Reatividade química dos sistemas Mn3O4/As(OH)3/AsO(OH)3 e ß-FeOOH/Li: investigação ab initio de mecanismos de oxirredução

Abstract

There is a wide variety of technological applications for manganese oxide known as hausmannite (Mn3O4), with special emphasis on the chemical adsorption processes of inorganic species, with potential for the treatment of aqueous environments contaminated by arsenic.Other materials of great technological importance are the iron oxides, which stand out the akaganeite (-FeOOH) in electrochemical applications. Although there are experimental studies of both oxides, chemical reactivity at the molecular level is the point of convergencethrough the oxidation properties of these systems. Thus, the present thesis reports the investigation of oxidation mechanisms in the two oxides in the light of the Density Functional Theory under different applications. The study of the structural, electronic and local propertiesof the Mn3O4 bulk was carried out. In the subsequent step, we characterized the energetically more favorable cleavage surface of this oxide, where the surface (001) presented the lowest estimated surface energy, exposing the Mn2+, Mn3+ and O2- ions in an asymmetric way. The reactivity was studied through the simulation of water, arsenite and arsenate adsorption processes at the two cationic sites (001) of Mn3O4. The water adsorption process occurs preferentially by the molecular mechanism, although the energy difference of the dissociativeprocess is about of 5 kcal mol-1. In the arsenite adsorption, our studies indicate that the complex forms in the monodentate-mononuclear configuration for the Mn3O4/As(OH)3 system. The bidentate-binuclear complex is the most stable form for the arsenate-containing system. Thewater contributes to the stability of the adsorption system of both arsenic species adsorption. We proposed an As3+/As5+ oxidation mechanism through the Mn3+/Mn2+ reduction in the presence of water, divided into three stages, where the highest transition state barrier energy was estimated at 33 kcal mol-1. We studied the structural and electronic properties of the Akaganeite system. From DFT calculations we investigated the influence of chloride ions and Fe3 + in the structural cavity of the solid through two computational templates of -FeOOH.The Li/Li+ oxidation process occurs by means of the Fe3+/Fe2+ reduction present in akaganeite through the insertion of Li atomic in the channel in order to simulate the cathodic reaction of a lithium ion battery, where we estimate the oxidation potential of the -FeOOH/Li.