Water storage capacity of closed-shell silicon oxyhydroxide cluster cations in the gas phase

Abstract

Abstract Stable compositions and H2O desorption processes of SinO2n+iH2i+1⁠+ (nominally, (SiO2)n(H2O)iH+; n = 3 to 10) clusters have been studied by gas-phase thermal desorption spectrometry coupled with density functional theory (DFT) calculations. Five or six H2O molecules were found to be stored (i = 5, 6) mainly in the clusters at room temperature. The clusters sequentially released H2O molecules upon heating to form the species with i = 2 (n = 3–5, 8) and 3 (n = 6, 7, 9, 10) as the most prominent compositions at 1,000 K. The desorption energies of H2O molecules from the clusters were evaluated from the temperature dependence data for n = 3 to 5. The experimental and theoretical results suggest that (SiO2)n(H2O)iH+ clusters with higher i (e.g. i ≥ 4 for n = 3) involve H2O molecules bound with hydrogen bonds and can easily release the H2O molecules (ΔE < ∼0.5 eV); species with lower i (e.g. i = 2 and 3 for n = 3) form H2O molecules from two hydroxide groups, whose desorption requires higher energies (ca. 1 to 2 eV); and H2O release is hampered if the product species does not maintain the structure with tetrahedrally coordinated Si atoms.