ABSTRACT
VI.ActivityofDifferentTypesofSurfaceSpecies45 A.Protolyticandionexchangeproperties46 B.Hydrogenbonding49 C.Activitybasedonabstractionofprotonfromsurfacesilanols52
VII .ModifiedSurfaceofSilica
References
I.INTRODUCTION
Silica,Si02,isagiantatomicstructureinwhicheachsiliconatomisbondedtofour oxygenatoms,andeachoxygenatomtotwosiliconatomsinsuchawaythateach siliconatomisatthecenterofaregulartetrahedronofoxygenatoms.Itisasolidof highmeltingpoint(1700°C),densitybetween2and3g/cm3andrefractiveindexin therange1.5-1.6.Theelectronicconfigurationofthesiliconatomsuggeststheidea oftheformationofbondsbasedontetrahedrallyorientedhybridsp3orbitals. Naturallyoccurringsilicacomprisesquartz,tridymite,cristobalite,coesite,keatite,
stishovite, and opal. These polymorphs exhibit a framework of tetrahedral motif; however, the pattern of linkage for each one is different, which appears in the different structural, physical, and chemical properties. Stishovite, a high-temperature form of silica, is unique among these polymorphs in being characterized by a skeleton in which silicon atoms are octahedrally 6-co-ordinated to oxygen atoms. Hybrid sp3 orbitals are present even if the silicon is bonded to three oxygen atoms only, but it is no longer stable when silicon remains bonded to only two oxygen atoms [1]. It is known, however, that owing to free orbital d, silicon can co-ordinate octahedrally, provided that the ligand is small and slightly polarizable.
