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

Abstract

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 th...