Ab initiosimulations of hydroxylation and dehydroxylation reactions at surfaces: amorphous silica and brucite

Hydroxylation and dehydroxylation reactions at the surfaces of amorphous silica and Mg(OH) 2 have been studied by ab initio Car-Parrinello molecular dynamics simulations. A particular topological defect on the amorphous silica surface, the two-membered (2M) silicon ring, is shown to react fast with water thus confirming the experimental assignment of the most reactive surface sites. The mechanism of the chemisorption of both water and ammonia on the 2M ring has been identified. Ab initio simulations on Mg(OH) 2 have provided information on the structural properties of the low-indices surfaces (0001), (11̄00) and (1000) of brucite. Comparison of the calculated dehydroxylation energy at surfaces and in the bulk suggests that the decomposition of brucite into MgO and water would preferentially occur at surfaces.