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 (⋮SiOH) 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 SiOSiH3 bond which mimics the SiO 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 SiOSi bond rupture followed by formation of a SiO• radical on the surface may provide a plausible mechanism for reactivation of chemically inert, silanol terminated, aged quartz surfaces.