Molybdena on titania : I. Preparation and characterization by Raman and Fourier Transform Infrared spectroscopy

Abstract

Titania-supported molybdenum and cobalt-molybdenum catalysts were prepared by the dry impregnation and equilibrium adsorption techniques. Raman and Fourier Transform Infrared spectroscopy were used to follow the changes the catalysts underwent during the drying and calcination stages. The results indicate that the nature of the surface species depends on the loading, pH of the starting solution and the method of catalyst loading. It was found, for coverages calculated to be less than a monolayer, that the main surface species are tetrahedral molybdates. As the loading increases, octahedrally coordinated polymeric surface species form. Above the monolayer coverage, MoO3 is also formed. After calcination the spectra of catalysts prepared by the equilibrium adsorption technique change much less than those for catalysts prepared by the dry impregnation technique. Addition of cobalt to titania-supported molybdenum catalysts suppresses the formation of bulk MoO3 and leads to the formation of a cobalt molybdate phase. The results show that surface molybdates interact strongly with the support, TiO2.