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 2p_3/2) 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 VO^4–₄ 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.