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Theoretical Study of the Reaction between HF Molecules and Hydroxyl Layers of Mg(OH) 2

2009; American Chemical Society; Volume: 113; Issue: 23 Linguagem: Inglês

10.1021/jp810047d

1520-5215

Viviane S. Vaiss, Rafael Arantes Berg, Ary R. Ferreira, Itamar Borges, Alexandre A. Leitão,

Fluoride Effects and Removal

The reaction of HF molecules with brucite, Mg(OH)2, leading to the formation of Mg(OH)2−xFx, was theoretically studied by ab initio density functional theory (DFT) with periodic boundary conditions. We proposed as mechanism for this reaction four elementary steps: adsorption of the HF molecule, OH− liberation from brucite as a water molecule, desorption of the newly formed H2O, and rearrangement of the F− anion into a hydroxyl position. For the Mg(OH)2−xFx formation, with x = 1/9, the final product, outcome from an initially adsorbed HF molecule, we computed the Helmholtz free energy variation ΔF = −23 kcal/mol. The calculated frequency for the most intense infrared band, a Mg−F stretching mode, was 342 cm−1. Two transition states, corresponding to the hydroxyl reacting with a proton forming a water molecule and migration of a fluoride anion into a hydroxyl vacancy, were computed. The calculated reaction barriers indicate that the reaction between Mg(OH)2 layers and HF molecules is slow and irreversible.