EXPERIMENTAL HYPERFINE CHARACTERIZATION OF ADSORBED MOLYBDATES ON OXIDE SURFACES

The dispersion of metal oxides on supports is a problem currently bein g investigated in supported catalysts. On the other hand, a nuclear sp ectroscopy technique, namely time differential perturbed angular corre lation (TDPAC), has been used in extensive investigations on Mo-based catalysts. This technique, through the measurement of the Ideal electr ic field gradient (EFG) at radioactive probes sites, can give informat ion about the characteristics (coordination, symmetry, distortions, et c.) of the different environments of the probes, their concentration a nd modifications related to in situ conditions (temperature, atmospher e, pressure, etc.) TDPAC technique was used in the present work to inv estigate the dispersion of molybdenum oxides on Al2O3, TiO2/Al2O3, and SiO2. The Mo-99 obtained by neutron irradiation of natural molybdenum present in the samples was used as TDPAC probe. Measurements at room temperature and higher ones as well as with different Mo concentration s in the samples were performed. The corresponding results allowed us through the fitted hyperfine parameters to find out the dispersion, ty pe of adsorbed Mo-O species, and their relative abundances. For Mo con centrations below the theoretical monolayer we were able to find bulkl ike MoO3 and two-dimensional-like Mo oxides. Previous results obtained with TDPAC and calculations based on a point charge model suggest dif ferent MoO3 crystallites sites for each support. The structures of the monolayer type molybdena are probably different in each catalyst.