Structures of the Interacting Species of Potassium Promoter and Molybdenum Components on Oxide Supports

Laser Raman spectroscopy and X-ray diffraction were used to study the interaction of potassium promoter with the molybdenum component supported on SiO₂, γ-Al₂O₃ and ZrO₂. It was found that the composition of the surface interacting species composed of potassium and molybdenum component strongly depend upon thd interaction of the Mo（Ⅵ） and supports and the content of the potassium. In the case of SiO₂ where the Mo（Ⅵ） weakly interacted with the support, the species of K₄Mo₈O₂₆, along with a small amount of K₆Mo₇O₂₄, formed at the low potassium loading and gradually degraded with the increase of the K / Mo atomic ratio, finally into K₂Mo₂O₂₄ at the K/ Mo ratio of 1.3. As a contrast to SiO₂ only the species of K₆Mo₇O₂₄ was detected on γ—Al₂O₃ over broad K/Mo ratio. The potassium ions interacted with the Mo（Ⅵ） to form the K₆Mo₇O₂₄ species was saturated stoichiometrically at the K/Mo ratio of 0.8. Beyond this ratio, the surplus potassium ions combined with the chloride anions to form the KCI crystallites. The interaction between the Mo（Ⅵ） and ZrO₂ was sufficiently strong. The main molybdenum species on ZrO₂ without the promotion of potassium were the tetrahedral and octahedral coordinated Mo（Ⅵ） as well as a small amount of Zr（MoO₄）₂ at a MoO₃ loading corresponding to about monolayer （8% wt）. With the promotion of potassium, however, though the amount was small, the species of K₄Mo₈O₂₆ could be detected by LRS and gradually degraded into K₆Mo₇O₂₆ with the increase of K/Mo ratios. This phenomenon became more obvious at the high loading of MoO₃（18% wt）.