In recent years, emerging pollutants have appeared as one of the main causes of global pollution of water resources, due to their persistence in aqueous media and their resistance to be eliminated by conventional water treatment methods. This has led to the development of more clean and efficient technologies for wastewater treatment based on the production and use of highly oxidizing species (Advanced Oxidation Processes), which are able to interact and degrade almost any pollutant at ambient conditions. In this work, heterogeneous photocatalysis has been investigated as advanced process for the degradation of rhodamine B, a hazardous organic compound used as a colorant and commonly present in industrial wastewaters. The major drawback of heterogeneous photocatalysis is that, up to now, the workability of high performance photocatalysts (mostly semiconductors) is limited to their use under ultraviolet light; this leads to high operational costs that hinder the implementation of this technique at large scales. Therefore, it would be desirable to increase the absorption features of those semiconductors in the visible spectrum, so as to increase the photocatalytic efficiency of these systems under sunlight. An approach to attain this goal is the use of carbon materials as additives to tungsten semiconductors (which present higher activity under sunlight than TiO2 benchmark). In order to explore this possibility in depth, the synthesis, characterization and evaluation of the photocatalytic performance of Bi2WO6/C and WO3/C photocatalysts were carried out in this thesis. Moreover, and aiming at elucidating the role of the additive, carbon materials with varied structural, textural and physico-chemical properties were investigated.

The results obtained showed that both the synthesis route and the amount of carbon additive are key factors in the photocatalytic performance exhibited by the binary semiconductor/carbon photocatalyst, and should be optimized for each semiconductor. Furthermore, the nature of the carbon material used as additive strongly influences the optical, textural and surface features of the resulting semiconductor/carbon photocatalysts, and thus determines the global efficiency of the photodegradation process. In this regard, it was also observed that the prevailing photooxidation mechanism of rhodamine B depends on the surface chemistry of the photocatalyst. Thus, acidic carbon additives favour degradation of the studied dye via coupled photosensitization and photocatalytic mechanisms. Contrarily, in carbon materials of basic nature, and due to the weak interactions between the pollutant and the surface of photocatalysts, the degradation occurred predominantly via the photocatalytic process. On the other hand, the self photoactivity exhibited by the carbon materials used as additives also contributed to the enhancement of the photoconversion of rhodamine B, due to their ability to generate O-radical species under illumination in aqueous suspensions. A thorough study and control of the above mentioned variables has pointed out that the use of carbon materials as additives to tungsten-based photocatalysts offers new possibilities in the water treatment processes, due to the enhanced performance of such hybrid catalysts towards the degradation of rhodamine B under solar radiation.

Tesis doctoral presentada en el Departamento de Ciencia de los Materiales e Ingeniería Metalúrgica de la Universidad de Oviedo, 2016. Tutor de la tesis, José Rubén García Menéndez.

Peer reviewed