improve the limited activity of commercial TiO2. Therefore, this thesis aims at the design, synthesis and characterization of materials based on TiO2 for the treatment of aqueous solutions of dichloroacetic acid (DCA) and the photocatalytic production of hydrogen, a promising energy vector, from glycerol.This thesis was conducted at the Advanced Separation Processes Group in the Department of Chemical and Biomolecular Engineering at the University of Cantabria. This research was financially supported by the Spanish Ministry of Economy and Competitiveness and the European Regional Development Fund through the projects: CTM2012-33917 “Advanced oxidation technologies for the improvement of landfill leachate treatment”, CTQ2012-31639 “New separation processes with kinetic control based on the use of functionalized materials”, CTM2015-69845-R “New developments in photocatalysis for environmental applications”, and CTQ2015-66078-R “Advanced separation applications. Modeling and experimental validation”. Paula Ribao has benefited from predoctoral research grant from University of Cantabria granted by resolution of January 11, 2016. Moreover, Paula Ribao has received a predoctoral mobility grant for short stays from University of Cantabria granted by resolution of April 27, 2017. Due to all of these, a warm thanks towards these institutions is extended

Influence of radiation and TiO2 concentration on the hydroxyl radicals generation in a photocatalytic LEDs reactor : application to dodecylbenzenesulfonate (DBS) degradation

ABSTRACT: One of the main issues associated to the development of photocatalysis is the lack of adequate indexes that allow the comparison of the results obtained in different experimental setup designs. Photocatalytic degradation of pollutants is based on the generation of strongly reactive species such as hydroxyl radicals (•OH) that attack and oxidize the target compound. Therefore, determining the generation rate and concentration of hydroxyl radicals in the reaction medium is the first step to describe their influence on the photocatalytic degradation and to compare the results obtained under different conditions and geometries. In this work, using a Light Emitting Diodes (LEDs) reactor aimed to maximize the light efficiency and minimize the energy consumption, the •OH concentration has been determined as a function of the radiation and catalyst concentration following an indirect method. Finally, the methodology has been applied to analyze the degradation kinetics of the anionic surfactant dodecylbenzenesulfonate (DBS), frequently used in shampoo formulations and detergents for washing machines.Máster en Ingeniería Química “Producción y consumo sostenible

Improvements in biodegradability of Bisphenol A synthetic solutions by advanced oxidation processes

Ingeniería químic

Comparative performance of TiO2-rGO photocatalyst in the degradation of dichloroacetic and perfluorooctanoic acids

Halogenated organic compounds are frequently characterized by their bioaccumulative nature, and long­term effects on human health. Their low biodegradability and their difficult removal by conventional technologies lead to their undesirable persistence in the environment. Titanium dioxide and reduced graphene oxide TiO2-rGO composites have shown promising results with enhanced photocatalytic degradation rates compared to bare TiO2 in the degradation of a good number of persistent pollutants. In this context, this work deepens on the influence of the type and number of halogen substitution on the photocatalytic degradation of halogenated organic compounds (HOCs). For the experimental analysis two HOCs, dichloroacetic acid (DCA) and perfluorooctanoic acid (PFOA), have been selected as both molecules differ in the type of halogen atoms, chlorine and fluorine, the number of halogen atoms, and in the length of the alkyl chain. The results showed that TiO2-rGO catalysts achieved a similar kinetic performance for the removal of the primary contaminant in terms of its degradation rate. Besides, TiO2-rGO composite successfully induced the release of all chlorine atoms from the DCA molecule, achieving its total mineralization. However, PFOA defluorination and mineralization rates were remarkably lower than its degradation rate. Although both contaminants released two halogen atoms step-by-step for the degradation of each molecule, PFOA dehalogenation was slowed down due to the generation of secondary products that retain fluorine. Therefore, we conclude that the composite photocatalysts showed a similar performance in terms of degradation rate of the primary contaminant independent on the length of the alkyl chain and on the number of halogen atoms, however, the mineralization and dehalogenation rates were strongly dependent on the number of halogen substituents and the length of the alkyl chain.This work was supported by the Spanish Ministry of Science, Innovation and Universities [grants number RTI2018-099407-B-I00 (MCIU/AEI/FEDER,UE) and CTM2016-75509-R (MINECO/FEDER, UE)