Model Phases for Selective Oxidation and Ammoxidation

Model Phases for Selective Oxidation and Ammoxidation III. Metastable Ion Distributions in Scheelite-Type Bi — Fe — Mo Oxides

“…Bi3FeMo2012-(3/l/2)-samples were prepared, as described in [5] and calcined at 450" C. They showed a tetragonal scheelite lattice, with Fe3 + mainly distributed on 4a-sites, generating a Mössbauer-doublet and a typical ESRsignal [5]. Calcination at 560°C and above induces Fe -Mo ion ordering on 4a-sites ( -Fe -Mo -Mo -), accompanied by the apparition of superstructure diffraction lines, monoclinic distortion and tripling of the cell parameter a (am 3at) [5,8].…”

“…Calcination at 560°C and above induces Fe -Mo ion ordering on 4a-sites ( -Fe -Mo -Mo -), accompanied by the apparition of superstructure diffraction lines, monoclinic distortion and tripling of the cell parameter a (am 3at) [5,8]. However, attempts to increase the Fe-content of the scheelite lattice by preparing Bi3Fe3Mo3018-(l/l/l)-samples have consistently failed, segregation of ferric molybdate Fe2(Mo04)3 (occasionally also a-Fe203) being noticed in all samples calcined at or above 450°C [2,5]. The question arises if these segregations are simply due to compositional inhomogeneity of the sample or reflect the relative stability of different ion distributions on the scheelite sublattices.…”

“…However, attempts to increase the Fe-content of the scheelite lattice by preparing Bi3Fe3Mo3018-(l/l/l)-samples have consistently failed, segregation of ferric molybdate Fe2(Mo04)3 (occasionally also a-Fe203) being noticed in all samples calcined at or above 450°C [2,5]. An additional conceptual difficulty is that the proportion of singlet (octahedrally coordinated) Fe3+ (as determined by MS) is generally much higher than that calculated from the amount of segregated Fe2(Mo04)3, as determined by X-ray diffraction (XRD) [5]. An additional conceptual difficulty is that the proportion of singlet (octahedrally coordinated) Fe3+ (as determined by MS) is generally much higher than that calculated from the amount of segregated Fe2(Mo04)3, as determined by X-ray diffraction (XRD) [5].…”

“…The experimental details and basic results will be published elsewhere [5]. These models should be able to explain simultaneously the XRD, MS and ESR results obtained for 3/1/2-and 1/1/1-compositions, calcined at different temperatures.…”

“…The relatively open scheelite lattice (tetragonal or with a slight monoclinic distortion [1,2]) can easily accommodate oxygen vacancies (caused by a disbalancing of cation valences) or interstitial cations. It was proven [5] that Bi3Fe3Mo3018 (1/1/1) is not a pure crystalline phase, but a mixture of Bi3FeMo2012 (3/1/2) and Fe2(Mo04)3. Other attempts to prove structural differences between them by IR, Raman and ESCA spectroscopies used samples prepared in several batches and subjected to different thermal and oxidation-reduction treatments.…”