Interaction of H-2 with strained rings at the silica surface from ab initio calculations

The interaction of H-2 with two-, three-, and four-member rings at the SiO2 surface has been studied by means of gradient-corrected Density Functional Theory calculations and cluster models. The mechanism of hydrogen dissociation, the energy of reactants and products, and the corresponding activation energies and transition states have been determined as a function of the angular strain and the bond strain ill the rings. Only two-member ring structures give an exothermic addition of H-2 to the Si-O bonds with formation of Si-H and O-H groups. The corresponding energy barrier, however, is relatively high. A similar result is obtained when the Si-O bonds are strained by about 4% with respect to their equilibrium position. (C) 2000 Elsevier Science B.V. All rights reserved.