Abstract
Hydroxylation and dissolution of well-structured silica bilayer films grown on a ruthenium single-crystal support (SiO2/Ru(0001)) was studied by temperature programmed desorption and X-ray photoelectron spectroscopy (XPS). Water desorption signals from SiO2/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 18O-labeled SiO2 and 16O-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 2p and O 1s XPS signal intensities upon exposure of bilayer SiO2/Ru(0001) to alkaline conditions at various temperatures are in agreement with the proposed rate model for bulk silica dissolution by OH− attack and provide further corroboration of the proposed hydroxylation mechanism.
Highlights
Because of its technological importance and prevalence in nature, silica is widely studied across a wide range of diverse fields, such as geology, electronic devices, sensors, optics, and heterogeneous catalysis
We recall that hydroxylated SiO2/Ru surfaces can be prepared by adsorption of water at 100 K followed by heating to room temperature [32], or, in order to enhance the hydroxyl concentration, with an additional electron irradiation step prior to heating [34, 35]
The more interesting data appears in the region between 200 and 1200 K, where molecular and recombinative desorption of silica-bound D2O and surface hydroxyls are expected [2]
Summary
Because of its technological importance and prevalence in nature, silica is widely studied across a wide range of diverse fields, such as geology, electronic devices, sensors, optics, and heterogeneous catalysis. The interaction of silica within its environment is strongly determined by the abundance and nature of surface functional groups, of which silanols (:Si–OH) are the most important [1]. Not surprisingly, studying the interaction of water with silica polymorphs (crystalline and amorphous) and establishing models for the hydroxylation state of silica surfaces, the surface potential, charge distribution, and dissolution mechanism continue to be active areas of experimental and theoretical research Silanol groups are classified into isolated [terminal, :Si–OH], geminal [=Si– (OH)2], vicinal [HO–Si–O–Si–OH], and hydrogen-bonded [Si–(OH)ÁÁÁ(HO)–Si] [2, 14], and their relative abundance is strongly temperature-dependent, with hydrogen bonded ones being the least, and terminal silanols the most
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