CHANGES IN THE STRUCTURE OF TIO2-SUPPORTED MOLYBDENA INDUCED BY NA-DOPING

The structure of well-dispersed molybdena moieties on the surface of a MoO3-TiO2 sample has been determined by extended X-ray absorption fine structure (EXAFS) spectroscopy. Octahedral coordination of molybdenum atoms is favored over the TiO2 support, and [MoO6] units join edges to form molybdenum rows that are similar to those found along the c-axis direction in bulk MoO3. The changes induced in this structure when the support is doped with different amounts (1, 3% w/w) of sodium have been studied by X-ray diffraction, temperature programmed reduction, and infrared and EXAFS spectroscopies. The addition of Na changes the coordination of Mo6+ ions from octahedral to tetrahedral, and simultaneously inhibits their reducibility. Mo-Mo bonds found in the Na-free sample are broken, and at low Na content isolated MoO42- units and ill-crystallized Mo2O72- chains are formed. Increasing the amount of the Na dopant increases the fraction of Mo atoms in tetrahedral coordination and crystalline Na2Mo2O7 coexists with dispersed phases in the 3% Na w/w sample. Also, an increase in the calcination temperature of the samples leads to the formation of crystalline Na-Mo-O compounds over the TiO2 surface. (C) 1994 Academic Press, Inc.