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

Calcined molybdenum-containing catalysts, prepared by impregnation of Al2O3 with heptamolybdate, were doped with different amounts of K+ ion s. Two other catalysts with a K/Mo ratio of 2 were prepared by impregn ation of Al2O3 with K2MoO4. The Mo surface structures of these catalys ts were characterized by X-ray absorption and Raman spectroscopy. The undoped samples contained the molybdenum in a 6-fold coordination as p olymolybdate. With increasing K+ content, these polymolybdate species depolymerized progressively. At the K/Mo ratio of 2, most of the Mo io ns were in a 4-fold coordination. The samples mainly contained K2MoO4 differently distorted than in crystalline K2MoO4, probably due to an i nteraction with the alumina support. The composition of the samples se emed to determine predominantly the nature of the Mo species stabilize d on the alumina in the catalysts with K/Mo = 2.