Investigation on the generation process of HO2 radicals by γ-ray irradiation in O2-loaded fumed silica

Abstract We report an experimental investigation on the effects of γ-ray irradiation in three types of fumed silica previously loaded with O 2 molecules. Our data indicate that the main effect of irradiation in these systems is to generate a very large concentration of HO 2 interstitial radicals (about 10 18 molecules/cm 3 ). Furthermore, the number of generated HO 2 was found to be larger in the samples with higher O 2 contents before irradiation. This correlation suggests that HO 2 radicals are induced by reaction of interstitial O 2 molecules with radiolytic H atoms, as previously suggested for O 2 -loaded bulk amorphous silicon dioxide (a-SiO 2 or silica) samples. However, at variance with respect to bulk materials, in fumed silica the radiolytic H does not arise from SiOH or SiOOH groups, as no EPR signal due to non-bridging oxygen hole centers (NBOHC) or to peroxy radicals (POR) is detected in the spectra of irradiated samples. As a reasonable alternative we propose that radiolytic hydrogen atoms could arise from a radiation induced breaking of interstitial H 2 O molecules, indicating that fumed silica in its pristine form could possess a very large concentration of interstitial water molecules.