Structural Model Proposition and Thermodynamic and Vibrational Analysis of Hydrotalcite-Like Compounds by DFT Calculations

Abstract

We propose a model to calculate the electronic structure of hydrotalcite-like compounds by using periodic boundary conditions and ab initio density functional theory (DFT). The proposed method to build up layered double hydroxides (LDHs) was tested for Zn 2/3 Al 1/3 (OH) 2 Cl 1/3 • 2 / 3 H 2 O, Zn 2/3 Al 1/3 (OH) 2 (CO 3 ) 1/6 • 4 / 6 H 2 O, and Mg 2/3 Al 1/3 (OH) 2 (CO 3 ) 1/6 • 4 / 6 H 2 O with 3R 1 polytype. In the model, the occupation of cationic sites in hydroxide layers is ordered and the interlayer anions and water molecules form a film between the layers. Direct comparison with experimental structural parameters shows good agreement. The a parameter is close to that in brucite for all three LDHs. The c parameter is smaller (about 1 Å) for the LDHs with CO 3 2as a consequence of its strong interaction with hydroxide layers. Those interactions were evidenced by the difference of density and vibrational analysis. The intercalated water molecules have small mobility and interact strongly with one another and with interlayer anions and hydroxide layers. These interactions cause the downshift in the calculated vibrational wavenumbers of water O-H stretching modes below 3420 cm -1 and are consistent with the reported infrared spectra of hydrotalcite-like compounds. The calculated formation enthalpies for LDHs with carbonate are in agreement with the previously reported trend. The biggest difference between theoretical and experimental values is 2 kcal/mol. The calculated formation Gibbs energies are negative. The zero point energy is important to evaluate ∆H, but the formation entropy does not affect the Gibbs free energy significantly.

Theoretical studies of the interaction of terephthalate anion in MgAl-layered double hydroxides

“…In this work ab initio calculations were done to provide a clear and more detailed picture of the arrangements of TA in the interlayer region and the interaction between the anion and the hydroxyl layers as the charge layer increases. This work is grounded in previous theoretical works based on DFT of layered materials produced recently (Costa et al, 2008), (Costa et al, 2010), (Costa et al, 2011), (Costa et al, 2012a), (Costa et al, 2012b), (Vaiss et al, 2009), (Vaiss & I., B. J., Leitão, A. A., 2011), (Vaiss et al, 2013).…”

“…The vector a=2; ffiffiffi 3 p a=6; c=3 (where a and c are the cell parameters for 3R 1 polytype) produces the correct stacking for the LDH structure and define a unit cell with only one layer and one interlayer space (Costa et al, 2010) Experimental values were used as initial c parameters for the three structures. Thus, a single layer model is used to reduce the number of atoms and the stacking sequence is recovered by a sloped c vector and the periodic boundary conditions allow the recovery of the 3R 1 polytype.…”

“…Due to the versatility for construction of the structure, the LDHs represent a class of materials with interesting and potentially useful properties (Vucelic et al, 1995), (Nishimura et al, 2013). Also, by reason of their capacity of expansion along the c vector, LDHs are suitable to intercalate a large number of organic species, making them useful in many fields such as medicine, photochemistry, molecular separation, catalysis, and anion-exchange (Kloprogge & Frost, 1999), (Kumar et al, 2007), (Costa et al, 2010), (Costa et al, 2011), (Costantino et al, 2013). Although the most common anion is carbonate, terephthalate (TA) has been widely studied most because of its use as swelling agent, to assist the incorporation of bulkier anions (Drezdzon, 1988), (Vucelic et al, 1995), and also because its relatively rigid nature enables studies on the dynamics of interlayer anions, both by spectroscopic, and computational studies (Greenwell et al, 2010).…”

A First-Principles Study of Oxygen Formation Over NiFe-Layered Double Hydroxides Surface

“…Costa et al studied the structural model proposition and thermodynamic and vibrational analysis of hydrotalcite-like compounds by DFT calculations [51], and we built the bulk used in our analysis referring to their method. The bulk contains four Ni atoms, two Fe atoms, 12 H atoms, 15 O atoms and one C atom, as shown in Fig.…”

Modelling the Structure and Vibrational Properties of Layered Double Hydroxides

“…In the case of a ratio R(Mg/Al) = 2 there is only one way to distribute the cations in order to avoid two consecutive Al(OH) 6 octahedra. This distribution can be described with a hexagonal supercell of a√3 length [15] as displayed in figure 1. Depending on the way the layers stack, different polytypes can be formed [16].…”