Experimental Hyperfine Characterization of Adsorbed Molybdates on Oxide Surfaces

Abstract

The dispersion of metal oxides on supports is a problem currently being investigated in supported catalysts. On the other hand, a nuclear spectroscopy technique, namely time differential perturbed angular correlation (TDPAC), has been used in extensive investigations on Mo-based catalysts. This technique, through the measurement of the local electric field gradient (EFG) at radioactive probes sites, can give information about the characteristics (coordination, symmetry, distortions, etc.) of the different environments ofthe probes, their concentration and modifications related to in situ conditions (temperature, atmosphere, pressure, etc.) TDPAC technique was used in the present work to investigate the dispersion of molybdenum oxides on Al 2 O 3 , TiO2/Al 2 O 3 , and SiO 2 . The 99 Mo 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 concentrations in the samples were performed. The corresponding results allowed us through the fitted hyperfine parameters to find out the dispersion, type of adsorbed Mo-O species, and their relative abundances. For Mo concentrations below the theoretical monolayer we were able to find bulklike MoO 3 and two-dimensional-like Mo oxides. Previous results obtained with TDPAC and calculations based on a point charge model suggest different MoO 3 crystallites sites for each support. The structures of the monolayer type molybdena are probably different in each catalyst.