Solid State Reaction Of CeO₂-Supported V₂O₅, MoO₃ Studied By Raman Spectroscopy

CeO2 is known to be an efficient promoter or a support. Its wide application in catalysis is not only related to its oxygen storage/release property, but also to the ability of improving dispersion of the activity phase. Therefore, CeO2 has attracted much attention in recent years. In this work, CeO2-supported V2O5, MoO3 were prepared. Raman spectroscopy, combined with X-ray diffraction (XRD), ultraviolet visible diffuse reflectance spectrum (UV-Vis DRS) techniques were used to analyze the solid-state reaction between V2O5, MoO3 and CeO2, as well as the effects of component content and treatments on the state of the oxide species. The detailed results are as follows:1. Raman spectroscopic study on the solid-state reaction of V2O5/CeO2 catalystRaman spectroscopy with 514 nm and 325 run excitation laser lines, X-ray diffraction (XRD), UV-visible diffuse reflectance spectroscopy (UV-Vis DRS) and N2 adsorption were used to study the solid-state reaction between V2O5 and CeO2. It was found that the vanadium oxidation species reacted with ceria and formed a CeVO4 phase on the surface of the sample that was calcined at 300℃, and the reaction was promoted at higher temperature. In addition, the absorption at 325 nm was stronger than at 514 nm for the sample and, therefore,325 nm Raman spectroscopy was more sensitive to surface information than 514 nm Raman spectroscopy. Calcination at low temperature led to unreacted V2O5 in the pores of CeO2 but it was hindered by CeVO4 on the sample surface. Therefore, the Raman band of V2O5 was present when using the 514 nm excitation laser line but it was absent when using the 325 nm excitation laser line.2. Surface phase composition of MoO3-CeO2 catalyst studied by Raman spectroscopyThe effects of MoO3 content and calcination temperature on the solid state reaction of MoO3-CeO2 complex oxide were studied by Raman spectroscopy and XRD. Depending on the MoO3 content, molybdenum oxide species were found to exist as the highly dispersed molybdate, crystalline Ce8Mo12O49 and Ce2Mo4O15 for the MoO3-CeO2 complex oxide calcined at 500℃resulting from the solid-state reaction between MoO3 and CeO2. Such a solid-state reaction was promoted at higher temperature. However, Ce2Mo3O13 was observed for 20%MoO3-CeO2 sample under O2 atmosphere, which indicated that Ce2Mo3O13 was formed firstly and then it was transformed into CegMo12O49 at higher temperature during the solid-state reaction.3. The effect of preparation method on the surface phase composition of MoO3-CeO25%,40% and 60% MoO3-CeO2 samples were prepared by mechanical mixing and wet impregnation method. It was found that the highly dispersed molybdate, crystalline Ce8Mo12O49 and Ce2Mo4O15 formed at lower calcination temperature for the samples prepared by wet impregnation method compared with the samples prepared by mechanical mixing method. This demonstrated that the solid-state reaction occurred more easyly between MoO3 and CeO2 for the samples prepared by wet impregnation method. However, the in situ Raman spectra of the samples prepared by two methods were analogous under O2 atmosphere, which could be due to the promotion of O2 during the solid state reaction.