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

Desorption processes of HO molecules from AlO(HO) ( = 3, 5, 7) and AlO(HO)H ( = 4, 6, 8) clusters were investigated using gas-phase thermal desorption spectrometry to evaluate the HO storage capacity and mechanisms of aluminum oxide clusters. The clusters stored approximately 10 HO molecules at ∼300 K, depending on the size (), and released them upon heating. Even after heating to ∼1000 K, 2-4 HO molecules remained bound. The HO desorption energy (Δ) from each composition of AlO(HO) or AlO(HO)H was determined based on the temperature dependence and compared with the DFT-calculated values. Based on the Δ and theoretically calculated structures, three modes of HO storage were found, i.e., hydrogen-bonded, coordination-bonded, and dissociated HO 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 HO adsorption and desorption, potentially enhancing HO storage capability when Al is incorporated into silica-based compounds.