Estudo do comportamento de catalisadores de CeO2 nas reações de transformação do etanol

Abstract

In Chapter 1, the synthesis results of nanostructured CeO2 based catalysts employing the hydrothermal method are shown, using two different Ce precursors and Ag as a dopant. The nanostructured catalysts were characterized by HRTEM, EPR, TPR, UV-vis, among others. The aim is to describe the role of O vacancies in the mechanism in acetone synthesis from ethanol, in order to support the MvK mechanism as one of the steps of this synthesis. In short, the acetone synthesis can be described as following: firstly, acetaldehyde is generated by ethanol oxidative dehydrogenation; then, this aldehyde reacts with the O of the oxide lattice forming acetate species; next, these carboxylate species condense generating acetone; finally, water dissociates on the vacancies and reoxidizes the catalyst. The O abstraction from the oxide lattice is slowest step of the acetone synthesis from acetaldehyde, which is the slowest step of the acetone synthesis from ethanol. The easier the vacancy generation, the higher the rate of the acetone synthesis is. This result is in line with the MvK mechanism. Considering the catalytic behavior of the nanostructured catalysts it can be proposed that the slowest step of the acetone synthesis from ethanol exhibits a redox characteristic. In Chapter 2, CeO2 catalysts synthesized employing conventional precipitation method are shown. Three different noble metals were impregnated in small percentages in order to verify the effect of the doping of these metals on the redox properties of CeO2 and to confirm the reaction mechanism for the acetone synthesis from ethanol. These catalysts were characterized by TPR, UV-vis, TPD-ethanol, TPD-CO, among other techniques. Pt was the most promising promoter for the acetone synthesis. From the characterizations results it was found that the catalysts show two Pt species. One very dispersed, for the catalysts with the low metal concentrations (from 0,005 % to 0.02%). The behavior of these catalysts is similar to the oxides, but much more active. The other specie, related to the highest metal percentage (0,2%) might contain probably PtO or Pt0 particles. The 0,2% impregnated Pt catalyst has metallic properties. The TPD-ethanol and the catalytic tests showed that these species follow different mechanisms for the acetone synthesis reaction. The lowest percentage of metal follows the MvK mechanism whereas highest percentage the acetyl species condensation mechanism.