Laser Raman and XPS analyses of molybdenum-rare earth oxide catalysts

Abstract

Laser Raman spectroscopy (LRS) and x-ray photoelectron spectroscopy (XPS), combined with x-ray diffraction (XRD), have been used to characterize the phases in Mo–RE–O (RE = Sm, Tb, Yb) systems. Two clearly differentiated composition regions were observed, namely the RE-rich region (x < 0.5–0.6, where x represents the Mo/(Mo + RE) atomic ratio), in which the RE oxide was always present as a separate phase, and the Mo-rich region (x > 0.6) in which MoO3 appeared as a separate phase. The broadening of the Raman bands in the RE-rich region is indicative of the presence of ill-defined structures. However, as the RE oxide content increased, crystalline molybdates with decreasing Mo/RE ratios were found in each system. XPS surface composition was roughly similar to that of the bulk for the Mo–Sm–O and Mo–Yb–O series, while relevance of MoO3 and molybdate phases in the Mo-rich composition regions is discussed in the context of allylic oxidation of light olefins.