Study of the local structure and oxidation state of iron in complex oxide catalysts for propylene ammoxidation

Abstract

Iron molybdate plays a crucial role in the complex oxide catalysts used for selective oxidation and ammoxidation of hydrocarbons but its structural and electronic properties and their changes in the process of the reaction are poorly understood. A combination of Raman, X-ray absorption, and UV-visible spectroscopy was applied to investigate a commercial catalyst as a function of the reaction time. The results show that an iron-containing compound exists predominantly as ferric molybdate in the fresh catalyst, which is reduced progressively in the process of reaction, forming predominantly ferrous molybdate. The irreversible transformation from Fe₂(MoO₄)₃ to FeMoO₄ was accompanied by formation of a small amount of Fe₂O₃. These two processes observed in our experiment shed light on the deactivation mechanism of this complex catalyst because they have a negative effect on the selectivity and activity. Specifically, they are responsible for the deterioration of the redox couple, blocking the transmission of lattice oxygen, and irreversibly changing the catalyst structure. Based on the results of the combined techniques, a refined procedure has been proposed to develop a more stable and efficient selective oxidation catalyst.