Computational Investigation of TiO₂-Supported Isolated Oxomolybdenum Species

Abstract

We present periodic density functional theory (DFT) calculations combined with thermodynamic analysis to study the structure of isolated molybdenum oxide entities supported on titania (anatase) under ambient and dehydrated conditions. The TiO 2 support is represented by the perfect and hydrated ( 101) and ( 001) surfaces. The calculation of the vibrational wavenumbers of the stable structures under various conditions allows us to access to structural information by comparison with the experimental data obtained in in-situ conditions. The calculation of the most stable model on the (101) surface indicates that in dry conditions molybdenum is in a distorted tetrahedral environment with a single molybdenyl bond whereas dioxo entities are more stable on the (001) surface. Furthermore, it appears that the ModO stretching wavenumber is strongly influenced by the hydration state of the surface through formation of hydrogen bonds with the surface OH or H 2 O groups that induce a shift to lower wavenumber (more than 60 cm -1 ) in agreement with the Raman shift observed during the hydration-dehydration process.

Molybdenum Trioxide on Anatase TiO2(101) - Formation of Monodispersed (MoO3)1 Monomers from Oligomeric (MoO3)n Clusters

“…During the last decade, Paul and co-workers investigated the structure of MoO3 and Mo2O6 species on anatase TiO2(101). [42][43] They found that the "distorted tetrahedral" mono-oxo species is the most stable monomeric (MoO3)1 structure, which is largely in agreement with the results presented here.…”

“…It is worth noting that the most stable (MoO3)1 structure we have identified here is the same as that found by Paul and co-workers using a purely GGA functional (i.e., without dispersion or correlation corrections). [42][43] Additionally, despite the different surface, the MoO3 structure on rutile TiO2(110) found by Metiu and co-workers 44 is also nearly identical, displaying the same coordination environment as the monomer shown in Figure 3c (two Mo-O2c and two O-Ti5c bonds, a single Mo=O). While the (MoO3)1 monomer shown in Figure 3b has slightly stronger adsorption energy (by ~5 kJ/mol/(MoO3)1) than the (MoO3)1 monomer shown in Figure 3c, the two are well within typically cited DFT errors and thus should be regarded as essentially isoenergetic.…”

“…From a computational standpoint, many studies have been carried out to elucidate the structure, reducibility and stability of supported molybdenum oxide clusters. [37][38][39][40][41][42][43][44] However, relatively few computational studies have dealt with the issue of molybdenum oxide clusters supported on TiO2.…”

Structure and reducibility of titania-supported monomeric and dimeric molybdenum oxide entities studied by DFT calculations

“…However, it is fully dehydrated above 500 K, which is a lower limit for the catalytic oxidation involving MoO x deposited on TiO 2 . Furthermore, in a previous work [27], we have studied the structure and hydration properties of the low loading (isolated) molybdenum oxide supported on anatase and demonstrated that the presence of adsorbed oxomolydemum species does not change significantly the dehydration temperature. The effect of water should be the negligible at the reaction temperature and we did not take into account.…”

“…In a previous work [27], we studied different models for the monomeric MoO 3 species on the (1 0 1) anatase surface. The most stable structure of isolated molybdenum oxide entities supported on the most stable anatase surface (1 0 1) in dehydrated environment is represented in Fig.…”