Mechanisms of carbon oxidation catalyzed by vanadium pentaoxide (V2O5) and molybdenum trioxide (MoO3) have been a subject of controversy. Two complementary in situ techniques, X-ray diffraction (XRD) and environmental scanning electron microscopy (ESEM), were used in this work to study the gasification of an activated charcoal catalyzed by the two metal oxides, their eutectic alloy and the binary mixture with the eutectic composition. Gasification experiments were carried out at relatively low temperatures (300-650°C) in an XRD cell (1 atm) and ESEM (2.2 Torr) to monitor phase transformations and morphological changes of oxide catalysts, respectively. The experimental results showed that MoO3 and V2O5 particles in contact with active carbon surfaces are reduced to oxides with lower oxidation states, e.g. MoO2 and V6O13, respectively. The reduction of MoO3 to MoO2 on the carbon surface prevents the sublimation of MoO3 which takes place readily on a quartz surface under the same conditions. The formation of V6O13, on the other hand, causes more extensive spreading of the catalyst on carbon surfaces, since V6O13 has a lower melting point than V2O5. Based on the XRD and ESEM observations, it is clear that phase transformations of metal oxides during gasification depend on their interactions with carbon surfaces. The phase transformations of the metal oxides, play, in turn, a significant role in carbon gasification, as evident from the kinetic data obtained for the catalytic gasification of the activated charcoal sample. The synergy observed between the components of the eutectic mixture is discussed, comparing the XRD and ESEM observations.
- A. Carbon
- B. gasification
- C. scanning electron microscopy (SEM)
- C. X-ray diffraction
- D. catalytic properties