RAMAN AND ULTRAVIOLET SPECTROSCOPIC CHARACTERIZATION OF MOLYBDENA ON ALUMINA CATALYSTS

Molybdena catalysts supported on γ- and η-Al2O3 have been prepared by impregnation (pore filling method) from aqueous molybdate solutions at pH 6 and pH 11, which contain respectively the Mo7O246- and MoO42- ion. The development of the final oxidic state of the catalysts during impregnation, drying, and calcination was followed by Raman and ultraviolet spectroscopy. An assignment of the vibrational bands and of the ligand charge transfer bands of the molybdena surface species was made on the basis of the corresponding solution spectra of a variety of isopolymolybdates. It is concluded that even during impregnation from solutions at pH 6 the initial species which ion exchanges with surface OH groups is the tetrahedral MoO42- ion. The degradation of the Mo7O246- ion is brought about by an increased pH at the support surface. Depending on the total loading, a more or less extensive formation of Mo-O-Mo bridges occurs during the drying and calcination procedure; a polymeric surface molybdate species is formed. The Mo coordination cannot unambiguously be determined. © 1979 American Chemical Society.