Molybdenum Oxide Clusters: Structure, Stability, and Electronic Properties.

Abstract

Ab initio calculations were carried out to understand the structural, electronic, and energetic properties of molybdenum oxide clusters, MomOn (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 MomO3m 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 MomO3m clusters is higher than 10 eV. MomOn clusters qualitatively and quantitatively exhibit a similar electronic structure to the bulk. The energy of the reduction reaction, MomOn → MomOn-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 MomOn 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.