SYSTEMATIC STUDY OF THE MICROSTRUCTURE OF DIFFERENT OXIDE CATALYSTS USING X-RAY POWDER DIFFRACTION AND HIGH RESOLUTION ELECTRON MICROSCOPY

G.N. Kryukova, S.V. Tsybulya

Boreskov Institute of Catalysis, pr. Lavrentieva 5, Novosibirsk 630090 Russia E-mail:kryu@catalysis.nsk.su

Keywords: microstructure, defects, oxides

Using X-ray powder diffraction (XRD) and high resolution electron microscopy (HREM) the peculiarities of microstructural arrangement of g-Fe₂O₃, vanadium doped TiO₂ and gamma-Al₂O₃, being the catalysts for the important industrial processes, were studied. XRD experiments were carried out with URD-63 diffractometer (CuKalfa-radiation, graphite monochromator on the reflected beam). HREM study was performed on JEM-2010 electron microscope (resolution limit 0.14 nm, accelerating potential 200 kV).

For g-Fe2O3 samples XRD patterns demonstrated the presence of numerous weak superstructural reflections, but at the same time, the tetragonal splitting of the main cubic reflections was not observed. HREM revealed also the well-developed superstructure in the oxide microcrystals. Data obtained allow us to consider a structure of gamma-Fe₂O₃ as superposition of two sublattices - basic spinel-like unit cell with cubic symmetry and tetragonal unit cell (P4₁2₁2 space group) formed due to the ordered vacancy distribution over eightfold (1/8, 1/8,2/3) octahedral positions.

In the system of TiO₂ (anatase) loaded with 15% wt of V₂O₅ the formation of regular interface between vanadia and titania microcrystals was found. Unit cells of titanium and vanadium oxides contact in such a way that the [100] axis is common for both phases. Since the basic structural unit of the both phases is an oxygen octahedron, stacking sequence of the oxygen positions along the [100] common line is almost perfect (misfit does not exceed 1.3%). It seems reasonable that vanadium and titanium atoms occupy octahedral sites in the interfacial area uniformly, thus forming an interface of mixed composition.

The main structural feature of g-Al₂O₃ crystallized from boehmite is a presence of the specific closed defects formed by prolongated dislocation walls those, in turn, appear as a result of the coalescence of the lattice vacancies during the formation of the structure of gamma-alumina upon calcination of the initial aluminum hydroxide at 600^oC.

This work made possible in part by grant N 97-03-33497 from RFBR.