Al Valence Controls the Coordination and Stability of Cationic Aluminum-Oxygen Clusters in Reactions of Al _n⁺ with Oxygen

The reactivity of cationic aluminum clusters with oxygen is studied via a customized time-of-flight mass spectrometer. Unlike the etching effect for anionic aluminum clusters exposed to oxygen, here, the cationic Al_n⁺ clusters react and produce a range of small Al_nO_m⁺ clusters. Relatively large-mass abundances are found for Al₃O₄⁺, Al₄O₅⁺, and Al₅O₇⁺ at lower O₂ reactivity, while at higher O₂ concentration, oxygen addition leads to Al₂O₇⁺, Al₃O_6,8-10⁺, and Al₄O_7,9⁺, showing relatively high abundance, and Al₅O₇⁺ remains as a stable species dominating the Al₅O_m⁺ distribution. To understand these results, we have investigated the structures and stabilities of the Al_nO_m⁺ clusters. First-principles theoretical investigations reveal the structures, highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gaps, fragmentation energies, ionization energies, and Hirshfield charge of the Al_nO_m⁺ clusters (2 ≤ n ≤ 7; 0 ≤ m ≤ 10). Energetically, Al₃O₄⁺, Al₄O₅⁺, and Al₅O₇⁺ are calculated to be most stable with high fragmentation energies; however, they still allow for the chemisorption of additional O₂ with large binding energies leading to clusters with higher O/Al ratios. The stability of the species is consistent with Al possessing three valence electrons, while O typically accepts two, leading to the expectation that Al₃O₄⁺, Al₅O₇⁺, and Al₇O₁₀⁺ are reasonably stable. In addition to this, Al₃O⁺, Al₅O₃⁺, and Al₇O₅⁺ are found to exhibit large HOMO-LUMO gaps associated with the different oxidation states of Al. The oxygen-rich species such as Al₂O₇⁺, Al₃O₁₀⁺, and Al₄O₉⁺ all display superoxide structures providing further insights into the oxidation of aluminum clusters.