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

Molybdenum oxide deposited on anatase can be used to catalyse mild oxidation of hydrocarbons or alcohols. The nature of the active site is still under debate even if the catalytic activity has been related to the reducibility of the deposited phase. After the determination of MoO3 and Mo2O6 adsorption geometries, we have investigated, by DFT calculations, the effect of the molecularity on the molybdenum reducibility. In dehydrated conditions, the monomeric species is in a rigid tetrahedral geometry and is less reducible than the support. Because of the interaction with the support's surface, the molybdenum atoms in a dimeric specie are not in a regular tetrahedral environment. This distorted geometry allows an important reconstruction upon reduction that favours the reduction of the molybdenum phase instead of the support and leads to the formation of paramagnetic centres upon reduction. As a consequence, the polymeric species may be active in oxidation reactions, whereas monomeric species are not. In order to strengthen the conclusions, the reduction of a tetrameric species has also been investigated and shows that it can be reduced by H2.