Alkaline-metal doped MoO3/TiO2 systems: Structure of supported molybdates

The structure of molybdenum oxocompounds formed in TiO2-supported MoO3 systems doped with alkaline metals have been studied by X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) and X-ray absorption (XAS) spectroscopies. XRD, FT-IR, and XAS data give a detailed picture of the structure of molybdenum oxocompounds formed in these systems. Mo atoms are mostly octahedrally coordinated in an undoped TiO2 support, as a dispersed MoO3 phase. However, a high fraction of molybdenum cations are in tetrahedral coordination after the addition of Li, Na, or K as dopants. Thus, only MoO42- species exist in the 1 wt% Li-doped sample in the form of crystalline Li(2)MoO4, while tetrahedrally and octahedrally coordinated molybdenum form dispersed Mo2O72- chains in the 1 wt% Na- and K-doped systems. Octahedral polymolybdates, Rb2Mo3O10. H2O and Rb2Mo4O13, whose structures are based in [MoO6] units forming chains, are the species detected in the 1 wt% Rb-doped sample. Increasing the molybdenum content in K-doped samples favors formation of octahedral species, although in this case the structure of the supported phase is affected by the impregnation steps during the preparation of the sample. (C) 1996 Academic Press, Inc.