Molybdenum Oxide Clusters: Structure, Stability, and Electronic Properties

Ab initio calculations were carried out to understand the structural, electronic, and energetic properties of molybdenum oxide clusters, MoO (m = 1-6; n = 1-3m), to understand the relationships between size, composition, and reactivity. In clusters with a low oxygen-to-molybdenum ratio, there are bridge-bonded and linearly bonded oxygen atoms on a molybdenum core, while at higher ratios, Mo atoms are separated from each other and oxygen atoms located between the molybdenum atoms. The energy gap between the highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) widens with n, i.e., at a high oxygen-to-molybdenum ratio. Stoichiometric MoO₃ clusters (m > 1) have a HOMO-LUMO gap that ranges from 2.6 to 3.4 eV in neutral conditions and less than 0.6 eV in ionic states. The ionization potential of MoO₃ clusters is higher than 10 eV. MoO clusters qualitatively and quantitatively exhibit a similar electronic structure to the bulk. The energy of the reduction reaction, MoO → MoO_-1 + 1/2O₂, is on average lower in clusters with high oxygen content; for example, the reduction energies of Mo₆O₁₈ and Mo₆O₉ are 2.23 and 5.19 eV, respectively. In the fragmentation of MoO clusters, the general trend for clusters with a low oxygen-to-molybdenum ratio is the separation of a Mo atom or a Mo₂ dimer from the cluster, while clusters with higher oxygen content mostly form stoichiometric MoO₃, Mo₂O₆, and Mo₃O₉ clusters.