Structure of Dimeric Molybdenum(VI) Oxide Species on γ-Alumina: A Periodic Density Functional Theory Study

The structure and stability of dimeric Mo(VI) oxide species on gamma-alumina are investigated with a periodic DFT approach. A large number of Mo dimers of various geometries, located on the (100) and (110) surfaces of gamma-Al2O3, are modeled. The most stable dimeric species on the (100) plane are mixed dioxo-monooxo or double-monooxo species that are 3-, 4-, or 5-fold bonded to the surface. In most eases, the molybdenum atoms are pseudotetrahedrally coordinated. In the case of the (110) surface, the most stable dimeric Mo(VI) forms are double-monooxo species, which are 6- or 5-fold bonded to the support. The coordination of the molybdenum atoms is most frequently a distorted square pyramidal, but distorted octahedral or distorted tetrahedral geometry also happens. The energetic stability of the dimeric Mo(VI) species, relative to the corresponding monomeric Mo(VI) centers, is determined. On the minority (100) surface, the complex-surface interaction is moderate and the dimeric Mo(VI) species are more stable than the monomeric Mo(VI) forms. Therefore, the formation of Mo oligomeric species on this surface is predicted. On the majority (110), in contrast, the interaction energy is strong, which favors dispersion, and hence, the monomers are more stable than the dimers so that, even at high Mo loadings, the monomeric Mo species can be still present on gamma-alumina, together with polymeric chains. The calculated Mo=O vibrational frequencies for the Mo(VI) dimers on the (100) surface are higher than for the corresponding monomeric Mo(VI) species and only slightly shifted in the case of the majority (110) surface. Theoretical Mo-O-Mo frequencies match well the experimental values attributed to surface polymeric Mo species on alumina.