Dehydration of goethite to hematite from molecular dynamics simulation

In Fischer-Tropsch synthesis (FTS), the prepared catalyst hematite (α-Fe2O3) originates from goethite (α-FeOOH) dehydration in calcinations process. The structure of α-FeOOH and its topotactic transformation into α-Fe2O3 under 300-900 K have been modeled within the framework of density functional theory. Molecular dynamics simulations show that the transformation begins preferentially on the surface and the dehydration is associated with the formation of voids. The loss of H2O in α-FeOOH does not modify the morphology extremely with respect to that of the original α-FeOOH. The transformation process involves hydrogen migration with the formation of adsorbed water, followed by desorption of the water molecule. The calculated barriers for hydrogen migration (118 kJ/mol) and water desorption (82 kJ/mol) suggest a direct reaction path without the formation of intermediates.