Mechanism of the oxidation-reduction of the MoVSbNbO catalyst: in operando X-ray absorption spectroscopy and electrical conductivity measurements.

The mechanism of the oxidation-reduction of the MoVSbNbO catalyst has been studied in dynamic conditions using X-ray absorption spectroscopy (XAS) and electrical conductivity measurements. XAS at Sb L1- and V/Mo K-edges permitted a better understanding of the chemical processes taking place in the M1 phase of the MoVSbNbO catalyst at different temperatures and atmosphere compositions. The reduction of antimony was already observed during the annealing of the M1 phase in He at 100 degrees C, which might be explained by the presence of hydrogen in the bronze-like structure of the M1 phase. Under operando conditions at 380 degrees C, we have found that Sb and V change their oxidation states depending on the C3H8/O2 ratio in the atmosphere. These changes occur simultaneously and with the same kinetics. Under the same conditions, variations in the oxidation state of Mo were not observed. These results prove that different types of oxygen (from the hexagonal channels and from the MO6 octahedral network) must be involved in the catalytic process although their relative contributions are different. It was found that the electrical conductance of the M1 phase correlates with the oxidation states of Sb and V and the concentration of oxygen vacancies.