Time-Resolved XAS Investigation of the Reduction/Oxidation of MoO3−x

Abstract

The bulk structure of molybdenum trioxide (MoO3−x) under reductive and oxidative 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). Reduction and reoxidation of MoO3−x at 773 K with H2 and O2 (100%, 1 atm), respectively, proceeded rapidly and a prompt response of the entire MoO3−x bulk structure to changes in the gas composition was observed. The Mo valence of the oxidized and the reduced phase was determined to be ∼6.0 and ∼5.4, respectively. Principal component analysis of Mo K-near-edge (XANES) spectra of reduction—oxidation cycles identified two constituent phases, namely MoO2 and MoO3. No intermediate was required to describe the experimental XANES spectra, indicating a rapid transition from the MoO3 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 sheer planes. This points toward a fairly disordered MoO3−x obtained from AHM which might be responsible for the observed high reaction rate. Furthermore, 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 alteration in the degree of distortion of MoO6 octahedra. Hydrogen pulses were found to perturb the periodic oscillations with the oscillation frequency exhibiting a linear dependence on the Mo valence.