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Characterization of mixed-metal oxides using synchrotron-based time-resolved X-ray diffraction and X-ray absorption spectroscopy

Abstract:

Experiments are described showing the utility of synchrotron-based time-resolved x-ray diffraction (TR-XRD) and x-ray absorption near-edge spectroscopy (XANES) for characterizing the physical and chemical properties of mixed-metal oxides that contain Mo and a second transition metal (Fe, Co or Ni) TR-XRD was used to study the transformations that occur during the heating of a FeMoO<inf>4</inf>/Fe<inf>2</inf>(MoO<inf>4</inf>)<inf>3</inf> mixture and the α→β phase transitions in CoMoO<inf>4</inf> and NiMoO<inf>4</inf>. The Mo L<inf>II</inf>- and O K-edges in XANES are very useful for probing the local symmetry of Mo atoms in mixed-metal oxides The results of XANES and density-functional calculations (DMol3, DFT-GGA) show large changes in the splitting of the empty Mo 4d levels when going from tetrahedral to octahedral coordinations. XANES is very useful for studying the reaction of H<inf>2</inf>, H<inf>2</inf>S and SO<inf>2</inf> with the mixed-metal oxides. Measurements at the S K-edge allow a clear identification of S, SO<inf>2</inf>, SO<inf>3</inf> or SO<inf>4</inf> on the oxide surfaces. Changes in the oxidation state of molybdenum produce substantial shifts in the position of the Mo L<inf>II</inf>- and M<inf>m</inf>-edges.