Síntese e caracterização de nanopartículas de CeO2-x e avaliação de sua atividade catalítica

Abstract

Cerium oxide CeO2 is widely used in catalytic reactions at high temperatures, such as heterogeneous catalyst for automotive emissions and emissions from oil refineries. The catalytic efficiency of CeO2 is related with the oxidation/reduction properties, its high capacity to store oxygen in its structure and its thermal and chemical stability during catalysis. The ceria removes the soot from incomplete combustion engine, which is generally caused by the variation in air/fuel ratio. After this process, CeO2 is simply regenerated by exposure to air. The motif of this work is the evaluation of the various factors which influence the catalytic efficiency of this material such as the surface area, formation of intrinsic and extrinsic defects, and the formation of reactive oxygen species in the presence of light and hydrogen peroxide. To this end, we tested the catalytic oxidation of soot as a function of temperature and the degradation of cationic dye in the presence of hydrogen peroxide and light at room temperature. In this study, two routes were used for chemical synthesis of nanostructured doped CeO2 : wet chemical route for doping with nitrogen and hydrothermal synthesis for doping with iron and copper. After synthesis, the samples were calcinated in oxidizing and reducing atmospheres at temperatures between 200 and 500 ° C. All samples produced were characterized by various experimental techniques such as X-ray diffraction, Raman spectroscopy and electron paramagnetic resonance (EPR). We were able to verify that doping with transition metals (Fe and Cu) is a more efficient process for the formation of oxygen vacancies compared with heat treatments in reducing atmospheres by green gas, a mixture of H2 and N2. The oxygen vacancies, the dominant defect species, are formed by two neighboring Ce3+ ions which transfer immediately their electrons to the vacancies forming reactive peroxide species (O22-). The formation of isolated Ce3+ defects in CeO2 is not a dominant process different from what happens in TiO2. Besides the introduction of oxygen vacancies, doping with transition metals ions (Fe and Cu) in CeO2 and treated in reducing atmospheres lead to formation of redox couples Fe2+ / Fe3+ and Cu2+ / Cu+ which increase the efficiency of catalytic reactions due to Fenton reaction mediated by the formation of reactive oxygen species such as O22- and OH. These species were verified by Raman spectroscopy and electron paramagnetic resonance using the spin trapping method. Our experimental findings indicate a significant improvement in catalytic tests performed with samples synthesized by us, compared to commercial cerium oxide.