Surface structure of highly dispersed MoO3 on MgO using in situ Mo L3-edge XANES

The evolution of the dispersed surface molybdate phase on high-surface-area magnesium oxide is studied as a function of weight loading of MoO3 and calcination temperature using in situ Mo Ls-edge X-ray absorption near edge spectroscopy, in situ laser Raman spectroscopy, and thermogravimetry-mass spectroscopy. Under ambient, hydrated conditions, the magnesium support is present as magnesium hydroxide, rather than MgO, and the molybdate species is present on the surface as an isolated tetrahedral species for weight loadings of 5-20 wt % MoO3. Under these hydrated conditions the form of the molybdate species is controlled by the net surface pH at the point of zero charge and is the same as that observed in aqueous solution. As the catalyst is calcined in air, the molybdate species transforms from a tetrahedral species, MoO4, to an octahedral one, MoO6. This transformation occurs at the same temperature, similar to 450 degrees C, as that at which the support changes from hydroxide to oxide. This transformation temperature is independent of the MoO3 loading. It is suggested that it is this support-induced structural change that drives the change in the form of the surface molybdate species and not the desorption of adsorbed water.