Effect of heat treatment on boron impurity in Vycor. Part II. Migration, reactivity with vapour water and dissolution in liquid water

With increasing temperature and time of heat pretreatment of Vycor porous glass, near infrared absorption related to the formation of surface boranol groups increased concurrently with the decrease of the absorption spectrum of silanol groups. This trend corresponds to the migration of boron toward the surface, as previously suggested in the literature. The surface boron is much more soluble than the surface silicon species in pure water at room temperature. One pretreatment of Vycor at 900 degrees C for 20 h allows the dissolution of 35% of the total pristine boron, corresponding to 1.6 soluble B atoms nm(-2). The thermal migration of boron is driven by a concentration gradient between surface and inner sites, since it is accelerated by dissolving surface boron between thermal stages at a given temperature. On preheated Vycor, a much quicker and more important formation of boranols than silanols by dissociative chemisorption of water is observed under a low vapour pressure and B2O3 aggregates have been detected.