Topics in Catalysis · 2016 · 25 citations · 34 references
Hydroxylation and dissolution of well-structured silica bilayer films grown on a ruthenium single-crystal support (SiO<sub>2</sub>/Ru(0001)) was studied by temperature programmed desorption and X-ray photoelectron spectroscopy (XPS). Water desorption signals from SiO<sub>2</sub>/Ru(0001) hydroxylated by electron-bombardment of adsorbed ice at 100 K were found to be comparable to those of hydroxylated bulk silica samples and attributed to adsorbed molecular water and silanol groups (vicinal and terminal). Isotopic exchange between <sup>18</sup>O-labeled SiO<sub>2</sub> and <sup>16</sup>O-labeled water suggests the occurrence of dynamic siloxane bond cleavage and re-formation during electron bombardment. Together with the observed strong dependence of hydroxylation activity on ice coverage, which is found to increase with increasing thickness of the ice layer, a hydroxylation mechanism based on the activation of siloxane bonds by water radiolysis products (e.g. hydroxyls) and subsequent water dissociation is proposed. Dissolution rates obtained from the attenuation of Si 2<i>p</i> and O 1<i>s</i> XPS signal intensities upon exposure of bilayer SiO<sub>2</sub>/Ru(0001) to alkaline conditions at various temperatures are in agreement with the proposed rate model for bulk silica dissolution by OH<sup>-</sup> attack and provide further corroboration of the proposed hydroxylation mechanism.
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