X-ray absorption (EXAFS/XANES) study of supported vanadium oxide catalysts. Structure of surface vanadium oxide species on silica and γ-alumina at a low level of vanadium loading

Abstract

X-Ray absorption spectroscopy has been employed to clarify the environmental structure around vanadium atoms in silica-supported and γ-alumina-supported vanadium oxide catalysts. Catalysts containing 2.8% of vanadium by weight were prepared with NH4VO3 and VO(acac)2 as impregnation agents. X.p.s. (V 2p3/2) of the catalysts showed that the vanadium atoms in these catalysts are pentavalent. E.s.r. signals from V4+ in the reduced catalysts indicated that paramagnetic VO4–4 and (VO)2+ ions are generated by the reduction on silica and alumina, respectively. Although the dispersion of vanadium oxide in the catalysts prepared with VO(acac)2 solution was found to be higher than those prepared with NH4VO3, the XANES and EXAFS spectra do not exhibit a significant difference for the two types of the catalyst, indicating that the dominant surface species are the same at such a low level of vanadium loading. Analysis of the XANES spectra suggests that VO4 tetrahedra are the dominant species on alumina and the vanadates on silica are square pyramidal; part of the vanadium species is present as V2O5 microcrystallites. EXAFS spectra of the catalysts and their Fourier-transforms show that VO4 units are isolated on alumina, and that the majority of vanadates on silica are polymeric. These findings by EXAFS/XANES spectroscopy indicate that VO4 on silica and VO5(or VO6) on γ-alumina, as detected by e.s.r., are only a minority species on each support even at a low level of loading of vanadium.