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

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 FeMoO₄/Fe₂(MoO₄)₃ mixture and the α→β phase transitions in CoMoO₄ and NiMoO₄. The Mo L_II- 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 H₂, H₂S and SO₂ with the mixed-metal oxides. Measurements at the S K-edge allow a clear identification of S, SO₂, SO₃ or SO₄ on the oxide surfaces. Changes in the oxidation state of molybdenum produce substantial shifts in the position of the Mo L_II- and M_m-edges.