Reaction Mechanisms in Irradiated, Precipitated, and Mesoporous Silica

Abstract

A matrix EPR spectroscopy study of the low temperature γ radiolysis of precipitated (Zeosil) and mesoporous high surface silica has afforded evidence of the formation of trapped H-atoms, H-atom centers, siloxy radicals ≡Si-O(•), anomalous silyl peroxy radicals ≡Si-OO(•) with reduced g tensor anisotropy, siloxy radical-cations (≡Si-O-Si≡)(+•), E' centers, and two species from Ge impurity. Coordination of peroxyl radicals with diamagnetic ≡Si(+) centers is proposed and tested by DFT computations in order to justify the observed g tensor. Coordination of H-atoms to ≡Si(+) centers is also proposed for the structure of the H-atom centers as an alternative model not requiring the intervention of Ge, Sn, or CO impurities. The DFT method has been employed to assess the electronic structure of siloxy radical-cations and its similarity with that of the carbon radical-cation analogues; the results have prompted a revision of the structures proposed in the literature for ST1 and ST2 centers. The comparison between the two types of silica has afforded evidence of different radiolysis mechanisms leading to a greater yield of trapped H-atoms and H-atom centers in zeosil silica, which is reckoned with the 4-fold greater concentration of silanol groups. Parallel radiolysis experiments carried out by using both types of silica with polybutadiene oligomers as adsorbate have afforded evidence of free valence and energy migration phenomena leading to irreversible linking of polybutadiene chains onto silica. Reaction mechanisms are proposed based on the detection of SiO2-bonded free radicals whose structure has been defined by EPR.

Strained surface siloxanes as a source of synthetically important radicals

“…The narrow, intense signal at 3336 G (g ¼ 2.0026) is typical of a CHbased radical that is part of an extensive hydrocarbon chain, 24 with the two smaller, broad signals at 3245 G (g ¼ 2.0569) and 3278 G (g ¼ 2.0329), attributable to the presence of low concentrations of silyl peroxy and organo peroxy radicals, respectively, in the PMA. 25 The EPR spectrum displays no distinct hyperne structure, suggesting the radical is delocalised throughout the polymer, while the experimental g-value of 2.0026 correlates well with other low-temperature results for PMA. Reports in the literature also indicate that the EPR spectra of PMA samples vary dramatically depending on instrument temperature and irradiation time.…”

Reinforced silica monoliths functionalised with metal hexacyanoferrates for cesium decontamination: a combination of a one-pot procedure and skeleton calcination

“…This is due to the fact that these monoliths are only made of silica and MHCF without additional organic compounds, such as graing molecules for example, that could induce hydrogen release when exposed to irradiation coming from radionuclides. Indeed, past studies have already shown that silica 59 and MHCF [60][61][62] were quite stable under gamma irradiation.…”

Towards controlled cationic polymer growth from inorganic oxide defects: Directing the mechanism of polystyrene grafting from γ-irradiated silica

“…In addition to radiolized silicainitiated polymerization, one should also keep in mind that styrene autopolymerizes thermally via a well-known radical initiation route, and that free radical species may be trapped by g-irradiated silica, independently of silica-induced initiation. Furthermore, other radiation-induced species [13] migrating from the bulk of silica to its surface, may initiate free polymerization independently of grafting. In Fig.…”

“…E 0 g centers can be long-lived enough to be involved in the reactivity of radiolized silica [14]. EPR studies of radiation-induced defects in silica trace back to the 1950's [17], while the mechanisms involved in their reactivity, their annealing by diffusing mobile species, and their stability have been the object of recent investigations [13,14].…”