On the Role of Vanadia Species for VO_x/SiO_2 in the Selective Oxidation of Methane

摘要

Various VO_x/SiO_2 catalysts were prepared by the methods of physical mixing, conventional wetness impregnation and ultrasonication-assistant impregnation. The catalysts were characterized by XRD, UV-Vis DRS, Raman, TPR, ESR and TPSR techniques and the nature of the vanadium species were correlated to their catalytic performance in the reaction of direct conversion of methane to formaldehyde. It is concluded that highly dispersed monomeric and low oligomeric vanadia species are formed on the sample prepared with both traditional wetness impregnation method and ultrasonication-assistant impregnation, whereas in the latter case, the amount of oligomeric vanadia species is much smaller. The V_2O_5 microcrystallines are the dominant species on the material prepared by physical mixing method. During the selective oxidation of methane, V~V species are reduced to V~(IV) paramagnetic species and both microcrystalline V_2O_5 species and oligomeric vanadia species are found to further disperse and transform into tetrahedral vanadia species. Based on the results of UV Raman spectroscopy and TPSR, CO_2 is suggested to be formed via two different routes, in which one is from the sequence reaction of CH_4 → HCHO → CO → CO_2 over monomeric vanadia species, and the other is from the direct oxidation of methane to CO_2 over oligomeric vanadia species. Oligomeric vanadia species is more active than monomeric vanadia species for methane activation.