Variability in the structure of supported MoO3 catalysts:: Studies using Raman and X-ray absorption spectroscopy with ab initio calculations

Molybdenum oxide catalysts (1 wt %) supported on SiO2 (L-90, 95 m(2)/g), SiO2 (EH-5, 350 m(2)/g), Al2O3 (96 m(2)/g), and TiO2 (52 m(2)/g) were characterized using Raman spectroscopy and near-edge and extended X-ray absorption fine structure (NEXAFS and EXAFS, respectively) spectroscopies. The structural possibilities (tetrahedral and distorted octahedral) for the Mo active center were explored using ab initio calculations through the Hartree-Fock method using a 3-21G(d) basis set. The Raman vibrational frequencies and bond lengths obtained from EXAFS were compared to ab initio calculations to arrive at the likely structures for the isolated Mo centers on each support. The studies revealed a mixture of tetrahedral Mo sites along with crystalline MoO3 for the MoO3/SiO2 (L-90) catalyst. The Mo sites for the MoO3/SiO2 (EH-5) catalyst were found to be a mixture of isolated, tetrahedral, and distorted octahedral sites. The MoO3/Al2O3 catalyst was found to have isolated, tetrahedral sites. Finally, the MoO3/TiO2 catalyst was found to have distorted octahedral Mo active centers. Complementary information from Raman, NEXAFS, and EXAFS spectra and ab initio calculations were required to arrive at the structural assignments for the Mo centers in the present study. Importantly, however, the calculations indicate the possible existence of other stable geometries and help explain the many diverging conclusions in the literature concerning the structure of supported molybdenum oxide catalysts.