Water Storage Forms of Aluminum-Silicon Composite Oxide Cluster Cations in the Gas Phase

The water (D2O) storage ability and mechanism of aluminum-silicon composite oxide clusters were investigated using gas-phase thermal desorption spectrometry and quantum chemical calculations. Thermally stable compositions were identified for closed-shell species containing one or two Al atoms, namely AlSimO2m+1(D2O)i+ and Al2SimO2m+3(D2O)iD+, respectively. The temperature dependence of D2O desorption from AlSimO2m+1(D2O)i+ was then analyzed to elucidate the nature of water binding. In these oxide clusters, Si and Al atoms preferentially adopt tetrahedral coordination geometries. Upon adsorption, D2O initially undergoes dissociative adsorption, followed by molecular adsorption involving coordination and hydrogen bonding. D2O molecules preferentially coordinate to Al atoms, whereas coordination to Si atoms is not observed. In some cases, different D2O adsorption structures are energetically competitive, with similar energies simulated for coordination and hydrogen bonding configurations. This suggests enhanced flexibility in D2O storage forms, probably originating from the greater structural complexity of the Al-Si mixed oxide system compared to those of the Al or Si oxide clusters. This behavior indicates a synergistic effect rather than a simple integration of the individual features of Al and Si atoms.