X-ray diffraction and photoelectron spectroscopy studies of MoO2 as catalyst for the partial oxidation of isooctane

X-ray diffraction (XRD), X-ray photoemission (XPS) as well as ultraviolet photoemission (UPS) spectroscopy experiments on MoO2 powders were carried out to examine the bulk, the core level energies, and the electronic structure of MoO2 samples that were employed as catalysts for the partial oxidation of isooctane. Five fresh 0.5-g MoO2 samples were exposed for 0, 0.5, 9, 20, and 43 h to identical reforming environments and their spent samples were analyzed using the techniques mentioned above. Our results indicate the rapid appearance of an intermediate Mo phase with a binding energy of 228.5 eV and whose concentration progressively increases with time. The oxidation state for this new phase was graphically estimated to approximately +2.6 and assigned to the compound Mo2O3, which forms on the catalyst surface as a result of its exposure to the reforming environment. The electronic structure probed by UPS reveals two bands, one at 1.62 eV and another at 0.55 eV below the Fermi level, that decrease with the increasing time on stream. These results correlate very well with the drop in the catalytic performance of MoO2 for the partial oxidation of isooctane and with the decline in the concentration of dioxide (Mo4+) detected not only on the catalyst surface, but also in the bulk structure, as confirmed by our XRD analysis.