Time-resolved XAS investigation of the reduction/oxidation of MoO3-x

The bulk structure of molybdenum trioxide (MoO3-x) under reductive and oxid ative reaction conditions was investigated in situ with time-resolved X-ray absorption spectroscopy (XAS). Alterations in the electronic and geometric structure of the molybdenum oxide systems were measured by in situ XAS at the Mo K-edge utilizing an energy-dispersive spectrometer (ESRF, ID24). Red uction and reoxidation of MoO3-x at 773 K with H-2 and O-2 (100%, 1 atm), r espectively, proceeded rapidly and a prompt response of the entire MoO3-x b ulk structure to changes in the gas composition was observed. The Mo valenc e of the oxidized and the reduced phase was determined to be similar to 6.0 and similar to 5.4, respectively. Principal component analysis of Mo K-nea r-edge (XANES) spectra of reduction-oxidation cycles identified two constit uent phases, namely MoO2 and MoO3. No intermediate was required to describe the experimental XANES spectra, indicating a rapid transition from the MoO 3 to the MoO2 structure without the detection of intermediates. Absorption edge fine structure analysis revealed the presence of edge-shared octahedra in the molybdenum trioxide structure presumably due to the occurrence of s heer planes. This points toward a fairly disordered MoO3-x obtained from AH M which might be responsible for the observed high reaction rate. Furthermo re, intriguing dynamic behavior in propene oxidation on MoO3-x at 773 K was observed. Periodic oscillations were found in the Mo 1s-4d pre-edge peak. The observed dynamic behavior appears to be related to an oscillating alter ation in the degree of distortion of MoO6 octahedra. Hydrogen pulses were f ound to perturb the periodic oscillations with the oscillation frequency ex hibiting a linear dependence on the Mo valence. (C) 2000 Academic Press.