Structural Characterization of Molybdenum Oxide Supported on Zirconia

Abstract

X-ray diffraction and X-ray absorption and Raman spectroscopies were used to determine the structure of dispersed and crystalline structures in MoOx/ZrO2 catalysts useful in the oxidative dehydrogenation of alkanes. The MoOx surface density on ZrO2 was varied over a wide range (0.35−50 Mo/nm2) by changing the Mo content (1−44 wt % MoO3) and the treatment temperature (393−973 K). Raman spectra showed that MoOx/ZrO2 samples with low surface density (<5 Mo/nm2) treated at temperatures below 873 K initially contain isolated tetrahedral MoOx species; these species oligomerize to form two-dimensional structures with bridging MoOMo bonds as the surface density increased to values typical for a polymolybdate monolayer (∼5 Mo/nm2). An increase in surface density led to a shift in the ν(MoO) 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 MoOMo bonds and to the formation of tetragonal−pyramidal OMoO4 species. For MoOx/ZrO2 samples with Mo surface densities greater than 5 Mo/nm2, MoO3 and Zr(MoO4)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 MoO3, while higher temperatures led to a solid-state reaction between MoO3 and ZrO2 to form Zr(MoO4)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(MoO4)2 contains Mo6+ 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(MoO4)2 at 750, 945, and 1003 cm-1 were assigned to νsym(OMoO), νasym(OMoO), and ν(MoO) vibrational modes, respectively, based on the analysis of the Raman bands observed after 18O2 exchange with lattice oxygen atoms. Bridging O atoms in MoOMo species exchanged with gas phase 18O2 more readily than terminal MoO species.