Investigação teórica do bulk e da superfície da δ-FeOOH dopada com Nb como potencial catalisador bifuncional

Abstract

The development of bifunctional catalysts has drawn the attention of researchers in recent years. The use of these materials can transform the mechanisms of various industrial processes, improving the production of inputs of great importance. Recent studies look for the highly efficient materials, constituting of abundant transition metals, to replace the noble metal-based ones. In this work was investigated the properties of ferroxyhyte (δ-FeOOH), pure and doped with niobium, Nb, through computational calculations of periodic systems, especially addressing the characteristics responsible for the bifunctional activity of these catalysts. Bulk and surface of materials calculations based on the Density Functional Theory, DFT, were performed, also using the Projector Augmented Wave approach, PAW. The δ-FeOOH structure was optimized considering the effects of van der Waals interactions, vdW, on the total energy of the DFT calculation, and the Hubbard correction, DFT+U. The calculated structural parameters are in good agreement with experimental data. The value of U for the iron atom in the structure of δ-FeOOH, was determined for the first time, in this study. The use of U = 4.9eV provided results with greater proximity to the experimental value of δ-FeOOH band gap (2.2 eV). The results also showed that the formation the Nb-doped bulk oxyhydroxide is thermodynamically stable and that the insertion of the dopant leads to the formation of stronger vdW interactions between the layers of the δ-FeOOH bulk, which can increase the thermal stability of the catalyst. In addition, characteristics that lead to the improvement of the bifunctional activity and other catalytic properties of feroxyhyte as a result of doping with Nb were evidenced. The Nb incorporation in the more stable surface model of δ-FeOOH was even more favorable than in bulk, suggesting that new surface active sites can be formed with doping. The results of the structural and electronic properties study of the doped δ-FeOOH surface, indicated that the presence of Nb increases Brønsted acidity and create new Lewis acid sites on the catalyst surface, which can enhance the bifunctional efficiency of the material.