Nanostructured CeO2-TiO2 for the photodegradation of Methylene Blue

In the latest years environmental protection and use of renewable energy sources are two major objectives in chemical research. Solar energy can be exploited for the photocatalytic degradation of organic molecules such as pollutants, hormones or drugs in air, water and surfaces because sunlight is able to decompose them [1]. Despite of the huge number of applications of titania (TiO2), a no-toxic, low cost and very promising photocatalyst [2], there are some critical factors that limit its photoactivity, first of all the fact that titania is a semiconductor only active in the UV region of the light spectrum. Several methods have been proposed to reduce its energy-gap value, among which noble metal deposition, surface modifications and doping with transition metal ions or rare earth elements. CeO2 has attracted great attentions due to its unique properties, such as biocompatibility, chemical inertness and strong oxidizing capability related to the formation of oxygen defects [3]. CeO2-TiO2 systems seem to exhibit improved photocatalytic activity due to the enhanced mobility of excitons and/or reduced band gaps. In the present work, nanostructured TiO2 with different CeO2 loadings were synthesized with a two-step hydrothermal route, the first one being a sol-gel process and the second one a dehydration with NaOH. CeO2-TiO2 samples were characterized by many techniques. Surface and bulk chemistry was evaluated using X-ray diffraction (XRD) and X-ray photoelectron Spectroscopy (XPS); morphological and textural characterization was carried out by high resolution transmission electron microscopy (HRTEM); porosity was measured by N2 physisorption. Cerium-doped TiO2 samples displayed a methylene blue (MB) degradation rate in aqueous suspension under UV light higher than that of pure TiO2, since the introduction of cationic ceria dopant led to the decrease of band gap energy, as found by DRIFT UV-vis spectroscopy (from 3.2 eV to about 2.8 eV). The CeO2-TiO2 photocatalysts were found active in the test reaction attaining very high MB degradation values of dye oxidation after the monitored reaction period (120 min).