STRUCTURE OF K-DOPED MOLYBDENA-ON-SILICA CATALYSTS AS STUDIED BY X-RAY-ABSORPTION AND RAMAN-SPECTROSCOPY

A calcined molybdenum catalyst prepared by impregnation of SiO2 with h eptamolybdate has been doped with different amounts of K+ ions. The ca talysts were characterized by X-ray absorption and Raman spectroscopy. Due to the lack of interaction of the polymolybdate species with the carrier in the Kf-free catalyst, MoO3 was formed after calcination. Wi th increasing K+ content in the catalyst, the surface area of the SiO2 support drastically decreased and structural deformations occurred. T he resulting molybdenum structures were well defined. In the uncalcine d samples with ratios K/Mo less than or equal to 1, the molybdenum was 6-fold coordinated. At a K/Mo ratio of 2, the coordination changed in to 4-fold and the samples contained mainly K2MoO4, which was different ly distorted than in crystalline K2MoO4. Impregnating SiO2 with K2MoO4 led to spectra, which indicated a weak interaction of the Mo species with the SiO2 carrier.