Ab initio simulations of hydroxylation anddehydroxylation reactions at surfaces: amorphous silica andbrucite

Abstract

Hydroxylation and dehydroxylation reactions at the surfaces ofamorphous silica and Mg(OH)2 have been studied by ab initioCar-Parrinello molecular dynamics simulations. A particulartopological defect on the amorphous silica surface, thetwo-membered (2M) silicon ring, is shown to react fast with waterthus confirming the experimental assignment of the most reactivesurface sites. The mechanism of the chemisorption of bothwater and ammonia on the 2M ring has been identified.Ab initio simulations on Mg(OH)2 have provided information onthe structural properties of the low-indices surfaces (0001),(11̄00) and (1000) of brucite. Comparison of the calculateddehydroxylation energy at surfaces and in the bulk suggests thatthe decomposition of brucite into MgO and water wouldpreferentially occur at surfaces.