Fotocatalizadores heterogêneos magnéticos baseados em óxidos de ferro para produção de hidrogênio via fotólise da água ou para oxidação de compostos orgânicos

Abstract

This thesis is divided into two major parts: (i) experimental and (ii) theoretical modeling. The experiments can be subdivided into four major parts: (a) Synthesis, characterization and study of the photocatalytic properties of -FeOOH to produce gaseous hydrogen via water photolysis (b) Synthesis, characterization and study of the photocatalytic properties of composites based on -FeOOH/Ni(OH)2 for the production of hydrogen gas via water photolysis (c) Synthesis, characterization and photocatalytic study of -FeOOH containing niobium in the oxidation of methylene blue dye as model molecule (d) Preparation of photocatalysts from niobium mining tailings, rich in iron oxides. The theoretical modeling of the band structure of the pure -FeOOH, the nickel doped -FeOOH and niobium doped -FeOOH shows that the charge transfer occurs from oxygen to iron atoms. The addition of nickel did not cause significant change in the value of band gap of the material. The -FeOOH was used for production of molecular hydrogen from photocatalytic decomposition of water under ultraviolet (UV) and visible light. Under UV irradiation, the material produces 548.4 micromolh-1 of gaseous hydrogen. However, in the dark, there is no production of hydrogen. For comparative purposes, TiO2 (Degussa P-25) was tested, yielding 52 molh-1 of hydrogen. The reaction was also conducted with Na2CO3 as sacrifice reagent. In these circumstances, hydrogen production increased significantly, reaching 1164.7 micromolh-1. The reaction was tested under visible light and hydrogen gas was produced at the rate of 10.9 molh-1. This result shows that the photocatalyst can also be effectively activated by visible light. The composite -FeOOH/Ni(OH)2 was studied in the production of hydrogen under visible radiation. It was observed that the incorporation of Ni(OH)2 is important to stimulate the production of H2. The pure -FeOOH produces 10 molh-1, while the material with 20% of Ni(OH)2 produces 30 molh-1.These results suggest that Ni(OH)2 can act as a co-catalyst in photocatalytic reaction. The niobium-doped -FeOOH was tested in photooxidation of methylene blue dye under ultraviolet radiation. The material without the presence of niobium removes 10.2 % of staining after 120 minutes of reaction, while the material doped with 10.0 wt% of niobium showed 92.6% of stain removal after 10 minutes of reaction. These data show the importance of the incorporation of Nb for photocatalytic degradation reactions of organic molecules by a delay in the recombination of the electron-hole pair, which would explain the high photocatalytic activity. In the fourth part of the experimental work, samples of tailing from niobium mine , provided by the Company of Metallurgy and Mining (CBMM; Araxá MG) was treated, in order to obtain photocatalytically active materials. The waste is primarily composed of hematite (-Fe2O3; approximately 59 wt%) and magnetite (Fe3O4; approximately 41 wt%). The materials were used in photooxidation of model molecules methylene blue, congo red and orange methyl. Under ultraviolet light, the natural material showed greater removal of methyl orange, with a reaction rate constant of 0.00205 min-1. The material treated at 500 °C provides greater removal of methylene blue molecule with a rate constant of 0.00751 min-1. The material treated at 1000 °C has the highest removal capacity for the congo red molecule with speed constant 0.00738 min-1.