Nanoestruturação de Ferritas de Cobalto (CoxFe3-xO4): efeito sobre a catálise e sobre a detecção de gases poluentes

Abstract

The present thesis aims to study the cobalt ferrite (CoxFe3-xO4) nanocrystallites morphology and composition effects on their catalytic and detection properties. Different synthesis methods such as hydrothermal, solvothermal and thermal decomposition were used to control nanoparticles shape and composition. Moreover, the effect of synthesis parameters variations, such as addition of different surfactants, different solvents, surfactant concentration, temperature and time of synthesis were also investigated. The powders were characterized by Xray Diffraction and Transmission Electron Microscopy, and analyzed by Dispersive Energy Spectroscopy. The catalytic and detection properties were evaluated in CO atmosphere in the range of 100 to 300°C and in NO2 atmosphere at 150 and 200°C, respectively. CoFe2O4 nanooctahedra with 15-20 nm were produced by hydrothermal synthesis using different surfactants (CTAB, SDS and PVP). CoxFe3-xO4 with x = 1.5 and x = 1.8 were also synthesized by hydrothermal method. However, the TEM analysis showed that the amount of Co present in CoxFe3-xO4 is limited at x = 1.5. When ethylene glycol was used as solvent, Co1,8Fe1,2O4 of 4 nm with irregular shape and homogeneous composition was obtained. A mixture of nanocubes (18 nm) and nanospheres (10 nm) of CoFe2O4 was produced by solvothermal synthesis using oleylamine as surfactant. In the thermal decomposition, nanospherical CoFe2O4 were produced in benzyl alcohol, while nanooctahedra of Co0.5Fe2.5O4 were produced in dibenzyl ether. The nanooctahedra with composition x = 1.5 and the mixture of nanocubes and nanospheres were tested in the CO catalysis. The nanocubes and nanospheres mixtures were also tested in the detection of NO2. The results were compared with those obtained for the nanooctahedra with x=1 existent in the literature. The applications results showed that the shape and composition have an effect on the nanoparticles catalytic activity. The powder of CoxFe3-xO4 with x = 1.5 has higher activity and lower temperature needed for the maximum CO conversion. For the detection test, the mixture of cubes and spheres presented lower sensitivity than the nanooctahedra for all temperatures analyzed. The maximum sensor response obtained was 52% when the sensor was subjected to 50 ppm of NO2 at 200 ° C.