UV Raman Characterization Of Supported Vanadium Oxide Catalysts And Iron Substituted Zeolites

UV Raman spectroscopy has drawn extensive attention for its high sensitivity, avoiding of fluorescence interference and the UV resonance Raman effect. In this thesis, we have characterized the supported vanadium oxides and iron substituted ZSM-5 by using UV Raman spectroscopy.The structure changes of surface vanadium species due to different vanadium loadings and different oxide supports were studied by UV Raman spectroscopy with the excitation line at 325 nm together with UV-vis-DRS. The results show that even under ambient conditions the out-of-plane bending and stretching modes of surface V-O-V bridging bonds could be selectively detected owing to the UV resonance Raman effect. The so-called monolayer content of vanadium oxide on the surface of oxide supports can be approximately measured based on the UV Raman spectra of V2O5/Y-Al2O3 with different vanadium loadings. It is also observed that the tetrahedral vanadate species tend to form octahedral structures with higher polymerization degree with the increasing of vanadium loading or the decreasing of surface pH value at the point of zero change (PZC).Iron-substituted zeolites (Fe-ZSM-5) of both different iron contents and crystallizing stages were synthesized by hydrothermal method and characterized by UV Raman spectroscopy with the excitation line at 244 nm, XRD and UV-vis-DRS.Tetrahedral framework iron species are confirmed by the d-d transition bands in the UV-vis-DRS spectra but can no longer be detected after the Fe-ZSM-5 is calcined at 500. This is partly due to the dislodgement of the framework iron species into extraframework positions by the calcination. However, the residence of tetrahedral iron can still be evidently identified by UV Raman spectroscopy because of the resonance Raman enhancement.UV Raman results also indicate that the tetrahedral iron species have been connected with the silica anions since the beginning of the hydrothermal crystallization. Fe(OSi(OH)3)4-n(OH)n species, which partly demonstrate framework iron characters in the UV Raman spectra, are formed as the original framework units. Subsequently, these connected species gradually form the long-distance ordered zeolites crystallites.