CO2 and C2H4 grafting to the native defects of ion-bombarded porous silica

The radiation damage imparted to highly dispersed silica by ion bombardment may be stabilized by performing the bombardment in a controlled atmosphere . The native defects resulting from the bombardment are the silicon-link va cancy (which in the absence of relaxation can be described as a tetraradica l center (=SiO.)(4), though it is expected to relate via formation of perox idic bridges), and the oxygen-bridge vacancy (which in the absence of relax ation can be described as a diradical center (=Si-.)(2) - the E " center). These radicals react with the residual atmosphere according to completely n ew pathways: the bombardment in a CO2 atmosphere results in the formation o f ester-like and carboxylate groups, stable up to 500 degrees C at least, i nserted in the SiO2 network at the oxygen-bridge vacancies. The bombardment in a C2H4 atmosphere results in more complex configurations: the oxygen-br idge vacancy reacts at room temperature with ethylene forming a Lewis adduc t which, after heating at 500 degrees C, presumably reverts to a -CH2-CH2- bridge in between silicon atoms; the silicon-link vacancy likely reacts wit h C2H4 forming CH3CHO. These conclusions, based on experimental data (mainl y infrared spectroscopy), are also supported by extended quantum mechanical calculations (density-functional methods and ab initio molecular dynamics) .