Structure of Isolated Molybdenum(VI) and Molybdenum(IV) Oxide Species on Silica: Periodic and Cluster DFT Studies

Abstract

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 Mo(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 Mo(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 Mo(VI) species, they should be dominant in the molybdena−silica system, whereas the monooxo Mo(VI) species are predicted to be in minority. The calculated frequencies of the MoO stretching mode for the monooxo Mo(VI) species are generally higher than the frequencies of the symmetric OMoO stretch for the dioxo species, corresponding to the strongest band observed experimentally. The relative energies of the reduced Mo(IV) species on silica are close to the relative energies of the corresponding Mo(VI) precursors.

Gas-Phase Selective Oxidation of Methane into Methane Oxygenates

“…These theoretical results are consistent with the Raman studies of Lee and Wachs [48,49]. Handzlik et al [50] studied the structure of monomeric molybdenum oxide species in amorphous silica systems using DFT. The results showed that as long as the local structure of silica could be well four-fold bonding to the surface, the monooxo Mo(VI) species would be more stable than the dioxospecies under dehydration conditions, which are rare.…”

Acid sites on silica-supported molybdenum oxides probed by ammonia adsorption: Experiment and theory

“…S5), was generated (Fig. 9B) [45]. Such a strained configuration resulted in an N-H distance of 1.09 Å in the energetically optimized adsorption complex, hence, the interaction of the ammonia molecule with the silanol group results in a nitrogen atom that is in contact with four H atoms in a distance < 1.1 Å, similar like in an ammonium ion.…”

“…Density functional theory (DFT) calculations were performed to simulate Lewis and Brønsted acidity of the silica-supported molybdenum oxides by applying cluster models adapted from previous studies [42][43][44][45][46]. The Becke three-parameter hybrid functional with Lee-Yang-Parr correlation functional (B3LYP) together with Ahlrichs' triple-zeta split-valence basis set augmented by polarization functions (def2TZVP or def2TZVPP, respectively) were chosen for the calculations [47,48].…”

Formation of intrinsic and silicon defects in MoO3 under varied oxygen partial pressure and temperature conditions: an ab initio DFT investigation

“…Previously, extrinsic defects in MoO 3 crystals such as Na, 47 Li 3,51 and H 52,53 have been studied with DFT, but so far DFT studies of the interaction between MoO 3 and silicon have only examined molybdenum oxides on silica surfaces. [54][55][56] In this paper, the possibility of silicon diffusion in crystalline MoO 3 is investigated and its effect on electronic properties is explored.…”