Estudo da adsorção de água em compostos tipo-­brucita por cálculos ab initio

Abstract

The anionic clays, also known as layered double hydroxides (LDH) or hydrotalcite-like compounds, constitute a family of materials whose chemical formula can be represented by: [M2+(1­x) Me3+x(OH)2]x+ . Am­x/m .nH2O. These compounds have hydroxide planes, or layers, which become permanently positively charged by the substitution of divalent cations M2+ by trivalent cations M3+. That charge is balanced by inter­layer anions and solvating water. In the last years, the LDH have been used to obtain catalyst supports, catalysts precursors or even acting as the catalyst itself. In spite of this, there is no information, at atomistic level, which helps in prediction and for searching for more efficient and selective catalysts. The lack of theoretical knowledge about hydrotalcites justifies a previous studies in brucite­-like compounds, which have a similar structure. A careful investigation of the interaction between layers can be done even without the presence of anions. It is also interesting to understand the way as the water molecules interact with the layered structure in order to provide a better description of the interlayer domain. Thus, in this work, ab initio DFT calculations were performed to obtain the interaction energy between water and brucite­like compounds with Mg2+ (brucite itself), Ca2+ e Zn2+, and the formation energies of these compounds with one, two and three water molecules in the interlayer domain (the so­called “hydrobrucite­like” compounds). The calculations were performed using the generalized gradient approximation for exchange and correlation corrections and Troullier­Martins pseudopotentials for core electrons. Plane waves were used as basis set for the wave function of the valence electrons. In general, the brucite­like Zn(OH)2 exhibited an intermediate behavior between Mg(OH)2 and Ca(OH)2 on the value of the adsorption energies. It was also noted that, in all cases, the water molecules prefer to interact among themselves instead of the layers of the brucite­like compounds. The calculations with many adsorbed water molecules shows that the orientation assumed by their hydrogen atoms is completely aleatory, depending on the

substrate. Considering the oxygen atoms, the water molecules places themselves forming squares with sides varying between 2.74 and 2.79Å. There is a tendency to physisorption in the zinc and calcium hydroxides, and to chemisorption in the magnesium hydroxide. For the hydrobrucites, it was verified that the formation energy of these compounds decreases with the increase of the number of water molecules in the interlayer. These results support the idea of the interactions between water molecules have a key role for the formation of this kind of compounds. Moreover, just small deviations from the original packing has been observed as consequences of the presence of water. This effect suggests that the presence of water in the interlayer domain is not responsible for the different packing of the hydrotalcite­-like compounds comparing to the brucite-­like ones.