A Theoretical Study On The Adsorption And Stability Of Water Molecules On Oxide Surfaces

Titanium dioxide（TiO₂）and silicon dioxide（SiO₂）are two common oxides,often used as catalyst carrier in heterogeneous catalysis.The properties and stabilities of TiO₂ and SiO₂ are often affected by the presence of water molecules or hydroxyl groups.In this work,five periodic crystalline surface models including TiO₂（101）,TiO₂（001）C（100）,C（101）and C（011）surfaces with different water and hydroxyl concentrations were investigated using first principles density functional theory（DFT）calculations.On the basis of these results,desilication processes over three fully hydroxylatedβ-cristobalite silica surfaces in the presence of methanol,were also studied.The main results are summarized as follows.（1）The interaction of water molecules with two low-index crystalline anatase surfaces,i.e.,TiO₂（101）and TiO₂（001）and three low-index crystallineβ-cristobalite silica surfaces under various water and hydroxyl coverages were studied using DFT calculations.It has been found that water molecules spontaneously dissociate into surface hydroxyls over TiO₂（001）,C（101）and C（011）surfaces.The completely dehydrated temperatures of TiO₂（101）and TiO₂（001）surfaces are 450 K and 1000 K,respectively.In addition,using BF₃ and NH₃ as probe molecules,the effects of hydroxyl coverages on the surface acidity and basicity of TiO₂（101）and TiO₂（001）were evaluated.（2）The Gibbs free energies of C（100）,C（101）and C（011）surfaces were calculated under different hydroxylation conditions.The morphology evolution and surface hydroxyl coverages ofβ-cristobalite nanoparticles in the temperature range of100～2100 K were predicted by using the Wulff construction principle.Under the temperature below 1000 K,the fully hydroxylated C（100）and C（101）surfaces are main surface structures over the rod-shapedβ-cristobalite.When the temperature is increased over 1700 K,theβ-cristobalite silica is in the octahedral shape with expsed fully dehydroxylated C（101）surface only.Finally,the surface desilication processes over three fully hydroxylatedβ-cristobalite silica surfaces in the presence of methanol molecules were studied.Compared with the fully hydroxylated C（100）and C（101）surfaces.Which surface desilication is laregely thermodynamically and kinetically hindered,the desilication process of the fully hydroxylated C（011）surface is relatively feasible.