Electron-Stimulated Hydroxylation of Silica Bilayer Films Grown on Ru(0001): A Combined HREELS and EPR Study

Abstract

Electron-assisted hydroxylation of single-crystalline silica bilayer films grown on Ru(0001) is studied by high-resolution electron energy loss spectroscopy (HREELS) and electron paramagnetic resonance (EPR) spectroscopy. The HREELS results reveal the formation of several hydroxyl species whose number and speciation depend on the defect structure of the film. For incomplete bilayer films, which exhibit nanometer-sized holes in the bilayer, the level of hydroxylation is significantly larger than for complete films. HREEL spectra taken in off-specular geometry provide evidence for the presence of hydroxyl groups with a transition dipole moment almost parallel to the surface for complete and incomplete bilayer films. Hydroxylation with isotopically labeled water (H2 18O) reveals a clear difference between the two casesOH species on the incomplete film almost exclusively contain oxygen from water, while the more ideal film exhibits OH groups with oxygen atoms stemming from both water and the silica film. These observations not only indicate that the degree of hydroxylation is significantly enhanced for the incomplete film but also that the reaction mechanism for hydroxylation at defect sites of this film is different. To gain insight into the reaction mechanism of electron-assisted hydroxylation, in situ EPR spectroscopy of electron-bombarded adsorbed ice layers was combined with infrared (IR) spectroscopy and mass spectrometry. We show that the electron bombardment removes a significant part of the water layer and produces different reactive paramagnetic species, namely, O2D, D, and solvated electrons, which may be trapped at low temperatures. The interaction of the silica film with such species may lead to splitting Si–O bonds even for covalently saturated silica structures as found in the ideal bilayer film and thus provide insight into possible reaction mechanisms.

Mixed Germania‐Silica Films on Ru(0001): A Combined Experimental and Theoretical Study

“…Specifically, silica is used as a support material for catalytically active materials. Moreover, two‐dimensional silica films may be hydroxylated [4] . Modified two‐dimensional silica films are very suitable model systems for catalysts, due to their reduced complexity compared to powder‐based catalysts.…”

“…Moreover, two-dimensional silica films may be hydroxylated. [4] Modified two-dimensional silica films are very suitable model systems for catalysts, due to their reduced complexity compared to powder-based catalysts. As these model systems are accessible to a variety of surface-science characterization methods and theoretical calculations, fundamental processes in catalysis can be studied at the atomic level.…”