Reactivity of Hydroxyl Radicals on Hydroxylated Quartz Surface. 1. Cluster Model Calculations

The interaction of a hydroxyl radical with cluster models of a hydroxylated α-quartz surface was studied by means of density functional calculations. Two molecular models representing isolated (⋮Si OH) and geminal ( Si(OH) 2 ) silanol-terminated quartz surfaces were used. Two reactive sites of the silanol models were investigated: the hydrogen of the silanol group and the surface-approximating silicon atom. The • OH binds weakly to the silanol hydrogen atoms in both models, with a reaction energy of about −7 kcal/mol. The silicon atom in the isolated silanol cluster is not reactive toward the • OH. In contrast, the • OH adsorbs on the Si in the geminal silanol cluster with a reaction energy of −4 kcal/mol. The Si−OSi bond in the resulting pentacoordinated silicon complex is weakened upon the • OH adsorption and can be dissociated. The energy barrier to the dissociation is 4 kcal/mol, and the overall reaction energy is −4 kcal/mol. The dissociation of the Si OSiH 3 bond which mimics the Si O subsurface bonding in the real quartz surface suggests a possibility of • OH activated quartz surface layer disintegration. The calculated energy barrier in this • OH radical activated process is significantly lower than the predicted energy barrier in the rate determining step in the OH - catalyzed quartz surface dissolution (19 kcal/mol). The Si OSi bond rupture followed by formation of a Si O • radical on the surface may provide a plausible mechanism for reactivation of chemically inert, silanol terminated, aged quartz surfaces.