Structural dynamics of an iron molybdate catalyst under redox cycling conditions studied with in situ multi edge XAS and XRD

The structural dynamics and phase transformations of an iron molybdate catalyst with excess molybdenum trioxide (Mo/Fe = 2.0) were studied during redox cycling of the catalyst using in situ multi-edge X-ray absorption spectroscopy (XAS) at the Mo K-edge (transmission mode) and Fe K-edge (fluorescence mode) in combination with X-ray diffraction (XRD). X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) analysis showed that heating under reducing conditions with methanol up to 400 °C produced MoO₂ and FeMoO₄. Linear combination fitting (LCF) analysis showed that iron was reduced completely, while molybdenum remained partly oxidized (60% as Mo(vi)). Complementary in situ XRD also supported the phase transformation due to reduction of Fe₂(MoO₄)₃ and MoO₃ to FeMoO₄ and MoO₂. Subsequent heating under oxidative conditions from 200 to 500 °C transformed the catalyst into its initial state via Fe₂O₃ and extra MoO₃ as intermediate phases. This underlines the segregation and iron enrichment during redox cycling. MoO₃ volatilization, observed under industrial reaction conditions of a methanol and oxygen containing atmosphere, causes this segregation to be permanent. Complete regeneration could only be achieved at 500 °C, which is significantly higher than industrial reaction temperatures. Overall, multi edge in situ XAS along with complementary XRD was found to be an ideal tool for tracing the different amorphous and crystalline phases present during redox cycling of the catalyst.