Structure and dispersion of molybdenum oxide supported on alumina and titania

Samples of molybdenum oxide supported on alumina and titania have been studied by oxygen chemisorption, X-ray diffraction and laser Raman spectroscopy. The three techniques gave a consistent picture of the dispersion of molybdenum oxide on the supports. For the alumina-supported samples it was found that reduction and reoxidation at 673 K were appropriate for titrating surface molybdenum atoms. This temperature was found to be lowered to 623 K for the titania-supported materials. The structure of the dispersed phase on both supports was found to be the same. In the absence of water, it consisted of a monomeric OMoO4 species with a single OMo double bond, four O—Mo links to the surface, and a possible long O⋯Mo bond opposite the double bond. With the introduction of water vapour, this species became a dioxo species with two OMo double bonds and two O—Mo links to the surface. The interaction of the oxide with the support was of an acid–base nature.