Structural Characterization of Molybdenum Oxide Supported on Zirconia

X-ray diffraction and X-ray absorption and Raman spectroscopies were used to determine the structure of dispersed and crystalline structures in MoO x /ZrO 2 catalysts useful in the oxidative dehydrogenation of alkanes. The MoO x surface density on ZrO 2 was varied over a wide range (0.35−50 Mo/nm 2 ) by changing the Mo content (1−44 wt % MoO 3 ) and the treatment temperature (393−973 K). Raman spectra showed that MoO x /ZrO 2 samples with low surface density (<5 Mo/nm 2 ) treated at temperatures below 873 K initially contain isolated tetrahedral MoO x species; these species oligomerize to form two-dimensional structures with bridging Mo O Mo bonds as the surface density increased to values typical for a polymolybdate monolayer (∼5 Mo/nm 2 ). An increase in surface density led to a shift in the ν(Mo O) Raman band to higher frequencies and to changes in the near-edge X-ray absorption spectra. Both of these are consistent with the growth of these polymolybdate domains with increasing Mo surface density, as also suggested by the concurrent decrease in the UV−visible absorption energy. Thermal treatment at 973 K led to the dissociation of Mo O Mo bonds and to the formation of tetragonal−pyramidal O MoO 4 species. For MoO x /ZrO 2 samples with Mo surface densities greater than 5 Mo/nm 2, MoO 3 and Zr(MoO 4 ) 2 were detected by Raman and for larger crystallites also by X-ray diffraction. Treatment of these samples in air at 723 K led to the predominant formation of MoO 3, while higher temperatures led to a solid-state reaction between MoO 3 and ZrO 2 to form Zr(MoO 4 ) 2 . This structural evolution was confirmed by the evolution of pre-edge and near edge features in the X-ray absorption spectra of these high surface density samples. Zr(MoO 4 ) 2 contains Mo 6+ cations in a distorted tetrahedral coordination with one oxygen bonded only to molybdenum and the other three shared by Zr and Mo atoms. The Raman bands observed for Zr(MoO 4 ) 2 at 750, 945, and 1003 cm -1 were assigned to ν sym (O Mo O), ν asym (O Mo O), and ν(Mo O) vibrational modes, respectively, based on the analysis of the Raman bands observed after 18 O 2 exchange with lattice oxygen atoms. Bridging O atoms in Mo O Mo species exchanged with gas phase 18 O 2 more readily than terminal Mo O species.