First-principles calculations of structural and electronic properties of Cu (II) doped magnetite for CrO42- adsorption

Abstract

Industrial effluents, containing the most diverse chemical species, are largely responsible for water contamination. In view of the harms that can be caused to human beings and the environment, technologies for remediation are the focus of research in several areas. In this work, the activity of magnetite (Fe3O4) for adsorption of Cr (VI) was optimized by doping with Cu (II). The structures of the materials were built and studied by means of computational calculations, which allowed for a detailed analysis of their electronic properties. The effects of doping were investigated for bulks and catalyst surfaces. Theoretical studies have shown that the bulk of magnetite is more stable when Cu (II) is inserted into octahedral sites, a position that causes less effect on its organization. The Density of States (DOS) profiles showed electronic differences and pointed to an increase in the magnetization of the iron atoms after the modification. The DOS provided a band gap of 0.2 eV for the oxide in its original form, a value consistent with that found in the literature. The cleavage planes (111) and (311) were investigated regarding the thermodynamic stability from the determination of their surface energies (Esurf). The slab (111) presented a lower Esurf value before and after doping, so it was defined for the chromate adsorption processes (CrO42-), one of the forms in which Cr (VI) is found in effluents. The Cu/Fe3O4 (111) catalyst showed a value of -1.21 eV for the adsorption energy (Eads), while the value of Eads for Fe3O4 (111) was -0.97 eV. Knowing that the mechanism depends on electrostatic molecule-surface attractions, we can infer that the better performance of the doped magnetite is related to the appearance of regions of lower electron density in its structure, which favors its interaction with the Cr anions. All simulations were performed by using the pseudopotential PAW (Projector-Augmented-Wave) implemented in the VASP program (Vienna Ab-initio Simulation Package). The set of k-points (5 x 5 x 5) was used and the method employed was DFT (Density Functional Theory) with the addition of the Hubbard parameter (DFT+U) and the Generalized Gradient Aproximation (GGA-PBE).