Microstructural Characterization of Gamma Ferric Oxides of Different Genesis

Using X-ray powder diffraction and high resolution electron microscopy the peculiarities of the structural arrangement of γ-ferric oxide microcrystals prepared with the thermal treatment of acircular magnetite and plasmochemistry were studied. Both samples were found to have a complete structural equivalence : distortion of the basic spinel-like unit cell is not observed, whereas ordered distribution of the cation vacancies on the octahedral positions results in the formation of the tetragonal superlattice. Different magnetic properties of the materials in question seem to be associated with their microstructural features, i.e. the orientation of the multiple twin domains of those every particles of both samples are composed with respect to the developed planes of oxide microcrystals

Reduction of mixed Mn Zr oxides in situ XPS and XRD studies

The reduction of the solid solutions MnxZr1−xO2−δ proceeds via two stages. During the first stage, the Mn cations incorporated into the solid solutions MnxZr1−xO2−δ undergo partial reduction. At the second stage, Mn cations segregate on the surface.</p

Cobalt Fischer-Tropsch synthesis : deactivation by oxidation?

Cobalt catalysts as used in the Fischer-Tropsch synthesis (FTS) are relatively expensive (as compared to iron) and need to have a high metal dispersion and long life to be able to offer a good balance between cost and performance. The oxidation of nano-sized metallic cobalt to cobalt oxide during Fischer-Tropsch synthesis has long been postulated as a major deactivation mechanism. However, to date there is no consistent picture. This paper presents an extensive overview of the literature on this topic of deactivation by means of oxidation for unsupported as well as silica-, alumina- and titania-supported cobalt catalysts. Furthermore, it presents results on the deactivation of an industrial Co/Al2O3 catalyst as obtained by pseudo in situ X-ray diffraction, magnetic measurements and X-ray absorption near-edge spectroscopy. These analyses were performed to study the oxidation state of spent industrial Co/Al2O3 catalyst samples withdrawn from a slurry reactor operating under realistic FTS conditions, and it was concluded that oxidation can be ruled out as a major deactivation mechanism. Finally, these data together with all relevant literature were used to create a common view on the oxidation behaviour of metallic cobalt during FTS. The apparent discrepancies in literature on the oxidation behaviour of cobalt are most likely due to the lack of direct characterisation of the cobalt oxidation state and due to the comparison of catalysts with varying cobalt crystallites sizes, compared at different reactor partial pressures of hydrogen and water (PH2O/PH2). It was shown that the oxidation of cobalt can be prevented by selecting the correct combination of the reactor partial pressures of hydrogen and water (PH2O/PH2) and the cobalt crystallite size

Cobalt Fischer-Tropsch synthesis : deactivation by oxidation?

Cobalt catalysts as used in the Fischer-Tropsch synthesis (FTS) are relatively expensive (as compared to iron) and need to have a high metal dispersion and long life to be able to offer a good balance between cost and performance. The oxidation of nano-sized metallic cobalt to cobalt oxide during Fischer-Tropsch synthesis has long been postulated as a major deactivation mechanism. However, to date there is no consistent picture. This paper presents an extensive overview of the literature on this topic of deactivation by means of oxidation for unsupported as well as silica-, alumina- and titania-supported cobalt catalysts. Furthermore, it presents results on the deactivation of an industrial Co/Al2O3 catalyst as obtained by pseudo in situ X-ray diffraction, magnetic measurements and X-ray absorption near-edge spectroscopy. These analyses were performed to study the oxidation state of spent industrial Co/Al2O3 catalyst samples withdrawn from a slurry reactor operating under realistic FTS conditions, and it was concluded that oxidation can be ruled out as a major deactivation mechanism. Finally, these data together with all relevant literature were used to create a common view on the oxidation behaviour of metallic cobalt during FTS. The apparent discrepancies in literature on the oxidation behaviour of cobalt are most likely due to the lack of direct characterisation of the cobalt oxidation state and due to the comparison of catalysts with varying cobalt crystallites sizes, compared at different reactor partial pressures of hydrogen and water (PH2O/PH2). It was shown that the oxidation of cobalt can be prevented by selecting the correct combination of the reactor partial pressures of hydrogen and water (PH2O/PH2) and the cobalt crystallite size

The structural genesis of a complex (MoVW)5O14(MoVW)_5O_{14} oxide during thermal treatments and its redox behavior at elevated temperatures

The structural genesis of a Mo0.68V0.23W0.09 oxide with Mo5O14-like structure has been examined. A precursor prepared by spray-drying of mixed aqueous metal salt solutions was calcined in air and subsequently treated in helium at different temperatures. X-ray diffraction, HRTEM, 51V MAS NMR, ESR, UV/Vis DR spectroscopy and oxygen and hydrogen adsorption measurements have been applied to monitor the preparation procedure. It was found that a structure closely related to that of Mo5O14 already appears at nano-scale level after calcination of the spray-dried precursor in air at 350°C. At this stage, the material comprises of crystalline particles less than 3 nm in size stabilized by an amorphous matrix. Further heating causes nano-structural rearrangements that lead to the formation of the final Mo0.68V0.23W0.09 oxide with phase-pure polycrystalline structure. Molybdenum and tungsten ions are hexavalent and coordinated in an octahedral environment. Furthermore, vanadium is present as V4+ and V5+ ions which partially occupy octahedral sites, whereas highly distorted trigonal pyramidal sites could be accommodated in pentagonal bipyramids of the Mo5O14 structure, however, displaced away from the center. According to the results of H2 and O2 adsorption the crystalline ternary oxide does not possess accessible micropores. Oxygen pulses at 450oC and reductive treatment with pure hydrogen at 300oC did not cause noticeable changes of the bulk structure thus indicating a remarkable structural stability of the complex MoVW oxide under redox conditions at elevated temperature