Investigations of alkali doped Fe2O3--V2O5 catalysts by transmission and conversion electron Mössbauer spectroscopy

Fe₂O₃--V₂O₅ catalysts doped with Cs₂SO₄ (molar ratio: V:Fe:Cs=1:0.74:0.06) were found to be a rather inhomogeneous mixture of various crystalline and amorphous iron vanadate phases. After calcination in air the catalyst was divided into three different parts which were analyzed separately revealing the formation of FeVO₄ in the top and bottom fraction of the crucible and Fe₂V₄O₁₃ in the middle fraction. As compared to the well crystallized FeVO₄ reference sample, the quadrupole splittings of FeVO₄ in the Cs-doped catalysts were larger pointing to more distorted iron sites which were assumed to be responsible for high catalytic selectivities. In contrast, the quadrupole splittings of FeVO₄ in the less selective K- and Rb-doped Fe₂O₃--V₂O₅ catalysts were smaller. Additional components in the bottom fraction were also α-Fe₂O₃ and Fe_1-xS. As indicated by the CEMS spectra the latter is located preferentially on the surface of the catalyst particles. This revised version was published online in August 2006 with corrections to the Cover Date.