Water Storage Capacity and Mechanism of Aluminum Oxide Cluster Cations in the Gas Phase

Desorption processes of H₂O molecules from Al_<i>n</i>O_{(3<i>n</i>-1)/2}(H₂O)_<i>i</i>⁺ (<i>n</i> = 3, 5, 7) and Al_<i>n</i>O_3<i>n</i>/2(H₂O)_<i>i</i>H⁺ (<i>n</i> = 4, 6, 8) clusters were investigated using gas-phase thermal desorption spectrometry to evaluate the H₂O storage capacity and mechanisms of aluminum oxide clusters. The clusters stored approximately 10 H₂O molecules at ∼300 K, depending on the size (<i>n</i>), and released them upon heating. Even after heating to ∼1000 K, 2-4 H₂O molecules remained bound. The H₂O desorption energy (Δ<i>E</i>_{<i>n</i>,<i>i</i>}) from each composition of Al_<i>n</i>O_{(3<i>n</i>-1)/2}(H₂O)_<i>i</i>⁺ or Al_<i>n</i>O_3<i>n</i>/2(H₂O)_<i>i</i>H⁺ was determined based on the temperature dependence and compared with the DFT-calculated values. Based on the Δ<i>E</i>_{<i>n</i>,<i>i</i>} and theoretically calculated structures, three modes of H₂O storage were found, i.e., hydrogen-bonded, coordination-bonded, and dissociated H₂O molecules, the binding energy of which tends to increase in this order. Compared with Si-oxide cluster cations, the formation of coordination bonds is a unique characteristic of Al atoms. While both Al and Si atoms prefer tetrahedral coordination, Al atoms can adopt higher coordination numbers (5 and 6). These properties of Al atoms confer greater geometric flexibility during H₂O adsorption and desorption, potentially enhancing H₂O storage capability when Al is incorporated into silica-based compounds.