1 research outputs found
Structure of Isolated Molybdenum(VI) and Molybdenum(IV) Oxide Species on Silica: Periodic and Cluster DFT Studies
The structure of monomeric molybdenum oxide species on
silica is
still a subject under debate. In this work, a large number of advanced
silica models are used to study molybdena–silica system with
density functional theory. The calculated relative energies of the
monooxo and dioxo MoÂ(VI) species depend on the location of the Mo
center on the surface and on the structure of the model. Periodic
and cluster calculations employing comparable models of silica give
similar results. It is shown that the monooxo MoÂ(VI) species can be
more stable than the dioxo species under dehydrated conditions, provided
that the local structure of silica enables preferable 4-fold bonding
to the surface. As most locations are favorable for the 2-fold bonded
dioxo MoÂ(VI) species, they should be dominant in the molybdena–silica
system, whereas the monooxo MoÂ(VI) species are predicted to be in
minority. The calculated frequencies of the Moî—»O stretching
mode for the monooxo MoÂ(VI) species are generally higher than the
frequencies of the symmetric Oî—»Moî—»O stretch for the
dioxo species, corresponding to the strongest band observed experimentally.
The relative energies of the reduced MoÂ(IV) species on silica are
close to the relative energies of the corresponding MoÂ(VI) precursors