Ab Initio Study of Reaction Pathways Related to Initial Steps of Thermal Decomposition of the Layered Double Hydroxide Compounds

We present density functional theory (DFT) calculations with periodic boundary conditions combined with thermodynamic and kinetic analyses to study the changes in the [Mg2/3Al1/3(OH)(2)](CO3)(1/6)center dot 2/3H(2)O layered double hydroxide (LDH) over a temperature interval of 25-350 degrees C. The inspection of five different hypotheses related to the decomposition reaction of this material suggests the mechanism: (1) The layered double hydroxide contains H2O molecules up to 180 degrees C. (2) The next step involves the formation of a lamellar structure in which the CO32- anions are monodentate grafted to the layers at about 280 degrees C. (3) At 350 degrees C is observed a reorganization of the carbonate, which leads to a bidentate bond of this anion with the layer. So, the occurrence of two partially dehydroxylated intermediates containing a grafted carbonate was identified. The study of the grafting mechanism path also allows us to access structural information of all the intermediates and the transition structures along the reaction pathway, which can be divided into three parts: the vacancy formation, the incorporation of carbonate as a monodentate ligand, and finally, its incorporation as a bidentate ligand.