| Abstract
| - The interaction of H2 with the defect sites of the SiO2 surface has been studied by means of gradient-correcteddensity functional theory calculations on cluster models. The mechanism of hydrogen dissociation, the energyof reactants and products, and the corresponding activation energies and transition states have been determinedfor the following defect sites: Si singly occupied sp3 dangling bonds (E‘ centers), ⋮Si•; nonbridging oxygencenters (NBO), ⋮Si−O•; divalent Si, Si:; and neutral oxygen vacancies, ⋮Si−Si⋮. H2 cracking on theNBO sites is exothermic by ∼0.4 eV and has an energy barrier of ∼0.1 eV (or less considering nonadiabaticeffects) which suggest the occurrence of the process even at low temperature. On Si dangling bonds theformation of ⋮Si−H and neutral H atom is endothermic and occurs with an activation energy of less than 0.5eV; the reaction can occur at room temperature. The interaction of molecular hydrogen with the diamagneticoxygen deficient centers, Si: and ⋮Si−Si⋮, leads to the formation of stable ⋮Si−H groups with exothermicprocesses and relatively high activation energies of about 2 eV. Thus, H2 cracking is predicted to occur atroom temperature on paramagnetic defects and only at high temperatures on the diamagnetic centers.
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