Nonstoichiometric molybdenum oxides with tunnel structure, doped with Group IV, V and VI metals, has become the great deal of scientific attention in the last time because of their catalytic activity in the selective oxidation of alkanes C₂-C₃. For instance, triple V-Mo-Nb oxide, which in crystalline state has the Mo₅O₁₄-like structure [1], acts as an active phase in ethane selective (amm) oxidation [2-4]. During the process of catalyst genesis XRD-amorphous phase forms in the temperature range 400-550^oC, crystalline phase forms at 550^oC.

In the present work structures of XRD-amorphous and crystalline oxides with composition Mo₁Nb_0,37V_0,30O_x are characterized by the complex of physical and chemical methods (EXAFS, HREM and XRD). Cation distribution in crystalline state is established and structure of crystalline oxide is refined by full profile analysis.

All diffraction data and EXAFS- spectra are obtained in Siberian Synchrotron Radiation Center, (Budker INP, Novosibirsk, Russia). Effect of anomalous scattering far and near the adsorption edge of niobium is used to distinguish the contribution of niobium and molybdenum to diffraction peaks. EXAFS spectra proceeding and modeling is made in VIPER software environment [5]. Rietveld refinement is performed using GSAS with EXPGUI user interface [6].

HREM investigation of the XRD-amorphous V-Mo-Nb oxide reveal 1D-ordered nanostructure with block size <10 nm and interlayer distance of 0,40 nm. . In perpendicular plane, networks are built by Me₅O₁₄-like subunits with “star shape”, which consist of MeO₇-bipiramides connected by edge with five octahedra. Cations of molybdenum and niobium are statistically distributed in the structure and located both in the centers of pentagonal bipiramides and octahedra. These Me₅O₁₄-like subunits are distributed in the distorted matrix of MeO₆ octahedra, where Me=V, Mo, Nb. Oxygen surrounding of molybdenum and niobium does not change noticeably when crystalline phase forms. The latter process is accompanied by changes in angles between polyhedra.

In the crystalline V-Mo-Nb oxide Me₅O₁₄-like subunits form tetragonal unite cell with statistical distribution of molybdenum and niobium in the structure. These subunits share corners with single octahedra, occupied either by vanadium cations or by molybdenum (niobium) ones.

Authors acknowledge the financial support of RFBR (project 07-03-00203)

References:

1. Kihlborg L., 1959, Acta Chem. Scand., 13, 95
2. Roussel M., Bouchard M. et. al., 2006, Appl. Catal. A, 308, 62 
3. Bondareva V. M., Andrushkevich T.V. et. al., 2006, React. Kinet. Catal. Lett., 87, 377
4. Merzuoki M., Taouk B., Monceaux L., et al., 1992, Stud. Surf. Sci. Catal, 72, 165.
5. Klementiev K. V., 2001, J. Phys. D: Appl. Phys., 34, 209.
6. Toby B. H., 2001, J. Appl. Cryst., 34, 210-221

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