Defect structure and transport properties of Fe2(MoO4)3 doped with Bi

Abstract

Measurements are presented of X-ray diffraction, differential scanning calorimetry and electrical conductivity performed in the temperature range 100–600 °C and in the oxygen partial-pressure region 1–10⁵ Pa on unpromoted and Bi-promoted Fe₂(MoO₄)₃ catalyst samples and on a pure iron molybate specimen as a reference. All the catalysts investigated are monophasic in nature, and no significant differences in the unit-cell parameters of the monoclinic Fe₂(MoO₄)₃ have been found. Results from differential scanning calorimetry show that the kinetics of the reversible monoclinic–orthorhombic phase transformation of Fe₂(MoO₄)₃ is complex in FeMo–O catalysts as opposed to pure iron molybdate. From electrical measurements it was concluded that (i) all the samples investigated present a majority-defect pair consisting of (V^2·₀, e′) at T > T_c(T_c≈ 500 °C), where Fe₂(MoO₄)₃ exists in the orthorhombic modification, (ii) FeMo–O catalysts present the same type of defect pair at T_v < T < T_c(T_v≈ 350 °C) and an extrinsic electronic conduction mechanism characterized by the presence of a level close to the band at T < T_v over the entire oxygen partial-pressure range investigated (this picture does not conform completely to that previously observed for pure iron molybdate) and (iii) the behaviour of FeMoBi–0.0064 is very similar to that of unpromoted FeMo–O catalysts, whereas that of FeMoBi–0.032 shows some relevant differences. The defect structure of the various catalysts investigated and their transport properties are discussed and compared with each other. The presence of Mo ions in the interstitial sites of the Fe₂(MoO₄)₃ structure, and of Bi ions in the same sites up to saturation, is proposed in the case of FeMo–O and FeMoBi catalysts, respectively.